Bone morphogenetic proteins such as BMP4 are essential for proper development of telencephalic forebrain structures and induce differentiation of telencephalic neural stem cells into a variety of cellular fates, including astrocytic, neuronal, and mesenchymal cells. Little is yet understood regarding the mechanisms that underlie the spatiotemporal differences in progenitor response to BMP4. In a screen designed to identify novel targets of BMP4 signaling in telencephalic neural stem cells, we found the mRNA levels of the previously uncharacterized factor CXXC5 reproducibly up-regulated upon BMP4 stimulation. In vivo, CXXC5 expression overlapped with BMP4 adjacent to Wnt3a expression in the dorsal regions of the telencephalon, including the developing choroid plexus. CXXC5 showed partial homology with Idax, a related protein previously shown to interact with the Wnt-signaling intermediate Dishevelled (Dvl). Indeed CXXC5 and Dvl co-localized in the cytoplasm and interacted in co-immunoprecipitation experiments. Moreover, fluorescence resonance energy transfer (FRET) experiments verified that CXXC5 and Dvl2 were located in close spatial proximity in neural stem cells. Studies of the functional role of CXXC5 revealed that overexpression of CXXC5 or exposure to BMP4 repressed the levels of the canonical Wnt signaling target Axin2, and CXXC5 attenuated Wnt3a-mediated increase in TOPflash reporter activity. Accordingly, RNA interference of CXXC5 attenuated the BMP4-mediated decrease in Axin2 levels and facilitated the response to Wnt3a in neural stem cells. We propose that CXXC5 is acting as a BMP4–induced inhibitor of Wnt signaling in neural stem cells.

The role of MYC proteins in somatic stem and progenitor cells during development is poorly understood. We have taken advantage of a chick in vivo model to examine their role in progenitor cells of the developing neural tube. Our results show that depletion of endogenous MYC in radial glial precursors (RGPs) is incompatible with differentiation and conversely, that overexpression of MYC induces neurogenesis independently of premature or upregulated expression of proneural gene programs. Unexpectedly, the neurogenic function of MYC depends on the integrity of the polarized neural tissue, in contrast to the situation in dissociated RGPs where MYC is mitogenic. Within the polarized RGPs of the neural tube, MYC drives differentiation by inhibiting Notch signaling and by increasing neurogenic cell division, eventually resulting in a depletion of progenitor cells. These results reveal an unexpected role of MYC in the control of stemness versus differentiation of neural stem cells in vivo. Synopsis This study reveals an unexpected role for MYC in the control of stemness versus differentiation of neural stem cells in vivo and shows that Myc represses Notch signaling and promotes asymmetric neurogenic cell divisions. Elevated MYC levels increase neurogenesis in the developing chick neural tube The neurogenic function of MYC depends on the integrity of the polarized neural tissue MYC promotes apico‐basal neurogenic divisions

CXCL12/CXCR4 signaling has been reported to regulate three essential processes for the establishment of neural networks in different neuronal systems: neuronal migration, cell positioning and axon wiring. However, it is not known whether it regulates the development of A9-A10 tyrosine hydroxylase positive (TH+) midbrain dopaminergic (mDA) neurons. We report here that Cxcl12 is expressed in the meninges surrounding the ventral midbrain (VM), whereas CXCR4 is present in NURR1+ mDA precursors and mDA neurons from E10.5 to E14.5. CXCR4 is activated in NURR1+ cells as they migrate towards the meninges. Accordingly, VM meninges and CXCL12 promoted migration and neuritogenesis of TH+ cells in VM explants in a CXCR4-dependent manner. Moreover, in vivo electroporation of Cxcl12 at E12.5 in the basal plate resulted in lateral migration, whereas expression in the midline resulted in retention of TH+ cells in the IZ close to the midline. Analysis of Cxcr4-/- mice revealed the presence of VM TH+ cells with disoriented processes in the intermediate zone (IZ) at E11.5 and marginal zone (MZ) at E14. Consistently, pharmacological blockade of CXCR4 or genetic deletion of Cxcr4 resulted in an accumulation of TH+ cells in the lateral aspect of the IZ at E14, indicating that CXCR4 is required for the radial migration of mDA neurons in vivo. Altogether, our findings demonstrate that CXCL12/CXCR4 regulates the migration and orientation of processes in A9-A10 mDA neurons.