Left ventricular (LV) structural remodeling, such as changes in LV mass, volume, and geometry, are important predictors of adverse functional and clinical outcomes. Advancing age, independent of concurrent cardiovascular disease, can be associated with significant LV remodeling. Studies in aged animals have shown that increasing age is associated with development of LV concentric remodeling, increased extracellular matrix (ECM) fibrillar collagen content, and significant abnormalities in diastolic function. However, cellular mechanisms by which advanced age leads to cardiac remodeling, particularly a net increase in myocardial collagen content and the development of diastolic dysfunction, have not been completely defined. Regulation of the structure and composition of the collagenous ECM of the myocardium is controlled by cardiac fibroblasts and is regulated at the levels of procollagen synthesis, post-synthetic procollagen processing, and collagen degradation. In aged animals, collagen synthesis is decreased, procollagen processing is increased, and mediators of collagen degradation are generally decreased. In addition, aged fibroblasts demonstrate reduced rates of proliferation consistent with a senescent fibroblast phenotype. In the previous funding period, we identified that expression of SPARC (Secreted Protein Acidic and Rich in Cysteine/osteonectin/BM40), a collagen-binding matricellular protein, was increased in aged myocardium and is a critical factor contributing to elevated collagen content in aged hearts. We also found that SPARC acts to decrease procollagen interaction with cardiac fibroblasts, presumably through decreased engagement of transmembrane collagen receptors. Based on preliminary data, we have identified Discoidin Domain Receptor 2 (DDR2) as a principle collagen receptor engaged in binding collagen and acting in opposition to SPARC. Because DDR2 and SPARC share the same binding site on procollagen, we propose that, in aged myocardium, increases in SPARC expression reduce DDR2 binding to procollagen thus limiting DDR2 binding activity and down-stream signaling pathways. Procollagen binding to DDR2 has been demonstrated to induce fibroblast proliferation and increase production of matrix metalloproteinases (MMP) -2 and -13. Decreased DDR2 activity in aged myocardium is therefore predicted to decrease fibroblast proliferation and production of MMP-2 and -13 thus contributing to the senescent phenotype indicative of aged cardiac fibroblasts.
In Aim 1, we will determine whether reductions in DDR2 activity contribute to decreased proliferation and decreased expression of MMP-2 and 13 in aged fibroblasts and whether DDR2 activity is enhanced by decreasing SPARC expression in aged cells. Experiments in Aim 2 will test whether increasing DDR2 activity in aged myocardium, either through over-expression of DDR2 or inhibition of SPARC expression, reverses the senescent fibroblast phenotype in vivo through increasing fibroblast proliferation and production of MMP-2 and 13. Previous studies that have addressed indices of procollagen processing and degradation in aged myocardium have been based primarily on indirect measurements of these processes such as levels of enzymes that process or degrade collagen.
In Aim 3, newly developed in vivo radiolabeling techniques will be used to directly quantify changes in procollagen processing and degradation in aged myocardium and determine whether altering DDR2 or SPARC activity influences these processes.
The structural and functional changes that occur during aging result in limitations in exercise capacity. In addition, these structural and functional changes create a substrate that is more vulnerable to the superimposition of specific cardiac disease processes. For this reason, hypertension and ischemia are more likely to cause heart failure in older patients. If we can more completely determine the cellular mechanisms that cause age-dependent changes in functional capacity and substrate vulnerability, more effective and comprehensive patient care management programs can be developed to limit the effects of aging on LV structure and function. Elucidation of cellular mechanisms that govern age-dependent increases in cardiac collagen has the potential to improve strategies in this regard. These issues are especially important in our Veterans who are older, have a high incidence of hypertension and ischemia, and have a disproportionately greater level of age-dependent functional limitations.