Neural stem/progenitor cells maintain their identity via continuous self-renewal and suppression of differentiation. Gain-of-function experiments in the chick revealed an involvement for Sox1-3 transcription factors in the maintenance of the undifferentiated neural progenitor (NP) identity. However, the mechanism(s) employed by each factor has not been resolved. Here, we derived cortical neural/stem progenitor cells from wild-type and Sox1-null mouse embryos and found that Sox1 plays a key role in the suppression of neurogenic cell divisions. Loss of Sox1 leads to progressive depletion of self-renewing cells, elongation of the cell cycle of proliferating cells, and significant increase in the number of cells exiting the cell cycle. In proliferating NP cells, Sox1 acts via a prospero-related homeobox 1 (Prox1)-mediated pathway to block cell cycle exit that leads to neuronal differentiation in vivo and in vitro. Thus, our results demonstrate that Sox1 regulates the size of the cortical NP pool via suppression of Prox1-mediated neurogenic cell divisions.
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