Matthew L. Johnson

Skeletal muscle aging and the mitochondrion

Decline in human muscle mass and strength (sarcopenia) is a hallmark of the aging process. A growing body of research in the areas of bioenergetics and protein turnover has placed the mitochondria at the center of this process. It is now clear that, unless an active lifestyle is rigorously followed, skeletal muscle mitochondrial decline occurs as humans age. Increasing research on mitochondrial biology has elucidated the regulatory pathways involved in mitochondrial biogenesis, many of which are potential therapeutic targets, and highlight the beneficial effects of vigorous physical activity on skeletal muscle health for an aging population.

Central obesity: Association with left ventricular dysfunction and mortality in the community

BACKGROUND We sought to measure the strength of association between 2 indices of obesity, waist-hip ratio (WHR) and body mass index (BMI), with left ventricular (LV) dysfunction and mortality in a community cohort. The distribution of body fat is thought to affect cardiovascular disease risk. The association of BMI (an index of overall obesity) and WHR (an index of central obesity) with LV systolic and diastolic dysfunction in a population-based cohort is unknown. METHODS Anthropomorphic measurements and echocardiographic LV function were measured in a cross-sectional population-based sample of 2,042 men and women>45 years old in the Olmsted County Heart Function Study. Five-year prospective mortality was measured. RESULTS Increased WHR had a stronger association than BMI with (1) lower LV ejection fraction (r=-0.24, P<.0001 vs r=-0.04, P=.09) and (2) LV diastolic dysfunction (r=0.18, P<.0001 vs r=0.05, P=.02). After adjustment for standard cardiovascular risk factors, WHR continued to be significantly associated with diastolic dysfunction but not with systolic dysfunction. Waist-hip ratio, but not BMI, was strongly predictive of all-cause mortality independent of age and sex (hazard ratio 23.6, CI 4.0-139.8, P=.0005). This relationship was attenuated on adjustment for diastolic dysfunction. CONCLUSIONS Waist-hip ratio is a stronger correlate of LV dysfunction and mortality than BMI. These cross-sectional data suggest that the increased risk of mortality from central obesity is mediated at least in part by LV dysfunction, especially diastolic dysfunction.

Influence of fish oil on skeletal muscle mitochondrial energetics and lipid metabolites during high-fat diet

Omega-3 polyunsaturated fatty acids (n-3 PUFAs) enhance insulin sensitivity and glucose homeostasis in rodent models of insulin resistance. These beneficial effects have been linked with anti-inflammatory properties, but emerging data suggest that the mechanisms may also converge on mitochondria. We evaluated the influence of dietary n-3 PUFAs on mitochondrial physiology and muscle lipid metabolites in the context of high-fat diet (HFD) in mice. Mice were fed control diets (10% fat), HFD (60% fat), or HFD with fish oil (HFD+FO, 3.4% kcal from n-3 PUFAs) for 10 wk. Body mass and fat mass increased similarly in HFD and HFD+FO, but n-3 PUFAs attenuated the glucose intolerance that developed with HFD and increased expression of genes that regulate glucose metabolism in skeletal muscle. Despite similar muscle triglyceride levels in HFD and HFD+FO, long-chain acyl-CoAs and ceramides were lower in the presence of fish oil. Mitochondrial abundance and oxidative capacity were similarly increased in HFD and HFD+FO compared with controls. Hydrogen peroxide production was similarly elevated in HFD and HFD+FO in isolated mitochondria but not in permeabilized muscle fibers, likely due to increased activity and expression of catalase. These results support a hypothesis that n-3 PUFAs protect glucose tolerance, in part by preventing the accumulation of bioactive lipid mediators that interfere with insulin action. Furthermore, the respiratory function of skeletal muscle mitochondria does not appear to be a major factor in sphingolipid accumulation, glucose intolerance, or the protective effects of n-3 PUFAs.

Defects in mitochondrial efficiency and H2O2 emissions in obese women are restored to a lean phenotype with aerobic exercise training

The notion that mitochondria contribute to obesity-induced insulin resistance is highly debated. Therefore, we determined whether obese (BMI 33 kg/m2), insulin-resistant women with polycystic ovary syndrome had aberrant skeletal muscle mitochondrial physiology compared with lean, insulin-sensitive women (BMI 23 kg/m2). Maximal whole-body and mitochondrial oxygen consumption were not different between obese and lean women. However, obese women exhibited lower mitochondrial coupling and phosphorylation efficiency and elevated mitochondrial H2O2 (mtH2O2) emissions compared with lean women. We further evaluated the impact of 12 weeks of aerobic exercise on obesity-related impairments in insulin sensitivity and mitochondrial energetics in the fasted state and after a high-fat mixed meal. Exercise training reversed obesity-related mitochondrial derangements as evidenced by enhanced mitochondrial bioenergetics efficiency and decreased mtH2O2 production. A concomitant increase in catalase antioxidant activity and decreased DNA oxidative damage indicate improved cellular redox status and a potential mechanism contributing to improved insulin sensitivity. mtH2O2 emissions were refractory to a high-fat meal at baseline, but after exercise, mtH2O2 emissions increased after the meal, which resembles previous findings in lean individuals. We demonstrate that obese women exhibit impaired mitochondrial bioenergetics in the form of decreased efficiency and impaired mtH2O2 emissions, while exercise effectively restores mitochondrial physiology toward that of lean, insulin-sensitive individuals.

