MURF-1 and MURF-2 target a specific subset of myofibrillar proteins redundantly: towards understanding MURF-dependent muscle ubiquitination

Rare variants in genes encoding MuRF1 and MuRF2 are modifiers of hypertrophic cardiomyopathy

Modifier genes contribute to the diverse clinical manifestations of hypertrophic cardiomyopathy (HCM), but are still largely unknown. Muscle ring finger (MuRF) proteins are a class of muscle-specific ubiquitin E3-ligases that appear to modulate cardiac mass and function by regulating the ubiquitin-proteasome system. In this study we screened all the three members of the MuRF family, MuRF1, MuRF2 and MuRF3, in 594 unrelated HCM patients and 307 healthy controls by targeted resequencing. Identified rare variants were confirmed by capillary Sanger sequencing. The prevalence of rare variants in both MuRF1 and MuRF2 in HCM patients was higher than that in control subjects (MuRF1 13/594 (2.2%) vs. 1/307 (0.3%), p = 0.04; MuRF2 22/594 (3.7%) vs. 2/307 (0.7%); p = 0.007). Patients with rare variants in MuRF1 or MuRF2 were younger (p = 0.04) and had greater maximum left ventricular wall thickness (p = 0.006) than those without such variants. Mutations in genes encoding sarcomere proteins were present in 19 (55.9%) of the 34 HCM patients with rare variants in MuRF1 and MuRF2. These data strongly supported that rare variants in MuRF1 and MuRF2 are associated with higher penetrance and more severe clinical manifestations of HCM. The findings suggest that dysregulation of the ubiquitin-proteasome system contributes to the pathogenesis of HCM.

Gigantic business: titin properties and function through thick and thin

The giant protein titin forms a unique filament network in cardiomyocytes, which engages in both mechanical and signaling functions of the heart. TTN, which encodes titin, is also a major human disease gene. In this review, we cover the roles of cardiac titin in normal and failing hearts, with a special emphasis on the contribution of titin to diastolic stiffness. We provide an update on disease-associated titin mutations in cardiac and skeletal muscles and summarize what is known about the impact of protein-protein interactions on titin properties and functions. We discuss the importance of titin-isoform shifts and titin phosphorylation, as well as titin modifications related to oxidative stress, in adjusting the diastolic stiffness of the healthy and the failing heart. Along the way we distinguish among titin alterations in systolic and in diastolic heart failure and ponder the evidence for titin stiffness as a potential target for pharmacological intervention in heart disease.

Muscle RING finger-1 promotes a maladaptive phenotype in chronic hypoxia-induced right ventricular remodeling

Exposure to chronic hypoxia (CH) induces elevated pulmonary artery pressure/resistance, leading to an eventual maladaptive right ventricular hypertrophy (RVH). Muscle RING finger-1 (MuRF1) is a muscle-specific ubiquitin ligase that mediates myocyte atrophy and has been shown to play a role in left ventricular hypertrophy and altered cardiac bioenergetics in pressure overloaded hearts. However, little is known about the contribution of MuRF1 impacting RVH in the setting of CH. Therefore, we hypothesized that MuRF1 deletion would enhance RVH compared to their wild-type littermates, while cardiac-specific overexpression would reduce hypertrophy following CH-induced pulmonary hypertension. We assessed right ventricular systolic pressure (RVSP), right ventricle to left ventricle plus septal weight ratio (RV/LV+S) and hematocrit (Hct) following a 3-wk isobaric CH exposure. Additionally, we conducted dual-isotope SPECT/CT imaging with cardiac function agent ²⁰¹Tl-chloride and cell death agent ^99mTc-annexin V. Predictably, CH induced pulmonary hypertension, measured by increased RVSP, RV/LV+S and Hct in WT mice compared to normoxic WT mice. Normoxic WT and MuRF1-null mice exhibited no significant differences in RVSP, RV/LV+S or Hct. CH-induced increases in RVSP were also similar between WT and MuRF1-null mice; however, RV/LV+S and Hct were significantly elevated in CH-exposed MuRF1-null mice compared to WT. In cardiac-specific MuRF1 overexpressing mice, RV/LV+S increased significantly due to CH exposure, even greater than in WT mice. This remodeling appeared eccentric, maladaptive and led to reduced systemic perfusion. In conclusion, these results are consistent with an atrophic role for MuRF1 regulating the magnitude of right ventricular hypertrophy following CH-induction of pulmonary hypertension.

MuRF1 activity is present in cardiac mitochondria and regulates reactive oxygen species production in vivo

MuRF1 is a previously reported ubiquitin-ligase found in striated muscle that targets troponin I and myosin heavy chain for degradation. While MuRF1 has been reported to interact with mitochondrial substrates in yeast two-hybrid studies, no studies have identified MuRF1's role in regulating mitochondrial function to date. In the present study, we measured cardiac mitochondrial function from isolated permeabilized muscle fibers in previously phenotyped MuRF1 transgenic and MuRF1-/- mouse models to determine the role of MuRF1 in intermediate energy metabolism and ROS production. We identified a significant decrease in reactive oxygen species production in cardiac muscle fibers from MuRF1 transgenic mice with increased α-MHC driven MuRF1 expression. Increased MuRF1 expression in ex vivo and in vitro experiments revealed no alterations in the respiratory chain complex I and II function. Working perfusion experiments on MuRF1 transgenic hearts demonstrated significant changes in glucose oxidation. However, total oxygen consumption was decreased [corrected]. This data provides evidence for MuRF1 as a novel regulator of cardiac ROS, offering another mechanism by which increased MuRF1 expression may be cardioprotective in ischemia reperfusion injury, in addition to its inhibition of apoptosis via proteasome-mediate degradation of c-Jun. The lack of mitochondrial function phenotype identified in MuRF1-/- hearts may be due to the overlapping interactions of MuRF1 and MuRF2 with energy regulating proteins found by yeast two-hybrid studies reported here, implying a duplicity in MuRF1 and MuRF2's regulation of mitochondrial function.

Molecular basis for the fold organization and sarcomeric targeting of the muscle atrogin MuRF1

MuRF1 is an E3 ubiquitin ligase central to muscle catabolism. It belongs to the TRIM protein family characterized by a tripartite fold of RING, B-box and coiled-coil (CC) motifs, followed by variable C-terminal domains. The CC motif is hypothesized to be responsible for domain organization in the fold as well as for high-order assembly into functional entities. But data on CC from this family that can clarify the structural significance of this motif are scarce. We have characterized the helical region from MuRF1 and show that, contrary to expectations, its CC domain assembles unproductively, being the B2- and COS-boxes in the fold (respectively flanking the CC) that promote a native quaternary structure. In particular, the C-terminal COS-box seemingly forms an α-hairpin that packs against the CC, influencing its dimerization. This shows that a C-terminal variable domain can be tightly integrated within the conserved TRIM fold to modulate its structure and function. Furthermore, data from transfected muscle show that in MuRF1 the COS-box mediates the in vivo targeting of sarcoskeletal structures and points to the pharmacological relevance of the COS domain for treating MuRF1-mediated muscle atrophy.

Diggin' on u(biquitin): a novel method for the identification of physiological E3 ubiquitin ligase substrates

The ubiquitin-proteasome system (UPS) plays a central role in maintaining protein homeostasis, emphasized by a myriad of diseases that are associated with altered UPS function such as cancer, muscle-wasting, and neurodegeneration. Protein ubiquitination plays a central role in both the promotion of proteasomal degradation as well as cellular signaling through regulation of the stability of transcription factors and other signaling molecules. Substrate-specificity is a critical regulatory step of ubiquitination and is mediated by ubiquitin ligases. Recent studies implicate ubiquitin ligases in multiple models of cardiac diseases such as cardiac hypertrophy, atrophy, and ischemia/reperfusion injury, both in a cardioprotective and maladaptive role. Therefore, identifying physiological substrates of cardiac ubiquitin ligases provides both mechanistic insights into heart disease as well as possible therapeutic targets. Current methods identifying substrates for ubiquitin ligases rely heavily upon non-physiologic in vitro methods, impeding the unbiased discovery of physiological substrates in relevant model systems. Here we describe a novel method for identifying ubiquitin ligase substrates utilizing tandem ubiquitin binding entities technology, two-dimensional differential in gel electrophoresis, and mass spectrometry, validated by the identification of both known and novel physiological substrates of the ubiquitin ligase MuRF1 in primary cardiomyocytes. This method can be applied to any ubiquitin ligase, both in normal and disease model systems, in order to identify relevant physiological substrates under various biological conditions, opening the door to a clearer mechanistic understanding of ubiquitin ligase function and broadening their potential as therapeutic targets.

