Regulation of Muscle Stem Cell Dynamics and Quiescence by Netrin-1 Cytoskeletal Signaling

The hallmark property of muscle stem cells (MuSCs) at homeostasis is quiescence. However, MuSCs have cellular protrusions that are heterogeneous, complex, and tipped with filopodia, all signs of motile structures. Such protrusions may serve as sensors of the MuSC niche. Through development of a novel ex vivo live imaging assay for MuSC protrusion dynamics, we report here the identification of a regulatory pathway in this process. The axon guidance cue Netrin-1 promotes MuSC protrusion outgrowth ex vivo in a manner dependent on its receptors Dcc and Neogenin, the small GTPase Rac1, and the actin-branching factor Arp2/3; this pathway is also required for Netrin-dependent axonal growth cone motility. Adult MuSC-specific genetic removal of Netrin-1 receptors, Rac1, and Arp2/3 each result in failure to maintain homeostatic protrusion lengths, spontaneous quiescence exit, and MuSC attrition in uninjured mice. These findings reveal an unanticipated level of morphological dynamism by a quiescent stem cell at homeostasis and link this phenomenon to preservation of the quiescent state.