Marcin Dobaczewski

The extracellular matrix as a modulator of the inflammatory and reparative response following myocardial infarction

The dynamic alterations in the cardiac extracellular matrix following myocardial infarction not only determine the mechanical properties of the infarcted heart, but also directly modulate the inflammatory and reparative response. During the inflammatory phase of healing, rapid activation of Matrix Metalloproteinases (MMP) causes degradation of the cardiac extracellular matrix. Matrix fragments exert potent pro-inflammatory actions, while MMPs process cytokines and chemokines altering their biological activity. In addition, vascular hyperpermeability results in extravasation of fibronectin and fibrinogen leading to formation of a plasma-derived provisional matrix that serves as a scaffold for leukocyte infiltration. Clearance of the infarct from dead cells and matrix debris is essential for resolution of inflammation and marks the transition to the proliferative phase. The fibrin-based provisional matrix is lysed and cellular fibronectin is secreted. ED-A fibronectin, mechanical tension and Transforming Growth Factor (TGF)-beta are essential for modulation of fibroblasts into myofibroblasts, the main collagen-secreting cells in the wound. The matricellular proteins thrombospondin-1 and -2, osteopontin, tenascin-C, periostin, and secreted protein acidic and rich in cysteine (SPARC) are induced in the infarct regulating cellular interactions and promoting matrix organization. As the infarct matures, matrix cross-linking results in formation of a dense collagen-based scar. At this stage, shielding of fibroblasts from external mechanical tension by the mature matrix network may promote deactivation and cellular quiescence. The components of the extracellular matrix do not passively follow the pathologic alterations of the infarcted heart but critically modulate inflammatory and reparative pathways by transducing signals that affect cell survival, phenotype and gene expression.

TGF-β signaling in fibrosis.

Transforming growth factor β (TGF-β) is a central mediator of fibrogenesis. TGF-β is upregulated and activated in fibrotic diseases and modulates fibroblast phenotype and function, inducing myofibroblast transdifferentiation while promoting matrix preservation. Studies in a wide range of experimental models have demonstrated the involvement of the canonical activin receptor-like kinase 5/Smad3 pathway in fibrosis. Smad-independent pathways may regulate Smad activation and, under certain conditions, may directly transduce fibrogenic signals. The profibrotic actions of TGF-β are mediated, at least in part, through induction of its downstream effector, connective tissue growth factor. In light of its essential role in the pathogenesis of fibrosis, TGF-β has emerged as an attractive therapeutic target. However, the pleiotropic and multifunctional effects of TGF-β and its role in tissue homeostasis, immunity and cell proliferation raise concerns regarding potential side effects that may be caused by TGF-β blockade. This minireview summarizes the role of TGF-β signaling pathways in the fibrotic response.

Essential Role of Smad3 in Infarct Healing and in the Pathogenesis of Cardiac Remodeling

Background— Postinfarction cardiac repair is regulated through timely activation and repression of inflammatory pathways, followed by transition to fibrous tissue deposition and formation of a scar. The transforming growth factor-&bgr;/Smad3 pathway is activated in healing infarcts and may regulate cellular events critical for the inflammatory and the fibrotic responses. Methods and Results— We examined the effects of Smad3 gene disruption on infarct healing and the pathogenesis of cardiac remodeling. In the absence of injury, Smad3-null hearts had comparable function to and similar morphology as wild-type hearts. Smad3-null animals had suppressed peak chemokine expression and decreased neutrophil recruitment in the infarcted myocardium but showed timely repression of inflammatory gene synthesis and resolution of the inflammatory infiltrate. Although myofibroblast density was higher in Smad3-null infarcts, interstitial deposition of collagen and tenascin-C in the remodeling myocardium was markedly reduced. Compared with wild-type animals, Smad3−/− mice exhibited decreased dilative remodeling and attenuated diastolic dysfunction; however, infarct size was comparable between groups. Transforming growth factor-&bgr;-mediated induction of procollagen type III and tenascin-C in isolated cardiac fibroblasts was dependent on Smad3, which suggests that decreased fibrotic remodeling in infarcted Smad3-null hearts may be due to abrogation of the profibrotic transforming growth factor-&bgr; responses. Conclusions— Smad3 loss does not alter the time course of resolution of inflammation in healing infarcts, but it prevents interstitial fibrosis in the noninfarcted myocardium and attenuates cardiac remodeling. Thus, the Smad3 cascade may be a promising therapeutic target for the treatment of myocardial infarction.