Cardiac troponin in ischemic cardiomyocytes: intracellular decrease before onset of cell death

AIM

Cardiac troponin I (cTnI) and T (cTnT) are the most important biomarkers in the diagnosis of acute myocardial infarction (AMI). Nevertheless, they can be elevated in the absence of AMI. It is unclear if such elevations represent irreversible cardiomyocyte-damage or leakage from viable cardiomyocytes. Our objective is to evaluate whether cTn is released from viable cardiomyocytes in response to ischemia and to identify differences in the release of cTn and its molecular forms.

METHODS AND RESULTS

HL-1 cardiomyocytes (mouse) were subjected to ischemia (modeled by anoxia with glucose deprivation). The total contents and molecular forms of cTn were determined in culture media and cell lysates. Cell viability was assessed from the release of lactate dehydrogenase (LDH). Before the release of LDH, the intracellular cTn content in ischemic cells decreased significantly compared to control (52% for cTnI; 23% for cTnT) and was not matched by a cTn increase in the medium. cTnI decreased more rapidly than cTnT, resulting in an intracellular cTnT/cTnI ratio of 25.5 after 24 h of ischemia. Western blots revealed changes in the relative amounts of fragmented cTnI and cTnT in ischemic cells.

CONCLUSIONS

HL-1 cardiomyocytes subjected to simulated ischemia released cTnI and cTnT only in combination with the release of LDH. We find no evidence of cTn release from viable cardiomyocytes, but did observe a significant decrease in cTn content, before the onset of cell death. Intracellular decrease of cTn in viable cardiomyocytes can have important consequences for the interpretation of cTn values in clinical practice.

Z-disc transcriptional coupling, sarcomeroptosis and mechanoptosis [corrected]

Cardiovascular diseases are the leading cause of morbidity and mortality worldwide. Heart failure, which contributes significantly to the incidence and prevalence of cardiovascular-related diseases, can be the result of a myriad of diverse aetiologies including viral infections, coronary heart disease and genetic abnormalities—just to name a few. Interestingly, almost every type of heart failure is characterized by the loss of cardiac myocytes, either via necrosis, apoptosis or autophagy. While the former for a long time mainly has been characterized by passive loss of cells and only the latter two have been regarded as active processes, a new view is now emerging, whereby all three forms of cell death are regarded as different types of programmed cell death which can be induced via different stimuli and pathways, most of which are probably not well understood (Kung et al., Circulation Research 108(8):1017–1036, 2011). Here, we focus on the sarcomeric Z-disc, Z-disc transcriptional coupling and its role in pro-survival pathways as well as in striated muscle specific forms of cell death (sarcomeroptosis) and mechanically induced apoptosis or mechanoptosis.

The role of ubiquitin ligases in cardiac disease

Rigorous surveillance of protein quality control is essential for the maintenance of normal cardiac function, while the dysregulation of protein turnover is present in a diverse array of common cardiac diseases. Central to the protein quality control found in all cells is the ubiquitin proteasome system (UPS). The UPS plays a critical role in protein trafficking, cellular signaling, and most prominently, protein degradation. As ubiquitin ligases (E3s) control the specificity of the UPS, their description in the cardiomyocyte has highlighted how ubiquitin ligases are critical to the turnover and function of the sarcomere complex, responsible for the heart's required continuous contraction. In this review, we provide an overview of the UPS, highlighting a comprehensive overview of the cardiac ubiquitin ligases identified to date. We then focus on recent studies of new cardiac ubiquitin ligases outlining their novel roles in protein turnover, cellular signaling, and the regulation of mitochondrial dynamics and receptor turnover in the pathophysiology of cardiac hypertrophy, cardiac atrophy, myocardial infarction, and heart failure. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".

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.

Suppression of atrogin-1 and MuRF1 prevents dexamethasone-induced atrophy of cultured myotubes

OBJECTIVE

The mechanistic role of the ubiquitin ligases atrogin-1 and MuRF1 in glucocorticoid-induced muscle wasting is not fully understood. Here, we tested the hypothesis that glucocorticoid-induced muscle atrophy is at least in part linked to atrogin-1 and MuRF1 expression and that the ubiquitin ligases are regulated by compensatory mechanisms.

METHODS

The expression of atrogin-1 and MuRF1 was suppressed individually or in combination in cultured L6 myotubes by using siRNA technique. Myotubes were treated with dexamethasone followed by determination of mRNA and protein levels for atrogin-1 and MuRF1, protein synthesis and degradation rates, and myotube morphology.

RESULTS

Suppression of atrogin-1 resulted in increased expression of MuRF1 and vice versa, suggesting that the ubiquitin ligases are regulated by compensatory mechanisms. Simultaneous suppression of atrogin-1 and MuRF1 resulted in myotube hypertrophy, mainly reflecting stimulated protein synthesis, and prevented dexamethasone-induced myotube atrophy, mainly reflecting inhibited protein degradation.

CONCLUSIONS

The results provide evidence for a link between upregulated atrogin-1 and MuRF1 expression and glucocorticoid-induced muscle atrophy. The study also suggests that atrogin-1 and MuRF1 levels are regulated by compensatory mechanisms and that inhibition of both ubiquitin ligases may be needed to prevent glucocorticoid-induced muscle proteolysis and atrophy.

Calpain-6 deficiency promotes skeletal muscle development and regeneration

Calpains are Ca(2+)-dependent modulator Cys proteases that have a variety of functions in almost all eukaryotes. There are more than 10 well-conserved mammalian calpains, among which eutherian calpain-6 (CAPN6) is unique in that it has amino acid substitutions at the active-site Cys residue (to Lys in humans), strongly suggesting a loss of proteolytic activity. CAPN6 is expressed predominantly in embryonic muscles, placenta, and several cultured cell lines. We previously reported that CAPN6 is involved in regulating microtubule dynamics and actin reorganization in cultured cells. The physiological functions of CAPN6, however, are still unclear. Here, to elucidate CAPN6's in vivo roles, we generated Capn6-deficient mice, in which a lacZ expression cassette was integrated into the Capn6 gene. These Capn6-deficient mouse embryos expressed lacZ predominantly in skeletal muscles, as well as in cartilage and the heart. Histological and biochemical analyses showed that the CAPN6 deficiency promoted the development of embryonic skeletal muscle. In primary cultured skeletal muscle cells that were induced to differentiate into myotubes, Capn6 expression was detected in skeletal myocytes, and Capn6-deficient cultures showed increased differentiation. Furthermore, we found that CAPN6 was expressed in the regenerating skeletal muscles of adult mice after cardiotoxin-induced degeneration. In this experimental system, Capn6-deficient mice exhibited more advanced skeletal-muscle regeneration than heterozygotes or wild-type mice at the same time point. These results collectively showed that a loss of CAPN6 promotes skeletal muscle differentiation during both development and regeneration, suggesting a novel physiological function of CAPN6 as a suppressor of skeletal muscle differentiation.

