The dynamics of fibroblast-myocyte-capillary interactions in the heart

Native mechano-regulative matrix properties stabilize alternans dynamics and reduce spiral wave stabilization in cardiac tissue

The stability of wave conduction in the heart is strongly related to the proper interplay between the electrophysiological activation and mechanical contraction of myocytes and extracellular matrix (ECM) properties. In this study, we statistically compare bioengineered cardiac tissues cultured on soft hydrogels ( E ≃ 12 kPa) and rigid glass substrates by focusing on the critical threshold of alternans, network-physiological tissue properties, and the formation of stable spiral waves that manifest after wave breakups. For the classification of wave dynamics, we use an improved signal oversampling technique and introduce simple probability maps to identify and visualize spatially concordant and discordant alternans as V- and X-shaped probability distributions. We found that cardiac tissues cultured on ECM-mimicking soft hydrogels show a lower variability of the calcium transient durations among cells in the tissue. This lowers the likelihood of forming stable spiral waves because of the larger dynamical range that tissues can be stably entrained with to form alternans and larger spatial spiral tip movement that increases the chance of self-termination on the tissue boundary. Conclusively, we show that a dysfunction in the excitation-contraction coupling dynamics facilitates life-threatening arrhythmic states such as spiral waves and, thus, highlights the importance of the network-physiological interplay between contractile myocytes and the ECM.

The Roles of Cardiac Fibroblasts and Endothelial Cells in Myocarditis

In myocarditis caused by various etiologies, activated immune cells and the immune regulatory factors released by them play important roles. But in this complex microenvironment, non-immune cells and non-cardiomyocytes in the heart, such as cardiomyocytes (CMs), cardiac fibroblasts (CFs) and endothelial cells (ECs), play the role of “sentinel”, amplify inflammation, and interact with the cardiomyocytes. The complex interactions between them are rarely paid attention to. This review will re-examine the functions of CFs and ECs in the pathological conditions of myocarditis and their direct and indirect interactions with CMs, in order to have a more comprehensive understanding of the pathogenesis of myocarditis and better guide the drug development and clinical treatment of myocarditis.

Fibroblast-mediated intercellular crosstalk in the healthy and diseased heart

Cardiac fibroblasts constitute a major cell population in the heart. They secrete extracellular matrix components and various other factors shaping the microenvironment of the heart. In silico analysis of intercellular communication based on single-cell RNA sequencing revealed that fibroblasts are the source of the majority of outgoing signals to other cell types. This observation suggests that fibroblasts play key roles in orchestrating cellular interactions that maintain organ homeostasis but that can also contribute to disease states. Here, we will review the current knowledge of fibroblast interactions in the healthy, diseased, and aging heart. We focus on the interactions that fibroblasts establish with other cells of the heart, specifically cardiomyocytes, endothelial cells and immune cells, and particularly those relying on paracrine, electrical, and exosomal communication modes.

Estrogen Deprivation and Myocardial Infarction: Role of Aerobic Exercise Training, Inflammation and Metabolomics

In general, postmenopausal women present higher mortality, and worse prognosis after myocardial infarction (MI) compared to men, due to estrogen deficiency. After MI, cardiovascular alterations occur such as the autonomic imbalance and the pro-inflammatory cytokines increase. In this sense, therapies that aim to minimize deleterious effects caused by myocardial ischemia are important. Aerobic training has been proposed as a promising intervention in the prevention of cardiovascular diseases. On the other hand, some studies have attempted to identify potential biomarkers for cardiovascular diseases or specifically for MI. For this purpose, metabolomics has been used as a tool in the discovery of cardiovascular biomarkers. Therefore, the objective of this work is to discuss the changes involved in ovariectomy, myocardial infarction, and aerobic training, with emphasis on inflammation and metabolism.

Aerobic training prevents cardiometabolic changes triggered by myocardial infarction in ovariectomized rats

This study aimed to evaluate the impact of aerobic training (AT) on autonomic, cardiometabolic, ubiquitin-proteasome activity, and inflammatory changes evoked by myocardial infarction (MI) in ovariectomized rats. Female Wistar rats were ovariectomized and divided into four groups: sedentary + sham (SS), sedentary + MI (SI), AT + sham surgery (TS), AT + MI (TI). AT was performed on a treadmill for 8 weeks before MI. Infarcted rats previously subjected to AT presented improved physical capacity, increased interleukin-10, and decreased pro-inflammatory cytokines. Metabolomic analysis identified and quantified 62 metabolites, 9 were considered significant by the Vip Score. SS, SI, and TS groups presented distinct metabolic profiles; however, TI could not be distinguished from the SS group. MI dramatically increased levels of dimethylamine, and AT prevented this response. Our findings suggest that AT may be useful in preventing the negative changes in functional, inflammatory, and metabolic parameters related to MI in ovariectomized rats.

Cardiac tissue remodeling in healthy aging: the road to pathology

This review aims to highlight the normal physiological remodeling that occurs in healthy aging hearts, including changes that occur in contractility, conduction, valve function, large and small coronary vessels, and the extracellular matrix. These "normal" age-related changes serve as the foundation that supports decreased plasticity and limited ability for tissue remodeling during pathophysiological states such as myocardial ischemia and heart failure. This review will identify populations at greater risk for poor tissue remodeling in advanced age along with present and future therapeutic strategies that may ameliorate dysfunctional tissue remodeling in aging hearts.

