Neutrophil roles in left ventricular remodeling following myocardial infarction

The role of Wnt signaling in the healing myocardium: a focus on cell specificity

Various cell types are involved in the healing process after myocardial infarction (MI). Besides cardiac resident cells (such as cardiomyocytes, fibroblasts and endothelial cells) already present at the lesion site, a massive influx of leukocytes (mainly monocytes and neutrophils) is observed within hours after the ischemic event. So far, little is known about modes of interaction of these cells. Wnt signaling is an evolutionary conserved signaling cassette known to play an important role in cell-cell communication. While the overall reactivation of Wnt signaling upon ischemic injury is well described, the precise expression pattern of Wnt proteins, however, is far from understood. We here describe known Wnt components that partake in MI healing and differentiate cell-specific aspects. The secretion of Wnt proteins and their antagonists in the context of cardiac inflammation after MI appear to be tightly regulated in a spatial-temporal manner. Overall, we aim to stress the importance of elucidating not only Wnt component-specific aspects, but also their sometimes contradicting effects in different target cells. A better understanding of Wnt signaling in MI healing may eventually lead to the development of successful therapeutic approaches in an often considered "un-druggable" pathway.

Immunomodulatory interventions in myocardial infarction and heart failure: a systematic review of clinical trials and meta-analysis of IL-1 inhibition

Following a myocardial infarction (MI), the immune system helps to repair ischaemic damage and restore tissue integrity, but excessive inflammation has been implicated in adverse cardiac remodelling and development towards heart failure (HF). Pre-clinical studies suggest that timely resolution of inflammation may help prevent HF development and progression. Therapeutic attempts to prevent excessive post-MI inflammation in patients have included pharmacological interventions ranging from broad immunosuppression to immunomodulatory approaches targeting specific cell types or factors with the aim to maintain beneficial aspects of the early post-MI immune response. These include the blockade of early initiators of inflammation including reactive oxygen species and complement, inhibition of mast cell degranulation and leucocyte infiltration, blockade of inflammatory cytokines, and inhibition of adaptive B and T-lymphocytes. Herein, we provide a systematic review on post-MI immunomodulation trials and a meta-analysis of studies targeting the inflammatory cytokine Interleukin-1. Despite an enormous effort into a significant number of clinical trials on a variety of targets, a striking heterogeneity in study population, timing and type of treatment, and highly variable endpoints limits the possibility for meaningful meta-analyses. To conclude, we highlight critical considerations for future studies including (i) the therapeutic window of opportunity, (ii) immunological effects of routine post-MI medication, (iii) stratification of the highly diverse post-MI patient population, (iv) the potential benefits of combining immunomodulatory with regenerative therapies, and at last (v) the potential side effects of immunotherapies.

Neutrophils orchestrate post-myocardial infarction healing by polarizing macrophages towards a reparative phenotype

Aims

Acute myocardial infarction (MI) is the leading cause of mortality worldwide. Anti-inflammatory strategies to reduce neutrophil-driven acute post-MI injury have been shown to limit acute cardiac tissue damage. On the other hand, whether neutrophils are required for resolving post-MI inflammation and repair is unknown.

Methods and Results

We show that neutrophil-depleted mice subjected to MI had worsened cardiac function, increased fibrosis, and progressively developed heart failure. Flow cytometry of blood, lymphoid organs and digested hearts revealed reduced numbers of Ly6Chigh monocytes in infarcts of neutrophil-depleted mice, whereas the number of macrophages increased, which was paralleled by reduced splenic Ly6Chigh monocyte mobilization but enhanced proliferation of cardiac macrophages. Macrophage subtype analysis revealed reduced cardiac expression of M1 markers, whereas M2 markers were increased in neutrophil-depleted mice. Surprisingly, we found reduced expression of phagocytosis receptor myeloid-epithelial-reproductive tyrosine kinase, a marker of reparative M2c macrophages which mediate clearance of apoptotic cells. In agreement with this finding, neutrophil-depleted mice had increased numbers of TUNEL-positive cells within infarcts. We identified neutrophil gelatinase-associated lipocalin (NGAL) in the neutrophil secretome as a key inducer of macrophages with high capacity to engulf apoptotic cells. The cardiac macrophage phenotype in neutrophil-depleted mice was restored by administration of neutrophil secretome or NGAL.

Conclusion

Neutrophils are crucially involved in cardiac repair after MI by polarizing macrophages towards a reparative phenotype. Therapeutic strategies to reduce acute neutrophil-driven inflammation after MI should be carefully balanced as they might interfere with the healing response and cardiac remodelling.

The Mouse Heart Attack Research Tool 1.0 database

The generation of big data has enabled systems-level dissections into the mechanisms of cardiovascular pathology. Integration of genetic, proteomic, and pathophysiological variables across platforms and laboratories fosters discoveries through multidisciplinary investigations and minimizes unnecessary redundancy in research efforts. The Mouse Heart Attack Research Tool (mHART) consolidates a large data set of over 10 yr of experiments from a single laboratory for cardiovascular investigators to generate novel hypotheses and identify new predictive markers of progressive left ventricular remodeling after myocardial infarction (MI) in mice. We designed the mHART REDCap database using our own data to integrate cardiovascular community participation. We generated physiological, biochemical, cellular, and proteomic outputs from plasma and left ventricles obtained from post-MI and no-MI (naïve) control groups. We included both male and female mice ranging in age from 3 to 36 mo old. After variable collection, data underwent quality assessment for data curation (e.g., eliminate technical errors, check for completeness, remove duplicates, and define terms). Currently, mHART 1.0 contains >888,000 data points and includes results from >2,100 unique mice. Database performance was tested, and an example is provided to illustrate database utility. This report explains how the first version of the mHART database was established and provides researchers with a standard framework to aid in the integration of their data into our database or in the development of a similar database. NEW & NOTEWORTHY The Mouse Heart Attack Research Tool combines >888,000 cardiovascular data points from >2,100 mice. We provide this large data set as a REDCap database to generate novel hypotheses and identify new predictive markers of adverse left ventricular remodeling following myocardial infarction in mice and provide examples of use. The Mouse Heart Attack Research Tool is the first database of this size that integrates data sets across platforms that include genomic, proteomic, histological, and physiological data.

Molecular imaging of myocardial infarction with Gadofluorine P - A combined magnetic resonance and mass spectrometry imaging approach

Background

Molecular MRI is becoming increasingly important for preclinical research. Validation of targeted gadolinium probes in tissue however has been cumbersome up to now. Novel methodology to assess gadolinium distribution in tissue after in vivo application is therefore needed.

Purpose

To establish combined Magnetic Resonance Imaging (MRI) and Mass Spectrometry Imaging (MSI) for improved detection and quantification of Gadofluorine P deposition in scar formation and myocardial remodeling.

Materials and methods

Animal studies were performed according to institutionally approved protocols. Myocardial infarction was induced by permanent ligation of the left ascending artery (LAD) in C57BL/6J mice. MRI was performed at 7T at 1 week and 6 weeks after myocardial infarction. Gadofluorine P was used for dynamic T₁ mapping of extracellular matrix synthesis during myocardial healing and compared to Gd-DTPA. After in vivo imaging contrast agent concentration as well as distribution in tissue were validated and quantified by spatially resolved Matrix-Assisted Laser Desorption Ionization (MALDI) MSI and Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS) imaging.

