HSPB7 prevents cardiac conduction system defect through maintaining intercalated disc integrity

Reduction of Filamin C Results in Altered Proteostasis, Cardiomyopathy, and Arrhythmias

BACKGROUND

Many cardiomyopathy-associated FLNC pathogenic variants are heterozygous truncations, and FLNC pathogenic variants are associated with arrhythmias. Arrhythmia triggers in filaminopathy are incompletely understood.

METHODS AND RESULTS

We describe an individual with biallelic FLNC pathogenic variants, p.Arg650X and c.970-4A>G, with peripartum cardiomyopathy and ventricular arrhythmias. We also describe clinical findings in probands with FLNC variants including Val2715fs87X, Glu2458Serfs71X, Phe106Leu, and c.970-4A>G with hypertrophic and dilated cardiomyopathy, atrial fibrillation, and ventricular tachycardia. Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated. The FLNC truncation, Arg650X/c.970-4A>G, showed a marked reduction in filamin C protein consistent with biallelic loss of function mutations. To assess loss of filamin C, gene editing of a healthy control iPSC line was used to generate a homozygous FLNC disruption in the actin binding domain. Because filamin C has been linked to protein quality control, we assessed the necessity of filamin C in iPSC-CMs for response to the proteasome inhibitor bortezomib. After exposure to low-dose bortezomib, FLNC-null iPSC-CMs showed an increase in the chaperone proteins BAG3, HSP70 (heat shock protein 70), and HSPB8 (small heat shock protein B8) and in the autophagy marker LC3I/II. FLNC null iPSC-CMs had prolonged electric field potential, which was further prolonged in the presence of low-dose bortezomib. FLNC null engineered heart tissues had impaired function after low-dose bortezomib.

CONCLUSIONS

FLNC pathogenic variants associate with a predisposition to arrhythmias, which can be modeled in iPSC-CMs. Reduction of filamin C prolonged field potential, a surrogate for action potential, and with bortezomib-induced proteasome inhibition, reduced filamin C led to greater arrhythmia potential and impaired function.

Functional Diversity of Mammalian Small Heat Shock Proteins: A Review

The small heat shock proteins (sHSPs), whose molecular weight ranges from 12∼43 kDa, are members of the heat shock protein (HSP) family that are widely found in all organisms. As intracellular stress resistance molecules, sHSPs play an important role in maintaining the homeostasis of the intracellular environment under various stressful conditions. A total of 10 sHSPs have been identified in mammals, sharing conserved α-crystal domains combined with variable N-terminal and C-terminal regions. Unlike large-molecular-weight HSP, sHSPs prevent substrate protein aggregation through an ATP-independent mechanism. In addition to chaperone activity, sHSPs were also shown to suppress apoptosis, ferroptosis, and senescence, promote autophagy, regulate cytoskeletal dynamics, maintain membrane stability, control the direction of cellular differentiation, modulate angiogenesis, and spermatogenesis, as well as attenuate the inflammatory response and reduce oxidative damage. Phosphorylation is the most significant post-translational modification of sHSPs and is usually an indicator of their activation. Furthermore, abnormalities in sHSPs often lead to aggregation of substrate proteins and dysfunction of client proteins, resulting in disease. This paper reviews the various biological functions of sHSPs in mammals, emphasizing the roles of different sHSPs in specific cellular activities. In addition, we discuss the effect of phosphorylation on the function of sHSPs and the association between sHSPs and disease.

HSPB7 oppositely regulates human mesenchymal stromal cell-derived osteogenesis and adipogenesis

BACKGROUND

Recent evidence suggests that accumulation of marrow adipose tissue induced by aberrant lineage allocation of bone marrow-derived mesenchymal stromal cells (BMSCs) contributes to the pathophysiologic processes of osteoporosis. Although master regulators of lineage commitment have been well documented, molecular switches between osteogenesis and adipogenesis are largely unknown.

METHODS

HSPB7 gene expression during osteogenic and adipogenic differentiation of BMSCs was evaluated by qPCR and Western blot analyses. Lentiviral-mediated knockdown or overexpression of HSPB7 and its deletion constructs were used to assess its function. The organization of cytoskeleton was examined by immunofluorescent staining. ALP activity, calcium assay, Alizarin Red S staining and Oil Red O staining were performed in vitro during osteoblast or adipocyte differentiation. SB431542 and Activin A antibody were used to identify the mechanism of Activin A in the regulation of osteogenic differentiation in BMSCs.

