Skeletal muscle mitochondria are essential for maintaining metabolic homeostasis in response to a myriad of physiologic or pathophysiological stresses. The folliculin (FLCN)-interacting protein 1 (FNIP1) forms a complex with AMP-activated protein kinase (AMPK), and we previously demonstrated that FNIP1 inhibits AMPK activity in myocytes. Using gain- and loss-of-function approaches in mice, we discovered that FNIP1 is necessary, but not sufficient, to suppress AMPK-dependent mitochondrial function in skeletal muscle. Muscles in FNIP1 knockout (KO) mice show enhanced red coloration, oxidative mitochondrial activity, and type I fiber transformation. Surprisingly, no change in mitochondrial oxidative capacity and fiber type switching was observed in transgenic mice overexpressing FNIP1 in muscle (FNIP1Tg), while restoration of the expression of FNIP1 in the KO muscle (FNIP1Tg/KO) reversed the mitochondrial oxidative metabolism program. RNA-seq profiling revealed that 3328 genes showed over 1.5-fold altered in FNIP1 KO muscle, and the majority number (2856) was regulated in a FNIP1-dependent manner. Gene ontology based analysis revealed that the primary FNIP1-altered genes were mitochondrial metabolic genes. Mitochondrial respiration analysis confirmed that both pyruvate- and succinate-driven state 3 respiration rates were significantly higher in FNIP1 KO muscle, whereas restoration of the expression of FNIP1 in KO muscle reduced the mitochondrial function. Importantly, AMPK, but not mTOR, was markedly induced in FNIP1 KO muscle but reduced in FNIP1Tg/KO muscle. Muscle-specific knockout of the two AMPKα subunits (AMPKα1α2 dmKO) reduced mitochondrial function in FNIP1 KO muscle. Similar results were obtained in skeletal myocytes in culture, using AMPK loss-of-function approaches in FNIP1 KO myocyte. Thus, FNIP1 represents a previously unrecognized negative regulator of muscle mitochondrial function and AMPK is vital for FNIP1 actions in skeletal muscle.
L. Xiao: None. J. Liu: None. Z. Sun: None. Y. Yin: None. Y. Mao: None.