Intercalated disc protein Xinβ is required for Hippo-YAP signaling in the heart

Animal Models and Molecular Pathogenesis of Arrhythmogenic Cardiomyopathy Associated with Pathogenic Variants in Intercalated Disc Genes

Arrhythmogenic cardiomyopathy (ACM) is a rare genetic cardiac disease characterized by the progressive substitution of myocardium with fibro-fatty tissue. Clinically, ACM shows wide variability among patients; symptoms can include syncope and ventricular tachycardia but also sudden death, with the latter often being its sole manifestation. Approximately half of ACM patients have been found with variations in one or more genes encoding cardiac intercalated discs proteins; the most involved genes are plakophilin 2 (PKP2), desmoglein 2 (DSG2), and desmoplakin (DSP). Cardiac intercalated discs provide mechanical and electro-metabolic coupling among cardiomyocytes. Mechanical communication is guaranteed by the interaction of proteins of desmosomes and adheren junctions in the so-called area composita, whereas electro-metabolic coupling between adjacent cardiac cells depends on gap junctions. Although ACM has been first described almost thirty years ago, the pathogenic mechanism(s) leading to its development are still only partially known. Several studies with different animal models point to the involvement of the Wnt/β-catenin signaling in combination with the Hippo pathway. Here, we present an overview about the existing murine models of ACM harboring variants in intercalated disc components with a particular focus on the underlying pathogenic mechanisms. Prospectively, mechanistic insights into the disease pathogenesis will lead to the development of effective targeted therapies for ACM.

A review on decoding the roles of YAP/TAZ signaling pathway in cardiovascular diseases: Bridging molecular mechanisms to therapeutic insights

Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) serve as transcriptional co-activators that dynamically shuttle between the cytoplasm and nucleus, resulting in either the suppression or enhancement of their downstream gene expression. Recent emerging evidence demonstrates that YAP/TAZ is strongly implicated in the pathophysiological processes that contribute to cardiovascular diseases (CVDs). In the cardiovascular system, YAP/TAZ is involved in the orchestration of a range of biological processes such as oxidative stress, inflammation, proliferation, and autophagy. Furthermore, YAP/TAZ has been revealed to be closely associated with the initiation and development of various cardiovascular diseases, including atherosclerosis, pulmonary hypertension, myocardial fibrosis, cardiac hypertrophy, and cardiomyopathy. In this review, we delve into recent studies surrounding YAP and TAZ, along with delineating their roles in contributing to the pathogenesis of CVDs with a link to various physiological processes in the cardiovascular system. Additionally, we highlight the current potential drugs targeting YAP/TAZ for CVDs therapy and discuss their challenges for translational application. Overall, this review may offer novel insights for understanding and treating cardiovascular disorders.

Proteomic and Phosphoproteomic Analysis of Right Ventricular Hypertrophy in the Pulmonary Hypertension Rat Model

Pulmonary arterial hypertension (PAH) is a progressive disease that affects both the lungs and heart. Right ventricle (RV) hypertrophy is a primary pathological feature of PAH; however, its underlying molecular mechanisms remain insufficiently studied. In this study, we employed tandem mass tag (TMT)-based quantitative proteomics for the integrative analysis of the proteome and phosphoproteome of the RV derived from monocrotaline-induced PAH model rats. Compared with control samples, 564 significantly upregulated proteins, 616 downregulated proteins, 622 downregulated phosphopeptides, and 683 upregulated phosphopeptides were identified (P < 0.05, abs (log2 (fold change)) > log2 1.2) in the MCT samples. The quantitative real-time polymerase chain reaction (qRT-PCR) validated the expression levels of top 20 significantly altered proteins, including Nppa (natriuretic peptides A), latent TGF-β binding protein 2 (Ltbp2), periostin, connective tissue growth factor 2 (Ccn2), Ncam1 (neural cell adhesion molecule), quinone reductase 2 (Nqo2), and tropomodulin 4 (Tmod4). Western blotting confirmed the upregulation of Ncam1 and downregulation of Nqo2 and Tmod4 in both MCT-induced and hypoxia-induced PH rat models. Pathway enrichment analyses indicated that the altered proteins are associated with pathways, such as vesicle-mediated transport, actin cytoskeleton organization, TCA cycle, and respiratory electron transport. These significantly changed phosphoproteins were enriched in pathways such as diabetic cardiomyopathy, hypertrophic cardiomyopathy, glycolysis/gluconeogenesis, and cardiac muscle contraction. In summary, this study provides an initial analysis of the RV proteome and phosphoproteome in the progression of PAH, highlighting several RV dysfunction-associated proteins and pathways.