Results

Both Gadofluorine P enhancement as well as local tissue content in the myocardial scar were highest at 15 minutes post injection. R₁ values increased from 1 to 6 weeks after MI (1.62 s⁻¹ vs 2.68 s⁻¹, p = 0.059) paralleled by an increase in Gadofluorine P concentration in the infarct from 0.019 mM at 1 week to 0.028 mM at 6 weeks (p = 0.048), whereas Gd-DTPA enhancement showed no differences (3.95 s⁻¹ vs 3.47 s⁻¹, p = 0.701). MALDI-MSI results were corroborated by elemental LA-ICP-MS of Gadolinium in healthy and infarcted myocardium. Histology confirmed increased extracellular matrix synthesis at 6 weeks compared to 1 week.

Conclusion

Adding quantitative MSI to MR imaging enables a quantitative validation of Gadofluorine P distribution in the heart after MI for molecular imaging.

Myocardial infarction remodeling that progresses to heart failure: a signaling misunderstanding

After myocardial infarction, remodeling of the left ventricle involves a wound-healing orchestra involving a variety of cell types. In order for wound healing to be optimal, appropriate communication must occur; these cells all need to come in at the right time, be activated at the right time in the right amount, and know when to exit at the right time. When this occurs, a new homeostasis is obtained within the infarct, such that infarct scar size and quality are sufficient to maintain left ventricular size and shape. The ideal scenario does not always occur in reality. Often, miscommunication can occur between infarct and remote spaces, across the temporal wound-healing spectrum, and across organs. When miscommunication occurs, adverse remodeling can progress to heart failure. This review discusses current knowledge gaps and recent development of the roles of inflammation and the extracellular matrix in myocardial infarction remodeling. In particular, the macrophage is one cell type that provides direct and indirect regulation of both the inflammatory and scar-forming responses. We summarize current research efforts focused on identifying biomarker indicators that reflect the status of each component of the wound-healing process to better predict outcomes.

Featured Article: TGF-β1 dominates extracellular matrix rigidity for inducing differentiation of human cardiac fibroblasts to myofibroblasts

Cardiac fibroblasts and their activated derivatives, myofibroblasts, play a critical role in wound healing after myocardial injury and often contribute to long-term pathological outcomes, such as excessive fibrosis. Thus, defining the microenvironmental factors that regulate the phenotype of cardiac fibroblasts and myofibroblasts could lead to new therapeutic strategies. Both chemical and biomechanical cues have previously been shown to induce myofibroblast differentiation in many organs and species. For example, transforming growth factor beta 1, a cytokine secreted by neutrophils, and rigid extracellular matrix environments have both been shown to promote differentiation. However, the relative contributions of transforming growth factor beta 1 and extracellular matrix rigidity, two hallmark cues in many pathological myocardial microenvironments, to the phenotype of human cardiac fibroblasts are unclear. We hypothesized that transforming growth factor beta 1 and rigid extracellular matrix environments would potentially have a synergistic effect on the differentiation of human cardiac fibroblasts to myofibroblasts. To test this, we seeded primary human adult cardiac fibroblasts onto coverslips coated with polydimethylsiloxane of various elastic moduli, introduced transforming growth factor beta 1, and longitudinally quantified cell phenotype by measuring expression of α-smooth muscle actin, the most robust indicator of myofibroblasts. Our data indicate that, although extracellular matrix rigidity influenced differentiation after one day of transforming growth factor beta 1 treatment, ultimately transforming growth factor beta 1 superseded extracellular matrix rigidity as the primary regulator of myofibroblast differentiation. We also measured expression of POSTN, FAP, and FSP1, proposed secondary indicators of fibroblast/myofibroblast phenotypes. Although these genes partially trended with α-smooth muscle actin expression, they were relatively inconsistent. Finally, we demonstrated that activated myofibroblasts incompletely revert to a fibroblast phenotype after they are re-plated onto new surfaces without transforming growth factor beta 1, suggesting differentiation is partially reversible. Our results provide new insights into how microenvironmental cues affect human cardiac fibroblast differentiation in the context of myocardial pathology, which is important for identifying effective therapeutic targets and dictating supporting cell phenotypes for engineered human cardiac disease models. Impact statement Heart disease is the leading cause of death worldwide. Many forms of heart disease are associated with fibrosis, which increases extracellular matrix (ECM) rigidity and compromises cardiac output. Fibrotic tissue is synthesized primarily by myofibroblasts differentiated from fibroblasts. Thus, defining the cues that regulate myofibroblast differentiation is important for understanding the mechanisms of fibrosis. However, previous studies have focused on non-human cardiac fibroblasts and have not tested combinations of chemical and mechanical cues. We tested the effects of TGF-β1, a cytokine secreted by immune cells after injury, and ECM rigidity on the differentiation of human cardiac fibroblasts to myofibroblasts. Our results indicate that differentiation is initially influenced by ECM rigidity, but is ultimately superseded by TGF-β1. This suggests that targeting TGF-β signaling pathways in cardiac fibroblasts may have therapeutic potential for attenuating fibrosis, even in rigid microenvironments. Additionally, our approach can be leveraged to engineer more precise multi-cellular human cardiac tissue models.

Anatomical-Molecular Distribution of EphrinA1 in Infarcted Mouse Heart Using MALDI Mass Spectrometry Imaging

EphrinA1 is a tyrosine kinase receptor localized in the cellular membrane of healthy cardiomyocytes, the expression of which is lost upon myocardial infarction (MI). Intra-cardiac injection of the recombinant form of ephrinA1 (ephrinA1-Fc) at the time of ligation in mice has shown beneficial effects by reducing infarct size and myocardial necrosis post-MI. To date, immunohistochemistry and Western blotting comprise the only experimental approaches utilized to localize and quantify relative changes of ephrinA1 in sections and homogenates of whole left ventricle, respectively. Herein, we used matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) coupled with a time-of-flight mass spectrometer (MALDI/TOF MS) to identify intact as well as tryptic fragments of ephrinA1 in healthy controls and acutely infarcted murine hearts. The purpose of the present study was 3-fold: (1) to spatially resolve the molecular distribution of endogenous ephrinA1, (2) to determine the anatomical expression profile of endogenous ephrinA1 after acute MI, and (3) to identify molecular targets of ephrinA1-Fc action post-MI. The tryptic fragments detected were identified as the ephrinA1-isoform with 38% and 34% sequence coverage and Mascot scores of 25 for the control and MI hearts, respectively. By using MALDI-MSI, we have been able to simultaneously measure the distribution and spatial localization of ephrinA1, as well as additional cardiac proteins, thus offering valuable information for the elucidation of molecular partners, mediators, and targets of ephrinA1 action in cardiac muscle. Graphical Abstract ᅟ.

