Plasticity in mitochondrial cristae density allows metabolic capacity modulation in human skeletal muscle

Stay Fit, Stay Young: Mitochondria in Movement: The Role of Exercise in the New Mitochondrial Paradigm

Skeletal muscles require the proper production and distribution of energy to sustain their work. To ensure this requirement is met, mitochondria form large networks within skeletal muscle cells, and during exercise, they can enhance their functions. In the present review, we discuss recent findings on exercise-induced mitochondrial adaptations. We emphasize the importance of mitochondrial biogenesis, morphological changes, and increases in respiratory supercomplex formation as mechanisms triggered by exercise that may increase the function of skeletal muscles. Finally, we highlight the possible effects of nutraceutical compounds on mitochondrial performance during exercise and outline the use of exercise as a therapeutic tool in noncommunicable disease prevention. The resulting picture shows that the modulation of mitochondrial activity by exercise is not only fundamental for physical performance but also a key point for whole-organism well-being.

Physical Activity and Sports-Real Health Benefits: A Review with Insight into the Public Health of Sweden

Positive effects from sports are achieved primarily through physical activity, but secondary effects bring health benefits such as psychosocial and personal development and less alcohol consumption. Negative effects, such as the risk of failure, injuries, eating disorders, and burnout, are also apparent. Because physical activity is increasingly conducted in an organized manner, sport’s role in society has become increasingly important over the years, not only for the individual but also for public health. In this paper, we intend to describe sport’s physiological and psychosocial health benefits, stemming both from physical activity and from sport participation per se. This narrative review summarizes research and presents health-related data from Swedish authorities. It is discussed that our daily lives are becoming less physically active, while organized exercise and training increases. Average energy intake is increasing, creating an energy surplus, and thus, we are seeing an increasing number of people who are overweight, which is a strong contributor to health problems. Physical activity and exercise have significant positive effects in preventing or alleviating mental illness, including depressive symptoms and anxiety- or stress-related disease. In conclusion, sports can be evolving, if personal capacities, social situation, and biological and psychological maturation are taken into account. Evidence suggests a dose–response relationship such that being active, even to a modest level, is superior to being inactive or sedentary. Recommendations for healthy sports are summarized.

High-Intensity Exercise and Mitochondrial Biogenesis: Current Controversies and Future Research Directions

It is well established that different types of exercise can provide a powerful stimulus for mitochondrial biogenesis. However, there are conflicting findings in the literature, and a consensus has not been reached regarding the efficacy of high-intensity exercise to promote mitochondrial biogenesis in humans. The purpose of this review is to examine current controversies in the field and to highlight some important methodological issues that need to be addressed to resolve existing conflicts.

The Role of Mineral and Trace Element Supplementation in Exercise and Athletic Performance: A Systematic Review

Minerals and trace elements (MTEs) are micronutrients involved in hundreds of biological processes. Deficiency in MTEs can negatively affect athletic performance. Approximately 50% of athletes have reported consuming some form of micronutrient supplement; however, there is limited data confirming their efficacy for improving performance. The aim of this study was to systematically review the role of MTEs in exercise and athletic performance. Six electronic databases and grey literature sources (MEDLINE; EMBASE; CINAHL and SportDISCUS; Web of Science and clinicaltrials.gov) were searched, in accordance with PRISMA guidelines. Results: 17,433 articles were identified and 130 experiments from 128 studies were included. Retrieved articles included Iron (n = 29), Calcium (n = 11), Magnesium, (n = 22), Phosphate (n = 17), Zinc (n = 9), Sodium (n = 15), Boron (n = 4), Selenium (n = 5), Chromium (n = 12) and multi-mineral articles (n = 5). No relevant articles were identified for Copper, Manganese, Iodine, Nickel, Fluoride or Cobalt. Only Iron and Magnesium included articles of sufficient quality to be assigned as 'strong'. Currently, there is little evidence to support the use of MTE supplementation to improve physiological markers of athletic performance, with the possible exception of Iron (in particular, biological situations) and Magnesium as these currently have the strongest quality evidence. Regardless, some MTEs may possess the potential to improve athletic performance, but more high quality research is required before support for these MTEs can be given. PROSPERO preregistered (CRD42018090502).

