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 primary gene mutated in Charcot-Marie-Tooth type 2A is mitofusin-2 (Mfn2). Mfn2 encodes a mitochondrial protein that participates in the maintenance of the mitochondrial network and that regulates mitochondrial metabolism and intracellular signaling. The potential for regulation of human Mfn2 gene expression in vivo is largely unknown. Based on the presence of mitochondrial dysfunction in insulin-resistant conditions, we have examined whether Mfn2 expression is dysregulated in skeletal muscle from obese or nonobese type 2 diabetic subjects, whether muscle Mfn2 expression is regulated by body weight loss, and the potential regulatory role of tumor necrosis factor (TNF)␣ or interleukin-6. We show that mRNA concentration of Mfn2 is decreased in skeletal muscle from both male and female obese subjects. Muscle Mfn2 expression was also reduced in lean or in obese type 2 diabetic patients. There was a strong negative correlation between the Mfn2 expression and the BMI in nondiabetic and type 2 diabetic subjects. A positive correlation between the Mfn2 expression and the insulin sensitivity was also detected in nondiabetic and type 2 diabetic subjects. To determine the effect of weight loss on Mfn2 mRNA expression, six morbidly obese subjects were subjected to weight loss by bilio-pancreatic diversion. Mean expression of muscle Mfn2 mRNA increased threefold after reduction in body weight, and a positive correlation between muscle Mfn2 expression and insulin sensitivity was again detected. In vitro experiments revealed an inhibitory effect of TNF␣ or interleukin-6 on Mfn2 expression in cultured cells. We conclude that body weight loss upregulates the expression of Mfn2 mRNA in skeletal muscle of obese humans, type 2 diabetes downregulates the expression of Mfn2 mRNA in skeletal muscle, Mfn2 expression in skeletal muscle is directly proportional to insulin sensitivity and is inversely proportional to the BMI, TNF␣ and interleukin-6 downregulate Mfn2 expression and may participate in the dysregulation of Mfn2 expression in obesity or type 2 diabetes, and the in vivo modulation of Mfn2 mRNA levels is an additional level of regulation for the control of muscle metabolism and could provide a molecular mechanism for alterations in mitochondrial function in obesity or type 2 diabetes. Diabetes 54: [2685][2686][2687][2688][2689][2690][2691][2692][2693] 2005

Defective regulation of gene expression may be involved in the pathogenesis of type 2 diabetes. We have characterized the concerted regulation by insulin (3-h hyperinsulinemic clamp) of the expression of 10 genes related to insulin action in skeletal muscle and in subcutaneous adipose tissue, and we have verified whether a defective regulation of some of them could be specifically encountered in tissues of type 2 diabetic patients. Basal mRNA levels (determined by reverse transcriptase-competitive polymerase chain reaction) of insulin receptor, insulin receptor substrate-1, p85␣ phosphatidylinositol 3-kinase (PI3K), p110␣PI3K, p110␤PI3K, GLUT4, glycogen synthase, and sterol regulatory-element-binding protein-1c (SREBP-1c) were similar in muscle of control (n ‫؍‬ 17), type 2 diabetic (n ‫؍‬ 9), type 1 diabetic (n ‫؍‬ 9), and nondiabetic obese (n ‫؍‬ 9) subjects. In muscle, the expression of hexokinase II was decreased in type 2 diabetic patients (P < 0.01). In adipose tissue, SREBP-1c (P < 0.01) mRNA expression was reduced in obese (nondiabetic and type 2 diabetic) subjects and was negatively correlated with the BMI of the subjects (r ‫؍‬ ؊0.63, P ‫؍‬ 0.02). Insulin (؎1,000 pmol/l) induced a two-to threefold increase (P < 0.05) in hexokinase II, p85␣PI3K, and SREBP-1c mRNA levels in muscle and in adipose tissue in control subjects, in insulin-resistant nondiabetic obese patients, and in hyperglycemic type 1 diabetic subjects. Upregulation of these genes was completely blunted in type 2 diabetic patients. This study thus provides evidence for a specific defect in the regulation of a group of important genes in response to insulin in peripheral tissues of type 2 diabetic patients. Diabetes 50: 1134 -1142, 2001 I nsulin resistance is the main metabolic feature of type 2 diabetes (1,2), and several studies indicate that it generally precedes the onset of the disease (2,3). In vivo, skeletal muscle is the major site for insulin-dependent glucose disposal, and type 2 diabetic patients are characterized by a marked decrease in insulinstimulated glucose utilization in muscle mainly due to reduced glucose uptake and storage (1,2). Insulin stimulates glucose uptake by increasing the translocation of GLUT4-containing vesicles to the plasma membrane and by modifying the activity of enzymes involved in glucose metabolism (4). Insulin action is initiated by binding of the hormone to cell membranes and activation of the insulin receptor tyrosine kinase that results in the stimulation of intracellular signaling cascades (4). Among these cascades, the phosphatidylinositol 3-kinase (PI3K) pathway is thought to play a crucial role in the effects of insulin on glucose metabolism (5). Several defects in the insulin signaling pathways have been identified in skeletal muscle of type 2 diabetic patients. Impaired phosphorylation of insulin receptor and insulin receptor substrate (IRS)-1 in response to insulin has been reported (6 -8), and the induction of PI3K and Akt kinase activities have been found to be reduced (8 -10). The stimulat...