The Heparan Sulfate Landscape in Complement Regulation

University Of California, San Diego, La Jolla CA

Investigators

Abstract

PROJECT SUMMARY The complement system is a complex cascade of proteins that bridges innate and adaptive immunity by circulating in the bloodstream and rapidly initiating and amplifying an immune response when pathogenic material is identified. Discrimination between pathogenic material and host tissue is a vital function for maintaining a healthy immune system. Tissue-specific damage that is driven by complement activation and subsequent protein deposition at the tissue surface is characteristic of complement-mediated diseases. One of the key protectors of complement activation on host tissue is the Factor H protein (FH). FH can bind both heparan sulfate and sialic acid in the glycocalyx of glycoproteins and proteoglycans that covers most cells. FH inactivates complement component C3, a central protein that drives complement activity. FH is a member of a protein family composed of an additional five FH-related proteins (FHRs). Although these FHR proteins carry highly similar heparan sulfate-binding domains, FHR proteins counteract the regulatory mechanisms of FH and promote C3 activation and amplification of the complement response. Heparan sulfate and sialic acid have been appreciated as key drivers of host recognition, but their specific interactions and their relative importance is not well-defined. Heparan sulfate is a highly diverse polysaccharide that varies in composition across tissues. Sialic acid is a single sugar molecule that is often found on terminal residue(s) of larger asparagine- or serine/threonine-linked glycans on glycoproteins and gangliosides. The central focus of this project is to define the contribution of heparan sulfate to the tissue-specific regulation of complement, using a combination of biochemical, cell biological and genetic models in mice. We will (i) use in vitro biochemical and cellular models to define binding affinities of FH and FHR proteins for heparan sulfate and to quantify complement activation on the cell surface when heparan sulfate or sialic acid biosynthesis is disrupted, (ii) examine the impact of heparan sulfate on complement activation in a murine model of FH deficiency, which in humans results in complement activation and chronic inflammatory diseases, such as C3 Glomerulopathy and Age-Related Macular Degeneration; and (iii) determine the effect of recombinant heparan sulfate oligosaccharides as agents for sequestering FH or FHR proteins and altering tissue-specific complement regulation both in vitro and in vivo. These studies address the central theme of this request for applications focused on tissue-specific regulation of complement activation and host recognition. Defining the role of heparan sulfate in complement regulation could open the door to a new class of therapeutics that would significantly impact a number of human diseases.