Bruce M. Spiegelman

Skeletal muscle is a central determinant of organismal health. Preserving muscle quality is therefore critical for preventing disease and sustaining quality of life across the lifespan. Despite its central role, the field lacks a unifying framework that defines the core properties of skeletal muscle health. Here, we propose a conceptual framework for muscle homeostasis built around seven interconnected hallmarks—metabolism and bioenergetics, proteostasis, genomics, excitability, structure, regeneration and cross-talk—that collectively govern muscle integrity, adaptability and resilience. Each hallmark is mechanistically grounded, quantifiable and potentially modifiable. This framework provides a unifying blueprint for the next generation of precision diagnostics and targeted therapies for preserving skeletal muscle health.

Irisin is an exercise-induced myokine that confers multiple physiological benefits, including browning of subcutaneous adipose tissue in mice. However, the underlying cellular and molecular mechanisms of irisin’s effects on obesity are unclear. Here, we show that irisin modulates adipose tissue inflammation by increasing interleukin (IL)-33 production and preserving ST2⁺ regulatory T cells in white adipose tissues. Administered chronically to high-fat-diet-fed male mice, irisin preserves visceral adipose tissue (VAT) levels of ST2⁺ regulatory T cells, an important immunomodulatory population that usually contracts after long-term high-fat-diet feeding. This protection results from increases in IL-33-producing mesenchymal stromal cells and IL-33 levels in VAT. These effects are primary, as irisin directly induces IL-33 expression in cultured VAT mesenchymal stromal cells. Irisin-mediated changes in VAT IL-33 dynamics are accompanied by IL-33-dependent upregulation of thermogenic gene expression in subcutaneous adipose tissue. These irisin-driven cell–cell and inter-tissue interactions improve obesity and glucose intolerance, and increase energy expenditure, with no reduced food intake and muscle loss in obese mice.

Endurance exercise promotes adaptive growth and improved function of myocytes, which is supported by increased mitochondrial activity. In skeletal muscle, these benefits are in part transcriptionally coordinated by peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α). The importance of PGC-1α to exercise-induced adaptations in the heart has been unclear. Here we show that deleting PGC-1α specifically in cardiomyocytes prevents the expected benefits from exercise training and instead leads to heart failure after just 6 weeks of training. Consistent with this, in humans, rare genetic variants in PPARGC1A, which encodes PGC-1α, are associated with increased risk of heart failure. In this model, we identify growth differentiation factor 15 (GDF15) as a key heart-secreted mediator that contributes to this dysfunction. Blocking cardiac Gdf15 expression improves cardiac performance and exercise capacity in these mice. Finally, in human heart tissue, lower cardiomyocyte PPARGC1A expression is associated with higher GDF15 expression and reduced cardiomyocyte density. These findings uncover a crucial role for cardiomyocyte PGC-1α in enabling healthy cardiac adaptation to exercise in part through suppression of GDF15.

Ergothioneine (EGT) is a diet-derived, atypical amino acid that accumulates to high levels in human tissues. Reduced EGT levels have been linked to age-related disorders, including neurodegenerative and cardiovascular diseases, while EGT supplementation is protective in a broad range of disease and aging models in mice. Despite these promising data, the direct and physiologically relevant molecular target of EGT has remained elusive. Here we use a systematic approach to identify how mitochondria remodel their metabolome in response to exercise training. From this data, we find that EGT accumulates in muscle mitochondria upon exercise training. Proteome-wide thermal stability studies identify 3-mercaptopyruvate sulfurtransferase (MPST) as a direct molecular target of EGT; EGT binds to and activates MPST, thereby boosting mitochondrial respiration and exercise training performance in mice. Together, these data identify the first physiologically relevant EGT target and establish the EGT-MPST axis as a molecular mechanism for regulating mitochondrial function and exercise performance.

Summary: Exercise training induces physiological cardiac hypertrophy, enhanced mitochondrial biogenesis and myocardial contractility. In skeletal muscle, the transcriptional coactivator PGC-1α is a key orchestrator of these responses. The heart expresses abundant and exercise-responsive PGC-1α, but it is unclear whether cardiomyocyte PGC-1α is necessary for cardiac adaptation to endurance training. Here we demonstrate that cardiomyocyte PGC-1α is required for physiological cardiac hypertrophy during exercise training in mice. In the absence of cardiomyocyte PGC-1α, voluntary wheel running does not improve exercise capacity and instead confers immune-fibrotic-atrophic heart failure after just 6 weeks of training. We identify cardiomyocyte PGC-1α as a negative regulator of stress-responsive senescence gene expression. The most enriched of these is the myomitokine GDF15. GDF15 is secreted locally but not systemically in PGC-1α-deficient mouse hearts and reduces cardiomyocyte size. Cardiomyocyte-specific reduction of GDF15 expression preserves exercise tolerance and cardiac contractility in PGC-1α-deficient mice during endurance training. Finally, we show that cardiomyocyte PPARGC1A expression correlates with cardiomyocyte number and negatively with GDF15 expression in human cardiomyopathies through single nucleus RNA sequencing. Our data implicate cardiomyocyte PGC-1α as a vital safeguard against stress-induced atrophy and local GDF15-induced dysfunction during exercise.

Irisin is a myokine released from muscle during exercise with distinct signaling effects on tissues throughout the body, including an influence on skeletal remodeling. Our previous work has shown that irisin stimulates resorption, a key first step in bone remodeling, by enhancing osteoclastogenesis. The present study further investigates the action of irisin on the metabolic function of osteoclast progenitors during differentiation. Fluorescent imaging showed increased mitochondrial content and reactive oxygen species production with irisin treatment in osteoclast progenitors after 48 h of osteoclastogenic culture. Mitochondrial stress testing demonstrated a significant increase in maximal oxygen consumption rate and spare capacity after 48 h of preconditioning with irisin treatment. Together, these findings further elucidate the stimulatory action of irisin on osteoclastogenesis, demonstrating an enhancement of metabolism through mitochondrial respiration in the progenitor to support the energy demands of their differentiation into mature osteoclasts.