Plakoglobin has both structural and signalling roles in zebrafish development

Molecular Pathways and Animal Models of Cardiomyopathies

Cardiomyopathies are a heterogeneous group of disorders of the heart muscle that ultimately result in congestive heart failure. Rapid progress in genetics, molecular and cellular biology with breakthrough innovative genetic-engineering techniques, such as next-generation sequencing and multiomics platforms, stem cell reprogramming, as well as novel groundbreaking gene-editing systems over the past 25 years has greatly improved the understanding of pathogenic signaling pathways in inherited cardiomyopathies. This chapter will focus on intracellular and intercellular molecular signaling pathways that are activated by a genetic insult in cardiomyocytes to maintain tissue and organ level regulation and resultant cardiac remodeling in certain forms of cardiomyopathies. In addition, animal models of different clinical forms of human cardiomyopathies with their summaries of triggered key molecules and signaling pathways will be described.

A novel DSP zebrafish model reveals training- and drug-induced modulation of arrhythmogenic cardiomyopathy phenotypes

Arrhythmogenic cardiomyopathy (AC) is an inherited disorder characterized by progressive loss of the ventricular myocardium causing life-threatening ventricular arrhythmias, syncope and sudden cardiac death in young and athletes. About 40% of AC cases carry one or more mutations in genes encoding for desmosomal proteins, including Desmoplakin (Dsp). We present here the first stable Dsp knock-out (KO) zebrafish line able to model cardiac alterations and cell signalling dysregulation, characteristic of the AC disease, on which environmental factors and candidate drugs can be tested. Our stable Dsp knock-out (KO) zebrafish line was characterized by cardiac alterations, oedema and bradycardia at larval stages. Histological analysis of mutated adult hearts showed reduced contractile structures and abnormal shape of the ventricle, with thinning of the myocardial layer, vessels dilation and presence of adipocytes within the myocardium. Moreover, TEM analysis revealed "pale", disorganized and delocalized desmosomes. Intensive physical training protocol caused a global worsening of the cardiac phenotype, accelerating the progression of the disease. Of note, we detected a decrease of Wnt/β-catenin signalling, recently associated with AC pathogenesis, as well as Hippo/YAP-TAZ and TGF-β pathway dysregulation. Pharmacological treatment of mutated larvae with SB216763, a Wnt/β-catenin agonist, rescued pathway expression and cardiac abnormalities, stabilizing the heart rhythm. Overall, our Dsp KO zebrafish line recapitulates many AC features observed in human patients, pointing at zebrafish as a suitable system for in vivo analysis of environmental modulators, such as the physical exercise, and the screening of pathway-targeted drugs, especially related to the Wnt/β-catenin signalling cascade.

Desmosomes in Cell Fate Determination: From Cardiogenesis to Cardiomyopathy

Desmosomes play a vital role in providing structural integrity to tissues that experience significant mechanical tension, including the heart. Deficiencies in desmosomal proteins lead to the development of arrhythmogenic cardiomyopathy (AC). The limited availability of preventative measures in clinical settings underscores the pressing need to gain a comprehensive understanding of desmosomal proteins not only in cardiomyocytes but also in non-myocyte residents of the heart, as they actively contribute to the progression of cardiomyopathy. This review focuses specifically on the impact of desmosome deficiency on epi- and endocardial cells. We highlight the intricate cross-talk between desmosomal proteins mutations and signaling pathways involved in the regulation of epicardial cell fate transition. We further emphasize that the consequences of desmosome deficiency differ between the embryonic and adult heart leading to enhanced erythropoiesis during heart development and enhanced fibrogenesis in the mature heart. We suggest that triggering epi-/endocardial cells and fibroblasts that are in different "states" involve the same pathways but lead to different pathological outcomes. Understanding the details of the different responses must be considered when developing interventions and therapeutic strategies.

Using Zebrafish Animal Model to Study the Genetic Underpinning and Mechanism of Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is largely an autosomal dominant genetic disorder manifesting fibrofatty infiltration and ventricular arrhythmia with predominantly right ventricular involvement. ACM is one of the major conditions associated with an increased risk of sudden cardiac death, most notably in young individuals and athletes. ACM has strong genetic determinants, and genetic variants in more than 25 genes have been identified to be associated with ACM, accounting for approximately 60% of ACM cases. Genetic studies of ACM in vertebrate animal models such as zebrafish (Danio rerio), which are highly amenable to large-scale genetic and drug screenings, offer unique opportunities to identify and functionally assess new genetic variants associated with ACM and to dissect the underlying molecular and cellular mechanisms at the whole-organism level. Here, we summarize key genes implicated in ACM. We discuss the use of zebrafish models, categorized according to gene manipulation approaches, such as gene knockdown, gene knock-out, transgenic overexpression, and CRISPR/Cas9-mediated knock-in, to study the genetic underpinning and mechanism of ACM. Information gained from genetic and pharmacogenomic studies in such animal models can not only increase our understanding of the pathophysiology of disease progression, but also guide disease diagnosis, prognosis, and the development of innovative therapeutic strategies.

Crosstalk Between β-CATENIN-Mediated Cell Adhesion and the WNT Signaling Pathway

Cell adhesion and stable signaling regulation are fundamental ways of maintaining homeostasis. Among them, the Wnt/β-CATENIN signaling plays a key role in embryonic development and maintenance of body dynamic homeostasis. At the same time, the key signaling molecule β-CATENIN in the Wnt signaling can also function as a cytoskeletal linker protein to regulate tissue barriers, cell migration, and morphogenesis. Dysregulation of the balance between Wnt signaling and adherens junctions can lead to disease. How β-CATENIN maintains the independence of these two functions, or mediates the interaction and balance of these two functions, has been explored and debated for a long time. In this study, we will focus on five aspects of β-CATENIN chaperone molecules, phosphorylation of β-CATENIN and related proteins, epithelial mesenchymal transition, β-CATENIN homolog protein γ-CATENIN and disease, thus deepening the understanding of the Wnt/β-CATENIN signaling and the homeostasis between cell adhesion and further addressing related disease problems.

