Fast-twitch Glycolytic Muscle Research Articles

383 Effect of Feeding Ractopamine to Finisher Lard-Type Mangalica Pigs on Longissimus Muscle Fiber Myosin-Heavy Chain Isoform Expression and Cross-Sectional Area

Abstract Heritage breeds such as Mangalica pigs are gaining popularity in US niche markets and currently sold as high-quality meat due to greater meat quality characteristics. Research has demonstrated that feeding the β-adrenergic agonist ractopamine hydrochloride (RAC), stimulates hypertrophic growth of type-2, fast-twitch glycolytic muscle fibers via mTOR signaling resulting in greater myofibrillar protein synthesis. Therefore, the current study evaluated the effect of feeding RAC to Mangalica pigs during the finisher phase on longissimus muscle fiber myosin heavy-chain (MHC) isoform expression and cross-sectional area. Mangalica pigs were randomly assigned finisher diets for 21 d with 20 ppm (RAC; n = 8) or without (CTL; n = 6). Longissimus muscle samples were collected during carcass fabrication, flash-frozen in liquid nitrogen and stored at -80 °C. Immunofluorescence staining of 5-µm-thick cryosections to identify Type-1 (Developmental Studies Hybridoma Bank (DSHB) antibody Ba-D5), Type-2a (DSHB SC-71), and 2x (DSHB BF-35) MHC positive fibers. Type 2b fibers were determined by subtraction. Fibers in each image (≥ 150) were assessed for fiber type and their cross-sectional area (CSA) at 200X magnification. Data were analyzed as a one-way ANOVA using the GLIMMIX procedure of SAS V9.4. Means were separated with the PDIFF option and declared different when P ≤ 0.05. Tendencies were declared when 0.0501 ≤ P ≤ 0.1000. Feeding RAC decreased the density of Type 2a (P = 0.0136) and Type 2b (P = 0.0186) fibers in the longissimus muscle of Mangalica pigs. A tendency for RAC to reduce the density of Type 1, slow-twitch (6.61 vs. 16.14 fiber per mm2; P = 0.0505) fibers and increase the density of primarily oxidative fibers expressing a combination of Type 1 + Type 2a MHC isoforms (10.23 vs. 2.88%; P = 0.0524) was also observed. When analyzed as a proportion of total fibers, RAC reduced the proportion of fibers expressing only Type 2a (11.8 vs. 6.70%; P = 0.0468) and Type 2b (0.19 vs. 1.32%; P = 0.0101) MHC isoforms but increased the percentage of fibers expressing both Type 1 + Type 2a MHC isoforms (4.66 vs. 1.13%; P = 0.0280). Feeding RAC did not impact the density or proportion of fibers expressing Type 2x MHC (P ≥ 0.386). Mean longissimus muscle fiber CSA was not impacted by feeding Mangalica pigs RAC for 21 d (P = 0.4815). The CSA of the smallest and largest fibers nor fiber distribution were influenced by RAC inclusion in the diet of Mangalica pigs (P ≥ 0.484). In conclusion, feeding RAC Mangalica pigs impacted fiber MHC isoform expression in the longissimus muscle without altering fiber CSA. Future studies will explore the impact of RAC on the CSA of the various fiber types as well as the impact on other muscles in the Mangalica pig.

Hydrogen Sulfide Regulates Glucose Uptake in Skeletal Muscles via S-Sulfhydration of AMPK in Muscle Fiber Type-Dependent Way

BackgroundThe effect of hydrogen sulfide (H2S) on glucose homeostasis remains to be elucidated, especially in the state of insulin resistance. ObjectivesIn the present study, we aimed to investigate H2S-regulated glucose uptake in the M. pectoralis major (PM) muscle (which mainly consists of fast-twitch glycolytic fibers) and M. biceps femoris (BF) muscle (which mainly consists of slow-twitch oxidative fibers) of the chicken, a potential model of insulin resistance. MethodsChicks were subjected to intraperitoneal injection of sodium hydrosulfide (NaHS, 50 μmol/kg body mass/day) twice a day to explore glucose homeostasis. In vitro, myoblasts from PM and BF muscles were used to detect glucose uptake and utilization. Effects of AMP-activated protein kinase (AMPK) phosphorylation, AMPK S-sulfhydration, and mitogen-activated protein kinase (MAPK) pathway induction by NaHS were detected. ResultsNaHS enhanced glucose uptake and utilization in chicks (P < 0.05). In myoblasts from PM muscle, NaHS (100 μM) increased glucose uptake by activating AMPK S-sulfhydration, AMPK phosphorylation, and the AMPK/p38 MAPK pathway (P < 0.05). However, NaHS decreased glucose uptake in myoblasts from BF muscle by suppressing the p38 MAPK pathway (P < 0.05). Moreover, NaHS increased S-sulfhydration and, in turn, the phosphorylation of AMPK (P < 0.05). ConclusionsThis study reveals the role of H2S in enhancing glucose uptake and utilization in chicks. The results suggest that NaHS is involved in glucose uptake in skeletal muscle in a fiber type-dependent way. The AMPK/p38 pathway and protein S-sulfhydration promote glucose uptake in fast-twitch glycolytic muscle fibers, which provides a muscle fiber-specific potential therapeutic target to ameliorate glucose metabolism.