Doxorubicin induces protein ubiquitination and inhibits proteasome activity during cardiotoxicity

Anthracycline-induced cardiotoxicity is a clinically complex syndrome that leads to substantial morbidity and mortality for cancer survivors. Despite several years of research, the underlying molecular mechanisms remain largely undefined and thus effective therapies to manage this condition are currently non-existent. This study therefore aimed to determine the contribution of the ubiquitin-proteasome pathway (UPP) and endoplasmic reticulum (ER)-stress within this context. Cardiotoxicity was induced with the use of doxorubicin (DXR) in H9C2 rat cardiomyoblasts (3 μM) for 24 h, whereas the tumour-bearing GFP-LC3 mouse model was treated with a cumulative dose of 20 mg/kg. Markers for proteasome-specific protein degradation were significantly upregulated in both models following DXR treatment, however proteasome activity was lost. Moreover, ER-stress as assessed by increased ER load was considerably augmented (in vitro) with modest binding of DXR with ER. These results suggest that DXR induces intrinsic activation of the UPP and ER stress which ultimately contributes to dysfunction of the myocardium during this phenomenon.

Recessive MYL2 mutations cause infantile type I muscle fibre disease and cardiomyopathy

A cardioskeletal myopathy with onset and death in infancy, morphological features of muscle type I hypotrophy with myofibrillar disorganization and dilated cardiomyopathy was previously reported in three Dutch families. Here we report the genetic cause of this disorder. Multipoint parametric linkage analysis of six Dutch patients identified a homozygous region of 2.1 Mb on chromosome 12, which was shared between all Dutch patients, with a log of odds score of 10.82. Sequence analysis of the entire linkage region resulted in the identification of a homozygous mutation in the last acceptor splice site of the myosin regulatory light chain 2 gene (MYL2) as the genetic cause. MYL2 encodes a myosin regulatory light chain (MLC-2V). The myosin regulatory light chains bind, together with the essential light chains, to the flexible neck region of the myosin heavy chain in the hexameric myosin complex and have a structural and regulatory role in muscle contraction. The MYL2 mutation results in use of a cryptic splice site upstream of the last exon causing a frameshift and replacement of the last 32 codons by 20 different codons. Whole exome sequencing of an Italian patient with similar clinical features showed compound heterozygosity for two other mutations affecting the same exon of MYL2, also resulting in mutant proteins with altered C-terminal tails. As a consequence of these mutations, the second EF-hand domain is disrupted. EF-hands, assumed to function as calcium sensors, can undergo a conformational change upon binding of calcium that is critical for interactions with downstream targets. Immunohistochemical staining of skeletal muscle tissue of the Dutch patients showed a diffuse and weak expression of the mutant protein without clear fibre specificity, while normal protein was absent. Heterozygous missense mutations in MYL2 are known to cause dominant hypertrophic cardiomyopathy; however, none of the parents showed signs of cardiomyopathy. In conclusion, the mutations in the last exon of MYL2 are responsible for a novel autosomal recessive lethal myosinopathy due to defects changing the C-terminal tail of the ventricular form of the myosin regulatory light chain. We propose 'light chain myopathy' as a name for this MYL2-associated myopathy.

Muscle ring finger 1 and muscle ring finger 2 are necessary but functionally redundant during developmental cardiac growth and regulate E2F1-mediated gene expression in vivo

AIMS

Muscle ring finger (MuRF) proteins have been implicated in the transmission of mechanical forces to nuclear cell signaling pathways through their association with the sarcomere. We recently reported that MuRF1, but not MuRF2, regulates pathologic cardiac hypertrophy in vivo. This was surprising given that MuRF1 and MuRF2 interact with each other and many of the same sarcomeric proteins experimentally.

METHODS AND RESULTS

Mice missing all four MuRF1 and MuRF2 alleles [MuRF1/MuRF2 double null (DN)] were born with a massive spontaneous hypertrophic cardiomyopathy and heart failure; mice that were null for one of the genes but heterozygous for the other (i.e. MuRF1(-/-) //MuRF2(+/-) or MuRF1(+/-) //MuRF2(-/-) ) were phenotypically identical to wild-type mice. Microarray analysis of genes differentially-expressed between MuRF1/MuRF2 DN, mice missing three of the four alleles and wild-type mice revealed a significant enrichment of genes regulated by the E2F transcription factor family. More than 85% of the differentially-expressed genes had E2F promoter regions (E2f:DP; P<0.001). Western analysis of E2F revealed no differences between MuRF1/MuRF2 DN hearts and wild-type hearts; however, chromatin immunoprecipitation studies revealed that MuRF1/MuRF2 DN hearts had significantly less binding of E2F1 in the promoter regions of genes previously defined to be regulated by E2F1 (p21, Brip1 and PDK4, P<0.01).

CONCLUSIONS

These studies suggest that MuRF1 and MuRF2 play a redundant role in regulating developmental physiologic hypertrophy, by regulating E2F transcription factors essential for normal cardiac development by supporting E2F localization to the nucleus, but not through a process that degrades the transcription factor.

Breaking down protein degradation mechanisms in cardiac muscle

Regulated protein degradation through the ubiquitin-proteasome and lysosomal/autophagy systems is critical for homeostatic protein turnover in cardiac muscle and for proper cardiac function. The discovery of muscle-specific components in these systems has illuminated how aberrations in their levels are pivotal to the development of cardiac stress and disease. New evidence suggests that equal importance in disease development should be given to ubiquitously expressed degradation components. These are compartmentalized within cardiac muscles and, when mislocalized, can be critical in the development of specific cardiac diseases. Here, we discuss how alterations in the compartmentalization of degradation components affect disease states, the tools available to investigate these mechanisms, as well as recent discoveries that highlight the therapeutic value of targeting these pathways in disease.

In vivo and in vitro investigations of heterozygous nebulin knock-out mice disclose a mild skeletal muscle phenotype

Nemaline myopathy is the most common congenital skeletal muscle disease, and mutations in the nebulin gene account for 50% of all cases. Recent studies suggest that the disease severity might be related to the nebulin expression levels. Considering that mutations in the nebulin gene are typically recessive, one would expect that a single functional nebulin allele would maintain nebulin protein expression which would result in preserved skeletal muscle function. We investigated skeletal muscle function of heterozygous nebulin knock-out (i.e., nebulin(+/-)) mice using a multidisciplinary approach including protein and gene expression analysis and combined in vivo and in vitro force measurements. Skeletal muscle anatomy and energy metabolism were studied strictly non-invasively using magnetic resonance imaging and 31P-magnetic resonance spectroscopy. Maximal force production was reduced by around 16% in isolated muscle of nebulin(+/-) mice while in vivo force generating capacity was preserved. Muscle weakness was associated with a shift toward a slower proteomic phenotype, but was not related to nebulin protein deficiency or to an impaired energy metabolism. Further studies would be warranted in order to determine the mechanisms leading to a mild skeletal muscle phenotype resulting from the expression of a single nebulin allele.

ICU-acquired weakness: mechanisms of disability

PURPOSE OF REVIEW

ICU-acquired weakness (ICUAW) is now recognized as a major complication of critical illness. There is no doubt that ICUAW is prevalent - some might argue ubiquitous - after critical illness, but its true role, the interaction with preexisting nerve and muscle lesions as well as its contribution to long-term functional disability, remains to be elucidated.

RECENT FINDINGS

In this article, we review the current state-of-the-art of the basic pathophysiology of nerve and muscle weakness after critical illness and explore the current literature on ICUAW with a special emphasis on the most important mechanisms of weakness.

SUMMARY

Variable contributions of structural and functional changes likely contribute to both early and late myopathy and neuropathy, although the specifics of the temporality of both processes, and the influence patient comorbidities, age, and nature of the ICU insult have on them, remain to be determined.