Animal Models to Study Cardiac Arrhythmias

Cardiac arrhythmias are a significant cause of morbidity and mortality worldwide, accounting for 10% to 15% of all deaths. Although most arrhythmias are due to acquired heart disease, inherited channelopathies and cardiomyopathies disproportionately affect children and young adults. Arrhythmogenesis is complex, involving anatomic structure, ion channels and regulatory proteins, and the interplay between cells in the conduction system, cardiomyocytes, fibroblasts, and the immune system. Animal models of arrhythmia are powerful tools for studying not only molecular and cellular mechanism of arrhythmogenesis but also more complex mechanisms at the whole heart level, and for testing therapeutic interventions. This review summarizes basic and clinical arrhythmia mechanisms followed by an in-depth review of published animal models of genetic and acquired arrhythmia disorders.

Guidance by followers ensures long-range coordination of cell migration through α-catenin mechanoperception

Morphogenesis, wound healing, and some cancer metastases depend upon the migration of cell collectives that need to be guided to their destination as well as coordinated with other cell movements. During zebrafish gastrulation, the extension of the embryonic axis is led by the mesendodermal polster that migrates toward the animal pole, followed by the axial mesoderm that undergoes convergence and extension. Here, we investigate how polster cells are guided toward the animal pole. Using a combination of precise laser ablations, advanced transplants, and functional as well as in silico approaches, we establish that each polster cell is oriented by its immediate follower cells. Each cell perceives the migration of followers, through E-cadherin/α-catenin mechanotransduction, and aligns with them. Therefore, directional information propagates from cell to cell over the whole tissue. Such guidance of migrating cells by followers ensures long-range coordination of movements and developmental robustness.

Inherited Ventricular Arrhythmia in Zebrafish: Genetic Models and Phenotyping Tools

In the last years, the field of inheritable ventricular arrhythmia disease modelling has changed significantly with a push towards the use of novel cellular cardiomyocyte based models. However, there is a growing need for new in vivo models to study the disease pathology at the tissue and organ level. Zebrafish provide an excellent opportunity for in vivo modelling of inheritable ventricular arrhythmia syndromes due to the remarkable similarity between their cardiac electrophysiology and that of humans. Additionally, many state-of-the-art methods in gene editing and electrophysiological phenotyping are available for zebrafish research. In this review, we give a comprehensive overview of the published zebrafish genetic models for primary electrical disorders and arrhythmogenic cardiomyopathy. We summarise and discuss the strengths and weaknesses of the different technical approaches for the generation of genetically modified zebrafish disease models, as well as the electrophysiological approaches in zebrafish phenotyping. By providing this detailed overview, we aim to draw attention to the potential of the zebrafish model for studying arrhythmia syndromes at the organ level and as a platform for personalised medicine and drug testing.

Mechanotransduction and Adrenergic Stimulation in Arrhythmogenic Cardiomyopathy: An Overview of in vitro and in vivo Models

Arrhythmogenic Cardiomyopathy (AC) is a rare inherited heart disease, manifesting with progressive myocardium degeneration and dysfunction, and life-threatening arrhythmic events that lead to sudden cardiac death. Despite genetic determinants, most of AC patients admitted to hospital are athletes or very physically active people, implying the existence of other disease-causing factors. It is recognized that AC phenotypes are enhanced and triggered by strenuous physical activity, while excessive mechanical stretch and load, and repetitive adrenergic stimulation are mechanisms influencing disease penetrance. Different approaches have been undertaken to recapitulate and study both mechanotransduction and adrenergic signaling in AC, including the use of in vitro cellular and tissue models, and the development of in vivo models (particularly rodents but more recently also zebrafish). However, it remains challenging to reproduce mechanical load stimuli and physical activity in laboratory experimental settings. Thus, more work to drive the innovation of advanced AC models is needed to recapitulate these subtle physiological influences. Here, we review the state-of-the-art in this field both in clinical and laboratory-based modeling scenarios. Specific attention will be focused on highlighting gaps in the knowledge and how they may be resolved by utilizing novel research methodology.

Genetic Animal Models for Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy has been clinically defined since the 1980s and causes right or biventricular cardiomyopathy associated with ventricular arrhythmia. Although it is a rare cardiac disease, it is responsible for a significant proportion of sudden cardiac deaths, especially in athletes. The majority of patients with arrhythmogenic cardiomyopathy carry one or more genetic variants in desmosomal genes. In the 1990s, several knockout mouse models of genes encoding for desmosomal proteins involved in cell-cell adhesion revealed for the first time embryonic lethality due to cardiac defects. Influenced by these initial discoveries in mice, arrhythmogenic cardiomyopathy received an increasing interest in human cardiovascular genetics, leading to the discovery of mutations initially in desmosomal genes and later on in more than 25 different genes. Of note, even in the clinic, routine genetic diagnostics are important for risk prediction of patients and their relatives with arrhythmogenic cardiomyopathy. Based on improvements in genetic animal engineering, different transgenic, knock-in, or cardiac-specific knockout animal models for desmosomal and nondesmosomal proteins have been generated, leading to important discoveries in this field. Here, we present an overview about the existing animal models of arrhythmogenic cardiomyopathy with a focus on the underlying pathomechanism and its importance for understanding of this disease. Prospectively, novel mechanistic insights gained from the whole animal, organ, tissue, cellular, and molecular levels will lead to the development of efficient personalized therapies for treatment of arrhythmogenic cardiomyopathy.