The interstitium in cardiac repair: role of the immune-stromal cell interplay

Cardiac regeneration, that is, restoration of the original structure and function in a damaged heart, differs from tissue repair, in which collagen deposition and scar formation often lead to functional impairment. In both scenarios, the early-onset inflammatory response is essential to clear damaged cardiac cells and initiate organ repair, but the quality and extent of the immune response vary. Immune cells embedded in the damaged heart tissue sense and modulate inflammation through a dynamic interplay with stromal cells in the cardiac interstitium, which either leads to recapitulation of cardiac morphology by rebuilding functional scaffolds to support muscle regrowth in regenerative organisms or fails to resolve the inflammatory response and produces fibrotic scar tissue in adult mammals. Current investigation into the mechanistic basis of homeostasis and restoration of cardiac function has increasingly shifted focus away from stem cell-mediated cardiac repair towards a dynamic interplay of cells composing the less-studied interstitial compartment of the heart, offering unexpected insights into the immunoregulatory functions of cardiac interstitial components and the complex network of cell interactions that must be considered for clinical intervention in heart diseases.

Mechanical regulation of gene expression in cardiac myocytes and fibroblasts

The intact heart undergoes complex and multiscale remodelling processes in response to altered mechanical cues. Remodelling of the myocardium is regulated by a combination of myocyte and non-myocyte responses to mechanosensitive pathways, which can alter gene expression and therefore function in these cells. Cellular mechanotransduction and its downstream effects on gene expression are initially compensatory mechanisms during adaptations to the altered mechanical environment, but under prolonged and abnormal loading conditions, they can become maladaptive, leading to impaired function and cardiac pathologies. In this Review, we summarize mechanoregulated pathways in cardiac myocytes and fibroblasts that lead to altered gene expression and cell remodelling under physiological and pathophysiological conditions. Developments in systems modelling of the networks that regulate gene expression in response to mechanical stimuli should improve integrative understanding of their roles in vivo and help to discover new combinations of drugs and device therapies targeting mechanosignalling in heart disease.

Contribution of left ventricular residual stress by myocytes and collagen: existence of inter-constituent mechanical interaction

We quantify the contribution of myocytes, collagen fibers and their interactions to the residual stress field found in the left ventricle (LV) using both experimental and theoretical methods. Ring tissue samples extracted from normal rat, male and female, LV were treated with collagenase and decellularization to isolate myocytes and collagen fibers, respectively. Opening angle tests were then performed on these samples as well as intact tissue samples containing both constituents that served as control. Our results show that the collagen fibers are the main contributor to the residual stress fields found in the LV. Specifically, opening angle measured in collagen-only samples (106.45[Formula: see text] ± 23.02[Formula: see text]) and myocytes-only samples (21.00[Formula: see text] ± 4.37[Formula: see text]) was significantly higher and lower than that of the control (57.88[Formula: see text] ± 12.29[Formula: see text]), respectively. A constrained mixture (CM) modeling framework was then used to infer these experimental results. We show that the framework cannot reproduce the opening angle found in the intact tissue with measurements made on the collagen-only and myocytes-only samples. Given that the CM framework assumes that each constituent contributes to the overall mechanics simply by their mere presence, this result suggests the existence of some myocyte-collagen mechanical interaction that cannot be ignored in the LV. We then propose an extended CM formulation that takes into account of the inter-constituent mechanical interaction in which constituents are deformed additionally when they are physically combined into a mixture. We show that the intact tissue opening angle can be recovered in this framework.

Versatile Fabrication Approach of Conductive Hydrogels via Copolymerization with Vinyl Monomers

Functionalized poly(ethylene dioxythiophene) (f-PEDOT) was copolymerized with two vinyl monomers of different hydrophilicity, acrylic acid and hydroxyethyl methacrylate, to produce electroconductive hydrogels with a range of physical and electronic properties. These hydrogels not only possessed tailored physical properties, such as swelling ratios and mechanical properties, but also displayed electroactivity dependent on the chemical composition of the network. Raman spectroscopy indicated that the functional PEDOT in the hydrogels is in an oxidized form, most likely accounting for the good electrochemical response of the hydrogels observed in physiological buffer. In vitro cell studies showed that cardiac cells respond differently when seeded on hydrogel substrates with different compositions. This study presents a facile approach for the fabrication of electroconductive hydrogels with a range of properties, paving the way for scaffolds that can meet the requirements of different electroresponsive tissues.

Luhong formula inhibits myocardial fibrosis in a paracrine manner by activating the gp130/JAK2/STAT3 pathway in cardiomyocytes

ETHNOPHARMACOLOGICAL RELEVANCE

Luhong formula (LHF)-a traditional Chinese medicine containing Cervus nippon Temminck, Carthamus tinctorius L., Cinnamomum cassia Presl, Codonopisis pilosula( Franch.) Nannf., Astragalus membranaceus ( Fisch.) Bge. var. mongholicus ( Bge.) Hsiao, Lepidium apetalum Willd-is used in the treatment of heart failure.

