Leucine attenuates skeletal muscle wasting via inhibition of ubiquitin ligases

A critical discussion on the relationship between E3 ubiquitin ligases, protein degradation, and skeletal muscle wasting: it's not that simple

Ubiquitination is an important post-translational modification (PTM) for protein substrates, whereby ubiquitin is added to proteins through the coordinated activity of activating (E1), ubiquitin-conjugating (E2), and ubiquitin ligase (E3) enzymes. The E3s provide key functions in the recognition of specific protein substrates to be ubiquitinated and aid in determining their proteolytic or nonproteolytic fates, which has led to their study as indicators of altered cellular processes. MuRF1 and MAFbx/Atrogin-1 were two of the first E3 ubiquitin ligases identified as being upregulated in a range of different skeletal muscle atrophy models. Since their discovery, the expression of these E3 ubiquitin ligases has often been studied as a surrogate measure of changes to bulk protein degradation rates. However, emerging evidence has highlighted the dynamic and complex regulation of the ubiquitin proteasome system (UPS) in skeletal muscle and demonstrated that protein ubiquitination is not necessarily equivalent to protein degradation. These observations highlight the potential challenges of quantifying E3 ubiquitin ligases as markers of protein degradation rates or ubiquitin proteasome system (UPS) activation. This perspective examines the usefulness of monitoring E3 ubiquitin ligases for determining specific or bulk protein degradation rates in the settings of skeletal muscle atrophy. Specific questions that remain unanswered within the skeletal muscle atrophy field are also identified, to encourage the pursuit of new research that will be critical in moving forward our understanding of the molecular mechanisms that govern protein function and degradation in muscle.

Sarcopenia in youth

Recent research has revealed causes other than aging that may induce sarcopenia in young people, contrary to the long-studied age-dependent reduction in muscular mass and function. The risk of sarcopenia begins in early adulthood, resulting in exaggerated muscle dysfunction in later life. Despite its clinical significance, research on youth-onset sarcopenia is still in its infancy. Due to a paucity of epidemiologic data and standardized criteria for sarcopenia in youth, determining the prevalence of sarcopenia in the young population remains challenging. Based on the evidence, >1 in every 10 young adults of most ethnicities is estimated to have sarcopenia. This review summarizes the possible etiologies of sarcopenia in young populations, including metabolic syndrome, physical inactivity, inadequate nutrition, inherent and perinatal factors, vitamin D deficiency, endocrinopathy, an imbalance of gut microbiota, neuromuscular diseases, organ failure, malignancy, and other inflammatory disorders. This is the first review of the current knowledge on the importance, prevalence, diagnosis, and causes of sarcopenia in youth.

Glutamine and leucine administration attenuates muscle atrophy in sepsis

AIMS

This study investigated whether l-glutamine (Gln) and/or l-leucine (Leu) administration could attenuate muscle atrophy in a mouse model of cecal ligation and puncture (CLP)-induced sepsis.

MATERIALS AND METHODS

Septic mice were given a daily intraperitoneal injection of Gln, Leu, or Gln plus Leu, and mice were sacrificed on either day 1 or 4 after CLP. Blood and muscles were collected for analysis of amino acid contents and markers related to protein degradation, muscle regeneration, and protein synthesis.

KEY FINDINGS

Leu treatment alone increased both muscle mass and total muscle protein content on day 4 after CLP. Gln administration reduced muscular Gln contents on day 1 and enhanced plasma Gln levels on day 4. Higher plasma branched-chain amino acid (BCAA) abundances and lower muscular BCAA levels were observed in Leu-treated mice on day 4. Gln and Leu individually suppressed muscle expressions of the E3 ubiquitin ligase genes, Trim63 and Fbxo32, on day 4 after CLP. As to muscle expressions of myogenic genes, both Gln and Leu upregulated Myog expression on day 1, but Leu alone enhanced Myf5 gene expression, whereas Gln plus Leu increased MyoD and Myog expression levels on day 4. Akt/mammalian target of rapamycin (mTOR) signaling was only activated by Gln and Leu when individually administered.

SIGNIFICANCE

Gln and/or Leu administration reduces sepsis-induced muscle degradation and promotes myogenic gene expressions. Leu treatment alone had more-pronounced effects on maintaining muscle mass during sepsis. A combination of Gln and Leu failed to show synergistic effects on alleviating sepsis-induced muscle atrophy.

Glucose supplementation improves intestinal amino acid transport and muscle amino acid pool in pigs during chronic cold exposure

Mammals in northern regions chronically suffer from low temperatures during autumn-winter seasons. The aim of this study was to investigate the response of intestinal amino acid transport and the amino acid pool in muscle to chronic cold exposure via Min pig models (cold adaptation) and Yorkshire pig models (non-cold adaptation). Furthermore, this study explored the beneficial effects of glucose supplementation on small intestinal amino acid transport and amino acid pool in muscle of cold-exposed Yorkshire pigs. Min pigs (Exp. 1) and Yorkshire pigs (Exp. 2) were divided into a control group (17 °C, n = 6) and chronic cold exposure group (7 °C, n = 6), respectively. Twelve Yorkshire pigs (Exp. 3) were divided into a cold control group and cold glucose supplementation group (8 °C). The results showed that chronic cold exposure inhibited peptide transporter protein 1 (PepT1) and excitatory amino acid transporter 3 (EAAT3) expression in ileal mucosa and cationic amino acid transporter-1 (CAT-1) in the jejunal mucosa of Yorkshire pigs (P < 0.05). In contrast, CAT-1, PepT1 and EAAT3 expression was enhanced in the duodenal mucosa of Min pigs (P < 0.05). Branched amino acids (BCAA) in the muscle of Yorkshire pigs were consumed by chronic cold exposure, accompanied by increased muscle RING-finger protein-1 (MuRF1) and muscle atrophy F-box (atrogin-1) expression (P < 0.05). More importantly, reduced concentrations of dystrophin were detected in the muscle of Yorkshire pigs (P < 0.05). However, glycine concentration in the muscle of Min pigs was raised (P < 0.05). In the absence of interaction between chronic cold exposure and glucose supplementation, glucose supplementation improved CAT-1 expression in the jejunal mucosa and PepT1 expression in the ileal mucosa of cold-exposed Yorkshire pigs (P < 0.05). It also improved BCAA and inhibited MuRF1 and atrogin-1 expression in muscle (P < 0.05). Moreover, dystrophin concentration was improved by glucose supplementation (P < 0.05). In summary, chronic cold exposure inhibits amino acid absorption in the small intestine, depletes BCAA and promotes protein degradation in muscle. Glucose supplementation ameliorates the negative effects of chronic cold exposure on amino acid transport and the amino acid pool in muscle.

Transcription factor NRF2 as potential therapeutic target for preventing muscle wasting in aging chronic kidney disease patients

Increased muscle protein catabolism leading to muscle wasting is a prominent feature of the syndrome of protein-energy wasting (PEW) in patients with chronic kidney disease (CKD). PEW and muscle wasting are induced by factors such as inflammation, oxidative stress and metabolic acidosis that activate the ubiquitin-proteasome system, the main regulatory mechanism of skeletal muscle degradation. Whether deficiency of nuclear factor erythroid 2-related factor 2 (NRF2), which regulates expression of antioxidant proteins protecting against oxidative damage triggered by inflammation, may exacerbate PEW has yet to be examined in aging patients with CKD. This review focuses on the hypothesis that NRF2 is involved in the maintenance of muscle mass and explores whether sustained activation of NRF2 by non-pharmacological interventions using nutraceutical activators to improve redox homeostasis could be a plausible strategy to prevent skeletal muscle disorders, including muscle wasting, sarcopenia and frailty associated with PEW in aging CKD patients.

