Importance of the ATP-ubiquitin-proteasome pathway in the degradation of soluble and myofibrillar proteins in rabbit muscle extracts

NANOG Restores Contractility of Mesenchymal Stem Cell-Based Senescent Microtissues

Mesenchymal stem cells (MSCs) have been extensively used in the field of tissue engineering as a source of smooth muscle cells (SMCs). However, recent studies showed deficits in the contractile function of SMCs derived from senescent MSCs and there are no available strategies to restore the contractile function that is impaired due to cellular or organismal senescence. In this study, we developed a tetracycline-regulatable system and employed micropost tissue arrays to evaluate the effects of the embryonic transcription factor, NANOG, on the contractility of senescent MSCs. Using this system, we show that expression of NANOG fortified the actin cytoskeleton and restored contractile function that was impaired in senescent MSCs. NANOG increased the expression of smooth muscle α-actin (ACTA2) as well as the contractile force generated by cells in three-dimensional microtissues. Interestingly, NANOG worked together with transforming growth factor-beta1 to further enhance the contractility of senescent microtissues. The effect of NANOG on contractile function was sustained for about 10 days after termination of its expression. Our results show that NANOG could reverse the effects of stem cell senescence and restore the myogenic differentiation potential of senescent MSCs. These findings may enable development of novel strategies to restore the function of senescent cardiovascular and other SMC-containing tissues.

Role of calpain in eccentric contraction-induced proteolysis of Ca2+-regulatory proteins and force depression in rat fast-twitch skeletal muscle

The aim of this study was to examine the in vivo effects of eccentric contraction (ECC) on calpain-dependent proteolysis of Ca2+-regulatory proteins and force production in fast-twitch skeletal muscles. Rat extensor digitorum longus muscles were exposed to 200 repeated ECC in situ and excised immediately [recovery 0 (REC0)] or 3 days [recovery 3 (REC3)] after cessation of ECC. Calpain inhibitor (CI)-treated rats were intraperitoneally injected with MDL-28170 before ECC and during REC3. Tetanic force was markedly reduced at REC0 and remained reduced at REC3. CI treatment ameliorated the ECC-induced force decline but only at REC3. No evidence was found for proteolysis of dihydropyridine receptor (DHPR), junctophilin (JP)1, JP2, ryanodine receptor (RyR), sarcoplasmic reticulum Ca2+-ATPase (SERCA)1a, or junctional face protein-45 at REC0. At REC3, ECC resulted in decreases in DHPR, JP1, JP2, RyR, and SERCA1a. CI treatment prevented the decreases in DHPR, JP1, and JP2, whereas it had little effect on RyR and SERCA1a. These findings suggest that DHPR, JP1, and JP2, but not RyR and SERCA1a, undergo calpain-dependent proteolysis in in vivo muscles subjected to ECC and that impaired function of DHPR and/or JP might cause prolonged force deficits with ECC.

NEW & NOTEWORTHY Calpain-dependent proteolysis is one of the contributing factors to muscle damage that occurs with eccentric contraction (ECC). It is unclear, however, whether calpains account for proteolysis of Ca2+-regulatory proteins in in vivo muscles subjected to ECC. Here, we provide evidence that dihydropyridine receptor and junctophilin, but not ryanodine receptor and sarcoplasmic reticulum Ca2+-ATPase, undergo calpain-dependent proteolysis.

Myofibril breakdown during atrophy is a delayed response requiring the transcription factor PAX4 and desmin depolymerization

A hallmark of muscle atrophy is the excessive degradation of myofibrillar proteins primarily by the ubiquitin proteasome system. In mice, during the rapid muscle atrophy induced by fasting, the desmin cytoskeleton and the attached Z-band-bound thin filaments are degraded after ubiquitination by the ubiquitin ligase tripartite motif-containing protein 32 (Trim32). To study the order of events leading to myofibril destruction, we investigated the slower atrophy induced by denervation (disuse). We show that myofibril breakdown is a two-phase process involving the initial disassembly of desmin filaments by Trim32, which leads to the later myofibril breakdown by enzymes, whose expression is increased by the paired box 4 (PAX4) transcription factor. After denervation of mouse tibialis anterior muscles, phosphorylation and Trim32-dependent ubiquitination of desmin filaments increased rapidly and stimulated their gradual depolymerization (unlike their rapid degradation during fasting). Trim32 down-regulation attenuated the loss of desmin and myofibrillar proteins and reduced atrophy. Although myofibrils and desmin filaments were intact at 7 d after denervation, inducing the dissociation of desmin filaments caused an accumulation of ubiquitinated proteins and rapid destruction of myofibrils. The myofibril breakdown normally observed at 14 d after denervation required not only dissociation of desmin filaments, but also gene induction by PAX4. Down-regulation of PAX4 or its target gene encoding the p97/VCP ATPase reduced myofibril disassembly and degradation on denervation or fasting. Thus, during atrophy, the initial loss of desmin is critical for the subsequent myofibril destruction, and over time, myofibrillar proteins become more susceptible to PAX4-induced enzymes that promote proteolysis.

Denervation-Induced Activation of the Standard Proteasome and Immunoproteasome

The standard 26S proteasome is responsible for the majority of myofibrillar protein degradation leading to muscle atrophy. The immunoproteasome is an inducible form of the proteasome. While its function has been linked to conditions of atrophy, its contribution to muscle proteolysis remains unclear. Therefore, the purpose of this study was to determine if the immunoproteasome plays a role in skeletal muscle atrophy induced by denervation. Adult male C57BL/6 wild type (WT) and immunoproteasome knockout lmp7^-/-/mecl-1^-/- (L7M1) mice underwent tibial nerve transection on the left hindlimb for either 7 or 14 days, while control mice did not undergo surgery. Proteasome activity (caspase-, chymotrypsin-, and trypsin- like), protein content of standard proteasome (β1, β5 and β2) and immunoproteasome (LMP2, LMP7 and MECL-1) catalytic subunits were determined in the gastrocnemius muscle. Denervation induced significant atrophy and was accompanied by increased activities and protein content of the catalytic subunits in both WT and L7M1 mice. Although denervation resulted in a similar degree of muscle atrophy between strains, the mice lacking two immunoproteasome subunits showed a differential response in the extent and duration of proteasome features, including activities and content of the β1, β5 and LMP2 catalytic subunits. The results indicate that immunoproteasome deficiency alters the proteasome’s composition and activities. However, the immunoproteasome does not appear to be essential for muscle atrophy induced by denervation.

MicroRNA deep sequencing in two adult stem cell populations identifies miR-501 as a novel regulator of myosin heavy chain during muscle regeneration

MicroRNAs (miRNAs) are important regulators of skeletal muscle regeneration, but the underlying mechanisms are still incompletely understood. Here, comparative miRNA sequencing analysis of myogenic progenitor cells (MPs) and non-myogenic fibroblast-adipocyte progenitors (FAPs) during cardiotoxin (CTX)-induced muscle injury uncovered miR-501 as a novel muscle-specific miRNA. miR-501 is an intronic miRNA and its expression levels in MPs correlated with its host gene, chloride channel, voltage-sensitive 5 (Clcn5). Pharmacological inhibition of miR-501 dramatically blunted the induction of embryonic myosin heavy chain (MYH3) and, to a lesser extent, adult myosin isoforms during muscle regeneration, and promoted small-diameter neofibers. An unbiased target identification approach in primary myoblasts validated gigaxonin as a target of miR-501 that mimicked the effect of miR-501 inhibition on MYH3 expression. In the mdx mouse model, which models a pathological disease state, not only was miR-501 induced in regenerating skeletal muscle, but also its serum levels were increased, which correlated with the disease state of the animals. Our results suggest that miR-501 plays a key role in adult muscle regeneration and might serve as a novel serum biomarker for the activation of adult muscle stem cells.