RESULTS

In this study, we identified HSPB7 capable of oppositely regulating osteogenic and adipogenic differentiation of BMSCs. HSPB7 silencing promoted adipogenesis while reducing osteogenic differentiation and mineralization. Conversely, overexpression of HSPB7 strongly enhanced osteogenesis, but no effect was observed on adipogenic differentiation. Deletion of the N-terminal or C-terminal domain of HSPB7 led to decreased osteoblastic potency and mineralization. Mechanistically, our data showed that Activin A is a downstream target participating in HSPB7 knockdown-mediated osteogenic inhibition.

CONCLUSIONS

Our findings suggest that HSPB7 plays a positive role in driving osteoblastic differentiation, and with the capability in maintaining the osteo-adipogenesis balance. It holds great promise as a potential therapeutic target in the treatment of bone metabolic diseases.

The Role of Small Heat Shock Proteins in Protein Misfolding Associated Motoneuron Diseases

Motoneuron diseases (MNDs) are neurodegenerative conditions associated with death of upper and/or lower motoneurons (MNs). Proteostasis alteration is a pathogenic mechanism involved in many MNDs and is due to the excessive presence of misfolded and aggregated proteins. Protein misfolding may be the product of gene mutations, or due to defects in the translation process, or to stress agents; all these conditions may alter the native conformation of proteins making them prone to aggregate. Alternatively, mutations in members of the protein quality control (PQC) system may determine a loss of function of the proteostasis network. This causes an impairment in the capability to handle and remove aberrant or damaged proteins. The PQC system consists of the degradative pathways, which are the autophagy and the proteasome, and a network of chaperones and co-chaperones. Among these components, Heat Shock Protein 70 represents the main factor in substrate triage to folding, refolding, or degradation, and it is assisted in this task by a subclass of the chaperone network, the small heat shock protein (sHSPs/HSPBs) family. HSPBs take part in proteostasis by bridging misfolded and aggregated proteins to the HSP70 machinery and to the degradative pathways, facilitating refolding or clearance of the potentially toxic proteins. Because of its activity against proteostasis alteration, the chaperone system plays a relevant role in the protection against proteotoxicity in MNDs. Here, we discuss the role of HSPBs in MNDs and which HSPBs may represent a valid target for therapeutic purposes.

Role of Small Heat Shock Proteins in the Remodeling of Actin Microfilaments

Small heat shock proteins (sHsps) play an important role in the maintenance of proteome stability and, particularly, in stabilization of the cytoskeleton and cell contractile apparatus. Cell exposure to different types of stress is accompanied by the translocation of sHsps onto actin filaments; therefore, it is commonly believed that the sHsps are true actin-binding proteins. Investigations of last years have shown that this assumption is incorrect. Stress-induced translocation of sHsp to actin filaments is not the result of direct interaction of these proteins with intact actin, but results from the chaperone-like activity of sHsps and their interaction with various actin-binding proteins. HspB1 and HspB5 interact with giant elastic proteins titin and filamin thus providing an integrity of the contractile apparatus and its proper localization in the cell. HspB6 binds to the universal adapter protein 14-3-3 and only indirectly affects the structure of actin filament. HspB7 interacts with filamin C and controls actin filament assembly. HspB8 forms tight complex with the universal regulatory and adapter protein Bag3 and participates in the chaperone-assisted selective autophagy (CASA) of actin-binding proteins (e.g., filamin), as well as in the actin-depending processes taking place in mitoses. Hence, the mechanisms of sHsp participation in the maintenance of the contractile apparatus and cytoskeleton are much more complicated and diverse than it has been postulated earlier and are not limited to direct interactions of sHsps with actin. The old hypothesis on the direct binding of sHsps to intact actin should be revised and further detailed investigation on the sHsp interaction with minor proteins participating in the formation and remodeling of actin filaments is required.