The influence of nutrients on mechanical overload-induced changes to skeletal muscle mRNA content

Mechanical overload and nutrients influence skeletal muscle phenotype, with the combination sometimes having a synergistic effect. Muscle phenotypes influenced by these stimuli are mediated in part by changes to the muscle mRNA signature. However, the mechanical overload-sensitive gene programs that are influenced by nutrients remain unclear. The purpose of this study was to identify mechanical overload-sensitive gene programs that are influenced by nutrients and identify potential transcription factors that may differentiate the change in mRNA in response to mechanical overload versus nutrients. Nutrient deprived 12-week-old male mice were randomized to remain fasted or allowed access to food. All mice underwent a single bout of unilateral high force contractions of the tibialis anterior (TA). Four hours post-contractions TA muscles were extracted and content of 12 contraction-sensitive mRNAs were analyzed. The mRNA content of genes associated with Transcription, PI3K-Akt Signaling Pathway, Z-Disc, Intracellular Signal Transduction, Cell Cycle, and Amino Acid Transport was altered by contractions without influence of nutrient consumption. Conversely, the mRNA content of genes associated with Transcription, Cell Cycle, FoxO Signaling Pathway, and Amino Acid Transport was altered by contractions with nutrition consumption influencing the change. We identified Signal transducer and activator of transcription 3 (STAT3) and Activator protein 1 (AP-1) as transcription factors common amongst mRNAs that were primarily altered by mechanical overload regardless of feeding. Overall, these data provide a deeper molecular basis for the specific muscle phenotypes exclusive to mechanical overload versus those regulated by the addition of nutrients.

Factors mediating spaceflight-induced skeletal muscle atrophy

Skeletal muscle atrophy is a well-known consequence of spaceflight. Because of the potential significant impact of muscle atrophy and muscle dysfunction on astronauts and to their mission, a thorough understanding of the mechanisms of this atrophy and the development of effective countermeasures is critical. Spaceflight-induced muscle atrophy is similar to atrophy seen in many terrestrial conditions, and therefore our understanding of this form of atrophy may also contribute to the treatment of atrophy in humans on Earth. The unique environmental features humans encounter in space include the weightlessness of microgravity, space radiation, and the distinctive aspects of living in a spacecraft. The disuse and unloading of muscles in microgravity are likely the most significant factors that mediate spaceflight-induced muscle atrophy, and have been extensively studied and reviewed. However, there are numerous other direct and indirect effects on skeletal muscle that may be contributing factors to the muscle atrophy and dysfunction seen as a result of spaceflight. This review offers a novel perspective on the issue of muscle atrophy in space by providing a comprehensive overview of the unique aspects of the spaceflight environment and the various ways in which they can lead to muscle atrophy. We systematically review the potential contributions of these different mechanisms of spaceflight-induced atrophy and include findings from both actual spaceflight and ground-based models of spaceflight in humans, animals, and in vitro studies.

Nutritional Considerations for Injury Prevention and Recovery in Combat Sports

Sports participation is not without risk, and most athletes incur at least one injury throughout their careers. Combat sports are popular all around the world, and about one-third of their injuries result in more than 7 days of absence from competition or training. The most frequently injured body regions are the head and neck, followed by the upper and lower limbs, while the most common tissue types injured are superficial tissues and skin, followed by ligaments and joint capsules. Nutrition has significant implications for injury prevention and enhancement of the recovery process due to its effect on the overall physical and psychological well-being of the athlete and improving tissue healing. In particular, amino acid and protein intake, antioxidants, creatine, and omega-3 are given special attention due to their therapeutic roles in preventing muscle loss and anabolic resistance as well as promoting injury healing. The purpose of this review is to present the roles of various nutritional strategies in reducing the risk of injury and improving the treatment and rehabilitation process in combat sports. In this respect, nutritional considerations for muscle, joint, and bone injuries as well as sports-related concussions are presented. The injury risk associated with rapid weight loss is also discussed. Finally, preoperative nutrition and nutritional considerations for returning to a sport after rehabilitation are addressed.

Regulation of Glucose Metabolism by MuRF1 and Treatment of Myopathy in Diabetic Mice with Small Molecules Targeting MuRF1

The muscle-specific ubiquitin ligase MuRF1 regulates muscle catabolism during chronic wasting states, although its roles in general metabolism are less-studied. Here, we metabolically profiled MuRF1-deficient knockout mice. We also included knockout mice for MuRF2 as its closely related gene homolog. MuRF1 and MuRF2-KO (knockout) mice have elevated serum glucose, elevated triglycerides, and reduced glucose tolerance. In addition, MuRF2-KO mice have a reduced tolerance to a fat-rich diet. Western blot and enzymatic studies on MuRF1-KO skeletal muscle showed perturbed FoxO-Akt signaling, elevated Akt-Ser-473 activation, and downregulated oxidative mitochondrial metabolism, indicating potential mechanisms for MuRF1,2-dependent glucose and fat metabolism regulation. Consistent with this, the adenoviral re-expression of MuRF1 in KO mice normalized Akt-Ser-473, serum glucose, and triglycerides. Finally, we tested the MuRF1/2 inhibitors MyoMed-205 and MyoMed-946 in a mouse model for type 2 diabetes mellitus (T2DM). After 28 days of treatment, T2DM mice developed progressive muscle weakness detected by wire hang tests, but this was attenuated by the MyoMed-205 treatment. While MyoMed-205 and MyoMed-946 had no significant effects on serum glucose, they did normalize the lymphocyte-granulocyte counts in diabetic sera as indicators of the immune response. Thus, small molecules directed to MuRF1 may be useful in attenuating skeletal muscle strength loss in T2DM conditions.

Ubiquitin Ligases at the Heart of Skeletal Muscle Atrophy Control

Skeletal muscle loss is a detrimental side-effect of numerous chronic diseases that dramatically increases mortality and morbidity. The alteration of protein homeostasis is generally due to increased protein breakdown while, protein synthesis may also be down-regulated. The ubiquitin proteasome system (UPS) is a master regulator of skeletal muscle that impacts muscle contractile properties and metabolism through multiple levers like signaling pathways, contractile apparatus degradation, etc. Among the different actors of the UPS, the E3 ubiquitin ligases specifically target key proteins for either degradation or activity modulation, thus controlling both pro-anabolic or pro-catabolic factors. The atrogenes MuRF1/TRIM63 and MAFbx/Atrogin-1 encode for key E3 ligases that target contractile proteins and key actors of protein synthesis respectively. However, several other E3 ligases are involved upstream in the atrophy program, from signal transduction control to modulation of energy balance. Controlling E3 ligases activity is thus a tempting approach for preserving muscle mass. While indirect modulation of E3 ligases may prove beneficial in some situations of muscle atrophy, some drugs directly inhibiting their activity have started to appear. This review summarizes the main signaling pathways involved in muscle atrophy and the E3 ligases implicated, but also the molecules potentially usable for future therapies.

Leucine Supplementation Decreases HDAC4 Expression and Nuclear Localization in Skeletal Muscle Fiber of Rats Submitted to Hindlimb Immobilization

In this study we surveyed a rat skeletal muscle RNA-Seq for genes that are induced by hindlimb immobilization and, in turn, become attenuated by leucine supplementation. This approach, in search of leucine-atrophy protection mediating genes, identified histone deacetylase 4 (HDAC4) as highly responsive to both hindlimb immobilization and leucine supplementation. We then examined the impact of leucine on HDAC4 expression, tissue localization, and target genes. A total of 76 male Wistar rats (~280 g) were submitted to hindlimb immobilization and/or leucine supplementation for 3, 7 and 12 days. These animals were euthanized, and soleus muscle was removed for further analysis. RNA-Seq analysis of hindlimb immobilized rats indicated a sharp induction (log2 = 3.4) of HDAC4 expression which was attenuated by leucine supplementation (~50%). Real-time PCR and protein expression analysis by Western blot confirmed increased HDAC4 mRNA after 7 days of hindlimb immobilization and mitigation of induction by leucine supplementation. Regarding the HDAC4 localization, the proportion of positive nuclei was higher in the immobilized group and decreased after leucine supplementation. Also, we found a marked decrease of myogenin and MAFbx-atrogin-1 mRNA levels upon leucine supplementation, while CAMKII and DACH2 mRNA levels were increased by leucine supplementation. Our data suggest that HDAC4 inhibition might be involved in the anti-atrophic effects of leucine.

