PDK4 Augments ER-Mitochondria Contact to Dampen Skeletal Muscle Insulin Signaling During Obesity

Abstract

Mitochondria-associated endoplasmic reticulum membrane (MAM) is a structural link between mitochondria and endoplasmic reticulum (ER). MAM regulates Ca²⁺ transport from the ER to mitochondria via an IP3R1-GRP75-VDAC1 complex-dependent mechanism. Excessive MAM formation may cause mitochondrial Ca²⁺ overload and mitochondrial dysfunction. However, the exact implication of MAM formation in metabolic syndromes remains debatable. Here, we demonstrate that PDK4 interacts with and stabilizes the IP3R1-GRP75-VDAC1 complex at the MAM interface. Obesity-induced increase in PDK4 activity augments MAM formation and suppresses insulin signaling. Conversely, PDK4 inhibition dampens MAM formation and improves insulin signaling by preventing MAM-induced mitochondrial Ca²⁺ accumulation, mitochondrial dysfunction, and ER stress. Furthermore, Pdk4^-/- mice exhibit reduced MAM formation and are protected against diet-induced skeletal muscle insulin resistance. Finally, forced formation and stabilization of MAMs with synthetic ER-mitochondria linker prevented the beneficial effects of PDK4 deficiency on insulin signaling. Overall, our findings demonstrate a critical mediatory role of PDK4 in the development of skeletal muscle insulin resistance via enhancement of MAM formation.