Markers of skeletal muscle mitochondrial function and lipid accumulation are moderately associated with the homeostasis model assessment index of insulin resistance in obese men

Mitochondrial puzzle in muscle: Linking the electron transport system to overweight

Human skeletal muscle mitochondria regulate energy expenditure. Research has shown that the functionality of muscle mitochondria is altered in subjects with overweight, as well as in response to nutrient excess and calorie restriction. Two metabolic features of obesity and overweight are (1) incomplete muscular fatty acid oxidation and (2) increased circulating lactate levels. In this study, I propose that these metabolic disturbances may originate from a common source within the muscle mitochondrial electron transport system. Specifically, a reorganization of the supramolecular structure of the electron transport chain could facilitate the maintenance of readily accessible coenzyme Q pools, which are essential for metabolizing lipid substrates. This approach is expected to maintain effective electron transfer, provided that there is sufficient complex III to support the Q-cycle. Such an adaptation could enhance fatty acid oxidation and prevent mitochondrial overload, thereby reducing lactate production. These insights advance our understanding of the molecular mechanisms underpinning metabolic dysregulation in overweight states. This provides a basis for targeted interventions in the quest for metabolic health.

Altered intramuscular network of lipid droplets and mitochondria in type 2 diabetes

Excessive storage of lipid droplets (LDs) in skeletal muscles is a hallmark of type 2 diabetes. However, LD morphology displays a high degree of subcellular heterogeneity and varies between single muscle fibers, which impedes the current understanding of lipid-induced insulin resistance. Using quantitative transmission electron microscopy (TEM), we conducted a comprehensive single-fiber morphological analysis to investigate the intramuscular network of LDs and mitochondria, and the effects of 8 wk of high-intensity interval training (HIIT) targeting major muscle groups, in patients with type 2 diabetes and nondiabetic obese and lean controls. We found that excessive storage of intramuscular lipids in patients with type 2 diabetes was exclusively explained by extremely large LDs situated in distinct muscle fibers with a location-specific deficiency in subsarcolemmal mitochondria. After HIIT, this intramuscular deficiency was improved by a remodeling of LD size and subcellular distribution and mitochondrial content. Analysis of LD morphology further revealed that individual organelles were better described as ellipsoids than spheres. Moreover, physical contact between LD and mitochondrial membranes indicated a dysfunctional interplay between organelles in the diabetic state. Taken together, type 2 diabetes should be recognized as a metabolic disease with high cellular heterogeneity in intramuscular lipid storage, underlining the relevance of single-cell technologies in clinical research. Furthermore, HIIT changed intramuscular LD storage toward nondiabetic characteristics.

Fat-Free Mass and Skeletal Muscle Mass Gain Are Associated with Diabetes Remission after Laparoscopic Sleeve Gastrectomy in Males but Not in Females

Besides massive body weight loss, laparoscopic sleeve gastrectomy (LSG) causes massive lean mass, including fat-free mass (FFM) and skeletal muscle mass (SM) that present higher metabolic rates in males. This study examines sex differences in FFM and SM changes of type 2 diabetes (T2D) remission at 12 months post-LSG. This cohort study recruited 119 patients (53.7% females) with T2D and obesity (body mass index 42.2 ± 7.0 kg/m²) who underwent LSG. Fat-mass (FM) loss was higher in males than in females (−12.8 ± 6.2% vs. −9.9 ± 5.0%, p = 0.02) after one-year post-operation. Regardless of the weight-loss difference, males had higher FFM and SM gain than did females (12.8 ± 8.0 vs. 9.9 ± 5.0% p = 0.02 and 6.5 ± 4.3% vs. 4.9 ± 6.2%, p = 0.03, respectively). Positive correlations of triglyceride reduction with FM loss (r = 0.47, p = 0.01) and SM gain (r = 0.44, p = 0.02) over 12 months post-operation were observed in males who achieved T2D remission. The T2D remission rate significantly increased 16% and 26% for each additional percentage of FFM and SM gain one year after LSG, which only happened in males. Increased FFM and SM were remarkably associated with T2D remission in males, but evidence lacks for females.

Fibro-Adipogenic Progenitors: Versatile keepers of skeletal muscle homeostasis, beyond the response to myotrauma

While Fibro-Adipogenic Progenitors (FAPs) have been originally identified as muscle-interstitial mesenchymal cells activated in response to muscle injury and endowed with inducible fibrogenic and adipogenic potential, subsequent studies have expanded their phenotypic and functional repertoire and revealed their contribution to skeletal muscle response to a vast range of perturbations. Here we review the emerging contribution of FAPs to skeletal muscle responses to motor neuron injuries and to systemic physiological (e.g., exercise) or pathological metabolic (e.g., diabetes) perturbations. We also provide an initial blueprint of discrete sub-clusters of FAPs that are activated by specific perturbations and discuss their role in muscle adaptation to these conditions.

The lifecycle of skeletal muscle mitochondria in obesity

Skeletal muscle possesses dramatic metabolic plasticity that allows for the rapid adaptation in cellular energy transduction to meet the demands of the organism. Obesity elicits changes in skeletal muscle structure and function, resulting in the accumulation of intramuscular lipids. The accumulation of intramuscular lipids in obesity is associated with impaired skeletal muscle mitochondrial content and function. Mitochondria exist as a dynamic network that is regulated by the processes of biogenesis, fusion, fission, and mitophagy. In this review, we outline adaptations in molecular pathways that regulate mitochondrial structure and function in obesity. We highlight the emerging role of dysregulated skeletal muscle macroautophagy and mitochondrial turnover in obesity. Future research should further elucidate the role of mitophagy in observed reductions in mitochondrial content and function during obesity.