Skeletal Muscle Anti-Atrophic Effects of Leucine Involve Myostatin Inhibition

Lack of mechanical load leads to skeletal muscle atrophy, and one major underlying mechanism involves the myostatin pathway that negatively regulates protein synthesis and also activates Atrogin-1/MAFbx and MuRF1 genes. In hindlimb immobilization, leucine was observed to attenuate the upregulation of the referred atrogenes, thereby shortening the impact on fiber cross-sectional area, nonetheless, the possible connection with myostatin is still elusive. This study sought to verify the impact of leucine supplementation on myostatin expression. Male Wistar rats were supplemented with leucine and hindlimb immobilized for 3 and 7 days, after which soleus muscles were removed for morphometric measurements and analyzed for gene and protein expression by real-time PCR and Western blotting, respectively. Muscle wasting was prominent 7 days after immobilization, as expected, leucine feeding mitigated this effect. Atrogin-1/MAFbx gene expression was upregulated only after 3 days of immobilization, and this effect was attenuated by leucine supplementation. Atrogin-1/MAFbx protein levels were elevated after 7 days of immobilization, which leucine supplementation was not able to lessen. On the other hand, myostatin gene expression was upregulated in immobilization for 3 and 7 days, which returned to normal levels after leucine supplementation. Myostatin protein levels followed gene expression at a 3-day time point only. Follistatin gene expression was upregulated during immobilization and accentuated by leucine after 3 days of supplementation. Concerning protein expression, follistatin was not altered neither by immobilization nor in immobilized animals treated with leucine. In conclusion, leucine protects against skeletal muscle mass loss during disuse, and the underlying molecular mechanisms appear to involve myostatin inhibition and Atrogin-1 normalization independently of follistatin signaling.

Small-Molecule Chemical Knockdown of MuRF1 in Melanoma Bearing Mice Attenuates Tumor Cachexia Associated Myopathy

: Patients with malignant tumors frequently suffer during disease progression from a syndrome referred to as cancer cachexia (CaCax): CaCax includes skeletal muscle atrophy and weakness, loss of bodyweight, and fat tissues. Currently, there are no FDA (Food and Drug Administration) approved treatments available for CaCax. Here, we studied skeletal muscle atrophy and dysfunction in a murine CaCax model by injecting B16F10 melanoma cells into mouse thighs and followed mice during melanoma outgrowth. Skeletal muscles developed progressive weakness as detected by wire hang tests (WHTs) during days 13-23. Individual muscles analyzed at day 24 had atrophy, mitochondrial dysfunction, augmented metabolic reactive oxygen species (ROS) stress, and a catabolically activated ubiquitin proteasome system (UPS), including upregulated MuRF1. Accordingly, we tested as an experimental intervention of recently identified small molecules, Myomed-205 and -946, that inhibit MuRF1 activity and MuRF1/MuRF2 expression. Results indicate that MuRF1 inhibitor fed attenuated induction of MuRF1 in tumor stressed muscles. In addition, the compounds augmented muscle performance in WHTs and attenuated muscle weight loss. Myomed-205 and -946 also rescued citrate synthase and complex-1 activities in tumor-stressed muscles, possibly suggesting that mitochondrial-metabolic and muscle wasting effects in this CaCax model are mechanistically connected. Inhibition of MuRF1 during tumor cachexia may represent a suitable strategy to attenuate skeletal muscle atrophy and dysfunction.

MuRF1/TRIM63, Master Regulator of Muscle Mass

The E3 ubiquitin ligase MuRF1/TRIM63 was identified 20 years ago and suspected to play important roles during skeletal muscle atrophy. Since then, numerous studies have been conducted to decipher the roles, molecular mechanisms and regulation of this enzyme. This revealed that MuRF1 is an important player in the skeletal muscle atrophy process occurring during catabolic states, making MuRF1 a prime candidate for pharmacological treatments against muscle wasting. Indeed, muscle wasting is an associated event of several diseases (e.g., cancer, sepsis, diabetes, renal failure, etc.) and negatively impacts the prognosis of patients, which has stimulated the search for MuRF1 inhibitory molecules. However, studies on MuRF1 cardiac functions revealed that MuRF1 is also cardioprotective, revealing a yin and yang role of MuRF1, being detrimental in skeletal muscle and beneficial in the heart. This review discusses data obtained on MuRF1, both in skeletal and cardiac muscles, over the past 20 years, regarding the structure, the regulation, the location and the different functions identified, and the first inhibitors reported, and aim to draw the picture of what is known about MuRF1. The review also discusses important MuRF1 characteristics to consider for the design of future drugs to maintain skeletal muscle mass in patients with different pathologies.

The Effect of Leucine-Enriched Essential Amino Acid Supplementation on Anabolic and Catabolic Signaling in Human Skeletal Muscle after Acute Resistance Exercise: A Randomized, Double-Blind, Placebo-Controlled, Parallel-Group Comparison Trial

Resistance exercise transiently activates anabolic and catabolic systems in skeletal muscle. Leucine-enriched essential amino acids (LEAAs) are reported to stimulate the muscle anabolic response at a lower dose than whey protein. However, little is known regarding the effect of LEAA supplementation on the resistance exercise-induced responses of the anabolic and catabolic systems. Here, we conducted a randomized, double-blind, placebo-controlled, parallel-group comparison trial to investigate the effect of LEAA supplementation on mechanistic target of rapamycin complex 1 (mTORC1), the ubiquitin-proteasome system and inflammatory cytokines after a single bout of resistance exercise in young men. A total of 20 healthy young male subjects were supplemented with either 5 g of LEAA or placebo, and then they performed 10 reps in three sets of leg extensions and leg curls (70% one-repetition maximum). LEAA supplementation augmented the phosphorylation of mTOR^Ser2448 (+77.1%, p < 0.05), p70S6K^Thr389 (+1067.4%, p < 0.05), rpS6^Ser240/244 (+171.3%, p < 0.05) and 4EBP1^Thr37/46 (+33.4%, p < 0.05) after resistance exercise. However, LEAA supplementation did not change the response of the ubiquitinated proteins, MuRF-1 and Atrogin-1 expression. Additionally, the mRNA expression of IL-1β and IL-6 did not change. These data indicated that LEAA supplementation augments the effect of resistance exercise by enhancing mTORC1 signal activation after exercise.

Nutritional interventions during bed rest and spaceflight: prevention of muscle mass and strength loss, bone resorption, glucose intolerance, and cardiovascular problems

Bed rest is necessary for many medical conditions but also used as a ground-based model for space flight (along with head-down tilt to simulate fluid shifts in microgravity). The purpose of this review is to examine nutritional interventions during bed rest and spaceflight for prevention of muscle and strength loss, glucose intolerance, bone resorption, and cardiovascular problems. Increased dietary protein intake and supplementation with amino acids, β-hydroxy-β-methylbutyrate, or cofactors with antioxidant properties are effective for ameliorating bed rest-induced loss of muscle mass and strength. Previous literature involving bed rest with dietary protein/amino acid supplementation had mixed findings, likely due to differences in dosage. Although high protein intake in some studies prevents bed rest-induced muscle loss, it also increases bone resorption. High calcium intake and vitamin D supplementation are not beneficial for preventing bone degradation during bed rest or spaceflight. Very few studies investigated countermeasures to prevent glucose intolerance and cardiovascular risks during bed rest/spaceflight. Low-glycemic index diets might be beneficial for the prevention of bed rest-induced glucose intolerance and cardiovascular problems. The present evidence warrants additional studies on the exact threshold of protein/amino acid intake to prevent the loss of muscle mass and strength during bed rest/spaceflight specifically to maintain the beneficial effects of proteins on muscle mass and function without increasing bone resorption. Furthermore, it is suggested to study the effects of vitamin K supplementation on bone health during bed rest/spaceflight and determine the role of long-term low-glycemic index diets on glucose regulation and cardiovascular health during extended bed rest.

