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Roles of the calcineurin and CaMK signaling pathways in fast-to-slow fiber type transformation of cultured adult mouse skeletal muscle fibers

¹ Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine Baltimore, Maryland
² Department of Biology, Morgan State University, Baltimore, Maryland

Two Ca²⁺-dependent signaling pathways, mediated by the Ca²⁺-activated phosphatase calcineurin and by the Ca²⁺-activated kinase Ca²⁺/calmodulin-dependent kinase (CaMK), are both believed to function in fast-to-slow skeletal muscle fiber type transformation, but questions about the relative importance of the two pathways still remain. Here, the differential gene expression during fast-to-slow fiber type transformation was studied using cultured adult flexor digitorum brevis (FDB) fibers and a custom minimicroarray system containing 21 fiber type-specific marker genes. After 3 days of culture, unstimulated fibers showed a generally slower gene expression profile; 3 days of electric field stimulation of cultured FDB fibers with a slow fiber-type pattern transformed the fibers to an even slower gene expression profile. Unstimulated FDB fibers overexpressing constitutively active calcineurin featured a slower gene expression profile, except four genes, indicating that transformation occurred, but was incomplete with activation of the calcineurin pathway alone. In both unstimulated FDB fibers and slow-type electrically stimulated FDB fibers, blocking of CaMK pathway with KN93 generated a faster gene expression profile compared with the negative control KN92, indicating that CaMK pathway functions during the transformation induced by both unstimulated culturing and slow fiber-type electrical stimulation. Moreover, neither the calcineurin nor the CaMK pathway alone could maximally activate the transformation, and coordination of the two pathways is required to accomplish a complete fast-to-slow fiber type transformation.

plasticity; electrical stimulation; calcium-dependent signaling pathways; Ca²⁺/calmodulin-dependent kinase

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