Loss of cardiac Wnt/β-catenin signalling in desmoplakin-deficient AC8 zebrafish models is rescuable by genetic and pharmacological intervention

Aims

Arrhythmogenic cardiomyopathy (AC) is an inherited heart disease characterized by life-threatening ventricular arrhythmias and fibro-fatty replacement of the myocardium. More than 60% of AC patients show pathogenic mutations in genes encoding for desmosomal proteins. By focusing our attention on the AC8 form, linked to the junctional protein desmoplakin (DSP), we present here a zebrafish model of DSP deficiency, exploited to identify early changes of cell signalling in the cardiac region.

Methods and results

To obtain an embryonic model of Dsp deficiency, we first confirmed the orthologous correspondence of zebrafish Dsp genes (dspa and dspb) to the human DSP counterpart. Then, we verified their cardiac expression, at embryonic and adult stages, and subsequently we targeted them by antisense morpholino strategy, confirming specific and disruptive effects on desmosomes, like those identified in AC patients. Finally, we exploited our Dsp-deficient models for an in vivo cell signalling screen, using pathway-specific reporter transgenes. Out of nine considered, three pathways (Wnt/β-catenin, TGFβ/Smad3, and Hippo/YAP-TAZ) were significantly altered, with Wnt as the most dramatically affected. Interestingly, under persistent Dsp deficiency, Wnt signalling is rescuable both by a genetic and a pharmacological approach.

Conclusion

Our data point to Wnt/β-catenin as the final common pathway underlying different desmosomal AC forms and support the zebrafish as a suitable model for detecting early signalling pathways involved in the pathogenesis of DSP-associated diseases, possibly responsive to pharmacological or genetic rescue.

Cell volume changes contribute to epithelial morphogenesis in zebrafish Kupffer's vesicle

How epithelial cell behaviors are coordinately regulated to sculpt tissue architecture is a fundamental question in biology. Kupffer’s vesicle (KV), a transient organ with a fluid-filled lumen, provides a simple system to investigate the interplay between intrinsic cellular mechanisms and external forces during epithelial morphogenesis. Using 3-dimensional (3D) analyses of single cells we identify asymmetric cell volume changes along the anteroposterior axis of KV that coincide with asymmetric cell shape changes. Blocking ion flux prevents these cell volume changes and cell shape changes. Vertex simulations suggest cell shape changes do not depend on lumen expansion. Consistent with this prediction, asymmetric changes in KV cell volume and shape occur normally when KV lumen growth fails due to leaky cell adhesions. These results indicate ion flux mediates cell volume changes that contribute to asymmetric cell shape changes in KV, and that these changes in epithelial morphology are separable from lumen-generated forces.

The canonical way to make a heart: β-catenin and plakoglobin in heart development and remodeling

The main mediator of the canonical Wnt pathway, β-catenin, is a major effector of embryonic development, postnatal tissue homeostasis, and adult tissue regeneration. The requirement for β-catenin in cardiogenesis and embryogenesis has been well established. However, many questions regarding the molecular mechanisms by which β-catenin and canonical Wnt signaling regulate these developmental processes remain unanswered. An interesting question that emerged from our studies concerns how β-catenin signaling is modulated through interaction with other factors. Recent experimental data implicate new players in canonical Wnt signaling, particularly those which modulate β-catenin function in many its biological processes, including cardiogenesis. One of the interesting candidates is plakoglobin, a little-studied member of the catenin family which shares several mechanistic and functional features with its close relative, β-catenin. Here we have focused on the function of β-catenin in cardiogenesis. We also summarize findings on plakoglobin signaling function and discuss possible interplays between β-catenin and plakoglobin in the regulation of embryonic heart development. Impact statement Heart development, function, and remodeling are complex processes orchestrated by multiple signaling networks. This review examines our current knowledge of the role of canonical Wnt signaling in cardiogenesis and heart remodeling, focusing primarily on the mechanistic action of its effector β-catenin. We summarize the generally accepted understanding of the field based on experimental in vitro and in vivo data, and address unresolved questions in the field, specifically relating to the role of canonical Wnt signaling in heart maturation and regeneration. What are the modulators of canonical Wnt, and particularly what are the potential roles of plakoglobin, a close relative of β-catenin, in regulating Wnt signaling?Answers to these questions will enhance our understanding of the mechanism by which the canonical Wnt signaling regulates development of the heart and its regeneration after damage.

Animal models of arrhythmogenic right ventricular cardiomyopathy: what have we learned and where do we go? Insight for therapeutics

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a rare genetically-determined cardiac heart muscle disorder characterized by fibro-fatty replacement of the myocardium that results in heart failure and sudden cardiac death (SCD), predominantly in young males. The disease is often caused by mutations in genes encoding proteins of the desmosomal complex, with a significant minority caused by mutations in non-desmosomal proteins. Existing treatment options are based on SCD prevention with the implantable cardioverter defibrillator, antiarrhythmic drugs, and anti-heart failure medication. Heart transplantation may also be required and there is currently no cure. Several genetically modified animal models have been developed to characterize the disease, assess its progression, and determine the influence of potential environmental factors. These models have also been very valuable for translational therapeutic approaches, to screen new treatment options that prevent and/or reverse the disease. Here, we review the available ARVC animal models reported to date, highlighting the most important pathophysiological findings and discussing the effect of treatments tested so far in this setting. We also describe gaps in our knowledge of the disease, with the goal of stimulating research and improving patient outcomes.

