Combined single-cell RNA sequencing and mendelian randomization to identify biomarkers associated with circadian rhythm in sarcopenia.

Sarcopenia refers to the involuntary loss of skeletal muscle mass and function with aging and is associated with multiple adverse health outcomes. Disruption of normal circadian rhythms due to shift work or nocturnal lifestyle is associated with the risk of several diseases such as metabolic syndrome and cancer. However, its role in sarcopenia remains unclear. The synergy of single-cell RNA sequencing and Mendelian randomization (MR) analysis provides an opportunity to reveal the important involvement of circadian rhythms in the pathogenesis of sarcopenia. Data quality control and normalisation were performed in the single-cell dataset, GSE167186. Then, different cell types were obtained by cell annotation through marker genes. Differentially expressed genes (DEGs) were further analyzed in different cell types. The intersection of DEGs and circadian-related genes in fast skeletal muscle cell were taken. Afterwards, the genes that had causal relationship with sarcopenia were selected as biomarkers by MR analysis. Variations in signaling pathways between different cell types were further analyzed. In GSE167186, 20 different cell populations identified by UMAP cluster analysis were further annotated to 8 cell types using maker genes. Afterwards, 44 DEGs were screened between fast skeletal muscle cell and circadian-related genes. Further MR Analysis yielded three genes with significant causal association with sarcopenia, which could be used as biomarkers in this study. SMARCD3 was found to have a protective effect against sarcopenia (OR = 0.9183, 95% CI = 0.8577-0.9832, p = 0.0144); in contrast, CPED1 (OR = 1.0292, 95% CI = 1.0089-1.0500, p = 0.0047) and FNBP4 (OR = 1.1096, 95% CI = 1.0316-1.1934, p = 0.0051) were associated with an increased risk of sarcopenia. The analysis of intercellular signaling revealed that the loss of the protective factor SMARCD3 in fast skeletal muscle cell triggers a specific upregulation of EGF signaling directed at FAPs. This study highlights the contribution of circadian rhythms in the pathogenesis of sarcopenia and further defines circadian rhythm-related biomarkers in sarcopenia, as demonstrated by MR analysis and scRNA-seq analysis. This suggests circadian rhythms as a focal point for pathogenesis research and potential therapeutic targeting in sarcopenia.