Organization Of Cardiomyocytes Research Articles

Expression of Smyd1b_tv1 by Alternative Splicing in Cardiac Muscle is Critical for Sarcomere Organization in Cardiomyocytes and Heart Function.

Smyd1, a member of the Smyd lysine methyltransferase family, plays an important role in myofibrillogenesis of skeletal and cardiac muscles. Loss of Smyd1b (a Smyd1 ortholog) function in zebrafish results in embryonic death from heart malfunction. smyd1b encodes two isoforms, Smyd1b_tv1 and Smyd1b_tv2, differing by 13 amino acids due to alternative splicing. While smyd1 alternative splicing is evolutionarily conserved, the isoform-specific expression and function of Smyd1b_tv1 and Smyd1b_tv2 remained unknown. Here we analyzed their expression and function in skeletal and cardiac muscles. Our analysis revealed expression of smyd1b_tv1 predominately in cardiac and smyd1b_tv2 in skeletal muscles. Using zebrafish models expressing only one isoform, we demonstrated that Smyd1b_tv1 is essential for cardiomyocyte differentiation and fish viability, whereas Smyd1b_tv2 is dispensable for heart development and fish survival. Cellular and biochemical analyses revealed that Smyd1b_tv1 differs from Smyd1b_tv2 in protein localization and binding with myosin chaperones. While Smyd1b_tv2 diffused in the cytosol of muscle cells, Smyd1b_tv1 was localized to M-lines and essential for sarcomere organization in cardiomyocytes. Co-IP analysis revealed a stronger binding of Smyd1b_tv1 with chaperones and cochaperones compared with Smyd1b_tv2. Collectively, these findings highlight the nonequivalence of Smyd1b isoforms in cardiomyocyte differentiation, emphasizing the critical role of Smyd1b_tv1 in cardiac function.

Abnormal circadian rhythms exacerbate dilated cardiomyopathy by reducing the ventricular mechanical strength.

Dilated cardiomyopathy (DCM) has etiological and pathophysiological heterogeneity. Abnormal circadian rhythm (ACR) is related to the development of DCM in animal models, but exploration based on clinical samples is lacking. Sleep apnea (SA) is the most common disease related to ACR, and we chose SA as the study object to explore ACR-DCM. We included a derivation cohort (n =105) and a validation cohort (n = 65). DCM patients were divided into SA and without SA group. RT-qPCR was used to determine the change of rhythm gene expression pattern of heart samples from different timepoints. We used single-nucleus RNA sequencing (snRNA-seq) to explore the abnormal transcriptional patterns in the ACR group, and we verified the findings by pathological staining, atomic force microscopy (AFM), and Rev-erbα/β knockout (KO) mice analysis. DCM patients with SA showed decreased amplitude of rhythm gene expression. SA group showed more severe dilation of left heart chambers. From snRNA-seq, ACR-DCM lost the morning transcriptional patterns, detailly, actin cytoskeleton organization of cardiomyocytes (CMs) disrupted and hypertrophy aggravated, and the proportion of activated fibroblasts (Fibs) decreased with the reduction of fibrotic area ratio. The results of pathological staining, mechanical experiments, and transcriptional feature of Rev-erbα/β KO mice supported the above findings. Compared with the non-SA group, left ventricular (LV) wall dilation was more severe and the structural strength was lower in DCM patients with SA, and phenotypic changes in CM and Fib were involved in this process. ACR-DCM was histopathologically characterized by a structurally weak ventricular wall.

