Fibro-adipogenic Progenitor and Macrophage Remodeling of the Aging Skeletal Muscle Niche During Exercise-Induced Hypertrophy

Fibro-adipogenic progenitors (FAPs) have emerged as central regulators of the skeletal muscle homeostasis and the muscle microenvironment. However, the role of FAPs in the context of exercise-induced muscle hypertrophy remains largely unexplored. Here, we utilized six weeks of blood flow restricted resistance exercise (BFRRE) to study cellular adaptations within the muscle microenvironment accompanying muscle hypertrophy in healthy older individuals. Using flow cytometry, we characterized global changes of key cell populations within the skeletal muscle microenvironment, including FAPs, muscle stem cells (MuSCs), and immune cells. BFRRE induced significant enlargement of both FAPs and MuSCs, consistent with cellular adaptation to exercise. Notably, exercise shifted the FAP pool toward an increased predominance of the CD90high FAP phenotype, without altering total FAP abundance. In addition to matrix and collagen-related genes, transcriptional analysis of genes associated with secretory proteins revealed enrichment of promyogenic factors in CD90high versus CD90low FAPs. Conditioned media experiments of freshly isolated FAPs demonstrated that CD90high FAPs promote myotube growth in vitro compared to CD90low counterparts, suggesting that this phenotypic shift may facilitate muscle hypertrophy. In parallel, BFRRE increased the proportion of pro-inflammatory (CD11c+) macrophages within the skeletal muscle niche, highlighting a dynamic immune response during adaptation. Finally, we show identified possible link between pro-inflammatory macrophages and FAPs, as TNF markedly reduced the proliferation of human primary FAPs ex vivo, suggesting that macrophage-derived signals may attenuate excessive FAP expansion during tissue remodelling. Together, these findings provide new insight into how the remodelling of the cellular niche may support muscle hypertrophy in response to exercise. The coordinated expansion and phenotypic remodeling of FAP and immune cell populations may represent an important mechanism through which exercise supports hypertrophy in older individuals.