In many cells and specially in muscle, mitochondria form elongated filaments or a branched reticulum. We show that Mfn2 (mitofusin 2), a mitochondrial membrane protein that participates in mitochondrial fusion in mammalian cells, is induced during myogenesis and contributes to the maintenance and operation of the mitochondrial network. Repression of Mfn2 caused morphological and functional fragmentation of the mitochondrial network into independent clusters. Concomitantly, repression of Mfn2 reduced glucose oxidation, mitochondrial membrane potential, cell respiration, and mitochondrial proton leak. We also show that the Mfn2-dependent mechanism of mitochondrial control is disturbed in obesity by reduced Mfn2 expression. In all, our data indicate that Mfn2 expression is crucial in mitochondrial metabolism through the maintenance of the mitochondrial network architecture, and reduced Mfn2 expression may explain some of the metabolic alterations associated with obesity.

The fiber type-specific expression of skeletal muscle GLUT4 and the effect of 2 weeks of low-intensity training were investigated in 8 young untrained male subjects. Single muscle fibers were dissected from a vastus lateralis biopsy sample. Based on myosin heavy chain (MHC) expression, fibers were pooled into 3 groups (MHC I, MHC IIA, and MHC IIX), and the GLUT4 content of 15-40 pooled fibers was determined using SDS-PAGE and immunological detection. The GLUT4 content in pooled muscle fibers expressing MHC I was ~20% higher (P < 0.05) than that in muscle fibers expressing MHC IIA or MHC IIX. No difference in GLUT4 could be detected between fibers expressing MHC IIA or MHC IIX. Two weeks of exercise training increased (P < 0.05) the peak power output of the knee extensors by 13%, the maximal activities of citrate synthase and 3-hydroxyacyl-CoA dehydrogenase by 21 and 18%, respectively, and the GLUT4 protein content by 26% in a muscle homogenate. Furthermore, a 23% increase (P < 0.05) in GLUT4 was seen in fibers expressing the MHC I isoform after exercise training for 2 weeks. No change was seen in fibers expressing MHC IIA or MHC IIX. In conclusion, our data directly demonstrate that GLUT4 is expressed in a fiber type-specific manner in human skeletal muscle, although fiber type differences are relatively small. In addition, low-intensity exercise training recruiting primarily fibers expressing MHC I increased GLUT4 content in these fibers but not in fibers expressing MHC IIA or MHC IIX, indicating that GLUT4 protein content is related more to activity level of the fiber than to its fiber type, which is defined by expression of contractile protein. Diabetes 49:1092-1095, 2000 I n both rats and humans, glucose transport has been shown to be the rate-limiting step in muscle glucose uptake under most conditions (1,2). Glucose transport in mammalian skeletal muscle is almost exclusively mediated by the insulin-and contraction-regulatable glucose transporter GLUT4 (3). In both rodents and humans, different muscles have been shown to exhibit large differences in their GLUT4 content (4-9), and in rats, this variation is often associated with differences in insulin-stimulated glucose uptake (4-6,8). Because different muscles are composed of a mixture of several different muscle fiber types in both rats and humans (10), it is possible that a relationship exists between fiber type composition and GLUT4 content. Such a relationship has been proposed in human muscle based on indirect evidence (11,12), but in reality, it is unknown at present if the GLUT4 content differs between fiber types in human skeletal muscle.It is also possible that the differences in GLUT4 content and insulin-stimulated glucose uptake are more related to activity level. Changes in the skeletal muscle activity level have been shown to be important regulators of the GLUT4 content in rats (13-16). In humans, athletes have more GLUT4 than untrained age-matched control subjects (11,17,18), and in both normal healthy control subjects and individuals with de...