Adaptations in Mitochondrial Enzymatic Activity Occurs Independent of Genomic Dosage in Response to Aerobic Exercise Training and Deconditioning in Human Skeletal Muscle

Mitochondrial DNA (mtDNA) replication is thought to be an integral part of exercise-training-induced mitochondrial adaptations. Thus, mtDNA level is often used as an index of mitochondrial adaptations in training studies. We investigated the hypothesis that endurance exercise training-induced mitochondrial enzymatic changes are independent of genomic dosage by studying mtDNA content in skeletal muscle in response to six weeks of knee-extensor exercise training followed by four weeks of deconditioning in one leg, comparing results to the contralateral untrained leg, in 10 healthy, untrained male volunteers. Findings were compared to citrate synthase activity, mitochondrial complex activities, and content of mitochondrial membrane markers (porin and cardiolipin). One-legged knee-extensor exercise increased endurance performance by 120%, which was accompanied by increases in power output and peak oxygen uptake of 49% and 33%, respectively (p < 0.01). Citrate synthase and mitochondrial respiratory chain complex I–IV activities were increased by 51% and 46–61%, respectively, in the trained leg (p < 0.001). Despite a substantial training-induced increase in mitochondrial activity of TCA and ETC enzymes, there was no change in mtDNA and mitochondrial inner and outer membrane markers (i.e., cardiolipin and porin). Conversely, deconditioning reduced endurance capacity by 41%, muscle citrate synthase activity by 32%, and mitochondrial complex I–IV activities by 29–36% (p < 0.05), without any change in mtDNA and porin and cardiolipin content in the previously trained leg. The findings demonstrate that the adaptations in mitochondrial enzymatic activity after aerobic endurance exercise training and the opposite effects of deconditioning are independent of changes in the number of mitochondrial genomes, and likely relate to changes in the rate of transcription of mtDNA.

The Mitochondrial Basis for Adaptive Variation in Aerobic Performance in High-Altitude Deer Mice

Mitochondria play a central role in aerobic performance. Studies aimed at elucidating how evolved variation in mitochondrial physiology contributes to adaptive variation in aerobic performance can therefore provide a unique and powerful lens to understanding the evolution of complex physiological traits. Here, we review our ongoing work on the importance of changes in mitochondrial quantity and quality to adaptive variation in aerobic performance in high-altitude deer mice. Whole-organism aerobic capacity in hypoxia (VO2max) increases in response to hypoxia acclimation in this species, but high-altitude populations have evolved consistently greater VO2max than populations from low altitude. The evolved increase in VO2max in highlanders is associated with an evolved increase in the respiratory capacity of the gastrocnemius muscle. This appears to result from highlanders having more mitochondria in this tissue, attributed to a higher proportional abundance of oxidative fiber-types and a greater mitochondrial volume density within oxidative fibers. The latter is primarily caused by an over-abundance of subsarcolemmal mitochondria in high-altitude mice, which is likely advantageous for mitochondrial O2 supply because more mitochondria are situated adjacent to the cell membrane and close to capillaries. Evolved changes in gastrocnemius phenotype appear to be underpinned by population differences in the expression of genes involved in energy metabolism, muscle development, and vascular development. Hypoxia acclimation has relatively little effect on respiratory capacity of the gastrocnemius, but it increases respiratory capacity of the diaphragm. However, the mechanisms responsible for this increase differ between populations: lowlanders appear to adjust mitochondrial quantity and quality (i.e., increases in citrate synthase [CS] activity, and mitochondrial respiration relative to CS activity) and they exhibit higher rates of mitochondrial release of reactive oxygen species, whereas highlanders only increase mitochondrial quantity in response to hypoxia acclimation. In contrast to the variation in skeletal muscles, the respiratory capacity of cardiac muscle does not appear to be affected by hypoxia acclimation and varies little between populations. Therefore, evolved changes in mitochondrial quantity and quality make important tissue-specific contributions to adaptive variation in aerobic performance in high-altitude deer mice.

