BAG3 and Hsc70 interact with actin capping protein CapZ to maintain myofibrillar integrity under mechanical stress

Corelating the molecular structure of BAG3 to its oncogenic role

BAG3 is a multifaceted protein characterised by having WW domain, PXXP motif and BAG domain. This protein gets upregulated during malignant transformation of cells and has been associated with poorer survival of patients. Procancerous activity of BAG domain of BAG3 is well documented. BAG domain interacts with ATPase domain of Hsp-70 preventing protein delivery to proteasome. This impediment results in enhanced cell survival, proliferation, resistance to apoptosis and chemoresistance. Besides BAG domain other two domains/motifs of BAG3 are under research vigilance to explore its further oncogenic role. This review summarises the role of different structural determinants of BAG3 in elevating oncogenesis. Based on the already existing findings, more interacting partners of BAG3 are anticipated. The anticipated partners of BAG3 can shed a wealth of information into the mechanistic insights of its proproliferative role. Proper insights into the mechanistic details adopted by BAG3 to curtail/elaborate activity of anticipated interacting partners can serve as a potent target for development of therapeutic interventions.

Hypertrophic cardiomyopathy in MYBPC3 carriers in aging

Hypertrophic cardiomyopathy (HCM) is characterized by abnormal thickening of the myocardium, leading to arrhythmias, heart failure, and elevated risk of sudden cardiac death, particularly among the young. This inherited disease is predominantly caused by mutations in sarcomeric genes, among which those in the cardiac myosin binding protein-C3 (MYBPC3) gene are major contributors. HCM associated with MYBPC3 mutations usually presents in the elderly and ranges from asymptomatic to symptomatic forms, affecting numerous cardiac functions and presenting significant health risks with a spectrum of clinical manifestations. Regulation of MYBPC3 expression involves various transcriptional and translational mechanisms, yet the destiny of mutant MYBPC3 mRNA and protein in late-onset HCM remains unclear. Pathogenesis related to MYBPC3 mutations includes nonsense-mediated decay, alternative splicing, and ubiquitin-proteasome system events, leading to allelic imbalance and haploinsufficiency. Aging further exacerbates the severity of HCM in carriers of MYBPC3 mutations. Advancements in high-throughput omics techniques have identified crucial molecular events and regulatory disruptions in cardiomyocytes expressing MYBPC3 variants. This review assesses the pathogenic mechanisms that promote late-onset HCM through the lens of transcriptional, post-transcriptional, and post-translational modulation of MYBPC3, underscoring its significance in HCM across carriers. The review also evaluates the influence of aging on these processes and MYBPC3 levels during HCM pathogenesis in the elderly. While pinpointing targets for novel medical interventions to conserve cardiac function remains challenging, the emergence of personalized omics offers promising avenues for future HCM treatments, particularly for late-onset cases.

Dysregulated Autophagy and Sarcomere Dysfunction in Patients With Heart Failure With Co-Occurrence of P63A and P380S BAG3 Variants

BACKGROUND

Mutations to the co-chaperone protein BAG3 (B-cell lymphoma-2-associated athanogene-3) are a leading cause of dilated cardiomyopathy (DCM). These mutations often impact the C-terminal BAG domain (residues 420-499), which regulates heat shock protein 70-dependent protein turnover via autophagy. While mutations in other regions are less common, previous studies in patients with DCM found that co-occurrence of 2 BAG3 variants (P63A, P380S) led to worse prognosis. However, the underlying mechanism for dysfunction is not fully understood.

METHODS AND RESULTS

In this study, we used proteomics, Western blots, and myofilament functional assays on left ventricular tissue from patients with nonfailing, DCM, and DCM with BAG363/380 to determine how these mutations impact protein quality control and cardiomyocyte contractile function. We found dysregulated autophagy and increased protein ubiquitination in patients with BAG363/380 compared with nonfailing and DCM, suggesting impaired protein turnover. Expression and myofilament localization of BAG3-binding proteins were also uniquely altered in the BAG3,63/380 including abolished localization of the small heat shock protein CRYAB (alpha-crystallin B chain) to the sarcomere. To determine whether these variants impacted sarcomere function, we used cardiomyocyte force-calcium assays and found reduced maximal calcium-activated force in DCM and BAG363/380. Interestingly, myofilament calcium sensitivity was increased in DCM but not with BAG363/380, which was not explained by differences in troponin I phosphorylation.

CONCLUSIONS

Together, our data support that the disease-enhancing mechanism for BAG3 variants outside of the BAG domain is through disrupted protein turnover leading to compromised sarcomere function. These findings suggest a shared mechanism of disease among pathogenic BAG3 variants, regardless of location.

Role of Actin-Binding Proteins in Skeletal Myogenesis

Maintenance of skeletal muscle quantity and quality is essential to ensure various vital functions of the body. Muscle homeostasis is regulated by multiple cytoskeletal proteins and myogenic transcriptional programs responding to endogenous and exogenous signals influencing cell structure and function. Since actin is an essential component in cytoskeleton dynamics, actin-binding proteins (ABPs) have been recognized as crucial players in skeletal muscle health and diseases. Hence, dysregulation of ABPs leads to muscle atrophy characterized by loss of mass, strength, quality, and capacity for regeneration. This comprehensive review summarizes the recent studies that have unveiled the role of ABPs in actin cytoskeletal dynamics, with a particular focus on skeletal myogenesis and diseases. This provides insight into the molecular mechanisms that regulate skeletal myogenesis via ABPs as well as research avenues to identify potential therapeutic targets. Moreover, this review explores the implications of non-coding RNAs (ncRNAs) targeting ABPs in skeletal myogenesis and disorders based on recent achievements in ncRNA research. The studies presented here will enhance our understanding of the functional significance of ABPs and mechanotransduction-derived myogenic regulatory mechanisms. Furthermore, revealing how ncRNAs regulate ABPs will allow diverse therapeutic approaches for skeletal muscle disorders to be developed.

Skeletal Muscle-Specific Bis Depletion Leads to Muscle Dysfunction and Early Death Accompanied by Impairment in Protein Quality Control

Bcl-2-interacting cell death suppressor (BIS), also called BAG3, plays a role in physiological functions such as anti-apoptosis, cell proliferation, autophagy, and senescence. Whole-body Bis-knockout (KO) mice exhibit early lethality accompanied by abnormalities in cardiac and skeletal muscles, suggesting the critical role of BIS in these muscles. In this study, we generated skeletal muscle-specific Bis-knockout (Bis-SMKO) mice for the first time. Bis-SMKO mice exhibit growth retardation, kyphosis, a lack of peripheral fat, and respiratory failure, ultimately leading to early death. Regenerating fibers and increased intensity in cleaved PARP1 immunostaining were observed in the diaphragm of Bis-SMKO mice, indicating considerable muscle degeneration. Through electron microscopy analysis, we observed myofibrillar disruption, degenerated mitochondria, and autophagic vacuoles in the Bis-SMKO diaphragm. Specifically, autophagy was impaired, and heat shock proteins (HSPs), such as HSPB5 and HSP70, and z-disk proteins, including filamin C and desmin, accumulated in Bis-SMKO skeletal muscles. We also found metabolic impairments, including decreased ATP levels and lactate dehydrogenase (LDH) and creatine kinase (CK) activities in the diaphragm of Bis-SMKO mice. Our findings highlight that BIS is critical for protein homeostasis and energy metabolism in skeletal muscles, suggesting that Bis-SMKO mice could be used as a therapeutic strategy for myopathies and to elucidate the molecular function of BIS in skeletal muscle physiology.