Studies in rats suggest that increases in fatty acid oxidation in skeletal muscle during exercise are related to the phosphorylation and inhibition of acetyl-CoA carboxylase (ACC), and secondary to this, a decrease in the concentration of malonyl-CoA. Studies in human muscle have not revealed a consistent decrease in the concentration of malonyl-CoA during exercise; however, measurements of ACC activity have not been reported. Thus, whether the same mechanism operates in human muscle in response to physical activity remains uncertain. To investigate this question, ACC was immunoprecipitated from muscle of human volunteers and its activity assayed in the same individual at rest and after one-legged kneeextensor exercise at 60, 85, and 100% of knee extensor VO 2max . ACC activity was diminished by 50-75% during exercise with the magnitude of the decrease generally paralleling exercise intensity. Treatment of the immunoprecipitated enzyme with protein phosphatase 2A restored activity to resting values, suggesting the decrease in activity was due to phosphorylation. The measurement of malonyl-CoA in the muscles revealed that its concentration is 1/10 of that in rats, and that it is diminished (12-17%) during the higher-intensity exercises. The respiratory exchange ratio increased with increasing exercise intensity from 0.84 ± 0.02 at 60% to 0.99 ± 0.04 at 100% VO 2max . Calculated rates of whole-body fatty acid oxidation were 121 mg/min at rest and 258 ± 35, 264 ± 63, and 174 ± 76 mg/min at 60, 85, and 100% VO 2max , respectively. The results show that ACC activity, and to a lesser extent malonyl-CoA concentration, in human skeletal muscle decrease during exercise. Although these changes may contribute to the increases in fat oxidation from rest to exercise, they do not appear to explain the shift from mixed fuel to predominantly carbohydrate utilization when exercise intensity is increased. Diabetes 49:1295-1300, 2000 P hysical activity is associated with substantial increases in both fatty acid and carbohydrate oxidation in skeletal muscle, with the relative use of the 2 fuels varying with exercise intensity (1). Thus, in overnight-fasted humans, during low-intensity exercise (30-40% VO 2max ), fatty acids are the principal oxidative substrate, whereas during somewhat more intense exercise (60-70% VO 2max ), the absolute rates of both fatty acid and carbohydrate oxidation are higher, but the oxidation of fatty acids relative to carbohydrate is decreased. Furthermore, during very intense (≥90% VO 2max ) versus moderate-intensity exercise, carbohydrate oxidation is still further increased, and even the rate of fatty acid oxidation may be diminished.Studies in both rats (2-6) and humans (7,8) indicate that the rate of carbohydrate oxidation in muscle is elevated during exercise by a coordinated series of events that lead to increases in glucose transport, glycogenolysis, glycolysis, and pyruvate dehydrogenase activity. In contrast, the mechanism by which fatty acid oxidation is increased is less clear. In rats, a rea...

1. Eccentric exercise causes impaired postexercise glycogen resynthesis. To study whether changes in muscle concentration of the glucose transporter (GLUT4) protein might be involved, seven healthy young men performed one-legged eccentric exercise by resisting knee flexion enforced by a motor-driven device. 2. The GLUT4 protein concentration in the exercised and in the control thigh was unchanged immediately after exercise. On days 1 and 2 after exercise, the GLUT4 protein concentration in the exercised muscle was 68 + 10 and 64 + 10% (means + S.E.M.; P < 0 05), respectively, of the concentration in the control muscle, and had returned to control values on days 4 and 7.3. The muscle glycogen concentration decreased from 404 + 44 to 336 + 44 mmol (kg dry wtf1 (P < 0 05) during exercise. The glycogen concentration remained significantly lower than in the control thigh on days 1 and 2 after exercise but on days 4 and 7 no differences were found. 4. Although no cause-effect relationship was established, these findings may suggest that decreased muscle concentrations of GLUT4 protein, and, hence, a decreased rate of glucose transport into muscle cells, may concentration seen after eccentric exercise.Eccentric exercise has been shown to cause muscle damage and impaired postexercise glycogen resynthesis (O'Reilly,

Training improves insulin sensitivity, which in turn may affect performance by modulation of fuel availability. Insulin action, in turn, has been linked to specific patterns of muscle structural lipids in skeletal muscle. This study investigated whether regular exercise training exerts an effect on the muscle membrane phospholipid fatty acid composition in humans. Seven male subjects performed endurance training of the knee extensors of one leg for 4 wk. The other leg served as a control. Before, after 4 days, and after 4 wk, muscle biopsies were obtained from the vastus lateralis. After 4 wk, the phospholipid fatty acid contents of oleic acid 18:1(n-9) and docosahexaenoic acid 22:6(n-3) were significantly higher in the trained (10.9 +/- 0.5% and 3.2 +/- 0.4% of total fatty acids, respectively) than the untrained leg (8.8 +/- 0.5% and 2.6 +/- 0.4%, P < 0.05). The ratio between n-6 and n-3 fatty acids was significantly lower in the trained (11.1 +/- 0.9) than the untrained leg (13.1 +/- 1.2, P < 0.05). In contrast, training did not affect muscle triacylglycerol fatty acid composition. Citrate synthase activity was increased by 17% in the trained compared with the untrained leg (P < 0.05). In this model, diet plays a minimal role, as the influence of dietary intake is similar on both legs. Regular exercise training per se influences the phospholipid fatty acid composition of muscle membranes but has no effect on the composition of fatty acids stored in triacylglycerols within the muscle.