The ubiquitin-proteasome system in cardiac proteinopathy: a quality control perspective

Paralogue-Specific Roles of SUMO1 and SUMO2/3 in Protein Quality Control and Associated Diseases

Small ubiquitin-related modifiers (SUMOs) function as post-translational protein modifications and regulate nearly every aspect of cellular function. While a single ubiquitin protein is expressed across eukaryotic organisms, multiple SUMO paralogues with distinct biomolecular properties have been identified in plants and vertebrates. Five SUMO paralogues have been characterized in humans, with SUMO1, SUMO2 and SUMO3 being the best studied. SUMO2 and SUMO3 share 97% protein sequence homology (and are thus referred to as SUMO2/3) but only 47% homology with SUMO1. To date, thousands of putative sumoylation substrates have been identified thanks to advanced proteomic techniques, but the identification of SUMO1- and SUMO2/3-specific modifications and their unique functions in physiology and pathology are not well understood. The SUMO2/3 paralogues play an important role in proteostasis, converging with ubiquitylation to mediate protein degradation. This function is achieved primarily through SUMO-targeted ubiquitin ligases (STUbLs), which preferentially bind and ubiquitylate poly-SUMO2/3 modified proteins. Effects of the SUMO1 paralogue on protein solubility and aggregation independent of STUbLs and proteasomal degradation have also been reported. Consistent with these functions, sumoylation is implicated in multiple human diseases associated with disturbed proteostasis, and a broad range of pathogenic proteins have been identified as SUMO1 and SUMO2/3 substrates. A better understanding of paralogue-specific functions of SUMO1 and SUMO2/3 in cellular protein quality control may therefore provide novel insights into disease pathogenesis and therapeutic innovation. This review summarizes current understandings of the roles of sumoylation in protein quality control and associated diseases, with a focus on the specific effects of SUMO1 and SUMO2/3 paralogues.

Integrated transcriptomics and metabolomics reveal protective effects on heart of hibernating Daurian ground squirrels

Hibernating mammals are natural models of resistance to ischemia, hypoxia-reperfusion injury, and hypothermia. Daurian ground squirrels (spermophilus dauricus) can adapt to endure multiple torpor-arousal cycles without sustaining cardiac damage. However, the molecular regulatory mechanisms that underlie this adaptive response are not yet fully understood. This study investigates morphological, functional, genetic, and metabolic changes that occur in the heart of ground squirrels in three groups: summer active (SA), late torpor (LT), and interbout arousal (IBA). Morphological and functional changes in the heart were measured using hematoxylin-eosin (HE) staining, Masson staining, echocardiography, and enzyme-linked immunosorbent assay (ELISA). Results showed significant changes in cardiac function in the LT group as compared with SA or IBA groups, but no irreversible damage occurred. To understand the molecular mechanisms underlying these phenotypic changes, transcriptomic and metabolomic analyses were conducted to assess differential changes in gene expression and metabolite levels in the three groups of ground squirrels, with a focus on GO and KEGG pathway analysis. Transcriptomic analysis showed that differentially expressed genes were involved in the remodeling of cytoskeletal proteins, reduction in protein synthesis, and downregulation of the ubiquitin-proteasome pathway during hibernation (including LT and IBA groups), as compared with the SA group. Metabolomic analysis revealed increased free amino acids, activation of the glutathione antioxidant system, altered cardiac fatty acid metabolic preferences, and enhanced pentose phosphate pathway activity during hibernation as compared with the SA group. Combining the transcriptomic and metabolomic data, active mitochondrial oxidative phosphorylation and creatine-phosphocreatine energy shuttle systems were observed, as well as inhibition of ferroptosis signaling pathways during hibernation as compared with the SA group. In conclusion, these results provide new insights into cardio-protection in hibernators from the perspective of gene and metabolite changes and deepen our understanding of adaptive cardio-protection mechanisms in mammalian hibernators.

S14-Phosphorylated RPN6 Mediates Proteasome Activation by PKA and Alleviates Proteinopathy

BACKGROUND

A better understanding of the regulation of proteasome activities can facilitate the search for new therapeutic strategies. A cell culture study shows that PKA (cAMP-dependent protein kinase or protein kinase A) activates the 26S proteasome by pS14-Rpn6 (serine14-phosphorylated Rpn6), but this discovery and its physiological significance remain to be established in vivo.

METHODS

Male and female mice with Ser14 of Rpn6 (regulatory particle non-ATPase 6) mutated to Ala (S14A [Rpn6/Psmd11S14A]) or Asp (S14D) to respectively block or mimic pS14-Rpn6 were created and used along with cells derived from them. cAMP/PKA were manipulated pharmacologically. Ubiquitin-proteasome system functioning was evaluated with the GFPdgn (green fluorescence protein with carboxyl fusion of the CL1 degron) reporter mouse and proteasomal activity assays. Impact of S14A and S14D on proteotoxicity was tested in mice and cardiomyocytes overexpressing the misfolded protein R120G-CryAB (R120G [arginine120 to glycine missense mutant alpha B-crystallin]).

RESULTS

PKA activation increased pS14-Rpn6 and 26S proteasome activities in wild-type but not S14A embryonic fibroblasts (mouse embryonic fibroblasts), adult cardiomyocytes, and mouse hearts. Basal 26S proteasome activities were significantly greater in S14D myocardium and adult mouse cardiomyocytes than in wild-type counterparts. S14D::GFPdgn mice displayed significantly lower myocardial GFPdgn protein but not mRNA levels than GFPdgn mice. In R120G mice, a classic model of cardiac proteotoxicity, basal myocardial pS14-Rpn6 was significantly lower compared with nontransgenic littermates, which was not always associated with reduction of other phosphorylated PKA substrates. Cultured S14D neonatal cardiomyocytes displayed significantly faster proteasomal degradation of R120G than wild-type neonatal cardiomyocytes. Compared with R120G mice, S14D/S14D::R120G mice showed significantly greater myocardial proteasome activities, lower levels of total and K48-linked ubiquitin conjugates, and of aberrant CryAB (alpha B-crystallin) protein aggregates, less fetal gene reactivation, and cardiac hypertrophy, and delays in cardiac malfunction.

CONCLUSIONS

This study establishes in animals that pS14-Rpn6 mediates the activation of 26S proteasomes by PKA and that the reduced pS14-Rpn6 is a key pathogenic factor in cardiac proteinopathy, thereby identifying a new therapeutic target to reduce cardiac proteotoxicity.

The H protein of attenuated canine distemper virus is degraded via endoplasmic reticulum-associated protein degradation

Canine distemper (CD) caused by canine distemper virus (CDV) is considered a highly contagious and acutely febrile disease in various animals around the world. Endoplasmic reticulum-associated protein degradation (ERAD) is an important biological effect induced by endoplasmic reticulum (ER) stress (ERS) for the degradation of unfolded/misfolded proteins in the ER of cells. CDV H glycoprotein is translocated into the ER for post-translational modifications. The effects of CDV H and ER on each other are unclear. In this study, we found that CDV H protein induced ERS through the PERK-mediated signaling pathway. The inhibition of ERS by 4-Phenylbutyric acid (4-PBA) increased the H protein amounts of an attenuated CDV, which was reduced by dithiothreitol (DTT)-induced ERS. Further, the H protein levels were increased when ERAD was inhibited by using Eeyarestatin I or interfering E3 ligase Hrd1 in ERAD, suggesting that the attenuated CDV H protein is degraded via ERAD. ERAD involved ubiquitin-dependent proteasome degradation (UPD) and/or autophagic-lysosome degradation (ALD). The attenuated CDV H protein was ubiquitinated and significantly increased after treatment with UPD inhibitor MG132 but not ALD inhibitor chloroquine (CQ), suggesting that ERAD degrading the attenuated CDV H protein selectively depends on UPD. Moreover, the inhibition of the degradation of CDV H protein with 4-PBA or MG132 treatment increased viral replication, whereas treatment with DTT promoting degradation of H protein was found to reduce viral replication. These findings suggest that the degradation of CDV H protein via ERAD negatively affects viral replication and provide a new idea for developing CDV prevention and control strategies.

Ser14-RPN6 Phosphorylation Mediates the Activation of 26S Proteasomes by cAMP and Protects against Cardiac Proteotoxic Stress in Mice

Background

A better understanding of the regulation of proteasome activities can facilitate the search for new therapeutic strategies. A cell culture study shows that cAMP-dependent protein kinase (PKA) activates the 26S proteasome by phosphorylating Ser14 of RPN6 (pS14-RPN6), but this discovery and its physiological significance remain to be established in vivo .

