Identifying the Structural Adaptations that Drive the Mechanically Induced Growth of Skeletal Muscle

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Project Summary / Abstract Mechanical signals play a major role in the regulation of skeletal muscle mass, and the maintenance of muscle mass contributes significantly to disease prevention and quality of life. Although the link between mechanical signals and the regulation of muscle mass has been recognized for decades, the mechanisms that control this process remain ill-defined. For instance, most studies indicate that the mechanically induced growth of skeletal muscle is driven by an increase in the size of the existing myofibers rather than an increase in the number of myofibers. Moreover, current models assert that the increase in myofiber size is mediated by an increase in the balance between the rates of protein synthesis and protein degradation which, in turn, leads to the accumulation of newly synthesized proteins (NSPs) and the concomitant structural changes that drive the growth response. For instance, it is well known that an increase in mechanical loading can lead to microstructural changes such as the radial growth of myofibers. Surprisingly, however, the ultrastructural adaptations that drive these microstructural changes have not been defined. Indeed, a number of foundationally important questions such as whether the radial growth of myofibers is driven by an increase in the size and/or the number of myofibrils have not been answered. Likewise, the location(s) in which NSPs accumulate during mechanically induced growth (i.e., the sites of growth) are not known. As such, one of the major goals of this project is to fill these gaps in knowledge. Another major goal is to develop a better understanding of the signaling events that control the different aspects of mechanically induced growth. For instance, our previous work has established that signaling through mTORC1 plays a central role in the process via which mechanical stimuli induce the radial growth of myofibers. However, our preliminary data indicate that the longitudinal growth of myofibers can also make a substantive contribution to the mechanically induced accretion of muscle mass, yet, unlike radial growth, the longitudinal growth of myofibers does not appear to require signaling by mTORC1. In other words, our preliminary data suggest that the radial and longitudinal growth of myofibers are regulated by distinct signaling pathways. Specifically, we propose that the radial growth of myofibers is driven by a mTORC1-dependent mechanism that we have coined as the “myofibril expansion cycle”, whereas the longitudinal growth of myofibers is mediated by a mTORC1-independent mechanism that involves transverse Z-line splitting of sarcomeres at regions called sphenodes. To test the validity of these hypotheses we will use advanced imaging techniques, various genetic interventions, two complementary models of mechanical load-induced growth, and our new state-of-the-art technology that enables us to visualize and quantify (with ≤10 nm resolution) where NSPs accumulate. Collectively, it is anticipated that the outcomes of this project will not only fill major gaps in our understanding of how mechanical stimuli regulate muscle mass, but they will also build the framework for future studies that are aimed at developing a better understanding of this highly important process.