Two weeks of single-leg immobilization alters intramyocellular lipid storage characteristics in healthy, young women

Muscle disuse rapidly induces insulin resistance (IR). Despite a relationship between intramyocellular lipid (IMCL) content and IR, during muscle-disuse IR develops before IMCL accumulation, suggesting that IMCL are not related to disuse-induced IR. However, recent studies show that it is not total IMCL content, but IMCL size and location that are related to IR. Changes in these IMCL parameters may occur prior to increases in IMCL content, thus contributing to disuse-induced IR. Omega-3 fatty acids may mitigate the effects of disuse on IR by preventing a decline in insulin signaling proteins. Twenty women (age 22 ± 3 yr) received either 5 g·day^-1 omega-3 fatty acid or isoenergetic sunflower oil for 4 wk prior to, throughout 2 wk of single-leg immobilization, and during 2 wk of recovery. Changes in IMCL characteristics and insulin signaling proteins were examined in vastus lateralis samples taken before supplementation and immobilization, and following immobilization and recovery. Omega-3 supplementation had no effect. IMCL area density decreased in the subsarcolemmal region during immobilization and recovery (-19% and -56%, respectively, P = 0.009). IMCL size increased in the central intermyofibrillar region during immobilization (43%, P = 0.007), returning to baseline during recovery. PLIN5 and AKT increased during immobilization (87%, P = 0.002; 30%, P = 0.007, respectively). PLIN 5 remained elevated and AKT increased further (15%) during recovery. IRS1, AS160, and GLUT4 decreased during immobilization (-35%, P = 0.001; -44%, P = 0.03; -56%, P = 0.02, respectively), returning to baseline during recovery. Immobilization alters IMCL storage characteristics while negatively affecting unstimulated insulin signaling protein content in young women.NEW & NOTEWORTHY We report that the subcellular storage location of IMCL is altered by limb immobilization, highlighting the need to evaluate IMCL storage location when assessing the effects of disuse on IMCL content. We also found that AKT content increased during immobilization in our female population, contrary to studies in males finding that AKT decreases during disuse, highlighting that men and women may respond differently to disuse and the necessity to include women in all research.

NLRP3 Inflammasome: Potential Role in Obesity Related Low-Grade Inflammation and Insulin Resistance in Skeletal Muscle

Among multiple mechanisms, low-grade inflammation is critical for the development of insulin resistance as a feature of type 2 diabetes. The nucleotide-binding oligomerization domain-like receptor family (NOD-like) pyrin domain containing 3 (NLRP3) inflammasome has been linked to the development of insulin resistance in various tissues; however, its role in the development of insulin resistance in the skeletal muscle has not been explored in depth. Currently, there is limited evidence that supports the pathological role of NLRP3 inflammasome activation in glucose handling in the skeletal muscle of obese individuals. Here, we have centered our focus on insulin signaling in skeletal muscle, which is the main site of postprandial glucose disposal in humans. We discuss the current evidence showing that the NLRP3 inflammasome disturbs glucose homeostasis. We also review how NLRP3-associated interleukin and its gasdermin D-mediated efflux could affect insulin-dependent intracellular pathways. Finally, we address pharmacological NLRP3 inhibitors that may have a therapeutical use in obesity-related metabolic alterations.

Umbilical Cord-Mesenchymal Stem Cell-Conditioned Medium Improves Insulin Resistance in C2C12 Cell

BACKGROUND

Umbilical cord-mesenchymal stem cell-conditioned medium (UC-MSC-CM) has emerged as a promising cell-free therapy. The aim of this study was to explore the therapeutic effects of UC-MSC-CM on insulin resistance in C2C12 cell.

METHODS

Insulin resistance was induced by palmitate. Effects of UC-MSC-CM on insulin resistance were evaluated using glucose uptake, glucose transporter type 4 (GLUT4) translocation, the insulin-signaling pathway, and mitochondrial contents and functions in C2C12 cell.

RESULTS

Glucose uptake was improved by UC-MSC-CM. UC-MSC-CM treatment increased only in membranous GLUT4 expression, not in cytosolic GLUT4 expression. It restored the insulin-signaling pathway in insulin receptor substrate 1 and protein kinase B. Mitochondrial contents evaluated by mitochondrial transcription factor A, mitochondrial DNA copy number, and peroxisome proliferator-activated receptor gamma coactivator 1-alpha were increased by UC-MSC-CM. In addition, UC-MSC-CM significantly decreased mitochondrial reactive oxygen species and increased fatty acid oxidation and mitochondrial membrane potential. There was no improvement in adenosine triphosphate (ATP) contents, but ATP synthesis was improved by UC-MSC-CM. Cytokine and active factor analysis of UC-MSC-CM showed that it contained many regulators inhibiting insulin resistance.

CONCLUSION

UC-MSC-CM improves insulin resistance with multiple mechanisms in C2C12 cell.