Wavelet Analysis of Respiratory Muscle sEMG Signals during the Physiological Breakpoint of Static Dry End-Expiratory Breath-Holding in Naive Apneists: A Pilot Study.

The wavelet spectral characteristics of three respiratory muscle signals (scalenus (SC), parasternal intercostal (IC), and rectus abdominis (RA)) and one locomotor muscle (brachioradialis (BR)) were analyzed in the time-frequency (T-F) domain during voluntary breath-holding (BH), with a focus on the physiological breakpoint that is commonly considered the first involuntary breathing movement (IBM) that signals the end of the easy-going phase of BH. The study was performed for an end-expiratory BH physiological breaking point maneuver on twelve healthy, physically active, naive breath-holders/apneists (six professional athletes; six recreational athletes, and two individuals in the post-COVID-19 period) using surface electromyography (sEMG). We observed individual effects that were dependent on muscle oxygenation and each person's fitness, which were consistent with the mechanism of motor unit (MU) recruitment and the transition of slow-twitch oxidative (type 1) to fast-twitch glycolytic (type 2) muscle fibers. Professional athletes had longer BH durations (BHDs) and strong hypercapnic responses regarding the expiratory RA muscle, which is activated abruptly at higher BHDs in a person-specific range below 250 Hz and is dependent on the BHD. This is in contrast with recreational athletes, who had strong hypoxic responses regarding inspiratory IC muscle, which is activated faster and gradually in the frequency range of 250-450 Hz (independent of the person and BHD). This pilot study preliminarily indicates that it is possible to noninvasively assess the physiological characteristics of skeletal muscles, especially oxygenation, and improve physical fitness tests by determining the T-F features of elevated myoelectric IC and RA activity during BH.

Fecal transplant from myostatin deletion pigs positively impacts the gut-muscle axis.

The host genome may influence the composition of the intestinal microbiota, and the intestinal microbiota has a significant effect on muscle growth and development. In this study, we found that the deletion of the myostatin (MSTN) gene positively regulates the expression of the intestinal tight junction-related genes TJP1 and OCLN through the myosin light-chain kinase/myosin light chain pathway. The intestinal structure of MSTN-/- pigs differed from wild-type, including by the presence of a thicker muscularis and longer plicae. Together, these changes affect the structure of intestinal microbiota. Mice transplanted with the intestinal microbiota of MSTN-/- pigs had myofibers with larger cross-sectional areas and higher fast-twitch glycolytic muscle mass. Microbes responsible for the production of short-chain fatty acids (SCFAs) were enriched in both the MSTN-/- pigs and recipient mice, and SCFAs levels were elevated in the colon contents. We also demonstrated that valeric acid stimulates type IIb myofiber growth by activating the Akt/mTOR pathway via G protein-coupled receptor 43 and ameliorates dexamethasone-induced muscle atrophy. This is the first study to identify the MSTN gene-gut microbiota-SCFA axis and its regulatory role in fast-twitch glycolytic muscle growth.

