Appetite for destruction: E3 ubiquitin-ligase protection in cardiac disease

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.

LRSAM1 E3 ubiquitin ligase: molecular neurobiological perspectives linked with brain diseases

Cellular protein quality control (PQC) plays a significant role in the maintenance of cellular homeostasis. Failure of PQC mechanism may lead to various neurodegenerative diseases due to accumulation of aberrant proteins. To avoid such fatal neuronal conditions PQC employs autophagy and ubiquitin proteasome system (UPS) to degrade misfolded proteins. Few quality control (QC) E3 ubiquitin ligases interplay an important role to specifically recognize misfolded proteins for their intracellular degradation. Leucine-rich repeat and sterile alpha motif-containing 1 (LRSAM1) is a really interesting new gene (RING) class protein that possesses E3 ubiquitin ligase activity with promising applications in PQC. LRSAM1 is also known as RING finger leucine repeat rich (RIFLE) or TSG 101-associated ligase (TAL). LRSAM1 has various cellular functions as it modulates the protein aggregation, endosomal sorting machinery and virus egress from the cells. Thus, this makes LRSAM1 interesting to study not only in protein conformational disorders such as neurodegeneration but also in immunological and other cancerous disorders. Furthermore, LRSAM1 interacts with both cellular protein degradation machineries and hence it can participate in maintenance of overall cellular proteostasis. Still, more research work on the quality control molecular functions of LRSAM1 is needed to comprehend its roles in various protein aggregatory diseases. Earlier findings suggest that in a mouse model of Charcot-Marie-Tooth (CMT) disease, lack of LRSAM1 functions sensitizes peripheral axons to degeneration. It has been observed that in CMT the patients retain dominant and recessive mutations of LRSAM1 gene, which encodes most likely a defective protein. However, still the comprehensive molecular pathomechanism of LRSAM1 in neuronal functions and neurodegenerative diseases is not known. The current article systematically represents the molecular functions, nature and detailed characterization of LRSAM1 E3 ubiquitin ligase. Here, we review emerging molecular mechanisms of LRSAM1 linked with neurobiological functions, with a clear focus on the mechanism of neurodegeneration and also on other diseases. Better understanding of LRSAM1 neurobiological and intracellular functions may contribute to develop promising novel therapeutic approaches, which can also propose new lines of molecular beneficial targets for various neurodegenerative diseases.

Increasing Cardiomyocyte Atrogin-1 Reduces Aging-Associated Fibrosis and Regulates Remodeling in Vivo

The muscle-specific ubiquitin ligase atrogin-1 (MAFbx) has been identified as a critical regulator of pathologic and physiological cardiac hypertrophy; it regulates these processes by ubiquitinating transcription factors [nuclear factor of activated T-cells and forkhead box O (FoxO) 1/3]. However, the role of atrogin-1 in regulating transcription factors in aging has not previously been described. Atrogin-1 cardiomyocyte-specific transgenic (Tg⁺) adult mice (α-major histocompatibility complex promoter driven) have normal cardiac function and size. Herein, we demonstrate that 18-month-old atrogin-1 Tg⁺ hearts exhibit significantly increased anterior wall thickness without functional impairment versus wild-type mice. Histologic analysis at 18 months revealed atrogin-1 Tg⁺ mice had significantly less fibrosis and significantly greater nuclei and cardiomyocyte cross-sectional analysis. Furthermore, by real-time quantitative PCR, atrogin-1 Tg⁺ had increased Col 6a4, 6a5, 6a6, matrix metalloproteinase 8 (Mmp8), and Mmp9 mRNA, suggesting a role for atrogin-1 in regulating collagen deposits and MMP-8 and MMP-9. Because atrogin-1 Tg⁺ mice exhibited significantly less collagen deposition and protein levels, enhanced Mmp8 and Mmp9 mRNA may offer one mechanism by which collagen levels are kept in check in the aged atrogin-1 Tg⁺ heart. In addition, atrogin-1 Tg⁺ hearts showed enhanced FoxO1/3 activity. The present study shows a novel link between atrogin-1-mediated regulation of FoxO1/3 activity and reduced collagen deposition and fibrosis in the aged heart. Therefore, targeting FoxO1/3 activity via the muscle-specific atrogin-1 ubiquitin ligase may offer a muscle-specific method to modulate aging-related cardiac fibrosis.