Differential Effect of Endurance Training on Mitochondrial Protein Damage, Degradation, and Acetylation in the Context of Aging

Acute aerobic exercise increases reactive oxygen species and could potentially damage proteins, but exercise training (ET) enhances mitochondrial respiration irrespective of age. Here, we report a differential impact of ET on protein quality in young and older participants. Using mass spectrometry we measured oxidative damage to skeletal muscle proteins before and after 8 weeks of ET and find that young but not older participants reduced oxidative damage to both total skeletal muscle and mitochondrial proteins. Young participants showed higher total and mitochondrial derived semitryptic peptides and 26S proteasome activity indicating increased protein degradation. ET however, increased the activity of the endogenous antioxidants in older participants. ET also increased skeletal muscle content of the mitochondrial deacetylase SIRT3 in both groups. A reduction in the acetylation of isocitrate dehydrogenase 2 was observed following ET that may counteract the effect of acute oxidative stress. In conclusion aging is associated with an inability to improve skeletal muscle and mitochondrial protein quality in response to ET by increasing degradation of damaged proteins. ET does however increase muscle and mitochondrial antioxidant capacity in older individuals, which provides increased buffering from the acute oxidative effects of exercise.

Effects of Dietary n-3 Fatty Acids on Hepatic and Peripheral Insulin Sensitivity in Insulin-Resistant Humans

OBJECTIVE Dietary n-3 polyunsaturated fatty acids, including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), prevent insulin resistance and stimulate mitochondrial biogenesis in rodents, but the findings of translational studies in humans are thus far ambiguous. The aim of this study was to evaluate the influence of EPA and DHA on insulin sensitivity, insulin secretion, and muscle mitochondrial function in insulin-resistant, nondiabetic humans using a robust study design and gold-standard measurements. RESEARCH DESIGN AND METHODS Thirty-one insulin-resistant adults received 3.9 g/day EPA+DHA or placebo for 6 months in a randomized double-blind study. Hyperinsulinemic-euglycemic clamp with somatostatin was used to assess hepatic and peripheral insulin sensitivity. Postprandial glucose disposal and insulin secretion were measured after a meal. Measurements were performed at baseline and after 6 months of treatment. Abdominal fat distribution was evaluated by MRI. Muscle oxidative capacity was measured in isolated mitochondria using high-resolution respirometry and noninvasively by magnetic resonance spectroscopy. RESULTS Compared with placebo, EPA+DHA did not alter peripheral insulin sensitivity, postprandial glucose disposal, or insulin secretion. Hepatic insulin sensitivity, determined from the suppression of endogenous glucose production by insulin, exhibited a small but significant improvement with EPA+DHA compared with placebo. Muscle mitochondrial function was unchanged by EPA+DHA or placebo. CONCLUSIONS This study demonstrates that dietary EPA+DHA does not improve peripheral glucose disposal, insulin secretion, or skeletal muscle mitochondrial function in insulin-resistant nondiabetic humans. There was a modest improvement in hepatic insulin sensitivity with EPA+DHA, but this was not associated with any improvements in clinically meaningful outcomes.

Mechanism by Which Caloric Restriction Improves Insulin Sensitivity in Sedentary Obese Adults.

Caloric restriction (CR) improves insulin sensitivity and reduces the incidence of diabetes in obese individuals. The underlying mechanisms whereby CR improves insulin sensitivity are not clear. We evaluated the effect of 16 weeks of CR on whole-body insulin sensitivity by pancreatic clamp before and after CR in 11 obese participants (BMI = 35 kg/m2) compared with 9 matched control subjects (BMI = 34 kg/m2). Compared with the control subjects, CR increased the glucose infusion rate needed to maintain euglycemia during hyperinsulinemia, indicating enhancement of peripheral insulin sensitivity. This improvement in insulin sensitivity was not accompanied by changes in skeletal muscle mitochondrial oxidative capacity or oxidant emissions, nor were there changes in skeletal muscle ceramide, diacylglycerol, or amino acid metabolite levels. However, CR lowered insulin-stimulated thioredoxin-interacting protein (TXNIP) levels and enhanced nonoxidative glucose disposal. These results support a role for TXNIP in mediating the improvement in peripheral insulin sensitivity after CR.