Methods

Male and female mice with Ser14 of Rpn6 mutated to Ala (S14A) or Asp (S14D) to respectively block or mimic pS14-Rpn6 were created and used along with cells derived from them. cAMP/PKA were manipulated pharmacologically. Ubiquitin-proteasome system (UPS) functioning was evaluated with the GFPdgn reporter mouse and proteasomal activity assays. Impact of S14A and S14D on proteotoxicity was tested in mice and cardiomyocytes overexpressing the misfolded protein R120G-CryAB (R120G).

Results

PKA activation increased pS14-Rpn6 and 26S proteasome activities in wild-type (WT) but not S14A embryonic fibroblasts (MEFs), adult cardiomyocytes (AMCMs), and mouse hearts. Basal 26S proteasome activities were significantly greater in S14D myocardium and AMCMs than in WT counterparts. S14D::GFPdgn mice displayed significantly lower myocardial GFPdgn protein but not mRNA levels than GFPdgn mice. In R120G mice, a classic model of cardiac proteotoxicity, basal myocardial pS14-Rpn6 was significantly lower compared with non- transgenic littermates, which was not always associated with reduction of other phosphorylated PKA substrates. Cultured S14D neonatal cardiomyocytes displayed significantly faster proteasomal degradation of R120G than WT neonatal cardiomyocytes. Compared with R120G mice, S14D/S14D::R120G mice showed significantly greater myocardial proteasome activities, lower levels of total and K48-linked ubiquitin conjugates and of aberrant CryAB protein aggregates, less reactivation of fetal genes and cardiac hypertrophy, and delays in cardiac malfunction.

Conclusions

This study establishes in animals that pS14-Rpn6 mediates the activation of 26S proteasomes by PKA and that the reduced pS14-Rpn6 is a key pathogenic factor in cardiac proteinopathy, thereby identifies a new therapeutic target to reduce cardiac proteotoxicity.

Analysis of a degron-containing reporter protein GFP-CL1 reveals a role for SUMO1 in cytosolic protein quality control

Misfolded proteins are recognized and degraded through protein quality control (PQC) pathways, which are essential for maintaining proteostasis and normal cellular functions. Defects in PQC can result in disease, including cancer, cardiovascular disease, and neurodegeneration. The small ubiquitin-related modifiers (SUMOs) were previously implicated in the degradation of nuclear misfolded proteins, but their functions in cytoplasmic PQC are unclear. Here, in a systematic screen of SUMO protein mutations in the budding yeast Saccharomyces cerevisiae, we identified a mutant allele (Smt3-K38A/K40A) that sensitizes cells to proteotoxic stress induced by amino acid analogs. Smt3-K38A/K40A mutant strains also exhibited a defect in the turnover of a soluble PQC model substrate containing the CL1 degron (NES-GFP-Ura3-CL1) localized in the cytoplasm, but not the nucleus. Using human U2OS SUMO1- and SUMO2-KO cell lines, we observed a similar SUMO-dependent pathway for degradation of the mammalian degron-containing PQC reporter protein, GFP-CL1, also only in the cytoplasm but not the nucleus. Moreover, we found that turnover of GFP-CL1 in the cytoplasm was uniquely dependent on SUMO1 but not the SUMO2 paralogue. Additionally, we showed that turnover of GFP-CL1 in the cytoplasm is dependent on the AAA-ATPase, Cdc48/p97. Cellular fractionation studies and analysis of a SUMO1-GFP-CL1 fusion protein revealed that SUMO1 promotes cytoplasmic misfolded protein degradation by maintaining substrate solubility. Collectively, our findings reveal a conserved and previously unrecognized role for SUMO1 in regulating cytoplasmic PQC and provide valuable insights into the roles of sumoylation in PQC-associated diseases.

Heart Failure And Type 2 Diabetes Mellitus: Neurohumoral, Histological And Molecular Interconnections

Heart failure (HF) is a global healthcare burden and a leading cause of morbidity and mortality worldwide. Type 2 diabetes mellitus (T2DM) appears to be one of the major risk factors that significantly worsen HF prognosis and increase the risk of fatal cardiovascular outcomes. Despite a great knowledge of pathophysiological mechanisms involved in HF development and progression, hospitalization rates in patients with HF and concomitant T2DM remain elevated. In this review, we discuss the complex interplay between systemic neurohumoral regulation and local cardiac mechanisms participating in myocardial remodeling and HF development in T2DM with special attention to cardiomyocyte energy metabolism, mitochondrial function and calcium metabolism, cardiomyocyte hypertrophy and death, extracellular matrix remodeling.

Keeping the beat against time: Mitochondrial fitness in the aging heart

The process of aging strongly correlates with maladaptive architectural, mechanical, and biochemical alterations that contribute to the decline in cardiac function. Consequently, aging is a major risk factor for the development of heart disease, the leading cause of death in the developed world. In this review, we will summarize the classic and recently uncovered pathological changes within the aged heart with an emphasis on the mitochondria. Specifically, we describe the metabolic changes that occur in the aging heart as well as the loss of mitochondrial fitness and function and how these factors contribute to the decline in cardiomyocyte number. In addition, we highlight recent pharmacological, genetic, or behavioral therapeutic intervention advancements that may alleviate age-related cardiac decline.

UCHL1 protects against ischemic heart injury via activating HIF-1α signal pathway

Ubiquitin carboxyl-terminal esterase L1 (UCHL1) has been thought to be a neuron specific protein and shown to play critical roles in Parkinson's Disease and stroke via de-ubiquiting and stabilizing key pathological proteins, such as α-synuclein. In the present study, we found that UCHL1 was significantly increased in both mouse and human cardiomyocytes following myocardial infarction (MI). When LDN-57444, a pharmacological inhibitor of UCHL1, was used to treat mice subjected to MI surgery, we found that administration of LDN-57444 compromised cardiac function when compared with vehicle treated hearts, suggesting a potential protective role of UCHL1 in response to MI. When UCHL1 was knockout by CRISPR/Cas 9 gene editing technique in human induced pluripotent stem cells (hiPSCs), we found that cardiomyocytes derived from UCHL1−/− hiPSCs were more susceptible to hypoxia/re-oxygenation induced injury as compared to wild type cardiomyocytes. To study the potential targets of UCHL1, a BioID based proximity labeling approach followed by mass spectrum analysis was performed. The result suggested that UCHL1 could bind to and stabilize HIF-1α following MI. Indeed, expression of HIF-1α was lower in UCHL1−/− cells as determined by Western blotting and HIF-1α target genes were also suppressed in UCHL1−/− cells as quantified by real time RT-PCR. Recombinant UCHL1 (rUCHL1) protein was purified by E. Coli fermentation and intraperitoneally (I.P.) delivered to mice. We found that administration of rUCHL1 could significantly preserve cardiac function following MI as compared to control group. Finally, adeno associated virus mediated cardiac specific UCHL1 delivery (AAV9-cTNT-m-UCHL1) was performed in neonatal mice. UCHL1 overexpressing hearts were more resistant to MI injury as compare to the hearts infected with control virus. In summary, our data revealed a novel protective role of UCHL1 on MI via stabilizing HIF-1α and promoting HIF-1α signaling.

The Role of HECT-Type E3 Ligase in the Development of Cardiac Disease

Despite advances in medicine, cardiac disease remains an increasing health problem associated with a high mortality rate. Maladaptive cardiac remodeling, such as cardiac hypertrophy and fibrosis, is a risk factor for heart failure; therefore, it is critical to identify new therapeutic targets. Failing heart is reported to be associated with hyper-ubiquitylation and impairment of the ubiquitin–proteasome system, indicating an importance of ubiquitylation in the development of cardiac disease. Ubiquitylation is a post-translational modification that plays a pivotal role in protein function and degradation. In 1995, homologous to E6AP C-terminus (HECT) type E3 ligases were discovered. E3 ligases are key enzymes in ubiquitylation and are classified into three families: really interesting new genes (RING), HECT, and RING-between-RINGs (RBRs). Moreover, 28 HECT-type E3 ligases have been identified in human beings. It is well conserved in evolution and is characterized by the direct attachment of ubiquitin to substrates. HECT-type E3 ligase is reported to be involved in a wide range of human diseases and health. The role of HECT-type E3 ligases in the development of cardiac diseases has been uncovered in the last decade. There are only a few review articles summarizing recent advancements regarding HECT-type E3 ligase in the field of cardiac disease. This study focused on cardiac remodeling and described the role of HECT-type E3 ligases in the development of cardiac disease. Moreover, this study revealed that the current knowledge could be exploited for the development of new clinical therapies.

