Three-dimensional structure of the intercalated disc reveals plicate domain and gap junction remodeling in heart failure

Mitochondrial Structure and Function in Human Heart Failure

Despite clinical and scientific advancements, heart failure is the major cause of morbidity and mortality worldwide. Both mitochondrial dysfunction and inflammation contribute to the development and progression of heart failure. Although inflammation is crucial to reparative healing following acute cardiomyocyte injury, chronic inflammation damages the heart, impairs function, and decreases cardiac output. Mitochondria, which comprise one third of cardiomyocyte volume, may prove a potential therapeutic target for heart failure. Known primarily for energy production, mitochondria are also involved in other processes including calcium homeostasis and the regulation of cellular apoptosis. Mitochondrial function is closely related to morphology, which alters through mitochondrial dynamics, thus ensuring that the energy needs of the cell are met. However, in heart failure, changes in substrate use lead to mitochondrial dysfunction and impaired myocyte function. This review discusses mitochondrial and cristae dynamics, including the role of the mitochondria contact site and cristae organizing system complex in mitochondrial ultrastructure changes. Additionally, this review covers the role of mitochondria-endoplasmic reticulum contact sites, mitochondrial communication via nanotunnels, and altered metabolite production during heart failure. We highlight these often-neglected factors and promising clinical mitochondrial targets for heart failure.

Mst4, a novel cardiac STRIPAK complex-associated kinase, regulates cardiomyocyte growth and survival and is upregulated in human cardiomyopathy

Myocardial failure is associated with adverse remodeling, including loss of cardiomyocytes, hypertrophy, and alterations in cell-cell contacts. Striatin-interacting phosphatase and kinase (STRIPAK) complexes and their mammalian STE20-like kinase 4 (Mst4) have been linked to development of different diseases. The role and targets of Mst4 in cardiomyocytes have not been investigated yet. Multitissue immunoblot experiments show highly enriched Mst4 expression in rodent hearts. Analyses of human biopsy samples from patients suffering from dilated cardiomyopathy revealed that Mst4 is upregulated (5- to 8-fold p < 0.001) compared with nonfailing controls. Increased abundance of Mst4 could also be detected in mouse models of cardiomyopathy. We confirmed that Mst4 interacts with STRIPAK components in neonatal rat ventricular cardiomyocytes, indicating that STRIPAK is present in the heart. Immunofluorescence stainings and molecular interaction studies revealed that Mst4 is localized to the intercalated disc and interacts with several intercalated disc proteins. Overexpression of Mst4 in cardiomyocytes results in hypertrophy compared with controls. In adult rat cardiomyocytes, Mst4 overexpression increases cellular and sarcomeric fractional shortening (p < 0.05), indicating enhanced contractility. Overexpression of Mst4 also inhibits apoptosis shown by reduction of cleaved caspase3 (-69%, p < 0.0001), caspase7 (-80%, p < 0.0001), and cleaved Parp1 (-27%, p < 0.001). To elucidate potential Mst4 targets, we performed phosphoproteomics analyses in neonatal rat cardiomyocytes after Mst4 overexpression and inhibition. The results revealed target candidates of Mst4 at the intercalated disc. We identified Mst4 as a novel cardiac kinase that is upregulated in cardiomyopathy-regulating cardiomyocyte growth and survival.

Realistic numerical simulations of diffusion tensor cardiovascular magnetic resonance: The effects of perfusion and membrane permeability

PURPOSE

To study the sensitivity of diffusion tensor cardiovascular magnetic resonance (DT-CMR) to microvascular perfusion and changes in cell permeability.

METHODS

Monte Carlo (MC) random walk simulations in the myocardium have been performed to simulate self-diffusion of water molecules in histology-based media with varying extracellular volume fraction (ECV) and permeable membranes. The effect of microvascular perfusion on simulations of the DT-CMR signal has been incorporated by adding the contribution of particles traveling through an anisotropic capillary network to the diffusion signal. The simulations have been performed considering three pulse sequences with clinical gradient strengths: monopolar stimulated echo acquisition mode (STEAM), monopolar pulsed-gradient spin echo (PGSE), and second-order motion-compensated spin echo (MCSE).

RESULTS

Reducing ECV intensifies the diffusion restriction and incorporating membrane permeability reduces the anisotropy of the diffusion tensor. Widening the intercapillary velocity distribution results in increased measured diffusion along the cardiomyocytes long axis when the capillary networks are anisotropic. Perfusion amplifies the mean diffusivity for STEAM while the opposite is observed for short diffusion encoding time sequences (PGSE and MCSE).

CONCLUSION

The effect of perfusion on the measured diffusion tensor is reduced using an increased reference b-value. Our results pave the way for characterization of the response of DT-CMR to microstructural changes underlying cardiac pathology and highlight the higher sensitivity of STEAM to permeability and microvascular circulation due to its longer diffusion encoding time.

Monte Carlo simulation of water diffusion through cardiac tissue models

Monte Carlo diffusion simulations are commonly used to establish a reliable ground truth of tissue microstructure, including for the validation of diffusion-weighted MRI. However, selecting simulation parameters is challenging and affects validity and reproducibility. We conducted experiments to investigate critical conditions in Monte Carlo simulations, such as tissue representation complexity, simulated molecules, update duration, and compartment size. Results show significant changes in microstructure characteristics when parameters are altered, emphasizing the importance of careful control for a reliable ground truth.

