Stabilised beta-catenin in postnatal ventricular myocardium leads to dilated cardiomyopathy and premature death

Evaluation of the Canonical Wnt Signaling Pathway in the Hearts of Hypertensive Rats of Various Etiologies

Wnt/β-catenin signaling dysregulation is associated with the pathogenesis of many human diseases, including hypertension and heart disease. The aim of this study was to immunohistochemically evaluate and compare the expression of the Fzd8, WNT1, GSK-3β, and β-catenin genes in the hearts of rats with spontaneous hypertension (SHRs) and deoxycorticosterone acetate (DOCA)-salt-induced hypertension. The myocardial expression of Fzd8, WNT1, GSK-3β, and β-catenin was detected by immunohistochemistry, and the gene expression was assessed with a real-time PCR method. In SHRs, the immunoreactivity of Fzd8, WNT1, GSK-3β, and β-catenin was attenuated in comparison to that in normotensive animals. In DOCA-salt-induced hypertension, the immunoreactivity of Fzd8, WNT1, GSK-3β, and β-catenin was enhanced. In SHRs, decreases in the expression of the genes encoding Fzd8, WNT1, GSK-3β, and β-catenin were observed compared to the control group. Increased expression of the genes encoding Fzd8, WNT1, GSK-3β, and β-catenin was demonstrated in the hearts of rats with DOCA-salt-induced hypertension. Wnt signaling may play an essential role in the pathogenesis of arterial hypertension and the accompanying heart damage. The obtained results may constitute the basis for further research aimed at better understanding the role of the Wnt/β-catenin pathway in the functioning of the heart.

The role of the Wnt / β-catenin pathway and the functioning of the heart in arterial hypertension - A review

Many factors and molecular pathways are involved in the pathogenesis of arterial hypertension. The increase in blood pressure may be determined by the properties of specific gene products and their associated action with environmental factors. In recent years, much attention has been paid to the Wnt/β-catenin signaling pathway which is essential for organ damage repair and homeostasis. Deregulation of the activity of the Wnt/β-catenin pathway may be directly or indirectly related to myocardial hypertrophy, as well as to cardiomyocyte remodeling and remodeling processes in pathological states of this organ. There are reports pointing to the role of the Wnt/β-catenin pathway in the course and development of organ complications in conditions of arterial hypertension. This paper presents the current state of knowledge of the role of the Wnt/β-catenin pathway in the regulation of arterial pressure and its impact on the physiology and the development of the complications of arterial hypertension in the heart.

Phosphoproteomic Analysis Reveals Downstream PKA Effectors of AKAP Cypher/ZASP in the Pathogenesis of Dilated Cardiomyopathy

Background: Dilated cardiomyopathy (DCM) is a major cause of heart failure worldwide. The Z-line protein Cypher/Z-band alternatively spliced PDZ-motif protein (ZASP) is closely associated with DCM, both clinically and in animal models. Our earlier work revealed Cypher/ZASP as a PKA-anchoring protein (AKAP) that tethers PKA to phosphorylate target substrates. However, the downstream PKA effectors regulated by AKAP Cypher/ZASP and their relevance to DCM remain largely unknown.

Methods and Results: For the identification of candidate PKA substrates, global quantitative phosphoproteomics was performed on cardiac tissue from wild-type and Cypher-knockout mice with PKA activation. A total of 216 phosphopeptides were differentially expressed in the Cypher-knockout mice; 31 phosphorylation sites were selected as candidates using the PKA consensus motifs. Bioinformatic analysis indicated that differentially expressed proteins were enriched mostly in cell adhesion and mRNA processing. Furthermore, the phosphorylation of β-catenin Ser675 was verified to be facilitated by Cypher. This phosphorylation promoted the transcriptional activity of β-catenin, and also the proliferative capacity of cardiomyocytes. Immunofluorescence staining demonstrated that Cypher colocalised with β-catenin in the intercalated discs (ICD) and altered the cytoplasmic distribution of β-catenin. Moreover, the phosphorylation of two other PKA substrates, vimentin Ser72 and troponin I Ser23/24, was suppressed by Cypher deletion.

Conclusions: Cypher/ZASP plays an essential role in β-catenin activation via Ser675 phosphorylation, which modulates cardiomyocyte proliferation. Additionally, Cypher/ZASP regulates other PKA effectors, such as vimentin Ser72 and troponin I Ser23/24. These findings establish the AKAP Cypher/ZASP as a signalling hub in the progression of DCM.

The titin N2B and N2A regions: biomechanical and metabolic signaling hubs in cross-striated muscles

Muscle specific signaling has been shown to originate from myofilaments and their associated cellular structures, including the sarcomeres, costameres or the cardiac intercalated disc. Two signaling hubs that play important biomechanical roles for cardiac and/or skeletal muscle physiology are the N2B and N2A regions in the giant protein titin. Prominent proteins associated with these regions in titin are chaperones Hsp90 and αB-crystallin, members of the four-and-a-half LIM (FHL) and muscle ankyrin repeat protein (Ankrd) families, as well as thin filament-associated proteins, such as myopalladin. This review highlights biological roles and properties of the titin N2B and N2A regions in health and disease. Special emphasis is placed on functions of Ankrd and FHL proteins as mechanosensors that modulate muscle-specific signaling and muscle growth. This region of the sarcomere also emerged as a hotspot for the modulation of passive muscle mechanics through altered titin phosphorylation and splicing, as well as tethering mechanisms that link titin to the thin filament system.

