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Sonic Hedgehog–Regulated Oligodendrocyte Lineage Genes Encoding bHLH Proteins in the Mammalian Central Nervous System
Abstract
During development, basic helix-loop-helix (bHLH) proteins regulate formation of neurons from multipotent progenitor cells. However, bHLH factors linked to gliogenesis have not been described. We have isolated a pair of oligodendrocyte lineage genes (Olg-1 and Olg-2) that encode bHLH proteins and are tightly associated with development of oligodendrocytes in the vertebrate central nervous system (CNS). Ectopic expression of Olg-1 in rat cortical progenitor cell cultures promotes formation of oligodendrocyte precursors. In developing mouse embryos, Olg gene expression overlaps but precedes the earliest known markers of the oligodendrocyte lineage. Olg genes are expressed at the telencephalon-diencephalon border and adjacent to the floor plate, a source of the secreted signaling molecule Sonic hedgehog (Shh). Gain- and loss-of-function analyses in transgenic mice demonstrate that Shh is both necessary and sufficient for Olg gene expression in vivo.
... The class II basic-helix-loop-helix (bHLH) TFs Ascl1 and Olig2 are regulators early in oligodendrogenesis as knockouts show robust defects in the embryonic generation of OPCs in brain and spinal cord [8][9][10][11][12][13][14][15]. In addition, stage specific conditional knockouts and in vitro cultures revealed that these genes as well as Olig1 play roles at later stages in postnatal OPC generation and/or OL differentiation [11,[16][17][18][19][20][21]. ...
... To assess for the generation of embryonic OPCs from the ventral telencephalon, we stained for the OPC markers Olig2, Pdgfrα, and Sox10 [8,9,[55][56][57][58][59]. Olig2 is first expressed in the VZ progenitors in the MGE and remains expressed in OPCs of the parenchyma [8,9,56]. ...
... To assess for the generation of embryonic OPCs from the ventral telencephalon, we stained for the OPC markers Olig2, Pdgfrα, and Sox10 [8,9,[55][56][57][58][59]. Olig2 is first expressed in the VZ progenitors in the MGE and remains expressed in OPCs of the parenchyma [8,9,56]. Pdgfrα and Sox10 are expressed in OPCs and are both downstream of Olig2 [10,58,60,61]. ...
Background
E-proteins encoded by Tcf3, Tcf4, and Tcf12 are class I basic helix-loop-helix (bHLH) transcription factors (TFs) that are thought to be widely expressed during development. However, their function in the developing brain, specifically in the telencephalon remains an active area of research. Our study examines for the first time if combined loss of two E-proteins (Tcf3 and Tcf12) influence distinct cell fates and oligodendrocyte development in the mouse telencephalon.
Methods
We generated Tcf3/12 double conditional knockouts (dcKOs) using Olig2Cre/+ or Olig1Cre/+ to overcome compensatory mechanisms between E-proteins and to understand the specific requirement for Tcf3 and Tcf12 in the ventral telencephalon and during oligodendrogenesis. We utilized a combination of in situ hybridization, immunohistochemistry, and immunofluorescence to address development of the telencephalon and oligodendrogenesis at embryonic and postnatal stages in Tcf3/12 dcKOs.
Results
We show that the E-proteins Tcf3 and Tcf12 are expressed in progenitors of the embryonic telencephalon and throughout the oligodendrocyte lineage in the postnatal brain. Tcf3/12 dcKOs showed transient defects in progenitor cells with an enlarged medial ganglionic eminence (MGE) region which correlated with reduced generation of embryonic oligodendrocyte progenitor cells (OPCs) and increased expression of MGE interneuron genes. Postnatal Tcf3/12 dcKOs showed a recovery of OPCs but displayed a sustained reduction in mature oligodendrocytes (OLs). Interestingly, Tcf4 remained expressed in the dcKOs suggesting that it cannot compensate for the loss of Tcf3 and Tcf12. Generation of Tcf3/12 dcKOs with Olig1Cre/+ avoided the MGE morphology defect caused by Olig2Cre/+ but dcKOs still exhibited reduced embryonic OPCs and subsequent reduction in postnatal OLs.
Conclusion
Our data reveal that Tcf3 and Tcf12 play a role in controlling OPC versus cortical interneuron cell fate decisions in MGE progenitors in addition to playing roles in the generation of embryonic OPCs and differentiation of postnatal OLs in the oligodendrocyte lineage.
... Modern studies involving the analysis of transcription factors that regulate the processes of cell differentiation and proliferation in the CNS have proven to be of exceptional importance. The first identified transcription factors that modulate the development of OLs are Olig1 and Olig2 [96,97]. Another transcription factor that determines oligodendroglial identity is Sox10. ...
... Olig2 and Sox10 are expressed at every stage of OL development, including mature forms [96,97,[103][104][105][106]. Olig2 is expressed in neuroepithelial cells even before their differentiation into OL cell lineage [104,107]. ...
... Аntigenic properties of OPCs, pre-OLs, mature OLs, and myelinating OLs as well as functions of OL cell lineage. The list of markers specific to each stage is limited to those referenced in the current review[96,108,112,114,118,123,124]. ...
There is a growing interest in glial cells in the central nervous system due to their important role in maintaining brain homeostasis under physiological conditions and after injury. A significant amount of evidence has been accumulated regarding their capacity to exert either pro-inflammatory or anti-inflammatory effects under different pathological conditions. In combination with their proliferative potential, they contribute not only to the limitation of brain damage and tissue remodeling but also to neuronal repair and synaptic recovery. Moreover, reactive glial cells can modulate the processes of neurogenesis, neuronal differentiation, and migration of neurons in the existing neural circuits in the adult brain. By discovering precise signals within specific niches, the regulation of sequential processes in adult neurogenesis holds the potential to unlock strategies that can stimulate the generation of functional neurons, whether in response to injury or as a means of addressing degenerative neurological conditions. Cerebral ischemic stroke, a condition falling within the realm of acute vascular disorders affecting the circulation in the brain, stands as a prominent global cause of disability and mortality. Extensive investigations into glial plasticity and their intricate interactions with other cells in the central nervous system have predominantly relied on studies conducted on experimental animals, including rodents and primates. However, valuable insights have also been gleaned from in vivo studies involving poststroke patients, utilizing highly specialized imaging techniques. Following the attempts to map brain cells, the role of various transcription factors in modulating gene expression in response to cerebral ischemia is gaining increasing popularity. Although the results obtained thus far remain incomplete and occasionally ambiguous, they serve as a solid foundation for the development of strategies aimed at influencing the recovery process after ischemic brain injury.
... Oligodendrogliale Vorläuferzellen (OVZ) werden hauptsächlich von den NEZ der pMN-Domäne gebildet und ein kleinerer Teil entspringt den dorsalen Domänen dP3-5 (Abbildung 1-2) (Lu et al., 2000;Kessaris et al., 2001;Lu et al., 2002;Zhou und Anderson, 2002;Cai et al., 2005;Vallstedt et al., 2005). ...
... Astrozyten können aus allen Vorläuferdomänen der VZ generiert werden (Abbildung 1-2) (Bignami und Dahl, 1974;Zhou und Anderson, 2002;Malatesta et al., 2003;Pringle et al., 2003;Masahira et al., 2006). Astrogliale und oligodendrogliale Zellspezifizierungen sind während der Embryonalentwicklung räumlich, und zum Teil auch zeitlich, voneinander getrennt (Rowitch, 2004 (Lu et al., 2000;Mizuguchi et al., 2001;Rowitch et al., 2002;Zhou und Anderson, 2002;S. K. Lee et al., 2005). ...
... Olig2 und Sox10 werden in diesem Zelltyp durchgehend exprimiert und die Proteine dienen als oligodendrogliale Marker (Lu et al., 2000;Zhou et al., 2000;Stolt et al., 2002). Darüber hinaus wird ...
Für den Übergang von der Neurogenese zur Gliogenese, der im Rückenmark der Maus am Embryonaltag 12 stattfindet, ist die Präsenz des Transkriptionsfaktors Sox9 unerlässlich. Die neuroepithelialen Zellen entlang der Ventrikularzone teilen sich weiterhin asymmetrisch und bringen nun Tochterzellen hervor, die anstelle von neuronalen Zellen nun zu Gliazellen spezifizieren. In den Zellen, die Olig2 zusammen mit Sox9 exprimieren, kommt es zur Induktion der Sox10-Expression und in Folge zur Spezifizierung dieser Zellen zu Oligodendroglia. Andere Zellen entlang der Ventrikularzone exprimieren Sox9 zusammen mit Nfia und entwickeln sich zu Astroglia. Eine Deletion von Sox9 im embryonalen Rückenmark führt zu einer stark verzögerten Gliogenese während die Neurogenese verlängert wird. Die Oligodendroglia-Population ist vorübergehend reduziert, wohingegen die Astroglia-Zahlen nachhaltig dezimiert sind. In der vorliegenden Arbeit wurde der komplementäre Ansatz einer gesteigerten Expression zur weiteren Funktionsbestimmung von Sox9 gewählt. Auch mit dieser Vorgehensweise wird gezeigt, dass die Regulation der Sox9-Expression in verschiedenen Zelltypen während der Entwicklung des Rückenmarks von großer Bedeutung ist. Eine durch Brn4-Cre-Rekombinase gesteuerte Sox9-Fehl- und Überexpression in den neuroepithelialen Zellen der Ventrikularzone verursacht eine starke Reduktion dieser Zellen und der daraus stammenden Neurone. Das Neuroepithel um den Ventrikel verschwindet, die Morphologie des Rückenmarks ist stark verändert und die Gesamtzahl der Zellen im Rückenmark ist drastisch reduziert. Nach einer Tetrazyklin-abhängigen Verzögerung der Sox9-Transgenexpression bis kurz vor Beginn der Gliogenese, können die Effekte auf die neuroepithelialen und neuronalen Zellen unterbunden werden. Allerdings werden unabhängig vom Start der Sox9-Transgen-Expression vermehrt Gliazellen im Rückenmark generiert. Eine Sox9-Transgenexpression unter der Kontrolle des humanen Gfap-Promotors führt im Rückenmark vor allem zu einer erhöhten Anzahl von Makroglia, die teilweise ein Reifungsdefizit aufweisen. Ab dem postnatalen Tag 14 lässt sich zusätzlich eine verstärkte Immunreaktion im Rückenmark sowie eine verringerte Überlebensrate der Tiere feststellen. Die Auswirkungen der Transgenexpression auf die Zellpopulationen des Rückenmarks können durch eine vorübergehende Tetrazyklin-Behandlung vollständig rückgängig gemacht werden. Zusammenfassend lässt sich sagen, dass Fehl- und Überexpression von Sox9 die Identität von neuroepithelialen Zellen stark beeinträchtigt, Apoptose in neuronalen Zellen induziert und Gliazellen in einem unreifen Stadium hält. In Astroglia ändert sich darüber hinaus das Expressionsmuster stark, sodass eine erhöhte Immunantwort und Tod der Tiere resultieren. Sox9 hat folglich einen großen Einfluss auf die Entwicklung des murinen Rückenmarks und die astrogliale Genexpression. Um negative Effekte auf das Rückenmark, dessen Zellpopulationen sowie das Überleben der Tiere zu vermeiden, muss die Sox9-Expression strikt kontrolliert sein.
... As shown in Figure 6d, in the presence of the highest concentration tested, that is 10 μM, GL-8-28 treatment led to significantly reduced mRNA levels for OLG expressed genes associated with OLG maturation and CNS myelination, namely 2,′3′-cyclic nucleotide 3′-phosphodiesterase (Cnp) (Gravel et al., 1998;Kurihara et al., 1992) and UDPglycosyltransferase8(Ugt8) (Bosio et al., 1996;Gard & Pfeiffer, 1990). This outcome was not found to be associated (Lu et al., 2000;Wegner, 2001;Zhou et al., 2000) and with the CC1 antibody to identify mature OLGs (Bin et al., 2016;Fuss et al., 2000); arrows mark CC1-Olig2 double-positive cells. Scale bar: 50 μm. ...
... Scale bar: 10 μm. (c) Representative confocal images of the 3-week-old mouse corpus callosum, triple labeled using RNAscope for mRNAs encoding Lpar6, Olig2 to mark all OLG lineage cells(Lu et al., 2000;Wegner, 2001;Zhou et al., 2000) and Plp1 to mark later OLG maturation stages (Dubois-Dalcq et al., 1986;Duchala et al., 1995). Stars indicate nuclei of Olig2-positive cells. ...
... Stars indicate nuclei of Olig2-positive cells. Scale bar: 20 μm.3-week-old mouse corpus callosum, Lpar6 mRNA was detected in all cells expressing Olig2, a marker for all stages of the OLG lineage in rodents(Lu et al., 2000;Wegner, 2001;Zhou et al., 2000), including later stages of maturing OLGs as identified by the presence of Plp1 mRNA(Dubois-Dalcq et al., 1986;Duchala et al., 1995) (Figure 1c). These findings are consistent with previously published mRNA profiling data(Marques et al., 2016;Suckau et al., 2019; ...
The developmental process of central nervous system (CNS) myelin sheath formation is characterized by well‐coordinated cellular activities ultimately ensuring rapid and synchronized neural communication. During this process, myelinating CNS cells, namely oligodendrocytes (OLGs), undergo distinct steps of differentiation, whereby the progression of earlier maturation stages of OLGs represents a critical step toward the timely establishment of myelinated axonal circuits. Given the complexity of functional integration, it is not surprising that OLG maturation is controlled by a yet fully to be defined set of both negative and positive modulators. In this context, we provide here first evidence for a role of lysophosphatidic acid (LPA) signaling via the G protein‐coupled receptor LPA6 as a negative modulatory regulator of myelination‐associated gene expression in OLGs. More specifically, cell surface accessibility of LPA6 was found to be restricted to the earlier maturation stages of differentiating OLGs, and OLG maturation was found to occur precociously in Lpar6 knockout mice. To further substantiate these findings, a novel small molecule ligand with selectivity for preferentially LPA6 and LPA6 agonist characteristics was functionally characterized in vitro in primary cultures of rat OLGs and in vivo in the developing zebrafish. Utilizing this approach, a negative modulatory role of LPA6 signaling in OLG maturation could be corroborated. During development, such a functional role of LPA6 signaling likely serves to ensure timely coordination of circuit formation and myelination. Under pathological conditions as seen in the major human demyelinating disease multiple sclerosis (MS), however, persistent LPA6 expression and signaling in OLGs can be seen as an inhibitor of myelin repair. Thus, it is of interest that LPA6 protein levels appear elevated in MS brain samples, thereby suggesting that LPA6 signaling may represent a potential new druggable pathway suitable to promote myelin repair in MS.
... Two classes of transcription families, the bHLH proteins Olig1 and Olig2, and homeodomain protein Nkx2.2, are highly Shh concentration-dependent and act in concert to control neurone versus glial fate (Fu et al., 2002;Lu et al., 2000Lu et al., , 2002Zhou & Anderson, 2002;Zhou et al., 2001). Olig2 is required for production of both motor neurones and OPCs, but OPC specification requires both Olig1 and Olig2. ...
... OPC specification is marked by the induction of the high mobility group (HMG) domain protein Sox10, which is a direct target of Olig2 in OPC (Kuspert et al., 2011), and Olig2 and Sox10 are retained throughout the lineage (Lu et al., 2000;Stolt et al., 2006;Zhou and Anderson, 2002). Sox10, together with Sox9, promote oligodendrocyte specification and are required to maintain PDGFRa in OPC (Finzsch et al., 2008). ...
... Modern studies involving the analysis of transcription factors that regulate the processes of cell differentiation and proliferation in the CNS have proven to be of exceptional importance. The first identified transcription factors that modulate the development of oligodendrocytes are Olig1 and Olig2 [93,94]. Another transcription factor that determines oligodendroglial identity is Sox10. ...
... Olig2 and Sox10 are expressed at every stage of oligodendrocyte development, including mature forms [93,94,[100][101][102][103]. Olig2 is expressed in neuroepithelial cells even before their differentiation into oligodendrocyte cell lineage [101,104]. ...