An updated review of YAP: A promising therapeutic target against cardiac aging?

The transcriptional co-activator Yes-associated protein (YAP) functions as a downstream effector of the Hippo signaling pathway and plays a crucial role in cardiomyocyte survival. In its non-phosphorylated activated state, YAP binds to transcription factors, activating the transcription of downstream target genes. It also regulates cell proliferation and survival by selectively binding to enhancers and activating target genes. However, the upregulation of the Hippo pathway in human heart failure inhibits cardiac regeneration and disrupts astrogenesis, thus preventing the nuclear translocation of YAP. Existing literature indicates that the Hippo/YAP axis contributes to inflammation and fibrosis, potentially playing a role in the development of cardiac, vascular and renal injuries. Moreover, it is a key mediator of myofibroblast differentiation and fibrosis in the infarcted heart. Given these insights, can we harness YAP's regenerative potential in a targeted manner? In this review, we provide a detailed discussion of the Hippo signaling pathway and consolidate concepts for the development and intervention of cardiac anti-aging drugs to leverage YAP signaling as a pivotal target.

PITX2 induction leads to impaired cardiomyocyte function in arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an inherited progressive disease characterized by electrophysiological and structural remodeling of the ventricles. However, the disease-causing molecular pathways, as a consequence of desmosomal mutations, are poorly understood. Here, we identified a novel missense mutation within desmoplakin in a patient clinically diagnosed with ACM. Using CRISPR-Cas9, we corrected this mutation in patient-derived human induced pluripotent stem cells (hiPSCs) and generated an independent knockin hiPSC line carrying the same mutation. Mutant cardiomyocytes displayed a decline in connexin 43, NaV1.5, and desmosomal proteins, which was accompanied by a prolonged action potential duration. Interestingly, paired-like homeodomain 2 (PITX2), a transcription factor that acts a repressor of connexin 43, NaV1.5, and desmoplakin, was induced in mutant cardiomyocytes. We validated these results in control cardiomyocytes in which PITX2 was either depleted or overexpressed. Importantly, knockdown of PITX2 in patient-derived cardiomyocytes is sufficient to restore the levels of desmoplakin, connexin 43, and NaV1.5.

The circular RNA circHelz enhances cardiac fibrosis by facilitating the nuclear translocation of YAP1

Cardiac fibrosis is a common pathological change in the development of heart disease. Circular RNA (circRNA) has been shown to be related to the occurrence and development of various cardiovascular diseases. This study aimed to evaluate the effects and potential mechanisms of circHelz in cardiac fibrosis. Knockdown of circHelz alleviated cardiac fibrosis and myocardial fibroblast activation induced by myocardial infarction (MI) or angiotensin II (AngII) in vivo and transforming growth factor-β (TGF-β) in vitro. Overexpression of circHelz exacerbated cell proliferation and differentiation. Mechanistically, nuclear factor of activated T cells, cytoplasmic 2 (NFATc2) was found to act as a transcriptional activator to upregulate the expression of circHelz. The increased circHelz was demonstrated to bind to Yes-associated protein (YAP) and facilitate its localization in the nucleus to promote cell proliferation and growth. Moreover, silencing YAP1 reversed the detrimental effects caused by circHelz in vitro, as indicated by the observed decreases in cell viability, fibrotic marker expression levels, proliferation and migration. Collectively, the protective effect of circHelz knockdown against cardiac fibrosis injury is accomplished by inhibiting the nuclear translocation of YAP1. Thus, circHelz may be a novel target for the prevention and treatment of cardiovascular disease.

