Continuous Large Maf Activity Maintains Fast Myofiber Identity and Multicellular Homeostasis in Adult Skeletal Muscle

Fast myofibers are preferentially affected during aging, exhibiting atrophy and shifts toward slower myofiber programs, yet the mechanisms preserving established fast-fiber identity remain unclear. Here, we show that the large Maf transcription factors Mafa, Mafb, and Maf are continuously required to maintain type IIb myofiber identity in adult skeletal muscle. Inducible deletion of large Mafs caused near-complete loss of type IIb fibers, a transition toward type IIx/IIa states, reduced strength, and increased fatigue resistance. RNA-seq and ATAC-seq revealed transcriptional and chromatin accessibility remodeling across the fast myosin locus, characterized by persistent loss of Myh4 expression and increased accessibility around Myh1. Single-nucleus RNA-seq identified transient myonuclei co-expressing multiple myosin isoforms before emergence of a Myh1-dominant state, accompanied by remodeling of non-myonuclear cell populations. These changes shared features with aging- and denervation-associated muscle remodeling. Thus, continuous large Maf activity maintains adult fast myofiber identity and coordinates skeletal muscle tissue homeostasis.