The Effects of APOE4 on Mitochondrial Dynamics and Proteins in vivo

This study examined the effects of apolipoprotein E4 (APOE4), the most prevalent genetic risk factor for Alzheimer's disease (AD), on proteins involved in mitochondrial dynamics and autophagy, in the hippocampus of targeted replacement mice. Immunohistochemical measurements revealed that the levels of the mitochondrial fusion-mediating protein, MFN1, were higher, whereas those of corresponding fission-regulating protein, DRP-1, were lower in the hippocampus of ApoE4 mice than in the corresponding ApoE3 mice, indicating that APOE4 is associated with increased mitochondrial fusion and decreased fission. A similar ApoE4-driven decrease in DRP-1 was also observed in AD brains. The levels of the mitochondrial proteins COX1 and Tom40, were higher in the ApoE4 mice, which is consistent with the increased fusion. Measurements of the levels of cleaved PINK1 and parkin, which mark and target mitochondria for mitophagic degradation, revealed lower levels of cleaved PINK1, suggesting reduced mitochondrial membrane potential, and higher levels of parkin in the hippocampus of ApoE4 compared with the ApoE3 mice, indicating altered mitophagy. The levels of the ubiquitin-binding scaffold protein, p62/SQSTM1, which directs selected cargo to the autophagosomes, were also higher in the ApoE4 mice. These findings suggest that APOE4 is associated with enhanced mitochondrial fusion and decreased fission. Additionally, the results indicate that mitophagy/autophagy is reduced in ApoE4 mice, resulting in higher levels of proteins such as parkin and p62, which are normally degraded during this process. Taken together, these results suggest a novel mechanism that may underlie the pathological effects of APOE4 and indicate that use of APOE4 genotyping could pave the way for identification of novel APOE4-related therapeutic targets.

Perspective: Mitochondria-ER Contacts in Metabolic Cellular Stress Assessed by Microscopy

The interplay of mitochondria with the endoplasmic reticulum and their connections, called mitochondria-ER contacts (MERCs) or mitochondria-associated ER membranes (MAMs), are crucial hubs in cellular stress. These sites are essential for the passage of calcium ions, reactive oxygen species delivery, the sorting of lipids in whole-body metabolism. In this perspective article, we focus on microscopic evidences of the pivotal role of MERCs/MAMs and their changes in metabolic diseases, like obesity, diabetes, and neurodegeneration.

Mitochondrial ultrastructural adaptations in fast muscles of mice lacking IL15RA

The pro-inflammatory cytokine interleukin-15 (IL15) and its receptor α (IL15RA) participate in the regulation of musculoskeletal function and metabolism. Deletion of the Il15ra gene in mice increases spontaneous activity, improves fatigue resistance in the glycolytic extensor digitorum longus (EDL) and protects from diet-induced obesity. In humans, IL15RA single-nucleotide polymorphisms (SNPs) have been linked to muscle strength, metabolism and performance in elite endurance athletes. Taken together, these features suggest a possible role for IL15RA in muscle mitochondrial structure and function. Here, we have investigated the consequences of loss of IL15RA on skeletal muscle fiber-type properties and mitochondrial ultrastructure. Immunostaining of the EDL for myosin heavy chain (MyHC) isoforms revealed no significant changes in fiber type. Electron microscopy (EM) analysis of the EDL indicated an overall higher mitochondria content, and increased cristae density in subsarcolemmal and A-band mitochondrial subpopulations. The higher cristae density in Il15ra ^-/- mitochondria was associated with higher OPA1 and cardiolipin levels. Overall, these data extend our understanding of the role of IL15RA signaling in muscle oxidative metabolism and adaptation to exercise.

Training-Induced Changes in Mitochondrial Content and Respiratory Function in Human Skeletal Muscle

A sedentary lifestyle has been linked to a number of metabolic disorders that have been associated with sub-optimal mitochondrial characteristics and an increased risk of premature death. Endurance training can induce an increase in mitochondrial content and/or mitochondrial functional qualities, which are associated with improved health and well-being and longer life expectancy. It is therefore important to better define how manipulating key parameters of an endurance training intervention can influence the content and functionality of the mitochondrial pool. This review focuses on mitochondrial changes taking place following a series of exercise sessions (training-induced mitochondrial adaptations), providing an in-depth analysis of the effects of exercise intensity and training volume on changes in mitochondrial protein synthesis, mitochondrial content and mitochondrial respiratory function. We provide evidence that manipulation of different exercise training variables promotes specific and diverse mitochondrial adaptations. Specifically, we report that training volume may be a critical factor affecting changes in mitochondrial content, whereas relative exercise intensity is an important determinant of changes in mitochondrial respiratory function. As a consequence, a dissociation between training-induced changes in mitochondrial content and mitochondrial respiratory function is often observed. We also provide evidence that exercise-induced changes are not necessarily predictive of training-induced adaptations, we propose possible explanations for the above discrepancies and suggestions for future research.