Desmosomes: emerging pathways and non-canonical functions in cardiac arrhythmias and disease

Desmosomes are critical adhesion structures in cardiomyocytes, with mutation/loss linked to the heritable cardiac disease, arrhythmogenic right ventricular cardiomyopathy (ARVC). Early studies revealed the ability of desmosomal protein loss to trigger ARVC disease features including structural remodeling, arrhythmias, and inflammation; however, the precise mechanisms contributing to diverse disease presentations are not fully understood. Recent mechanistic studies demonstrated the protein degradation component CSN6 is a resident cardiac desmosomal protein which selectively restricts cardiomyocyte desmosomal degradation and disease. This suggests defects in protein degradation can trigger the structural remodeling underlying ARVC. Additionally, a subset of ARVC-related mutations show enhanced vulnerability to calpain-mediated degradation, further supporting the relevance of these mechanisms in disease. Desmosomal gene mutations/loss has been shown to impact arrhythmogenic pathways in the absence of structural disease within ARVC patients and model systems. Studies have shown the involvement of connexins, calcium handling machinery, and sodium channels as early drivers of arrhythmias, suggesting these may be distinct pathways regulating electrical function from the desmosome. Emerging evidence has suggested inflammation may be an early mechanism in disease pathogenesis, as clinical reports have shown an overlap between myocarditis and ARVC. Recent studies focus on the association between desmosomal mutations/loss and inflammatory processes including autoantibodies and signaling pathways as a way to understand the involvement of inflammation in ARVC pathogenesis. A specific focus will be to dissect ongoing fields of investigation to highlight diverse pathogenic pathways associated with desmosomal mutations/loss.

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

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

Induction of Wnt signaling antagonists and p21-activated kinase enhances cardiomyocyte proliferation during zebrafish heart regeneration

Heart regeneration occurs by dedifferentiation and proliferation of pre-existing cardiomyocytes (CMs). However, the signaling mechanisms by which injury induces CM renewal remain incompletely understood. Here, we find that cardiac injury in zebrafish induces expression of the secreted Wnt inhibitors, including Dickkopf 1 (Dkk1), Dkk3, secreted Frizzled-related protein 1 (sFrp1), and sFrp2, in cardiac tissue adjacent to injury sites. Experimental blocking of Wnt activity via Dkk1 overexpression enhances CM proliferation and heart regeneration, whereas ectopic activation of Wnt8 signaling blunts injury-induced CM dedifferentiation and proliferation. Although Wnt signaling is dampened upon injury, the cytoplasmic β-catenin is unexpectedly increased at disarrayed CM sarcomeres in myocardial wound edges. Our analyses indicated that p21-activated kinase 2 (Pak2) is induced at regenerating CMs, where it phosphorylates cytoplasmic β-catenin at Ser 675 and increases its stability at disassembled sarcomeres. Myocardial-specific induction of the phospho-mimetic β-catenin (S675E) enhances CM dedifferentiation and sarcomere disassembly in response to injury. Conversely, inactivation of Pak2 kinase activity reduces the Ser 675-phosphorylated β-catenin (pS675-β-catenin) and attenuates CM sarcomere disorganization and dedifferentiation. Taken together, these findings demonstrate that coordination of Wnt signaling inhibition and Pak2/pS675-β-catenin signaling enhances zebrafish heart regeneration by supporting CM dedifferentiation and proliferation.

Ageing-related changes in the levels of β-catenin, CacyBP/SIP, galectin-3 and immunoproteasome subunit LMP7 in the heart of men

Aging is a major risk factor for morbidity and mortality from cardiovascular causes in men. To better understand the cellular processes related to age-related cardiac complications, we undertook research aimed at comparative evaluation of genes expression and distribution of β-catenin, CacyBP/SIP, galectin-3 and LMP7 in the heart of healthy men in different age groups. The study was conducted on the hearts of 12 men (organ donors) without a history of cardiovascular disease, who were divided into two age groups: men under and men over 45 years of age. On paraffin sections, immunohistochemical reactions were performed to detect β-catenin, CacyBP/SIP, galectin-3 and immunoproteasome subunit LMP7. The expression of genes coding β-catenin, CacyBP/SIP, galectin-3 and LMP7 was also evaluated by real-time PCR method. In the heart of men over 45 years old, both gene expression and immunoreactivity of β-catenin, CacyBP/SIP, galectin-3 and LMP7 were stronger compared to younger individuals. The results of the presented studies suggest that β-catenin, CacyBP/SIP, galectin-3 and immunoproteasomes might be involved in the internal regulation of heart homeostasis during ageing.

The M-band: The underestimated part of the sarcomere

The sarcomere is the basic unit of the myofibrils, which mediate skeletal and cardiac Muscle contraction. Two transverse structures, the Z-disc and the M-band, anchor the thin (actin and associated proteins) and thick (myosin and associated proteins) filaments to the elastic filament system composed of titin. A plethora of proteins are known to be integral or associated proteins of the Z-disc and its structural and signalling role in muscle is better understood, while the molecular constituents of the M-band and its function are less well defined. Evidence discussed here suggests that the M-band is important for managing force imbalances during active muscle contraction. Its molecular composition is fine-tuned, especially as far as the structural linkers encoded by members of the myomesin family are concerned and depends on the specific mechanical characteristics of each particular muscle fibre type. Muscle activity signals from the M-band to the nucleus and affects transcription of sarcomeric genes, especially via serum response factor (SRF). Due to its important role as shock absorber in contracting muscle, the M-band is also more and more recognised as a contributor to muscle disease.

Enhancement of β-catenin/T-cell factor 4 signaling causes susceptibility to cardiac arrhythmia by suppressing NaV1.5 expression in mice

BACKGROUND

β-Catenin/T-cell factor 4 (TCF4) signaling is enhanced in ischemic heart disease in which ventricular tachycardia (VT)/ventricular fibrillation occurs frequently. How this signaling links to arrhythmogenesis remains unclear.

OBJECTIVE

The purpose of this study was to investigate the role of β-catenin gain of function in the development of arrhythmia.