There is a growing interest in glial cells in the central nervous system due to their important role in maintaining brain homeostasis both under physiological conditions and after injury. A significant amount of evidence has been accumulated regarding their capacity to exert either pro-inflammatory or anti-inflammatory effects under different pathological conditions. In combination with their known proliferative potential, they contribute not only to the limitation of brain damage and tissue remodeling but also to neuronal repair and synaptic recovery. Moreover, reactive glial cells can modulate the processes of neurogenesis, proliferation, and migration of neurons in the existing neural circuits in the adult brain. By discovering precise signals within specific niches, the regulation of sequential processes in adult neurogenesis holds the potential to unlock strategies that can stimulate the generation of functional neurons, whether in response to injury or as a means of addressing degenerative neurological conditions. Cerebral ischemic stroke, a condition falling within the realm of acute vascular disorders affecting the circulation of the brain, stands as a prominent global cause of disability and mortality. Extensive investigations into glial plasticity and their intricate interactions with other cells in the central nervous system have predominantly relied on studies conducted on experimental animals, including rodents and primates. However, valuable insights have also been gleaned from in vivo studies involving post-stroke patients, utilizing highly specialized imaging techniques. Following the attempts to map brain cells, the role of various transcription factors in modulating gene expression in response to cerebral ischemia is gaining increasing popularity. Although the results obtained thus far remain incomplete and occasionally ambiguous, they serve as a solid foundation for the development of strategies aimed at influencing the recovery process after ischemic brain injury.
... The bHLH-type transcription factors, Olig1 and Olig2, whose expression is induced by Sonic Hedgehog (Shh) signaling, have been shown to regulate differentiation into oligodendrocytes and induce oligodendrocyte maturation [4]. bHLH-type transcription factors are inhibited by HES and ID family molecules [5,6]; furthermore, Notch and bone morphogenetic protein (BMP) signals induce the expression of these inhibitory bHLH factors [7,8], thereby promoting the neuronal differentiation of NSCs and oligodendrogenesis. It is involved in maintaining the undifferentiated state of NSCs and suppressing cell differentiation [7,8]. ...
... Astrocytic differentiation is induced by extracellular factors, such as interleukin-6 family cytokines (IL-6FCs), leukemia inhibitory factor (LIF), and bone morphogenetic protein (BMP), and intracellular factors, such as Notch, STAT3, and small mother against decapentaplegic (Smad). Oligodendrocytic differentiation is induced by extracellular factors, such as Sonic Hedgehog (Shh), PDGF, fibroblast growth factor (FGF), and bone morphogenetic protein (BMP), and intracellular factors, such as VHL and Olig1 and 2. SOCS and VHL inhibit STAT3 through the Janus kinase-signal transduction and activator of transcription (JAK-STAT) pathway [2][3][4][5][6][7][8][9][10][11][12]. ...
The basic helix–loop–helix factors play a central role in neuronal differentiation and nervous system development, which involve the Notch and signal transducer and activator of transcription (STAT)/small mother against decapentaplegic signaling pathways. Neural stem cells differentiate into three nervous system lineages, and the suppressor of cytokine signaling (SOCS) and von Hippel-Lindau (VHL) proteins are involved in this neuronal differentiation. The SOCS and VHL proteins both contain homologous structures comprising the BC-box motif. SOCSs recruit Elongin C, Elongin B, Cullin5(Cul5), and Rbx2, whereas VHL recruits Elongin C, Elongin B, Cul2, and Rbx1. SOCSs form SBC-Cul5/E3 complexes, and VHL forms a VBC-Cul2/E3 complex. These complexes degrade the target protein and suppress its downstream transduction pathway by acting as E3 ligases via the ubiquitin–proteasome system. The Janus kinase (JAK) is the main target protein of the E3 ligase SBC-Cul5, whereas hypoxia-inducible factor is the primary target protein of the E3 ligase VBC-Cul2; nonetheless, VBC-Cul2 also targets the JAK. SOCSs not only act on the ubiquitin–proteasome system but also act directly on JAKs to suppress the Janus kinase–signal transduction and activator of transcription (JAK-STAT) pathway. Both SOCS and VHL are expressed in the nervous system, predominantly in brain neurons in the embryonic stage. Both SOCS and VHL induce neuronal differentiation. SOCS is involved in differentiation into neurons, whereas VHL is involved in differentiation into neurons and oligodendrocytes; both proteins promote neurite outgrowth. It has also been suggested that the inactivation of these proteins may lead to the development of nervous system malignancies and that these proteins may function as tumor suppressors. The mechanism of action of SOCS and VHL involved in neuronal differentiation and nervous system development is thought to be mediated through the inhibition of downstream signaling pathways, JAK-STAT, and hypoxia-inducible factor–vascular endothelial growth factor pathways. In addition, because SOCS and VHL promote nerve regeneration, they are expected to be applied in neuronal regenerative medicine for traumatic brain injury and stroke.
... OPCs are actively dividing cells (5-10%) in developing and adult brains and represent the largest pool of proliferative progenitor cells ($ 70%) in the brain parenchyma (20). Olig1 and Olig2 mark the oligodendrocyte lineage cells during development (21,22), and the ablation of mitotically active Olig2 þ cells inhibits the growth of actively progressing GBM (23). Recent single-cell profiling of murine and human developing brains identified Olig1/2 þ intermediate progenitors known as primitive OPCs (pri-OPCs) or precursor OPCs (pre-OPC; refs. ...
... 24,25). Olig1 or Olig2 marks pri-OPCs at early developmental stages prior to OPC commitment (22,25). In contrast to relative quiescent NSC, Olig1/2 þ pri-OPCs are rapidly dividing progenitor cells (25) and represent a population of the transit-amplifying cells in the developing and adult brain that are susceptible to malignant transformation (23,25,26). ...
Malignant gliomas such as glioblastoma are highly heterogeneous with distinct cells of origin and varied genetic alterations. It remains elusive whether the specific states of neural cell lineages are differentially susceptible to distinct genetic alterations during malignant transformation. Here, an analysis of The Cancer Genome Atlas databases revealed that comutations of PTEN and TP53 are most significantly enriched in human high-grade gliomas. Therefore, we selectively ablated Pten and Trp53 in different progenitors to determine which cell lineage states are susceptible to malignant transformation. Mice with PTEN/p53 ablation mediated by multilineage-expressing human GFAP (hGFAP) promoter–driven Cre developed glioma but with incomplete penetrance and long latency. Unexpectedly, ablation of Pten and Trp53 in Nestin+ neural stem cells (NSC) or Pdgfra+/NG2+ committed oligodendrocyte precursor cells (OPC), two major cells of origin in glioma, did not induce glioma formation in mice. Strikingly, mice lacking Pten and Trp53 in Olig1+/Olig2+ intermediate precursors (pri-OPC) prior to the committed OPCs developed high-grade gliomas with 100% penetrance and short latency. The resulting tumors exhibited distinct tumor phenotypes and drug sensitivities from NSC- or OPC-derived glioma subtypes. Integrated transcriptomic and epigenomic analyses revealed that PTEN/p53-loss induced activation of oncogenic pathways, including HIPPO-YAP and PI3K signaling, to promote malignant transformation. Targeting the core regulatory circuitries YAP and PI3K signaling effectively inhibited tumor cell growth. Thus, our multicell state in vivo mutagenesis analyses suggests that transit-amplifying states of Olig1/2 intermediate lineage precursors are predisposed to PTEN/p53-loss–induced transformation and gliomagenesis, pointing to subtype-specific treatment strategies for gliomas with distinct genetic alterations.
Significance
Multiple progenitor-state mutagenesis reveal that Olig1/2-expressing intermediate precursors are highly susceptible to PTEN/p53-loss–mediated transformation and impart differential drug sensitivity, indicating tumor-initiating cell states and genetic drivers dictate glioma phenotypes and drug responses.
See related commentary by Zamler and Hu, p. 807
... OPC differentiation is tightly regulated by intrinsic and extrinsic factors (Rowitch & Kriegstein, 2010). The basic helixloop-helix transcription factor Olig2, is one of the pivotal intrinsic determinants for oligodendrocyte specification (Liu et al., 2007;Lu et al., 2000;Lu et al., 2002;Maire et al., 2010;Zhou et al., 2000;Zhou & Anderson, 2002). In Olig2 null mice, OPC formation fails at embryonic and perinatal stages (Ligon et al., 2006;Mei et al., 2013), while overexpression of Olig2 triggers OPC differentiation and precocious myelination (Maire et al., 2010;Wegener et al., 2015). ...
... At the early postnatal brain Olig2 is mainly expressed by oligodendrocyte lineage cells and a subtype of astrocytes Wang et al., 2021). Subsequently, astrocytes progressively downregulate Olig2 during their postnatal development, thereby Olig2 becomes restricted to the oligodendrocyte lineage (Lu et al., 2000;Takebayashi et al., 2002;Zhou et al., 2000). However, under pathological conditions, Olig2 expression alters and the expressing population even extends to other cell types, for example, astrocytes (Chen et al., 2008). ...
Oligodendrocyte precursor cells (OPCs) are uniformly distributed in the mammalian brain; however, their function is rather heterogeneous in respect to their origin, location, receptor/channel expression and age. The basic helix–loop–helix transcription factor Olig2 is expressed in all OPCs as a pivotal determinant of their differentiation. Here, we identified a subset (2%–26%) of OPCs lacking Olig2 in various brain regions including cortex, corpus callosum, CA1 and dentate gyrus. These Olig2 negative (Olig2neg) OPCs were enriched in the juvenile brain and decreased subsequently with age, being rarely detectable in the adult brain. However, the loss of this population was not due to apoptosis or microglia-dependent phagocytosis. Unlike Olig2pos OPCs, these subset cells were rarely labeled for the mitotic marker Ki67. And, accordingly, BrdU was incorporated only by a three-day long-term labeling but not by a 2-hour short pulse, suggesting these cells do not proliferate any more but were derived from proliferating OPCs. The Olig2neg OPCs exhibited a less complex morphology than Olig2pos ones. Olig2neg OPCs preferentially remain in a precursor stage rather than differentiating into highly branched oligodendrocytes. Changing the adjacent brain environment, for example, by acute injuries or by complex motor learning tasks, stimulated the transition of Olig2pos OPCs to Olig2neg cells in the adult. Taken together, our results demonstrate that OPCs transiently suppress Olig2 upon changes of the brain activity.
... To find new factors involved in OL differentiation, we screened for target genes of Olig2, Chd7, and Chd8, key regulators of oligodendrogenesis (Lu et al., 2000;Lu et al., 2002;Yu et al., 2013;He et al., 2016;Marie et al., 2018;Zhao et al., 2018;Parras et al., 2020). We generated and compared the genome-wide binding profiles for these factors in acutely purified oligodendroglial cells from postnatal mouse brain cortices by magnetic cell sorting (MACS) of O4 + cells . ...
... The observation that OPCs are present within demyelinating MS lesions but fail to efficiently differentiate into myelinating cells with age and disease progression (Chang et al., 2002;Neumann et al., 2019), together with the strong sensitivity of iOLs to survival/apoptotic signals (Hughes and Stockton, 2021), suggests that efforts to foster OPC differentiation and survival of iOLs are a critical events for healthy aging and successful remyelination in MS patients. In this study, we combined the genome-wide binding profile of key regulators of OL differentiation, Olig2, Chd7, and Chd8 (Lu et al., 2000;Zhou et al., 2000;Lu et al., 2002;Zhou and Anderson, 2002;He et al., 2016;Küspert and Wegner, 2016;Marie et al., 2018;Zhao et al., 2018), to identify their common gene targets, and focused our analysis on Tensin3 (Tns3), whose expression matched the onset of OL differentiation. To study Tns3 expression and function, we generated several genetic tools, including CRISPR/Cas9 vectors, to induce Tns3 mutations both in vivo and in vitro, a Tns3 Tns3-V5 knock-in mouse, two constitutive Tns3 knockout mice, and finally an inducible knockout (Tns3 Flox ) mouse. ...
The differentiation of oligodendroglia from oligodendrocyte precursor cells (OPCs) to complex and extensive myelinating oligodendrocytes (OLs) is a multistep process that involves largescale morphological changes with significant strain on the cytoskeleton. While key chromatin and transcriptional regulators of differentiation have been identified, their target genes responsible for the morphological changes occurring during OL myelination are still largely unknown. Here, we show that the regulator of focal adhesion, Tensin3 (Tns3), is a direct target gene of Olig2, Chd7, and Chd8, transcriptional regulators of OL differentiation. Tns3 is transiently upregulated and localized to cell processes of immature OLs, together with integrin-b1, a key mediator of survival at this transient stage. Constitutive Tns3 loss-of-function leads to reduced viability in mouse and humans, with surviving knockout mice still expressing Tns3 in oligodendroglia. Acute deletion of Tns3 in vivo, either in postnatal neural stem cells (NSCs) or in OPCs, leads to a two-fold reduction in OL numbers. We find that the transient upregulation of Tns3 is required to protect differentiating OPCs and immature OLs from cell death by preventing the upregulation of p53, a key regulator of apoptosis. Altogether, our findings reveal a specific time window during which transcriptional upregulation of Tns3 in immature OLs is required for OL differentiation likely by mediating integrin-b1 survival signaling to the actin cytoskeleton as OL undergo the large morphological changes required for their terminal differentiation.
... Shh is expressed in the ventral motor neuron progenitor domain of the developing spinal cord and is required for the specification of OPCs that are derived from this region (Danesin & Soula, 2017). Shh signaling appears to be necessary for the expression of genes encoding Olig1 and Olig2, which play essential roles in NPC specification and OPC differentiation, as described above (Lu et al., 2000). The Wnt pathway appears to fulfill multiple roles during oligodendrocyte development in a stage-specific manner (Guo et al., 2015). ...
... During early CNS development, NPCs can give rise to neuron, astrocyte, or oligodendrocyte precursors. At some point during development, Olig1+/2+ NPCs stop producing neurons and begin to specifically generate OPCs (Lu et al., 2000). The process of fate specification in NPCs is regulated by several extrinsic and intrinsic cues (Namihira et al., 2008). ...
Oligodendrocytes are the glial cells responsible for the formation of myelin around axons of the central nervous system (CNS). Myelin is an insulating layer that allows electrical impulses to transmit quickly and efficiently along neurons. If myelin is damaged, as in chronic demyelinating disorders such as multiple sclerosis (MS), these impulses slow down. Remyelination by oligodendrocytes is often ineffective in MS, in part because of the failure of oligodendrocyte precursor cells (OPCs) to differentiate into mature, myelinating oligodendrocytes. The process of oligodendrocyte differentiation is tightly controlled by several regulatory networks involving transcription factors, intracellular signaling pathways, and extrinsic cues. Understanding the factors that regulate oligodendrocyte development is essential for the discovery of new therapeutic strategies capable of enhancing remyelination. Over the past decade, microRNAs (miRNAs) have emerged as key regulators of oligodendrocyte development, exerting effects on cell specification, proliferation, differentiation, and myelination. This article will review the role of miRNAs on oligodendrocyte biology and discuss their potential as promising therapeutic tools for remyelination. The process of oligodendrocyte differentiation is tightly controlled by several regulatory networks involving transcription factors, intracellular signaling pathways, and extrinsic cues. Another higher level of regulation is provided by microRNAs which target the expression of several downstream components. In the recent past, several microRNAs have emerged as key regulators of oligodendrocyte development, exerting effects on cell specification, proliferation, differentiation, and myelination. This article reviews the role of miRNAs on oligodendrocyte biology and discusses their potential as promising therapeutic targets for remyelination.
... Increased amount of GABAergic interneuron (Lu et al., 2000;Silbereis et al., 2014) Down Syndrome (Haydar and Reeves, 2012) Olig2 ...
... Absence of OPCs (Furusho et al., 2006;Petryniak et al., 2007;Ono et al., 2008) Down Syndrome; DMG (Lu et al., 2000;Filbin et al., 2018) Forkhead ...
Forebrain development in vertebrates is regulated by transcription factors encoded by homeobox, bHLH and forkhead gene families throughout the progressive and overlapping stages of neural induction and patterning, regional specification and generation of neurons and glia from central nervous system (CNS) progenitor cells. Moreover, cell fate decisions, differentiation and migration of these committed CNS progenitors are controlled by the gene regulatory networks that are regulated by various homeodomain-containing transcription factors, including but not limited to those of the Pax (paired), Nkx, Otx (orthodenticle), Gsx/Gsh (genetic screened), and Dlx (distal-less) homeobox gene families. This comprehensive review outlines the integral role of key homeobox transcription factors and their target genes on forebrain development, focused primarily on the telencephalon. Furthermore, links of these transcription factors to human diseases, such as neurodevelopmental disorders and brain tumors are provided.
