Fast Myosin Binding Protein-C Is a Vital Regulator in Young and Aged Fast Skeletal Muscle Homeostasis

Skeletal muscle plays a vital role in voluntary movement and locomotion. Fast-twitch muscle fibres are characterized by their rapid contraction kinetics, high-force generation and a distinct gene expression profile compared to slow-twitch fibres. These fibres have a predominant expression of fast skeletal myosin binding protein-C (fMyBP-C). The role of fMyBP-C in skeletal muscle disease and aging remains poorly understood. To address this, our study employs mouse models with fMyBP-C ablation to investigate its significance in skeletal muscle physiology.

Skeletal muscle samples from wild-type, db/db, MDX and ECC injury model (2-7 months) were analysed to determine the fMyBP-C levels. Next, male Mybpc2 knockout (C2^-/-) mice, both young (3-5 months) and old (22 months), were utilized to investigate the role of fMyBP-C in aging. The effects of C2^-/- and aging on the fibre type, size and number, as well as the overall muscle structure, were evaluated using immunohistochemistry and electron microscopy. In vivo and ex vivo muscle force generation was assessed to determine the functional impact of C2^-/- and aging. RNA sequencing was conducted to identify the altered molecular pathways causing the muscle dysfunction in young and old C2^-/- mice.

The expression of fMyBP-C was reduced (0.25-fold, p < 0.05) in the fast-twitch muscles of db/db mice, with a modest compensatory upregulation of slow skeletal MyBP-C (sMyBP-C) (~1.15-fold, p < 0.05). In MDX mice, fMyBP-C levels remain unchanged, whereas sMyBP-C levels were upregulated (~1.2-fold, p < 0.01). The fMyBP-C expression was 75% higher in the male skeletal muscles (p < 0.01) compared to females. Studies in young male C2^-/- mice revealed a reduction in isometric tetanic force generation by 25% (p < 0.01) and relaxation rate by 42% (p < 0.001). The C2^-/- mice also had 12.8% fewer type IIb fibres (p < 0.01), and a 20% reduction in type IIb fibre size (p < 0.01). Similarly, aged male C2^-/- mice exhibited significant deficits in muscle strength, endurance and survival rate relative to their wild-type counterparts. The aged male C2^-/- mice displayed a reduced size of type IIa, IIx and IIb muscle fibres compared to aged wild-type mice. RNA sequencing revealed that assembly and trimerization of collagen fibril pathway-related genes were altered in C2^-/- mice.

fMyBP-C is a critical regulator of muscle function and homeostasis in young male fast-twitch muscle fibres. Its absence exacerbates the impact of aging on muscle structure and function. These findings suggest that fMyBP-C could serve as a promising therapeutic target for mitigating muscle wasting associated with aging and disease.