Msps governs acentrosomal microtubule assembly and reactivation of quiescent neural stem cells

Abstract

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. Here, we show that in the protrusion of quiescent NSCs, microtubules are predominantly acentrosomal and oriented plus‐end‐out toward the tip of the primary protrusion. We have identified Mini Spindles (Msps)/XMAP215 as a key microtubule regulator in quiescent NSCs that governs NSC reactivation via regulating acentrosomal microtubule growth and orientation. We show that quiescent NSCs form membrane contact with the neuropil and E‐cadherin, a cell adhesion molecule, localizes to these NSC‐neuropil junctions. Msps and a plus‐end directed motor protein Kinesin‐2 promote NSC cell cycle re‐entry and target E‐cadherin to NSC‐neuropil contact during NSC reactivation. Together, this work establishes acentrosomal microtubule organization in the primary protrusion of quiescent NSCs and the Msps‐Kinesin‐2 pathway that governs NSC reactivation, in part, by targeting E‐cad to NSC‐neuropil contact sites.

Golgi-dependent Reactivation and Regeneration of Quiescent Neural Stem Cells

“…We found that Ace-tub intensity in the protrusion gradually declines over the course of larval development (Figure S1C-F). Msps, an XMAP215/ch-TOG family protein, is a key regulator of microtubule growth in both quiescent and dividing NSCs 33,42 , Interestingly, msps depletion led to significantly lower levels of Ace-Tub in the primary protrusions of qNSCs as compared to control brains at 0h and 6h ALH (Figure S1C-E), and almost similar levels at 16h ALH (Figure S1C and F). This data is consistent with age-dependent decline of the regeneration capability of qNSCs and suggests that the microtubules may play a role in the regeneration of qNSC primary protrusions.…”

“…In most dividing cells, including active NSCs, the centrosomes function as the microtubule-organizing centre (MTOC), the major site of microtubule nucleation and anchoring and the central point for the formation of microtubule arrays 31,32 . Interestingly, a recent study from our lab has demonstrated that microtubules in the primary protrusion of qNSCs are predominantly acentrosomal and oriented plus-end-out, distal to the cell body 33 . However, the microtubule-organizing centre of qNSCs is still unknown.…”

“…Recently, we showed that the cell-adhesion molecule E-cadherin/Shotgun localizes to the NSCneuropil contact sites in an Msps-dependent manner and is required for NSC reactivation (Deng et al, 2021). To determine the role of functional Golgi in this process, we examined E-cad localization and intensity at the NSC-neuropil contact sites in arf1 and sec71 loss-of-function conditions.…”

“…Mini spindles (Msps), an XMAP215/ch-TOG/Msps family protein, is a key regulator of microtubule growth in dividing cells 40 ; it functions as a microtubule polymerase that binds to microtubule plus ends 40 . Our lab has previously demonstrated that Msps plays a vital role in NSC reactivation by regulating microtubule dynamics and orientation 33 . We also showed that the cell adhesion molecule E-cadherin (E-cad), which localizes to NSC-neuropil contact sites in an Msps-dependent manner and with the help of the kinesin-2 complex (plus-end-directed microtubule motor proteins), is intrinsically required for NSC reactivation 33 .…”

“…Our lab has previously demonstrated that Msps plays a vital role in NSC reactivation by regulating microtubule dynamics and orientation 33 . We also showed that the cell adhesion molecule E-cadherin (E-cad), which localizes to NSC-neuropil contact sites in an Msps-dependent manner and with the help of the kinesin-2 complex (plus-end-directed microtubule motor proteins), is intrinsically required for NSC reactivation 33 .…”