Z-Disk-Associated Plectin (Isoform 1d): Spatial Arrangement, Interaction Partners, and Role in Filamin C Homeostasis

Plectin, a highly versatile cytolinker protein, is crucial for myofiber integrity and function. Accordingly, mutations in the human gene (PLEC) cause several rare diseases, denoted as plectinopathies, with most of them associated with progressive muscle weakness. Of several plectin isoforms expressed in skeletal muscle and the heart, P1d is the only isoform expressed exclusively in these tissues. Using high-resolution stimulated emission depletion (STED) microscopy, here we show that plectin is located within the gaps between individual α-actinin-positive Z-disks, recruiting and bridging them to desmin intermediate filaments (IFs). Loss of plectin in myofibril bundles led to a complete loss of desmin IFs. Loss of Z-disk-associated plectin isoform P1d led to disorganization of muscle fibers and slower relaxation of myofibrils upon mechanical strain, in line with an observed inhomogeneity of muscle ultrastructure. In addition to binding to α-actinin and thereby providing structural support, P1d forms a scaffolding platform for the chaperone-assisted selective autophagy machinery (CASA) by directly interacting with HSC70 and synpo2. In isoform-specific knockout (P1d-KO) mouse muscle and mechanically stretched plectin-deficient myoblasts, we found high levels of undigested filamin C, a bona fide substrate of CASA. Similarly, subjecting P1d-KO mice to forced swim tests led to accumulation of filamin C aggregates in myofibers, highlighting a specific role of P1d in tension-induced proteolysis activated upon high loads of physical exercise and muscle contraction.

The chaperone-assisted selective autophagy complex dynamics and dysfunctions

Each protein must be synthesized with the correct amino acid sequence, folded into its native structure, and transported to a relevant subcellular location and protein complex. If any of these steps fail, the cell has the capacity to break down aberrant proteins to maintain protein homeostasis (also called proteostasis). All cells possess a set of well-characterized protein quality control systems to minimize protein misfolding and the damage it might cause. Autophagy, a conserved pathway for the degradation of long-lived proteins, aggregates, and damaged organelles, was initially characterized as a bulk degradation pathway. However, it is now clear that autophagy also contributes to intracellular homeostasis by selectively degrading cargo material. One of the pathways involved in the selective removal of damaged and misfolded proteins is chaperone-assisted selective autophagy (CASA). The CASA complex is composed of three main proteins (HSPA, HSPB8 and BAG3), essential to maintain protein homeostasis in muscle and neuronal cells. A failure in the CASA complex, caused by mutations in the respective coding genes, can lead to (cardio)myopathies and neurodegenerative diseases. Here, we summarize our current understanding of the CASA complex and its dynamics. We also briefly discuss how CASA complex proteins are involved in disease and may represent an interesting therapeutic target.Abbreviation ALP: autophagy lysosomal pathway; ALS: amyotrophic lateral sclerosis; AMOTL1: angiomotin like 1; ARP2/3: actin related protein 2/3; BAG: BAG cochaperone; BAG3: BAG cochaperone 3; CASA: chaperone-assisted selective autophagy; CMA: chaperone-mediated autophagy; DNAJ/HSP40: DnaJ heat shock protein family (Hsp40); DRiPs: defective ribosomal products; EIF2A/eIF2α: eukaryotic translation initiation factor 2A; EIF2AK1/HRI: eukaryotic translation initiation factor 2 alpha kinase 1; GABARAP: GABA type A receptor-associated protein; HDAC6: histone deacetylase 6; HSP: heat shock protein; HSPA/HSP70: heat shock protein family A (Hsp70); HSP90: heat shock protein 90; HSPB8: heat shock protein family B (small) member 8; IPV: isoleucine-proline-valine; ISR: integrated stress response; KEAP1: kelch like ECH associated protein 1; LAMP2A: lysosomal associated membrane protein 2A; LATS1: large tumor suppressor kinase 1; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOC: microtubule organizing center; MTOR: mechanistic target of rapamycin kinase; NFKB/NF-κB: nuclear factor kappa B; NFE2L2: NFE2 like bZIP transcription factor 2; PLCG/PLCγ: phospholipase C gamma; polyQ: polyglutamine; PQC: protein quality control; PxxP: proline-rich; RAN translation: repeat-associated non-AUG translation; SG: stress granule; SOD1: superoxide dismutase 1; SQSTM1/p62: sequestosome 1; STUB1/CHIP: STIP1 homology and U-box containing protein 1; STK: serine/threonine kinase; SYNPO: synaptopodin; TBP: TATA-box binding protein; TARDBP/TDP-43: TAR DNA binding protein; TFEB: transcription factor EB; TPR: tetratricopeptide repeats; TSC1: TSC complex subunit 1; UBA: ubiquitin associated; UPS: ubiquitin-proteasome system; WW: tryptophan-tryptophan; WWTR1: WW domain containing transcription regulator 1; YAP1: Yes1 associated transcriptional regulator.

Takotsubo Syndrome: The Secret Crosstalk between Heart and Brain

An acute, transient episode of left ventricular dysfunction characterizes Takotsubo syndrome. It represents about 2% of all cases of acute coronary syndrome (ACS), and occurs predominantly in postmenopausal women, generally following a significant physical or emotional stressor. It can be diagnosed based on clinical symptoms and the absence of coronary artery disease on angiography. Ventriculography remains the gold standard for the diagnosis. Despite its transitory characteristic Takotsubo syndrome should not be considered a benign condition since complications occur in almost half of the patients, and the mortality rate reaches 4-5%. Lately, it has been revealed that Takotsubo syndrome can also lead to permanent myocardial damage due to the massive release of catecholamines that leads to myocardial dysfunction. Different mechanisms have been advanced to explain this fascinating syndrome, such as plaque rupture and thrombosis, coronary spasm, microcirculatory dysfunction, catecholamine toxicity, and activation of myocardial survival pathways. Here are still several issues with Takotsubo syndrome that need to be investigated: the complex relationship between the heart and the brain, the risk of permanent myocardial damage, and the impairment of cardiomyocyte. Our review aims to elucidate the pathophysiology and the mechanisms underlying this complex disease to manage the diagnostic and therapeutic algorithms to create a functional synergy between physicians and patients.

Genetics of BAG3: A Paradigm for Developing Precision Therapies for Dilated Cardiomyopathies

Nonischemic dilated cardiomyopathy is a common form of heart muscle disease in which genetic factors play a critical etiological role. In this regard, both rare disease-causing mutations and common disease-susceptible variants, in the Bcl-2-associated athanogene 3 (BAG3) gene have been reported, highlighting the critical role of BAG3 in cardiomyocytes and in the development of dilated cardiomyopathy. The phenotypic effects of the BAG3 mutations help investigators understand the structure and function of the BAG3 gene. Indeed, we report herein that all of the known pathogenic/likely pathogenic variants affect at least 1 of 3 protein functional domains, ie, the WW domain, the second IPV (Ile-Pro-Val) domain, or the BAG domain, whereas none of the missense nontruncating pathogenic/likely pathogenic variants affect the proline-rich repeat (PXXP) domain. A common variant, p.Cys151Arg, associated with reduced susceptibility to dilated cardiomyopathy demonstrated a significant difference in allele frequencies among diverse human populations, suggesting evolutionary selective pressure. As BAG3-related therapies for heart failure move from the laboratory to the clinic, the ability to provide precision medicine will depend in large part on having a thorough understanding of the potential effects of both common and uncommon genetic variants on these target proteins. The current review article provides a roadmap that investigators can utilize to determine the potential interactions between a patient's genotype, their phenotype, and their response to therapeutic interventions with both gene delivery and small molecules.

