Studies on the Role of Vitronectin and Plasminogen-Activator Inhibitor-1 Complexes Beyond Inhibiting Proteases: Binding to the Extracellular Matrix, Cell Interactions and Pathogenesis

Plasminogen activator inhibitor-1 (PAI-1), a member of the serine protease inhibitor (serpin) superfamily of proteins, circulates in blood in a complex with vitronectin (VN). These two proteins are also found localized together in the extracellular matrix in many different pathophysiological conditions. Both of these proteins are involved with a number of physiologically important processes. Though PAI-1 is a well-known inhibitor of serine proteases, more emphasis is now geared towards its protease independent functions. VN, on the other hand, is a binding protein that exists in the circulation in a preferred monomeric conformation. However, in the extracellular matrix, VN exists as multimer with altered conformation. Though the exact reason for such conformational alterations and compartmentalization is unknown, there are a number of biomolecules, including PAI-1 that are proposed to cause such alterations. In last few years, sufficient experimental evidence has been gathered to confirm this protease- independent effect of PAI-1 by which it induces multimerization of VN in a concentration-dependent fashion. It has been observed also that PAI-1 remains associated with this multimeric complex for several hours. A major focus of this dissertation work was to extend our understanding of the mechanism of the interaction between these proteins and to explore the physiological relevance of the multimeric complexes formed by their interaction on cellular adhesion and migration. In our study, emphasis has been given to the presence of an appropriate microenvironment so that the role of the multimeric complexes could be investigated in a relevant biological setting. Our findings indicate the importance of the surrounding microenvironment in establishing the specific role of the VN/PAI-1 complex in cell-matrix interactions. In a previous study from our lab, it was found that vitronectin knock-out mice were more resistant to Candida infection compared to wild type C57Bl/6 mice. One of the goals of this dissertation work was to provide a mechanistic explanation for their increased survival of the vitronectin knock-out mice upon Candida infection. Another important aspect of this work was to establish biophysical methods for understanding the structural changes that happen in PAI-1 naturally or due to ligand binding.