Merry L. Lindsey

Targeted deletion of matrix metalloproteinase-9 attenuates left ventricular enlargement and collagen accumulation after experimental myocardial infarction

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.

Macrophage roles following myocardial infarction

Following myocardial infarction (MI), circulating blood monocytes respond to chemotactic factors, migrate into the infarcted myocardium, and differentiate into macrophages. At the injury site, macrophages remove necrotic cardiac myocytes and apoptotic neutrophils; secrete cytokines, chemokines, and growth factors; and modulate phases of the angiogenic response. As such, the macrophage is a primary responder cell type that is involved in the regulation of post-MI wound healing at multiple levels. This review summarizes what is currently known about macrophage functions post-MI and borrows literature from other injury and inflammatory models to speculate on additional roles. Basic science and clinical avenues that remain to be explored are also discussed.

Matrix-dependent mechanism of neutrophil-mediated release and activation of matrix metalloproteinase 9 in myocardial ischemia/reperfusion

BackgroundA key component of reperfusion of myocardial infarction is an immediate inflammatory response, which enhances tissue repair. Matrix turnover is crucial to tissue repair, and matrix metalloproteinases (MMPs) are key enzymes involved in matrix degradation. The hypothesis tested is that one inflammation-based effector of tissue repair is the secretion and activation of MMP-9 by infiltrating neutrophils. Methods and ResultsCardiac lymph and tissue were assayed for latent and active MMP-2 and MMP-9 by zymography and immunochemistry. Dual-labeling immunofluorescence determined the cellular source of MMP-9 protein. Isolated canine neutrophils were incubated with preischemic and postischemic cardiac lymph in the presence and absence of collagen-fibronectin pads, and the supernatants were assayed for latent and active MMP-9. MMP-9 increased during the first hours of reperfusion in both lymph supernatants and myocardial extracts, and this increase was of neutrophil origin. MMP-9 in the cardiac lymph remained latent but was activatable. In contrast, MMP-9 in the myocardium was in both latent and active forms. In situ zymography demonstrated that activated MMP-9 surrounded the infiltrated neutrophils. When postischemic cardiac lymph was incubated with neutrophils in vitro, MMP-9 secretion and activation occurred only in the presence of a collagen-fibronectin substrate; preischemic cardiac lymph did not induce significant secretion or activation. ConclusionsInfiltrating neutrophils are an early source of MMP-9 after reperfusion, and a portion of MMP-9 in the myocardium is active. Infiltrating neutrophils may localize MMP-9 activation by secreting MMP-9 and as a source of activating proteases.

Extracellular matrix remodeling following myocardial injury

While current therapeutic strategies restore blood flow to the ischemic myocardium and limit infarct size, adverse left ventricular (LV) remodeling that progresses to dysfunction remains a significant complication following myocardial infarction (MI). The extracellular matrix (ECM) is a key component in the remodeling process, and increases in collagen occur in the infarct area to replace necrotic myocytes and form a scar. The ECM is coupled to the cell through cell surface receptors, primary of which are the integrins. In addition, the matrix metalloproteinases coordinate ECM turnover through degradation of ECM components. Several laboratories have demonstrated matrix metalloproteinase (MMP) participation in remodeling events that lead to LV dilation, and inhibition or targeted deletion of specific MMPs has beneficial effects post-MI. MMP inhibition is a particular focus of recent studies designed to understand the underlying mechanisms of LV remodeling and to evaluate pharmacologic strategies that target the ECM to affect adverse LV remodeling following MI.

Matrix Metalloproteinase-7 Affects Connexin-43 Levels, Electrical Conduction, and Survival After Myocardial Infarction

Background— Matrix metalloproteinases (MMPs) contribute to left ventricular remodeling after myocardial infarction (MI). Specific causative roles of particular MMPs, however, remain unclear. MMP-7 is abundant in cardiomyocytes and macrophages, but MMP-7 function after MI has not been defined. Methods and Results— Wild-type (WT; n=55) and MMP-7–null (MMP-7−/−; n=32) mice underwent permanent coronary artery ligation for 7 days. MI sizes were similar, but survival was greatly improved in MMP-7−/− mice. The survival difference could not be attributed to differences in left ventricular dilation because end-diastolic volumes increased similarly. ECG analysis revealed a prolonged PR interval in WT but not in MMP-7−/− post-MI mice. Post-MI conduction velocity, determined by optically mapping electrical wavefront propagation, decreased to 78±6% of control for WT and was normalized in MMP-7−/− mice. In WT mice, slower conduction velocity correlated with a 53% reduction in the gap junction protein connexin-43. Direct binding of MMP-7 to connexin-43, determined by surface plasmon resonance technology, occurred in a dose-dependent manner. Connexin-43 processing by MMP-7 was confirmed by in silico and in vitro substrate analyses and MMP-7 infusion induced arrhythmias in vivo. Conclusions— MMP-7 deletion results in improved survival and myocardial conduction patterns after MI. This is the first report to implicate MMP-7 in post-MI remodeling and to demonstrate that connexin-43 is a novel MMP-7 substrate.