Role of HSP70 chaperone in protein aggregate phenomenon of GNE mutant cells: Therapeutic lead for GNE Myopathy

Limited treatment options and research in understanding the pathomechanisms of rare diseases has raised concerns about their therapeutic development. One such poorly understood ultra-rare neuromuscular disorder is GNE Myopathy (GNEM) which is caused due to mutation in key sialic acid biosynthetic enzyme, GNE. Treatment with sialic acid or its derivatives/precursors slows the disease progression, but curative strategies need to be explored further. Pathologically, muscle biopsy samples of GNEM patients reveal rimmed vacuole formation due to aggregation of β-amyloid, Tau, presenilin proteins with unknown mechanism. The present study aims to understand the mechanism of protein aggregate formation in GNE mutant cells to decipher role of chaperones in disease phenotype. The pathologically relevant GNE mutations expressed as recombinant proteins in HEK cells was used as a model system for GNEM to estimate extent of protein aggregation. We identified HSP70, a chaperone, as binding partner of GNE. Downregulation of HSP70 with altered BAG3, JNK, BAX expression levels was observed in GNE mutant cells. The cell apoptosis was observed in GNE mutation specific manner. An activator of HSP70 chaperone, BGP-15, rescued the phenotypic defects due to GNE mutation, thereby, reducing protein aggregation significantly. The results were further validated in rat skeletal muscle cell lines carrying single Gne allele. Our study suggests that HSP70 activators can be a promising therapeutic target in the treatment of ultra-rare GNE Myopathy disease.

Mob4-dependent STRIPAK involves the chaperonin TRiC to coordinate myofibril and microtubule network growth

Myofibrils of the skeletal muscle are comprised of sarcomeres that generate force by contraction when myosin-rich thick filaments slide past actin-based thin filaments. Surprisingly little is known about the molecular processes that guide sarcomere assembly in vivo, despite deficits within this process being a major cause of human disease. To overcome this knowledge gap, we undertook a forward genetic screen coupled with reverse genetics to identify genes required for vertebrate sarcomere assembly. In this screen, we identified a zebrafish mutant with a nonsense mutation in mob4. In Drosophila, mob4 has been reported to play a role in spindle focusing as well as neurite branching and in planarians mob4 was implemented in body size regulation. In contrast, zebrafish mob4geh mutants are characterised by an impaired actin biogenesis resulting in sarcomere defects. Whereas loss of mob4 leads to a reduction in the amount of myofibril, transgenic expression of mob4 triggers an increase. Further genetic analysis revealed the interaction of Mob4 with the actin-folding chaperonin TRiC, suggesting that Mob4 impacts on TRiC to control actin biogenesis and thus myofibril growth. Additionally, mob4geh features a defective microtubule network, which is in-line with tubulin being the second main folding substrate of TRiC. We also detected similar characteristics for strn3-deficient mutants, which confirmed Mob4 as a core component of STRIPAK and surprisingly implicates a role of the STRIPAK complex in sarcomerogenesis.

Inhibitors of the Ubiquitin Proteasome System block myofibril assembly in cardiomyocytes derived from chick embryos and human pluripotent stem cells

Details of sarcomeric protein assembly during de novo myofibril formation closely resemble myofibrillogenesis in skeletal and cardiac myocytes in birds, rodents and zebrafish. The arrangement of proteins during myofibrillogenesis follows a three-step process: beginning with premyofibrils, followed by nascent myofibrils, and concluding with mature myofibrils (reviewed in Sanger et al., 2017). Our aim is to determine if the same pathway is followed in human cardiomyocytes derived from human inducible pluripotent stem cells. We found that the human cardiomyocytes developed patterns of protein organization identical to the three-step series seen in the model organisms cited above. Further experiments showed that myofibril assembly can be blocked at the nascent myofibril by five different inhibitors of the Ubiquitin Proteasome System (UPS) stage in both avian and human cardiomyocytes. With the exception of Carfilzomib, removal of the UPS inhibitors allows nascent myofibrils to proceed to mature myofibrils. Some proteasomal inhibitors, such as Bortezomib and Carfilzomib, used to treat multiple myeloma patients, have off-target effects of damage to hearts in three to six percent of these patients. These cardiovascular adverse events may result from prevention of mature myofibril formation in the cardiomyocytes. In summary, our results support a common three-step model for the formation of myofibrils ranging from avian to human cardiomyocytes. The Ubiquitin Proteasome System is required for progression from nascent myofibrils to mature myofibrils. Our experiments suggest a possible explanation for the cardiac and skeletal muscle off-target effects reported in multiple myeloma patients treated with proteasome inhibitors. This article is protected by copyright. All rights reserved.

An unexpected role for BAG3 in regulating PARP1 ubiquitination in oxidative stress-related endothelial damage

Oxidative stress-associated endothelial damage is the initiation factor of cardiovascular disease, and protein posttranslational modifications play critical roles in this process. Bcl-2-associated athanogene 3 (BAG3) is a molecular chaperone regulator of the BAG family, which interacts with various proteins and influences cell survival by activating multiple pathways. BAG3 undergoes posttranslational modifications; however, research evaluating BAG3 acetylation and its regulatory mechanism is lacking. In addition, the interacting protein and regulatory mechanism of BAG3 in oxidative stress-associated endothelial damage remain unclear. Here, key molecular interactions and protein modifications of BAG3 were identified in oxidative stress-associated endothelial damage. Endothelial-specific BAG3 knockout in the mouse model starkly enhances oxidative stress-associated endothelial damage and vascular remodeling, while BAG3 overexpression in mice significantly relieves this process. Mechanistically, poly(ADP-ribose) polymerase 1 (PARP1), causing oxidative stress, was identified as a novel physiological substrate of BAG3. Indeed, BAG3 binds to PARP1's BRCT domain to promote its ubiquitination (K249 residue) by enhancing the E3 ubiquitin ligase WWP2, which leads to proteasome-induced PARP1 degradation. Furthermore, we surprisingly found that BAG3 represents a new substrate of the acetyltransferase CREB-binding protein (CBP) and the deacetylase Sirtuin 2 (SIRT2) under physiological conditions. CBP/SIRT2 interacted with BAG3 and acetylated/deacetylated BAG3's K431 residue. Finally, deacetylated BAG3 promoted the ubiquitination of PARP1. This work reveals a novel regulatory system, with deacetylation-dependent regulation of BAG3 promoting PARP1 ubiquitination and degradation via enhancing WWP2, which is one possible mechanism to decrease vulnerability of oxidative stress in endothelial cells.

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Endothelial-specific BAG3 knockout in mice aggravates oxidative stress endothelial injury.

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BAG3 transgenic mice relieves oxidative stress endothelial injury.

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BAG3 promotes ubiquitination at the K249 residue of PARP1 via mobilization of the E3 ubiquitin ligase WWP2.

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CBP/SIRT2 interacted with BAG3 and acetylated/deacetylated BAG3's K431 residue.

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Deacetylated BAG3 promoted the ubiquitination of PARP1.

Genetic dissection of novel myopathy models reveals a role of CapZα and Leiomodin 3 during myofibril elongation

Myofibrils within skeletal muscle are composed of sarcomeres that generate force by contraction when their myosin-rich thick filaments slide past actin-based thin filaments. Although mutations in components of the sarcomere are a major cause of human disease, the highly complex process of sarcomere assembly is not fully understood. Current models of thin filament assembly highlight a central role for filament capping proteins, which can be divided into three protein families, each ascribed with separate roles in thin filament assembly. CapZ proteins have been shown to bind the Z-disc protein α-actinin to form an anchoring complex for thin filaments and actin polymerisation. Subsequent thin filaments extension dynamics are thought to be facilitated by Leiomodins (Lmods) and thin filament assembly is concluded by Tropomodulins (Tmods) that specifically cap the pointed end of thin filaments. To study thin filament assembly in vivo, single and compound loss-of-function zebrafish mutants within distinct classes of capping proteins were analysed. The generated lmod3- and capza1b-deficient zebrafish exhibited aspects of the pathology caused by variations in their human orthologs. Although loss of the analysed main capping proteins of the skeletal muscle, capza1b, capza1a, lmod3 and tmod4, resulted in sarcomere defects, residual organised sarcomeres were formed within the assessed mutants, indicating that these proteins are not essential for the initial myofibril assembly. Furthermore, detected similarity and location of myofibril defects, apparent at the peripheral ends of myofibres of both Lmod3- and CapZα-deficient mutants, suggest a function in longitudinal myofibril growth for both proteins, which is molecularly distinct to the function of Tmod4.

