Troy+ brain stem cells cycle through quiescence and regulate their number by sensing niche occupancy

The adult mouse subependymal zone (SEZ) provides a niche for mammalian neural stem cells (NSCs). However, the molecular signature, self-renewal potential and fate behavior of NSCs remain poorly defined. Here we propose a model in which the fate of active NSCs is coupled to the total number of neighboring NSCs in a shared niche. Using knock-in reporter alleles and single-cell RNA sequencing, we show that the Wnt target Tnfrsf19/Troy identifies both active and quiescent NSCs. Quantitative analysis of genetic lineage tracing of individual NSCs under homeostasis or in response to injury reveals rapid expansion of stem cell number before some return to quiescence. This behavior is best explained by stochastic fate decisions, where stem cell number within a shared niche fluctuates over time. Fate-mapping proliferating cells using a novel Ki67iresCreER allele confirms that active NSCs reversibly return to quiescence, achieving long-term self-renewal. Our findings suggest a novel niche-based mechanism for the regulation of NSC fate and number.