METHODS

A mouse model with a conditional deletion of CTNNB1 exon 3 resulting in cardiac exon 3-deleted and stabilized β-catenin (β-catΔE3) was used to determine the role of β-catenin gain of function in the regulation of cardiac rhythm.

RESULTS

Western blotting showed β-catΔE3 expression and significantly decreased NaV1.5 protein in CTNNB1 E3-/- and CTNNB1 E3+/- mouse hearts. Real-time qRT-PCR revealed significantly decreased NaV1.5 messenger RNA with no changes in Na+ channel β1 to β4 expression in these hearts. Immunofluorescence revealed accumulation of β-catΔE3 in the nuclei of CTNNB1 E3-/- cardiomyocytes. Immunohistochemistry demonstrated nuclear localization of β-catenin in cardiomyocytes, which was associated with significantly decreased NaV1.5 messenger RNA in human ischemic hearts. Immunoprecipitation revealed that β-catΔE3 interacted with TCF4 in CTNNB1 E3-/- cardiomyocytes. Whole-cell recordings showed that Na+ currents and depolarization and amplitude of action potentials were significantly decreased in CTNNB1 E3-/- ventricular myocytes. Electrocardiographic recordings demonstrated that in mice with cardiac CTNNB1 E3-/-, the QRS complex was prolonged and VT was induced by the Na+ channel blocker flecainide. However, cardiac function, as determined by echocardiography and heart/body weight ratios, remained unchanged.

CONCLUSION

Enhancement of β-catenin/TCF4 signaling led to the prolongation of the QRS complex and increase in susceptibility to VT by suppression of NaV1.5 expression and Na+ channel activity in mice.

Molecular and functional characterization of Arabidopsis thaliana VPNB1 gene involved in plant vascular development

Armadillo (ARM) repeat containing proteins constitute a large family in plants and are involved in diverse cellular functions, like signal transduction, proliferation and differentiation. In animals, ARM repeat proteins have been implicated in cancer development. In this study, we aimed in characterizing the VPNB1 gene from Arabidopsis thaliana and its role in plant development, by implementing a number of genetic and molecular approaches. AtVPNB1 encodes for an ARM repeat protein of unknown function, exclusively expressed in the cambium as well as in the differentiating xylem and phloem cells of the vascular system. Subcellular localization experiments showed that VPNB is confined in nucleoplasmic speckle-like structures unrelated to cajal bodies. Transgenic VPNB-impaired plants exhibit a slower growing phenotype and a non-canonical pattern of xylem tissue. On the contrary, VPNB overexpression lines display an inverted phenotype of increased growth, accompanied by an increased deposition of phloem and xylem cell layers. In line with the above data, qPCR analysis revealed a deregulation of several key master genes of secondary wall biosynthesis, underlining the involvement of VPNB1 in the regulation and differentiation of the root and shoot vascular tissue.

Comparative evaluation of CacyBP/SIP protein, β-catenin, and immunoproteasome subunit LMP7 in the heart of rats with hypertension of different etiology

Calcyclin-binding protein/Siah-1-interacting protein (CacyBP/SIP) is the recently discovered peptide, which participates in various intracellular processes. Recent reports indicated that CacyBP/SIP activates the ubiquitin ligases and promotes proteasomal degradation of proteins. One of the most important proteins degraded in CacyBP/SIP-dependent pathway is β-catenin. Considering the key importance of β-catenin in the functioning of the cardiovascular system and in the view of the close relationship between CacyBP/SIP, β-catenin, and proteasomal activity, we have decided to undertake research to identify and evaluate the distribution of CacyBP/SIP, β-catenin and the LMP7 subunit of the immunoproteasome in the heart of rats with hypertension of various etiology. The studies were carried out on the hearts of rats with spontaneous hypertension (SHR), renovascular hypertension, and DOCA-salt hypertension. The myocardial expression of CacyBP/SIP, β-catenin, and LMP7 was detected by immunohistochemistry using the EnVision method. The hypertension significantly increased the immunoreactivity to CacyBP/SIP and LMP-7, while weakening the β-catenin immunoreaction. The intensity of the observed changes depends on the type of hypertension. Our results show an innovative and important network of interactions between proteins potentially involved in the development and progression of heart problems in various types of hypertension. This report might contribute to deeper understanding of the role of the CacyBP/SIP protein, β-catenin, and immunoproteasomes in heart function, as well as to bringing new information concerning pathophysiologic mechanisms leading to cardiac dysfunction in the state of elevated blood pressure.

Impact statement

Despite extensive research into the pathogenesis of hypertension and disease-related end organ damage, the mechanisms leading to cardiac complications of hypertensive patients are still not fully elucidated. The aim of the presented research was immunodetection and evaluation of CacyBP/SIP, β-catenin, and proteasomes in the hearts of rats with hypertension of different etiology. Our results show an innovative and important network of interactions between proteins potentially involved in the development and progression of heart problems in various types of hypertension. This report might contribute to deeper understanding of the role of the CacyBP/SIP protein, β-catenin, and proteasomes in heart function. Our results might also bring new information concerning the intracellular processes and signal pathways involved in the regulation of cardiomyocytes functioning in hypertension state. In addition to cognitive significance, the results of presented studies may contribute to further successes in preventing and treatment of cardiac complications associated with hypertension.

Riding the waves of the intercalated disc of the heart

Cardiomyocytes interact with each other at their ends through the specialised membrane complex, the intercalated disck (ID). It is a fascinating structure. It allows cardiomyocytes to interact with several neighbouring cells, thereby allowing the complex structure of the heart to develop. It acts as tension transducer, structural prop, and multi signalling domain as well as a regulator of growth. It achieves its many functions through a number of specialised domains and intercellular junctions associated with its complex folded membrane. This review outlines the results of some 20 years of fascination with the ups and downs of the ID. These include locating the spectrin-associated membrane cytoskeleton in the ID and investigating the role of Protein 4.1R in calcium signalling; structural studies of the relationship of the ID to myofibrils, sarcoplasmic reticulum and mitochondria and, finally, consideration of the role of the ID in cardiomyocyte growth and heart disease.

