Molecular mechanisms regulating neurogenesis in the developing mouse cerebral cortex

Radial glial cells are a widespread non-neuronal cell type in the
developing central nervous system (CNS) of all vertebrates. In the
cortex, distinct subsets of radial glial cells coexist that are
either multipotent or specified towards the generation of neurons
or glial cells (Malatesta et al., 2000). Radial glial cells in the
cerebral cortex are also the source of a second type of neurogenic
progenitors, called basal progenitors. However, whether the
generation of basal progenitors occurs in a stochastic manner or
whether a specific lineage of radial glial cells is specified
towards the generation of these progenitors has not been previously
known. To identify functionally distinct lineages of cortical
radial glial cells, I developed a new strategy using
fluorescence-activated cell sorting (FACS) to isolate them and
study their progeny. I isolated radial glial cells by FACS from a
transgenic mouse line in which green fluorescent protein (GFP)
expression is under the control of the human GFAP promoter.
Strikingly, GFP intensity was correlated with cell fate. Selective
enrichment of cells with a higher GFP intensity separated a largely
non-neurogenic from a neurogenic (low GFP-intensity) subsets of
radial glial cells. Notable differences on the progeny of these
distinct sets of radial glia were found. The neurogenic radial
glial cells subset generated neurons directly and those that are
largely non-neurogenic also gave rise to a small proportion of
Tbr2-positive basal progenitors that are then neurogenic. Thus,
this last subset comprises an indirect neurogenic population of
radial glial cells present in the developing cortex. Microarray
analysis of these distinct sets of radial glial cells revealed
profound differences in their gene expression. Genes related to
gliogenesis, proliferation and cell-cycle regulation were expressed
at higher levels in the largely non-neurogenic set of radial glia
while genes related to neurogenesis, cell adhesion,
neurotransmitter secretion and axon guidance were expressed mostly
in the neurogenic subset. Moreover, the set of genes expressed at
higher levels in the neurogenic radial glia was down-regulated at
later stages (cortical radial glia at E18). Thus, this analysis
reveals differences at the transcriptional level between direct
neurogenic and largely non-neurogenic radial glial cells,
supporting their intrinsic lineage differences. The functional
analysis of a key fate determinant for neurogenesis from radial
glia discovered in this transcriptome analysis will also be
presented. This gene is the transcription factor AP2γ which was
expressed at significantly higher levels in radial glial cells
generating basal progenitors. AP2γ is restricted to the ventricular
zone (VZ)/subventricular zone (SVZ) regions of the developing
cerebral cortex in the entire nervous system and is also highly
expressed in primate and human cortical progenitors. Its genetic
deletion within the mouse cerebral cortex results in the molecular
misspecification of basal progenitors with decreased levels of Tbr2
and Math3 expression, as well as their overproliferation associated
with increased cell death specifically in the occipital cortex.
This causes a reduction in upper layer neuron generation with
intriguing functional defects in visual acuity. Gain-of-function
studies also revealed the important role of AP2γ for the adequate
specification and development of basal progenitors in the cerebral
cortex, while apical progenitors were not affected by the loss- and
gain-of-function of this transcription factor. Thus, I show for the
first time the prospective isolation of distinct radial glia
subtypes in the mouse developing cortex demonstrated at the
molecular and functional level.