... To find new factors involved in oligodendrocyte differentiation, we screened for target genes of Olig2, Chd7, and Chd8, key regulators of oligodendrogenesis (Lu et al., 2000;Lu et al., 2002;Yu et al., 2013;He et al., 2016;Marie et al., 2018;Zhao et al., 2018;Parras et al., 2020). We generated and compared the genome-wide binding profiles for these factors in acutely purified oligodendroglial cells from postnatal mouse brain cortices by magnetic cell sorting (MACS) of O4 + cells (Marie et al., 2018). ...
... The observation that OPCs are present within demyelinating MS lesions, but fail to efficiently differentiate into myelinating cells with age and disease progression (Chang et al., 2002;Neumann et al., 2019), together with the strong sensitivity of immature oligodendrocytes to survival/apoptotic signals (Hughes and Stockton, 2021), suggests that efforts to foster OPC differentiation and survival of immature oligodendrocytes are a critical events for healthy aging and successful remyelination in MS patients. In this study, we combined the genome-wide binding profile of key regulators of oligodendrocyte differentiation, Olig2, Chd7, and Chd8 (Lu et al., 2000;Zhou et al., 2000;Lu et al., 2002;Zhou and Anderson, 2002;He et al., 2016;Küspert and Wegner, 2016;Marie et al., 2018;Zhao et al., 2018), to identify their common gene targets, and focused our analysis on Tensin3 (Tns3), whose expression matched the onset of oligodendrocyte differentiation. ...
The differentiation of oligodendroglia from oligodendrocyte precursor cells (OPCs) to complex and extensive myelinating oligodendrocytes (OLs) is a multistep process that involves largescale morphological changes with significant strain on the cytoskeleton. While key chromatin and transcriptional regulators of differentiation have been identified, their target genes responsible for the morphological changes occurring during OL myelination are still largely unknown. Here, we show that the regulator of focal adhesion, Tensin3 (Tns3), is a direct target gene of Olig2, Chd7, and Chd8, transcriptional regulators of OL differentiation. Tns3 is transiently upregulated and localized to cell processes of immature OLs, together with integrin-β1, a key mediator of survival at this transient stage. Constitutive Tns3 loss-of-function leads to reduced viability in mouse and humans, with surviving knockout mice still expressing Tns3 in oligodendroglia. Acute deletion of Tns3 in vivo, either in postnatal neural stem cells (NSCs) or in OPCs, leads to a two-fold reduction in OL numbers. We find that the transient upregulation of Tns3 is required to protect differentiating OPCs and immature OLs from cell death by preventing the upregulation of p53, a key regulator of apoptosis. Altogether, our findings reveal a specific time window during which transcriptional upregulation of Tns3 in immature OLs is required for OL differentiation likely by mediating integrin-β1 survival signaling to the actin cytoskeleton as OL undergo the large morphological changes required for their terminal differentiation.
... The most common primary brain tumours are those with a glial cell origin, which cannot be fully classified by cellular morphology only [12]. The molecular analysis became crucial for glioma differentiation, grading and prognosis. ...
... The association of brain gliomas with 1p19q co-deletion and Olig2 expression has never been explored. Olig2, a member of the group of basic helixloop-helix transcription factors, is essential for the development of neural progenitors and oligodendrocytes [12]. Several studies have detected prominent Olig2 expression in oligodendrogliomas and oligoastrocytomas [2,[5][6][7]11,13]. ...
Aim of the study:
Oligodendrocyte transcriptional factor-2 (Olig2) is an essential marker for oligodendrocytes expression. We aimed to explore the expression of Olig2 in different glial neoplasms and to investigate if diffuse Olig2 expression can replace 1p19q co-deletion for the diagnosis of oligodendroglioma.
Material and methods:
Olig2 was performed on 53 samples of different glial neoplasms using immunohistochemistry (IHC). 1p/19q deletions were investigated using fluorescence in situ hybridization (FISH).
Results:
Olig2 labelling of different glial neoplasms revealed various expressions, in which 26 tumours showed diffuse expression (≥ 60%) and 23 tumours showed partial focal expression (< 50%). Four tumours showed no expression. Of the 26 tumours, 6 oligodendrogliomas had 1p19q co-deletion and the remaining 3 oligodendrogliomas showed no co-deletion. Three non-oligodendroglial tumours were found to have 19q deletion. The FISH of the remaining tumours (14/26) showed no aberrations. There was no significant difference in the final diagnosis by using 1p19q co-deletion test among glial neoplasms with diffuse Olig2 expression (p = 0.248).
Conclusions:
Olig2 marker cannot be used as an alternative diagnostic method for 1p19q co-deletion to distinguish oligodendrogliomas from other glial neoplasms. Although some glial tumours showed diffuse Olig2 expression, 1p19q co-deletion testing is the best diagnostic method.
... During vertebrate development, OPCs are specified from gliogenic precursors in the CNS, which consists of the brain and spinal cord (Barres et al., 1993;Dawson et al., 2000). Most OPCs are specified from ventral precursor cells marked by expression of the basic helix-loop-helix transcription factor Olig2, which differentiate into Sox10-positive OPCs (Warf et al., 1991;Noll and Miller, 1993;Lu et al., 2000;Zhou et al., 2001;Park et al., 2002). Following the specification, OPCs rapidly disperse throughout the CNS until they occupy distinct, non-overlapping territories (Cai et al., 2005;Kirby et al., 2006;De Biase et al., 2017;Ravanelli et al., 2018). ...
... In mammals, OPCs are also specified in the forebrain and utilize vasculature during initial migration, which is reviewed in Xia and Fancy (2021). Ventral precursors that give rise to OPCs are marked by their expression of the transcription factor Olig2, and subsequently, expression of Sox10 beginning around 36 h post fertilization (hpf) in zebrafish, embryonic day 12.5 (E12.5) in mouse, and 10 weeks gestational age during human fetal development (Lu et al., 2000;Zhou et al., 2001;Ravanelli et al., 2018;van Tilborg et al., 2018). Based on in vivo studies, OPC tiling begins immediately following specification, with OPCs migrating out of the pMN domain both dorsally and ventrally ( Figure 1A). ...
Tiling is a developmental process where cell populations become evenly distributed throughout a tissue. In this review, we discuss the developmental cellular tiling behaviors of the two major glial populations in the central nervous system (CNS)—oligodendrocyte progenitor cells (OPCs) and astrocytes. First, we discuss OPC tiling in the spinal cord, which is comprised of the three cellular behaviors of migration, proliferation, and contact-mediated repulsion (CMR). These cellular behaviors occur simultaneously during OPC development and converge to produce the emergent behavior of tiling which results in OPCs being evenly dispersed and occupying non-overlapping domains throughout the CNS. We next discuss astrocyte tiling in the cortex and hippocampus, where astrocytes migrate, proliferate, then ultimately determine their exclusive domains by gradual removal of overlap rather than sustained CMR. This results in domains that slightly overlap, allowing for both exclusive control of “synaptic islands” and astrocyte-astrocyte communication. We finally discuss the similarities and differences in the tiling behaviors of these glial populations and what remains unknown regarding glial tiling and how perturbations to this process may impact injury and disease.
... We focused on progenitor cells with three spatial identities, p0-p1, p2 and pMN. The three progenitor domains generate a variety of temporally distinct neurons (motor neurons (MNs) and interneurons) and later glial cell types (oligodendrocytes and astrocytes) (Briscoe et al., 2000;Deneen et al., 2006;Hayashi et al., 2018;Hochstim et al., 2008;Lu et al., 2000;Novitch et al., 2001;Worthy et al., 2023;Zhou et al., 2000). This suggests a temporal programme must intersect with the spatial identity of progenitors to determine the fate of progeny. ...
The vertebrate neural tube generates a large diversity of molecularly and functionally distinct neurons and glia from a small progenitor pool. While the role of spatial patterning in organising cell fate specification has been extensively studied, temporal patterning, which controls the timing of cell type generation, is equally important. Here we define a global temporal programme in the spinal cord. This governs cell fate choices by regulating chromatin accessibility in neural progenitors. Perturbation of this cis-regulatory programme affects sequential transitions in spinal cord progenitors and the identity of progeny. The temporal programme operates in parallel to spatial patterning, ensuring the timely availability of regulatory elements for spatial determinants to direct cell-type specific gene expression. These findings identify a chronotopic spatiotemporal integration strategy in which a global temporal chromatin programme determines the output of a spatial gene regulatory network resulting in the temporally and spatially ordered allocation of cell type identity.
... Oligodendrocytes, the myelinating cells of the central nervous system (CNS), are chiefly responsible for myelin ensheathment of axons, providing trophic support and protection to neurons [1][2][3][4], and regulating iron homeostasis [3,5,6]. In the spinal cord, oligodendrocytes are produced by neuroepithelial (NE) cells in the ventral portion of the neural tube and are dependent on the proximity to the source of the morphogen sonic hedgehog (SHH) [7,8]. These ventrally-derived neural progenitor cells committing to oligodendrocyte lineage express basic helix-loop-helix (bHLH) oligodendrocyte transcription factor 2 (OLIG2) and NK2 Homeobox 2 (NKX2.2), ...
Oligodendrocytes originating in the brain and spinal cord as well as in the ventral and dorsal domains of the neural tube are transcriptomically and functionally distinct. These distinctions are also reflected in the ultrastructure of the produced myelin, and the susceptibility to myelin-related disorders, which highlights the significance of the choice of patterning protocols in the differentiation of induced pluripotent stem cells (iPSCs) into oligodendrocytes. Thus, our first goal was to survey the different approaches applied to the generation of iPSC-derived oligodendrocytes in 2D culture and in organoids, as well as reflect on how these approaches pertain to the regional and spatial fate of the generated oligodendrocyte progenitors and myelinating oligodendrocytes. This knowledge is increasingly important to disease modeling and future therapeutic strategies. Our second goal was to recap the recent advances in the development of oligodendrocyte-enriched organoids, as we explore their relevance to a regional specification alongside their duration, complexity, and maturation stages of oligodendrocytes and myelin biology. Finally, we discuss the shortcomings of the existing protocols and potential future explorations.
... Further, the phenotype of nearly 50% of BrdU + cells studied at the time point 2h after BrdU could not be classified with cell markers listed above (Fig. 7 A), thus raising the possibility of the existence of undifferentiated cells which could potentially express such embryonic transcription factors. We therefore performed double-labeling for BrdU and transcription factors which are expressed in embryonic progenitors: Sox1, Sox2, Sox9, Sox10, Smo, Emx2, Ngn1, Olig1, Olig2, Olig3 [26,28,[34][35][36][39][40][41][42][43][44][45][46][47]. ...
The existence of proliferating cells in the intact spinal cord, their distribution and phenotype, are well studied in rodents. A limited number of studies also address the proliferation after spinal cord injury, in non-human primates. However, a detailed description of the quantity, distribution and phenotype of proliferating cells at different anatomical levels of the intact adult non-human primate spinal cord is lacking at present. In the present study, we analyzed normal spinal cord tissues from adult macaque monkeys (Macaca fuscata), infused with Bromo-2′-deoxyuridine (BrdU), and euthanized at 2h, 2 weeks, 5 weeks and 10 weeks after BrdU. We found a significantly higher density of BrdU + cells in the gray matter of cervical segments as compared to thoracic or lumbar segments, and a significantly higher density of proliferating cells in the posterior as compared to the anterior horn of the gray matter. BrdU + cells exhibited phenotype of microglia or endothelial cells (∼50%) or astroglial and oligodendroglial cells (∼40%), including glial progenitor phenotypes marked by the transcription factors Sox9 and Sox10. BrdU + cells also co-expressed other transcription factors known for their involvement in embryonic development, including Emx2, Sox1, Sox2, Ngn1, Olig1, Olig2, Olig3. In the central canal, BrdU + cells were located along the dorso-ventral axis and co-labeled for the markers Vimentin and Nestin. These results reveal the extent of cellular plasticity in the spinal cord of non-human primates under normal conditions.
... Concomitantly with this pro-astroglial effect, BMP activation antagonizes oligodendrocyte progenitor cell (OPC) differentiation by inducing ID2/ID4 expression. Indeed, ID2/ID4 prevent the nuclear translocation of Olig1/2 transcription factors that specifically guide oligodendrocyte differentiation by inducing oligogenic gene expression, such as myelin basic protein (MBP) or 2′,3′-cyclic nucleotide 3′-phosphodiesterase (CNP) [57][58][59][60]. BMP signaling also plays a critical role in maintaining adult NSC niches in the subventricular zone (SVZ) and subgranular zone (SGZ) to allow neuronal and glial regeneration, indicating that the role of BMPs in the CNS is not limited to antenatal development. ...
The BMP pathway is one of the major signaling pathways in embryonic development, ontogeny and homeostasis, identified many years ago by pioneers in developmental biology. Evidence of the deregulation of its activity has also emerged in many cancers, with complex and sometimes opposing effects. Recently, its role has been suspected in Diffuse Midline Gliomas (DMG), among which Diffuse Intrinsic Pontine Gliomas (DIPG) are one of the most complex challenges in pediatric oncology. Genomic sequencing has led to understanding part of their molecular etiology, with the identification of histone H3 mutations in a large proportion of patients. The epigenetic remodeling associated with these genetic alterations has also been precisely described, creating a permissive context for oncogenic transcriptional program activation. This review aims to describe the new findings about the involvement of BMP pathway activation in these tumors, placing their appearance in a developmental context. Targeting the oncogenic synergy resulting from this pathway activation in an H3K27M context could offer new therapeutic perspectives based on targeting treatment-resistant cell states.
... Olig2 mutants have higher HES5 levels in pMN progenitor cells, whereas Neurog2 expression is downregulated. Olig2 expression in turn is induced by Shh signaling emanating from the floor plate of the spinal cord (Lu et al., 2000). Thus, the regulatory network of OLIG2, HES5 and NEUROG2 integrates patterning and neurogenesis signals to control motor neuron pool size. ...
Dopaminergic neurons develop in distinct neural domains by integrating local patterning and neurogenesis signals. While the proneural proteins Neurog1 and Olig2 have been previously linked to development of dopaminergic neurons, their dependence on local prepatterning and specific contributions to dopaminergic neurogenesis are not well understood. Here, we show that both transcription factors are differentially required for the development of defined dopaminergic glutamatergic subpopulations in the zebrafish posterior tuberculum, which are homologous to A11 dopaminergic neurons in mammals. Both Olig2 and Neurog1 are expressed in otpa expressing progenitor cells and appear to act upstream of Otpa during dopaminergic neurogenesis. Our epistasis analysis confirmed that Neurog1 acts downstream of Notch signaling, while Olig2 acts downstream of Shh, but upstream and/or in parallel to Notch signaling during neurogenesis of A11-type dopaminergic clusters. Furthermore, we identified Olig2 to be an upstream regulator of neurog1 in dopaminergic neurogenesis. This regulation occurs through Olig2-dependent repression of the proneural repressor and Notch target gene her2. Our study reveals how Neurog1 and Olig2 integrate local patterning signals, including Shh, with Notch neurogenic selection signaling, to specify the progenitor population and initiate neurogenesis and differentiation of A11-type dopaminergic neurons.
... Olig2, described initially as a transcription factor important in the specification of oligodendrocytes (Zhou et al., 2000), (Lu et al., 2000) has since then been demonstrated to be linked to the specification of motoneurons and oligodendrocytes (Takebayashi et al., 2000), (Zhou and Anderson, 2002), (Lu et al., 2002) in the ventral spinal cord, as markers in neoplastic disease (Lu et al., 2001), in the early stages of specification of astroglia in the neural precursors in the subventricular zone (SVZ) (Marshall et al., 2005) and ependymal cells (Masahira et al., 2006). Several groups have attempted the strategy of overexpression of Olig2 to specify oligodendroglia from stem cell populations such as olfactory ensheathing cells (Zhang et al., 2005), and mouse neural stem cells (Copray et al., 2006). ...