AIM OF THE STUDY

To investigate the antifibrotic efficacy of LHF in a myocardial infarction-induced rat model of heart failure and to determine its mechanism of action.

MATERIAL AND METHODS

Myocardial infarction was induced in rats by coronary artery ligation, and cardiac fibroblasts were isolated. Neonatal rat cardiomyocytes (NRCMs) were isolated from 2 to 3-day-old Sprague-Dawley male rats, and cardiomyocyte hypertrophy was induced by isoprenaline. Histological examination was carried out to estimate the degree of myocardial fibrosis. Expression of gp130/JAK2/STAT3 pathway proteins was measured by western blot. The mRNA levels of downstream genes of gp130/JAK2/STAT3 pathway (i.e., CTGF, TSP-1, and TIMP1) were determined by RT-PCR; while CTGF, TSP-1, and TIMP1 protein levels were measured by ELISA. To investigate paracrine effects, cell proliferation and collagen synthesis was measured after treating cardiac fibroblasts with the conditioned media from isoprenaline-treated NRCMs.

RESULTS

Histopathological changes showed that LHF inhibited myocardial fibrosis in heart failure rats. Treatment with LHF up-regulated gp130, JAK2, and STAT3 protein expression in heart tissue, and down-regulated CTGF, TSP-1, and TIMP1 gene expression. Isoprenaline-treated NRCMs displayed lower expression of the gp130, JAK2, and STAT3 pathway proteins and higher secretion of its downstream signaling molecules (CTGF, TSP-1, TIMP1). LHF inhibited cardiac fibroblast proliferation and collagen synthesis after treatment with the conditioned media from isoprenaline-treated NRCMs.

CONCLUSION

LHF treatment attenuates myocardial fibrosis in vivo. LHF inhibits cardiac fibroblasts proliferation and collagen synthesis in a paracrine manner by activating the gp130/JAK2/STAT3 pathway in cardiomyocytes, thereby inhibiting the secretion of downstream profibrogenic cytokines.

Human cardiac fibroblasts adaptive responses to controlled combined mechanical strain and oxygen changes in vitro

Upon cardiac pathological conditions such as ischemia, microenvironmental changes instruct a series of cellular responses that trigger cardiac fibroblasts-mediated tissue adaptation and inflammation. A comprehensive model of how early environmental changes may induce cardiac fibroblasts (CF) pathological responses is far from being elucidated, partly due to the lack of approaches involving complex and simultaneous environmental stimulation. Here, we provide a first analysis of human primary CF behavior by means of a multi-stimulus microdevice for combined application of cyclic mechanical strain and controlled oxygen tension. Our findings elucidate differential human CFs responses to different combinations of the above stimuli. Individual stimuli cause proliferative effects (PHH3⁺ mitotic cells, YAP translocation, PDGF secretion) or increase collagen presence. Interestingly, only the combination of hypoxia and a simulated loss of contractility (2% strain) is able to additionally induce increased CF release of inflammatory and pro-fibrotic cytokines and matrix metalloproteinases.

DOI:http://dx.doi.org/10.7554/eLife.22847.001

When the supply of oxygen to the heart is reduced, its cells start to die within hours, the heart muscle becomes less able to contract, and the area becomes inflamed. This inflammation is accompanied by an influx of immune cells. It also activates other cells known as cardiac fibroblasts that help to break down the framework of molecules that supported the damaged heart tissue and replace it with a scar. This response is part of the normal repair process, but it can lead to the formation of scar tissue in non-damaged areas of the heart. Excess scar tissue makes the heart muscle less able to contract and increases the affected individual’s chance of dying.

Understanding how this repair process works is an important step in developing strategies to minimise the damage caused by coronary artery disease or heart attacks. However, existing laboratory models are only partly able to recreate the conditions seen in real heart tissue. To properly understand the response at the level of living cells, a more complete model is needed.

Ugolini et al. now report improvements to a small device, referred to as a lab-on-chip, that can subject cells to mechanical strain. The improvements mean the device could also recreate other conditions seen early on in damaged heart tissue, specifically the reduced supply of oxygen. Replicating combinations of mechanical changes and oxygen supplies meant that the impact of these conditions on human cardiac fibroblasts could be directly observed in the laboratory for the first time. Ugolini et al. found that a lack of contraction and low oxygen levels triggered the cardiac fibroblasts to produce inflammatory molecules and molecules associated with the formation of scar tissue. This resembles the response seen in living hearts.

The next step is to improve the lab-on-chip device further by adding other cell types, including heart muscle cells and immune cells. A more complete model may aid future research into how our hearts operate in both health and disease.

DOI:http://dx.doi.org/10.7554/eLife.22847.002

Tissue engineering the cardiac microenvironment: Multicellular microphysiological systems for drug screening

The ability to accurately detect cardiotoxicity has become increasingly important in the development of new drugs. Since the advent of human pluripotent stem cell-derived cardiomyocytes, researchers have explored their use in creating an in vitro drug screening platform. Recently, there has been increasing interest in creating 3D microphysiological models of the heart as a tool to detect cardiotoxic compounds. By recapitulating the complex microenvironment that exists in the native heart, cardiac microphysiological systems have the potential to provide a more accurate pharmacological response compared to current standards in preclinical drug screening. This review aims to provide an overview on the progress made in creating advanced models of the human heart, including the significance and contributions of the various cellular and extracellular components to cardiac function.