Smad3 signaling critically regulates fibroblast phenotype and function in healing myocardial infarction

Rationale Cardiac fibroblasts are key effector cells in the pathogenesis of cardiac fibrosis. Transforming growth factor (TGF)-&bgr;/Smad3 signaling is activated in the border zone of healing infarcts and induces fibrotic remodeling of the infarcted ventricle contributing to the development of diastolic dysfunction. Objective The present study explores the mechanisms responsible for the fibrogenic effects of Smad3 by dissecting its role in modulating cardiac fibroblast phenotype and function. Methods and Results Smad3 null mice and corresponding wild-type controls underwent reperfused myocardial infarction protocols. Surprisingly, reduced collagen deposition in Smad3−/− infarcts was associated with increased infiltration with myofibroblasts. In vitro studies demonstrated that TGF-&bgr;1 inhibited murine cardiac fibroblast proliferation; these antiproliferative effects were mediated via Smad3. Smad3−/− fibroblasts were functionally defective, exhibiting impaired collagen lattice contraction when compared with wild-type cells. Decreased contractile function was associated with attenuated TGF-&bgr;–induced expression of &agr;-smooth muscle actin. In addition, Smad3−/− fibroblasts had decreased migratory activity on stimulation with serum, and exhibited attenuated TGF-&bgr;1–induced upregulation of extracellular matrix protein synthesis. Upregulation of connective tissue growth factor, an essential downstream mediator in TGF-&bgr;–induced fibrosis, was in part dependent on Smad3. Connective tissue growth factor stimulation enhanced extracellular matrix protein expression by cardiac fibroblasts in a Smad3-independent manner. Conclusions Disruption of Smad3 results in infiltration of the infarct with abundant hypofunctional fibroblasts that exhibit impaired myofibroblast transdifferentiation, reduced migratory potential, and suppressed expression of fibrosis-associated genes.

Interleukin-1 Receptor Type I Signaling Critically Regulates Infarct Healing and Cardiac Remodeling

The proinflammatory cytokine interleukin (IL)-1 signals exclusively through the type I IL-1 receptor (IL-1RI). IL-1 expression is markedly induced in the infarcted heart; however, its role in cardiac injury and repair remains controversial. We examined the effects of disrupted IL-1 signaling on infarct healing and cardiac remodeling using IL-1RI(-/-) mice. After reperfused infarction IL-1RI-null mice exhibited decreased infiltration of the infarcted myocardium with neutrophils and macrophages and reduced chemokine and cytokine expression. In the absence of IL-1 signaling, suppressed inflammation was followed by an attenuated fibrotic response. Infarcted IL-1RI(-/-) mice had decreased myofibroblast infiltration and reduced collagen deposition in the infarcted and remodeling myocardium. IL-1RI deficiency protected against the development of adverse remodeling; however, infarct size was comparable between groups suggesting that the beneficial effects of IL-1RI gene disruption were not attributable to decreased cardiomyocyte injury. Reduced chamber dilation in IL-1RI-null animals was associated with decreased collagen deposition and attenuated matrix metalloproteinase (MMP)-2 and MMP-3 expression in the peri-infarct area, suggesting decreased fibrotic remodeling of the noninfarcted heart. IL-1beta stimulated MMP mRNA synthesis in wild-type, but not in IL-1RI-null cardiac fibroblasts. In conclusion, IL-1 signaling is essential for activation of inflammatory and fibrogenic pathways in the healing infarct, playing an important role in the pathogenesis of remodeling after infarction. Thus, interventional therapeutics targeting the IL-1 system may have great benefits in myocardial infarction.