Poor hygiene of housing conditions influences energy metabolism in a muscle type-dependent manner in growing pigs differing in feed efficiency

The ability of pigs to cope with inflammatory challenges may by modified by selection for residual feed intake (RFI), a measure of feed efficiency. In the current study, we evaluated skeletal muscle metabolic responses to degraded hygiene conditions in pigs divergently selected for RFI. At 82 d of age, low RFI and high RFI pigs were housed in either poor or good hygiene conditions. After a 6-week challenge, the poor hygiene conditions induced a decrease in growth performance (P < 0.001) and in plasma IGF-I concentrations (P < 0.003) in both lines. In the slow-twitch oxidative semispinalis muscle, poor hygiene conditions induced a shift towards a more oxidative metabolism and an activation of the AMPK pathway in pigs of both RFI lines. In the fast-twitch glycolytic longississimus muscle, poor hygiene conditions were associated to a less glycolytic metabolism in the HRFI line only. Poor hygiene conditions also increased the protein level of lipidation of microtubule-associated protein 1 light-chain 3β (LC3-II) in both RFI lines, suggesting an activation of the autophagy pathway. Altogether, the data revealed muscle-type specific metabolic adaptations to poor hygiene conditions, which may be related to different strategies to fuel the activated immune system.

L-Arginine/nitric oxide regulates skeletal muscle development via muscle fibre-specific nitric oxide/mTOR pathway in chickens.

L-Arginine (L-Arg), the precursor of nitric oxide (NO), plays an important role in muscle function. Fast-twitch glycolytic fibres are more susceptible to age-related atrophy than slow-twitch oxidative fibres. The effect of L-Arg/NO on protein metabolism of fast- and slow-twitch muscle fibres was evaluated in chickens. In Exp. 1, 48 chicks at 1 day old were divided into 4 groups of 12 birds and subjected to 4 treatments: basal diet without supplementation or supplemented with 1% L-Arg, and water supplemented with or without L-nitro-arginine methyl ester (L-NAME, 18.5 mM). In Exp. 2, 48 chicks were divided into 4 groups of 12 birds fed with the basal diet and subjected to the following treatments: tap water (control), tap water supplemented with L-NAME (18.5 mM), or molsidomine (MS, 0.1 mM), or 18.5 mM L-NAME + 0.1 mM MS (NAMS). The regulatory effect of L-Arg/NO was further investigated in vitro with myoblasts obtained from chicken embryo pectoralis major (PM) and biceps femoris (BF). In vivo, dietary L-Arg supplementation increased breast (+14.94%, P < 0.05) and thigh muscle mass (+23.40%, P < 0.05); whereas, MS treatment had no detectable influence. However, L-NAME treatment blocked the beneficial influence of L-Arg on muscle development. L-Arg decreased (P < 0.05) protein synthesis rate, phosphorylated mTOR and ribosomal protein S6 kinase beta-1 (p70S6K) levels in breast muscle, which was recovered by L-NAME treatment. In vitro, L-Arg or sodium nitroprusside (SNP) reduced protein synthesis rate, suppressed phosphorylated mTOR/p70S6K and decreased atrogin-1 and muscle RING finger 1 (MuRF1) in myoblasts from PM muscle (P < 0.05). L-NAME abolished the inhibitory effect of L-Arg on protein synthesis and the mTOR/p70S6K pathway. However, myoblasts from BF muscle showed the weak influence. Moreover, blocking the mTOR/p70S6K pathway with rapamycin suppressed protein synthesis of the 2 types of myoblasts; whereas, the protein expression of atrogin-1 and MuRF1 levels were restricted only in myoblasts from PM muscle. In conclusion, L-Arg/NO/mTOR/p70S6K pathway enhances protein accumulation and muscle development in fast-twitch glycolytic muscle in chickens. L-Arg/NO regulates protein turnover in a muscle fibre specific way, which highlights the potential clinical application in fast-twitch glycolytic muscle fibres.

Alteration of the Neuromuscular Junction and Modifications of Muscle Metabolism in Response to Neuron-Restricted Expression of the CHMP2Bintron5 Mutant in a Mouse Model of ALS-FTD Syndrome.

CHMP2B is a protein that coordinates membrane scission events as a core component of the ESCRT machinery. Mutations in CHMP2B are an uncommon cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), two neurodegenerative diseases with clinical, genetic, and pathological overlap. Different mutations have now been identified across the ALS-FTD spectrum. Disruption of the neuromuscular junction is an early pathogenic event in ALS. Currently, the links between neuromuscular junction functionality and ALS-associated genes, such as CHMP2B, remain poorly understood. We have previously shown that CHMP2B transgenic mice expressing the CHMP2Bintron5 mutant specifically in neurons develop a progressive motor phenotype reminiscent of ALS. In this study, we used complementary approaches (behavior, histology, electroneuromyography, and biochemistry) to determine the extent to which neuron-specific expression of CHMP2Bintron5 could impact the skeletal muscle characteristics. We show that neuronal expression of the CHMP2Bintron5 mutant is sufficient to trigger progressive gait impairment associated with structural and functional changes in the neuromuscular junction. Indeed, CHMP2Bintron5 alters the pre-synaptic terminal organization and the synaptic transmission that ultimately lead to a switch of fast-twitch glycolytic muscle fibers to more oxidative slow-twitch muscle fibers. Taken together these data indicate that neuronal expression of CHMP2Bintron5 is sufficient to induce a synaptopathy with molecular and functional changes in the motor unit reminiscent of those found in ALS patients.

