Muscle-specific RING finger 1 is a bona fide ubiquitin ligase that degrades cardiac troponin I

Uncovering the mechanisms of MuRF1-induced ubiquitylation and revealing similarities with MuRF2 and MuRF3

MuRF1 (Muscle-specific RING finger protein 1; gene name TRIM63) is a ubiquitin E3 ligase, associated with the progression of muscle atrophy. As a RING (Really Interesting New Gene) type E3 ligase, its unique activity of ubiquitylation is driven by a specific interaction with a UBE2 (ubiquitin conjugating enzyme). Our understanding of MuRF1 function remains unclear as candidate UBE2s have not been fully elucidated. In the present study, we screened human ubiquitin dependent UBE2s in vitro and found that MuRF1 engages in ubiquitylation with UBE2D, UBE2E, UBE2N/V families and UBE2W. MuRF1 can cause mono-ubiquitylation, K48- and K63-linked polyubiquitin chains in a UBE2 dependent manner. Moreover, we identified a two-step UBE2 dependent mechanism whereby MuRF1 is monoubiquitylated by UBE2W which acts as an anchor for UBE2N/V to generate polyubiquitin chains. With the in vitro ubiquitylation assay, we also found that MuRF2 and MuRF3 not only share the same UBE2 partners as MuRF1 but can also directly ubiquitylate the same substrates: Titin (A168-A170), Desmin, and MYLPF (Myosin Light Chain, Phosphorylatable, Fast Skeletal Muscle; also called Myosin Light Regulatory Chain 2). In summary, our work presents new insights into the mechanisms that underpin MuRF1 activity and reveals overlap in MuRF-induced ubiquitylation which could explain their partial redundancy in vivo.

Upregulation of TRIM16 mitigates doxorubicin-induced cardiotoxicity by modulating TAK1 and YAP/Nrf2 pathways in mice

The clinic application of doxorubicin (DOX) is severely limited by its severe cardiotoxicity. Tripartite motif-containing protein 16 (TRIM16) has E3 ubiquitin ligase activity and is upregulated in cardiomyocytes under pathological stress, yet its role in DOX-induced cardiotoxicity remains elusive. This study aims to investigate the role and mechanism of TRIM16 in DOX cardiotoxicity. Following TRIM16 overexpression in hearts with AAV9-TRIM16, mice were intravenously administered DOX at a dose of 4 mg/kg/week for 4 weeks to assess the impact of TRIM16 on doxorubicin-induced cardiotoxicity. Transfection of OE-TRIM16 plasmids and siRNA-TRIM16 was performed in neonatal rat cardiomyocytes (NRCMs). Our results revealed that DOX challenge elicited a significant upregulation of TRIM16 proteins in cardiomyocytes. TRIM16 overexpression efficiently ameliorated cardiac function while suppressing inflammation, ROS generation, apoptosis and fibrosis provoked by DOX in the myocardium. TRIM16 knockdown exacerbated these alterations caused by DOX in NRCMs. Mechanistically, OE-TRIM16 augmented the ubiquitination and degradation of p-TAK1, thereby arresting JNK and p38MAPK activation evoked by DOX in cardiomyocytes. Furthermore, DOX enhanced the interaction between p-TAK1 and YAP1 proteins, resulting in a reduction in YAP and Nrf2 proteins in cardiomyocytes. OE-TRIM16 elevated YAP levels and facilitated its nuclear translocation, thereby promoting Nrf2 expression and mitigating oxidative stress and inflammation. This effect was nullified by siTRIM16 or TAK1 inhibitor Takinib. Collectively, the current study elaborates that upregulating TRIM16 mitigates DOX-induced cardiotoxicity through anti-inflammation and anti-oxidative stress by modulating TAK1-mediated p38 and JNK as well as YAP/Nrf2 pathways, and targeting TRIM16 may provide a novel strategy to treat DOX-induced cardiotoxicity.

Systemic Inflammation but not Oxidative Stress Is Associated with Physical Performance in Moderate Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) patients manifest muscle dysfunction and impaired muscle oxidative capacity, which result in reduced exercise capacity and poor health status. The aim of this study was to compare the physical performance, systemic inflammation, and oxidative stress of patients with moderate COPD, and to associate physical performance with inflammatory and oxidative stress plasma markers. Twenty CONTROL (n = 10) and moderate COPD (n = 10) patients participated in this study. Systematic inflammation and oxidative stress plasma markers, maximal aerobic capacity (VO2peak), and maximal isometric strength (MVIC) of the knee extensor (KE) muscles were measured. VO2peak was 31.3% greater in CONTROL compared to COPD (P = 0.006). The MVIC strength of the KE was 43.9% greater in CONTROL compared to COPD (P = 0.002). Tumor necrosis factor-alpha (TNF-α) was 79.6% greater in COPD compared to CONTROL (P < 0.001). Glutathione peroxidase activity (GPx) activity was 27.5% lesser in COPD compared to CONTROL (P = 0.05). TNF-α concentration was correlated with KE MVC strength (R = -0.48; P = 0.045) and VO2peak (R = -0.58; P = 0.01). Meanwhile, malondialdehyde (MDA) and GPx activity were not associated with KE strength or VO2peak (P = 0.74 and P = 0.14, respectively). COPD patients showed lesser muscle strength and aerobic capacity than healthy control individuals. Furthermore, patients with COPD showed greater systemic inflammation and lesser antioxidant capacity than healthy counterparts. A moderate association was evident between levels of systemic inflammation and physical performance variables.

Mitochondrial dysfunction and skeletal muscle atrophy: Causes, mechanisms, and treatment strategies

Skeletal muscle, which accounts for approximately 40% of total body weight, is one of the most dynamic and plastic tissues in the human body and plays a vital role in movement, posture and force production. More than just a component of the locomotor system, skeletal muscle functions as an endocrine organ capable of producing and secreting hundreds of bioactive molecules. Therefore, maintaining healthy skeletal muscles is crucial for supporting overall body health. Various pathological conditions, such as prolonged immobilization, cachexia, aging, drug-induced toxicity, and cardiovascular diseases (CVDs), can disrupt the balance between muscle protein synthesis and degradation, leading to skeletal muscle atrophy. Mitochondrial dysfunction is a major contributing mechanism to skeletal muscle atrophy, as it plays crucial roles in various biological processes, including energy production, metabolic flexibility, maintenance of redox homeostasis, and regulation of apoptosis. In this review, we critically examine recent knowledge regarding the causes of muscle atrophy (disuse, cachexia, aging, etc.) and its contribution to CVDs. Additionally, we highlight the mitochondrial signaling pathways involvement to skeletal muscle atrophy, such as the ubiquitin-proteasome system, autophagy and mitophagy, mitochondrial fission-fusion, and mitochondrial biogenesis. Furthermore, we discuss current strategies, including exercise, mitochondria-targeted antioxidants, in vivo transfection of PGC-1α, and the potential use of mitochondrial transplantation as a possible therapeutic approach.

Platinum-based anticancer drugs-induced downregulation of myosin heavy chain isoforms in skeletal muscle of mouse

Cisplatin, a platinum-based anticancer drug used frequently in cancer treatment, causes skeletal muscle atrophy. It was predicted that the proteolytic pathway is enhanced as the mechanism of this atrophy. Therefore, we investigated whether a platinum-based anticancer drug affects the expression of the major proteins of skeletal muscle, myosin heavy chain (MyHC). Mice were injected with cisplatin or oxaliplatin for four consecutive days. C2C12 myotubes were treated using cisplatin and oxaliplatin. Administration of platinum-based anticancer drug reduced quadriceps mass and muscle strength compared to the control group. Protein levels of all MyHC isoforms were reduced in the platinum-based anticancer drug groups. However, only Myh2 (MyHC-IIa) gene expression in skeletal muscle of mice treated with platinum-based anticancer drugs was found to be reduced. Treatment of C2C12 myotubes with platinum-based anticancer drugs reduced the protein levels of all MyHCs, and treatment with the proteasome inhibitor MG-132 restored this reduction. The expression of Mef2c, which was predicted to act upstream of Myh2, was reduced in the skeletal muscle of mice treated systemically with platinum-based anticancer drug. Degradation of skeletal muscle MyHCs by proteasomes may be a factor that plays an important role in muscle mass loss in platinum-based anticancer drug-induced muscle atrophy.

