[Show abstract][Hide abstract] ABSTRACT: Neurotrophins are key players of neural development by controlling cell death programs. However, the signaling pathways that mediate their selective responses in different populations of neurons remain unclear. In the mammalian cochlea, sensory neurons differentiate perinatally into type I and II populations both expressing TrkB and TrkC, which bind respectively brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3). How these two neuronal populations respond differentially to these two neurotrophins remains unknown. Here, we report in rat the segregation of the nuclear factor-κB (NFκB) subunit p65 specifically within the type II population postnatally. Using dissociated cultures of embryonic and postnatal spiral ganglion neurons, we observed a specific requirement of NFκB for BDNF but not NT3-dependent neuronal survival during a particular postnatal time window that corresponds to a period of neuronal cell death and hair cell innervation refinement in the developing cochlea. Consistently, postnatal p65 knockout mice showed a specific decreased number in type II spiral ganglion neurons. Taken together, these results identify NFκB as a type II neuron-specific factor that participates in the selective survival effects of BDNF and NT3 signaling on developing spiral ganglion neurons.
Frontiers in Cellular Neuroscience 11/2013; 7:242. · 4.18 Impact Factor
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[Show abstract][Hide abstract] ABSTRACT: The principle by which unmyelinated primary sensory neurons transducing thermal, itch and pain perception are specified in early development is unknown. These classes of sensory neurons diversify from a common population of late-born neurons, which initiate expression of Runt homology domain transcription factor RUNX1 and the nerve growth factor receptor TrkA. Here, we report that signals emanating from within the mouse dorsal root ganglion mediated partly by early-born neurons destined to a myelinated phenotype participate in fating late-born RUNX1(+)/TrkA(+) neurons. Inductive factors include FGFs via activation of FGF receptor 1 (FGFR1). Consistently, FGF2 is sufficient to induce expression of RUNX1, and Fgfr1 conditional mutant mice display deficits in fating of the common population of late-born RUNX1(+)/TrkA(+) neurons that develop into unmyelinated neurons. Thus, the distinct lineages of sensory neurons are acquired in response to increasing FGF levels provided by a rising number of born neurons.
Journal of Neuroscience 11/2013; 33(45):17656-66. · 6.75 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The sympathetic nervous system relies on distinct populations of neurons that use noradrenaline or acetylcholine as neurotransmitter. We show that fating of the sympathetic lineage at early stages results in hybrid precursors from which, genetic cell-lineage tracing reveals, all types progressively emerge by principal mechanisms of maintenance, repression and induction of phenotypes. The homeobox transcription factor HMX1 represses Tlx3 and Ret, induces TrkA and maintains tyrosine hydroxylase (Th) expression in precursors, thus driving segregation of the noradrenergic sympathetic fate. Cholinergic sympathetic neurons develop through cross-regulatory interactions between TRKC and RET in precursors, which lead to Hmx1 repression and sustained Tlx3 expression, thereby resulting in failure of TrkA induction and loss of maintenance of Th expression. Our results provide direct evidence for a model in which diversification of noradrenergic and cholinergic sympathetic neurons is based on a principle of cross-repressive functions in which the specific cell fates are directed by an active suppression of the expression of transcription factors and receptors that direct the alternative fate.
[Show abstract][Hide abstract] ABSTRACT: Hearing requires an optimal afferent innervation of sensory hair cells by spiral ganglion neurons in the cochlea. Here we report that complementary expression of ephrin-A5 in hair cells and EphA4 receptor among spiral ganglion neuron populations controls the targeting of type I and type II afferent fibres to inner and outer hair cells, respectively. In the absence of ephrin-A5 or EphA4 forward signalling, a subset of type I projections aberrantly overshoot the inner hair cell layer and invade the outer hair cell area. Lack of type I afferent synapses impairs neurotransmission from inner hair cells to the auditory nerve. By contrast, radial shift of type I projections coincides with a gain of presynaptic ribbons that could enhance the afferent signalling from outer hair cells. Ephexin-1, cofilin and myosin light chain kinase act downstream of EphA4 to induce type I spiral ganglion neuron growth cone collapse. Our findings constitute the first identification of an Eph/ephrin-mediated mutual repulsion mechanism responsible for specific sorting of auditory projections in the cochlea.
