Mechanisms for Insulin-dependent Regulation of Skeletal Muscle Mitochondria by OPA-1

Augmentation of mitochondrial oxidative metabolism by insulin might be mediated in part by induction of the inner mitochondrial membrane protein, Optic Atrophy-1 (OPA-1). Decreased OPA-1 in skeletal muscle has been reported in both murine and human type II diabetic models. Reduced OPA-1 in these contexts correlates with mitochondrial content, oxidative capacity, and dynamics. In addition to increasing mitochondrial fusion, OPA-1 mediates cristae remodeling toenhance mitochondrial bioenergetics through oligomer formation. Therefore, we hypothesized that insulin stimulation increases OPA-1 protein levels and mitochondrial respiration by increasing OPA-1 oligomerization and cristae remodeling in skeletal muscle cells. To investigate the role of insulin on mitochondrial dynamics and cristae organization, Cre-LoxP technology was used to ablate OPA-1 from skeletal myoblasts isolated from floxed OPA-1 mice. Confocal microscopy and MitoTracker orange was employed to obtain three-dimensional images of mitochondria networks, and transmission electron microscopy (TEM) was used to analyze ultrastructure of mitochondrial cristae. Two hours of insulin treatment increased OPA-1 protein levels, promoted mitochondrial fusion, elevated mitochondrial respiration, increased cristae number along with volume density and area, decreased the distance of ER-mitochondria contacts, and it may have even changed OPA-1 oligomerization. Depletion of OPA-1 and MFN-2 blocked the metabolic effects of insulin stimulation. OPA-1 depletion reduced cristae number, volume density, and area. Loss of OPA-1 resulted in a compensatory elevation of mitochondrial associated membrane proteins (MAMs), increasing the number of ER-mitochondria contacts, blocking OPA-1 oligomerization, and decreasing the ER-mitochondria contact distance. Together, ablation of OPA-1 alters metabolic responses to insulin in skeletal muscle cells. Furthermore, insulin stimulation promotes changes in cristae morphology and triggers mitochondrial fusion through an IR-pAKT pathway that increases OPA-1. ER-mitocontacts increase after ablation of OPA-1 due to an increase in MAMs. Lastly, Loss of OPA-1 inhibits OPA-1 oligomerization formation.