Introduction: The ability to switch stem cell differentiation fate in-vitro, is a powerful tool that may allow the generation of large numbers of cells that may be required in order to develop biologically engineered tissues and cells required for therapeutic applications such as pharmacological testing of new medications. The transcription factor (“master switch”) Olig2, alone or in conjunction with Nkx2.2, has been implicated as a key cell fate decider for emerging neuro-glial precursors derived from both embryonic stem (ES) cells and from foetal neural stem (FNS) cells. Methods: The in-vitro system of stem cells devoid of exogenous signaling was developed. Stem cells were manipulated by pIRES plasmid vector driven, constitutively expressed Olig-2 or Olig-2/Nkx2.2 transcription factor system introduced into proliferating embryonic or foetal neural stem cells, following a similar embryological temporal p at te r ni n g s e q ue nc e s e e n i n -vi v o. Findings: Successful stem cell fate modification could be achieved in-vitro using the transcription factor overexpression system. Substantially different cell fates were noted in the presence of Olig-2 alone and in combination with Nkx2.2, with the achievement of premature glial differentiation. Conclusion: This method, therefore, may be useful to generate rare live human cells (such as Oligodendroglia or specialised myocardial cells) in-vitro.
... More transcriptional control mechanisms that are only active during the repair phase have since been discovered, such as Merlin and STAT3 [11]. Schwann cells involved in repair after injury express protein molecules, such as Sonic Hedgehog (Shh) and Olig 1, which are also not found in the embryonic stage [12]. Schwann cells also develop properties of an inflammatory cell expressing cytokines attracting macrophages [13]. ...
Work-related injuries are common. The cost of these injuries is around USD 176 billion to USD 350 billion a year. A significant number of work-related injuries involve nerve damage or dysfunction. Injuries may heal with full recovery of function, but those involving nerve damage may result in significant loss of function or very prolonged recovery. While many factors can predispose a person to suffer nerve damage, in most cases, it is a multifactorial issue that involves both intrinsic and extrinsic factors. This makes preventing work-related injuries hard. To date, no evidence-based guidelines are available to clinicians to evaluate work-related nerve dysfunction. While the symptoms range from poor endurance to cramping to clear loss of motor and sensory functions, not all nerves are equally vulnerable. The common risk factors for nerve damage are a superficial location, a long course, an acute change in trajectory along the course, and coursing through tight spaces. The pathophysiology of acute nerve injury is well known, but that of chronic nerve injury is much less well understood. The two most common mechanisms of nerve injury are stretching and compression. Chronic mild to moderate compression is the most common mechanism of nerve injury and it elicits a characteristic response from Schwann cells, which is different from the one when nerve is acutely injured. It is important to gain a better understanding of work-related nerve dysfunction, both from health and from regulatory standpoints. Currently, management depends upon etiology of nerve damage, recovery is often poor if nerves are badly damaged or treatment is not instituted early. This article reviews the current pathophysiology of chronic nerve injury. Chronic nerve injury animal models have contributed a lot to our understanding but it is still not complete. Better understanding of chronic nerve injury pathology will result in identification of novel and more effective targets for pharmacological interventions.
... Olig2 mutants have higher HES5 levels in pMN progenitor cells, whereas Neurog2 expression is downregulated. Olig2 expression in turn is induced by Shh signaling emanating from the floor plate of the spinal cord (Lu et al., 2000). Thus, the regulatory network of OLIG2, HES5 and NEUROG2 integrates patterning and neurogenesis signals to control motor neuron pool size. ...
Dopaminergic neurons develop in distinct neural domains by integrating local patterning and neurogenesis signals. While the proneural proteins Neurog1 and Olig2 have been previously linked to development of dopaminergic neurons, their dependence on local prepatterning and specific contributions to dopaminergic neurogenesis are not well understood. Here, we show that both transcription factors are differentially required for the development of defined dopaminergic glutamatergic subpopulations in the zebrafish posterior tuberculum, which are homologous to A11 dopaminergic neurons in mammals. Both Olig2 and Neurog1 are expressed in otpa expressing progenitor cells and appear to act upstream of Otpa during dopaminergic neurogenesis. Our epistasis analysis confirmed that Neurog1 acts downstream of Notch signaling, while Olig2 acts downstream of Shh, but upstream and/or in parallel to Notch signaling. Furthermore, we identified Olig2 to be an upstream regulator of neurog1 in dopaminergic neurogenesis. This regulation occurs through Olig2-dependent repression of the proneural repressor and Notch target gene her2 . Our study reveals how Neurog1 and Olig2 integrate local patterning signals, including Shh, with Notch neurogenic selection signaling, to specify the progenitor population and initiate neurogenesis and differentiation of A11-type dopaminergic neurons.
... Nonetheless, there are also regional differences that exist across oligodendrogenic cells found in different areas of the CNS [40,41]. In this regard, we believe that using Olig2 induction is particularly relevant for the generation of cells that are suitable for a variety of demyelinating conditions, as Olig2 is ubiquitously expressed in oligodendrogenic lineage cells throughout the CNS [42][43][44][45]. ...
Oligodendrocytes are the myelinating cells of the central nervous system that facilitate efficient signal transduction. The loss of these cells and the associated myelin sheath can lead to profound functional deficits. Moreover, oligodendrocytes also play key roles in mediating glial-neuronal interactions, which further speaks to their importance in health and disease. Neural progenitor cells (NPCs) are a promising source of cells for the treatment of oligodendrocyte-related neurological diseases due to their ability to differentiate into a variety of cell types, including oligodendrocytes. However, the efficiency of oligodendrocyte differentiation is often low. In this study, we induced the expression of the Olig2 transcription factor in tripotent NPCs using a doxycycline-inducible promoter, such that the extent of oligodendrocyte differentiation could be carefully regulated. We characterized the differentiation profile and the transcriptome of these inducible oligodendrogenic NPCs (ioNPCs) using a combination of qRT-PCR, immunocytochemistry and RNA sequencing with gene ontology (GO) and gene set enrichment analysis (GSEA). Our results show that the ioNPCs differentiated into a significantly greater proportion of oligodendrocytes than the NPCs. The induction of Olig2 expression was also associated with the upregulation of genes involved in oligodendrocyte development and function, as well as the downregulation of genes involved in other cell lineages. The GO and GSEA analyses further corroborated the oligodendrocyte specification of the ioNPCs.
... This was achieved by crossing two transgenic mouse lines, the Pdgfra-CreER T2 line 12 and the Sox10-lox-GFP-STOPlox-DTA (Sox10-DTA) line, 20 to enable diphtheria toxin A (DTA) expression in adult OPCs upon delivery of tamoxifen (TAM) (Figure 1A). As SOX10 is expressed exclusively by oligodendroglia in the postnatal CNS, 21,22 this ensures that DTA expression is restricted to OPCs and is excluded from VLMCs and choroid plexus epithelial cells, which express PDGFRA but not SOX10. DTA expression is not expected to target Schwann cells in the peripheral nervous system since most Schwann cells express SOX10 but not PDGFRA. ...
Approaches to investigate adult oligodendrocyte progenitor cells (OPCs) by targeted cell ablation in the rodent CNS have limitations in the extent and duration of OPC depletion. We have developed a pharmacogenetic approach for conditional OPC ablation, eliminating >98% of OPCs throughout the brain. By combining recombinase-based transgenic and viral strategies for targeting OPCs and ventricular-subventricular zone (V-SVZ)-derived neural precursor cells (NPCs), we found that new PDGFRA-expressing cells born in the V-SVZ repopulated the OPC-deficient brain starting 12 days after OPC ablation. Our data reveal that OPC depletion induces V-SVZ-derived NPCs to generate vast numbers of PDGFRA+NG2+ cells with the capacity to proliferate and migrate extensively throughout the dorsal anterior forebrain. Further application of this approach to ablate OPCs will advance knowledge of the function of both OPCs and oligodendrogenic NPCs in health and disease. Motivation: To investigate the function of oligodendrocyte progenitor cells (OPCs), several groups have developed strategies to deplete OPCs within the adult CNS. However, these methods have significant limitations in achieving complete or long-term OPC ablation. We developed a pharmacogenetic method that achieves near-complete ablation of OPCs via the inducible and conditional expression of diphtheria toxin A (DTA) in adult OPCs followed by delivery of an anti-mitotic agent into the CNS to ablate dividing OPCs that escape genetic targeting.
... ET-1 then binds the endothelin receptor B on oligodendrocyte progenitor cells (OPCs), in vitro and in vivo, to enhance myelination (Swire et al., 2019;Yuen et al., 2013). OPCs are characterised by their expression of the mitogenic receptor platelet-derived growth factor receptor α (PDGFRα) (Stallcup and Beasley, 1987;Hart et al., 1989;Pringle et al., 1992;Rivers et al., 2008), the NG2 proteoglycan (Zhu et al., 2008) and the transcription factors SOX10 (SEY-box transcription factor 10) (Kuhlbrodt et al., 1998) and OLIG2 (oligodendrocyte transcription factor 2) (Lu et al., 2000;Zhou et al., 2000;Dimou et al., 2008). While OPCs are best known for their role in generating myelinating oligodendrocytes in the developing and mature CNS (Pepper et al., 2018), they closely associate with the vasculature as they migrate and are a cellular constituent of the neurovascular unit in adulthood, regulating angiogenesis and BBB integrity (Fig. 2). ...
Multiple sclerosis is an inflammatory demyelinating disease of the central nervous system (CNS) and the most common non-traumatic cause of neurological disability in young adults. Multiple sclerosis clinical care has improved considerably due to the development of disease-modifying therapies that effectively modulate the peripheral immune response and reduce relapse frequency. However, current treatments do not prevent neurodegeneration and disease progression, and efforts to prevent multiple sclerosis will be hampered so long as the cause of this disease remains unknown. Risk factors for multiple sclerosis development or severity include vitamin D deficiency, cigarette smoking and youth obesity, which also impact vascular health. People with multiple sclerosis frequently experience blood-brain barrier breakdown, microbleeds, reduced cerebral blood flow and diminished neurovascular reactivity, and it is possible that these vascular pathologies are tied to multiple sclerosis development. The neurovascular unit is a cellular network that controls neuroinflammation, maintains blood-brain barrier integrity, and tightly regulates cerebral blood flow, matching energy supply to neuronal demand. The neurovascular unit is composed of vessel-associated cells such as endothelial cells, pericytes and astrocytes, however neuronal and other glial cell types also comprise the neurovascular niche. Recent single-cell transcriptomics data, indicate that neurovascular cells, particular cells of the microvasculature, are compromised within multiple sclerosis lesions. Large-scale genetic and small-scale cell biology studies also suggest that neurovascular dysfunction could be a primary pathology contributing to multiple sclerosis development. Herein we revisit multiple sclerosis risk factors and multiple sclerosis pathophysiology and highlight the known and potential roles of neurovascular unit dysfunction in multiple sclerosis development and disease progression. We also evaluate the suitability of the neurovascular unit as a potential target for future disease modifying therapies for multiple sclerosis.
... OPCs produce oligodendrocytes from multiple germinal neuroepithelial cells and radial glial (RG) neuronal progenitor cells. The specialized oligodendrogenic domain in the ventral neuroepithelium is under the influence of Shh signaling [42][43][44]; thus, OPC specification is influenced by the Shh signaling pathway [45][46][47]. Other growth factors are involved in OPC development, such as platelet-derived growth factor A (Pdgf-A), which mediates OPC proliferation and migration [44,[48][49][50]. ...
The developmental functions of primary cilia and the downstream signaling pathways have been widely studied; however, the roles of primary cilia in the developing neurovascular system are not clearly understood. In this study, we found that ablation of genes encoding ciliary transport proteins such as intraflagellar transport homolog 88 (Ift88) and kinesin family member 3a (Kif3a) in cortical radial progenitors led to periventricular heterotopia during late mouse embryogenesis. Conditional mutation of primary cilia unexpectedly caused breakdown of both the neuroepithelial lining and the blood‐choroid plexus barrier. Choroidal leakage was partially caused by enlargement of the choroid plexus in the cilia mutants. We found that the choroid plexus expressed platelet‐derived growth factor A (Pdgf‐A) and that Pdgf‐A expression was ectopically increased in cilia‐mutant embryos. Cortices obtained from embryos in utero electroporated with Pdgfa mimicked periventricular heterotopic nodules of the cilia mutant. These results suggest that defective ciliogenesis in both cortical progenitors and the choroid plexus leads to breakdown of cortical and choroidal barriers causing forebrain neuronal dysplasia, which may be related to developmental cortical malformation. Forebrain heterotopia development caused by combined barrier breakdown.
... Olig2 is expressed throughout the oligodendrocyte lineage from oligodendrocyte precursor cells (OPCs) to mature myelinating cells (Lu et al., 2000;Zhou et al., 2000). OPCs are derived from neural stem cells and become positive for plate-derived growth factor receptor α (PDGFRα) (Hall et al., 1996) and NG2 (polydendrocytes) (Nishiyama et al., 1999). ...
Microtubule‐associated protein Tau is primarily expressed in axons of neurons, but also in Olig2‐positive oligodendrocytes in adult rodent and monkey brains. In this study, we sought to determine at what cell stage Tau becomes expressed in the oligodendrocyte lineage. We performed immunostaining of adult mouse brain sections using well‐known markers of oligodendrocyte lineage and found that Tau is expressed in mature oligodendrocytes, but not in oligodendrocyte progenitors and immature pre‐oligodendrocytes. We also investigated Tau expression in developing mouse brain. Surprisingly, Tau expression occurred after the peak of myelination and even exceeded GSTπ expression, which has been considered as a marker of myelinating oligodendrocytes. These results suggest Tau as a novel marker of oligodendrocyte maturation. We then investigated whether Tau is important for oligodendrocyte development and/or myelination and how Tau changes in demyelination. First, we found no changes in myelination and oligodendrocyte markers in Tau knockout mice, suggesting that Tau is dispensable. Next, we analyzed the proteolipid protein 1 transgenic model of Pelizaeus‐Merzbacher disease, which is a rare leukodystrophy. In hemizygous transgenic mice, the number of Tau‐positive cells were significantly increased as compared with wild type mice. These cells were also positive for Olig2, CC1, and GSTπ, but not PDGFRα and GPR17. In stark contrast, the expression level of Tau, as well as GSTπ, was dramatically decreased in the cuprizone‐induced model of multiple sclerosis. Taken together, we propose Tau as a new marker of oligodendrocyte lineage and for investigating demyelination lesions. Main Point Microtubule‐binding protein Tau is expressed in oligodendrocytes during late stages of cell maturation. Tau increases in oligodendrocytes in a model of Pelizaeus‐Merzbacher disease, but not in the cuprizone‐induced model of multiple sclerosis.
... Like GABAergic interneurons, OPCs are induced in the ventral telencephalon and migrate into the dorsal telencephalon during embryogenesis [50]. In the ventral telencephalon, Olig1 and Olig2 are expressed in OPCs and sustained during migration [54][55][56]. Additionally, the fate specification of oligodendrocytes needs Olig1/2 in the ventral telencephalon [57]. ...
The development of functional neural circuits in the central nervous system (CNS) requires the production of sufficient numbers of various types of neurons and glial cells, such as astrocytes and oligodendrocytes, at the appropriate periods and regions. Hence, severe neuronal loss of the circuits can cause neurodegenerative diseases such as Huntington’s disease (HD), Parkinson’s disease (PD), Alzheimer’s disease (AD), and Amyotrophic Lateral Sclerosis (ALS). Treatment of such neurodegenerative diseases caused by neuronal loss includes some strategies of cell therapy employing stem cells (such as neural progenitor cells (NPCs)) and gene therapy through cell fate conversion. In this report, we review how bHLH acts as a regulator in neuronal differentiation, reprogramming, and cell fate determination. Moreover, several different researchers are conducting studies to determine the importance of bHLH factors to direct neuronal and glial cell fate specification and differentiation. Therefore, we also investigated the limitations and future directions of conversion or transdifferentiation using bHLH factors.
... To do this, we modified the FANS NEUN + /-method for isolating neuronal nuclei from fresh-frozen cortex [22,23] by incorporating positive selection nuclear markers for astrocyte and OPC nuclei. For OPC isolation, we used OLIG2, a known marker of adult oligodendroglial lineage cells, which shows stronger expression in OPCs compared to mature oligodendrocytes [43][44][45]. To find a suitable nuclear astrocytic marker, we searched for astrocyte-enriched transcription factor (TF) genes within the HepaCAM-purified resting human astrocyte transcriptome database [46] and found PAX6 and SOX9 to be among the top upregulated astrocyte-specific genes. ...
