Understanding cellular and molecular interactions of gC1qR, a receptor for the globular domain of complement protein C1q

Abstract

gC1qR was originally discovered as a C1q receptor specific to the globular head domain of C1q, the first subcomponent of the classical pathway of complement activation. During the same period, calreticulin (CRT), formerly called as cC1qR, was described as a receptor for the collagen region of C1q and collectins. Although much work has been carried out with relation to CRT-CD91 complex, the biological implications and structure-function studies of C1q-gC1qR interaction has not been further explored. With passage of time since 1994, it has become evident that gC1qR is also a multi-functional pattern recognition receptor that can recognise pathogens in addition to acting as a modulator of inflammation at the site of injury or infection. In this thesis, a recombinant form of gC1qR using a T7 promotor expression system was expressed and examined for its interaction with individual globular head modules of C1q A, B and C chains (ghA, ghB and ghC, respectively). A number of single residue substitution mutants of ghA, ghB and ghC modules were also analysed for their interaction with gC1qR in order to map complementary binding sites. Concomitant expression of gC1qR and C1q in the adherent monocytes with, and without proinflammatory stimuli was analysed by qPCR in order to establish autocrine/paracrine basis of C1q-gC1qR interaction. In addition, experiments were carried out to examine if C1q-mediated anti-lymphoproliferative effect can be altered by gC1qR. Subsequently, using the wild type and mutants of ghA, ghB and ghC modules, the interaction of DC-SIGN and SIGN-R, a newly discovered partner of C1q and gC1qR on the dendritic cell surface, was examined. Experiments are underway to understand how a trimolecular complex involving C1q, gC1qR and DC-SIGN participate during HIV-1 infection. Structure-function studies involving gC1qR and HCV core protein and HIV-1 gp41 have also been carried out to localise domains of gC1qR responsible for viral pathogenesis. The last chapter dwells on a newly discovered ability of gC1qR to upregulate bradykinin 1 receptor on the endothelial cell surface, thus its role in altering vascular permeability and the contact system. The thesis describes (1) localisation of interacting sites between C1q and gC1qR and their togetherness in co-expression under pro-inflammatory conditions and possibly suppression of immune cell proliferative response; (2) localisation of complementary binding sites between DC-SIGN, gC1qR and C1q and its possible implications in HIV-1 infection and antigen presenting cells such as dendritic cells; (3) localisation of interacting sites between gC1qR and HCV core protein as well as HIV-1 gp41 peptides with potential to propose a therapeutic peptide; and (4) ability of gC1qR to upregulate bradykinin 1 and 2 receptors on endothelial cells and its newly identified function as a modifier of inflammation.