Delivry, L. | Search bioRxiv

bioRxiv2026

Identification of a new population of myonuclei during skeletal muscle hypertrophy

BackgroundSkeletal muscle represents around 40% of total human body weight and exhibits remarkable plasticity. It can hypertrophy, atrophy, or regenerate in response to changes in activity, nutrient availability, or injury. The main component of striated muscle, the myofiber, is a post-mitotic, multinucleated cell that contains the muscles contractile unit, the sarcomere. The myonuclei within these fibers are specialized and differ in terms of gene expression and localization. Adult muscles also contain various other cell types, including adult muscle stem cells (MuSCs), macrophages, fibro-adipogenic progenitors (FAPs), and endothelial cells. MuSCs are central to muscle plasticity, and are capable of activation, proliferation, differentiation, and fusion to form new myofibers during regeneration, or to fuse with existing myofibers during hypertrophy. Muscle hypertrophy and myofibers enlargement involve increased protein synthesis and reduced protein degradation, as well as myonuclear accretion following satellite cell activation. Multiple signaling pathways, such as the mTOR pathway and the RhoA/SRF mechanotransduction pathway, are involved in these processes. MethodsWe performed single-nucleus RNA sequencing (snRNA-seq) on plantaris muscles of adult mice, comparing samples 7 days after hypertrophy induction (overload, 7OV) to non-hypertrophied controls (Ctl). RNAscope experiments on isolated myofibers identified the heterogeneity of myonuclei along the myofiber. ResultsSnRNA-seq analysis revealed a previously unknown population of myonuclei (UM). UM-Ctl, which is present only in the Ctl condition, and UM-7OV, only in the 7OV condition. These myonuclei are localised at the tips of myofibres. Furthermore, we determined that UM-7OV are not newly fused myonuclei from activated satellite cells. Trajectory analyses suggest that UM-Ctl transition into UM-7OV during hypertrophy, returning to a near-basal homeostatic state after 21 days of overload (21OV). Gene expression analysis showed that UM-Ctl and UM-7OV have distinct gene expression profiles compared to other myonuclei and respond differently to hypertrophy. ConclusionOur findings suggest the existence of a specific population of myonuclei with unique localization and gene expression profiles, which play distinct roles at baseline and during hypertrophy. These results highlight the differential properties of myonuclei in the myofiber and their potential specific functions in muscle homeostasis and adaptation.

molecular biology↗

bioRxiv2026

SIX1-dependent myofiber typology and metabolism controls muscle hypertrophy

The different types of muscle fibres respond in a specific way to hypertrophy or atrophy. The mechanisms underlying these heterogeneous adaptations remain poorly understood. Using single-nucleus RNA sequencing, we propose that fast glycolytic fibres show genetic limitations to hypertrophy induced by mechanical overload. We show that a prior fibre transition, achieved by reducing SIX1 protein expression (hypomorphism), enhances and accelerates overload-induced hypertrophy, bypassing the genetic limitations of fast glycolytic fibres. In contrast and unexpectedly, Six1 knockout in myofibers abolished overload-induced hypertrophy and instead caused atrophy of IIb/IIx fibers, despite the induction of a strong slow oxidative phenotype. In particular, Six1 deletion leads to metabolic defects caused by inhibition of glycolysis, AMPK and mitochondrial biogenesis. Our findings highlight the critical role of SIX1/AMPK/glycolysis-dependent aerobic metabolism in muscle growth and suggest that fibre type transitions, coupled with preserved metabolic function, may optimise hypertrophic responses.

physiology↗

bioRxiv2021

RhoA within myofibers controls satellite cell microenvironment to allow hypertrophic growth

Adult skeletal muscle is a plastic tissue that can adapt its size to workload. Here, we show that RhoA within myofibers is needed for overload-induced hypertrophy by controlling satellite cell fusion to the growing myofibers without affecting protein synthesis. At the molecular level, we demonstrate that, in response to increased workload, RhoA controls in a cell autonomous manner Erk1/2 activation and the expressions of extracellular matrix (ECM) regulators such as Mmp9/Mmp13/Adam8 and of macrophage chemo-attractants such as Ccl3/Cx3cl1. Their decreased expression in RhoA mutant is associated with ECM and fibrillar collagen disorganization and lower macrophage infiltration. Moreover, Mmps inhibition and macrophage depletion in controls phenocopied the lack of growth of RhoA mutants. These findings unravel the implication of RhoA within myofibers, in response to increase load, in the building of a permissive microenvironment for muscle growth and for satellite cell accretion through ECM remodeling and inflammatory cell recruitment.

physiology↗