Nutritional status affects Igf1 regulation of skeletal muscle myogenesis, myostatin, and myofibrillar protein degradation pathways in gopher rockfish (Sebastes carnatus)

Insulin-like growth factor-1 (Igf1) regulates skeletal muscle growth in fishes by increasing protein synthesis and promoting muscle hypertrophy. When fish experience periods of insufficient food intake, they undergo slower muscle growth or even muscle wasting, and those changes emerge in part from nutritional modulation of Igf1 signaling. Here, we examined how food deprivation (fasting) modulates Igf1 regulation of liver and skeletal muscle gene expression in gopher rockfish (Sebastes carnatus), a nearshore rockfish of importance for commercial and recreational fisheries in the northeastern Pacific Ocean, to understand how food limitation impacts Igf regulation of muscle growth pathways. Rockfish were either fed or fasted for 14 d, after which a subset of fish from each group was treated with recombinant Igf1 from sea bream (Sparus aurata). Fish that were fasted lost body mass and had lower body condition, reduced hepatosomatic index, and lower plasma Igf1 concentrations, as well as a decreased abundance of igf1 gene transcripts in the liver, increased hepatic mRNAs for Igf binding proteins igfbp1a, igfbp1b, and igfbp3a, and decreased mRNA abundances for igfbp2b and a putative Igf acid labile subunit (igfals) gene. In skeletal muscle, fasted fish showed a reduced abundance of intramuscular igf1 mRNAs but elevated gene transcripts encoding Igf1 receptors A (igf1ra) and B (igf1rb), which also showed downregulation by Igf1. Fasting increased skeletal muscle mRNAs for myogenin and myostatin1, as well as ubiquitin ligase F-box only protein 32 (fbxo32) and muscle RING-finger protein-1 (murf1) genes involved in muscle atrophy, while concurrently downregulating mRNAs for myoblast determination protein 2 (myod2), myostatin2, and myogenic factors 5 (myf5) and 6 (myf6 encoding Mrf4). Treatment with Igf1 downregulated muscle myostatin1 and fbxo32 under both feeding conditions, but showed feeding-dependent effects on murf1, myf5, and myf6/Mrf4 gene expression indicating that Igf1 effects on muscle growth and atrophy pathways is contingent on recent food consumption experience.

Secreted Protein Acidic and Rich in Cysteine (SPARC)-Mediated Exercise Effects: Illustrative Molecular Pathways against Various Diseases

The strong benefits of exercise, in addition to the development of both the therapeutic applications of physical activity and molecular biology tools, means that it has become very important to explore the underlying molecular patterns linking exercise and its induced phenotypic changes. Within this context, secreted protein acidic and rich in cysteine (SPARC) has been characterized as an exercise-induced protein that would mediate and induce some important effects of exercise. Herein, we suggest some underlying pathways to explain such SPARC-induced exercise-like effects. Such mechanistic mapping would not only allow us to understand the molecular processes of exercise and SPARC effects but would also highlight the potential to develop novel molecular therapies. These therapies would be based on mimicking the exercise benefits via either introducing SPARC or pharmacologically targeting the SPARC-related pathways to produce exercise-like effects. This is of a particular importance for those who do not have the ability to perform the required physical activity due to disabilities or diseases. The main objective of this work is to highlight selected potential therapeutic applications deriving from SPARC properties that have been reported in various publications.

Cardiac metabolism in HFpEF: from fuel to signalling

Heart failure (HF) is marked by distinctive changes in myocardial uptake and utilization of energy substrates. Among the different types of HF, HF with preserved ejection fraction (HFpEF) is a highly prevalent, complex, and heterogeneous condition for which metabolic derangements seem to dictate disease progression. Changes in intermediate metabolism in cardiometabolic HFpEF-among the most prevalent forms of HFpEF-have a large impact both on energy provision and on a number of signalling pathways in the heart. This dual, metabolic vs. signalling, role is played in particular by long-chain fatty acids (LCFAs) and short-chain carbon sources [namely, short-chain fatty acids (SCFAs) and ketone bodies (KBs)]. LCFAs are key fuels for the heart, but their excess can be harmful, as in the case of toxic accumulation of lipid by-products (i.e. lipotoxicity). SCFAs and KBs have been proposed as a potential major, alternative source of energy in HFpEF. At the same time, both LCFAs and short-chain carbon sources are substrate for protein post-translational modifications and other forms of direct and indirect signalling of pivotal importance in HFpEF pathogenesis. An in-depth molecular understanding of the biological functions of energy substrates and their signalling role will be instrumental in the development of novel therapeutic approaches to HFpEF. Here, we summarize the current evidence on changes in energy metabolism in HFpEF, discuss the signalling role of intermediate metabolites through, at least in part, their fate as substrates for post-translational modifications, and highlight clinical and translational challenges around metabolic therapy in HFpEF.

Circadian regulation of cardiac muscle function and protein degradation

The circadian clock plays a fundamental role in physiology. In particular, the heart is a target organ where the clock orchestrates various aspects of cardiac function. At the molecular level, the clock machinery governs daily rhythms of gene expression. Such circadian regulation is in tune with the dynamic nature of heart structure and function, and provides the foundation for chronotherapeutic applications in cardiovascular diseases. In comparison, a regulatory role of the clock in cardiac protein degradation is poorly documented. Sarcomere is the structural and functional unit responsible for cardiac muscle contraction, and sarcomere components are closely regulated by protein folding and proteolysis. Emerging evidence supports a role of the circadian clock in governing sarcomere integrity and function. Particularly, recent studies uncovered a circadian regulation of a core sarcomere component TCAP. It is possible that circadian regulation of the cardiac muscle protein turnover is a key regulatory mechanism underlying cardiac remodeling in response to physiological and environmental stimuli. While the detailed regulatory mechanisms and the molecular links to cardiac (patho)physiology remain to be further studied, therapeutic strategies targeting circadian control in the heart may markedly enhance intervention outcomes against cardiovascular disease.

Identification of hypertrophy-modulating Cullin-RING ubiquitin ligases in primary cardiomyocytes

Cullin-RING ubiquitin ligases (CRL) regulate numerous biological processes in the heart and have been implicated in regulating cardiac hypertrophy. This study aimed to identify novel hypertrophy-modulating CRLs in cardiomyocytes (CM). A functional genomic approach using siRNA-mediated depletion and automated microscopy was employed to screen for cell size-modulating CRLs in neonatal rat CM. Screening hits were confirmed by ³H-isoleucine incorporation. Of 43 targets screened, siRNA-mediated depletion of Fbxo6, Fbxo45, and Fbxl14 resulted in decreased cell size, whereas depletion of Fbxo9, Fbxo25, Fbxo30, Fbxo32, Fbxo33, Cullin1, Roc1, Ddb1, Fbxw4, and Fbxw5 led to a markedly increased cell size under basal conditions. In CM stimulated with phenylephrine (PE), depletion of Fbxo6, Fbxo25, Fbxo33, Fbxo45, and Fbxw4 further augmented PE-induced hypertrophy. As a proof-of-concept, the CRL^Fbox25 was analysed by transverse aortic constriction (TAC) resulting in a 4.5-fold increase in Fbxo25 protein concentrations compared to control animals. In cell culture, siRNA-mediated depletion of Fbxo25 resulted in a ∼ 37% increase in CM cell size and ∼41% increase in ³H-isoleucine incorporation. Depleting Fbxo25 resulted in upregulation of Anp and Bnp. In summary, we identified 13 novel CRLs as positive or negative regulators of CM hypertrophy. Of these, CRL^Fbox25 was further characterized, as a potential modulator of cardiac hypertrophy.

The accumulation of muscle RING finger-1 in regenerating myofibers: Implications for muscle repair in immune-mediated necrotizing myopathy

Background

Muscle RING finger-1 (MuRF-1) plays a key role in the degradation of skeletal muscle proteins. We hypothesize the involvement of MuRF-1 in immune-mediated necrotizing myopathy (IMNM).

Methods

Muscle biopsies from patients with IMNM (n = 37) were analyzed and compared to biopsies from patients with dermatomyositis (DM, n = 13), dysferlinopathy (n = 9) and controls (n = 7) using immunostaining.