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.

MMP Induction and Inhibition in Myocardial Infarction

Short-term survival following a myocardial infarction (MI) has greatly improved, due in part to therapeutic interventions that restore blood flow and limit infarct size. The increased incidence of infarct-stimulated left ventricular (LV) remodeling that advances to congestive heart failure (CHF), however, is a significant long-term complication and a leading cause of mortality. Changes to ECM structure and function are primary components of LV remodeling and are precipitated by the early increase in infarct area collagen levels that replace necrotic myocytes and form a scar. ECM turnover is coordinated through the synthesis and degradation of ECM and non-ECM components, particularly the matrix metalloproteinases (MMP), a family of proteolytic enzymes that cleave ECM. MMPs have multiple roles in remodeling events that lead to LV dilation. The inhibition or targeted deletion of specific MMPs attenuates LV remodeling events post-MI. MMP inhibitors have been used in animal models to delineate LV remodeling mechanisms and to evaluate the pharmacologic potential of targeting the ECM to modify LV remodeling post-MI. This review summarizes the current knowledge and limitations of MMP inhibition in the post-MI myocardium.

Matrix Metalloproteinase-28 Deletion Exacerbates Cardiac Dysfunction and Rupture After Myocardial Infarction in Mice by Inhibiting M2 Macrophage Activation

Rationale: Matrix metalloproteinase (MMP)-28 regulates the inflammatory and extracellular matrix responses in cardiac aging, but the roles of MMP-28 after myocardial infarction (MI) have not been explored. Objective: To determine the impact of MMP-28 deletion on post-MI remodeling of the left ventricle (LV). Methods and Results: Adult C57BL/6J wild-type (n=76) and MMP null (MMP-28−/−, n=86) mice of both sexes were subjected to permanent coronary artery ligation to create MI. MMP-28 expression decreased post-MI, and its cell source shifted from myocytes to macrophages. MMP-28 deletion increased day 7 mortality because of increased cardiac rupture post-MI. MMP-28−/− mice exhibited larger LV volumes, worse LV dysfunction, a worse LV remodeling index, and increased lung edema. Plasma MMP-9 levels were unchanged in the MMP-28−/− mice but increased in wild-type mice at day 7 post-MI. The mRNA levels of inflammatory and extracellular matrix proteins were attenuated in the infarct regions of MMP-28−/− mice, indicating reduced inflammatory and extracellular matrix responses. M2 macrophage activation was impaired when MMP-28 was absent. MMP-28 deletion also led to decreased collagen deposition and fewer myofibroblasts. Collagen cross-linking was impaired as a result of decreased expression and activation of lysyl oxidase in the infarcts of MMP-28−/− mice. The LV tensile strength at day 3 post-MI, however, was similar between the 2 genotypes. Conclusions: MMP-28 deletion aggravated MI-induced LV dysfunction and rupture as a result of defective inflammatory response and scar formation by suppressing M2 macrophage activation.

Temporal and Spatial Expression of Matrix Metalloproteinases and Tissue Inhibitors of Metalloproteinases Following Myocardial Infarction

Following a myocardial infarction (MI), the homeostatic balance between matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) is disrupted as part of the left ventricle (LV) response to injury. The full complement of responses to MI has been termed LV remodeling and includes changes in LV size, shape and function. The following events encompass the LV response to MI: (1) inflammation and LV wall thinning and dilation, (2) infarct expansion and necrotic myocyte resorption, (3) accumulation of fibroblasts and scar formation, and (4) endothelial cell activation and neovascularization. In this review, we will summarize MMP and TIMP roles during these events, focusing on the spatiotemporal localization and MMP and TIMP effects on cellular and tissue-level responses. We will review MMP and TIMP structure and function, and discuss specific MMP roles during both the acute and chronic phases post-MI, which may provide insight into novel therapeutic targets to limit adverse remodeling in the MI setting.

The history of matrix metalloproteinases: milestones, myths, and misperceptions

Since the discovery of tadpole collagenase in 1962, the matrix metalloproteinase (MMP) family has emerged as a significant proteinase group with recognized effects on the cardiovascular system. Over the last 40 years, many milestones have been achieved, from the identification of the first MMP, to the generation of the first MMP cDNA clone and null mouse, to the clinical approval of the first MMP inhibitor. Over the years, a few myths and misunderstandings have interwoven into the truths. In this review, we will discuss the major milestones of MMP research, as well as review the misinterpretations and misperceptions that have evolved. Clarifying the confusions and dispelling the myths will both provide a better understanding of MMP properties and functions and focus the cardiovascular field on the outstanding research questions that need to be addressed.