Exercise-mediated Rescue of Mitochondrial Derangements Driving Insulin Resistance in Humans (EX-MITO-DYS-IR)

Background: Peripheral insulin resistance is a major risk factor for metabolic diseases such as type 2 diabetes. Skeletal muscle accounts for the majority of insulin-stimulated glucose disposal, hence restoring insulin action in skeletal muscle is key in the prevention of type 2 diabetes. Mitochondrial dysfunction is implicated in the etiology of muscle insulin resistance. Also, as mitochondrial function is determined by its proteome quantity and quality, alterations in the muscle mitochondrial proteome may play a critical role in the pathophysiology of insulin resistance. However, insulin resistance is multifactorial in nature and whether mitochondrial derangements are a cause or a consequence of impaired insulin action is unclear. In recent years, the study of humans with genetic mutations has shown enormous potential to establish the mechanistic link between two physiological variables; indeed, if the mutation has a functional impact on one of those variables, then the direction of causality can be readily ascribed. Mitochondrial myopathies are genetic disorders of the mitochondrial respiratory chain affecting predominantly skeletal muscle. Mitochondrial myopathies are caused by pathogenic mutations in either nuclear or mitochondrial DNA (mtDNA), which ultimately lead to mitochondrial dysfunction. Although the prevalence of mtDNA mutations is just 1 in 5,000, the study of patients with mtDNA defects has the potential to provide unique information on the pathogenic role of mitochondrial derangements that is disproportionate to the rarity of affected individuals. The m.3243A>G mutation in the MT-TL1 gene encoding the mitochondrial leucyl-tRNA 1 gene is the most common mutation leading to mitochondrial myopathy in humans. The m.3243A>G mutation is associated with impaired glucose tolerance and insulin resistance in skeletal muscle. Most importantly, insulin resistance precedes impairments of β-cell function in carriers of the m.3243A>G mutation, making these patients an ideal human model to study the causative nexus between muscle mitochondrial dysfunction and insulin resistance. Exercise training is a potent stimulus to enhance muscle insulin action, improve mitochondrial function, and promote mitochondrial proteome remodeling. Accordingly, rescue of mitochondrial dysfunction has been proposed to play a role in the insulin-sensitizing effect of exercise. Yet, numerous mechanisms may contribute to the pathophysiology of insulin resistance and the beneficial effects of exercise may be linked to amelioration of multiple factors, thus challenging the interpretation of the functional significance of improved muscle mitochondrial function per se. Nevertheless, since mitochondrial dysfunction is likely the primary cause of muscle insulin resistance in carriers of the m.3243A>G mutation, prospective studies including an in-depth analysis of the mitochondrial adaptations elicited by exercise training in this cohort of patients may offer a unique opportunity to identify those mitochondrial derangements that, once rescued, drive enhancements in insulin sensitivity.