Three-dimensional micro-structurally informed in silico myocardium-Towards virtual imaging trials in cardiac diffusion weighted MRI

In silico tissue models (viz. numerical phantoms) provide a mechanism for evaluating quantitative models of magnetic resonance imaging. This includes the validation and sensitivity analysis of imaging biomarkers and tissue microstructure parameters. This study proposes a novel method to generate a realistic numerical phantom of myocardial microstructure. The proposed method extends previous studies by accounting for the variability of the cardiomyocyte shape, water exchange between the cardiomyocytes (intercalated discs), disorder class of myocardial microstructure, and four sheetlet orientations. In the first stage of the method, cardiomyocytes and sheetlets are generated by considering the shape variability and intercalated discs in cardiomyocyte-cardiomyocyte connections. Sheetlets are then aggregated and oriented in the directions of interest. The morphometric study demonstrates no significant difference (p>0.01) between the distribution of volume, length, and primary and secondary axes of the numerical and real (literature) cardiomyocyte data. Moreover, structural correlation analysis validates that the in-silico tissue is in the same class of disorderliness as the real tissue. Additionally, the absolute angle differences between the simulated helical angle (HA) and input HA (reference value) of the cardiomyocytes (4.3°±3.1°) demonstrate a good agreement with the absolute angle difference between the measured HA using experimental cardiac diffusion tensor imaging (cDTI) and histology (reference value) reported by (Holmes et al., 2000) (3.7°±6.4°) and (Scollan et al. 1998) (4.9°±14.6°). Furthermore, the angular distance between eigenvectors and sheetlet angles of the input and simulated cDTI is much smaller than those between measured angles using structural tensor imaging (as a gold standard) and experimental cDTI. Combined with the qualitative results, these results confirm that the proposed method can generate richer numerical phantoms for the myocardium than previous studies.

Three-dimensional visualization of heart-wide myocardial architecture and vascular network simultaneously at single-cell resolution

Obtaining various structures of the entire mature heart at single-cell resolution is highly desired in cardiac studies; however, effective methodologies are still lacking. Here, we propose a pipeline for labeling and imaging myocardial and vascular structures. In this pipeline, the myocardium is counterstained using fluorescent dyes and the cardiovasculature is labeled using transgenic markers. High-definition dual-color fluorescence micro-optical sectioning tomography is used to perform heart-wide tissue imaging, enabling the acquisition of whole-heart data at a voxel resolution of 0.32 × 0.32 × 1 μm³. Obtained structural data demonstrated the superiority of the pipeline. In particular, the three-dimensional morphology and spatial arrangement of reconstructed cardiomyocytes were revealed, and high-resolution vascular data helped determine differences in the features of endothelial cells and complex coiled capillaries. Our pipeline can be used in cardiac studies for examining the structures of the entire heart at the single-cell level.

Electron microscopy of cardiac 3D nanodynamics: form, function, future

The 3D nanostructure of the heart, its dynamic deformation during cycles of contraction and relaxation, and the effects of this deformation on cell function remain largely uncharted territory. Over the past decade, the first inroads have been made towards 3D reconstruction of heart cells, with a native resolution of around 1 nm³, and of individual molecules relevant to heart function at a near-atomic scale. These advances have provided access to a new generation of data and have driven the development of increasingly smart, artificial intelligence-based, deep-learning image-analysis algorithms. By high-pressure freezing of cardiomyocytes with millisecond accuracy after initiation of an action potential, pseudodynamic snapshots of contraction-induced deformation of intracellular organelles can now be captured. In combination with functional studies, such as fluorescence imaging, exciting insights into cardiac autoregulatory processes at nano-to-micro scales are starting to emerge. In this Review, we discuss the progress in this fascinating new field to highlight the fundamental scientific insight that has emerged, based on technological breakthroughs in biological sample preparation, 3D imaging and data analysis; to illustrate the potential clinical relevance of understanding 3D cardiac nanodynamics; and to predict further progress that we can reasonably expect to see over the next 10 years.

Neutrophil extracellular traps promote cancer-associated inflammation and myocardial stress

Cancer is associated with systemic pathologies that contribute to mortality, such as thrombosis and distant organ failure. The aim of this study was to investigate the potential role of neutrophil extracellular traps (NETs) in myocardial inflammation and tissue damage in treatment-naïve individuals with cancer. Mice with mammary carcinoma (MMTV-PyMT) had increased plasma levels of NETs measured as H3Cit-DNA complexes, paralleled with elevated coagulation, compared to healthy littermates. MMTV-PyMT mice displayed upregulation of pro-inflammatory markers in the heart, myocardial hypertrophy and elevated cardiac disease biomarkers in the blood, but not echocardiographic heart failure. Moreover, increased endothelial proliferation was observed in hearts from tumor-bearing mice. Removal of NETs by DNase I treatment suppressed the myocardial inflammation, expression of cardiac disease biomarkers and endothelial proliferation. Compared to a healthy control group, treatment-naïve cancer patients with different malignant disorders had increased NET formation, which correlated to plasma levels of the inflammatory marker CRP and the cardiac disease biomarkers NT-proBNP and sTNFR1, in agreement with the mouse data. Altogether, our data indicate that NETs contribute to inflammation and myocardial stress during malignancy. These findings suggest NETs as potential therapeutic targets to prevent cardiac inflammation and dysfunction in cancer patients.

Differential remodelling of mitochondrial subpopulations and mitochondrial dysfunction are a feature of early stage diabetes

Mitochondrial dysfunction is a feature of type I and type II diabetes, but there is a lack of consistency between reports and links to disease development. We aimed to investigate if mitochondrial structure–function remodelling occurs in the early stages of diabetes by employing a mouse model (GENA348) of Maturity Onset Diabetes in the Young, exhibiting hyperglycemia, but not hyperinsulinemia, with mild left ventricular dysfunction. Employing 3-D electron microscopy (SBF-SEM) we determined that compared to wild-type, WT, the GENA348 subsarcolemma mitochondria (SSM) are ~ 2-fold larger, consistent with up-regulation of fusion proteins Mfn1, Mfn2 and Opa1. Further, in comparison, GENA348 mitochondria are more irregular in shape, have more tubular projections with SSM projections being longer and wider. Mitochondrial density is also increased in the GENA348 myocardium consistent with up-regulation of PGC1-α and stalled mitophagy (down-regulation of PINK1, Parkin and Miro1). GENA348 mitochondria have more irregular cristae arrangements but cristae dimensions and density are similar to WT. GENA348 Complex activity (I, II, IV, V) activity is decreased but the OCR is increased, potentially linked to a shift towards fatty acid oxidation due to impaired glycolysis. These novel data reveal that dysregulated mitochondrial morphology, dynamics and function develop in the early stages of diabetes.