The role of the Hippo pathway in autophagy in the heart

The Hippo pathway, an evolutionarily conserved signalling mechanism, controls organ size and tumourigenesis. Increasing lines of evidence suggest that autophagy, an important mechanism of lysosome-mediated cellular degradation, is regulated by the Hippo pathway, which thereby profoundly affects cell growth and death responses in various cell types. In the heart, Mst1, an upstream component of the Hippo pathway, not only induces apoptosis but also inhibits autophagy through phosphorylation of Beclin 1. YAP/TAZ, transcription factor co-factors and the terminal effectors of the Hippo pathway, affect autophagy through transcriptional activation of TFEB, a master regulator of autophagy and lysosomal biogenesis. The cellular abundance of YAP is negatively regulated by autophagy and suppression of autophagy induces accumulation of YAP, which, in turn, acts as a feedback mechanism to induce autophagosome formation. Thus, the Hippo pathway and autophagy regulate each other, thereby profoundly affecting cardiomyocyte survival and death. This review discusses the interaction between the Hippo pathway and autophagy and its functional significance during stress conditions in the heart and the cardiomyocytes therein.

Extracellular Non-Coding RNAs in Cardiovascular Diseases

Cardiovascular diseases (CVDs) remain the world's leading cause of death despite the best available healthcare and therapy. Emerging as a key mediator of intercellular and inter-organ communication in CVD pathogenesis, extracellular vesicles (EVs) are a heterogeneous group of membrane-enclosed nano-sized vesicles released by virtually all cells, of which their RNA cargo, especially non-coding RNAs (ncRNA), has been increasingly recognized as a promising diagnostic and therapeutic target. Recent evidence shows that ncRNAs, such as small ncRNAs, circular RNAs, and long ncRNAs, can be selectively sorted into EVs or other non-vesicular carriers and modulate various biological processes in recipient cells. In this review, we summarize recent advances in the literature regarding the origin, extracellular carrier, and functional mechanisms of extracellular ncRNAs with a focus on small ncRNAs, circular RNAs, and long ncRNAs. The pathophysiological roles of extracellular ncRNAs in various CVDs, including atherosclerosis, ischemic heart diseases, hypertension, cardiac hypertrophy, and heart failure, are extensively discussed. We also provide an update on recent developments and challenges in using extracellular ncRNAs as biomarkers or therapeutical targets in these CVDs.

Single-cell and spatial transcriptomics of the infarcted heart define the dynamic onset of the border zone in response to mechanical destabilization

The border zone (BZ) of the infarcted heart is a geographically complex and biologically enigmatic interface separating poorly perfused infarct zones (IZs) from remote zones (RZs). The cellular and molecular mechanisms of myocardial BZs are not well understood because microdissection inevitably combines them with uncontrolled amounts of RZs and IZs. Here, we use single-cell/nucleus RNA sequencing, spatial transcriptomics and multiplexed RNA fluorescence in situ hybridization to redefine the BZ based on cardiomyocyte transcriptomes. BZ1 (Nppa + Xirp2 -) forms a hundreds-of-micrometer-thick layer of morphologically intact cells adjacent to RZs that are detectable within an hour of injury. Meanwhile, BZ2 (Nppa + Xirp2 +) forms a near-single-cell-thick layer of morphologically distorted cardiomyocytes at the IZ edge that colocalize with matricellular protein-expressing myofibroblasts and express predominantly mechanotransduction genes. Surprisingly, mechanical injury alone is sufficient to induce BZ genes. We propose a 'loss of neighbor' hypothesis to explain how ischemic cell death mechanically destabilizes the BZ to induce its transcriptional response.

The Hippo-YAP pathway in various cardiovascular diseases: Focusing on the inflammatory response

The Hippo pathway was initially discovered in Drosophila melanogaster and mammals as a key regulator of tissue growth both in physiological and pathological states. Numerous studies depict the vital role of the Hippo pathway in cardiovascular development, heart regeneration, organ size and vascular remodeling through the regulation of YAP (yes-associated protein) translocation. Recently, an increasing number of studies have focused on the Hippo-YAP pathway in inflammation and immunology. Although the Hippo-YAP pathway has been revealed to play controversial roles in different contexts and cell types in the cardiovascular system, the mechanisms regulating tissue inflammation and the immune response remain to be clarified. In this review, we summarize findings from the past decade on the function and mechanism of the Hippo-YAP pathway in CVDs (cardiovascular diseases) such as myocardial infarction, cardiomyopathy and atherosclerosis. In particular, we emphasize the role of the Hippo-YAP pathway in regulating inflammatory cell infiltration and inflammatory cytokine activation.

