Fibroblast progenitor cells are recruited into the myocardium prior to the development of myocardial fibrosis

Coronary microvascular dysfunction in hypertrophy and heart failure

Left ventricular (LV) hypertrophy (LVH) is a growth in left myocardial mass mainly caused by increased cardiomyocyte size. LVH can be a physiological adaptation to physical exercise or a pathological condition either primary, i.e. genetic, or secondary to LV overload. Patients with both primary and secondary LVH have evidence of coronary microvascular dysfunction (CMD). The latter is mainly due to capillary rarefaction and adverse remodelling of intramural coronary arterioles due to medial wall thickening with an increased wall/lumen ratio. An important feature of this phenomenon is the diffuse nature of this remodelling, which generally affects the coronary microvessels in the whole of the left ventricle. Patients with LVH secondary to arterial hypertension can develop both heart failure with preserved ejection fraction (HFpEF) and heart failure with reduced ejection fraction (HFrEF). These patients can develop HFrEF via a 'direct pathway' with an interval myocardial infarction and also in its absence. On the other hand, patients can develop HFpEF that can then progress to HFrEF with or without interval myocardial infarction. A similar evolution towards LV dysfunction and both HFpEF and HFrEF can occur in patients with hypertrophic cardiomyopathy, the most common genetic cardiomyopathy with a phenotype characterized by massive LVH. In this review article, we will discuss both the experimental and clinical studies explaining the mechanisms responsible for CMD in LVH as well as the evidence linking CMD with HFpEF and HFrEF.

The Vascular Involvement in Soft Tissue Fibrosis-Lessons Learned from Pathological Scarring

Soft tissue fibrosis in important organs such as the heart, liver, lung, and kidney is a serious pathological process that is characterized by excessive connective tissue deposition. It is the result of chronic but progressive accumulation of fibroblasts and their production of extracellular matrix components such as collagens. Research on pathological scars, namely, hypertrophic scars and keloids, may provide important clues about the mechanisms that drive soft tissue fibrosis, in particular the vascular involvement. This is because these dermal fibrotic lesions bear all of the fibrotic characteristics seen in soft tissue fibrosis. Moreover, their location on the skin surface means they are readily observable and directly treatable and therefore more accessible to research. We will focus here on the roles that blood vessel-associated cells play in cutaneous scar pathology and assess from the literature whether these cells also contribute to other soft tissue fibroses. These cells include endothelial cells, which not only exhibit aberrant functions but also differentiate into mesenchymal cells in pathological scars. They also include pericytes, hepatic stellate cells, fibrocytes, and myofibroblasts. This article will review with broad strokes the roles that these cells play in the pathophysiology of different soft tissue fibroses. We hope that this brief but wide-ranging overview of the vascular involvement in fibrosis pathophysiology will aid research into the mechanisms underlying fibrosis and that this will eventually lead to the development of interventions that can prevent, reduce, or even reverse fibrosis formation and/or progression.

KCa3.1 Channels Promote Cardiac Fibrosis Through Mediating Inflammation and Differentiation of Monocytes Into Myofibroblasts in Angiotensin II -Treated Rats

Background Cardiac fibrosis is a core pathological process associated with heart failure. The recruitment and differentiation of primitive fibroblast precursor cells of bone marrow origin play a critical role in pathological interstitial cardiac fibrosis. The K_{C a}3.1 channels are expressed in both ventricular fibroblasts and circulating mononuclear cells in rats and are upregulated by angiotensin II . We hypothesized that K_{C a}3.1 channels mediate the inflammatory microenvironment in the heart, promoting the infiltrated bone marrow-derived circulating mononuclear cells to differentiate into myofibroblasts, leading to myocardial fibrosis. Methods and Results We established a cardiac fibrosis model in rats by infusing angiotensin II to evaluate the impact of the specific K_{C a}3.1 channel blocker TRAM -34 on cardiac fibrosis. At the same time, mouse CD 4⁺ T cells and rat circulating mononuclear cells were separated to investigate the underlying mechanism of the TRAM -34 anti-cardiac fibrosis effect. TRAM -34 significantly attenuated cardiac fibrosis and the inflammatory reaction and reduced the number of fibroblast precursor cells and myofibroblasts. Inhibition of K_{C a}3.1 channels suppressed angiotensin II -stimulated expression and secretion of interleukin-4 and interleukin-13 in CD 4⁺ T cells and interleukin-4- or interleukin-13-induced differentiation of monocytes into fibrocytes. Conclusions K_{C a}3.1 channels facilitate myocardial inflammation and the differentiation of bone marrow-derived monocytes into myofibroblasts in cardiac fibrosis caused by angiotensin II infusion.