Mechanisms for the development of heart failure and improvement of cardiac function by angiotensin-converting enzyme inhibitors

Angiotensin-converting enzyme (ACE) inhibitors, which prevent the conversion of angiotensin I to angiotensin II, are well-known for the treatments of cardiovascular diseases, such as heart failure, hypertension and acute coronary syndrome. Several of these inhibitors including captopril, enalapril, ramipril, zofenopril and imidapril attenuate vasoconstriction, cardiac hypertrophy and adverse cardiac remodeling, improve clinical outcomes in patients with cardiac dysfunction and decrease mortality. Extensive experimental and clinical research over the past 35 years has revealed that the beneficial effects of ACE inhibitors in heart failure are associated with full or partial prevention of adverse cardiac remodeling. Since cardiac function is mainly determined by coordinated activities of different subcellular organelles, including sarcolemma, sarcoplasmic reticulum, mitochondria and myofibrils, for regulating the intracellular concentration of Ca2+ and myocardial metabolism, there is ample evidence to suggest that adverse cardiac remodelling and cardiac dysfunction in the failing heart are the consequence of subcellular defects. In fact, the improvement of cardiac function by different ACE inhibitors has been demonstrated to be related to the attenuation of abnormalities in subcellular organelles for Ca2+-handling, metabolic alterations, signal transduction defects and gene expression changes in failing cardiomyocytes. Various ACE inhibitors have also been shown to delay the progression of heart failure by reducing the formation of angiotensin II, the development of oxidative stress, the level of inflammatory cytokines and the occurrence of subcellular defects. These observations support the view that ACE inhibitors improve cardiac function in the failing heart by multiple mechanisms including the reduction of oxidative stress, myocardial inflammation and Ca2+-handling abnormalities in cardiomyocytes.

Quantification of Myocyte Disarray in Human Cardiac Tissue

Proper three-dimensional (3D)-cardiomyocyte orientation is important for an effective tension production in cardiac muscle. Cardiac diseases can cause severe remodeling processes in the heart, such as cellular misalignment, that can affect both the electrical and mechanical functions of the organ. To date, a proven methodology to map and quantify myocytes disarray in massive samples is missing. In this study, we present an experimental pipeline to reconstruct and analyze the 3D cardiomyocyte architecture in massive samples. We employed tissue clearing, staining, and advanced microscopy techniques to detect sarcomeres in relatively large human myocardial strips with micrometric resolution. Z-bands periodicity was exploited in a frequency analysis approach to extract the 3D myofilament orientation, providing an orientation map used to characterize the tissue organization at different spatial scales. As a proof-of-principle, we applied the proposed method to healthy and pathologically remodeled human cardiac tissue strips. Preliminary results suggest the reliability of the method: strips from a healthy donor are characterized by a well-organized tissue, where the local disarray is log-normally distributed and slightly depends on the spatial scale of analysis; on the contrary, pathological strips show pronounced tissue disorganization, characterized by local disarray significantly dependent on the spatial scale of analysis. A virtual sample generator is developed to link this multi-scale disarray analysis with the underlying cellular architecture. This approach allowed us to quantitatively assess tissue organization in terms of 3D myocyte angular dispersion and may pave the way for developing novel predictive models based on structural data at cellular resolution.

Localization of Na+ channel clusters in narrowed perinexi of gap junctions enhances cardiac impulse transmission via ephaptic coupling: a model study

Abstract

It has been proposed that when gap junctional coupling is reduced in cardiac tissue, action potential propagation can be supported via ephaptic coupling, a mechanism mediated by negative electric potentials occurring in narrow intercellular clefts of intercalated discs (IDs). Recent studies showed that sodium (Na⁺) channels form clusters near gap junction plaques in nanodomains called perinexi, where the ID cleft is even narrower. To examine the electrophysiological relevance of Na⁺ channel clusters being located in perinexi, we developed a 3D finite element model of two longitudinally abutting cardiomyocytes, with a central Na⁺ channel cluster on the ID membranes. When this cluster was located in the perinexus of a closely positioned gap junction plaque, varying perinexal width greatly modulated impulse transmission from one cell to the other, with narrow perinexi potentiating ephaptic coupling. This modulation occurred via the interplay of Na⁺ currents, extracellular potentials in the cleft and patterns of current flow within the cleft. In contrast, when the Na⁺ channel cluster was located remotely from the gap junction plaque, this modulation by perinexus width largely disappeared. Interestingly, the Na⁺ current in the ID membrane of the pre‐junctional cell switched from inward to outward during excitation, thus contributing ions to the activating channels on the post‐junctional ID membrane. In conclusion, these results indicate that the localization of Na⁺ channel clusters in the perinexi of gap junction plaques is crucial for ephaptic coupling, which is furthermore greatly modulated by perinexal width. These findings are relevant for a comprehensive understanding of cardiac excitation.

Key points

Ephaptic coupling is a cardiac conduction mechanism involving nanoscale‐level interactions between the sodium (Na⁺) current and the extracellular potential in narrow intercalated disc clefts.

When gap junctional coupling is reduced, ephaptic coupling acts in conjunction with the classical cardiac conduction mechanism based on gap junctional current flow.

In intercalated discs, Na⁺ channels form clusters that are preferentially located in the periphery of gap junction plaques, in nanodomains known as perinexi, but the electrophysiological role of these perinexi has never been examined.

In our new 3D finite element model of two cardiac cells abutting each other with their intercalated discs, a Na⁺ channel cluster located inside a narrowed perinexus facilitated impulse transmission via ephaptic coupling.

Our simulations demonstrate the role of narrowed perinexi as privileged sites for ephaptic coupling in pathological situations when gap junctional coupling is decreased.

Intercalated disk nanoscale structure regulates cardiac conduction

The intercalated disk (ID) is a specialized subcellular region that provides electrical and mechanical connections between myocytes in the heart. The ID has a clearly defined passive role in cardiac tissue, transmitting mechanical forces and electrical currents between cells. Recent studies have shown that Na+ channels, the primary current responsible for cardiac excitation, are preferentially localized at the ID, particularly within nanodomains such as the gap junction-adjacent perinexus and mechanical junction-associated adhesion-excitability nodes, and that perturbations of ID structure alter cardiac conduction. This suggests that the ID may play an important, active role in regulating conduction. However, the structures of the ID and intercellular cleft are not well characterized and, to date, no models have incorporated the influence of ID structure on conduction in cardiac tissue. In this study, we developed an approach to generate realistic finite element model (FEM) meshes replicating nanoscale of the ID structure, based on experimental measurements from transmission electron microscopy images. We then integrated measurements of the intercellular cleft electrical conductivity, derived from the FEM meshes, into a novel cardiac tissue model formulation. FEM-based calculations predict that the distribution of cleft conductances is sensitive to regional changes in ID structure, specifically the intermembrane separation and gap junction distribution. Tissue-scale simulations predict that ID structural heterogeneity leads to significant spatial variation in electrical polarization within the intercellular cleft. Importantly, we found that this heterogeneous cleft polarization regulates conduction by desynchronizing the activation of postjunctional Na+ currents. Additionally, these heterogeneities lead to a weaker dependence of conduction velocity on gap junctional coupling, compared with prior modeling formulations that neglect or simplify ID structure. Further, we found that disruption of local ID nanodomains can either slow or enhance conduction, depending on gap junctional coupling strength. Our study therefore suggests that ID nanoscale structure can play a significant role in regulating cardiac conduction.