Desmosomal COP9 regulates proteome degradation in arrhythmogenic right ventricular dysplasia/cardiomyopathy

Dysregulated protein degradative pathways are increasingly recognized as mediators of human disease. This mechanism may have particular relevance to desmosomal proteins that play critical structural roles in both tissue architecture and cell-cell communication, as destabilization/breakdown of the desmosomal proteome is a hallmark of genetic-based desmosomal-targeted diseases, such as the cardiac disease arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C). However, no information exists on whether there are resident proteins that regulate desmosomal proteome homeostasis. Here, we uncovered a cardiac constitutive photomorphogenesis 9 (COP9) desmosomal resident protein complex, composed of subunit 6 of the COP9 signalosome (CSN6), that enzymatically restricted neddylation and targeted desmosomal proteome degradation. CSN6 binding, localization, levels, and function were affected in hearts of classic mouse and human models of ARVD/C affected by desmosomal loss and mutations, respectively. Loss of desmosomal proteome degradation control due to junctional reduction/loss of CSN6 and human desmosomal mutations destabilizing junctional CSN6 were also sufficient to trigger ARVD/C in mice. We identified a desmosomal resident regulatory complex that restricted desmosomal proteome degradation and disease.

A Transgenic Mouse Model of Eccentric Left Ventricular Hypertrophy With Preserved Ejection Fraction Exhibits Alterations in the Autophagy-Lysosomal Pathway

The ubiquitin-proteasome system (UPS) and the autophagy-lysosomal pathway (ALP) are the main proteolytic systems involved in cellular homeostasis. Since cardiomyocytes, as terminally differentiated cells, lack the ability to share damaged proteins with their daughter cells, they are especially reliant on these protein degradation systems for their proper function. Alterations of the UPS and ALP have been reported in a wide range of cardiac diseases, including cardiomyopathies. In this study, we determined whether the UPS and ALP are altered in a mouse model of eccentric left ventricular (LV) hypertrophy expressing both cyclin T1 and Gαq under the control of the cardiac-specific α-myosin heavy chain promoter (double transgenic; DTG). Compared to wild-type (WT) littermates, DTG mice showed higher end-diastolic (ED) LV wall thicknesses and diameter with preserved ejection fraction (EF). The cardiomyopathic phenotype was further confirmed by an upregulation of the fetal gene program and genes associated with fibrosis as well as a downregulation of genes involved in Ca²⁺ handling. Likewise, higher NT-proBNP levels were detected in DTG mice. Investigation of the UPS showed elevated steady-state levels of (poly)ubiquitinated proteins without alterations of all proteasomal activities in DTG mice. Evaluation of ALP key marker revealed a mixed pattern with higher protein levels of microtubule-associated protein 1 light chain 3 beta (LC3)-I and lysosomal-associated membrane protein-2, lower protein levels of beclin-1 and FYVE and coiled-coil domain-containing protein 1 (FYCO1) and unchanged protein levels of p62/SQSTM1 in DTG mice when compared to WT. At transcriptional level, a > 1.2-fold expression was observed for Erbb2, Hdac6, Lamp2, Nrg1, and Sqstm1, while a < 0.8-fold expression was revealed for Fyco1 in DTG mice. The results related to the ALP suggested overall a repression of the ALP during the initiation process, but an induction of the ALP at the level of autophagosome-lysosome fusion and the delivery of ubiquitinated cargo to the ALP for degradation.

Tumor Susceptibility Gene 101 facilitates rapamycin-induced autophagic flux in neuron cells

Tumor Susceptibility Gene 101 (TSG101) is a member of endosomal sorting complexes responsible for endocytic pathway, which is associated with autophagic process. However, the role of TSG101 in autophagy remains unclear. To investigate the effect of TSG101 on the membrane-bound MAP1LC3-II, p62 and ubiquitinated protein levels in neuron cells, immunoblotting was used to evaluate the effects in cells silenced with siRNA against TSG101 and treated with autophagy inducer rapamycin. GFP-MAP1LC3 and tandem fluorescent-tagged LC3 (mTagRFP-mWasabi-MAP1LC3) reporter vectors were used to monitor autophagy in cells using confocal microcopy. The autophagic vacuoles were further validated with transmission electron microscopy. Our results showed that TSG101 expression was slightly increased in neuron cells when exposed to rapamycin. Depletion of TSG101 with siRNA lead to accumulation of MAP1LC3-II, GFP-MAP1LC3 puncta and autophagic vacuoles in the cells. Rapamycin-elevated MAP1LC3-II turnover and RFP⁺Wasabi^- puncta were repressed in TSG101 silenced cells, indicating that TSG101 is involved in rapamycin-induced autophagic flux in cells. Moreover, silencing TSG101 reduced colocalization of Rab7, MAP1LC3 and cell viability, increased p62, ubiquitinated proteins in the neuron cells. Taken together, our results suggested that TSG101 might be required for amphisome formation to promote autophagic flux in neuron cells when exposed to rapamycin.

Neddylation, an Emerging Mechanism Regulating Cardiac Development and Function

Defects in protein quality control have been increasingly recognized as pathogenic factors in the development of heart failure, a persistent devastating disease lacking efficacious therapies. Ubiquitin and ubiquitin-like proteins, a family of post-translational modifying polypeptides, play important roles in controlling protein quality by maintaining the stability and functional diversity of the proteome. NEDD8 (neural precursor cell expressed, developmentally downregulated 8), a small ubiquitin-like protein, was discovered two decades ago but until recently the biological significance of NEDD8 modifications (neddylation) in the heart has not been appreciated. In this review, we summarize the current knowledge of the biology of neddylation, highlighting several mechanisms by which neddylation regulates the function of its downstream targets, and discuss the expanding roles for neddylation in cardiac physiology and disease, with an emphasis on cardiac protein quality control. Finally, we outline challenges linked to the study of neddylation in health and disease.

Phosphorylation Modifications Regulating Cardiac Protein Quality Control Mechanisms

Many forms of cardiac disease, including heart failure, present with inadequate protein quality control (PQC). Pathological conditions often involve impaired removal of terminally misfolded proteins. This results in the formation of large protein aggregates, which further reduce cellular viability and cardiac function. Cardiomyocytes have an intricately collaborative PQC system to minimize cellular proteotoxicity. Increased expression of chaperones or enhanced clearance of misfolded proteins either by the proteasome or lysosome has been demonstrated to attenuate disease pathogenesis, whereas reduced PQC exacerbates pathogenesis. Recent studies have revealed that phosphorylation of key proteins has a potent regulatory role, both promoting and hindering the PQC machinery. This review highlights the recent advances in phosphorylations regulating PQC, the impact in cardiac pathology, and the therapeutic opportunities presented by harnessing these modifications.

NSAIDs-dependent adaption of the mitochondria-proteasome system in immortalized human cardiomyocytes

The progressive consumption growth of non-steroidal anti-inflammatory drugs (NSAIDs) has progressively raised the attention toward the gastrointestinal, renal, and cardiovascular toxicity. Increased risk of cardiovascular diseases was strictly associated with the usage of COX-2 selective NSAIDs. Other studies allowed to clarify that the cardiovascular risk is not limited to COX-2 selective but also extended to non-selective NSAIDs, such as Diclofenac and Ketoprofen. To date, although a less favorable cardiovascular risk profile for Diclofenac as compared to Ketoprofen is reported, the mechanisms through which NSAIDs cause adverse cardiovascular events are not entirely understood. The present study aimed to evaluate the effects of Ketoprofen in comparison with Diclofenac in immortalized human cardiomyocytes. The results obtained highlight the dose-dependent cardiotoxicity of Diclofenac compared to Ketoprofen. Despite both drugs induce the increase in ROS production, decrease of mitochondrial membrane potential, and proteasome activity modulation, only Diclofenac exposure shows a marked alteration of these intracellular parameters, leading to cell death. Noteworthy, Diclofenac decreases the proteasome 26S DC and this scenario may be dependent on the intracellular overload of oxidized proteins. The data support the hypothesis that immortalized human cardiomyocytes exposed to Ketoprofen are subjected to tolerable stress events, conversely Diclofenac exposition triggers cell death.

Targeting Protein Kinase G to Treat Cardiac Proteotoxicity

Impaired or insufficient protein kinase G (PKG) signaling and protein quality control (PQC) are hallmarks of most forms of cardiac disease, including heart failure. Their dysregulation has been shown to contribute to and exacerbate cardiac hypertrophy and remodeling, reduced cell survival and disease pathogenesis. Enhancement of PKG signaling and PQC are associated with improved cardiac function and survival in many pre-clinical models of heart disease. While many clinically used pharmacological approaches exist to stimulate PKG, there are no FDA-approved therapies to safely enhance cardiomyocyte PQC. The latter is predominantly due to our lack of knowledge and identification of proteins regulating cardiomyocyte PQC. Recently, multiple studies have demonstrated that PKG regulates PQC in the heart, both during physiological and pathological states. These studies tested already FDA-approved pharmacological therapies to activate PKG, which enhanced cardiomyocyte PQC and alleviated cardiac disease. This review examines the roles of PKG and PQC during disease pathogenesis and summarizes the experimental and clinical data supporting the utility of stimulating PKG to target cardiac proteotoxicity.