The ubiquitin ligase Nedd4-1 participates in denervation-induced skeletal muscle atrophy in mice

Skeletal muscle atrophy is a consequence of muscle inactivity resulting from denervation, unloading and immobility. It accompanies many chronic disease states and also occurs as a pathophysiologic consequence of normal aging. In all these conditions, ubiquitin-dependent proteolysis is a key regulator of the loss of muscle mass, and ubiquitin ligases confer specificity to this process by interacting with, and linking ubiquitin moieties to target substrates through protein∶protein interaction domains. Our previous work suggested that the ubiquitin-protein ligase Nedd4-1 is a potential mediator of skeletal muscle atrophy associated with inactivity (denervation, unloading and immobility). Here we generated a novel tool, the Nedd4-1 skeletal muscle-specific knockout mouse (myo^Cre;Nedd4-1^flox/flox) and subjected it to a well validated model of denervation induced skeletal muscle atrophy. The absence of Nedd4-1 resulted in increased weights and cross-sectional area of type II fast twitch fibres of denervated gastrocnemius muscle compared with wild type littermates controls, at seven and fourteen days following tibial nerve transection. These effects are not mediated by the Nedd4-1 substrates MTMR4, FGFR1 and Notch-1. These results demonstrate that Nedd4-1 plays an important role in mediating denervation-induced skeletal muscle atrophy in vivo.

Inhibition of xanthine oxidase by allopurinol prevents skeletal muscle atrophy: role of p38 MAPKinase and E3 ubiquitin ligases

Alterations in muscle play an important role in common diseases and conditions. Reactive oxygen species (ROS) are generated during hindlimb unloading due, at least in part, to the activation of xanthine oxidase (XO). The major aim of this study was to determine the mechanism by which XO activation causes unloading-induced muscle atrophy in rats, and its possible prevention by allopurinol, a well-known inhibitor of this enzyme. For this purpose we studied one of the main redox sensitive signalling cascades involved in skeletal muscle atrophy i.e. p38 MAPKinase, and the expression of two well known muscle specific E3 ubiquitin ligases involved in proteolysis, the Muscle atrophy F-Box (MAFbx; also known as atrogin-1) and Muscle RING (Really Interesting New Gene) Finger-1 (MuRF-1). We found that hindlimb unloading induced a significant increase in XO activity and in the protein expression of the antioxidant enzymes CuZnSOD and Catalase in skeletal muscle. The most relevant new fact reported in this paper is that inhibition of XO with allopurinol, a drug widely used in clinical practice, prevents soleus muscle atrophy by ∼20% after hindlimb unloading. This was associated with the inhibition of the p38 MAPK-MAFbx pathway. Our data suggest that XO was involved in the loss of muscle mass via the activation of the p38MAPK-MAFbx pathway in unloaded muscle atrophy. Thus, allopurinol may have clinical benefits to combat skeletal muscle atrophy in bedridden, astronauts, sarcopenic, and cachexic patients.

Catabolic signaling pathways, atrogenes, and ubiquitinated proteins are regulated by the nutritional status in the muscle of the fine flounder

A description of the intracellular mechanisms that modulate skeletal muscle atrophy in early vertebrates is still lacking. In this context, we used the fine flounder, a unique and intriguing fish model, which exhibits remarkably slow growth due to low production of muscle-derived IGF-I, a key growth factor that has been widely acknowledged to prevent and revert muscle atrophy. Key components of the atrophy system were examined in this species using a detailed time-course of sampling points, including two contrasting nutritional periods. Under basal conditions high amounts of the atrogenes MuRF-1 and Atrogin-1 were observed. During fasting, the activation of the P38/MAPK and Akt/FoxO signaling pathways decreased; whereas, the activation of the IκBα/NFκB pathway increased. These changes in signal transduction activation were concomitant with a strong increase in MuRF-1, Atrogin-1, and protein ubiquitination. During short-term refeeding, the P38/MAPK and Akt/FoxO signaling pathways were strongly activated, whereas the activation of the IκBα/NFκB pathway decreased significantly. The expression of both atrogenes, as well as the ubiquitination of proteins, dropped significantly during the first hour of refeeding, indicating a strong anti-atrophic condition during the onset of refeeding. During long-term refeeding, Akt remained activated at higher than basal levels until the end of refeeding, and Atrogin-1 expression remained significantly lower during this period. This study shows that the components of the atrophy system in skeletal muscle appeared early in the evolution of vertebrates and some mechanisms have been conserved, whereas others have not. These results represent an important achievement for the area of fish muscle physiology, showing an integrative view of the atrophy system in a non-mammalian species and contributing to novel insights on the molecular basis of muscle growth regulation in earlier vertebrates.

Titin-based tension in the cardiac sarcomere: molecular origin and physiological adaptations

The passive stiffness of cardiac muscle plays a critical role in ventricular filling during diastole and is determined by the extracellular matrix and the sarcomeric protein titin. Titin spans from the Z-disk to the M-band of the sarcomere and also contains a large extensible region that acts as a molecular spring and develops passive force during sarcomere stretch. This extensible segment is titin's I-band region, and its force-generating mechanical properties determine titin-based passive tension. The properties of titin's I-band region can be modulated by isoform splicing and post-translational modification and are intimately linked to diastolic function. This review discusses the physical origin of titin-based passive tension, the mechanisms that alter titin stiffness, and titin's role in stress-sensing signaling pathways.

Histone deacetylase inhibition suppresses myogenin-dependent atrogene activation in spinal muscular atrophy mice

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disease caused by mutations in the survival of motor neuron 1 (SMN1) gene and deficient expression of the ubiquitously expressed SMN protein. Pathologically, SMA is characterized by motor neuron loss and severe muscle atrophy. During muscle atrophy, the E3 ligase atrogenes, atrogin-1 and muscle ring finger 1 (MuRF1), mediate muscle protein breakdown through the ubiquitin proteasome system. Atrogene expression can be induced by various upstream regulators. During acute denervation, they are activated by myogenin, which is in turn regulated by histone deacetylases 4 and 5. Here we show that atrogenes are induced in SMA model mice and in SMA patient muscle in association with increased myogenin and histone deacetylase-4 (HDAC4) expression. This activation during both acute denervation and SMA disease progression is suppressed by treatment with a histone deacetylase inhibitor; however, this treatment has no effect when atrogene induction occurs independently of myogenin. These results indicate that myogenin-dependent atrogene induction is amenable to pharmacological intervention with histone deacetylase inhibitors and help to explain the beneficial effects of these agents on SMA and other denervating diseases.

Obscurin and KCTD6 regulate cullin-dependent small ankyrin-1 (sAnk1.5) protein turnover

Small ankyrin-1 isoform 5 (sAnk1.5) turnover is regulated by posttranslational modification (ubiquitylation, neddylation, and acetylation), the presence of obscurin, and KCTD6 (a novel tissue-specific interaction partner). KCTD6 links sAnk1.5 to cullin-3. The absence of obscurin results in translocation of sAnk1.5/KCTD6 to the Z-disk and loss of sAnk1.5 on the protein level.

Protein turnover through cullin-3 is tightly regulated by posttranslational modifications, the COP9 signalosome, and BTB/POZ-domain proteins that link cullin-3 to specific substrates for ubiquitylation. In this paper, we report how potassium channel tetramerization domain containing 6 (KCTD6) represents a novel substrate adaptor for cullin-3, effectively regulating protein levels of the muscle small ankyrin-1 isoform 5 (sAnk1.5).

Binding of sAnk1.5 to KCTD6, and its subsequent turnover is regulated through posttranslational modification by nedd8, ubiquitin, and acetylation of C-terminal lysine residues. The presence of the sAnk1.5 binding partner obscurin, and mutation of lysine residues increased sAnk1.5 protein levels, as did knockdown of KCTD6 in cardiomyocytes. Obscurin knockout muscle displayed reduced sAnk1.5 levels and mislocalization of the sAnk1.5/KCTD6 complex. Scaffolding functions of obscurin may therefore prevent activation of the cullin-mediated protein degradation machinery and ubiquitylation of sAnk1.5 through sequestration of sAnk1.5/KCTD6 at the sarcomeric M-band, away from the Z-disk–associated cullin-3. The interaction of KCTD6 with ankyrin-1 may have implications beyond muscle for hereditary spherocytosis, as KCTD6 is also present in erythrocytes, and erythrocyte ankyrin isoforms contain its mapped minimal binding site.

