Background: Skeletal muscle plays a vital role in voluntary motion and locomotion. Fast-twitch muscle fibers are characterized by rapid contraction kinetics, high force generation capacity, and a distinct gene expression profile compared to slow-twitch fibers. Skeletal myosin binding protein-C (MyBP-C) paralogs, slow (sMyBP-C) and fast (fMyBP-C), interact with myosin and actin filaments within sarcomeres to modulate force development during contraction. These paralogs are differentially expressed in muscle fibers, with fMyBP-C predominantly expressed in the fast-twitch fibers. However, the role of fMyBP-C in diseased states and skeletal muscle aging remains poorly understood. This study employs mouse models with fMyBP-C ablation to investigate its significance in skeletal muscle physiology. Methods: Adult skeletal muscle samples aged 2~7 months from male and female wild-type, db/db, MDX, ECC injury model, were used to determine the differential expression of fMyBP-C. Next, Mybpc2 knockout (C2-/-) young (3~5 months ) and old (22 months) male mice were used to define the role of fMyBP-C in aging. Western immunoblotting was employed to analyze the expression of fMyBP-C and sMyBP-C and the phosphorylation status of sMyBP-C. The impact of C2-/- and aging on the fiber type, size, and number as well as general muscle structure was assessed by immunohistochemistry and electron microscopy. The functional effect of C2-/- and aging was measured in terms of in vivo and ex vivo muscle force generation. Lastly, RNA sequencing was performed to identify the molecular pathways dysregulated in the C2-/- mediated muscle dysfunction in young and old mice. Results: fMyBP-C was significantly reduced with a modest compensatory upregulation of sMyBP-C in the diseased fast-twitch muscles. fMyBP-C has a significantly higher expression in the male skeletal muscles compared to females. Conversely, the female muscles demonstrated significantly higher sMyBP-C phosphorylation. Whereas, C2-/- muscles display resistance to PKA-mediated sMyBP-C phosphorylation. Further studies using young male C2-/- mice showed a significant reduction in isometric tetanic force generation and relaxation rate, fiber type switching, atrophy, and altered gene expressions related to muscle function and metabolism compared to wild-type mice. Similarly, compared to their wild-type counterparts, aged male C2-/- mice display significant deficits in muscle strength and endurance, accompanied by changes in muscle fiber size and molecular signaling pathways critical for muscle homeostasis. Conclusion: fMyBP-C is an important regulator of muscle function and homeostasis in young and aged male fast-twitch muscle fibers. The absence of fMyBP-C aggravates the effect of aging on muscle structure and function. fMyBP-C has the potential to be a therapeutic target to modulate muscle wasting caused by aging and disease.
