Left Ventricular Remodeling: One Small Step for the Extracellular Matrix Will Translate to a Giant Leap for the Myocardium

Authors

  • Andriy Yabluchanskiy PhD,

    1. San Antonio Cardiovascular Proteomics Center, The University of Texas Health Science Center, San Antonio, TX
    2. Barshop Institute for Longevity and Aging Studies, The University of Texas Health Science Center, San Antonio, TX
    3. Division of Geriatrics, The University of Texas Health Science Center, San Antonio, TX
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  • Robert J. Chilton DO, FACC,

    1. San Antonio Cardiovascular Proteomics Center, The University of Texas Health Science Center, San Antonio, TX
    2. Division of Cardiology, Gerontology and Palliative Medicine, Department of Medicine, The University of Texas Health Science Center, San Antonio, TX
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  • Merry L. Lindsey PhD

    Corresponding author
    1. Barshop Institute for Longevity and Aging Studies, The University of Texas Health Science Center, San Antonio, TX
    2. Division of Geriatrics, The University of Texas Health Science Center, San Antonio, TX
    • San Antonio Cardiovascular Proteomics Center, The University of Texas Health Science Center, San Antonio, TX
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Address for correspondence: Merry L. Lindsey, PhD, Department of Medicine, Division of Geriatrics, Gerontology and Palliative Medicine, The University of Texas Health Science Center at San Antonio, 15355 Lambda Drive, MC 7755, San Antonio, TX 78245

E-mail: lindseym@uthscsa.edu

Myocardial infarction (MI) is initiated by an acute disruption of blood supply to the heart. Prolonged ischemia in the region downstream of the occlusion culminates with tissue necrosis. Inflammation follows the induction of necrosis and orchestrates the wound healing response. Key components of wound healing are the recruitment and engagement of leukocytes and the reorganization of the extracellular matrix (ECM). As a result of the poor regenerative capacity of the heart, cardiomyocyte loss is compensated by the generation of a collagen-based ECM scar.

The early post-MI scar is mainly composed of collagen III, but as the collagen scar maturates, the ratio of collagen types changes toward a predominance of collagen I. Long-term outcomes post-MI depend on the quality of scar formed, in terms of quantity, quality, and extent of cross-linking. The irreversible changes in the biomechanics and geometry of the left ventricle are collectively known as left ventricular (LV) remodeling, and adverse LV remodeling stimulates the progression to congestive heart failure.

The most promising treatments for patients with MI involve pre-MI detection to avert ischemia, early diagnosis to limit myocyte loss during ischemia, or targeting mechanisms that occur during resolution of the inflammatory and fibrotic responses to prevent the progression to heart failure (Figure). As such, understanding how the ECM influences remodeling will identify novel targets for clinical intervention.

Figure 1.

Phases of the left ventricular response to myocardial infarction. ECM indicates extracellular matrix.

Reorganization of the ECM During Post-MI Remodeling

The adult mammalian heart is characterized by a myocardium that is highly organized with an overlapping layered structure, and the ECM scaffold surrounds and supports the cells of the myocardium. ECM is mainly composed of fibrillar collagen that provides structural strength through its 3-dimensional structure that interdigitates between cardiac muscle fibers. As such, the ECM plays a pivotal role in the transmission of contractile forces. Collagen I accounts for approximately 70% to 85% of total cardiac collagen in the left ventricle and provides tensile strength, whereas collagen III accounts for about 10% of total cardiac collagen. The thin collagen III fibers maintain the elasticity of the ECM network.[1] In addition to collagen and other glycoproteins, glycosaminoglycans, basement membrane components, and proteoglycans are components of the ECM.[2] Besides providing an environment for cells, ECM also serves as a reservoir for growth factors and proteases. Under normal conditions, ECM homeostasis is maintained by signaling interactions between cardiac cells and matrix proteins through cell surface receptors, eg, integrins.

