Habitual Physical Activity Preserves Energetically Constrained Mitochondrial Programs During Human Skeletal Muscle Aging

Technical Summary: Habitual Physical Activity Preserves Energetically Constrained Mitochondrial Programs During Human Skeletal Muscle Aging

Problem Statement
Aging is characterized by a decline in aerobic capacity, driven in part by impaired mitochondrial oxidative phosphorylation in skeletal muscle. While prior proteomic studies have established that aging reduces the abundance of mitochondrial proteins, the functional coupling between mitochondrial proteome remodeling and energetic capacity remains poorly understood. Furthermore, it is unclear whether habitual physical activity (PA) merely increases mitochondrial content or selectively preserves specific mitochondrial programs tightly linked to bioenergetic function. Existing literature often treats molecular signatures and physiological phenotypes separately or focuses on narrow age ranges, lacking a lifespan framework that links age-related proteomic remodeling to in vivo energetic capacity across multiple tissues.

Methodology
This study utilized data from the Genetic and Epigenetic Study of Aging Laboratory Testing (GESTALT), a cohort of 96 healthy individuals aged 22–89 years. The researchers employed a multi-scale, multi-tissue approach:

• Proteomics: Quantitative tandem mass tag (TMT) proteomics was performed on skeletal muscle biopsies, peripheral blood mononuclear cells (PBMCs), naïve CD4⁺ and CD8⁺ T cells, and skin. A total of 4,895 proteins were quantified in muscle, with 741 mapped to the high-confidence MitoCoP mitochondrial reference set.
• Physiological Measures: In vivo mitochondrial oxidative capacity was assessed via 31P-magnetic resonance spectroscopy (kPCr, the phosphocreatine recovery rate constant). Cardiorespiratory fitness was measured as peak oxygen uptake (VO₂peak). Habitual physical activity was quantified via self-reported questionnaires (validated against wearable accelerometry in a subset) and categorized into four ordinal levels.
• Statistical Analysis: Linear regression models adjusted for sex, race, BMI, fiber ratio, and batch effects were used to estimate age, PA, and VO₂peak associations for each protein. The study analyzed not only protein abundance but also the correlation structure (stoichiometry) within functional pathways. "Rescue" signatures were defined as proteins where the direction of association with PA or VO₂peak opposed the direction of association with age. Network density and cross-tissue correlation matrices were used to assess systemic coordination.

Key Contributions and Results

1. Selective Remodeling of the Mitochondrial Proteome:
   Aging does not cause a uniform loss of all mitochondrial proteins. Instead, it drives a selective depletion of core bioenergetic modules (Oxidative Phosphorylation [OxPhos] complex subunits, TCA cycle, mitochondrial ribosomes, and protein import machinery) while altering the correlation structure within pathways. Specifically, aging was associated with a pathway-specific reorganization where core bioenergetic pathways showed higher internal correlation but lower heterogeneity in older adults, suggesting a rigidification or convergence of these networks, while proteostasis pathways became more variable.

2. Physical Activity as a Selective Modifier:
   Habitual physical activity was associated with a proteomic signature that directly opposed age-related changes.

   • Inverse Correlation: The regression coefficients for age and PA were inversely correlated (Spearman ρ = −0.35, p < 3.9 × 10⁻⁷).
   • Rescue of Core Pathways: PA preferentially "rescued" (maintained higher abundance) proteins in core bioenergetic categories, including OxPhos, TCA cycle, and mitochondrial ribosomes. Proteins that declined with age (e.g., CYCS, COX20, COX6C) were found at higher levels in physically active individuals.
   • Distinction from VO₂peak: While cardiorespiratory fitness (VO₂peak) showed a stronger inverse correlation with age-associated proteins (ρ = −0.83), the PA-rescued signature was more tightly coupled to in vivo muscle energetic function (kPCr). The PA-rescued score remained significantly associated with kPCr even after adjusting for age and VO₂peak, whereas the VO₂peak-rescued score did not. This suggests PA captures muscle-intrinsic energetic adaptations more directly than the systemic VO₂peak phenotype.

3. Functional Coupling to Energetic Capacity:
   The PA-rescued mitochondrial proteomic signature was strongly linked to physiological function. Individuals with higher PA-rescued scores exhibited greater skeletal muscle oxidative capacity (kPCr). Furthermore, 93% of the 76 PA-rescued mitochondrial proteins were also significantly associated with kPCr, indicating that this signature reflects functionally relevant components of the mitochondrial proteome rather than secondary correlations.

4. Cross-Tissue Coordination:
   Mitochondrial proteomic profiles showed modest but structured coordination across tissues (muscle, PBMC, T cells, skin). Aging was associated with a decline in a composite peripheral mitochondrial score. Notably, high PA-rescue status did not result in uniformly stronger cross-tissue synchronization. Instead, it was associated with selective changes in tissue-to-tissue correlations, particularly involving skin and immune cell compartments, suggesting that physical activity may promote tissue-specific specialization of mitochondrial programs rather than global systemic coupling.

Significance and Claims
The paper claims that mitochondrial aging in human skeletal muscle is not merely a quantitative loss of mitochondrial content but a progressive breakdown of energetic coordination and a selective disruption of core bioenergetic systems. The study reframes habitual physical activity as a selective stabilizer that preserves the mitochondrial programs most tightly linked to oxidative function.

The authors conclude that the PA-rescued signature identifies a biologically meaningful subset of the mitochondrial proteome that is directly coupled to in vivo energetic capacity. By demonstrating that physical activity opposes age-related remodeling specifically in core bioenergetic pathways and maintains their functional coupling to muscle energetics, the study provides a molecular basis for the protective effects of exercise against sarcopenia and mobility limitation. The findings suggest that mitochondrial aging involves a disorganization of the bioenergetic network, which habitual physical activity helps to stabilize.