Distinct Adaptations of Mitochondrial Dynamics to Electrical Pulse Stimulation in Lean and Severely Obese Primary Myotubes

BACKGROUND Skeletal muscle from lean and obese subjects elicit differential adaptations in response to exercise/muscle contractions. In order to determine whether obesity alters the adaptations in mitochondrial dynamics in response to exercise/muscle contractions and whether any of these distinct adaptations are linked to alterations in insulin sensitivity, we compared the effects of electrical pulse stimulation (EPS) on mitochondrial network structure and regulatory proteins in mitochondrial dynamics in myotubes from lean humans and humans with severe obesity and evaluated the correlations between these regulatory proteins and insulin signaling. METHODS Myotubes from human skeletal muscle cells obtained from lean humans (BMI 23.8 ± 1.67 kg/m) and humans with severer obesity (45.5 ± 2.26 kg/m) (n=8/group) were electrically stimulated for 24 hours. Four-hours after EPS, mitochondrial network structure, protein markers of insulin signaling and mitochondrial dynamics were assessed. RESULTS EPS enhanced insulin-stimulated Akt phosphorylation, reduced the number of non-networked individual mitochondria and increased the mitochondrial network size in both groups (P<0.05). Mitochondrial fusion marker mitofusin 2 was significantly increased in myotubes from the lean subjects (P<0.05), but reduced in subjects with severe obesity (P<0.05). In contrast, fission marker dynamin-related protein 1 (Drp1) was reduced in myotubes from subjects with severe obesity (P<0.05), but remained unchanged in lean subjects. Reductions in Drp phosphorylation were correlated with improvements in insulin-stimulated Akt phosphorylation following EPS (r = -0.679, P = 0.004). CONCLUSION Our data demonstrated that EPS induces more fused mitochondrial networks, which are associated with differential adaptations in mitochondrial dynamic processes in myotubes from lean humans and human with severe obesity. It also suggests that improved insulin signaling following muscle contractions may be linked to the reduction in Drp1 activity.