Prolonged ischemia that turns into irreversible ischemia ultimately leads to cell death, and this event requires ECM network reorganization. Key players in the degradation of ECM are the matrix metalloproteinases (MMPs). Neutrophils are a rich source of MMPs, and neutrophil infiltration occurs as early as 15 minutes after the initiation of reperfusion.[3] MMPs cleave collagens and other ECM proteins at site-specific locations to generate ECM fragments.[4] Early post-MI, this degradation of ECM is accompanied by the continuous generation of a fibrin-based provisional matrix that provides structural support for the infarct region.

Necrotic cell debris and matrix fragments activate membrane-bound toll-like receptors, the complement system, high-mobility group box 1, and the receptor for advanced glycation end-products, all of which play important roles in the early steps of the wound healing response by stimulating the generation of reactive oxygen species and initiating the nuclear factor–kB pathway to produce inflammatory cytokines.[5, 6] Cytokines attract inflammatory cells into the infarcted region, where these cells are activated to initiate scavenging and repair processes. Macrophages infiltrate beginning at day 3 post-MI, serving to engulf necrotic myocytes and apoptotic neutrophils, as well as promote anti-inflammatory processes. Activated macrophages also release growth factors to stimulate collagen synthesis and fibroblast activation, which stimulates formation of the ECM scar.[7] Over time, the provisional fibrin-based matrix is degraded by proteolytic enzymes and replaced by a secondary fibroblast-derived matrix containing several ECM proteins, including fibronectin and hyaluronan. At this point, a number of matricellular proteins are released by both fibroblasts and macrophages to orchestrate the signaling pathways responsible for infarct repair.

Matricellular proteins, unlike fibrillar ECM components, are not involved in structural roles but rather modulate cell function by interacting with cell-surface receptors, proteases, hormones, and other bioeffector molecules, as well as with structural matrix proteins such as collagens.[3] Among the most studied matricellular proteins are thrombospondin (TSP)-1, -2, and -4; secreted protein acidic and rich in cysteine; tenascin-C and -X; osteopontin; periostin; and members of the CCN family (including CCN1 and CCN2/connective tissue growth factor).3

The expression of matricellular proteins in the normal adult myocardium is low and generally is only observed during development or as a response to injury. In the infarct environment, matricellular protein expression rapidly increases. Matricellular proteins stimulate cell migration, proliferation, and adhesion, but also serve crucial roles in mediating the inflammatory response. For example, TSP-1, an important transforming growth factor β activator with potent angiostatic and anti-inflammatory effects, is significantly increased in the border area between the infarct and remote areas and serves to limit infarct expansion.[8]

In the infarct environment, cardiac fibroblast cells are important sources of ECM, MMPs, and growth factors that stimulate angiogenesis.[9] While the source of the cardiac myofibroblast is controversial, resident and infiltrating cells can undergo phenotypic change to differentiate into myofibroblasts.[10, 11] Myofibroblasts in particular produce robust amounts of ECM that provides structural support to the infarcted region. Fibroblasts also initiate the contraction of the provisionary matrix deposited in the affected region. Alterations in the composition of ECM over time mediate multiple components of the inflammatory and fibrotic responses.

Targeting ECM Proteins

Treatment strategies for the MI patient have dramatically improved over the past 40 years. The current optimal approach includes reperfusion of the ischemic myocardium within 1 hour of symptoms onset. For approximately 20% of patients, however, reperfusion is either not successful or not possible, and this translates to approximately 250,000 patients each year with nonreperfused infarcts.[12] In addition, adverse LV remodeling progressing to heart failure occurs in approximately 5% of MI patients who have received optimum therapy, and this percentage approaches 40% for MI patients with comorbidities such as advanced age or diabetes.[13] Five-year mortality rates for congestive heart failure remain at 50%, which underscores the need for additional therapies to impede the progression to heart failure.

Understanding ECM turnover during LV remodeling and targeting ECM proteins will likely provide fruitful targets to develop novel treatment strategies for post-MI patients. A list of ECM proteins known to be altered post-MI is provided in Table 1. To date, very limited progress has been achieved in targeting specific ECM components as a therapeutic option post-MI.