LITTLE FISH, BIG DATA: ZEBRAFISH AS A MODEL FOR CARDIOVASCULAR AND METABOLIC DISEASE

The burden of cardiovascular and metabolic diseases worldwide is staggering. The emergence of systems approaches in biology promises new therapies, faster and cheaper diagnostics, and personalized medicine. However, a profound understanding of pathogenic mechanisms at the cellular and molecular levels remains a fundamental requirement for discovery and therapeutics. Animal models of human disease are cornerstones of drug discovery as they allow identification of novel pharmacological targets by linking gene function with pathogenesis. The zebrafish model has been used for decades to study development and pathophysiology. More than ever, the specific strengths of the zebrafish model make it a prime partner in an age of discovery transformed by big-data approaches to genomics and disease. Zebrafish share a largely conserved physiology and anatomy with mammals. They allow a wide range of genetic manipulations, including the latest genome engineering approaches. They can be bred and studied with remarkable speed, enabling a range of large-scale phenotypic screens. Finally, zebrafish demonstrate an impressive regenerative capacity scientists hope to unlock in humans. Here, we provide a comprehensive guide on applications of zebrafish to investigate cardiovascular and metabolic diseases. We delineate advantages and limitations of zebrafish models of human disease and summarize their most significant contributions to understanding disease progression to date.

Beta-Catenin and Plakoglobin Expression during Zebrafish Tooth Development and Replacement

We analyzed the protein distribution of two cadherin-associated molecules, plakoglobin and β-catenin, during the different stages of tooth development and tooth replacement in zebrafish. Plakoglobin was detected at the plasma membrane already at the onset of tooth development in the epithelial cells of the tooth. This pattern remained unaltered during further tooth development. The mesenchymal cells only showed plakoglobin from cytodifferentiation onwards. Plakoglobin 1a morpholino-injected embryos showed normal tooth development with proper initiation and differentiation. Although plakoglobin is clearly present during normal odontogenesis, the loss of plakoglobin 1a does not influence tooth development. β-catenin was found at the cell borders of all cells of the successional lamina but also in the nuclei of surrounding mesenchymal cells. Only membranous, not nuclear, β-catenin, was found during morphogenesis stage. However, during cytodifferentiation stage, both nuclear and membrane-bound β-catenin was detected in the layers of the enamel organ as well as in the differentiating odontoblasts. Nuclear β-catenin is an indication of an activated Wnt pathway, therefore suggesting a possible role for Wnt signalling during zebrafish tooth development and replacement.

The V-ATPase accessory protein Atp6ap1b mediates dorsal forerunner cell proliferation and left-right asymmetry in zebrafish

Asymmetric fluid flows generated by motile cilia in a transient 'organ of asymmetry' are involved in establishing the left-right (LR) body axis during embryonic development. The vacuolar-type H(+)-ATPase (V-ATPase) proton pump has been identified as an early factor in the LR pathway that functions prior to cilia, but the role(s) for V-ATPase activity are not fully understood. In the zebrafish embryo, the V-ATPase accessory protein Atp6ap1b is maternally supplied and expressed in dorsal forerunner cells (DFCs) that give rise to the ciliated organ of asymmetry called Kupffer's vesicle (KV). V-ATPase accessory proteins modulate V-ATPase activity, but little is known about their functions in development. We investigated Atp6ap1b and V-ATPase in KV development using morpholinos, mutants and pharmacological inhibitors. Depletion of both maternal and zygotic atp6ap1b expression reduced KV organ size, altered cilia length and disrupted LR patterning of the embryo. Defects in other ciliated structures-neuromasts and olfactory placodes-suggested a broad role for Atp6ap1b during development of ciliated organs. V-ATPase inhibitor treatments reduced KV size and identified a window of development in which V-ATPase activity is required for proper LR asymmetry. Interfering with Atp6ap1b or V-ATPase function reduced the rate of DFC proliferation, which resulted in fewer ciliated cells incorporating into the KV organ. Analyses of pH and subcellular V-ATPase localizations suggested Atp6ap1b functions to localize the V-ATPase to the plasma membrane where it regulates proton flux and cytoplasmic pH. These results uncover a new role for the V-ATPase accessory protein Atp6ap1b in early development to maintain the proliferation rate of precursor cells needed to construct a ciliated KV organ capable of generating LR asymmetry.

Circulating tumor cell clusters-associated gene plakoglobin and breast cancer survival

Breast cancer recurrence is a major cause of the disease-specific death. Circulating tumor cells (CTCs) are negatively associated with breast cancer survival. Plakoglobin, a cell adhesion protein, was recently reported as a determinant of CTCs types, single or clustered ones. Here, we aim to summarize the studies on the roles of plakoglobin and evaluate the association of plakoglobin and breast cancer survival. Plakoglobin as a key component in both cell adhesion and the signaling pathways was briefly reviewed first. Then the double-edge functions of plakoglobin in tumors and its association with CTCs and breast cancer metastasis were introduced. Finally, based on an open-access database, the association between plakoglobin and breast cancer survival was investigated using univariate and multivariate survival analyses. Plakoglobin may be a molecule functioning as a double-edge sword. Loss of plakoglobin expression leads to increased motility of epithelial cells, thereby promoting epithelial-mesenchymal transition and further metastasis of cancer. However, studies also show that plakoglobin can function as an oncogene. High expression of plakoglobin results in clustered tumor cells in circulation with high metastatic potential in breast cancer and shortened patient survival. Plakoglobin may be a potential prognostic biomarker that can be exploited to develop as a therapeutic target for breast cancer.

