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.