Abstract We081: β1 integrins regulate cellular behavior and cardiomyocyte organization during ventricular wall formation

Introduction: Cardiomyocytes are surrounded by extracellular matrix (ECM) and interact with ECM via integrins. However, the mechanisms through which these cells organize themselves to establish the tissue architecture of trabeculae and the ventricular wall remain inadequately elucidated. Integrins are the primary cell surface receptors for adhesion to the ECM and mediate both inside-out and outside-in signal transduction pathways that control various cell functions, including proliferation, survival, migration, and gene expression. We found that the β1 integrin subunit (β1), encoded by Itgb1 , is highly expressed in all cardiac cell types. Hypothesis: β1 integrins play a crucial role in regulating cardiomyocyte behavior and organization during ventricular wall morphogenesis in mice. Methods and Results: We applied mRNA deep sequencing and immunostaining to determine the expression repertoires of α/β integrins and their ligands in the embryonic heart. β1 and some of its ECM ligands are asymmetrically distributed and enriched in the luminal side of cardiomyocytes, and fibronectin surrounds cardiomyocytes, creating a network for them. Itgb1 , which encodes the β1, was deleted via Nkx2.5 Cre/+ to generate myocardial-specific Itgb1 knockout (B1KO) mice. B1KO hearts lack a trabecular zone but a thicker compact zone. The levels of hyaluronic acid and versican, essential for trabecular initiation, were not significantly different between control and B1KO. Instead, fibronectin, a ligand of β1, was absent in the myocardium of B1KO hearts. Furthermore, B1KO cardiomyocytes display a random cellular orientation and fail to undergo perpendicular cell division, be organized properly, and establish the proper tissue architecture to form trabeculae. Mosaic clonal lineage tracing showed that Itgb1 regulates cardiomyocyte transmural migration and proliferation autonomously. Conclusions: β1 is asymmetrically localized in the cardiomyocytes, and some of its ECM ligands are enriched along the luminal side of the myocardium, and fibronectin surrounds cardiomyocytes. β1 integrins are required for cardiomyocytes to attach to the ECM network. This engagement provides structural support for cardiomyocytes to maintain shape, undergo perpendicular division, and establish cellular organization. Deletion of Itgb1 leads to loss of β1 and fibronectin and prevents cardiomyocytes from engaging the ECM network, resulting in failure to establish tissue architecture to form trabeculae.

β1 integrins regulate cellular behaviour and cardiomyocyte organization during ventricular wall formation.

The mechanisms regulating the cellular behaviour and cardiomyocyte organization during ventricular wall morphogenesis are poorly understood. Cardiomyocytes are surrounded by extracellular matrix (ECM) and interact with ECM via integrins. This study aims to determine whether and how β1 integrins regulate cardiomyocyte behaviour and organization during ventricular wall morphogenesis in the mouse. We applied mRNA deep sequencing and immunostaining to determine the expression repertoires of α/β integrins and their ligands in the embryonic heart. Integrin β1 subunit (β1) and some of its ECM ligands are asymmetrically distributed and enriched in the luminal side of cardiomyocytes, and fibronectin surrounds cardiomyocytes, creating a network for them. Itgb1, which encodes the β1, was deleted via Nkx2.5Cre/+ to generate myocardial-specific Itgb1 knockout (B1KO) mice. B1KO hearts display an absence of a trabecular zone but a thicker compact zone. The levels of hyaluronic acid and versican, essential for trabecular initiation, were not significantly different between control and B1KO. Instead, fibronectin, a ligand of β1, was absent in the myocardium of B1KO hearts. Furthermore, B1KO cardiomyocytes display a random cellular orientation and fail to undergo perpendicular cell division, be organized properly, and establish the proper tissue architecture to form trabeculae. Mosaic clonal lineage tracing showed that Itgb1 regulates cardiomyocyte transmural migration and proliferation autonomously. β1 is asymmetrically localized in the cardiomyocytes, and some of its ECM ligands are enriched along the luminal side of the myocardium, and fibronectin surrounds cardiomyocytes. β1 integrins are required for cardiomyocytes to attach to the ECM network. This engagement provides structural support for cardiomyocytes to maintain shape, undergo perpendicular division, and establish cellular organization. Deletion of Itgb1 leads to loss of β1 and fibronectin and prevents cardiomyocytes from engaging the ECM network, resulting in failure to establish tissue architecture to form trabeculae.