Emerging Strategies Targeting Catabolic Muscle Stress Relief

Skeletal muscle wasting represents a common trait in many conditions, including aging, cancer, heart failure, immobilization, and critical illness. Loss of muscle mass leads to impaired functional mobility and severely impedes the quality of life. At present, exercise training remains the only proven treatment for muscle atrophy, yet many patients are too ill, frail, bedridden, or neurologically impaired to perform physical exertion. The development of novel therapeutic strategies that can be applied to an in vivo context and attenuate secondary myopathies represents an unmet medical need. This review discusses recent progress in understanding the molecular pathways involved in regulating skeletal muscle wasting with a focus on pro-catabolic factors, in particular, the ubiquitin-proteasome system and its activating muscle-specific E3 ligase RING-finger protein 1 (MuRF1). Mechanistic progress has provided the opportunity to design experimental therapeutic concepts that may affect the ubiquitin-proteasome system and prevent subsequent muscle wasting, with novel advances made in regards to nutritional supplements, nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) inhibitors, myostatin antibodies, β₂ adrenergic agonists, and small-molecules interfering with MuRF1, which all emerge as a novel in vivo treatment strategies for muscle wasting.

Testosterone suppression does not exacerbate disuse atrophy and impairs muscle recovery that is not rescued by high protein

Androgen deprivation therapy (ADT) decreases muscle mass, force, and physical activity levels, but it is unclear whether disuse atrophy and testosterone suppression are additive. Additionally, conflicting reports exist on load-mediated hypertrophy during ADT and if protein supplementation offsets these deficits. This study sought to determine the role of testosterone suppression and a high-protein diet on 1) immobilization-induced atrophy and 2) muscle regrowth during reloading. Eight-week-old male Fischer 344 rats underwent sham surgery (Sham), castration surgery (ORX), or ORX and a high-casein diet supplemented with branched-chain amino acids (BCAA) (ORX+CAS/AA) followed by 10 days of unilateral immobilization (IMM) and 0, 6, or 14 days of reloading. With IMM, body mass gains were ~8% greater than ORX and ORX+CAS/AA that increased to 15% during reloading (both P < 0.01). IMM reduced muscle mass by 11-34% (all P < 0.01) and extensor digitorum longus and soleus (SOL) force by 21% and 49% (both P < 0.01), respectively, with no group differences. During reloading, castration reduced gastrocnemius mass (~12%) at 6 days and SOL mass (~20%) and SOL force recovery (~46%) at 14 days relative to Sham (all P < 0.05). Specific force reduced castration deficits, indicating that muscle atrophy was a key contributor. IMM decreased SOL cross-sectional area by 30.3% (P < 0.001), with a trend for reduced regrowth in ORX and ORX+CAS/AA following reloading (P = 0.083). Castration did not exacerbate disuse atrophy but may impair recovery of muscle function, with no benefit from a CAS/AA diet during reloading. Examining functional outcomes in addition to muscle mass during dietary interventions provides novel insights into muscle regrowth during ADT.NEW & NOTEWORTHY Low testosterone levels during skeletal muscle disuse did not worsen declines in muscle mass and function, although hypogonadism may attenuate recovery during subsequent reloading. Diets high in casein did not improve outcomes during immobilization or reloading. Practical strategies are needed that do not compromise caloric intake yet provide effective protein doses to augment these adverse effects.

Muscle Disuse Atrophy Caused by Discord of Intracellular Signaling

Significance: Regular contractile activity plays a critical role in maintaining skeletal muscle morphological integrity and physiological function. If the muscle is forced to stop contraction, such as during limb immobilization (IM), the IGF/Akt/mTOR signaling pathway that normally stimulates protein synthesis and inhibits proteolysis will be suppressed, whereas the FoxO-controlled catabolic pathways such as ubiquitin-proteolysis and autophagy/mitophagy will be activated and dominate, resulting in muscle fiber atrophy. Recent Advances: Mitochondria occupy a central position in the regulation of both protein synthesis and degradation through several redox-sensitive pathways, including peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), mitochondrial fusion and fission proteins, mitophagy, and sirtuins. Prolonged IM downregulates PGC-1α due to AMPK (5'-AMP-activated protein kinase) and FoxO activation, thus decreasing mitochondrial biogenesis and causing oxidative damage. Decrease of mitochondrial inner membrane potential and increase of mitochondrial fission can trigger cascades of mitophagy leading to loss of mitochondrial homeostasis (mitostasis), inflammation, and apoptosis. The phenotypic outcomes of these disorders are compromised muscle function and fiber atrophy. Critical Issues: Given the molecular mechanism of the pathogenesis, it is imperative that the integrity of intracellular signaling be restored to prevent the deterioration. So far, overexpression of PGC-1α via transgene and in vivo DNA transfection has been found to be effective in ameliorating mitostasis and reduces IM-induced muscle atrophy. Nutritional supplementation of select amino acids and phytochemicals also provides mechanistic and practical insights into the prevention of muscle disuse atrophy. Future Directions: In light of the importance of mitochondria in regulating the various critical signaling pathways, future work should focus on exploring new epigenetic strategies to restore mitostasis and redox balance.

Exercise Mitigates the Loss of Muscle Mass by Attenuating the Activation of Autophagy during Severe Energy Deficit

The loss of skeletal muscle mass with energy deficit is thought to be due to protein breakdown by the autophagy-lysosome and the ubiquitin-proteasome systems. We studied the main signaling pathways through which exercise can attenuate the loss of muscle mass during severe energy deficit (5500 kcal/day). Overweight men followed four days of caloric restriction (3.2 kcal/kg body weight day) and prolonged exercise (45 min of one-arm cranking and 8 h walking/day), and three days of control diet and restricted exercise, with an intra-subject design including biopsies from muscles submitted to distinct exercise volumes. Gene expression and signaling data indicate that the main catabolic pathway activated during severe energy deficit in skeletal muscle is the autophagy-lysosome pathway, without apparent activation of the ubiquitin-proteasome pathway. Markers of autophagy induction and flux were reduced by exercise primarily in the muscle submitted to an exceptional exercise volume. Changes in signaling are associated with those in circulating cortisol, testosterone, cortisol/testosterone ratio, insulin, BCAA, and leucine. We conclude that exercise mitigates the loss of muscle mass by attenuating autophagy activation, blunting the phosphorylation of AMPK/ULK1/Beclin1, and leading to p62/SQSTM1 accumulation. This includes the possibility of inhibiting autophagy as a mechanism to counteract muscle loss in humans under severe energy deficit.

The role of cigarette smoke-induced epigenetic alterations in inflammation

Background

Exposure to cigarette smoke (CS) is a major threat to human health worldwide. It is well established that smoking increases the risk of respiratory diseases, cardiovascular diseases and different forms of cancer, including lung, liver, and colon. CS-triggered inflammation is considered to play a central role in various pathologies by a mechanism that stimulates the release of pro-inflammatory cytokines. During this process, epigenetic alterations are known to play important roles in the specificity and duration of gene transcription.

Main text

Epigenetic alterations include three major modifications: DNA modifications via methylation; various posttranslational modifications of histones, namely, methylation, acetylation, phosphorylation, and ubiquitination; and non-coding RNA sequences. These modifications work in concert to regulate gene transcription in a heritable fashion. The enzymes that regulate these epigenetic modifications can be activated by smoking, which further mediates the expression of multiple inflammatory genes. In this review, we summarize the current knowledge on the epigenetic alterations triggered by CS and assess how such alterations may affect smoking-mediated inflammatory responses.

Conclusion

The recognition of the molecular mechanisms of the epigenetic changes in abnormal inflammation is expected to contribute to the understanding of the pathophysiology of CS-related diseases such that novel epigenetic therapies may be identified in the near future.