Gene deletion of γ-actin impairs insulin-stimulated skeletal muscle glucose uptake in growing mice but not in mature adult mice.

The cortical cytoskeleton, consisting of the cytoplasmic actin isoforms β and/or γ‐actin, has been implicated in insulin‐stimulated GLUT4 translocation and glucose uptake in muscle and adipose cell culture. Furthermore, transgenic inhibition of multiple actin‐regulating proteins in muscle inhibits insulin‐stimulated muscle glucose uptake. The current study tested if γ‐actin was required for insulin‐stimulated glucose uptake in mouse skeletal muscle. Based on our previously reported age‐dependent phenotype in muscle‐specific β‐actin gene deletion (−/−) mice, we included cohorts of growing 8–14 weeks old and mature 18–32 weeks old muscle‐specific γ‐actin−/− mice or wild‐type littermates. In growing mice, insulin significantly increased the glucose uptake in slow‐twitch oxidative soleus and fast‐twitch glycolytic EDL muscles from wild‐type mice, but not γ‐actin−/−. In relative values, the maximal insulin‐stimulated glucose uptake was reduced by ~50% in soleus and by ~70% in EDL muscles from growing γ‐actin−/− mice compared to growing wild‐type mice. In contrast, the insulin‐stimulated glucose uptake responses in mature adult γ‐actin−/− soleus and EDL muscles were indistinguishable from the responses in wild‐type muscles. Mature adult insulin‐stimulated phosphorylations on Akt, p70S6K, and ULK1 were not significantly affected by genotype. Hence, insulin‐stimulated muscle glucose uptake shows an age‐dependent impairment in young growing but not in fully grown γ‐actin−/− mice, bearing phenotypic resemblance to β‐actin−/− mice. Overall, γ‐actin does not appear required for insulin‐stimulated muscle glucose uptake in adulthood. Furthermore, our data emphasize the need to consider the rapid growth of young mice as a potential confounder in transgenic mouse phenotyping studies.

Effects of metabolizable protein concentration, amino acid profile, and fiber source on the messenger RNA expression of skeletal muscle in peripartum dairy cows

After parturition, dairy cows mobilize AA from skeletal muscle to meet metabolizable protein (MP) requirements. High mobilization may compromise cow health and longer-term milk production. Postpartum diets with higher MP concentrations, improved AA profiles, or MP increased at the expense of forages rather than nonforage fiber sources may attenuate muscle catabolism; however, the molecular mechanisms responsible need investigation. We evaluated mRNA expression in the longissimus dorsi of cows fed postpartum diets differing in MP concentration, AA profile, and fiber source. From 0 to 25 d after parturition, 40 multiparous cows received the following diets: (1) 13% deficient in MP (D-MP), (2) adequate in MP using primarily soy protein (A-MP), (3) adequate in MP using blends of proteins and individual AA to improve the AA profile (Blend), or (4) similar to Blend except additional protein replaced forage (Blend-fNDF). Biopsies were taken approximately -5, 7, and 25 d relative to parturition. Greater dietary MP concentration (D-MP vs. A-MP and Blend) decreased expression of genes related to protein synthesis (MTOR, RPS6KB1) and degradation (FOXO1), inflammation (IFNG, TLR4), and endoplasmic reticulum (ER) stress (HSPA5, DDIT) and increased genes associated with lipogenesis (PPARG) and glucose oxidation (LDH, MB). In Blend versus A-MP (i.e., effect of AA profile), expression related to apoptosis (CASP8) and inflammation (TNFA) decreased and genes associated with cell cycle progression (E2F1) and fast-twitch glycolytic muscle fiber type (MYH4) increased. Less forage (Blend-fNDF vs. Blend) decreased genes associated with lipogenesis (PPARG, ACACA) and ER stress (BCL2, DDIT3, EIF2AK3, PPP1R15A) and increased genes associated with inflammation (TNF), inhibition of myogenesis (MSTN), and autophagy (PEBP1). In summary and based on mRNA expression, increasing MP supply may attenuate muscle turnover and ER stress. However, an unbalanced AA supply reduced cell cycle progression and protein synthesis. Lower energy supplies may reduce cell growth and cause autophagy.

