Cellular and molecular mechanisms regulating cell proliferation during the forebrain development of the mouse.

The predominant precursor cell type during cortical neurogenesis are radial glia cells, which receive extrinsic and intrinsic signals that might influence cell proliferation and neurogenesis. These radial glia cells have direct contact to the growth factor rich basement membrane throughout cell division. However, it is not known, how the signals received from the basal cell attachment influence the behavior of radial glia cells in regard to the regulation of cell proliferation and neurogenesis. Therefore, I examined the lamininγ1 (LNγ1) mutant, lacking the contact of radial glial endfeet to the basement membrane, and the α6 integrin-/- with a disturbed assembly of the basement membrane. The analysis of the LNγ1 mutant and the α6 integrin-/- showed no defects in the radial glia progeny, cell proliferation or their orientation of cell division. Thus, these results strongly suggest that the direct contact of radial glia cells to the basement membrane is not required for these aspects. Radial glia cells of the dorsal telencephalon are also known to be specified by the expression of the transcription factor Pax6, which plays a pivotal role in the regulation of cell proliferation, neurogenesis and regionalisation during development of the telencephalon. In order to understand how Pax6 coordinates these diverse functions at the molecular level, the roles of the different DNA-binding domains of Pax6, the paired domain (PD), the splice variant of the paired domain (PD5a) and the homeodomain (HD) were analyzed in loss- and gain-of-function approaches. The analysis of the specific paired domain mutant Pax6Aey18-/-, that lacks large parts of the paired domain, but contains an intact homeodomain and transactivating domain (TAD), showed that the paired domain is required for the regulation of neurogenesis, cell proliferation and regionalisation in the developing telencephalon and eye. The homeodomain plays only a minor role during telencephalic development, in contrast to its function in the eye, as shown by the analysis of Pax64Neu-/- mice, which have a point mutation in the DNA-binding domain of the homeodomain, while paired domain and transactivating domain are still functional. Moreover retrovirus-mediated overexpression of Pax6 and Pax6(5a) in cortical cells showed that splicing of the paired domain regulates between a Pax6 form that affects neurogenesis, and cell proliferation, while the other Pax6 form, containing exon5a, regulates exclusively cell proliferation