NAD+ precursor increases aerobic performance in mice

PURPOSE

Nicotinamide riboside (NR) acts as a potent NAD⁺ precursor and improves mitochondrial oxidative capacity and mitochondrial biogenesis in several organisms. However, the effects of NR supplementation on aerobic performance remain unclear. Here, we evaluated the effects of NR supplementation on the muscle metabolism and aerobic capacity of sedentary and trained mice.

METHODS

Male C57BL/6 J mice were supplemented with NR (400 mg/Kg/day) over 5 and 10 weeks. The training protocol consisted of 5 weeks of treadmill aerobic exercise, for 60 min a day, 5 days a week. Bioinformatic and physiological assays were combined with biochemical and molecular assays to evaluate the experimental groups.

RESULTS

NR supplementation by itself did not change the aerobic performance, even though 5 weeks of NR supplementation increased NAD⁺ levels in the skeletal muscle. However, combining NR supplementation and aerobic training increased the aerobic performance compared to the trained group. This was accompanied by an increased protein content of NMNAT3, the rate-limiting enzyme for NAD + biosynthesis and mitochondrial proteins, including MTCO1 and ATP5a. Interestingly, the transcriptomic analysis using a large panel of isogenic strains of BXD mice confirmed that the Nmnat3 gene in the skeletal muscle is correlated with several mitochondrial markers and with different phenotypes related to physical exercise. Finally, NR supplementation during aerobic training markedly increased the amount of type I fibers in the skeletal muscle.

CONCLUSION

Taken together, our results indicate that NR may be an interesting strategy to improve mitochondrial metabolism and aerobic capacity.

Skeletal muscle atrogenes: From rodent models to human pathologies

Skeletal muscle atrophy is a common side effect of most human diseases. Muscle loss is not only detrimental for the quality of life but it also dramatically impairs physiological processes of the organism and decreases the efficiency of medical treatments. While hypothesized for years, the existence of an atrophying programme common to all pathologies is still incompletely solved despite the discovery of several actors and key regulators of muscle atrophy. More than a decade ago, the discovery of a set of genes, whose expression at the mRNA levels were similarly altered in different catabolic situations, opened the way of a new concept: the presence of atrogenes, i.e. atrophy-related genes. Importantly, the atrogenes are referred as such on the basis of their mRNA content in atrophying muscles, the regulation at the protein level being sometimes more complicate to elucidate. It should be noticed that the atrogenes are markers of atrophy and that their implication as active inducers of atrophy is still an open question for most of them. While the atrogene family has grown over the years, it has mostly been incremented based on data coming from rodent models. Whether the rodent atrogenes are valid for humans still remain to be established. An "atrogene" was originally defined as a gene systematically up- or down-regulated in several catabolic situations. Even if recent works often restrict this notion to the up-regulation of a limited number of proteolytic enzymes, it is important to keep in mind the big picture view. In this review, we provide an update of the validated and potential rodent atrogenes and the metabolic pathways they belong, and based on recent work, their relevance in human physio-pathological situations. We also propose a more precise definition of the atrogenes that integrates rapid recovery when catabolic stimuli are stopped or replaced by anabolic ones.

Leucine attenuates muscle atrophy and autophagosome formation by activating PI3K/AKT/mTOR signaling pathway in rotator cuff tears

Rotator cuff tears (RCTs), the most common tendon injury, are always accompanied by muscle atrophy, which is characterized by excessive protein degradation. Autophagy-lysosome systems are the crucial proteolytic pathways and are activated in atrophying muscle. Thus, inhibition of the autophagy-lysosome pathway might be an alternative way to minimize skeletal muscle atrophy. In this present study, combined with a tendon transection-induced rat model of massive rotator cuff tears, HE staining and transmission electron microscopy methods, we found leucine supplementation effectively prevented muscle atrophy, muscle injury and autophagosome formation. Utilizing immunoblotting, we discovered that leucine supplementation is able to inhibit the rise in autophagy-related protein expression (including LC3, Atrogin-1, MuRF1, Bnip3 and FoxO3) driven by tendon transection. The PI3K/AKT/mTOR pathway that was essential in autophagosome formation and autophagy was blocked in atrophying muscle and reactivated in the presence of leucine. Once administrated with LY294002 (PI3K inhibitor) and Rapamycin (mTOR inhibitor), leucine mediated by the anti-atrophic effects was nearly blunted. These results suggest that leucine potentially attenuates tendon transection-induced muscle atrophy through autophagy inhibition via activating the PI3K/AKT/mTOR pathway.

The neurophysiology of deforming spastic paresis: A revised taxonomy

This paper revisits the taxonomy of the neurophysiological consequences of a persistent impairment of motor command execution in the classic environment of sensorimotor restriction and muscle hypo-mobilization in short position. Around each joint, the syndrome involves 2 disorders, muscular and neurologic. The muscular disorder is promoted by muscle hypo-mobilization in short position in the context of paresis, in the hours and days after paresis onset: this genetically mediated, evolving myopathy, is called spastic myopathy. The clinician may suspect it by feeling extensibility loss in a resting muscle, although long after the actual onset of the disease. The neurologic disorder, promoted by sensorimotor restriction in the context of paresis and by the muscle disorder itself, comprises 4 main components, mostly affecting antagonists to desired movements: the first is spastic dystonia, an unwanted, involuntary muscle activation at rest, in the absence of stretch or voluntary effort; spastic dystonia superimposes on spastic myopathy to cause visible, gradually increasing body deformities; the second is spastic cocontraction, an unwanted, involuntary antagonist muscle activation during voluntary effort directed to the agonist, aggravated by antagonist stretch; it is primarily due to misdirection of the supraspinal descending drive and contributes to reducing movement amplitude; and the third is spasticity, one form of hyperreflexia, defined by an enhancement of the velocity-dependent responses to phasic stretch, detected and measured at rest (another form of hyperreflexia is "nociceptive spasms", following flexor reflex afferent stimulation, particularly after spinal cord lesions). The 3 main forms of overactivity, spastic dystonia, spastic cocontraction and spasticity, share the same motor neuron hyperexcitability as a contributing factor, all being predominant in the muscles that are more affected by spastic myopathy. The fourth component of the neurologic disorder affects the agonist: it is stretch-sensitive paresis, which is a decreased access of the central command to the agonist, aggravated by antagonist stretch. Improved understanding of the pathophysiology of deforming spastic paresis should help clinicians select meaningful assessments and refined treatments, including the utmost need to preserve muscle tissue integrity as soon as paresis sets in.

Interplay between hypoactivity, muscle properties and motor command: How to escape the vicious deconditioning circle?

Activity-dependent processes addressing the central nervous system (CNS) and musculoskeletal structures are critical for maintaining motor performance. Chronic reduction in activity, whether due to a sedentary lifestyle or extended bed rest, results in impaired performance in motor tasks and thus decreased quality of life. In the first part of this paper, we give a narrative review of the effects of hypoactivity on the neuromuscular system and behavioral outcomes. Motor impairments arise from a combination of factors including altered muscle properties, impaired afferent input, and plastic changes in neural structure and function throughout the nervous system. There is a reciprocal interplay between the CNS and muscle properties, and these sensorimotor loops are essential for controlling posture and movement. As a result, patients under hypoactivity experience a self-perpetuating cycle, in with sedentarity leading to decreased motor activity and thus a progressive worsening of a situation, and finally deconditioning. Various rehabilitation strategies have been studied to slow down or reverse muscle alteration and altered motor performance. In the second part of the paper, we review representative protocols directed toward the muscle, the sensory input and/or the cerebral cortex. Improving an understanding of the loss of motor function under conditions of disuse (such as extended bed rest) as well as identifying means to slow this decline may lead to therapeutic strategies to preserve quality of life for a range of individuals. The most efficient strategies seem multifactorial, using a combination of approaches targeting different levels of the neuromuscular system.