Extracellular matrix-mediated cellular communication in the heart

The extracellular matrix (ECM) is a complex and dynamic scaffold that maintains tissue structure and dynamics. However, the view of the ECM as an inert architectural support has been increasingly challenged. The ECM is a vibrant meshwork, a crucial organizer of cellular microenvironments. It plays a direct role in cellular interactions regulating cell growth, survival, spreading, proliferation, differentiation and migration through the intricate relationship among cellular and acellular tissue components. This complex interrelationship preserves cardiac function during homeostasis; however it is also responsible for pathologic remodeling following myocardial injury. Therefore, enhancing our understanding of this cross-talk may provide mechanistic insights into the pathogenesis of heart failure and suggest new approaches to novel, targeted pharmacologic therapies. This review explores the implications of ECM-cell interactions in myocardial cell behavior and cardiac function at baseline and following myocardial injury.

Cellular Communications in the Heart

Heart failure is one of the leading causes of morbidity and mortality worldwide. Cardiac remodelling is first an adaptive, becoming a maladaptive, compensatory mechanism that finally causes ventricular dysfunction independently of the etiology of the initial insult. In the present article the authors describe the elements of the human heart, examining their basic functions and their inter-communication under both normal and pathological circumstances. Cardiac myocytes carry out mechanical and electrical functions of the heart and cardiac fibroblasts maintain its structural integrity. Several factors can affect fibroblast activation and under pathological stress they transdifferentiate into myofibroblasts. Endothelial cells have complex biological functions, including the control of vascular permeability, vasomotion, regulation of haemostasis, immune responses and angiogenesis. The extracellular matrix is a complex architectural network consisting of a variety of proteins. Various routes using a plethora of products and mediators contribute to the cross-talk of the myocytes with endothelial cells, extracellular matrix and cardiac fibroblasts. A better understanding of the entire mechanism of cellular communication by the established or the more recently discovered agents will certainly emerge promising new perspectives when looking at the prevention of heart failure and leading to more substantial therapeutic interventions.

The role of extracellular matrix in age-related conduction disorders: a forgotten player?

Cardiovascular aging is a physiological process gradually leading to structural degeneration and functional loss of all the cardiac and vascular components. Conduction system is also deeply influenced by the aging process with relevant reflexes in the clinical side. Age-related arrhythmias carry significant morbidity and mortality and represent a clinical and economical burden. An important and unjustly unrecognized actor in the pathophysiology of aging is represented by the extracellular matrix (ECM) that not only structurally supports the heart determining its mechanical and functional properties, but also sends a biological signaling regulating cellular function and maintaining tissue homeostasis. At the biophysical level, cardiac ECM exhibits a peculiar degree of anisotropy, which is among the main determinants of the conductive properties of the specialized electrical conduction system. Age-associated alterations of cardiac ECM are therefore able to profoundly affect the function of the conduction system with striking impact on the patient clinical conditions. This review will focus on the ECM changes that occur during aging in the heart conduction system and on their translation to the clinical scenario. Potential diagnostic and therapeutical perspectives arising from the knowledge on ECM age-associated alterations are further discussed.

Passive ventricular remodeling in cardiac disease: focus on heterogeneity

Passive ventricular remodeling is defined by the process of molecular ventricular adaptation to different forms of cardiac pathophysiology. It includes changes in tissue architecture, such as hypertrophy, fiber disarray, alterations in cell size and fibrosis. Besides that, it also includes molecular remodeling of gap junctions, especially those composed by Connexin43 proteins (Cx43) in the ventricles that affect cell-to-cell propagation of the electrical impulse, and changes in the sodium channels that modify excitability. All those alterations appear mainly in a heterogeneous manner, creating irregular and inhomogeneous electrical and mechanical coupling throughout the heart. This can predispose to reentry arrhythmias and adds to a further deterioration into heart failure. In this review, passive ventricular remodeling is described in Hypertrophic Cardiomyopathy (HCM), Dilated Cardiomyopathy (DCM), Ischemic Cardiomyopathy (ICM), and Arrhythmogenic Cardiomyopathy (ACM), with a main focus on the heterogeneity of those alterations mentioned above.

Klotho and phosphate are modulators of pathologic uremic cardiac remodeling

Cardiac dysfunction in CKD is characterized by aberrant cardiac remodeling with hypertrophy and fibrosis. CKD is a state of severe systemic Klotho deficiency, and restoration of Klotho attenuates vascular calcification associated with CKD. We examined the role of Klotho in cardiac remodeling in models of Klotho deficiency-genetic Klotho hypomorphism, high dietary phosphate intake, aging, and CKD. Klotho-deficient mice exhibited cardiac dysfunction and hypertrophy before 12 weeks of age followed by fibrosis. In wild-type mice, the induction of CKD led to severe cardiovascular changes not observed in control mice. Notably, non-CKD mice fed a high-phosphate diet had lower Klotho levels and greatly accelerated cardiac remodeling associated with normal aging compared with those on a normal diet. Chronic elevation of circulating Klotho because of global overexpression alleviated the cardiac remodeling induced by either high-phosphate diet or CKD. Regardless of the cause of Klotho deficiency, the extent of cardiac hypertrophy and fibrosis correlated tightly with plasma phosphate concentration and inversely with plasma Klotho concentration, even when adjusted for all other covariables. High-fibroblast growth factor-23 concentration positively correlated with cardiac remodeling in a Klotho-deficient state but not a Klotho-replete state. In vitro, Klotho inhibited TGF-β1-, angiotensin II-, or high phosphate-induced fibrosis and abolished TGF-β1- or angiotensin II-induced hypertrophy of cardiomyocytes. In conclusion, Klotho deficiency is a novel intermediate mediator of pathologic cardiac remodeling, and fibroblast growth factor-23 may contribute to cardiac remodeling in concert with Klotho deficiency in CKD, phosphotoxicity, and aging.

