Differences in Type 2 Fiber Composition in the Vastus Lateralis and Gluteus Maximus of Patients with Hip Fractures.

BACKGRUOUND: Aging leads to sarcopenia, which is characterized by reduced muscle mass and strength. Many factors, including altered muscle protein turnover, diminished neuromuscular function, hormonal changes, systemic inflammation, and the structure and composition of muscle fibers, play a crucial role in age-related muscle decline. This study explored differences in muscle fiber types contributing to overall muscle function decline in aging, focusing on individuals with hip fractures from falls. METHODS: A pilot study at Chungnam National University Hospital collected muscle biopsies from hip fracture patients aged 20 to 80 undergoing surgical treatment. Muscle biopsies from the vastus lateralis and gluteus maximus were obtained during hip arthroplasty or internal fixation. Handgrip strength, calf and thigh circumference, and bone mineral density were evaluated in individuals with hip fractures from falls. We analyzed the relationships between each clinical characteristic and muscle fiber type. RESULTS: In total, 26 participants (mean age 67.9 years, 69.2% male) were included in this study. The prevalence of sarcopenia was 53.8%, and that of femoral and lumbar osteoporosis was 19.2% and 11.5%, respectively. Vastus lateralis analysis revealed an age-related decrease in type IIx fibers, a higher proportion of type IIa fibers in women, and an association between handgrip strength and type IIx fibers in men. The gluteus maximus showed no significant correlations with clinical parameters. CONCLUSION: This study identified complex associations between age, sex, handgrip strength, and muscle fiber composition in hip fracture patients, offering insights crucial for targeted interventions combating age-related muscle decline and improving musculoskeletal health.

Integrative Metabolomics, Enzymatic Activity, and Gene Expression Analysis Provide Insights into the Metabolic Profile Differences between the Slow-Twitch Muscle and Fast-Twitch Muscle of Pseudocaranx dentex.

The skeletal muscles of teleost fish encompass heterogeneous muscle types, termed slow-twitch muscle (SM) and fast-twitch muscle (FM), characterized by distinct morphological, anatomical, histological, biochemical, and physiological attributes, driving different swimming behaviors. Despite the central role of metabolism in regulating skeletal muscle types and functions, comprehensive metabolomics investigations focusing on the metabolic differences between these muscle types are lacking. To reveal the differences in metabolic characteristics between the SM and FM of teleost, we conducted an untargeted metabolomics analysis using Pseudocaranx dentex as a representative model and identified 411 differential metabolites (DFMs), of which 345 exhibited higher contents in SM and 66 in FM. KEGG enrichment analysis showed that these DFMs were enriched in the metabolic processes of lipids, amino acids, carbohydrates, purines, and vitamins, suggesting that there were significant differences between the SM and FM in multiple metabolic pathways, especially in the metabolism of energy substances. Furthermore, an integrative analysis of metabolite contents, enzymatic activity assays, and gene expression levels involved in ATP-PCr phosphate, anaerobic glycolysis, and aerobic oxidative energy systems was performed to explore the potential regulatory mechanisms of energy metabolism differences. The results unveiled a set of differential metabolites, enzymes, and genes between the SM and FM, providing compelling molecular evidence of the FM achieving a higher anaerobic energy supply capacity through the ATP-PCr phosphate and glycolysis energy systems, while the SM obtains greater energy supply capacity via aerobic oxidation. These findings significantly advance our understanding of the metabolic profiles and related regulatory mechanisms of skeletal muscles, thereby expanding the knowledge of metabolic physiology and ecological adaptation in teleost fish.

Levosimendan's Effects on Length-Dependent Activation in Murine Fast-Twitch Skeletal Muscle.

Levosimendan's calcium sensitizing effects in heart muscle cells are well established; yet, its potential impact on skeletal muscle cells has not been evidently determined. Despite controversial results, levosimendan is still expected to interact with skeletal muscle through off-target sites (further than troponin C). Adding to this debate, we investigated levosimendan's acute impact on fast-twitch skeletal muscle biomechanics in a length-dependent activation study by submersing single muscle fibres in a levosimendan-supplemented solution. We employed our MyoRobot technology to investigate the calcium sensitivity of skinned single muscle fibres alongside their stress-strain response in the presence or absence of levosimendan (100 µM). While control data are in agreement with the theory of length-dependent activation, levosimendan appears to shift the onset of the 'descending limb' of active force generation to longer sarcomere lengths without notably improving myofibrillar calcium sensitivity. Passive stretches in the presence of levosimendan yielded over twice the amount of enlarged restoration stress and Young's modulus in comparison to control single fibres. Both effects have not been described before and may point towards potential off-target sites of levosimendan.

