Abstract
A detailed analysis of how muscle fiber nuclei (myonuclei) respond to a hypertrophic stimulus would provide a critical step toward understanding compromised skeletal muscle plasticity with age. We used recombination-independent doxycycline-inducible myonucleus-specific fluorescent labelling, tissue RNA-sequencing, myonuclear DNA methylation analysis, multi-omic integration, and single myonucleus RNA-sequencing to define the molecular characteristics of adult (6-8 month) and aged (24 month) murine skeletal muscle after acute mechanical overload (MOV). In adult and aged MOV muscles, we found that: 1) similarities in the transcriptional response to loading - specifically in metabolism genes - were partly explained by a post-transcriptional microRNA-mediated mechanism, which we corroborated using an inducible muscle fiber-specific miR-1 knockout model, 2) differences in age-dependent transcriptional responses were linked to the magnitude and location of differential DNA methylation in resident myonuclei, specifically around hypertrophy-associated genes such as Myc, Runx1, Mybph, Ankrd1, collagen genes, and minichromosome maintenance genes, 3) adult and aged resident myonuclear transcriptomes had differing enrichment for innervation-related transcripts as well as unique transcriptional profiles in an Atf3+ "sarcomere assembly" population after MOV, and 4) cellular deconvolution analysis supports a role for neuromuscular junction regulation in age-specific hypertrophic adaptation. These data are a roadmap for uncovering molecular targets to enhance aged muscle adaptability.