GelMA micropattern enhances cardiomyocyte organization, maturation, and contraction via contact guidance.

Cardiac tissue engineering has emerged as a promising approach for restoring the functionality of damaged cardiac tissues following myocardial infarction. To effectively replicate the native anisotropic structure of cardiac tissues in vitro, this study focused on the fabrication of micropatterned gelatin methacryloyl hydrogels with varying geometric parameters. These substrates were evaluated for their ability to guide induced pluripotent stem cell-derived cardiomyocytes (CMs). The findings demonstrate that the mechanical properties of this hydrogel closely resemble those of native cardiac tissues, and it exhibits high fidelity in micropattern fabrication. Micropatterned hydrogel substrates lead to enhanced organization, maturation, and contraction of CMs. A microgroove with 20-μm-width and 20-μm-spacing was identified as the optimal configuration for maximizing the contact guidance effect, supported by analyses of nuclear orientation and F-actin organization. Furthermore, this specific micropattern design was found to promote CMs' maturation, as evidenced by increased expression of connexin 43 and vinculin, along with extended sarcomere length. It also enhanced CMs' contraction, resulting in larger contractile amplitudes and greater contractile motion anisotropy. In conclusion, these results underscore the significant benefits of optimizing micropatterned gelatin methacryloyl for improving CMs' organization, maturation, and contraction. This valuable insight paves the way for the development of highly organized and functionally mature cardiac tissues in vitro.

RNA-Binding Proteins in Cardiomyopathies.

The post-transcriptional regulation of gene expression plays an important role in heart development and disease. Cardiac-specific alternative splicing, mediated by RNA-binding proteins, orchestrates the isoform switching of proteins that are essential for cardiomyocyte organization and contraction. Dysfunctions of RNA-binding proteins impair heart development and cause the main types of cardiomyopathies, which represent a heterogenous group of abnormalities that severely affect heart structure and function. In particular, mutations of RBM20 and RBFOX2 are associated with dilated cardiomyopathy, hypertrophic cardiomyopathy, or hypoplastic left heart syndrome. Functional analyses in different animal models also suggest possible roles for other RNA-binding proteins in cardiomyopathies because of their involvement in organizing cardiac gene programming. Recent studies have provided significant insights into the causal relationship between RNA-binding proteins and cardiovascular diseases. They also show the potential of correcting pathogenic mutations in RNA-binding proteins to rescue cardiomyopathy or promote cardiac regeneration. Therefore, RNA-binding proteins have emerged as promising targets for therapeutic interventions for cardiovascular dysfunction. The challenge remains to decipher how they coordinately regulate the temporal and spatial expression of target genes to ensure heart function and homeostasis. This review discusses recent advances in understanding the implications of several well-characterized RNA-binding proteins in cardiomyopathies, with the aim of identifying research gaps to promote further investigation in this field.

Abstract P3051: Master Splicing Regulator Rbpms2 Is Not Required For Murine Cardiac Development Or Contractile Function