Developmental regulation of MURF E3 ubiquitin ligases in skeletal muscle

The striated muscle-specific tripartite motif (TRIM) proteins TRIM63/MURF1, TRIM55/MURF2 and TRIM54/MURF3 can function as E3 ubiquitin ligases in ubiquitin-mediated muscle protein turnover. Despite the well-characterised role of MURF1 in skeletal muscle atrophy, the dynamics of MURF isogene expression in the development and early postnatal adaptation of skeletal muscle is unknown. Here, we show that MURF2 is the isogene most highly expressed in embryonic skeletal muscle at E15.5, with the 50 kDa A isoform predominantly expressed. MURF1 and MURF3 are upregulated only postnatally. Knockdown of MURF2 p50A by isoform-specific siRNA results in delayed myogenic differentiation and myotube formation in vitro, with perturbation of the stable, glutamylated microtubule population. This underscores that MURF2 plays an important role in the earliest stages of skeletal muscle differentiation and myofibrillogenesis. During further development, there is a shift towards the 60 kDa A isoform, which dominates postnatally. Analysis of the fibre-type expression shows that MURF2 A isoforms are predominantly slow-fibre associated, whilst MURF1 is largely excluded from these fibres, and MURF3 is ubiquitously distributed in both type I and II fibres.

In vivo characterization of mutant myotilins

Myofibrillar myopathy (MFM) is a group of disorders that are pathologically defined by the disorganization of the myofibrillar alignment associated with the intracellular accumulation of Z-disk-associated proteins. MFM is caused by mutations in genes encoding Z-disk-associated proteins, including myotilin. Although a number of MFM mutations have been identified, it has been difficult to elucidate the precise roles of the mutant proteins. Here, we present a useful method for the characterization of mutant proteins associated with MFM. Expression of mutant myotilins in mouse tibialis anterior muscle by in vivo electroporation recapitulated both the pathological changes and the biochemical characteristics observed in patients with myotilinopathy. In mutant myotilin-expressing muscle fibers, myotilin aggregates and is costained with polyubiquitin, and Z-disk-associated proteins and myofibrillar disorganization were commonly seen. In addition, the expressed S60C mutant myotilin protein displayed marked detergent insolubility in electroporated mouse muscle, similar to that observed in human MFM muscle with the same mutation. Thus, in vivo electroporation can be a useful method for evaluating the pathogenicity of mutations identified in MFM.

Daunorubicin therapy is associated with upregulation of E3 ubiquitin ligases in the heart

Daunorubicin (DNR) and doxorubicin (DOX) are two of the most effective anthracycline drugs known for the treatment of systemic neoplasms and solid tumors. However, their clinical use is hampered due to profound cardiotoxicity. The mechanism by which DNR injures the heart remains to be fully elucidated. Recent reports have indicated that DOX activates ubiquitin proteasome-mediated degradation of specific transcription factors; however, no reports exist on the effect of DNR on the E3 ubiquitin ligases, MURF-1 (muscle ring finger 1) and MAFbx (muscle atrophy F-box). The aim of this study was to investigate the effect of DNR treatment on the protein and organelle degradation systems in the heart and to elucidate some of the signalling mechanisms involved. Adult rats were divided into two groups where one group received six intraperitoneal injections of 2 mg/kg DNR on alternate days and the other group received saline injections as control. Hearts were excised and perfused on a working heart system the day after the last injection and freeze-clamped for biochemical analysis. DNR treatment significantly attenuated cardiac function and increased apoptosis in the heart. DNR-induced cardiac cytotoxicity was associated with upregulation of the E3 ligases, MURF-1 and MAFbx and also caused significant increases in two markers of autophagy, beclin-1 and LC3. These changes observed in the heart were also associated with attenuation of the phosphoinositide 3-kinase/Akt signalling pathway.

Various jobs of proteolytic enzymes in skeletal muscle during unloading: facts and speculations

Skeletal muscles, namely, postural muscles, as soleus, suffer from atrophy under disuse. Muscle atrophy development caused by unloading differs from that induced by denervation or other stimuli. Disuse atrophy is supposed to be the result of shift of protein synthesis/proteolysis balance towards protein degradation increase. Maintaining of the balance involves many systems of synthesis and proteolysis, whose activation leads to muscle adaptation to disuse rather than muscle degeneration. Here, we review recent data on activity of signaling systems involved in muscle atrophy development under unloading and muscle adaptation to the lack of support.

MuRFs: specialized members of the TRIM/RBCC family with roles in the regulation of the trophic state of muscle and its metabolism

MuRFs, brief for muscle specific RING finger proteins, correspond to a subfamily of the TRIM/RBCC protein family. Here, we review recent progress on the structural biology of MuRF1, the MuRF family member being most clearly associated with muscle diseases. The emerging understanding of the structural biology of MuRFs and their interaction with their numerous myocellular proteins, at least in part representing ubiquitination targets for MuRFs, is likely to provide future rationales to modulate their activity, thus affecting their roles in muscle disease progression.

Analysis of myotilin turnover provides mechanistic insight into the role of myotilinopathy-causing mutations

MFM (myofibrillar myopathies) are caused by mutations in several sarcomeric components, including the Z-disc protein myotilin. The morphological changes typical of MFM include Z-disc alterations and aggregation of dense filamentous sarcomeric material. The causes and mechanisms of protein aggregation in myotilinopathies and other forms of MFM remain unknown, although impaired degradation may explain, in part, the abnormal protein accumulation. In the present paper we have studied the mechanisms regulating myotilin turnover, analysed the consequences of defective myotilin degradation and tested whether disease-causing myotilin mutations result in altered protein turnover. The results indicate that myotilin is a substrate for the Ca(2+)-dependent protease calpain and identify two calpain cleavage sites in myotilin by MS. We further show that myotilin is degraded by the proteasome system in transfected COS7 cells and in myotubes, and that disease-causing myotilinopathy mutations result in reduced degradation. Finally, we show that proteolysis-inhibitor-induced reduction in myotilin turnover results in formation of intracellular myotilin and actin-containing aggregates, which resemble those seen in diseased muscle cells. These findings identify for the first time biological differences between wt (wild-type) and mutant myotilin. The present study provides novel information on the pathways controlling myotilin turnover and on the molecular defects associated with MFM.

Effects of alfacalcidol on circulating cytokines and growth factors in rat skeletal muscle

Supra-physiological levels of vitamin D induce skeletal muscle atrophy, which may be particularly detrimental in already sarcopaenic elderly. Neither the cause nor whether the atrophy is fibre type specific are known. To obtain supraphysiological levels of circulating vitamin D (1,25(OH)(2)D(3)) 27.5-month-old female Fischer(344) × Brown Norway F1 rats were orally treated for 6 weeks with vehicle or the vitamin D analogue alfacalcidol. Alfacalcidol treatment induced a 22% decrease in body mass and 17% muscle atrophy. Fibre atrophy was restricted to type IIb fibres in the low-oxidative part of the gastrocnemius medialis only (-22%; P < 0.05). There was a concomitant 1.6-fold increase in mRNA expression of the ubiquitin ligase MuRF-1 (P < 0.001), whereas those of insulin-like growth factor 1 and myostatin were not affected. Circulating IL-6 was unaltered, but leptin and adiponectin were decreased (-39%) and increased (64%), respectively. The treated rats also exhibited a reduced food intake. In conclusion, supraphysiological levels of circulating 1,25(OH)(2)D(3) cause preferential atrophy of type IIb fibres, which is associated with an increased expression of MuRF-1 without evidence of systemic inflammation. The atrophy and loss of body mass in the presence of supra-physiological levels of vitamin D are primarily due to a reduced food intake.