At the heart of inter- and intracellular signaling: the intercalated disc

Proper cardiac function requires the synchronous mechanical and electrical coupling of individual cardiomyocytes. The intercalated disc (ID) mediates coupling of neighboring myocytes through intercellular signaling. Intercellular communication is highly regulated via intracellular signaling, and signaling pathways originating from the ID control cardiomyocyte remodeling and function. Herein, we present an overview of the inter- and intracellular signaling that occurs at and originates from the intercalated disc in normal physiology and pathophysiology. This review highlights the importance of the intercalated disc as an integrator of signaling events regulating homeostasis and stress responses in the heart and the center of several pathophysiological processes mediating the development of cardiomyopathies.

Cardiomyocyte-specific deletion of GSK-3β leads to cardiac dysfunction in a diet induced obesity model

BACKGROUND AND RATIONALE

Obesity, an independent risk factor for the development of myocardial diseases is a growing healthcare problem worldwide. It's well established that GSK-3β is critical to cardiac pathophysiology. However, the role cardiomyocyte (CM) GSK-3β in diet-induced cardiac dysfunction is unknown.

METHODS

CM-specific GSK-3β knockout (CM-GSK-3β-KO) and littermate controls (WT) mice were fed either a control diet (CD) or high-fat diet (HFD) for 55weeks. Cardiac function was assessed by transthoracic echocardiography.

RESULTS

At baseline, body weights and cardiac function were comparable between the WT and CM-GSK-3β-KOs. However, HFD-fed CM-GSK-3β-KO mice developed severe cardiac dysfunction. Consistently, both heart weight/tibia length and lung weight/tibia length were significantly elevated in the HFD-fed CM-GSK-3β-KO mice. The impaired cardiac function and adverse ventricular remodeling in the CM-GSK-3β-KOs were independent of body weight or the lean/fat mass composition as HFD-fed CM-GSK-3β-KO and controls demonstrated comparable body weight and body masses. At the molecular level, on a CD, CM-GSK-3α compensated for the loss of CM-GSK-3β, as evident by significantly reduced GSK-3αs21 phosphorylation (activation) resulting in a preserved canonical β-catenin ubiquitination pathway and cardiac function. However, this protective compensatory mechanism is lost with HFD, leading to excessive accumulation of β-catenin in HFD-fed CM-GSK-3β-KO hearts, resulting in adverse ventricular remodeling and cardiac dysfunction.

CONCLUSION

In summary, these results suggest that cardiac GSK-3β is crucial to protect against obesity-induced adverse ventricular remodeling and cardiac dysfunction.

Activation of nuclear β-catenin/c-Myc axis promotes oxidative stress injury in streptozotocin-induced diabetic cardiomyopathy

Myocardial oxidative stress injury plays a crucial role in the pathogenesis of diabetic cardiomyopathy (DCM). Wnt/β-catenin signaling has been reported to involve in various heart diseases. However, the underlying mechanism associated with β-catenin in DCM remains elusive. This study intended to explore the effect of β-catenin on oxidative damage of DCM by establishing streptozotocin (STZ)-induced diabetic mouse model and hydrogen peroxide (H₂O₂)-treated myocardial cell model. Cardiac oxidative stress in DCM was detected by measurements of lipid peroxidation and anti-oxidative enzyme activities as well as DHE staining. Nuclear β-catenin activity and oxidative damage degree were measured by western blotting, qPCR, MTT assay and TUNEL staining. Cardiac function and morphology were evaluated by echocardiography and histopathology. Under diabetic oxidative stress or H₂O₂ stimulation, nuclear β-catenin accumulation upregulated downstream c-Myc and further facilitated DNA damage and p53-mediated apoptosis as well as cell viability reduction, followed by phenotypic changes of cardiac dysfunction, interstitial fibrosis deposition and myocardial atrophy. Conversely, through directly inhibiting nuclear β-catenin/c-Myc axis, not only did siRNA knockdown of β-catenin or c-Myc attenuate cell injury in H₂O₂-stimulated cardiomyocytes, but also diabetic cardiac-specific β-catenin-knockout mice displayed the same prevention of heart injury as insulin-treated diabetic mice. The present study demonstrated that activated nuclear β-catenin/c-Myc axis was responsible for oxidative cardiac impairment of DCM. Therefore, repressing functional nuclear β-catenin may provide a hopeful therapeutic strategy for DCM.

Disturbed Desmoglein-2 in the intercalated disc of pediatric patients with dilated cardiomyopathy

Dilated cardiomyopathy (DCM) leads to disturbed contraction and force transduction, and is associated with substantial mortality in all age groups. Involvement of a disrupted composition of the intercalated disc (ID) has been reported. However, in children, little is established about such subcellular changes during disease, because of the pathological mix-up with the ongoing cardiac maturation. This leaves maladaptive remodeling often undetected. We aimed at illustrating subcellular alterations in children diagnosed with DCM compared to age-matched controls, focusing on ID proteins known to be crucially stable under healthy conditions and destabilized during cardiac injury in adults. Left ventricular or septal pediatric specimens were collected from 7 individuals diagnosed with DCM (age: 23 weeks in utero to 8 weeks postnatal) and age-matched controls that died of non-cardiovascular cause. We determined the amount of fibrosis and localization of ID proteins by immunohistochemistry. In pediatric DCM, most ID proteins follow similar spatiotemporal changes in localization as in controls. However, although no mutations were found, the signal of the desmosomal protein Desmoglein-2 was reduced in all pediatric DCM specimens, but not in controls or adult DCM patients. Endocardial and transmural fibrosis was increased in all pediatric DCM patients compared to age-matched controls. Composition of the ID in pediatric DCM patients is similar to controls, except for the localization of Desmoglein-2 and presence of severe fibrosis. This suggests that the architecture of desmosomes is already disturbed in the early stages of DCM. These findings contribute to the understanding of pediatric DCM.