Distribution of formins in cardiac muscle: FHOD1 is a component of intercalated discs and costameres

The formin homology domain-containing protein1 (FHOD1) suppresses actin polymerization by inhibiting nucleation, but bundles actin filaments and caps filament barbed ends. Two polyclonal antibodies against FHOD1 were generated against (i) its N-terminal sequence (residues 1-339) and (ii) a peptide corresponding the sequence from position 358-371, which is unique for FHOD1 and does not occur in its close relative FHOD3. After affinity purification both antibodies specifically stain purified full length FHOD1 and a band of similar molecular mass in homogenates of cardiac muscle. The antibody against the N-terminus of FHOD1 was used for immunostaining cells of established lines, primary neonatal (NRC) and adult (ARC) rat cardiomyocytes and demonstrated the presence of FHOD1 in HeLa and fibroblastic cells along stress fibers and within presumed lamellipodia and actin arcs. In NRCs and ARCs we observed a prominent staining of presumed intercalated discs (ICD). Immunostaining of sections of hearts with both anti-FHOD1 antibodies confirmed the presence of FHOD1 in ICDs and double immunostaining demonstrated its colocalisation with cadherin, plakoglobin and a probably slightly shifted localization to connexin43. Similarly, immunostaining of isolated mouse or pig ICDs corroborated the presence of FHOD1 and its colocalisation with the mentioned cell junctional components. Anti-FHOD1 immunoblots of isolated ICDs demonstrated the presence of an immunoreactive band comigrating with purified FHOD1. Conversely, an anti-peptide antibody specific for FHOD3 with no cross-reactivity against FHOD1 immunostained on sections of cardiac muscle and ARCs the myofibrils in a cross-striated pattern but not the ICDs. In addition, the anti-peptide-FHOD1 antibody stained the lateral sarcolemma of ARCs in a banded pattern. Double immunostaining with anti-cadherin and -integrin-ß1 indicated the additional localization of FHOD1 in costameres. Immunostaining of cardiac muscle sections or ARCs with antibodies against mDia3-FH2-domain showed colocalisation with cadherin along the lateral border of cardiomyocytes suggesting also its presence in costameres.

Desmosomal adhesion in vivo

Desmosomes are intercellular junctions that provide strong adhesion or hyper-adhesion in tissues. Here, we discuss the molecular and structural basis of this with particular reference to the desmosomal cadherins (DCs), their isoforms and evolution. We also assess the role of DCs as regulators of epithelial differentiation. New data on the role of desmosomes in development and human disease, especially wound healing and pemphigus, are briefly discussed, and the importance of regulation of the adhesiveness of desmosomes in tissue dynamics is considered.

Zebrafish as a model of cardiac disease

The zebrafish has been rapidly adopted as a model for cardiac development and disease. The transparency of the embryo, its limited requirement for active oxygen delivery, and ease of use in genetic manipulations and chemical exposure have made it a powerful alternative to rodents. Novel technologies like TALEN/CRISPR-mediated genome engineering and advanced imaging methods will only accelerate its use. Here, we give an overview of heart development and function in the fish and highlight a number of areas where it is most actively contributing to the understanding of cardiac development and disease. We also review the current state of research on a feature that we only could wish to be conserved between fish and human; cardiac regeneration.

The hippo pathway is activated and is a causal mechanism for adipogenesis in arrhythmogenic cardiomyopathy

RATIONALE

Mutations in the intercalated disc proteins, such as plakophilin 2 (PKP2), cause arrhythmogenic cardiomyopathy (AC). AC is characterized by the replacement of cardiac myocytes by fibro-adipocytes, cardiac dysfunction, arrhythmias, and sudden death.

OBJECTIVE

To delineate the molecular pathogenesis of AC.

METHODS AND RESULTS

Localization and levels of selected intercalated disc proteins, including signaling molecules, were markedly reduced in human hearts with AC. Altered protein constituents of intercalated discs were associated with activation of the upstream Hippo molecules in the human hearts, in Nkx2.5-Cre:Dsp(W/F) and Myh6:Jup mouse models of AC, and in the PKP2 knockdown HL-1 myocytes (HL-1(PKP2:shRNA)). Level of active protein kinase C-α isoform, which requires PKP2 for activity, was reduced. In contrast, neurofibromin 2 (or Merlin), a molecule upstream of the Hippo pathway and that is inactivated by protein kinase C-α isoform, was activated. Consequently, the downstream Hippo molecules mammalian STE20-like protein kinases 1/2 (MST1/2), large tumor suppressor kinases 1/2 (LATS1/2), and Yes-associated protein (YAP) (the latter is the effector of the pathway) were phosphorylated. Coimmunoprecipitation detected binding of phosphorylated YAP, phosphorylated β-catenin, and junction protein plakoglobin (the latter translocated from the junction). RNA sequencing, transcript quantitative polymerase chain reaction, and reporter assays showed suppressed activity of SV40 transcriptional enhancer factor domain (TEAD) and transcription factor 7-like 2 (TCF7L2), which are transcription factors of the Hippo and the canonical Wnt signaling, respectively. In contrast, adipogenesis was enhanced. Simultaneous knockdown of Lats1/2, molecules upstream to YAP, rescued inactivation of YAP and β-catenin and adipogenesis in the HL-1(PKP2:shRNA) myocytes.

CONCLUSIONS

Molecular remodeling of the intercalated discs leads to pathogenic activation of the Hippo pathway, suppression of the canonical Wnt signaling, and enhanced adipogenesis in AC. The findings offer novel mechanisms for the pathogenesis of AC.

Cadmium(Cd)-induced oxidative stress down-regulates the gene expression of DNA mismatch recognition proteins MutS homolog 2 (MSH2) and MSH6 in zebrafish (Danio rerio) embryos