Piperine enhances contractile force in slow- and fast-twitch muscle.

Piperine has been shown to bind to myosin and shift the distribution of conformational states of myosin molecules from the super-relaxed state to the disordered relaxed state. However, little is known about the implications for muscle force production and potential underlying mechanisms. Muscle contractility experiments were performed using isolated muscles and single fibres from rats and mice. The dose-response effect of piperine on muscle force was assessed at several stimulation frequencies. The potentiation of muscle force was also tested in muscles fatigued by eccentric contractions. Potential mechanisms of force potentiation were assessed by measuring Ca2+ levels during stimulation in enzymatically dissociated muscle fibres, while myofibrillar Ca2+ sensitivity was assessed in chemically skinned muscle fibres. Piperine caused a dose-dependent increase in low-frequency force with no effect on high-frequency force in both slow- and fast-twitch muscle, with similar relative increases in twitch force, rate of force development and relaxation rate. The potentiating effect of piperine on low-frequency force was reversible, and piperine partially recovered low-frequency force in fatigued muscle. Piperine had no effect on myoplasmic free [Ca2+] levels in mouse muscle fibres, whereas piperine substantially augmented the force response to submaximal levels of [Ca2+] in rat MyHCII fibres and MyHCI fibres along with a minor increase in maximum Ca2+-activated force. Piperine enhances low-frequency force production in both fast- and slow-twitch muscle. The effects are reversible and can counteract muscle fatigue. The primary underlying mechanism appears to be an increase in Ca2+ sensitivity. KEY POINTS: Piperine is a plant alkaloid derived from black pepper. It is known to bind to skeletal muscle myosin and enhance resting ATP turnover but its effects on contractility are not well known. We showed for the first time a piperine-induced force potentiation that was pronounced during low-frequency electrical stimulation of isolated muscles. The effect of piperine was observed in both slow and fast muscle types, was reversible, and could counteract the force decrements observed after fatiguing muscle contractions. Piperine treatment caused an increase in myofibrillar Ca2+ sensitivity in chemically skinned muscle fibres, while we observed no effect on intracellular Ca2+ concentrations during electrical stimulation in enzymatically dissociated muscle fibres.

Differences in thick filament activation in fast rodent skeletal muscle and slow porcine cardiac muscle.

There is a growing appreciation that regulation of muscle contraction requires both thin filament and thick filament activation in order to fully activate the sarcomere. The prevailing mechano-sensing model for thick filament activation was derived from experiments on fast-twitch muscle. We address the question whether, or to what extent, this mechanism can be extrapolated to the slow muscle in the hearts of large mammals, including humans. We investigated the similarities and differences in structural signatures of thick filament activation in porcine myocardium as compared to fast rat extensor digitorum longus (EDL) skeletal muscle under relaxed conditions and sub-maximal contraction using small angle X-ray diffraction. Thick and thin filaments were found to adopt different structural configurations under relaxing conditions, and myosin heads showed different changes in configuration upon sub-maximal activation, when comparing the two muscle types. Titin was found to have an X-ray diffraction signature distinct from those of the overall thick filament backbone, and its spacing change appeared to be positively correlated to the force exerted on the thick filament. Structural changes in fast EDL muscle were found to be consistent with the mechano-sensing model. In porcine myocardium, however, the structural basis of mechano-sensing is blunted suggesting the need for additional activation mechanism(s) in slow cardiac muscle. These differences in thick filament regulation can be related to their different physiological roles where fast muscle is optimized for rapid, burst-like, contractions, and the slow cardiac muscle in large mammalian hearts adopts a more finely tuned, graded response to allow for their substantial functional reserve. KEY POINTS: Both thin filament and thick filament activation are required to fully activate the sarcomere. Thick and thin filaments adopt different structural configurations under relaxing conditions, and myosin heads show different changes in configuration upon sub-maximal activation in fast extensor digitorum longus muscle and slow porcine cardiac muscle. Titin has an X-ray diffraction signature distinct from those of the overall thick filament backbone and this titin reflection spacing change appeared to be directly proportional to the force exerted on the thick filament. Mechano-sensing is blunted in porcine myocardium suggesting the need for additional activation mechanism(s) in slow cardiac muscle. Fast skeletal muscle is optimized for rapid, burst-like contractions, and the slow cardiac muscle in large mammalian hearts adopts a more finely tuned graded response to allow for their substantial functional reserve.