Effects of sex steroids on indices of protein turnover in rainbow trout (Oncorhynchusmykiss) white muscle

Effects of 17β-estradiol (E2), testosterone, and 5α-dihydrotestosterone (DHT) on protein turnover and proteolytic gene expression were determined in rainbow trout (Oncorhynchus mykiss) primary myocytes and white muscle tissue. E2 reduced rates of protein synthesis and increased rates of protein degradation in primary myocytes by 45% and 27%, respectively. DHT reduced rates of protein synthesis by 27%. Testosterone did not affect protein synthesis and neither testosterone nor DHT affected rates of protein degradation. Single injections of E2 increased expression of ubiquitin ligase genes fbxo32, fbxo25, and murf1, and the proteasome subunit psmd6 by 24h after injection. Within the cathepsin-lysosome pathway, E2 increased expression of cathepsins ctsd and ctsl, as well as autophagy-related genes atg4b and lc3b. Additionally, E2 injection up-regulated the expression of casp3 and casp9 caspase genes. Incubation of primary myocytes with E2 also increased expression of ubiquitin ligase genes. Therefore, catabolic effects of E2 on protein turnover result in part from E2-induced increases in proteolytic gene expression directly in muscle. Injection of testosterone increased milli-calpain (capn2) and casp3 expression, and DHT increased ctsd expression in vivo, whereas both androgens up-regulated fbxo32 expression in primary myocytes. These results suggest that effects of androgens on protein turnover in muscle are not driven primarily by direct effects of these hormones in this tissue.

Cardiac proteasome activity in muscle ring finger-1 null mice at rest and following synthetic glucocorticoid treatment

Muscle ring finger-1 (MuRF1) is a muscle-specific E3 ubiquitin ligase that has been implicated in the regulation of cardiac mass through its control of the ubiquitin proteasome system. While it has been suggested that MuRF1 is required for cardiac atrophy, a resting cardiac phenotype has not been reported in mice with a null deletion [knockout (KO)] of MuRF1. Here, we report that MuRF1 KO mice have significantly larger hearts than age-matched wild-type (WT) littermates at ≥ 6 mo of age and that loss of cardiac mass can occur in the absence of MuRF1. The objective of this study was to determine whether changes in proteasome activity were responsible for the cardiac phenotypes observed in MuRF1 KO mice. Cardiac function, architecture, and proteasome activity were analyzed at rest and following 28 days of dexamethasone (Dex) treatment in 6-mo-old WT and MuRF1 KO mice. Echocardiography demonstrated normal cardiac function in the enlarged hearts in MURF1 KO mice. At rest, heart mass and cardiomyocyte diameter were significantly greater in MuRF1 KO than in WT mice. The increase in cardiac size in MuRF1 KO mice was related to a decrease in proteasome activity and an increase in Akt signaling relative to WT mice. Dex treatment induced a significant loss of cardiac mass in MuRF1 KO, but not WT, mice. Furthermore, Dex treatment resulted in an increase in proteasome activity in KO, but a decrease in WT, mice. In contrast, Akt/mammalian target of rapamycin signaling decreased in MuRF1 KO mice and increased in WT mice in response to Dex treatment. These findings demonstrate that MuRF1 plays an important role in regulating cardiac size through alterations in protein turnover and that MuRF1 is not required to induce cardiac atrophy.

Effects of fasting and refeeding on expression of MAFbx and MuRF1 in chick skeletal muscle

The present study investigated the effects of fasting and refeeding on the expression of proteasome-related genes and their downstream targets in the skeletal muscles of chicks. Seven-day-old chicks were fasted for 24 or 48 h and then refed for 4 h. The expression levels of MAFbx and MuRF1, which function as E3 ligases in the ubiquitin-proteasome system, were investigated at the mRNA and protein levels. MAFbx and MuRF1 expression were increased by fasting and these increases were downregulated by refeeding. The expression of the target proteins of these E3 ligases, MyoD and M-CK, was also analyzed. The levels of these proteins were downregulated by fasting, and these decreases were rescued by refeeding. The results of this study indicate that fasting stimulates MAFbx and MuRF1 expression in chicks, possibly leading to increased degradation of their corresponding target proteins.

The sarcomeric Z-disc and Z-discopathies

The sarcomeric Z-disc defines the lateral borders of the sarcomere and has primarily been seen as a structure important for mechanical stability. This view has changed dramatically within the last one or two decades. A multitude of novel Z-disc proteins and their interacting partners have been identified, which has led to the identification of additional functions and which have now been assigned to this structure. This includes its importance for intracellular signalling, for mechanosensation and mechanotransduction in particular, an emerging importance for protein turnover and autophagy, as well as its molecular links to the t-tubular system and the sarcoplasmic reticulum. Moreover, the discovery of mutations in a wide variety of Z-disc proteins, which lead to perturbations of several of the above-mentioned systems, gives rise to a diverse group of diseases which can be termed Z-discopathies. This paper provides a brief overview of these novel aspects as well as points to future research directions.

Insulin-like growth factor (IGF) signalling and genome-wide transcriptional regulation in fast muscle of zebrafish following a single-satiating meal

Male zebrafish (Danio rerio) were fasted for 7 days and fed to satiation over 3 h to investigate the transcriptional responses to a single meal. The intestinal content at satiety (6.3% body mass) decreased by 50% at 3 h and 95% at 9 h following food withdrawal. Phosphorylation of the insulin-like growth factor (IGF) signalling protein Akt peaked within 3 h of feeding and was highly correlated with gut fullness. Retained paralogues of IGF hormones genes were regulated with feeding, with igf1a showing a pronounced peak in expression after 3 h and igf2b after 6 h. Igf-I receptor transcripts were markedly elevated with fasting, and decreased to their lowest levels 45 min after feeding. igf1rb transcripts increased more quickly than igf1ra transcripts as the gut emptied. Paralogues of the insulin-like growth factor binding proteins (IGFBPs) were constitutively expressed, except for igfbp1a and igfbp1b transcripts, which were significantly elevated with fasting. Genome-wide transcriptional responses were analysed using the Agilent 44K oligonucleotide microarray and selected genes validated by qPCR. Fasting was associated with the upregulation of genes for the ubiquitin-proteasome degradation pathway, anti-proliferative and pro-apoptotic genes. Protein chaperones (unc45b, hspd1, hspa5, hsp90a.1, hsp90a.2) and chaperone interacting proteins (ahsa1 and stip1) were upregulated 3 h after feeding along with genes for the initiation of protein synthesis and mRNA processing. Transcripts for the enzyme ornithine decarboxylase 1 showed the largest increase with feeding (11.5-fold) and were positively correlated with gut fullness. This study demonstrates the fast nature of the transcriptional responses to a meal and provides evidence for differential regulation of retained paralogues of IGF signalling pathway genes.

Growth and the regulation of myotomal muscle mass in teleost fish

Teleost muscle first arises in early embryonic life and its development is driven by molecules present in the egg yolk and modulated by environmental stimuli including temperature and oxygen. Several populations of myogenic precursor cells reside in the embryonic somite and external cell layer and contribute to muscle fibres in embryo, larval, juvenile and adult stages. Many signalling proteins and transcription factors essential for these events are known. In all cases, myogenesis involves myoblast proliferation, migration, fusion and terminal differentiation. Maturation of the embryonic muscle is associated with motor innervation and the development of a scaffold of connective tissue and complex myotomal architecture needed to generate swimming behaviour. Adult muscle is a heterogeneous tissue composed of several cell types that interact to affect growth patterns. The development of capillary and lymphatic circulations and extramuscular organs--notably the gastrointestinal, endocrine, neuroendocrine and immune systems--serves to increase information exchange between tissues and with the external environment, adding to the complexity of growth regulation. Teleosts often exhibit an indeterminate growth pattern, with body size and muscle mass increasing until mortality or senescence occurs. The dramatic increase in myotomal muscle mass between embryo and adult requires the continuous production of muscle fibres until 40-50% of the maximum body length is reached. Sarcomeric proteins can be mobilised as a source of amino acids for energy metabolism by other tissues and for gonad generation, requiring the dynamic regulation of muscle mass throughout the life cycle. The metabolic and contractile phenotypes of muscle fibres also show significant plasticity with respect to environmental conditions, migration and spawning. Many genes regulating muscle growth are found as multiple copies as a result of paralogue retention following whole-genome duplication events in teleost lineages. The extent to which indeterminate growth, ectothermy and paralogue preservation have resulted in modifications of the genetic pathways regulating muscle growth in teleosts compared to mammals largely remains unknown. This review describes the use of compensatory growth models, transgenesis and tissue culture to explore the mechanisms of muscle growth in teleosts and provides some perspectives on future research directions.