Multiscale cardiac imaging spanning the whole heart and its internal cellular architecture in a small animal model

Cardiac pumping depends on the morphological structure of the heart, but also on its subcellular (ultrastructural) architecture, which enables cardiac contraction. In cases of congenital heart defects, localized ultrastructural disruptions that increase the risk of heart failure are only starting to be discovered. This is in part due to a lack of technologies that can image the three-dimensional (3D) heart structure, to assess malformations; and its ultrastructure, to assess organelle disruptions. We present here a multiscale, correlative imaging procedure that achieves high-resolution images of the whole heart, using 3D micro-computed tomography (micro-CT); and its ultrastructure, using 3D scanning electron microscopy (SEM). In a small animal model (chicken embryo), we achieved uniform fixation and staining of the whole heart, without losing ultrastructural preservation on the same sample, enabling correlative multiscale imaging. Our approach enables multiscale studies in models of congenital heart disease and beyond.

Radixin Relocalization and Nonmuscle α-Actinin Expression Are Features of Remodeling Cardiomyocytes in Adult Patients with Dilated Cardiomyopathy

Background

Pediatric patients show an impressive capacity of cardiac regeneration. In contrast, severely deteriorated adult hearts do usually not recover. Since cardiac remodeling—involving the expression of fetal genes—is regarded as an adaptation to stress, we compared hearts of adult patients suffering from dilated cardiomyopathy (DCM) with remodeling of cultured neonatal (NRC) as well as adult (ARC) rat cardiomyocytes and the developing postnatal myocardium.

Methods

NRC and ARC were stimulated with serum and cardiac morphogens derived from DCM hearts. Protein synthesis (PS) as well as protein accumulation (PA) was measured, and cell survival was determined under ischemic conditions. Fetal markers were investigated by Western blot. Biomarkers of remodeling were analyzed in controls, DCM, and 2- to 6-month-old children with tetralogy of Fallot as well as in neonatal and adult rats by immunofluorescence.

Results

In NRC, serum and morphogens strongly stimulated PS and PA and the reestablishment of cell-cell contacts (CCC). In ARC, both stimulants increased PS and CCC, but PA was only elevated after serum stimulation. In contrast to serum, morphogen treatment resulted in the expression of fetal genes in ARC as determined by nonmuscle α-actinin-1 and α-actinin-4 expression (NM-actinins) and was associated with increased survival under ischemia. NM-actinins were present in cardiomyocytes of DCM in a cross-striated pattern reminiscent of sarcomeres as well as in extensions of the area of the intercalated disc (ID). NM-actinins are expressed in NRC and in the developing heart. Radixin staining revealed remodeling of the area of the ID in DCM almost identical to stimulated cultured ARC.

Conclusions

Remodeling was similar in ARC and in cardiomyocytes of DCM suggesting evolutionary conserved mechanisms of regeneration. Despite activation of fetal genes, the atrophy of ARC indicates differences in their regenerative capacity from NRC. Cardiac-derived factors induced NM-actinin expression and increased survival of ischemic ARC while circulating molecules were less effective. Identification of these cardiac-derived factors and determination of their individual capacity to heal or damage are of particular importance for a biomarker-guided therapy in adult patients.

DREADD technology reveals major impact of Gq signalling on cardiac electrophysiology

AIMS

Signalling via Gq-coupled receptors is of profound importance in many cardiac diseases such as hypertrophy and arrhythmia. Nevertheless, owing to their widespread expression and the inability to selectively stimulate such receptors in vivo, their relevance for cardiac function is not well understood. We here use DREADD technology to understand the role of Gq-coupled signalling in vivo in cardiac function.

METHODS AND RESULTS

We generated a novel transgenic mouse line that expresses a Gq-coupled DREADD (Dq) in striated muscle under the control of the muscle creatine kinase promotor. In vivo injection of the DREADD agonist clozapine-N-oxide (CNO) resulted in a dose-dependent, rapid mortality of the animals. In vivo electrocardiogram data revealed severe cardiac arrhythmias including lack of P waves, atrioventricular block, and ventricular tachycardia. Following Dq activation, electrophysiological malfunction of the heart could be recapitulated in the isolated heart ex vivo. Individual ventricular and atrial myocytes displayed a positive inotropic response and arrhythmogenic events in the absence of altered action potentials. Ventricular tissue sections revealed a strong co-localization of Dq with the principal cardiac connexin CX43. Western blot analysis with phosphor-specific antibodies revealed strong phosphorylation of a PKC-dependent CX43 phosphorylation site following CNO application in vivo.

CONCLUSION

Activation of Gq-coupled signalling has a major impact on impulse generation, impulse propagation, and coordinated impulse delivery in the heart. Thus, Gq-coupled signalling does not only modulate the myocytes' Ca2+ handling but also directly alters the heart's electrophysiological properties such as intercellular communication. This study greatly advances our understanding of the plethora of modulatory influences of Gq signalling on the heart in vivo.