[Show abstract][Hide abstract] ABSTRACT: The formation of functional connectivity in the nervous system is governed by axon guidance that instructs nerve growth and branching during development, implying a similarity between neuronal subtypes in terms of nerve extension. We demonstrate the molecular mechanism of another layer of complexity in vertebrates by defining a transcriptional program underlying growth differences between positionally different neurons. The rate of axon extension of the early subset of embryonic dorsal root ganglion sensory neurons is encoded in neurons at different axial levels. This code is determined by a segmental pattern of axial levels of Runx family transcription factor Runx3. Runx3 in turn determines transcription levels of genes encoding cytoskeletal proteins involved in axon extension, including Rock1 and Rock2 which have ongoing activities determining axon growth in early sensory neurons and blocking Rock activity reverses axon extension deficits of Runx3(-/-) neurons. Thus, Runx3 acts to regulate positional differences in axon extension properties apparently without affecting nerve guidance and branching, a principle that could be relevant to other parts of the nervous system.
The EMBO Journal 08/2012; 31(18):3718-29. · 10.75 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sensory neurons of the dorsal root ganglion (DRG) respond to many different kinds of stimulus. The ability to discriminate between the diverse types of sensation is reflected by the existence of functionally and morphologically specialized sensory neurons. This neuronal diversity is created in a step-wise process extending well into postnatal life. Here, we review the hierarchical organization and the molecular process involving interactions between environmental growth factors, used and reused in different developmental contexts in self-reinforcing and cross-inhibitory mechanisms, and intrinsic gene programs that underlie the progressive diversification of sensory progenitors into specialized neurons. The recent advance in knowledge of sensory neuron specification may provide mechanistic principles that could extend to other parts of the nervous system.
Trends in Neurosciences 04/2012; 35(6):373-81. · 12.90 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The cellular origin and molecular mechanisms regulating pigmentation of head and neck are largely unknown. Melanocyte specification is controlled by the transcriptional activity of Mitf, but no general logic has emerged to explain how Mitf and progenitor transcriptional activities consolidate melanocyte and progenitor cell fates. We show that cranial melanocytes arise from at least two different cellular sources: initially from nerve-associated Schwann cell precursors (SCPs) and later from a cellular source that is independent of nerves. Unlike the midbrain-hindbrain cluster from which melanoblasts arise independently of nerves, a large center of melanocytes in and around cranial nerves IX-X is derived from SCPs, as shown by genetic cell-lineage tracing and analysis of ErbB3-null mutant mice. Conditional gain- and loss-of-function experiments show genetically that cell fates in the neural crest involve both the SRY transcription factor Sox2 and Mitf, which consolidate an SCP progenitor or melanocyte fate by cross-regulatory interactions. A gradual downregulation of Sox2 in progenitors during development permits the differentiation of both neural crest- and SCP-derived progenitors into melanocytes, and an initial small pool of nerve-associated melanoblasts expands in number and disperses under the control of endothelin receptor B (Ednrb) and Wnt5a signaling.
Development 01/2012; 139(2):397-410. · 6.27 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Touch sensation is mediated by specific subtypes of sensory neurons which develop in a hierarchical process from common early progenitor neurons, but the molecular mechanism that underlies diversification of touch-sensitive mechanoreceptive neurons is not fully known. Here, we use genetically manipulated mice to examine whether the transcription factor short stature homeobox 2 (Shox2) participates in the acquisition of neuronal subtypes conveying touch sensation. We show that Shox2 encodes the development of category I low-threshold mechanoreceptive neurons in glabrous skin, i.e. discriminative touch-sensitive neurons which form innervations of epidermal Merkel cell and Meissner corpuscles. In contrast, other sensory fiber endings, including those innervating Pacinian corpuscles, are not dependent on Shox2. Shox2 is expressed in neurons of most or all classes of sensory neurons at early embryonic stages and is later confined to touch-sensitive neurons expressing Ret and/or TrkB. Conditional deletion of Shox2 and analysis of Runx3(-/-);Bax(-/-) mutant mice reveals that Runx3 is suppressing Shox2 while Shox2 is necessary for TrkB expression, and that these interactions are necessary for diversification of TrkB(+) and TrkC(+) mechanoreceptive neurons. In particular, development of TrkB(+)/Ret(+) and TrkB(+)/Ret(-) touch-sensitive neurons is critically dependent on Shox2. Consistently, Shox2 conditional mutant mice demonstrate a dramatic impairment of light touch responses. These results show that Shox2 is required for specification of a subclass of TrkB(+) sensory neurons which convey the sensation of discriminative touch arising from stimuli of the skin.