Skeletal muscle proteome of piglets is affected in a muscle-dependent manner by a limiting total sulfur amino acid supply.

A total sulfur amino acid (TSAA) deficient diet can affect the amino acid composition of skeletal muscles. However, it is unknown how the different muscle proteins are affected by the TSAA deficiency. The proteomic profiles of the fast-twitch glycolytic longissimus (LM) and the slow-twitch oxidative rhomboideus (RM) muscles were compared in 42-day-old piglets fed either a 28% deficient (TSAA-) or a sufficient (TSAA+) diet in TSAA for 10 days. Differentially regulated proteins were identified and submitted to Gene Ontology Pathways Analysis to identify biological processes affected by TSAA deficiency. A total of 36 proteins in LM and 24 proteins in RM differed in abundance between the two dietary treatments. In both muscles, an increased oxidative energy metabolism was observed in TSAA- piglets. However, a greater mitochondrial oxidation of pyruvate generated from glycolysis was observed in LM of TSAA- piglets, whereas fatty acid β-oxidation and glycogen sparing were favored in RM. This suggests a muscle-specific reorientation of energy metabolism in response to a TSAA- deficiency. In both muscles, the protein abundance and enzyme activity of superoxide dismutase were increased in TSAA- piglets. Other enzymes involved in antioxidant defense, heat shock proteins coping with cellular stress, and annexins involved in the regulation of apoptosis were generally found to be more expressed in the LM of TSAA- piglets, with no or minor changes in RM. Skeletal muscle proteome in young growing piglets was modulated in a muscle-dependent manner by a deficient TSAA supply, with accentuated changes in fast-twitch glycolytic muscle.

From muscle to food: oxidative challenges and developmental anomalies in poultry breast muscle

The glycolytic capacity in the "white muscle fibers" found predominantly in broiler and turkey breast muscle have long been recognized as being key components contributing to optimal pH changes post-harvest and the quality of breast muscle products. The pectoralis major (PMAJ) is the predominant breast muscle and is almost exclusively comprised of fast-twitch glycolytic (anaerobic) type IIb muscle fibers (white). In nature, the pectoralis muscles are designed to meet the intense, periodic energy demands of flight in contrast to the continual, oxidative metabolism (aerobic) inherent to the type I slow twitch fibers (red) in the thigh and drum. Within the past decade, it has been recognized that increased breast muscle yield in modern broiler strains may have contributed to muscle myopathies characterized by visible white striations (white striping; WS) often accompanied by surface hemorrhages and areas of extreme firmness (woody breast; WB). The research to date has primarily addressed differences in muscle physiology and processing characteristics in "normal" and "abnormal" muscle samples from older broilers. One hypothesis is that the extensive accretion of protein in a poorly vascularized breast muscle overwhelms the capacity for coping with excessive production of reactive oxygen species and the term "oxidative stress" has been used to describe this condition. There are few recent reports in poultry, however, which have addressed the potential negative physiological effects imposed by multiple layers of fascia surrounding individual muscle fibers, fiber bundles, and intact muscles on circulation and oxidative stability. The objective of this review is to support our hypothesis that the extensive accretion of breast muscle in a poorly vascularized and tightly bound PMAJ could be creating a "compartment syndrome" like condition leading to the development of the WS and WB anomalies in heavy, high yielding birds. Recent gene expression data support this hypothesis.

Geranylgeraniol prevents statin-induced skeletal muscle fatigue without causing adverse effects in cardiac or vascular smooth muscle performance