Monocyte-derived fibrocytes elimination had little contribution on liver fibrosis

BACKGROUND

Monocyte-derived fibrocytes play an important role in the progression of fibrosis in the skin, lungs, heart and kidney. However, the contribution of fibrocytes to liver fibrosis is unclear. The aim of this study was to investigate whether fibrocytes contributed to fibrosis progression in the livers of carbon tetrachloride (CCl₄)-treated mice.

METHODS

C57BL/6J mice were divided into 4 groups: normal control group, CCl₄-treated group, CCl₄ + control liposome-treated group, and CCl₄ + clodronate liposome-treated group. For the elimination of systemic monocyte and monocyte-derived fibrocyte, one group was treated with clodronate liposome, and another group with control liposome as a control. After 4 weeks of treatment, hepatic mononuclear cells were subjected to immunofluorescent (IF) staining and fluorescence-activated cell sorter (FACS) analysis to detect fibrocytes. Measurement of collagen-positive Sirius red stained area and collagen-I mRNA expression in the liver were performed to evaluate the degree of liver fibrosis quantitatively.

RESULTS

In the liver of the CCl₄-treated and CCl₄ + control liposome-treated groups, the number of fibrocytes, the area positive for Sirius red staining and collagen-I mRNA expression significantly increased compared with those in the normal control group. In the liver of the CCl₄ + clodronate liposome-treated group, few fibrocytes was observed as in the normal control group, but Sirius red staining positive area and collagen-I mRNA expression were increased and equivalent to the CCl₄-treated and CCl₄ + control liposome-treated groups.

CONCLUSION

Monocyte-derived fibrocytes play a minimal role in CCl₄-induced liver fibrosis. Cells other than fibrocytes such as hepatic stellate cells play a central role in liver fibrosis.

Fibrocyte migration, differentiation and apoptosis during the corneal wound healing response to injury

The aim of this study was to determine whether bone marrow-derived fibrocytes migrate into the cornea after stromal scar-producing injury and differentiate into alpha-smooth muscle actin (αSMA) + myofibroblasts. Chimeric mice expressing green fluorescent protein (GFP) bone marrow cells had fibrosis (haze)-generating irregular phototherapeutic keratectomy (PTK). Multiplex immunohistochemistry (IHC) for GFP and fibrocyte markers (CD34, CD45, and vimentin) was used to detect fibrocyte infiltration into the corneal stroma and the development of GFP+ αSMA+ myofibroblasts. IHC for activated caspase-3, GFP and CD45 was used to detect fibrocyte and other hematopoietic cells undergoing apoptosis. Moderate haze developed in PTK-treated mouse corneas at 14 days after surgery and worsened, and persisted, at 21 days after surgery. GFP+ CD34+ CD45+ fibrocytes, likely in addition to other CD34+ and/or CD45+ hematopoietic and stem/progenitor cells, infiltrated the cornea and were present in the stroma in high numbers by one day after PTK. The fibrocytes and other bone marrow-derived cells progressively decreased at four days and seven days after surgery. At four days after PTK, 5% of the GFP+ cells expressed activated caspase-3. At 14 days after PTK, more than 50% of GFP+ CD45+ cells were also αSMA+ myofibroblasts. At 21 days after PTK, few GFP+ αSMA+ cells persisted in the stroma and more than 95% of those remaining expressed activated caspase-3, indicating they were undergoing apoptosis. GFP+ CD45+ SMA+ cells that developed from 4 to 21 days after irregular PTK were likely developed from fibrocytes. After irregular PTK in the strain of C57BL/6-C57/BL/6-Tg(UBC-GFP)30Scha/J chimeric mice, however, more than 95% of fibrocytes and other hematopoietic cells underwent apoptosis prior to the development of mature αSMA+ myofibroblasts. Most GFP+ CD45+ αSMA+ myofibroblasts that did develop subsequently underwent apoptosis-likely due to epithelial basement membrane regeneration and deprivation of epithelium-derived TGFβ requisite for myofibroblast survival.