European Journal of Neuroscience 11/2011; 34(10):1529-41. · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In mammals, sensorineural deafness results from damage to the auditory receptors of the inner ear, the nerve pathways to the brain or the cortical area that receives sound information. In this review, we first focused on the cellular and molecular events taking part to spiral ganglion axon growth, extension to the organ of Corti, and refinement. In the second half, we considered the functional maturation of synaptic contacts between sensory hair cells and their afferent projections. A better understanding of all these processes could open insights into novel therapeutic strategies aimed to re-establish primary connections from sound transducers to the ascending auditory nerve pathways.
[Show abstract][Hide abstract] ABSTRACT: Melanocytes and Schwann cells are derived from the multipotent population of neural crest cells. Although both cell types were thought to be generated through completely distinct pathways and molecular processes, a recent study has revealed that these different cell types are intimately interconnected far beyond previously postulated limits in that they share a common post-neural crest progenitor, i.e. the Schwann cell precursor. This finding raises interesting questions about the lineage relationships of hitherto unrelated cell types such as melanocytes and Schwann cells, and may provide clinical insights into mechanisms of pigmentation disorders and for cancer involving Schwann cells and melanocytes.
Cellular and Molecular Life Sciences CMLS 05/2010; 67(18):3037-55. · 5.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Current opinion holds that pigment cells, melanocytes, are derived from neural crest cells produced at the dorsal neural tube and that migrate under the epidermis to populate all parts of the skin. Here, we identify growing nerves projecting throughout the body as a stem/progenitor niche containing Schwann cell precursors (SCPs) from which large numbers of skin melanocytes originate. SCPs arise as a result of lack of neuronal specification by Hmx1 homeobox gene function in the neural crest ventral migratory pathway. Schwann cell and melanocyte development share signaling molecules with both the glial and melanocyte cell fates intimately linked to nerve contact and regulated in an opposing manner by Neuregulin and soluble signals including insulin-like growth factor and platelet-derived growth factor. These results reveal SCPs as a cellular origin of melanocytes, and have broad implications on the molecular mechanisms regulating skin pigmentation during development, in health and pigmentation disorders.
[Show abstract][Hide abstract] ABSTRACT: Cell migration is essential for the development of numerous structures derived from embryonic neural crest cells (NCCs), however the underlying molecular mechanisms are incompletely understood. NCCs migrate long distances in the embryo and contribute to many different cell types, including peripheral neurons, glia and pigment cells. In the present work we report expression of Nedd9, a scaffolding protein within the integrin signaling pathway, in non-lineage-restricted neural crest progenitor cells. In particular, Nedd9 was found to be expressed in the dorsal neural tube at the time of neural crest delamination and in early migrating NCCs. To analyze the role of Nedd9 in neural crest development we performed loss- and gain-of-function experiments and examined the subsequent effects on delamination and migration in vitro and in vivo. Our results demonstrate that loss of Nedd9 activity in chick NCCs perturbs cell spreading and the density of focal complexes and actin filaments, properties known to depend on integrins. Moreover, a siRNA dose-dependent decrease in Nedd9 activity results in a graded reduction of NCC's migratory distance while forced overexpression increases it. Retinoic acid (RA) was found to regulate Nedd9 expression in NCCs. Our results demonstrate in vivo that Nedd9 promotes the migration of NCCs in a graded manner and suggest a role for RA in the control of Nedd9 expression levels.
[Show abstract][Hide abstract] ABSTRACT: NEDD9 is a scaffolding protein in the integrin signaling pathway that is involved in cell adhesion dynamics. Little is known of the cellular localization of NEDD9 expression during embryonic development. In the present study, we have analyzed NEDD9 mRNA expression in the mouse and identified new relevant expression sites. In addition, we have characterized NEDD9 protein expression pattern for the first time in mammals. At E9.5-E10.5, high levels of Nedd9 and the neurogenic transcription factor neurogenin-2 (Ngn2) were found to largely overlap in two discrete domains of the trunk neural tube along its dorso-ventral axis, with Nedd9 extending to more ventral regions of the ventricular zone and Ngn2 differentially expressed in neuronally committed progenitors of the intermediate zone. At encephalic and trunk levels of the neural tube, NEDD9 was present in Sox2(+) progenitor cell populations mostly generating Ngn2(+) and/or Nurr1(+) cells. A sharp down-regulation of NEDD9 expression was found in cells upon lineage commitment, as observed in Nurr1(+) and Ngn2(+) mesencephalic dopaminergic and brainstem neuronal progenitors. In other tissues/organs, i.e. prospective heart, retina, olfactory epithelium, gonads, cartilage, gut and pituitary gland, NEDD9 was found to be co-expressed with Sox2, RXR alpha and/or Nurr1-like proteins, suggesting that NEDD9 expression is confined to early progenitors involved in diverse organogenesis and that it may depend on the repertoire and levels of retinoic acid co-receptors expressed by those cells.