Results

MuRF-1 staining could be observed in IMNM, DM and dysferlinopathy biopsies, whereas the percentage of MuRF-1 positive myofibers was significantly higher in IMNM than in dysferlinopathy (p = 0.0448), and positively correlated with muscle weakness and disease activity in IMNM and DM. Surprisingly, MuRF-1 staining predominantly presented in regenerating fibers but not in atrophic fibers. Moreover, MuRF-1-positive fibers tended to be distributed around necrotic myofibers and myofibers with sarcolemma membrane attack complex deposition. Abundant MuRF-1 expression in IMNM and DM was associated with rapid activation of myogenesis after muscle injury, whereas relatively low expression of MuRF-1 in dysferlinopathy may be attributed to damaged muscle regeneration.

Conclusions

MuRF-1 accumulated in regenerating myofibers, which may contribute to muscle injury repair in IMNM and DM. MuRF-1 staining may help clinicians differentiate IMNM and dysferlinopathy.

Tripartite motif 25 ameliorates doxorubicin-induced cardiotoxicity by degrading p85α

Doxorubicin (DOX)-based chemotherapy is widely used to treat malignant tumors; however, the cardiotoxicity induced by DOX restricts its clinical usage. A therapeutic dose of DOX can activate ubiquitin-proteasome system. However, whether and how ubiquitin-proteasome system brings out DOX-induced cardiotoxicity remains to be investigated. Here we conducted a proteomics analysis of a DOX-induced cardiotoxicity model to screen the potentially ubiquitination-related molecules. Dysregulated TRIM25 was found to contribute to the cardiotoxicity. In vivo and in vitro cardiotoxicity experiments revealed that TRIM25 ameliorated DOX-induced cardiotoxicity. Electron microscopy and endoplasmic reticulum stress markers revealed that TRIM25 mitigated endoplasmic reticulum stress and apoptosis in DOX-induced cardiomyocytes. Mechanistically, the Co-immunoprecipitation assays and CHX pulse-chase experiment determined that TRIM25 affected p85α stability and promoted its ubiquitination and degradation. This leads to increase of nuclear translocation of XBP-1s, which mitigates endoplasmic reticulum stress. These findings reveal that TRIM25 may have a therapeutic role for DOX-induced cardiotoxicity.

Protein Quality Control at the Sarcomere: Titin Protection and Turnover and Implications for Disease Development

Sarcomeres are mainly composed of filament and signaling proteins and are the smallest molecular units of muscle contraction and relaxation. The sarcomere protein titin serves as a molecular spring whose stiffness mediates myofilament extensibility in skeletal and cardiac muscle. Due to the enormous size of titin and its tight integration into the sarcomere, the incorporation and degradation of the titin filament is a highly complex task. The details of the molecular processes involved in titin turnover are not fully understood, but the involvement of different intracellular degradation mechanisms has recently been described. This review summarizes the current state of research with particular emphasis on the relationship between titin and protein quality control. We highlight the involvement of the proteasome, autophagy, heat shock proteins, and proteases in the protection and degradation of titin in heart and skeletal muscle. Because the fine-tuned balance of degradation and protein expression can be disrupted under pathological conditions, the review also provides an overview of previously known perturbations in protein quality control and discusses how these affect sarcomeric proteins, and titin in particular, in various disease states.

Hidden Agenda - The Involvement of Endoplasmic Reticulum Stress and Unfolded Protein Response in Inflammation-Induced Muscle Wasting

Critically ill patients at the intensive care unit (ICU) often develop a generalized weakness, called ICU-acquired weakness (ICUAW). A major contributor to ICUAW is muscle atrophy, a loss of skeletal muscle mass and function. Skeletal muscle assures almost all of the vital functions of our body. It adapts rapidly in response to physiological as well as pathological stress, such as inactivity, immobilization, and inflammation. In response to a reduced workload or inflammation muscle atrophy develops. Recent work suggests that adaptive or maladaptive processes in the endoplasmic reticulum (ER), also known as sarcoplasmic reticulum, contributes to this process. In muscle cells, the ER is a highly specialized cellular organelle that assures calcium homeostasis and therefore muscle contraction. The ER also assures correct folding of proteins that are secreted or localized to the cell membrane. Protein folding is a highly error prone process and accumulation of misfolded or unfolded proteins can cause ER stress, which is counteracted by the activation of a signaling network known as the unfolded protein response (UPR). Three ER membrane residing molecules, protein kinase R-like endoplasmic reticulum kinase (PERK), inositol requiring protein 1a (IRE1a), and activating transcription factor 6 (ATF6) initiate the UPR. The UPR aims to restore ER homeostasis by reducing overall protein synthesis and increasing gene expression of various ER chaperone proteins. If ER stress persists or cannot be resolved cell death pathways are activated. Although, ER stress-induced UPR pathways are known to be important for regulation of skeletal muscle mass and function as well as for inflammation and immune response its function in ICUAW is still elusive. Given recent advances in the development of ER stress modifying molecules for neurodegenerative diseases and cancer, it is important to know whether or not therapeutic interventions in ER stress pathways have favorable effects and these compounds can be used to prevent or treat ICUAW. In this review, we focus on the role of ER stress-induced UPR in skeletal muscle during critical illness and in response to predisposing risk factors such as immobilization, starvation and inflammation as well as ICUAW treatment to foster research for this devastating clinical problem.

Primary skeletal muscle cells from chronic kidney disease patients retain hallmarks of cachexia in vitro

BACKGROUND

Skeletal muscle wasting and dysfunction are common characteristics noted in people who suffer from chronic kidney disease (CKD). The mechanisms by which this occurs are complex, and although progress has been made, the key underpinning mechanisms are not yet fully elucidated. With work to date primarily conducted in nephrectomy-based animal models, translational capacity to our patient population has been challenging. This could be overcome if rationale developing work could be conducted in human based models with greater translational capacity. This could be achieved using cells derived from patient biopsies, if they retain phenotypic traits noted in vivo.

METHODS

Here, we performed a systematic characterization of CKD derived muscle cells (CKD; n = 10; age: 54.40 ± 15.53 years; eGFR: 22.25 ± 13.22 ml/min/1.73 m² ) in comparison with matched controls (CON; n = 10; age: 58.66 ± 14.74 years; eGFR: 85.81 ± 8.09 ml/min/1.73 m² ). Harvested human derived muscle cells (HDMCs) were taken through proliferative and differentiation phases and investigated in the context of myogenic progression, inflammation, protein synthesis, and protein breakdown. Follow up investigations exposed HDMC myotubes from each donor type to 0, 0.4, and 100 nM of IGF-1 in order to investigate any differences in anabolic resistance.

RESULTS

Harvested human derived muscle cells isolated from CKD patients displayed higher rates of protein degradation (P = 0.044) alongside elevated expression of both TRIM63 (2.28-fold higher, P = 0.054) and fbox32 (6.4-fold higher, P < 0.001) in comparison with CONs. No differences were noted in rates of protein synthesis under basal conditions (P > 0.05); however, CKD derived cells displayed a significant degree of anabolic resistance in response to IGF-1 stimulation (both doses) in comparison with matched CONs (0.4 nm: P < 0.001; 100 nM: P < 0.001).

CONCLUSIONS

In summary, we report for the first time that HDMCs isolated from people suffering from CKD display key hallmarks of the well documented in vivo phenotype. Not only do these findings provide further mechanistic insight into CKD specific cachexia, but they also demonstrate this is a reliable and suitable model in which to perform targeted experiments to begin to develop novel therapeutic strategies targeting the CKD associated decline in skeletal muscle mass and function.

Amphioxus muscle transcriptomes reveal vertebrate-like myoblast fusion genes and a highly conserved role of insulin signalling in the metabolism of muscle

BACKGROUND

The formation and functioning of muscles are fundamental aspects of animal biology, and the evolution of 'muscle genes' is central to our understanding of this tissue. Feeding-fasting-refeeding experiments have been widely used to assess muscle cellular and metabolic responses to nutrition. Though these studies have focused on vertebrate models and only a few invertebrate systems, they have found similar processes are involved in muscle degradation and maintenance. Motivation for these studies stems from interest in diseases whose pathologies involve muscle atrophy, a symptom also triggered by fasting, as well as commercial interest in the muscle mass of animals kept for consumption. Experimentally modelling atrophy by manipulating nutritional state causes muscle mass to be depleted during starvation and replenished with refeeding so that the genetic mechanisms controlling muscle growth and degradation can be understood.