Summary: MicroRNA 501 is a novel muscle-specific microRNA that is induced during muscle regeneration and regulates the transition of myosin heavy chains during early myogenesis.

The impact of postexercise essential amino acid ingestion on the ubiquitin proteasome and autophagosomal-lysosomal systems in skeletal muscle of older men

Essential amino acid (EAA) ingestion enhances postexercise muscle protein synthesis, and, in particular, the anabolic response of older adults appears sensitive to the quantity of ingested leucine. The effect of leucine ingestion on muscle breakdown following resistance exercise (RE) is less understood. The purpose of this study was to identify the impact of postexercise leucine ingestion on the ubiquitin proteasome and autophagosomal-lysosomal systems following acute RE in older men. Subjects (72 ± 2 yr) performed RE and 1 h postexercise ingested 10 g of EAA containing a leucine quantity similar to quality protein (control, 1.8 g leucine, n = 7) or enriched in leucine (leucine, 3.5 g leucine, n = 8). Stable isotope infusion and muscle biopsies (vastus lateralis) obtained at rest and 2, 5, and 24 h postexercise were used to examine protein content (Western blot), mRNA expression (RT-quantitative PCR), and muscle protein fractional breakdown rate (FBR). Muscle-specific RING finger 1 mRNA increased in both groups at 2 and 5 h (P < 0.05). LC3 mRNA increased, and the LC3BII-to-LC3BI ratio decreased at all postexercise time points in control (P < 0.05). Conversely, LC3 mRNA only increased at 2 h, and the LC3BII-to-LC3BI ratio only decreased at 2 and 5 h in leucine (P < 0.05). Tumor necrosis factor receptor-associated factor-6 mRNA increased (P < 0.05) in control at 5 h. FBR was not statistically different between groups or from basal 24 h postexercise (P > 0.05). These data indicate that ingesting a larger quantity of leucine following RE may further reduce postexercise skeletal muscle autophagy in older men; however, it does not appear to influence the acute postexercise elevation in markers of the ubiquitin proteasome system or the breakdown of intact proteins.NEW & NOTEWORTHY The impact of postexercise leucine ingestion on processes of skeletal muscle breakdown in older adults is not well understood. Additional postexercise leucine ingestion appears to further reduce autophagy, but it does not interfere with the increase in ubiquitin proteasome system markers or the breakdown of intact proteins in skeletal muscle of older men. Postexercise leucine ingestion may promote a healthier protein pool and favorable muscle adaptations in older adults through greater accretion of myofibrillar proteins.

Role of proteases in the pathophysiology of cardiac disease

Cardiovascular disease is a major cause of death and thus a great deal of effort has been made in salvaging the diseased myocardium. Although various factors have been identified as possible causes of different cardiac diseases such as heart failure and ischemic heart disease, there is a real need to elucidate their role for the better understanding of the cardiac disease pathology and formulation of strategies for developing newer therapeutic interventions. In view of the intimate involvement of different types of proteases in maintaining cellular structure, the role of proteases in various cardiac diseases has become the focus of recent research. Proteases are present in the cytosol as well as are localized in a number of subcellular organelles in the cell. These are known to use extracellular matrix, cytoskeletal, sarcolemmal, sarcoplasmic reticular, mitochondrial and myofibrillar proteins as substrates. Work from different laboratories using a wide variety of techniques has shown that the activation of proteases causes alterations of a number of specific proteins leading to subcellular remodeling and cardiac dysfunction. Inhibition of protease action by different drugs and agents, therefore, has a clinical relevance and is expected to form a part of new treatment paradigm for improving heart function. This review examines the biochemistry and localization of some of the proteases in the cardiac tissue in addition to identification of the sites of action of some protease inhibitors. (Mol Cell Biochem 263: 241-256, 2004).

The Molecular Mechanisms of Calpains Action on Skeletal Muscle Atrophy

Skeletal muscle atrophy is associated with a loss of muscle protein which may result from both increased proteolysis and decreased protein synthesis. Investigations on cell signaling pathways that regulate muscle atrophy have promoted our understanding of this complicated process. Emerging evidence implicates that calpains play key roles in dysregulation of proteolysis seen in muscle atrophy. Moreover, studies have also shown that abnormally activated calpain results muscle atrophy via its downstream effects on ubiquitin-proteasome pathway (UPP) and Akt phosphorylation. This review will discuss the role of calpains in regulation of skeletal muscle atrophy mainly focusing on its collaboration with either UPP or Akt in atrophy conditions in hope to stimulate the interest in development of novel therapeutic interventions for skeletal muscle atrophy.

Denervation-Induced Activation of the Ubiquitin-Proteasome System Reduces Skeletal Muscle Quantity Not Quality

It is well known that the ubiquitin-proteasome system is activated in response to skeletal muscle wasting and functions to degrade contractile proteins. The loss of these proteins inevitably reduces skeletal muscle size (i.e., quantity). However, it is currently unknown whether activation of this pathway also affects function by impairing the muscle’s intrinsic ability to produce force (i.e., quality). Therefore, the purpose of this study was twofold, (1) document how the ubiquitin-proteasome system responds to denervation and (2) identify the physiological consequences of these changes. To induce soleus muscle atrophy, C57BL6 mice underwent tibial nerve transection of the left hindlimb for 7 or 14 days (n = 6–8 per group). At these time points, content of several proteins within the ubiquitin-proteasome system were determined via Western blot, while ex vivo whole muscle contractility was specifically analyzed at day 14. Denervation temporarily increased several key proteins within the ubiquitin-proteasome system, including the E3 ligase MuRF1 and the proteasome subunits 19S, α7 and β5. These changes were accompanied by reductions in absolute peak force and power, which were offset when expressed relative to physiological cross-sectional area. Contrary to peak force, absolute and relative forces at submaximal stimulation frequencies were significantly greater following 14 days of denervation. Taken together, these data represent two keys findings. First, activation of the ubiquitin-proteasome system is associated with reductions in skeletal muscle quantity rather than quality. Second, shortly after denervation, it appears the muscle remodels to compensate for the loss of neural activity via changes in Ca²⁺ handling.

Proteasome regulates turnover of toxic human amylin in pancreatic cells

Toxic human amylin (hA) oligomers and aggregates are implicated in the pathogenesis of type 2 diabetes mellitus (T2DM). Although recent studies demonstrated a causal connection between hA uptake and toxicity in pancreatic cells, the mechanism of amylin's clearance following its internalization and its relationship to toxicity is yet to be determined, and hence was investigated here. Using pancreatic rat insulinoma β-cells and human islets as model systems, we show that hA, following its internalization, first accumulates in the cytosol followed by its translocation into nucleus, and to a lesser extent lysosomes, keeping the net cytosolic amylin content low. An increase in hA accumulation in the nucleus of pancreatic cells correlated with its cytotoxicity, suggesting that its excessive accumulation in the nucleus is detrimental. hA interacted with 20S core and 19S lid subunits of the β-cell proteasomal complex, as suggested by immunoprecipitation and confocal microscopy studies, which subsequently resulted in a decrease in the proteasome's proteolytic activity in these cells. In vitro binding and activity assays confirmed an intrinsic and potent ability of amylin to interact with the 20S core complex thereby modulating its proteolytic activity. Interestingly, less toxic and aggregation incapable rat amylin (rA) showed a comparable inhibitory effect on proteasome activity and protein ubiquitination, decoupling amylin aggregation/ toxicity and amylin-induced protein stress. In agreement with these studies, inhibition of proteasomal proteolytic activity significantly increased intracellular amylin content and toxicity. Taken together, our results suggest a pivotal role of proteasomes in amylin's turnover and detoxification in pancreatic cells.