DNA mismatch repair (MMR) of simple base mismatches and small insertion-deletion loops in eukaryotes is initiated by the binding of the MutS homolog 2 (MSH2)-MSH6 heterodimer to mismatched DNA. Cadmium (Cd) is a genotoxic heavy metal that has been recognized as a human carcinogen. Oxidant stress and inhibition of DNA repair have been proposed as major factors underlying Cd genotoxicity. Our previous studies indicated the ability of Cd to disturb the gene expression of MSH6 in zebrafish (Danio rerio) embryos. This study was undertaken to explore if Cd-induced oxidative stress down-regulated MSH gene activities. Following the exposure of zebrafish embryos at 1 h post fertilization (hpf) to sublethal concentrations of Cd at 3-5 μM for 4 or 9 h, a parallel down-regulation of MSH2, MSH6 and Cu/Zn superoxide dismutase (Cu/Zn-SOD) gene expression was detected by real-time RT-PCR and the expression levels were 40-50% of control after a 9-h exposure. Cd exposure also induced oxidative stress, yet no inhibition of catalase gene activity was observed. Whole mount in situ hybridization revealed a wide distribution of msh6 mRNA in the head regions of 10 hpf embryos and pretreatment of embryos with antioxidants butylhydroxytoluene (BHT), d-mannitol or N-acetylcysteine (NAC) at 1-10 μM restored Cd-suppressed msh6 expression. QPCR confirmed the protective effects of antioxidants on Cd-suppressed msh2/msh6 mRNA production. Down-regulated MSH gene activities reaching about 50% of control were also induced in embryos exposed to paraquat, a reactive oxygen species (ROS)-generating herbicide, or hydrogen peroxide at 200 μM. Hence, Cd at sublethal levels down-regulates msh2/msh6 expression primarily via ROS as signaling molecules. The transcriptional activation of human msh6 is known to be fully dependent on the specificity factor 1 (Sp1). Cd failed to inhibit the DNA binding activity of zebrafish Sp1 unless at lethal concentrations based on band shift assay, therefore excluding the involvement of Sp1 inactivation in Cd-induced MSH gene inhibition in zebrafish embryos.

A disintegrin and metalloproteinase 15 contributes to atherosclerosis by mediating endothelial barrier dysfunction via Src family kinase activity

OBJECTIVE

Endothelium dysfunction is an initiating factor in atherosclerosis. A disintegrin and metalloproteinase 15 (ADAM 15) is a multidomain metalloprotease recently identified as a regulator of endothelial permeability. However, whether and how ADAM15 contributes to atherosclerosis remains unknown.

METHODS AND RESULTS

Genetic ablation of ADAM15 in apolipoprotein E-deficient mice led to a significant reduction in aortic atherosclerotic lesion size (by 52%), plaque macrophage infiltration (by 69%), and smooth muscle cell deposition (by 82%). In vitro studies implicated endothelial-derived ADAM15 in barrier dysfunction and monocyte transmigration across mouse aortic and human umbilical vein endothelial cell monolayers. This role of ADAM15 depended on intact functioning of the cytoplasmic domain, as evidenced in experiments with site-directed mutagenesis targeting the metalloprotease active site (E349A), the disintegrin domain (Arginine-Glycine-Aspartic acid→Threonine-Aspartic acid-Aspartic acid), or the cytoplasmic tail. Further investigations revealed that ADAM15-induced barrier dysfunction was concomitant with dissociation of endothelial adherens junctions (vascular endothelial [VE]-cadherin/γ-catenin), an effect that was sensitive to Src family kinase inhibition. Through small interfering RNA-mediated knockdown of distinct Src family kinase members, c-Src and c-Yes were identified as important mediators of these junctional effects of ADAM15.

CONCLUSIONS

These results suggest that endothelial cell-derived ADAM15, signaling through c-Src and c-Yes, contributes to atherosclerotic lesion development by disrupting adherens junction integrity and promoting monocyte transmigration.

Desmosomal adhesiveness is developmentally regulated in the mouse embryo and modulated during trophectoderm migration

During embryonic development tissues remain malleable to participate in morphogenetic movements but on completion of morphogenesis they must acquire the toughness essential for independent adult life. Desmosomes are cell-cell junctions that maintain tissue integrity especially where resistance to mechanical stress is required. Desmosomes in adult tissues are termed hyper-adhesive because they adhere strongly and are experimentally resistant to extracellular calcium chelation. Wounding results in weakening of desmosomal adhesion to a calcium-dependent state, presumably to facilitate cell migration and wound closure. Since desmosomes appear early in mouse tissue development we hypothesised that initial weak adhesion would be followed by acquisition of hyper-adhesion, the opposite of what happens on wounding. We show that epidermal desmosomes are calcium-dependent until embryonic day 12 (E12) and become hyper-adhesive by E14. Similarly, trophectodermal desmosomes change from calcium-dependence to hyper-adhesiveness as blastocyst development proceeds from E3 to E4.5. In both, development of hyper-adhesion is accompanied by the appearance of a midline between the plasma membranes supporting previous evidence that hyper-adhesiveness depends on the organised arrangement of desmosomal cadherins. By contrast, adherens junctions remain calcium-dependent throughout but tight junctions become calcium-independent as desmosomes mature. Using protein kinase C (PKC) activation and PKCα-/- mice, we provide evidence suggesting that conventional PKC isoforms are involved in developmental progression to hyper-adhesiveness. We demonstrate that modulation of desmosomal adhesion by PKC can regulate migration of trophectoderm. It appears that tissue stabilisation is one of several roles played by desmosomes in animal development.

Mutations with pathogenic potential in proteins located in or at the composite junctions of the intercalated disk connecting mammalian cardiomyocytes: a reference thesaurus for arrhythmogenic cardiomyopathies and for Naxos and Carvajal diseases

In the past decade, an avalanche of findings and reports has correlated arrhythmogenic ventricular cardiomyopathies (ARVC) and Naxos and Carvajal diseases with certain mutations in protein constituents of the special junctions connecting the polar regions (intercalated disks) of mature mammalian cardiomyocytes. These molecules, apparently together with some specific cytoskeletal proteins, are components of (or interact with) composite junctions. Composite junctions contain the amalgamated fusion products of the molecules that, in other cell types and tissues, occur in distinct separate junctions, i.e. desmosomes and adherens junctions. As the pertinent literature is still in an expanding phase and is obviously becoming important for various groups of researchers in basic cell and molecular biology, developmental biology, histology, physiology, cardiology, pathology and genetics, the relevant references so far recognized have been collected and are presented here in the following order: desmocollin-2 (Dsc2, DSC2), desmoglein-2 (Dsg2, DSG2), desmoplakin (DP, DSP), plakoglobin (PG, JUP), plakophilin-2 (Pkp2, PKP2) and some non-desmosomal proteins such as transmembrane protein 43 (TMEM43), ryanodine receptor 2 (RYR2), desmin, lamins A and C, striatin, titin and transforming growth factor-β3 (TGFβ3), followed by a collection of animal models and of reviews, commentaries, collections and comparative studies.