Structure and regulation of desmosomes in intercalated discs: Lessons from epithelia

For electromechanical coupling of cardiomyocytes, intercalated discs (ICDs) are pivotal as highly specialized intercellular contact areas. ICD consists of adhesive contacts, such as desmosomes and adherens junctions (AJs) that are partially intermingled and thereby form an area composita to provide mechanical strength, as well as gap junctions (GJ) and sodium channels for excitation propagation. In contrast, in epithelia, mixed junctions with features of desmosomes and AJs are regarded as transitory primarily during the formation of desmosomes. The anatomy of desmosomes is defined by a typical ultrastructure with dense intracellular plaques anchoring the cadherin-type adhesion molecules to the intermediate filament cytoskeleton. Desmosomal diseases characterized by impaired adhesive and signalling functions of desmosomal contacts lead to arrhythmogenic cardiomyopathy when affecting cardiomyocytes and cause pemphigus when manifesting in keratinocytes or present as cardiocutaneous syndromes when both cell types are targeted by the disease, which underscores the high biomedical relevance of these cell contacts. Therefore, comparative analyses regarding the structure and regulation of desmosomal contacts in cardiomyocytes and epithelial cells are helpful to better understand disease pathogenesis. In this brief review, we describe the structural properties of ICD compared to epithelial desmosomes and suggest that mechanisms regulating adhesion may at least in part be comparable. Also, we discuss whether phenomena such as hyperadhesion or the bidirectional regulation of desmosomes to serve as signalling hubs in epithelial cells may also be relevant for ICD.

lncExACT1 and DCHS2 Regulate Physiological and Pathological Cardiac Growth

BACKGROUND

The heart grows in response to pathological and physiological stimuli. The former often precedes cardiomyocyte loss and heart failure; the latter paradoxically protects the heart and enhances cardiomyogenesis. The mechanisms underlying these differences remain incompletely understood. While long noncoding RNAs (lncRNAs) are important in cardiac development and disease, less is known about their roles in physiological hypertrophy or cardiomyogenesis.

METHODS

RNA sequencing was applied to hearts from mice after eight weeks voluntary exercise-induced physiological hypertrophy and cardiomyogenesis or transverse aortic constriction (TAC) for two or eight weeks to induce pathological hypertrophy or heart failure. The top lncRNA candidate was overexpressed in hearts with adeno-associated virus (AAV) vectors and inhibited with antisense locked nucleic acid (LNA)-GapmeRs to examine its function. Downstream effectors were identified through promoter analyses and binding assays. The functional roles of a novel downstream effector, dachsous cadherin-related 2 (DCHS2), were examined through transgenic overexpression in zebrafish and cardiac-specific deletion in Cas9-knockin mice.

RESULTS

We identified exercise-regulated cardiac lncRNAs, termed lncExACTs. lncExACT1 was evolutionarily conserved and decreased in exercised hearts but increased in human and experimental heart failure. Cardiac lncExACT1 overexpression caused pathological hypertrophy and heart failure, while lncExACT1 inhibition induced physiological hypertrophy and cardiomyogenesis, protecting against cardiac fibrosis and dysfunction. lncExACT1 functioned by regulating microRNA-222, calcineurin signaling, and Hippo/Yap1 signaling through DCHS2. Cardiomyocyte DCHS2 overexpression in zebrafish induced pathological hypertrophy and impaired cardiac regeneration, promoting scarring after injury. In contrast, murine DCHS2 deletion induced physiological hypertrophy and promoted cardiomyogenesis.

CONCLUSIONS

These studies identify lncExACT1-DCHS2 as a novel pathway regulating cardiac hypertrophy and cardiomyogenesis. lncExACT1-DCHS2 acts as a master switch toggling the heart between physiological and pathological growth to determine functional outcomes, providing a potentially tractable therapeutic target for harnessing the beneficial effects of exercise.

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.