[Show abstract][Hide abstract] ABSTRACT: Peripherin is an intermediate filament protein that is expressed in peripheral and enteric neurons. In the cochlear nervous system, peripherin expression has been extensively used as a differentiation marker by preferentially labeling the type II neuronal population at adulthood, but yet without knowing its function. Since the expression of peripherin has been associated in time with the process of axonal extension and during regeneration of nerve fibers in other systems, it was of interest to determine whether peripherin expression in cochlear neurons was a static phenotypic trait or rather prone to modifications following nerve injury. In the present study, we first compared the expression pattern of peripherin and beta III-tubulin from late embryonic stages to the adult in rat cochlea. The staining for both proteins was seen before birth within all cochlear neurons. By birth, and for 2 or 3 days, peripherin expression was gradually restricted to the type II neuronal population and their projections. In contrast, from postnatal day (P) 10 onwards, while the expression of beta III-tubulin was still found in projections of all cochlear neurons, only the type I population had beta III-tubulin immunoreactivity in their cell bodies. We next investigated the expression of peripherin in axotomized cochlear neurons using an organotypic explant model. Peripherin expression was surprisingly re-expressed in a vast majority of neurons after axotomy. In parallel, the expression and localization of beta III-tubulin and peripherin in dissociated cultures of cochlear neurons were studied. Both proteins were distributed along the entire neuronal length but exhibited complementary distribution, especially within the projections. Moreover, peripherin immunoreactivity was still abundant in the growth cone, whereas that of beta III-tubulin was decreasing at this compartment. Our findings are consistent with a model in which peripherin plays an important structural role in cochlear neurons and their projections during both development and regenerative processes and which is compatible with the assumption that frequently developmentally regulated factors are reactivated during neuronal regeneration.
[Show abstract][Hide abstract] ABSTRACT: In mammals, degeneration of peripheral auditory neurons constitutes one of the main causes of sensorineural hearing loss. Unfortunately, to date, pharmacological interventions aimed at counteracting this condition have not presented complete effectiveness in protecting the integrity of cochlear neural elements. In this context, the protein kinase C (PKC) family of enzymes are important signalling molecules that play a role in preventing neurodegeneration after nervous system injury. The present study demonstrates, for the first time, that the PKC signalling pathway is directly neurotrophic to axotomised spiral ganglion neurons (SGNs). We found that PKCbetaI was strictly expressed by postnatal and adult SGNs both in situ and in vitro. In cultures of SGNs, we observed that activators of PKC, such as phorbol esters and bryostatin 1, induced neuronal survival and neurite regrowth in a manner dependent on the activation of PKCbetaI. The neuroprotective effects of PKC activators were suppressed by pre-treatment with LY294002 (a PI3K inhibitor) and with U0126 (a MEK inhibitor), indicating that PKC activators promote the survival and neurite outgrowth of SGNs by both PI3K/Akt and MEK/ERK-dependent mechanisms. In addition, whereas combining the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) was shown to provide only an additive effect on SGN survival, the interaction between PKC and neurotrophin signalling gave rise to a synergistic increase in SGN survival. Taken together, the data indicate that PKCbetaI activation represents a key factor for the protection of the integrity of neural elements in the cochlea.
[Show abstract][Hide abstract] ABSTRACT: Some synthetic ligands of the peripheral-type benzodiazepine receptor (PBR), an 18 kDa protein of the outer mitochondrial membrane, are cytotoxic for several tumor cell lines and arise as promising chemotherapeutic candidates. However, conflicting results were reported regarding the actual effect of these drugs on cellular survival ranging from protection to toxicity. Moreover, the concentrations needed to observe such a toxicity were usually high, far above the affinity range for their receptor, hence questioning its specificity. In the present study, we have shown that micromolar concentrations of FGIN-1-27 and Ro 5-4864, two chemically unrelated PBR ligands are toxic for both PBR-expressing SK-N-BE neuroblastoma cells and PBR-deficient Jurkat lymphoma cells. We have thereby demonstrated that the cytotoxicity of these drugs is unrelated to their PBR-binding activity. Moreover, Ro 5-4864-induced cell death differed strikingly between both cell types, being apoptotic in Jurkat cells while necrotic in SK-N-BE cells. Again, this did not seem to be related to PBR expression since Ro 5-4864-induced death of PBR-transfected Jurkat cells remained apoptotic. Taken together, our results show that PBR is unlikely to mediate all the effects of these PBR ligands. They however confirm that some of these ligands are very effective cytotoxic drugs towards various cancer cells, even for reputed chemoresistant tumors such as neuroblastoma, and, surprisingly, also for PBR-lacking tumor cells.
