Phosphorylation of CARM1 at Ser572 preserves muscle stem cell self-renewal by restricting premature myogenic commitment
Skeletal muscle regeneration depends on satellite cells, which must balance myogenic commitment with self-renewal to maintain regenerative capacity throughout life. The methyltransferase CARM1 is a key regulator of satellite cell fate, acting through methylation of PAX7 to promote Myf5 expression and myogenic commitment. The dystrophin glycoprotein complex regulates this pathway through p38{gamma}-mediated phosphorylation of CARM1 at Ser572, however the physiological role of this phosphorylation event in vivo remains unknown. Here, we generated a phosphorylation-deficient CARM1 knock-in mouse (CARM1 SA) to determine how CARM1 Ser572 phosphorylation regulates satellite cell fate during regenerative myogenesis. Satellite cell-derived myoblasts from CARM1 SA mice exhibited reduced PAX7 expression, impaired fusion, and abnormal myotube formation despite transcriptional signatures consistent with accelerated myogenic progression. Ex vivo single-myofiber assays revealed precocious acquisition of a committed progenitor identity accompanied by a reduction in the self-renewing satellite cell population. While muscle development and regeneration following a single injury were largely preserved, repeated regenerative challenge resulted in progressive depletion of the satellite cell pool in CARM1 SA mice. Transcriptomic analysis demonstrated minimal effects under homeostatic conditions but identified substantial alterations during regeneration, including reduced expression of extracellular matrix-associated genes, impaired TGF{beta}-SMAD signaling, decreased Tgfbr3 expression, and increased MYC activity, consistent with a loss of self-renewal programs and enhanced activation. Collectively, these findings demonstrate that phosphorylation of CARM1 at Ser572 is a critical mechanism to delay satellite cell commitment, thereby maintaining a balance between self-renewal and commitment to ensure long-term maintenance of the muscle stem cell compartment.