The administration of geranylgeranyl pyrophosphate (GGPP) (or its precursor, geranylgeraniol [GGOH]) has been shown by several in vitro studies to be capable of abrogating statin-induced myotoxicity. Nonetheless, the potential of GGPP repletion to prevent statin-associated muscle symptoms (SAMS) in vivo is yet to be investigated. Therefore, this study aimed to evaluate the ability of GGOH to prevent SAMS in rodents. Female Wistar rats (12 weeks of age) were randomised to 1 of 4 treatment groups: control, control with GGOH, simvastatin or simvastatin with GGOH. Ex vivo assessment of force production was conducted in skeletal muscles of varying fiber composition. Ex vivo left ventricular performance and blood vessel function was also assessed to determine if the administration of GGOH caused adverse changes in these parameters. Statin administration was associated with reduced force production in fast-twitch glycolytic muscle, but coadministration with GGOH completely abrogated this effect. Additionally, GGOH improved the performance of muscles not adversely affected by simvastatin (ie, those with a greater proportion of slow-twitch oxidative fibers), and increased force production in the control animals. Neither control nor statin-treated rodents given GGOH exhibited adverse changes in cardiac function. Vascular relaxation was also maintained following treatment with GGOH. The findings of this study demonstrate that GGOH can prevent statin-induced skeletal muscle fatigue in rodents without causing adverse changes in cardiovascular function. Further studies to elucidate the exact mechanisms underlying the effects observed in this investigation are warranted.

Endogenous CSE/Hydrogen Sulfide System Regulates the Effects of Glucocorticoids and Insulin on Muscle Protein Synthesis.

Aims Insulin and glucocorticoids play crucial roles in skeletal muscle protein turnover. Fast-twitch glycolytic fibres are more susceptible to atrophy than slow-twitch oxidative fibres. Based on accumulating evidence, hydrogen sulfide (H2S) is a physiological mediator of this process. The regulatory effect of H2S on protein synthesis in fast-twitch fibres was evaluated. Results A NaHS (sodium hydrosulfide) injection simultaneously increased the diameter of M. pectoralis major (i.e., fast-twitch glycolytic fibres) and activated the mammalian target of the rapamycin (mTOR)/p70S6 kinase (p70S6K) pathway. Dexamethasone (DEX) inhibited protein synthesis, downregulated mTOR and p70S6K phosphorylation, and suppressed the expression of the cystathionine γ-lyase (CSE) protein in myoblasts. The precursor of H2S, L-cysteine, completely abolished the inhibitory effects of DEX. The CSE inhibitor DL-propargylglycine (PAG) completely abrogated the effects of RU486 on blocking the suppressive effects of DEX. The H2S donor NaHS increased the H2S concentrations and abrogated the inhibitory effects of DEX on protein synthesis. Insulin increased protein synthesis and upregulated CSE expression. However, PAG abrogated the stimulatory effects of insulin on protein synthesis and the activity of the mTOR/p70S6K pathway. Innovation These results demonstrated that CSE/H2S regulated protein synthesis in fast-twitch muscle fibres, and glucocorticoids and insulin regulated protein synthesis in an endogenous CSE/H2S system-dependent manner. Conclusions The results from the present study suggest that the endogenous CSE/H2S system regulates fast-twitch glycolytic muscle degeneration and regeneration.

Biological Background of Block Periodized Endurance Training: A Review.

Block periodized (BP) training is an innovative and prospective approach that is drawing increasing attention from coaching scientists and practitioners. However, its further dissemination and implementation demands serious scientific biological underpinnings. More specifically, the fundamental scientific concepts of homeostatic regulation, stress adaptation and the law of supercompensation determine the biological essence and content of appropriate block mesocycles, i.e., the accumulation, transmutation and realization cycles, respectively. Such a separation is intended to prevent conflicting physiological responses and provide a favorable interaction for training effects. Several studies have evaluated the metabolic effects of various training programs, and the superiority of the BP model has been confirmed in terms of significant gains of maximal oxygen uptake, maximal power output and positive trends in athletic performance. It was found that the endocrine status of athletes is strictly dependent on appropriate blocks such as voluminous extensive workloads combined with resistance training (accumulation), lower-volume intense training (transmutation), and event-specific precompetitive training (realization). Evidence from molecular biology indicates the major regulators that determine meaningful adaptive events within specific block mesocycles. Specifically, voluminous extensive accumulation blocks stimulate mitochondrial biogenesis and protein synthesis in slow-twitch muscle fibres, whereas lower-volume intense workloads of the transmutation blocks evoke adaptive modifications in fast-twitch glycolytic and oxidative-glycolytic muscle fibers. Furthermore, such a training program causes a remarkable elevation of myonuclear content in muscle fibers that enables athletes to regain previously acquired abilities. The precompetitive realization block produces accentuated expression of stress-related and myogenic genes that affect protein synthesis and increase muscle glycogen. In addition, such a program stimulates and increases the size, force and power of fast-twitch fibers.

