Long-term maintenance of the adult neurogenic niche depends on proper regulation of entry and exit from quiescence. Neural stem cell (NSC) transition from quiescence to activation is a complex process requiring precise cell-cycle control coordinated with transcriptional and morphological changes. How NSC fate transitions in coordination with the cell-cycle machinery remains poorly understood. Here we show that the Rb/E2F axis functions by linking the cell-cycle machinery to pivotal regulators of NSC fate. Deletion of Rb family proteins results in activation of NSCs, inducing a transcriptomic transition toward activation. Deletion of their target activator E2Fs1/3 results in intractable quiescence and cessation of neurogenesis. We show that the Rb/E2F axis mediates these fate transitions through regulation of factors essential for NSC function, including REST and ASCL1. Thus, the Rb/E2F axis is an important regulator of NSC fate, coordinating cell-cycle control with NSC activation and quiescence fate transitions.

Adult neural stem cells (aNSCs) are relatively quiescent populations that give rise to distinct neuronal subtypes throughout life, yet, at a very low rate and restricted differentiation potential. Thus, identifying the molecular mechanisms that control their cellular expansion is critical for regeneration after brain injury. Loss of the Retinoblastoma protein, Rb, leads to several defects in cell cycle as well as neuronal differentiation and migration during brain development. Here, we investigated the role of Rb during adult neurogenesis in the olfactory bulb (OB) by inducing its temporal deletion in aNSCs and progenitors. Loss of Rb was associated with increased proliferation of adult progenitors in the subventricular zone (SVZ) and the rostral migratory stream (RMS) but did not alter self-renewal of aNSCs or neuroblasts subsequent migration and terminal differentiation. Hence, one month after their birth, Rb-null neuroblasts were able to differentiate into distinct subtypes of GABAergic OB interneurons but were gradually lost after 3 months. Similarly, Rb controlled aNSCs/progenitors proliferation in vitro without affecting their differentiation capacity. This enhanced SVZ/OB neurogenesis associated with loss of Rb was only transient and negatively affected by increased apoptosis indicating a critical requirement for Rb in the long-term survival of adult-born OB interneurons.

Previous research has shown that cyclin-dependent kinases (Cdks) that play physiological roles in cell cycle regulation become activated in post-mitotic neurons after ischemic stroke, resulting in apoptotic neuronal death. In this article, we report our results using the widely used oxygen–glucose deprivation (OGD) in vitro model of ischemic stroke on primary mouse cortical neurons to investigate whether Cdk7, as part of the Cdk-activating kinase (CAK) complex that activates cell cycle Cdks, might be a regulator of ischemic neuronal death and may potentially constitute a therapeutic target for neuroprotection. We found no evidence of neuroprotection with either pharmacological or genetic invalidation of Cdk7. Despite the well-established idea that apoptosis contributes to cell death in the ischemic penumbra, we also found no evidence of apoptosis in the OGD model. This could explain the absence of neuroprotection following Cdk7 invalidation in this model. Neurons exposed to OGD seem predisposed to die in an NMDA receptor-dependent manner that could not be prevented further downstream. Given the direct exposure of neurons to anoxia or severe hypoxia, it is questionable how relevant OGD is for modeling the ischemic penumbra. Due to remaining uncertainties about cell death after OGD, caution is warranted when using this in vitro model to identify new stroke therapies.

The Retinoblastoma (Rb) family of pocket proteins (p107, Rb, and p130) controls all aspects of neurogenesis from stem cell activation to long-term neuronal survival in the brain. Previous studies have reported non-overlapping, often complementary, roles for these cell cycle regulators with possibility for functional compensation. Yet the extent to which each protein might compensate for other family members and whether synergistic effects exist during neural stem cell (NSC) lineage development remain unclear. Fong et al. recently revealed that a triple knock-out (TKO) of all pocket proteins results in a transcriptomic switch from NSC quiescence to activation, followed by niche depletion in the adult hippocampus. Here, we investigated whether pocket proteins are equally critical in NSC fate regulation in the adult subventricular zone (aSVZ) and during embryogenesis. We report that TKO of these proteins results in NSC activation coupled to ectopic progenitor proliferation and massive apoptosis, leading to niche depletion and premature loss of neurogenesis inside the olfactory bulb (OB). Notably, a p107–p130 double knockout carrying a single wild-type Rb allele (DKO) substantially rescues the above defects and maintains adult neurogenesis. In comparison, TKO embryos display severe disruptions in all stages of neurogenesis at E14.5, leading to embryonic lethality. Similar defects are detected when any five out of the six alleles of pocket proteins are lost, with only partial rescue of the proliferation defects observed in DKO embryos. The above TKO phenotypes are partially mediated by opposed deregulations in the Notch-Hes signaling pathway in the embryonic versus the adult brain. Such deregulation is linked to opposite changes in E2F3a and E2F3b embryonic gene expressions. Our data identifies Rb as a critical pocket protein in the control and maintenance of adult OB neurogenesis, and uncovers interchangeable, dose-dependent roles for pocket proteins in the control of neuronal differentiation and survival during development.