Single cell RNA sequencing analysis of mice hindlimb muscles identifies transcriptional heterogeneity in aging and physical frailty.

Frailty, a geriatric syndrome, is characterized by the age-related deterioration of physical capabilities and multiple organ systems. However, its age-associated and age-independent mechanisms remain vague, impeding prevention and clinical intervention. Here, the physical frailty status of young and old mice estimated using the frailty phenotype and frailty index values was used to divide mice into non-frail young/old (NF-Y/NF-O) and frail old (F-O) groups. Age-associated and age-independent transcriptional changes in frailty were investigated using single-cell RNA sequencing to profile transcriptomes in various cell types in limb muscles. We investigated the ratio of cell types, transcriptional regulation networks, and cell-cell communications in 15 major cell types in mice during relatively healthy aging (RHA), age-associated frailty (AAF), and age-independent frailty (AIF). Each group of RHA, AAF or AIF genes exhibited one major expression pattern and transcriptional regulation network. Besides its unique pattern, genes in the AAF group faintly exhibited the two major patterns seen in the AIF and RHA groups. B cells and satellite cells in both the AIF and AAF groups showed the most down-regulated and up-regulated differentially expressed genes, respectively. The transcriptional pattern of B cells, which showed stronger transcriptional changes than satellite cells in the AIF process, was validated by sorting B cells and performing SMART-sequencing. Thus, by analyzing these molecular events at the single-cell level, our study revealed the specific expression patterns and transcriptional heterogeneities of candidate cell types involved in relatively healthy aging and physical frailty, laying a foundation to characterize the detailed mechanisms and presenting possible therapeutic strategies for physical frailty.