Maintenance of Skeletal Muscle Mitochondria in Health, Exercise, and Aging

Mitochondria are critical organelles responsible for regulating the metabolic status of skeletal muscle. These organelles exhibit remarkable plasticity by adapting their volume, structure, and function in response to chronic exercise, disuse, aging, and disease. A single bout of exercise initiates signaling to provoke increases in mitochondrial biogenesis, balanced by the onset of organelle turnover carried out by the mitophagy pathway. This accelerated turnover ensures the presence of a high functioning network of mitochondria designed for optimal ATP supply, with the consequence of favoring lipid metabolism, maintaining muscle mass, and reducing apoptotic susceptibility over the longer term. Conversely, aging and disuse are associated with reductions in muscle mass that are in part attributable to dysregulation of the mitochondrial network and impaired mitochondrial function. Therefore, exercise represents a viable, nonpharmaceutical therapy with the potential to reverse and enhance the impaired mitochondrial function observed with aging and chronic muscle disuse.

Three-Dimensional Analysis of Mitochondrial Crista Ultrastructure in a Patient with Leigh Syndrome by In Situ Cryoelectron Tomography

Mitochondrial diseases produce profound neurological dysfunction via mutations affecting mitochondrial energy production, including the relatively common Leigh syndrome (LS). We recently described an LS case caused by a pathogenic mutation in USMG5, encoding a small supernumerary subunit of mitochondrial ATP synthase. This protein is integral for ATP synthase dimerization, and patient fibroblasts revealed an almost total loss of ATP synthase dimers. Here, we utilize in situ cryoelectron tomography (cryo-ET) in a clinical case-control study of mitochondrial disease to directly study mitochondria within cultured fibroblasts from a patient with LS and a healthy human control subject. Through tomographic analysis of patient and control mitochondria, we find that loss of ATP synthase dimerization due to the pathogenic mutation causes profound disturbances of mitochondrial crista ultrastructure. Overall, this work supports the crucial role of ATP synthase in regulating crista architecture in the context of human disease.

Superior Intrinsic Mitochondrial Respiration in Women Than in Men

Sexual dimorphism is apparent in humans, however, to date no studies have investigated mitochondrial function focusing on intrinsic mitochondrial respiration (i.e., mitochondrial respiration for a given amount of mitochondrial protein) and mitochondrial oxygen affinity (p50_mito) in relation to biological sex in human. A skeletal muscle biopsy was donated by nine active women, and ten men matched for maximal oxygen consumption (VO_2max) and by nine endurance trained men. Intrinsic mitochondrial respiration, assessed in isolated mitochondria, was higher in women compared to men when activating complex I (CI_P) and complex I+II (CI+II_P) (p < 0.05), and was similar to trained men (CI_P, p = 0.053; CI+II_P, p = 0.066). Proton leak and p50_mito were higher in women compared to men independent of VO_2max. In conclusion, significant novel differences in mitochondrial oxidative function, intrinsic mitochondrial respiration and p50_mito exist between women and men. These findings may represent an adaptation in the oxygen cascade in women to optimize muscle oxygen uptake to compensate for a lower oxygen delivery during exercise.

The Muscle Fiber Profiles, Mitochondrial Content, and Enzyme Activities of the Exceptionally Well-Trained Arm and Leg Muscles of Elite Cross-Country Skiers