Adaptations of cytoarchitecture in human dilated cardiomyopathy

Hypertrophic cardiomyopathy is characterised by a histological phenotype of myocyte disarray, but heart tissue samples from patients with dilated cardiomyopathy (DCM) often look comparatively similar to those from healthy individuals apart from conspicuous regions of fibrosis and necrosis. We have previously investigated subcellular alterations in the cytoarchitecture of mouse models of dilated cardiomyopathy and found that both the organisation and composition of the intercalated disc, i.e. the specialised type of cell-cell contact in the heart, is altered. There is also is a change in the composition of the M-band of the sarcomere due to an expression shift towards the more extensible embryonic heart (EH)-myomesin isoform. Analysis of human samples from the Sydney Human Heart Tissue Bank have revealed similar structural findings and also provided evidence for a dramatic change in overall cardiomyocyte size control, which has also been seen in the mouse. Together these changes in cytoarchitecture probably contribute to the decreased functional output that is seen in DCM.

Overexpression of protein phosphatase 2A in a murine model of chronic myocardial infarction leads to increased adverse remodeling but restores the regulation of β-catenin by glycogen synthase kinase 3β

BACKGROUND/OBJECTIVES

Increased activity of cardiac protein phosphatases is an important feature in human heart failure. Several different protein phosphatases (PP) are involved in the regulation of excitation-contraction-coupling of the myocardium. Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase consisting of a dimeric core enzyme and tissue-specific subunits. In this study we used transgenic mice overexpressing PP2A to further investigate the role of PP2A in cardiac remodeling after myocardial infarction.

METHODS AND RESULTS

Adult male CD-1 mice overexpressing the catalytic subunit α of PP2A (αMHC-PP2A; TG) underwent chronic LAD-ligation or sham surgery, respectively; wildtype littermates (WT) were used as controls. Cardiac function was determined by echocardiography before and 28 days after LAD-ligation. 28 days after MI, the animals were sacrificed and cardiac remodeling was analyzed in histological sections and by Western blots. PP2A overexpression leads to dilated cardiomyopathy in mice, and increased cardiomyocyte hypertrophy and fibrosis of the remote myocardium can be seen after myocardial infarction. However, we found an improved survival of TG in the subacute phase after MI in comparison to WT. On the molecular level, TG shows reduced expression of SERCA and CaMKII alpha both under basal condition as well 28 days after MI. Additionally, the regulation of the Akt/GSK3β/β-catenin pathway is severely disturbed in TG at baseline where a significant activation of Akt is found that coincides with the typical phosphorylation of GSK3β. However, this does not lead to the accumulation of β-catenin - on the contrary: phosphorylation-induced degradation of β-catenin is significantly enhanced.

CONCLUSION

Transgenic overexpression of myocardial PP2A causes adverse remodeling which coincides with a disruption of the classical Akt/GSK3/β-catenin pathway under baseline conditions that is restored to normal values in chronic myocardial infarction. Even so overall survival of TG after myocardial infarction was not constrained and survival after day 2 post MI was improved.

Attenuation of Wnt/β-catenin activity reverses enhanced generation of cardiomyocytes and cardiac defects caused by the loss of emerin

Mutations in EMD, encoding emerin cause skeletal muscle and heart defects in patients with X-linked Emery-Dreifuss muscular dystrophy (X-EDMD) but the underlying mechanisms leading to cardiac defects are poorly understood. Here, we investigated the role of emerin in controlling cardiomyocyte proliferation and cardiac remodeling and explored its function in regulation of the Wnt/β-catenin pathway. We observed a remarkable increase of cardiomyocytes in emerin-null adult mice accompanied with decreased numbers of multinucleated cells. Depletion of emerin in mouse ES cell-derived cardiomyocytes by shRNA caused hyperactivation of Wnt/β-catenin signaling, increased proliferation and abrogated timely cardiac differentiation. Likewise, emerin-null mice exhibited increased Wnt/β-catenin signaling, cardiac dysfunction and perturbed hypertrophic remodeling following pressure overload. Pharmacological inhibition of β-catenin normalized proliferation and differentiation of ES cell-derived cardiomyocytes while inactivation of a single allele of β-catenin efficiently rescued cardiac dysfunction in emerin-null mice. We conclude that emerin constrains β-catenin signaling in the heart providing tight control of cardiomyocyte numbers. Enhanced Wnt/β-catenin signaling seems to contribute to cardiac defects observed in X-EDMD. Hence, therapeutic inhibition of Wnt/β-catenin signaling might be beneficial for X-EDMD patients.

Alterations in cell adhesion proteins and cardiomyopathy

Cell adhesive junction is specialized intercellular structure composed of cell adhesion proteins. They are essential to connect adjacent heart muscle cell and make heart contraction effectively and properly. Clinical and genetic studies have revealed close relationship between cell adhesive proteins and the occurrence of various cardiomyopathies. Here we will review recent development on the disease phenotype, potential cellular and molecular mechanism related to cell adhesion molecules, with particular disease pathogenesis learned from genetic manipulated murine models.

