Faculty Bibliography

At present little is known about the developmental mechanisms that give rise to inhibitory gamma-aminobutyric acidergic interneurons of the neocortex or the timing of their subtype specification. As such, we performed a gene expression microarray analysis on cortical interneuron precursors isolated through their expression of a Dlx5/6(Cre-IRES-EGFP) transgene. We purified these precursors from the embryonic mouse neocortex at E13.5 and E15.5 by sorting of enhanced green fluorescent protein-expressing cells. We identified novel transcription factors, neuropeptides, and cell surface genes whose expression is highly enriched in embryonic cortical interneuron precursors. Our identification of many of the genes known to be selectively enriched within cortical interneurons validated the efficacy of our approach. Surprisingly, we find that subpopulations of migrating cortical interneurons express genes encoding for proteins characteristic of mature interneuron subtypes as early as E13.5. These results provide support for the idea that many of the genes characteristic of specific cortical interneuron subtypes are evident prior to their functional integration into cortical microcircuitry. They suggest interneurons are already relegated to specific genetic subtypes shortly after they become postmitotic. Moreover, our work has revealed that many of the genes expressed in cortical interneuron precursors have been independently linked to neurological disorders in both mice and humans

Medulloblastoma is the most common malignant brain tumor in children, but the cells from which it arises remain unclear. Here we examine the origin of medulloblastoma resulting from mutations in the Sonic hedgehog (Shh) pathway. We show that activation of Shh signaling in neuronal progenitors causes medulloblastoma by 3 months of age. Shh pathway activation in stem cells promotes stem cell proliferation but only causes tumors after commitment to-and expansion of-the neuronal lineage. Notably, tumors initiated in stem cells develop more rapidly than those initiated in progenitors, with all animals succumbing by 3-4 weeks. These studies suggest that medulloblastoma can be initiated in progenitors or stem cells but that Shh-induced tumorigenesis is associated with neuronal lineage commitment

Olfactory bulb (OB) interneurons are a heterogeneous population produced beginning in embryogenesis and continuing through adulthood. Understanding how this diversity arises will provide insight into how OB microcircuitry is established as well as adult neurogenesis. Particular spatial domains have been shown to contribute specific interneuron subtypes. However, the temporal profile by which OB interneuron subtypes are produced is unknown. Using inducible genetic fate mapping of Dlx1/2 precursors, we analyzed the production of seven OB interneuron subtypes and found that the generation of each subpopulation has a unique temporal signature. Within the glomerular layer, the production of tyrosine hydroxylase-positive interneurons is maximal during early embryogenesis and decreases thereafter. In contrast, the generation of calbindin interneurons is maximal during late embryogenesis and declines postnatally, whereas calretinin (CR) cell production is low during embryogenesis and increases postnatally. Parvalbumin interneurons within the external plexiform layer are produced only perinatally, whereas the generation of 5T4-positive granule cells in the mitral cell layer does not change significantly over time. CR-positive granule cells are not produced at early embryonic time points, but constitute a large percentage of the granule cells born after birth. Blanes cells in contrast are produced in greatest number during embryogenesis. Together we provide the first comprehensive analysis of the temporal generation of OB interneuron subtypes and demonstrate that the timing by which these populations are produced is tightly orchestrated

ABSTRACT: BACKGROUND: During the embryonic development of the cerebellum, neurons are produced from progenitor cells located along a ventricular zone within dorsal rhombomere 1 that extends caudally to the roof plate of the fourth ventricle. The apposition of the caudal neuroepithelium and roof plate results in a unique inductive region termed the cerebellar rhombic lip, which gives rise to granule cell precursors and other glutamatergic neuronal lineages. Recently, we and others have shown that, at early embryonic stages prior to the emergence of granule cell precursors (E12), waves of neurogenesis in the cerebellar rhombic lip produce specific hindbrain nuclei followed by deep cerebellar neurons. How the induction of rhombic lip-derived neurons from cerebellar progenitors is regulated during this phase of cerebellar development to produce these temporally discrete neuronal populations while maintaining a progenitor pool for subsequent neurogenesis is not known. RESULTS: Employing both gain- and loss-of-function methods, we find that Notch1 signaling in the cerebellar primordium regulates the responsiveness of progenitor cells to bone morphogenetic proteins (BMPs) secreted from the roof plate that stimulate the production of rhombic lip-derived neurons. In the absence of Notch1, cerebellar progenitors are depleted during the early production of hindbrain neurons, resulting in a severe decrease in the deep cerebellar nuclei that are normally born subsequently. Mechanistically, we demonstrate that Notch1 activity prevents the induction of Math1 by antagonizing the BMP receptor-signaling pathway at the level of Msx2 expression. CONCLUSION: Our results provide a mechanism by which a balance between neural induction and maintenance of neural progenitors is achieved in the rhombic lip throughout embryonic development

We have utilized an in vivo-inducible genetic-fate-mapping strategy to permanently label cohorts of Math1-positive cells and their progeny that arise in the rhombic lip of the cerebellar primordium during embryogenesis. At stages prior to E12.5, with the exception of the deep cerebellar nuclei, we find that Math1 cells migrate out of the cerebellar primordium into the rostral hindbrain to populate specific nuclei that include cholinergic neurons of the mesopontine tegmental system. Moreover, analysis of Math1-null embryos shows that this gene is required for the formation of some of these nuclei. Around E12.5, granule cell precursors begin to be labeled: first, ones that give rise to granule cells that predominantly populate the anterior lobes of the adult cerebellum and later, those that populate progressing more caudally lobes until labeling of all granule cell precursors is complete by E17. Thus, we demonstrate that the cerebellar rhombic lip gives rise to multiple cell types within rhombomere 1

The existence of stem cells in the adult nervous system is well recognized; however, the potential of these cells is still widely debated. We demonstrate that neural stem cells exist within the embryonic and adult cerebellum. Comparing the potential of neural stem cells derived from the forebrain and cerebellum, we find that progeny derived from each of these brain regions retain regional character in vitro as well as after homotopic transplantation. However, when ectopically transplanted, neurosphere-derived cells from either region are largely unable to generate neurons. With regard specifically to embryonic and adult cerebellar stem cells, we observe that they are able to give rise to neurons that resemble different select classes of cerebellar subclasses when grafted into the perinatal host cerebellum. Most notably, upon transplantation to the perinatal cerebellum, cerebellar stem cells from all ages are able to acquire the position and mature electrophysiological properties of cerebellar granule cells