Independent trafficking of flavocytochrome b558 and myeloperoxidase to phagosomes during phagocytosis visualised by energy-filtering and energy-dispersive spectroscopy-scanning transmission electron microscopy

When polymorphonuclear leukocytes (PMNs) phagocytose opsonised zymosan particles (OPZ), free radicals and reactive oxygen species (ROS) are formed in the phagosomes. ROS production is mediated by NADPH oxidase (Nox), which transfers electrons in converting oxygen to superoxide (O₂^- ). Nox-generated O₂^- is rapidly converted to other ROS. Free radical-forming secretory vesicles containing the Nox redox center flavocytochrome b558, a membrane protein, and azurophil granules with packaged myeloperoxidase (MPO) have been described. Presuming the probable fusion of these vesicular and granular organelles with phagosomes, the translation process of the enzymes was investigated using energy-filtering and energy-dispersive spectroscopy-scanning transmission electron microscopy. In this work, the primary method for imaging cerium (Ce) ions demonstrated the localisation of H₂ O₂ generated by phagocytosing PMNs. The MPO activity of the same PMNs was continuously monitored using 0.1% 3,3'-diaminobenzidine-tetrahydrochloride (DAB) and 0.01% H₂ O₂ . A detailed view of these vesicular and granular structures was created by overlaying each electron micrograph with pseudocolors: blue for Ce and green for nitrogen (N).

RORγt-expressing cells attenuate cardiac remodeling after myocardial infarction

Aims

Retinoic acid receptor-related orphan nuclear receptor γt (RORγt) is a transcriptional factor responsible for IL-17-producing T-cell differentiation. Although it was demonstrated that RORγt plays essential roles in the onset of autoimmune myocarditis, pathophysiological significance of RORγt in cardiac remodeling after myocardial infarction (MI) remains to be fully elucidated.

Methods and results

MI was generated by ligating coronary artery. The expression of RORγt and IL-17A transcripts increased in murine hearts after MI. Additionally, immunohistochemical staining revealed that RORγt-expressing cells infiltrated in the border zone after MI. Flow cytometric analysis showed that RORγt-expressing cells were released from the spleen at day 1 after MI. Though RORγt-expressing cells in spleen expressed γδTCR or CD4, γδTCR⁺ cells were major population of RORγt-expressing cells that infiltrated into post-infarct myocardium. To address the biological functions of RORγt-expressing cells in infarcted hearts, we used mice with enhanced GFP gene heterozygously knocked-in at RORγt locus (RORγt^+/- mice), which physiologically showed reduced expression of RORγt mRNA in thymus. Kaplan-Meier analysis showed that MI-induced mortality was higher in RORγt^+/- mice than wild-type (WT) mice. Masson’s trichrome staining demonstrated that cardiac injury was exacerbated in RORγt^+/- mice 7 days after MI (Injured area: RORγt^+/-; 42.1±6.5%, WT; 34.0±3.7%, circumference of injured myocardium: RORγt^+/-; 61.8±4.8%, WT; 49.6±5.1%), accompanied by exacerbation of cardiac function (fractional shortening: RORγt^+/-; 32.9±2.9%, WT; 38.3±3.6%). Moreover, immunohistochemical analyses revealed that capillary density in border zone was significantly reduced in RORγt^+/- mice after MI, compared with WT mice, associated with the reduced expression of angiopoietin 2. Finally, the mRNA expression of RORγt, IL-17A, IL-17F and IL-23 receptor (IL-23R) mRNA and protein expression of IL-10 were decreased in RORγt^+/- hearts.

Conclusions

Heterozygous deletion of RORγt gene resulted in aggravated cardiac remodeling, accompanied by reduced capillary density, after MI, suggesting that RORγt-expressing cells contribute to tissue repair in infarcted myocardium.

Leukocyte diversity in resolving and nonresolving mechanisms of cardiac remodeling

In response to myocardial infarction (MI), time-dependent leukocyte infiltration is critical to program the acute inflammatory response. Post-MI leukocyte density, residence time in the infarcted area, and exit from the infarcted injury predict resolving or nonresolving inflammation. Overactive or unresolved inflammation is the primary determinant in heart failure pathology post-MI. Here, our review describes supporting evidence that the acute inflammatory response also guides the generation of healing and regenerative mediators after cardiac damage. Time-dependent leukocyte density and diversity and the magnitude of myocardial injury is responsible for the resolving and nonresolving pathway in myocardial healing. Post MI, the diversity of leukocytes, such as neutrophils, macrophages, and lymphocytes, has been explored that regulate the clearance of deceased cardiomyocytes by using the classic and reparative pathways. Among the innovative factors and intermediates that have been recognized as essential in acute the self-healing and clearance mechanism, we highlight specialized proresolving mediators as the emerging factor for post-MI reparative mechanisms-translational leukocyte modifiers, such as aging, the source of leukocytes, and the milieu around the leukocytes. In the clinical setting, it is possible that leukocyte diversity is more prominent as a result of risk factors, such as obesity, diabetes, and hypertension. Pharmacologic agents are critical modifiers of leukocyte diversity in healing mechanisms that may impair or stimulate the clearance mechanism. Future research is needed, with a focused approach to understand the molecular targets, cellular effectors, and receptors. A clear understanding of resolving and nonresolving inflammation in myocardial healing will help to develop novel targets with major emphasis on the resolution of inflammation in heart failure pathology.-Tourki, B., Halade, G. Leukocyte diversity in resolving and nonresolving mechanisms of cardiac remodeling.

11β-HSD1 suppresses cardiac fibroblast CXCL2, CXCL5 and neutrophil recruitment to the heart post MI

We have previously demonstrated that neutrophil recruitment to the heart following myocardial infarction (MI) is enhanced in mice lacking 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) that regenerates active glucocorticoid within cells from intrinsically inert metabolites. The present study aimed to identify the mechanism of regulation. In a mouse model of MI, neutrophil mobilization to blood and recruitment to the heart were higher in 11β-HSD1-deficient (Hsd11b1^-/- ) relative to wild-type (WT) mice, despite similar initial injury and circulating glucocorticoid. In bone marrow chimeric mice, neutrophil mobilization was increased when 11β-HSD1 was absent from host cells, but not when absent from donor bone marrow-derived cells. Consistent with a role for 11β-HSD1 in 'host' myocardium, gene expression of a subset of neutrophil chemoattractants, including the chemokines Cxcl2 and Cxcl5, was selectively increased in the myocardium of Hsd11b1^-/- mice relative to WT. SM22α-Cre directed disruption of Hsd11b1 in smooth muscle and cardiomyocytes had no effect on neutrophil recruitment. Expression of Cxcl2 and Cxcl5 was elevated in fibroblast fractions isolated from hearts of Hsd11b1^-/- mice post MI and provision of either corticosterone or of the 11β-HSD1 substrate, 11-dehydrocorticosterone, to cultured murine cardiac fibroblasts suppressed IL-1α-induced expression of Cxcl2 and Cxcl5 These data identify suppression of CXCL2 and CXCL5 chemoattractant expression by 11β-HSD1 as a novel mechanism with potential for regulation of neutrophil recruitment to the injured myocardium, and cardiac fibroblasts as a key site for intracellular glucocorticoid regeneration during acute inflammation following myocardial injury.