A novel murine model for arrhythmogenic cardiomyopathy points to a pathogenic role of Wnt signalling and miRNA dysregulation

AIMS

Arrhythmogenic cardiomyopathy (AC) is one of the most common inherited cardiomyopathies, characterized by progressive fibro-fatty replacement in the myocardium. Clinically, AC manifests itself with ventricular arrhythmias, syncope, and sudden death and shows wide inter- and intra-familial variability. Among the causative genes identified so far, those encoding for the desmosomal proteins plakophilin-2 (PKP2), desmoplakin (DSP), and desmoglein-2 (DSG2) are the most commonly mutated. So far, little is known about the molecular mechanism(s) behind such a varied spectrum of phenotypes, although it has been shown that the causative mutations not only lead to structural abnormalities but also affect the miRNA profiling of cardiac tissue. Here, we aimed at studying the pathogenic effects of a nonsense mutation of the desmoglein-2 gene, both at the structural level and in terms of miRNA expression pattern.

METHODS AND RESULTS

We generated transgenic mice with cardiomyocyte-specific overexpression of a FLAG-tagged human desmoglein-2 harbouring the Q558* nonsense mutation found in an AC patient. The hearts of these mice showed signs of fibrosis, decrease in desmosomal size and number, and reduction of the Wnt/β-catenin signalling. Genome-wide RNA-Seq performed in Tg-hQ hearts and non-transgenic hearts revealed that 24 miRNAs were dysregulated in transgenic animals. Further bioinformatic analyses for selected miRNAs suggested that miR-217-5p, miR-499-5p, and miR-708-5p might be involved in the pathogenesis of the disease.

CONCLUSION

Down-regulation of the canonical Wnt/β-catenin signalling might be considered a common key event in the AC pathogenesis. We identified the miRNA signature in AC hearts, with miR-708-5p and miR-217-5p being the most up-regulated and miR-499-5p the most down-regulated miRNAs. All of them were predicted to be involved in the regulation of the Wnt/β-catenin pathway and might reveal the potential pathophysiology mechanisms of AC, as well as be useful as therapeutic targets for the disease.

Unravelling the ultrastructural details of αT-catenin-deficient cell-cell contacts between heart muscle cells by the use of FIB-SEM

The intercalated disc is an important structure in cardiomyocytes, as it is essential to maintain correct contraction and proper functioning of the heart. Adhesion and communication between cardiomyocytes are mediated by three main types of intercellular junctions, all residing in the intercalated disc: gap junctions, desmosomes and the areae compositae. Mutations in genes that encode junctional proteins, including αT-catenin (encoded by CTNNA3), have been linked to arrhythmogenic cardiomyopathy and sudden cardiac death. In mice, the loss of αT-catenin in cardiomyocytes leads to impaired heart function, fibrosis, changed expression of desmosomal proteins and increased risk for arrhythmias following ischemia-reperfusion. Currently, it is unclear how the intercalated disc and the intercellular junctions are organised in 3D in the hearts of this αT-catenin knockout (KO) mouse model. In order to scrutinise this, ventricular cardiac tissue of αT-catenin KO mice was used for volume electron microscopy (VEM), making use of Focused Ion Beam Scanning Electron Microscopy (FIB-SEM), allowing a careful 3D reconstruction of the intercalated disc, including gap junctions and desmosomes. Although αT-catenin KO and control mice display a comparable organisation of the sarcomere and the different intercalated disc regions, the folds of the plicae region of the intercalated disc are longer and more narrow in the KO heart, and the pale region between the sarcomere and the intercalated disc is larger. In addition, αT-catenin KO intercalated discs appear to have smaller gap junctions and desmosomes in the plicae region, while gap junctions are larger in the interplicae region of the intercalated disc. Although the reason for this remodelling of the ultrastructure after αT-catenin deletion remains unclear, the excellent resolution of the FIB-SEM technology allows us to reconstruct details that were not reported before. LAY DESCRIPTION: Cardiomyocytes are cells that make up the heart muscle. As the chief cell type of the heart, cardiomyocytes are primarily involved in the contractile function of the heart that enables the pumping of blood around the body. Cardiac muscle cells are connected to each other at their short end by numerous intercellular junctions forming together a structure called the intercalated disc. These intercellular junctions comprise specific protein complexes, which are crucial for both intercellular adhesion and correct contraction of the heart. Imaging by conventional electron microscopy (EM) revealed a heavily folded intercalated disc with apparently random organization of the intercellular junctions. However, this conclusion was based on analysis in two dimensions (2D). 3D information of these structures is needed to unravel their true organization and function. In the present study, we used a more contemporary technique, called volume EM, to image and reconstruct the intercalated discs in 3D. By this approach, EM images are made from a whole block of tissue what differs significantly from classical EM methods that uses only one very thin slice for imaging. Further, we analyzed in comparison to normal mice also a mouse model for cardiomyopathy in which a specific protein of the cardiac intercellular junctions, αT-catenin, is absent. Volume EM revealed that in the hearts of these mice with cardiomyopathy, the finger-like folds of the intercalated disc are longer and thinner compared to control hearts. Also the intercellular junctions on the folded parts of the intercalated disc are smaller and their connection to the striated cytoskeleton seems further away. In conclusion, our volume EM study has expanded our understanding of 3D structures at the intercalated discs and will pave the way for more detailed models of disturbed cell-cell contacts associated with heart failure.

Automated segmentation of cardiomyocyte Z-disks from high-throughput scanning electron microscopy data

Background

With the advent of new high-throughput electron microscopy techniques such as serial block-face scanning electron microscopy (SBF-SEM) and focused ion-beam scanning electron microscopy (FIB-SEM) biomedical scientists can study sub-cellular structural mechanisms of heart disease at high resolution and high volume. Among several key components that determine healthy contractile function in cardiomyocytes are Z-disks or Z-lines, which are located at the lateral borders of the sarcomere, the fundamental unit of striated muscle. Z-disks play the important role of anchoring contractile proteins within the cell that make the heartbeat. Changes to their organization can affect the force with which the cardiomyocyte contracts and may also affect signaling pathways that regulate cardiomyocyte health and function. Compared to other components in the cell, such as mitochondria, Z-disks appear as very thin linear structures in microscopy data with limited difference in contrast to the remaining components of the cell.

Methods

In this paper, we propose to generate a 3D model of Z-disks within single adult cardiac cells from an automated segmentation of a large serial-block-face scanning electron microscopy (SBF-SEM) dataset. The proposed fully automated segmentation scheme is comprised of three main modules including “pre-processing”, “segmentation” and “refinement”. We represent a simple, yet effective model to perform segmentation and refinement steps. Contrast stretching, and Gaussian kernels are used to pre-process the dataset, and well-known “Sobel operators” are used in the segmentation module.