Effects of whey protein on skeletal muscle microvascular and mitochondrial plasticity following 10 weeks of exercise training in men with type 2 diabetes

Skeletal muscle microvascular dysfunction and mitochondrial rarefaction feature in type 2 diabetes mellitus (T2DM) linked to low tissue glucose disposal rate (GDR). Exercise training and milk protein supplementation independently promote microvascular and metabolic plasticity in muscle associated with improved nutrient delivery, but combined effects are unknown. In a randomised-controlled trial, 24 men (55.6 y, SD 5.7) with T2DM ingested whey protein drinks (protein/carbohydrate/fat: 20/10/3 g; WHEY) or placebo (carbohydrate/fat: 30/3 g; CON) before/after 45 mixed-mode intense exercise sessions over 10 weeks, to study effects on insulin-stimulated (hyperinsulinemic clamp) skeletal-muscle microvascular blood flow (mBF) and perfusion (near-infrared spectroscopy), and histological, genetic, and biochemical markers (biopsy) of microvascular and mitochondrial plasticity. WHEY enhanced insulin-stimulated perfusion (WHEY-CON 5.6%; 90% CI -0.1, 11.3), while mBF was not altered (3.5%; -17.5, 24.5); perfusion, but not mBF, associated (regression) with increased GDR. Exercise training increased mitochondrial (range of means: 40%-90%) and lipid density (20%-30%), enzyme activity (20%-70%), capillary:fibre ratio (∼25%), and lowered systolic (∼4%) and diastolic (4%-5%) blood pressure, but without WHEY effects. WHEY dampened PGC1α -2.9% (90% compatibility interval: -5.7, -0.2) and NOS3 -6.4% (-1.4, -0.2) expression, but other messenger RNA (mRNA) were unclear. Skeletal muscle microvascular and mitochondrial exercise adaptations were not accentuated by whey protein ingestion in men with T2DM. ANZCTR Registration Number: ACTRN12614001197628. Novelty: Chronic whey ingestion in T2DM with exercise altered expression of several mitochondrial and angiogenic mRNA. Whey added no additional benefit to muscle microvascular or mitochondrial adaptations to exercise. Insulin-stimulated perfusion increased with whey but was without impact on glucose disposal.

Pathophysiological role of metabolic flexibility on metabolic health

Glucose, fatty acids, and amino acids among others are oxidized to generate adenosine triphosphate (ATP). These fuels are supplied from the environment (through food intake) and internal depots (through lipolysis, glycogenolysis, and proteolysis) at different rates throughout the day. Complex adaptive systems permit to accommodate fuel oxidation according to fuel availability. This capacity of a cell, tissue, or organism to adapt fuel oxidation to fuel availability is defined as metabolic flexibility (MetF). There are conditions, such as insulin resistance, diabetes, and obesity, in which MetF seems to be impaired. The observation that those conditions are accompanied by mitochondrial dysfunction has set the basis to propose a link between mitochondrial dysfunction, metabolic inflexibility, and metabolic health. We here highlight the evidence about the notion that MetF influences metabolic health.

Subcellular localisation and composition of intramuscular triacylglycerol influence insulin sensitivity in humans

AIMS/HYPOTHESIS

Subcellular localisation is an important factor in the known impact of bioactive lipids, such as diacylglycerol and sphingolipids, on insulin sensitivity in skeletal muscle; yet, the role of localised intramuscular triacylglycerol (IMTG) is yet to be described. Excess accumulation of IMTG in skeletal muscle is associated with insulin resistance, and we hypothesised that differences in subcellular localisation and composition of IMTG would relate to metabolic health status in humans.

METHODS

We evaluated subcellular localisation of IMTG in lean participants, endurance-trained athletes, individuals with obesity and individuals with type 2 diabetes using LC-MS/MS of fractionated muscle biopsies and insulin clamps.

RESULTS

Insulin sensitivity was significantly different between each group (athletes>lean>obese>type 2 diabetes; p < 0.001). Sarcolemmal IMTG was significantly greater in individuals with obesity and type 2 diabetes compared with lean control participants and athletes, but individuals with type 2 diabetes were the only group with significantly increased saturated IMTG. Sarcolemmal IMTG was inversely related to insulin sensitivity. Nuclear IMTG was significantly greater in individuals with type 2 diabetes compared with lean control participants and athletes, and total and saturated IMTG localised in the nucleus had a significant inverse relationship with insulin sensitivity. Total cytosolic IMTG was not different between groups, but saturated cytosolic IMTG species were significantly increased in individuals with type 2 diabetes compared with all other groups. There were no significant differences between groups for IMTG concentration in the mitochondria/endoplasmic reticulum.

CONCLUSIONS/INTERPRETATION

These data reveal previously unknown differences in subcellular IMTG localisation based on metabolic health status and indicate the influence of sarcolemmal and nuclear IMTG on insulin sensitivity. Additionally, these studies suggest saturated IMTG may be uniquely deleterious for muscle insulin sensitivity. Graphical abstract.

Are Alterations in Skeletal Muscle Mitochondria a Cause or Consequence of Insulin Resistance?

As a major site of glucose uptake following a meal, skeletal muscle has an important role in whole-body glucose metabolism. Evidence in humans and animal models of insulin resistance and type 2 diabetes suggests that alterations in mitochondrial characteristics accompany the development of skeletal muscle insulin resistance. However, it is unclear whether changes in mitochondrial content, respiratory function, or substrate oxidation are central to the development of insulin resistance or occur in response to insulin resistance. Thus, this review will aim to evaluate the apparent conflicting information placing mitochondria as a key organelle in the development of insulin resistance in skeletal muscle.

Exercising your fat (metabolism) into shape: a muscle-centred view

Fatty acids are an important energy source during exercise. Training status and substrate availability are determinants of the relative and absolute contribution of fatty acids and glucose to total energy expenditure. Endurance-trained athletes have a high oxidative capacity, while, in insulin-resistant individuals, fat oxidation is compromised. Fatty acids that are oxidised during exercise originate from the circulation (white adipose tissue lipolysis), as well as from lipolysis of intramyocellular lipid droplets. Moreover, hepatic fat may contribute to fat oxidation during exercise. Nowadays, it is clear that myocellular lipid droplets are dynamic organelles and that number, size, subcellular distribution, lipid droplet coat proteins and mitochondrial tethering of lipid droplets are determinants of fat oxidation during exercise. This review summarises recent insights into exercise-mediated changes in lipid metabolism and insulin sensitivity in relation to lipid droplet characteristics in human liver and muscle. Graphical abstract.

