Cellular Mechanisms of Cardiac ECM Structure and Function

点击翻译按钮获取中文摘要

Chronic heart failure (CHF) has a designated Quality Enhancement Research Initiative (QUERI) in the VA system to address ways to improve cardiovascular healthcare for Veteran’s suffering from CHF. Success in treatment of CHF associated with chronic pressure overload (hypertension, aortic valve stenosis) is limited by the presence of persistent interstitial fibrosis despite our ability to normalize hemodynamic load. The proposed studies will define abnormalities in cellular mechanisms that cause this critical clinical unmet need. Filling this need will depend on defining the fundamental causal determinants that control both initial ECM degradation and persistence of interstitial myocardial fibrosis following normalization of hemodynamic load. Primary cellular regulators of ECM homeostasis are postulated to be myocardial macrophages and fibroblasts. Our previous studies and preliminary data have led to our central hypothesis: Chronic hemodynamic overload causes fundamental changes in both macrophage and fibroblast phenotype, the hallmark of which is dysregulated protease homeostasis that in turn impedes cellular response to unloading and limits complete regression of fibrosis even after normalization of hemodynamic load. To test this hypothesis, innovations in in vivo animal models and in vitro fibroblast culture were developed. In vivo, a clinically relevant reversal of LVPO (unloading) was created in mice by surgical removal of the transverse aortic constriction (unTAC). UnTAC was found to initiate but lead to an incomplete regression of cardiac fibrosis. Preliminary data indicate that a significant increase in myocardial macrophages coincides with initiation of collagen degradation following hemodynamic unloading but these increases in macrophages are not sustained at later times after unTAC. To address whether load-dependent changes in fibroblast phenotype were a key factor in this remodeling, a fibroblast culture systems that mimics clinically relevant myocardial stiffness was established. In vivo, measurements of myocardial stiffness demonstrated that fibroblasts are exposed to a force of ~8 kPA in PO myocardium and ~2 kPA in normal myocardium. Physiologically relevant, stiffness-dependent changes in fibroblasts phenotype were observed in fibroblasts from normal myocardium whereas fibroblasts from TAC and unTAC myocardium exhibited a pro-fibrotic non-responsive phenotype to changes in stiffness. Preliminary data indicate that Tissue inhibitor of metalloproteinase (TIMP)-1 was a causal factor in this pro-fibrotic persistent phenotype. Our strong preliminary data gave rise to the following Specific Aims to test our central hypothesis: Aim 1: Test the hypothesis that reversal of sustained hemodynamic overload shifts myocardial macrophage phenotype to a distinct but transient anti-fibrotic (ECM-degradation) phenotype that initiates, but does not complete, a load-dependent regression of accumulated interstitial ECM. Aim 2: Test the hypothesis that sustained in vivo increases in hemodynamic load change myocardial fibroblasts to a profibrotic, TIMP-1 dependent phenotype that remains profibrotic even when hemodynamic load is reversed.