Skeletal muscle is the largest metabolic organ system in the human body. As such, metabolic dysfunction occurring in skeletal muscle impacts whole-body nutrient homeostasis. Macronutrient metabolism changes within the skeletal muscle with aging, and these changes are associated in part with age-related skeletal muscle remodeling. Moreover, age-related changes in skeletal muscle metabolism are affected differentially between males and females and are likely driven by changes in sex hormones. Intrinsic and extrinsic factors impact observed age-related changes and sex-related differences in skeletal muscle metabolism. Despite some support for sex-specific differences in skeletal muscle metabolism with aging, more research is necessary to identify underlying differences in mechanisms. Understanding sex-specific aging skeletal muscle will assist with the development of therapies to attenuate adverse metabolic and functional outcomes.

Muscle-specific adult stem cells (MuSCs) are required for skeletal muscle regeneration. To ensure efficient skeletal muscle regeneration after injury, MuSCs must undergo state transitions as they are activated from quiescence, give rise to a population of proliferating myoblasts, and continue either to terminal differentiation, to repair or replace damaged myofibers, or self-renewal to repopulate the quiescent population. Changes in MuSC/myoblast state are accompanied by dramatic shifts in their transcriptional profile. Previous reports in other adult stem cell systems have identified alterations in the most abundant internal mRNA modification, N6-methyladenosine (m6A), conferred by its active writer, METTL3, to regulate cell state transitions through alterations in the transcriptional profile of these cells. Our objective was to determine if m6A-modification deposition via METTL3 is a regulator of MuSC/myoblast state transitions in vitro and in vivo. Using liquid chromatography/mass spectrometry we identified that global m6A levels increase during the early stages of skeletal muscle regeneration, in vivo, and decline when C2C12 myoblasts transition from proliferation to differentiation, in vitro. Using m6A-specific RNA-sequencing (MeRIP-seq), a distinct profile of m6A-modification was identified, distinguishing proliferating from differentiating C2C12 myoblasts. RNAi studies show that reducing levels of METTL3, the active m6A methyltransferase, reduced global m6A levels and forced C2C12 myoblasts to prematurely differentiate. Reducing levels of METTL3 in primary mouse MuSCs prior to transplantation enhanced their engraftment capacity upon primary transplantation, however their capacity for serial transplantation was lost. In conclusion, METTL3 regulates m6A levels in MuSCs/myoblasts and controls the transition of MuSCs/myoblasts to different cell states. Furthermore, the first transcriptome wide map of m6A-modifications in proliferating and differentiating C2C12 myoblasts is provided and reveals a number of genes that may regulate MuSC/myoblast state transitions which had not been previously identified.

Objective Skeletal muscle regeneration relies on muscle-specific adult stem cells (MuSCs), MuSC progeny, muscle progenitor cells (MPCs), and a coordinated myogenic program that is influenced by the extracellular environment. Following injury, MPCs undergo a transient and rapid period of population expansion, which is necessary to repair damaged myofibers and restore muscle homeostasis. Certain pathologies (e.g., metabolic diseases and muscle dystrophies) and advanced age are associated with dysregulated muscle regeneration. The availability of serine and glycine, two nutritionally non-essential amino acids, is altered in humans with these pathologies, and these amino acids have been shown to influence the proliferative state of non-muscle cells. Our objective was to determine the role of serine/glycine in MuSC/MPC function. Methods Primary human MPCs ( h MPCs) were used for in vitro experiments, and young (4–6 mo) and old (>20 mo) mice were used for in vivo experiments. Serine/glycine availability was manipulated using specially formulated media in vitro or dietary restriction in vivo followed by downstream metabolic and cell proliferation analyses. Results We identified that serine/glycine are essential for h MPC proliferation. Dietary restriction of serine/glycine in a mouse model of skeletal muscle regeneration lowered the abundance of MuSCs 3 days post-injury. Stable isotope-tracing studies showed that h MPCs rely on extracellular serine/glycine for population expansion because they exhibit a limited capacity for de novo serine/glycine biosynthesis. Restriction of serine/glycine to h MPCs resulted in cell cycle arrest in G0/G1. Extracellular serine/glycine was necessary to support glutathione and global protein synthesis in h MPCs. Using an aged mouse model, we found that reduced serine/glycine availability augmented intermyocellular adipocytes 28 days post-injury. Conclusions These studies demonstrated that despite an absolute serine/glycine requirement for MuSC/MPC proliferation, de novo synthesis was inadequate to support these demands, making extracellular serine and glycine conditionally essential for efficient skeletal muscle regeneration.

Background Skeletal muscle progenitor cells (MPCs) repair damaged muscle postinjury. Pyruvate kinase M2 (PKM2) is a glycolytic enzyme (canonical activity) that can also interact with other proteins (noncanonical activity) to modify diverse cellular processes. Recent evidence links PKM2 to MPC proliferation. Objectives This study aimed to understand cellular roles for PKM2 in MPCs and the necessity of PKM2 in MPCs for muscle regeneration postinjury. Methods Cultured, proliferating MPCs (C2C12 cells) were treated with a short hairpin RNA targeting PKM2 or small molecules that selectively affect canonical and noncanonical PKM2 activity (shikonin and TEPP-46). Cell number was measured, and RNA-sequencing and metabolic assays were used in follow-up experiments. Immunoprecipitation coupled to proteomics was used to identify binding partners of PKM2. Lastly, an MPC-specific PKM2 knockout mouse was generated and challenged with a muscle injury to determine the impact of PKM2 on regeneration. Results When the noncanonical activity of PKM2 was blocked or impaired, there was an increase in reactive oxygen species concentrations (1.6–2.0-fold, P < 0.01). Blocking noncanonical PKM2 activity also increased lactate excretion (1.2–1.6-fold, P < 0.05) and suppressed mitochondrial oxygen consumption (1.3–1.6-fold, P < 0.01). Glutamate dehydrogenase 1 (GLUD1) was identified as a PKM2 binding partner and blocking noncanonical PKM2 activity increased GLUD activity (1.5–1.6-fold, P < 0.05). Mice with an MPC-specific PKM2 deletion did not demonstrate impaired muscle regeneration. Conclusions The results suggest that the noncanonical activity of PKM2 is important for MPC proliferation in vitro and demonstrate GLUD1 as a PKM2 binding partner. Because no impairments in muscle regeneration were detected in a mouse model, the endogenous environment may compensate for loss of PKM2.

Purpose: In Canada, few men are dietitians. Literature is sparse regarding why so few men are drawn to dietetics. This study, part of a larger qualitative study, explores the experiences of men who are dietitians throughout their training and careers using a phenomenology framework. The study examines the meanings participants make about dietetics in relation to recruitment. Methods: Semi-structured individual interviews with 6 men who are dietitians were completed, transcribed, and analyzed. Results: An overarching theme, “experiences and outcomes of a gendered profession”, was related to the participants’ perspectives concerning recruitment into the dietetic profession. Four sub-themes are reported: (i) societal gender division, (ii) gender division within the profession, (iii) isolation from men who are mentors and other men, and (iv) the need to deconstruct and change. The results provide insight into recruitment barriers and potential approaches for increasing the number of men within dietetics, including changing the perceptions of the profession, increasing role models for men, and dismantling gendered practices. Conclusion: Participants believed that increasing men within dietetics would be beneficial and would increase diversity. It is unlikely that recruitment of men will increase if the status quo and gender norms of the profession are not disrupted and challenged.