Endogenous calcium/calmodulin-dependent protein kinase II gamma and delta maintain Type II myofiber identity in adult mice.

Skeletal muscle fiber identity is commonly characterized by coordinated contractile and metabolic programs, but the mechanisms that preserve this coordination in adult muscle remain unclear. We tested whether endogenous calcium/calmodulin-dependent protein kinase II γ and δ (CaMKIIγ/δ) maintain type II myofiber identity. Tamoxifen-inducible, skeletal muscle-specific Camk2g/Camk2d double-knockout mice were analyzed 1 and 3 mo after deletion using soleus fiber typing, laser microdissection proteomics, and immunofluorescence. CaMKIIγ/δ deletion was not associated with detectable changes in body weight or muscle mass but increased type I fibers and reduced total type II fibers at both time points, with a transient increase in type I/II hybrid fibers. Within mKO muscles, the type IIa fiber proportion declined over time. Fiber type-resolved proteomics revealed remodeling within fibers retaining a type II myosin heavy chain profile. Proteins enriched in control type I fibers shifted upward in knockout type II fibers, whereas type II-enriched proteins shifted downward. Consistently, a type I-like score derived from an independent single-myofiber proteomic dataset increased selectively in knockout type II fibers, supporting remodeling beyond myosin isoform switching. CaMKII abundance was higher in type II than in type I fibers. Unexpectedly, this type I-like remodeling was not accompanied by an oxidative shift: oxidative phosphorylation-related protein programs were reduced across type I, type II, and hybrid fibers. These findings identify endogenous CaMKIIγ/δ as a homeostatic regulator of adult type II myofiber identity and show that fiber identity-associated proteomic features and mitochondrial oxidative programs can be remodeled in divergent directions.