Effects of a Proteasome Inhibitor on Cardiomyocytes in a Pressure-Overload Hypertrophy Rat Model: An Animal Study

Background

The ubiquitin-proteasome system (UPS) is an important pathway of proteolysis in pathologic hypertrophic cardiomyocytes. We hypothesize that MG132, a proteasome inhibitor, might prevent hypertrophic cardiomyopathy (CMP) by blocking the UPS. Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and androgen receptor (AR) have been reported to be mediators of CMP and heart failure. This study drew upon pathophysiologic studies and the analysis of NF-κB and AR to assess the cardioprotective effects of MG132 in a left ventricular hypertrophy (LVH) rat model.

Methods

We constructed a transverse aortic constriction (TAC)-induced LVH rat model with 3 groups: sham (TAC-sham, n=10), control (TAC-cont, n=10), and MG132 administration (TAC-MG132, n=10). MG-132 (0.1 mg/kg) was injected for 4 weeks in the TAC-MG132 group. Pathophysiologic evaluations were performed and the expression of AR and NF-κB was measured in the left ventricle.

Results

Fibrosis was prevalent in the pathologic examination of the TAC-cont model, and it was reduced in the TAC-MG132 group, although not significantly. Less expression of AR, but not NF-κB, was found in the TAC-MG132 group than in the TAC-cont group (p<0.05).

Conclusion

MG-132 was found to suppress AR in the TAC-CMP model by blocking the UPS, which reduced fibrosis. However, NF-κB expression levels were not related to UPS function.

Allele-specific expression in the human heart and its application to postoperative atrial fibrillation and myocardial ischemia

BACKGROUND

Allele-specific expression (ASE) is differential expression of each of the two chromosomal alleles of an autosomal gene. We assessed ASE patterns in the human left atrium (LA, n = 62) and paired samples from the left ventricle (LV, n = 76) before and after ischemia, and tested the utility of differential ASE to identify genes associated with postoperative atrial fibrillation (poAF) and myocardial ischemia.

METHODS

Following genotyping from whole blood and whole-genome sequencing of LA and LV samples, we called ASE using sequences overlapping heterozygous SNPs using rigorous quality control to minimize false ASE calling. ASE patterns were compared between cardiac chambers and with a validation cohort from cadaveric tissue. ASE patterns in the LA were compared between patients who had poAF and those who did not. Changes in ASE in the LV were compared between paired baseline and post-ischemia samples.

RESULTS

ASE was found for 3404 (5.1%) and 8642 (4.0%) of SNPs analyzed in the LA and LV, respectively. Out of 6157 SNPs with ASE analyzed in both chambers, 2078 had evidence of ASE in both LA and LV (p < 0.0001). The SNP with the greatest ASE difference in the LA of patients with and without postoperative atrial fibrillation was within the gelsolin (GSN) gene, previously associated with atrial fibrillation in mice. The genes with differential ASE in poAF were enriched for myocardial structure genes, indicating the importance of atrial remodeling in the pathophysiology of AF. The greatest change in ASE between paired post-ischemic and baseline samples of the LV was in the zinc finger and homeodomain protein 2 (ZHX2) gene, a modulator of plasma lipids. Genes with differential ASE in ischemia were enriched in the ubiquitin ligase complex pathway associated with the ischemia-reperfusion response.

CONCLUSIONS

Our results establish a pattern of ASE in the human heart, with a high degree of shared ASE between cardiac chambers as well as chamber-specific ASE. Furthermore, ASE analysis can be used to identify novel genes associated with (poAF) and myocardial ischemia.