RESULTS

Using amphioxus, the earliest branching chordate lineage, we address the gap in previous work stemming from comparisons between distantly related vertebrate and invertebrate models. Our amphioxus feeding-fasting-refeeding muscle transcriptomes reveal a highly conserved myogenic program and that the pro-orthologues of many vertebrate myoblast fusion genes were present in the ancestral chordate, despite these invertebrate chordates having unfused mononucleate myocytes. We found that genes differentially expressed between fed and fasted amphioxus were orthologous to the genes that respond to nutritional state in vertebrates. This response is driven in a large part by the highly conserved IGF/Akt/FOXO pathway, where depleted nutrient levels result in activation of FOXO, a transcription factor with many autophagy-related gene targets.

CONCLUSION

Reconstruction of these gene networks and pathways in amphioxus muscle provides a key point of comparison between the distantly related groups assessed thus far, significantly refining the reconstruction of the ancestral state for chordate myoblast fusion genes and identifying the extensive role of duplicated genes in the IGF/Akt/FOXO pathway across animals. Our study elucidates the evolutionary trajectory of muscle genes as they relate to the increased complexity of vertebrate muscles and muscle development.

Smad2 and Smad3 expressed in skeletal muscle promote immobilization-induced bone atrophy in mice

Skeletal muscle is known to regulate bone homeostasis through muscle-bone interaction, although factors that control this activity remain unclear. Here, we newly established Smad3-flox mice, and then generated skeletal muscle-specific Smad2/Smad3 double conditional knockout mice (DcKO) by crossing Smad3-flox with skeletal muscle-specific Ckmm Cre and Smad2-flox mice. We show that immobilization-induced gastrocnemius muscle atrophy occurring due to sciatic nerve denervation was partially but significantly inhibited in DcKO mice, suggesting that skeletal muscle cell-intrinsic Smad2/3 is required for immobilization-induced muscle atrophy. Also, tibial bone atrophy seen after sciatic nerve denervation was partially but significantly inhibited in DcKO mice. Bone formation rate in wild-type mouse tibia was significantly inhibited by immobilization, but inhibition was abrogated in DcKO mice. We propose that skeletal muscle regulates immobilization-induced bone atrophy via Smad2/3, and Smad2/3 represent potential therapeutic targets to prevent both immobilization-induced bone and muscle atrophy.

Out of Control: The Role of the Ubiquitin Proteasome System in Skeletal Muscle during Inflammation

The majority of critically ill intensive care unit (ICU) patients with severe sepsis develop ICU-acquired weakness (ICUAW) characterized by loss of muscle mass, reduction in myofiber size and decreased muscle strength leading to persisting physical impairment. This phenotype results from a dysregulated protein homeostasis with increased protein degradation and decreased protein synthesis, eventually causing a decrease in muscle structural proteins. The ubiquitin proteasome system (UPS) is the predominant protein-degrading system in muscle that is activated during diverse muscle atrophy conditions, e.g., inflammation. The specificity of UPS-mediated protein degradation is assured by E3 ubiquitin ligases, such as atrogin-1 and MuRF1, which target structural and contractile proteins, proteins involved in energy metabolism and transcription factors for UPS-dependent degradation. Although the regulation of activity and function of E3 ubiquitin ligases in inflammation-induced muscle atrophy is well perceived, the contribution of the proteasome to muscle atrophy during inflammation is still elusive. During inflammation, a shift from standard- to immunoproteasome was described; however, to which extent this contributes to muscle wasting and whether this changes targeting of specific muscular proteins is not well described. This review summarizes the function of the main proinflammatory cytokines and acute phase response proteins and their signaling pathways in inflammation-induced muscle atrophy with a focus on UPS-mediated protein degradation in muscle during sepsis. The regulation and target-specificity of the main E3 ubiquitin ligases in muscle atrophy and their mode of action on myofibrillar proteins will be reported. The function of the standard- and immunoproteasome in inflammation-induced muscle atrophy will be described and the effects of proteasome-inhibitors as treatment strategies will be discussed.

[Metabolic pathways controlled by E3 ligases: an opportunity for therapeutic targeting]

Since its discovery, the Ubiquitin Proteasome System (UPS) has been recognized for its major role in controlling most of the cell's metabolic pathways. In addition to its essential role in the degradation of proteins, it is also involved in the addressing, signaling or repair of DNA, which makes it a key player in cellular homeostasis. Although other control systems exist in the cell, the UPS is often referred to as the conductor. In view of its importance, any dysregulation of the UPS leads to more or less severe disorders for the cell and therefore the body, which accounts for UPS implication in many pathologies (cancer, Alzheimer's disease, Huntington's disease, etc.). UPS is made up of more than 1000 different proteins, the combinations of which allow the fine targeting of virtually all proteins in the body. UPS uses an enzymatic cascade (E1, 2 members; E2 > 35; E3 > 800) which allows the transfer of ubiquitin, a small protein of 8.5 kDa onto the protein to be targeted either for its degradation or to modify its activity. This ubiquitinylation signal is reversible and many deubiquitinylases (DUB, ∼ 80 isoforms) also have an important role. E3 enzymes are the most numerous and their function is to recognize the target protein, which makes them important players in the specific action of UPS. The very nature of E3 and the complexity of their interactions with different partners offer a very broad field of investigation and therefore significant potential for the development of therapeutic approaches. Without being exhaustive, this review illustrates the different strategies that have already been implemented to fight against different pathologies (excluding bacterial or viral infections).

UBE2L3, a Partner of MuRF1/TRIM63, Is Involved in the Degradation of Myofibrillar Actin and Myosin

The ubiquitin proteasome system (UPS) is the main player of skeletal muscle wasting, a common characteristic of many diseases (cancer, etc.) that negatively impacts treatment and life prognosis. Within the UPS, the E3 ligase MuRF1/TRIM63 targets for degradation several myofibrillar proteins, including the main contractile proteins alpha-actin and myosin heavy chain (MHC). We previously identified five E2 ubiquitin-conjugating enzymes interacting with MuRF1, including UBE2L3/UbcH7, that exhibited a high affinity for MuRF1 (K_D = 50 nM). Here, we report a main effect of UBE2L3 on alpha-actin and MHC degradation in catabolic C2C12 myotubes. Consistently UBE2L3 knockdown in Tibialis anterior induced hypertrophy in dexamethasone (Dex)-treated mice, whereas overexpression worsened the muscle atrophy of Dex-treated mice. Using combined interactomic approaches, we also characterized the interactions between MuRF1 and its substrates alpha-actin and MHC and found that MuRF1 preferentially binds to filamentous F-actin (K_D = 46.7 nM) over monomeric G-actin (K_D = 450 nM). By contrast with actin that did not alter MuRF1–UBE2L3 affinity, binding of MHC to MuRF1 (K_D = 8 nM) impeded UBE2L3 binding, suggesting that differential interactions prevail with MuRF1 depending on both the substrate and the E2. Our data suggest that UBE2L3 regulates contractile proteins levels and skeletal muscle atrophy.

Identification of the MuRF1 Skeletal Muscle Ubiquitylome Through Quantitative Proteomics

MuRF1 (TRIM63) is a muscle-specific E3 ubiquitin ligase and component of the ubiquitin proteasome system. MuRF1 is transcriptionally upregulated under conditions that cause muscle loss, in both rodents and humans, and is a recognized marker of muscle atrophy. In this study, we used in vivo electroporation to determine whether MuRF1 overexpression alone can cause muscle atrophy and, in combination with ubiquitin proteomics, identify the endogenous MuRF1 substrates in skeletal muscle. Overexpression of MuRF1 in adult mice increases ubiquitination of myofibrillar and sarcoplasmic proteins, increases expression of genes associated with neuromuscular junction instability, and causes muscle atrophy. A total of 169 ubiquitination sites on 56 proteins were found to be regulated by MuRF1. MuRF1-mediated ubiquitination targeted both thick and thin filament contractile proteins, as well as, glycolytic enzymes, deubiquitinases, p62, and VCP. These data reveal a potential role for MuRF1 in not only the breakdown of the sarcomere but also the regulation of metabolism and other proteolytic pathways in skeletal muscle.