Heterogeneous cardiac proteasomes: mandated by diverse substrates?

Proteasome biology has taken central stage in cardiac physiology and pathophysiology. The molecular heterogeneity of proteasome subpopulations supports the specificity of proteasome function to degrade diverse substrate repertoires. Unveiling the dynamics of proteasome function should inspire new therapeutic strategies for combating cardiac disease.

Increased cardiac alpha-myosin heavy chain in left atria and decreased myocardial insulin-like growth factor (Igf-I) expression accompany low heart rate in hibernating grizzly bears

Grizzly bears (Ursus arctos horribilis) tolerate extended periods of extremely low heart rate during hibernation without developing congestive heart failure or cardiac chamber dilation. Left ventricular atrophy and decreased left ventricular compliance have been reported in this species during hibernation. We evaluated the myocardial response to significantly reduced heart rate during hibernation by measuring relative myosin heavy-chain (MyHC) isoform expression and expression of a set of genes important to muscle plasticity and mass regulation in the left atria and left ventricles of active and hibernating bears. We supplemented these data with measurements of systolic and diastolic function via echocardiography in unanesthetized grizzly bears. Atrial strain imaging revealed decreased atrial contractility, decreased expansion/reservoir function (increased atrial stiffness), and decreased passive-filling function (increased ventricular stiffness) in hibernating bears. Relative MyHC-α protein expression increased significantly in the atrium during hibernation. The left ventricle expressed 100% MyHC-β protein in both groups. Insulin-like growth factor (IGF-I) mRNA expression was reduced by ∼50% in both chambers during hibernation, consistent with the ventricular atrophy observed in these bears. Interestingly, mRNA expression of the atrophy-related ubiquitin ligases Muscle Atrophy F-box (MAFBx) and Muscle Ring Finger 1 did not increase, nor did expression of myostatin or hypoxia-inducible factor 1α (HIF-1α). We report atrium-specific decreases of 40% and 50%, respectively, in MAFBx and creatine kinase mRNA expression during hibernation. Decreased creatine kinase expression is consistent with lowered energy requirements and could relate to reduced atrial emptying function during hibernation. Taken together with our hemodynamic assessment, these data suggest a potential downregulation of atrial chamber function during hibernation to prevent fatigue and dilation due to excessive work against an optimally filled ventricle, a response unpredicted by the Frank-Starling mechanism.

Endoplasmic reticulum stress markers and ubiquitin–proteasome pathway activity in response to a 200-km run

PURPOSE

This study investigated whether a 200-km run modulates signaling pathways implicated in cellular stress in skeletal muscle, with special attention paid to the endoplasmic reticulum (ER) stress and to the activation of the ubiquitin-proteasome pathway.

METHODS

Eight men ran 200 km (28 h 03 min ± 2 h 01 min). Two muscle biopsies were obtained from the vastus lateralis muscle 2 wk before and 3 h after the race. Mitogen-activated protein kinase, ubiquitin-proteasome pathway, ER stress, inflammation, and oxidative stress markers were assayed by Western blot analysis or by quantitative real-time polymerase chain reaction. Chymotrypsin-like activity of the proteasome was measured by a fluorimetric assay.

RESULTS

Phosphorylation states of extracellular signal-related kinase 1/2 (+401% ± 173.8%, P = 0.027) and c-Jun N-terminal (+149% ± 61.9%, P = 0.023) increased after the race, whereas p38 phosphorylation remained unchanged. Increases in BiP (+235% ± 94.7%, P = 0.021) and in the messenger RNA level of total (+138% ± 31.2%, P = 0.002) and spliced X-box binding protein 1 (+241% ± 53.3%, P = 0.001) indicated the presence of ER stress. Transcripts of inflammatory markers interleukin-6 (+403% ± 96.1%, P = 0.002) and tumor necrosis factor-α (+233% ± 58.4%, P = 0.003) as well as oxidative stress markers metallothionein 1F (+519% ± 258.3%, P = 0.042), metallothionein 1H (+666% ± 157.5%, P = 0.002), and nicotinamide adenine dinucleotide phosphate-oxidase (NADPH oxidase) (+162% ± 60.5%, P = 0.016) were increased. The messenger RNA level of the ubiquitin ligases muscle-specific RING finger 1 (+583% ± 244.3%, P = 0.024) and muscle atrophy F-box (+249% ± 83.8%, P = 0.011) and the C2 proteasome subunit (+116% ± 40.6%, P = 0.012) also increased. Surprisingly, the amount of ubiquitin-conjugated proteins and the chymotrypsin-like activity of the proteasome were decreased by 20% ± 8.3% (P = 0.025) and 21% ± 4.4% (P = 0.001), respectively. The expression of ubiquitin-specific protease 28 deubiquitinase was increased (+81% ± 37.9%, P = 0.034).

CONCLUSIONS

In the skeletal muscle, a 200-km run activates the expression of ubiquitin ligases muscle-specific RING finger 1 and muscle atrophy F-box as well as various cellular stresses, among which are ER stress, oxidative stress, and inflammation. Meanwhile, compensatory mechanisms seem also triggered: the unfolded protein response is up-regulated, and the chymotrypsin-like activity of the proteasome is repressed.

Telethonin deficiency is associated with maladaptation to biomechanical stress in the mammalian heart

RATIONALE

Telethonin (also known as titin-cap or t-cap) is a 19-kDa Z-disk protein with a unique β-sheet structure, hypothesized to assemble in a palindromic way with the N-terminal portion of titin and to constitute a signalosome participating in the process of cardiomechanosensing. In addition, a variety of telethonin mutations are associated with the development of several different diseases; however, little is known about the underlying molecular mechanisms and telethonin's in vivo function.

OBJECTIVE

Here we aim to investigate the role of telethonin in vivo and to identify molecular mechanisms underlying disease as a result of its mutation.

METHODS AND RESULTS

By using a variety of different genetically altered animal models and biophysical experiments we show that contrary to previous views, telethonin is not an indispensable component of the titin-anchoring system, nor is deletion of the gene or cardiac specific overexpression associated with a spontaneous cardiac phenotype. Rather, additional titin-anchorage sites, such as actin-titin cross-links via α-actinin, are sufficient to maintain Z-disk stability despite the loss of telethonin. We demonstrate that a main novel function of telethonin is to modulate the turnover of the proapoptotic tumor suppressor p53 after biomechanical stress in the nuclear compartment, thus linking telethonin, a protein well known to be present at the Z-disk, directly to apoptosis ("mechanoptosis"). In addition, loss of telethonin mRNA and nuclear accumulation of this protein is associated with human heart failure, an effect that may contribute to enhanced rates of apoptosis found in these hearts.

CONCLUSIONS

Telethonin knockout mice do not reveal defective heart development or heart function under basal conditions, but develop heart failure following biomechanical stress, owing at least in part to apoptosis of cardiomyocytes, an effect that may also play a role in human heart failure.