Ube2v1 Positively Regulates Protein Aggregation by Modulating Ubiquitin Proteasome System Performance Partially Through K63 Ubiquitination

RATIONALE

Compromised protein quality control can result in proteotoxic intracellular protein aggregates in the heart, leading to cardiac disease and heart failure. Defining the participants and understanding the underlying mechanisms of cardiac protein aggregation is critical for seeking therapeutic targets. We identified Ube2v1 (ubiquitin-conjugating enzyme E2 variant 1) in a genome-wide screen designed to identify novel effectors of the aggregation process. However, its role in the cardiomyocyte is undefined.

OBJECTIVE

To assess whether Ube2v1 regulates the protein aggregation caused by cardiomyocyte expression of a mutant αB crystallin (CryAB^R120G) and identify how Ube2v1 exerts its effect.

METHODS AND RESULTS

Neonatal rat ventricular cardiomyocytes were infected with adenoviruses expressing either wild-type CryAB (CryAB^WT) or CryAB^R120G. Subsequently, loss- and gain-of-function experiments were performed. Ube2v1 knockdown decreased aggregate accumulation caused by CryAB^R120G expression. Overexpressing Ube2v1 promoted aggregate formation in CryAB^WT and CryAB^R120G-expressing neonatal rat ventricular cardiomyocytes. Ubiquitin proteasome system performance was analyzed using a ubiquitin proteasome system reporter protein. Ube2v1 knockdown improved ubiquitin proteasome system performance and promoted the degradation of insoluble ubiquitinated proteins in CryAB^R120G cardiomyocytes but did not alter autophagic flux. Lys (K) 63-linked ubiquitination modulated by Ube2v1 expression enhanced protein aggregation and contributed to Ube2v1's function in regulating protein aggregate formation. Knocking out Ube2v1 exclusively in cardiomyocytes by using AAV9 (adeno-associated virus 9) to deliver multiplexed single guide RNAs against Ube2v1 in cardiac-specific Cas9 mice alleviated CryAB^R120G-induced protein aggregation, improved cardiac function, and prolonged lifespan in vivo.

CONCLUSIONS

Ube2v1 plays an important role in protein aggregate formation, partially by enhancing K63 ubiquitination during a proteotoxic stimulus. Inhibition of Ube2v1 decreases CryAB^R120G-induced aggregate formation through enhanced ubiquitin proteasome system performance rather than autophagy and may provide a novel therapeutic target to treat cardiac proteinopathies.

A novel phenotype with splicing mutation identified in a Chinese family with desminopathy

BACKGROUND

Desminopathy, a hereditary myofibrillar myopathy, mainly results from the desmin gene (DES) mutations. Desminopathy involves various phenotypes, mainly including different cardiomyopathies, skeletal myopathy, and arrhythmia. Combined with genotype, it helps us precisely diagnose and treat for desminopathy.

METHODS

Sanger sequencing was used to characterize DES variation, and then a minigene assay was used to verify the effect of splice-site mutation on pre-mRNA splicing. Phenotypes were analyzed based on clinical characteristics associated with desminopathy.

RESULTS

A splicing mutation (c.735+1G>T) in DES was detected in the proband. A minigene assay revealed skipping of the whole exon 3 and transcription of abnormal pre-mRNA lacking 32 codons. Another affected family member who carried the identical mutation, was identified with a novel phenotype of desminopathy, non-compaction of ventricular myocardium. There were 2 different phenotypes varied in cardiomyopathy and skeletal myopathy among the 2 patients, but no significant correlation between genotype and phenotype was identified.

CONCLUSIONS

We reported a novel phenotype with a splicing mutation in DES, enlarging the spectrum of phenotype in desminopathy. Molecular studies of desminopathy should promote our understanding of its pathogenesis and provide a precise molecular diagnosis of this disorder, facilitating clinical prevention and treatment at an early stage.

The role of post-translational modifications in cardiac hypertrophy

Pathological cardiac hypertrophy involves excessive protein synthesis, increased cardiac myocyte size and ultimately the development of heart failure. Thus, pathological cardiac hypertrophy is a major risk factor for many cardiovascular diseases and death in humans. Extensive research in the last decade has revealed that post‐translational modifications (PTMs), including phosphorylation, ubiquitination, SUMOylation, O‐GlcNAcylation, methylation and acetylation, play important roles in pathological cardiac hypertrophy pathways. These PTMs potently mediate myocardial hypertrophy responses via the interaction, stability, degradation, cellular translocation and activation of receptors, adaptors and signal transduction events. These changes occur in response to pathological hypertrophy stimuli. In this review, we summarize the roles of PTMs in regulating the development of pathological cardiac hypertrophy. Furthermore, PTMs are discussed as potential targets for treating or preventing cardiac hypertrophy.

Proteasome biology and therapeutics in cardiac diseases

The ubiquitin proteasome system (UPS) is the major pathway for intracellular protein degradation in most organs, including the heart. UPS controls many fundamental biological processes such as cell cycle, cell division, immune responses, antigen presentation, apoptosis, and cell signaling. The UPS not only degrades substrates but also regulates activity of gene transcription at the post-transcription level. Emerging evidence suggests that impairment of UPS function is sufficient to cause a number of cardiac diseases, including heart failure, cardiomyopathies, hypertrophy, atrophy, ischemia-reperfusion, and atherosclerosis. Alterations in the expression of UPS components, changes in proteasomal peptidase activities and increased ubiquitinated and oxidized proteins have also been detected in diabetic cardiomyopathy (DCM). However, the pathophysiological role of the UPS in DCM has not been examined. Recently, in vitro and in vivo studies have proven highly valuable in assessing effects of various stressors on the UPS and, in some cases, suggesting a causal link between defective protein clearance and disease phenotypes in different cardiac diseases, including DCM. Translation of these findings to human disease can be greatly strengthened by corroboration of discoveries from experimental model systems using human heart tissue from well-defined patient populations. This review will summarize the general role of the UPS in different cardiac diseases, with major focus on DCM, and on recent advances in therapeutic development.

A new gold(I) complex-Au(PPh3)PT is a deubiquitinase inhibitor and inhibits tumor growth

Background

Ubiquitin-proteasome system (UPS) is integral to cell survival by maintaining protein homeostasis, and its dysfunction has been linked to cancer and several other human diseases. Through counteracting ubiquitination, deubiquitinases (DUBs) can either positively or negatively regulate UPS function, thereby representing attractive targets of cancer therapies. Previous studies have shown that metal complexes can inhibit tumor growth through targeting the UPS; however, novel metal complexes with higher specificity for cancer therapy are still lacking.

Methods

We synthesized a new gold(I) complex, Au(PPh₃)PT. The inhibitory activity of Au(PPh₃)PT on the UPS and the growth of multiple cancer cell types were tested in vitro, ex vivo, and in vivo. Furthermore, we compared the efficacy of Au(PPh₃)PT with other metal compounds in inhibition of UPS function and tumor growth.

Findings

Here we report that (i) a new gold(I) complex-pyrithione, i.e., Au(PPh₃)PT, induced apoptosis in two lung cancer cell lines A549 and NCI-H1299; (ii) Au(PPh₃)PT severely impaired UPS proteolytic function; (iii) Au(PPh₃)PT selectively inhibited 19S proteasome-associated DUBs (UCHL5 and USP14) and other non-proteasomal DUBs with minimal effects on the function of 20S proteasome; (iv) Au(PPh₃)PT induced apoptosis in cancer cells from acute myeloid leukemia patients; (v) Au(PPh₃)PT effectively suppressed the growth of lung adenocarcinoma xenografts in nude mice; and (vi) Au (PPh₃)PT elicited less cytotoxicity in normal cells than several other metal compounds.

Interpretation

Together, this study discovers a new gold(I) complex to be an effective inhibitor of the DUBs and a potential anti-cancer drug.

Fund

The National High Technology Research and Development Program of China, the project of Guangdong Province Natural Science Foundation, the projects from Foundation for Higher Education of Guangdong, the project from Guangzhou Medical University for Doctor Scientists, the Medical Scientific Research Foundation of Guangdong Province, and the Guangzhou Key Medical Discipline Construction Project Fund.