Pathophysiological roles of canstatin on myofibroblasts after myocardial infarction in rats

Myofibroblasts play an important role during remodeling process after myocardial infarction through proliferation, migration, production and degradation of extracellular matrix (ECM) and contraction. Canstatin, a 24kDa polypeptide, is cleaved from α2 chain of type IV collagen, which is a major component of basement membrane around cardiomyocytes. We examined the effects of canstatin on myofibroblasts isolated from the areas of myocardial infarction. Myocardial infarction model was made by ligating left anterior descending artery of Wistar rats. Two weeks after the operation, the cells were isolated by an explant method and identified as myofibroblasts with immunofluorescence staining. Cell counting assay was performed to examine cell proliferation. Boyden chamber assay was performed to examine cell migration. Expression and phosphorylation of proteins were detected by Western blotting. Collagen gel contraction assay was performed to measure cell contractility. Canstatin stimulated proliferation, secretion of matrix metalloproteinases, expression of cyclooxygenase (COX)-2, and inhibited collagen gel contraction in myofibroblasts. Canstatin increased Akt phosphorylation. LY294002, a phosphoinositide-3-kinase/Akt inhibitor, inhibited the canstatin-induced proliferation. NS-398, a COX-2 inhibitor, suppressed the inhibitory effect of canstatin on collagen gel contraction. Canstatin expression in areas of myocardial infarction 2 weeks after surgery decreased. We for the first time demonstrate that canstatin is an endogenous bioactive molecule regulating the various functions of myofibroblasts after myocardial infarction. The decrease of canstatin expression in the maturated areas of myocardial infarction might lead to stabilization of scar tissues perhaps in part through the reduction of proliferation and ECM degradation as well as the stimulation of contractility in myofibroblasts.

Mechanisms of action of sacubitril/valsartan on cardiac remodeling: a systems biology approach

Sacubitril/Valsartan, proved superiority over other conventional heart failure management treatments, but its mechanisms of action remains obscure. In this study, we sought to explore the mechanistic details for Sacubitril/Valsartan in heart failure and post-myocardial infarction remodeling, using an in silico, systems biology approach. Myocardial transcriptome obtained in response to myocardial infarction in swine was analyzed to address post-infarction ventricular remodeling. Swine transcriptome hits were mapped to their human equivalents using Reciprocal Best (blast) Hits, Gene Name Correspondence, and InParanoid database. Heart failure remodeling was studied using public data available in gene expression omnibus (accession GSE57345, subseries GSE57338), processed using the GEO2R tool. Using the Therapeutic Performance Mapping System technology, dedicated mathematical models trained to fit a set of molecular criteria, defining both pathologies and including all the information available on Sacubitril/Valsartan, were generated. All relationships incorporated into the biological network were drawn from public resources (including KEGG, REACTOME, INTACT, BIOGRID, and MINT). An artificial neural network analysis revealed that Sacubitril/Valsartan acts synergistically against cardiomyocyte cell death and left ventricular extracellular matrix remodeling via eight principal synergistic nodes. When studying each pathway independently, Valsartan was found to improve cardiac remodeling by inhibiting members of the guanine nucleotide-binding protein family, while Sacubitril attenuated cardiomyocyte cell death, hypertrophy, and impaired myocyte contractility by inhibiting PTEN. The complex molecular mechanisms of action of Sacubitril/Valsartan upon post-myocardial infarction and heart failure cardiac remodeling were delineated using a systems biology approach. Further, this dataset provides pathophysiological rationale for the use of Sacubitril/Valsartan to prevent post-infarct remodeling.

The new wonder drug in heart failure management, Sacubitril/Valsartan, rejuvenates the heart by preventing its dilation. Using data from myocardial infarction and heart failure samples, we generated a mathematical model to better understand how Sacubitril/Valsartan modulates pathological heart resize and the combined effect of the drug. Our analysis revealed that Sacubitril/Valsartan mainly acts by blocking both, cell death and the pathological makeover of the outer-membrane of the cardiac cells. These two major processes occur after a heart attack. Most importantly, we discovered a core of 8 proteins that emerge as key players in this process. A better understanding of the mechanism of novel cardiovascular drugs at the most basic level may help decipher future therapies and indications.

Modeling the Human Scarred Heart In Vitro: Toward New Tissue Engineered Models

Cardiac remodeling is critical for effective tissue healing, however, excessive production and deposition of extracellular matrix components contribute to scarring and failing of the heart. Despite the fact that novel therapies have emerged, there are still no lifelong solutions for this problem. An urgent need exists to improve the understanding of adverse cardiac remodeling in order to develop new therapeutic interventions that will prevent, reverse, or regenerate the fibrotic changes in the failing heart. With recent advances in both disease biology and cardiac tissue engineering, the translation of fundamental laboratory research toward the treatment of chronic heart failure patients becomes a more realistic option. Here, the current understanding of cardiac fibrosis and the great potential of tissue engineering are presented. Approaches using hydrogel-based tissue engineered heart constructs are discussed to contemplate key challenges for modeling tissue engineered cardiac fibrosis and to provide a future outlook for preclinical and clinical applications.

Optimal Environmental Stiffness for Stem Cell Mediated Ischemic Myocardium Repair

Cardiovascular diseases related to myocardial infarction (MI) contribute significantly to morbidity and mortality worldwide. The loss of cardiomyocytes during MI is a key factor in the impairment of cardiac-pump functions. Employing cell transplantation has shown great potential as a therapeutic approach in regenerating ischemic myocardium. Several studies have suggested that the therapeutic effects of stem cells vary based on the timing of cell administration. It has been clearly established that the myocardium post-infarction experiences a time-dependent stiffness change, and many studies have highlighted the importance of stiffness (elasticity) of microenvironment on modulating the fate and function of stem cells. Therefore, this chapter outlines our studies and other experiments designed to establish the optimal stiffness of microenvironment that maximizes benefits for maintaining cell survival, promoting phenotypic plasticity, and improving functional specification of the engrafted stem cells.

The Immune System in Tissue Environments Regaining Homeostasis after Injury: Is "Inflammation" Always Inflammation?

Inflammation is a response to infections or tissue injuries. Inflammation was once defined by clinical signs, later by the presence of leukocytes, and nowadays by expression of "proinflammatory" cytokines and chemokines. But leukocytes and cytokines often have rather anti-inflammatory, proregenerative, and homeostatic effects. Is there a need to redefine "inflammation"? In this review, we discuss the functions of "inflammatory" mediators/regulators of the innate immune system that determine tissue environments to fulfill the need of the tissue while regaining homeostasis after injury.

Systematic Evaluation of the Cellular Innate Immune Response During the Process of Human Atherosclerosis

Background

The concept of innate immunity is well recognized within the spectrum of atherosclerosis, which is primarily dictated by macrophages. Although current insights to this process are largely based on murine models, there are fundamental differences in the atherosclerotic microenvironment and associated inflammatory response relative to humans. In this light, we characterized the cellular aspects of innate immune response in normal, nonprogressive, and progressive human atherosclerotic plaques.