Delineation of Molecular Pathways Involved in Cardiomyopathies Caused by Troponin T Mutations

Familial hypertrophic cardiomyopathy (FHC) is associated with mild to severe cardiac problems and is the leading cause of sudden death in young people and athletes. Although the genetic basis for FHC is well-established, the molecular mechanisms that ultimately lead to cardiac dysfunction are not well understood. To obtain important insights into the molecular mechanism(s) involved in FHC, hearts from two FHC troponin T models (Ile79Asn [I79N] and Arg278Cys [R278C]) were investigated using label-free proteomics and metabolomics. Mutations in troponin T are the third most common cause of FHC, and the I79N mutation is associated with a high risk of sudden cardiac death. Most FHC-causing mutations, including I79N, increase the Ca(2+) sensitivity of the myofilament; however, the R278C mutation does not alter Ca(2+) sensitivity and is associated with a better prognosis than most FHC mutations. Out of more than 1200 identified proteins, 53 and 76 proteins were differentially expressed in I79N and R278C hearts, respectively, when compared with wild-type hearts. Interestingly, more than 400 proteins were differentially expressed when the I79N and R278C hearts were directly compared. The three major pathways affected in I79N hearts relative to R278C and wild-type hearts were the ubiquitin-proteasome system, antioxidant systems, and energy production pathways. Further investigation of the proteasome system using Western blotting and activity assays showed that proteasome dysfunction occurs in I79N hearts. Metabolomic results corroborate the proteomic data and suggest the glycolytic, citric acid, and electron transport chain pathways are important pathways that are altered in I79N hearts relative to R278C or wild-type hearts. Our findings suggest that impaired energy production and protein degradation dysfunction are important mechanisms in FHCs associated with poor prognosis and that cardiac hypertrophy is not likely needed for a switch from fatty acid to glucose metabolism.

Stable atrogin-1 (Fbxo32) and MuRF1 (Trim63) gene expression is involved in the protective mechanism in soleus muscle of hibernating Daurian ground squirrels (Spermophilus dauricus)

Understanding the mechanisms that protect against or limit muscle atrophy in hibernators during prolonged inactivity has important implications for its treatment. We examined whether external factors influence the pathways regulating protein synthesis and degradation, leading to muscle atrophy prevention in Daurian ground squirrels (Spermophilus dauricus). We investigated the effects of 14-day hindlimb-unloading (HU) in different seasons and two-month hibernation on the soleus (SOL) muscle wet mass, muscle-to-body mass ratio, fiber cross sectional area (CSA), fiber distribution and muscle ultrastructure. We also measured changes in the protein expression and activation states of Akt, mTOR and FoxO1 and the mRNA expression of atrogin-1 and MuRF1. Compared with the control groups, autumn and winter HU significantly lowered SOL muscle wet mass and muscle-to-body mass ratio, decreased type I and II fiber CSA and induced ultrastructural anomalies. However, these measured indices were unchanged between Pre-hibernation and Hibernation groups. Furthermore, phosphorylation levels of Akt and mTOR significantly decreased, while the phosphorylation level of FoxO1 and mRNA expression of atrogin-1 and MuRF1 increased after HU. During hibernation, the phosphorylation levels of Akt and mTOR significantly decreased, but the phosphorylation level of FoxO1 and mRNA expression of atrogin-1 and MuRF1 remained unchanged. Overall, our findings suggest that disuse and seasonality may not be sufficient to initiate the innate protective mechanism that prevents SOL atrophy during prolonged periods of hibernation inactivity. The stable expression of atrogin-1 and MuRF1 may facilitate to prevent SOL atrophy via controlling ubiquitination of muscle proteins during hibernation.

Summary: mRNA expression of atrogin-1 and MuRF1 remains unchanged during hibernation of Daurian ground squirrels, suggesting that stable expression of these genes may facilitate the prevention of SOL atrophy via controlling ubiquitylation of muscle proteins during hibernation.

Effects of ageing on expression of the muscle-specific E3 ubiquitin ligases and Akt-dependent regulation of Foxo transcription factors in skeletal muscle

Controversy exists as to whether the muscle-specific E3 ubiquitin ligases MAFbx and MuRF1 are transcriptionally upregulated in the process of sarcopenia. In the present study, we investigated the effects of ageing on mRNA/protein expression of muscle-specific E3 ubiquitin ligases and Akt/Foxo signalling in gastrocnemius muscles of female mice. Old mice exhibited a typical sarcopenic phenotype, characterized by loss of muscle mass and strength, decreased amount of myofibrillar proteins, incidence of aberrant muscle fibres, and genetic signature to sarcopenia. Activation levels of Akt were lower in adult and old mice than in young mice. Consequently, Akt-mediated phosphorylation levels of Foxo1 and Foxo3 proteins were decreased. Nuclear levels of Foxo1 and Foxo3 proteins showed an overall increasing trend in old mice. MAFbx mRNA expression was decreased in old mice relative to adult mice, whereas MuRF1 mRNA expression was less affected by ageing. At the protein level, MAFbx was less affected by ageing, whereas MuRF1 was increased in old mice relative to adult mice, with ubiquitin-protein conjugates being increased with ageing. In conclusion, we provided evidence for no mRNA upregulation of muscle-specific E3 ubiquitin ligases and disconnection between their expression and Akt/Foxo signalling in sarcopenic mice. Their different responsiveness to ageing may reflect different roles in sarcopenia.

Degradation systems in heart failure

Heart failure is a complex clinical syndrome that results from any structural or functional impairment of ventricular filling or the ejection of blood, and is a leading cause of morbidity and mortality in industrialized countries. The mechanisms underlying the development of heart failure are multiple, complex and not well understood. Cardiac mass and its homeostasis are maintained by the balance between protein synthesis and degradation, and an imbalance is likely to result in cellular dysfunction and disease. The protein degradation systems are the principle mechanisms for maintaining cellular homeostasis via protein quality control. Three major protein degradation systems have been identified, namely the calpain system, autophagy, and the ubiquitin proteasome system. Proinflammatory mediators involve the development and progression of heart failure. DNA and RNA degradation systems play a critical role in regulating inflammation and maintaining cellular homeostasis mediated by damaged DNA clearance and posttranscriptional regulation, respectively. This review discusses some recent advances in understanding the role of these degradation systems in heart failure.

Inhibition of Stat3 activation suppresses caspase-3 and the ubiquitin-proteasome system, leading to preservation of muscle mass in cancer cachexia

Cachexia occurs in patients with advanced cancers. Despite the adverse clinical impact of cancer-induced muscle wasting, pathways causing cachexia are controversial, and clinically reliable therapies are not available. A trigger of muscle protein loss is the Jak/Stat pathway, and indeed, we found that conditioned medium from C26 colon carcinoma (C26) or Lewis lung carcinoma cells activates Stat3 (p-Stat3) in C2C12 myotubes. We identified two proteolytic pathways that are activated in muscle by p-Stat3; one is activation of caspase-3, and the other is p-Stat3 to myostatin, MAFbx/Atrogin-1, and MuRF-1 via CAAT/enhancer-binding protein δ (C/EBPδ). Using sequential deletions of the caspase-3 promoter and CHIP assays, we determined that Stat3 activation increases caspase-3 expression in C2C12 cells. Caspase-3 expression and proteolytic activity were stimulated by p-Stat3 in muscles of tumor-bearing mice. In mice with cachexia caused by Lewis lung carcinoma or C26 tumors, knock-out of p-Stat3 in muscle or with a small chemical inhibitor of p-Stat3 suppressed muscle mass losses, improved protein synthesis and degradation in muscle, and increased body weight and grip strength. Activation of p-Stat3 stimulates a pathway from C/EBPδ to myostatin and expression of MAFbx/Atrogin-1 and increases the ubiquitin-proteasome system. Indeed, C/EBPδ KO decreases the expression of MAFbx/Atrogin-1 and myostatin, while increasing muscle mass and grip strength. In conclusion, cancer stimulates p-Stat3 in muscle, activating protein loss by stimulating caspase-3, myostatin, and the ubiquitin-proteasome system. These results could lead to novel strategies for preventing cancer-induced muscle wasting.

