In the embryonic mouse cerebral cortex, progenitors in the ventricular zone (VZ) undergo a developmental change between embryonic day 13 (E13) and E15. This results in the generation of a secondary proliferative population and the appearance of a second germinal layer, the subventricular zone (SVZ). We have shown previously that bone morphogenetic proteins (BMPs) and fibroblast growth factor 2 (FGF2) act antagonistically to regulate the development of a subset of SVZ progenitors that normally express a high level of epidermal growth factor (EGF) receptors and divide in response to EGF. In the present study, we show that Wnt 7a, Wnt 7b, and Sonic hedgehog (Shh) promote progenitor maturation in explant cultures, as reported for FGF2. Wnts 7a and 7b also stimulate the proliferation of neurogenic progenitors and increase the number of cells that can generate primary neurospheres. To determine whether Wnts, FGF2, and Shh act independently or in a common pathway, each factor was inhibited in cortical explants. This revealed that endogenous Wnts, FGF2, and Shh normally contribute to progenitor maturation. Moreover, Wnt 7a depends on FGF2 or Shh to promote maturation but not proliferation. Maturation induced by blocking BMPs also depends on Shh. In contrast, FGF2 promotes maturation by a Shh-independent mechanism. In vivo, progenitors infected with a Wnt 7a retrovirus at E10.5 were found preferentially in the SVZ at E16.5. These findings suggest that Wnts depend on Shh or FGF2 to promote progenitor maturation to an SVZ state in the embryonic cortex.

Cortical progenitors begin to interpret leukemia inhibitory factor (LIF) and bone morphogenetic protein (BMP) as astrocyte-inducing signals during late embryonic cortical development, coincident with an increase in their expression of epidermal growth factor receptors (EGFRs). To determine whether the developmental change in EGFRs regulates the change in responsiveness to LIF and BMP, we analyzed cortical progenitors induced to express EGFRs prematurely and progenitors from late embryonic EGFR-null cortex. Premature elevation of EGFRs conferred premature competence to interpret LIF, but not BMP, as an astrocyte-inducing signal. EGFR-null progenitors from late embryonic cortex did not interpret LIF as an astrocyte-inducing signal but responded to BMP4. LIF responsiveness in EGFR-null cells was rescued by the addition of EGFRs but not by the stimulation of fibroblast growth factor receptors. Astrocyte differentiation induced by LIF depends on signal transducer and activator of transcription 3 (STAT3). We show that the level of STAT3 increases during late embryonic development in a subset of progenitors. EGFRs regulate this change in STAT3 and increase STAT3 phosphorylation in response to LIF. Increasing STAT3 prematurely with a retrovirus also increased the phosphorylation of STAT3 by LIF. In contrast to the finding with EGFRs, however, increasing STAT3 did not cause LIF to induce astrocytes, although it reduced expression of the neurogenic factor PAX6 (paired box gene 6 ). Our findings show that developmental changes in EGFRs regulate the competence of progenitors to interpret LIF as an astrocyte-inducing signal. EGFRs elevate STAT3 expression and increase its phosphorylation by LIF, but this is not sufficient to change LIF responsiveness to astrocyte induction, suggesting that EGFRs also regulate LIF responsiveness downstream of STAT3.

Epidermal growth factor receptors (EGFRs) have been implicated in the control of migration in the telencephalon, but the mechanism underlying their contribution is unclear. We show that expression of a threshold level of EGFRs confers chemotactic competence in stem cells, neurons and astrocytes in cortical explants. This level of receptor expression is normally achieved by a subpopulation of cells during mid-embryonic development. Cells that express high levels of EGFR are located in migration pathways, including the tangential pathway to the olfactory bulb via the rostral migratory stream (RMS), the lateral cortical stream (LCS) leading to ventrolateral cortex and the radial pathway from proliferative zones to cortical plate. The targets of these pathways express the ligands HB-EGF and/or TGFα. To test the idea that EGFRs mediate chemotactic migration these pathways, we increased the size of the population of cells expressing threshold levels of EGFRs in vivo by viral transduction. Our results suggest that EGFRs mediate migration radially to the cortical plate and ventrolaterally in the LCS, but not tangentially in the RMS. Within the bulb, however, EGFRs also mediate radial migration. Our findings suggest that developmental changes in EGFR expression, together with changes in ligand expression regulate the migration of specific populations of cells in the telencephalon by a chemoattractive mechanism.