N-cadherin/catenin complex as a master regulator of intercalated disc function

Intercellular adhesive junctions are essential for maintaining the physical integrity of tissues; this is particularly true for the heart that is under constant mechanical load. The correct functionality of the heart is dependent on the electrical and mechanical coordination of its constituent cardiomyocytes. The intercalated disc (ID) structure located at the termini of the rod-shaped adult cardiomyocyte contains various junctional proteins responsible for the integration of structural information and cell-cell communication. According to the classical description, the ID consists of three distinct junctional complexes: adherens junction (AJ), desmosome (Des), and gap junction (GJ) that work together to mediate mechanical and electrical coupling of cardiomyocytes. However, recent morphological and molecular studies indicate that AJ and Des components are capable of mixing together resulting in a "hybrid adhering junction" or "area composita." This review summarizes recent progress in understanding the in vivo function(s) of AJ components in cardiac homeostasis and disease.

Cardiomyocyte growth and sarcomerogenesis at the intercalated disc

Cardiomyocytes grow during heart maturation or disease-related cardiac remodeling. We present evidence that the intercalated disc (ID) is integral to both longitudinal and lateral growth: increases in width are accommodated by lateral extension of the plicate tread regions and increases in length by sarcomere insertion within the ID. At the margin between myofibril and the folded membrane of the ID lies a transitional junction through which the thin filaments from the last sarcomere run to the ID membrane and it has been suggested that this junction acts as a proto Z-disc for sarcomere addition. In support of this hypothesis, we have investigated the ultrastructure of the ID in mouse hearts from control and dilated cardiomyopathy (DCM) models, the MLP-null and a cardiac-specific β-catenin mutant, cΔex3, as well as in human left ventricle from normal and DCM samples. We find that the ID amplitude can vary tenfold from 0.2 μm up to a maximum of ~2 μm allowing gradual expansion during heart growth. At the greatest amplitude, equivalent to a sarcomere length, A-bands and thick filaments are found within the ID membrane loops together with a Z-disc, which develops at the transitional junction position. Here, also, the tops of the membrane folds, which are rich in αII spectrin, become enlarged and associated with junctional sarcoplasmic reticulum. Systematically larger ID amplitudes are found in DCM samples. Other morphological differences between mouse DCM and normal hearts suggest that sarcomere inclusion is compromised in the diseased hearts.

Expression and redistribution of β-catenin in the cardiac myocytes of left ventricle of spontaneously hypertensive rat

Beta-catenin is not only an adhering junction protein, but also the central player of the canonical Wnt signalling pathway. In order to investigate the roles of β-catenin in the mechanism of myocardial hypertrophy, we determined the expression and distribution of β-catenin in the cardiomyocytes of spontaneously hypertensive heart failure (SHHF) rats and age-matched Wistar-Kyoto (WKY) rats. We identified the reducing of β-catenin expression in the membrane protein fraction but increasing in the nuclear protein in the 6 and 12 month-old SHHF rats as compared with the age-matched WKY rats by Western blotting. Immunolabeling of β-catenin colocalized with cadherin at the intercalated disc sites and showed nuclear accumulation in myocytes of SHHF rats. We also revealed that the association between glycogen synthase kinase-3β and β-catenin had weakened in the 6 month-old SHHF rats as compared with the age-matched WKY rats by immunoprecipitation. These findings suggested that nuclear translocation of β-catenin might play important roles in regulating signal transduction in the decompensated hypertrophy stage.

Dapper-1 induces myocardial remodeling through activation of canonical Wnt signaling in cardiomyocytes

Heart failure has an increasing contribution to cardiovascular disease burden and is governed by the myocardial remodeling process. The contribution of Wnt signaling to cardiac remodeling has recently drawn significant attention. Here, we report that upregulation of Dapper-1 in a transgenic mouse model activates the canonical/β-catenin-dependent Wnt pathway through dishevelled-2. These mice exhibited increased heart weight/tibia length ratio, myocyte cross-sectional area, and upregulation of hypertrophic marker genes compared with wild-type mice. Furthermore, impairment of left ventricular systolic and diastolic function was observed in all indicating features of myocardial remodeling. Depletion of Dapper-1 and dishevelled-2 in cardiomyocytes demonstrated that Dapper-1 functions upstream of dishevelled-2 and that activity of both Dapper-1 and dishevelled-2 is essential for activating canonical Wnt signaling. Moreover, Dapper-1 depletion alleviated Wnt3a- and phenylephrine-induced cardiomyocyte hypertrophy. These observations provide evidence that Dapper-1-mediated activation of canonical Wnt signaling is necessary and sufficient to induce cardiomyocyte hypertrophy. Inhibition of this pathway may thus serve as a novel therapeutic strategy for alleviating cardiac hypertrophy.

Breaking down protein degradation mechanisms in cardiac muscle

Regulated protein degradation through the ubiquitin-proteasome and lysosomal/autophagy systems is critical for homeostatic protein turnover in cardiac muscle and for proper cardiac function. The discovery of muscle-specific components in these systems has illuminated how aberrations in their levels are pivotal to the development of cardiac stress and disease. New evidence suggests that equal importance in disease development should be given to ubiquitously expressed degradation components. These are compartmentalized within cardiac muscles and, when mislocalized, can be critical in the development of specific cardiac diseases. Here, we discuss how alterations in the compartmentalization of degradation components affect disease states, the tools available to investigate these mechanisms, as well as recent discoveries that highlight the therapeutic value of targeting these pathways in disease.

Ablation of cyclase-associated protein 2 (CAP2) leads to cardiomyopathy

Cyclase-associated proteins are highly conserved proteins that have a role in the regulation of actin dynamics. Higher eukaryotes have two isoforms, CAP1 and CAP2. To study the in vivo function of CAP2, we generated mice in which the CAP2 gene was inactivated by a gene-trap approach. Mutant mice showed a decrease in body weight and had a decreased survival rate. Further, they developed a severe cardiac defect marked by dilated cardiomyopathy (DCM) associated with drastic reduction in basal heart rate and prolongations in atrial and ventricular conduction times. Moreover, CAP2-deficient myofibrils exhibited reduced cooperativity of calcium-regulated force development. At the microscopic level, we observed disarrayed sarcomeres with development of fibrosis. We analyzed CAP2's role in actin assembly and found that it sequesters G-actin and efficiently fragments filaments. This activity resides completely in its WASP homology domain. Thus CAP2 is an essential component of the myocardial sarcomere and is essential for physiological functioning of the cardiac system, and a deficiency leads to DCM and various cardiac defects.