Constant heat stress reduces skeletal muscle protein deposition in broilers

BACKGROUND

This experiment was conducted to evaluate the effects of constant heat stress on growth performance and protein metabolism in skeletal muscle of Arbor Acres broilers.

RESULTS

Two hundred and seventy 21-day-old Arbor Acres broilers with similar body weight (1298 ± 28 g) were selected for a 3-week trial (29-49 days of age). The broilers were randomly assigned to three groups including the control group, constant heat stress group and pair-fed group. Up-regulation of the rectal temperature and the mRNA expression of heat shock protein 70 in liver indicate that the model for constant heat stress was success. The average daily gain, feed conversion ratio, breast and thigh muscle weight, percentage of breast muscle, crude protein content in breast and thigh muscle in constant heat stress group were significantly lower than in control group and pair-fed group. Serum uric acid content and the glutamic-oxaloacetic transaminase activity were significantly higher, while protein content and glutamic-pyruvate transaminase activity were significantly lower in liver of heat stress group than of the control and pair-fed groups. The expression of insulin-like growth factor 1, phosphatidylinositol 3-kinase and p70S6 kinase associated with protein synthesis were lower in breast muscle but higher in thigh muscle in heat stress group compared to the control or fed-pair groups. In thigh muscles, the expression of muscle ring-finger protein-1 and MAFbx associated with protein degradation were higher in the heat stress group than in the control and pair-fed groups.

CONCLUSION

Poor performance of the birds under heat stress may be due to lower synthesis and increased degradation of proteins.

Autophagy modulates amino acid signaling network in myotubes: differential effects on mTORC1 pathway and the integrated stress response

Induction of autophagy and the integrated stress response is important for amino acid homeostasis. It remains unknown whether the autophagy coregulates both mechanistic target of rapamycin complex 1 (mTORC1) signaling and the integrated stress response. In mouse C2C12 myotubes, we found that amino acid limitation induced autophagy and that the subsequent release of amino acid is required to sustain mTORC1 signaling. Inhibition of autophagy by bafilomycin A1 or chloroquine treatment during amino acid scarcity abolished mTORC1 signaling, an effect that could be rescued by inhibiting protein synthesis or amino acid supplementation, respectively. Autophagy is required to sustain the balance of both essential and nonessential amino acids during amino acid starvation, and it has a predominant role over the ubiquitin-proteasome system in the regulation of mTORC1. Inhibition of autophagy was found to activate the integrated stress response, as well as eukaryotic initiation factor 2α (eIF2α) and its target genes independent of amino acid availability. Conversely, autophagy induction via mTOR inhibition is sufficient to reduce eIF2α phosphorylation. Thus, autophagy protects the eIF2α-mediated stress response independent of amino acid supply in cultured myotubes. Our results showed that autophagy uniquely modulates mTORC1 and the integrated stress response in an amino acid-dependent and -independent manner, respectively.

Inhibition of the ubiquitin-proteasome pathway does not protect against ventilator-induced accelerated proteolysis or atrophy in the diaphragm

BACKGROUND

Mechanical ventilation (MV) is a life-saving intervention in patients with acute respiratory failure. However, prolonged MV results in ventilator-induced diaphragm dysfunction (VIDD), a condition characterized by both diaphragm fiber atrophy and contractile dysfunction. Previous work has shown that calpain, caspase-3, and the ubiquitin-proteasome pathway (UPP) are all activated in the diaphragm during prolonged MV. However, although it is established that both calpain and caspase-3 are important contributors to VIDD, the role that the UPP plays in the development of VIDD remains unknown. These experiments tested the hypothesis that inhibition of the UPP will protect the diaphragm against VIDD.

METHODS

The authors tested this prediction in an established animal model of MV using a highly specific UPP inhibitor, epoxomicin, to prevent MV-induced activation of the proteasome in the diaphragm (n = 8 per group).

RESULTS

The results of this study reveal that inhibition of the UPP did not prevent ventilator-induced diaphragm muscle fiber atrophy and contractile dysfunction during 12 h of MV. Also, inhibition of the UPP does not affect MV-induced increases in calpain and caspase-3 activity in the diaphragm. Finally, administration of the proteasome inhibitor did not protect against the MV-induced increases in the expression of the E3 ligases, muscle ring finger-1 (MuRF1), and atrogin-1/MaFbx.

CONCLUSION

Collectively, these results indicate that proteasome activation does not play a required role in VIDD development during the first 12 h of MV.

NADPH oxidase hyperactivity induces plantaris atrophy in heart failure rats

BACKGROUND

Skeletal muscle wasting is associated with poor prognosis and increased mortality in heart failure (HF) patients. Glycolytic muscles are more susceptible to catabolic wasting than oxidative ones. This is particularly important in HF since glycolytic muscle wasting is associated with increased levels of reactive oxygen species (ROS). However, the main ROS sources involved in muscle redox imbalance in HF have not been characterized. Therefore, we hypothesized that NADPH oxidases would be hyperactivated in the plantaris muscle of infarcted rats, contributing to oxidative stress and hyperactivation of the ubiquitin-proteasome system (UPS), ultimately leading to atrophy.

METHODS

Rats were submitted to myocardial infarction (MI) or Sham surgery. Four weeks after surgery, MI and Sham groups underwent eight weeks of treatment with apocynin, a NADPH oxidase inhibitor, or placebo. NADPH oxidase activity, oxidative stress markers, NF-κB activity, p38 MAPK phosphorylation, mRNA and sarcolemmal protein levels of NADPH oxidase components, UPS activation and fiber cross-sectional area were assessed in the plantaris muscle.

RESULTS

The plantaris of MI rats displayed atrophy associated with increased Nox2 mRNA and sarcolemmal protein levels, NADPH oxidase activity, ROS production, lipid hydroperoxides levels, NF-κB activity, p38 MAPK phosphorylation and UPS activation. NADPH oxidase inhibition by apocynin prevented MI-induced skeletal muscle atrophy by reducing ROS production, NF-κB hyperactivation, p38 MAPK phosphorylation and proteasomal hyperactivity.

CONCLUSION

Our data provide evidence for NADPH oxidase hyperactivation as an important source of ROS production leading to plantaris atrophy in heart failure rats, suggesting that this enzyme complex plays key role in skeletal muscle wasting in HF.

Decreased rate of protein synthesis, caspase-3 activity, and ubiquitin–proteasome proteolysis in soleus muscles from growing rats fed a low-protein, high-carbohydrate diet

The aim of this study was to investigate the changes in the rates of both protein synthesis and breakdown, and the activation of intracellular effectors that control these processes in soleus muscles from growing rats fed a low-protein, high-carbohydrate (LPHC) diet for 15 days. The mass and the protein content, as well as the rate of protein synthesis, were decreased in the soleus from LPHC-fed rats. The availability of amino acids was diminished, since the levels of various essential amino acids were decreased in the plasma of LPHC-fed rats. Overall rate of proteolysis was also decreased, explained by reductions in the mRNA levels of atrogin-1 and MuRF-1, ubiquitin conjugates, proteasome activity, and in the activity of caspase-3. Soleus muscles from LPHC-fed rats showed increased insulin sensitivity, with increased levels of insulin receptor and phosphorylation levels of AKT, which probably explains the inhibition of both the caspase-3 activity and the ubiquitin–proteasome system. The fall of muscle proteolysis seems to represent an adaptive response that contributes to spare proteins in a condition of diminished availability of dietary amino acids. Furthermore, the decreased rate of protein synthesis may be the driving factor to the lower muscle mass gain in growing rats fed the LPHC diet.

