Skeletal muscle mitochondrial inertia is associated with carnitine acetyltransferase activity and physical function in humans.

BACKGROUNDAt the onset of exercise, the speed at which phosphocreatine (PCr) decreases toward a new steady state (PCr on-kinetics) reflects the readiness to activate mitochondrial ATP synthesis, which is secondary to Acetyl-CoA availability in skeletal muscle. We hypothesized that PCr on-kinetics are slower in metabolically compromised and older individuals and are associated with low carnitine acetyltransferase (CrAT) protein activity and compromised physical function.METHODSWe applied 31P-magnetic resonance spectroscopy (31P-MRS) to assess PCr on-kinetics in 2 cohorts of volunteers. Cohort 1 included patients who had type 2 diabetes, were obese, were lean trained (VO2max > 55 mL/kg/min), and were lean untrained (VO2max < 45 mL/kg/min). Cohort 2 included young (20-30 years) and older (65-80 years) individuals with normal physical activity and older, trained individuals. Previous results of CrAT protein activity and acetylcarnitine content in muscle tissue were used to explore the underlying mechanisms of PCr on-kinetics, along with various markers of physical function.RESULTSPCr on-kinetics were significantly slower in metabolically compromised and older individuals (indicating mitochondrial inertia) as compared with young and older trained volunteers, regardless of in vivo skeletal muscle oxidative capacity (P < 0.001). Mitochondrial inertia correlated with reduced CrAT protein activity, low acetylcarnitine content, and functional outcomes (P < 0.001).CONCLUSIONPCr on-kinetics are significantly slower in metabolically compromised and older individuals with normal physical activity compared with young and older trained individuals, regardless of in vivo skeletal muscle oxidative capacity, indicating greater mitochondrial inertia. Thus, PCr on-kinetics are a currently unexplored signature of skeletal muscle mitochondrial metabolism, tightly linked to functional outcomes. Skeletal muscle mitochondrial inertia might emerge as a target of intervention to improve physical function.TRIAL REGISTRATIONNCT01298375 and NCT03666013 (clinicaltrials.gov).FUNDINGRM and MH received an EFSD/Lilly grant from the European Foundation for the Study of Diabetes (EFSD). VS was supported by an ERC starting grant (grant 759161) "MRS in Diabetes."

Carnitine supplementation improves metabolic flexibility and skeletal muscle acetylcarnitine formation in volunteers with impaired glucose tolerance: A randomised controlled trial.

BACKGROUND: Type 2 diabetes patients and individuals at risk of developing diabetes are characterized by metabolic inflexibility and disturbed glucose homeostasis. Low carnitine availability may contribute to metabolic inflexibility and impaired glucose tolerance. Here, we investigated whether carnitine supplementation improves metabolic flexibility and insulin sensitivity in impaired glucose tolerant (IGT) volunteers. METHODS: Eleven IGT- volunteers followed a 36-day placebo- and L-carnitine treatment (2 g/day) in a randomised, placebo-controlled, double blind crossover design. A hyperinsulinemic-euglycemic clamp (40 mU/m2/min), combined with indirect calorimetry (ventilated hood) was performed to determine insulin sensitivity and metabolic flexibility. Furthermore, metabolic flexibility was assessed in response to a high-energy meal. Skeletal muscle acetylcarnitine concentrations were measured in vivo using long echo time proton magnetic resonance spectroscopy (1H-MRS, TE=500 ms) in the resting state (7:00AM and 5:00PM) and after a 30-min cycling exercise. Twelve normal glucose tolerant (NGT) volunteers were included without any intervention as control group. RESULTS: Metabolic flexibility of IGT-subjects completely restored towards NGT control values upon carnitine supplementation, measured during a hyperinsulinemic-euglycemic clamp and meal test. In muscle, carnitine supplementation enhanced the increase in resting acetylcarnitine concentrations over the day (delta 7:00 AM versus 5:00 PM) in IGT-subjects. Furthermore, carnitine supplementation increased post-exercise acetylcarnitine concentrations and reduced long-chain acylcarnitine species in IGT-subjects, suggesting the stimulation of a more complete fat oxidation in muscle. Whole-body insulin sensitivity was not affected. CONCLUSION: Carnitine supplementation improves acetylcarnitine formation and rescues metabolic flexibility in IGT-subjects. Future research should investigate the potential of carnitine in prevention/treatment of type 2 diabetes.