The pathophysiology of epilepsy underlies a complex network dysfunction between neurons and glia, the molecular cell type-specific contributions of which remain poorly defined in the human disease. In this study, we validated a method that simultaneously isolates neuronal (NEUN +), astrocyte (PAX6 + NEUN–), and oligodendroglial progenitor (OPC) (OLIG2 + NEUN–) enriched nuclei populations from non-diseased, fresh-frozen human neocortex and then applied it to characterize the distinct transcriptomes of such populations isolated from electrode-mapped temporal lobe epilepsy (TLE) surgical samples. Nuclear RNA-seq confirmed cell type specificity and informed both common and distinct pathways associated with TLE in astrocytes, OPCs, and neurons. Compared to postmortem control, the transcriptome of epilepsy astrocytes showed downregulation of mature astrocyte functions and upregulation of development-related genes. To gain further insight into glial heterogeneity in TLE, we performed single cell transcriptomics (scRNA-seq) on four additional human TLE samples. Analysis of the integrated TLE dataset uncovered a prominent subpopulation of glia that express a hybrid signature of both reactive astrocyte and OPC markers, including many cells with a mixed GFAP + OLIG2 + phenotype. A further integrated analysis of this TLE scRNA-seq dataset and a previously published normal human temporal lobe scRNA-seq dataset confirmed the unique presence of hybrid glia only in TLE. Pseudotime analysis revealed cell transition trajectories stemming from this hybrid population towards both OPCs and reactive astrocytes. Immunofluorescence studies in human TLE samples confirmed the rare presence of GFAP + OLIG2 + glia, including some cells with proliferative activity, and functional analysis of cells isolated directly from these samples disclosed abnormal neurosphere formation in vitro. Overall, cell type-specific isolation of glia from surgical epilepsy samples combined with transcriptomic analyses uncovered abnormal glial subpopulations with de-differentiated phenotype, motivating further studies into the dysfunctional role of reactive glia in temporal lobe epilepsy.
... In the spinal cord, OLIG2 expression is initially activated in neural progenitors of the pMN domain at ~ E8.5, but is rapidly lost from their motor neuron progeny during neurogenesis [33,34]. As development proceeds, OLIG2 expression is retained in OPCs but is slightly downregulated in differentiated oligodendrocytes [35,36]. In the brain, OLIG2 is broadly expressed in the VZ of the ventral telencephalon, including the LGE, MGE, and AEP regions at embryonic stages [37][38][39]. ...
Glial cells in the central nervous system (CNS) are composed of oligodendrocytes, astrocytes and microglia. They contribute more than half of the total cells of the CNS, and are essential for neural development and functioning. Studies on the fate specification, differentiation, and functional diversification of glial cells mainly rely on the proper use of cell- or stage-specific molecular markers. However, as cellular markers often exhibit different specificity and sensitivity, careful consideration must be given prior to their application to avoid possible confusion. Here, we provide an updated overview of a list of well-established immunological markers for the labeling of central glia, and discuss the cell-type specificity and stage dependency of their expression.
... The remaining 2 clusters of cells (OPC1 and OPC2) expressed at least 2 of the 5 canonical OPC markers Ptprz, PDGFRα, Olig1, Olig2, and Cspg4(Fig. 3B)18,[38][39][40][41] . Importantly, each OPC population expressed a unique transcriptional signature distinct from the gene expression in every other cluster(Fig. ...
Oligodendrocyte progenitor cells (OPCs) account for approximately 5% of the adult brain and have been historically studied for their role in myelination. In the adult brain, OPCs maintain their proliferative capacity and ability to differentiate into oligodendrocytes throughout adulthood, even though relatively few mature oligodendrocytes are produced post-developmental myelination. Recent work has begun to demonstrate that OPCs likely perform multiple functions in both homeostasis and disease and can significantly impact behavioral phenotypes such as food intake and depressive symptoms. However, the exact mechanisms through which OPCs might influence brain function remain unclear. The first step in further exploration of OPC function is to profile the transcriptional repertoire and assess the heterogeneity of adult OPCs. In this work, we demonstrate that adult OPCs are transcriptionally diverse and separate into two distinct populations in the homeostatic brain. These two groups show distinct transcriptional signatures and enrichment of biological processes unique to individual OPC populations. We have validated these OPC populations using multiple methods, including multiplex RNA in situ hybridization and RNA flow cytometry. This study provides an important resource that profiles the transcriptome of adult OPCs and will provide a toolbox for further investigation into novel OPC functions.
... Glial cell production also follows DV regional organization. Oligodendrocytes, which populate the entire spinal cord, derive from the ventral pMN/pOL and dorsal domains (Zhou et al., 2000;Lu et al., 2000;Cai et al., 2005;Vallstedt et al., 2005;Fogarty et al., 2005). Astrocytes, however, are produced from vz territories spanning the whole DV axis (Rowitch and Kriegstein, 2010;Pringle et al., 2003). ...
Significant progress has been made in elucidating the basic principles that govern neuronal specification in the developing central nervous system. In contrast, much less is known about the origin of astrocytic diversity. Here we demonstrate that a restricted pool of progenitors in the mouse spinal cord, expressing the transcription factor Dbx1, produces a subset of astrocytes, in addition to interneurons. Ventral p0-derived astrocytes (vA0) exclusively populate intermediate regions of spinal cord with extraordinary precision. Postnatal vA0 population comprises gray matter protoplasmic and white matter fibrous astrocytes and a group of cells with strict radial morphology contacting the pia. We identified that vA0 cells in the lateral funiculus are distinguished by the expression of Reelin and Kcnmb4. We show that Dbx1 mutants have increased vA0 cells at the expense of p0-derived interneurons. Manipulation of the Notch pathway, together with the alteration in their ligands seen in Dbx1 knock-outs, suggest that Dbx1 controls neuron-glial balance by modulating Notch-dependent cell interactions. In summary, this study highlights that restricted progenitors in dorsal-ventral neural tube produce region-specific astrocytic subgroups and that progenitor transcriptional programs highly influence glial fate and are instrumental in creating astrocyte diversity.
... A careful comparison of PDGFRα and NG2 expression by double immunofluorescence labelling demonstrated a complete overlap between glial cells that expressed NG2 and those that expressed PDGFRα both in vitro and in the developing rat brain in vivo [62,63]. OPCs are characterized by a bipolar morphology and a combination of cell surface markers, including NG2, PDGFRα and A2B5, and transcriptional markers such as OLIG1 and 2, MYT1, NKX2.2, NKX2.6, SOX9 and SOX10 [64][65][66] (reviewed in [37,67]). Although most of the transcription factors expression is maintained in pre-myelinating and myelinating oligodendrocytes, the cell surface markers change as the cell differentiate (reviewed in [37,67,68]). ...
A2B5 IgM recognizes c-series gangliosides with three sialic acids. The aim of this review was to focus on A2B5 expression in the central nervous system and gliomas. In brain development, A2B5+ cells are recorded in areas containing multipotent neural stem cells (NSC). In adults, A2B5+ cells persist in neurogenic areas and in white matter where it identifies oligodendrocyte precursor cells (OPCs) but also cells with NSC properties. Although the expression of A2B5 has been widely studied in culture, where it characterizes bipotential glial progenitor cells, its expression in vivo is less characterized mainly because of technical issues. A new interest was given to the NSCs and OPCs since the discovery of cancer stem cells (CSC) in gliomas. Among other cell surface molecules, A2B5 has been identified as an accurate marker to identify glioma CSCs. We and others have shown that all types of gliomas express A2B5, and that only A2B5+ cells, and not A2B5- cells, can generate a tumor after orthotopic implantation in immunocompromised animals. Moreover, A2B5 epitope expression is positively correlated with stemness and tumor growth. This review highlights that A2B5 is an attractive target to tackle glioma CSCs, and a better characterization of its expression in the developing and adult CNS will benefit to a better understanding of gliomagenesis.
... Ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF) initiate differentiation of the neural stem cells into astrocytes (Johe et al., 1996;Rajan and McKay, 1998). Whereas the signaling molecules sonic hedgehog (Shh), Notch, and Bone Morphogenic Proteins are essential for brain patterning (i.e., ventralization) and for expression of basic helix-loop-helix factors Olig1 and 2 which are required for oligodendrocyte differentiation (Lu et al., 2000;Zhou et al., 2000;Nery et al., 2001;Wang and Almazan, 2016). Secretion of platelet-derived growth factor (PDGF) by astrocytes triggers differentiation of oligodendrocyte precursor cells to oligodendrocytes when simultaneous with neuronal firing (Johe et al., 1996;Williams et al., 1997). ...
Early-life adversity (ELA), often clinically referred to as “adverse childhood experiences (ACE),” is the exposure to stress-inducing events in childhood that can result in poor health outcomes. ELA negatively affects neurodevelopment in children and adolescents resulting in several behavioral deficits and increasing the risk of developing a myriad of neuropsychiatric disorders later in life. The neurobiological mechanisms by which ELA alters neurodevelopment in childhood have been the focus of numerous reviews. However, a comprehensive review of the mechanisms affecting adolescent neurodevelopment (i.e., synaptic pruning and myelination) is lacking. Synaptic pruning and myelination are glia-driven processes that are imperative for brain circuit refinement during the transition from adolescence to adulthood. Failure to optimize brain circuitry between key brain structures involved in learning and memory, such as the hippocampus and prefrontal cortex, leads to the emergence of maladaptive behaviors including increased anxiety or reduced executive function. As such, we review preclinical and clinical literature to explore the immediate and lasting effects of ELA on brain circuit development and refinement. Finally, we describe a number of therapeutic interventions best-suited to support adolescent neurodevelopment in children with a history of ELA.
Laminin, a major component of the basal lamina in the CNS, is also expressed in oligodendrocytes (OLs). However, the function of OL-derived laminin remains largely unknown. Here, we performed loss-of-function studies using two OL-specific laminin-α5 conditional knockout mouse lines. Both mutants were grossly normal and displayed intact blood-brain barrier (BBB) integrity. In a mouse model of intracerebral hemorrhage (ICH), control mice and both mutants exhibited comparable hematoma size and neurological dysfunction. In addition, similar levels of hemoglobin and IgG leakage were detected in the mutant brains compared to the controls, indicating comparable BBB damage. Consistent with this finding, subsequent studies revealed no differences in tight junction protein (TJP) and caveolin-1 expression among control and knockout mice, suggesting that neither paracellular nor transcellular mechanism was affected in the mutants. Furthermore, compared to the controls, both mutant lines showed comparable oligodendrocyte number, oligodendrocyte proliferation rate, MBP/MAG levels, and SMI-32 expression, highlighting a minimal role of OL-derived laminin-α5 in OL biology. Together, these findings highlight a dispensable role of OL-derived laminin-α5 in both brain homeostasis and ICH pathogenesis.
The emergence of myelinating oligodendrocytes represents a pivotal developmental milestone in vertebrates, given their capacity to ensheath axons and facilitate the swift conduction of action potentials. It is widely accepted that cortical oligodendrocyte progenitor cells (OPCs) arise from medial ganglionic eminence (MGE), lateral/caudal ganglionic eminence (LGE/CGE) and cortical radial glial cells (RGCs). Here, we used two different fate mapping strategies to challenge the established notion that the LGE generates cortical OPCs. Furthermore, we used a Cre-loxP-dependent exclusion strategy to reveal that the LGE/CGE does not give rise to cortical OPCs. Additionally, we showed that specifically eliminating MGE-derived OPCs leads to a significant reduction of cortical OPCs. Together, our findings indicate that the LGE does not generate cortical OPCs, contrary to previous beliefs. These findings provide a new view of the developmental origins of cortical OPCs and a valuable foundation for future research on both normal development and oligodendrocyte-related disease.
Oligodendrocytes (OLs) are the myelin-forming cells in the CNS that support neurons through the insulating sheath of axons. This unique feature and developmental processes are achieved by extrinsic and intrinsic gene expression programs, where RNA-binding proteins can contribute to dynamic and fine-tuned post-transcriptional regulation. Here, we identified SECIS-binding protein 2-like (Sbp2l), which is specifically expressed in OLs by integrated transcriptomics. Histological analysis revealed that Sbp2l is a molecular marker of OL maturation. Sbp2l knockdown (KD) led to suppression of matured OL markers, but not a typical selenoprotein, Gpx4. Transcriptome analysis demonstrated that Sbp2l KD decreased cholesterol-biosynthesis-related genes regulated by Tcf7l2 transcription factor. Indeed, we confirmed the downregulation of Tcf7l2 protein without changing its mRNA in Sbp2l KD OPCs. Furthermore, Sbp2l KO mice showed the decrease of Tcf7l2 protein and deficiency of OL maturation. These results suggest that Sbp2l contributes to OL maturation by translational control of Tcf7l2.
White matter injuries (WMIs) are the leading cause of neurologic impairment in infants born premature. There are no treatment options available. The most common forms of WMIs in infants occur prior to the onset of normal myelination, making its pathophysiology distinctive, thus requiring a tailored approach to treatment. Neonates present a unique opportunity to repair WMIs due to a transient abundance of neural stem/progenitor cells (NSPCs) present in the germinal matrix with oligodendrogenic potential. We identified an endogenous oxysterol, 20-αHydroxycholesterol (20HC), in human maternal breast milk that induces oligodendrogenesis through a sonic hedgehog (shh), Gli-dependent mechanism. Following WMI in neonatal mice, injection of 20HC induced subventricular zone-derived oligodendrogenesis and improved myelination in the periventricular white matter, resulting in improved motor outcomes. Targeting the oligodendrogenic potential of postnatal NSPCs in neonates with WMIs may be further developed into a novel approach to mitigate this devastating complication of preterm birth.
The precise timing of neural progenitor development and the correct balance between proliferation and differentiation are crucial to generating a functional brain. The number, survival, and differentiation of neural progenitors during postnatal neurogenesis and gliogenesis is a highly regulated process. Postnatally, the majority of brain oligodendrocytes are generated from progenitors residing in the subventricular zone (SVZ), the germinal niche surrounding the lateral ventricles. In this study, we demonstrate that the p75 neurotrophin receptor (p75NTR) is highly expressed by OPCs in the postnatal male and female rat SVZ. Whereas the p75NTR is known to initiate apoptotic signaling after brain injury, it is highly expressed by proliferating progenitors in the SVZ, suggesting that it may have a different function during development. Lack of p75NTR reduced progenitor proliferation and caused premature oligodendrocyte differentiation and maturation both in vitro and in vivo, leading to aberrant early myelin formation. Our data reveal a novel role for p75NTR as a rheostat for oligodendrocyte production and maturation during myelin formation in the postnatal rat brain.
Oligodendrocytes have elaborate arbors of microtubules that extend toward axons and spiral around myelin sheaths. Oligodendrocytes rely on satellite organelles called Golgi outposts to nucleate new microtubules at sites far from the cell body. We now show that the Golgi outpost marker TPPP (tubulin polymerization promoting protein) forms liquid condensates that co-partition with tubulin in order to nucleate microtubules. In oligodendrocytes, TPPP forms either dynamic puncta or aberrant microtubule-associated aggregates. In Multiple System Atrophy (MSA), a sequela of histological events initiates with TPPP aggregation in myelin sheaths and terminates in perinuclear TPPP co-aggregation with alpha-synuclein (aSyn). Finally, recombinant TPPP aggregates are toxic to primary oligodendrocytes. Thus, while the liquid condensate property of TPPP facilitates microtubule nucleation, it also predisposes TPPP to aggregate in disease.