Post-transcriptional gene regulation (PTGR) through RNA-binding proteins (RBP) is an understudied area of cardiac biology. Emerging research describes important roles for Rbpms and Rbpms2 in RNA splicing, cardiomyocyte diploidy and proliferation, cytoskeletal reorganization and translation initiation during cardiomyocyte differentiation and cardiac development. However, the role of Rbpms2 in adult heart is largely unknown. We identified Rbpms2 as a cardiovascular-enriched RBP that is consistently downregulated in failing hearts from humans and mice. Though prior research has focused on roles in splicing, a large portion Rbpms2 protein is extranuclear, and >35% of empirically determined Rbpms binding sites localize in 3’UTRs, suggesting alternative PTGR roles. To assess Rbpms2’s role in cardiomyocyte PTGR, we assessed mRNA-seq changes in neonatal rat cardiomyocytes after overexpression and knockdown of Rbpms2. Pathway analyses revealed significant upregulation of genes involved in cell cycle control, and downregulation of genes involved in viral host defense pathway after Rbpms2 knockdown. We next characterized Rbpms2 knockout (KO) mice, which are viable without overt phenotypes, to provide initial descriptions of adult mouse cardiac function after Rbpms2 deletion. Baseline echocardiography data of male mice at 6 months of age shows normal cardiac size, structure, and function without significant difference between KO and wildtype (WT) mice (n=10). To assess the role of Rbpms2 in cardiac hypertrophy, we induced pressure overload via transverse aortic constriction (TAC) on mix-sex WT and Rbpms2-KO mice, which resulted in robust hypertrophic responses and reduced ejection fraction in both groups, but no significant differences between WT and KO mice (n=11 female, n=7-10 male). Overall, our findings are surprising given recently published data showing that Rbpms2 is a master regulator of RNA splicing affecting sarcomere organization and cardiomyocyte contractile function in zebrafish and stem cell-derived cardiomyocytes, supporting the need for follow-up studies on cardiac Rbpms2. It is possible that Rbpms compensates for the loss of Rbpms2 in the mouse heart and would require double KOs to assess synergistic phenotypes.

A chromEM-staining protocol optimized for cardiac tissue.

Three-dimensional (3D) chromatin organization has a key role in defining the transcription program of cells during development. Its alteration is the cause of gene expression changes responsible for several diseases. Thus, we need new tools to study this aspect of gene expression regulation. To this end, ChromEM was recently developed: this is an electron-microscopy staining technique that selectively marks nuclear DNA without altering its structure and, thus, allows better visualization of 3D chromatin conformation. However, despite increasingly frequent application of this staining technique on cells, it has not yet been applied to visualize chromatin ultrastructure in tissues. Here, we provide a protocol to carry out ChromEM on myocardial tissue harvested from the left ventricles of C57BL/6J mice and use this in combination with transmission electron microscopy (TEM) to measure some morphological parameters of peripheral heterochromatin in cardiomyocytes. This protocol could also be used, in combination with electron tomography, to study 3D chromatin organization in cardiomyocytes in different aspects of heart pathobiology (e.g., heart development, cardiac aging, and heart failure) as well as help to set-up ChromEM in other tissues.

Centering Margins in Organ Assembly

Centering Margins in Organ Assembly

Affimers targeting proteins in the cardiomyocyte Z-disc: Novel tools that improve imaging of heart tissue.

Dilated Cardiomyopathy is a common form of heart failure. Determining how this disease affects the structure and organization of cardiomyocytes in the human heart is important in understanding how the heart becomes less effective at contraction. Here we isolated and characterised Affimers (small non-antibody binding proteins) to Z-disc proteins ACTN2 (α-actinin-2), ZASP (also known as LIM domain binding protein 3 or LDB3) and the N-terminal region of the giant protein titin (TTN Z1-Z2). These proteins are known to localise in both the sarcomere Z-discs and the transitional junctions, found close to the intercalated discs that connect adjacent cardiomyocytes. We use cryosections of left ventricles from two patients diagnosed with end-stage Dilated Cardiomyopathy who underwent Orthotopic Heart Transplantation and were whole genome sequenced. We describe how Affimers substantially improve the resolution achieved by confocal and STED microscopy compared to conventional antibodies. We quantified the expression of ACTN2, ZASP and TTN proteins in two patients with dilated cardiomyopathy and compared them with a sex- and age-matched healthy donor. The small size of the Affimer reagents, combined with a small linkage error (the distance from the epitope to the dye label covalently bound to the Affimer) revealed new structural details in Z-discs and intercalated discs in the failing samples. Affimers are thus useful for analysis of changes to cardiomyocyte structure and organisation in diseased hearts.

High-speed 2D light-sheet fluorescence microscopy enables quantification of spatially varying calcium dynamics in ventricular cardiomyocytes.