Endospanin-2 enhances skeletal muscle energy metabolism and running endurance capacity.

Metabolic stresses such as dietary energy restriction or physical activity exert beneficial metabolic effects. In the liver, endospanin-1 and endospanin-2 cooperatively modulate calorie restriction-mediated (CR-mediated) liver adaptations by controlling growth hormone sensitivity. Since we found CR to induce endospanin protein expression in skeletal muscle, we investigated their role in this tissue. In vivo and in vitro endospanin-2 triggers ERK phosphorylation in skeletal muscle through an autophagy-dependent pathway. Furthermore, endospanin-2, but not endospanin-1, overexpression decreases muscle mitochondrial ROS production, induces fast-to-slow fiber-type switch, increases skeletal muscle glycogen content, and improves glucose homeostasis, ultimately promoting running endurance capacity. In line, endospanin-2-/- mice display higher lipid peroxidation levels, increased mitochondrial ROS production under mitochondrial stress, decreased ERK phosphorylation, and reduced endurance capacity. In conclusion, our results identify endospanin-2 as a potentially novel player in skeletal muscle metabolism, plasticity, and function.

Quantum coherence spectroscopy to measure dietary fat retention in the liver.

The prevalence of fatty liver reaches alarming proportions. Fatty liver increases the risk for insulin resistance, cardiovascular disease, and nonalcoholic steatohepatitis (NASH). Although extensively studied in a preclinical setting, the lack of noninvasive methodologies hampers our understanding of which pathways promote hepatic fat accumulation in humans. Dietary fat retention is one of the pathways that may lead to fatty liver. The low (1.1%) natural abundance (NA) of carbon-13 (13C) allows use of 13C-enriched lipids for in vivo MR studies. Successful implementation of such methodology, however, is challenging due to low sensitivity of 13C-magnetic resonance spectroscopy (13C-MRS). Here, we investigated the use of 1-dimensional gradient enhanced heteronuclear single quantum coherence (ge-HSQC) spectroscopy for the in vivo detection of hepatic 1H-[13C]-lipid signals after a single high-fat meal with 13C-labeled fatty acids in 5 lean and 6 obese subjects. Postprandial retention of orally administered 13C-labeled fatty acids was significant (P < 0.01). Approximately 1.5% of the tracer was retained in the liver after 6 hours, and retention was similar in both groups (P = 0.92). Thus, a substantial part of the liver fat can originate directly from storage of meal-derived fat. The ge-HSQC can be used to noninvasively reveal the contribution of dietary fat to the development of hepatic steatosis over time.

Muscular mitochondrial dysfunction and type 2 diabetes mellitus.

PURPOSE OF REVIEW: Muscular mitochondrial dysfunction, leading to the accumulation of fat in skeletal muscle, has been proposed to be involved in the development of type 2 diabetes mellitus. Here, we review human studies that investigated various aspects of mitochondrial function in relation to muscular insulin sensitivity and/or diabetes. RECENT FINDINGS: In-vivo magnetic resonance spectroscopy allows assessment of mitochondrial functionality from adenosine triphosphate flux in the nonexercising state and from phosphocreatine recovery from (sub)maximal exercising. Application of both approaches revealed reduced mitochondrial oxidative capacity in insulin-resistant (pre)diabetic humans. Reductions in mitochondrial density may contribute to, or even underlie, these findings as well as intrinsic defects in mitochondrial respiration. So far, only two studies reported measurements of mitochondrial respiratory capacity in intact mitochondria in diabetic patients, with inconsistent findings. SUMMARY: Muscular mitochondrial aberrations in type 2 diabetes mellitus can be detected, but it is so far unclear if these aberrations are causally related to the development of the disease. Alternatively, mitochondrial dysfunction may simply be the consequence of elevated plasma fatty acids or glucose levels.