Brain patterning mechanisms of neurogenesis in mice
Our laboratory is interested in the mechanisms that pattern the telencephalon. Comparison of the E9.5 to E13.5 telencephalon provides illustration of the two fundamental issues we are addressing. Over this four day time period the telencephalon goes from an apparently homogeneous pseudostratified epithelium to a well-defined structure. For this to occur two coordinated processes must occur: 1) distinct regions of the telencephalon must to allocated and 2) from these regions specific cell types must be generated in appropriate numbers. Over the past four years we have investigated a number of distinct signaling pathways that appear elemental in the control of each of these processes. With regard to regional patterning we have found that Shh acts to induce or repress a set of transcription factors that in turn act to pattern the dorsal/ventral axis of the telencephalon. Concomitant with the establishment of regional patterning, lateral signaling acts to maintain the balance between stem cell populations and differentiation. Notably this latter work has implicated radial glia as a stem cell population with the CNS and suggested that Notch and FGF signaling pathways interact in the establishment and maintenance of stem cell populations.
Shh-dependent expression of three homeobox genes act to
Analysis of the Shh null mutant demonstrates that ventral patterning in the telencephalon is dependent on Shh signaling. Work from our laboratory suggests that Shh can modulate the expression of a number of regionally expressed transcription factors implicated in dorsal ventral patterning of the telencephalon (Gaiano et al., 1999; Corbin et al., 2000; Nery et al., 2001). To further our understanding of these transcription factors we are taking a conventional loss of function approach and in parallel using UBM-guided virally-mediated gain of function methods that we developed in our laboratory (Gaiano et al., 1999). An intriguing aspect of this analysis has come from the realization that certain neuronal populations generated within specific dorsoventral domains of the telencephalon undergo tangential migration to assume their position in the mature telencephalon. We are interested in using a combination of transplantation, in combination with the use of transgenic donor cells from Dlx2, Pax6 and Emx1 knockin lines to fate maps how early patterns of gene expression map relate to the ultimate fate of cells transiently expressing these factors during development.
Notch and FGF signaling cooperate to promote neural progenitors to
Notch signaling is known to play a decisive role in keeping neural progenitors in an undifferentiated state in numerous species including mammals. Analysis of the role of Notch signaling in telencephalic development has been hampered by the early lethality of Notch null animals. Using our gain of function approach, we recently demonstrated that Notch signaling promotes neural progenitors to adopt a radial glial identity (Gaiano et al., 2000). Previous observations that radial glia share numerous markers with neural progenitors prompted us to explore the idea that radial glia may be a stem cell population and that the Notch pathway may be utilized in the specification of neural progenitors. We find that cells expressing activated Notch show a four-fold increase in their ability to proliferate in response to FGF signaling. Furthermore, examination of the phenotype generated in vivo by expression of an activated form of the FGF receptor 2 demonstrated that, as with activated Notch, activation of the FGF signaling pathway promotes the formation of radial glia. Together our recent work suggests that the Notch and FGF signaling pathways contribute to the formation and maintenance of neural stem cell populations. Furthermore, our work suggests that in addition to providing a substrate for neuronal migration, radial glia represent an embryonic stem cell population.
Our work to date has explored both the mechanism of regional and cell determination within the telencephalon, as well as the molecular pathways that regulate the maintenance of progenitor populations. A challenge for the future will be understanding how these two processes are cross regulated. The laboratory is taking a combined loss and gain of function genetic approach to address these questions through both in vivo and in vitro analysis.
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