Introduction: Reduced synchrony of calcium release and t-tubule structure organization in individual cardiomyocytes has been linked to loss of contractile strength and arrhythmia. Compared to confocal scanning techniques widely used for imaging calcium dynamics in cardiac muscle cells, light-sheet fluorescence microscopy enables fast acquisition of a 2D plane in the sample with low phototoxicity. Methods: A custom light-sheet fluorescence microscope was used to achieve dual-channel 2D timelapse imaging of calcium and the sarcolemma, enabling calcium sparks and transients in left and right ventricle cardiomyocytes to be correlated with the cell microstructure. Imaging electrically stimulated dual-labelled cardiomyocytes immobilized with para-nitroblebbistatin, a non-phototoxic, low fluorescence contraction uncoupler, with sub-micron resolution at 395fps over a 38μm × 170µm FOV allowed characterization of calcium spark morphology and 2D mapping of the calcium transient time-to-half-maximum across the cell. Results: Blinded analysis of the data revealed sparks with greater amplitude in left ventricle myocytes. The time for the calcium transient to reach half-maximum amplitude in the central part of the cell was found to be, on average, 2ms shorter than at the cell ends. Sparks co-localized with t-tubules were found to have significantly longer duration, larger area and spark mass than those further away from t-tubules. Conclusion: The high spatiotemporal resolution of the microscope and automated image-analysis enabled detailed 2D mapping and quantification of calcium dynamics of n = 60 myocytes, with the findings demonstrating multi-level spatial variation of calcium dynamics across the cell, supporting the dependence of synchrony and characteristics of calcium release on the underlying t-tubule structure.

Automated Sarcomere Structure Analysis for Studying Cardiotoxicity in Human Pluripotent Stem Cell-Derived Cardiomyocytes

Abstract Drug-induced cardiotoxicity is one of the main causes of heart failure (HF), a worldwide major and growing public health issue. Extensive research on cardiomyocytes has shown that two crucial features of the mechanisms involved in HF are the disruption of striated sarcomeric organization and myofibril deterioration. However, most studies that worked on extracting these sarcomere features have only focused on animal models rather than the more representative human pluripotent stem cells (hPSCs). Currently, there are limited established image analysis systems to specifically assess and quantify the sarcomeric organization of hPSC-derived cardiomyocytes (hPSC-CMs). Here, we report a fully automated and robust image analysis pipeline to detect z-lines and myofibrils from hPSC-CMs with a high-throughput live-imaging setup. Phenotype measurements were further quantified to evaluate the cardiotoxic effect of the anticancer drug Doxorubicin. Our findings show that this pipeline is able to capture z-lines and myofibrils. The pipeline filters out disrupted sarcomere structures and irrelevant noisy signals, which allows us to perform automated high-throughput imaging for accurate quantification of cardiomyocyte injury.

On the structural origin of the anisotropy in the myocardium: Multiscale modeling and analysis

Due to structural heterogeneities within the tissue, the myocardium displays an orthotropic material behavior. However, the link between the microstructure and the macroscopic mechanical properties is still not fully established. In particular, if it is admitted that the cardiomyocyte organization induces a transversely isotropic symmetry, the relative role in the observed orthotropic symmetry of cardiomyocyte orientation variation and perimysium collagen “sheetlet” structure, two mechanisms occurring at different scales, is still a matter of debate. In order to shed light on this question, we designed a multiscale model of the myocardium, bridging the cell, sheetlet and tissue scales. More precisely, we compared the macroscopic anisotropy obtained by homogenization of different mesostructures consisting in cardiomyocytes and extracellular collageneous layers, also taking into account the variation of cardiomyocyte and sheetlet orientations on the macroscale, to available experimental data. This study confirms the importance of sheetlets layers in assuring the tissue’s anisotropic response, as cardiomyocytes-only mesostructures cannot reproduce the observed anisotropy. Moreover, our model shows the existence of a size effect in the myocardial tissue shear properties, which will require further experimental analysis.

RBPMS2 Is a Myocardial-Enriched Splicing Regulator Required for Cardiac Function.