Eicosapentaenoic acid but not docosahexaenoic acid restores skeletal muscle mitochondrial oxidative capacity in old mice

Mitochondrial dysfunction is often observed in aging skeletal muscle and is implicated in age‐related declines in physical function. Early evidence suggests that dietary omega‐3 polyunsaturated fatty acids (n‐3 PUFAs) improve mitochondrial function. Here, we show that 10 weeks of dietary eicosapentaenoic acid (EPA) supplementation partially attenuated the age‐related decline in mitochondrial function in mice, but this effect was not observed with docosahexaenoic acid (DHA). The improvement in mitochondrial function with EPA occurred in the absence of any changes in mitochondrial abundance or biogenesis, which was evaluated from RNA sequencing, large‐scale proteomics, and direct measurements of muscle mitochondrial protein synthesis rates. We find that EPA improves muscle protein quality, specifically by decreasing mitochondrial protein carbamylation, a post‐translational modification that is driven by inflammation. These results demonstrate that EPA attenuated the age‐related loss of mitochondrial function and improved mitochondrial protein quality through a mechanism that is likely linked with anti‐inflammatory properties of n‐3 PUFAs. Furthermore, we demonstrate that EPA and DHA exert some common biological effects (anticoagulation, anti‐inflammatory, reduced FXR/RXR activation), but also exhibit many distinct biological effects, a finding that underscores the importance of evaluating the therapeutic potential of individual n‐3 PUFAs.

Exercise Effects on Postprandial Glucose Metabolism in Type 1 Diabetes: A Triple Tracer Approach

To determine the effects of exercise on postprandial glucose metabolism and insulin action in type 1 diabetes (T1D), we applied the triple tracer technique to study 16 T1D subjects on insulin pump therapy before, during, and after 75 min of moderate-intensity exercise (50% V̇o2max) that started 120 min after a mixed meal containing 75 g of labeled glucose. Prandial insulin bolus was administered as per each subjects customary insulin/carbohydrate ratio adjusted for meal time meter glucose and the level of physical activity. Basal insulin infusion rates were not altered. There were no episodes of hypoglycemia during the study. Plasma dopamine and norepinephrine concentrations rose during exercise. During exercise, rates of endogenous glucose production rose rapidly to baseline levels despite high circulating insulin and glucose concentrations. Interestingly, plasma insulin concentrations increased during exercise despite no changes in insulin pump infusion rates, implying increased mobilization of insulin from subcutaneous depots. Glucagon concentrations rose before and during exercise. Therapeutic approaches for T1D management during exercise will need to account for its effects on glucose turnover, insulin mobilization, glucagon, and sympathetic response and possibly other blood-borne feedback and afferent reflex mechanisms to improve both hypoglycemia and hyperglycemia.

Transpulmonary Pyruvate kinetics

Shuttling of intermediary metabolites, such as pyruvate, contributes to the dynamic energy and biosynthetic needs of tissues. Tracer kinetic studies offer a powerful tool to measure the metabolism of substrates like pyruvate that are simultaneously taken up from and released into the circulation by organs. However, we understood that during each circulatory passage, the entire cardiac output transits the pulmonary circulation. Therefore, we examined the transpulmonary pyruvate kinetics in an anesthetized rat model during an unstimulated (Con), lactate clamp (LC), and epinephrine infusion (Epi) conditions using a primed-continuous infusion of [U-¹³C]pyruvate. Compared with Con and Epi stimulation, LC significantly increased mixed central venous ([v]) and arterial ([a]) pyruvate concentrations (P < 0.05). We hypothesized that the lungs, specifically the pulmonary capillary beds are sites of simultaneous production and removal of pyruvate and contributes significantly to whole body carbohydrate intermediary metabolism. Transpulmonary net pyruvate balances were positive during all three conditions, indicating net pyruvate uptake. Net balance was significantly greater during epinephrine stimulation compared with the unstimulated control (P < 0.05). Tracer-measured pyruvate fractional extraction averaged 42.8 ± 5.8% for all three conditions and was significantly higher during epinephrine stimulation (P < 0.05) than during either Con or LC conditions, that did not differ from each other. Pyruvate total release (tracer measured uptake - net balance) was significantly higher during epinephrine stimulation (400 ± 100 μg/min) vs. Con (30 ± 20 μg/min) (P < 0.05). These data are interpreted to mean that significant pyruvate extraction occurs during circulatory transport across lung parenchyma. The extent of pulmonary parenchymal pyruvate extraction predicts high expression of monocarboxylate (lactate/pyruvate) transporters (MCTs) in the tissue. Western blot analysis of whole lung homogenates detected three isoforms, MCT1, MCT2, and MCT4. We conclude that a major site of circulating pyruvate extraction resides with the lungs and that during times of elevated circulating lactate, pyruvate, or epinephrine stimulation, pyruvate extraction is increased.