Cardiovascular adverse events in multiple myeloma patients

Multiple myeloma is a malignant disease, caused by an uncontrolled clonal proliferation of a specific group of white blood cells, the plasma cells. Clinical manifestations include bone pain due to osteolysis, hypercalcemia, anemia, and renal insufficiency. Proteasome inhibitors have substantially improved survival of patients suffering from multiple myeloma, providing an efficient treatment option mainly for relapsed and refractory multiple myeloma. Although constituting one substance class, bortezomib, carfilzomib, and ixazomib differ greatly regarding their non-hematologic side effects. This article reviews the clinical and preclinical data on approved proteasome inhibitors in an attempt to decipher the underlying pathomechanisms related to cardiovascular adverse events seen in clinical trials.

Treatment with crocin improves cardiac dysfunction by normalizing autophagy and inhibiting apoptosis in STZ-induced diabetic cardiomyopathy

BACKGROUND AND AIM

The association of diabetes mellitus (DM) and poor metabolic control with high incidence of cardiovascular diseases is well established. The aim of this study was to investigate the potential cardioprotective effect of crocin (Crocus sativus L. extract) on diabetic heart dysfunction and to elucidate the mediating molecular mechanisms.

METHODS AND RESULTS

Streptozotocin (STZ)-induced diabetic rats were treated with two different concentrations of crocin (10 or 20 mg/kg), while isolated cardiac myocytes exposed to 25 mM glucose, were treated with 1 or 10 μM of crocin. Treatment of STZ-diabetic rats with crocin resulted in normalization of plasma glucose levels, inhibition of cardiac hypertrophy and fibrosis, and improvement of cardiac contractile function. Heat Shock Response was enhanced. Myocardial AMPK phosphorylation was increased after treatment with crocin, resulting in normalization of autophagy marker proteins (LC3BII/LC3BI ratio, SQSTM1/p62 and Beclin-1), while the diabetes-induced myocardial apoptosis was decreased. Similar results regarding the effect of crocin on autophagy and apoptosis pathways were obtained in isolated cardiac myocytes exposed to high concentration of glucose.

CONCLUSION

The results suggest that crocin improves the deteriorated cardiac function in diabetic animals by enhancing the heat shock response, inhibiting apoptosis and normalizing autophagy in cardiac myocytes. Thus, treatment with crocin may represent a novel approach for treating diabetic cardiomyopathy.

Cardiac fibrosis in mouse expressing DsRed tetramers involves chronic autophagy and proteasome degradation insufficiency

Proteinopathy in the heart which often manifests excessive misfolded/aggregated proteins in cardiac myocytes can result in severe fibrosis and heart failure. Here we developed a mouse model, which transgenically express tetrameric DsRed, a red fluorescent protein (RFP), in an attempt to mimic the pathological mechanisms ofcardiac fibrosis. Whilst DsRed is expressed and forms aggregation in most mouse organs, certain pathological defects are specifically recapitulated in cardiac muscle cells including mitochondria damages, aggresome-like residual bodies, excessive ubiquitinated proteins, and the induction of autophagy. The proteinopathy and cellular injuries caused by DsRed aggregates may be due to impaired or overburdened ubiquitin-proteasome system and autophagy-lysosome systems. We further identified that DsRed can be ubiquitinated and associated with MuRF1, a muscle-specific E3 ligase. Concomitantly, an activation of NF-κB signaling and a strong TIMP1 induction were noted, suggesting that RFP-induced fibrosis was augmented by a skewed balance between TIMP1 and MMPs. Taken together, our study highlights the molecular consequences of uncontrolled protein aggregation leading to congestive heart failure, and provides novel insights into fibrosis formation that can be exploited for improved therapy.

Milder degenerative effects of Carfilzomib vs. Bortezomib in the Drosophila model: a link to clinical adverse events

Proteasome inhibitors, e.g. Bortezomib (BTZ) and Carfilzomib (CFZ), have demonstrated clinical efficacy against haematological cancers. Interestingly, several adverse effects are less common, compared to BTZ, in patients treated with CFZ. As the molecular details of these observations remain not well understood we assayed the pathophysiological effects of CFZ vs. BTZ in the Drosophila experimental model. Mass Spectrometry analyses showed that neither CFZ nor BTZ are hydrolysed in flies’ tissues, while at doses inducing similar inhibition of the rate limiting for protein breakdown chymotrypsin-like (CT-L) proteasomal activity, CFZ treatment resulted in less intense increase of oxidative stress or activation of antioxidant and proteostatic modules. Also, despite comparable cardiotoxicity likely due to disrupted mitochondrial function, CFZ did not affect developmental processes, showed minimal neuromuscular defects and reduced to a lesser extent flies’ healthspan. Studies in flies, human cancer cell lines and blood cells isolated from Multiple Myeloma patients treated with CFZ or BTZ revealed, that the increased BTZ toxicity likely relates to partial co-inhibition of the caspase-like (C-L) proteasomal activity Supportively, co-treating flies with CFZ and a C-L selective proteasome inhibitor exacerbated CFZ-mediated toxicity. Our findings provide a reasonable explanation for the differential adverse effects of CFZ and BTZ in the clinic.

Chemotherapy-Induced Tissue Injury: An Insight into the Role of Extracellular Vesicles-Mediated Oxidative Stress Responses

The short- and long-term side effects of chemotherapy limit the maximum therapeutic dose and impair quality of life of survivors. Injury to normal tissues, especially chemotherapy-induced cardiomyopathy, is an unintended outcome that presents devastating health impacts. Approximately half of the drugs approved by the Food and Drug Administration for cancer treatment are associated with the generation of reactive oxygen species, and Doxorubicin (Dox) is one of them. Dox undergoes redox cycling by involving its quinone structure in the production of superoxide free radicals, which are thought to be instrumental to the role it plays in cardiomyopathy. Dox-induced protein oxidation changes protein function, translocation, and aggregation that are toxic to cells. To maintain cellular homeostasis, oxidized proteins can be degraded intracellularly by ubiquitin-proteasome pathway or by autophagy, depending on the redox status of the cell. Alternatively, the cell can remove oxidized proteins by releasing extracellular vesicles (EVs), which can be transferred to neighboring or distant cells, thereby instigating an intercellular oxidative stress response. In this article, we discuss the role of EVs in oxidative stress response, the potential of EVs as sensitive biomarkers of oxidative stress, and the role of superoxide dismutase in attenuating EV-associated oxidative stress response resulting from chemotherapy.

Transcriptome analysis of IFM-specific actin and myosin nulls in Drosophila melanogaster unravels lesion-specific expression blueprints across muscle mutations

Muscle contraction is a highly fine-tuned process that requires the precise and timely construction of large protein sub-assemblies to form sarcomeres. Mutations in many genes encoding constituent proteins of this macromolecular machine result in defective functioning of the muscle tissue. However, the pathways underlying muscle degeneration, and manifestation of myopathy phenotypes are not well understood. In this study, we explored transcriptional alterations that ensue from the absence of the two major muscle proteins - myosin and actin - using the Drosophila indirect flight muscles. Our aim was to understand how the muscle tissue responds as a whole to the absence of either of the major scaffold proteins, whether the responses are generic to the tissue; or unique to the thick versus thin filament systems. Our results indicated that muscles respond by altering gene transcriptional levels in multiple systems active in muscle remodelling, protein degradation and heat shock responses. However, there were some responses that were filament-specific signatures of muscle degeneration, like immune responses, metabolic alterations and alterations in expression of muscle structural genes and mitochondrial ribosomal genes. These general and filament-specific changes in gene expression may be of relevance to human myopathies.

Endoplasmic Reticulum Stress Induces the Early Appearance of Pro-apoptotic and Anti-apoptotic Proteins in Neurons of Five Familial Alzheimer's Disease Mice

Background:

Amyloid β (Aβ) deposits and the endoplasmic reticulum stress (ERS) are both well established in the development and progression of Alzheimer's disease (AD). However, the mechanism and role of Aβ-induced ERS in AD-associated pathological progression remain to be elucidated.

Methods:

The five familial AD (5×FAD) mice and wild-type (WT) mice aged 2, 7, and 12 months were used in the present study. Morris water maze test was used to evaluate their cognitive performance. Immunofluorescence and Western blot analyses were used to examine the dynamic changes of pro-apoptotic (CCAAT/enhancer-binding protein homologous protein [CHOP] and cleaved caspase-12) and anti-apoptotic factors (chaperone glucose-regulated protein [GRP] 78 and endoplasmic reticulum-associated protein degradation-associated ubiquitin ligase synovial apoptosis inhibitor 1 [SYVN1]) in the ERS-associated unfolded protein response (UPR) pathway.