The force-generating contractile apparatus is a highly organised structure mainly composed of thick and thin filaments of uniform length. Three families of capping proteins are described to play a role in the regulation of thin filament length. Current models suggest that thin filament assembly is initiated by CapZ, extended by Leiomodins (Lmods) and concluded by Tropomodulins (Tmods). To better understand the role of these capping proteins, we analysed single and double loss-of-function zebrafish mutants for these capping proteins. We find that lmod3- and capza1b-deficient zebrafish model aspects of the human disorders caused by variations in their orthologs. Surprisingly, although pivotal for sarcomere formation, our results reveal that none of the analysed capping proteins, capza1b, capza1a, lmod3 and tmod4, are absolutely required for thin filament assembly, as suggested by current models. Our study further indicates that the roles of CapZ and Lmod3 are distinct from Tmod4. Both Lmod3- as well as CapZα-deficient mutants feature specific defects at the peripheral ends of muscle cells. We conclude that, in addition to their non-essential role during thin filament assembly, both Lmod3- and CapZα proteins may function in the longitudinal growth of the contractile apparatus.

Understanding the molecular basis of cardiomyopathy

Inherited cardiomyopathies are a major cause of mortality and morbidity worldwide and can be caused by mutations in a wide range of proteins located in different cellular compartments. The present review is based on Dr. Ju Chen's 2021 Robert M. Berne Distinguished Lectureship of the American Physiological Society Cardiovascular Section, in which he provided an overview of the current knowledge on the cardiomyopathy-associated proteins that have been studied in his laboratory. The review provides a general summary of the proteins in different compartments of cardiomyocytes associated with cardiomyopathies, with specific focus on the proteins that have been studied in Dr. Chen's laboratory.

Loss-of-function mutations in the co-chaperone protein BAG5 cause dilated cardiomyopathy requiring heart transplantation

Dilated cardiomyopathy (DCM) is a major cause of heart failure, characterized by ventricular dilatation and systolic dysfunction. Familial DCM is reportedly caused by mutations in more than 50 genes, requiring precise disease stratification based on genetic information. However, the underlying genetic causes of 60 to 80% of familial DCM cases remain unknown. Here, we identified that homozygous truncating mutations in the gene encoding Bcl-2-associated athanogene (BAG) co-chaperone 5 (BAG5) caused inherited DCM in five patients among four unrelated families with complete penetrance. BAG5 acts as a nucleotide exchange factor for heat shock cognate 71 kDa protein (HSC70), promoting adenosine diphosphate release and activating HSC70-mediated protein folding. Bag5 mutant knock-in mice exhibited ventricular dilatation, arrhythmogenicity, and poor prognosis under catecholamine stimulation, recapitulating the human DCM phenotype, and administration of an adeno-associated virus 9 vector carrying the wild-type BAG5 gene could fully ameliorate these DCM phenotypes. Immunocytochemical analysis revealed that BAG5 localized to junctional membrane complexes (JMCs), critical microdomains for calcium handling. Bag5-mutant mouse cardiomyocytes exhibited decreased abundance of functional JMC proteins under catecholamine stimulation, disrupted JMC structure, and calcium handling abnormalities. We also identified heterozygous truncating mutations in three patients with tachycardia-induced cardiomyopathy, a reversible DCM subtype associated with abnormal calcium homeostasis. Our study suggests that loss-of-function mutations in BAG5 can cause DCM, that BAG5 may be a target for genetic testing in cases of DCM, and that gene therapy may potentially be a treatment for this disease.

Global identification of phospho-dependent SCF substrates reveals a FBXO22 phosphodegron and an ERK-FBXO22-BAG3 axis in tumorigenesis

SKP1-CUL1-F-box (SCF) ubiquitin ligases play fundamental roles in cellular functions. Typically, substrate phosphorylation is required for SCF recognition and subsequent degradation. However, phospho-dependent substrates remain largely unidentified. Here, using quantitative phoshoproteome approach, we performed a system-wide investigation of phospho-dependent SCF substrates. This strategy identified diverse phospho-dependent candidates. Biochemical verification revealed a mechanism by which SCF^FBXO22 recognizes the motif XXPpSPXPXX as a conserved phosphodegron to target substrates for destruction. We further demonstrated BAG3, a HSP70 co-chaperone, is a bona fide substrate of SCF^FBXO22. FBXO22 mediates BAG3 ubiquitination and degradation that requires ERK-dependent BAG3 phosphorylation at S377. FBXO22 depletion or expression of a stable BAG3 S377A mutant promotes tumor growth via defects in apoptosis and cell cycle progression in vitro and in vivo. In conclusion, our study identified broad phosphorylation-dependent SCF substrates and demonstrated a phosphodegron recognized by FBXO22 and a novel ERK-FBXO22-BAG3 axis involved in tumorigenesis.

Pharmacological inhibition of BAG3-HSP70 with the proposed cancer therapeutic JG-98 is toxic for cardiomyocytes

The co-chaperone Bcl2-associated athanogene-3 (BAG3) maintains cellular protein quality control through the regulation of heat shock protein 70 (HSP70). Cancer cells manipulate BAG3-HSP70-regulated pathways for tumor initiation and proliferation, which has led to the development of promising small molecule therapies, such as JG-98, which inhibit the BAG3-HSP70 interaction and mitigate tumor growth. However, it is not known how these broad therapies impact cardiomyocytes, where the BAG3-HSP70 complex is a key regulator of protein turnover and contractility. Here, we show that JG-98 exposure is toxic in neonatal rat ventricular myocytes (NRVMs). Using immunofluorescence microscopy to assess cell death, we found that apoptosis increased in NRVMs treated with JG-98 doses as low as 10 nM. JG-98 treatment also reduced autophagy flux and altered expression of BAG3 and several binding partners involved in BAG3-dependent autophagy, including SYNPO2 and HSPB8. We next assessed protein half-life with disruption of the BAG3-HSP70 complex by treating with JG-98 in the presence of cycloheximide and found BAG3, HSPB5, and HSPB8 half-lives were reduced, indicating that complex formation with HSP70 is important for their stability. Next, we assessed sarcomere structure using super-resolution microscopy and found that disrupting the interaction with HSP70 leads to sarcomere structural disintegration. To determine whether the effects of JG-98 could be mitigated by pharmacological autophagy induction, we cotreated NRVMs with rapamycin, which partially reduced the extent of apoptosis and sarcomere disarray. Finally, we investigated whether the effects of JG-98 extended to skeletal myocytes using C2C12 myotubes and found again increased apoptosis and reduced autophagic flux. Together, our data suggest that nonspecific targeting of the BAG3-HSP70 complex to treat cancer may be detrimental for cardiac and skeletal myocytes.

The role of BAG3 in health and disease: A "Magic BAG of Tricks"

The multi-domain structure of Bcl-2-associated athanogene 3 (BAG3) facilitates its interaction with many different proteins that participate in regulating a variety of biological pathways. After revisiting the BAG3 literature published over the past ten years with Citespace software, we classified the BAG3 research into several clusters, including cancer, cardiomyopathy, neurodegeneration, and viral propagation. We then highlighted recent key findings in each cluster. To gain greater insight into the roles of BAG3, we analyzed five different published mass spectrometry data sets of proteins that co-immunoprecipitate with BAG3. These data gave us insight into universal, as well as cell-type-specific BAG3 interactors in cancer cells, cardiomyocytes, and neurons. Finally, we mapped variable BAG3 SNPs and also mutation data from previous publications to further explore the link between the domains and function of BAG3. We believe this review will provide a better understanding of BAG3 and direct future studies towards understanding BAG3 function in physiological and pathological conditions.

