Neural stem cells comprise a small population of subependymal cells in the adult brain that divide asymmetrically under baseline conditions to maintain the stem cell pool and divide symmetrically in response to injury to increase their numbers. Using in vivo and in vitro models, we demonstrate that Wnt signaling plays a role in regulating the symmetric divisions of adult neural stem cells with no change in the proliferation kinetics of the progenitor population. Using BAT-gal transgenic reporter mice to identify cells with active Wnt signaling, we demonstrate that Wnt signaling is absent in stem cells in conditions where they are dividing asymmetrically and that it is upregulated when stem cells are dividing symmetrically, such as (a) during subependymal regeneration in vivo, (b) in response to stroke, and (c) during colony formation in vitro. Moreover, we demonstrate that blocking Wnt signaling in conditions where neural stem cells are dividing symmetrically inhibits neural stem cell expansion both in vivo and in vitro. Together, these findings reveal that the mechanism by which Wnt signaling modulates the size of the stem cell pool is by regulating the symmetry of stem cell division.
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