The transcription factor GLI1 modulates the inflammatory response during pancreatic tissue remodeling

Pancreatic cancer, one of the deadliest human malignancies, is almost uniformly associated with a mutant, constitutively active form of the oncogene Kras. Studies in genetically engineered mouse models have defined a requirement for oncogenic KRAS in both the formation of pancreatic intraepithelial neoplasias, the most common precursor lesions to pancreatic cancer, and in the maintenance and progression of these lesions. Previous work using an inducible model allowing tissue-specific and reversible expression of oncogenic Kras in the pancreas indicates that inactivation of this GTPase at the pancreatic intraepithelial neoplasia stage promotes pancreatic tissue repair. Here, we extend these findings to identify GLI1, a transcriptional effector of the Hedgehog pathway, as a central player in pancreatic tissue repair upon Kras inactivation. Deletion of a single allele of Gli1 results in improper stromal remodeling and perdurance of the inflammatory infiltrate characteristic of pancreatic tumorigenesis. Strikingly, this partial loss of Gli1 affects activated fibroblasts in the pancreas and the recruitment of immune cells that are vital for tissue recovery. Analysis of the mechanism using expression and chromatin immunoprecipitation assays identified a subset of cytokines, including IL-6, mIL-8, Mcp-1, and M-csf (Csf1), as direct GLI1 target genes potentially mediating this phenomenon. Finally, we demonstrate that canonical Hedgehog signaling, a known regulator of Gli1 activity, is required for pancreas recovery. Collectively, these data delineate a new pathway controlling tissue repair and highlight the importance of GLI1 in regulation of the pancreatic microenvironment during this cellular process.

Regulatory RNAs and paracrine networks in the heart

Cardiac hypertrophy and fibrosis are two closely related adaptive response mechanisms of the myocardium to mechanical, metabolic, and genetic stress that finally contribute to the development of heart failure (HF). This relation is based on a dynamic interplay between many cell types including cardiomyocytes and fibroblasts during disease progression. Both cell types secrete a variety of growth factors, cytokines, and hormones that influence hypertrophic cardiomyocyte growth and fibrotic fibroblast activation in a paracrine and autocrine manner. It has become evident that, aside proteinous signals, microRNAs (miRNAs) and possible other RNA species such as long non-coding RNAs are potential players in such a cell-to-cell communication. By directly acting as paracrine signals or by modulating downstream intercellular signalling mediators, miRNAs can act as moderators of the intercellular crosstalk. These small regulators can potentially be secreted in a 'mircrine' fashion, so that miRNAs can be assumed as the message itself. This review will summarize the recent findings about the paracrine crosstalk between cardiac fibroblasts and cardiomyocytes and addresses how miRNAs may be involved in this interplay. It also highlights therapeutic strategies targeting factors of pathological communication for the treatment of HF.

Fibroblast-myocyte electrotonic coupling: does it occur in native cardiac tissue?

Heterocellular electrotonic coupling between cardiac myocytes and non-excitable connective tissue cells has been a long-established and well-researched fact in vitro. Whether or not such coupling exists in vivo has been a matter of considerable debate. This paper reviews the development of experimental insight and conceptual views on this topic, describes evidence in favour of and against the presence of such coupling in native myocardium, and identifies directions for further study needed to resolve the riddle, perhaps less so in terms of principal presence which has been demonstrated, but undoubtedly in terms of extent, regulation, patho-physiological context, and actual relevance of cardiac myocyte–non-myocyte coupling in vivo. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

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Electrical coupling of cardiomyocytes and fibroblasts is well-established in vitro

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Whether such hetero-cellular coupling exists in vivo has been a matter of debate

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We review the development of experimental and conceptual insight into the topic

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Conclusion 1: hetero-cellular coupling in heart tissue has been shown in principle

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Conclusion 2: extent, regulation, context, and relevance remain to be established

Origin, development, and differentiation of cardiac fibroblasts

Cardiac fibroblasts are the most abundant cell in the mammalian heart. While they have been historically underappreciated in terms of their functional contributions to cardiac development and physiology, they and their activated form, myofibroblasts, are now known to play key roles in both development and disease through structural, paracrine, and electrical interactions with cardiomyocytes. The lack of specific markers for fibroblasts currently convolutes the study of this dynamic cell lineage, but advances in marker analysis and lineage mapping technologies are continuously being made. Understanding how to best utilize these tools, both individually and in combination, will help to elucidate the functional significance of fibroblast-cardiomyocyte interactions in vivo. Here we review what is currently known about the diverse roles played by cardiac fibroblasts and myofibroblasts throughout development and periods of injury with the intent of emphasizing the duality of their nature. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium ".