The Ubr2 Gene is Expressed in Skeletal Muscle Atrophying as a Result of Hind Limb Suspension, but not Merg1a Expression Alone

Skeletal muscle (SKM) atrophy is a potentially debilitating condition induced by muscle disuse, denervation, many disease states, and aging. The ubiquitin proteasome pathway (UPP) contributes greatly to the protein loss suffered in muscle atrophy. The MERG1a K⁺ channel is known to induce UPP activity and atrophy in SKM. It has been further demonstrated that the mouse ether-a-gogo-related gene (Merg)1a channel modulates expression of MURF1, an E3 ligase component of the UPP, while it does not affect expression of the UPP E3 ligase Mafbx/ATROGIN1. Because the UBR2 E3 ligase is known to participate in SKM atrophy, we have investigated the effect of Merg1a expression and hind limb suspension on Ubr2 expression. Here, we report that hind limb suspension results in a significant 25.6% decrease in mouse gastrocnemius muscle fiber cross sectional area (CSA) and that electro-transfer of Merg1a alone into gastrocnemius muscles yields a 15.3% decrease in CSA after 7 days. More interestingly, we discovered that hind limb suspension caused a significant 8-fold increase in Merg1a expression and a significant 4.7-fold increase in Ubr2 transcript after 4 days, while electro-transfer of Merg1a into gastrocnemius muscles resulted in a significant 6.2-fold increase in Merg1a transcript after 4 days but had no effect on Ubr2 expression. In summary, the MERG1a K⁺ channel, known to induce atrophy and MURF1 E3 ligase expression, does not affect UBR2 E3 ligase transcript levels. Therefore, to date, the MERG1a channel’s contribution to UPP activity appears mainly to be through up-regulation of Murf1 gene expression.

Lack of β2 -adrenoceptors aggravates heart failure-induced skeletal muscle myopathy in mice

Skeletal myopathy is a hallmark of heart failure (HF) and has been associated with a poor prognosis. HF and other chronic degenerative diseases share a common feature of a stressed system: sympathetic hyperactivity. Although beneficial acutely, chronic sympathetic hyperactivity is one of the main triggers of skeletal myopathy in HF. Considering that β2 -adrenoceptors mediate the activity of sympathetic nervous system in skeletal muscle, we presently evaluated the contribution of β2 -adrenoceptors for the morphofunctional alterations in skeletal muscle and also for exercise intolerance induced by HF. Male WT and β2 -adrenoceptor knockout mice on a FVB genetic background (β2 KO) were submitted to myocardial infarction (MI) or SHAM surgery. Ninety days after MI both WT and β2 KO mice presented to cardiac dysfunction and remodelling accompanied by significantly increased norepinephrine and epinephrine plasma levels, exercise intolerance, changes towards more glycolytic fibres and vascular rarefaction in plantaris muscle. However, β2 KO MI mice displayed more pronounced exercise intolerance and skeletal myopathy when compared to WT MI mice. Skeletal muscle atrophy of infarcted β2 KO mice was paralleled by reduced levels of phosphorylated Akt at Ser 473 while increased levels of proteins related with the ubiquitin--proteasome system, and increased 26S proteasome activity. Taken together, our results suggest that lack of β2 -adrenoceptors worsen and/or anticipate the skeletal myopathy observed in HF.

Three calpain isoforms are autolyzed in rat fast-twitch muscle after eccentric contractions

The present study investigated changes in autolysis of three calpain isoforms in skeletal muscles undergoing eccentric contractions (ECC), leading to prolonged force deficits. Rat extensor digitorum longus and tibialis anterior muscles were exposed to 200-repeated ECC in situ, excised immediately after or 3 or 6 days after cessation of ECC, and used for measures of force output and for biochemical analyses. Full restoration of tetanic force in ECC-treated muscles was not attained until 6 days of recovery. Maximal calpain activity determined by a fluorogenic substrate was unaltered immediately after ECC, but increased to 313 and 450 % after 3 and 6 days, respectively. Increases in the amount of autolyzed calpain-3 were apparent immediately and developed progressively with recovery time, whereas elevations of autolyzed μ- and m-calpain occurred after 3 and 6 days, respectively. The protein content was augmented only in m-calpain. It is suggested that the three calpain isoforms may be involved in the dismantling, repair, remodeling and/or regeneration processes in ECC-treated muscles.

Supplementation of ketoacids contributes to the up-regulation of the Wnt7a/Akt/p70S6K pathway and the down-regulation of apoptotic and ubiquitin–proteasome systems in the muscle of 5/6 nephrectomised rats

Ketoacids (KA) are known to improve muscle mass among patients with chronic kidney disease (CKD) on a low-protein diet (CKD-LPD), but the mechanism of its preventive effects on muscle atrophy still remains unclear. Since muscle atrophy in CKD may be attributable to the down-regulation of the Wnt7a/Akt/p70S6K pathway and the activation of the ubiquitin–proteasome system (UPS) and the apoptotic signalling pathway, a hypothesis can readily be drawn that KA supplementation improves muscle mass by up-regulating the Wnt7a/Akt/p70S6K pathway and counteracting the activation of the UPS and caspase-3-dependent apoptosis in the muscle of CKD-LPD rats. Rats with 5/6 nephrectomy were randomly divided into three groups, and fed with either 22 % protein (normal-protein diet; NPD), 6 % protein (LPD) or 5 % protein plus 1 % KA for 24 weeks. Sham-operated rats with NPD intake were used as the control. The results demonstrated that KA supplementation improved protein synthesis and increased related mediators such as Wnt7a, phosphorylated Akt and p70S6K in the muscle of CKD-LPD rats. It also inhibited protein degradation, withheld the increase in ubiquitin and its ligases MAFbx (muscle atrophy F-box) and MuRF1 (muscle ring finger-1) as well as attenuated proteasome activity in the muscle of CKD-LPD rats. Moreover, KA supplementation gave rise to a reduction in DNA fragment, cleaved caspase-3 and 14 kDa actin fragment via the down-regulation of the Bax:Bcl-2 ratio in the muscle of CKD-LPD rats. The beneficial effects unveiled herein further consolidate that KA may be a better therapeutic strategy for muscle atrophy in CKD-LPD.

Mechanisms of muscle growth and atrophy in mammals and Drosophila

BACKGROUND

The loss of skeletal muscle mass (atrophy) that accompanies disuse and systemic diseases is highly debilitating. Although the pathogenesis of this condition has been primarily studied in mammals, Drosophila is emerging as an attractive system to investigate some of the mechanisms involved in muscle growth and atrophy.

RESULTS

In this review, we highlight the outstanding unsolved questions that may benefit from a combination of studies in both flies and mammals. In particular, we discuss how different environmental stimuli and signaling pathways influence muscle mass and strength and how a variety of disease states can cause muscle wasting.

CONCLUSIONS

Studies in Drosophila and mammals should help identify molecular targets for the treatment of muscle wasting in humans.