Methods and Results

A systematic analysis of innate immune response was performed on 110 well‐characterized human perirenal aortic plaques with immunostaining for specific macrophage subtypes (M1 and M2 lineage) and their activation markers, neopterin and human leukocyte antigen–antigen D related (HLA‐DR), together with dendritic cells (DCs), natural killer (NK) cells, mast cells, neutrophils, and eosinophils. Normal aortae were devoid of low‐density lipoprotein, macrophages, DCs, NK cells, mast cells, eosinophils, and neutrophils. Early, atherosclerotic lesions exhibited heterogeneous populations of (CD68⁺) macrophages, whereby 25% were double positive “M1” (CD68⁺/ inducible nitric oxide synthase [iNOS]⁺/CD163⁻), 13% “M2” double positive (CD68⁺/iNOS⁻/CD163⁺), and 17% triple positive for (M1) iNOS (M2)/CD163 and CD68, with the remaining (≈40%) only stained for CD68. Progressive fibroatheromatous lesions, including vulnerable plaques, showed increasing numbers of NK cells and fascin‐positive cells mainly localized to the media and adventitia whereas the M1/M2 ratio and level of macrophage activation (HLA‐DR and neopterin) remained unchanged. On the contrary, stabilized (fibrotic) plaques showed a marked reduction in macrophages and cell activation with a concomitant decrease in NK cells, DCs, and neutrophils.

Conclusions

Macrophage “M1” and “M2” subsets, together with fascin‐positive DCs, are strongly associated with progressive and vulnerable atherosclerotic disease of human aorta. The observations here support a more complex theory of macrophage heterogeneity than the existing paradigm predicated on murine data and further indicate the involvement of (poorly defined) macrophage subtypes or greater dynamic range of macrophage plasticity than previously considered.

Endoplasmic reticulum stress in bone marrow-derived cells prevents acute cardiac inflammation and injury in response to angiotensin II

Inflammation plays an important role in hypertensive cardiac injury. The endoplasmic reticulum (ER) stress pathway is involved in the inflammatory response. However, the role of ER stress in elevated angiotensin II (Ang II)-induced cardiac injury remains unclear. In this study, we investigated the role of ER stress in Ang II-induced hypertensive cardiac injury. Transcriptome analysis and quantitative real-time PCR showed that Ang II infusion in mice increased ER stress-related genes expression in the heart. C/EBP homologous protein (CHOP) deficiency, a key mediator of ER stress, increased infiltration of inflammatory cells, especially neutrophils, the production of inflammatory cytokines, chemokines in Ang II-infused mouse hearts. CHOP deficiency increased Ang II-induced cardiac fibrotic injury: (1) Masson trichrome staining showed increased fibrotic areas, (2) immunohistochemistry staining showed increased expression of α-smooth muscle actin, transforming growth factor β1 and (3) quantitative real-time PCR showed increased expression of collagen in CHOP-deficient mouse heart. Bone marrow transplantation experiments indicated that CHOP deficiency in bone marrow cells was responsible for Ang II-induced cardiac fibrotic injury. Moreover, TUNEL staining and flow cytometry revealed that CHOP deficiency decreased neutrophil apoptosis in response to Ang II. Taken together, our study demonstrated that hypertension induced ER stress after Ang II infusion. ER stress in bone marrow-derived cells protected acute cardiac inflammation and injury in response to Ang II.

Plasma-derived Extracellular Vesicles Contain Predictive Biomarkers and Potential Therapeutic Targets for Myocardial Ischemic (MI) Injury

Myocardial infarction (MI) triggers a potent inflammatory response via the release of circulatory mediators, including extracellular vesicles (EVs) by damaged cardiac cells, necessary for myocardial healing. Timely repression of inflammatory response are critical to prevent and minimize cardiac tissue injuries, nonetheless, progression in this aspect remains challenging. The ability of EVs to trigger a functional response upon delivery of carried bioactive cargos, have made them clinically attractive diagnostic biomarkers and vectors for therapeutic interventions. Using label-free quantitative proteomics approach, we compared the protein cargo of plasma EVs between patients with MI and from patients with stable angina (NMI). We report, for the first time, the proteomics profiling on 252 EV proteins that were modulated with >1.2-fold after MI. We identified six up-regulated biomarkers with potential for clinical applications; these reflected post-infarct pathways of complement activation (Complement C1q subcomponent subunit A (C1QA), 3.23-fold change, p = 0.012; Complement C5 (C5), 1.27-fold change, p = 0.087), lipoprotein metabolism (Apoliporotein D (APOD), 1.86-fold change, p = 0.033; Apolipoprotein C-III (APOCC3), 2.63-fold change, p = 0.029) and platelet activation (Platelet glycoprotein Ib alpha chain (GP1BA), 9.18-fold change, p < 0.0001; Platelet basic protein (PPBP), 4.72-fold change, p = 0.027). The data have been deposited to the ProteomeXchange with identifier PXD002950. This novel biomarker panel was validated in 43 patients using antibody-based assays (C1QA (p = 0.005); C5 (p = 0.0047), APOD (p = 0.0267); APOC3 (p = 0.0064); GP1BA (p = 0.0031); PPBP (p = 0.0465)). We further present that EV-derived fibrinogen components were paradoxically down-regulated in MI, suggesting that a compensatory mechanism may suppress post-infarct coagulation pathways, indicating potential for therapeutic targeting of this mechanism in MI. Taken together, these data demonstrated that plasma EVs contain novel diagnostic biomarkers and therapeutic targets that can be further developed for clinical use to benefit patients with coronary artery diseases (CADs).

Knowledge gaps to understanding cardiac macrophage polarization following myocardial infarction

Following myocardial infarction (MI), macrophages coordinate both pro-inflammatory and reparative responses of the left ventricle (LV) by reacting to and secreting cytokines, chemokines, and growth factors and by stimulating endothelial cells and fibroblasts to modulate neovascularization and scar formation. Healing of the infarcted LV can be divided into three distinct, but overlapping phases: inflammatory, proliferative, and maturation. Macrophages are involved in all phases. Despite macrophages being a major leukocyte cell type in the post-MI LV, how this cell type regulates LV remodeling over the post-MI time continuum is not completely understood. In this review, we summarize the current literature as a foundation to discuss the major knowledge gaps that remain. Defining the post-MI temporal macrophage phenotypes to establish a classification system is the first step in exploring how macrophage phenotypes are regulated, how temporal stimulation and secretion profiles evolve, and how best to modify stimuli to yield predictable cell responses. This article is part of a Special Issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck & Jan F.C. Glatz.

Endothelial Actions of ANP Enhance Myocardial Inflammatory Infiltration in the Early Phase After Acute Infarction

RATIONALE

In patients after acute myocardial infarction (AMI), the initial extent of necrosis and inflammation determine clinical outcome. One early event in AMI is the increased cardiac expression of atrial natriuretic peptide (NP) and B-type NP, with their plasma levels correlating with severity of ischemia. It was shown that NPs, via their cGMP-forming guanylyl cyclase-A (GC-A) receptor and cGMP-dependent kinase I (cGKI), strengthen systemic endothelial barrier properties in acute inflammation.

OBJECTIVE

We studied whether endothelial actions of local NPs modulate myocardial injury and early inflammation after AMI.