Detection of sieric BAG3 in patients affected by cardiovascular diseases: State of art and perspectives

BAG3 is highly expressed in the heart and its functions are essential in maintaining cardiac muscle cells homeostasis. In the past, BAG3 was detected in serum from advanced heart failure patients and its higher levels were correlated to an increased death risk. Moreover, it has also been reported that BAG3 levels in serum are increased in patients with hypertension, a known cardiovascular risk marker. Evidence from different laboratories suggested the possibility to use BAG3-based strategies to improve the clinical outcome of cardiovascular disease patients. This review aims to highlight the biological roles of intracellular or secreted BAG3 in myocardiocytes and propose additional new data on the levels of sieric BAG3 in patients with acute myocardial infarction (AMI), never assessed before. We evaluated BAG3 serum levels in relation to cardiovascular risk parameters in 64 AMI patients aged ≥18 years of either sex. We observed significant (p < .01) correlations of BAG3 positivity with dyslipidemic status and diabetic disease. We did not observe any significant correlations of BAG3 levels with smoking habit, hypertension or familiarity for AMI, although BAG3-positive seemed to be more numerous than BAG3-negative patients among hypertensives and among patients with familiarity for AMI. Furthermore, a significant (p < .001) correlation of BAG3 positivity with diuretics assumption was also noted. In conclusion, 32.8% of the patients were BAG3-positive and were characterized by some particular features as comorbidity presence or concomitant therapies. The significance of these observations needs to be verified by more extensive studies and could help in the validation of the use of BAG3 as a biomarker in heart attack risk stratification.

Therapeutic targeting of BAG3: considering its complexity in cancer and heart disease

Bcl2-associated athanogene-3 (BAG3) is expressed ubiquitously in humans, but its levels are highest in the heart, the skeletal muscle, and the central nervous system; it is also elevated in many cancers. BAG3's diverse functions are supported by its multiple protein-protein binding domains, which couple with small and large heat shock proteins, members of the Bcl2 family, other antiapoptotic proteins, and various sarcomere proteins. In the heart, BAG3 inhibits apoptosis, promotes autophagy, couples the β-adrenergic receptor with the L-type Ca2+ channel, and maintains the structure of the sarcomere. In cancer cells, BAG3 binds to and supports an identical array of prosurvival proteins, and it may represent a therapeutic target. However, the development of strategies to block BAG3 function in cancer cells may be challenging, as they are likely to interfere with the essential roles of BAG3 in the heart. In this Review, we present the current knowledge regarding the biology of this complex protein in the heart and in cancer and suggest several therapeutic options.

Identification and Quantification of Glutathionylated Cysteines under Ischemic Stress

Ischemia reperfusion injury contributes to adverse cardiovascular diseases in part by producing a burst of reactive oxygen species that induce oxidations of many muscular proteins. Glutathionylation is one of the major protein cysteine oxidations that often serve as molecular mechanisms behind the pathophysiology associated with ischemic stress. Despite the biological significance of glutathionylation in ischemia reperfusion, identification of specific glutathionylated cysteines under ischemic stress has been limited. In this report, we have analyzed glutathionylation under oxygen-glucose deprivation (OGD) or repletion of nutrients after OGD (OGD/R) by using a clickable glutathione approach that specifically detects glutathionylated proteins. Our data find that palmitate availability induces a global level of glutathionylation and decreases cell viability during OGD/R. We have then applied a clickable glutathione-based proteomic quantification strategy, which enabled the identification and quantification of 249 glutathionylated cysteines in response to palmitate during OGD/R in the HL-1 cardiomyocyte cell line. The subsequent bioinformatic analysis found 18 glutathionylated cysteines whose genetic variants are associated with muscular disorders. Overall, our data report glutathionylated cysteines under ischemic stress that may contribute to adverse outcomes or muscular disorders.

Different roles of BAG3 in cardiac physiological hypertrophy and pathological remodeling

Heart failure is one of the leading causes of death worldwide. A stimulated heart undergoes either adaptive physiological hypertrophy, which can maintain a normal heart function, or maladaptive pathological remodeling, which can deteriorate heart function. These 2 kinds of remodeling often co-occur at the early stages of many heart diseases and have important effects on cardiac function. The Bcl2-associated athanogene 3 (BAG3) protein is highly expressed in the heart and has many functions. However, it is unknown how BAG3 is regulated and what its function is during physiological hypertrophy and pathological remodeling. We generated tamoxifen-induced, heart-specific heterozygous and homozygous BAG3 knockout mouse models (BAG3 protein level decreased by approximately 40% and 80% in the hearts after tamoxifen administration). BAG3 knockout models were subjected to swimming training or phenylephrine (PE) infusion to induce cardiac physiological hypertrophy and pathological remodeling. Neonatal rat ventricular cardiomyocytes (NRVCs) were used to study BAG3 functions and mechanisms in vitro. We found that BAG3 was upregulated in physiological hypertrophy and in pathological remodeling both in vivo and in vitro. Heterozygous or homozygous knockout BAG3 in mouse hearts and knockdown of BAG3 in the NRVCs blunted physiological hypertrophy and aggravated pathological remodeling, while overexpression of BAG3 promoted physiological hypertrophy and inhibited pathological remodeling in NRVCs. Mechanistically, BAG3 overexpression in NRVCs promoted physiological hypertrophy by activating the protein kinase B (AKT)/mammalian (or mechanistic) target of rapamycin (mTOR) pathway. BAG3 knockdown in NRVCs aggravated pathological remodeling through activation of the calcineurin/nuclear factor of activated T cells 2 (NFATc2) pathway. Because BAG3 has a dual role in cardiac remodeling, heart-specific regulation of BAG3 may be an effective therapeutic strategy to protect against deterioration of heart function and heart failure caused by many heart diseases.

Cardiomyocyte contractile impairment in heart failure results from reduced BAG3-mediated sarcomeric protein turnover

The association between reduced myofilament force-generating capacity (F_max) and heart failure (HF) is clear, however the underlying molecular mechanisms are poorly understood. Here, we show impaired F_max arises from reduced BAG3-mediated sarcomere turnover. Myofilament BAG3 expression decreases in human HF and positively correlates with F_max. We confirm this relationship using BAG3 haploinsufficient mice, which display reduced F_max and increased myofilament ubiquitination, suggesting impaired protein turnover. We show cardiac BAG3 operates via chaperone-assisted selective autophagy (CASA), conserved from skeletal muscle, and confirm sarcomeric CASA complex localization is BAG3/proteotoxic stress-dependent. Using mass spectrometry, we characterize the myofilament CASA interactome in the human heart and identify eight clients of BAG3-mediated turnover. To determine if increasing BAG3 expression in HF can restore sarcomere proteostasis/F_max, HF mice were treated with rAAV9-BAG3. Gene therapy fully rescued F_max and CASA protein turnover after four weeks. Our findings indicate BAG3-mediated sarcomere turnover is fundamental for myofilament functional maintenance.

Decreased expression of BAG3 in the heart is associated with contractile dysfunction and heart failure. Here the authors show that this is due to decreased BAG3-dependent sarcomere protein turnover, which impairs mechanical function, and that sarcomere force-generating capacity is restored with BAG3 gene therapy.