Genetic ablation of 12/15-lipoxygenase but not 5-lipoxygenase protects against denervation-induced muscle atrophy

Skeletal muscle atrophy is a debilitating outcome of a number of chronic diseases and conditions associated with loss of muscle innervation by motor neurons, such as aging and neurodegenerative diseases. We previously reported that denervation-induced loss of muscle mass is associated with activation of cytosolic phospholipase A2 (cPLA2), the rate-limiting step for the release of arachidonic acid from membrane phospholipids, which then acts as a substrate for metabolic pathways that generate bioactive lipid mediators. In this study, we asked whether 5- and 12/15-lipoxygenase (LO) lipid metabolic pathways downstream of cPLA2 mediate denervation-induced muscle atrophy in mice. Both 5- and 12/15-LO were activated in response to surgical denervation; however, 12/15-LO activity was increased ~2.5-fold versus an ~1.5-fold increase in activity of 5-LO. Genetic and pharmacological inhibition of 12/15-LO (but not 5-LO) significantly protected against denervation-induced muscle atrophy, suggesting a selective role for the 12/15-LO pathway in neurogenic muscle atrophy. The activation of the 12/15-LO pathway (but not 5-LO) during muscle atrophy increased NADPH oxidase activity, protein ubiquitination, and ubiquitin-proteasome-mediated proteolytic degradation. In conclusion, this study reveals a novel pathway for neurogenic muscle atrophy and suggests that 12/15-LO may be a potential therapeutic target in diseases associated with loss of innervation and muscle atrophy.

TWEAK/Fn14 Signaling Axis Mediates Skeletal Muscle Atrophy and Metabolic Dysfunction

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) through binding to its receptor fibroblast growth factor inducible 14 (Fn14) has been shown to regulate many cellular responses including proliferation, differentiation, apoptosis, inflammation, and fibrosis, under both physiological and pathological conditions. Emerging evidence suggests that TWEAK is also a major muscle wasting cytokine. TWEAK activates nuclear factor-κB signaling and proteolytic pathways such as ubiquitin–proteasome system, autophagy, and caspases to induce muscle proteolysis in cultured myotubes. Fn14 is dormant or expressed in minimal amounts in normal healthy muscle. However, specific atrophic conditions, such as denervation, immobilization, and starvation stimulate the expression of Fn14 leading to activation of TWEAK/Fn14 signaling and eventually skeletal muscle atrophy. TWEAK also causes slow- to fast-type fiber transition in skeletal muscle. Furthermore, recent studies suggest that TWEAK diminishes mitochondrial content and represses skeletal muscle oxidative phosphorylation capacity. TWEAK mediates these effects through affecting the expression of a number of genes and microRNAs. In this review article, we have discussed the recent advancements toward understanding the role and mechanisms of action of TWEAK/Fn14 signaling in skeletal muscle with particular reference to different models of atrophy and oxidative metabolism.

Autophagy signaling in skeletal muscle of infarcted rats

Background

Heart failure (HF)-induced skeletal muscle atrophy is often associated to exercise intolerance and poor prognosis. Better understanding of the molecular mechanisms underlying HF-induced muscle atrophy may contribute to the development of pharmacological strategies to prevent or treat such condition. It has been shown that autophagy-lysosome system is an important mechanism for maintenance of muscle mass. However, its role in HF-induced myopathy has not been addressed yet. Therefore, the aim of the present study was to evaluate autophagy signaling in myocardial infarction (MI)-induced muscle atrophy in rats.

Methods/Principal Findings

Wistar rats underwent MI or Sham surgeries, and after 12 weeks were submitted to echocardiography, exercise tolerance and histology evaluations. Cathepsin L activity and expression of autophagy-related genes and proteins were assessed in soleus and plantaris muscles by fluorimetric assay, qRT-PCR and immunoblotting, respectively. MI rats displayed exercise intolerance, left ventricular dysfunction and dilation, thereby suggesting the presence of HF. The key findings of the present study were: a) upregulation of autophagy-related genes (GABARAPL1, ATG7, BNIP3, CTSL1 and LAMP2) was observed only in plantaris while muscle atrophy was observed in both soleus and plantaris muscles, and b) Cathepsin L activity, Bnip3 and Fis1 protein levels, and levels of lipid hydroperoxides were increased specifically in plantaris muscle of MI rats.

Conclusions

Altogether our results provide evidence for autophagy signaling regulation in HF-induced plantaris atrophy but not soleus atrophy. Therefore, autophagy-lysosome system is differentially regulated in atrophic muscles comprising different fiber-types and metabolic characteristics.

Simulated microgravity contributes to autophagy induction by regulating AMP-activated protein kinase

Exposure to microgravity is supposed to affect almost all biological systems, and we speculated that microgravity is potentially involved in autophagy regulation. A clinostat was used to simulate microgravity, and HEK293 cells that stably express GFP-LC3 were used for sensitive monitoring of autophagy induction. The clinorotation of GFP-LC3 cells resulted in autophagosome formation in the cytoplasm and a change in autophagosomal marker expression. Autophagy induction was accompanied by phosphorylation of AMPK (Thr 172) and by the dephosphorylation of mammalian target of rapamycin. To elucidate the role of AMPK in microgravity-induced autophagy, we suppressed AMPK expression by knockdown via siRNA, which inhibited the induction of autophagy upon exposure to microgravity. In addition, the clinorotation of C2C12 myotube cells resulted in the enlarged and distinctive LC3 spots in the cytoplasm and AMPK activation. These results indicate that simulated microgravity possibly contributes to autophagy induction by regulating AMPK.

Disruptive influence

Cachexia, a condition that kills about one-fifth of cancer patients, may be linked to Rb—a protein that is already linked to various cancers—moving from the cell nucleus to the cytoplasm.

MURF2B, a novel LC3-binding protein, participates with MURF2A in the switch between autophagy and ubiquitin proteasome system during differentiation of C2C12 muscle cells

The ubiquitin proteasome system and macroautophagy are proteolytic pathways essential in the maintenance of cellular homeostasis during differentiation and remodelling of skeletal muscle. In both pathways, proteins to be degraded are tagged with polyubiquitin. In skeletal muscles, the MURF2 proteins display E3 ubiquitin ligase structure suggesting that they may covalently attach ubiquitin polypeptides to still unknown target proteins. So far only MURF2A isoforms were studied and shown to interact with p62/SQSTM1, a protein implicated in macroautophagic and ubiquitin proteasome system degradations. Here, we analyzed the MURF2B and MURF2A proteins and show that the ratio of the isoforms changes during differentiation of muscle C2C12 cells and that the shift of the isoforms expression follows the sequential activation of autophagic or proteasomal degradation. We also show that MURF2B has a functional domain needed for its interaction with LC3, a protein needed for autophagic vesicles formation. Using specific MURF2 RNAi cells we observed that MURF2A and MURF2B are both needed for the formation of autophagosomes and that in the absence of MURF2B, the cells expressing MURF2A display an activated ubiquitin proteasome system implicated in the degradation of p62/SQSTM1 by UPS. Altogether, our results indicate that MURF2A and MURF2B proteins could participate in the molecular switch between the two ubiquitin degradative pathways.

Mechanisms of skeletal muscle aging: insights from Drosophila and mammalian models

A characteristic feature of aged humans and other mammals is the debilitating, progressive loss of skeletal muscle function and mass that is known as sarcopenia. Age-related muscle dysfunction occurs to an even greater extent during the relatively short lifespan of the fruit fly Drosophila melanogaster. Studies in model organisms indicate that sarcopenia is driven by a combination of muscle tissue extrinsic and intrinsic factors, and that it fundamentally differs from the rapid atrophy of muscles observed following disuse and fasting. Extrinsic changes in innervation, stem cell function and endocrine regulation of muscle homeostasis contribute to muscle aging. In addition, organelle dysfunction and compromised protein homeostasis are among the primary intrinsic causes. Some of these age-related changes can in turn contribute to the induction of compensatory stress responses that have a protective role during muscle aging. In this Review, we outline how studies in Drosophila and mammalian model organisms can each provide distinct advantages to facilitate the understanding of this complex multifactorial condition and how they can be used to identify suitable therapies.