METHODS AND RESULTS

Necrosis and inflammation after experimental AMI were compared between control mice and littermates with endothelial-restricted inactivation of GC-A (knockout mice with endothelial GC-A deletion) or cGKI (knockout mice with endothelial cGKI deletion). Unexpectedly, myocardial infarct size and neutrophil infiltration/activity 2 days after AMI were attenuated in knockout mice with endothelial GC-A deletion and unaltered in knockout mice with endothelial cGKI deletion. Molecular studies revealed that hypoxia and tumor necrosis factor-α, conditions accompanying AMI, reduce the endothelial expression of cGKI and enhance cGMP-stimulated phosphodiesterase 2A (PDE2A) levels. Real-time cAMP measurements in endothelial microdomains using a novel fluorescence resonance energy transfer biosensor revealed that PDE2 mediates NP/cGMP-driven decreases of submembrane cAMP levels. Finally, intravital microscopy studies of the mouse cremaster microcirculation showed that tumor necrosis factor-α-induced endothelial NP/GC-A/cGMP/PDE2 signaling impairs endothelial barrier functions.

CONCLUSIONS

Hypoxia and cytokines, such as tumor necrosis factor-α, modify the endothelial postreceptor signaling pathways of NPs, with downregulation of cGKI, induction of PDE2A, and altered cGMP/cAMP cross talk. Increased expression of PDE2 can mediate hyperpermeability effects of paracrine endothelial NP/GC-A/cGMP signaling and facilitate neutrophil extravasation during the early phase after MI.

PKM2 released by neutrophils at wound site facilitates early wound healing by promoting angiogenesis

Neutrophils infiltration/activation following wound induction marks the early inflammatory response in wound repair. However, the role of the infiltrated/activated neutrophils in tissue regeneration/proliferation during wound repair is not well understood. Here, we report that infiltrated/activated neutrophils at wound site release pyruvate kinase M2 (PKM2) by its secretive mechanisms during early stages of wound repair. The released extracellular PKM2 facilitates early wound healing by promoting angiogenesis at wound site. Our studies reveal a new and important molecular linker between the early inflammatory response and proliferation phase in tissue repair process.

Temporal neutrophil polarization following myocardial infarction

AIMS

Although macrophage phenotypes have been well studied in the myocardial infarction (MI) setting, this study investigated temporal neutrophil polarization and activation mechanisms.

METHODS AND RESULTS

Neutrophils isolated from the infarcted left ventricle (LV) of mice showed high expression of proinflammatory markers at Day 1 and anti-inflammatory markers at Days 5 and 7 post-MI, indicating distinct neutrophil phenotypes along the post-MI time continuum. Flow cytometry analysis revealed that although proinflammatory N1 neutrophils were always predominant (>80% of total neutrophils at each time point), the percentage of N2 neutrophils increased post-MI from 2.4 ± 0.6% at Day 1 to 18.1 ± 3.0% at Day 7. In vitro, peripheral blood neutrophils were polarized to proinflammatory N1 by lipopolysaccharide and interferon-γ or anti-inflammatory N2 by interleukin-4, indicating high plasticity potential. The in vivo post-MI relevant LV damage-associated molecular patterns (DAMPs) polarized neutrophils to a proinflammatory N1 phenotype by activating toll-like receptor-4. Transforming growth factor-β1 inhibited proinflammatory production in neutrophils. N1 neutrophils positively correlated with infarct wall thinning at Day 7 post-MI, possibly due to high production of matrix metalloproteinases-12 and -25.

CONCLUSION

This study is the first to identify the existence of N1 and N2 neutrophils in the infarct region and reveals that N1 polarization could be mediated by DAMPs.

CD36 Is a Matrix Metalloproteinase-9 Substrate That Stimulates Neutrophil Apoptosis and Removal During Cardiac Remodeling

BACKGROUND

After myocardial infarction, the left ventricle undergoes a wound healing response that includes the robust infiltration of neutrophils and macrophages to facilitate removal of dead myocytes as well as turnover of the extracellular matrix. Matrix metalloproteinase (MMP)-9 is a key enzyme that regulates post-myocardial infarction left ventricular remodeling.

METHODS AND RESULTS

Infarct regions from wild-type and MMP-9 null mice (n=8 per group) analyzed by glycoproteomics showed that of 541 N-glycosylated proteins quantified, 45 proteins were at least 2-fold upregulated or downregulated with MMP-9 deletion (all P<0.05). Cartilage intermediate layer protein and platelet glycoprotein 4 (CD36) were identified as having the highest fold increase in MMP-9 null mice. By immunoblotting, CD36 but not cartilage intermediate layer protein decreased steadily during the time course post-myocardial infarction, which identified CD36 as a candidate MMP-9 substrate. MMP-9 was confirmed in vitro and in vivo to proteolytically degrade CD36. In vitro stimulation of day 7 post-myocardial infarction macrophages with MMP-9 or a CD36-blocking peptide reduced phagocytic capacity. Dual immunofluorescence revealed concomitant accumulation of apoptotic neutrophils in the MMP-9 null group compared with wild-type group. In vitro stimulation of isolated neutrophils with MMP-9 decreased neutrophil apoptosis, indicated by reduced caspase-9 expression.

CONCLUSIONS

Our data reveal a new cell-signaling role for MMP-9 through CD36 degradation to regulate macrophage phagocytosis and neutrophil apoptosis.

Deficiency of ataxia telangiectasia mutated kinase delays inflammatory response in the heart following myocardial infarction

Background

Ataxia‐telangiectasia results from mutations in ataxia telangiectasia mutated kinase (ATM) gene. We recently reported that ATM deficiency attenuates left ventricular (LV) dysfunction and dilatation 7 days after myocardial infarction (MI) with increased apoptosis and fibrosis. Here we investigated the role of ATM in the induction of inflammatory response, and activation of survival signaling molecules in the heart acute post‐MI.

Methods and Results

LV structure, function, inflammatory response, and biochemical parameters were measured in wild‐type (WT) and ATM heterozygous knockout (hKO) mice 1 and 3 days post‐MI. ATM deficiency had no effect on infarct size. MI‐induced decline in heart function, as measured by changes in percent fractional shortening, ejection fraction and LV end systolic and diastolic volumes, was lower in hKO‐MI versus WT‐MI (n=10 to 12). The number of neutrophils and macrophages was significantly lower in the infarct LV region of hKO versus WT 1 day post‐MI. Fibrosis and expression of α‐smooth muscle actin (myofibroblast marker) were higher in hKO‐MI, while active TGF‐β1 levels were higher in the WT‐MI 3 days post‐MI. Myocyte cross‐sectional area was higher in hKO‐sham with no difference between the two MI groups. MMP‐9 protein levels were similarly increased in the infarct LV region of both MI groups. Apoptosis was significantly higher in the infarct LV region of hKO at both time points. Akt activation was lower, while Bax expression was higher in hKO‐MI infarct.

Conclusion

ATM deficiency results in decreased dilative remodeling and delays inflammatory response acute post‐MI. However, it associates with increased fibrosis and apoptosis.

Avemar and Echinacea extracts enhance mobilization and homing of CD34(+) stem cells in rats with acute myocardial infarction

Introduction

Activation of endogenous stem cell mobilization can contribute to myocardial regeneration after ischemic injury. This study aimed to evaluate the possible role of Avemar or Echinacea extracts in inducing mobilization and homing of CD34⁺ stem cells in relation to the inflammatory and hematopoietic cytokines in rats suffering from acute myocardial infarction (AMI).

