Investigating the molecular regulation of adult neural stem cell quiescence

Quiescence is essential for long-term maintenance of adult stem cells and tissue homeostasis. The adult mammalian brain was recently discovered to harbour populations of neural stem cells in at least two neurogenic regions: the subventricular zone (SVZ) of the lateral ventricles, and the subgranular zone (SGZ) of the hippocampus. Adult hippocampal neural stem cells (AHNSCs) are a mostly quiescent population, whose activation is tightly controlled by a complex range of signals derived from their niche. How exactly these signals are interpreted by AHNSCs to regulate quiescence is not fully understood. The proneural bHLH transcription factor Ascl1 is crucial for AHNSC activation, and degradation of Ascl1 protein by the E3 ubiquitin ligase Huwe1 is an important mechanism to allow active neural stem cells to return to quiescence. Here I present the discovery that Ascl1 is unexpectedly expressed and transcribed in quiescent NSCs in vivo, and is suppressed at the protein level to maintain quiescence of NSCs. In order to investigate the molecular mechanisms regulating AHNSC quiescence, and to circumvent the complexities of the neurogenic niche, we have developed an in vitro model of AHNSC quiescence. We have found that the in vitro model robustly reproduces the quiescent stem cell state of NSCs in vivo. We undertook a candidate approach to identify regulators of Ascl1 protein and NSC quiescence and identified the inhibitor of differentiation (Id) protein, Id4, as a novel quiescence factor. Functional studies in vitro in combination with genetic approaches in vivo confirmed that Id4 is expressed in quiescent hippocampal NSCs, maintains NSC quiescence, and mediates the inhibition of Ascl1 protein in quiescent NSCs, by sequestration of its E-protein binding partners. Finally, we investigated the niche signals regulating Id4 expression, and discovered that it is regulated in a complex manner likely by more than one signalling pathway

Id4 promotes the elimination of the pro-activation factor ascl1 to maintain quiescence of adult hippocampal stem cells

Quiescence is essential for the long-term maintenance of adult stem cells but how stem cells maintain quiescence is poorly understood. Here we show that neural stem cells in the adult mouse hippocampus actively transcribe the pro-activation factor Ascl1 regardless of their activated or quiescent states. We found that the inhibitor of DNA binding protein Id4 is enriched in quiescent neural stem cells and that elimination of Id4 results in abnormal accumulation of Ascl1 protein and premature stem cell activation. Accordingly, Id4 and other Id proteins promote elimination of Ascl1 protein in neural stem cell cultures. Id4 sequesters Ascl1 heterodimerisation partner E47, promoting Ascl1 protein degradation and stem cell quiescence. Our results highlight the importance of non-transcriptional mechanisms for the maintenance of neural stem cell quiescence and reveal a role for Id4 as a quiescence-inducing factor, in contrast with its role of promoting the proliferation of embryonic neural progenitors