[Show abstract][Hide abstract] ABSTRACT: N-butyl-beta-carboline-3-carboxylate (betaCCB) is, together with 2-methyl-norharmanium and 2,9-dimethylnorharmanium ions, an endogenously occurring beta-carboline. Due to their structural similarities with the synthetic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), harman and norharman compounds have been proposed to be involved in the pathogenesis of Parkinson's disease. While also structurally related, betaCCB has received much less interest in that respect although we had previously demonstrated that it induces the apoptotic cell death of cultured cerebellar granule neurons (CGNs). Herein, we have investigated the molecular events leading to CGN apoptosis upon betaCCB treatment. We first demonstrated that betaCCB-induced apoptosis occurs in neurons only, most likely as a consequence of a specific neuronal uptake as shown using binding/uptake experiments. Then we observed that, in betaCCB-treated CGNs, caspases 9, 3 and 8 were successively activated, suggesting an activation of the mitochondrial pathway. Consistently, betaCCB also induced the release from the mitochondrial intermembrane space of two pro-apoptotic factors, i.e. cytochrome c and apotptosis inducing factor (AIF). Interestingly, no mitochondrial membrane depolarisation was associated with this release, suggesting a mitochondrial permeability transition pore-independent mechanism. The absence of any neuroprotective effect provided by two mPTP inhibitors, i.e. cyclosporine A and bongkrekic acid, further supported this hypothesis. Together, these results show that betaCCB is specifically taken up by neuronal cells where it triggers a specific permeabilization of the outer mitochondrial membrane and a subsequent apoptotic cell death.
[Show abstract][Hide abstract] ABSTRACT: Most hearing loss results from lesions of the sensory cells and/or neurons of the auditory portion of the inner ear. To date, only the cochlear implantation offers long-term hearing-aid benefit, but still with limited performance and expensive cost. While the underlying causes of deafness are not clear, the death or hair cells and/or neurons and the loss of neuronal contacts are key pathological features. Pinpointing molecular events that control cell death in the cochlea is critical for the development of new strategies to prevent and treat deafness, whether in combination or not with cochlear implant therapy.
Current Pharmaceutical Design 02/2005; 11(17):2257-75. · 3.29 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This review covers the general roles of members of the cysteine protease family of caspases in the process of apoptosis (programmed cell death) looking at their participation in both the "extrinsic" cell death receptor and the "intrinsic" mitochondrial cell death pathways. It defines the difference between initiator and effector caspases and shows the progression of caspase activations that ends up in the apoptotic cell death and elimination of a damaged cell. The review then presents what is currently know about the participation of caspases in the programmed cell death of inner ear sensory cells during the process of normal development and maturation of the inner ear and their importance in this process as illustrated by the results of caspase-3 gene knockout experiments. The participation of specific caspases and the sequence of their activation in the elimination (apoptosis) of damaged sensory cells from adult inner ears after an injury that generates oxidative stress are reviewed. Both the possibility and the potential efficacy of caspase inhibition with a broad-spectrum pancaspase inhibitor as an interventional therapy to treat and rescue oxidative stress-damaged inner ear sensory cells from apoptosis are presented and discussed.
[Show abstract][Hide abstract] ABSTRACT: Immunocytochemical analysis showed that ionotropic glycine receptors are expressed in neurogenic progenitors purified from the newborn rat striatum and expressing the polysialylated form of the neural cell adhesion molecule, both in vitro and in situ. To ascertain whether glycine receptors were functional in vitro, whole-cell patch-clamp recordings demonstrated that glycine triggers inward strychnine-sensitive currents in the majority of these cells. Moreover, we found that glycine receptors expressed by these neurogenic progenitors display intermediate electrophysiological characteristics between those of glycine receptors expressed by neural stem cells and by mature interneurons from the rat striatum. Altogether, the present data show that functional strychnine-sensitive glycine receptors are expressed in neurogenic progenitors purified from the newborn rat striatum.