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Research
Interests
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.
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Fig.
1 Between
E9.5 and E13.5 days of development the telencephalon goes from a
single layer of proliferating cells to a structure with a well developed
pallial cortex underlain with two prominent basal pallidal structures,
the striatum and the globus pallidus. |
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Fig.
2 Within the telencephalon Shh positively regulates Nkx2.1 (red)
and Gsh2 (green) and negatively regulates Pax6 (blue) expression.
Subsequently mutual repressive interactions between these homeodomain
genes results in the three complementary domains of expression seen
here schematically in the E9.5 telencephalon. In addition, each
of these transcription factors acts to specify appropriate regional
phenotypes within their domains of expression. |
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Fig.
3 In panels A-C are shown the morphology of cells (infected
either individually [A] or in clusters [B & C]) with a retrovirus
expressing the intracellular portion of the Notch1 receptor (an
constitutatively active form of the Notch receptor). We observed
that under these conditions forebrain cells adopt a radial glial
phenotype. In the lower panels of this figure a schematic of the
viral vector system (top left) and UBM guided delivery system we
are using to do Gain of Function experiments in vivo. |
Over this four day time
period the telencephalon goes from an apparently homogeneous pseudostratified
epithelium to a well-defined structure (Fig.1). 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 pattern the dorso-ventral axis
of the telencephalon.
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; Fig.2). 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 become radial glia
and acquire stem cell properties.
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.
Future Directions: 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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