Results:

Compared with age-matched WT mice, 5×FAD mice showed higher cleaved caspase-3, lower neuron-positive staining at the age of 12 months, but earlier cognitive deficit at the age of 7 months (all P < 0.05). Interestingly, for 2-month-old 5×FAD mice, the related proteins involved in the ERS-associated UPR pathway, including CHOP, cleaved caspase-12, GRP 78, and SYVN1, were significantly increased when compared with those in age-matched WT mice (all P < 0.05). Moreover, ERS occurred mainly in neurons, not in astrocytes.

Conclusions:

These findings suggest that compared with those of age-matched WT mice, ERS-associated pro-apoptotic and anti-apoptotic proteins are upregulated in 2-month-old 5×FAD mice, consistent with intracellular Aβ aggregation in neurons.

A retrospective analysis of 3954 patients in phase 2/3 trials of bortezomib for the treatment of multiple myeloma: towards providing a benchmark for the cardiac safety profile of proteasome inhibition in multiple myeloma

This retrospective analysis aimed to establish the overall cardiac safety profile of bortezomib using patient-level data from one phase 2 and seven phase 3 studies in previously untreated and relapsed/refractory multiple myeloma (MM). Seven clinically relevant primary [congestive heart failure (CHF), arrhythmias, ischaemic heart disease (IHD), cardiac death] and secondary (hypertension, dyspnoea, oedema) cardiac endpoints were defined based on MedDRA v16.0 preferred terms. 2509 bortezomib-treated patients and 1445 patients in non-bortezomib-based control arms were included. The incidence of grade ≥3 CHF was 1·3-4·0% in studies in relapsed/refractory MM and 1·2-4·7% in previously untreated MM (2·0-7·6% all grades), with no significant differences between bortezomib- and non-bortezomib-based arms in comparative studies. Incidences of arrhythmias (1·3-5·9% grade ≥2; 0·6-4·1% grade ≥3), IHD (1·2-2·9% all grades; 0·4-2·7% grade ≥3) and cardiac death (0-1·4%) were low, with no differences between bortezomib-based and non-bortezomib-based arms. Higher rates of oedema (mostly grade 1/2) were seen in bortezomib-based versus non-bortezomib-based arms in one study and a pooled transplant study analysis. Logistic regression analyses of comparative studies showed no impact on cardiac risk with bortezomib-based versus non-bortezomib-based treatment. Bortezomib-based treatment was associated with low incidences of cardiac events.

Amelioration of X-Linked Related Autophagy Failure in Danon Disease With DNA Methylation Inhibitor

BACKGROUND

Danon disease is an X-linked disorder that leads to fatal cardiomyopathy caused by a deficiency in lysosome-associated membrane protein-2 (LAMP2). In female patients, a later onset and less severe clinical phenotype have been attributed to the random inactivation of the X chromosome carrying the mutant diseased allele. We generated a patient-specific induced pluripotent stem cell (iPSCs)-based model of Danon disease to evaluate the therapeutic potential of Xi-chromosome reactivation using a DNA methylation inhibitor.

METHODS

Using whole-exome sequencing, we identified a nonsense mutation (c.520C>T, exon 4) of the LAMP2 gene in a family with Danon disease. We generated iPSC lines from somatic cells derived from the affected mother and her 2 sons, and we then differentiated them into cardiomyocytes (iPSC-CMs) for modeling the histological and functional signatures, including autophagy failure of Danon disease.

RESULTS

Our iPSC-CM platform provides evidence that random inactivation of the wild-type and mutant LAMP2 alleles on the X chromosome is responsible for the unusual phenotype in female patients with Danon disease. In vitro, iPSC-CMs from these patients reproduced the histological features and autophagy failure of Danon disease. Administration of the DNA demethylating agent 5-aza-2'-deoxycytidine reactivated the silent LAMP2 allele in iPSCs and iPSC-CMs in female patients with Danon disease and ameliorated their autophagy failure, supporting the application of a patient-specific iPSC platform for disease modeling and drug screening.

CONCLUSIONS

Our iPSC-CM platform provides novel mechanistic and therapeutic insights into the contribution of random X chromosome inactivation to disease phenotype in X-linked Danon disease.

Ginkgolide K protects the heart against endoplasmic reticulum stress injury by activating the inositol-requiring enzyme 1α/X box-binding protein-1 pathway

BACKGROUND AND PURPOSE

Endoplasmic reticulum (ER) stress is increasingly recognized as an important causal factor of many diseases. Targeting ER stress has now emerged as a new therapeutic strategy for treating cardiovascular diseases. Here, we investigated the effects and underlying mechanism of ginkgolide K (1,10-dihydroxy-3,14-didehydroginkgolide, GK) on cardiac ER stress.

EXPERIMENTAL APPROACH

Cell death, apoptosis and ER stress-related signalling pathways were measured in cultured neonatal rat cardiomyocytes, treated with the ER stress inducers tunicamycin, hydrogen peroxide and thapsigargin. Acute myocardial infarction was established using left coronary artery occlusion in mice, and infarct size was measured by triphenyltetrazolium chloride staining. Echocardiography was used to assess heart function and transmission electron microscopy for evaluating ER expansion.

KEY RESULTS

Ginkgolide K (GK) significantly decreased ER stress-induced cell death in both in vitro and in vivo models. In ischaemic injured mice, GK treatment reduced infarct size, rescued heart dysfunction and ameliorated ER dilation. Mechanistic studies revealed that the beneficial effects of GK occurred through enhancement of inositol-requiring enzyme 1α (IRE1α)/X box-binding protein-1 (XBP1) activity, which in turn led to increased ER-associated degradation-mediated clearance of misfolded proteins and autophagy. In addition, GK was also able to partly repress the pro-apoptotic action of regulated IRE1-dependent decay and JNK pathway.

CONCLUSIONS AND IMPLICATIONS

In conclusion, GK acts through selective activation of the IRE1α/XBP1 pathway to limit ER stress injury. GK is revealed as a promising therapeutic agent to ameliorate ER stress for treating cardiovascular diseases.

The role of AMPK in cardiomyocyte health and survival

Cellular energy homeostasis is a fundamental process that governs the overall health of the cell and is paramount to cell survival. Central to this is the control of ATP generation and utilization, which is regulated by a complex myriad of enzymatic reactions controlling cellular metabolism. In the cardiomyocyte, ATP generated from substrate catabolism is used for numerous cellular processes including maintaining ionic homeostasis, cell repair, protein synthesis and turnover, organelle turnover, and contractile function. In many instances, cardiovascular disease is associated with impaired cardiac energetics and thus the signalling that regulates pathways involved in cardiomyocyte metabolism may be potential targets for pharmacotherapy designed to help treat cardiovascular disease. An important regulator of cardiomyocyte energy homeostasis is adenosine monophosphate-activated protein kinase (AMPK). AMPK is a serine-threonine kinase that functions primarily as a metabolic sensor to coordinate anabolic and catabolic activities in the cell via the phosphorylation of multiple proteins involved in metabolic pathways. In addition to the direct role that AMPK plays in the regulation of cardiomyocyte metabolism, AMPK can also either directly or indirectly influence other cellular processes such as regulating mitochondrial function, post-translation acetylation, autophagy, mitophagy, endoplasmic reticulum stress, and apoptosis. Thus, AMPK is implicated in the control of a wide variety of cellular processes that can influence cardiomyocyte health and survival. In this review, we will discuss the important role that AMPK plays in regulating cardiac metabolism, as well as the additional cellular processes that may contribute to cardiomyocyte function and survival in the healthy and the diseased heart. This article is part of a Special Issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck & Jan. F.C. Glatz.

UBC9-Mediated Sumoylation Favorably Impacts Cardiac Function in Compromised Hearts

RATIONALE

SUMOylation plays an important role in cardiac function and can be protective against cardiac stress. Recent studies show that SUMOylation is an integral part of the ubiquitin proteasome system, and expression of the small ubiquitin-like modifier (SUMO) E2 enzyme UBC9 improves cardiac protein quality control. However, the precise role of SUMOylation on other protein degradation pathways, particularly autophagy, remains undefined in the heart.

OBJECTIVE

To determine whether SUMOylation affects cardiac autophagy and whether this effect is protective in a mouse model of proteotoxic cardiac stress.