As one of the most physically demanding sports in the Olympic Games, cross-country skiing poses considerable challenges with respect to both force generation and endurance during the combined upper- and lower-body effort of varying intensity and duration. The isoforms of myosin in skeletal muscle have long been considered not only to define the contractile properties, but also to determine metabolic capacities. The current investigation was designed to explore the relationship between these isoforms and metabolic profiles in the arms (triceps brachii) and legs (vastus lateralis) as well as the range of training responses in the muscle fibers of elite cross-country skiers with equally and exceptionally well-trained upper and lower bodies. The proportion of myosin heavy chain (MHC)-1 was higher in the leg (58 ± 2% [34-69%]) than arm (40 ± 3% [24-57%]), although the mitochondrial volume percentages [8.6 ± 1.6 (leg) and 9.0 ± 2.0 (arm)], and average number of capillaries per fiber [5.8 ± 0.8 (leg) and 6.3 ± 0.3 (arm)] were the same. In these comparable highly trained leg and arm muscles, the maximal citrate synthase (CS) activity was the same. Still, 3-hydroxy-acyl-CoA-dehydrogenase (HAD) capacity was 52% higher (P < 0.05) in the leg compared to arm muscles, suggesting a relatively higher capacity for lipid oxidation in leg muscle, which cannot be explained by the different fiber type distributions. For both limbs combined, HAD activity was correlated with the content of MHC-1 (r² = 0.32, P = 0.011), whereas CS activity was not. Thus, in these highly trained cross-country skiers capillarization of and mitochondrial volume in type 2 fiber can be at least as high as in type 1 fibers, indicating a divergence between fiber type pattern and aerobic metabolic capacity. The considerable variability in oxidative metabolism with similar MHC profiles provides a new perspective on exercise training. Furthermore, the clear differences between equally well-trained arm and leg muscles regarding HAD activity cannot be explained by training status or MHC distribution, thereby indicating an intrinsic metabolic difference between the upper and lower body. Moreover, trained type 1 and type 2A muscle fibers exhibited similar aerobic capacity regardless of whether they were located in an arm or leg muscle.

A trade-off between thickness and length in the zebra finch sperm mid-piece

The sperm mid-piece has traditionally been considered to be the engine that powers sperm. Larger mid-pieces have therefore been assumed to provide greater energetic capacity. However, in the zebra finch Taeniopygia guttata, a recent study showed a surprising negative relationship between mid-piece length and sperm energy content. Using a multi-dimensional approach to study mid-piece structure, we tested whether this unexpected relationship can be explained by a trade-off between mid-piece length and mid-piece thickness and/or cristae density inside the mitochondrial helix. We used selective plane illumination microscopy to study mid-piece structure from three-dimensional images of zebra finch sperm and used high-resolution transmission electron microscopy to quantify mitochondrial density. Contrary to the assumption that longer mid-pieces are larger and therefore produce or contain a greater amount of energy, our results indicate that the amount of mitochondrial material is consistent across mid-pieces of varying lengths, and longer mid-pieces are simply proportionately ‘thinner’.

Adaptation of motor unit contractile properties in rat medial gastrocnemius to treadmill endurance training: Relationship to muscle mitochondrial biogenesis

This study aimed at investigating the effects of 2, 4 and 8 weeks of endurance training on the contractile properties of slow (S), fast fatigue resistant (FR) and fast fatigable (FF) motor units (MUs) in rat medial gastrocnemius (MG) in relation to the changes in muscle mitochondrial biogenesis. The properties of functionally isolated MUs were examined in vivo. Mitochondrial biogenesis was judged based on the changes in mitochondrial DNA copy number (mtDNA), the content of the electron transport chain (ETC) proteins and PGC-1α in the MG. Moreover, the markers of mitochondria remodeling mitofusins (Mfn1, Mfn2) and dynamin-like protein (Opa1) were studied using qPCR. A proportion of FR MUs increased from 37.9% to 50.8% and a proportion of FF units decreased from 44.7% to 26.6% after 8 weeks of training. The increased fatigue resistance, shortened twitch duration, and increased ability to potentiate force were found as early as after 2 weeks of endurance training, predominantly in FR MUs. Moreover, just after 2 weeks of the training an enhancement of the mitochondrial network remodeling was present as judged by an increase in expression of Mfn1, Opa1 and an increase in PGC-1α in the slow part of MG. Interestingly, no signs of intensification of mitochondrial biogenesis assessed by ETC proteins content and mtDNA in slow and fast parts of gastrocnemius were found at this stage of the training. Nevertheless, after 8 weeks of training an increase in the ETC protein content was observed, but mainly in the slow part of gastrocnemius. Concluding, the functional changes in MUs’ contractile properties leading to the enhancement of muscle performance accompanied by an activation of signalling that controls the muscle mitochondrial network reorganisation and mitochondrial biogenesis belong to an early muscle adaptive responses that precede an increase in mitochondrial ETC protein content.

