Fibroblast Growth Factor 21 Regulation Of Skeletal Muscle Fiber Types And Mitochondrial Function And The Underlying Mechanism

The purpose of this study was to investigate the effects of FGF21 on skeletal muscle fiber types and mitochondrial functions,and to explore its possible mechanism.Three experiments were included in this study.Exp.1 Effects of feed restriction on FGF21 expression,muscle fiber types and mitochondrial functions in DLY piglets.The objective of this experiment was to investigate the effects of feed restriction on FGF21 expression,skeletal muscle fiber types and mitochondrial functions in DLY piglets.A total of 16 health male DLY barrows at 45-d old age were selected and divided into control group and feeding restriction group with 8 replicates in each.Pigs in control group had free access to the feed,and pigs in feeding restriction group were fed the amount equals to 2%of body weight.The trial period was 14d.The growth performance,FGF21level,muscle fiber types,the expressions of mitochondrial function related genes and muscle fiber type transformation key genes were investigated.The results showed that:feed restriction reduced the average daily gain of piglets and increased the ratio of feed to gain（P<0.05）.Feed restriction increased FGF21 levels in serum,liver and fat,and decreased the expressions of FGF21 in skeletal muscles（P<0.05）;reduced the muscle fiber diameters of soleus（SOL）and extensor digitorum longus（EDL）,and increased muscle fiber density of SOL（P<0.05）.Feed restriction increased the ratio of type I muscle fiber in SOL and EDL（P<0.05）;increased the expressions of oxidative muscle fiber in different skeletal muscles,and reduced the expressions of glycolytic muscle fiber in semitendinosus（SM）,psoas muscle（PM）,SOL and gastrocnemius（GAS）（P<0.05）.Feed restriction increased the expressions of mitochondrial function related genes,such as CS,CPT-1b,SDHB and Nrf-1 in different skeletal muscles（P<0.05）.Feed restriction increased the expressions of muscle fiber type transformation key genes including AMPK,SIRT1 and PGC-1αin skeletal muscles（P<0.05）.Therefore,feed restriction improved the FGF21 level,the proportion of oxidative muscle fibers in skeletal muscles,mitochondrial functions,and increased the expressions of AMPK,SIRT1 and PGC-1αin skeletal muscles of DLY piglets.Exp.2 Effects of FGF21 on skeletal muscle fiber types and mitochondrial functions in mice.The purpose of this experiment was to verify the effects of FGF21 on the muscle fiber types and mitochondrial functions in different skeletal muscles and explore the possible mechanism.Eighteen health 6 weeks old C57BL/6 male mice were selected.According to the body weight,mice were divided into 2 groups,with 9 repelicates in each.The micro osmotic pump was implanted subcutaneously by operation.The treatment group was administrated with 100μL 0.5 mg/kg BW/d recombinant FGF21 protein using 100μL saline as control,the trial period was 14d.The growth of mice,organ index,FGF21 level,muscle fiber types,the expressions of mitochondrial function related genes,FGFR and muscle fiber type transformation key genes were investigated.The results showed that:subcutaneous implanted 0.5 mg/kg BW/d FGF21 reduced body weight gain in mice（P<0.05）,but did not affect feed intake;decreased the relative weights of liver,perirenal and epididymal fat（P<0.05）;raised the levels of FGF21 in serum and liver.FGF21 reduced the muscle fiber diameters and increased muscle fiber densities of SOL and EDL（P<0.05）.The expressions of oxidative muscle fiber in different skeletal muscles were increased,and the expressions of glycolytic muscle fiber were decreased in SOL（P<0.05）.FGF21 increased the expressions of mitochondrial function related genes,such as CS,CPT-1b,SDHB and Nrf-1 in different skeletal muscles（P<0.05）.The expressions of KLB,FGFR1,FGFR2c and FGFR4 in different skeletal muscles were increased（P<0.05）.FGF21 increased the expressions of AMPK,SIRT1 and PGC-1αin different skeletal muscles（P<0.05）.In addition,the expressions of Calcineurin,NFATc3,Ca MKII,HDAC4,PPARδin SOL were increased and the expression of PPARδin EDL was increased（P<0.05）.It was suggested that FGF21 increased the proportion of the oxidative muscle fibers and mitochondrial functions in different skeletal muscles,and reduced the proportion of the glycolytic muscle fiber in SOL.The possible mechanism might be that FGF21 enhanced mitochondrial function through AMPK-SIRT1-PGC1αpathway,and then increased the proportion of oxidative muscle fibers.In addition,the role of FGF21 in the transformation from glycolysis to oxidation in the slow skeletal muscle may also exist of other signaling pathways,such as the Ca N-NFATc3 and the Ca²⁺/Ca MK pathway.Exp.3 Effects of hepatic FGF21 specific knockout on skeletal muscle fiber types and mitochondrial functions in mice.The purpose of this experiment was to verify the effect of FGF21 on skeletal muscle fiber types and mitochondrial functions,and the role of related signaling pathways using hepatic FGF21-deficiency mice.Eight female hepatic FGF21 specific knockout mice and 8female wild type littermates were selected.The measurements were same to experiment 2.The results showed that:in hepatic FGF21 specific knockout mice,liver FGF21m RNA was not expressed（P<0.05）,and had no significant effect on the expressions of FGF21 m RNA in different skeletal muscles.Muscle color was shallow,muscle fiber diameter was increased and muscle fiber density was decreased in SOL（P<0.05）.The expressions of oxidative muscle fibers in different skeletal muscles were decreased,and the expressions of glycolytic muscle fiber were increased in SOL（P<0.05）.Hepatic FGF21specific knockout reduced the expressions of mitochondrial function related genes,such as CS,CPT-1b,SDHB and Nrf-1 in different skeletal muscles（P<0.05）.The expressions of KLB,FGFR1,FGFR2c and FGFR4 in skeletal muscles were decreased（P<0.05）.The expressions of AMPK,SIRT1 and PGC-1αin different skeletal muscles were decreased（P<0.05）.In addition,the expressions of Calcineurin,NFATc3,Ca MKII,HDAC4,PPARδin SOL were decreased and the expression of PPARδin EDL was decreased（P<0.05）.It was suggested that hepatic FGF21 specific knockout reversed the endocrine effect of FGF21 on skeletal muscle,and FGF21 might regulate the muscle fiber types by altering mitochondrial functions in different skeletal muscles.In conclusion,FGF21 may increase mitochondrial functions in skeletal muscles through AMPK-SIRT1-PGC1αpathway by endocrine action,thus improve the proportion of oxidative muscle fiber in different skeletal muscles.In addition,FGF21 is likely to promote the transformation from glycolytic fiber to oxidative fiber in the slow skeletal muscle by Calcineurin-NFATc3 and Ca²⁺/Ca MK pathway.