The Coordinate Control Mechanism Of Skeletal Muscle Energy Metabolism And Structural Programs

Skeletal muscle is the largest metabolic demanding organ in human body,it comprises about 40%of the total body weight.Skeletal muscle metabolic abnormalities play an important role in the development of many metabolic diseases.Muscle fiber type and mitochondrial energy production are two determinants of skeletal muscle function.According to their contraction characteristics,muscle fibers are generally classified into slow-twitch myofibers and fast-twitch myofibers.According to their metabolic characteristics,muscle fibers are also classified into oxidative and glycolytic muscle fibers.Slow-twitch myofibers are mitochondria-rich and produce ATP mainly dependent on mitochondria oxidative metabolism.Fast-twitch myofibers have less mitochondria,relying largely on glycolytic metabolism to produce energy.Upon adaption of skeletal muscle to physiological and pathophysiological stimuli,skeletal muscle fiber type and mitochondrial function must be coordinately regulated.Once this coordination mechanism is disrupted,skeletal muscle function decline,leading to the body's energy metabolism imbalance,contributing to the progression of many metabolic disorders and muscular diseases.However,the precise mechanism for coupling of mitochondria function and muscle contraction during fiber type transition is still unclear.Previous studies have demonstrated that nuclear receptors such as PPARs and ERRs,and their co-activator PGC-la,and the key energy sensor AMPK play important role in regulating skeletal muscle energy metabolism and muscle fiber specification.Previous study from our laboratory also found that Type I myofiber genes Myh7 and Myh7b encod miR-208b and miR-499,which play dominant role in the control of slow-twitch muscle fiber program.In addition,the expression level of miR-499 in human muscle is positively correlated with the ratio of slow muscle fibers and metabolic parameters,suggesting that miR-499 can regulate the energy metabolism of skeletal muscle.Thus,we propose a hypothesis that miR-499 and miR-208b may be an important factor in regulating the tight coupling of skeletal muscle energy metabolism and muscle fiber contraction.In order to explore this hypothesis,we first examined the expression of miR-499 and miR-208b and mitochondrial oxidative respiration capability during muscle fiber type transition.We found the dynamic expression of miR-499 and miR-208b was parallel with mitochondrial function.Muscle specific miR-499 transgenic mice have increased mitochondrial function,leading to increase muscle fat oxidative metabolism during exercise.Mitochondrial enzyme activity and mitochondrial respiration capacity are all increased in miR-499 transgenic mice compared to controls.Furthermore,we found miR-499 increased mitochondrial oxidative function was due to the dramatic induction of PGC-la protein.Muscle specific PGC-1? loss-of-function in miR-499 transgenic mice reversed the increased mitochondria oxidative capacity in miR-499 transgenic mice.Further studies have shown that miR-499 directly target an AMPK interacting protein,folliculin interacting protein-1(Fnip1).In vivo and in vitro studies demonstrated that miR-499 directly suppresses Fnipl,thus activating AMPK-PGC-1?signaling to promote mitochondria oxidative metabolism.In addition,we also simultaneously inhibit miRNAs and Fnip1 in myocytes to confirm that miR-499 increases the mitochondrial function through inhibiting Fnipl protein.Based on the experimental results of mice and cells,we have found a novel miR-499/Fnip1/AMPK signaling pathway that regulates the coupling of mitochondrial function with muscle fiber type during muscle fiber transition.Duchenne muscular dystrophy(DMD)is the most common and severe form of muscular dystrophy.We have found that the levels of miR-499 and miR-208b were significantly down-regulated in mdx mice muscle compared to wild-type control mice.The reduction of mitochondria function and slow muscle fibers proportion in mdx mice were consistent with our findings that miR-499 coordinately regulate mitochondrial function and muscle fiber contraction.To determine whether miR-499 could ameliorate the severity of muscle dystrophy in the mdx mice,we re-activated the miR-499 in the muscle of mdx mice,restoration of the expression of miR-499 lead to significant increases in mitochondrial function and ameliorate the hallmarks of dystrophic phenotype.In summary,we have identified a novel mechanism for coupling of skeletal muscle fiber type and mitochondria function by a miR-499/Fnipl/AMPK circuit,and activation of this circuit can ameliorate muscular dystrophy.