Short-term muscle disuse has demonstrated the numerous adverse effects it causes on skeletal muscle, including reductions in function and mass. Previous work has suggested that inflammation may increase during disuse and contribute to muscle atrophy, however it is currently unknown how different cell types in skeletal muscle respond to short term disuse. The objective of this study was to determine which cell types are responsible for increased inflammation-associated gene expression in middle-aged human skeletal muscle. We hypothesized that macrophage infiltration into muscle during disuse would be responsible for the increase in inflammatory gene expression. We enrolled 37 healthy late-midlife adults (15 males 58±1 yr, BMI = 29.4±0.8 kg·m-2, and 22 females, 55±1 yr, BMI = 29.1±1.1 kg·m-2) that underwent 7 days of leg immobilization via unilateral lower limb suspension (ULLS). Muscle biopsies from the vastus lateralis were collected prior to and following immobilization. We then performed single nucleus RNA sequencing and bulk RNA sequencing to assess the transcriptomic changes in men and women. 2576 and 757 individual transcripts increased, and 154 and 788 individual transcripts decreased in females and males respectively after 7 days of ULLS (FDR or < 0.4). Gene set enrichment analysis (GSEA) showed that similar gene sets were downregulated in males and females post-ULLS, particularly related to mitochondria, namely aerobic respiration and respiratory electron transport. In contrast, extracellular matrix organization and collagen formation gene sets were upregulated. However, gene sets linked to immune function (i.e., neutrophil degranulation and interleukin 4 and 13 signaling) had a greater upregulation in females. Single nucleus RNA sequencing in a subset of subjects identified at least 8 different cell types including fast twitch muscle cells, slow twitch muscle cells, satellite cells, fibro-adipogenic precursor cells (FAPs), endothelial cells, macrophages, neuromuscular junction and smooth muscle. Post-disuse the number of macrophages within muscle increased by ~12%. We then performed differential gene expression in each cell type and found that macrophage gene expression supported a strong inflammatory response during disuse atrophy. We conclude that short-term disuse atrophy in middle-aged adults decreases gene expression of TCA cycle and electron transport genes and increases inflammation-associated genes which may be driven by macrophages. Support: NIH R01 AG064386. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
Kilroe et al. (Fri,) studied this question.