Subpopulation-specific differences in skeletal muscle mitochondria in humans with obesity: insights from studies employing acute nutritional and exercise stimuli

Mitochondria from skeletal muscle of humans with obesity often display alterations with respect to their morphology, proteome, biogenesis, and function. These changes in muscle mitochondria are considered to contribute to metabolic abnormalities observed in humans with obesity. Most of the evidence describing alterations in muscle mitochondria in humans with obesity, however, lacks reference to a specific subcellular location. This is despite data over the years showing differences in the morphology and function of subsarcolemmal (found near the plasma membrane) and intermyofibrillar (nested between the myofibrils) mitochondria in skeletal muscle. Recent studies reveal that impairments in mitochondrial function in obesity with respect to the subcellular location of the mitochondria in muscle are more readily evident following exposure of the skeletal muscle to physiological stimuli. In this review, we highlight the need to understand skeletal muscle mitochondria metabolism in obesity in a subpopulation-specific manner and in the presence of physiological stimuli that modify mitochondrial function in vivo. Experimental approaches employed under these conditions will allow for more precise characterization of impairments in skeletal muscle mitochondria and their implications in inducing metabolic dysfunction in human obesity.

The Influence of Age and Cardiorespiratory Fitness on Bioactive Lipids in Muscle

Reduced insulin sensitivity is observed with aging and often explained by decreased physical activity. The mechanisms involved are not clarified, but bioactive lipids may play a role. We aimed to evaluate the influence of age and cardiorespiratory fitness on ceramide and diacylglycerol content in muscle and key proteins in lipid metabolism and insulin signaling. Healthy males were stratified by age into trained and untrained groups including 27 young (23.2 ± 0.3 years) and 33 aged (65.2 ± 0.6 years). Maximal oxygen uptake and body composition were measured and fasting blood samples and muscle biopsies obtained. Muscle ceramide and diacylglycerol were determined by thin-layer and gas-liquid chromatography and proteins by western blotting. We show that HOMA-IR was higher and VO2 peak lower in aged compared with young. Total, saturated, C16:0 and C18:0 ceramide content were lower in muscle from aged compared with young. Intramuscular C18:1n9 and C20:4n6 content were higher in trained versus untrained. Content of total unsaturated and C16:1n7 diacylglycerol fatty acids were higher and C24:0 lower in muscle of aged versus young. Cardiorespiratory fitness had no impact on total diacylglycerol content. In conclusion, these data argue against intramuscular ceramide or diacylglycerol accumulation as driver of age-related insulin resistance in lean individuals.

Influence of Exercise Training on Skeletal Muscle Insulin Resistance in Aging: Spotlight on Muscle Ceramides

Intramuscular lipid accumulation has been associated with insulin resistance (IR), aging, diabetes, dyslipidemia, and obesity. A substantial body of evidence has implicated ceramides, a sphingolipid intermediate, as potent antagonists of insulin action that drive insulin resistance. Indeed, genetic mouse studies that lower ceramides are potently insulin sensitizing. Surprisingly less is known about how physical activity (skeletal muscle contraction) regulates ceramides, especially in light that muscle contraction regulates insulin sensitivity. The purpose of this review is to critically evaluate studies (rodent and human) concerning the relationship between skeletal muscle ceramides and IR in response to increased physical activity. Our review of the literature indicates that chronic exercise reduces ceramide levels in individuals with obesity, diabetes, or hyperlipidemia. However, metabolically healthy individuals engaged in increased physical activity can improve insulin sensitivity independent of changes in skeletal muscle ceramide content. Herein we discuss these studies and provide context regarding the technical limitations (e.g., difficulty assessing the myriad ceramide species, the challenge of obtaining information on subcellular compartmentalization, and the paucity of flux measurements) and a lack of mechanistic studies that prevent a more sophisticated assessment of the ceramide pathway during increased contractile activity that lead to divergences in skeletal muscle insulin sensitivity.

Role of Metabolic Stress and Exercise in Regulating Fibro/Adipogenic Progenitors

Obesity is a major public health concern and is associated with decreased muscle quality (i.e., strength, metabolism). Muscle from obese adults is characterized by increases in fatty, fibrotic tissue that decreases the force producing capacity of muscle and impairs glucose disposal. Fibro/adipogenic progenitors (FAPs) are muscle resident, multipotent stromal cells that are responsible for muscle fibro/fatty tissue accumulation. Additionally, they are indirectly involved in muscle adaptation through their promotion of myogenic (muscle-forming) satellite cell proliferation and differentiation. In conditions similar to obesity that are characterized by chronic muscle degeneration, FAP dysfunction has been shown to be responsible for increased fibro/fatty tissue accumulation in skeletal muscle, and impaired satellite cell function. The role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle is just beginning to be unraveled. Thus, the present review aims to summarize the recent literature on the role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle, and the mechanisms responsible for these effects. Furthermore, we will review the role of physical activity in reversing or ameliorating the detrimental effects of obesity on FAP function.

Sphingolipid Metabolism and Signaling in Skeletal Muscle: From Physiology to Physiopathology

Sphingolipids represent one of the major classes of eukaryotic lipids. They play an essential structural role, especially in cell membranes where they also possess signaling properties and are capable of modulating multiple cell functions, such as apoptosis, cell proliferation, differentiation, and inflammation. Many sphingolipid derivatives, such as ceramide, sphingosine-1-phosphate, and ganglioside, have been shown to play many crucial roles in muscle under physiological and pathological conditions. This review will summarize our knowledge of sphingolipids and their effects on muscle fate, highlighting the role of this class of lipids in modulating muscle cell differentiation, regeneration, aging, response to insulin, and contraction. We show that modulating sphingolipid metabolism may be a novel and interesting way for preventing and/or treating several muscle-related diseases.

