Comprehensive interrogation of human skeletal muscle reveals a dissociation between insulin resistance and mitochondrial capacity

Abstract

Aims/HypothesisInsulin resistance and blunted mitochondrial capacity in skeletal muscle are often synonymous; however, this association remains controversial. The aim of this study was to perform an in-depth multi-factorial comparison of skeletal muscle mitochondrial capacity between individuals who were lean and active (Active), individuals with obesity (Obese) and individuals with Obesity, insulin resistance and type 2 diabetes (T2D). MethodsSkeletal muscle biopsies were obtained from the Vastus Lateralis of individuals who were lean and active (Active- n = 9), individuals with obesity (Obese- n = 9) and individuals with obesity insulin resistance and T2D (T2D- n =22) in this cross-sectional design. Mitochondrial capacity was assessed by ex vivo mitochondrial respiration with fatty-acid and glycolytic supported protocols adjusted for mitochondrial content (mtDNA and citrate synthase activity). Supercomplex assembly was measured by BN-PAGE and immunoblot. TCA cycle intermediates were assessed with targeted metabolomics. Exploratory transcriptomics and DNA methylation analyses were performed to uncover molecular differences affecting mitochondrial function among the three groups. ResultsActive had greater mitochondrial capacity compared to both Obese and T2D for ex vivo mitochondrial respiration with fatty-acid and glycolytic supported protocols adjusted for mitochondrial content (P < 0.05). Complex IV supercomplex assembley was greater in Active compared to Obese and T2D (P < 0.05) whereas Complex I and III supercomplex assembly was greater in Active compared to T2D only (P < 0.05). TCA cycle intermediates; citrate, succinate, fumarate and malate were all significantly greater in Active compared to Obese and T2D (P < 0.05). Strikingly, Obese and T2D do not differ in any of the skeletal muscle mitochondrial measurements. Active had an upregulation of genes related to respiration/mitochondrial capacity compared to both Obese and T2D. Transcriptional differences between Obese and T2D were not driven by mitochondrial related process. Active had reduced methylation correlated with increased gene expression for important mitochondrial-related genes, including ATP5PD and MFN2. Conclusions/InterpretationsWe reveal no discernable differences in skeletal muscle mitochondrial content, mitochondrial capacity and mitochondrial molecular profiles between obese individuals with and without T2D that had comparable levels of confounding factors (BMI, age, aerobic capacity) that affect mitochondrial capacity. We highlight that lean, active individuals have enhanced skeletal muscle mitochondrial capacity that is also reflected at the level of DNA methylation and gene transcription. The collective observation of comparable muscle mitochondrial capacity in individuals with obesity and T2D (vs. individuals without T2D) underscores a dissociation from skeletal muscle insulin resistance. Clinical trial numberNCT0191110