Stretch-shortening cycles protect against the age-related loss of power generation in rat single muscle fibres.

Aging is associated with impaired strength and power during isometric and shortening contractions, however, during lengthening (i.e., eccentric) contractions, strength is maintained. During daily movements, muscles undergo stretch-shortening cycles (SSCs). It is unclear whether the age-related maintenance of eccentric strength offsets age-related impairments in power generation during SSCs owing to the utilization of elastic energy or other cross-bridge based mechanisms. Here we investigated how aging influences SSC performance at the single muscle fibre level and whether performing active lengthening prior to shortening protects against age-related impairments in power generation. Single muscle fibres from the psoas major of young (∼8 months; n = 31 fibres) and old (∼32 months; n = 41 fibres) male F344BN rats were dissected and chemically permeabilized. Fibres were mounted between a force transducer and length controller and maximally activated (pCa 4.5). For SSCs, fibres were lengthened from average sarcomere lengths of 2.5 to 3.0 µm and immediately shortened back to 2.5 µm at both fast and slow (0.15 and 0.60 Lo/s) lengthening and shortening speeds. The magnitude of the SSC effect was calculated by comparing work and power during shortening to an active shortening contraction not preceded by active lengthening. Absolute isometric force was ∼37 % lower in old compared to young rat single muscle fibres, however, when normalized to cross-sectional area (CSA), there was no longer a significant difference in isometric force between age groups, meanwhile there was an ∼50 % reduction in absolute power in old as compared with young. We demonstrated that SSCs significantly increased power production (75-110 %) in both young and old fibres when shortening occurred at a fast speed and provided protection against power-loss with aging. Therefore, in older adults during everyday movements, power is likely 'protected' in part due to the stretch-shortening cycle as compared with isolated shortening contractions.

Grape seed proanthocyanidin extract promotes skeletal muscle fiber type transformation through modulation of cecal microbiota and enhanced butyric acid production.

The conversion of fast-twitch fibers into slow-twitch fibers within skeletal muscle plays a crucial role in improving physical stamina and safeguarding against metabolic disorders in individuals. Grape seed proanthocyanidin extract (GSPE) possesses numerous pharmacological and health advantages, effectively inhibiting the onset of chronic illnesses. However, there is a lack of research on the specific mechanisms by which GSPE influences muscle physiology and gut microbiota. This study aims to investigate the role of gut microbiota and their metabolites in GSPE regulation of skeletal muscle fiber type conversion. In this experiment, 54 male BALB/c mice were randomly divided into three groups: basal diet, basal diet supplemented with GSPE, and basal diet supplemented with GSPE and antibiotics. During the feeding period, glucose tolerance and forced swimming tests were performed. After euthanasia, samples of muscle and feces were collected for analysis. The results showed that GSPE increased the muscle mass and anti-fatigue capacity of the mice, as well as the expression of slow-twitch fibers. However, the beneficial effects of GSPE on skeletal muscle fibers disappeared after adding antibiotics to eliminate intestinal microorganisms, suggesting that GSPE may play a role by regulating intestinal microbial structure. In addition, GSPE increased the relative abundance of Blautia, Muribaculaceae, and Enterorhabdus, as well as butyrate production. Importantly, these gut microbes exhibited a significant positive correlation with the expression of slow-twitch muscle fibers. In conclusion, supplementation with GSPE can increase the levels of slow-twitch fibers by modulating the gut microbiota, consequently prolonging the duration of exercise before exhaustion. PRACTICAL APPLICATION: This research suggests that grape seed proanthocyanidin extract (GSPE) has potential applications in improving physical stamina and preventing metabolic disorders. By influencing the gut microbiota and increasing butyric acid production, GSPE contributes to the conversion of fast-twitch muscle fibers into slow-twitch fibers, thereby enhancing anti-fatigue capacity and exercise endurance. While further studies are needed, incorporating GSPE into dietary supplements or functional foods could support individuals seeking to optimize their exercise performance and overall metabolic health.