A Combined Human in Silico and CRISPR/Cas9-Mediated in Vivo Zebrafish Based Approach to Provide Phenotypic Data for Supporting Early Target Validation

The clinical heterogeneity of heart failure has challenged our understanding of the underlying genetic mechanisms of this disease. In this respect, large-scale patient DNA sequencing studies have become an invaluable strategy for identifying potential genetic contributing factors. The complex aetiology of heart failure, however, also means that in vivo models are vital to understand the links between genetic perturbations and functional impacts as part of the process for validating potential new drug targets. Traditional approaches (e.g., genetically-modified mice) are optimal for assessing small numbers of genes, but less practical when multiple genes are identified. The zebrafish, in contrast, offers great potential for higher throughput in vivo gene functional assessment to aid target prioritisation, by providing more confidence in target relevance and facilitating gene selection for definitive loss of function studies undertaken in mice. Here we used whole-exome sequencing and bioinformatics on human patient data to identify 3 genes (API5, HSPB7, and LMO2) suggestively associated with heart failure that were also predicted to play a broader role in disease aetiology. The role of these genes in cardiovascular system development and function was then further investigated using in vivo CRISPR/Cas9-mediated gene mutation analysis in zebrafish. We observed multiple impacts in F0 knockout zebrafish embryos (crispants) following effective somatic mutation, including changes in ventricle size, pericardial oedema, and chamber malformation. In the case of lmo2, there was also a significant impact on cardiovascular function as well as an expected reduction in erythropoiesis. The data generated from both the human in silico and zebrafish in vivo assessments undertaken supports further investigation of the potential roles of API5, HSPB7, and LMO2 in human cardiovascular disease. The data presented also supports the use of human in silico genetic variant analysis, in combination with zebrafish crispant phenotyping, as a powerful approach for assessing gene function as part of an integrated multi-level drug target validation strategy.

Insights on Human Small Heat Shock Proteins and Their Alterations in Diseases

The family of the human small Heat Shock Proteins (HSPBs) consists of ten members of chaperones (HSPB1-HSPB10), characterized by a low molecular weight and capable of dimerization and oligomerization forming large homo- or hetero-complexes. All HSPBs possess a highly conserved centrally located α-crystallin domain and poorly conserved N- and C-terminal domains. The main feature of HSPBs is to exert cytoprotective functions by preserving proteostasis, assuring the structural maintenance of the cytoskeleton and acting in response to cellular stresses and apoptosis. HSPBs take part in cell homeostasis by acting as holdases, which is the ability to interact with a substrate preventing its aggregation. In addition, HSPBs cooperate in substrates refolding driven by other chaperones or, alternatively, promote substrate routing to degradation. Notably, while some HSPBs are ubiquitously expressed, others show peculiar tissue-specific expression. Cardiac muscle, skeletal muscle and neurons show high expression levels for a wide variety of HSPBs. Indeed, most of the mutations identified in HSPBs are associated to cardiomyopathies, myopathies, and motor neuropathies. Instead, mutations in HSPB4 and HSPB5, which are also expressed in lens, have been associated with cataract. Mutations of HSPBs family members encompass base substitutions, insertions, and deletions, resulting in single amino acid substitutions or in the generation of truncated or elongated proteins. This review will provide an updated overview of disease-related mutations in HSPBs focusing on the structural and biochemical effects of mutations and their functional consequences.

Linking metabolic dysfunction with cardiovascular diseases: Brn-3b/POU4F2 transcription factor in cardiometabolic tissues in health and disease

Metabolic and cardiovascular diseases are highly prevalent and chronic conditions that are closely linked by complex molecular and pathological changes. Such adverse effects often arise from changes in the expression of genes that control essential cellular functions, but the factors that drive such effects are not fully understood. Since tissue-specific transcription factors control the expression of multiple genes, which affect cell fate under different conditions, then identifying such regulators can provide valuable insight into the molecular basis of such diseases. This review explores emerging evidence that supports novel and important roles for the POU4F2/Brn-3b transcription factor (TF) in controlling cellular genes that regulate cardiometabolic function. Brn-3b is expressed in insulin-responsive metabolic tissues (e.g. skeletal muscle and adipose tissue) and is important for normal function because constitutive Brn-3b-knockout (KO) mice develop profound metabolic dysfunction (hyperglycaemia; insulin resistance). Brn-3b is highly expressed in the developing hearts, with lower levels in adult hearts. However, Brn-3b is re-expressed in adult cardiomyocytes following haemodynamic stress or injury and is necessary for adaptive cardiac responses, particularly in male hearts, because male Brn-3b KO mice develop adverse remodelling and reduced cardiac function. As a TF, Brn-3b regulates the expression of multiple target genes, including GLUT4, GSK3β, sonic hedgehog (SHH), cyclin D1 and CDK4, which have known functions in controlling metabolic processes but also participate in cardiac responses to stress or injury. Therefore, loss of Brn-3b and the resultant alterations in the expression of such genes could potentially provide the link between metabolic dysfunctions with adverse cardiovascular responses, which is seen in Brn-3b KO mutants. Since the loss of Brn-3b is associated with obesity, type II diabetes (T2DM) and altered cardiac responses to stress, this regulator may provide a new and important link for understanding how pathological changes arise in such endemic diseases.

