Consecutive skeletal muscle PGC-1α overexpression: A double-edged sword for mitochondrial health in the aging brain

Mitochondrial dysfunction is a critical contributor to age-related functional declines in skeletal muscle and brain. Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) is essential for mitochondrial biogenesis and function during aging. While skeletal muscle-specific overexpression of PGC-1α is known to mimic exercise-induced benefits in young animals, its chronic systemic effects on aging tissues remain unclear. This study aimed to determine the lifelong impact of skeletal muscle-specific PGC-1α overexpression on mitochondrial health, oxidative stress, inflammation, and cognitive function in aged mice. We established three experimental groups: young wild-type mice (3-4 months old), aged wild-type mice (25-27 months old), and aged mice with skeletal muscle-specific PGC-1α overexpression (24-27 months old). In skeletal muscle, aging led to significant reductions in mitochondrial biogenesis markers, including PGC-1α, FNDC5, and mtDNA content. PGC-1α overexpression reversed this decline, elevating the expression of PGC-1α, SIRT1, LONP1, SDHA, CS, TFAM, eNOS, and mtDNA levels, suggesting preserved mitochondrial biogenesis. However, FNDC5 and SIRT3 were paradoxically suppressed, indicating potential compensatory feedback mechanisms. PGC-1α overexpression also enhanced anabolic signaling, as evidenced by increased phosphorylation of mTOR and S6, and reduced FOXO1 expression, favoring a muscle growth-promoting environment. Moreover, aging impaired mitochondrial dynamics by downregulating MFN1, MFN2, OPA1, FIS1, and PINK1. While PGC-1α overexpression did not restore fusion-related proteins, it further reduced fission-related protein and enhanced mitophagy proteins, as evidenced by increased PINK1 phosphorylation. In contrast, in the hippocampus, muscle-specific PGC-1α overexpression exacerbated age-associated mitochondrial biogenesis decline. Expression levels of key mitochondrial markers, including PGC-1α, SIRT1, CS, FNDC5, Cytochrome C, and TFAM, were further reduced compared to aged wild-type controls. mTOR phosphorylation was also significantly suppressed, whereas cognition-related proteins (BDNF, VEGF, eNOS) and performance in behavioral tests remained unchanged. Importantly, skeletal muscle-specific PGC-1α overexpression triggered pronounced oxidative stress and inflammatory responses in both skeletal muscle and the hippocampus. In skeletal muscle, elevated levels of protein carbonyls, IκB-α, NF-κB, TNF-α, SOD2, and NRF2 were observed, accompanied by a reduction in the DNA repair enzyme OGG1. Notably, similar patterns were detected in the hippocampus, including increased expression of protein carbonyls, iNOS, NF-κB, TNF-α, SOD2, GPX1, and NRF2, alongside decreased OGG1 levels. These findings suggest that the overexpression of PGC-1α in skeletal muscle may have contributed to systemic oxidative stress and inflammation. In conclusion, skeletal muscle-specific PGC-1α overexpression preserves mitochondrial biogenesis and enhances anabolic signaling in aging muscle but concurrently induces oxidative stress and inflammatory responses, which may adversely affect mitochondrial health in the brain. These results emphasize the complex role of the skeletal muscle PGC-1α during aging.