Table 1. ECM Proteins Known to Change Post-MI[28-47]
ECM Protein
  1. Abbreviations: ECM, extracellular matrix; MI, myocardial infarction; SPARC, secreted protein acidic and rich in cysteine.

Collagen I
Collagen III
Fibronectin
Hyaluronan
Laminin
Osteopontin
Periostin
SPARC
Tenascin C
Thrombospondin-1

A search for direct MMP inhibition began in the late 1970s.[14] At that time, however, only a few MMPs had been identified, which made selectivity difficult to assess. The first several generations of MMP inhibitors also had very poor bioavailability and numerous side effects, which tempered enthusiasm for this therapeutic approach.[15, 16] Over the past 2 decades, a number of MMP inhibitors have been tested. Of all MMP inhibitors tested to date, only doxycycline has been approved by the US Food and Drug Administration.[17-19]

Most of the therapies, such as angiotensin-converting enzyme inhibition or β-adrenergic receptor blockade, have both direct and indirect effects on ECM.

ECM-derived peptide fragments provide one such novel therapeutic option in the treatment of post-MI patients (Table 2). ECM fragments affect cellular response and behavior in other models of wound healing; however, little is known about their roles post-MI.[20-23] Lee and colleagues, for example, have shown that ECM fragments stimulate angiogenesis and significantly improve cardiac function in a mouse model of MI.[24] Further exploration of the role of ECM fragments in stimulating cardiac inflammatory cells, fibroblasts, or other endogenous cell types will likely provide new strategies to optimize scar formation post-MI.

Table 2. Effects of ECM-Derived Fragments In Vitro and In Vivo in Models of Wound Healing and Myocardial Infarction18,19,22–27
ECMPeptideEffects
  1. Abbreviations: ECM, extracellular matrix; LV, left ventricular.

Collagen IV

Hep I

[TAGSCLRKFSTMY-OH]

Promotes endothelial cell adhesion, migration, and proliferation

Interacts with α2, α3, and β1 integrin subunits

Interacts with full-length collagen IV

Increases capillary density in the infarcted region

Reduces infarct size

Hep III

[GEFYFDLRLKGDKY-OH]

Promotes endothelial cell adhesion, migration, and proliferation

Interacts with α2β1 and α3β1 integrins

Increases capillary density in the infarcted region

Reduces infarct size and improves LV function

ElastinVaI-Gly-VaI-Ala-Pro-Gly Stimulates chemotaxis of fibroblasts and monocytes
Fibronectin

RGD

[GRGDSPASSPISC-OH]

Promotes endothelial cell adhesion, migration, and proliferation

Interacts with αvβ3 integrin

Increases capillary density in the infarcted region

Reduces infarct size

120-kDa cell-binding FN fragmentsDecreases VLA-5 receptor expression Reduces monocyte migration
LamininLGTIPG and YIGSRInhibits macrophage migration
A119 [LSNIDYILIKAS]Promotes cell attachment, neurite, outgrowth, and amyloid-like fibril formation and binds to syndecans
A13 and C16Highly angiogenic, binds to integrins αβ3 and α5β1 and stimulates fibroblast migration and wound contraction

Several ECM-derived fragments from collagen IV have been used as scaffolds for the infarcted myocardium to provide structural support. These ECM fragments were shown to interact with themselves and the infarct scar to form an enhanced scaffold matrix.[25] Future methods of injecting polymers into the infarcted hearts will likely also be a promising strategy. For example, intramyocardial injection of fibrin significantly improved cardiac function after MI, indicating that the formation of a scaffold provides support for the infarct region.[26, 27] Whether the injection of ECM fragments also stimulate cell signaling has not been explored. Understanding the role of exogenously administered ECM fragments has significant potential to improve current treatment schemes. Further insights into the molecular and signaling pathways may bring new breakthroughs and capabilities to influence the early generation of stable scar.

Acknowledgments

We acknowledge support from NIH/NHLBI HHSN 268201000036C (N01-HV-00244) for the San Antonio Cardiovascular Proteomics Center and R01 HL075360, the Max and Minnie Tomerlin Voelcker Fund, and the Veteran's Administration (Merit) to MLL.

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