Chapter 2 Extracellular Matrix and Cardiac Remodeling
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Chapter 2 Extracellular Matrix and Cardiac Remodeling Bodh I. Jugdutt University of Alberta‚ Edmonton‚ Alberta 1. Introduction Cumulative evidence over the last 3 decades suggests that the cardiac interstitium or extracellular matrix (ECM)‚ especially the extracellular collagen matrix (ECCM)‚ plays an important role in cardiac remodeling during various cardiac diseases including myocardial infarction (MI) and heart failure [1-4]. A unique feature of the healthy heart is the ability of the pumping chambers to return to the ideal shape at end-diastole despite repetitive changes during every cardiac cycle throughout life. This physiologic remodeling or property of resuming functional shape depends largely on the integrity of the ECCM‚ the intricate network of fibrillar collagens found in the ECM. In a broad sense‚ pathophysiologic cardiac remodeling refers to changes in cardiac structure‚ shape and function following stress or damage [5-7]. To remodel is defined in the Webster dictionary as “ to alter the structure‚ to remake” and in the Oxford dictionary as “to model again and differently‚ reconstruct and reorganize.” The Oxford dictionary defines “model” as “representation in three dimensions of the proposed structure” and “to model” as “to fashion and shape.” This review will focus on the role of the ECCM in pathologic cardiac remodeling and specifically left ventricular (LV) remodeling. 2. Cardiac extracellular matrix 2.1. Cardiac ECM‚ ECCM and fibroblasts In the heart‚ as in other organs [8‚ 9]‚ the specialized parenchymal cells (cardiomyocytes) are supported by an ECM made up of an intricate macromolecular network of fibers and different cell types of mesenchymal origin including fibroblasts‚ endothelial cells (cardiac and vascular)‚ smooth 24 muscle cells‚ blood-borne cells (macrophages and others)‚ pericytes and neurons‚ bathed in a gel-type ground substance (Table 1). The supporting extracellular network‚ composed mainly of fibrillar collagen [2]‚ is referred to as the ECCM in this review. In the healthy heart‚ it is important to note that cardiomyocytes make up 25 to 35% of the cell number [2‚10‚11]‚ of myocardial mass [12-14] and 67 to 75 % of the myocardial volume [2‚ 15] while nonmyocytes account for about 67 to 75% of the cell number [11‚ 15‚ 16] and of the myocardial volume [2‚15]. The nonmyocytes make up 75% of the total number of cells [11] or 90 to 95 % of the non-myocyte fraction of cardiac cells [12-14]. Vascular tissue‚ together with the lumens‚ was reported to occupy about 60% of the extracellular space in one study [17] and 13% of the volume fraction in another study [18]. Although there are differences among various studies‚ probably related to the use of different methodologies‚ it is clear that fibroblasts are one of the dominant cell types in mammalian myocardium (Table 2) and they play a major role in cardiac remodeling. Most of the matrix macromolecules (of which collagen is the principal structural protein) are produced by fibroblasts [8‚ 9]. The cardiomyocytes which mediate the heart’s pump function throughout life are still the center of attention. However‚ the cardiac fibroblasts which mediate the formation of the ECCM‚ essential for maintaining structural and functional integrity [19-21]‚ are gaining increasing attention as potential targets. 2.2. The cardiac ECCM and collagen types The collagen molecule typically consists of a central core of long‚ stiff‚ triple-stranded helices in which 3 chains (composed of triplet amino acid sequences‚ usually glycine-proline-hydroxyproline) are wound around each 25 other to form a superhelix [9]. Since each collagen alpha chain is encoded by a separate gene‚ there are several collagen types (Table 3). The major fibrillar collagens in the heart are collagen types I and III and they are synthesized in cardiac fibroblasts [10]. Approximately 85% of the total myocardial collagen is type I and 11% is type III [1]. Collagen type I‚ the dominant fibrillar collagen‚ is associated mainly with thick fibers which confer tensile strength and resistance to stretch and deformation [3]. Collagen type III is associated with thin fibers which confer resilience [22]. Types I and III collagen maintain the structural integrity of the myocytes [3]. They maintain alignment of myofibrils within myocytes via collagen- integrin-cytoskeleton-myofibril connections‚ and thereby allow for the cyclic translation of myocyte shortening into coordinated contraction of the ventricular chambers. It follows that ECCM remodeling may result in significant LV dysfunction. Other components of the ECM (such as the polysaccharide gel‚ elastin‚ fibronectin‚ laminin and proteoglycans) also mediate important functions that have been reviewed [8‚ 9]. The elastin content in myocardium is modest compared to that in distensible blood vessels [2‚ 22]. 