Slow-Myofiber Commitment by Semaphorin 3A Secreted from Myogenic Stem Cells.

Recently, we found that resident myogenic stem satellite cells upregulate a multi‐functional secreted protein, semaphorin 3A (Sema3A), exclusively at the early‐differentiation phase in response to muscle injury; however, its physiological significance is still unknown. Here we show that Sema3A impacts slow‐twitch fiber generation through a signaling pathway, cell‐membrane receptor (neuropilin2‐plexinA3) → myogenin‐myocyte enhancer factor 2D → slow myosin heavy chain. This novel axis was found by small interfering RNA‐transfection experiments in myoblast cultures, which also revealed an additional element that Sema3A‐neuropilin1/plexinA1, A2 may enhance slow‐fiber formation by activating signals that inhibit fast‐myosin expression. Importantly, satellite cell‐specific Sema3A conditional‐knockout adult mice (Pax7CreERT2‐Sema3Afl°x activated by tamoxifen‐i.p. injection) provided direct in vivo evidence for the Sema3A‐driven program, by showing that slow‐fiber generation and muscle endurance were diminished after repair from cardiotoxin‐injury of gastrocnemius muscle. Overall, the findings highlight an active role for satellite cell‐secreted Sema3A ligand as a key “commitment factor” for the slow‐fiber population during muscle regeneration. Results extend our understanding of the myogenic stem‐cell strategy that regulates fiber‐type differentiation and is responsible for skeletal muscle contractility, energy metabolism, fatigue resistance, and its susceptibility to aging and disease. Stem Cells 2017;35:1815–1834