BAG3 Proteomic Signature under Proteostasis Stress

The multifunctional HSP70 co-chaperone BAG3 (BCL-2-associated athanogene 3) represents a key player in the quality control of the cellular proteostasis network. In response to stress, BAG3 specifically targets aggregation-prone proteins to the perinuclear aggresome and promotes their degradation via BAG3-mediated selective macroautophagy. To adapt cellular homeostasis to stress, BAG3 modulates and functions in various cellular processes and signaling pathways. Noteworthy, dysfunction and deregulation of BAG3 and its pathway are pathophysiologically linked to myopathies, cancer, and neurodegenerative disorders. Here, we report a BAG3 proteomic signature under proteostasis stress. To elucidate the dynamic and multifunctional action of BAG3 in response to stress, we established BAG3 interactomes under basal and proteostasis stress conditions by employing affinity purification combined with quantitative mass spectrometry. In addition to the identification of novel potential BAG3 interactors, we defined proteins whose interaction with BAG3 was altered upon stress. By functional annotation and protein-protein interaction enrichment analysis of the identified potential BAG3 interactors, we confirmed the multifunctionality of BAG3 and highlighted its crucial role in diverse cellular signaling pathways and processes, ensuring cellular proteostasis and cell viability. These include protein folding and degradation, gene expression, cytoskeleton dynamics (including cell cycle and transport), as well as granulostasis, in particular.

Multiple Progressive Thermopreconditioning Improves Cardiac Ischemia/Reperfusion-induced Left Ventricular Contractile Dysfunction and Structural Abnormality in Rat

BACKGROUND

Triple progressive thermopreconditioning (3PTP) may induce high Hsp-70 expression to maintain cardiac function. We suggest that 3PTP may reduce myocardial ischemia/reperfusion (I/R) injury during organ transplantation through Bag3/Hsp-70 mediated defense mechanisms.

METHODS

Male Wistar rats were divided into sham control group and 72 h after 3PTP in a 42°C water bath (3PTP) group. Rats underwent 60 min of ischemia by occlusion of the left anterior descending coronary artery followed by 240 min reperfusion. Hemodynamic parameters, including the electrocardiogram, microcirculation, heart rate, left ventricular end-diastolic pressure, maximal rate of rise (+dp/dt), and fall (-dp/dt) in the left ventricular pressure for index of contraction and relaxation were determined. Myocardial infarct size was evaluated by the Evans blue-2,3,5-triphenyltetrazolium chloride method. 3PTP-induced protective mechanisms were determined by Western blot and immunohistochemistry.

RESULTS

Cardiac I/R depressed cardiac microcirculation, induced S-T segment elevation, and R-R and P-R interval elongation increased infarct size associated with erythrocyte extravasation, leukocytes and macrophage/monocyte infiltration, granulocyte colony-stimulating factor, poly(ADP-ribose) polymerase 1 stain, and transferase-mediated dUTP-biotin nick end labeling positive cells. However, 3PTP evoked significant cardioprotection against I/R injury, characterized by the increased +dp/dt value and the decreased elevated left ventricular end-diastolic pressure, erythrocyte extravasation, leukocyte and macrophage/monocyte infiltration, granulocyte colony-stimulating factor expression, poly(ADP-ribose) polymerase 1 expression, transferase-mediated dUTP-biotin nick end labeling positive cells, and fragmentation and infarct area. In addition, 3PTP increased Hsp-70 and Bag3 expression and decreased Bax/Bcl-2 ratio, but did not affect the Beclin-1 and LC3-II/LC3-I ratio in the heart with I/R injury.

CONCLUSIONS

3PTP therapies may through Bag3 upregulation alleviate I/R injury-induced left ventricular structural deterioration and dysfunction.

Traceable stimulus-dependent rapid molecular changes in dendritic spines in the brain

Dendritic spines function as microcompartments that can modify the efficiency of their associated synapses. Here, we analyzed stimulus-dependent molecular changes in spines. The F-actin capping protein CapZ accumulates in parts of dendritic spines within regions where long-term potentiation has been induced. We produced a transgenic mouse line, AiCE-Tg, in which CapZ tagged with enhanced green fluorescence protein (EGFP-CapZ) is expressed. Twenty minutes after unilateral visual or somatosensory stimulation in AiCE-Tg mice, relative EGFP-CapZ signal intensification was seen in a subset of dendritic spines selectively in stimulated-side cortices; this right-left difference was abolished by NMDA receptor blockade. Immunolabeling of α-actinin, a PSD-95 binding protein that can recruit AMPA receptors, showed that the α-actinin signals colocalized more frequently in spines with the brightest EGFP-CapZ signals (top 100) than in spines with more typical EGFP-CapZ signal strength (top 1,000). This stimulus-dependent in vivo redistribution of EGFP-CapZ represents a novel molecular event with plasticity-like characteristics, and bright EGFP-CapZ in AiCE-Tg mice make high-CapZ spines traceable in vivo and ex vivo. This mouse line has the potential to be used to reveal sequential molecular events, including synaptic tagging, and to relate multiple types of plasticity in these spines, extending knowledge related to memory mechanisms.

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

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

Modeling Cardiac Disease Mechanisms Using Induced Pluripotent Stem Cell-Derived Cardiomyocytes: Progress, Promises and Challenges

Cardiovascular diseases (CVDs) are a class of disorders affecting the heart or blood vessels. Despite progress in clinical research and therapy, CVDs still represent the leading cause of mortality and morbidity worldwide. The hallmarks of cardiac diseases include heart dysfunction and cardiomyocyte death, inflammation, fibrosis, scar tissue, hyperplasia, hypertrophy, and abnormal ventricular remodeling. The loss of cardiomyocytes is an irreversible process that leads to fibrosis and scar formation, which, in turn, induce heart failure with progressive and dramatic consequences. Both genetic and environmental factors pathologically contribute to the development of CVDs, but the precise causes that trigger cardiac diseases and their progression are still largely unknown. The lack of reliable human model systems for such diseases has hampered the unraveling of the underlying molecular mechanisms and cellular processes involved in heart diseases at their initial stage and during their progression. Over the past decade, significant scientific advances in the field of stem cell biology have literally revolutionized the study of human disease in vitro. Remarkably, the possibility to generate disease-relevant cell types from induced pluripotent stem cells (iPSCs) has developed into an unprecedented and powerful opportunity to achieve the long-standing ambition to investigate human diseases at a cellular level, uncovering their molecular mechanisms, and finally to translate bench discoveries into potential new therapeutic strategies. This review provides an update on previous and current research in the field of iPSC-driven cardiovascular disease modeling, with the aim of underlining the potential of stem-cell biology-based approaches in the elucidation of the pathophysiology of these life-threatening diseases.

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

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

Neuromuscular Diseases Due to Chaperone Mutations: A Review and Some New Results

Skeletal muscle and the nervous system depend on efficient protein quality control, and they express chaperones and cochaperones at high levels to maintain protein homeostasis. Mutations in many of these proteins cause neuromuscular diseases, myopathies, and hereditary motor and sensorimotor neuropathies. In this review, we cover mutations in DNAJB6, DNAJB2, αB-crystallin (CRYAB, HSPB5), HSPB1, HSPB3, HSPB8, and BAG3, and discuss the molecular mechanisms by which they cause neuromuscular disease. In addition, previously unpublished results are presented, showing downstream effects of BAG3 p.P209L on DNAJB6 turnover and localization.

Assembly and Maintenance of Sarcomere Thin Filaments and Associated Diseases

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

BAG3P215L/KO Mice as a Model of BAG3P209L Myofibrillar Myopathy

BCL-2-associated athanogene 3 (BAG3) is a co-chaperone to heat shock proteins important in degrading misfolded proteins through chaperone-assisted selective autophagy. The recurrent dominant BAG3-P209L mutation results in a severe childhood-onset myofibrillar myopathy (MFM) associated with progressive muscle weakness, cardiomyopathy, and respiratory failure. Because a homozygous knock-in (KI) strain for the mP215L mutation homologous to the human P209L mutation did not have a gross phenotype, compound heterozygote knockout (KO) and KI mP215L mice were generated to establish whether further reduction in BAG3 expression would lead to a phenotype. The KI/KO mice have a significant decrease in voluntary movement compared with wild-type and KI/KI mice in the open field starting at 7 months. The KI/KI and KI/KO mice both have significantly smaller muscle fiber cross-sectional area. However, only the KI/KO mice have clear skeletal muscle histologic changes in MFM. As in patient muscle, there are increased levels of BAG3-interacting proteins, such as p62, heat shock protein B8, and αB-crystallin. The KI/KO mP215L strain is the first murine model of BAG3 myopathy that resembles the human skeletal muscle pathologic features. The results support the hypothesis that the pathologic development of MFM requires a significant decrease in BAG3 protein level and not only a gain of function caused by the dominant missense mutation.