Methods

AMI was developed by two consecutive subcutaneous injections of isoprenaline (85 mg/kg). AMI rats were either post-treated or pre- and post-treated daily with oral doses of Avemar (121 mg/kg) or Echinacea (130 mg/kg). In whole blood, the number of CD34⁺ cells was measured by flow cytometry and their homing to the myocardium was immunohistochemically assessed. Serum creatine kinase, vascular endothelial growth factor, interleukin-8 and granulocyte macrophage colony stimulating factor were determined on days 1, 7 and 14 after AMI. Sections of the myocardium were histopathologically assessed.

Results

Rats pre- and post-treated with Avemar or Echinacea exhibited substantial increases in the number of circulating CD34⁺ cells, peaking on the first day after AMI to approximately 13-fold and 15-fold, respectively, with a decline in their level on day 7 followed by a significant increase on day 14 compared to their corresponding AMI levels. Only post-treatment with Echinacea caused a time-dependent increase in circulating CD34⁺ cells on days 7 and 14. Such increases in circulating CD34⁺ cells were accompanied by increased homing to myocardial tissue 14 days after AMI. Interestingly, pre- and post-treatment with Avemar or Echinacea substantially increased serum creatine kinase on day 1, normalized its activity on day 7 and, on continued treatment, only Echinacea markedly increased its activity on day 14 compared to the corresponding AMI values. Moreover, both treatments modified differently the elevated serum vascular endothelial growth factor and the lowered granulocyte macrophage colony stimulating factor levels of the AMI group but did not affect the level of interleukin-8. These results were supported histopathologically by reduced inflammatory reactions and enhanced neovascularization.

Conclusion

Avemar and Echinacea extracts can effectively induce mobilization and homing of CD34⁺ stem cells to the myocardial tissue and thus may help in stem cell-based regeneration of the infarcted myocardium.

The circular relationship between matrix metalloproteinase-9 and inflammation following myocardial infarction

Matrix metalloproteinase-9 (MMP-9) regulates remodeling of the left ventricle after myocardial infarction (MI) and is tightly linked to the inflammatory response. The inflammatory response serves to recruit leukocytes as part of the wound healing reaction to the MI injury, and infiltrated leukocytes produce cytokines and chemokines that stimulate MMP-9 production and release. In turn, MMP-9 proteolyzes cytokines and chemokines. Although in most cases, MMP-9 cleavage of the cytokine or chemokine substrate serves to increase activity, there are cases where cleavage results in reduced activity. Global MMP-9 deletion in mouse MI models has proven beneficial, suggesting inhibition of some aspects of MMP-9 activity may be valuable for clinical use. At the same time, overexpression of MMP-9 in macrophages has also proven beneficial, indicating that we still do not fully understand the complexity of MMP-9 mechanisms of action. In this review, we summarize the cycle of MMP-9 effects on cytokine production and cleavage to regulate leukocyte functions. Although we use MI as the example process, similar events occur in other inflammatory and wound healing conditions.

Embolization of the first diagonal branch of the left anterior descending coronary artery as a porcine model of chronic trans-mural myocardial infarction

Background

Although the incidence of acute death related to coronary artery disease has decreased with the advent of new interventional therapies, myocardial infarction remains one of the leading causes of death in the US. Current animal models developed to replicate this phenomenon have been associated with unacceptably high morbidity and mortality. A new model utilizing the first diagonal branch of the left anterior descending artery (D1-LAD) was developed to provide a clinically relevant lesion, while attempting to minimize the incidence of adverse complications associated with infarct creation.

Methods

Eight Yucatan miniature pigs underwent percutaneous embolization of the D1-LAD via injection of 90 µm polystyrene micro-spheres. Cardiac structure and function were monitored at baseline, immediately post-operatively, and at 8-weeks post-infarct using transthoracic echocardiography. Post-mortem histopathology and biochemical analyses were performed to evaluate for changes in myocardial structure and extracellular matrix (ECM) composition respectively. Echocardiographic data were evaluated using a repeated measures analysis of variance followed by Tukey’s HSD post hoc test. Biochemical analyses of infarcted to non-infarcted myocardium were compared using analysis of variance.

Results

All eight pigs successfully underwent echocardiography prior to catheterization. Overall procedural survival rate was 83% (5/6) with one pig excluded due to failure of infarction and another due to deviation from protocol. Ejection fraction significantly decreased from 69.7 ± 7.8% prior to infarction to 50.6 ± 14.7% immediately post-infarction, and progressed to 48.7 ± 8.9% after 8-weeks (p = 0.011). Left ventricular diameter in systole significantly increased from 22.6 ± 3.8 mm pre-operatively to 30.9 ± 5.0 mm at 8 weeks (p = 0.016). Histopathology showed the presence of disorganized fibrosis on hematoxylin and eosin and Picro Sirius red stains. Collagen I and sulfated glycosaminoglycan content were significantly greater in the infarcted region than in normal myocardium (p = 0.007 and p = 0.018, respectively); however, pyridinoline crosslink content per collagen I content in the infarcted region was significantly less than normal myocardium (p = 0.048).

Conclusion

Systolic dysfunction and changes in ECM composition induced via embolization of the D1-LAD closely mimic those found in individuals with chronic myocardial infarction (MI), and represents a location visible without the need for anesthesia. As a result, this method represents a useful model for studying chronic MI.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-015-0547-4) contains supplementary material, which is available to authorized users.

Splenic leukocytes mediate the hyperglycemic exacerbation of myocardial infarct size in mice

Acute hyperglycemia during acute myocardial infarction is associated with worse myocardial injury and increased mortality. Using a mouse model of myocardial ischemia/reperfusion injury, we tested the hypothesis that acute hyperglycemia activates splenic leukocytes and subsequently exacerbates myocardial infarct size. We then examined whether the adverse effects of hyperglycemia could be attenuated by a potent anti-inflammatory agent (an agonist of the adenosine A2A receptor) administered immediately prior to reperfusion. C57BL6 (WT) mice underwent 30-min LAD occlusion and 60-min reperfusion with or without prior splenectomy. Acute hyperglycemia before ischemia increased myocardial infarct size (IS) by 43% (p < 0.05). Splenectomy before ischemia did not change IS (vs. control, p = NS) but did serve to prevent the exacerbation of IS by hyperglycemia. Acute hyperglycemia activated splenic leukocytes by increasing formyl peptide receptor expression and reactive oxygen species production before ischemia, and enhanced splenic neutrophil release with resultant peripheral neutrophilia and increased myocardial neutrophil infiltration during reperfusion. Acute adoptive transfer of splenic leukocytes to splenectomized mice before ischemia restored the hyperglycemic exacerbation of infarct size. ATL146e, an adenosine 2A receptor (A2AR) agonist, abolished neutrophilia during reperfusion and reduced IS in hyperglycemic mice. ATL146e also reduced IS in splenectomized hyperglycemic mice with transfer of WT splenic leukocytes, but not with transfer of splenic leukocytes from A2AR knockout mice. Acute hyperglycemia prior to myocardial ischemia and reperfusion exacerbates IS by activating splenic leukocytes. ATL146e administered at reperfusion suffices to abrogate the hyperglycemic exacerbation of IS by acting on A2ARs on splenic leukocytes.