Biallelic FRA10AC1 variants cause a neurodevelopmental disorder with growth retardation

The major spliceosome mediates pre-mRNA splicing by recognizing the highly conserved sequences at the 5' and 3' splice sites and the branch point. More than 150 proteins participate in the splicing process and are organized in the spliceosomal A, B, and C complexes. FRA10AC1 is a peripheral protein of the spliceosomal C complex and its ortholog in the green alga facilitates recognition or interaction with splice sites. We identified biallelic pathogenic variants in FRA10AC1 in five individuals from three consanguineous families. The two unrelated patients 1 and 2 with loss-of-function variants showed developmental delay, intellectual disability, and no speech, while three siblings with the c.494_496delAAG (p.Glu165del) variant had borderline to mild intellectual disability. All patients had microcephaly, hypoplasia or agenesis of the corpus callosum, growth retardation, and craniofacial dysmorphism. FRA10AC1 transcripts and proteins were drastically reduced or absent in fibroblasts of patients 1 and 2. In a heterologous expression system, the p. Glu165del variant impacts intrinsic stability of FRA10AC1 but does not affect its nuclear localization. By co-immunoprecipitation, we found ectopically expressed HA-FRA10AC1 in complex with endogenous DGCR14, another component of the spliceosomal C complex, while the splice factors CHERP, NKAP, RED, and SF3B2 could not be co-immunoprecipitated. Using an in vitro splicing reporter assay, we did not obtain evidence for FRA10AC1 deficiency to suppress missplicing events caused by mutations in the highly conserved dinucleotides of 5' and 3' splice sites in an in vitro splicing assay in patient-derived fibroblasts. Our data highlight the importance of specific peripheral spliceosomal C complex proteins for neurodevelopment. It remains possible that FRA10AC1 may have other and/or additional cellular functions, such as coupling of transcription and splicing reactions.

Targeting the Hippo pathway in heart repair

The Hippo pathway is an evolutionarily and functionally conserved signaling pathway that controls organ size by regulating cell proliferation, apoptosis, and differentiation. Emerging evidence has shown that the Hippo pathway plays critical roles in cardiac development, homeostasis, disease, and regeneration. Targeting the Hippo pathway has tremendous potential as a therapeutic strategy for treating intractable cardiovascular diseases such as heart failure. In this review, we summarize the function of the Hippo pathway in the heart. Particularly, we highlight the posttranslational modification of Hippo pathway components, including the core kinases LATS1/2 and their downstream effectors YAP/TAZ, in different contexts, which has provided new insights and avenues in cardiac research.

YAP/TEAD1 Complex Is a Default Repressor of Cardiac Toll-Like Receptor Genes

Toll-like receptors (TLRs) are a family of pattern recognition receptors (PRRs) that modulate innate immune responses and play essential roles in the pathogenesis of heart diseases. Although important, the molecular mechanisms controlling cardiac TLR genes expression have not been clearly addressed. This study examined the expression pattern of Tlr1, Tlr2, Tlr3, Tlr4, Tlr5, Tlr6, Tlr7, Tlr8, and Tlr9 in normal and disease-stressed mouse hearts. Our results demonstrated that the expression levels of cardiac Tlr3, Tlr7, Tlr8, and Tlr9 increased with age between neonatal and adult developmental stages, whereas the expression of Tlr5 decreased with age. Furthermore, pathological stress increased the expression levels of Tlr2, Tlr4, Tlr5, Tlr7, Tlr8, and Tlr9. Hippo-YAP signaling is essential for heart development and homeostasis maintenance, and YAP/TEAD1 complex is the terminal effector of this pathway. Here we found that TEAD1 directly bound genomic regions adjacent to Tlr1, Tlr2, Tlr3, Tlr4, Tlr5, Tlr6, Tlr7, and Tlr9. In vitro, luciferase reporter data suggest that YAP/TEAD1 repression of Tlr4 depends on a conserved TEAD1 binding motif near Tlr4 transcription start site. In vivo, cardiomyocyte-specific YAP depletion increased the expression of most examined TLR genes, activated the synthesis of pro-inflammatory cytokines, and predisposed the heart to lipopolysaccharide stress. In conclusion, our data indicate that the expression of cardiac TLR genes is associated with age and activated by pathological stress and suggest that YAP/TEAD1 complex is a default repressor of cardiac TLR genes.