Results

We have validated our model by comparing segmentation results with ground-truth annotated Z-disks in terms of pixel-wise accuracy. The results show that our model correctly detects Z-disks with 90.56% accuracy. We also compare and contrast the accuracy of the proposed algorithm in segmenting a FIB-SEM dataset against the accuracy of segmentations from a machine learning program called Ilastik and discuss the advantages and disadvantages that these two approaches have.

Conclusions

Our validation results demonstrate the robustness and reliability of our algorithm and model both in terms of validation metrics and in terms of a comparison with a 3D visualisation of Z-disks obtained using immunofluorescence based confocal imaging.

Cardiomyocyte damage control in heart failure and the role of the sarcolemma

The cardiomyocyte plasma membrane, termed the sarcolemma, is fundamental for regulating a myriad of cellular processes. For example, the structural integrity of the cardiomyocyte sarcolemma is essential for mediating cardiac contraction by forming microdomains such as the t-tubular network, caveolae and the intercalated disc. Significantly, remodelling of these sarcolemma microdomains is a key feature in the development and progression of heart failure (HF). However, despite extensive characterisation of the associated molecular and ultrastructural events there is a lack of clarity surrounding the mechanisms driving adverse morphological rearrangements. The sarcolemma also provides protection, and is the cell's first line of defence, against external stresses such as oxygen and nutrient deprivation, inflammation and oxidative stress with a loss of sarcolemma viability shown to be a key step in cell death via necrosis. Significantly, cumulative cell death is also a feature of HF, and is linked to disease progression and loss of cardiac function. Herein, we will review the link between structural and molecular remodelling of the sarcolemma associated with the progression of HF, specifically considering the evidence for: (i) Whether intrinsic, evolutionary conserved, plasma membrane injury-repair mechanisms are in operation in the heart, and (ii) if deficits in key 'wound-healing' proteins (annexins, dysferlin, EHD2 and MG53) may play a yet to be fully appreciated role in triggering sarcolemma microdomain remodelling and/or necrosis. Cardiomyocytes are terminally differentiated with very limited regenerative capability and therefore preserving cell viability and cardiac function is crucially important. This review presents a novel perspective on sarcolemma remodelling by considering whether targeting proteins that regulate sarcolemma injury-repair may hold promise for developing new strategies to attenuate HF progression.

The SLMAP/Striatin complex: An emerging regulator of normal and abnormal cardiac excitation-contraction coupling

The excitation-contraction (E-C) module involves a harmonized correspondence between the sarcolemma and the sarcoplasmic reticulum. This is provided by membrane proteins, which primarily shape the caveolae, the T-tubule/Sarcoplasmic reticulum (TT/SR) junction, and the intercalated discs (ICDs). Distortion of either one of these structures impairs myocardial contraction, and subsequently translates into cardiac failure. Thus, detailed studies on the molecular cues of the E-C module are becoming increasingly necessary to pharmacologically eradicate cardiac failure Herein we reviewed the organization of caveolae, TT/SR junctions, and the ICDs in the heart, with special attention to the Sarcolemma Membrane Associated Protein (SLMAP) and striatin (STRN) in cardiac membranes biology and cardiomyocyte contraction. We emphasized on their in vivo and in vitro signaling in cardiac function/dysfunction. SLMAP is a cardiac membrane protein that plays an important role in E-C coupling and the adrenergic response of the heart. Similarly, STRN is a dynamic protein that is also involved in cardiac E-C coupling and ICD-related cardiomyopathies. Both SLMAP and STRN are linked to cardiac conditions, including heart failure, and their role in cardiomyocyte function was elucidated in our laboratory. They interact together in a protein complex that holds therapeutic potentials for cardiac dysfunction. This review is the first of its kind to conceptualize the role of the SLMAP/STRN complex in cardiac function and failure. It provides in depth information on the signaling of these two proteins and projects their interaction as a novel therapeutic target for cardiac failure.

Riding the waves of the intercalated disc of the heart

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

Three-dimensional reconstruction of the intercalated disc including the intercellular junctions by applying volume scanning electron microscopy

The intercalated disc (ID) contains different kinds of intercellular junctions: gap junctions (GJs), desmosomes and areae compositae, essential for adhesion and communication between adjacent cardiomyocytes. The junctions can be identified based on their morphology when imaged using transmission electron microscopy (TEM), however, only with very limited information in the z-dimension. The application of volume EM techniques can give insight into the three-dimensional (3-D) organization of complex biological structures. In this study, we generated 3-D datasets using serial block-face scanning electron microscopy (SBF-SEM) and focused ion beam SEM (FIB-SEM), the latter resulting in datasets with 5 nm isotropic voxels. We visualized cardiomyocytes in murine ventricular heart tissue and, for the first time, we could three-dimensionally reconstruct the ID including desmosomes and GJs with 5 nm precision in a large volume. Results show in three dimensions a highly folded structure of the ID, with the presence of GJs and desmosomes in both plicae and interplicae regions. We observed close contact of GJs with mitochondria and a variable spatial distribution of the junctions. Based on measurements of the shape of the intercellular junctions in 3-D, it is seen that GJs and desmosomes vary in size, depending on the region within the ID. This demonstrates that volume EM is essential to visualize morphological changes and its potential to quantitatively determine structural changes between normal and pathological conditions, e.g., cardiomyopathies.

An automated workflow for segmenting single adult cardiac cells from large-volume serial block-face scanning electron microscopy data

This paper presents a new algorithm to automatically segment the myofibrils, mitochondria and nuclei within single adult cardiac cells that are part of a large serial-block-face scanning electron microscopy (SBF-SEM) dataset. The algorithm only requires a set of manually drawn contours that roughly demarcate the cell boundary at routine slice intervals (every 50th, for example). The algorithm correctly classified pixels within the single cell with 97% accuracy when compared to manual segmentations. One entire cell and the partial volumes of two cells were segmented. Analysis of segmentations within these cells showed that myofibrils and mitochondria occupied 47.5% and 51.6% on average respectively, while the nuclei occupy 0.7% of the cell for which the entire volume was captured in the SBF-SEM dataset. Mitochondria clustering increased at the periphery of the nucleus region and branching points of the cardiac cell. The segmentations also showed high area fraction of mitochondria (up to 70% of the 2D image slice) in the sub-sarcolemmal region, whilst it was closer to 50% in the intermyofibrillar space. We finally demonstrate that our segmentations can be turned into 3D finite element meshes for cardiac cell computational physiology studies. We offer our large dataset and MATLAB implementation of the algorithm for research use at www.github.com/CellSMB/sbfsem-cardiac-cell-segmenter/. We anticipate that this timely tool will be of use to cardiac computational and experimental physiologists alike who study cardiac ultrastructure and its role in heart function.