Dynamic cell-cell and cell-ECM interactions in the heart

Recent studies have placed an increasing amount of emphasis on the cardiovascular system and understanding how the heart and its vasculature can be regenerated following pathological stresses, such as hypertension and myocardial infarction. The remodeling process involves the permanent cellular constituents of the heart including myocytes, fibroblasts, endothelial cells, pericytes, smooth muscle cells and stem cells. It also includes transient cell populations, such as immune cells (e.g. lymphocytes, mast cells and macrophages) and circulating stem cells. Following injury, there are dramatic shifts in the various cardiac cell populations that can affect cell-cell and cell-extracellular matrix interactions and cardiac function. Cardiac fibroblasts are a key component in normal heart function, as well as during the remodeling process through dynamic cell-cell interactions and synthesis and degradation of the extracellular matrix. Fibroblasts dynamically interact with the various cardiac cell populations through mechanical, chemical (autocrine and/or paracrine) and electrophysiological means to alter gene and protein expression, cellular processes and ultimately cardiac function. Better understanding these cell-cell and cell-extracellular matrix interactions and their biological consequences should provide novel therapeutic targets for the treatment of heart disease. In this review we discuss the nature of these interactions and the importance of these interactions in maintaining normal heart function, as well as their role in the cardiac remodeling process. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

Placental growth factor as a protective paracrine effector in the heart

In response to injury, the myocardium hypertrophies in an attempt to maintain or augment function, which is associated with ventricular remodeling and changes in capillary density. During the compensatory phase of the hypertrophic response, the myocardium maintains output and is characterized by a coordinated neo-angiogenic and fibrotic response that supports cardiomyocyte health and survival. Emerging evidence shows that paracrine-mediated cross talk between cardiac myocytes and nonmyocytes within the heart is critical for cardiac adaptation to stress, including the extent of hypertrophy and angiogenesis. This review discusses recent results indicating that placental growth factor (PGF; also called PlGF), a secreted factor within the vascular endothelial growth factor superfamily, is a pivotal mediator of adaptive cardiac hypertrophy and beneficial angiogenesis through its ability to coordinate the intercellular communication between different cell types in the heart.

Cross talk between cardiac myocytes and fibroblasts: from multiscale investigative approaches to mechanisms and functional consequences

The heart is comprised of a syncytium of cardiac myocytes (CM) and surrounding nonmyocytes, the majority of which are cardiac fibroblasts (CF). CM and CF are highly interspersed in the myocardium with one CM being surrounded by one or more CF. Bidirectional cross talk between CM and CF plays important roles in determining cardiac mechanical and electrical function in both normal and diseased hearts. Genetically engineered animal models and in vitro studies have provided evidence that CM and CF can regulate each other's function. Their cross talk contributes to structural and electrical remodeling in both atria and ventricles and appears to be involved in the pathogenesis of various heart diseases that lead to heart failure and arrhythmia disorders. Mechanisms of CM-CF cross talk, which are not yet fully understood, include release of paracrine factors, direct cell-cell interactions via gap junctions and potentially adherens junctions and nanotubes, and cell interactions with the extracellular matrix. In this article, we provide an overview of the existing multiscale experimental and computational approaches for the investigation of cross talk between CM and CF and review recent progress in our understanding of the functional consequences and underlying mechanisms. Targeting cross talk between CM and CF could potentially be used therapeutically for the modulation of the cardiac remodeling response in the diseased heart and may lead to new strategies for the treatment of heart failure or rhythm disturbances.

Cardiac intercellular communication: are myocytes and fibroblasts fair-weather friends?

The cardiac fibroblast (CF) has historically been thought of as a quiescent cell of the heart, passively maintaining the extracellular environment for the cardiomyocytes (CM), the functional cardiac cell type. The increasingly appreciated role of the CF, however, extends well beyond matrix production, governing many aspects of cardiac function including cardiac electrophysiology and contractility. Importantly, its contributions to cardiac pathophysiology and pathologic remodeling have created a shift in the field's focus from the CM to the CF as a therapeutic target in the treatment of cardiac diseases. In response to cardiac injury, the CF undergoes a pathologic phenotypic transition into a myofibroblast, characterized by contractile smooth muscle proteins and upregulation of collagens, matrix proteins, and adhesion molecules. Further, the myofibroblast upregulates expression and secretion of a variety of pro-inflammatory, profibrotic mediators, including cytokines, chemokines, and growth factors. These mediators act in both an autocrine fashion to further activate CFs, as well as in a paracrine manner on both CMs and circulating inflammatory cells to induce myocyte dysfunction and chronic inflammation, respectively. Together, cell-specific cytokine-induced effects exacerbate pathologic remodeling and progression to HF. A better understanding of this dynamic intercellular communication will lead to novel targets for the attenuation of cardiac remodeling. Current strategies aimed at targeting cytokines have been largely unsuccessful in clinical trials, lending insights into ways that such intercellular cross talk can be more effectively attenuated. This review will summarize the current knowledge regarding CF functions in the heart and will discuss the regulation and signaling behind CF-mediated cytokine production and function. We will then highlight clinical trials that have exploited cytokine cross talk in the treatment of heart failure and provide novel strategies currently under investigation that may more effectively target pathologic CF-CM communication for the treatment of cardiac disease. This review explores novel mechanisms to directly attenuate heart failure progression through inhibition of signaling downstream of pro-inflammatory cytokines that are elevated after cardiac injury.