2.3. Collagen synthesis In general‚ collagen biosynthesis [8‚ 9] involves at least 8 enzymatic steps: i) intracellular synthesis of pro-alpha chains; ii) hydroxylation of selected prolines and lysines; iii) glycosylation of selected hydroxyl serines; iv) formation of procollagen triple helixes; v) secretion into the extracellular space; vi) conversion into less soluble molecules by removal of propeptides; vii) self- assembly into collagen fibrils; and viii) aggregation into fibers. 26 A key enzyme in collagen synthesis is prolyl 4-hydroxylase (P4H)‚ which catalyzes hydroxylation of proline residues on monomers to yield thermally stable triple helical protocollagen molecules that are then secreted into the ECCM [8‚ 9] . Co-factors for the enzyme leading to the formation of 4- hydroxyproline include 2-oxoglutarate‚ and ascorbic acid or vitamin C. Vitamin C regulates collagen synthesis and stimulates synthesis of pro-collagen mRNAs [23]. Several growth factors and cytokines also influence collagen gene expression [8‚ 9] and synthesis (Table 4). Transforming growth stimulates collagen synthesis at transcriptional and post-transcriptional levels as well as overall protein synthesis [24]. is a mitogen and growth factor in various tissues including the myocardium [25]‚ stimulates cardiac fibroblasts and their conversion to myofibroblasts (MyoFbs) [14]‚ plays a major role in ECCM synthesis [4]‚ and mediates several of its mitogenic and growth effects by inducing connective tissue growth factor (CTGF) expression [26]. Insulin growth factor (IGF-1‚ IGF-2) increases collagen and overall protein synthesis 27 [8]. Interleukin-1 (IL-1) can both increase and decrease collagen synthesis [8]. Tumor necrosis and inhibit collagen gene transcription [27]. Several hormones and enzymes also influence collagen synthesis (Table 4). Glucocorticoids inhibit procollagen gene transcription [8]. Other steroid hormones and parathyroid hormone also decrease collagen synthesis [28]. Inhibition of P4H prevents collagen synthesis [29]. It is important to note that even a mild reduction of about 20% in 4-hydroxyproline content is sufficient to reduce the ‘melting’ temperature of the helices below the physiological level of 37°C [30]‚ thereby decreasing the physical stability of collagen‚ its resistance to proteolysis‚ its secretion with ECCM‚ and its ability to interact with other ECCM components. 2.4. Collagen degradation In general‚ the orderly degradation of ECCM is critical for growth‚ development‚ morphogenesis‚ remodeling and repair [8‚ 9]. Two main classes of locally secreted extracellular proteolytic enzymes control ECCM degradation: or dependent matrix metalloproteinases (MMPs) and serine proteases. In the heart‚ collagen degradation is mediated mainly by MMPs‚ most of whom have been cloned [31‚ 32]. Matrix metalloproteinases include 28 collagenases‚ gelatinases‚ stromelysins‚ membrane type MMPs (MT-MMPs) and putative MMPs such as PUMP-1 (Table 5). The collagenases include interstitial collagenase 1 (MMP-1)‚ neutrophil collagenase 2 (MMP-8)‚ and collagenase 3 (MMP-13‚ a homolog of human MMP-1). These collagenases are highly specific in their action and cleave particular proteins (primarily fibrillar collagen) at specific sites thereby destroying structural integrity with a minimum amount of proteolysis. The major substrates for MMP-1 and MMP-8 are fibrillar collagen types I‚ II and III. The major substrate for MMP-13 is collagen type I. The gelatinases include MMP-2 and MMP-9 and degrade denatured fibrillar collagens and collagen types IV and V. An important concept is that once collagenase is bound to the fibril and has begun its attack of the ECCM‚ it would continue to act until all the substrate has been completely degraded unless it is inhibited or controlled [33]. A natural protective mechanism against uncontrolled collagenase degradation is provided by tissue mediated inhibition of collagenase activity [33]. In the heart‚ endogenous inhibitors of MMPs termed tissue inhibitors of metalloproteinases (TIMPs) are co-expressed and form tight complexes [34]. 29 Both MMPs and TIMPs are secreted by cardiac fibroblasts and their gene expression is tightly controlled at the transcription level [34]. Several studies [34] have demonstrated: i) transcription of MMP-1 and TIMP mRNAs in fibroblast-like cells; ii) MMP genes (particularly those for collagenase) in fibroblasts‚ endothelial cells and polymorphonuclear cells; iii) involvement of TIMPs in MMP activation and stimulation of fibroblast growth; and iv) modulation of the synthesis and secretion of pro-MMPs and TIMPs by cytokines‚ polypeptide growth factors‚ hormones‚ steroids‚ and phorbol esters. Collagenase produced by cardiac fibroblasts