CAIII expression in skeletal muscle is regulated by Ca²⁺-CaMKII-MEF2C signaling

Carbonic anhydrase III (CAIII) is selectively expressed in slow-twitch myofibers in skeletal muscle. The fast-twitch to slow-twitch transformation of myofibers following denervation is accompanied by increased CAIII expression, suggesting that the effects of nerve impulses on skeletal-muscle remodeling influence CAIII expression. Here, we determined the molecular mechanisms underlying the effects of nerve conduction on CAIII expression. The results indicated that changes in skeletal-muscle [Ca²⁺]_i altered CAIII expression. Moreover, results from the RNA-interference and over-expression experiments identified myocyte enhancer factor 2C (MEF2C) as the key transcription factor regulating [Ca²⁺]_i-mediated changes in CAIII transcription. Additionally, chromatin immunoprecipitation experiments and luciferase assays confirmed MEF2C interaction and direct binding of the CAIII promoter between -416 and -200 base pair. Investigations of upstream cytoplasmic signaling pathways responsible for MEF2C activation revealed Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) as the key factor involved in MEF2C-mediated regulation of CAIII expression. This study demonstrates that the Ca²⁺-CaMKII-MEF2C signaling pathway is the key factor involved in regulating CAIII expression in skeletal muscle. These results provide a theoretical basis supporting further investigations of changes in CAIII levels under different pathophysiological conditions and will facilitate a broader understanding of the biological functions of CAIII.