Early matrix metalloproteinase-12 inhibition worsens post-myocardial infarction cardiac dysfunction by delaying inflammation resolution

RATIONALE

Matrix metalloproteinases (MMPs) regulate remodeling of the left ventricle (LV) post-myocardial infarction (MI). MMP-12 has potent macrophage-dependent remodeling properties in the atherosclerotic plaque; however, post-MI roles have not been examined.

OBJECTIVE

The goal was to determine MMP-12 post-MI mechanisms.

METHODS AND RESULTS

Male C57BL/6J mice (3-6 months old) were subjected to left coronary artery ligation. Saline or the RXP 470.1 MMP-12 inhibitor (MMP-12i; 0.5mg/kg/day) was delivered by osmotic mini-pump beginning 3h post-MI, and mice were sacrificed at day (d)1, 3, 5 or 7 post-MI and compared to d0 controls (mice without MI; n=6-12/group/time). MMP-12 expression increased early post-MI, and contrary to expected, neutrophils were a surprising early cellular source for MMP-12. MMP-12i reduced MMP-12 activity 33 ± 1% at d1 post-MI. Despite similar infarct areas and survival rates, MMP-12i led to greater LV dilation and worsened LV function. At d7 post-MI, MMP-12i prolonged pro-inflammatory cytokine upregulation (IL1r1, IL6ra, IL11, and Cxcr5) and decreased CD44 (both gene and protein levels). Hyaluronan (HA), a CD44 ligand, was elevated at d1 and d7 post-MI with MMP12i, as a result of decreased fragmentation. Because CD44-HA regulates neutrophil removal, apoptosis markers were evaluated. Caspase 3 increased, while cleaved caspase 3 levels decreased in MMP-12i group at d7 post-MI, indicating reduced neutrophil apoptosis. In isolated neutrophils, active MMP-12 directly stimulated CD44, caspase 3, and caspase 8 expression.

CONCLUSION

Our results reveal a novel protective mechanism for MMP-12 in neutrophil biology. Post-MI, MMP-12i impaired CD44-HA interactions to suppress neutrophil apoptosis and prolong inflammation, which worsened LV function.

Using systems biology approaches to understand cardiac inflammation and extracellular matrix remodeling in the setting of myocardial infarction

Inflammation and extracellular matrix (ECM) remodeling are important components regulating the response of the left ventricle to myocardial infarction (MI). Significant cellular- and molecular-level contributors can be identified by analyzing data acquired through high-throughput genomic and proteomic technologies that provide expression levels for thousands of genes and proteins. Large-scale data provide both temporal and spatial information that need to be analyzed and interpreted using systems biology approaches in order to integrate this information into dynamic models that predict and explain mechanisms of cardiac healing post-MI. In this review, we summarize the systems biology approaches needed to computationally simulate post-MI remodeling, including data acquisition, data analysis for biomarker classification and identification, data integration to build dynamic models, and data interpretation for biological functions. An example for applying a systems biology approach to ECM remodeling is presented as a reference illustration.

Stem cell mechanisms during left ventricular remodeling post-myocardial infarction: Repair and regeneration

Post-myocardial infarction (MI), the left ventricle (LV) undergoes a series of events collectively referred to as remodeling. As a result, damaged myocardium is replaced with fibrotic tissue consequently leading to contractile dysfunction and ultimately heart failure. LV remodeling post-MI includes inflammatory, fibrotic, and neovascularization responses that involve regulated cell recruitment and function. Stem cells (SCs) have been transplanted post-MI for treatment of LV remodeling and shown to improve LV function by reduction in scar tissue formation in humans and animal models of MI. The promising results obtained from the application of SCs post-MI have sparked a massive effort to identify the optimal SC for regeneration of cardiomyocytes and the paradigm for clinical applications. Although SC transplantations are generally associated with new tissue formation, SCs also secrete cytokines, chemokines and growth factors that robustly regulate cell behavior in a paracrine fashion during the remodeling process. In this review, the different types of SCs used for cardiomyogenesis, markers of differentiation, paracrine factor secretion, and strategies for cell recruitment and delivery are addressed.

The spatiotemporal cellular dynamics of lung immunity

The lung is a complex structure that is interdigitated with immune cells. Understanding the 4D process of normal and defective lung function and immunity has been a centuries-old problem. Challenges intrinsic to the lung have limited adequate microscopic evaluation of its cellular dynamics in real time, until recently. Because of emerging technologies, we now recognize alveolar-to-airway transport of inhaled antigen. We understand the nature of neutrophil entry during lung injury and are learning more about cellular interactions during inflammatory states. Insights are also accumulating in lung development and the metastatic niche of the lung. Here we assess the developing technology of lung imaging, its merits for studies of pathophysiology and areas where further advances are needed.

Myofibroblasts and the extracellular matrix network in post-myocardial infarction cardiac remodeling

The cardiac extracellular matrix (ECM) fills the space between cells, supports tissue organization, and transduces mechanical, chemical, and biological signals to regulate homeostasis of the left ventricle (LV). Following myocardial infarction (MI), a multitude of ECM proteins are synthesized to replace myocyte loss and form a reparative scar. Activated fibroblasts (myofibroblasts) are the primary source of ECM proteins, thus playing a key role in cardiac repair. A balanced turnover of ECM through regulation of synthesis by myofibroblasts and degradation by matrix metalloproteinases (MMPs) is critical for proper scar formation. In this review, we summarize the current literature on the roles of myofibroblasts, MMPs, and ECM proteins in MI-induced LV remodeling. In addition, we discuss future research directions that are needed to further elucidate the molecular mechanisms of ECM actions to optimize cardiac repair.

Crosstalk between fibroblasts and inflammatory cells

Fibroblasts, which are traditionally recognized as a quiescent cell responsible for extracellular matrix production, are more and more appreciated as an active key player of the immune system. This review describes how fibroblasts and immune cells reciprocally influence the pathogenesis of fibrosis. An overview is given how fibroblasts are triggered by components of the innate and adaptive immunity on the one hand and how fibroblasts modulate immune cell behaviour via conditioning the cellular and cytokine microenvironment on the other hand. Finally, latest insights into the role of cardiac fibroblasts in the orchestration of inflammatory cell infiltration in the heart, and their impact on heart failure, are outlined.

Matrix metalloproteinase-9: Many shades of function in cardiovascular disease

Matrix metalloproteinase (MMP)-9, one of the most widely investigated MMPs, regulates pathological remodeling processes that involve inflammation and fibrosis in cardiovascular disease. MMP-9 directly degrades extracellular matrix (ECM) proteins and activates cytokines and chemokines to regulate tissue remodeling. MMP-9 deletion or inhibition has proven overall beneficial in multiple animal models of cardiovascular disease. As such, MMP-9 expression and activity is a common end point measured. MMP-9 cell-specific overexpression, however, has also proven beneficial and highlights the fact that little information is available on the underlying mechanisms of MMP-9 function. In this review, we summarize our current understanding of MMP-9 physiology, including structure, regulation, activation, and downstream effects of increased MMP-9. We discuss MMP-9 roles during inflammation and fibrosis in cardiovascular disease. By concentrating on the substrates of MMP-9 and their roles in cardiovascular disease, we explore the overall function and discuss future directions on the translational potential of MMP-9 based therapies.