Macroscopic to ultrastructural analyses identify the loss of myofibrils as the primary mediator of ageing‐ and disuse‐induced muscle fibre atrophy

Ageing 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. Indeed, even the most basic questions, such as whether radial atrophy of muscle fibres is driven primarily by reductions in myofibril size, and/or the loss of myofibrils, remain unanswered. To address this gap, skeletal muscle structure was assessed at the macroscopic, microscopic and ultrastructural levels in young and older humans and mice. In humans, magnetic resonance imaging was used to measure quadriceps muscle volume and cross-sectional area (CSA), whereas vastus lateralis biopsies underwent standard immunohistochemistry for microscopic evaluations, coupled with a next-generation fluorescence imaging pipeline for ultrastructural analyses. Parallel experiments were conducted in mice, including a unilateral immobilization model of disuse-induced atrophy. Ageing in humans was associated with lower muscle volume and CSA, along with radial atrophy of SERCA1-positive fibres, whereas SERCA2-positive fibre CSA was preserved. Notably, the radial atrophy of SERCA1 fibres was largely explained by a lower number of myofibrils, rather than a smaller size of the myofibrils. Similar alterations were observed in aged mice, although SERCA1 fibres also exhibited slightly smaller myofibril size. In mice, disuse likewise caused radial muscle fibre atrophy that was again almost exclusively associated with a lower number of myofibrils. Collectively, these findings identify the loss of myofibrils as a central and conserved mechanism that mediates radial muscle fibre atrophy during ageing and disuse, highlighting a potential therapeutic target for preserving skeletal muscle mass. KEY POINTS: Skeletal muscles atrophy with ageing and disuse, and the loss of muscle mass increases the risk of falls, disability and all-cause mortality. Skeletal muscles are composed of long cells called muscle fibres, and each fibre is densely packed with thread-like structures called myofibrils; however, whether muscle fibre atrophy is primarily driven by reductions in the size of the myofibrils, and/or the loss of myofibrils, is not known. Using a next-generation imaging pipeline, this study examined muscle samples from young and older adults, as well as from young and old mice. The results show that muscle fibre atrophy during both ageing and disuse is mainly associated with a lower number of myofibrils, rather than a smaller size of the myofibrils. Identifying the loss of myofibrils as the primary mechanism underlying muscle fibre atrophy highlights a specific therapeutic target for preserving muscle as people age or recover from injury.