Conformation-regulated mechanosensory control via titin domains in cardiac muscle

The giant filamentous protein titin is ideally positioned in the muscle sarcomere to sense mechanical stimuli and transform them into biochemical signals, such as those triggering cardiac hypertrophy. In this review, we ponder the evidence for signaling hotspots along the titin filament involved in mechanosensory control mechanisms. On the way, we distinguish between stress and strain as triggers of mechanical signaling events at the cardiac sarcomere. Whereas the Z-disk and M-band regions of titin may be prominently involved in sensing mechanical stress, signaling hotspots within the elastic I-band titin segment may respond primarily to mechanical strain. Common to both stress and strain sensor elements is their regulation by conformational changes in protein domains.

Molecular characterization of skeletal muscle atrophy in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD), a neurodegenerative disorder caused by mutant huntingtin, is characterized by a catabolic phenotype. To determine the mechanisms underlying muscle wasting, we examined key signal transduction pathways governing muscle protein metabolism, apoptosis, and autophagy in R6/2 mice, a well-characterized transgenic model of HD. R6/2 mice exhibited increased adiposity, elevated energy expenditure, and decreased body weight and lean mass without altered food intake. Severe skeletal muscle wasting accounted for a majority of the weight loss. Protein synthesis was unexpectedly increased 19% in gastrocnemius muscle, which was associated with overactivation of basal and refeeding-stimulated mammalian target of rapamycin (mTOR) signaling, elevated Akt expression and Ser(473) phosphorylation, and decreased AMPK Thr(172) phosphorylation. Moreover, mRNA abundance of atrogenes muscle ring finger-1 and atrophy F-box, was markedly attenuated during fasting and refeeding, and the urinary excretion of 3-methylhistidine was decreased, arguing against a role for the ubiquitin proteasome-mediated proteolysis in the atrophy. In contrast, mRNA expression of several caspase genes and genes involved in the extrinsic or intrinsic apoptotic pathway, caspase-3/7, -8, and -9 activity, protein abundance of caspase-3 and -9, Fas, and Fadd, and cytochrome c release were elevated. Protein expressions of LC3B-I and -II, beclin-I, and atg5 and -7 in muscle were upregulated. Thus, mutant huntingtin in skeletal muscle results in increased protein synthesis and mTOR signaling, which is countered by activation of the apoptotic and autophagic pathways, contributing to an overall catabolic phenotype and the severe muscle wasting.

Proteolysis in illness-associated skeletal muscle atrophy: from pathways to networks

Improvements in health in the past decades have resulted in increased numbers of the elderly in both developed and developing regions of the world. Advances in therapy have also increased the prevalence of patients with chronic and degenerative diseases. Muscle wasting, a feature of most chronic diseases, is prominent in the elderly and contributes to both morbidity and mortality. A major research goal has been to identify the proteolytic system(s) that is responsible for the degradation of proteins that occurs in muscle atrophy. Findings over the past 20 years have clearly confirmed an important role of the ubiquitin proteasome system in mediating muscle proteolysis, particularly that of myofibrillar proteins. However, recent observations have provided evidence that autophagy, calpains and caspases also contribute to the turnover of muscle proteins in catabolic states, and furthermore, that these diverse proteolytic systems interact with each other at various levels. Importantly, a number of intracellular signaling pathways such as the IGF1/AKT, myostatin/Smad, PGC1, cytokine/NFκB, and AMPK pathways are now known to interact and can regulate some of these proteolytic systems in a coordinated manner. A number of loss of function studies have identified promising therapeutic approaches to the prevention and treatment of wasting. However, additional biomarkers and other approaches to improve early identification of patients who would benefit from such treatment need to be developed. The current data suggests a network of interacting proteolytic and signaling pathways in muscle. Future studies are needed to improve understanding of the nature and control of these interactions and how they work to preserve muscle function under various states of growth and atrophy.

The ubiquitin ligase MuRF1 protects against cardiac ischemia/reperfusion injury by its proteasome-dependent degradation of phospho-c-Jun

Despite improvements in interventions of acute coronary syndromes, primary reperfusion therapies restoring blood flow to ischemic myocardium leads to the activation of signaling cascades that induce cardiomyocyte cell death. These signaling cascades, including the mitogen-activated protein kinase signaling pathways, activate cardiomyocyte death in response to both ischemia and reperfusion. We have previously identified muscle ring finger-1 (MuRF1) as a cardiac-specific protein that regulates cardiomyocyte mass through its ubiquitin ligase activity, acting to degrade sarcomeric proteins and inhibit transcription factors involved in cardiac hypertrophy signaling. To determine MuRF1's role in cardiac ischemia/reperfusion (I/R) injury, cardiomyocytes in culture and intact hearts were challenged with I/R injury in the presence and absence of MuRF1. We found that MuRF1 is cardioprotective, in part, by its ability to prevent cell death by inhibiting Jun N-terminal kinase (JNK) signaling. MuRF1 specifically targets JNK's proximal downstream target, activated phospho-c-Jun, for degradation by the proteasome, effectively inhibiting downstream signaling and the induction of cell death. MuRF1's inhibitory affects on JNK signaling through its ubiquitin proteasome-dependent degradation of activated c-Jun is the first description of a cardiac ubiquitin ligase inhibiting mitogen-activated protein kinase signaling. MuRF1's cardioprotection in I/R injury is attenuated in the presence of pharmacologic JNK inhibition in vivo, suggesting a prominent role of MuRF1's regulation of c-Jun in the intact heart.

Exercise training has beneficial anti-atrophy effects by inhibiting oxidative stress-induced MuRF1 upregulation in rats with diabetes

AIMS

MuRF1 E3 ubiquitin ligase has been identified as a mediator of skeletal muscle wasting in various skeletal muscle atrophy models, and its expression is upregulated by oxidative stress. Exercise training could decrease oxidative stress and restore the atrophied skeletal muscle. Here, our aim was to investigate whether exercise training has any effect on MuRF1 expression in rats with diabetes.

MAIN METHODS

Rats with streptozotocin-induced diabetes were subjected to exercise training, after which oxidative stress was determined, and MuRF1 expression was analyzed by immunohistochemistry, real-time RT-PCR and Western blot analysis. In addition, we analyzed C2C12 myotubes in an in vitro model to examine the effects of oxidative stress on the protein levels of MuRF1 and myosin heavy chain (MHC).

KEY FINDINGS

While oxidative stress and MuRF1 expression were increased in rats with diabetes, exercise training diminished the skeletal muscle wasting in diabetic rats by decreasing oxidative stress and inhibiting MuRF1 expression at both the mRNA and protein levels. In addition, oxidative stress-induced MuRF1 upregulation promoted proteasome dependent degradation of the myosin heavy chain (MHC) in C2C12 myotubes.

SIGNIFICANCE

Our study provides the first evidence that the beneficial anti-atrophy effects of exercise training on diabetes might be mediated by inhibiting oxidative stress-induced MuRF1 upregulation and preventing MuRF1-mediated degradation of MHC.

Titin visualization in real time reveals an unexpected level of mobility within and between sarcomeres

Contrary to prior models in which titin serves as a stable scaffold in sarcomeres, sarcomeric and soluble titin exchange dynamically in myofibers when calcium levels are low.

The giant muscle protein titin is an essential structural component of the sarcomere. It forms a continuous periodic backbone along the myofiber that provides resistance to mechanical strain. Thus, the titin filament has been regarded as a blueprint for sarcomere assembly and a prerequisite for stability. Here, a novel titin-eGFP knockin mouse provided evidence that sarcomeric titin is more dynamic than previously suggested. To study the mobility of titin in embryonic and neonatal cardiomyocytes, we used fluorescence recovery after photobleaching and investigated the contribution of protein synthesis, contractility, and calcium load to titin motility. Overall, the kinetics of lateral and longitudinal movement of titin-eGFP were similar. Whereas protein synthesis and developmental stage did not alter titin dynamics, there was a strong, inhibitory effect of calcium on titin mobility. Our results suggest a model in which the largely unrestricted movement of titin within and between sarcomeres primarily depends on calcium, suggesting that fortification of the titin filament system is activity dependent.