Skeletal Muscle PGC-1α Remodels Mitochondrial Phospholipidome but Does Not Alter Energy Efficiency for ATP Synthesis

The coupling of oxygen consumption to ATP synthesis via oxidative phosphorylation (OXPHOS) is central for cellular energy homeostasis. Some studies suggest exercise training increases the efficiency of ATP synthesis, but the molecular mechanisms are unclear. We have previously shown that exercise remodels the lipid composition of mitochondrial membranes, and some of these changes in mitochondrial lipids might influence OXPHOS efficiency (ATP produced per O₂ consumed, referred to as P/O). Peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) is a transcriptional co-activator that coordinately regulates exercise-induced adaptations, including mitochondria. We hypothesized that increased PGC-1α activity might remodel mitochondrial membrane lipids and promote energy efficiency.

Mice with skeletal muscle-specific overexpression of PGC-1α (MCK-PGC-1α) and their wildtype littermates were used for this study. Lipid mass spectrometry and quantitative PCR were used to assess muscle mitochondrial lipid composition and their biosynthesis pathway. The abundance of OXPHOS enzymes was determined by Western blotting. High-resolution respirometry and fluorometry analyses were performed to characterize mitochondrial bioenergetics (ATP production, O₂ consumption and P/O) for permeabilized fibres and isolated mitochondria. Respiratory supercomplexes were assessed by blue native PAGE.

Lipidomic analyses of skeletal muscle mitochondria from wildtype and MCK-PGC-1α mice revealed that PGC-1α increases the concentrations of cone-shaped lipids such as phosphatidylethanolamine (PE; +25%, p < 0.0001), cardiolipin (CL; +184%, p < 0.0001) and lysophospholipids (+34%-94%, all p < 0.01), while decreasing the concentrations of phosphatidylcholine (PC; -4%, p = 0.0020), phosphatidylinositol (PI; -17%, p < 0.0001) and phosphatidic acid (PA; -35%, p < 0.0001). However, while PGC-1α overexpression increased the abundance of OXPHOS enzymes (two- to fourfold, p < 0.0001), the rate of O₂ consumption (1.5-fold, p = 0.0030), or the respiratory supercomplexes (~1.5-fold, p < 0.01), P/O values were unaffected by PGC-1α overexpression in permeabilized fibres or isolated mitochondria.

Collectively, overexpression of PGC-1α promotes the biosynthesis of mitochondrial PE and CL, but neither PGC-1α nor the mitochondrial membrane lipid remodelling induced in MCK-PGC-1α mice is sufficient to increase the efficiency of mitochondrial ATP synthesis. These findings indicate that PGC-1α-dependent mechanisms or changes in mitochondrial membrane lipids may be insufficient to alter P/O. While muscles from MCK-PGC-1α mice are known not to completely phenocopy adaptations with exercise training, our findings also highlight that there is a need to examine whether exercise training indeed improves P/O in mouse skeletal muscle.