HSPB7 regulates osteogenic differentiation of human adipose derived stem cells via ERK signaling pathway

Background

Heat shock protein B7 (HSPB7), which belongs to small heat shock protein family, has been reported to be involved in diverse biological processes and diseases. However, whether HSPB7 regulates osteogenic differentiation of human adipose derived stem cells (hASCs) remains unexplored.

Methods

The expression level of HSPB7 during the osteogenesis of hASCs was examined by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and Western blot analysis. Lentivirus transfection was used to knock down or overexpress HSPB7, which enabled us to investigate the effect of HSPB7 on osteogenic differentiation of hASCs. U0126 and extracellular signal-regulated protein kinase 1/2 (ERK1/2) siRNA were used to identify the mechanism of the HSPB7/ERK1/2 axis in regulating osteogenic differentiation of hASCs. Moreover, ectopic bone formation in nude mice and osteoporosis mice model was used to investigate the effect of HSPB7 on osteogenesis in vivo.

Results

In this study, we found the expression of HSPB7 was significantly downregulated during the osteogenic differentiation of hASCs. HSPB7 knockdown remarkably promoted osteogenic differentiation of hASCs, while HSPB7 overexpression suppressed osteogenic differentiation of hASCs both in vitro and in vivo. Moreover, we discovered that the enhancing effect of HSPB7 knockdown on osteogenic differentiation was related to the activation of extracellular signal-regulated protein kinase (ERK) signaling pathway. Inhibition of ERK signaling pathway with U0126 or silencing ERK1/2 effectively blocked the stimulation of osteogenic differentiation induced by HSPB7 knockdown. Additionally, we found that HSPB7 expression was markedly increased in mouse bone marrow mesenchymal stem cells (mBMSCs) from the osteoporotic mice which suggested that HSPB7 might be utilized as a potential target in the development of effective therapeutic strategies to treat osteoporosis and other bone diseases.

Conclusion

Taken together, these findings uncover a previously unrecognized function of HSPB7 in regulating osteogenic differentiation of hASCs, partly via the ERK signaling pathway.

The Genetic Makeup of the Electrocardiogram

The electrocardiogram (ECG) is one of the most useful non-invasive diagnostic tests for a wide array of cardiac disorders. Traditional approaches to analyzing ECGs focus on individual segments. Here, we performed comprehensive deep phenotyping of 77,190 ECGs in the UK Biobank across the complete cycle of cardiac conduction, resulting in 500 spatial-temporal datapoints, across 10 million genetic variants. In addition to characterizing polygenic risk scores for the traditional ECG segments, we identified over 300 genetic loci that are statistically associated with the high-dimensional representation of the ECG. We established the genetic ECG signature for dilated cardiomyopathy, associated the BAG3, HSPB7/CLCNKA, PRKCA, TMEM43, and OBSCN loci with disease risk and confirmed this association in an independent cohort. In total, our work demonstrates that a high-dimensional analysis of the entire ECG provides unique opportunities for studying cardiac biology and disease and furthering drug development. A record of this paper's transparent peer review process is included in the Supplemental Information.

Small heat-shock proteins and their role in mechanical stress

The ability of cells to respond to stress is central to health. Stress can damage folded proteins, which are vulnerable to even minor changes in cellular conditions. To maintain proteostasis, cells have developed an intricate network in which molecular chaperones are key players. The small heat-shock proteins (sHSPs) are a widespread family of molecular chaperones, and some sHSPs are prominent in muscle, where cells and proteins must withstand high levels of applied force. sHSPs have long been thought to act as general interceptors of protein aggregation. However, evidence is accumulating that points to a more specific role for sHSPs in protecting proteins from mechanical stress. Here, we briefly introduce the sHSPs and outline the evidence for their role in responses to mechanical stress. We suggest that sHSPs interact with mechanosensitive proteins to regulate physiological extension and contraction cycles. It is likely that further study of these interactions - enabled by the development of experimental methodologies that allow protein contacts to be studied under the application of mechanical force - will expand our understanding of the activity and functions of sHSPs, and of the roles played by chaperones in general.