Mechanical properties of respiratory muscles

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

Disruption of calcium homeostasis in cardiomyocytes underlies cardiac structural and functional changes in severe sepsis

Sepsis, a major cause of morbidity/mortality in intensive care units worldwide, is commonly associated with cardiac dysfunction, which worsens the prognosis dramatically for patients. Although in recent years the concept of septic cardiomyopathy has evolved, the importance of myocardial structural alterations in sepsis has not been fully explored. This study offers novel and mechanistic data to clarify subcellular events that occur in the pathogenesis of septic cardiomyopathy and myocardial dysfunction in severe sepsis. Cultured neonatal mice cardiomyocytes subjected to serum obtained from mice with severe sepsis presented striking increment of [Ca²⁺]_i and calpain-1 levels associated with decreased expression of dystrophin and disruption and derangement of F-actin filaments and cytoplasmic bleb formation. Severe sepsis induced in mice led to an increased expression of calpain-1 in cardiomyocytes. Moreover, decreased myocardial amounts of dystrophin, sarcomeric actin, and myosin heavy chain were observed in septic hearts associated with depressed cardiac contractile dysfunction and a very low survival rate. Actin and myosin from the sarcomere are first disassembled by calpain and then ubiquitinated and degraded by proteasome or sequestered inside specialized vacuoles called autophagosomes, delivered to the lysosome for degradation forming autophagolysosomes. Verapamil and dantrolene prevented the increase of calpain-1 levels and preserved dystrophin, actin, and myosin loss/reduction as well cardiac contractile dysfunction associated with strikingly improved survival rate. These abnormal parameters emerge as therapeutic targets, which modulation may provide beneficial effects on future vascular outcomes and mortality in sepsis. Further studies are needed to shed light on this mechanism, mainly regarding specific calpain inhibitors.

Muscle wasting from kidney failure-a model for catabolic conditions

PURPOSE

Muscle atrophy is a frequent complication of chronic kidney disease (CKD) and is associated with increased morbidity and mortality. The processes causing loss of muscle mass are also present in several catabolic conditions. Understanding the pathogenesis of CKD-induced muscle loss could lead to therapeutic interventions that prevent muscle wasting in CKD and potentially, other catabolic conditions.

MAJOR FINDINGS

Insulin or IGF-1 resistance caused by CKD, acidosis, inflammation, glucocorticoids or cancer causes defects in insulin-stimulated intracellular signaling that suppresses IRS-1 activity leading to decreased phosphorylation of Akt (p-Akt). A low p-Akt activates caspase-3 which provides muscle proteins substrates of the ubiquitin-proteasome system (UPS). A low p-Akt also leads to decreased phosphorylation of forkhead transcription factors which enter the nucleus to stimulate the expression of atrogin-1/MAFbx and MuRF1, E3 ubiquitin ligases that can be associated with proteolysis of muscle cells by the UPS. Caspase-3 also stimulates proteasome-dependent proteolysis in muscle.

SUMMARY

In CKD, diabetes, inflammatory conditions or in response to acidosis or excess glucocorticoids, insulin resistance develops, initiating reduced IRS-1/PI3K/Akt signaling. In CKD, this reduces p-Akt which stimulates muscle proteolysis by activating caspase-3 and the UPS. Second, caspase-3 cleaves actomyosin yielding substrates for the UPS and increased proteasome-mediated proteolysis. Third, p-Akt down-regulation suppresses myogenesis in CKD. Fourth, exercise in CKD stimulates insulin/IGF-1 signaling to reduce muscle atrophy. Lastly, there is evidence that microRNAs influence insulin signaling providing a potential opportunity to design therapeutic interventions. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Disuse-induced muscle wasting

Loss of skeletal muscle mass occurs frequently in clinical settings in response to joint immobilization and bed rest, and is induced by a combination of unloading and inactivity. Disuse-induced atrophy will likely affect every person in his or her lifetime, and can be debilitating especially in the elderly. Currently there are no good therapies to treat disuse-induced muscle atrophy, in part, due to a lack of understanding of the cellular and molecular mechanisms responsible for the induction and maintenance of muscle atrophy. Our current understanding of disuse atrophy comes from the investigation of a variety of models (joint immobilization, hindlimb unloading, bed rest, spinal cord injury) in both animals and humans. Under conditions of unloading, it is widely accepted that there is a decrease in protein synthesis, however, the role of protein degradation, especially in humans, is debated. This review will examine the current understanding of the molecular and cellular mechanisms regulating muscle loss under disuse conditions, discussing the similarities and areas of dispute between the animal and human literature. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Role of the ubiquitin proteasome system in the heart

Proper protein turnover is required for cardiac homeostasis and, accordingly, impaired proteasomal function appears to contribute to heart disease. Specific proteasomal degradation mechanisms underlying cardiovascular biology and disease have been identified, and such cellular pathways have been proposed to be targets of clinical relevance. This review summarizes the latest literature regarding the specific E3 ligases involved in heart biology, and the general ways that the proteasome regulates protein quality control in heart disease. The potential for therapeutic intervention in Ubiquitin Proteasome System function in heart disease is discussed.

Calpain and caspase-3 play required roles in immobilization-induced limb muscle atrophy

Prolonged skeletal muscle inactivity results in a rapid decrease in fiber size, primarily due to accelerated proteolysis. Although several proteases are known to contribute to disuse muscle atrophy, the ubiquitin proteasome system is often considered the most important proteolytic system during many conditions that promote muscle wasting. Emerging evidence suggests that calpain and caspase-3 may also play key roles in inactivity-induced atrophy of respiratory muscles, but it remains unknown if these proteases are essential for disuse atrophy in limb skeletal muscles. Therefore, we tested the hypothesis that activation of both calpain and caspase-3 is required for locomotor muscle atrophy induced by hindlimb immobilization. Seven days of immobilization (i.e., limb casting) promoted significant atrophy in type I muscle fibers of the rat soleus muscle. Independent pharmacological inhibition of calpain or caspase-3 prevented this casting-induced atrophy. Interestingly, inhibition of calpain activity also prevented caspase-3 activation, and, conversely, inhibition of caspase-3 prevented calpain activation. These findings indicate that a regulatory cross talk exists between these proteases and provide the first evidence that the activation of calpain and caspase-3 is required for inactivity-induced limb muscle atrophy.

Investigation of wild-type and mycolactone-negative mutant Mycobacterium ulcerans on skeletal muscle: IGF-1 protects against mycolactone-induced muscle catabolism

Buruli ulcer (BU), which is caused by Mycobacterium ulcerans (MU), is an endemic and neglected tropical disease that affects mostly subcutaneous tissues. Skeletal muscle under infected skin is also subject to serious dysfunctions and contractures. The goal of this study was to investigate the effects of an infection with the wild-type M. ulcerans (WT-MU) or the mycolactone-negative Mycobacterium ulcerans (Mneg-MU) mutant strains on myotubes or fully differentiated skeletal muscles. WT-MU infection decreased by 22% and 29% the maximal muscle force at days 7 and 42 postinfection, respectively, while Mneg-MU induced no decrease at day 7 postinfection and a small but significant 13% decrease in muscle force at day 42. A 13.2-fold and 4.3-fold increase in neutrophil and macrophage concentrations, respectively, was observed on day 42 following the injection of WT-MU. However, the increases in neutrophil and macrophage concentrations were 2.4-fold and 5.5-fold in Mneg-MU. Myoblast proliferation decreased by 20%, myotube diameter by 45%, MyHC levels by 32%, while MuRF-1 levels increased by 22.8% when C2C12 cells and WT-MU were cocultured for 48 h at a multiplicity of infection of 5:1. In contrast, Mneg-MU had no significant effect. Interestingly, the addition of 1,000 ng/ml of IGF-1 to the WT-MU/C2C12 coculture significantly improved all of these biological parameters. The present investigation clearly established that muscle dysfunction and chronic inflammation in the presence of WT-MU are largely caused by the release of mycolactone, and the addition of recombinant IGF-1 was sufficient to alleviate some of the antiproliferative and atrophic effects of mycolactone.