Histological and ultrastructural abnormalities in murine desmoglein 2-mutant hearts

Mice carrying a deletion of the adhesive extracellular domain of the desmosomal cadherin desmoglein 2 develop an arrhythmogenic right ventricular cardiomyopathylike phenotype with ventricular dilation, fibrosis and arrhythmia. To unravel the sequence of myocardial alterations and to identify potential pathomechanisms, histological analyses were performed on mutant hearts from the juvenile to the adult state, i.e., between 2 and 13 weeks. At an age of 2 weeks 30% of mutants presented lesions,which were visible as white plaques on the heart surface or in the septum. From 4 weeks onwards, all mutants displayed a cardiac phenotype. Dying cardiomyocytes with calcification were found in lesions of all ages. But lesions of young mutant animals contained high amounts of CD45+ immune cells and little collagen fibers, whereas lesions of the older animals were collagen-rich and harbored only a small but still significantly increased number of CD45+ cells. Electron microscopy further showed that distinct desmosomes cannot be distinguished in intercalated discs of mutant hearts. Widening of the intercellular cleft and even complete dissociation of intercalated discs were often observed close to lesions. Disturbed sarcomer structure, altered Z-discs, multiple autophagic vacuoles and swollen mitochondria were other prominent pathological features. Taken together, the following scenario is suggested: mutant desmoglein 2 cannot fully support the increased mechanical requirements placed on intercalated disc adhesion during postnatal heart development, resulting in compromised adhesion and cell stress. This induces cardiomyocyte death, aseptic inflammation and fibrotic replacement. The acute stage of scar formation is followed by permanent impairment of the cardiac function.

Beyond cell adhesion: the role of armadillo proteins in the heart

Plakoglobin (PG, γ-Catenin, JUP), a member of the armadillo protein family and close homolog of β-catenin, functions to link cell surface cadherin molecules with the cytoskeleton. PG is the only junctional component found in both desmosomes and adherens junctions and thus plays a critical role in the regulation of cell-cell adhesion. Similar to β-catenin, PG is able to interact with components of the Wnt signaling pathway and directly affect gene expression by binding with LEF/TCF transcription factors. In addition, it has been proposed that PG functions primarily as a competitive inhibitor of β-catenin transcriptional activity by sequestering LEF/TCF. Compared to β-catenin, the contribution of PG as a transcriptional regulator in either physiological or pathological conditions is poorly understood. There is increasing clinical interest in PG as both a structural protein as well as a signaling molecule as mutations have been identified in the human PG gene that cause Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC) and cutaneous syndromes. This review will discuss the connection between altered cell adhesion and gene expression and its contribution to disease pathogenesis.

Stimulation of HERG channel activity by β-catenin

The multifunctional protein ß-catenin governs as transcription factor the expression of a wide variety of genes relevant for cell proliferation and cell survival. In addition, ß-catenin is localized at the cell membrane and may influence the function of channels. The present study explored the possibility that ß-catenin participates in the regulation of the HERG K⁺ channel. To this end, HERG was expressed in Xenopus oocytes with or without ß-catenin and the voltage-gated current determined utilizing the dual electrode voltage clamp. As a result, expression of ß-catenin markedly upregulated HERG channel activity, an effect not sensitive to inhibition of transcription with actinomycin D (10 µM). According to chemiluminescence, ß-catenin may increase HERG channel abundance within the oocyte cell membrane. Following inhibition of channel insertion into the cell membrane by brefeldin A (5 µM) the decay of current was similar in oocytes expressing HERG together with ß-catenin to oocytes expressing HERG alone. The experiments uncover a novel function of APC/ß-catenin, i.e. the regulation of HERG channels.

Analysis of a Jup hypomorphic allele reveals a critical threshold for postnatal viability

Mutations in the human Jup gene cause arrhythmogenic right ventricular cardiomyopathy (ARVC), a heart muscle disease that often leads to sudden cardiac death. Inactivation of the murine Jup gene (also known as plakoglobin) results in embryonic lethality due to cardiac rupture. In an effort to generate a conditional knockout allele, a neomycin cassette was introduced into the murine plakoglobin (PG) gene. This allele (PG F(N)) functions as a hypomorph when combined with a null allele (PG Δ). About half of the PG F(N)/Δ animals were smaller than their littermates and died before weaning age, whereas the remaining PG F(N)/Δ animals survived. Despite the reduced levels of PG in the heart, there were no signs of cardiomyopathy or cardiac dysfunction as determined by echocardiography. Importantly, the PG homolog, β-catenin (CTNNB1), was increased in the PG F(N)/Δ hearts. In addition to its structural role as part of the N-cadherin/catenin adhesion complex, β-catenin is a downstream effector of Wnt signaling. However, no change in β-catenin/TCF reporter activity was observed in PG F(N)/Δ embryos suggesting that excess β-catenin was not likely causing increased transcription of Wnt/β-catenin target genes. These data suggest novel function(s) for PG beyond the heart and define a critical threshold of PG expression that is necessary for postnatal survival.