Mechanisms of Small Heat Shock Proteins

Small heat shock proteins (sHSPs) are ATP-independent chaperones that delay formation of harmful protein aggregates. sHSPs' role in protein homeostasis has been appreciated for decades, but their mechanisms of action remain poorly understood. This gap in understanding is largely a consequence of sHSP properties that make them recalcitrant to detailed study. Multiple stress-associated conditions including pH acidosis, oxidation, and unusual availability of metal ions, as well as reversible stress-induced phosphorylation can modulate sHSP chaperone activity. Investigations of sHSPs reveal that sHSPs can engage in transient or long-lived interactions with client proteins depending on solution conditions and sHSP or client identity. Recent advances in the field highlight both the diversity of function within the sHSP family and the exquisite sensitivity of individual sHSPs to cellular and experimental conditions. Here, we will present and highlight current understanding, recent progress, and future challenges.

Role of Bcl2-associated Athanogene 3 in Turnover of Gap Junction Protein, Connexin 43, in Neonatal Cardiomyocytes

Any pathological stress that impairs expression, turnover and phosphorylation of connexin 43 (Cx43), one of the major proteins of gap junctions, can adversely impact myocardial cell behavior, thus leading to the development of cardiac arrhythmias and heart failure. Our results in primary neonatal rat ventricular cardiomyocytes (NRVCs) show that impairment of the autophagy-lysosome pathway dysregulates degradation of Cx43, either by inhibiting lysosomal activity or suppressing the level of Bcl2-associated athanogene 3 (BAG3), a stress-induced pleiotropic protein that is involved in protein quality control (PQC) via the autophagy pathway. Inhibition of lysosomal activity leads to the accumulation of Cx43 aggregates and suppression of BAG3 significantly diminished turnover of Cx43. In addition, knock-down of BAG3 reduced the levels of Cx43 by dysregulating Cx43 protein stability. Under stress conditions, expression of BAG3 affected the state of Cx43 phosphorylation and its degradation. Furthermore, we found that BAG3 co-localized with the cytoskeleton protein, α-Tubulin, and depolymerization of α-Tubulin led to the intracellular accumulation of Cx43. These observations ascribe a novel function for BAG3 that involves control of Cx43 turnover under normal and stress conditions and potentially for optimizing communication of cardiac muscle cells through gap junctions.

The BAG3-dependent and -independent roles of cardiac small heat shock proteins

Small heat shock proteins (sHSPs) comprise an important protein family that is ubiquitously expressed, is highly conserved among species, and has emerged as a critical regulator of protein folding. While these proteins are functionally important for a variety of tissues, an emerging field of cardiovascular research reveals sHSPs are also extremely important for maintaining normal cardiac function and regulating the cardiac stress response. Notably, numerous mutations in genes encoding sHSPs have been associated with multiple cardiac diseases. sHSPs (HSPB5, HSPB6, and HSPB8) have been described as mediating chaperone functions within the heart by interacting with the cochaperone protein BCL-2-associated anthanogene 3 (BAG3); however, recent reports indicate that sHSPs (HSPB7) can perform other BAG3-independent functions. Here, we summarize the cardiac functions of sHSPs and present the notion that cardiac sHSPs function via BAG3-dependent or -independent pathways.

HSPB7 is indispensable for heart development by modulating actin filament assembly

Small heat shock protein HSPB7 is highly expressed in the heart. Several mutations within HSPB7 are associated with dilated cardiomyopathy and heart failure in human patients. However, the precise role of HSPB7 in the heart is still unclear. In this study, we generated global as well as cardiac-specific HSPB7 KO mouse models and found that loss of HSPB7 globally or specifically in cardiomyocytes resulted in embryonic lethality before embryonic day 12.5. Using biochemical and cell culture assays, we identified HSPB7 as an actin filament length regulator that repressed actin polymerization by binding to monomeric actin. Consistent with HSPB7's inhibitory effects on actin polymerization, HSPB7 KO mice had longer actin/thin filaments and developed abnormal actin filament bundles within sarcomeres that interconnected Z lines and were cross-linked by α-actinin. In addition, loss of HSPB7 resulted in up-regulation of Lmod2 expression and mislocalization of Tmod1. Furthermore, crossing HSPB7 null mice into an Lmod2 null background rescued the elongated thin filament phenotype of HSPB7 KOs, but double KO mice still exhibited formation of abnormal actin bundles and early embryonic lethality. These in vivo findings indicated that abnormal actin bundles, not elongated thin filament length, were the cause of embryonic lethality in HSPB7 KOs. Our findings showed an unsuspected and critical role for a specific small heat shock protein in directly modulating actin thin filament length in cardiac muscle by binding monomeric actin and limiting its availability for polymerization.