Matrix metalloproteinase-9 (MMP-9) is prominently overexpressed after myocardial infarction (MI). We tested the hypothesis that mice with targeted deletion of MMP9 have less left ventricular (LV) dilation after experimental MI than do sibling wild-type (WT) mice. Animals that survived ligation of the left coronary artery underwent echocardiographic studies after MI; all analyses were performed without knowledge of mouse genotype. By day 8, MMP9 knockout (KO) mice had significantly smaller increases in end-diastolic and end-systolic ventricular dimensions at both midpapillary and apical levels, compared with infarcted WT mice; these differences persisted at 15 days after MI. MMP-9 KO mice had less collagen accumulation in the infarcted area than did WT mice, and they showed enhanced expression of MMP-2, MMP-13, and TIMP-1 and a reduced number of macrophages. We conclude that targeted deletion of the MMP9 gene attenuates LV dilation after experimental MI in mice. The decrease in collagen accumulation and the enhanced expression of other MMPs suggest that MMP-9 plays a prominent role in extracellular matrix remodeling after MI.

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.

Myocardial infarction is a prevalent major cardiovascular event that arises from myocardial ischemia with or without reperfusion, and basic and translational research is needed to better understand its underlying mechanisms and consequences for cardiac structure and function. Ischemia underlies a broad range of clinical scenarios ranging from angina to hibernation to permanent occlusion, and while reperfusion is mandatory for salvage from ischemic injury, reperfusion also inflicts injury on its own. In this consensus statement, we present recommendations for animal models of myocardial ischemia and infarction. With increasing awareness of the need for rigor and reproducibility in designing and performing scientific research to ensure validation of results, the goal of this review is to provide best practice information regarding myocardial ischemia-reperfusion and infarction models.Listen to this article’s corresponding podcast at ajpheart.podbean.com/e/guidelines-for-experimental-models-of-myocardial-ischemia-and-infarction/.

Background —Neutrophil-induced cardiomyocyte injury requires the expression of myocyte intercellular adhesion molecule (ICAM)-1 and ICAM-1–CD11b/CD18 adhesion. We have previously demonstrated interleukin (IL)-6 activity in postischemic cardiac lymph; IL-6 is the primary stimulus for myocyte ICAM-1 induction. Furthermore, we found that induction of IL-6 mRNA occurred very early on reperfusion of the infarcted myocardium. We hypothesized that the release of a preformed upstream cytokine induced IL-6 in leukocytes infiltrating on reperfusion. Methods and Results —Constitutive expression of TNF-α and not IL-1β was demonstrated in the normal canine myocardium and was localized predominantly in cardiac mast cells. Mast cell degranulation in the ischemic myocardium was documented by demonstration of a rapid release of histamine and TNF-α in the cardiac lymph after myocardial ischemia. Histochemical studies with FITC-labeled avidin demonstrated degranulating mast cells only in ischemic samples of canine myocardium. Immunohistochemistry suggested that degranulating mast cells were the primary source of TNF-α in the ischemic myocardium. In situ hybridization studies of reperfused myocardium localized IL-6 mRNA in infiltrating mononuclear cells and in mononuclear cells appearing in the postischemic cardiac lymph within the first 15 minutes of reperfusion. Furthermore, isolated canine mononuclear cells incubated with postischemic cardiac lymph demonstrated significant induction of IL-6 mRNA, which was partially blocked with a neutralizing antibody to TNF-α. Conclusions —Cardiac mast cells degranulate after myocardial ischemia, releasing preformed mediators, such as histamine and TNF-α. We suggest that mast cell–derived TNF-α may be a crucial factor in upregulating IL-6 in infiltrating leukocytes and initiating the cytokine cascade responsible for myocyte ICAM-1 induction and subsequent neutrophil-induced injury.

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