Therapeutic Targeting of Skeletal Muscle Nix in Early‐Onset Insulin Resistance

Abstract

Fetal exposure to diabetes during pregnancy increases the risk for early‐onset insulin resistance in the offspring; however, the key molecular regulators responsible for fetal metabolic programming have not been characterized in muscle tissue. Previously, we demonstrated that the expression of a mitochondrial death gene Nix, was elevated in the skeletal muscle of rats exposed to gestational diabetes. Through detailed phospho‐peptide mapping of Nix, we identified a novel protein kinase‐A (PKA) phosphorylation residue within the transmembrane domain, suggesting that Nix function could be regulated post‐translationally. Therefore, we hypothesized that pharmacological activation of PKA by the adrenergic agonist clenbuterol could prevent Nix‐induced mitochondrial dysfunction in muscle cells. To investigate the cellular role of this phospho‐acceptor site, we engineered both neutral (S212A) and phospho‐mimetic (S212D) Nix mutants, and generated a custom phospho‐specific antibody targeted to serine‐212 (S212) of Nix. We confirmed that PKA activating agents led to Nix phosphorylation in intact cells, but not in the presence of S212A. In cultured C2C12 cells, either clenbuterol or PKA restored mitochondrial membrane potential following palmitate exposure and Nix expression. However, clenbuterol did not restore the mitochondria membrane potential in presence of the S212A mutant. Consistent with this finding, C2C12 cells transfected with Nix wild‐type and the S212A mutation decreased mitochondrial membrane potential, but the S212D mutant had no effect. Using organelle‐targeted calcium biosensors, we demonstrated that Nix wild‐type and the S212A mutation led to sarcoplasmic reticulum (SR) calcium release and mitochondrial calcium accumulation, which contribute to mitochondrial depolarization, whereas the phospho‐mimetic mutant of Nix had no effect on these biosensors. While investigating the effects of Nix phosphorylation on cellular function, we observed that Nix and S212 mutants equally contributed to induce autophagosome formation, determined by LC3‐GFP. Suggesting that Nix phosphorylation at S212 does not impact macro‐autophagy activation, which has been shown to be an important mechanism to maintain insulin sensitivity in muscle. Additionally, we showed that phosphorylated Nix is exclusively localized in the cytosol, and not in mitochondria. Furthermore, we demonstrate that Nix translocation to cytosol is dependent on a physical interaction with the molecular chaperone 14‐3‐3b, which is enhanced by PKA phosphorylation of Nix. Finally, we observed that Nix wild‐type and S212A mutation decreased insulin stimulated glucose uptake; however, the S212D mutant had no effect on insulin sensitivity. Our data supports the hypothesis that Nix regulates mitochondrial function and insulin sensitivity in differentiated myotubes and implicates PKA activating agents as possible therapeutic approach to restore mitochondrial dysfunction and insulin sensitivity in skeletal muscle of offspring exposed to gestational diabetes.