TGFβ-Smad3 signaling restores cell-autonomous Srsf1-mediated splicing of fibronectin in aged skeletal muscle stem cells.

Loss of Fibronectin (FN) from the skeletal muscle stem cell (MuSC) niche represents a root cause of regenerative failure in aging. While FN has pleiotropic functions during healthy skeletal muscle regeneration, it remains unclear how aging affects its spatiotemporal specificity for MuSCs. Here, we demonstrate that activated MuSCs secrete an autoregulatory FN splice variant containing the EDB extra domain (EDB(+) FN), which is not expressed by accessory cells in the niche. EDB(+) FN splicing in MuSCs depends on serine/arginine-rich splicing factor 1 (Srsf1) whose promoter is controlled by Smad3. EDB(+) FN knockdown or downregulation in aging affects MuSC proliferation through aberrant integrin signaling and impairs skeletal muscle regeneration. During a defined regeneration interval in aged mice, Smad3 activation using transforming growth factor-beta 1 (TGFβ1) improves MuSC function and skeletal muscle repair by stimulating EDB(+) FN secretion. Altogether, we identify and characterize the TGFβ1-Smad3-Srsf1-EDB(+) FN pathway as a therapeutic target for age-associated regenerative failure.