Nonmyocytes as electrophysiological contributors to cardiac excitation and conduction

Nanoparticle-assisted targeting of heart lesions with cardiac myofibroblasts: Combined gene and cell therapy

Rationale: The cardiac scar is an area rich in collagen. It is populated by myofibroblasts and lacks Connexin 43 expressing cardiomyocytes. Myocardial infarctions have so far proven little amenable to gene- and cell-based therapeutic interventions. Our aim was to establish an experimental approach with translational potential for effective cell-based gene therapy of the cardiac scar. Methods: We have developed a targeting strategy for myocardial infarctions by grafting ex vivo lentivirus-transduced and magnetic nanoparticle-loaded embryonic cardiac myofibroblasts into mouse hearts with magnetic steering. Results: Our approach yielded highly efficient targeting and cell grafting into the cardiac scar. Engraftment rates of myofibroblasts proved very high (30% of injected cells) due to cell proliferation and a low apoptosis rate. We also demonstrate that grafting lentivirus-transduced Connexin 43 overexpressing myofibroblasts into the lesion resulted in increased Connexin 43 protein content and strong protection against ventricular arrhythmias in vivo, as their incidence was reduced by ~ 50% at 2- and 8 weeks after myocardial infarction. Conclusion: The combination of ex vivo gene and in vivo cell therapy, along with magnetic steering of cardiac myofibroblasts, enables, efficient targeting of the cardiac scar and can even modulate its functional properties.

Engineered Strategies to Interfere with Macrophage Fate in Myocardial Infarction

Myocardial infarction (MI), a severe cardiovascular condition, is typically triggered by coronary artery disease, resulting in ischemic damage and the subsequent necrosis of the myocardium. Macrophages, known for their remarkable plasticity, are capable of exhibiting a range of phenotypes and functions as they react to diverse stimuli within their local microenvironment. In recent years, there has been an increasing number of studies on the regulation of macrophage behavior based on tissue engineering strategies, and its regulatory mechanisms deserve further investigation. This review first summarizes the effects of key regulatory factors of engineered biomaterials (including bioactive molecules, conductivity, and some microenvironmental factors) on macrophage behavior, then explores specific methods for inducing macrophage behavior through tissue engineering materials to promote myocardial repair, and summarizes the role of macrophage-host cell crosstalk in regulating inflammation, vascularization, and tissue remodeling. Finally, we propose some future challenges in regulating macrophage-material interactions and tailoring personalized biomaterials to guide macrophage phenotypes.

The Macrophage-Fibroblast Dipole in the Context of Cardiac Repair and Fibrosis

Stromal and immune cells and their interactions have gained the attention of cardiology researchers and clinicians in recent years as their contribution in cardiac repair is increasingly recognized. The repair process in the heart is a particularly critical constellation of complex molecular and cellular events and interactions that characteristically fail to ensure adequate recovery following injury, insult, or exposure to stress conditions in this regeneration-hostile organ. The tremendous consequence of this pronounced inability to maintain homeostatic states is being translated in numerous ways promoting progress into heart failure, a deadly, irreversible condition requiring organ transplantation. Fibrosis is in fact a repair response eventually promoting cardiac dysfunction and cardiac fibroblasts are the major cellular players in this process, overproducing collagens and other extracellular matrix components when activated. On the other hand, macrophages may differentially affect fibroblasts and cardiac repair depending on their status and subsets. The opposite interaction is also probable. We discuss here the multifaceted aspects and crosstalk of this cell dipole and the opportunities it may offer for beneficial manipulation approaches that will hopefully lead to progress in heart disease interventions.

Cardiac macrophages in maintaining heart homeostasis and regulating ventricular remodeling of heart diseases

Macrophages are most important immune cell population in the heart. Cardiac macrophages have broad-spectrum and heterogeneity, with two extreme polarization phenotypes: M1 pro-inflammatory macrophages (CCR2-ly6Chi) and M2 anti-inflammatory macrophages (CCR2-ly6Clo). Cardiac macrophages can reshape their polarization states or phenotypes to adapt to their surrounding microenvironment by altering metabolic reprogramming. The phenotypes and polarization states of cardiac macrophages can be defined by specific signature markers on the cell surface, including tumor necrosis factor α, interleukin (IL)-1β, inducible nitric oxide synthase (iNOS), C-C chemokine receptor type (CCR)2, IL-4 and arginase (Arg)1, among them, CCR2+/- is one of most important markers which is used to distinguish between resident and non-resident cardiac macrophage as well as macrophage polarization states. Dedicated balance between M1 and M2 cardiac macrophages are crucial for maintaining heart development and cardiac functional and electric homeostasis, and imbalance between macrophage phenotypes may result in heart ventricular remodeling and various heart diseases. The therapy aiming at specific target on macrophage phenotype is a promising strategy for treatment of heart diseases. In this article, we comprehensively review cardiac macrophage phenotype, metabolic reprogramming, and their role in maintaining heart health and mediating ventricular remodeling and potential therapeutic strategy in heart diseases.