Myelin is an essential part of the nervous system as it allows for increased and more efficient signal transmission through saltatory conduction. In the central nervous system, myelin sheaths are produced by oligodendrocytes subsequent to the activation of several genes, including the Olig1 and Olig2 transcription factors and Plp1 and Mbp encoding proteins that make up the myelin sheath and which also play roles in oligodendrocyte development from oligodendroglial progenitor cells. Interruption to the function of these genes, such as through inherited mutations, leads to defects in myelin production and abnormal nervous system function that is evident in several demyelinating diseases. However, the regenerative processes of remyelination and innovative therapeutic approaches under clinical development may help to partially alleviate these demyelinating disorders.KeywordsMyelinOligodendroglial progenitor cellOligodendrocyteRemyelination
Jimpy
Rumpshaker
Shiverer
Leukodystrophy
Astrocytes play diverse roles in the central nervous system (CNS) in both physiological and pathological conditions. Previous studies have identified many markers of astrocytes to analyze their complicated roles. Recently, closure of the critical period by mature astrocytes has been revealed, and the need for finding mature astrocyte-specific markers has been growing. We previously found that Ethanolamine phosphate phospholyase (Etnppl) was almost not expressed in the developing neonatal spinal cord, and its expression level slightly decreased after pyramidotomy in adult mice, which showed weak axonal sprouting, suggesting that its expression level negatively correlates with axonal elongation. Although the expression of Etnppl in astrocytes in adult is known, its utility as an astrocytic marker has not yet been investigated in detail. Here, we showed that Etnppl was selectively expressed in astrocytes in adult. Re-analyses using published RNA-sequencing datasets revealed changes in Etnppl expression in spinal cord injury, stroke, or systemic inflammation models. We produced high-quality monoclonal antibodies against ETNPPL and characterized ETNPPL localization in neonatal and adult mice. Expression of ETNPPL was very weak in neonatal mice, except in the ventricular and subventricular zones, and it was heterogeneously expressed in adult mice, with the highest expression in the cerebellum, olfactory bulb, and hypothalamus and the lowest in white matter. Subcellular localization of ETNPPL was dominant in the nuclei with weak expression in the cytosol in the minor population. Using the antibody, astrocytes in adult were selectively labeled in the cerebral cortex or spinal cord, and changes in astrocytes were detected in the spinal cord after pyramidotomy. ETNPPL is expressed in a subset of Gjb6+ astrocytes in the spinal cord. The monoclonal antibodies we created, as well as fundamental knowledge characterized in this study, will be valuable resources in the scientific community and will expand our understanding of astrocytes and their complicated responses in many pathological conditions in future analyses.
In the present study, we examined neural circuit formation in the forebrain of the Olig2 knockout (Olig2‐KO) mouse model and found disruption of the anterior commissure at the late fetal stage. Axon bundles of the anterior commissure encountered the wall of the third ventricle and ceased axonal extension. L1‐CAM immunohistochemistry showed that Olig2‐KO mice lose decussation formation in the basal forebrain. DiI tracing revealed that the thin bundles of the anterior commissure axons crossed the midline but ceased further extension into the deep part of the contralateral side. Furthermore, some fractions of DiI‐labeled axons were oriented dorsolaterally, which was not observed in the control mouse forebrain. The rostral part of the third ventricle was much wider in the Olig2‐KO mice than in wild‐type mice, which likely resulted in the delay of midline fusion and subsequent delay and malformation of the anterior commissure. We analyzed gene expression alterations in the Olig2‐KO mice using a public database and found multiple genes, which are related to axon guidance and epithelial‐mesenchymal transition, showing subtle expression changes. These results suggest that Olig2 is essential for anterior commissure formation, likely by regulating multiple biological processes.
This resource is the long-awaited new revision of the most highly regarded reference volume on glial cells, and has been completely revised, greatly enlarged, and enhanced with full color figures throughout. Neglected in research for years, it is now evident that the brain only functions in a concerted action of all the cells, namely glia and neurons. Seventy one chapters comprehensively discuss virtually every aspect of normal glial cell anatomy, physiology, biochemistry and function, and consider the central roles of these cells in neurological diseases including stroke, Alzheimer disease, multiple sclerosis, Parkinson's disease, neuropathy, and psychiatric conditions. With more than 20 new chapters it addresses the massive growth of knowledge about the basic biology of glia and the sophisticated manner in which they partner with neurons in the course of normal brain function.
This resource is the long-awaited new revision of the most highly regarded reference volume on glial cells, and has been completely revised, greatly enlarged, and enhanced with full color figures throughout. Neglected in research for years, it is now evident that the brain only functions in a concerted action of all the cells, namely glia and neurons. Seventy one chapters comprehensively discuss virtually every aspect of normal glial cell anatomy, physiology, biochemistry and function, and consider the central roles of these cells in neurological diseases including stroke, Alzheimer disease, multiple sclerosis, Parkinson's disease, neuropathy, and psychiatric conditions. With more than 20 new chapters it addresses the massive growth of knowledge about the basic biology of glia and the sophisticated manner in which they partner with neurons in the course of normal brain function.
This resource is the long-awaited new revision of the most highly regarded reference volume on glial cells, and has been completely revised, greatly enlarged, and enhanced with full color figures throughout. Neglected in research for years, it is now evident that the brain only functions in a concerted action of all the cells, namely glia and neurons. Seventy one chapters comprehensively discuss virtually every aspect of normal glial cell anatomy, physiology, biochemistry and function, and consider the central roles of these cells in neurological diseases including stroke, Alzheimer disease, multiple sclerosis, Parkinson's disease, neuropathy, and psychiatric conditions. With more than 20 new chapters it addresses the massive growth of knowledge about the basic biology of glia and the sophisticated manner in which they partner with neurons in the course of normal brain function.
Despite recent advances leading to the suppression of the auto-immune attack during Multiple Sclerosis, efficient remyelinating therapies are still lacking. In MS, the spontaneous remyelination from oligodendrocyte precursor cells (OPCs), present all over the brain, is inefficient and diminishes with age. We identified Tns3 (Tensin 3) as a target of key oligodendrogenic factors (Chd7, Chd8 or Olig2) and showed that it was strongly induced at the onset of oligodendrocytes (OLs) differentiation while downregulated in mature OLs, constituting a good marker for immature OLs (iOLs). I characterized Tns3 expression in oligodendroglia as: 1) mainly restricted to the iOLs, 2) localized in the OLs cytoplasm and processes, and 3) overlapping with known iOLs markers (Itpr2 and Nkx2.2). Tns3 expression is also found during adult brain remyelination after LPC injection, especially in newly formed OLs, therefore constituting a novel marker for iOLs.In vivo Tns3 loss-of-function (LOF) by CRISPR/Cas9 technology in neonatal neural stem cells (NSCs) of the subventricular zone blocks OL differentiation without affecting OPC survival or proliferation. It seems that a total deletion of the locus and the frameshift of the full-length isoform induces similar effects on the OL population. Moreover, OPCs specific deletion of Tns3 in a floxed Tns3 model also impair OL production without affecting the OPC population and the Nkx2.2 positive iOLs. Finally, preliminary data on MACSorted OPCs for Tns3flox mice suggest that Tns3 KO in OPCs delay iOLs differentiation and decrease their survival on the first 72h.All of these data show the involvement of Tns3 in the oligodendrocyte generation
Nervous system development is a dynamic process that follows a highly constrained and genetically organised pattern leading to the generation of a complex framework for guiding human behaviour. Generation of an appropriate number of neurons and glia at the correct time and location is crucial for the proper development of the neural circuitry and mental functions. The neural epithelial cells (NEPs) undergo a stereotyped programme of cell division to generate both neurons and macroglia through progressive differentiation and commitment. A number of evolutionarily conserved signalling factors direct the progenitors to decide whether to self-renew or generate neurons or various glial cells via activating a specified programme of transcription factor expression in target cells. In the present chapter, we will provide an overview regarding the cellular and molecular basis of the generation of various macroglia (astrocytes, oligodendrocytes and NG2 glia) as well as the developmental mechanisms that direct these cells to acquire specific glial identities in the developing central nervous system. This will strengthen our understanding to explore how these mechanisms can be used to design curative therapeutic strategies for various neurodevelopmental disorders and promote the regeneration and repair of the central nervous system.KeywordsNeural epithelial cellsAstrocytogenesisOligodendrocyte precursor cellsOligodendrogliogenesisNG2 glia
5-azacytidine treatment of mouse C3H10T1/2 embryonic fibroblasts converts them to myoblasts at a frequency suggesting alteration of one or only a few closely linked regulatory loci. Assuming such loci to be differentially expressed as poly(A)+ RNA in proliferating myoblasts, we prepared proliferating myoblast-specific, subtracted cDNA probes to screen a myocyte cDNA library. Based on a number of criteria, three cDNAs were selected and characterized. We show that expression of one of these cDNAs transfected into C3H10T1/2 fibroblasts, where it is not normally expressed, is sufficient to convert them to stable myoblasts. Myogenesis also occurs, but to a lesser extent, when this cDNA is expressed in a number of other cell lines. The major open reading frame encoded by this cDNA contains a short protein segment similar to a sequence present in the myc protein family.
Background:
The differentiation of floor plate cells and motor neurons in the vertebrate neural tube appears to be induced by signals from the notochord. The secreted protein encoded by the Sonic hedgehog (Shh) gene is expressed by axial midline cells and can induce floor plate cells in vivo and in vitro. Motor neurons can also be induced in vitro by cells that synthesize Sonic hedgehog protein (Shh). It remains unclear, however, if the motor-neuron-inducing activity of Shh depends on the synthesis of a distinct signaling molecule by floor plate cells. To resolve this issue, we have developed an in vitro assay which uncouples the notochord-mediated induction of motor neurons from floor plate differentiation, and have used this assay to examine whether Shh induces motor neurons in the absence of floor plate differentiation.
Results:
Floor plate cells and motor neurons were induced in neural plate explants grown in contact with the notochord, but only motor neurons were induced when explants were separated from the notochord. COS cells transfected with Shh induced both floor plate cells and motor neurons when grown in contact with neural plate explants, whereas only motor neurons were induced when the explants were grown at a distance from Shh-transfected COS cells. Direct transfection of neural plate cells with an Shh-expression construct induced both floor plate cells and motor neurons, with motor neuron differentiation occurring prior to, or coincidentally with, floor plate differentiation. The induction of motor neurons appears, therefore, not to depend on floor plate differentiation.
Conclusions:
The induction of motor neurons by Shh does not depend on distinct floor-plate-derived signaling molecules. Shh can, therefore, initiate the differentiation of two cell types that are generated in the ventral region of the neural tube. These results show that the early development of motor neurons involves the inductive action of Shh, whereas the survival of motor neurons at later stages of embryonic development requires neurotrophic factors.
Oligodendrocytes, the myelinating cells of the vertebrate CNS, originally develop from cells of the neuroepithelium. Recent studies suggest that spinal cord oligodendrocyte precursors are initially localized in the region of the ventral ventricular zone and subsequently disperse throughout the spinal cord. The characteristics of these early oligodendrocyte precursors and their subsequent migration has been difficult to assay directly in the rodent spinal cord due to a lack of appropriate reagents. In the developing chick spinal cord, we show that oligodendrocyte precursors can be specifically identified by labeling with O4 monoclonal antibody. In contrast to rodent oligodendrocyte precursors, which express O4 immunoreactivity only during the later stages of maturation, in the chick O4 immunoreactivity appears very early and its expression is retained through cellular maturation. In embryos older than stage 35, O4+ cells represent the most immature, self-renewing, cells of the chick spinal cord oligodendrocyte lineage. In the intact chick spinal cord, the earliest O4+ cells are located at the ventral ventricular zone where they actually contribute to the ventricular lining of the central canal. The subsequent migration of O4+ cells into the dorsal region of the spinal cord temporally correlates with the capacity of isolated dorsal spinal cord to generate oligodendrocytes in vitro. Biochemical analysis suggests O4 labels a POA-like antigen on the surface of chick spinal cord oligodendrocyte precursors. These studies provide direct evidence for the ventral ventricular origin of spinal cord oligodendrocytes, and suggest that this focal source of oligodendrocytes is a general characteristic of vertebrate development.
Sonic hedgehog (Shh) encodes a signal that is implicated in both short- and long-range interactions that pattern the vertebrate central nervous system (CNS), somite and limb. Studies in vitro indicate that Shh protein undergoes an internal cleavage to generate two secreted peptides. We have investigated the distribution of Shh peptides with respect to these patterning events using peptide-specific antibodies. Immunostaining of chick and mouse embryos indicates that Shh peptides are expressed in the notochord, floor plate and posterior mesenchyme of the limb at the appropriate times for their postulated patterning functions. The amino peptide that is implicated in intercellular signaling is secreted but remains tightly associated with expressing cells. The distribution of peptides in the ventral CNS is polarized with the highest levels of protein accumulating towards the luminal surface. Interestingly, Shh expression extends beyond the floor plate, into ventrolateral regions from which some motor neuron precursors are emerging. In the limb bud, peptides are restricted to a small region of posterior-distal mesenchyme in close association with the apical ectodermal ridge; a region that extends 50-75 microns along the anterior-posterior axis. Temporal expression of Shh peptides is consistent with induction of sclerotome in somites and floor plate and motor neurons in the CNS, as well as the regulation of anterior-posterior polarity in the limb. However, we can find no direct evidence for long-range diffusion of the 19 x 10(3) Mr peptide which is thought to mediate both short- and long-range cell interactions. Thus, either long-range signaling is mediated indirectly by the activation of other signals, or alternatively the low levels of diffusing peptide are undetectable using available techniques.
The differentiation of floor plate cells and motor neurons can be induced by Sonic hedgehog (SHH), a secreted signaling protein that undergoes autoproteolytic cleavage to generate amino- and carboxy-terminal products. We have found that both floor plate cells and motor neurons are induced by the amino-terminal cleavage product of SHH (SHH-N). The threshold concentration of SHH-N required for motor neuron induction is about 5-fold lower than that required for floor plate induction. Higher concentrations of SHH-N can induce floor plate cells at the expense of motor neuron differentiation. Our results suggest that the induction of floor plate cells and motor neurons by the notochord in vivo is mediated by exposure of neural plate cells to different concentrations of the amino-terminal product of SHH autoproteolytic cleavage.
Products of the PLP gene, proteolipid protein and its isoform DM-20, are the most abundant proteins in CNS myelin, and are markers of the oligodendrocyte, the myelin-forming cell in the CNS. The DM-20 transcript has previously been reported to be expressed in newborn oligodendrocyte progenitor cells and during embryonic development. We have therefore used a DM-20 cRNA probe to follow, by in situ hybridization, the oligodendrocyte lineage during embryonic development. DM-20-expressing cells were first detected at E9.5 in the ventricular germinal layer of the laterobasal plate of the diencephalon. At E14.5, DM-20+ cells had largely disappeared from the diencephalic ventricular germinal layer and had colonized the ventral mantle layer at the posterior part of the basal diencephalon. Between E17.5 and P1, the number of DM-20+ cells increased and progressively invaded the major white matter tracts. In the hindbrain, DM-20+ cells appeared at E12.5 in the caudal part of the rhombencephalon, and at E14.5 all along the ventral spinal cord. Between E14.5 and P1, DM-20+ cells progressively colonized, first ventrally then dorsally, all the spinal cord and more extensively the white matter tracts. At E14.5, a large gap separated, rostrally, the medullary columns from the mantle layer cells in the prosencephalon, suggesting that oligodendrocytes in the mid- and forebrain originate from a different pool of precursors than in the rhombencephalon and the spinal cord. Together, these observations suggest that expression of the DM-20 transcript is an early marker of commitment to the oligodendrocyte lineage, and that oligodendrocyte precursors originate in a ventrally restricted region.
The protooncogene Wnt-1 encodes a short-range signal which is first expressed in, and appears to demarcate, the presumptive midbrain. Absence of Wnt-1 expression leads to the loss of this region of the brain. By the end of neural tube closure, expression of Wnt-1 extends down much of the dorsal midline of the central nervous system (CNS). Expression is exclusively limited to the CNS at this and later stages. We have investigated the regulation of Wnt-1 during mouse development. Analysis of the embryonic expression of Wnt-1-lacZ reporter constructs spanning nearly 30 kb of the Wnt-1 locus identified a 5.5 kb cis-acting 3' enhancer element which confers correct temporal and spatial expression on the lacZ gene. Interestingly embryos express Wnt-1-lacZ transgenes in migrating neural crest cells which are derived from the dorsal CNS. Ectopic expression of the Wnt-1-lacZ transgenes may result from perdurance of beta-galactosidase activity in migrating neural crest cells originating from a Wnt-1-expressing region of the dorsal CNS. Alternatively, ectopic expression may arise from transient de novo activation of the transgenes in this cell population. These results are a first step towards addressing how regional cell signaling is established in the mammalian CNS. In addition, transgene expression provides a new tool for the analysis of neural crest development in normal and mutant mouse embryos.