Desmosomal cadherins in zebrafish epiboly and gastrulation

Background

The desmosomal cadherins (DCs), desmocollin (Dsc) and desmoglein (Dsg), are the adhesion molecules of desmosomes, intercellular adhesive junctions of epithelia and cardiac muscle. Both the DCs and desmosomes have demonstrably essential roles in mammalian development. In order to initiate their study in a more tractable developmental system we have characterised zebrafish DCs and examined their roles in early zebrafish development.

Results

We find that zebrafish possess one Dsc, the orthologue of mammalian Dsc1, which we designate zfDsc. Unlike mammalian Dscs, zfDsc exists only as the "a" form since it lacks the alternatively-spliced mini-exon that shortens the cytoplasmic domain to produce the "b" form. Zebrafish possess two Dsgs, designated zfDsgα and zfDsgβ, orthologues of mammalian Dsg2. They show 43.8% amino acid identity and the α form has a 43 amino acid glycine-rich sequence of unknown function in its extracellular domain. Both zfDsc and zfDsgα were present as maternal and zygotic transcripts whereas zfDsgβ was first expressed from 8 hours post-fertilisation (hpf). All three transcripts were present throughout subsequent stages of development. Morpholino knockdown of both zfDsc and zfDsgα expression produced similar defects in epiboly, axis elongation and somite formation, associated with abnormal desmosomes or reduced desmosome numbers.

Conclusions

These results demonstrate an important role for DCs and desmosomes in the early morphogenesis of the zebrafish embryo, provide a basis for more detailed analysis of their role and raise interesting questions relating to the evolution and functional significance of DC isoforms.

Patterning and development of the atrioventricular canal in zebrafish

Proper atrioventricular canal (AVC) patterning and subsequent valvulogenesis is a complex process, and defects can result in disease or early death. The zebrafish Danio rerio has become a useful model system for studying AVC development, and much progress has been made in dissecting out the critical steps. Here, we review the recent advances in the field and highlight the cellular and molecular changes observed during zebrafish AVC development.

Desmosomes: new perpetrators in tumour suppression

Adherens junctions, which are intercellular adhesive complexes that are crucial for maintaining epithelial homeostasis, are downregulated in many cancers to promote tumour progression. However, the role of desmosomes - adhesion complexes that are related to adherens junctions - in carcinogenesis has remained elusive. Recent studies using mouse genetic approaches have uncovered a role for desmosomes in tumour suppression, demonstrating that desmosome downregulation occurs before that of adherens junctions to drive tumour development and early invasion, suggesting a two-step model of adhesion dysfunction in cancer progression.

Dissection of cardiovascular development and disease pathways in zebrafish

The use of animal models in medicine has contributed significantly to the development of drug treatments and surgical procedures for the last century, in particular for cardiovascular disease. In order to model human disease in an animal, an appreciation of the strengths and limitations of the system are required to interpret results and design the logical sequence of steps toward clinical translation. As the world's population ages, cardiovascular disease will become even more prominent and further progress will be essential to stave off what seems destined to become a massive public health issue. Future treatments will require the imaginative application of current models as well as the generation of new ones. In this review, we discuss the resources available for modeling cardiovascular disease in zebrafish and the varied attributes of this system. We then discuss current zebrafish disease models and their potential that has yet to be exploited.

Sublethal levels of cadmium down-regulate the gene expression of DNA mismatch recognition protein MutS homolog 6 (MSH6) in zebrafish (Danio rerio) embryos

MutS homolog 6 (MSH6) is the major mismatch contacting component of the MSH2-MSH6 heterodimeric complex (MutSα) that mediates DNA mismatch repair (MMR) of simple mispairs and small insertion-deletion loops in eukaryotes. This study examined the potential of cadmium (Cd) to disturb the gene expression of MSH6 in vertebrates using zebrafish (Danio rerio) embryo as a model organism. Semiquantitative RT-PCR indicated that msh2 and msh6 expressions were suppressed in embryos at 1h post fertilization (hpf), then drastically up-regulated in 2 hpf embryos and actively expressed in 3-25 hpf embryos. In the presence of a constitutive β-actin expression, exposure of 1 hpf embryos to sublethal concentrations of CdCl(2) at 0.5-3 μM for 4 or 9h caused a time and concentration-dependent down-regulation of msh6 transcription. Cd failed to inhibit msh2 transcription except at 3 μM, reflecting the higher sensitivity of msh6 than msh2 transcription to Cd. Whole mount in situ hybridization showed a wide distribution of msh6 transcripts in the front body portions of 10 hpf embryos and Cd-induced a general suppression of msh6 expression in zebrafish tissues. Cd-induced down-regulation of msh6 transcription paralleled with reduced levels of MSH6 protein synthesis and MSH6-mediated G-T mismatch binding activities identified by band shift assay using recombinant zebrafish MSH6 and an anti-human MSH6 antibody. Our results revealed the inhibition of Cd on MSH6 expression at both mRNA and protein levels and this mechanism may play a role in Cd genotoxicity.