Cardiac fibroblasts: answering the call

Cardiac fibroblasts play a pivotal role in maintaining heart homeostasis by depositing extracellular matrix (ECM) to provide structural support for the myocardium, vasculature, and neuronal network and by contributing to essential physiological processes. In response to injury such as myocardial infarction or pressure overload, fibroblasts become activated, leading to increased ECM production that can ultimately drive left ventricular remodeling and progress to heart failure. Recently, the American Journal of Physiology-Heart and Circulatory Physiology issued a call for papers on cardiac fibroblasts that yielded articles with topics spanning fibroblast physiology, technical considerations, signaling pathways, and interactions with other cell types. This mini-review summarizes those articles and places the new findings in the context of what is currently known for cardiac fibroblasts and what future directions remain.

Re-entry in models of cardiac ventricular tissue with scar represented as a Gaussian random field

Introduction: Fibrotic scar in the heart is known to act as a substrate for arrhythmias. Regions of fibrotic scar are associated with slowed or blocked conduction of the action potential, but the detailed mechanisms of arrhythmia formation are not well characterised and this can limit the effective diagnosis and treatment of scar in patients. The aim of this computational study was to evaluate different representations of fibrotic scar in models of 2D 10 × 10 cm ventricular tissue, where the region of scar was defined by sampling a Gaussian random field with an adjustable length scale of between 1.25 and 10.0 mm. Methods: Cellular electrophysiology was represented by the Ten Tusscher 2006 model for human ventricular cells. Fibrotic scar was represented as a spatially varying diffusion, with different models of the boundary between normal and fibrotic tissue. Dispersion of activation time and action potential duration (APD) dispersion was assessed in each sample by pacing at an S1 cycle length of 400 ms followed by a premature S2 beat with a coupling interval of 323 ms. Vulnerability to reentry was assessed with an aggressive pacing protocol. In all models, simulated fibrosis acted to delay activation, to increase the dispersion of APD, and to generate re-entry. Results: A higher incidence of re-entry was observed in models with simulated fibrotic scar at shorter length scale, but the type of model used to represent fibrotic scar had a much bigger influence on the incidence of reentry. Discussion: This study shows that in computational models of fibrotic scar the effects that lead to either block or propagation of the action potential are strongly influenced by the way that fibrotic scar is represented in the model, and so the results of computational studies involving fibrotic scar should be interpreted carefully.

Nonpreferential but Detrimental Accumulation of Macrophages With Clonal Hematopoiesis-Driver Mutations in Cardiovascular Tissues-Brief Report

BACKGROUND

Clonal hematopoiesis of indeterminate potential (CHIP) is an acquired genetic risk factor for both leukemia and cardiovascular disease. It results in proinflammatory myeloid cells in the bone marrow and blood; however, how these cells behave in the cardiovascular tissue remains unclear. Our study aimed at investigating whether CHIP-mutated macrophages accumulate preferentially in cardiovascular tissues and examining the transcriptome of tissue macrophages from DNMT3A (DNA methyltransferase 3 alpha) or TET2 (Tet methylcytosine dioxygenase 2) mutation carriers.

METHODS

We recruited patients undergoing carotid endarterectomy or heart surgeries to screen for CHIP mutation carriers using targeted genomic sequencing. Myeloid and lymphoid cells were isolated from blood and cardiovascular tissue collected during surgeries using flow cytometry. DNA and RNA extracted from these sorted cells were subjected to variant allele frequency measurement using droplet digital polymerase chain reaction and transcriptomic profiling using bulk RNA sequencing, respectively.

RESULTS

Using droplet digital polymerase chain reaction, we detected similar variant allele frequency of CHIP in monocytes from blood and macrophages from atheromas and heart tissues, even among heart macrophages with and without CCR2 (C-C motif chemokine receptor 2) expression. Bulk RNA sequencing revealed a proinflammatory gene profile of myeloid cells from DNMT3A or TET2 mutation carriers compared with those from noncarriers.

CONCLUSIONS

Quantitatively, CHIP-mutated myeloid cells did not preferentially accumulate in cardiovascular tissues, but qualitatively, they expressed a more disease-prone phenotype.

Implications for neutrophils in cardiac arrhythmias

Cardiac arrhythmias commonly occur as a result of aberrant electrical impulse formation or conduction in the myocardium. Frequently discussed triggers include underlying heart diseases such as myocardial ischemia, electrolyte imbalances, or genetic anomalies of ion channels involved in the tightly regulated cardiac action potential. Recently, the role of innate immune cells in the onset of arrhythmic events has been highlighted in numerous studies, correlating leukocyte expansion in the myocardium to increased arrhythmic burden. Here, we aim to call attention to the role of neutrophils in the pathogenesis of cardiac arrhythmias and their expansion during myocardial ischemia and infectious disease manifestation. In addition, we will elucidate molecular mechanisms associated with neutrophil activation and discuss their involvement as direct mediators of arrhythmogenicity.

