Crossregulation between Neurogenin2 and pathways specifying neuronal identity in the spinal cord

We have examined how genetic pathways that specify neuronal identity and regulate neurogenesis interface in the vertebrate neural tube. Here, we demonstrate that expression of the proneural gene Neurogenin2 (Ngn2) in the ventral spinal cord results from the modular activity of three enhancers active in distinct progenitor domains, suggesting that Ngn2 expression is controlled by dorsoventral patterning signals. Consistent with this hypothesis, Ngn2 enhancer activity is dependent on the function of Pax6, a homeodomain factor involved in specifying the identity of ventral spinal cord progenitors. Moreover, we show that Ngn2 is required for the correct expression of Pax6 and several homeodomain proteins expressed in defined neuronal populations. Thus, neuronal differentiation involves crossregulatory interactions between a bHLH-driven program of neurogenesis and genetic pathways specifying progenitor and neuronal identity in the spinal cord.     

Zic1 promotes the expansion of dorsal neural progenitors in spinal cord by inhibiting neuronal differentiation

The role of Zic1 was investigated by altering its expression status in developing spinal cords. Zic genes encode zinc finger proteins homologous to Drosophila Odd-paired. In vertebrate neural development, they are generally expressed in the dorsal neural tube. Chick Zic1 was initially expressed evenly along the dorsoventral axis and its expression became increasingly restricted dorsally during the course of neurulation. The dorsal expression of Zic1 was regulated by Sonic hedgehog, BMP4, and BMP7, as revealed by their overexpressions in the spinal cord. When Zic1 was misexpressed on the ventral side of the chick spinal cord, neuronal differentiation was inhibited irrespective of the dorsoventral position. In addition, dorsoventral properties were not grossly affected as revealed by molecular markers. Concordantly, when Zic1 was overexpressed in the dorsal spinal cord in transgenic mice, we observed hypercellularity in the dorsal spinal cord. The transgene-expressing cells were increased in comparison to those of truncated mutant Zic1-bearing mice. Conversely, we observed a significant cell number reduction without loss of dorsal properties in the dorsal spinal cords of Zic1-deficient mice. Taken together, these findings suggest that Zic1 controls the expansion of neuronal precursors by inhibiting the progression of neuronal differentiation. Notch-mediated inhibition of neuronal differentiation is likely to act downstream of Zic genes since Notch1 is upregulated in Zic1-overexpressing spinal cords in both the mouse and the chick.     

Canonical BMP7 activity is required for the generation of discrete neuronal populations in the dorsal spinal cord

BMP activity is essential for many steps of neural development, including the initial role in neural induction and the control of progenitor identities along the dorsal-ventral axis of the neural tube. Taking advantage of chick in ovo electroporation, we show a novel role for BMP7 at the time of neurogenesis initiation in the spinal cord. Using in vivo loss-of-function experiments, we show that BMP7 activity is required for the generation of three discrete subpopulations of dorsal interneurons: dI1-dI3-dI5. Analysis of the BMP7 mouse mutant shows the conservation of this activity in mammals. Furthermore, this BMP7 activity appears to be mediated by the canonical Smad pathway, as we demonstrate that Smad1 and Smad5 activities are similarly required for the generation of dI1-dI3-dI5. Moreover, we show that this role is independent of the patterned expression of progenitor proteins in the dorsal spinal cord, but depends on the BMP/Smad regulation of specific proneural proteins, thus narrowing this BMP7 activity to the time of neurogenesis. Together, these data establish a novel role for BMP7 in primary neurogenesis, the process by which a neural progenitor exits the cell cycle and enters the terminal differentiation pathway.     

Neurogenic differentiation of human adipose-derived stem cells: Relevance of different signaling molecules,transcription factors,and key marker genes

Since numerous diseases affect the central nervous system and it has limited self-repair capability, a great interest in using stem cells as an alternative cell source is generated. Previous reports have shown the differentiation of adipose-derived stem cells in neuron-like cells and it has also been proved that the expression pattern of patterning, proneural, and neural factors, such as Pax6, Mash1, Ngn2, NeuroD1, Tbr2 and Tbr1, regulates and defines adult neurogenesis. Regarding this, we hypothesize that a functional parallelism between adult neurogenesis and neuronal differentiation of human adipose-derived stem cells exists. In this study we differentiate human adipose-derived stem cells into neuron-like cells and analyze the expression pattern of different patterning, proneural, neural and neurotransmitter genes, before and after neuronal differentiation. The neuron-like cells expressed neuronal markers, patterning and proneural factors characteristics of intermediate stages of neuronal differentiation. Thus we demonstrated that it is possible to differentiate adipose-derived stem cells in vitro into immature neuron-like cells and that this process is regulated in a similar way to adult neurogenesis. This may contribute to elucidate molecular mechanisms involved in neuronal differentiation of adult human non-neural cells, in aid of the development of potential therapeutic tools for diseases of the nervous system.     

Neural bHLH genes control the neuronal versus glial fate decision in cortical progenitors

We have addressed the role of the proneural bHLH genes Neurogenin2 (Ngn2) and Mash1 in the selection of neuronal and glial fates by neural stem cells. We show that mice mutant for both genes present severe defects in development of the cerebral cortex, including a reduction of neurogenesis and a premature and excessive generation of astrocytic precursors. An analysis of wild-type and mutant cortical progenitors in culture showed that a large fraction of Ngn2; Mash1 double-mutant progenitors failed to adopt a neuronal fate, instead remaining pluripotent or entering an astrocytic differentiation pathway. Together, these results demonstrate that proneural genes are involved in lineage restriction of cortical progenitors, promoting the acquisition of the neuronal fate and inhibiting the astrocytic fate.