A Decade of Boon or Burden: What Has the CHIP Ever Done for Cellular Protein Quality Control Mechanism Implicated in Neurodegeneration and Aging?

Cells regularly synthesize new proteins to replace old and abnormal proteins for normal cellular functions. Two significant protein quality control pathways inside the cellular milieu are ubiquitin proteasome system (UPS) and autophagy. Autophagy is known for bulk clearance of cytoplasmic aggregated proteins, whereas the specificity of protein degradation by UPS comes from E3 ubiquitin ligases. Few E3 ubiquitin ligases, like C-terminus of Hsc70-interacting protein (CHIP) not only take part in protein quality control pathways, but also plays a key regulatory role in other cellular processes like signaling, development, DNA damage repair, immunity and aging. CHIP targets misfolded proteins for their degradation through proteasome, as well as autophagy; simultaneously, with the help of chaperones, it also regulates folding attempts for misfolded proteins. The broad range of CHIP substrates and their associations with multiple pathologies make it a key molecule to work upon and focus for future therapeutic interventions. E3 ubiquitin ligase CHIP interacts and degrades many protein inclusions formed in neurodegenerative diseases. The presence of CHIP at various nodes of cellular protein-protein interaction network presents this molecule as a potential candidate for further research. In this review, we have explored a wide range of functionality of CHIP inside cells by a detailed presentation of its co-chaperone, E3 and E4 enzyme like functions, with central focus on its protein quality control roles in neurodegenerative diseases. We have also raised many unexplored but expected fundamental questions regarding CHIP functions, which generate hopes for its future applications in research, as well as drug discovery.

The E3 ubiquitin ligase Asb2β is downregulated in a mouse model of hypertrophic cardiomyopathy and targets desmin for proteasomal degradation

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is an autosomal-dominant disease with mutations in genes encoding sarcomeric proteins. Previous findings suggest deregulation of the ubiquitin proteasome system (UPS) in HCM in humans and in a mouse model of HCM (Mybpc3-targeted knock-in (KI) mice). In this study we investigated transcript levels of several muscle-specific E3 ubiquitin ligases in KI mice and aimed at identifying novel protein targets.

METHODS AND RESULTS

Out of 9 muscle-specific E3 ligases, Asb2β was found with the lowest mRNA level in KI compared to wild-type (WT) mice. After adenoviral-mediated Asb2β transduction of WT neonatal mouse cardiomyocytes with either a WT or inactive Asb2β mutant, desmin was identified as a new target of Asb2β by mass spectrometry, co-immunoprecipitation and immunoblotting. Immunofluorescence analysis revealed a co-localization of desmin with Asb2β at the Z-disk of the sarcomere. Knock-down of Asb2β in cardiomyocytes resulted in higher desmin protein levels. Furthermore, desmin levels were higher in ventricular samples of HCM mice and patients than controls.

CONCLUSIONS

This study identifies desmin as a new Asb2β target for proteasomal degradation in cardiomyocytes and suggests that accumulation of desmin could contribute to UPS impairment in HCM mice and patients.

Targeting the ubiquitin-proteasome system in heart disease: the basis for new therapeutic strategies

SIGNIFICANCE

Novel therapeutic strategies to treat heart failure are greatly needed. The ubiquitin-proteasome system (UPS) affects the structure and function of cardiac cells through targeted degradation of signaling and structural proteins. This review discusses both beneficial and detrimental consequences of modulating the UPS in the heart.

RECENT ADVANCES

Proteasome inhibitors were first used to test the role of the UPS in cardiac disease phenotypes, indicating therapeutic potential. In early cardiac remodeling and pathological hypertrophy with increased proteasome activities, proteasome inhibition prevented or restricted disease progression and contractile dysfunction. Conversely, enhancing proteasome activities by genetic manipulation, pharmacological intervention, or ischemic preconditioning also improved the outcome of cardiomyopathies and infarcted hearts with impaired cardiac and UPS function, which is, at least in part, caused by oxidative damage.

