Unveiling neural stem cell quiescence: a crosstalk with the extracellular matrix niche

Neural stem cells (NSCs) in the subependymal zone (SEZ) reside in a very specialized microenvironment that tightly regulates their activation state. Three NSC populations coexist in the niche, quiescent (qNSCs), primed and activated (aNSCs), which display unique, actively modulated, molecular identities. One of the specialized extrinsic properties that affect NSC quiescence-to-activation transitions is the extracellular matrix (ECM). The SEZ is the only region in the whole mouse brain that has two basement membrane (BM) structures: vascular and speckled BMs. Further, it displays higher stiffness compared to non-neurogenic areas, suggesting that both composition and ECM properties could participate in NSC maintenance. In this thesis, we have studied the role of the ECM in regulating NSC behavior. However, differently from more classical studies, we have evaluated this topic considering the potential diverse responses in specific NSC populations. NSC-ECM interactions have been approached at multiple levels. In the first place, we have assessed the role of NSCs as active regulators of the ECM composition. Both our transcriptomic and proteomic data support that qNSCs have a distinguished function regarding the modulation of their local ECM niche. We have shown that qNSCs display a very specific ECM-related signature compared to aNSCs. Furthermore, we have identified BMP4 as a molecular inducer that triggers the acquisition of this signature and favors the secretion of ECM components by quiescent-like cells in vitro. Using a novel microvessel-NSC co-isolation strategy, we have revealed that qNSCs are highly attached to vascular BM in vivo compared to aNSCs. Further, our data unveiled that this interaction favors quiescence maintenance both through direct ECM binding and through EC-derived BMP4 secretion. Using BMP4 NSC conditioned media as a coating, we have shown that NSC-derived ECM also induces a reversible quiescent state, revealing a positive feed-back loop that enhances BMP4-induced quiescence. Regarding the molecular mechanisms underlying this process, we have uncovered the role of ROCK-modulated actin cytoskeleton in adhesion and proliferation in response to NSC self-assembled ECM. Besides, we have revealed that YAP transcriptional activity is enhanced in NSCs in response to adhesion to self-assembled ECM and that its constitutive activation disturbed NSC proliferation, advocating for its role in the acquisition of the adhesion mediated quiescent NSC state.