Gene Mutations Resulting in the Development of ARVC/D Could Affect Cells of the Cardiac Conduction System

In contrast to epithelial cells, cardiomyocytes are connected by complex hybrid-type adhering junctions, termed composite junctions (areae compositae). Composite junctions are found to be composed of typical desmosomal as well as adherens junction proteins. Therefore, in adult mammalian cardiomyocytes desmosomal proteins are not restricted to the relatively small desmosomes but are indirectly involved in anchoring the myofibrillar actin filaments. Subsequent investigations revealed that the formation of composite junctions is a rather late event during mammalian heart development and vertebrate heart evolution. Nascent, more round shaped cardiomyocytes of early developmental stages are connected by desmosomes and separate adherens junctions quite similar to cells of epithelial origin. During progression of development both types of adhering junctions seem to gradually fuse at the two poles of the mature mammalian cardiomyocytes to establish the hybrid-type composite junctions. Recently, we demonstrated that the specialized cardiomyocytes of the cardiac conduction system exhibit high amounts of desmosomes, not fully established composite junctions and adherens junctions. This underlines the fact that cells of the cardiac conduction system are known to resemble cardiomyocytes in their nascent state and do not undergo working myocardial differentiation. However, the astonishing high amount of desmosomal protein containing adhering junctions connecting, e.g., Purkinje fibers raises the possibility that pacemaker and conductive tissue may be affected by desmosomal gene mutations in ARVC/D patients.

Loss of cadherin-binding proteins β-catenin and plakoglobin in the heart leads to gap junction remodeling and arrhythmogenesis

Arrhythmic right ventricular cardiomyopathy (ARVC) is a hereditary heart muscle disease that causes sudden cardiac death (SCD) in young people. Almost half of ARVC patients have a mutation in genes encoding cell adhesion proteins of the desmosome, including plakoglobin (JUP). We previously reported that cardiac tissue-specific plakoglobin (PG) knockout (PG CKO) mice have no apparent conduction abnormality and survive longer than expected. Importantly, the PG homolog, β-catenin (CTNNB1), showed increased association with the gap junction protein connexin43 (Cx43) in PG CKO hearts. To determine whether β-catenin is required to maintain cardiac conduction in the absence of PG, we generated mice lacking both PG and β-catenin specifically in the heart (i.e., double knockout [DKO]). The DKO mice exhibited cardiomyopathy, fibrous tissue replacement, and conduction abnormalities resulting in SCD. Loss of the cadherin linker proteins resulted in dissolution of the intercalated disc (ICD) structure. Moreover, Cx43-containing gap junction plaques were reduced at the ICD, consistent with the arrhythmogenicity of the DKO hearts. Finally, ambulatory electrocardiogram monitoring captured the abrupt onset of spontaneous lethal ventricular arrhythmia in the DKO mice. In conclusion, these studies demonstrate that the N-cadherin-binding partners, PG and β-catenin, are indispensable for maintaining mechanoelectrical coupling in the heart.

Desmoglein 2 mutant mice develop cardiac fibrosis and dilation

Desmosomes are cell–cell adhesion sites and part of the intercalated discs, which couple adjacent cardiomyocytes. The connection is formed by the extracellular domains of desmosomal cadherins that are also linked to the cytoskeleton on the cytoplasmic side. To examine the contribution of the desmosomal cadherin desmoglein 2 to cardiomyocyte adhesion and cardiac function, mutant mice were prepared lacking a part of the extracellular adhesive domain of desmoglein 2. Most live born mutant mice presented normal overall cardiac morphology at 2 weeks. Some animals, however, displayed extensive fibrotic lesions. Later on, mutants developed ventricular dilation leading to cardiac insufficiency and eventually premature death. Upon histological examination, cardiomyocyte death by calcifying necrosis and replacement by fibrous tissue were observed. Fibrotic lesions were highly proliferative in 2-week-old mutants, whereas the fibrotic lesions of older mutants showed little proliferation indicating the completion of local muscle replacement by scar tissue. Disease progression correlated with increased mRNA expression of c-myc, ANF, BNF, CTGF and GDF15, which are markers for cardiac stress, remodeling and heart failure. Taken together, the desmoglein 2-mutant mice display features of dilative cardiomyopathy and arrhythmogenic right ventricular cardiomyopathy, an inherited human heart disease with pronounced fibrosis and ventricular arrhythmias that has been linked to mutations in desmosomal proteins including desmoglein 2.

EH-myomesin splice isoform is a novel marker for dilated cardiomyopathy

The M-band is the prominent cytoskeletal structure that cross-links the myosin and titin filaments in the middle of the sarcomere. To investigate M-band alterations in heart disease, we analyzed the expression of its main components, proteins of the myomesin family, in mouse and human cardiomyopathy. Cardiac function was assessed by echocardiography and compared to the expression pattern of myomesins evaluated with RT-PCR, Western blot, and immunofluorescent analysis. Disease progression in transgenic mouse models for dilated cardiomyopathy (DCM) was accompanied by specific M-band alterations. The dominant splice isoform in the embryonic heart, EH-myomesin, was strongly up-regulated in the failing heart and correlated with a decrease in cardiac function (R = −0.86). In addition, we have analyzed the expressions of myomesins in human myocardial biopsies (N = 40) obtained from DCM patients, DCM patients supported by a left ventricular assist device (LVAD), hypertrophic cardiomyopathy (HCM) patients and controls. Quantitative RT-PCR revealed that the EH-myomesin isoform was up-regulated 41-fold (P < 0.001) in the DCM patients compared to control patients. In DCM hearts supported by a LVAD and HCM hearts, the EH-myomesin expression was comparable to controls. Immunofluorescent analyses indicate that EH-myomesin was enhanced in a cell-specific manner, leading to a higher heterogeneity of the myocytes’ cytoskeleton through the myocardial wall. We suggest that the up-regulation of EH-myomesin denotes an adaptive remodeling of the sarcomere cytoskeleton in the dilated heart and might serve as a marker for DCM in mouse and human myocardium.