Prepubertal skeletal muscle growth requires Pax7-expressing satellite cell-derived myonuclear contribution

Abstract

The functional role of Pax7-expressing satellite cells (SCs) in postnatal skeletal muscle development beyond weaning remains obscure. Therefore, the relevance of SCs during prepubertal growth, a period after weaning but prior to the onset of puberty, has not been examined. Here, we have characterized mouse skeletal muscle growth during prepuberty and found significant increases in myofiber cross-sectional area that correlated with SC-derived myonuclear number. Remarkably, genome-wide RNA-sequencing analysis established that post-weaning juvenile and early adolescent skeletal muscle have markedly different gene expression signatures. These distinctions are consistent with extensive skeletal muscle maturation during this essential, albeit brief, developmental phase. Indelible labeling of SCs with Pax7CreERT2/+; Rosa26nTnG/+ mice demonstrated SC-derived myonuclear contribution during prepuberty, with a substantial reduction at puberty onset. Prepubertal depletion of SCs in Pax7CreERT2/+; Rosa26DTA/+ mice reduced myofiber size and myonuclear number, and caused force generation deficits to a similar extent in both fast and slow-contracting muscles. Collectively, these data demonstrate SC-derived myonuclear accretion as a cellular mechanism that contributes to prepubertal hypertrophic skeletal muscle growth.

Exercise promotes satellite cell contribution to myofibers in a load-dependent manner

“…Thus, the high number of nTom + myonuclei, which we observed after voluntary running is a vast overestimation of SC fusion. Importantly, these data warrant against the use of nuclear reporter mice in multinucleated cells such as myofibers and suggest a careful re-interpretation of previous data published with Pax7 nuclear reporter mice (Bachman et al, 2018;Liu et al, 2017;Tierney et al, 2017).…”

Dysregulated cell signalling and reduced satellite cell potential in ageing muscle

“…The work of White et al has shown that the myonuclear content of mouse muscle fibres increases up to 3 weeks in mice (White RB et al, 2010). However others have shown that myonuclei continue to be added at least into adulthood and support not only the muscle fibre size but also their muscle phenotype (specific Myosin Heavy chain expression) (Bachman JF et al, 2018; Pawlikowski B et al, 2015). These studies show that satellite cells are activated in un-injured muscle during phases of myonuclei accretion (up to postnatal day 21) and thereafter to replace nuclei that undergo apoptosis.…”

Dual Energy X-ray Absorptiometry (DEXA) as a Longitudinal Outcome Measure of Cancer-Related Muscle Wasting in Mice

“…TA muscles were cut from the most distal and proximal TA tendon attachment, cleaned of extraneous tissue, blotted, and weighed. After which, TA, muscles were stored in a 30% mixture of sucrose in PBS for 24 hours, at which time, muscles were embedded in OCT (Tissue Tek), flash-frozen using dry ice and 2-methylbutane (Sigma-Aldrich), and processed for fresh-frozen histology as previously described (21). 10μm sections were cut and stained with H&E and representative micrographs were obtained for descriptive analyses.…”

“…Immunohistochemistry for laminin (extracellular matrix) and nuclei determination were also performed as previously described (21, 22), in which TA muscles were cryosectioned at 10 μm, to obtain transverse sections. Muscle sections were fixed in 4% paraformaldehyde (PFA) for 10 min.…”

“…Single myofiber size and myonuclear analysis was performed, as previously described (21). For single myofiber size and myonuclear analysis, whole limbs (n=10) were fixed in 4% PFA for 48 h prior to EDL and SOL muscle dissection.…”