METHODS AND RESULTS

We modulated expression of UBC9, a SUMO E2 ligase, using gain- and loss-of-function in neonatal rat ventricular cardiomyocytes. UBC9 expression seemed to directly alter autophagic flux. To confirm this effect in vivo, we generated transgenic mice overexpressing UBC9 in cardiomyocytes. These mice have an increased level of SUMOylation at baseline and, in confirmation of the data obtained from neonatal rat ventricular cardiomyocytes, demonstrated increased autophagy, suggesting that increased UBC9-mediated SUMOylation is sufficient to upregulate cardiac autophagy. Finally, we tested the protective role of SUMOylation-mediated autophagy by expressing UBC9 in a model of cardiac proteotoxicity, induced by cardiomyocyte-specific expression of a mutant α-B-crystallin, mutant CryAB (CryAB(R120G)), which shows impaired autophagy. UBC9 overexpression reduced aggregate formation, decreased fibrosis, reduced hypertrophy, and improved cardiac function and survival.

CONCLUSIONS

The data showed that increased UBC9-mediated SUMOylation is sufficient to induce relatively high levels of autophagy and may represent a novel strategy for increasing autophagic flux and ameliorating morbidity in proteotoxic cardiac disease.

Derailed Proteostasis as a Determinant of Cardiac Aging

Age comprises the single most important risk factor for cardiac disease development. The incidence and prevalence of cardiac diseases, which represents the main cause of death worldwide, will increase even more because of the aging population. A hallmark of aging is that it is accompanied by a gradual derailment of proteostasis (eg, the homeostasis of protein synthesis, folding, assembly, trafficking, function, and degradation). Loss of proteostasis is highly relevant to cardiomyocytes, because they are postmitotic cells and therefore not constantly replenished by proliferation. The derailment of proteostasis during aging is thus an important factor that preconditions for the development of age-related cardiac diseases, such as atrial fibrillation. In turn, frailty of proteostasis in aging cardiomyocytes is exemplified by its accelerated derailment in multiple cardiac diseases. Here, we review 2 major components of the proteostasis network, the stress-responsive and protein degradation pathways, in healthy and aged cardiomyocytes. Furthermore, we discuss the relation between derailment of proteostasis and age-related cardiac diseases, including atrial fibrillation. Finally, we introduce novel therapeutic targets that might possibly attenuate cardiac aging and thus limit cardiac disease progression.

Age- and Hypertension-Associated Protein Aggregates in Mouse Heart Have Similar Proteomic Profiles

Neurodegenerative diseases are largely defined by protein aggregates in affected tissues. Aggregates contain some shared components as well as proteins thought to be specific for each disease. Aggregation has not previously been reported in the normal, aging heart or the hypertensive heart. Detergent-insoluble protein aggregates were isolated from mouse heart and characterized on 2-dimensional gels. Their levels increased markedly and significantly with aging and after sustained angiotensin II-induced hypertension. Of the aggregate components identified by high-resolution proteomics, half changed in abundance with age (392/787) or with sustained hypertension (459/824), whereas 30% (273/901) changed concordantly in both, each P<0.05. One fifth of these proteins were previously associated with age-progressive neurodegenerative or cardiovascular diseases, or both (eg, ApoE, ApoJ, ApoAIV, clusterin, complement C3, and others involved in stress-response and protein-homeostasis pathways). Because fibrosis is a characteristic of both aged and hypertensive hearts, we posited that aging of fibroblasts may contribute to the aggregates observed in cardiac tissue. Indeed, as cardiac myofibroblasts "senesced" (approached their replicative limit) in vitro, they accrued aggregates with many of the same constituent proteins observed in vivo during natural aging or sustained hypertension. In summary, we have shown for the first time that compact (detergent-insoluble) protein aggregates accumulate during natural aging, chronic hypertension, and in vitro myofibroblast senescence, sharing many common proteins. Thus, aggregates that arise from disparate causes (aging, hypertension, and replicative senescence) may have common underlying mechanisms of accrual.

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.

Antioxidant Treatment and Induction of Autophagy Cooperate to Reduce Desmin Aggregation in a Cellular Model of Desminopathy

Desminopathies, a subgroup of myofibrillar myopathies (MFMs), the progressive muscular diseases characterized by the accumulation of granulofilamentous desmin-positive aggregates, result from mutations in the desmin gene (DES), encoding a muscle-specific intermediate filament. Desminopathies often lead to severe disability and premature death from cardiac and/or respiratory failure; no specific treatment is currently available. To identify drug-targetable pathophysiological pathways, we performed pharmacological studies in C2C12 myoblastic cells expressing mutant DES. We found that inhibition of the Rac1 pathway (a G protein signaling pathway involved in diverse cellular processes), antioxidant treatment, and stimulation of macroautophagy reduced protein aggregation by up to 75% in this model. Further, a combination of two or three of these treatments was more effective than any of them alone. These results pave the way towards the development of the first treatments for desminopathies and are potentially applicable to other muscle or brain diseases associated with abnormal protein aggregation.

Impact of leucine supplementation on exercise training induced anti-cardiac remodeling effect in heart failure mice

Leucine supplementation potentiates the effects of aerobic exercise training (AET) on skeletal muscle; however, its potential effects associated with AET on cardiac muscle have not been clarified yet. We tested whether leucine supplementation would potentiate the anti-cardiac remodeling effect of AET in a genetic model of sympathetic hyperactivity-induced heart failure in mice (α2A/α2CARKO). Mice were assigned to five groups: wild type mice treated with placebo and sedentary (WT, n = 11), α2A/α2CARKO treated with placebo and sedentary (KO, n = 9), α2A/α2CARKO treated with leucine and sedentary (KOL, n = 11), α2A/α2CARKO treated with placebo and AET (KOT, n = 12) or α2A/α2CARKO treated with leucine and AET (KOLT, n = 12). AET consisted of four weeks on a treadmill with 60 min sessions (six days/week, 60% of maximal speed) and administration by gavage of leucine (1.35 g/kg/day) or placebo (distilled water). The AET significantly improved exercise capacity, fractional shortening and re-established cardiomyocytes' diameter and collagen fraction in KOT. Additionally, AET significantly prevented the proteasome hyperactivity, increased misfolded proteins and HSP27 expression. Isolated leucine supplementation displayed no effect on cardiac function and structure (KOL), however, when associated with AET (KOLT), it increased exercise tolerance to a higher degree than isolated AET (KOT) despite no additional effects on AET induced anti-cardiac remodeling. Our results provide evidence for the modest impact of leucine supplementation on cardiac structure and function in exercised heart failure mice. Leucine supplementation potentiated AET effects on exercise tolerance, which might be related to its recognized impact on skeletal muscle.

The protein quality control machinery regulates its misassembled proteasome subunits

Cellular toxicity introduced by protein misfolding threatens cell fitness and viability. Failure to eliminate these polypeptides is associated with various aggregation diseases. In eukaryotes, the ubiquitin proteasome system (UPS) plays a vital role in protein quality control (PQC), by selectively targeting misfolded proteins for degradation. While the assembly of the proteasome can be naturally impaired by many factors, the regulatory pathways that mediate the sorting and elimination of misassembled proteasomal subunits are poorly understood. Here, we reveal how the dysfunctional proteasome is controlled by the PQC machinery. We found that among the multilayered quality control mechanisms, UPS mediated degradation of its own misassembled subunits is the favored pathway. We also demonstrated that the Hsp42 chaperone mediates an alternative pathway, the accumulation of these subunits in cytoprotective compartments. Thus, we show that proteasome homeostasis is controlled through probing the level of proteasome assembly, and the interplay between UPS mediated degradation or their sorting into distinct cellular compartments.

The accumulation of misfolded proteins threatens cell fitness and viability and their aggregation is commonly associated with numerous neurodegenerative disorders. Cells therefore rely on a number of protein quality control (PQC) pathways to prevent protein aggregation. In eukaryotes, the ubiquitin proteasome system (UPS), a supramolecular machinery that mediates the proteolysis of damaged or misfolded proteins, plays a vital role in PQC by selectively targeting proteins for degradation. Although the critical role-played by the UPS in PQC, and the severe consequences of impairing this pathway are well established, little was known about the mechanisms that control dysfunctional proteasome subunits. Here, we reveal that the interplay between UPS mediated degradation of its own misassembled subunits, and sorting them into cytoprotective compartments, a process that is mediated by the Hsp42 chaperone, determines how proteasome homeostasis is controlled in yeast cells. We believe that the mechanism of proteasome regulation by the PCQ in yeast may serve as a paradigm for understanding how homeostasis of this essential complex is controlled in major chronic neurodegenerative disorders in higher eukaryotes.