Sex-based differences in hepatic and skeletal muscle triglyceride storage and metabolism 1

Women and men store lipid differently within the body with men storing more fat in the android region and women storing more fat in the gynoid region. Fat is predominately stored in adipose tissue as triacylglycerides (TG); however, TG are also stored in other tissues including the liver and skeletal muscle. Excess hepatic TG storage, defined as a TG concentration >5% of liver weight and known as nonalcoholic fatty liver disease (NAFLD), is related to the metabolic syndrome. Similarly, elevated skeletal muscle TG, termed intramyocellular lipids (IMCL), are related to insulin resistance in obesity and type II diabetes. Men store more hepatic TG than women and, unsurprisingly, NAFLD is more prevalent in men than women. Women store more IMCL than men, yet type II diabetes risk is not greater, which is likely due to the manner in which women store TG within muscle. Sex-based differences in TG storage between men and women are underpinned by differences in messenger RNA expression, protein content, and enzyme activities of skeletal muscle and hepatic lipid metabolic pathways. Furthermore, women have a greater reliance on lipid during exercise because of upregulation of lipid oxidative pathways. The purpose of this review is to discuss the role of sex in mediating lipid storage and metabolism within skeletal muscle and the liver at rest and during exercise and its relationship with metabolic disease.

High-intensity interval, but not endurance, training induces muscle fiber type-specific subsarcolemmal lipid droplet size reduction in type 2 diabetic patients

This study compared the effects of moderate-intensity endurance training and high-intensity interval training on fiber type-specific subcellular volumetric content and morphology of lipid droplets and mitochondria in skeletal muscles of type 2 diabetic patients. Sixteen sedentary type 2 diabetic patients (57 ± 7 yr old) were randomized to complete 11 wk of either 40-min cycling at 50% peak workload (Endurance, n = 8) or 10 1-min cycling intervals at 95% peak workload separated by 1 min of recovery (High-Intensity Interval, n = 8), three times per week. Assessments for cardiorespiratory fitness, body composition, glycemic control, together with muscle biopsies were performed before and after the intervention. Morphometric analyses of lipid droplets and mitochondria were conducted in the subcellular fractions of biopsied muscle fibers using quantitative electron microscopy. The training intervention increased cardiorespiratory fitness, lowered fat mass, and improved nonfasting glycemic control ( P < 0.05), with no difference between training modalities. In the subsarcolemmal space, training decreased lipid droplet volume ( P = 0.003), and high-intensity interval, but not endurance, training reduced the size of lipid droplets, specifically in type 2 fibers ( P < 0.001). No training-induced change in intermyofibrillar lipid droplets was observed in both fiber types. Subsarcolemmal mitochondrial volume was increased by high-intensity interval ( P = 0.02), but not endurance, training ( P = 0.79). Along with improvement in glycemic control, low-volume high-intensity interval training is an alternative time-saving training modality that affects subcellular morphology and volumetric content of lipid droplets in skeletal muscle of type 2 diabetic patients.

The effect of diet and exercise on lipid droplet dynamics in human muscle tissue

The majority of fat in the human body is stored as triacylglycerols in white adipose tissue. In the obese state, adipose tissue mass expands and excess lipids are stored in non-adipose tissues, such as skeletal muscle. Lipids are stored in skeletal muscle in the form of small lipid droplets. Although originally viewed as dull organelles that simply store lipids as a consequence of lipid overflow from adipose tissue, lipid droplets are now recognized as key components in the cell that exert a variety of relevant functions in multiple tissues (including muscle). Here, we review the effect of diet and exercise interventions on myocellular lipid droplets and their putative role in insulin sensitivity from a human perspective. We also provide an overview of lipid droplet biology and identify gaps for future research.

Plasma Amino Acids Stimulate Uncoupled Respiration of Muscle Subsarcolemmal Mitochondria in Lean but Not Obese Humans

CONTEXT

Obesity is associated with mitochondrial dysfunction in skeletal muscle. Increasing the plasma amino acid (AA) concentrations stimulates mitochondrial adenosine triphosphate (ATP) production in lean individuals.

OBJECTIVE

To determine whether acute elevation in plasma AAs enhances muscle mitochondrial respiration and ATP production in subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria in obese adults.

DESIGN

Assessment of SS and IMF mitochondrial function during saline (i.e., control) and AA infusions.

PARTICIPANTS

Eligible participants were healthy lean (body mass index, <25 kg/m2; age, 37 ± 3 years; n = 10) and obese (body mass index >30 kg/m2; age 35 ± 3 years; n = 11) subjects.

INTERVENTION

Single trial of saline infusion followed by AA infusion. SS and IMF mitochondria were isolated from muscle biopsies collected at the end of the saline and AA infusions.

MAIN OUTCOMES

Mitochondrial respiration and ATP production.

RESULTS

AA infusion increased adenosine 5'-diphosphate (ADP)-stimulated respiration and ATP production rates of SS mitochondria in the lean (P < 0.05), but not obese, subjects. Furthermore, AA infusion increased the uncoupled (i.e., non-ADP-stimulated) respiration of SS mitochondria in the lean subjects only (P < 0.05). AA infusion had no effect on any of these parameters in IMF mitochondria in either lean or obese subjects (P > 0.05).

