Rac1 deficiency reduces mitochondrial respiratory capacity, impairs fatty acid metabolism and causes muscle wasting

BackgroundThe age-related progressive decline in skeletal muscle function is characterised by declining mitochondrial quality control and perturbed fatty acid metabolism, contributing to frailty and increased mortality. The actin cytoskeleton, a key structural component of skeletal muscle, has recently been implicated in mitochondrial anchoring and dynamics. However, the role of actin-regulating proteins, including the Rho GTPase Rac1, in mitochondrial function and age-associated metabolic and functional muscle deterioration remains undefined. MethodsSkeletal muscle from mice with inducible muscle-specific deletion of Rac1 (Rac1 imKO) underwent unbiased mass spectrometry-based proteomic profiling. Mitochondrial morphology was assessed by transmission electron microscopy, and physiological parameters, including muscle mass and contraction-stimulated palmitate oxidation in isolated soleus muscle, were evaluated. Mitochondrial respiratory function was determined by high-resolution respirometry in permeabilised gastrocnemius skeletal muscle fibre bundles. Biochemically, muscular triacylglycerol (TG) content, mRNA (qPCR) and protein (immunoblotting) content were determined. In vastus lateralis muscle biopsies from healthy, untrained young (20-30 years) and old, sarcopenic (83-94 years) men, Rac1 and mitochondrial respiratory protein abundances were measured. A complementary human genetic association analysis was performed using the FinnGen dataset. ResultsRac1 deficiency triggered muscle wasting in middle-aged mice (Gastrocnemius: -10%; Quadriceps: -7%). Preceding muscle wasting, gene set enrichment analysis identified enrichment in fatty acid metabolism and oxidative phosphorylation pathways, consistent with increased mitochondrial volume density in Rac1 imKO muscle (subsarcolemmal: +467%; intermyofibrillar: +166%). Despite mitochondrial expansion at this stage, Rac1 deficiency attenuated the increase in palmitate oxidation in response to muscle contraction (-62%). At the muscle-wasting stage, Rac1 imKO muscle exhibited reduced mitochondrial respiratory capacity (-25-32%). Additionally, the mitochondrial dysfunction was associated with an accumulation of muscle TG (+78%, p = 0.096) and upregulation of fatty acid transporter, CD36 protein content (+25%), indicative of altered fatty acid handling. In humans, Rac1 muscle protein content was increased in old, sarcopenic subjects compared to young (+41%), and negatively correlated with quadriceps cross-sectional area (CSA) (r = -0.475) and type II fibre CSA (r = -0.466). In old, sarcopenic muscle, Rac1 protein content correlated negatively with protein content of multiple mitochondrial respiratory complexes (CI: r = -0.690, CIV: r = -0.938, CV: r = -0.704). GWAS further identified associations between Rac1 SNP variants and lipid metabolic and muscle-wasting diseases. ConclusionsMuscle Rac1 deficiency reduces mitochondrial respiratory capacity and metabolic flexibility through impaired fatty acid metabolism, leading to muscle wasting and highlighting a potential therapeutic target in age-related functional decline.