Sayed, R. K. A. | bioRxiv Science

bioRxiv2026

Mechanical Loading Induces the Radial Growth of Myofibrils and Myofibrillogenesis via an mTORC1-Dependent Mechanism

Increased mechanical loading induces skeletal muscle growth and, at the ultrastructural level, promotes myofibrillogenesis and the radial growth of myofibrils. However, the mechanisms regulating these ultrastructural adaptations are not known. Here, we sought to determine whether the mechanistic target of rapamycin complex 1 (mTORC1) regulates these processes. To accomplish this, muscle-specific, tamoxifen-inducible raptor knockout (iRAmKO) mice were used to inhibit signaling through mTORC1, and growth was induced with a model of chronic mechanical overload (MOV). Using a next-generation fluorescence imaging pipeline for ultrastructural analyses, we found that mTORC1 is a critical regulator of the myofibrillogenesis and radial growth of myofibrils that occur in response to MOV. Together with other recent advances in the field, we propose a model in which mTORC1 acts as a gatekeeper that permits the retention, rather than the synthesis, of proteins that drive the ultrastructural adaptations.

cell biology↗

bioRxiv2026

Macroscopic to Ultrastructural Analyses Identify the Loss of Myofibrils as the Primary Mediator of Muscle Fiber Atrophy in Aging and Disuse

BackgroundAging and disuse are two of the most clinically relevant conditions associated with the loss of skeletal muscle mass, yet the ultrastructural adaptations that drive these losses remain poorly defined. In particular, it is unclear whether radial atrophy of muscle fibers is driven by a reduction in the size of the existing myofibrils, and/or the loss of myofibrils. Accordingly, the objective of this study was to define the macro-to-ultrastructural adaptations that mediate aging- and disuse-induced loss of muscle mass. MethodsSkeletal muscle structure was assessed at the macroscopic, microscopic, and ultrastructural levels in humans and mice. In humans, magnetic resonance imaging was used to quantify knee extensor muscle volume and cross-sectional area (CSA) in young (19 - 40 years) and old (65 - 84 years) adults, and vastus lateralis biopsies were analyzed for microscopic and ultrastructural adaptations using immunohistochemistry and fluorescence imaging of myofibrils with image deconvolution (FIM-ID). Parallel studies were performed in young (4 months) and aged (24 months) mice, along with the use of unilateral hindlimb immobilization to model disuse. ResultsAging led to a robust loss of skeletal muscle mass that was mediated by coordinated macro-to-ultrastructural adaptations. In humans, aging reduced knee extensor muscle volume (34%, P < 0.005) and CSA (32%, P < 0.001) in a sex-independent manner, and these effects were associated with radial atrophy of SERCA1-positive fibers (23%, P < 0.05). Ultrastructural analyses revealed that the radial atrophy was driven by a reduction in the number of myofibrils per fiber (23%, P < 0.05) without changes in myofibril CSA. In mice, aging produced similar macro-to-ultrastructural adaptations in various flexor muscles; however, radial atrophy of the highly glycolytic/Type IIb fibers, which are not present in human limb muscles, was also associated with a decrease in the CSA of the myofibrils (9%, P < 0.005). We also determined that disuse led to radial atrophy of SERCA1-positive fibers (24%, P < 0.001), and this was mediated by a decrease in both the number (22%, P < 0.005) and size of the myofibrils (4%, P < 0.05). Notably, the results also revealed that the magnitude of the disuse-induced adaptations was significantly blunted with aging. ConclusionThis study identifies the loss of myofibrils as a central and conserved mediator of the radial atrophy of muscle fibers that occurs in response to disuse and aging, while also highlighting smaller context-dependent contributions that can arise from changes in myofibril size.

physiology↗

bioRxiv2025

Mechanical Loading Induces the Longitudinal Growth of Muscle Fibers via an mTORC1-Independent Mechanism

Mechanical loading drives skeletal muscle growth, yet the mechanisms that regulate this process remain undefined. Here, we show that an increase in mechanical loading induces muscle fiber growth through two distinct mechanisms. Radial growth, reflected by an increase in fiber cross-sectional area, is mediated through an mTORC1-dependent signaling pathway, whereas longitudinal growth, marked by the in-series addition of sarcomeres, is mediated through an mTORC1-independent signaling pathway. To gain further insight into the events that drive longitudinal growth, we combined BONCAT-based labeling of newly synthesized proteins with high-resolution imaging and determined that the in-series addition of sarcomeres is mediated by a process that involves transverse splitting at the Z-lines of pre-existing sarcomeres. Collectively, our findings not only challenge the long-standing view that mechanically induced growth is uniformly governed by mTORC1, but they also lay the framework for a new understanding of the molecular and structural events that drive this process. TeaserUnlocking the Mechanical Load-Induced Growth of Skeletal Muscle: mTORC1 Doesnt Always Hold the Key.

physiology↗