RBPs (RNA-binding proteins) perform indispensable functions in the post-transcriptional regulation of gene expression. Numerous RBPs have been implicated in cardiac development or physiology based on gene knockout studies and the identification of pathogenic RBP gene mutations in monogenic heart disorders. The discovery and characterization of additional RBPs performing indispensable functions in the heart will advance basic and translational cardiovascular research. We performed a differential expression screen in zebrafish embryos to identify genes enriched in nkx2.5-positive cardiomyocytes or cardiopharyngeal progenitors compared to nkx2.5-negative cells from the same embryos. We investigated the myocardial-enriched gene RNA-binding protein with multiple splicing (variants) 2 [RBPMS2)] by generating and characterizing rbpms2 knockout zebrafish and human cardiomyocytes derived from RBPMS2-deficient induced pluripotent stem cells. We identified 1848 genes enriched in the nkx2.5-positive population. Among the most highly enriched genes, most with well-established functions in the heart, we discovered the ohnologs rbpms2a and rbpms2b, which encode an evolutionarily conserved RBP. Rbpms2 localizes selectively to cardiomyocytes during zebrafish heart development and strong cardiomyocyte expression persists into adulthood. Rbpms2-deficient embryos suffer from early cardiac dysfunction characterized by reduced ejection fraction. The functional deficit is accompanied by myofibril disarray, altered calcium handling, and differential alternative splicing events in mutant cardiomyocytes. These phenotypes are also observed in RBPMS2-deficient human cardiomyocytes, indicative of conserved molecular and cellular function. RNA-sequencing and comparative analysis of genes mis-spliced in RBPMS2-deficient zebrafish and human cardiomyocytes uncovered a conserved network of 29 ortholog pairs that require RBPMS2 for alternative splicing regulation, including RBFOX2, SLC8A1, and MYBPC3. Our study identifies RBPMS2 as a conserved regulator of alternative splicing, myofibrillar organization, and calcium handling in zebrafish and human cardiomyocytes.

Genome organization in cardiomyocytes expressing mutated A-type lamins.

Cardiomyopathy is a myocardial disorder, in which the heart muscle is structurally and functionally abnormal, often leading to heart failure. Dilated cardiomyopathy is characterized by a compromised left ventricular function and contributes significantly to the heart failure epidemic, which represents a staggering clinical and public health problem worldwide. Gene mutations have been identified in 35% of patients with dilated cardiomyopathy. Pathogenic variants in LMNA, encoding nuclear A-type lamins, are one of the major causative causes of dilated cardiomyopathy (i.e. CardioLaminopathy). A-type lamins are type V intermediate filament proteins, which are the main components of the nuclear lamina. The nuclear lamina is connected to the cytoskeleton on one side, and to the chromatin on the other side. Among the models proposed to explain how CardioLaminopathy arises, the “chromatin model” posits an effect of mutated A-type lamins on the 3D genome organization and thus on the transcription activity of tissue-specific genes. Chromatin contacts with the nuclear lamina via specific genomic regions called lamina-associated domains lamina-associated domains. These LADs play a role in the chromatin organization and gene expression regulation. This review focuses on the identification of LADs and chromatin remodeling in cardiac muscle cells expressing mutated A-type lamins and discusses the methods and relevance of these findings in disease.

Tubulator: an automated approach to analysis of t-tubule and dyadic organization in cardiomyocytes.