Advances in the role and mechanism of BAG3 in dilated cardiomyopathy

The B cell lymphoma 2-associated anthanogene (BAG3) is an anti-apoptotic co-chaperone protein. Previous reports suggest that mutations in BAG3 are associated with dilated cardiomyopathy. This review aims to summarize the current understanding of the relationship between BAG3 mutations and dilated cardiomyopathy, primarily focusing on the role and protective mechanism of BAG3 in cardiomyocytes from individuals with dilated cardiomyopathy. The results of published studies show that BAG3 is critically important for reducing cardiomyocyte apoptosis, maintaining protein homeostasis, regulating mitochondrial stability, modulating myocardial contraction, and reducing cardiac arrhythmia, which suggests an indispensable protective mechanism of BAG3 in dilated cardiomyopathy. The significant role of BAG3 in protecting cardiomyocytes provides a new direction for the diagnosis and treatment of dilated cardiomyopathy. However, further research is required to explore the molecular mechanisms that regulate BAG3 expression, to identify a novel therapy for patients with dilated cardiomyopathy.

Investigation of a dilated cardiomyopathy-associated variant in BAG3 using genome-edited iPSC-derived cardiomyocytes

Mutations in B cell lymphoma 2-associated athanogene 3 (BAG3) are recurrently associated with dilated cardiomyopathy (DCM) and muscular dystrophy. Using isogenic genome-edited human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs), we examined how a DCM-causing BAG3 mutation (R477H), as well as complete loss of BAG3 (KO), impacts myofibrillar organization and chaperone networks. Although unchanged at baseline, fiber length and alignment declined markedly in R477H and KO iPSC-CMs following proteasome inhibition. RNA sequencing revealed extensive baseline changes in chaperone- and stress response protein-encoding genes, and protein levels of key BAG3 binding partners were perturbed. Molecular dynamics simulations of the BAG3-HSC70 complex predicted a partial disengagement by the R477H mutation. In line with this, BAG3-R477H bound less HSC70 than BAG3-WT in coimmunoprecipitation assays. Finally, myofibrillar disarray triggered by proteasome inhibition in R477H cells was mitigated by overexpression of the stress response protein heat shock factor 1 (HSF1). These studies reveal the importance of BAG3 in coordinating protein quality control subsystem usage within the cardiomyocyte and suggest that augmenting HSF1 activity might be beneficial as a means to mitigate proteostatic stress in the context of BAG3-associated DCM.

Haploinsufficiency of mechanistic target of rapamycin ameliorates bag3 cardiomyopathy in adult zebrafish

The adult zebrafish is an emerging vertebrate model for studying human cardiomyopathies; however, whether the simple zebrafish heart can model different subtypes of cardiomyopathies, such as dilated cardiomyopathy (DCM), remains elusive. Here, we generated and characterized an inherited DCM model in adult zebrafish and used this model to search for therapeutic strategies. We employed transcription activator-like effector nuclease (TALEN) genome editing technology to generate frame-shift mutants for the zebrafish ortholog of human BCL2-associated athanogene 3 (BAG3), an established DCM-causative gene. As in mammals, the zebrafish bag3 homozygous mutant (bag3e2/e2 ) exhibited aberrant proteostasis, as indicated by impaired autophagy flux and elevated ubiquitinated protein aggregation. Through comprehensive phenotyping analysis of the mutant, we identified phenotypic traits that resembled DCM phenotypes in mammals, including cardiac chamber enlargement, reduced ejection fraction characterized by increased end-systolic volume/body weight (ESV/BW), and reduced contractile myofibril activation kinetics. Nonbiased transcriptome analysis identified the hyperactivation of the mechanistic target of rapamycin (mTOR) signaling in bag3e2/e2 mutant hearts. Further genetic studies showed that mtorxu015/+ , an mTOR haploinsufficiency mutant, repaired abnormal proteostasis, improved cardiac function and rescued the survival of the bag3e2/e2 mutant. This study established the bag3e2/e2 mutant as a DCM model in adult zebrafish and suggested mtor as a candidate therapeutic target gene for BAG3 cardiomyopathy.

Dilated Cardiomyopathy Due to BLC2-Associated Athanogene 3 (BAG3) Mutations

BACKGROUND

The BAG3 (BLC2-associated athanogene 3) gene codes for an antiapoptotic protein located on the sarcomere Z-disc. Mutations in BAG3 are associated with dilated cardiomyopathy (DCM), but only a small number of cases have been reported to date, and the natural history of BAG3 cardiomyopathy is poorly understood.

OBJECTIVES

This study sought to describe the phenotype and prognosis of BAG3 mutations in a large multicenter DCM cohort.

METHODS

The study cohort comprised 129 individuals with a BAG3 mutation (62% males, 35.1 ± 15.0 years of age) followed at 18 European centers. Localization of BAG3 in cardiac tissue was analyzed in patients with truncating BAG3 mutations using immunohistochemistry.

RESULTS

At first evaluation, 57.4% of patients had DCM. After a median follow-up of 38 months (interquartile range: 7 to 95 months), 68.4% of patients had DCM and 26.1% who were initially phenotype-negative developed DCM. Disease penetrance in individuals >40 years of age was 80% at last evaluation, and there was a trend towards an earlier onset of DCM in men (age 34.6 ± 13.2 years vs. 40.7 ± 12.2 years; p = 0.053). The incidence of adverse cardiac events (death, left ventricular assist device, heart transplantation, and sustained ventricular arrhythmia) was 5.1% per year among individuals with DCM. Male sex, decreased left ventricular ejection fraction. and increased left ventricular end-diastolic diameter were associated with adverse cardiac events. Myocardial tissue from patients with a BAG3 mutation showed myofibril disarray and a relocation of BAG3 protein in the sarcomeric Z-disc.

CONCLUSIONS

DCM caused by mutations in BAG3 is characterized by high penetrance in carriers >40 years of age and a high risk of progressive heart failure. Male sex, decreased left ventricular ejection fraction, and enlarged left ventricular end-diastolic diameter are associated with adverse outcomes in patients with BAG3 mutations.

Evidence for the impact of BAG3 on electrophysiological activity of primary culture of neonatal cardiomyocytes

Homeostasis of proteins involved in contractility of individual cardiomyocytes and those coupling adjacent cells is of critical importance as any abnormalities in cardiac electrical conduction may result in cardiac irregular activity and heart failure. Bcl2-associated athanogene 3 (BAG3) is a stress-induced protein whose role in stabilizing myofibril proteins as well as protein quality control pathways, especially in the cardiac tissue, has captured much attention. Mutations of BAG3 have been implicated in the pathogenesis of cardiac complications such as dilated cardiomyopathy. In this study, we have used an in vitro model of neonatal rat ventricular cardiomyocytes to investigate potential impacts of BAG3 on electrophysiological activity by employing the microelectrode array (MEA) technology. Our MEA data showed that BAG3 plays an important role in the cardiac signal generation as reduced levels of BAG3 led to lower signal frequency and amplitude. Our analysis also revealed that BAG3 is essential to the signal propagation throughout the myocardium, as the MEA data-based conduction velocity, connectivity degree, activation time, and synchrony were adversely affected by BAG3 knockdown. Moreover, BAG3 deficiency was demonstrated to be connected with the emergence of independently beating clusters of cardiomyocytes. On the other hand, BAG3 overexpression improved the activity of cardiomyocytes in terms of electrical signal amplitude and connectivity degree. Overall, by providing more in-depth analyses and characterization of electrophysiological parameters, this study reveals that BAG3 is of critical importance for electrical activity of neonatal cardiomyocytes.

