ABSTRACT
Skeletal muscles exert remarkable regenerative or adaptive capacities in response to injuries or mechanical loads. However, the cellular networks underlying muscle adaptation are poorly understood compared to those underlying muscle regeneration. We employed single-cell RNA sequencing to investigate the gene expression patterns and cellular networks activated in overloaded muscles and compared these results with those observed in regenerating muscles. The cellular composition of the 4-day overloaded muscle, when macrophage infiltration peaked, closely resembled that of the 10-day regenerating muscle. In addition to the mesenchymal progenitor-muscle satellite cell (MuSC) axis, interactome analyses or targeted depletion experiments revealed communications between mesenchymal progenitors-macrophages and macrophages-MuSCs. Furthermore, granulin, a macrophage-derived factor, inhibited MuSC differentiation, and Granulin-knockout mice exhibited blunted muscle hypertrophy due to the premature differentiation of overloaded MuSCs. These findings reveal the critical role of granulin through the relayed communications of mesenchymal progenitors, macrophages, and MuSCs in facilitating efficient muscle hypertrophy.
Subject(s)
Cell Differentiation , Hypertrophy , Macrophages , Mesenchymal Stem Cells , Mice, Knockout , Satellite Cells, Skeletal Muscle , Animals , Satellite Cells, Skeletal Muscle/metabolism , Satellite Cells, Skeletal Muscle/pathology , Macrophages/metabolism , Mesenchymal Stem Cells/metabolism , Mice , Granulins , Cell Communication , Mice, Inbred C57BL , Muscle, Skeletal/metabolism , Muscle, Skeletal/pathology , Male , RegenerationABSTRACT
Skeletal muscle homeostasis and function are ensured by orchestrated cellular interactions among several types of cells. A noticeable aspect of skeletal muscle biology is the drastic cell-cell communication changes that occur in multiple scenarios. The process of recovering from an injury, which is known as regeneration, has been relatively well investigated. However, the cellular interplay that occurs in response to mechanical loading, such as during resistance training, is poorly understood compared to regeneration. During muscle regeneration, muscle satellite cells (MuSCs) rebuild multinuclear myofibers through a stepwise process of proliferation, differentiation, fusion, and maturation, whereas during mechanical loading-dependent muscle hypertrophy, MuSCs do not undergo such stepwise processes (except in rare injuries) because the nuclei of MuSCs become directly incorporated into the mature myonuclei. In this review, six specific examples of such differences in MuSC dynamics between regeneration and hypertrophy processes are discussed.
Subject(s)
Muscle, Skeletal , Myoblasts , Cell Differentiation , Humans , Hypertrophy/metabolism , Muscle, Skeletal/metabolism , RegenerationABSTRACT
Muscle satellite cells (MuSCs) supply nuclei to existing myofibers in response to mechanical loading. This myonuclear accretion is critical for efficient muscle hypertrophy. Herein, we present protocols for the detection of MuSC-derived new myonuclei in loaded mouse muscle, including procedures for EdU injection to stain myonuclei, followed by surgery and skeletal muscle fixation. We then describe immunostaining for EdU+ myonuclei and image acquisition for quantitative analyses. For complete details on the use and execution of this protocol, please refer to Kaneshige et al. (2022).