CaMKIIγ/δ contributes to mitochondrial metabolic adaptation in skeletal muscle during endurance training

BACKGROUND: Calcium/calmodulin-dependent protein kinase II (CaMKII) is activated in skeletal muscle with exercise, yet its physiological role in endurance training adaptation remains unclear. This study determined whether endogenous CaMKIIγ/δ in skeletal muscle is required for endurance training-induced metabolic remodeling and exercise adaptation. METHODS: We generated male skeletal muscle-specific CaMKIIγ/δ knockout (CaMKII mKO) mice and assessed muscle phenotype, exercise capacity, and training adaptation. Acute exercise-induced CaMKII activation was evaluated by phosphorylation status. Transcriptomic changes were analyzed by RNA sequencing before and after 4 weeks of treadmill endurance training. Mitochondrial protein abundance, ultrastructure, and bioenergetics were examined by immunoblotting, transmission electron microscopy, and Seahorse extracellular flux analysis in myotubes with acute CaMKIIγ/δ deletion. RESULTS: Acute treadmill exercise induced CaMKII phosphorylation in muscle without altering total CaMKII abundance. CaMKII mKO mice showed normal muscle mass, grip strength, and baseline performance. However, endurance training-induced improvement in running capacity was significantly blunted. Transcriptomic analyses revealed downregulation of oxidative phosphorylation and glycolytic gene programs in CaMKII-deficient muscle at baseline and after training. OXPHOS complex protein abundance was partially reduced at baseline and markedly reduced across complexes I-V after training. CaMKII deficiency increased ultrastructurally abnormal mitochondria without reducing mitochondrial number. Consistently, CaMKII-deficient myotubes showed lower absolute per-well oxygen consumption and extracellular acidification rate. CONCLUSIONS: In male mice, endogenous CaMKIIγ/δ in muscle is dispensable for baseline locomotor performance but essential for endurance training-induced metabolic remodeling and mitochondrial integrity. These findings support a role for CaMKII in linking contraction-induced calcium signaling to metabolic adaptation in muscle.