The transcriptomic signature of age and sex is not conserved in human primary myocytes

Human primary muscle cell (HPMC) lines derived from skeletal muscle biopsies are potentially powerful tools to interrogate the molecular pathways underlying fundamental muscle mechanisms. HPMCs retain their genome in culture, but many endogenous circulating factors are not present in the in vitro environment or at concentrations that do not mirror physiological levels. To address the assumption that HPMCs are valid models of age and sex-specificity in human muscle research, we examined to what extent HPMC lines maintain their source phenotypes in culture. A comparison of the transcriptomic signature of 30 human muscle biopsies (N = 10 young males, N = 10 young females, N = 10 old males) and their corresponding HPMCs indicated a near-complete lack of retention of the genes and pathways that were differentially regulated in vivo when compared to their in vitro equivalent, except for some genes encoded on the Y-chromosome. The diversity of the muscle cell population and the lack of sex- and age-dependent factors in the cellular milieu likely contribute to these observations, which call for caution when using HPMCs as an experimental model of human muscle sex or age. Nuclear single cell sequencing or HMPCs grown in 3D culture as organoids under strain might therefore provide a closer alternative to human muscle tissue.