CCR5 Signaling Suppresses Inflammation and Reduces Adverse Remodeling of the Infarcted Heart, Mediating Recruitment of Regulatory T Cells

Myocardial infarction triggers an inflammatory reaction that is involved in cardiac remodeling. Cardiac repair is dependent on regulatory mechanisms that suppress inflammation and prevent excessive matrix degradation. Chemokine induction in the infarcted heart mediates recruitment of leukocyte subsets with distinct properties. We demonstrate that signaling through the CC chemokine receptor 5 (CCR5) prevents uncontrolled postinfarction inflammation and protects from adverse remodeling by recruiting suppressive mononuclear cells. CCR5 and its ligands macrophage inflammatory protein (MIP)−1α and MIP-1β were markedly induced in the infarcted mouse myocardium. In addition, almost 40% of the mononuclear cells infiltrating the infarct expressed CCR5. CCR5−/− mice exhibited marked upregulation of proinflammatory cytokine and chemokine expression in the infarct. In wild-type infarcts CCR5+ mononuclear cells had anti-inflammatory properties, expressing higher levels of IL-10 than CCR5− cells. In contrast, mononuclear cells isolated from CCR5−/− infarcts had reduced IL-10 expression. Moreover, enhanced inflammation in the absence of CCR5 was associated with impaired recruitment of CD4+/foxp3+ regulatory T cells (Tregs). The CCR5+ Treg subset exhibited increased IL-10 expression, reflecting potent anti-inflammatory activity. Accentuated inflammation in CCR5−/− infarcts was associated with increased matrix metalloproteinase (MMP) expression, reduced TIMP levels, and enhanced MMP-2 and MMP-9 activity, resulting in worse cardiac dilation. These results suggest that CCR5-mediated Treg recruitment may restrain postinfarction inflammation, preventing excessive matrix degradation and attenuating adverse remodeling.

Extracellular matrix remodeling in canine and mouse myocardial infarcts

Extracellular matrix proteins not only provide structural support, but also modulate cellular behavior by activating signaling pathways. Healing of myocardial infarcts is associated with dynamic changes in the composition of the extracellular matrix; these changes may play an important role in regulating cellular phenotype and gene expression. We examined the time course of extracellular matrix deposition in a canine and mouse model of reperfused infarction. In both models, myocardial infarction resulted in fragmentation and destruction of the cardiac extracellular matrix, extravasation of plasma proteins, such as fibrinogen and fibronectin, and formation of a fibrin-based provisional matrix providing the scaffold for the infiltration of granulation tissue cells. Lysis of the plasma-derived provisional matrix was followed by the formation of a cell-derived network of provisional matrix composed of cellular fibronectin, laminin, and hyaluronic acid and containing matricellular proteins, such as osteopontin and osteonectin/SPARC. Finally, collagen was deposited in the infarct, and the wound matured into a collagen-based scar with low cellular content. Although the canine and mouse infarcts exhibited a similar pattern of extracellular matrix deposition, deposition of the provisional matrix was more transient in the mouse infarct and was followed by earlier formation of a mature collagen-based scar after 7–14 days of reperfusion; at the same timepoint, the canine infarct was highly cellular and evolving. In addition, mature mouse infarcts showed limited collagen deposition and significant tissue loss leading to the formation of a thin scar. In contrast, dogs exhibited extensive collagen accumulation in the infarcted area. These species-specific differences in infarct wound healing should be taken into account when interpreting experimental infarction studies and when attempting to extrapolate the findings to the human pathological process.

Endogenous Thrombospondin 1 Protects the Pressure-Overloaded Myocardium by Modulating Fibroblast Phenotype and Matrix Metabolism