Type selective ablation of postnatal slow and fast fatigue-resistant motor neurons in mice induces late onset kinetic and postural tremor following fiber-type transition and myopathy.

Animals on Earth need to hold postures and execute a series of movements under gravity and atmospheric pressure. VAChT-Cre is a transgenic Cre driver mouse line that expresses Cre recombinase selectively in motor neurons of S-type (slow-twitch fatigue-resistant) and FR-type (fast-twitch fatigue-resistant). Sequential motor unit recruitment is a fundamental principle for fine and smooth locomotion; smaller-diameter motor neurons (S-type, FR-type) first contract low-intensity oxidative type I and type IIa muscle fibers, and thereafter larger-diameter motor neurons (FInt-type, FF-type) are recruited to contract high-intensity glycolytic type IIx and type IIb muscle fibers. To selectively eliminate S- and FR-type motor neurons, VAChT-Cre mice were crossbred with NSE-DTA mice in which the cytotoxic diphtheria toxin A fragment (DTA) was expressed in Cre-expressing neurons. The VAChT-Cre;NSE-DTA mice were born normally but progressively manifested various characteristics, including body weight loss, kyphosis, kinetic and postural tremor, and muscular atrophy. The progressive kinetic and postural tremor was remarkable from around 20 weeks of age and aggravated. Muscular atrophy was apparent in slow muscles, but not in fast muscles. The increase in motor unit number estimation was detected by electromyography, reflecting compensatory re-innervation by remaining FInt- and FF-type motor neurons to the orphaned slow muscle fibers. The muscle fibers gradually manifested fast/slow hybrid phenotypes, and the remaining FInt-and FF-type motor neurons gradually disappeared. These results suggest selective ablation of S- and FR-type motor neurons induces progressive muscle fiber-type transition, exhaustion of remaining FInt- and FF-type motor neurons, and late-onset kinetic and postural tremor in mice.

Effects of sporadic inclusion body myositis on skeletal muscle fibre type specific morphology and markers of regeneration and inflammation.

Sporadic inclusion body myositis (sIBM) is a subgroup of idiopathic inflammatory myopathies characterised by progressive muscle weakness and skeletal muscle inflammation. Quantitative data on the myofibre morphology in sIBM remains scarce. Further, no previous study has examined fibre type association of satellite cells (SC), myonuclei number, macrophages, capillaries, and myonuclear domain (MD) in sIBM patients. Muscle biopsies from sIBM patients (n = 18) obtained previously (NCT02317094) were included in the analysis for fibre type-specific myofibre cross-sectional area (mCSA), SCs, myonuclei and macrophages, myonuclear domain, and capillarisation. mCSA (p < 0.001), peripheral myonuclei (p < 0.001) and MD (p = 0.005) were higher in association with type 1 (slow-twitch) than type 2 (fast-twitch) fibres. Conversely, quiescent SCs (p < 0.001), centrally placed myonuclei (p = 0.03), M1 macrophages (p < 0.002), M2 macrophages (p = 0.013) and capillaries (p < 0.001) were higher at type 2 fibres compared to type 1 fibres. In contrast, proliferating (Pax7+/Ki67+) SCs (p = 0.68) were similarly associated with each fibre type. Type 2 myofibres of late-phase sIBM patients showed marked signs of muscle atrophy (i.e. reduced mCSA) accompanied by higher numbers of associated quiescent SCs, centrally placed myonuclei, macrophages and capillaries compared to type 1 fibres. In contrast, type 1 fibres were suffering from pathological enlargement with larger MDs as well as fewer nuclei and capillaries per area when compared with type 2 fibres. More research is needed to examine to which extent different therapeutic interventions including targeted exercise might alleviate these fibre type-specific characteristics and countermeasure their consequences in impaired functional performance.

circUSP13 facilitates the fast-to-slow myofiber shift via the MAPK/ERK signaling pathway in goat skeletal muscles.