CONCLUSIONS

Increasing the plasma AA concentrations enhances the capacity for respiration and ATP production of muscle SS, but not IMF, mitochondria in lean individuals, in parallel with increases in uncoupled respiration. However, neither of these parameters increases in muscle SS or IMF mitochondria in obese individuals.

Training and acute exercise modulates mitochondrial dynamics in football players' blood mononuclear cells

PURPOSE

Regular physical activity induces oxidative stress but also causes adaptations in antioxidant defences including the nuclear factor κB (NF-κB) pathway, which activates target genes related to antioxidant defences such as uncoupling proteins (UCPs), and mitochondrial biogenesis mediated by peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α). The aim of the study was to determine the effect of long-term training and acute exercise on oxidant/antioxidant status and the expression of mitochondrial biogenesis genes in peripheral blood mononuclear cells (PBMCs).

METHODS

Twelve professional football players performed an 8-week exercise programme comprising a daily 2-h football training session. Blood samples were taken before and after the training season.

RESULTS

The results reported a significant increase in antioxidant protein levels and in mitochondrial proteins in resting conditions after the 8-week training period. PGC1α, UCP-2 and mitofusin 2 protein levels also increased after acute exercise compared to pre-exercise levels. After the training, the expression of PGC1α, cytochrome c oxidase subunit IV and mitochondrial NADH dehydrogenase subunit 5 messenger RNA (mRNA) significantly augmented after the acute physical activity compared to pre-exercise levels; while no changes occurred in these mRNA in basal conditions. NF-κB activation and ROS production reported a significant increase after acute exercise.

CONCLUSIONS

Training increases the levels of proteins related to mitochondrial biogenesis and improves the antioxidant capabilities of mitochondria in PBMCs among well-trained football players. Acute exercise may act as an inducer of mitochondrial biogenesis through NF-κB activation and PGC1α gene expression.

Intramyocellular lipid droplets and insulin sensitivity, the human perspective

Skeletal muscle can store excess fat as subcellular lipid droplets (LDs). While originally viewed as uninteresting static balls of triacylglycerol, it is now clear that myocellular LDs play an active role in myocellular (patho)physiology. In this review we aim to discuss the role of LDs in muscle cell insulin sensitivity and identify parameters which appear to affect this relationship. Moreover, we discuss the application of novel tools permitting detailed examination of these parameters. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Skeletal muscle insulin resistance: role of mitochondria and other ROS sources

At present, obesity is one of the most important public health problems in the world because it causes several diseases and reduces life expectancy. Although it is well known that insulin resistance plays a pivotal role in the development of type 2 diabetes mellitus (the more frequent disease in obese people) the link between obesity and insulin resistance is yet a matter of debate. One of the most deleterious effects of obesity is the deposition of lipids in non-adipose tissues when the capacity of adipose tissue is overwhelmed. During the last decade, reduced mitochondrial function has been considered as an important contributor to 'toxic' lipid metabolite accumulation and consequent insulin resistance. More recent reports suggest that mitochondrial dysfunction is not an early event in the development of insulin resistance, but rather a complication of the hyperlipidemia-induced reactive oxygen species (ROS) production in skeletal muscle, which might promote mitochondrial alterations, lipid accumulation and inhibition of insulin action. Here, we review the literature dealing with the mitochondria-centered mechanisms proposed to explain the onset of obesity-linked IR in skeletal muscle. We conclude that the different pathways leading to insulin resistance may act synergistically because ROS production by mitochondria and other sources can result in mitochondrial dysfunction, which in turn can further increase ROS production leading to the establishment of a harmful positive feedback loop.

Training Enhances Immune Cells Mitochondrial Biosynthesis, Fission, Fusion, and Their Antioxidant Capabilities Synergistically with Dietary Docosahexaenoic Supplementation

Exercise training induces adaptations in mitochondrial metabolism, dynamics, and oxidative protection. Omega-3 fatty acids change membrane lipid composition and modulate mitochondrial function. The aim was to investigate the effect of 8-week training and docosahexaenoic acid (DHA) supplementation (1.14 g/day) on the mitochondria dynamics and antioxidant status in peripheral blood mononuclear cells (PBMCs) from sportsmen. Subjects were assigned to an intervention (N = 9) or placebo groups (N = 7) in a randomized double-blind trial. Nutritional intervention significantly increased the DHA content in erythrocyte membranes from the experimental group. No significant differences were reported in terms of circulating PBMCs, Mn-superoxide dismutase protein levels, and their capability to produce reactive oxygen species. The proteins related to mitochondrial dynamics were, in general, increased after an 8-week training and this increase was enhanced by DHA supplementation. The content in mitofusins Mtf-1 and Mtf-2, optic atrophy protein-1 (Opa-1), and mitochondrial transcription factor A (Tfam) were significantly higher in the DHA-supplemented group after intervention. Cytochrome c oxidase (COX-IV) activity and uncoupling proteins UCP-2 and UCP-3 protein levels were increased after training, with higher UCP-3 levels in the supplemented group. In conclusion, training induced mitochondrial adaptations which may contribute to improved mitochondrial function. This mitochondrial response was modulated by DHA supplementation.