The dynamic role of cardiac fibroblasts in development and disease

Cardiac fibroblasts are the most abundant cell in the mammalian heart. While they have been historically overlooked in terms of functional contributions to development and physiology, cardiac fibroblasts are now front and center. They are currently recognized as key protagonists during both normal development and cardiomyopathy disease, and work together with cardiomyocytes through paracrine, structural, and potentially electrical interactions. However, the lack of specific biomarkers and fibroblast heterogeneous nature currently convolutes the study of this dynamic cell lineage; though, efforts to advance marker analysis and lineage mapping technologies are ongoing. These tools will help elucidate the functional significance of fibroblast-cardiomyocyte interactions in vivo and delineate the dynamic nature of normal and pathological cardiac fibroblasts. Since therapeutic promise lies in understanding the interface between developmental biology and the postnatal injury response, future studies to understand the divergent roles played by cardiac fibroblasts both in utero and following cardiac insult are essential.

Self-interference fluorescence microscopy: three dimensional fluorescence imaging without depth scanning

We present a new method for high-resolution, three-dimensional fluorescence imaging. In contrast to beam-scanning confocal microscopy, where the laser focus must be scanned both laterally and axially to collect a volume, we obtain depth information without the necessity of depth scanning. In this method, the emitted fluorescence is collected in the backward direction and is sent through a phase plate that encodes the depth information into the phase of a spectrally resolved interference pattern. We demonstrate that decoding this phase information allows for depth localization accuracy better than 4 µm over a 500 µm depth-of-field. In a high numerical aperture configuration with a much smaller depth of field, a localization accuracy of tens of nanometers can be achieved. This approach is ideally suited for miniature endoscopes, where space limitations at the endoscope tip render depth scanning difficult. We illustrate the potential for 3D visualization of complex biological samples by constructing a three-dimensional volume of the microvasculature of ex vivo murine heart tissue from a single 2D scan.

Reduced Cx43 Expression Triggers Increased Fibrosis Due to Enhanced Fibroblast Activity

Background—

Arrhythmogenic ventricular remodeling is hallmarked by both reduced gap junction expression and increased collagen deposition. We hypothesized that reduced connexin43 (Cx43) expression is responsible for enhanced fibrosis in the remodeled heart, resulting in an arrhythmogenic substrate. Therefore, we investigated the effect of normal or reduced Cx43 expression on the formation of fibrosis in a physiological (aging) and pathophysiological (transverse aortic constriction [TAC]) mouse model.

Methods and Results—

The Cx43 fl/fl and Cx43 CreER(T)/fl mice were aged 18 to 21 months or, at the age of 3 months, either TAC or sham operated and euthanized after 16 weeks. Epicardial activation mapping of the right and left ventricles was performed on Langendorff perfused hearts. Sustained ventricular arrhythmias were induced in 0 of 11 aged Cx43 fl/fl and 10 of 15 Cx43 Cre-ER(T)/fl mice ( P <0.01). Cx43 expression was reduced by half in aged Cx43 CreER(T)/fl compared with aged Cx43 fl/fl mice, whereas collagen deposition was significantly increased from 1.1±0.2% to 7.4±1.3%. Aged Cx43 CreER(T)/fl mice with arrhythmias had significantly higher levels of fibrosis and conduction heterogeneity than aged Cx43 CreER(T)/fl mice without arrhythmias. The TAC operation significantly increased fibrosis in control compared with sham (4.0±1.2% versus 0.4±0.06%), but this increase was significantly higher in Cx43 CreER(T)/fl mice (10.8±1.4%). Discoidin domain receptor 2 expression was unchanged, but procollagen peptide I and III expression and collagen type 1α2 mRNA levels were higher in TAC–operated Cx43HZ mice.

Conclusions—

Reduced cellular coupling results in more excessive collagen deposition during aging or pressure overload in mice due to enhanced fibroblast activity, leading to increased conduction in homogeneity and proarrhythmia.

Genomes, proteomes, and the central dogma

Systems biology, with its associated technologies of proteomics, genomics, and metabolomics, is driving the evolution of our understanding of cardiovascular physiology. Rather than studying individual molecules or even single reactions, a systems approach allows integration of orthogonal data sets from distinct tiers of biological data, including gene, RNA, protein, metabolite, and other component networks. Together these networks give rise to emergent properties of cellular function, and it is their reprogramming that causes disease. We present 5 observations regarding how systems biology is guiding a revisiting of the central dogma: (1) It deemphasizes the unidirectional flow of information from genes to proteins; (2) it reveals the role of modules of molecules as opposed to individual proteins acting in isolation; (3) it enables discovery of novel emergent properties; (4) it demonstrates the importance of networks in biology; and (5) it adds new dimensionality to the study of biological systems.