The haplolethality paradox of the wupA gene in Drosophila

Haplolethals (HL) are regions of diploid genomes that in one dose are fatal for the organism. Their biological meaning is obscure because heterozygous loss-of-function mutations result in dominant lethality (DL) and, consequently, should be under strong negative selection. We report an in depth study of the HL associated to the gene wings up A (wupA). It encodes 13 transcripts (A-M) that yield 11 protein isoforms (A-K) of Troponin I (TnI). They are functionally diverse in their control of muscle contraction, cell polarity and cell proliferation. Isoform K transfers to the nucleus where it increases transcription of the cell proliferation related genes CDK2, CDK4, Rap and Rab5. The nuclear translocation of isoform K is prevented by the co-expression of A or B isoforms, which illustrates isoform interactions. The corresponding DL mutations are, either DNA rearrangements clustered towards the gene 3’ end, thus affecting the genomic organization of all transcripts, or CRISPR-induced mutations in one of the two ATG sites which eliminate a subset of wupA products. The joint elimination of isoforms C, F, G and H, however, do not cause DL phenotypes. Genetically driven expression of single isoforms rescue neither DL nor any of the mutants known in the gene, suggesting that normal function requires properly regulated expression of specific combinations, rather than single, TnI isoforms. We conclude that the wupA associated HL results from the combined haploinsufficiency of a large set of TnI isoforms. The qualitative and quantitative normal expression of which, requires the chromosomal integrity of the wupA genomic region. Since all fly TnI isoforms are encoded in the same gene, its HL condition becomes unavoidable. These wupA features are comparable to those of dpp, the only other HL studied to some extent, and reveal a scenario of strict dosage dependence with implications for gene expression regulation and splitting.

Most species contain two copies of their genetic endowment, each received from each progenitor. If one of the duplicated genes is non-functional, the remaining copy may supply enough product as to cover the requirements for normal function or, alternatively, may reflect the insufficiency through a visible phenotype. In rare occasions, however, having one copy is so deleterious that causes lethality. These so called “haplolethal regions”, exist across species and represent an evolutionary paradox since they should have been subject to intense negative selection. The inherent difficulties to study haplolethals have precluded their study so far. Here, we analyzed the case of one of the five haplolethal regions of Drosophila, the one associated to the Troponin I encoding gene wupA, by measuring the transcriptional effects of mutations and chromosomal rearrangements affecting this gene. The data show that this haplolethality results from the combined insufficiency of a large number of Troponin I isoforms, which are functionally specialized, show interference and require the integrity of the native chromatin structure for their quantitatively regulated expression. These features unveil novel aspects of gene expression and, possibly, on evolutionary gene splitting. Also, haplolethals underscore the biological significance of protein dosage, in particular for functionally related products.

Mechanosignaling pathways alter muscle structure and function by post-translational modification of existing sarcomeric proteins to optimize energy usage

A transduced mechanical signal arriving at its destination in muscle alters sarcomeric structure and function. A major question addressed is how muscle mass and tension generation are optimized to match actual performance demands so that little energy is wasted. Three cases for improved energy efficiency are examined: the troponin complex for tuning force production, control of the myosin heads in a resting state, and the Z-disc proteins for sarcomere assembly. On arrival, the regulation of protein complexes is often controlled by post-translational modification (PTM), of which the most common are phosphorylation by kinases, deacetylation by histone deacetylases and ubiquitination by E3 ligases. Another branch of signals acts not through peptide covalent bonding but via ligand interactions (e.g. Ca²⁺ and phosphoinositide binding). The myosin head and the regulation of its binding to actin by the troponin complex is the best and earliest example of signal destinations that modify myofibrillar contractility. PTMs in the troponin complex regulate both the efficiency of the contractile function to match physiologic demand for work, and muscle mass via protein degradation. The regulation of sarcomere assembly by integration of incoming signaling pathways causing the same PTMs or ligand binding are discussed in response to mechanical loading and unloading by the Z-disc proteins CapZ, α-actinin, telethonin, titin N-termini, and others. Many human mutations that lead to cardiomyopathy and heart disease occur in the proteins discussed above, which often occur at their PTM or ligand binding sites.

Activation of Tripartite Motif Containing 63 Expression by Transcription Factor EB and Transcription Factor Binding to Immunoglobulin Heavy Chain Enhancer 3 Is Regulated by Protein Kinase D and Class IIa Histone Deacetylases

Rationale

The ubiquitin–proteasome system (UPS) is responsible for skeletal muscle atrophy. We showed earlier that the transcription factor EB (TFEB) plays a role by increasing E3 ubiquitin ligase muscle really interesting new gene-finger 1(MuRF1)/tripartite motif-containing 63 (TRIM63) expression. MuRF 1 ubiquitinates structural proteins and mediates their UPS-dependent degradation. We now investigated how TFEB-mediated TRIM63 expression is regulated.

Objective

Because protein kinase D1 (PKD1), histone deacetylase 5 (HDAC5), and TFEB belong to respective families with close structural, regulatory, and functional properties, we hypothesized that these families comprise a network regulating TRIM63 expression.

Methods and Results

We found that TFEB and transcription factor for immunoglobulin heavy-chain enhancer 3 (TFE3) activate TRIM63 expression. The class IIa HDACs HDAC4, HDAC5, and HDAC7 inhibited this activity. Furthermore, we could map the HDAC5 and TFE3 physical interaction. PKD1, PKD2, and PKD3 reversed the inhibitory effect of all tested class IIa HDACs toward TFEB and TFE3. PKD1 mediated nuclear export of all HDACs and lifted TFEB and TFE3 repression. We also mapped the PKD2 and HDAC5 interaction. We found that the inhibitory effect of PKD1 and PKD2 toward HDAC4, HDAC5, and HDAC7 was mediated by their phosphorylation and 14-3-3 mediated nuclear export.

Conclusion

TFEB and TFE3 activate TRIM63 expression. Both transcription factors are controlled by HDAC4, HDAC5, HDAC7, and all PKD-family members. We propose that the multilevel PKD/HDAC/TFEB/TFE3 network tightly controls TRIM63 expression.

How progressive cancer endangers the heart: an intriguing and underestimated problem

Since it came into being as a discipline, cardio-oncology has focused on the prevention and treatment of cardiotoxicity induced by antitumor chemotherapy and radiotherapy. Over time, it has been proved that even more detrimental is the direct effect generated by cancer cells that release pro-cachectic factors in the bloodstream. Secreted molecules target different organs at a distance, including the heart. Inflammatory and neuronal modulators released by the tumor bulk, either as free molecules or through exosomes, contribute to the pathogenesis of cardiac disease. Progressive cancer causes cachexia and severe cardiac muscle wasting accompanied by cardiomyocyte atrophy, tissue fibrosis, and several functional impairments up to heart failure. The molecular mechanisms responsible for such a cardiac muscle wasting have been partially elucidated in animal models, but minimally investigated in humans, although severe cardiac dysfunction exacerbates global cachexia and hampers efficient anti-cancer treatments. This review provides an overview of cancer-induced structural cardiac and functional damage, drawing on both clinical and scientific research. We start by looking at the pathophysiological mechanisms and evolving epidemiology and go on to discuss prevention, diagnosis, and a multimodal policy of intervention aimed at providing overall prognosis and global care for patients. Despite much interest in the cardiotoxicity of cancer therapies, the direct tumor effect on the heart remains poorly explored. There is still a lack of diagnostic criteria for the identification of the early stages of cardiac disease in cancer patients, while the possibilities that there are for effective prevention are largely underestimated. Research on innovative therapies has claimed considerable advances in preclinical studies, but none of the molecular targets suitable for clinical application has been approved for therapy. These issues are critically discussed here.

