Construction of the vertebrate nervous system begins with the decision of a group of ectoderm cells to take on a neural fate. Studies using Xenopus ectodermal explants, or with mouse ectoderm cells or embryonic stem (ES) cells, demonstrated that this process of neural determination occurred by default – the ectoderm cells became neural after the removal of inhibitory signals. Whether ectoderm or ES cells directly differentiate into bona fide neural stem cells is not clear. One model suggests that there is an intermediate stage where “primitive” neural stem cells (pNSC) emerge harbouring properties of both ES cells and neural stem cells. The goal of my research was to address this question by evaluating the role of growth factor signaling pathways and their impact on the function of the zinc-finger homeobox transcription factor, Zfhx1b, during mouse neural stem cell development. I tested whether FGF and Wnt signaling pathways could regulate Zfhx1b expression to control early neural stem cell development. Inhibition of FGF signaling at a time when the ectoderm is acquiring a neural fate resulted in the accumulation of too many pNSCs, at the expense of the definitive neural stem cells. Interestingly, over-expression of Zfhx1b was sufficient to rescue the transition from a pNSC to definitive NSC. These data suggested that definitive NSC fate specification in the mouse ectoderm was facilitated by FGF activation of Zfhx1b, whereas canonical Wnt signaling repressed Zfhx1b expression. Next I sought to determine whether Zfhx1b was similarly required during neural lineage development in ES cells. FGF and Wnt signaling modulated expression of Zfhx1b in ES cell cultures in manner resembling my observations from similar experiments using mouse ectoderm. Knockdown of Zfhx1b in ES cells using siRNA did not affect the initial transition of ES cells to pNSC fate, but did limit the ability of these neural cells to further develop into definitive NSCs. Thus, my findings using ES cells were congruent with evidence from mouse embryos and supported a model whereby intercellular signaling induced Zfhx1b, required for the development of definitive NSCs, subsequent to an initial neural specification event that was independent of this pathway.

Construction of the vertebrate nervous system begins with the decision of a group of ectoderm cells to take on a neural fate. Studies using Xenopus ectodermal explants, or with mouse ectoderm cells or embryonic stem (ES) cells, demonstrated that this process of neural determination occurred by default – the ectoderm cells became neural after the removal of inhibitory signals. Whether ectoderm or ES cells directly differentiate into bona fide neural stem cells is not clear. One model suggests that there is an intermediate stage where “primitive” neural stem cells (pNSC) emerge harbouring properties of both ES cells and neural stem cells. The goal of my research was to address this question by evaluating the role of growth factor signaling pathways and their impact on the function of the zinc-finger homeobox transcription factor, Zfhx1b, during mouse neural stem cell development. I tested whether FGF and Wnt signaling pathways could regulate Zfhx1b expression to control early neural stem cell development. Inhibition of FGF signaling at a time when the ectoderm is acquiring a neural fate resulted in the accumulation of too many pNSCs, at the expense of the definitive neural stem cells. Interestingly, over-expression of Zfhx1b was sufficient to rescue the transition from a pNSC to definitive NSC. These data suggested that definitive NSC fate specification in the mouse ectoderm was facilitated by FGF activation of Zfhx1b, whereas canonical Wnt signaling repressed Zfhx1b expression. Next I sought to determine whether Zfhx1b was similarly required during neural lineage development in ES cells. FGF and Wnt signaling modulated expression of Zfhx1b in ES cell cultures in manner resembling my observations from similar experiments using mouse ectoderm. Knockdown of Zfhx1b in ES cells using siRNA did not affect the initial transition of ES cells to pNSC fate, but did limit the ability of these neural cells to further develop into definitive NSCs. Thus, my findings using ES cells were congruent with evidence from mouse embryos and supported a model whereby intercellular signaling induced Zfhx1b, required for the development of definitive NSCs, subsequent to an initial neural specification event that was independent of this pathway.