The effects of acute leucine or leucine-glutamine co-ingestion on recovery from eccentrically biased exercise

This study investigated the effects of leucine or leucine + glutamine supplementation on recovery from eccentric exercise. In a double-blind independent groups design, 23 men were randomly assigned to a leucine (0.087 g/kg; n = 8), leucine + glutamine (0.087 g/kg + glutamine 0.3 g/kg; n = 8) or placebo (0.3 g/kg maltodextrin; n = 7) group. Participants performed 5 sets of drop jumps, with each set comprising 20 repetitions. Isometric knee-extensor strength, counter-movement jump (CMJ) height, delayed-onset muscle soreness (DOMS) and creatine kinase (CK) were measured at baseline, 1, 24, 48 h and 72 h post-exercise. There was a time × group interaction for isometric strength, CMJ and CK (P < 0.05), with differences between the leucine + glutamine and placebo group at 48 h and 72 h for strength (P = 0.013; d = 1.43 and P < 0.001; d = 2.06), CMJ (P = 0.008; d = 0.87 and P = 0.019; d = 1.17) and CK at 24 h (P = 0.012; d = 0.54) and 48 h (P = 0.010; d = 1.37). The leucine group produced higher strength at 72 h compared to placebo (P = 0.007; d = 1.65) and lower CK at 24 h (P = 0.039; d = 0.63) and 48 h (P = 0.022; d = 1.03). Oral leucine or leucine + glutamine increased the rate of recovery compared to placebo after eccentric exercise. These findings highlight potential benefits of co-ingesting these amino acids to ameliorate recovery.

FoxO3a suppression and VPS34 activity are essential to anti-atrophic effects of leucine in skeletal muscle

Our aim is to gain insight into the mechanisms underlying the anti-atrophic effects of leucine, namely, the way that this amino acid can restrain the up-regulation of MuRF1 and Mafbx/Atrogin-1 in muscle atrophy. Male rats received dietary leucine supplementation for 1-3 days, during which time their hind limbs were immobilized. Our results showed that leucine inhibited Forkhead Box O3 (FoxO3a) translocation to cell nuclei. In addition, leucine was able to reverse the expected reduction of FoXO3a ubiquitination caused by immobilization. Unexpectedly, leucine promoted these effects independently of the Class I PI3K/Akt pathway. Vacuolar protein sorting 34 (VPS34; a Class III PI3K) was strongly localized in nuclei after immobilization and leucine supplementation was able to prevent this effect. In experiments on cultured primary myotubes, dexamethasone led to the localization of VPS34 in the nucleus. In addition, the pharmacological inhibition of VPS34 blocked VPS34 nuclear localization and impaired the protective effect of leucine upon myotube trophicity. Finally, the pharmacological inhibition of VPS34 in primary myotubes prevented the protective effects of leucine upon MuRF1 and Mafbx/Atrogin-1 gene expression. Autophagy-related target genes were not responsive to leucine. Thus, we demonstrate that the anti-atrophic effect of leucine is dependent upon FoxO3a suppression and VPS34 activity.

Effects of Supplementation of Branched-Chain Amino Acids to Reduced-Protein Diet on Skeletal Muscle Protein Synthesis and Degradation in the Fed and Fasted States in a Piglet Model

Supplementation of branched-chain amino acids (BCAA) has been demonstrated to promote skeletal muscle mass gain, but the mechanisms underlying this observation are still unknown. Since the regulation of muscle mass depends on a dynamic equilibrium (fasted losses–fed gains) in protein turnover, the aim of this study was to investigate the effects of BCAA supplementation on muscle protein synthesis and degradation in fed/fasted states and the related mechanisms. Fourteen 26- (Experiment 1) and 28-day-old (Experiment 2) piglets were fed reduced-protein diets without or with supplemental BCAA. After a four-week acclimation period, skeletal muscle mass and components of anabolic and catabolic signaling in muscle samples after overnight fasting were determined in Experiment 1. Pigs in Experiment 2 were implanted with carotid arterial, jugular venous, femoral arterial and venous catheters, and fed once hourly along with the intravenous infusion of NaH¹³CO₃ for 2 h, followed by a 6-h infusion of [1-¹³C]leucine. Muscle leucine kinetics were measured using arteriovenous difference technique. The mass of most muscles was increased by BCAA supplementation. During feeding, BCAA supplementation increased leucine uptake, protein synthesis, protein degradation and net transamination. The greater increase in protein synthesis than in protein degradation resulted in elevated protein deposition. Protein synthesis was strongly and positively correlated with the intramuscular net production of α-ketoisocaproate (KIC) and protein degradation. Moreover, BCAA supplementation enhanced the fasted-state phosphorylation of protein translation initiation factors and inhibited the protein-degradation signaling of ubiquitin-proteasome and autophagy-lysosome systems. In conclusion, supplementation of BCAA to reduced-protein diet increases fed-state protein synthesis and inhibits fasted-state protein degradation, both of which could contribute to the elevation of skeletal muscle mass in piglets. The effect of BCAA supplementation on muscle protein synthesis is associated with the increase in protein degradation and KIC production in the fed state.

Free Amino Acid Profile and Expression of Genes Implicated in Protein Metabolism in Skeletal Muscle of Growing Pigs Fed Low-Protein Diets Supplemented with Branched-Chain Amino Acids

Revealing the expression patterns of genes involved in protein metabolism as affected by diets would be useful for further clarifying the importance of the balance among the branched-chain amino acids (BCAAs), which include leucine (Leu), isoleucine (Ile), and valine (Val). Therefore, we used growing pigs to explore the effects of different dietary BCAA ratios on muscle protein metabolism. The Leu:Ile:Val ratio was 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. Results showed that compared with the control group, low-protein diets with the BCAA ratio ranging from 1:0.75:0.75 to 1:0.25:0.25 elevated muscle free amino acid (AA) concentrations and AA transporter expression, significantly activated the mammalian target of rapamycin complex 1 pathway, and decreased serum urea nitrogen content and the mRNA expression of genes related to muscle protein degradation (P < 0.05). In conclusion, these results indicated that maintaining the dietary Leu:Ile:Val ratio within 1:0.25:0.25-1:0.75:0.75 in low-protein diets (17% CP) would facilitate the absorption and utilization of free AA and result in improved protein metabolism and muscle growth.

Effect of branched-chain amino acid ratio on the proliferation, differentiation, and expression levels of key regulators involved in protein metabolism of myocytes

OBJECTIVES

Branched-chain amino acids (BCAAs), including leucine (Leu), isoleucine (Ile), and valine (Val), are key regulators of protein synthesis in muscle. The aim of this study was to investigate the effect of different BCAA ratios (Leu:Ile:Val) on the proliferation, differentiation, and expression levels of the regulators related to protein metabolism of C2 C12 myocytes.

METHODS

Studies were conducted in C2C12 myocytes exposed to different BCAA ratios (Leu: Ile: Val = 0, 1:0.25:0.25, 1:1:1).

RESULTS

The ratio of 1:0.25:0.25 increased cell viability and promoted cell cycle progression from G0/G1 phase to S phase, which was an indicator of proliferation enhancement (P < 0.05). Moreover, this optimal ratio (1:0.25:0.25) promoted the differentiation of myocytes into myotubes by upregulating myogenin and interleukin-15 gene expression, and differently regulated the expression of L-type amino acid transporter 1 and 4 and system ASC amino acid transporters 2. Furthermore, the ratio stimulated mTOR expression at the mRNA and phosphorylated protein levels, as well as ribosomal protein S6 kinase and regulatory-associated protein of mTOR (raptor). In contrast, the optimal ratio decreased the amount of ubiquitin ligase muscle-specific RING finger 1 and muscle atrophy F-box during proliferation and differentiation (P < 0.05). No change was observed in the expression of key genes related to energy metabolism except for uncoupling protein 3 (P > 0.05).

CONCLUSIONS

The results suggested that appropriate BCAA ratios could enhance proliferation and differentiation of the C2 C12 myocytes, also mediate the key regulators related to protein metabolism including the mTORC1 pathway. A proper utilization of balanced BCAA ratio in food would be beneficial to human and animal nutrition.

