The growth of blood vessels during injury, repair, or cyclic renewal is controlled in part by extracellular matrix (ECM) proteins (e.g., collagen) and matricellular proteins (e.g., SPARC, secreted protein acidic and rich in cysteine). Strains of mice lacking collagen I and SPARC each exhibit defects in the morphogenesis of vessels. In this proposal we ask whether collagen I, an adhesive protein secreted by cells of the vessel wall, and SPARC, a counteradhesive protein that affects the cell cycle, cooperatively modulate a) cell adhesion, b) invasion/proteolysis of the ECM, and c) cell proliferation. SPARC knockout (KO) mice are deficient in dermal wound repair and in its associated angiogenesis. Cells from SPARC KO mice contract collagen I gels poorly and produce significantly diminished levels of collagen I in vitro. Mov-13 mouse embryos that lack collagen I exhibit diminished angiogenesis in the brain and do not retain SPARC in the ECM. Moreover, collagen gel contraction by their fibroblasts is compromised but can be rescued by exogenous SPARC. Our recent data also show an accelerated cell cycle in cells isolated from SPARC KO mice. These results support a role for SPARC in the regulation of cell cycle that might contribute, in part, to the enhanced production of collagen by growth-arrested cells. We thus address the following questions: 1) How do SPARC and collagen I promote collagen gel contraction (an assay of the capacity of cells to remodel and migrate through collagen)? 2) Does SPARC affect the interaction of native or proteolyzed collagen I with endothelial integrins? 3) How does SPARC regulate the production of collagen I? 4) Does SPARC inhibit stages of the cell cycle that coincide with an increased production of collagen I?. And 5) Which elements of the collagen 1(I) gene regulate collagen gel contraction and vascular morphogenesis; does SPARC influence promoter activity in either context? These studies will utilize mov 13 and SPARC KO mice, mice transgenic for different collagen I promoter sequences, and cells derived from these animals, and will involve assays in vivo, ex vivo, and in vitro. We have only a superficial understanding of angiogenesis: one level at which this process is regulated is the invasion of connective tissue by endothelial cells. Completion of the Aims should clarify a mechanism by which cells interact with collagen I and how this interaction is regulated by SPARC.