Fibroblast mediated dynamics in diffusively uncoupled myocytes: a simulation study using 2-cell motifs

In healthy hearts myocytes are typically coupled to nearest neighbours through gap junctions. Under pathological conditions such as fibrosis, or in scar tissue, or across ablation lines myocytes can uncouple from their neighbours. Electrical conduction may still occur via fibroblasts that not only couple proximal myocytes but can also couple otherwise unconnected regions. We hypothesise that such coupling can alter conduction between myocytes via introduction of delays or by initiation of premature stimuli that can potentially result in reentry or conduction blocks. To test this hypothesis we have developed several 2-cell motifs and investigated the effect of fibroblast mediated electrical coupling between uncoupled myocytes. We have identified various regimes of myocyte behaviour that depend on the strength of gap-junctional conductance, connection topology, and parameters of the myocyte and fibroblast models. These motifs are useful in developing a mechanistic understanding of long-distance coupling on myocyte dynamics and enable the characterisation of interaction between different features such as myocyte and fibroblast properties, coupling strengths and pacing period. They are computationally inexpensive and allow for incorporation of spatial effects such as conduction velocity. They provide a framework for constructing scar tissue boundaries and enable linking of cellular level interactions with scar induced arrhythmia.

Cardiovascular aging: spotlight on mitochondria

Mitochondria are cellular organelles critical for ATP production and are particularly relevant to cardiovascular diseases including heart failure, atherosclerosis, ischemia-reperfusion injury, and cardiomyopathies. With advancing age, even in the absence of clinical disease, mitochondrial homeostasis becomes disrupted (e.g., redox balance, mitochondrial DNA damage, oxidative metabolism, and mitochondrial quality control). Mitochondrial dysregulation leads to the accumulation of damaged and dysfunctional mitochondria, producing excessive reactive oxygen species and perpetuating mitochondrial dysfunction. In addition, mitochondrial DNA, cardiolipin, and N-formyl peptides are potent activators of cell-intrinsic and -extrinsic inflammatory pathways. These age-related mitochondrial changes contribute to the development of cardiovascular diseases. This review covers the impact of aging on mitochondria and links these mechanisms to therapeutic implications for age-associated cardiovascular diseases.

An increase in intercellular crosstalk and electrotonic coupling between cardiomyocytes and nonmyocytes reshapes the electrical conduction in the metabolic heart characterized by short QT intervals in ECGs

Cardiac conduction abnormalities are disorders in metabolic syndrome (MetS), however, their mechanisms are unknown. Although ventricular arrhythmia reflects the changes in QT-interval of electrocardiograms associated with the changes in cardiomyocyte action potential durations (APDs), recent studies emphasize role of intercellular crosstalk between cardiomyocytes and nonmyocytes via passive (electrotonic)-conduction. Therefore, considering the possible increase in intercellular interactions of nonmyocytes with cardiomyocytes, we hypothesized an early-cardiac-remodeling characterized by short QT-interval via contributions and modulations of changes by nonmyocytes to the ventricular APs in an early-stage MetS hearts. Following the feeding of 8-week-old rats with a high-sucrose diet (32%; MetS rats) and validation of insulin resistance, there was a significant increase in heart rate and changes in the electrical characteristics of the hearts, especially a shortening in action potential (AP) duration of the papillary muscles. The patch-clamp analysis of ventricular cardiomyocytes showed an increase in the Na+ -channel currents while there were decreases in  l-type Ca2+ -channel (LTCC) currents with unchanged K+ -channel currents. There was an increase in the phosphorylated form of connexin 43 (pCx43), mostly with lateral localization on sarcolemma, while its unphosphorylated form (Cx43) exhibited a high degree of localization within intercalated discs. A high-level positively-stained α-SMA and CD68 cells were prominently localized and distributed in interfibrillar spaces of the heart, implying the possible contributions of myofibroblasts and macrophages to both shortened APDs and abnormal electrical conduction in MetS hearts. Our data propose a previously unrecognized pathway for SQT induction in the heart. This pathway includes not only the contribution of short ventricular-APDs via ionic mechanisms but also increasing contributions of the electrotonic-cardiomyocyte depolarization, spontaneous electrical activity-associated fast heterogeneous impulse conduction in the heart via increased interactions and relocations between cardiomyocytes and nonmyocytes, which may be an explanation for the development of an SQT in early-cardiac-remodeling.

Cellular coupling in the heart

Fibroblasts in scar tissue elicit myocyte excitation and promote arrhythmia in mouse hearts.

The heterocellular heart: identities, interactions, and implications for cardiology

The heterocellular nature of the heart has been receiving increasing attention in recent years. In addition to cardiomyocytes as the prototypical cell type of the heart, non-myocytes such as endothelial cells, fibroblasts, or immune cells are coming more into focus. The rise of single-cell sequencing technologies enables  identification of ever more subtle differences and has reignited the question of what defines a cell's identity. Here we provide an overview of the major cardiac cell types, describe their roles in homeostasis, and outline recent findings on non-canonical functions that may be of relevance for cardiology. We highlight modes of biochemical and biophysical interactions between different cardiac cell types and discuss the potential implications of the heterocellular nature of the heart for basic research and therapeutic interventions.