Entanglement of GSK-3β, β-catenin and TGF-β1 signaling network to regulate myocardial fibrosis

Nearly every form of the heart disease is associated with myocardial fibrosis, which is characterized by the accumulation of activated cardiac fibroblasts (CFs) and excess deposition of extracellular matrix (ECM). Although, CFs are the primary mediators of myocardial fibrosis in a diseased heart, in the traditional view, activated CFs (myofibroblasts) and resulting fibrosis were simply considered the secondary consequence of the disease, not the cause. Recent studies from our lab and others have challenged this concept by demonstrating that fibroblast activation and fibrosis are not simply the secondary consequence of a diseased heart, but are crucial for mediating various myocardial disease processes. In regards to the mechanism, the vast majority of literature is focused on the direct role of canonical SMAD-2/3-mediated TGF-β signaling to govern the fibrogenic process. Herein, we will discuss the emerging role of the GSK-3β, β-catenin and TGF-β1-SMAD-3 signaling network as a critical regulator of myocardial fibrosis in the diseased heart. The underlying molecular interactions and cross-talk among signaling pathways will be discussed. We will primarily focus on recent in vivo reports demonstrating that CF-specific genetic manipulation can lead to aberrant myocardial fibrosis and sturdy cardiac phenotype. This will allow for a better understanding of the driving role of CFs in the myocardial disease process. We will also review the specificity and limitations of the currently available genetic tools used to study myocardial fibrosis and its associated mechanisms. A better understanding of the GSK-3β, β-catenin and SMAD-3 signaling network may provide a novel therapeutic target for the management of myocardial fibrosis in the diseased heart.

Cardiac Fibrosis and Arrhythmogenesis

Myocardial injury, mechanical stress, neurohormonal activation, inflammation, and/or aging all lead to cardiac remodeling, which is responsible for cardiac dysfunction and arrhythmogenesis. Of the key histological components of cardiac remodeling, fibrosis either in the form of interstitial, patchy, or dense scars, constitutes a key histological substrate of arrhythmias. Here we discuss current research findings focusing on the role of fibrosis, in arrhythmogenesis. Numerous studies have convincingly shown that patchy or interstitial fibrosis interferes with myocardial electrophysiology by slowing down action potential propagation, initiating reentry, promoting after-depolarizations, and increasing ectopic automaticity. Meanwhile, there has been increasing appreciation of direct involvement of myofibroblasts, the activated form of fibroblasts, in arrhythmogenesis. Myofibroblasts undergo phenotypic changes with expression of gap-junctions and ion channels thereby forming direct electrical coupling with cardiomyocytes, which potentially results in profound disturbances of electrophysiology. There is strong evidence that systemic and regional inflammatory processes contribute to fibrogenesis (i.e., structural remodeling) and dysfunction of ion channels and Ca2+ homeostasis (i.e., electrical remodeling). Recognizing the pivotal role of fibrosis in the arrhythmogenesis has promoted clinical research on characterizing fibrosis by means of cardiac imaging or fibrosis biomarkers for clinical stratification of patients at higher risk of lethal arrhythmia, as well as preclinical research on the development of antifibrotic therapies. At the end of this review, we discuss remaining key questions in this area and propose new research approaches. © 2017 American Physiological Society. Compr Physiol 7:1009-1049, 2017.

Role of Circulating Fibrocytes in Cardiac Fibrosis

OBJECTIVE

It is revealed that circulating fibrocytes are elevated in patients/animals with cardiac fibrosis, and this review aims to provide an introduction to circulating fibrocytes and their role in cardiac fibrosis.

DATA SOURCES

This review is based on the data from 1994 to present obtained from PubMed. The search terms were "circulating fibrocytes " and "cardiac fibrosis ".

STUDY SELECTION

Articles and critical reviews, which are related to circulating fibrocytes and cardiac fibrosis, were selected.

