Extracellular protease activation and unraveling of the myocardial interstitium: critical steps toward clinical applications

Abstract
Congestive heart failure is a complex pathophysiological process in which a significant biological underpinning is left ventricular (LV) failure. Part of the natural advancement to failure is myocardial remodeling, most commonly from adaptation to a pathological stimulus. LV remodeling can be defined as molecular, cellular, and interstitial changes within the myocardium that result in alterations in LV geometry and function ([2][1], [4][2], [11][3], [21][4], [63][5], [71][6], [78][7]). In patients with intensely progressive LV remodeling, morbidity and mortality are increased ([17][8], [57][9], [79][10]). This editorial is focused on the relationship between extracellular proteases and myocardial matrix remodeling. It will also explore basic research targets and analyze critical steps necessary in developing basic research into therapeutic and prognostic realizations. There are a number of underlying factors that contribute to the development and progression of myocardial LV remodeling. Specifically, alterations in myocardial tissue structure occur after myocardial infarction (MI), with the development of cardiomyopathies, and in myocardial hypertrophy ([2][1], [4][2], [11][3], [21][4], [63][5], [71][6], [78][7]). Although the initiating stimulus and symptoms may appear comparable among the three etiologies, the cellular remodeling processes are distinctly different and the underlying pathologies diverse. One area of research focus is how the myocardial extracellular matrix (ECM) may contribute to this remodeling process. Of particular note is the activation of extracellular proteases, which cause LV remodeling. The purpose of this editorial is to place into context the different patterns of myocardial remodeling to that of extracellular protease activity. It is now recognized that the myocardial remodeling process is not a uniform sequence of events but rather a diverse set of molecular and cellular triggers, which are likely to be distinctive to underlying pathological stimuli ([16][11]). This editorial attempts to place into context the clinically encountered etiologies of myocardial remodeling (infarction, cardiomyopathy, and hypertrophy) to a unique pattern of extracellular protease activation. However, it must be recognized that summating the complex process of myocardial remodeling under these categories fails to address the different magnitude and duration of the pathological stimuli. In an attempt to address the complex issue, this editorial will examine both clinical and relevant animal model data in each of the distinct forms of myocardial remodeling with respect to extracellular protease activation. The matrix metalloproteinases (MMPs) are a family of zinc-dependent proteases that are essential for normal tissue remodeling in processes such as bone growth, wound healing, and reproduction ([48][12], [52][13], [76][14], [83][15]). MMPs are responsible for turnover of the ECM, which in turn facilitates tissue remodeling. MMPs are synthesized as inactive zymogens and are secreted into the extracellular space in a proenzyme form ([48][12], [52][13], [76][14], [83][15]). The pro-MMP binds specific ECM proteins and remains enzymatically quiescent until the propeptide domain is cleaved. Cleavage results in exposure of the zinc active site in the catalytic domain and subsequent activation ([49][16]). The disruption of the cysteine-zinc interaction in the MMP active site is essential in the activation of MMPs. This “cysteine switch” hypothesis of MMP activation is likely the mechanism by which most MMPs are activated ([49][16]). Instead of a sporadic distribution of pro-MMPs throughout the ECM, there is a specific allotment of these proteolytic enzymes within the extracellular space. Moreover, a large reservoir of recruitable MMPs exists, which upon activation can result in a rapid surge of ECM proteolytic activity. ### MMP Classification MMPs are classified into subgroups according to substrate specificity and/or structure ([48][12], [52][13], [76][14], [83][15]). MMPs that have been identified in the myocardium are listed in [Table 1][17]. Interstitial collagenase (MMP-1) and collagenase-3 (MMP-13) possess high substrate specificity for fibrillar collagens, whereas the gelatinases (MMP-2 and MMP-9) demonstrate high substrate affinity for basement membrane proteins ([48][12], [52][13], [76][14], [83][15]). The substrate portfolio for stromelysin (MMP-3) includes important myocardial ECM proteins such as aggrecan, fibronectin, and fibrillar collagens ([48][12], [52][13], [76][14], [83][15]). Furthermore, past in vitro studies have demonstrated MMP-3 can proteolytically process pro-MMP species ([47][18], [49][16]). For example, Murphy and colleagues ([47][18]) reported a 12-fold increase in the conversion of pro-MMP-1 to active MMP-1 in the presence of MMP-3. In addition, other MMPs such as MMP-1, MMP-2, and the membrane-type MMPs (MT-MMPs) can also activate pro-MMPs ([24][19], [47][18]–[49][16], [52][13], [53][20], [76][14], [83][15]). MT-MMPs contain a transmembrane domain and are likely activated intracellularly through a proprotein convertase pathway ([24][19], [47][18]–[49][16], [52][13], [53][20], [76][14], [83][15]). Thus, unlike the secretable MMPs, MT-MMPs are activated once positioned in the cell membrane. It has been demonstrated that MT1-MMP degrades fibrillar collagens and a wide range of ECM components as well as proteolytically processes pro-MMP-2 and pro-MMP-13 ([24][19], [47][18]–[49][16], [52][13], [53][20], [76][14], [83][15]). There is emerging evidence that MMPs also degrade nonmatrix substrates, in turn affecting cell proliferation, migration, and apoptosis ([43][21]). View this table: Table 1. MMPs identified in human myocardium ### MMP Activation/Inhibition There are presently over 20 members of the MMP family of proteolytic enzymes, and several MMPs are substrate specific for other pro-MMPs. Therefore, activation of a few...