Intracellular localization of diacylglycerols and sphingolipids influences insulin sensitivity and mitochondrial function in human skeletal muscle

BACKGROUND

Accumulation of diacylglycerol (DAG) and sphingolipids is thought to promote skeletal muscle insulin resistance by altering cellular signaling specific to their location. However,the subcellular localization of bioactive lipids in human skeletal muscle is largely unknown.

METHODS

We evaluated subcellular localization of skeletal muscle DAGs and sphingolipids in lean individuals (n = 15), endurance-trained athletes (n = 16), and obese men and women with (n = 12) and without type 2 diabetes (n = 15). Muscle biopsies were fractionated into sarcolemmal, cytosolic, mitochondrial/ER, and nuclear compartments. Lipids were measured using liquid chromatography tandem mass spectrometry, and insulin sensitivity was measured using hyperinsulinemic-euglycemic clamp.

RESULTS

Sarcolemmal 1,2-DAGs were not significantly related to insulin sensitivity. Sarcolemmal ceramides were inversely related to insulin sensitivity, with a significant relationship found for the C18:0 species. Sarcolemmal sphingomyelins were also inversely related to insulin sensitivity, with the strongest relationships found for the C18:1, C18:0, and C18:2 species. In the mitochondrial/ER and nuclear fractions, 1,2-DAGs were positively related to, while ceramides were inversely related to, insulin sensitivity. Cytosolic lipids as well as 1,3-DAG, dihydroceramides, and glucosylceramides in any compartment were not related to insulin sensitivity. All sphingolipids but only specific DAGs administered to isolated mitochondria decreased mitochondrial state 3 respiration.

CONCLUSION

These data reveal previously unknown differences in subcellular localization of skeletal muscle DAGs and sphingolipids that relate to whole-body insulin sensitivity and mitochondrial function in humans. These data suggest that whole-cell concentrations of lipids obscure meaningful differences in compartmentalization and suggest that subcellular localization of lipids should be considered when developing therapeutic interventions to treat insulin resistance.

FUNDING

National Institutes of Health General Clinical Research Center (RR-00036), National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) (R01DK089170), NIDDK (T32 DK07658), and Colorado Nutrition Obesity Research Center (P30DK048520).

Mitochondrial Ultrastructure Is Coupled to Synaptic Performance at Axonal Release Sites

Mitochondrial function in neurons is tightly linked with metabolic and signaling mechanisms that ultimately determine neuronal performance. The subcellular distribution of these organelles is dynamically regulated as they are directed to axonal release sites on demand, but whether mitochondrial internal ultrastructure and molecular properties would reflect the actual performance requirements in a synapse-specific manner, remains to be established. Here, we examined performance-determining ultrastructural features of presynaptic mitochondria in GABAergic and glutamatergic axons of mice and human. Using electron-tomography and super-resolution microscopy we found, that these features were coupled to synaptic strength: mitochondria in boutons with high synaptic activity exhibited an ultrastructure optimized for high rate metabolism and contained higher levels of the respiratory chain protein cytochrome-c (CytC) than mitochondria in boutons with lower activity. The strong, cell type-independent correlation between mitochondrial ultrastructure, molecular fingerprints and synaptic performance suggests that changes in synaptic activity could trigger ultrastructural plasticity of presynaptic mitochondria, likely to adjust their performance to the actual metabolic demand.

Impact of Resistance Training on Skeletal Muscle Mitochondrial Biogenesis, Content, and Function

Skeletal muscle metabolic and contractile properties are reliant on muscle mitochondrial and myofibrillar protein turnover. The turnover of these specific protein pools is compromised during disease, aging, and inactivity. Oppositely, exercise can accentuate muscle protein turnover, thereby counteracting decay in muscle function. According to a traditional consensus, endurance exercise is required to drive mitochondrial adaptations, while resistance exercise is required to drive myofibrillar adaptations. However, concurrent practice of traditional endurance exercise and resistance exercise regimens to achieve both types of muscle adaptations is time-consuming, motivationally demanding, and contended to entail practice at intensity levels, that may not comply with clinical settings. It is therefore of principle interest to identify effective, yet feasible, exercise strategies that may positively affect both mitochondrial and myofibrillar protein turnover. Recently, reports indicate that traditional high-load resistance exercise can stimulate muscle mitochondrial biogenesis and mitochondrial respiratory function. Moreover, fatiguing low-load resistance exercise has been shown capable of promoting muscle hypertrophy and expectedly entails greater metabolic stress to potentially enhance mitochondrial adaptations. Consequently, fatiguing low-load resistance exercise regimens may possess the ability to stimulate muscle mitochondrial adaptations without compromising muscle myofibrillar accretion. However, the exact ability of resistance exercise to drive mitochondrial adaptations is debatable, not least due to some methodological challenges. The current review therefore aims to address the evidence on the effects of resistance exercise on skeletal muscle mitochondrial biogenesis, content and function. In prolongation, a perspective is taken on the specific potential of low-load resistance exercise on promoting mitochondrial adaptations.