The end-stage failing human myocardium - Where changes in ultrastructure of human cardiac muscle cells do not appear to dictate clinical outcomes

Heart failure is the end stage of cardiovascular abnormalities. Studies have primarily focused on the functional changes of cardiomyocytes in the failing heart from different animal models with very little information in the human condition. In addition little is known about the ultrastructural changes that proceed in cardiomyocytes in route to failure. The aim of this study was to examine the ultrastructural changes in the myocardium of human with end-stage heart failure. Left ventricular myocardial tissue samples from 7 patients with end-stage heart failure were examined with transmission and scanning electron microscopy. All heart failure patients were of New York Heart Association (NYHA) class III-IV. The data indicated normal three-dimensional arrangement of cardiac muscle cells in failing myocardium. The various organelles in cardiomyocytes including the nucleus, mitochondria, myofibrils, T-tubules and intercalated discs did not exhibit any remarkable morphological changes. We did observe the appearance of small membrane bound vesicles which appear to be associated with the intercalated discs. The nearly normal ultrastructure and arrangement of cardiomyocytes was remarkable in contrast to the dramatic clinical status of these patients in heart failure. These observations support the hypothesis, that there are no dramatic changes in the ultrastructure or three-dimensional architecture of cardiomyocytes in end-stage failing human myocardium.

Sodium Channel Remodeling in Subcellular Microdomains of Murine Failing Cardiomyocytes

BACKGROUND

Cardiac sodium channel (NaV1.5) dysfunction contributes to arrhythmogenesis during pathophysiological conditions. Nav1.5 localizes to distinct subcellular microdomains within the cardiomyocyte, where it associates with region-specific proteins, yielding complexes whose function is location specific. We herein investigated sodium channel remodeling within distinct cardiomyocyte microdomains during heart failure.

METHODS AND RESULTS

Mice were subjected to 6 weeks of transverse aortic constriction (TAC; n=32) to induce heart failure. Sham-operated on mice were used as controls (n=20). TAC led to reduced left ventricular ejection fraction, QRS prolongation, increased heart mass, and upregulation of prohypertrophic genes. Whole-cell sodium current (INa) density was decreased by 30% in TAC versus sham-operated on cardiomyocytes. On macropatch analysis, INa in TAC cardiomyocytes was reduced by 50% at the lateral membrane (LM) and by 40% at the intercalated disc. Electron microscopy and scanning ion conductance microscopy revealed remodeling of the intercalated disc (replacement of [inter-]plicate regions by large foldings) and LM (less identifiable T tubules and reduced Z-groove ratios). Using scanning ion conductance microscopy, cell-attached recordings in LM subdomains revealed decreased INa and increased late openings specifically at the crest of TAC cardiomyocytes, but not in groove/T tubules. Failing cardiomyocytes displayed a denser, but more stable, microtubule network (demonstrated by increased α-tubulin and Glu-tubulin expression). Superresolution microscopy showed reduced average NaV1.5 cluster size at the LM of TAC cells, in line with reduced INa.

CONCLUSIONS

Heart failure induces structural remodeling of the intercalated disc, LM, and microtubule network in cardiomyocytes. These adaptations are accompanied by alterations in NaV1.5 clustering and INa within distinct subcellular microdomains of failing cardiomyocytes.

TGF-β1 affects cell-cell adhesion in the heart in an NCAM1-dependent mechanism

The contractile property of the myocardium is maintained by cell-cell junctions enabling cardiomyocytes to work as a syncytium. Alterations in cell-cell junctions are observed in heart failure, a disease characterized by the activation of Transforming Growth Factor beta 1 (TGFβ1). While TGFβ1 has been implicated in diverse biologic responses, its molecular function in controlling cell-cell adhesion in the heart has never been investigated. Cardiac-specific transgenic mice expressing active TGFβ1 were generated to model the observed increase in activity in the failing heart. Activation of TGFβ1 in the heart was sufficient to drive ventricular dysfunction. To begin to understand the function of this important molecule we undertook an extensive structural analysis of the myocardium by electron microscopy and immunostaining. This approach revealed that TGFβ1 alters intercalated disc structures and cell-cell adhesion in ventricular myocytes. Mechanistically, we found that TGFβ1 induces the expression of neural adhesion molecule 1 (NCAM1) in cardiomyocytes in a p38-dependent pathway, and that selective targeting of NCAM1 was sufficient to rescue the cell adhesion defect observed when cardiomyocytes were treated with TGFβ1. Importantly, NCAM1 was upregulated in human heart samples from ischemic and non-ischemic cardiomyopathy patients and NCAM1 protein levels correlated with the degree of TGFβ1 activity in the human cardiac ventricle. Overall, we found that TGFβ1 is deleterious to the heart by regulating the adhesion properties of cardiomyocytes in an NCAM1-dependent mechanism. Our results suggest that inhibiting NCAM1 would be cardioprotective, counteract the pathological action of TGFβ1 and reduce heart failure severity.

Post-Myocardial Infarction T-tubules Form Enlarged Branched Structures With Dysregulation of Junctophilin-2 and Bridging Integrator 1 (BIN-1)

BACKGROUND

Heart failure is a common secondary complication following a myocardial infarction (MI), characterized by impaired cardiac contraction and t-tubule (t-t) loss. However, post-MI nano-scale morphological changes to the remaining t-ts are poorly understood.