The potential of endurance exercise-derived exosomes to treat metabolic diseases

Endurance exercise-mediated multisystemic adaptations are known to mitigate metabolism-related disorders such as obesity and type 2 diabetes mellitus (T2DM). However, the underlying molecular mechanisms that promote crosstalk between organs and orchestrate the pro-metabolic effects of endurance exercise remain unclear. Exercise-induced release of peptides and nucleic acids from skeletal muscle and other organs (collectively termed 'exerkines') has been implicated in mediating these systemic adaptations. Given that the extracellular milieu is probably not a hospitable environment for labile exerkines, a lipid vehicle-based mode of delivery has originated over the course of evolution. Two types of extracellular vesicles, exosomes and microvesicles, have been shown to contain proteins and nucleic acids that participate in a variety of physiological and pathological processes. Exosomes, in particular, have been shown to facilitate the exchange of peptides, microRNA, mRNA and mitochondrial DNA between cells and tissues. Intriguingly, circulatory extracellular vesicle content increases in an intensity-dependant manner in response to endurance exercise. We propose that the systemic benefits of exercise are modulated by exosomes and/or microvesicles functioning in an autocrine, paracrine and/or endocrine manner. Furthermore, we posit that native or modified exosomes, and/or microvesicles enriched with exerkines will have therapeutic utility in the treatment of obesity and T2DM.

Skeletal Muscle Mitochondrial Content, Oxidative Capacity, and Mfn2 Expression Are Reduced in Older Patients With Heart Failure and Preserved Ejection Fraction and Are Related to Exercise Intolerance

OBJECTIVES

The aim of this study was to examine skeletal muscle mitochondria content, oxidative capacity, and the expression of key mitochondrial dynamics proteins in patients with heart failure with preserved ejection fraction (HFpEF), as well as to determine potential relationships with measures of exercise performance.

BACKGROUND

Multiple lines of evidence indicate that severely reduced peak exercise oxygen uptake (peak VO2) in older patients with HFpEF is related to abnormal skeletal muscle oxygen utilization. Mitochondria are key regulators of skeletal muscle metabolism; however, little is known about how these organelles are affected in HFpEF.

METHODS

Both vastus lateralis skeletal muscle citrate synthase activity and the expression of porin and regulators of mitochondrial fusion were examined in older patients with HFpEF (n = 20) and healthy, age-matched control subjects (n = 17).

RESULTS

Compared with age-matched healthy control subjects, mitochondrial content assessed by porin expression was 46% lower (p = 0.01), citrate synthase activity was 29% lower (p = 0.01), and Mfn2 (mitofusin 2) expression was 54% lower (p <0.001) in patients with HFpEF. Expression of porin was significantly positively correlated with both peak VO2 and 6-min walk distance (r = 0.48, p = 0.003 and r = 0.33, p = 0.05, respectively). Expression of Mfn2 was also significantly positively correlated with both peak VO2 and 6-min walk distance (r = 0.40, p = 0.02 and r = 0.37, p = 0.03 respectively).

CONCLUSIONS

These findings suggest that skeletal muscle oxidative capacity, mitochondrial content, and mitochondrial fusion are abnormal in older patients with HFpEF and might contribute to their severe exercise intolerance.

Decoration of intramyocellular lipid droplets with PLIN5 modulates fasting-induced insulin resistance and lipotoxicity in humans

AIMS/HYPOTHESIS

In contrast to insulin-resistant individuals, insulin-sensitive athletes possess high intramyocellular lipid content (IMCL), good mitochondrial function and high perilipin 5 (PLIN5) levels, suggesting a role for PLIN5 in benign IMCL storage. We hypothesised a role for PLIN5 in modulating fasting-mediated insulin resistance.

METHODS

Twelve men were fasted for 60 h, before and after which muscle biopsies were taken and stained for lipid droplets (LDs), PLIN5 and laminin. Confocal microscopy images were analysed for LD size, number, PLIN5 association and subcellular distribution.

RESULTS

Fasting elevated IMCL content 2.8-fold and reduced insulin sensitivity (by 55%). Individuals with the most prominent increase in IMCL showed the least reduction in insulin sensitivity (r = 0.657; p = 0.028) and mitochondrial function (r = 0.896; p = 0.006). During fasting, PLIN5 gene expression or PLIN5 protein content in muscle homogenates was unaffected, microscopy analyses revealed that the fraction of PLIN5 associated with LDs (PLIN5+) increased significantly (+26%) upon fasting, suggesting PLIN5 redistribution. The significant increase in LD number (+23%) and size (+23%) upon fasting was entirely accounted for by PLIN5+ LDs, not by LDs devoid of PLIN5. Also the association between IMCL storage capacity and insulin resistance and mitochondrial dysfunction was only apparent for PLIN5+ LDs.

CONCLUSIONS/INTERPRETATION

Fasting results in subcellular redistribution of PLIN5 and promotes the capacity to store excess fat in larger and more numerous PLIN5-decorated LDs. This associates with blunting of fasting-induced insulin resistance and mitochondrial dysfunction, suggesting a role for PLIN5 in the modulation of fasting-mediated lipotoxicity.

TRIAL REGISTRATION

trialregister.nl NTR 2042.

Training Does Not Alter Muscle Ceramide and Diacylglycerol in Offsprings of Type 2 Diabetic Patients Despite Improved Insulin Sensitivity

Ceramide and diacylglycerol (DAG) may be involved in the early phase of insulin resistance but data are inconsistent in man. We evaluated if an increase in insulin sensitivity after endurance training was accompanied by changes in these lipids in skeletal muscle. Nineteen first-degree type 2 diabetes Offsprings (Offsprings) (age: 33.1 ± 1.4 yrs; BMI: 26.4 ± 0.4 kg/m²) and sixteen matched Controls (age: 31.3 ± 1.5 yrs; BMI: 25.3 ± 0.7 kg/m²) performed 10 weeks of endurance training three times a week at 70% of VO₂max on a bicycle ergometer. Before and after the intervention a hyperinsulinemic-euglycemic clamp and VO₂max test were performed and muscle biopsies obtained. Insulin sensitivity was significantly lower in Offsprings compared to control subjects (p < 0.01) but improved in both groups after 10 weeks of endurance training (Off: 17 ± 6%; Con: 12 ± 9%, p < 0.01). The content of muscle ceramide, DAG, and their subspecies were similar between groups and did not change in response to the endurance training except for an overall reduction in C22:0-Cer (p < 0.05). Finally, the intervention induced an increase in AKT protein expression (Off: 27 ± 11%; Con: 20 ± 24%, p < 0.05). This study showed no relation between insulin sensitivity and ceramide or DAG content suggesting that ceramide and DAG are not major players in the early phase of insulin resistance in human muscle.

