An Insulin Upstream Open Reading Frame (INSU) Is Present in Skeletal Muscle Satellite Cells: Changes with Age

Insulin resistance, stem cell dysfunction, and muscle fiber dystrophy are all age-related events in skeletal muscle (SKM). However, age-related changes in insulin isoforms and insulin receptors in myogenic progenitor satellite cells have not been studied. Since SKM is an extra-pancreatic tissue that does not express mature insulin, we investigated the levels of insulin receptors (INSRs) and a novel human insulin upstream open reading frame (INSU) at the mRNA, protein, and anatomical levels in Baltimore Longitudinal Study of Aging (BLSA) biopsied SKM samples of 27-89-year-old (yrs) participants. Using RT-qPCR and the MS-based selected reaction monitoring (SRM) assay, we found that the levels of INSR and INSU mRNAs and the proteins were positively correlated with the age of human SKM biopsies. We applied RNAscope fluorescence in situ hybridization (FISH) and immunofluorescence (IF) to SKM cryosections and found that INSR and INSU were co-localized with PAX7-labeled satellite cells, with enhanced expression in SKM sections from an 89 yrs old compared to a 27 yrs old. We hypothesized that the SKM aging process might induce compensatory upregulation of INSR and re-expression of INSU, which might be beneficial in early embryogenesis and have deleterious effects on proliferative and myogenic satellite cells with advanced age.