METHOD AND RESULTS

We utilized a porcine model of MI, using a nonlethal microembolization method to generate controlled microinfarcts. Using serial block face scanning electron microscopy, we report that post-MI, after mild left-ventricular dysfunction has developed, t-ts are not only lost in the peri-infarct region, but also the remnant t-ts form enlarged, highly branched disordered structures, containing a dense intricate inner membrane. Biochemical and proteomics analyses showed that the calcium release channel, ryanodine receptor 2 (RyR2), abundance is unchanged, but junctophilin-2 (JP2), important for maintaining t-t trajectory, is depressed (-0.5×) in keeping with the t-ts being disorganized. However, immunolabeling shows that populations of RyR2 and JP2 remain associated with the remodeled t-ts. The bridging integrator 1 protein (BIN-1), a regulator of tubulogensis, is upregulated (+5.4×), consistent with an overdeveloped internal membrane system, a feature not present in control t-ts. Importantly, we have determined that t-ts, in the remote region, are narrowed and also contain dense membrane folds (BIN-1 is up-regulated +3.4×), whereas the t-ts have a radial organization comparable to control JP2 is upregulated +1.7×.

CONCLUSIONS

This study reveals previously unidentified remodeling of the t-t nano-architecture in the post-MI heart that extends to the remote region. Our findings highlight that targeting JP2 may be beneficial for preserving the orientation of the t-ts, attenuating the development of hypocontractility post-MI.

New Cell Adhesion Molecules in Human Ischemic Cardiomyopathy. PCDHGA3 Implications in Decreased Stroke Volume and Ventricular Dysfunction

Background

Intercalated disks are unique structures in cardiac tissue, in which adherens junctions, desmosomes, and GAP junctions co-localize, thereby facilitating cardiac muscle contraction and function. Protocadherins are involved in these junctions; however, their role in heart physiology is poorly understood. We aimed to analyze the transcriptomic profile of adhesion molecules in patients with ischemic cardiomyopathy (ICM) and relate the changes uncovered with the hemodynamic alterations and functional depression observed in these patients.

Methods and Results

Twenty-three left ventricular tissue samples from patients diagnosed with ICM (n = 13) undergoing heart transplantation and control donors (CNT, n = 10) were analyzed using RNA sequencing. Forty-two cell adhesion genes involved in cellular junctions were differentially expressed in ICM myocardium. Notably, the levels of protocadherin PCDHGA3 were related with the stroke volume (r = –0.826, P = 0.003), ejection fraction (r = –0.793, P = 0.004) and left ventricular end systolic and diastolic diameters (r = 0.867, P = 0.001; r = 0.781, P = 0.005, respectively).

Conclusions

Our results support the importance of intercalated disks molecular alterations, closely involved in the contractile function, highlighting its crucial significance and showing gene expression changes not previously described. Specifically, altered PCDHGA3 gene expression was strongly associated with reduced stroke volume and ventricular dysfunction in ICM, suggesting a relevant role in hemodynamic perturbations and cardiac performance for this unexplored protocadherin.

Propofol attenuated liver transplantation-induced acute lung injury via connexin43 gap junction inhibition

BACKGROUND

Postoperative acute lung injury (ALI) is a severe complication after liver transplantation, which influences patient survival rate obviously. However, its mechanisms are unclear and effective therapies are still lacking. The current study focused on effects of propofol on liver transplantation-induced ALI and whether its underlying mechanism was relative with connexin43 (Cx43) alternation. The authors postulated that endotoxin induced enhancement of Cx43 gap junction (GJ) plays a critical role in mediating post liver transplantation ALI and that pretreatment with the anesthetic propofol, known to inhibit gap junction, can confer effective protection.

METHODS

Male Sprague-Dawley rats underwent autologous orthotopic liver transplantation (AOLT) in the absence or presence of treatments with the selective Cx43 inhibitor, enanthol (0.1 mg/kg) and propofol (50 mg/kg), a commonly used anesthetic in clinical anesthesia. In vitro study, BEAS-2B cells, a kind of lung epithelial cell line expressing Cx43, exposed to lipopolysaccharide (LPS), which mainly contributed to ALI. Function of Cx43 GJ was regulated by Cx43 specific inhibitors, gap26 (300 μM) or enhancer, retinoic acid (10 μM) and two specific siRNAs.

RESULTS

Compared with the sham group, AOLT results in ALI obviously with plasma endotoxin increase. Cx43 inhibition decreased ALI through inflammatory reaction reduction. In vitro studies, LPS-induced BEAS-2B cells damage was attenuated by Cx43 function inhibition, but amplified by enhancement. Another important finding was propofol reduced Cx43 function and protected against LPS-mediated BEAS-2B cells damage or AOLT-induced ALI, mechanisms of which were also associated with inflammatory reaction decrease.

CONCLUSION

Cx43 plays a vital role in liver transplantation-induced ALI. Propofol decreased Cx43 function and protected against ALI in vivo and in vitro. This finding provide a new basis for targeted intervention of organ protection in liver transplantation, even in other kinds of operations.

Sarcoplasmic reticulum is an intermediary of mitochondrial and myofibrillar growth at the intercalated disc

In cardiomyocytes columns of intermyofibrillar mitochondria run up to the intercalated disc (ID); half are collinear with those in the neighbouring cell, suggesting coordinated addition of sarcomeres and mitochondria both within and between cells during cardiomyocyte growth. Recent evidence for an association between sarcoplasmic reticulum (SR) and mitochondria indicates that the SR may be an intermediary in this coordinated behaviour. For this reason we have investigated the arrangement of SR and t tubules with respect to mitochondria and myofibrils, particularly at the ID. In the body of the cardiomyocyte the mitochondrial columns are frequently intersected by transverse tubules. In addition, we find that a majority of axial tubules are sandwiched between mitochondria and myofibril. No tubules are found at the ID. SR coats mitochondrial columns and fibrils throughout their length and reaches towards the peaks of the ID membrane where it attaches in the form of junctional (j)SR. These peripheral ID couplings are often situated between mitochondria and ID membrane, suggesting an SR connection between the two. In dilated cardiomyopathy (DCM) the mitochondria are somewhat disordered and clumped. In a mouse model for DCM, the muscle LIM protein KO, we find that there is a lack of mitochondria near the ID, suggesting the uncoupling of the myofibril/mitochondria organisation during growth. SR still coats the fibrils and reaches the ID folds in a jSR coupling. Unlike in control tissue, however, loops and long fingers of ID membrane penetrate into the proximal sarcomere suggesting a possible intermediary state in cardiomyocyte growth.