Breakdown of Filamentous Myofibrils by the UPS-Step by Step

Protein degradation maintains cellular integrity by regulating virtually all biological processes, whereas impaired proteolysis perturbs protein quality control, and often leads to human disease. Two major proteolytic systems are responsible for protein breakdown in all cells: autophagy, which facilitates the loss of organelles, protein aggregates, and cell surface proteins; and the ubiquitin-proteasome system (UPS), which promotes degradation of mainly soluble proteins. Recent findings indicate that more complex protein structures, such as filamentous assemblies, which are not accessible to the catalytic core of the proteasome in vitro, can be efficiently degraded by this proteolytic machinery in systemic catabolic states in vivo. Mechanisms that loosen the filamentous structure seem to be activated first, hence increasing the accessibility of protein constituents to the UPS. In this review, we will discuss the mechanisms underlying the disassembly and loss of the intricate insoluble filamentous myofibrils, which are responsible for muscle contraction, and whose degradation by the UPS causes weakness and disability in aging and disease. Several lines of evidence indicate that myofibril breakdown occurs in a strictly ordered and controlled manner, and the function of AAA-ATPases is crucial for their disassembly and loss.

Ubiquitin Ligases at the Heart of Skeletal Muscle Atrophy Control

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

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

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

The ubiquitin-proteasome system in regulation of the skeletal muscle homeostasis and atrophy: from basic science to disorders

Skeletal muscle is one of the most abundant and highly plastic tissues. The ubiquitin-proteasome system (UPS) is recognised as a major intracellular protein degradation system, and its function is important for muscle homeostasis and health. Although UPS plays an essential role in protein degradation during muscle atrophy, leading to the loss of muscle mass and strength, its deficit negatively impacts muscle homeostasis and leads to the occurrence of several pathological phenotypes. A growing number of studies have linked UPS impairment not only to matured muscle fibre degeneration and weakness, but also to muscle stem cells and deficiency in regeneration. Emerging evidence suggests possible links between abnormal UPS regulation and several types of muscle diseases. Therefore, understanding of the role of UPS in skeletal muscle may provide novel therapeutic insights to counteract muscle wasting, and various muscle diseases. In this review, we focussed on the role of proteasomes in skeletal muscle and its regeneration, including a brief explanation of the structure of proteasomes. In addition, we summarised the recent findings on several diseases and elaborated on how the UPS is related to their pathological states.

MuRF1/TRIM63, Master Regulator of Muscle Mass

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

Under construction: The dynamic assembly, maintenance, and degradation of the cardiac sarcomere

The sarcomere is the basic contractile unit of striated muscle and is a highly ordered protein complex with the actin and myosin filaments at its core. Assembling the sarcomere constituents into this organized structure in development, and with muscle growth as new sarcomeres are built, is a complex process coordinated by numerous factors. Once assembled, the sarcomere requires constant maintenance as its continuous contraction is accompanied by elevated mechanical, thermal, and oxidative stress, which predispose proteins to misfolding and toxic aggregation. To prevent protein misfolding and maintain sarcomere integrity, the sarcomere is monitored by an assortment of protein quality control (PQC) mechanisms. The need for effective PQC is heightened in cardiomyocytes which are terminally differentiated and must survive for many years while preserving optimal mechanical output. To prevent toxic protein aggregation, molecular chaperones stabilize denatured sarcomere proteins and promote their refolding. However, when old and misfolded proteins cannot be salvaged by chaperones, they must be recycled via degradation pathways: the calpain and ubiquitin-proteasome systems, which operate under basal conditions, and the stress-responsive autophagy-lysosome pathway. Mutations to and deficiency of the molecular chaperones and associated factors charged with sarcomere maintenance commonly lead to sarcomere structural disarray and the progression of heart disease, highlighting the necessity of effective sarcomere PQC for maintaining cardiac function. This review focuses on the dynamic regulation of assembly and turnover at the sarcomere with an emphasis on the chaperones involved in these processes and describes the alterations to chaperones - through mutations and deficient expression - implicated in disease progression to heart failure.

Ubiquitin Proteasome System Activity is Suppressed by Curcumin following Exercise-Induced Muscle Damage in Human Skeletal Muscle

PURPOSE

Curcumin is a polyphenolic compound that is suggested to dysregulate the ubiquitin-proteasome system (UPS). This study investigated the effects of curcumin supplementation on markers of UPS activity in response to muscle damage.

METHODS

Twenty-three recreationally active male and females between the ages of 18-30 were randomized into a curcumin (CUR) or placebo (PLA) group. Both groups ingested 2 g of their respective supplement and 20 mg of piperine for 11 consecutive days. Following 8 consecutive days of supplementation, participants performed a 45-minute eccentrically-biased treadmill protocol at 60% VO2max. Muscle biopsies and delayed onset muscle soreness (DOMS) assessments were performed 30 minutes prior and 3, 24, 48, and 72 hours following exercise. Skeletal muscle ubiquitin, MAFbx/Atrogin-1, ubiquitin specific peptidase 19 (USP19), and chymotrypsin-like protease concentrations were measured using ELISA. A 3-way repeated measures ANOVA with pairwise comparisons was conducted with significance set at p ≤ 0.05.

RESULTS

Compared to baseline, DOMS for both groups was significantly increased (p < 0.05) at all time points except 72 hours following exercise. No significant differences were found for USP19 between groups. Ubiquitin (p=.016) and MAFbx/Atrogin-1 (p=.006) were significantly lower for CUR compared to PLA. Additionally, MAFbx/Atrogin-1 was significantly greater for females (p=.013) compared to males. In males, curcumin resulted in significant reductions (p = .049) in chymotrypsin-like protease (p = .049).

CONCLUSION

While elevations in UPS activity were not observed in response to muscle damage, curcumin supplementation in humans does appear to dysregulate basal UPS activity in the presence of exercise-induced muscle damage.

Muscle RING-finger protein-1 (MuRF1) functions and cellular localization are regulated by SUMO1 post-translational modification

The muscle RING-finger protein-1 (MuRF1) is an E3 ubiquitin ligase expressed in skeletal and cardiac muscle tissues and it plays important roles in muscle remodeling. Upregulation of MuRF1 gene transcription participates in skeletal muscle atrophy, on contrary downregulation of protein expression leads to cardiac hypertrophy. MuRF1 gene point mutations have been found to generate protein aggregate myopathies defined as muscle disorder characterized by protein accumulation in muscle fibers. We have discovered that MuRF1 turned out to be also a target for a new post-translational modification arbitrated by conjugation of SUMO1 and it is mediated by the SUMO ligases E2 UBC9 and the E3 PIASγ/4. SUMOylation takes place at lysine 238 localized at the second coiled-coil protein domain that is required for efficient substrate interaction for polyubiquitination. We provided evidence that SUMOylation is essential for MuRF1 nuclear translocation and its mitochondria accumulation is enhanced in hyperglycemic conditions delivering a stabilization of the overall SUMOylated proteins in cultured myocytes. Thus, our findings add this SUMO1 post-translational modification as a new concept to understand muscle disorders related to the defect in MuRF1 activity.

Ubiquitin ligase Cbl-b and inhibitory Cblin peptides

This review focuses on the Cbl-b muscle atrophy-associated ubiquitin ligase and its inhibitors. Herein, the role of E3 ubiquitin ligase-associated muscle atrophy genes (atrogenes), including MAFbx-1/agrogin-1 and MuRF-1, as well as another ubiquitin ligase, Cbl-b and its inhibitors, is discussed. Cbl-b plays an important role in unloading muscle atrophy caused by spaceflight and in bedridden patients: Cbl-b ubiquitinated and induced the degradation of IRS-1, a key intermediate in the IGF-1 signaling. Furthermore, a pentapetpide (DGpYMP), inhibited Cbl-b-mediated IRS-1 ubiquitination. This peptide-based Cbl-b inhibitor Cblin and its homologous peptides in foods presumably affect muscle atrophy under such conditions.