Sarcolipin Signaling Promotes Mitochondrial Biogenesis and Oxidative Metabolism in Skeletal Muscle

SUMMARYThe major objective of this study was to understand the molecular basis of how sarcolipin uncoupling of SERCA regulates muscle oxidative metabolism. Using genetically engineered sarcolipin (SLN) mouse models and primary muscle cells, we demonstrate that SLN plays a crucial role in mitochondrial biogenesis and oxidative metabolism in muscle. Loss of SLN severely compromised muscle oxidative capacity without affecting fiber-type composition. Mice overexpressing SLN in fast-twitch glycolytic muscle reprogrammed mitochondrial phenotype, increasing fat utilization and protecting against high-fat dietinduced lipotoxicity. We show that SLN affects cytosolic Ca2+ transients and activates the Ca2+/ calmodulin-dependent protein kinase II (CamKII) and PGC1α axis to increase mitochondrial biogenesis and oxidative metabolism. These studies provide a fundamental framework for understanding the role of sarcoplasmic reticulum (SR)-Ca2+ cycling as an important factor in mitochondrial health and muscle metabolism. We propose that SLN can be targeted to enhance energy expenditure in muscle and prevent metabolic disease.

Alteration of muscle fiber characteristics and the AMPK-SIRT1-PGC-1α axis in skeletal muscle of growing pigs fed low-protein diets with varying branched-chain amino acid ratios.

There mainly exists four major myosin heavy chains (MyHC) (i.e., I, IIa, IIx, and IIb) in growing pigs. The current study aimed to explore the effects of low-protein diets supplemented with varying branched-chain amino acids (BCAAs) on muscle fiber characteristics and the AMPK-SIRT1-PGC-1α axis in skeletal muscles. Forty growing pigs (9.85 ± 0.35 kg) were allotted to 5 groups and fed with diets supplemented with varying leucine: isoleucine: valine ratios: 1:0.51:0.63 (20% crude protein, CP), 1:1:1 (17% CP), 1:0.75:0.75 (17% CP), 1:0.51:0.63 (17% CP), and 1:0.25:0.25 (17% CP), respectively. The skeletal muscles of different muscle fiber composition, that is, longissimus dorsi muscle (LM, a fast-twitch glycolytic muscle), biceps femoris muscle (BM, a mixed slow- and fast-twitch oxido-glycolytic muscle), and psoas major muscle (PM, a slow-twitch oxidative muscle) were collected and analyzed. Results showed that relative to the control group (1:0.51:0.63, 20% CP), the low-protein diets with the leucine: isoleucine: valine ratio ranging from 1:0.75:0.75 to 1:0.25:0.25 especially augmented the mRNA and protein abundance of MyHC I fibers in BM and lowered the mRNA abundance of MyHC IIb particularly in LM (P < 0.05), with a concurrent increase in the activation of AMPK and the mRNA abundance of SIRT and PGC-1α in BM (P < 0.05). The results reveal that low-protein diets supplemented with optimal BCAA ratio, i.e. 1:0.75:0.75-1:0.25:0.25, induce muscle more oxidative especially in oxido-glycolytic skeletal muscle of growing pigs. These effects are likely associated with the activation of the AMPK-SIRT1-PGC-1α axis.

Exercise myopathy: changes in myofibrils of fast-twitch muscle fibres.

The purpose of the present study was to determine the relationships between the changes of myofibrils in fast-twitch oxidative-glycolytic (type IIA) fibres and fast-twitch glycolytic (type IIB) muscle fibres, protein synthesis and degradation rate in exercise-induced myopathic skeletal muscle. Exhaustive exercise was used to induce myopathy in Wistar rats. Intensity of glycogenolysis in muscle fibres during exercise, protein synthesis rate, degradation rate and structural changes of myofibrils were measured using morphological and biochemical methods. Myofibril cross sectional area (CSA) in type IIA fibres decreased 33% and type IIB fibres 44%. Protein degradation rate increased in both type IIA and IIB fibres, 63% and 69% respectively in comparison with the control group. According to the intensity of glycogenolysis, fast oxidative-glycolytic fibres are recruited more frequently during overtraining. Myofibrils in both types of fast-twitch myopathic muscle fibres are significantly thinner as the result of more intensive protein degradation. Regeneration capacity according to the presence of satellite cells is higher in type IIA fibres than in type IIB fibres in myopathic muscle.

Impacts of the Callipyge mutation on ovine plasma metabolites and muscle fibre type.