Adiposity and immune-muscle crosstalk in South Asians &Europeans: A cross-sectional study

South Asians (SA) are at higher risk of cardiometabolic disorders than Europeans (EU), yet the potential determinants of this risk are poorly understood. We tested the hypotheses that 1) South Asians (SA) have greater muscle inflammation compared to Europeans (EU) at similar fat mass 2) differential regional adiposity in SA compared to EU is associated with enhanced muscle inflammation in SA. This cross-sectional study was conducted at a tertiary academic center in Hamilton, Ontario, Canada. The study included 29 EU and 26 SA. Quantitative real-time PCR and western blot were used to measure muscle inflammation. Statistical analysis was done using a General Linear Model. Despite having similar macrophage content to EU, SA muscle had lower levels of chemokine CCL2 compared to EU at gene expression (β -1.099, SE β 0.521, p-value 0.04) and protein (0.84 ± 0.69 versus 1.10 ± 0.60, p-value 0.052) levels. SA had more pronounced abdominal and hepatic adiposity, with smaller Intramyocellular lipid particles compared to EU (0.26 ± 0.12 μm² versus 0.15 ± 0.06 μm², p-value 0.02). In conclusion, CCL2 downregulation in SA may be an attempt to protect muscle against macrophage infiltration, and defects in fatty acid partitioning to muscle may lead to the disproportionate adiposity and adverse cardiometabolic profile in SA.

Exercise Training and Insulin Resistance: A Current Review

There is a general perception that increased physical activity will improve glucose homeostasis in all individuals. While this is an attractive concept, this conclusion may be overly simplistic and even misleading. The topic was reviewed extensively over 30 years ago and it was concluded that acute exercise enhances glucose uptake. However, in some cases the chronic influence of interventions utilizing exercise may have little effect on glucose metabolism. Moreover, insulin resistance often returns to near baseline levels within a couple of days following cessation of the exercise bout; leaving the overall effectiveness of the intervention in question. Since improving glucose homeostasis should be the focal endpoint of any intervention designed to mitigate the overwhelming degree of insulin resistance in individuals at risk for metabolic disease, it is essential to evaluate the key components of a successful approach.

The effect of pre-existing maternal obesity and diabetes on placental mitochondrial content and electron transport chain activity

INTRODUCTION

Mitochondria dysfunction has been extensively implicated in the progression of these metabolic disorders, their role in placental tissue of diabetic and/or obese pregnant women is yet to be investigated. The aim of this study was to determine the effect of pre-existing type 1 and type 2 diabetes mellitus (DM), and pre-existing maternal obesity on placental mitochondrial function as assessed by mitochondrial content, electron transport chain (ETC) complex activities and oxidative stress.

METHODS

Human placenta was obtained at the time of term Caesarean section from (i) non-obese (n = 19) and obese (n = 23) normal glucose tolerant (NGT) pregnant women; (ii) women with type 1 DM (n = 14) and BMI-matched NGT women (n = 14); and (iii) women with type 2 DM (n = 11) and BMI-matched NGT women (n = 11). The following endpoints were assessed: placental mitochondrial content by citrate synthase activity and mitochondrial DNA (mtDNA content); mitochondrial respiratory chain activity (complexes I, II, II & III, III and IV), and mitochondrial ROS (as assessed by mitochondrial hydrogen peroxide (H2O2) levels).

RESULTS

When compared to placenta from NGT non-obese women, there was significantly lower mitochondrial DNA (mtDNA) content and electron transport chain complex I activity, and significantly higher mitochondrial H2O2 levels in placenta from NGT obese women (P < 0.05). Placental tissue from type 1 DM women showed significant reductions in ETC complex I, II & III, and III activity and increased H2O2 levels when compared to BMI-matched NGT women (P < 0.05). Type 2 DM women only exhibited significantly reduced ETC complex II & III activity when compared to BMI-matched NGT women (P < 0.05).

DISCUSSION AND CONCLUSIONS

Women with pre-existing obesity or diabetes have decreased placental mitochondrial respiratory chain enzyme activities which may have detrimental consequences on placental function and therefore fetal growth and development.

An acetylation rheostat for the control of muscle energy homeostasis

In recent years, the role of acetylation has gained ground as an essential modulator of intermediary metabolism in skeletal muscle. Imbalance in energy homeostasis or chronic cellular stress, due to diet, aging, or disease, translate into alterations in the acetylation levels of key proteins which govern bioenergetics, cellular substrate use, and/or changes in mitochondrial content and function. For example, cellular stress induced by exercise or caloric restriction can alter the coordinated activity of acetyltransferases and deacetylases to increase mitochondrial biogenesis and function in order to adapt to low energetic levels. The natural duality of these enzymes, as metabolic sensors and effector proteins, has helped biologists to understand how the body can integrate seemingly distinct signaling pathways to control mitochondrial biogenesis, insulin sensitivity, glucose transport, reactive oxygen species handling, angiogenesis, and muscle satellite cell proliferation/differentiation. Our review will summarize the recent developments related to acetylation-dependent responses following metabolic stress in skeletal muscle.