Desmoplakin is important for proper cardiac cell-cell interactions

Normal cardiac function is maintained through dynamic interactions of cardiac cells with each other and with the extracellular matrix. These interactions are important for remodeling during cardiac growth and pathophysiological conditions. However, the precise mechanisms of these interactions remain unclear. In this study we examined the importance of desmoplakin (DSP) in cardiac cell-cell interactions. Cell-cell communication in the heart requires the formation and preservation of cell contacts by cell adhesion junctions called desmosome-like structures. A major protein component of this complex is DSP, which plays a role in linking the cytoskeletal network to the plasma membrane. Our laboratory previously generated a polyclonal antibody (1611) against the detergent soluble fraction of cardiac fibroblast plasma membrane. In attempting to define which proteins 1611 recognizes, we performed two-dimensional electrophoresis and identified DSP as one of the major proteins recognized by 1611. Immunoprecipitation studies demonstrated that 1611 was able to directly pulldown DSP. We also demonstrate that 1611 and anti-DSP antibodies co-localize in whole heart sections. Finally, using a three-dimensional in vitro cell-cell interaction assay, we demonstrate that 1611 can inhibit cell-cell interactions. These data indicate that DSP is an important protein for cell-cell interactions and affects a variety of cellular functions, including cytokine secretion.

SPARC mediates early extracellular matrix remodeling following myocardial infarction

Secreted protein, acidic, and rich in cysteine (SPARC) is a matricellular protein that functions in the extracellular processing of newly synthesized collagen. Collagen deposition to form a scar is a key event following a myocardial infarction (MI). Because the roles of SPARC in the early post-MI setting have not been defined, we examined age-matched wild-type (WT; n=22) and SPARC-deficient (null; n=25) mice at day 3 post-MI. Day 0 WT (n=28) and null (n=20) mice served as controls. Infarct size was 52 ± 2% for WT and 47 ± 2% for SPARC null (P=NS), indicating that the MI injury was comparable in the two groups. By echocardiography, WT mice increased end-diastolic volumes from 45 ± 2 to 83 ± 5 μl (P < 0.05). SPARC null mice also increased end-diastolic volumes but to a lesser extent than WT (39 ± 3 to 63 ± 5 μl; P < 0.05 vs. day 0 controls and vs. WT day 3 MI). Ejection fraction fell post-MI in WT mice from 57 ± 2 to 19 ± 1%. The decrease in ejection fraction was attenuated in the absence of SPARC (65 ± 2 to 28 ± 2%). Fibroblasts isolated from SPARC null left ventricle (LV) showed differences in the expression of 22 genes encoding extracellular matrix and adhesion molecule genes, including fibronectin, connective tissue growth factor (CTGF; CCN2), matrix metalloproteinase-3 (MMP-3), and tissue inhibitor of metalloproteinase-2 (TIMP-2). The change in fibroblast gene expression levels was mirrored in tissue protein extracts for fibronectin, CTGF, and MMP-3 but not TIMP-2. Combined, the results of this study indicate that SPARC deletion preserves LV function at day 3 post-MI but may be detrimental for the long-term response due to impaired fibroblast activation.

Cyclophilin A promotes cardiac hypertrophy in apolipoprotein E-deficient mice

OBJECTIVE

Cyclophilin A (CyPA, encoded by Ppia) is a proinflammatory protein secreted in response to oxidative stress in mice and humans. We recently demonstrated that CyPA increased angiotensin II (Ang II)-induced reactive oxygen species (ROS) production in the aortas of apolipoprotein E (Apoe)-/- mice. In this study, we sought to evaluate the role of CyPA in Ang II-induced cardiac hypertrophy.

METHODS AND RESULTS

Cardiac hypertrophy was not significantly different between Ppia+/+ and Ppia-/- mice infused with Ang II (1000 ng/min per kg for 4 weeks). Therefore, we investigated the effect of CyPA under conditions of high ROS and inflammation using the Apoe-/- mice. In contrast to Apoe-/- mice, Apoe-/-Ppia-/- mice exhibited significantly less Ang II-induced cardiac hypertrophy. Bone marrow cell transplantation showed that CyPA in cells intrinsic to the heart plays an important role in the cardiac hypertrophic response. Ang II-induced ROS production, cardiac fibroblast proliferation, and cardiac fibroblast migration were markedly decreased in Apoe-/-Ppia-/- cardiac fibroblasts. Furthermore, CyPA directly induced the hypertrophy of cultured neonatal cardiac myocytes.

CONCLUSIONS

CyPA is required for Ang II-mediated cardiac hypertrophy by directly potentiating ROS production, stimulating the proliferation and migration of cardiac fibroblasts, and promoting cardiac myocyte hypertrophy.

Extracellular matrix roles during cardiac repair

The cardiac extracellular matrix (ECM) provides a platform for cells to maintain structure and function, which in turn maintains tissue function. In response to injury, the ECM undergoes remodeling that involves synthesis, incorporation, and degradation of matrix proteins, with the net outcome determined by the balance of these processes. The major goals of this review are a) to serve as an initial resource for students and investigators new to the cardiac ECM remodeling field, and b) to highlight a few of the key exciting avenues and methodologies that have recently been explored. While we focus on cardiac injury and responses of the left ventricle (LV), the mechanisms reviewed here have pathways in common with other wound healing models.