Lhx2 and Lhx9 Determine Neuronal Differentiation and Compartition in the Caudal Forebrain by Regulating Wnt Signaling

Karlsruhe Institute of Technology, Institute of Toxicology and Genetics, Karlsruhe, Germany.
PLoS Biology (Impact Factor: 9.34). 12/2011; 9(12):e1001218. DOI: 10.1371/journal.pbio.1001218
Source: PubMed


Author Summary
The thalamus is the interface between the body and the brain. It connects sensory organs with higher brain areas and modulates processes such as sleep, alertness, and consciousness. Our knowledge about the embryonic development of this central relay station is still fragmented. Here, we show that the transcription factors Lhx2 and Lhx9 are essential for the development of the relay thalamus. Zebrafish embryos lacking Lhx2/Lhx9 have stalled neurogenesis - neuronal progenitor cells accumulate but do not complete their differentiation into thalamic neurons. In addition, we find that the neighboring Wnt-expressing epithalamus expands into the space containing mis-specified thalamus in these embryos. We identified a thalamus-specific cell adhesion modulator, Pcdh10b, which is controlled by canonical Wnt signaling. Altered Wnt-dependent Pcdh10b function in Lhx2/Lhx9-deficient embryos leads to intermingling of the thalamus and adjacent brain compartments and consequently regionalization within the caudal forebrain is lost. Organization of the developing CNS into molecularly distinct but transient segments and the implications for regional differentiation are well established for the developing hindbrain. We conclude that this applies to caudal forebrain too: Lhx2 and Lhx9 emerge as crucial factors driving neurogenesis and maintaining the regional integrity of the caudal forebrain. These are two prerequisites for the formation of this important relay station in the brain.