RESULTS

Circulating fibrocytes, which are derived from hematopoietic stem cells, represent a subset of peripheral blood mononuclear cells exhibiting mixed morphological and molecular characteristics of hematopoietic and mesenchymal cells (CD34+/CD45+/collagen I+). They can produce extracellular matrix and many cytokines. It is shown that circulating fibrocytes participate in many fibrotic diseases, including cardiac fibrosis. Evidence accumulated in recent years shows that aging individuals and patients with hypertension, heart failure, coronary heart disease, and atrial fibrillation have more circulating fibrocytes in peripheral blood and/or heart tissue, and this elevation of circulating fibrocytes is correlated with the degree of fibrosis in the hearts.

CONCLUSIONS

Circulating fibrocytes are effector cells in cardiac fibrosis.

Implications for the role of macrophages in a model of myocardial fibrosis: CCR2(-/-) mice exhibit an M2 phenotypic shift in resident cardiac macrophages

BACKGROUND

Macrophages (MΦ) are functionally diverse and dynamic. Until recently, cardiac MΦ were assumed to be monocyte derived; however, resident cardiac MΦ (rCMΦ), present at baseline, were identified in myocardia and have been implicated in cardiac healing. Previously, we demonstrated that CCR2(-/-) mice are protected from myocardial fibrosis - an observation initially attributed to changes in infiltrating monocytes. Here, we reexplored this observation in the context of our new understanding of rCMΦ.

METHODS

Male CCR2(-/-) and C57BL/6 hearts were digested and purified to a single cell suspension, incubated with fluorophore-linked antibodies (CCR2, CX3CR1, CD11b, Ly6C, TNF-α, and IL-10), and assessed by flow cytometry. Differentiated MΦ were cocultured with fibroblasts in order to characterize how MΦ phenotype influences fibroblast activation. Fibroblasts were characterized for their expression of smooth muscle cell actin (SMA).

RESULTS

A significant decrease in Ly6C expression was observed in the CCR2(-/-) cardiac MΦ population relative to WT, which corresponded with significantly lower TNF-α expression and significantly higher IL-10 expression. Using in vitro coculture system, classical MΦ promoted fibroblast activation relative to nonclassical MΦ.

CONCLUSION

CCR2(-/-) rCMΦ favor a more antiinflammatory phenotype relative to WT controls. Moreover, a shift toward the antiinflammatory promotes proliferation, but not activation in vitro. Together, these observations suggest that antiinflammatory cardiac MΦ populations may inhibit myocardial fibrosis in a pathological setting by preventing the activation of fibroblasts.

NEWS AND NOTEWORTHY

Here, we provide novel evidence for baseline differences in rCMΦ phenotypes (i.e. classical vs. nonclassical) and how these differences could modulate cardiac healing. Importantly, we observed differences in how classical vs. nonclassical MΦ influenced fibroblast activation, which could, in turn, affect fibrosis.

Chemokines and Heart Disease: A Network Connecting Cardiovascular Biology to Immune and Autonomic Nervous Systems

Among the chemokines discovered to date, nineteen are presently considered to be relevant in heart disease and are involved in all stages of cardiovascular response to injury. Chemokines are interesting as biomarkers to predict risk of cardiovascular events in apparently healthy people and as possible therapeutic targets. Moreover, they could have a role as mediators of crosstalk between immune and cardiovascular system, since they seem to act as a “working-network” in deep linkage with the autonomic nervous system. In this paper we will describe the single chemokines more involved in heart diseases; then we will present a comprehensive perspective of them as a complex network connecting the cardiovascular system to both the immune and the autonomic nervous systems. Finally, some recent evidences indicating chemokines as a possible new tool to predict cardiovascular risk will be described.

Anthracycline-dependent cardiotoxicity and extracellular matrix remodeling

The mechanisms of anthracycline-dependent cardiotoxicity have been studied widely, with the suggested principal mechanism of anthracycline damage being the generation of reactive oxygen species by iron-anthracycline complexes, leading to lipid peroxidation and membrane damage. An increasing number of researchers studying cardiovascular events associated with anthracycline-based chemotherapy are addressing cardiac extracellular matrix (ECM) remodeling. The heart is an efficient muscular pump, with the cardiomyocytes and intramural coronary vasculature of the heart tethered in an ECM consisting of a network of fibrillar, structural proteins, mostly collagens. Increasing evidence suggests that the ECM plays a complex and diverse role in the processes initiated by anthracycline-class drugs that lead to cardiac damage. This review discusses adverse myocardial remodeling induced by anthracyclines and focuses on their mechanisms of action.