Insulin resistance and muscle metabolism in chronic kidney disease

Insulin resistance is a common finding in chronic kidney disease (CKD) and is manifested by mild fasting hyperglycemia and abnormal glucose tolerance testing. Circulating levels of glucocorticoids are high. In muscle, changes in the insulin signaling pathway occur. An increase in the regulatory p85 subunit of Class I phosphatidylinositol 3-Kinase enzyme leads to decreased activation of the downstream effector protein kinase B (Akt). Mechanisms promoting muscle proteolysis and atrophy are unleashed. The link of Akt to the ubiquitin proteasome pathway, a major degradation pathway in muscle, is discussed. Another factor associated with insulin resistance in CKD is angiotensin II (Ang II) which appears to induce its intracellular effects through inflammatory cytokines or reactive oxygen species. Skeletal muscle ATP is depleted and the ability of AMP-activated protein kinase (AMPK) to replenish energy stores is blocked. How this can be reversed is discussed. Interleukin-6 (IL-6) levels are elevated in CKD and impair insulin signaling at the level of IRS-1. With exercise, IL-6 levels are reduced; glucose uptake and utilization are increased. For patients with CKD, exercise may improve insulin signaling and build up muscle. Treatment strategies for preventing muscle atrophy are discussed.

Immunological detection of m- and µ-calpains in the skeletal muscle of Marchigiana cattle

Calpains are Ca²⁺-dependent proteases able to cleave a large number of proteins involved in many biological functions. Particularly, in skeletal muscle they are involved in meat tenderizing during post mortem storage. In this report we analyzed the presence and expression of µ- and m-calpains in two skeletal muscles of the Marchigiana cattle soon after slaughter, using immunocytochemical and immunohistochemical techniques, Western blotting analysis and Casein Zymography. Therefore, the presence and the activity of these proteases was investigated until 15^th day post mortem during normal process of meat tenderizing. The results showed m- and µ-calpain immunosignals in the cytoplasm both along the Z disk/I band regions and in the form of intracellular stores. Moreover, the expression level of µ-calpain but not m-calpain decreased after 10 days of storage. Such a decrease in µ-calpain was accompanied by a gradual reduction of activity. On the contrary, m-calpain activity persisted up to 15 days of post mortem storage. Such data indicate that expression and activity of both µ-calpain and m-calpain analyzed in the Marchigiana cattle persist longer than reported in literature for other bovines and may be related to both the type of muscle and breed examined.

Mechanisms stimulating muscle wasting in chronic kidney disease: the roles of the ubiquitin-proteasome system and myostatin

Catabolic conditions including chronic kidney disease (CKD), cancer, and diabetes cause muscle atrophy. The loss of muscle mass worsens the burden of disease because it is associated with increased morbidity and mortality. To avoid these problems or to develop treatment strategies, the mechanisms leading to muscle wasting must be identified. Specific mechanisms uncovered in CKD generally occur in other catabolic conditions. These include stimulation of protein degradation in muscle arising from activation of caspase-3 and the ubiquitin-proteasome system (UPS). These proteases act in a coordinated fashion with caspase-3 initially cleaving the complex structure of proteins in muscle, yielding fragments that are substrates that are degraded by the UPS. Fortunately, the UPS exhibits remarkable specificity for proteins to be degraded because it is the major intracellular proteolytic system. Without a high level of specificity cellular functions would be disrupted. The specificity is accomplished by complex reactions that depend on recognition of a protein substrate by specific E3 ubiquitin ligases. In muscle, the specific ligases are Atrogin-1 and MuRF-1, and their expression has characteristics of a biomarker of accelerated muscle proteolysis. Specific complications of CKD (metabolic acidosis, insulin resistance, inflammation, and angiotensin II) activate caspase-3 and the UPS through mechanisms that include glucocorticoids and impaired insulin or IGF-1 signaling. Mediators activate myostatin, which functions as a negative growth factor in muscle. In models of cancer or CKD, strategies that block myostatin prevent muscle wasting, suggesting that therapies that block myostatin could prevent muscle wasting in catabolic conditions.

Age-related loss of nitric oxide synthase in skeletal muscle causes reductions in calpain S-nitrosylation that increase myofibril degradation and sarcopenia

Sarcopenia, the age-related loss of muscle mass, is a highly-debilitating consequence of aging. In this investigation, we show sarcopenia is greatly reduced by muscle-specific overexpression of calpastatin, the endogenous inhibitor of calcium-dependent proteases (calpains). Further, we show that calpain cleavage of specific structural and regulatory proteins in myofibrils is prevented by covalent modification of calpain by nitric oxide (NO) through S-nitrosylation. We find that calpain in adult, non-sarcopenic muscles is S-nitrosylated but that aging leads to loss of S-nitrosylation, suggesting that reduced S-nitrosylation during aging leads to increased calpain-mediated proteolysis of myofibrils. Further, our data show that muscle aging is accompanied by loss of neuronal nitric oxide synthase (nNOS), the primary source of muscle NO, and that expression of a muscle-specific nNOS transgene restores calpain S-nitrosylation in aging muscle and prevents sarcopenia. Together, the findings show that in vivo reduction of calpain S-nitrosylation in muscle may be an important component of sarcopenia, indicating that modulation of NO can provide a therapeutic strategy to slow muscle loss during old age.

Post-translational modification and regulation of actin

Many of the best-studied actin regulatory proteins use non-covalent means to modulate the properties of actin. Yet, actin is also susceptible to covalent modifications of its amino acids. Recent work is increasingly revealing that actin processing and its covalent modifications regulate important cellular events. In addition, numerous pathogens express enzymes that specifically use actin as a substrate to regulate their hosts' cells. Actin post-translational alterations have been linked to different normal and disease processes and the effects associated with metabolic and environmental stressors. Herein, we highlight specific co-translational and post-translational modifications of actin and discuss the current understanding of the role that these modifications play in regulating actin.

Ubiquitylation by Trim32 causes coupled loss of desmin, Z-bands, and thin filaments in muscle atrophy

During muscle atrophy, myofibrillar proteins are degraded in an ordered process in which MuRF1 catalyzes ubiquitylation of thick filament components (Cohen et al. 2009. J. Cell Biol. http://dx.doi.org/10.1083/jcb.200901052). Here, we show that another ubiquitin ligase, Trim32, ubiquitylates thin filament (actin, tropomyosin, troponins) and Z-band (α-actinin) components and promotes their degradation. Down-regulation of Trim32 during fasting reduced fiber atrophy and the rapid loss of thin filaments. Desmin filaments were proposed to maintain the integrity of thin filaments. Accordingly, we find that the rapid destruction of thin filament proteins upon fasting was accompanied by increased phosphorylation of desmin filaments, which promoted desmin ubiquitylation by Trim32 and degradation. Reducing Trim32 levels prevented the loss of both desmin and thin filament proteins. Furthermore, overexpression of an inhibitor of desmin polymerization induced disassembly of desmin filaments and destruction of thin filament components. Thus, during fasting, desmin phosphorylation increases and enhances Trim32-mediated degradation of the desmin cytoskeleton, which appears to facilitate the breakdown of Z-bands and thin filaments.