Understanding how skeletal muscle fiber proportions are regulated is essential for understanding muscle function and improving the quality of mutton. While circular RNA (circRNA) has a critical function in myofiber type transformation, the specific mechanisms are not yet fully understood. Prior evidence indicates that circular ubiquitin-specific peptidase 13 (circUSP13) can promote myoblast differentiation by acting as a ceRNA, but its potential role in myofiber switching is still unknown. Herein, we found that circUSP13 enhanced slow myosin heavy chain (MyHC-slow) and suppressed MyHC-fast expression in goat primary myoblasts (GPMs). Meanwhile, circUSP13 evidently enhanced the remodeling of the mitochondrial network while inhibiting the autophagy of GPMs. We obtained fast-dominated myofibers, via treatment with rotenone, and further demonstrated the positive role of circUSP13 in the fast-to-slow transition. Mechanistically, activation of the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway significantly impaired the slow-to-fast shift in fully differentiated myotubes, which was restored by circUSP13 or IGF1 overexpression. In conclusion, circUSP13 promoted the fast-to-slow myofiber type transition through MAPK/ERK signaling in goat skeletal muscle. These findings provide novel insights into the role of circUSP13 in myofiber type transition and contribute to a better understanding of the genetic mechanisms underlying meat quality.

Affinity Purification-Mass Spectrometry and Single Fiber Physiology/Proteomics Reveals Mechanistic Insights of C18ORF25.

C18ORF25 was recently shown to be phosphorylated at S67 by AMP-activated protein kinase (AMPK) in the skeletal muscle, following acute exercise in humans. Phosphorylation was shown to improve the ex vivo skeletal muscle contractile function in mice, but our understanding of the molecular mechanisms is incomplete. Here, we profiled the interactome of C18ORF25 in mouse myotubes using affinity purification coupled to mass spectrometry. This analysis included an investigation of AMPK-dependent and S67-dependent protein/protein interactions. Several nucleocytoplasmic and contractile-associated proteins were identified, which revealed a subset of GTPases that associate with C18ORF25 in an AMPK- and S67 phosphorylation-dependent manner. We confirmed that C18ORF25 is localized to the nucleus and the contractile apparatus in the skeletal muscle. Mice lacking C18Orf25 display defects in calcium handling specifically in fast-twitch muscle fibers. To investigate these mechanisms, we developed an integrated single fiber physiology and single fiber proteomic platform. The approach enabled a detailed assessment of various steps in the excitation-contraction pathway including SR calcium handling and force generation, followed by paired single fiber proteomic analysis. This enabled us to identify >700 protein/phenotype associations and 36 fiber-type specific differences, following loss of C18Orf25. Taken together, our data provide unique insights into the function of C18ORF25 and its role in skeletal muscle physiology.

Transcriptome and Metabolome Profiling Provide New Insights into Disuse Muscle Atrophy in Chicken: The Potential Role of Fast-Twitch Muscle Fibers.

Disuse muscle atrophy is a disease caused by restricted activity, affecting human health and animal protein quality. While extensive research on its mechanism has been studied in mammals, comparatively little is known about this process in chickens, which are a significant source of protein for human consumption worldwide. Understanding the mechanisms underlying skeletal muscle atrophy in chickens is crucial for improving poultry health and productivity, as well as for developing strategies to mitigate muscle loss. In this study, two groups of chickens were subjected to limb immobilization for two and four weeks, respectively, in order to induce disuse muscle atrophy and uniformly sampled gastrocnemius muscle at the fourth week. A combined analysis of the transcriptome and metabolome was conducted to investigate the mechanisms of disuse-induced muscle atrophy. Through H&E staining and immunofluorescence, we found that, compared to slow-twitch muscle fibers, the fast-twitch muscle fibers showed a greater reduction in cross-sectional area in the immobilized leg, and were also the main driver of changes in cross-sectional area observed in the non-immobilized leg. Integrated analysis revealed that differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) were mainly enriched in pathways related to energy metabolism, such as fatty acid metabolism, oxidative phosphorylation (OXPHOS), and glycolysis. These results provide important insights for further research on disuse muscle atrophy.

Mechanism of post-tetanic depression of slow muscle fibres.

