Antagonizing niche signals regulate the emergence of postnatal neural stem cells and allow enlargement of their pool

Adult neural stem cells (aNSCs) of the lateral ventricular sub-ventricular zone (V-SVZ) are set aside during embryogenesis from a pool of slowly dividing neural stem/progenitor cells (NSPCs) residing mainly in the lateral ganglionic eminence (LGE). The time of aNSCs specification coincides with the peak of embryonic neurogenesis, raising the question of which mechanisms allow only a subpopulation of NSPCs to retain stem cell identity while others proceed to divide and differentiate into neurons. To address this, we isolated rapidly and slowly dividing NSPCs from the mouse LGE at mid-neurogenesis, when aNSCs are specified, and profiled the transcriptome at the single cell level. We find that slowly dividing NSPCs constitute a heterogeneous population encompassing both radial glia cells (RGCs) and intermediate progenitors (IPs) and characterized by different gene signatures. Focusing on RGCs, we show that slowly and rapidly dividing RGCs differentially express genes involved in the formation - such as Lamb2 - or degradation - such as Mmp15 - of the extracellular matrix (ECM), pointing to opposite niche-remodelling strategies as a potential mechanism of fate determination. In vivo perturbation of Lamb2 and Mmp15 affects the seeding and maintenance of NSCs in an opposing manner, with knockdown of Mmp15 increasing the NSC pool as also shown by single-cell RNA-sequencing. These findings suggest that opposite ECM remodelling by slowly and rapidly dividing RGCs constitutes a novel cross-regulatory niche-level mechanism regulating aNSCs emergence, opening new avenues for regulating their numbers and thereby long-term maintenance of the brain's regenerative potential.