Prolonged mechanical muscle loading increases mechanosensor gene and protein levels and causes a moderate fast-to-slow fiber type switch in mice.

ABSTRACT

Mechanosensing and subsequent mechanotransduction are indispensable for muscle plasticity. Nevertheless, a scarcity of literature exists regarding an all-encompassing understanding of the muscle mechanosensing machinery's response to prolonged loading, especially in conditions that resemble a natural physiological state of skeletal muscle. This study aimed to comprehensively explore the effects of prolonged mechanical loading on mechanosensitive components, skeletal muscle characteristics, and metabolism-related gene clusters. Twenty male C57BL/6J mice were randomly divided into two groups control and prolonged mechanical loading. To induce prolonged mechanical loading on the triceps brachii (TRI) and biceps brachii (BIC) muscles, a 14-day period of tail suspension was implemented. In TRI only, prolonged mechanical loading caused a mild fast-to-slow fiber type shift together with increased mechanosensor gene and protein levels. It also increased transcription factors associated with slow muscle fibers while decreasing those related to fast-type muscle gene expression. Succinate dehydrogenase activity, a marker of muscle oxidative capacity, and genes involved in oxidative and mitochondrial turnover increased, whereas glycolytic-related genes decreased. Moreover, prolonged mechanical loading stimulated markers of muscle protein synthesis. Taken together, our data show a collective muscle-specific increase in mechanosensor gene and protein levels upon a period of prolonged mechanical loading in conditions that reflect a more natural physiological state of skeletal muscle in mice. We provide additional proof-of-concept that prolonged tail suspension-induced loading of the forelimbs triggers a muscle-specific fast-to-slow fiber type switch, and this coincides with increased protein synthesis-related signaling.NEW & NOTEWORTHY This study provides a comprehensive overview of the effects of prolonged loading on mechanosensitive components in conditions that better reflect the natural physiological state of skeletal muscle. Although the muscle mechanosensing machinery has been widely acknowledged for its responsiveness to altered loading, an inclusive understanding of its response to prolonged loading remains scarce. Our results show a fast-to-slow fiber type shift and an upregulation of mechanosensor gene and protein levels following prolonged loading.