A brief tetanic stimulation has a very different effect on the subsequent isometric twitch force of fast and slow skeletal muscles. Fast muscle responds with an enhanced twitch force which doubles that of the pre-tetanic value, whereas slow muscle depresses the post-tetanic twitch by about 20%. Twitch potentiation of fast muscle has long been known to be due to myosin light chain 2 phosphorylation. It is proposed that post-tetanic twitch depression in slow muscle is due to the dephosphorylation of the slow isoform of the thick filament protein, myosin-binding protein-C, by Ca2+/calmodulin-activated phosphatase calcineurin, whilst its phosphorylation underlies the force enhancement due to ß-adrenergic stimulation in slow and fast muscle.

Fast and slow muscle fiber transcriptome dynamics with lifelong endurance exercise.

We investigated fast and slow muscle fiber transcriptome exercise dynamics among three groups of men: lifelong exercisers (LLE, n = 8, 74 ± 1 yr), old healthy nonexercisers (OH, n = 9, 75 ± 1 yr), and young exercisers (YE, n = 8, 25 ± 1 yr). On average, LLE had exercised ∼4 day·wk-1 for ∼8 h·wk-1 over 53 ± 2 years. Muscle biopsies were obtained pre- and 4 h postresistance exercise (3 × 10 knee extensions at 70% 1-RM). Fast and slow fiber size and function were assessed preexercise with fast and slow RNA-seq profiles examined pre- and postexercise. LLE fast fiber size was similar to OH, which was ∼30% smaller than YE (P < 0.05) with contractile function variables among groups, resulting in lower power in LLE (P < 0.05). LLE slow fibers were ∼30% larger and more powerful compared with YE and OH (P < 0.05). At the transcriptome level, fast fibers were more responsive to resistance exercise compared with slow fibers among all three cohorts (P < 0.05). Exercise induced a comprehensive biological response in fast fibers (P < 0.05) including transcription, signaling, skeletal muscle cell differentiation, and metabolism with vast differences among the groups. Fast fibers from YE exhibited a growth and metabolic signature, with LLE being primarily metabolic, and OH showing a strong stress-related response. In slow fibers, only LLE exhibited a biological response to exercise (P < 0.05), which was related to ketone and lipid metabolism. The divergent exercise transcriptome signatures provide novel insight into the molecular regulation in fast and slow fibers with age and exercise and suggest that the ∼5% weekly exercise time commitment of the lifelong exercisers provided a powerful investment for fast and slow muscle fiber metabolic health at the molecular level.NEW & NOTEWORTHY This study provides the first insights into fast and slow muscle fiber transcriptome dynamics with lifelong endurance exercise. The fast fibers were more responsive to exercise with divergent transcriptome signatures among young exercisers (growth and metabolic), lifelong exercisers (metabolic), and old healthy nonexercisers (stress). Only lifelong exercisers had a biological response in slow fibers (metabolic). These data provide novel insights into fast and slow muscle fiber health at the molecular level with age and exercise.

Effects of sclerostin injection on soleus and extensor digitorum longus muscle tissue in male mice.

Sclerostin, a potent inhibitor of the Wnt signaling pathway, plays a critical role in bone homeostasis. Evidence suggests that sclerostin may also be involved in crosstalk between other tissues, including muscle. This pilot study attempted to examine the effects of sclerostin on soleus and extensor digitorum longus (EDL) muscle tissue from male mice that were given continuous recombinant sclerostin injections for 4 weeks. A total of 48 10-week-old male C57BL/6J mice were assigned to be sedentary or perform 1 h treadmill running per day for 4 weeks and administered subcutaneous injections of either saline or recombinant sclerostin 5 days/week. Sclerostin injection led to a reduction in the soleus myosin heavy chain (MHC) I, MHC I/IIA, MHC IIA/X, and MHC IIB cross-sectional area (p < 0.05) with no exercise effects on these reductions. In contrast, there were no effects of sclerostin injections or exercise on the fast-twitch EDL muscle in terms of size, MHC protein, or markers of Wnt signaling. These findings provide preliminary evidence of sclerostin's endocrine role in muscle via decreases in myofiber cross-sectional area, which seems to be independent of fiber type but muscle type-specific. More studies, however, are needed to confirm these preliminary results.

Need for speed: Human fast-twitch mitochondria favor power over efficiency.