In Vivo Function of the Chaperonin TRiC in α-Actin Folding during Sarcomere Assembly

The TCP-1 ring complex (TRiC) is a multi-subunit group II chaperonin that assists nascent or misfolded proteins to attain their native conformation in an ATP-dependent manner. Functional studies in yeast have suggested that TRiC is an essential and generalized component of the protein-folding machinery of eukaryotic cells. However, TRiC's involvement in specific cellular processes within multicellular organisms is largely unknown because little validation of TRiC function exists in animals. Our in vivo analysis reveals a surprisingly specific role of TRiC in the biogenesis of skeletal muscle α-actin during sarcomere assembly in myofibers. TRiC acts at the sarcomere's Z-disk, where it is required for efficient assembly of actin thin filaments. Binding of ATP specifically by the TRiC subunit Cct5 is required for efficient actin folding in vivo. Furthermore, mutant α-actin isoforms that result in nemaline myopathy in patients obtain their pathogenic conformation via this function of TRiC.

Development of an anti-BAG3 humanized antibody for treatment of pancreatic cancer

We have previously shown that secreted BAG3 is a potential target for the treatment of pancreatic ductal adenocarcinoma and that pancreatic tumor growth and metastatic dissemination can be reduced by treatment with an anti-BAG3 murine antibody. Here, we used complementarity-determining region (CDR) grafting to generate a humanized version of the anti-BAG3 antibody that may be further developed for possible clinical use. We show that the humanized anti-BAG3 antibody, named BAG3-H2L4, abrogates BAG3 binding to macrophages and subsequent release of IL-6. Furthermore, it specifically localizes into tumor tissues and significantly inhibits the growth of Mia PaCa-2 pancreatic cancer cell xenografts. We propose BAG3-H2L4 antibody as a potential clinical candidate for BAG3-targeted therapy in pancreatic cancer.

Differential expression of circulating miRNAs as a novel tool to assess BAG3-associated familial dilated cardiomyopathy

A new familial dilated cardiomyopathy (FDCM) was found related to mutations in BAG3 gene. MicroRNAs (miRNAs) represent new targets of FDCM, although no studies have assessed clinical association between Bcl2-associated athanogene 3 (BAG3)-related DCM and miRNAs. Here, we studied whether a clinical association between BAG3-related FDCM and circulating miRNAs may have diagnostic and prognostic value in a small cohort of familial related individuals carrying a BAG3 mutation (BAG3+) and/or diagnosed of dilated cardiomyopathy (DCM) (DCM+). The analysis of 1759 circulating miRNAs showed significant differences between BAG3+ and BAG3- individuals for miRNAs mir-3191-3p, 6769b-3p, 1249-ep, 154-5p, 6855-5p, and 182-5p, while comparisons between BAG3+/DCM+ versus BAG3+/DCM- were restricted to miRNAs mir-154-5p, 6885-5p, and 182-5p, showing significant correlation with systolic and diastolic blood pressure, A wave, left atrium length, and left atrium area. Additionally, when stratified by gender and age, miRNAs were statistically correlated with critical parameters, including left ventricle ejection fraction (LVEF) and ventricular diameter, in women and young men. Likewise, 56% of BAG3+/DCM+, significantly co-expressed mir-154-5p and mir-182-5p, and a slight 4% did not express such combination, suggesting that co-expression of mir-154-5p and mir-182-5p may potentially show diagnostic value. Further studies will require long-term follow-up, and validation in larger populations.

The N-cadherin interactome in primary cardiomyocytes as defined using quantitative proximity proteomics

The junctional complexes that couple cardiomyocytes must transmit the mechanical forces of contraction while maintaining adhesive homeostasis. The adherens junction (AJ) connects the actomyosin networks of neighboring cardiomyocytes and is required for proper heart function. Yet little is known about the molecular composition of the cardiomyocyte AJ or how it is organized to function under mechanical load. Here, we define the architecture, dynamics and proteome of the cardiomyocyte AJ. Mouse neonatal cardiomyocytes assemble stable AJs along intercellular contacts with organizational and structural hallmarks similar to mature contacts. We combine quantitative mass spectrometry with proximity labeling to identify the N-cadherin (CDH2) interactome. We define over 350 proteins in this interactome, nearly 200 of which are unique to CDH2 and not part of the E-cadherin (CDH1) interactome. CDH2-specific interactors comprise primarily adaptor and adhesion proteins that promote junction specialization. Our results provide novel insight into the cardiomyocyte AJ and offer a proteomic atlas for defining the molecular complexes that regulate cardiomyocyte intercellular adhesion. This article has an associated First Person interview with the first authors of the paper.

Left ventricular remodelling in chronic primary mitral regurgitation: implications for medical therapy

Surgical repair or replacement of the mitral valve is currently the only recommended therapy for severe primary mitral regurgitation. The chronic elevation of wall stress caused by the resulting volume overload leads to structural remodelling of the muscular, vascular and extracellular matrix components of the myocardium. These changes are initially compensatory but in the long term have detrimental effects, which ultimately result in heart failure. Understanding the changes that occur in the myocardium due to volume overload at the molecular and cellular level may lead to medical interventions, which potentially could delay or prevent the adverse left ventricular remodelling associated with primary mitral regurgitation. The pathophysiological changes involved in left ventricular remodelling in response to chronic primary mitral regurgitation and the evidence for potential medical therapy, in particular beta-adrenergic blockers, are the focus of this review.

Myopathy associated BAG3 mutations lead to protein aggregation by stalling Hsp70 networks

BAG3 is a multi-domain hub that connects two classes of chaperones, small heat shock proteins (sHSPs) via two isoleucine-proline-valine (IPV) motifs and Hsp70 via a BAG domain. Mutations in either the IPV or BAG domain of BAG3 cause a dominant form of myopathy, characterized by protein aggregation in both skeletal and cardiac muscle tissues. Surprisingly, for both disease mutants, impaired chaperone binding is not sufficient to explain disease phenotypes. Recombinant mutants are correctly folded, show unaffected Hsp70 binding but are impaired in stimulating Hsp70-dependent client processing. As a consequence, the mutant BAG3 proteins become the node for a dominant gain of function causing aggregation of itself, Hsp70, Hsp70 clients and tiered interactors within the BAG3 interactome. Importantly, genetic and pharmaceutical interference with Hsp70 binding completely reverses stress-induced protein aggregation for both BAG3 mutations. Thus, the gain of function effects of BAG3 mutants act as Achilles heel of the HSP70 machinery.

Mitochondrial quality control in cardiac cells: Mechanisms and role in cardiac cell injury and disease

Mitochondria play an important role in maintaining cardiac homeostasis by supplying the major energy required for cardiac excitation-contraction coupling as well as controlling the key intracellular survival and death pathways. Healthy mitochondria generate ATP molecules through an aerobic process known as oxidative phosphorylation (OXPHOS). Mitochondrial injury during myocardial infarction (MI) impairs OXPHOS and results in the excessive production of reactive oxygen species (ROS), bioenergetic insufficiency, and contributes to the development of cardiovascular diseases. Therefore, mitochondrial biogenesis along with proper mitochondrial quality control machinery, which removes unhealthy mitochondria is pivotal for mitochondrial homeostasis and cardiac health. Upon damage to the mitochondrial network, mitochondrial quality control components are recruited to segregate the unhealthy mitochondria and target aberrant mitochondrial proteins for degradation and elimination. Impairment of mitochondrial quality control and accumulation of abnormal mitochondria have been reported in the pathogenesis of various cardiac disorders and heart failure. Here, we provide an overview of the recent studies describing various mechanistic pathways underlying mitochondrial homeostasis with the main focus on cardiac cells. In addition, this review demonstrates the potential effects of mitochondrial quality control dysregulation in the development of cardiovascular disease.