The matricellular protein thrombospondin (TSP) 1 is induced after tissue injury and may regulate reparative responses by activating transforming growth factor-&bgr;, by suppressing angiogenesis and by modulating inflammation and matrix metabolism. We hypothesized that endogenous TSP-1 may be involved in the pathogenesis of cardiac remodeling in the pressure-overloaded heart. Myocardial TSP-1 expression was increased in a mouse model of pressure overload because of transverse aortic constriction. TSP-1−/− mice exhibited increased early hypertrophy and enhanced late dilation in response to pressure overload. Pressure-overloaded TSP-1 null mice had intense degenerative cardiomyocyte changes, exhibiting more extensive sarcomeric loss and sarcolemmal disruption when compared with wild-type hearts. Accentuated hypertrophy and cardiomyocyte injury in TSP-1−/− hearts was accompanied by increased myofibroblast density. However, despite a 2-fold higher infiltration of the cardiac interstitium with myofibroblasts, pressure-overloaded TSP-1 null hearts did not exhibit significantly increased collagen content when compared with wild-type hearts. The disproportionately low collagen content in TSP-1 null hearts was attributed to infiltration with abundant, but functionally defective, fibroblasts that exhibited impaired myofibroblast differentiation and reduced collagen expression in comparison with wild-type fibroblasts. Impaired myofibroblast activation in TSP-1 null hearts was associated with reduced Smad2 phosphorylation reflecting defective transforming growth factor-&bgr; signaling. Moreover, TSP-1 null hearts had increased myocardial matrix metalloproteinase 3 expression and enhanced matrix metalloproteinase 9 activation after pressure overload. TSP-1 upregulation in the pressure-overloaded heart critically regulates fibroblast phenotype and matrix remodeling by activating transforming growth factor-&bgr; signaling and by promoting matrix preservation, thus preventing chamber dilation.

Induction of the CXC Chemokine Interferon-γ–Inducible Protein 10 Regulates the Reparative Response Following Myocardial Infarction

Rationale: Interferon-&ggr;–inducible protein (IP)-10/CXCL10, an angiostatic and antifibrotic chemokine with an important role in T-cell trafficking, is markedly induced in myocardial infarcts, and may regulate the reparative response. Objective: To study the role of IP-10 in cardiac repair and remodeling. Methods and Results: We studied cardiac repair in IP-10–null and wild-type (WT) mice undergoing reperfused infarction protocols and examined the effects of IP-10 on cardiac fibroblast function. IP-10–deficient and WT animals had comparable acute infarct size. However, the absence of IP-10 resulted in a hypercellular early reparative response and delayed contraction of the scar. Infarcted IP-10−/− hearts exhibited accentuated early dilation, followed by rapid wall thinning during infarct maturation associated with systolic dysfunction. Although IP-10–null and WT mice had comparable cytokine expression, the absence of IP-10 was associated with marked alterations in the cellular content of the infarct. IP-10−/− infarcts had more intense infiltration with CD45+ leukocytes, Mac-2+ macrophages, and &agr;-smooth muscle actin (&agr;-SMA)+ myofibroblasts than WT infarcts but exhibited reduced recruitment of the subpopulations of leukocytes, T lymphocytes and &agr;-SMA+ cells that expressed CXCR3, the IP-10 receptor. IP-10 did not modulate cardiac fibroblast proliferation and apoptosis but significantly inhibited basic fibroblast growth factor–induced fibroblast migration. In addition, IP-10 enhanced growth factor–mediated wound contraction in fibroblast-populated collagen lattices. Conclusions: Endogenous IP-10 is an essential inhibitory signal that regulates the cellular composition of the healing infarct and promotes wound contraction, attenuating adverse remodeling. IP-10–mediated actions may be due, at least in part, to direct effects on fibroblast migration and function.

The extracellular matrix modulates fibroblast phenotype and function in the infarcted myocardium

Cardiac fibroblasts are key cellular effectors of cardiac repair; their phenotype and function are modulated by interactions with extracellular matrix proteins. This review manuscript discusses the effects of the extracellular matrix on the inflammatory and reparative properties of fibroblasts in the infarcted myocardium. Early generation of matrix fragments in the infarct induces a pro-inflammatory and matrix-degrading fibroblast phenotype. Formation of a fibrin/fibronectin-rich provisional matrix serves as a conduit for migration of fibroblasts into the infarcted area. Induction of ED-A fibronectin and nonfibrillar collagens may contribute to myofibroblast transdifferentiation. Upregulation of matricellular proteins promotes transduction of growth factor and cytokine-mediated signals. As the scar matures, matrix cross-linking, clearance of matricellular proteins, and reduced growth factor signaling cause deactivation and apoptosis of reparative infarct fibroblasts. Understanding the effects of matrix components on infarct fibroblasts may guide the design of peptides that reproduce, or inhibit, specific matricellular functions, attenuating adverse remodeling.