The comparison of methods to identify the presence of fibrocytes in formalin-fixed, paraffin-embedded archival cardiac tissue with coronary heart disease

The purpose of this study was to find the optimal technical approach to identify the presence of fibrocytes in formalin-fixed, paraffin-embedded archival cardiac tissue with CHD (coronary heart disease). Using the coexpression markers CD45 and αSMA, the presence of fibrocytes was examined by three different methods, including double immunohistochemistry staining, combination labeling of immunohistochemistry and immunofluorescence and double immunofluorescence labeling. Double immunohistochemistry staining was very difficult to identify the CD45(+)/αSMA(+) fibrocytes. Although combination staining of immunohistochemistry and immunofluorescence has made it possible to evaluate the co-localization of CD45 and αSMA in the fibrocytes, this method was prone to produce many false positive cells. In contrast, CD45(+)/αSMA(+) fibrocytes could be clearly recognized by double immunofluorescence labeling. In conclusion, double immunofluorescence labeling is the optimal technical approach to identify the presence of fibrocytes in routinely processed cardiac tissue with CHD.

CXCR6 plays a critical role in angiotensin II-induced renal injury and fibrosis

OBJECTIVE

Recent studies have shown that angiotensin II (Ang II) plays a critical role in the pathogenesis and progression of hypertensive kidney disease. However, the signaling mechanisms are poorly understood. In this study, we investigated the role of CXCR6 in Ang II-induced renal injury and fibrosis.

APPROACH AND RESULTS

Wild-type and CXCR6-green fluorescent protein (GFP) knockin mice were treated with Ang II via subcutaneous osmotic minipumps at 1500 ng/kg per minute after unilateral nephrectomy for ≤ 4 weeks. Wild-type and CXCR6-GFP knockin mice had virtually identical blood pressure at baseline. Ang II treatment led to an increase in blood pressure that was similar between wild-type and CXCR6-GFP knockin mice. CXCR6-GFP knockin mice were protected from Ang II-induced renal dysfunction, proteinuria, and fibrosis. CXCR6-GFP knockin mice accumulated fewer bone marrow-derived fibroblasts and myofibroblasts and produced less extracellular matrix protein in the kidneys after Ang II treatment. Furthermore, CXCR6-GFP knockin mice exhibited fewer F4/80(+) macrophages and CD3(+) T cells and expressed less proinflammatory cytokines in the kidneys after Ang II treatment. Finally, wild-type mice engrafted with CXCR6(-/-) bone marrow cells displayed fewer bone marrow-derived fibroblasts, macrophages, and T cells in the kidney after Ang II treatment when compared with wild-type mice engrafted with CXCR6(+/+) bone marrow cells.

CONCLUSIONS

Our results indicate that CXCR6 plays a pivotal role in the development of Ang II-induced renal injury and fibrosis through regulation of macrophage and T-cell infiltration and bone marrow-derived fibroblast accumulation.

Targeting cardiac fibroblasts to treat fibrosis of the heart: focus on HDACs

Cardiac fibrosis is implicated in numerous physiologic and pathologic conditions, including scar formation, heart failure and cardiac arrhythmias. However the specific cells and signaling pathways mediating this process are poorly understood. Lysine acetylation of nucleosomal histone tails is an important mechanism for the regulation of gene expression. Additionally, proteomic studies have revealed that thousands of proteins in all cellular compartments are subject to reversible lysine acetylation, and thus it is becoming clear that this post-translational modification will rival phosphorylation in terms of biological import. Acetyl groups are conjugated to lysine by histone acetyltransferases (HATs) and removed from lysine by histone deacetylases (HDACs). Recent studies have shown that pharmacologic agents that alter lysine acetylation by targeting HDACs have the remarkable ability to block pathological fibrosis. Here, we review the current understanding of cardiac fibroblasts and the fibrogenic process with respect to the roles of lysine acetylation in the control of disease-related cardiac fibrosis. Potential for small molecule HDAC inhibitors as anti-fibrotic therapeutics that target cardiac fibroblasts is highlighted. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

Antibody therapy can enhance AngiotensinII-induced myocardial fibrosis

Background

Myocardial fibrosis is a pathological process that is characterized by disrupted regulation of extracellular matrix proteins resulting in permanent scarring of the heart tissue and eventual diastolic heart failure. Pro-fibrotic molecules including transforming growth factor-β and connective tissue growth factor are expressed early in the AngiotensinII (AngII)-induced and other models of myocardial fibrosis. As such, antibody-based therapies against these and other targets are currently under development.