Dual-specificity phosphatase 29 is induced during neurogenic skeletal muscle atrophy and attenuates glucocorticoid receptor activity in muscle cell culture

Skeletal muscle atrophy is caused by a decrease in muscle size and strength and results from a range of physiological conditions, including denervation, immobilization, corticosteroid exposure and aging. Newly named dual-specificity phosphatase 29 (Dusp29) has been identified as a novel neurogenic atrophy-induced gene in skeletal muscle. Quantitative PCR analysis revealed that Dusp29 expression is significantly higher in differentiated myotubes compared with proliferating myoblasts. To determine how Dusp29 is transcriptionally regulated in skeletal muscle, fragments of the promoter region of Dusp29 were cloned, fused to a reporter gene, and found to be highly inducible in response to ectopic expression of the myogenic regulatory factors (MRF), MyoD and myogenin. Furthermore, site-directed mutagenesis of conserved E-box elements within the proximal promoter of Dusp29 rendered a Dusp29 reporter gene unresponsive to MRF overexpression. Dusp29, an atypical Dusp also known as Dupd1/Dusp27, was found to attenuate the ERK1/2 branch of the MAP kinase signaling pathway in muscle cells and inhibit muscle cell differentiation when ectopically expressed in proliferating myoblasts. Interestingly, Dusp29 was also found to destabilize AMPK protein while simultaneously enriching the phosphorylated pool of AMPK in muscle cells. Additionally, Dusp29 overexpression resulted in a significant increase in the glucocorticoid receptor (GR) protein and elevation in GR phosphorylation. Finally, Dusp29 was found to significantly impair the ability of the glucocorticoid receptor to function as a transcriptional activator in muscle cells treated with dexamethasone. Identifying and characterizing the function of Dusp29 in muscle provides novel insights into the molecular and cellular mechanisms for skeletal muscle atrophy.

Emerging Strategies Targeting Catabolic Muscle Stress Relief

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

Come Together: Protein Assemblies, Aggregates and the Sarcostat at the Heart of Cardiac Myocyte Homeostasis

Homeostasis in vertebrate systems is contingent on normal cardiac function. This, in turn, depends on intricate protein-based cellular machinery, both for contractile function, as well as, durability of cardiac myocytes. The cardiac small heat shock protein (csHsp) chaperone system, highlighted by αB-crystallin (CRYAB), a small heat shock protein (sHsp) that forms ∼3–5% of total cardiac mass, plays critical roles in maintaining proteostatic function via formation of self-assembled multimeric chaperones. In this work, we review these ancient proteins, from the evolutionarily preserved role of homologs in protists, fungi and invertebrate systems, as well as, the role of sHsps and chaperones in maintaining cardiac myocyte structure and function. We propose the concept of the “sarcostat” as a protein quality control mechanism in the sarcomere. The roles of the proteasomal and lysosomal proteostatic network, as well as, the roles of the aggresome, self-assembling protein complexes and protein aggregation are discussed in the context of cardiac myocyte homeostasis. Finally, we will review the potential for targeting the csHsp system as a novel therapeutic approach to prevent and treat cardiomyopathy and heart failure.

Muscle Disuse Atrophy Caused by Discord of Intracellular Signaling

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

Loss of Asb2 Impairs Cardiomyocyte Differentiation and Leads to Congenital Double Outlet Right Ventricle

Defining the pathways that control cardiac development facilitates understanding the pathogenesis of congenital heart disease. Herein, we identify enrichment of a Cullin5 Ub ligase key subunit, Asb2, in myocardial progenitors and differentiated cardiomyocytes. Using two conditional murine knockouts, Nkx^+/Cre.Asb2^fl/fl and AHF-Cre.Asb2^fl/fl, and tissue clarifying technique, we reveal Asb2 requirement for embryonic survival and complete heart looping. Deletion of Asb2 results in upregulation of its target Filamin A (Flna), and concurrent Flna deletion partially rescues embryonic lethality. Conditional AHF-Cre.Asb2 knockouts harboring one Flna allele have double outlet right ventricle (DORV), which is rescued by biallelic Flna excision. Transcriptomic and immunofluorescence analyses identify Tgfβ/Smad as downstream targets of Asb2/Flna. Finally, using CRISPR/Cas9 genome editing, we demonstrate Asb2 requirement for human cardiomyocyte differentiation suggesting a conserved mechanism between mice and humans. Collectively, our study provides deeper mechanistic understanding of the role of the ubiquitin proteasome system in cardiac development and suggests a previously unidentified murine model for DORV.

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Flna removal partially rescues embryonic lethality of Asb2-heart-specific knockout

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AHF-Asb2 knockouts harboring one Flna allele have double outlet right ventricle

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Asb2-Flna regulate TGFβ-Smad2 signaling in the heart

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Conserved role of Asb2 in heart morphogenesis between mice and humans

The autophagic-lysosomal and ubiquitin proteasome systems are simultaneously activated in the skeletal muscle of gastric cancer patients with cachexia

BACKGROUND

Cancer cachexia is characterized by weight loss, especially ongoing skeletal muscle loss, and is associated with poor patient outcomes. However, the molecular mechanism of skeletal muscle wasting is not fully understood.

OBJECTIVES

We aimed to investigate muscle fiber morphology and proteolysis system activity changes that may account for cancer cachexia and to relate these changes to patients' clinical phenotypes.

METHODS

We divided 39 patients with resectable gastric cancer into 4 groups based on the presence of cachexia (weight loss) and/or sarcopenia (low muscularity), including a noncachexia/nonsarcopenia group (N, n = 10), a cachexia/sarcopenia group (CS, n = 13), a cachexia/nonsarcopenia group (C, n = 9), and a noncachexia/sarcopenia group (S, n = 7). Rectus abdominis muscle biopsy specimens were obtained intraoperatively. Muscle fiber size, ultrastructural architecture, and the expression of autophagic-lysosomal system (ALS) and ubiquitin proteasome system (UPS) markers were assayed.

RESULTS

Mean ± SD muscle fiber cross-sectional areas were significantly decreased in the CS (460 ± 120 μm2) and S groups (480 ± 135 μm2) compared with the N (1615 ± 388 μm2, both P < 0.05) and C groups (1219 ± 302 μm2, both P < 0.05). In the C, S, and CS groups, the muscle exhibited tissue disorganization and autophagosome formation to different degrees. The levels of ALS and UPS markers were significantly increased in the CS, C, and S groups compared with the N group. Alterations in muscle fiber morphology and increased ALS and UPS activity were related to severe muscle loss, but not weight loss.

CONCLUSIONS

The ALS and UPS are simultaneously activated in cancer cachexia and may play coordinated roles in cachexia-induced muscle loss.

Fam83d modulates MAP kinase and AKT signaling and is induced during neurogenic skeletal muscle atrophy

Skeletal muscle atrophy is a serious health condition that can arise due to aging, cancer, corticosteroid exposure, and denervation. Previous work comparing gene expression profiles in control and denervated muscle tissue revealed for the first time that Fam83d is expressed in skeletal muscle and is significantly induced in response to denervation. Quantitative PCR and Western blot analysis found that Fam83d is more highly expressed in proliferating myoblasts compared to differentiated myotubes. Characterization of the transcriptional regulation of Fam83d showed that ectopic expression of myogenic regulatory factors inhibits Fam83d reporter gene activity. To assess where Fam83d is localized in the cell, Fam83d was fused with green fluorescent protein, expressed in C₂C₁₂ cells, and found to localize in a punctate manner to the cytoplasm of muscle cells. To assess function, Fam83d was ectopically expressed in cultured muscle cells and markers of muscle cell differentiation, the MAP Kinase signaling pathway, and the AKT signaling pathway were analyzed. Fam83d overexpression resulted in significant repression of myosin heavy chain and myogenin expression, while phosphorylated ERK and AKT were also significantly repressed. Interestingly, inhibition of the 26S proteasome and the MAP kinase signaling pathway both resulted in stabilization of Fam83d during muscle cell differentiation. Finally, Fam83d has a putative phospholipase D-like domain that appears to be necessary for destabilizing casein kinase Iα and inhibiting ERK phosphorylation in cultured myoblasts. The discovery that Fam83d is expressed in skeletal muscle combined with the observation that Fam83d, a potential modulator of MAP kinase and AKT signaling, is induced in response to neurogenic atrophy helps further our understanding of the molecular and cellular events of skeletal muscle wasting.

Assembly and Maintenance of Sarcomere Thin Filaments and Associated Diseases

Sarcomere assembly and maintenance are essential physiological processes required for cardiac and skeletal muscle function and organism mobility. Over decades of research, components of the sarcomere and factors involved in the formation and maintenance of this contractile unit have been identified. Although we have a general understanding of sarcomere assembly and maintenance, much less is known about the development of the thin filaments and associated factors within the sarcomere. In the last decade, advancements in medical intervention and genome sequencing have uncovered patients with novel mutations in sarcomere thin filaments. Pairing this sequencing with reverse genetics and the ability to generate patient avatars in model organisms has begun to deepen our understanding of sarcomere thin filament development. In this review, we provide a summary of recent findings regarding sarcomere assembly, maintenance, and disease with respect to thin filaments, building on the previous knowledge in the field. We highlight debated and unknown areas within these processes to clearly define open research questions.