OBJECTIVE: Human skeletal muscle consists of a mixture of slow- and fast-twitch fibers with distinct capacities for contraction mechanics, fermentation, and oxidative phosphorylation. While the divergence in mitochondrial volume favoring slow-twitch fibers is well established, data on the fiber type-specific intrinsic mitochondrial function and morphology are highly limited with existing data mainly being generated in animal models. This highlights the need for more human data on the topic. METHODS: Here, we utilized THRIFTY, a rapid fiber type identification protocol to detect, sort, and pool fast- and slow-twitch fibers within 6 h of muscle biopsy sampling. Respiration of permeabilized fast- and slow-twitch fiber pools was then analyzed with high-resolution respirometry. Using standardized western blot procedures, muscle fiber pools were subsequently analyzed for control proteins and key proteins related to respiratory capacity. RESULTS: Maximal complex I+II respiration was 25% higher in human slow-twitch fibers compared to fast-twitch fibers. However, per mitochondrial volume, the respiratory rate of mitochondria in fast-twitch fibers was approximately 50% higher for complex I+II, which was primarily mediated through elevated complex II respiration. Furthermore, the abundance of complex II protein and proteins regulating cristae structure were disproportionally elevated in mitochondria of the fast-twitch fibers. The difference in intrinsic respiratory rate was not reflected in fatty acid-or complex I respiration. CONCLUSION: Mitochondria of human fast-twitch muscle fibers compensate for their lack of volume by substantially elevating intrinsic respiratory rate through increased reliance on complex II.

Azilsartan prevents muscle loss and fast- to slow-twitch muscle fiber shift in natural ageing sarcopenic rats.

Sarcopenia is a musculoskeletal disease that reduces muscle mass and strength in older individuals. The study investigates the effects of azilsartan (AZL) on skeletal muscle loss in natural sarcopenic rats. Male Sprague-Dawley rats aged 4-6 months and 18-21 months were selected as young-matched control and natural-aged (sarcopenic) rats, respectively. Rats were allocated into young and old control (YC and OC) and young and old AZL treatment (YT and OT) groups, which received vehicles and AZL (8 mg/kg, orally) for 6 weeks. Rats were then sacrificed after muscle function analysis. Serum and gastrocnemius (GN) muscles were isolated for further endpoints. AZL significantly improved muscle grip strength and antioxidant levels in sarcopenic rats. AZL also restored the levels of insulin, testosterone, and muscle biomarkers such as myostatin and creatinine kinase in sarcopenic rats. Furthermore, AZL treatment improved the cellular and ultrastructure of GN muscle and prevented the shift of type II (glycolytic) myofibers to type I (oxidative) myofibers. The results showed that AZL intervention restored protein synthesis in natural sarcopenic rats by increasing p-Akt-1 and decreasing muscle RING-finger protein-1 and tumor necrosis factor alpha immunoexpressions. In conclusion, the present findings showed that AZL could be an effective intervention in treating age-related muscle impairments.

The impact of life-long strength versus endurance training on muscle fiber morphology and phenotype composition in older men.

Aging is typically associated with decreased muscle strength and rate of force development (RFD), partly explained by motor unit remodeling due to denervation, and subsequent loss of fast-twitch type II myofibers. Exercise is commonly advocated to counteract this detrimental loss. However, it is unclear how life-long strength versus endurance training may differentially affect markers of denervation and reinnervation of skeletal myofibers and, in turn, affect the proportion and morphology of fast-twitch type II musculature. Thus, we compared fiber type distribution, fiber type grouping, and the prevalence of atrophic myofibers (≤1,494 µm2) in strength-trained (OS) versus endurance-trained (OE) master athletes and compared the results to recreationally active older adults (all >70 yr, OC) and young habitually active references (<30 yr, YC). Immunofluorescent stainings were performed on biopsy samples from vastus lateralis, along with leg press maximal strength and RFD measurements. OS demonstrated similar type II fiber distribution (OS: 52.0 ± 16.4%; YC: 51.1 ± 14.4%), fiber type grouping, maximal strength (OS: 170.0 ± 18.9 kg, YC: 151.0 ± 24.4 kg), and RFD (OS: 3,993 ± 894 N·s-1, YC: 3,470 ± 1,394 N·s-1) as young, and absence of atrophic myofibers (OS: 0.2 ± 0.7%; YC: 0.1 ± 0.4%). In contrast, OE and OC exhibited more atrophic fibers (OE: 1.2 ± 1.0%; OC: 1.1 ± 1.4%), more grouped fibers, and smaller proportion of type II fibers (OE: 39.3 ± 11.9%; OC: 35.0 ± 12.4%) than OS and YC (all P < 0.05). In conclusion, strength-trained master athletes were characterized by similar muscle morphology as young, which was not the case for recreationally active or endurance-trained old. These results indicate that strength training may preserve type II fibers with advancing age in older men, likely as a result of chronic use of high contractile force generation.NEW & NOTEWORTHY Aging is associated with loss of fast-twitch type II myofibers, motor unit remodeling, and grouping of myofibers. This study reveals, for the first time, that strength training preserves neural innervation of type II fibers, resulting in similar myofiber type distribution and grouping in life-long strength-trained master athletes as young moderately active adults. In contrast, life-long endurance-trained master athletes and recreationally active old adults demonstrated higher proportion of type I fibers accompanied by more marked grouping of type I myofibers, and more atrophic fibers compared with strength-trained master athletes and young individuals. Thus, strength training should be utilized as a training modality for preservation of fast-twitch musculature, maximal muscle strength, and rapid force capacity (RFD) with advancing age.