During cardiac disease, t-tubules and dyads are remodelled and disrupted within cardiomyocytes, thereby reducing cardiac performance. Given the pathological implications of such dyadic remodelling, robust and versatile tools for characterizing these sub-cellular structures are needed. While analysis programs for continuous and regular structures such as rodent ventricular t-tubules are available, at least in two dimensions, these approaches are less appropriate for assessment of more irregular structures, such as dyadic proteins and non-rodent t-tubules. Here, we demonstrate versatile, easy-to-use software that performs such analyses. This software, called Tubulator, enables automated analysis of t-tubules and dyadic proteins alike, in both tissue sections and isolated myocytes. The program measures densities of subcellular structures and proteins in individual cells, quantifies their distribution into transversely and longitudinally oriented elements, and supports detailed co-localization analyses. Importantly, Tubulator provides tools for three-dimensional assessment and rendering of image stacks, extending examinations from the single plane to the whole-myocyte level. To provide insight into the consequences of dyadic organization for synchrony of Ca2+ handling, Tubulator also creates ‘distance maps', by calculating the distance from all cytosolic positions to the nearest t-tubule and/or dyad. In conclusion, this freely accessible program provides detailed automated analysis of the three-dimensional nature of dyadic and t-tubular structures.This article is part of the theme issue ‘The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease’.

Regulation of cardiomyocyte adhesion and mechanosignalling through distinct nanoscale behaviour of integrin ligands mimicking healthy or fibrotic extracellular matrix.

The stiffness of the cardiovascular environment changes during ageing and in disease and contributes to disease incidence and progression. Changing collagen expression and cross-linking regulate the rigidity of the cardiac extracellular matrix (ECM). Additionally, basal lamina glycoproteins, especially laminin and fibronectin regulate cardiomyocyte adhesion formation, mechanics and mechanosignalling. Laminin is abundant in the healthy heart, but fibronectin is increasingly expressed in the fibrotic heart. ECM receptors are co-regulated with the changing ECM. Owing to differences in integrin dynamics, clustering and downstream adhesion formation this is expected to ultimately influence cardiomyocyte mechanosignalling; however, details remain elusive. Here, we sought to investigate how different cardiomyocyte integrin/ligand combinations affect adhesion formation, traction forces and mechanosignalling, using a combination of uniformly coated surfaces with defined stiffness, polydimethylsiloxane nanopillars, micropatterning and specifically designed bionanoarrays for precise ligand presentation. Thereby we found that the adhesion nanoscale organization, signalling and traction force generation of neonatal rat cardiomyocytes (which express both laminin and fibronectin binding integrins) are strongly dependent on the integrin/ligand combination. Together our data indicate that the presence of fibronectin in combination with the enhanced stiffness in fibrotic areas will strongly impact on the cardiomyocyte behaviour and influence disease progression.This article is part of the theme issue ‘The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease’.

CASK is a key organizer of the structural and electrical polarity of the cardiomyocyte

Although the organization of the cardiomyocyte membrane into functional microdomains directly influences the anisotropic propagation of the action potential, the mechanisms underlying the rod-like structural organization of cardiomyocytes during postnatal development are poorly understood. We have previously shown in the adult cardiomyocyte that CASK regulates sodium channel expression at the lateral membrane in interaction with focal adhesion complexes. In addition, preliminary results have shown that the expression level of CASK evolves during postnatal development and is altered during post-infarction myocardial remodeling in rat. To unravel the implication of CASK in postnatal cardiomyocyte differentiation and in the acquisition of structural and electrical function. CASK viral constructs were designed to inhibit the expression of CASK in neonatal and adult cardiac myocytes, in vitro (Ad-shCASK) and in vivo (AAV-shCASK). Proteomic analysis shows that CASK invalidation differentially regulates the expression of different protein families between neonatal and adult myocytes, suggesting a different role for CASK during postnatal development. Immunostaining experiments on isolated neonatal cardiomyocytes showed that CASK is required for the correct organization of the cardiomyocyte cytoskeleton, and impact both mechanical and electrical junction formation. Neonatal cardiomyocytes invalidated for CASK display abnormal contractility, focal adhesion fragility, and anoikis. Echocardiography phenotyping of CASK-invalidated rats at the neonatal and adult stages reveals eccentric ventricular remodeling and decreased systolic performance. However, in neonatal CASK-invalidated rats, the contractile reserve is preserved and the cardiac output is increased. In addition, mild electrical remodeling is observed since only neonatal CASK-invalidated rats display QT prolongation and slowed heart rhythm. This study demonstrates that CASK confers the ability to orchestrate both cardiomyocyte molecular organization and tissue cohesion. CASK appears to be a key modulator in molecular arrangement of electrical and structural polarity of cardiomyocytes during postnatal development.