Cellular mechanism of immobilization-induced muscle atrophy: A mini review

It is well-established that regular contraction maintains morphological and functional integrity of skeletal muscle, whereas rigorous exercise training can upregulate muscle metabolic and contractile function. However, when muscles stop contraction, such as during immobilization (IM) and denervation, withdrawal of IGF/Akt/mTOR signaling allows FoxO-controlled protein degradation pathways to dominate. Mitochondria play an important role in regulating both protein synthesis and degradation via several redox sensitive signaling pathways such as mitochondrial biogenesis, fusion and fission dynamics, ubiquitin-proteolysis, autophagy/mitophagy, and apoptosis. During prolonged IM, downregulation of PGC-1α and increased mitochondrial oxidative damage facilitate fission protein and inflammatory cytokine production and activate mitophagic process, leading to a vicious cycle of protein degradation. This “mitostasis theory of muscle atrophy” is the opposite pathway of hormesis, which defines enhanced muscle function with contractile overload. The demonstration that PGC-1α overexpression via transgene or in vivo DNA transfection can successfully restore mitochondrial homeostasis and reverse myocyte atrophy supports such a proposition. Understanding the mechanism governing mitostasis can be instrumental to the treatment of muscle atrophy associated with bedrest, cancer cachexia and sarcopenia.

Exploring the Interface between Inflammatory and Therapeutic Glucocorticoid Induced Bone and Muscle Loss

Due to their potent immunomodulatory anti-inflammatory properties, synthetic glucocorticoids (GCs) are widely utilized in the treatment of chronic inflammatory disease. In this review, we examine our current understanding of how chronic inflammation and commonly used therapeutic GCs interact to regulate bone and muscle metabolism. Whilst both inflammation and therapeutic GCs directly promote systemic osteoporosis and muscle wasting, the mechanisms whereby they achieve this are distinct. Importantly, their interactions in vivo are greatly complicated secondary to the directly opposing actions of GCs on a wide array of pro-inflammatory signalling pathways that underpin catabolic and anti-anabolic metabolism. Several clinical studies have attempted to address the net effects of therapeutic glucocorticoids on inflammatory bone loss and muscle wasting using a range of approaches. These have yielded a wide array of results further complicated by the nature of inflammatory disease, underlying the disease management and regimen of GC therapy. Here, we report the latest findings related to these pathway interactions and explore the latest insights from murine models of disease aimed at modelling these processes and delineating the contribution of pre-receptor steroid metabolism. Understanding these processes remains paramount in the effective management of patients with chronic inflammatory disease.

Perturbation of the titin/MURF1 signaling complex is associated with hypertrophic cardiomyopathy in a fish model and in human patients

Hypertrophic cardiomyopathy (HCM) is a hereditary disease characterized by cardiac hypertrophy with diastolic dysfunction. Gene mutations causing HCM have been found in about half of HCM patients, while the genetic etiology and pathogenesis remain unknown for many cases of HCM. To identify novel mechanisms underlying HCM pathogenesis, we generated a cardiovascular-mutant medaka fish, non-spring heart (nsh), which showed diastolic dysfunction and hypertrophic myocardium. The nsh homozygotes had fewer myofibrils, disrupted sarcomeres and expressed pathologically stiffer titin isoforms. In addition, the nsh heterozygotes showed M-line disassembly that is similar to the pathological changes found in HCM. Positional cloning revealed a missense mutation in an immunoglobulin (Ig) domain located in the M-line–A-band transition zone of titin. Screening of mutations in 96 unrelated patients with familial HCM, who had no previously implicated mutations in known sarcomeric gene candidates, identified two mutations in Ig domains close to the M-line region of titin. In vitro studies revealed that the mutations found both in medaka fish and in familial HCM increased binding of titin to muscle-specific ring finger protein 1 (MURF1) and enhanced titin degradation by ubiquitination. These findings implicate an impaired interaction between titin and MURF1 as a novel mechanism underlying the pathogenesis of HCM.

Summary: The authors identified and characterized a medaka mutation in titin that leads to a phenotype similar to hypertrophic cardiomyopathy. Similar mutations were also observed in human patients.

Generation of MuRF-GFP transgenic zebrafish models for investigating murf gene expression and protein localization in Smyd1b and Hsp90α1 knockdown embryos

Muscle-specific RING-finger proteins (MuRFs) are E3 ubiquitin ligases that play important roles in protein quality control in skeletal and cardiac muscles. Here we characterized murf gene expression and protein localization in zebrafish embryos. We found that the zebrafish genome contains six murf genes, including murf1a, murf1b, murf2a, murf2b, murf3 and a murf2-like gene that are specifically expressed in skeletal and cardiac muscles of zebrafish embryos. To analyze the subcellular localization, we generated transgenic zebrafish models expressing MurF1a-GFP or MuRF2a-GFP fusion proteins. MuRF1a-GFP and MuRF2a-GFP showed distinct patterns of subcellular localization. MuRF1a-GFP displayed a striated pattern of localization in myofibers, whereas MuRF2a-GFP mainly exhibited a random pattern of punctate distribution. The MuRF1a-GFP signal appeared as small dots aligned along the M-lines of the sarcomeres in skeletal myofibers. To determine whether knockdown of smyd1b or hsp90α1 that increased myosin protein degradation could alter murf gene expression or MuRF protein localization, we knocked down smyd1b or hsp90α1 in wild type, Tg(ef1a:MurF1a-GFP) and Tg(ef1a:MuRF2a-GFP) transgenic zebrafish embryos. Knockdown of smyd1b or hsp90α1 had no effect on murf gene expression. However, the sarcomeric distribution of MuRF1a-GFP was abolished in the knockdown embryos. This was accompanied by an increased random punctate distribution of MuRF1a-GFP in muscle cells of zebrafish embryos. Collectively, these studies demonstrate that MuRFs are specifically expressed in developing muscles of zebrafish embryos. The M-line localization MuRF1a is altered by sarcomere disruption in smyd1b or hsp90α1 knockdown embryos.

Ubiquitin and Ubiquitin-like proteins in cardiac disease and protection

Post-translational modification represents an important mechanism to regulate protein function in cardiac cells. Ubiquitin (Ub) and ubiquitin-like proteins (UBLs) are a family of protein modifiers that share a certain extent of sequence and structure similarity. Conjugation of Ub or UBLs to target proteins is dynamically regulated by a set of UBL-specific enzymes and modulates the physical and physiological properties of protein substrates. Ub and UBLs control a strikingly wide spectrum of cellular processes and not surprisingly are involved in the development of multiple human diseases including cardiac diseases. Further identification of novel UBL targets will expand our understanding of the functional diversity of UBL pathways in physiology and pathology. Here we review recent findings on the mechanisms, proteome and functions of a subset of UBLs and highlight their potential impacts on the development and progression of various forms of cardiac diseases.

Mitochondrial dysregulation and muscle disuse atrophy

It is well established that mitochondria play a critical role in the metabolic and physiological adaptation of skeletal muscle to enhanced contractile activity. Several redox-sensitive signaling pathways such as PGC-1α, AMPK, IGF/Akt/mTOR, SIRT, NFκB, and FoxO are involved with extensive crosstalk to regulate vital cellular functions such as mitochondrial biogenesis, mitochondrial fusion and fission dynamics, autophagy/mitophagy, and apoptosis under altered demand and stress. However, when muscles cease contraction, such as during immobilization and denervation, mitochondria undergo a series of detrimental changes characterized by downregulation of PGC-1α and antioxidant defense, increased ROS generation, activated FoxO, NFκB, and inflammation, enhanced ubiquitination, and finally mitophagy and apoptotic cascades. The phenotypic outcome of the discord of mitochondrial homeostasis is elevated proteolysis and muscle atrophy. The demonstration that PGC-1α overexpression via transgene or in vivo DNA transfection can restore mitochondrial homeostasis and reverse myocyte atrophy supports the "mitostasis theory of muscle atrophy".