CRITICAL ISSUES

An understanding of the UPS status and the underlying mechanisms for its potential deregulation in cardiac disease is critical for targeted interventions. Several studies indicate that type and stage of cardiac disease influence the dynamics of UPS regulation in a nonlinear and multifactorial manner. Proteasome inhibitors targeting all proteasome complexes are associated with cardiotoxicity in humans. Furthermore, the type and dosage of proteasome inhibitor impact the pathogenesis in nonuniform ways.

FUTURE DIRECTIONS

Systematic analysis and targeting of individual UPS components with established and innovative tools will unravel and discriminate regulatory mechanisms that contribute to and protect against the progression of cardiac disease. Integrating this knowledge in drug design may reduce adverse effects on the heart as observed in patients treated with proteasome inhibitors against noncardiac diseases, especially cancer.

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.

Chaperoning myosin assembly in muscle formation and aging

The activity and assembly of various myosin subtypes is coordinated by conserved UCS (UNC-45/CRO1/She4p) domain proteins. One founding member of the UCS family is the Caenorhabditis elegans UNC-45 protein important for the organization of striated muscle filaments. Our recent structural and biochemical results demonstrated that UNC-45 forms a protein chain with defined periodicity of myosin interaction domains. Intriguingly, the UNC-45 chain serves as docking platform for myosin molecules, which promotes ordered spacing and incorporation of myosin into contractile muscle sarcomeres. The physiological relevance of this observation was demonstrated in C. elegans by transgenic expression of UNC-45 chain formation mutants, which provokes defects in muscle structure and size. Collaborating with the molecular chaperones, Hsp70 and Hsp90, chain formation of UNC-45 links myosin folding with myofilament assembly. Here, we discuss our recent findings on the dynamic regulation of UNC-45 structure and stability in the context of muscle regeneration mechanisms that are affected in myopathic diseases and during aging.

Metabolic derangements in the gastrocnemius and the effect of Compound A therapy in a murine model of cancer cachexia

BACKGROUND

Cancer cachexia is a severe wasting syndrome characterized by the progressive loss of lean body mass and systemic inflammation. Inhibiting the signaling of the transcription factor nuclear factor kappa B (NF-κB) largely prevents cancer-induced muscle wasting in murine models. We have previously shown the utility of Compound A, a highly selective novel NF-κB inhibitor that targets the IκB kinase complex, to provide clinical benefit in cancer-induced skeletal muscle and cardiac atrophy.

METHODS

Using a metabolomics approach, we describe the changes found between cachectic and noncachectic gastrocnemius muscles before and after Compound A treatment at various doses.

RESULTS

Of the 234 metabolites in the gastrocnemius, cachexia-induced changes in gastrocnemius metabolism reset the steady-state abundances of 42 metabolites (p < 0.05). These changes, not evenly distributed across biochemical categories, are concentrated in amino acids, peptides, carbohydrates and energetics intermediates, and lipids. The gastrocnemius glycolytic pathway is markedly altered-changes consistent with tumor Warburg physiology. This is the first account of a Warburg effect that is not exclusively restricted to cancer cells or rapidly proliferating nonmalignant cells. Cachectic gastrocnemius also displays tricarboxylic acid cycle disruptions, signs of oxidative stress, and impaired redox homeostasis. Compound A only partially rescues the phenotype of the cachectic gastrocnemius, failing to restore the gastrocnemius' baseline metabolic profile.

CONCLUSIONS

The findings in the present manuscript enumerate the metabolic consequences of cachexia in the gastrocnemius and demonstrate that NF-kB targeted treatment only partly rescues the cachectic metabolic phenotype. These data strengthen the previous findings from metabolomic characterization of serum in cachectic animals, suggesting that many of the metabolic alterations observed in the blood originate in the diseased muscle. These findings provide significant insight into the complex pathophysiology of cancer cachexia and provide objective criteria for evaluating future therapeutics.

