Mitochondrial Derangements Driving Muscle Insulin Resistance in Humans

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. Thus, a comprehensive characterization of mitochondrial functional defects and the associated proteome alterations in patients harboring a mtDNA mutation associated with an insulin-resistant phenotype may elucidate the causal nexus between mitochondrial derangements and insulin resistance. Also, as mitochondrial dysfunction exhibits many faces (e.g. reduced oxygen consumption rate, impaired ATP synthesis, overproduction of reactive oxygen species, altered membrane potential), such an approach may clarify which features of mitochondrial dysfunction play a prominent role in the pathogenesis of insulin resistance.