Cell cycle heterogeneity directs the timing of neural stem cell activation from quiescence

Quiescent stem cells in adult tissues can be activated for homeostasis or repair. Neural stem cells (NSCs) in Drosophila are reactivated from quiescence in response to nutrition by the insulin signaling pathway. It is widely accepted that quiescent stem cells are arrested in Go. In this study, however, we demonstrate that quiescent NSCs (qNSCs) are arrested in either G(2) or G(0). G(2)-G(0) heterogeneity directs NSC behavior: G(2) qNSCs reactivate before Go qNSCs. In addition, we show that the evolutionarily conserved pseudokinase Tribbles (Trbl) induces G(2) NSCs to enter quiescence by promoting degradation of Cdc25(string) and that it subsequently maintains quiescence by inhibiting Akt activation. Insulin signaling overrides repression of Akt and silences trbl transcription, allowing NSCs to exit quiescence. Our results have implications for identifying and manipulating quiescent stem cells for regenerative purposes.