The timing of oligodendrocyte differentiation is thought to depend on an intrinsic clock in oligodendrocyte precursor cells that counts time or cell divisions and limits precursor cell proliferation. We show here that this clock mechanism can be separated into a counting component and an effector component that stops cell proliferation: whereas the counting mechanism is driven by mitogens that activate cell-surface receptors, the effector mechanism depends on hydrophobic signals that activate intracellular receptors, such as thyroid hormones, glucocorticoids and retinoic acid. When purified oligodendrocyte precursor cells are cultured at clonal density in serum-free medium in the presence of mitogens but in the absence of these hydrophobic signals, the cells divide indefinitely and do not differentiate into postmitotic oligodendrocytes. In the absence of mitogens, the precursor cells stop dividing and differentiate prematurely into oligodendrocytes even in the absence of these hydrophobic signals, indicating that these signals are not required for differentiation. The levels of these signals in vivo may normally regulate the timing of oligodendrocyte differentiation, as the maximum number of precursor cell divisions in culture depends on the concentration of such signals and injections of thyroid hormone into newborn rats accelerates oligodendrocyte development. As thyroid hormone, glucocorticoids and retinoic acid have been shown to promote the differentiation of many types of vertebrate cells, it is possible that they help coordinate the timing of differentiation by signalling clocks in precursor cells throughout a developing animal.
We have examined the transcript distribution of six members of the murine paired box-containing gene family (Pax-gene family) in midgestation embryo and adult brain using in situ hybridization analysis. The expression domains of several Pax-genes in the embryo brain were found to correspond with anatomical boundaries that coincide with neuromere landmarks and therefore respect former neuromere territories in the forebrain. The results are consistent with the concept of brain segmentation and suggest a role for Pax-genes in the brain regionalization. In the adult brain the expression of Pax-genes was observed in discreet areas, with a caudal to rostral restriction in the number of the expressed genes. In general the distribution of transcripts along the anterior-posterior axis was similar to that found in midgestation embryo brain, suggesting a role for Pax-genes in the commitment of the precursor cells to different neuronal cell fates and in the maintenance of specific brain cell subtypes. In the cerebellar cortex, the granular cell layer was found to express high levels of the Pax-6 gene, while putative Bergmann glia and cells surrounding the Purkinje cells contained Pax-3 transcripts. The main adult brain structures that expressed distinct Pax-mRNAs were the periglomerular and granular cell layer of olfactory bulb, nuclei of the septum, amygdala, and isthmus, which suggests a role for the Pax-gene family in the specification of the subcortical domains of the evolutionary old limbic system.
The precursors for oligodendrocytes, the myelinating cells of the vertebrate CNS, appear to be initially restricted to ventral regions of the embryonic rat spinal cord. These cells subsequently populate dorsal spinal cord regions where they acquire the mature characteristics of oligodendrocytes. To determine the location and timing of proliferation of oligodendrocyte precursors in the ventral spinal cord, and to map their pathways of migration in vivo, an assay that identifies mitotic cells was used in conjunction with antibodies that distinguish astrocytes, oligodendrocytes and their precursors. Between E16.5 and E18.5, two hours after a maternal injection of BrdU, the majority of proliferating cells were located in a discrete cluster at the ventral ventricular zone dorsal to the ventral midline region of the developing spinal cord. By contrast, 12-24 hours following a BrdU injection at E16.5, increasing numbers of labeled cells were seen in the dorsal and more lateral locations of the spinal cord. These observations suggest that BrdU-labeled ventral ventricular cells, or their progeny migrate dorsally and laterally during subsequent spinal cord development. To determine the nature of these proliferating cells, cultures of dorsal and ventral spinal cord from BrdU-labeled animals were double-labeled with antibodies that identify oligodendrocytes or astrocytes and anti-BrdU. In dorsal spinal cord cultures derived from animals that had received a single injection of BrdU at E16.5, the majority of proliferating cells differentiated into astrocytes while, in ventrally derived cultures from the same animals, the majority of proliferating cells differentiated into oligodendrocytes. In dorsal cultures prepared from animals that received multiple injections of BrdU between E16.5 and E18.5, many more cells were labeled with BrdU and approximately half of these differentiated into oligodendrocytes. These observations suggest that during embryonic development proliferating oligodendrocyte precursors are initially located at the ventral ventricular zone dorsal to the ventral midline region of the spinal cord and during subsequent maturation these cells or their progeny migrated dorsally in the ventricular region of the spinal cord, and laterally to reside in the developing white matter.
During rat embryogenesis, PDGF alpha receptor (PDGF-alpha R) mRNA is expressed in the ventral half of the spinal cord in two longitudinal columns, one each side of the central canal. Initially, these columns are only two cells wide but the cells subsequently appear to proliferate and disseminate throughout the spinal cord. Our previous studies of PDGF-alpha R expression in the developing CNS suggested that PDGF-alpha R may be a useful marker of the oligodendrocyte lineage in situ. The data presented here complement those studies and lead us to propose that the earliest oligodendrocyte precursors in the spinal cord originate in a very restricted region of the ventricular zone during a brief window of time around embryonic day 14 (E14). In the embryonic brain, migrating PDGF-alpha R+ cells appear to originate in a localized germinal zone in the ventral diencephalon (beneath the foramen of Monro). Our data demonstrate that gene expression and cell fate can be regulated with exquisite spatial resolution along the dorsoventral axis of the mammalian neural tube.
Motor neuron differentiation is accompanied by the expression of a LIM homeodomain transcription factor, Islet1 (ISL1). To assess the involvement of ISL1 in the generation of motor neurons, we analyzed cell differentiation in the neural tube of embryos in which ISL1 expression has been eliminated by gene targeting. Motor neurons are not generated without ISL1, although many other aspects of cell differentiation in the neural tube occur normally. A population of interneurons that express Engrailed1 (EN1), however, also fails to differentiate in Isl1 mutant embryos. The differentiation of EN1+ interneurons can be induced in both wild-type and mutant neural tissue by regions of the neural tube that contain motor neurons. These results show that ISL1 is required for the generation of motor neurons and suggest that motor neuron generation is required for the subsequent differentiation of certain interneurons.
Identifying the signals that regulate stem cell differentiation is fundamental to understanding cellular diversity in the brain. In this paper we identify factors that act in an instructive fashion to direct the differentiation of multipotential stem cells derived from the embryonic central nervous system (CNS). CNS stem cell clones differentiate to multiple fates: neurons, astrocytes, and oligodendrocytes. The differentiation of cells in a clone is influenced by extracellular signals: Platelet-derived growth factor (PDGF-AA, -AB, and -BB) supports neuronal differentiation. In contrast, ciliary neurotrophic factor and thyroid hormone T3 act instructively on stem cells to generate clones of astrocytes and oligodendrocytes, respectively. Adult stem cells had remarkably similar responses to these growth factors. These results support a simple model in which transient exposure to extrinsic factors acting through known pathways initiates fate decisions by multipotential CNS stem cells.
Platelet-derived growth factor alpha-receptors (PDGFR alpha) are expressed by a subset of neuroepithelial cells in the ventral half of the embryonic day 14 (E14) rat spinal cord. The progeny of these cells subsequently proliferate and migrate into the dorsal parts of the cord after E16. Here, we show that E14 ventral cells are able to generate oligodendrocytes in culture but that dorsal cells acquire this ability only after E16, coinciding with the appearance of PDGFR alpha-immunoreactive cells in the starting population. PDGFR alpha-positive cells in optic nerve and spinal cord cultures co-labelled with antibody markers of oligodendrocyte progenitors. When PDGFR alpha-positive cells were purified from embryonic rat spinal cords by immunoselection and cultured in defined medium, they all differentiated into oligodendrocytes. Very few oligodendrocytes developed in cultures of embryonic spinal cord cells that had been depleted of PDGFR alpha-expressing cells by antibody-mediated complement lysis. These data demonstrate that all PDGFR alpha-positive cells in the embryonic rat spinal cord are oligodendrocyte progenitors and that most or all early-developing oligodendrocytes are derived from these ventrally-derived precursors.
The monoclonal antibody O4 has been used to define a biologically distinct stage of the oligodendroglial lineage in vitro. Furthermore, O4+ oligodendroglial progenitors have been found in cell cultures derived from mature tissue, leading to speculation about the presence of oligodendroglial progenitors in the adult central nervous system (CNS). However, the existence of adult oligodendroglial progenitors has yet to be conclusively demonstrated in the intact animal. We have investigated the expression of O4 immunoreactivity in the developing and mature rat forebrain and the relationship of these cells to cells expressing the early oligodendroglial progenitor markers GD3 ganglioside and NG2 chondroitin sulfate proteoglycan, and to differentiated galactocerebroside expressing oligodendroglia. By the day of birth O4+ cells were already widely distributed throughout the formative corpus callosum and increased in number in the white matter and cortical gray matter over the first 2 postnatal weeks. In contrast to cell culture observations, most O4+ cells seen over this period failed to express GD3, although the majority did express NG2. Beginning at postnatal day 4, NG2+/O4− progenitors in the corpus callosum and cerebral cortical gray matter underwent a wave of differentiation into NG2+/O4+ cells and then into galactocerebroside-positive oligodendroglia. Interestingly, not all cells underwent this progression: a population remained as O4+/NG2+ progenitors. Furthermore, this O4+/NG2+ population persisted into adulthood and failed to express either GD3, galactocerebroside, RIP, or myelin basic protein (MBP). They were also distinguishable from glial fibrillary acidic protein+ and glutamine synthetase+ astrocytes and OX-42+ microglia. We therefore propose that these O4+/NG2+ cells represent adult oligodendroglial progenitors hitherto only described in cell culture. © 1997 Wiley-Liss Inc.
Mutations of the adenomatous polyposis coli (APC) tumor suppressor gene have been linked to familial polyposis, an inherited predisposition to colon cancer, and a high percentage of sporadic colon adenomas. Although this gene is best known for its role in development of bowel neoplasms, in recent studies we have found that APC mRNA levels are greatly enriched in brain compared with peripheral tissues. To help define its role in the nervous system, in this study we have determined its cellular localization immunohistochemically in adult rat brain sections and have detected intense APC immunoreactivity in oligodendrocytes. Since prominent APC immunostaining is detected in cell bodies of mature oligodendrocytes, these antibodies may provide a useful addition to available oligodendrocyte markers. Although the cellular function of APC remains undefined, previous biochemical studies have demonstrated that APC is associated with catenins, cytoplasmic proteins involved in regulating cell-cell adhesion. We propose that, in addition to its critical role in ensuring normal maturation of colonic epithelial cells, the APC tumor suppressor protein also regulates the adhesive properties of oligodendrocytes. © 1996 Wiley-Liss, Inc.
The glial cells missing (gcm) gene in Drosophila encodes a novel nuclear protein that is transiently expressed early in the development of nearly all glia. In loss-of-function gcm mutant alleles, nearly all glia fail to differentiate, and, where we can follow them in the PNS, are transformed into neurons. In gain-of-function gcm conditions using transgenic constructs that drive ectopic gcm expression, many presumptive neurons are transformed into glia. Thus, gcm appears to function as a binary genetic switch for glia versus neurons. In the presence of gcm protein, presumptive neurons become glia, while in its absence, presumptive glia become neurons.
Platelet-derived growth factor (PDGF) has mitogenic or trophic effects on fibroblasts, smooth muscle cells, glial cells, capillary endothelial cells, and neurons. Structurally, PDGF is a family of disulfide-bonded dimers of different combinations of A- and B-polypeptide chains. The PDGF isoforms are stored in the α-granules of platelets and are also produced by a number of other cell types; they exert their cellular effects by binding with different affinities to two related tyrosine kinase receptors.PDGF has a functional role during embryonal development, and also stimulates wound healing in the adult. Another important function of PDGF may be to regulate platelet aggregability. Overactivity of PDGF may be part of the development of several disorders characterized by excessive cell growth, for example, malignancies, atherosclerosis, fibrotic conditions, and glomerulonephritis.
Steady-state levels of rat central nervous system (CNS) platelet-derived growth factor (PDGF) A- and B-chain mRNAs were measured by a polymerase chain reaction method employing a synthetic gene internal standard, and the rates of transcription of PDGF A- and B-chain genes in CNS were estimated by a nuclear runoff assay. The abundance of PDGF B-chain mRNA was an order of magnitude below that of PDGF A-chain mRNA, while the rate of PDGF B-chain transcription was only slightly below that for the PDGF A-chain gene, indicating that the half-life of PDGF B-chain in CNS is shorter than that of PDGF A-chain mRNA. No developmental alterations in expression of the PDGF A- and B-chain genes were detected. By contrast, Northern blots showed that steady-state levels of mRNAs encoding the two PDGF receptor proteins, alpha and beta, were markedly higher in embryonic day 15 and postnatal day 6 rat brains than in later life. These results suggest that the actions of PDGF on the brain in vivo are regulated not at the level of PDGF A and B-chain gene expression, but rather by changes in the level of expression of PDGF alpha- and beta-receptor genes.
A novel mouse homeobox-containing gene, Nkx-2.2, has been isolated. Nkx-2.2 is a member of a family of genes whose homeodomains are homologous to that of the Drosophila NK-2 gene. Nkx-2.2 transcripts are found in localized domains of the brain during mouse embryogenesis. Nkx-2.2 expression in the brain abuts and partially overlaps with the expression domains of two other related homeobox-containing genes, TTF-1 and Dlx. The expression domains of the three genes in the developing prosencephalon coincide with anatomical boundaries, particularly apparent in the diencephalon. This result raises the possibility that these genes may specify regional differentiation of the developing diencephalon into its anatomically and functionally defined subregions. Nkx-2.2 may be involved in specifying diencephalic neuromeric boundaries.
Platelet-derived growth factor (PDGF) has been proposed to be one of the growth factors that drive proliferation during normal development and in various pathological conditions. Support for these hypotheses has been largely circumstantial. We discuss the pros and cons of the different experimental approaches that have been taken to test these hypotheses, and evaluate the information to be gained by characterizing the consequences of deletion of one of the PDGF receptor genes in the Patch mutant mouse.
We have developed a binary transgenic system that activates an otherwise silent transgene in the progeny of a simple genetic cross. The system consists of two types of transgenic mouse strains, targets and transactivators. A target strain bears a transgene controlled by yeast regulatory sequences (UAS) that respond only to the yeast transcriptional activator GAL4. A transactivator strain expresses an active GAL4 gene that can be driven by any selected promoter. The current paradigm uses the murine growth factor int-2 cDNA as the target gene and the GAL4 gene driven by the mouse mammary tumor virus long terminal repeat as the transactivator. Both target and transactivator strains are phenotypically normal. By contrast, the bigenic offspring of these two strains express high levels of the target int-2 gene in each organ expressing the GAL4 transactivator. They also display a characteristic dominant int-2 phenotype that consists of epithelial hyperplasia in mammary and salivary glands, as well as prostatic and epididymal hypertrophy, which results in male sterility.
The myoD gene converts many differentiated cell types into muscle. MyoD is a member of the basic-helix-loop-helix family of
proteins; this 68-amino acid domain in MyoD is necessary and sufficient for myogenesis. MyoD binds cooperatively to muscle-specific
enhancers and activates transcription. The helix-loop-helix motif is responsible for dimerization, and, depending on its dimerization
partner, MyoD activity can be controlled. MyoD senses and integrates many facets of cell state. MyoD is expressed only in
skeletal muscle and its precursors; in nonmuscle cells myoD is repressed by specific genes. MyoD activates its own transcription;
this may stabilize commitment to myogenesis.
Murine homologs of the PDGF A, PDGF B, and PDGF receptor alpha subunit genes were cloned. These were used, together with a mouse PDGF receptor beta subunit cDNA clone, to monitor gene expression in early postimplantation mouse embryos and in F9 embryonal carcinoma cells. RNAse protection analysis shows that PDGF A chain, but not B chain, mRNA is expressed in 6.5- to 8.5-day embryonic and extraembryonic tissues. Both alpha and beta receptor subunit mRNAs are expressed in early embryos, however, alpha subunit mRNA appears earlier and is more abundant than beta subunit mRNA. Undifferentiated F9 embryonal carcinoma stem cells express abundant levels of A chain, but not B chain, mRNA. Neither of the PDGF receptor genes is expressed in stem cells. Treatment with retinoic acid stimulates expression of both PDGF receptor genes. As in postimplantation mouse embryos, alpha receptor subunit mRNA appears earlier and is substantially more abundant than beta subunit mRNA. Collectively, these data demonstrate that the genes encoding the two chains of PDGF and their receptors are regulated independently during development and suggest that the two systems have some nonoverlapping functions in vivo. PDGF A, but not PDGF B, may be particularly important in modulating early events in mouse embryonic development.