Results

In the present study, C57Bl/6 mice were subcutaneously implanted with a mini-osmotic pump containing either AngII (2.0 μg/kg/min) or saline control for 3 days in combination with mIgG (1 mg/kg/d) injected through the tail vein. Fibrosis was assessed after picosirius red staining of myocardial cross-sections and was significantly increased after AngII exposure compared to saline control (11.37 ± 1.41%, 4.94 ± 1.15%; P <0.05). Non-specific mIgG treatment (1 mg/kg/d) significantly increased the amount of fibrosis (26.34 ± 3.03%; P <0.01). However, when AngII exposed animals were treated with a Fab fragment of the mIgG or mIgM, this exacerbation of fibrosis was no longer observed (14.49 ± 2.23%; not significantly different from AngII alone).

Conclusions

These data suggest that myocardial fibrosis was increased by the addition of exogenous non-specific antibodies in an Fc-mediated manner. These findings could have substantial impact on the future experimental design of antibody-based therapeutics.

Class I HDACs regulate angiotensin II-dependent cardiac fibrosis via fibroblasts and circulating fibrocytes

Fibrosis, which is defined as excessive accumulation of fibrous connective tissue, contributes to the pathogenesis of numerous diseases involving diverse organ systems. Cardiac fibrosis predisposes individuals to myocardial ischemia, arrhythmias and sudden death, and is commonly associated with diastolic dysfunction. Histone deacetylase (HDAC) inhibitors block cardiac fibrosis in pre-clinical models of heart failure. However, which HDAC isoforms govern cardiac fibrosis, and the mechanisms by which they do so, remains unclear. Here, we show that selective inhibition of class I HDACs potently suppresses angiotensin II (Ang II)-mediated cardiac fibrosis by targeting two key effector cell populations, cardiac fibroblasts and bone marrow-derived fibrocytes. Class I HDAC inhibition blocks cardiac fibroblast cell cycle progression through derepression of the genes encoding the cyclin-dependent kinase (CDK) inhibitors, p15 and p57. In contrast, class I HDAC inhibitors block agonist-dependent differentiation of fibrocytes through a mechanism involving repression of ERK1/2 signaling. These findings define novel roles for class I HDACs in the control of pathological cardiac fibrosis. Furthermore, since fibrocytes have been implicated in the pathogenesis of a variety of human diseases, including heart, lung and kidney failure, our results suggest broad utility for isoform-selective HDAC inhibitors as anti-fibrotic agents that function, in part, by targeting these circulating mesenchymal cells.

Regulation and role of connective tissue growth factor in AngII-induced myocardial fibrosis

Exposure of rodents to angiotensin II (AngII) is a common model of fibrosis. We have previously shown that cellular infiltration of bone marrow-derived progenitor cells (fibrocytes) occurs before deposition of extracellular matrix and is associated with the production of connective tissue growth factor (CTGF). In the present study, we characterized the role of CTGF in promoting fibrocyte accumulation and regulation after AngII exposure. In animals exposed to AngII using osmotic minipumps (2.0 μg/kg per min), myocardial CTGF mRNA peaked at 6 hours (21-fold; P < 0.01), whereas transforming growth factor-β (TGF-β) peaked at 3 days (fivefold; P < 0.05) compared with saline control. Early CTGF expression occurred before fibrocyte migration (1 day) into the myocardium or ECM deposition (3 days). CTGF protein expression was evident by day 3 of AngII exposure and seemed to be localized to resident cells. Isolated cardiomyocytes and microvascular endothelial cells responded to AngII with increased CTGF production (2.1-fold and 2.8-fold, respectively; P < 0.05), which was abolished with the addition of anti-TGF-β neutralizing antibody. The effect of CTGF on isolated fibrocytes suggested a role in fibrocyte proliferation (twofold; P < 0.05) and collagen production (2.3-fold; P < 0.05). In summary, we provide strong evidence that AngII exposure first resulted in Smad2-dependent production of CTGF by resident cells (6 hours), well before the accumulation of fibrocytes or TGF-β mRNA up-regulation. In addition, CTGF contributes to fibrocyte proliferation in the myocardium and enhances fibrocyte differentiation into a myofibroblast phenotype responsible for ECM deposition.