The ovine Callipyge mutation causes postnatal muscle hypertrophy localized to the pelvic limbs and torso, as well as body leanness. The mechanism underpinning enhanced muscle mass is unclear, as is the systemic impact of the mutation. Using muscle fibre typing immunohistochemistry, we confirmed muscle specific effects and demonstrated that affected muscles had greater prevalence and hypertrophy of type 2X fast twitch glycolytic fibres and decreased representation of types 1, 2C, 2A and/or 2AX fibres. To investigate potential systemic effects of the mutation, proton NMR spectra of plasma taken from lambs at 8 and 12 weeks of age were measured. Multivariate statistical analysis of plasma metabolite profiles demonstrated effects of development and genotype but not gender. Plasma from Callipyge lambs at 12 weeks of age, but not 8 weeks, was characterized by a metabolic profile consistent with contributions from the affected hypertrophic fast twitch glycolytic muscle fibres. Microarray analysis of the perirenal adipose tissue depot did not reveal a transcriptional effect of the mutation in this tissue. We conclude that there is an indirect systemic effect of the Callipyge mutation in skeletal muscle in the form of changes of blood metabolites, which may contribute to secondary phenotypes such as body leanness.

The effects of Capn1 gene inactivation on skeletal muscle growth, development, and atrophy, and the compensatory role of other proteolytic systems.

Myofibrillar protein turnover is a key component of muscle growth and degeneration, requiring proteolytic enzymes to degrade the skeletal muscle proteins. The objective of this study was to investigate the role of the calpain proteolytic system in muscle growth development using μ-calpain knockout (KO) mice in comparison with control wild-type (WT) mice, and evaluate the subsequent effects of silencing this gene on other proteolytic systems. No differences in muscle development between genotypes were observed during the early stages of growth due to the up regulation of other proteolytic systems. The KO mice showed significantly greater m-calpain protein abundance (P < 0.01) and activity (P < 0.001), and greater caspase 3/7 activity (P < 0.05). At 30 wk of age, KO mice showed increased protein:DNA (P < 0.05) and RNA:DNA ratios (P < 0.01), greater protein content (P < 0.01) at the expense of lipid deposition (P < 0.05), and an increase in size and number of fast-twitch glycolytic muscle fibers (P < 0.05), suggesting that KO mice exhibit an increased capacity to accumulate and maintain protein in their skeletal muscle. Also, expression of proteins associated with muscle regeneration (neural cell adhesion molecule and myoD) were both reduced in the mature KO mice (P < 0.05 and P < 0.01, respectively), indicating less muscle regeneration and, therefore, less muscle damage. These findings indicate the concerted action of proteolytic systems to ensure muscle protein homeostasis in vivo. Furthermore, these data contribute to the existing evidence of the importance of the calpain system's involvement in muscle growth, development, and atrophy. Collectively, these data suggest that there are opportunities to target the calpain system to promote the growth and/or restoration of skeletal muscle mass.

Baf60c drives glycolytic metabolism in the muscle and improves systemic glucose homeostasis through Deptor-mediated Akt activation

A shift from oxidative to glycolytic metabolism has been associated with skeletal muscle insulin resistance in type 2 diabetes1–5. However, whether this metabolic switch is deleterious or adaptive remains controversial6–8, in part due to limited understanding of the regulatory network that directs the metabolic and contractile specification of fast-twitch glycolytic muscle. Here we show that BAF60c, a transcriptional cofactor enriched in fast-twitch muscle, promotes a switch from oxidative to glycolytic myofiber type through Deptor-mediated AKT activation. Muscle-specific transgenic expression of BAF60c activates a program of molecular, metabolic, and contractile changes characteristic of glycolytic muscle. In addition, BAF60c is required for maintaining glycolytic capacity in adult skeletal muscle in vivo. BAF60c expression is significantly decreased in skeletal muscle from obese mice. Unexpectedly, transgenic activation of the glycolytic muscle program by BAF60c protects mice from diet-induced insulin resistance and glucose intolerance. Further mechanistic studies revealed that Deptor is induced by the BAF60c/Six4 transcriptional complex and mediates activation of AKT and glycolytic metabolism by BAF60c in a cell-autonomous manner. This work defines a fundamental mechanism underlying the specification of fast glycolytic muscle and illustrates that the oxidative to glycolytic metabolic shift in skeletal muscle is potentially adaptive and beneficial in the diabetic state.