Neddylation and deneddylation in cardiac biology

Neddylation is a post-translational protein modification that conjugates a ubiquitin-like protein NEDD8 to target proteins. Similar to ubiquitination, neddylation is mediated by a cascade of three NEDD8 specific enzymes, an E1 activating enzyme, an E2 conjugating enzyme and one of the several E3 ligases. Neddylation is countered by the action of deneddylases via a process termed deneddylation. By altering the substrate's conformation, stability, subcellular localization or binding affinity to DNA or proteins, neddylation regulates diverse cellular processes including the ubiquitin-proteasome system-mediated protein degradation, protein transcription, cell signaling etc. Dysregulation of neddylation has been linked to cancer, neurodegenerative disorders, and more recently, cardiac disease. Here we comprehensively overview the biochemistry, the proteome and the biological function of neddylation. We also summarize the recent progress in revealing the physiological and pathological role of neddylation and deneddylation in the heart.

Role of endoplasmic reticulum stress in atherosclerosis and diabetic macrovascular complications

Age-related changes in endoplasmic reticulum (ER) are associated with stress of this cell organelle. Unfolded protein response (UPR) is a normal physiological reaction of a cell in order to prevent accumulation of unfolded and misfolded proteins in the ER and improve the normal ER function. However, in pathologic conditions such as atherosclerosis, obesity, and diabetes, ER function becomes impaired, leading to the development of ER stress. In chronic ER stress, defective posttranslational protein folding results in deposits of aberrantly folded proteins in the ER and the induction of cell apoptosis mediated by UPR sensors C/EBPα-homologous protein (CHOP) and inositol requiring protein-1 (IRE1). Since ER stress and ER-induced cell death play a nonredundant role in the pathogenesis of atherosclerosis and diabetic macrovascular complications, pharmaceutical targeting of ER stress components and pathways may be beneficial in the treatment and prevention of cardiovascular pathology.

Regulation of cardiac proteasomes by ubiquitination, SUMOylation, and beyond

The ubiquitin-proteasome system (UPS) is the major intracellular degradation system, and its proper function is critical to the health and function of cardiac cells. Alterations in cardiac proteasomes have been linked to several pathological phenotypes, including cardiomyopathies, ischemia-reperfusion injury, heart failure, and hypertrophy. Defects in proteasome-dependent cellular protein homeostasis can be causal for the initiation and progression of certain cardiovascular diseases. Emerging evidence suggests that the UPS can specifically target proteins that govern pathological signaling pathways for degradation, thus altering downstream effectors and disease outcomes. Alterations in UPS-substrate interactions in disease occur, in part, due to direct modifications of 19S, 11S or 20S proteasome subunits. Post-translational modifications (PTMs) are one facet of this proteasomal regulation, with over 400 known phosphorylation sites, over 500 ubiquitination sites and 83 internal lysine acetylation sites, as well as multiple sites for caspase cleavage, glycosylation (such as O-GlcNAc modification), methylation, nitrosylation, oxidation, and SUMOylation. Changes in cardiac proteasome PTMs, which occur in ischemia and cardiomyopathies, are associated with changes in proteasome activity and proteasome assembly; however several features of this regulation remain to be explored. In this review, we focus on how some of the less common PTMs affect proteasome function and alter cellular protein homeostasis. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".

Proteasome activation delays aging in vitro and in vivo

Aging is a natural biological process that is characterized by a progressive accumulation of macromolecular damage. In the proteome, aging is accompanied by decreased protein homeostasis and function of the major cellular proteolytic systems, leading to the accumulation of unfolded, misfolded, or aggregated proteins. In particular, the proteasome is responsible for the removal of normal as well as damaged or misfolded proteins. Extensive work during the past several years has clearly demonstrated that proteasome activation by either genetic means or use of compounds significantly retards aging. Importantly, this represents a common feature across evolution, thereby suggesting proteasome activation to be an evolutionarily conserved mechanism of aging and longevity regulation. This review article reports on the means of function of these proteasome activators and how they regulate aging in various species.

Nrf2 enhances myocardial clearance of toxic ubiquitinated proteins

Nuclear factor erythroid-2 related factor 2 (Nrf2) is a master transcription factor that controls the basal and inducible expression of a battery of antioxidant genes and other cytoprotective phase II detoxifying enzymes. While knockout of Nrf2 exaggerates cardiac pathological remodeling and dysfunction in diverse pathological settings, pharmacological activation of Nrf2 protects against cardiomyocyte injury and cardiac dysfunction. In contrast, there is also a concern that the chronic activation of Nrf2 secondary to oxidative stress is a contributing mechanism for the reductive stress-mediated heart failure. However, a direct link between cardiac specific activation of Nrf2 and cardiac protection or dysfunction in vivo remains to be established. Therefore, we investigated the effect of cardiomyocyte-specific transgenic activation of Nrf2 (Nrf2(ctg)) on cardiac pathological remodeling and dysfunction. We found that the cardiomyocyte-specific activation of Nrf2 suppressed myocardial oxidative stress as well as cardiac apoptosis, fibrosis, hypertrophy, and dysfunction in a setting of sustained pressure overload induced by transverse aortic arch constriction (TAC) in mice. Notably, the constitutive activation of Nrf2 increased the steady level of autophagosomes while decreasing the ubiquitinated protein aggregates in the heart after TAC. Nrf2 gene gain- and loss-of-function approaches revealed that Nrf2 enhances autophagosome formation and autophagic flux in cardiomyocytes. Unexpectedly, while Nrf2 minimally regulated apoptosis, it suppressed significantly the proteotoxic necrosis in cardiomyocytes. In addition, Nrf2 attenuated the proteocytotoxicity presumably via enhancing autophagy-mediated clearance of ubiquitinated protein aggregates in cardiomyocytes. Taken together, we demonstrated for the first time that cardiac specific activation of Nrf2 suppresses cardiac maladaptive remodeling and dysfunction most likely by enhancing autophagic clearance of toxic protein aggregates in the heart.

Muscarinic 2 receptors modulate cardiac proteasome function in a protein kinase G-dependent manner

Proteasome function insufficiency and inadequate protein quality control are strongly implicated in a large subset of cardiovascular disease and may play an important role in their pathogenesis. Protein degradation by the ubiquitin proteasome system can be physiologically regulated. Cardiac muscarinic 2 (M2) receptors were pharmacologically interrogated in intact mice and cultured neonatal rat ventricular myocytes (NRVMs). Proteasome-mediated proteolysis was measured with a surrogate misfolded protein, proteasome peptidase assay, and by characterizing key proteasome subunits. Successful M2 receptor manipulation in cardiomyocytes was determined by measuring an endogenous protein substrate, and in mice, the cardiovascular physiological response. M2 receptor stimulation was associated with increased proteasome-mediated proteolysis and enhanced peptidase activities, while M2 receptor inhibition yielded opposing results. Additionally, M2 receptor manipulation did not alter abundance of the key proteasome subunits, Rpt6 and β5, but significantly shifted their isoelectric points. Inhibition of protein kinase G abrogated the stimulatory effects on proteasome-mediated proteolysis from M2 receptor activation. We conclude that M2 receptor stimulation enhances, whereas M2 receptor inhibition reduces, proteasome-mediated proteolysis likely through posttranslational modifications. Protein kinase G appears to be the mediator of the M2 receptors actions.

Mice with cardiac-restricted overexpression of Myozap are sensitized to biomechanical stress and develop a protein-aggregate-associated cardiomyopathy

The intercalated disc (ID) is a major component of the cell-cell contact structures of cardiomyocytes and has been recognized as a hot spot for cardiomyopathy. We have previously identified Myozap as a novel cardiac-enriched ID protein, which interacts with several other ID proteins and is involved in RhoA/SRF signaling in vitro. To now study its potential role in vivo we generated a mouse model with cardiac overexpression of Myozap. Transgenic (Tg) mice developed cardiomyopathy with hypertrophy and LV dilation. Consistently, these mice displayed upregulation of the hypertrophy-associated and SRF-dependent gene expression. Pressure overload (transverse aortic constriction, TAC) caused exaggerated cardiac hypertrophy, further loss of contractility and LV dilation. Similarly, a physiological stimulus (voluntary running) also led to significant LV dysfunction. On the ultrastructural level, Myozap-Tg mouse hearts exhibited massive protein aggregates composed of Myozap, desmoplakin and other ID proteins. This aggregate-associated pathology closely resembled the alterations observed in desmin-related cardiomyopathy. Interestingly, desmin was not detectable in the aggregates, yet was largely displaced from the ID. Molecular analyses revealed induction of autophagy and dysregulation of the unfolded protein response (UPR), associated with apoptosis. Taken together, cardiac overexpression of Myozap leads to cardiomyopathy, mediated, at least in part by induction of Rho-dependent SRF signaling in vivo. Surprisingly, this phenotype was also accompanied by protein aggregates in cardiomyocytes, UPR alteration, accelerated autophagy and apoptosis. Thus, this mouse model may also offer additional insight into the pathogenesis of protein-aggregate-associated cardiomyopathies and represents a new candidate gene itself.