The role of cell-cell interactions in the development of bipotential glial progenitor cells in cultures of rat cerebellum and optic nerve was studied. In the cerebellar cultures, progenitor cells divide slowly and most of their progeny develop into additional progenitor cells. Progenitor cells isolated from postconfluent cultures of cerebellum, however, develop rapidly into oligodendrocytes when grown in a serum-free medium. Factors secreted or shed into the medium by young cerebellar interneurons stimulate optic nerve progenitor cells to divide and promote the survival of progenitor cells. These factors appear to alter the function of the internal clock that regulates the timing of oligodendrocyte differentiation. These results suggest that the neuronal microenvironment can influence the lineage decisions of multipotential glial progenitor cells.
Sensory neurons are a major derivative of the neural crest for which there have been no definitive molecular markers in mammals. We have developed a method that combines differential hybridization with degenerate RT-PCR to rapidly screen gene families for members exhibiting differential expression among tissues or cell types. We used this approach to search for transcription factor-encoding genes specifically expressed in mammalian sensory neurons. A novel paired homeodomain protein, called DRG11, was identified. DRG11 is expressed in most sensory neurons, including trkA-expressing neurons, but not in glia or sympathetic neurons. Unexpectedly, it is also expressed in the dorsal horn of the spinal cord, a region to which NGF-dependent sensory neurons project. These data suggest that DRG11 is not only a useful marker for sensory neurons, but may also function in the establishment or maintenance of connectivity between some of these neurons and their central nervous system targets.
2',3'-cyclic-nucleotide 3'-phosphodiesterase (CNP) is an abundant protein of myelinating oligodendrocytes. We report that one of the alternatively spliced CNP mRNAs is also expressed in cultured oligodendrocyte progenitor cells. In situ hybridization revealed a thin longitudinal column of CNP-positive cells in the ventral ventricular zone of the embryonic day 14 rat spinal cord, coincident in time and space with cells that express the platelet-derived growth factor alpha receptor, another putative marker of the oligodendrocyte lineage. These data support the hypothesis that the oligodendrocyte lineage originates at a discrete location in the ventral ventricular zone of the embryonic day 14 rat spinal cord. We further report that transcripts encoding the myelin proteolipid protein (PLP/DM-20) are expressed in an unidentified population of neural progenitors in the ventricular zone abutting the floor plate. Our results support the idea that the ventricular zone is a mosaic of specialized progenitor cells.
In the Drosophila CNS, both neurons and glial are derived from neuroblasts. We have identified a gene, glial cells missing (gcm), that encodes a novel nuclear protein expressed transiently in early glial cells. Its mutation causes presumptive glial cells to differentiate into neurons, whereas its ectopic expression forces virtually all CNS cells to become glial cells. Thus, gcm functions as a binary switch that turns on glial fate while inhibiting default neuronal fate of the neuroblasts and their progeny. Similar results are also obtained in the PNS. Analyses of the mutant revealed that "pioneer neurons" can find correct pathways without glial cells and that neurons and glia have a common molecular basis for individual identity.
Cell lineage studies of the rat cerebral cortex suggest that by midneurogenesis, most precursor cells of the ventricular zone are specified to produce a single cell type. Yet there is also evidence for multipotential precursor cells. We used a retroviral vector to follow the developmental potential of cortical precursor cells by labeling cortical cells in cultures from embryos between 12 and 18 days of gestation. We found specified precursor cells as early as embryonic day 12, in addition to bipotential cells that generate neurons and astrocytes. Most importantly, we discovered a type of neural precursor cell, a neuroepithelial cell, that predominates earlier in development, differs distinctly from the specified precursor cells, and as a population, appears to be multipotential. These data suggest that corticogenesis progresses from an early phase dominated by neuroepithelial cells to a later phase characterized by multiple populations of specified precursor cells.
We have examined the regulation of transcription factor gene expression and phenotypic markers in developing chick sympathetic neurons. Sympathetic progenitor cells first express the bHLH transcriptional regulator Cash-1 (a chicken achaete-scute homologue), followed by coordinate expression of Phox2, a paired homeodomain protein, and GATA-2, a zinc finger protein. SCG10, a pan-neuronal membrane protein, is first detected one stage later, followed by the catecholaminergic neurotransmitter enzyme tyrosine hydroxylase (TH). We have used these markers to ask two questions: (1) is their expression dependent upon inductive signals derived from the notochord or floor plate?; (2) does their sequential expression reflect a single linear pathway or multiple parallel pathways? Notochord ablation experiments indicate that the floor plate is essential for induction of GATA-2, Phox2 and TH, but not for that of Cash-1 and SCG10. Taken together these data suggest that the development of sympathetic neurons involves multiple transcriptional regulatory cascades: one, dependent upon notochord or floor plate-derived signals and involving Phox2 and GATA-2, is assigned to the expression of the neurotransmitter phenotype; the other, independent of such signals and involving Cash-1, is assigned to the expression of pan-neuronal properties. The parallel specification of different components of the terminal neuronal phenotype is likely to be a general feature of neuronal development.
Basic helix-loop-helix (bHLH) proteins are instrumental in determining cell type during development. A bHLH protein, termed NeuroD, for neurogenic differentiation, has now been identified as a differentiation factor for neurogenesis because (i) it is expressed transiently in a subset of neurons in the central and peripheral nervous systems at the time of their terminal differentiation into mature neurons and (ii) ectopic expression of neuroD in Xenopus embryos causes premature differentiation of neuronal precursors. Furthermore, neuroD can convert presumptive epidermal cells into neurons and also act as a neuronal determination gene. However, unlike another previously identified proneural gene (XASH-3), neuroD seems competent to bypass the normal inhibitory influences that usually prevent neurogenesis in ventral and lateral ectoderm and is capable of converting most of the embryonic ectoderm into neurons. The data suggest that neuroD may participate in the terminal differentiation step during vertebrate neuronal development.
We have identified three members of a mouse gene family related to the Drosophila segment polarity gene, hedgehog (hh). Like hh, they encode putative secreted proteins and are thus implicated in cell-cell interactions. One of these, Sonic hh (Shh), is expressed in the notochord, the floor plate, and the zone of polarizing activity, signaling centers that are thought to mediate central nervous system (CNS) and limb polarity. Ectopic expression of Shh in the mouse CNS leads to the activation of floor plate-expressed genes. These results suggest that Shh may play a role in the normal inductive interactions that pattern the ventral CNS.
The Drosophila gene pointed (pnt) encodes two putative transcription factors (P1 and P2) of the Ets family, which in the embryonic CNS are found exclusively in glial cells. Loss of pnt function leads to poorly differentiated glial cells and a marked decrease in the expression of the neuronal antigen 22C10 in the MP2 neurons, which are known to interact intimately with the pntP1-expressing longitudinal glial cells. Ectopic expression of pntP1 RNA forces additional CNS cells to enter the glial differentiation pathway. Interestingly, the additional glial-like cells are often flanked by cells that ectopically express the neuronal antigen 22C10. Therefore, both the pnt loss-of-function as well as the gain-of-function phenotype suggest that glial cells are able to induce 22C10 expression on neighboring neurons. This was further verified by cell transplantation experiments. Thus, pnt is not only required but also sufficient for several aspects of glial differentiation.
The differentiation of distinct cell types in the ventral neural tube depends on local inductive signals from the notochord. We have isolated a vertebrate homolog of the Drosophila segment polarity gene hedgehog (hh) from zebrafish and rat, termed vhh-1. vhh-1 is expressed in the node, notochord, floor plate, and posterior limb bud mesenchyme. Each of these cell groups has floor plate inducing activity, suggesting that the vhh-1 gene may encode a floor plate-inducing molecule. Widespread expression of rat vhh-1 in frog embryos leads to ectopic floor plate differentiation in the neural tube. In vitro tests for the signaling functions of vhh-1 demonstrate that COS cells expressing the rat vhh-1 gene induce floor plate and motor neuron differentiation in neural plate explants. vhh-1 may, therefore, contribute to the floor plate and motor neuron inducing activities of the notochord.
Near the floor plate of the embryonic neural tube there is a group of neuroepithelial precursor cells that are specialized for production of the oligodendrocyte lineage. We performed experiments to test whether specification of these neuroepithelial oligodendrocyte precursors, like other ventral neural cell types, depends on signals from the notochord and/or floor plate. We analyzed heterozygous Danforth's short tail (Sd/+) mutant mice, which lack a notochord and floor plate in caudal regions of the neural tube, and found that oligodendrocyte precursors did not appear at the ventricular surface where there was no floor plate. Moreover, oligodendrocytes did not develop in explant cultures of Sd/+ spinal cord in the absence of a floor plate. When a second notochord was grafted into an ectopic position dorsolateral to the endogenous notochord of a chicken embryo, an additional floor plate was induced along with an ectopic focus of oligodendrocyte precursors at the ventricular surface. Oligodendrocytes developed in explants of intermediate neural tube only when they were cocultured with fragments of notochord or in the presence of purified Sonic hedgehog (Shh) protein. Thus, signals from the notochord/floor plate, possibly involving Shh, are necessary and sufficient to induce the development of ventrally derived oligodendroglia. These signals appear to act by specifying the future fate(s) of neuroepithelial cells at the ventricular surface rather than by influencing the proliferation or differentiation of prespecified progenitor cells in the parenchyma of the cord.
Targeted gene disruption in the mouse shows that the Sonic hedgehog (Shh) gene plays a critical role in patterning of vertebrate embryonic tissues, including the brain and spinal cord, the axial skeleton and the limbs. Early defects are observed in the establishment or maintenance of midline structures, such as the notochord and the floorplate, and later defects include absence of distal limb structures, cyclopia, absence of ventral cell types within the neural tube, and absence of the spinal column and most of the ribs. Defects in all tissues extend beyond the normal sites of Shh transcription, confirming the proposed role of Shh proteins as an extracellular signal required for the tissue-organizing properties of several vertebrate patterning centres.
Several bHLH proteins are involved in vertebrate neurogenesis, but those controlling early stages of neuronal determination have not yet been identified. Here we describe a novel, NeuroD-related bHLH protein, NEUROGENIN, whose expression precedes that of NeuroD in both mouse and Xenopus. Expression of Xenopus NEUROGENIN-related-1 (X-NGNR-1) defines the three prospective territories of primary neurogenesis. Overexpression of X-NGNR-1 (or NEUROGENIN) induces ectopic neurogenesis and ectopic expression of XNeuroD mRNA. Endogenous X-ngnr-1 expression becomes restricted to subsets of cells by lateral inhibition, mediated by X-Delta-1 and X-Notch. The properties of X-NGNR-1 are thus analogous to those of the Drosophila proneural genes, suggesting that it functions as a vertebrate neuronal determination factor.
The epigenetic signals that regulate lineage development in the embryonic mammalian brain are poorly understood. Here we demonstrate that a specific subclass of the transforming growth factor beta superfamily, the bone morphogenetic proteins (BMPs), cause the selective, dose-dependent elaboration of the astroglial lineage from murine embryonic subventricular zone (SVZ) multipotent progenitor cells. The astroglial inductive effect is characterized by enhanced morphological complexity and expression of glial fibrillary acidic protein, with concurrent suppression of neuronal and oligodendroglial cell fates. SVZ progenitor cells express transcripts for the appropriate BMP-specific type I and II receptor subunits and selective BMP ligands, suggesting the presence of paracrine or autocrine developmental signaling pathways (or both). These observations suggest that the BMPs have a selective role in determining the cell fate of SVZ multipotent progenitor cells or their more developmentally restricted progeny.
The generation of distinct neuronal cell types in appropriate numbers and at precise positions underlies the assembly of neural circuits that encode animal behavior. Despite the complexity of the vertebrate central nervous system, advances have been made in defining the principles that control the diversification and patterning of its component cells. A combination of molecular genetic, biochemical, and embryological assays has begun to reveal the identity and mechanism of action of molecules that induce and pattern neural tissue and the role of transcription factors in establishing generic and specific neuronal fates. Some of these advances are discussed here, focusing on the spinal cord as a model system for analyzing the molecular control of central nervous system development in vertebrates.
Although ciliary neurotrophic factor (CNTF) is a potent survival factor for many types of neurons and glial cells in vitro, there is currently no evidence that it participates in normal development. Here we show that CNTF greatly enhances the rate of oligodendrocyte generation. Proliferation of oligodendrocyte precursor cells purified from rodent optic nerves and cultured in platelet-derived growth factor-containing medium is significantly increased by CNTF. Similarly, the number of proliferating oligodendrocyte precursor cells in developing optic nerves of transgenic mice lacking CNTF is decreased by up to threefold and the number of oligodendrocytes is transiently decreased; proliferation is restored to normal by the delivery of exogenous CNTF into the developing optic nerve. Both oligodendrocyte number and myelination ultimately attain wild-type values in CNTF-deficient adult mice, indicating that CNTF is not necessary for either oligodendrocyte differentiation or myelination, although it normally accelerates oligodendrocyte development by enhancing the proliferation of oligodendrocyte precursor cells.
The embryonic cerebral cortex contains a population of stem-like founder cells capable of generating large, mixed clones of neurons and glia in vitro. We report that the default state of early cortical stem cells is neuronal, and that stem cells are heterogeneous in the number of neurons that they generate. In low fibroblast growth factor (FGF2) concentrations, most maintain this specification, generating solely neuronal progeny. Oligodendroglial production within these clones is stimulated by a higher, threshold level of FGF2, and astrocyte production requires additional environmental factors. Because most cortical neurons are born before glia in vivo, these data support a model in which the scheduled production of cortical cells involves an intrinsic neuronal program in the early stem cells and exposure to environmental, glia-inducing signals.
Recent evidence indicates that oligodendrocytes originate initially from the ventral neural tube. We have documented in chick embryos the effect of early ventralization of the dorsal neural tube on oligodendrocyte differentiation. Notochord or floor plate grafted at stage 10 in dorsal position induced the development of oligodendrocyte precursors in the dorsal spinal cord. In vitro, oligodendrocytes differentiated from medial but not intermediate neural plate explants, suggesting that the ventral restriction of oligodendrogenesis is established early. Furthermore, quail fibroblasts overexpressing the ventralizing signal Sonic Hedgehog induced oligodendrocyte differentiation in both the intermediate neural plate and the E4 dorsal spinal cord. These results strongly suggest that the emergence of the oligodendrocyte lineage is related to the establishment of the dorso-ventral polarity of the neural tube.
Stem cells are a subject of intense and increasing interest because of their biological properties and potential medical importance. Unfortunately, the field has been difficult for the nonspecialist to penetrate, in part because of ambiguity about what exactly constitutes a stem cell. A working definition is useful in order to pose the important questions in stem cell biology. However, since different people define stem cells in different ways (for examples, see37 and 76), formulating a generally acceptable definition can lead to a conclusion similar to that of U. S. Supreme Court Justice Byron White's in regard to pornography: “It's hard to define, but I know it when I see it.” A minimalist definition is that stem cells have the capacity both to self-renew and to generate differentiated progeny. Although this is in many respects inadequate, it immediately highlights some important problems: How at each cell division is a stem cell able to pass on its “stem” properties to at least one of its two daughters? And what determines whether stem cell divisions will be self-renewing, or differentiating?
In this review, we discuss the properties of the NG2 chondroitin sulfate proteoglycan, a structurally unique, integral membrane proteoglycan that is found on the surfaces of several different types of immature cells. NG2 is associated with multipotential glial precursor cells (O2A progenitor cells), chondroblasts of the developing cartilage, brain capillary endothelial cells, aortic smooth muscle cells, skeletal myoblasts and human melanoma cells. One common feature of these diverse cell types is that they retain the ability to divide throughout the life of the organism. The NG2 proteoglycan is a multifunctional protein; in vitro studies have shown that NG2 binds type VI collagen, interacts with and modulates the activity of the platelet-derived growth factor-alpha receptor, and inhibits neurite outgrowth. These functional properties are analogous to those of other proteoglycans such as syndecan, betaglycan, and neurocan, suggesting that structurally divergent proteoglycans can carry out similar functions within the organism.