Effects of 4 Weeks of High-Intensity Interval Training and β-Hydroxy-β-Methylbutyric Free Acid Supplementation on the Onset of Neuromuscular Fatigue

This study investigated the effects of high-intensity interval training (HIIT) and β-hydroxy-β-methylbutyric free acid (HMB) supplementation on physical working capacity at the onset of neuromuscular fatigue threshold (PWC(FT)). Thirty-seven participants (22 men, 15 women; 22.8 ± 3.4 years) completed an incremental cycle ergometer test (graded exercise test [GXT]); electromyographic amplitude from the right vastus lateralis was recorded. Assessments occurred preceding (PRE) and after 4 weeks of supplementation (POST). Participants were randomly assigned to control (C, n = 9), placebo (P, n = 14), or supplementation (S, n = 14) groups. Both P and S completed 12 HIIT sessions, whereas C maintained normal diet and activity patterns. The PWC(FT) (W) was determined using the maximal perpendicular distance (D(MAX)) method. Electromyographic amplitude (μVrms) over time was used to generate a cubic regression. Onset of fatigue (TF) was the x-value of the point on the regression that was at D(MAX) from a line between the first and last data points. The PWC(FT) was estimated using TF and GXT power-output increments. The 2-way analysis of variance (ANOVA) (group × time) resulted in a significant interaction for PWC(FT) (F = 6.69, p = 0.004). Post hoc analysis with 1-way ANOVA resulted in no difference in PWC(FT) among groups at PRE (F = 0.87, p = 0.43); however, a difference in PWC(FT) was shown for POST (F = 5.46, p = 0.009). Post hoc analysis among POST values revealed significant differences between S and both P (p = 0.034) and C (p = 0.003). No differences (p = 0.226) were noted between P and C. Paired samples t-tests detected significant changes after HIIT for S (p < 0.001) and P (p = 0.016), but no change in C (p = 0.473). High-intensity interval training increased PWC(FT), but HMB with HIIT was more effective than HIIT alone. Furthermore, it seems that adding HMB supplementation with HIIT in untrained men and women may further improve endurance performance measures.

The role of E3 ubiquitin-ligases MuRF-1 and MAFbx in loss of skeletal muscle mass

The ubiquitin-proteasome system (UPS) is the main regulatory mechanism of protein degradation in skeletal muscle. The ubiquitin-ligase enzymes (E3s) have a central role in determining the selectivity and specificity of the UPS. Since their identification in 2001, the muscle specific E3s, muscle RING finger-1 (MuRF-1) and muscle atrophy F-box (MAFbx), have been shown to be implicated in the regulation of skeletal muscle atrophy in various pathological and physiological conditions. This review aims to explore the involvement of MuRF-1 and MAFbx in catabolism of skeletal muscle during various pathologies, such as cancer cachexia, sarcopenia of aging, chronic kidney disease (CKD), diabetes, and chronic obstructive pulmonary disease (COPD). In addition, the effects of various lifestyle and modifiable factors (e.g. nutrition, exercise, cigarette smoking, and alcohol) on MuRF-1 and MAFbx regulation will be discussed. Finally, evidence of potential strategies to protect against skeletal muscle wasting through inhibition of MuRF-1 and MAFbx expression will be explored.

Nutritional Support for Exercise-Induced Injuries

Nutrition is one method to counter the negative impact of an exercise-induced injury. Deficiencies of energy, protein and other nutrients should be avoided. Claims for the effectiveness of many other nutrients following injuries are rampant, but the evidence is equivocal. The results of an exercise-induced injury may vary widely depending on the nature of the injury and severity. Injuries typically result in cessation, or at least a reduction, in participation in sport and decreased physical activity. Limb immobility may be necessary with some injuries, contributing to reduced activity and training. Following an injury, an inflammatory response is initiated and while excess inflammation may be harmful, given the importance of the inflammatory process for wound healing, attempting to drastically reduce inflammation may not be ideal for optimal recovery. Injuries severe enough for immobilization of a limb result in loss of muscle mass and reduced muscle strength and function. Loss of muscle results from reductions in basal muscle protein synthesis and the resistance of muscle to anabolic stimulation. Energy balance is critical. Higher protein intakes (2-2.5 g/kg/day) seem to be warranted during immobilization. At the very least, care should be taken not to reduce the absolute amount of protein intake when energy intake is reduced. There is promising, albeit preliminary, evidence for the use of omega-3 fatty acids and creatine to counter muscle loss and enhance hypertrophy, respectively. The overriding nutritional recommendation for injured exercisers should be to consume a well-balanced diet based on whole, minimally processed foods or ingredients made from whole foods. The diet composition should be carefully assessed and changes considered as the injury heals and activity patterns change.

Immobilization induces nuclear accumulation of HDAC4 in rat skeletal muscle

The study described herein aimed to examine changes in HDAC4 and its downstream targets in immobilization-induced rat skeletal muscle atrophy. Eleven male Wistar rats were used, and one hindlimb was immobilized in the plantar flexion position using a plaster cast. The contralateral, non-immobilized leg served as an internal control. After 10 days, the gastrocnemius muscles were removed from both hindlimbs. Ten days of immobilization resulted in a significant reduction (-27.3 %) in gastrocnemius muscle weight. A significant decrease in AMPK phosphorylation was also observed in nuclear fractions from immobilized legs relative to the controls. HDAC4 expression was significantly increased in immobilized legs in both the cytoplasmic and nuclear fractions. Moreover, Myogenin and MyoD mRNA levels were upregulated in immobilized legs, resulting in increased Atrogin-1 mRNA expression. Our data suggest that nuclear HDAC4 accumulation is partly related to immobilization-induced muscle atrophy.

The Emerging Role of Branched-Chain Amino Acids in Insulin Resistance and Metabolism

Insulin is required for maintenance of glucose homeostasis. Despite the importance of insulin sensitivity to metabolic health, the mechanisms that induce insulin resistance remain unclear. Branched-chain amino acids (BCAAs) belong to the essential amino acids, which are both direct and indirect nutrient signals. Even though BCAAs have been reported to improve metabolic health, an increased BCAA plasma level is associated with a high risk of metabolic disorder and future insulin resistance, or type 2 diabetes mellitus (T2DM). The activation of mammalian target of rapamycin complex 1 (mTORC1) by BCAAs has been suggested to cause insulin resistance. In addition, defective BCAA oxidative metabolism might occur in obesity, leading to a further accumulation of BCAAs and toxic intermediates. This review provides the current understanding of the mechanism of BCAA-induced mTORC1 activation, as well as the effect of mTOR activation on metabolic health in terms of insulin sensitivity. Furthermore, the effects of impaired BCAA metabolism will be discussed in detail.

Differential effects of leucine supplementation in young and aged mice at the onset of skeletal muscle regeneration

Aging decreases the ability of skeletal muscle to respond to injury. Leucine has been demonstrated to target protein synthetic pathways in skeletal muscle thereby enhancing this response. However, the effect of aging on leucine-induced alterations in protein synthesis at the onset of skeletal muscle regeneration has not been fully elucidated. The purpose of this study was to determine if aging alters skeletal muscle regeneration and leucine-induced alterations in markers of protein synthesis. The tibialis anterior of young (3 months) and aged (24 months) female C57BL/6J mice were injected with either bupivacaine or PBS, and the mice were given ad libitum access to leucine-supplemented or normal drinking water. Protein and gene expression of markers of protein synthesis and degradation, respectively, were analyzed at three days post-injection. Following injury in young mice, leucine supplementation was observed to elevate only p-p70S6K. In aged mice, leucine was shown to elicit higher p-mTOR content with and without injury, and p-4EBP-1 content post-injury. Additionally in aged mice, leucine was shown to elicit higher content of relative p70S6K post-injury. Our study shows that leucine supplementation affects markers of protein synthesis at the onset of skeletal muscle regeneration differentially in young and aged mice.

