Progressive accumulation of resting cells ensures the lifelong maintenance of the hippocampal stem cell population

Neural stem cell numbers fall rapidly in the hippocampus of juvenile mice but stabilise during adulthood, ensuring lifelong hippocampal neurogenesis. We show that this reduction in stem cell depletion rate in young adults is the result of coordinated changes in stem cell behaviour. In particular, while proliferating neural stem cells in juveniles differentiate rapidly, they increasingly return to a resting state of shallow quiescence and progress through additional self-renewing divisions in adulthood. Single-cell transcriptomic, modelling and label-retention analyses indicate that resting cells have a higher activation rate and greater contribution to neurogenesis than dormant cells, which have not left quiescence. These progressive changes in stem cell behaviour result from reduced expression of the pro-activation protein ASCL1 due to increased post-translational degradation. These cellular mechanisms help reconcile current contradictory models of hippocampal NSC dynamics and may contribute to the different rates of decline of hippocampal neurogenesis in mammalian species including humans. Overall design: In vivo hippocampal neural stem cells were purified through a transgenic mouse line. The mouse line expressed Nestin-GFP, and was injected with tamoxifen for 6 days, before culling on the 8th day to recombine tdTomato expression in proliferating cells, using the Ki67-creERT2 transgene. Cells were analysed by 10x Genomics Chromium Single Cell 3' sequencing. 8 samples were analysed in total, across 3 age groups: 1 month, 2 months and 6-8 months old. In total dataset there are 26,036 cells of which 2,947 are hippocampal neural stem cells.