Abstract 13459: Human iPS-Derived Pre-Epicardial Cells Direct Cardiomyocyte Aggregation Expansion and Organization in vitro

During heart maturation, epicardial-derived cells emanating from the pro-epicardial organ envelope the developing heart, then integrate within the myocardium and undergo epithelial-mesenchymal transition (EMT) to become fibroblasts, smooth muscle cells, and endothelial cells that enable several processes of healthy heart morphogenesis (i.e., ventricular thickening, compaction, and angiogenesis). Directed cardiac differentiation of human induced-pluripotent stem cells (hiPSCs) holds great potential for generating several cardiovascular cell types, which can theoretically be combined to model cardiac development more completely and engineer sophisticated myocardial grafts. Here, we show that differentiating hiPSC-derived lateral plate mesoderm (LPM) with BMP4, RA and VEGF (BVR) for 96 hours can generate a premature form of epicardial cells (termed pre-epicardial cells, PECs) with high efficiency (86.8 ± 4.1% WT1 + by FACS), that also show significant increases in WT1, TBX18, SEMA3D , TBX5, BNC, and SCX transcript expression within 7 days. This protocol was successfully reproduced in three commercial lines of hiPSCs. BVR stimulation after Wnt inhibition of LPM demonstrated efficient co-differentiation and spatial organization of PECs and cardiomyocytes (CMs) in a single 2D culture. Co-culture consolidated CMs into dense aggregates, which then formed a connected beating syncytium with significantly enhanced contractility (0.029 ± 0.002 mN/mm 2 ; p<0.05 vs. CMs-alone and co-culture controls), improved calcium handling (i.e. amplitude, upstroke, and decay), sarcomere length (1.82 ± 0.02 μm vs. 1.72 ± 0.03 μm, p=0.00264) and mitochondrial density (79.4 ± 3% vs. 54.9 ± 6%, p=0.0029). Co-cultured PECs demonstrated enhanced maturity to epicardial cells with significant upregulation of UPK1B, ITGA4, and ALDH1A2 transcript expressions. Our study also demonstrated that PECs secrete IGF2 and stimulate CM proliferation in co-culture (37.2 ± 2.1% vs. 28.1 ± 1%, p=0.016). These characteristics suggest PECs could play a key role in enhancing cell-cell signaling and tissue organization, which may help to recapitulate important aspects of cardiac morphogenesis in vitro and advance the maturation of bioengineered cardiac constructs.

Overexpression of Lifeact-GFP Disrupts F-Actin Organization in Cardiomyocytes and Impairs Cardiac Function.

Lifeact-GFP is a frequently used molecular probe to study F-actin structure and dynamic assembly in living cells. In this study, we generated transgenic zebrafish models expressing Lifeact-GFP specifically in cardiac muscles to investigate the effect of Lifeact-GFP on heart development and its application to study cardiomyopathy. The data showed that transgenic zebrafish with low to moderate levels of Lifeact-GFP expression could be used as a good model to study contractile dynamics of actin filaments in cardiac muscles in vivo. Using this model, we demonstrated that loss of Smyd1b, a lysine methyltransferase, disrupted F-actin filament organization in cardiomyocytes of zebrafish embryos. Our studies, however, also demonstrated that strong Lifeact-GFP expression in cardiomyocytes was detrimental to actin filament organization in cardiomyocytes that led to pericardial edema and early embryonic lethality of zebrafish embryos. Collectively, these data suggest that although Lifeact-GFP is a good probe for visualizing F-actin dynamics, transgenic models need to be carefully evaluated to avoid artifacts induced by Lifeact-GFP overexpression.