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    • "Using genetic fate mapping, we have recently shown that thalamic neuron precursor cells express homeobox gene Gbx2, and the Gbx2 lineage defines a sharp compartment boundary between the habenula and thalamus in the mouse embryo [6], demonstrating a clear segregation of habenular and thalamic neurons. In parallel, we have demonstrated that the segregation between p1 and p2 cells is regulated by the cell adhesion factor protocadherin 10b in zebrafish [7]. However, how the epithalamus diverges from the thalamus and pretectum is largely unknown. "
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    ABSTRACT: The habenula and the thalamus are two critical nodes in the forebrain circuitry and they connect the midbrain and the cerebral cortex in vertebrates. The habenula is derived from the epithalamus and rests dorsally to the thalamus. Both epithalamus and thalamus arise from a single diencephalon segment called prosomere (p)2. Shh is expressed in the ventral midline of the neural tube and in the mid-diencephalic organizer (MDO) at the zona limitans intrathalamica between thalamus and prethalamus. Acting as a morphogen, Shh plays an important role in regulating cell proliferation and survival in the diencephalon and thalamic patterning. The molecular regulation of the MDO Shh expression and the potential role of Shh in development of the habenula remain largely unclear. We show that deleting paired-box and homeobox-containing gene Pax6 results in precocious and expanded expression of Shh in the prospective MDO in fish and mouse, whereas gain-of-function of pax6 inhibits MDO shh expression in fish. Using gene expression and genetic fate mapping, we have characterized the expression of molecular markers that demarcate the progenitors and precursors of habenular neurons. We show that the thalamic domain is shifted dorsally and the epithalamus is missing in the alar plate of p2 in Pax6 mutant mouse. Conversely, the epithalamus is expanded ventrally at the expanse of the thalamus in mouse embryos with reduced Shh activity. Significantly, attenuating Shh signaling largely rescues the patterning of p2 and restores the epithalamus in Pax6 mouse mutants, suggesting that Shh acts downstream of Pax6 in controlling the formation of the habenula. Similar to that found in the mouse, we show that pax6 controls the formation of the epithalamus mostly via the regulation of MDO shh expression in zebrafish. Our findings demonstrate that Pax6 has an evolutionarily conserved function in establishing the temporospatial expression of Shh in the MDO in vertebrates. Furthermore, Shh mediates Pax6 function in regulating the partition of the p2 domain into the epithalamus and thalamus.
    Full-text · Article · Feb 2014 · BMC Biology
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    • "A number of ligands such as Wnt1, Wnt3a, Wnt7 genes and Wnt8b are expressed in, or in close proximity to, developing dHb neurons [18,44] and may regulate the development of dHb neurons. Wnt3a is also expressed in the roof plate of prosomere 2 and influences the patterning and neurogenesis in the thalamus and the related thalamic mantle zone, which expresses downstream genes of Wnt signaling and harbors post-mitotic thalamic neurons [39]. It will be intriguing to determine whether indeed Wnt3a signaling influences vHb neuron development. "
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    ABSTRACT: The conserved habenular neural circuit relays cognitive information from the forebrain into the ventral mid- and hindbrain. In zebrafish, the bilaterally formed habenulae in the dorsal diencephalon are made up of the asymmetric dorsal and symmetric ventral habenular nuclei, which are homologous to the medial and lateral nuclei respectively, in mammals. These structures have been implicated in various behaviors related to the serotonergic/dopaminergic neurotransmitter system. The dorsal habenulae develop adjacent to the medially positioned pineal complex. Their precursors differentiate into two main neuronal subpopulations which differ in size across brain hemispheres as signals from left-sided parapineal cells influence their differentiation program. Unlike the dorsal habenulae and despite their importance, the ventral habenulae have been poorly studied. It is not known which genetic programs underlie their development and why they are formed symmetrically, unlike the dorsal habenulae. A main reason for this lack of knowledge is that the vHb origin has remained elusive to date. To address these questions, we applied long-term 2-photon microscopy time-lapse analysis of habenular neural circuit development combined with depth color coding in a transgenic line, labeling all main components of the network. Additional laser ablations and cell tracking experiments using the photoconvertible PSmOrange system in GFP transgenic fish show that the ventral habenulae develop in prosomere 2, posterior and lateral to the dorsal habenulae in the dorsal thalamus. Intriguingly, mutant analysis demonstrates that the ventral habenular nuclei only develop in the presence of functional Tcf7l2, a downstream modulator of the Wnt signaling cascade. Consistently, photoconverted thalamic tcf7l2exl/exl mutant cells do not contribute to habenula formation. We show in vivo that dorsal and ventral habenulae develop in different regions of prosomere 2. In the process of ventral habenula formation, functional tcf7l2 gene activity is required and in its absence, ventral habenular neurons do not develop. Influenced by signals from parapineal cells, dorsal habenular neurons differentiate at a time at which ventral habenular cells are still on their way towards their final destination. Thus, our finding may provide a simple explanation as to why only neuronal populations of the dorsal habenulae differ in size across brain hemispheres.
    Full-text · Article · Sep 2013 · Neural Development
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    • "For example, Lhx2 has been suggested to couple with BMP signaling to promote optic cup ventral identity in mice (21). Recently, it has been suggested that Lhx2 may play a role in the regionalization of thalamus formation, through attenuating WNT signaling emanating from the epithalamus in zebrafish (53). Additionally, expression of Lhx2 in the presumptive hippocampus during mouse neocortex formation attenuates WNT and BMP signaling emanating from the cortical hem, thereby restricting cortical hem extension and thereafter forming a sharp border between the hippocampus and cortical hem (18). "
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    ABSTRACT: The LIM homeobox 2 transcription factor Lhx2 is known to control crucial aspects of neural development in various species. However, its function in human neural development is still elusive. Here, we demonstrate that LHX2 plays a critical role in human neural differentiation, using human embryonic stem cells (hESCs) as a model. In hESC-derived neural progenitors (hESC-NPs), LHX2 was found to be expressed before PAX6, and co-expressed with early neural markers. Conditional ectopic expression of LHX2 promoted neural differentiation, whereas disruption of LHX2 expression in hESCs significantly impaired neural differentiation. Furthermore, we have demonstrated that LHX2 regulates neural differentiation at two levels: first, it promotes expression of PAX6 by binding to its active enhancers, and second, it attenuates BMP and WNT signaling by promoting expression of the BMP and WNT antagonist Cerberus 1 gene (CER1), to inhibit non-neural differentiation. These findings indicate that LHX2 regulates the transcription of downstream intrinsic and extrinsic molecules that are essential for early neural differentiation in human.
    Full-text · Article · Jun 2013 · Nucleic Acids Research
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