Skeletal muscle has remarkable capacity for regeneration. This capability derives from resident muscle stem cells, called satellite cells (SCs). During adult muscle homeostasis, SCs are quiescent. Upon injury, they are “activated” to generate myoblasts for muscle repair. SCs are localized between myofibers and their surround- ding basal lamina; this niche promotes SC quiescence. Proper regulation of quiescence is necessary for long- term SC function and for successful engraftment of SCs. Therefore, knowledge of how the niche promotes quiescence is critical to harnessing SCs for therapeutic purposes. However, the mechanisms that underlie SC quiescence and the quiescence-to-activation (Q-to-A) transition remain poorly understood. In uninjured muscles in vivo, SCs have long projections. Little is known of these structures, as they are lost during prepara- tion of SCs for study in vitro. We have developed a single myofiber isolation procedure that allows mainten- ance of projections. QSC projections are microtubule (MT)-rich and ringed with cortical F-actin, resembling neuronal axons. Our findings indicate that SC projections are motile, potentially allowing QSCs to scan the surface of the myofiber for damage and/or signals that regulate quiescence vs. activation. Retraction of project- tions occurs upon SC activation and is a very early step in the Q-to-A transition, preceding other known early steps. Cadherins, which are required for quiescence and regulate the Q-to-A transition, are important factors for maintenance of projections. We hypothesize that SC quiescence is a dynamic state, characterized by motile projections regulated by cadherins and the cytoskeleton. We have identified novel factors as candidate regulators of this process, including the GTPase, Rac1; the MT minus-end binding protein, CAMSAP3; and the RhoA GEF, LFC. We will test our hypotheses with a combination of genetic studies in mice and cell biological studies with single myofiber preparations. The following aims are proposed: 1) to determine the roles of Rac1, CAMSAP3, and LFC on SC quiescence and Q-to-A transition, we will construct mice conditionally lacking these factors in adult SCs and assess SC homeostasis, function, and structure in vivo and on single myofibers prepared with our new methods; and 2) to investigate cytoskeletal dynamics of retracting SC projections during the Q-to-A transition, we will adapt live imaging protocols to our single myofiber preparations. Mice with SC- specific expression of fluorescent probes for F-actin and MT dynamics will be employed with time-lapse micro- scopy. The effects of perturbing cadherins, the cytoskeleton, and specific signaling proteins on this initial step of SC activation will be determined. The ability to exploit SCs in muscle therapies requires detailed molecular and cell biological knowledge of SC quiescence and the Q-to-A transition, but they are poorly understood. The proposed aims are highly novel and involve a synergist...