Adult mouse muscle satellite cells (MuSCs) are quiescent in uninjured muscles. Upon injury, MuSCs exit quiescence to become activated, re-enter the cell cycle to proliferate, and differentiate to repair the damaged muscles. It remains unclear which extrinsic cues and intrinsic signaling pathways regulate quiescence exit during MuSC activation. Here, we demonstrated that inducible MuSC-specific deletion of, a catalytic subunit of phosphatidylinositol 3-kinase (PI3K), rendered MuSCs unable to exit quiescence, resulting in severely impaired MuSC proliferation and muscle regeneration. Genetic reactivation of mTORC1, or knockdown of s, in-null MuSCs partially rescued the above defects, making them key effectors downstream of PI3K in regulating quiescence exit. was found to be a key transcriptional target of the PI3K/mTORC1 signaling axis essential for MuSC quiescence exit. Moreover, induction of a constitutively active PI3K in quiescent MuSCs resulted in spontaneous MuSC activation in uninjured muscles and subsequent depletion of the MuSC pool. Thus, PI3K-p110α is both necessary and sufficient for MuSCs to exit quiescence in response to activating signals.

The brain-specific angiogenesis inhibitor (BAI) subfamily of adhesion G protein-coupled receptors (aGPCRs) plays crucial roles in diverse cellular processes including phagocytosis, myoblast fusion, and synaptic development through the ELMO/DOCK/Rac signaling pathway, although the underlying molecular mechanism is not well understood. Here, we demonstrate that an evolutionarily conserved fragment located in the C-terminal cytoplasmic tail of BAI-aGPCRs is specifically recognized by the RBD-ARR-ELMO (RAE) supramodule of the ELMO family scaffolds. The crystal structures of ELMO2-RAE and its complex with BAI1 uncover the molecular basis of BAI/ELMO interactions. Based on the complex structure we identify aGPCR-GPR128 as another upstream receptor for the ELMO family scaffolds, most likely with a recognition mode similar to that of BAI/ELMO interactions. Finally, we map disease-causing mutations of BAI and ELMO and analyze their effects on complex formation.

We developed a label-free nonlinear optical (NLO) microscope integrating the stimulated Raman scattering, multi-color two-photon excited fluorescence and second harmonic generation. The system produces multimodal images of protein content, mitochondria distribution and sarcomere structure of fresh muscle samples. With the advanced imaging technique, we studied the mal-development of skeletal muscle caused by sarcomeric gene deficiency. In addition, important development processes of normal muscle from neonatal to adult stage were also clearly revealed based on the changing sarcomere structure, mitochondria distribution and muscle fiber size. The results demonstrate that the newly developed multimodal NLO microscope is a powerful tool to assess the muscle integrity and function.