Enteral β-hydroxy-β-methylbutyrate supplementation increases protein synthesis in skeletal muscle of neonatal pigs

Many low-birth weight infants are at risk for poor growth due to an inability to achieve adequate protein intake. Administration of the amino acid leucine stimulates protein synthesis in skeletal muscle of neonates. To determine the effects of enteral supplementation of the leucine metabolite β-hydroxy-β-methylbutyrate (HMB) on protein synthesis and the regulation of translation initiation and degradation pathways, overnight-fasted neonatal pigs were studied immediately (F) or fed one of five diets for 24 h: low-protein (LP), high-protein (HP), or LP diet supplemented with 4 (HMB4), 40 (HMB40), or 80 (HMB80) μmol HMB·kg body wt(-1)·day(-1) Cell replication was assessed from nuclear incorporation of BrdU in the longissimus dorsi (LD) muscle and jejunum crypt cells. Protein synthesis rates in LD, gastrocnemius, rhomboideus, and diaphragm muscles, lung, and brain were greater in HMB80 and HP and in brain were greater in HMB40 compared with LP and F groups. Formation of the eIF4E·eIF4G complex and S6K1 and 4E-BP1 phosphorylation in LD, gastrocnemius, and rhomboideus muscles were greater in HMB80 and HP than in LP and F groups. Phosphorylation of eIF2α and eEF2 and expression of SNAT2, LAT1, MuRF1, atrogin-1, and LC3-II were unchanged. Numbers of BrdU-positive myonuclei in the LD were greater in HMB80 and HP than in the LP and F groups; there were no differences in jejunum. The results suggest that enteral supplementation with HMB increases skeletal muscle protein anabolism in neonates by stimulation of protein synthesis and satellite cell proliferation.

A critical reappraisal of dietary practices in methylmalonic acidemia raises concerns about the safety of medical foods. Part 1: isolated methylmalonic acidemias

PURPOSE

Medical foods for methylmalonic and propionic acidemias (MMA/PA) contain minimal valine, isoleucine, methionine and threonine, but have been formulated with increased leucine. We aimed to assess the effects of imbalanced branched-chain amino acid intake on metabolic and growth parameters in a cohort of MMA patients ascertained via a natural history study.

METHODS

Cross-sectional anthropometric and body composition measurements were correlated with diet content and disease-related biomarkers in 61 patients with isolated MMA (46 mut, 9 cblA and 6 cblB).

RESULTS

Patients with MMA tolerated close to the recommended daily allowance (RDA) of complete protein (mut⁰: 99.45 ± 32.05% RDA). However, 85% received medical foods, the protein-equivalent in which often exceeded complete protein intake (35%). Medical food consumption resulted in low plasma valine and isoleucine concentrations, prompting paradoxical supplementation with these propiogenic amino acids. Weight and height–for age Z-scores correlated negatively with the leucine/valine intake ratio (r=−0.453, P=0.014, R²=0.209 and r=−0.341, P=0.05, R²=0.123, respectively).

CONCLUSION

Increased leucine intake in patients with MMA resulted in iatrogenic amino acid deficiencies and was associated with adverse growth outcomes. Medical foods for propionate oxidation disorders need to be redesigned and studied prospectively, to ensure efficacy and safety.

TRIAL REGISTRATION

This clinical study is registered in www.clinicaltrials.gov with the ID: NCT00078078. Study URL: http://clinicaltrials.gov/ct2/show/NCT00078078

Effects of different doses of leucine ingestion following eight weeks of resistance exercise on protein synthesis and hypertrophy of skeletal muscle in rats

[Purpose]

This study was designed to determine the appropriate Leucine intake volume to obtain the effects of restoring damaged muscle through the synthesis of muscle proteins to increase skeletal muscle and improve exercise performance, and to achieve enhanced muscle hypertrophy.

[Methods]

To clarify the effects of leucine on skeletal muscle hypertrophy of SD rats, following eight weeks of resistance exercise (climbing ladder), the mass of the FHL (Flexor hallucis longus) was measured after extraction, after which change in the activity of muscle signaling proteins (PKB/Akt, mTOR, p70S6K, 4EBP1) was analyzed.

[Results]

The expressions of PKB/Akt, mTOR and p70S6K were increased in L5 (Leucine 50% administration group) compared with the control group (CON) and exercise group (Ex, exercise training group); EL1 (exercise + 10% leucine administration group) and EL5 (exercise + 50% Leucine administration) also exhibited increased expressions of PKB/Akt, mTOR, and p70S6K, while no difference between EL1 and EL5 were observed. No significant differences in 4EBP1 were found among any of the groups. In addition, there were no differences in FHL mass, while relative mass (FHL/body mass) was increased in the exercise group (Ex, EL1, EL5) compared with the control group. No differences were observed among the exercise groups.

[Conclusion]

The present study demonstrated that the relative body mass was increased in the EX group compared with the CON group, while no significant differences in muscle mass could be found among the groups. Even though some signaling proteins were increased, or some differences existed among groups, there were no differences in muscle mass between the leucine administration and exercise training combined with leucine administration groups in the present study.

The delayed recovery of the remobilized rat tibialis anterior muscle reflects a defect in proliferative and terminal differentiation that impairs early regenerative processes

BACKGROUND

The immobilization-induced tibialis anterior (TA) muscle atrophy worsens after cast removal and is associated with altered extracellular matrix (ECM) composition. The secreted protein acidic and rich in cysteine (Sparc) is an ECM component involved in Akt activation and in β-catenin stabilization, which controls protein turnover and induces muscle regulatory factors (MRFs), respectively. We hypothesized that ECM alterations may influence these intracellular signalling pathways controlling TA muscle mass.

METHODS

Six-month-old Wistar rats were subjected to hindlimb cast immobilization for 8 days (I8) or not (I0) and allowed to recover for 1 to 10 days (R1-10).

RESULTS

The TA atrophy during remobilization correlated with reduced fibre cross-sectional area and thickening of endomysium. mRNA levels for Sparc increased during remobilization until R10 and for integrin-α7 and -β1 at I8 and R1. Integrin-linked kinase protein levels increased during immobilization and remobilization until R10. This was inversely correlated with changes in Akt phosphorylation. β-Catenin protein levels increased in the remobilized TA at R1 and R10. mRNA levels of the proliferative MRFs (Myf5 and MyoD) increased at I8 and R1, respectively, without changes in Myf5 protein levels. In contrast, myogenin mRNA levels (a terminal differentiation MRF) decreased at R1, but only increased at R10 in remobilized muscles, as for protein levels.

CONCLUSIONS

Altogether, this suggests that the TA inefficiently attempted to preserve regeneration during immobilization by increasing transcription of proliferative MRFs, and that the TA could engage recovery during remobilization only when the terminal differentiation step of regeneration is enhanced.

Sarcopenia and critical illness: a deadly combination in the elderly

Sarcopenia is the age-associated loss of lean skeletal muscle mass. It is the result of multiple physiologic derangements, ultimately resulting in an insidious functional decline. Frailty, the clinical manifestation of sarcopenia and physical infirmity, is associated with significant morbidity and mortality in the elderly population. The underlying pathology results in a disruption of the individual's ability to tolerate internal and external stressors such as injury or illness. This infirmity results in a markedly increased risk of falls and subsequent morbidity and mortality from the resulting traumatic injury, as well as an inability to recover from medical insults, resulting in critical illness. The increasing prevalence of sarcopenia and critical illness in the elderly has resulted in a deadly intersection of disease processes. The lethality of this combination appears to be the result of altered muscle metabolism, decreased mitochondrial energetics needed to survive critical illness, and a chronically activated catabolic state likely mediated by tumor necrosis factor-α. Furthermore, these underlying derangements are independently associated with an increased incidence of critical illness, resulting in a progressive downward spiral. Considerable evidence has been gathered supporting the role of aggressive nutrition support and physical therapy in improving outcomes. Critical care practitioners must consider sarcopenia and the resulting frailty phenotype a comorbid condition so that the targeted interventions can be instituted and research efforts focused.