A Nuclear Role for miR-9 and Argonaute Proteins in Balancing Quiescent and Activated Neural Stem Cell States

Abstract

SummaryThroughout life, adult neural stem cells (NSCs) produce new neurons and glia that contribute to crucial brain functions. Quiescence is an essential protective feature of adult NSCs; however, the establishment and maintenance of this state remain poorly understood. We demonstrate that in the adult zebrafish pallium, the brain-enriched miR-9 is expressed exclusively in a subset of quiescent NSCs, highlighting a heterogeneity within these cells, and is necessary to maintain NSC quiescence. Strikingly, miR-9, along with Argonaute proteins (Agos), is localized to the nucleus of quiescent NSCs, and manipulating their nuclear/cytoplasmic ratio impacts quiescence. Mechanistically, miR-9 permits efficient Notch signaling to promote quiescence, and we identify the RISC protein TNRC6 as a mediator of miR-9/Agos nuclear localization in vivo. We propose a conserved non-canonical role for nuclear miR-9/Agos in controlling the balance between NSC quiescence and activation, a key step in maintaining adult germinal pools.

Involvement of sonic hedgehog and notch signaling in regenerative neurogenesis in adult zebrafish optic tectum after stab injury

“…These results suggest that Notch signaling mechanisms regulate RG activity in the optic tectum in both regenerative and physiological conditions, and are like those in the retina, but not in the telencephalon. Previous studies report that under physiological conditions, inhibition of Notch signaling by a γ‐secretase inhibitor induced ascl1a expression and then ~80% of RG to become proliferative in the telencephalon (about 15% of RG are proliferative in the adult telencephalon) (Chapouton et al, ; Katz et al, ) and that under regenerative conditions, up‐regulation of Notch signaling was induced and inhibition of Notch signaling suppressed RG proliferation and neuronal regeneration (Kishimoto et al, ). Thus, response of Notch signaling to stab injury seems to be opposite between telencephalon and optic tectum.…”

Notch/Hes signaling and miR-9 engage in complex feedback interactions controlling neural progenitor cell proliferation and differentiation

“…45). Very recently, it was shown that miR-9 potentiated Notch signaling to maintain quiescence in adult zebrafish NSCs 46 . In the context of this process, miR-9 is actively shuttled into the nucleus, a phenomenon mediated by binding of the miR-9-Ago complex to the shuttle protein TNRC6 46 .…”

“…Very recently, it was shown that miR-9 potentiated Notch signaling to maintain quiescence in adult zebrafish NSCs 46 . In the context of this process, miR-9 is actively shuttled into the nucleus, a phenomenon mediated by binding of the miR-9-Ago complex to the shuttle protein TNRC6 46 . The function of these nuclear Ago-miR-9 complexes stands in apparent contrast to the function of cytoplasmatic miR-9 in embryonic zebrafish NSCs, where miR-9 is expressed in cycling NSCs and serves to promote neuronal differentiation, an effect at least partially mediated by interference with Notch signaling 17 …”

Aggregated spatio-temporal division patterns emerge from reoccurring divisions of neural stem cells

“…Some levels of heterogeneity have also been observed molecularly, for instance, variable levels of the Zinc finger protein Fezf2 regulate Notch activity levels and quiescence of NSCs (Berberoglu et al, 2014) . Likewise, the expression of miR9 involved in keeping quiescence upstream of Notch signaling is found only in a subset of quiescent cells (Katz et al, 2016) several days after a division. In a recent study, a subpopulation of NSCs that expresses low levels of Elavl3 has been characterized as mostly non-dividing cells in transit towards neuronal differentiation (Lange et al, 2020).…”

“…Besides signals from the environment, cell-intrinsic modulations have been shown to impact on the proliferative activity, for instance the metabolic control of lipogenesis that can induce proliferation (Knobloch et al, 2013(Knobloch et al, , 2017 . Conversely, the expression of miR-9 (Katz et al, 2016) or the degradation of Ascl1 via the ubiquitin ligase Huwe1 are factors inducing quiescence. While a combination of signals received from the environment and intrinsic to the cells seem to influence proliferating activity, it remains to be precisely understood whether and how cells coordinate their activity with their neighbors.…”