Cadherins as targets for genetic diseases

The 6-billion human population provides a vast reservoir of mutations, which, in addition to the opportunity of detecting very subtle defects, including specific cognitive dysfunctions as well as late appearing disorders, offers a unique background in which to investigate the roles of cell-cell adhesion proteins. Here we focus on inherited human disorders involving members of the cadherin superfamily. Most of the advances concern monogenic disorders. Yet, with the development of single nucleotide polymorphism (SNP) association studies, cadherin genes are emerging as susceptibility genes in multifactorial disorders. Various skin and heart disorders revealed the critical role played by desmosomal cadherins in epidermis, hairs, and myocardium, which experience high mechanical stress. Of particular interest in that respect is the study of Usher syndrome type 1 (USH1), a hereditary syndromic form of deafness. Studies of USH1 brought to light the crucial role of transient fibrous links formed by cadherin 23 and protocadherin 15 in the cohesion of the developing hair bundle, the mechanoreceptive structure of the auditory sensory cells, as well as the involvement of these cadherins in the formation of the tip-link, a key component of the mechano-electrical transduction machinery. Finally, in line with the well-established role of cadherins in synaptic formation, maintenance, strength, and plasticity, a growing number of cadherin family members, especially protocadherins, have been found to be involved in neuropsychiatric disorders.

Junctional music that the nucleus hears: cell-cell contact signaling and the modulation of gene activity

Cell-cell junctions continue to capture the interest of cell and developmental biologists, with an emerging area being the molecular means by which junctional signals relate to gene activity in the nucleus. Although complexities often arise in determining the direct versus indirect nature of such signal transduction, it is clear that such pathways are essential for the function of tissues and that alterations may contribute to many pathological outcomes. This review assesses a variety of cell-cell junction-to-nuclear signaling pathways, and outlines interesting areas for further study.

Armadillo-repeat protein functions: questions for little creatures

Armadillo (ARM)-repeat proteins form a large family with diverse and fundamental functions in many eukaryotes. ARM-repeat proteins have largely been characterised in multicellular organisms and much is known about how a subset of these proteins function. The structure of ARM-repeats allows proteins containing them to be functionally very versatile. Are the ARM-repeat proteins in 'little creatures' as multifunctional as their better-studied relatives? The time is now right to start analysing ARM-repeat proteins in these new systems to better understand their cell biology. Here, we review recent advances in understanding the many cellular roles of both well-known and novel ARM-repeat proteins.

Animal models of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathies are a heterogeneous group of pathological conditions that give rise to myocardial dysfunction with an increased risk for atrial or ventricular arrhythmias. Inherited defects in cardiomyocyte proteins in the sarcomeric contractile apparatus, the cytoskeleton and desmosomal cell-cell contact junctions are becoming recognized increasingly as major causes of sudden cardiac death in the general population. Animal models have been developed for the systematic dissection of the genetic pathways involved in the pathogenesis of arrhythmogenic cardiomyopathies. This review presents an overview of current animal models for arrhythmogenic right ventricular cardiomyopathy (ARVC), hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) associated with cardiac arrhythmias and sudden cardiac death.

Compound heterozygous desmoplakin mutations result in a phenotype with a combination of myocardial, skin, hair, and enamel abnormalities

Desmoplakin (DP) anchors the intermediate filament cytoskeleton to the desmosomal cadherins and thereby confers structural stability to tissues. In this study, we present a patient with extensive mucocutaneous blisters, epidermolytic palmoplantar keratoderma, nail dystrophy, enamel dysplasia, and sparse woolly hair. The patient died at the age of 14 years from undiagnosed cardiomyopathy. The skin showed hyperplasia and acantholysis in the mid- and lower epidermal layers, whereas the heart showed extensive fibrosis and fibrofatty replacement in both ventricles. Immunofluorescence microscopy showed a reduction in the C-terminal domain of DP in the skin and oral mucosa. Sequencing of the DP gene showed undescribed mutations in the maternal and paternal alleles. Both mutations affected exon 24 encoding the C-terminal domain. The paternal mutation, c.6310delA, leads to a premature stop codon. The maternal mutation, c.7964 C to A, results in a substitution of an aspartic acid for a conserved alanine residue at amino acid 2655 (A2655D). Structural modeling indicated that this mutation changes the electrostatic potential of the mutated region of DP, possibly altering functions that depend on intermolecular interactions. To conclude, we describe a combination of DP mutation phenotypes affecting the skin, heart, hair, and teeth. This patient case emphasizes the importance of heart examination of patients with desmosomal genodermatoses.

The intercellular organization of the two muscular systems in the adult salmonid heart, the compact and the spongy myocardium

The ventricle of the salmonid heart consists of an outer compact layer of circumferentially arranged cardiomyocytes encasing a spongy myocardium that spans the lumen of the ventricle with a fine arrangement of muscular trabeculae. While many studies have detailed the anatomical structure of fish hearts, few have considered how these two cardiac muscle architectures are attached to form a functional working unit. The present study considers how the spindle-like cardiomyocytes, unlike the more rectangular structure of adult mammalian cardiomyocytes, form perpendicular connections between the two muscle layers that withstand the mechanical forces generated during cardiac systole and permit a simultaneous, coordinated contraction of both ventricular components. Therefore, hearts of rainbow trout (Oncorhynchus mykiss) and sockeye salmon (Oncorhynchus nerka) were investigated in detail using scanning electron microscopy (SEM) and various light microscopic techniques. In contrast to earlier suggestions, we found no evidence for a distinct connective tissue layer between the two muscle architectures that might 'glue' together the compact and the spongy myocardium. Instead, the contact layer between the compact and the spongy myocardium was characterized by a significantly higher amount of desmosome-like (D) and fascia adhaerens-like (FA) adhering junctions compared with either region alone. In addition, we observed that the trabeculae form muscular sheets of fairly uniform thickness and variable width rather than thick cylinders of variable diameter. This sheet-like trabecular anatomy would minimize diffusion distance while maximizing the area of contact between the trabecular muscle and the venous blood as well as the muscle tension generated by a single trabecular sheet.