Epigenetic Evidence for Distinct Contributions of Resident and Acquired Myonuclei During Long‐Term Exercise Adaptation Using Timed In Vivo Myonuclear Labeling

Abstract

Muscle fibers are syncytial post‐mitotic cells that can acquire exogenous nuclei from resident muscle stem cells, called satellite cells. Myonuclei are added to muscle fibers by satellite cells during conditions such as load‐induced hypertrophy. It is difficult to dissect the molecular contributions of resident versus satellite cell‐derived myonuclei during adaptation due to the complexity of labeling distinct nuclear populations in multinuclear cells without label transference between nuclei. To sidestep this barrier, we utilized a genetic mouse model where myonuclear DNA can be specifically and stably labeled via non‐constitutive H2B‐GFP at any point in the lifespan. Resident myonuclei (Mn) were GFP‐tagged in vivo before eight weeks of progressive weighted wheel running (PoWeR) in adult mice (>4‐month‐old). Resident+satellite cell‐derived myonuclei (Mn+SC Mn) were labeled at the end of PoWeR in a separate cohort. Following myonuclear isolation, promoter DNA methylation profiles acquired with low‐input RRBS were compared to deduce epigenetic contributions of satellite cell‐derived myonuclei during adaptation. Resident myonuclear DNA has hypomethylated promoters in genes related to protein turnover, while the addition of satellite cell‐derived myonuclei shifts myonuclear methylation profiles to favor transcription factor regulation and cell‐cell signaling. By comparing myonucleus‐specific methylation profiling to previously published single‐nucleus transcriptional analysis in the absence (Mn) versus presence of satellite cells (Mn+SC Mn) with PoWeR, we provide evidence that satellite cell‐derived myonuclei may preferentially supply ribosomal proteins to growing myofibers and retain an epigenetic “memory” of prior stem cell identity. These data offer insights on distinct epigenetic myonuclear characteristics and contributions during adult muscle growth.