Inherited myogenic abilities in muscle precursor cells defined by the mitochondrial complex I-encoding protein.

Skeletal muscle comprises different muscle fibers, including slow- and fast-type muscles, and satellite cells (SCs), which exist in individual muscle fibers and possess different myogenic properties. Previously, we reported that myoblasts (MBs) from slow-type enriched soleus (SOL) had a high potential to self-renew compared with cells derived from fast-type enriched tibialis anterior (TA). However, whether the functionality of myogenic cells in adult muscles is attributed to the muscle fiber in which they reside and whether the characteristics of myogenic cells derived from slow- and fast-type fibers can be distinguished at the genetic level remain unknown. Global gene expression analysis revealed that the myogenic potential of MBs was independent of the muscle fiber type they reside in but dependent on the region of muscles they are derived from. Thus, in this study, proteomic analysis was conducted to clarify the molecular differences between MBs derived from TA and SOL. NADH dehydrogenase (ubiquinone) iron-sulfur protein 8 (Ndufs8), a subunit of NADH dehydrogenase in mitochondrial complex I, significantly increased in SOL-derived MBs compared with that in TA-derived cells. Moreover, the expression level of Ndufs8 in MBs significantly decreased with age. Gain- and loss-of-function experiments revealed that Ndufs8 expression in MBs promoted differentiation, self-renewal, and apoptosis resistance. In particular, Ndufs8 suppression in MBs increased p53 acetylation, followed by a decline in NAD/NADH ratio. Nicotinamide mononucleotide treatment, which restores the intracellular NAD+ level, could decrease p53 acetylation and increase myogenic cell self-renewal ability in vivo. These results suggested that the functional differences in MBs derived from SOL and TA governed by the mitochondrial complex I-encoding gene reflect the magnitude of the decline in SC number observed with aging, indicating that the replenishment of NAD+ is a possible approach for improving impaired cellular functions caused by aging or diseases.

The Ca2+/Calmodulin-dependent Calcineurin/NFAT Signaling Pathway in the Pathogenesis of Insulin Resistance in Skeletal Muscle.

Skeletal muscle is the tissue directly involved in insulin-stimulated glucose uptake. Glucose is the primary energy substrate for contracting muscles, and proper metabolism of glucose is essential for health. Contractile activity and the associated Ca2+signaling regulate functional capacity and muscle mass. A high concentration of Ca2+and the presence of calmodulin (CaM) leads to the activation of calcineurin (CaN), a protein with serine-threonine phosphatase activity. The signaling pathway linked with CaN and transcription factors like the nuclear factor of activated T cells (NFAT) is essential for skeletal muscle development and reprogramming of fast-twitch to slow-twitch fibers. CaN activation may promote metabolic adaptations in muscle cells, resulting in better insulin-stimulated glucose transport. The molecular mechanisms underlying the altered insulin response remain unclear. The role of the CaN/NFAT pathway in regulating skeletal muscle hypertrophy is better described than its involvement in the pathogenesis of insulin resistance. Thus, there are opportunities for future research in that field. This review presents the role of CaN/NFAT signaling and suggests the relationship with insulin-resistant muscles.