Altered mitochondrial network morphology and regulatory proteins in mitochondrial quality control in myotubes from severely obese humans with or without type 2 diabetes.

Healthy mitochondrial networks are maintained via balanced integration of mitochondrial quality control processes (biogenesis, fusion, fission, and mitophagy). The purpose of this study was to investigate the effects of severe obesity and type 2 diabetes (T2D) on mitochondrial network morphology and expression of proteins regulating mitochondrial quality control processes in cultured human myotubes. Primary human skeletal muscle cells were isolated from biopsies from lean, severely obese nondiabetic individuals and severely obese type 2 diabetic individuals (n = 8-9/group) and were differentiated to myotubes. Mitochondrial network morphology was determined in live cells via confocal microscopy and protein markers of mitochondrial quality control were measured by immunoblotting. Myotubes from severely obese nondiabetic and type 2 diabetic humans exhibited fragmented mitochondrial networks (P < 0.05). Mitochondrial fission protein Drp1 (Ser616) phosphorylation was higher in myotubes from severely obese nondiabetic humans when compared with the lean controls (P < 0.05), while mitophagy protein Parkin expression was lower in myotubes from severely obese individuals with T2D in comparison to the other groups (P < 0.05). These data suggest that regulatory proteins in mitochondrial quality control processes, specifically mitochondrial fission protein Drp1 (Ser616) phosphorylation and mitophagy protein Parkin, are intrinsically dysregulated at cellular level in skeletal muscle from severely obese nondiabetic and type 2 diabetic humans, respectively. These differentially expressed mitochondrial quality control proteins may play a role in mitochondrial fragmentation evident in skeletal muscle from severely obese and type 2 diabetic humans. Novelty Mitochondrial network morphology and mitochondrial quality control proteins are intrinsically dysregulated in skeletal muscle cells from severely obese humans with or without T2D.

Roux-en-Y gastric bypass surgery restores insulin-mediated glucose partitioning and mitochondrial dynamics in primary myotubes from severely obese humans.

BACKGROUND/OBJECTIVES: Impaired insulin-mediated glucose partitioning is an intrinsic metabolic defect in skeletal muscle from severely obese humans (BMI ≥ 40 kg/m2). Roux-en-Y gastric bypass (RYGB) surgery has been shown to improve glucose metabolism in severely obese humans. The purpose of the study was to determine the effects of RYGB surgery on glucose partitioning, mitochondrial network morphology, and the markers of mitochondrial dynamics skeletal muscle from severely obese humans. SUBJECT/METHODS: Human skeletal muscle cells were isolated from muscle biopsies obtained from RYGB patients (BMI = 48.0 ± 2.1, n = 7) prior to, 1 month and 7 months following surgery and lean control subjects (BMI = 22.4 ± 1.1, n = 7). Complete glucose oxidation, non-oxidized glycolysis rates, mitochondrial respiratory capacity, mitochondrial network morphology, and the regulatory proteins of mitochondrial dynamics were determined in differentiated human myotubes. RESULTS: Myotubes derived from severely obese humans exhibited enhanced glucose oxidation (13.5%; 95% CI [7.6, 19.4], P = 0.043) and reduced non-oxidized glycolysis (-1.3%; 95% CI [-11.1, 8.6]) in response to insulin stimulation at 7 months after RYGB when compared with the presurgery state (-0.6%; 95% CI [-5.2, 4.0] and 19.5%; 95% CI [4.0, 35.0], P = 0.006), and were not different from the lean controls (16.7%; 95% CI [11.8, 21.5] and 1.9%; 95% CI [-1.6, 5.4], respectively). Further, the number of fragmented mitochondria and Drp1(Ser616) phosphorylation were trended to reduce/reduced (0.0104, 95% CI [0.0085, 0.0126], P = 0.091 and 0.0085, 95% CI [0.0068, 0.0102], P = 0.05) in myotubes derived from severely obese humans at 7 months after RYGB surgery in comparison with the presurgery state. Finally, Drp1(Ser616) phosphorylation was negatively correlated with insulin-stimulated glucose oxidation (r = -0.49, P = 0.037). CONCLUSION/INTERPRETATION: These data indicate that an intrinsic metabolic defect of glucose partitioning in skeletal muscle from severely obese humans is restored by RYGB surgery. The restoration of glucose partitioning may be regulated through reduced mitochondrial fission protein Drp1 phosphorylation.