Mining the TRAF6/p62 interactome for a selective ubiquitination motif

A new approach is described here to predict ubiquitinated substrates of the E3 ubiquitin ligase, TRAF6, which takes into account its interaction with the scaffold protein SQSTM1/p62. A novel TRAF6 ubiquitination motif defined as [–(hydrophobic)–k–(hydrophobic)–x–x–(hydrophobic)– (polar)–(hydrophobic)–(polar)–(hydrophobic)] was identified and used to screen the TRAF6/p62 interactome composed of 155 proteins, that were either TRAF6 or p62 interactors, or a negative dataset, composed of 54 proteins with no known association to either TRAF6 or p62. NRIF (K19), TrkA (K485), TrkB (K811), TrkC (K602 and K815), NTRK2 (K828), NTRK3 (K829) and MBP (K169) were found to possess a perfect match for the amino acid consensus motif for TRAF6/p62 ubiquitination. Subsequent analyses revealed that this motif was biased to the C-terminal regions of the protein (nearly 50% the sites), and had preference for loops (~50%) and helices (~37%) over beta-strands (15% or less). In addition, the motif was observed to be in regions that were highly solvent accessible (nearly 90%). Our findings suggest that specific Lysines may be selected for ubiquitination based upon an embedded code defined by a specific amino acid motif with structural determinants. Collectively, our results reveal an unappreciated role for the scaffold protein in targeting ubiquitination. The findings described herein could be used to aid in identification of other E3/scaffold ubiquitination sites.

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.

NF-κB inhibition protects against tumor-induced cardiac atrophy in vivo

Cancer cachexia is a severe wasting syndrome characterized by the progressive loss of lean body mass and systemic inflammation. It occurs in approximately 80% of patients with advanced malignancy and is the cause of 20% to 30% of all cancer-related deaths. The mechanism by which striated muscle loss occurs is the tumor release of pro-inflammatory cytokines, such as IL-1, IL-6, and TNF-α. These cytokines interact with their cognate receptors on muscle cells to enhance NF-κB signaling, which then mediates muscle loss and significant cardiac dysfunction. Genetic inhibition of NF-κB signaling has demonstrated its predominant role in skeletal muscle loss. Therefore, we tested two novel drugs designed to specifically inhibit NF-κB by targeting the IκB kinase (IKK) complex: Compound A and NEMO binding domain (NBD) peptide. Using an established mouse model of cancer cachexia (C26 adenocarcinoma), we determined how these drugs affected the development of tumor-induced cardiac atrophy and function. Echocardiographic and histological analysis revealed that both Compound A and NBD inhibit cardiac NF-κB activity and prevent the development of tumor-induced systolic dysfunction and atrophy. This protection was independent of any effects of the tumor itself (Compound A) or tumor-secreted cytokines (NBD). This study identifies for the first time, to our knowledge, that drugs targeting the IKK complex are cardioprotective against cancer cachexia-induced cardiac atrophy and systolic dysfunction, suggesting therapies that may help reduce cardiac-associated morbidities found in patients with advanced malignancies.

Sent to destroy: the ubiquitin proteasome system regulates cell signaling and protein quality control in cardiovascular development and disease

The ubiquitin proteasome system (UPS) plays a crucial role in biological processes integral to the development of the cardiovascular system and cardiovascular diseases. The UPS prototypically recognizes specific protein substrates and places polyubiquitin chains on them for subsequent destruction by the proteasome. This system is in place to degrade not only misfolded and damaged proteins, but is essential also in regulating a host of cell signaling pathways involved in proliferation, adaptation to stress, regulation of cell size, and cell death. During the development of the cardiovascular system, the UPS regulates cell signaling by modifying transcription factors, receptors, and structural proteins. Later, in the event of cardiovascular diseases as diverse as atherosclerosis, cardiac hypertrophy, and ischemia/reperfusion injury, ubiquitin ligases and the proteasome are implicated in protecting and exacerbating clinical outcomes. However, when misfolded and damaged proteins are ubiquitinated by the UPS, their destruction by the proteasome is not always possible because of their aggregated confirmations. Recent studies have discovered how these ubiquitinated misfolded proteins can be destroyed by alternative "specific" mechanisms. The cytosolic receptors p62, NBR, and histone deacetylase 6 recognize aggregated ubiquitinated proteins and target them for autophagy in the process of "selective autophagy." Even the ubiquitination of multiple proteins within whole organelles that drive the more general macro-autophagy may be due, in part, to similar ubiquitin-driven mechanisms. In summary, the crosstalk between the UPS and autophagy highlight the pivotal and diverse roles the UPS plays in maintaining protein quality control and regulating cardiovascular development and disease.

