Msps Governs Acentrosomal Microtubule Assembly and Reactivation of Quiescent Neural Stem Cells

SUMMARY The ability of stem cells to switch between quiescence and proliferation is crucial for tissue homeostasis and regeneration. Drosophila quiescent neural stem cells (NSCs) extend a primary cellular protrusion from the cell body prior to their reactivation. However, the structure and function of this protrusion are not well established. In this study, we show that in the primary protrusion of quiescent NSCs microtubules are predominantly acentrosomal and oriented plus-end-out, distal to the cell body. We have identified Mini Spindles (Msps)/XMAP215 as a key regulator of NSC reactivation and acentrosomal microtubule assembly in quiescent NSCs. We show that E-cadherin, a cell adhesion molecule, is localized to NSC-neuropil contact points, in a Msps-dependent manner, and is intrinsically required for NSC reactivation. Our study demonstrates a novel mechanism by which Msps-dependent microtubule assembly in the primary protrusion of quiescent NSCs targets E-cadherin to NSC-neuropil contact sites to promote NSC reactivation. We propose that the neuropil functions as a new niche for promoting NSC reactivation, which may be a general paradigm in mammalian systems.