Fibrocytes are associated with the fibrosis of coronary heart disease

Fibrocytes contribute significantly to fibrosis in many cardiac diseases. However, it is not clear whether fibrocytes are associated with the fibrosis in coronary heart disease (CHD). The aim of this study was to determine whether fibrocytes are involved in cardiac fibrosis in CHD. We identified the presence of fibrocytes in CHD heart by immunofluorescence and confocal microscopy, examined the collagen volume fraction by Masson's Trichrome staining, and evaluated the correlation between fibrocytes and cardiac fibrosis. In conjunction, we examined the location of CXCL12, a homing factor and specific ligand for CXCR4, by immunohistochemistry. Fibrocytes were identified in 26 out of 27 CHD hearts and in 10 out of 11 normal hearts. Combinations, including CD34/αSMA, CD34/procollagen-I, CD45/αSMA, CXCR4/procollagen-I and CXCR4/αSMA, stained significantly more fibrocytes in CHD hearts as compared with those in normal hearts (p<0.05). There were positive correlations between the collagen volume fraction and the amount of fibrocytes (r=0.558; p=0.003<0.01) and between the number of CXCR4(+) fibrocytes and the CXCL12(+) cells (r=0.741; p=0.000<0.01) in CHD hearts. Based upon these findings, we conclude that fibrocytes, likely recruited through the CXCR4/CXCL12 axis, may contribute to the increase in the fibroblast population in CHD heart.

Treatment with activated protein C (aPC) is protective during the development of myocardial fibrosis: an angiotensin II infusion model in mice

Aims

Myocardial fibrosis contributes to the development of heart failure. Activated Protein C (aPC) is a circulating anticoagulant with anti-inflammatory and cytoprotective properties. Using a model of myocardial fibrosis second to Angiotensin II (AngII) infusion, we investigated the novel therapeutic function aPC in the development of fibrosis.

Methods and Results

C57Bl/6 and Tie2-EPCR mice were infused with AngII (2.0 µg/kg/min), AngII and aPC (0.4 µg/kg/min) or saline for 3d. Hearts were harvested and processed for analysis or used for cellular isolation. Basic histology and collagen deposition were assessed using histologic stains. Transcript levels of molecular mediators were analyzed by quantitative RT-PCR. Mice infused with AngII exhibited multifocal areas of myocardial cellular infiltration associated with significant collagen deposition compared to saline control animals (p<0.01). AngII-aPC infusion inhibited this cellular infiltration and the corresponding collagen deposition. AngII-aPC infusion also inhibited significant expression of the pro-fibrotic cytokines TGF-β1, CTGF and PDGF found in AngII only infused animals (p<0.05). aPC signals through its receptor, EPCR. Using Tie2-EPCR animals, where endothelial cells over-express EPCR and exhibit enhanced aPC-EPCR signaling, no significant reduction in cellular infiltration or fibrosis was evident with AngII infusion suggesting aPC-mediate protection is endothelial cell independent. Isolated infiltrating cells expressed significant EPCR transcripts suggesting a direct effect on infiltrating cells.

Conclusions

This data indicates that aPC treatment abrogates the fibrogenic response to AngII. aPC does not appear to confer protection by stimulating the endothelium but by acting directly on the infiltrating cells, potentially inhibiting migration or activation.

Regeneration in heart disease-Is ECM the key?

The heart possesses a regeneration potential derived from endogenous and exogenous stem and progenitor cell populations, though baseline regeneration appears to be sub-therapeutic. This limitation was initially attributed to a lack of cells with cardiomyogenic potential following an insult to the myocardium. Rather, recent studies demonstrate increased numbers of cardiomyocyte progenitor cells in diseased hearts. Given that the limiting factor does not appear to be cell quantity but rather repletion of functional cardiomyocytes, it is crucial to understand potential mechanisms inhibiting progenitor cell differentiation. One of the extensively studied areas in heart disease is extracellular matrix (ECM) remodeling, with both the composition and mechanical properties of the ECM undergoing changes in diseased hearts. This review explores the influence of ECM properties on cardiomyogenesis and adult cardiac progenitor cells.