Seek and destroy: the ubiquitin----proteasome system in cardiac disease

The ubiquitin-proteasome system (UPS) is a major proteolytic system that regulates the degradation of intracellular proteins in the heart. The UPS regulates the turnover of misfolded and damaged proteins, in addition to numerous cellular processes, by affecting the stability of short-lived proteins such as transcription factors and cell signaling pathways. The UPS is tightly regulated by the specificity of ubiquitin ligases that recognize specific substrates and direct the addition of ubiquitin, targeting the substrates for degradation by the 26S proteasome. An increasing number of cardiac ubiquitin ligases have been identified, and the number of substrates each one is known to recognize also has increased, expanding their roles. Although mainly cardioprotective roles have been attributed to ubiquitin ligases, new studies have identified exceptions to this rule. This review discusses the mechanisms of cardiac ubiquitin ligases and identifies their role in common cardiac diseases including cardiac hypertrophy, cardiac atrophy, ischemic heart disease, and diabetic cardiomyopathy.

Muscle ring finger 1 mediates cardiac atrophy in vivo

Pathological cardiac hypertrophy, induced by various etiologies such as high blood pressure and aortic stenosis, develops in response to increased afterload and represents a common intermediary in the development of heart failure. Understandably then, the reversal of pathological cardiac hypertrophy is associated with a significant reduction in cardiovascular event risk and represents an important, yet underdeveloped, target of therapeutic research. Recently, we determined that muscle ring finger-1 (MuRF1), a muscle-specific protein, inhibits the development of experimentally induced pathological; cardiac hypertrophy. We now demonstrate that therapeutic cardiac atrophy induced in patients after left ventricular assist device placement is associated with an increase in cardiac MuRF1 expression. This prompted us to investigate the role of MuRF1 in two independent mouse models of cardiac atrophy: 1) cardiac hypertrophy regression after reversal of transaortic constriction (TAC) reversal and 2) dexamethasone-induced atrophy. Using echocardiographic, histological, and gene expression analyses, we found that upon TAC release, cardiac mass and cardiomyocyte cross-sectional areas in MuRF1(-/-) mice decreased approximately 70% less than in wild type mice in the 4 wk after release. This was in striking contrast to wild-type mice, who returned to baseline cardiac mass and cardiomyocyte size within 4 days of TAC release. Despite these differences in atrophic remodeling, the transcriptional activation of cardiac hypertrophy measured by beta-myosin heavy chain, smooth muscle actin, and brain natriuretic peptide was attenuated similarly in both MuRF1(-/-) and wild-type hearts after TAC release. In the second model, MuRF1(-/-) mice also displayed resistance to dexamethasone-induced cardiac atrophy, as determined by echocardiographic analysis. This study demonstrates, for the first time, that MuRF1 is essential for cardiac atrophy in vivo, both in the setting of therapeutic regression of cardiac hypertrophy and dexamethasone-induced atrophy.

Protein quality control and degradation in cardiomyocytes

The heart is constantly under stress and cardiomyocytes face enormous challenges to correctly fold nascent polypeptides and keep mature proteins from denaturing. To meet the challenge, cardiomyocytes have developed multi-layered protein quality control (PQC) mechanisms which are carried out primarily by chaperones and ubiquitin-proteasome system mediated proteolysis. Autophagy may also participate in PQC in cardiomyocytes, especially under pathological conditions. Cardiac PQC often becomes inadequate in heart disease, which may play an important role in the development of congestive heart failure.