Pronounced limb and fibre type differences in subcellular lipid droplet content and distribution in elite skiers before and after exhaustive exercise

KEY POINTS

Although lipid droplets in skeletal muscle are an important energy source during endurance exercise, our understanding of lipid metabolism in this context remains incomplete. Using transmission electron microscopy, two distinct subcellular pools of lipid droplets can be observed in skeletal muscle - one beneath the sarcolemma and the other between myofibrils. At rest, well-trained leg muscles of cross-country skiers contain 4- to 6-fold more lipid droplets than equally well-trained arm muscles, with a 3-fold higher content in type 1 than in type 2 fibres. During exhaustive exercise, lipid droplets between the myofibrils but not those beneath the sarcolemma are utilised by both type 1 and 2 fibres. These findings provide insight into compartmentalisation of lipid metabolism within skeletal muscle fibres.

ABSTRACT

Although the intramyocellular lipid pool is an important energy store during prolonged exercise, our knowledge concerning its metabolism is still incomplete. Here, quantitative electron microscopy was used to examine subcellular distribution of lipid droplets in type 1 and 2 fibres of the arm and leg muscles before and after 1 h of exhaustive exercise. Intermyofibrillar lipid droplets accounted for 85-97% of the total volume fraction, while the subsarcolemmal pool made up 3-15%. Before exercise, the volume fractions of intermyofibrillar and subsarcolemmal lipid droplets were 4- to 6-fold higher in leg than in arm muscles (P < 0.001). Furthermore, the volume fraction of intermyofibrillar lipid droplets was 3-fold higher in type 1 than in type 2 fibres (P < 0.001), with no fibre type difference in the subsarcolemmal pool. Following exercise, intermyofibrillar lipid droplet volume fraction was 53% lower (P = 0.0082) in both fibre types in arm, but not leg muscles. This reduction was positively associated with the corresponding volume fraction prior to exercise (R²  = 0.84, P < 0.0001). No exercise-induced change in the subsarcolemmal pool could be detected. These findings indicate clear differences in the subcellular distribution of lipid droplets in the type 1 and 2 fibres of well-trained arm and leg muscles, as well as preferential utilisation of the intermyofibrillar pool during prolonged exhaustive exercise. Apparently, the metabolism of lipid droplets within a muscle fibre is compartmentalised.

Cardiolipin content, linoleic acid composition, and tafazzin expression in response to skeletal muscle overload and unload stimuli

Cardiolipin (CL) is a unique mitochondrial phospholipid that, in skeletal muscle, is enriched with linoleic acid (18:2n6). Together, CL content and CL 18:2n6 composition are critical determinants of mitochondrial function. Skeletal muscle is comprised of slow and fast fibers that have high and low mitochondrial content, respectively. In response to overloading and unloading stimuli, these muscles undergo a fast-to-slow oxidative fiber type shift and a slow-to-fast glycolytic fiber type shift, respectively, with a concomitant change in mitochondrial content. Here, we examined changes in CL content and CL 18:2n6 composition under these conditions along with tafazzin (Taz) protein, which is a transacylase enzyme that generates CL lipids enriched with 18:2n6. Our results show that CL content, CL 18:2n6 composition, and Taz protein content increased with an overload stimulus in plantaris. Conversely, CL content and CL 18:2n6 composition was reduced with an unloaded stimulus in soleus. Interestingly, Taz protein was increased in the unloaded soleus, suggesting that Taz may provide some form of compensation for decreased CL content and CL 18:2n6 composition. Together, this study highlights the dynamic nature of CL and Taz in skeletal muscle, and future studies will examine the physiological significance behind the changes in CL content, CL 18:2n6 and Taz.