FHR5 Binds to Laminins, Uses Separate C3b and Surface-Binding Sites, and Activates Complement on Malondialdehyde-Acetaldehyde Surfaces

Abstract

Factor H related-protein 5 (CFHR5) is a surface-acting complement activator and variations in the CFHR5 gene are linked to CFHR glomerulonephritis. In this study, we show that FHR5 binds to laminin-521, the major constituent of the glomerular basement membrane, and to mesangial laminin-211. Furthermore, we identify malondialdehyde-acetaldehyde (MAA) epitopes, which are exposed on the surface of human necrotic cells (), as new FHR5 ligands. Using a set of novel deletion fragments, we show that FHR5 binds to laminin-521, MAA epitopes, heparin, and human necrotic cells (HUVECs) via the middle region [short consensus repeats (SCRs) 5-7]. In contrast, surface-bound FHR5 contacts C3b via the C-terminal region (SCRs8-9). Thus, FHR5 uses separate domains for C3b binding and cell surface interaction. MAA epitopes serve as a complement-activating surface by recruiting FHR5. The complement activator FHR5 and the complement inhibitor factor H both bind to oxidation-specific MAA epitopes and FHR5 competes with factor H for binding. The C3 glomerulopathy-associated FHR2-FHR5 hybrid protein is more potent in MAA epitope binding and activation compared with wild-type FHR5. The implications of these results for pathology of CFHR glomerulonephritis are discussed. In conclusion, we identify laminins and oxidation-specific MAA epitopes as novel FHR5 ligands and show that the surface-binding site of FHR5 (SCRs5-7) is separated from the C3b binding site (SCRs8-9). Furthermore, FHR5 competes with factor H for binding to MAA epitopes and activates complement on these modified structures.

A genome-wide association study identifies key modulators of complement factor H binding to malondialdehyde-epitopes

“…It has been shown that MDA-epitopes mediate the clearance of damaged structures, such as oxidized low-density lipoproteins and dead cells (2). Various components of the innate immune system (e.g., scavenger receptors, natural immunoglobulin M (IgM) antibodies, CFH, and FHL-1) either directly or indirectly facilitate this process (14,(19)(20)(21). On the other hand, an impaired removal of MDA-carrying metabolic waste will promote inflammation (14,19,22).…”

“…One of the major carriers of MDA-epitopes in vivo are apoptotic and necrotic cells (20,35). CFH as well as FHR5 have been found to bind to MDA-epitopes on necrotic cells (14,21). Therefore, we also tested the binding of recombinant FHR1 and FHR3 to necrotic human umbilical vein endothelial cells (HUVECs) by fluorescence microscopy.…”

Cryoglobulinemic HCV-related membranoproliferative glomerulonephritis with TMA – secondary HUS or infection-triggered aHUS?

“…The CFHR5 protein is a surface -acting complement activator and variations in the CFHR5 gene are linked to CFHR glomerulonephritis. 11 The CFHR5 protein binds to laminin -521, the major constituent of the glomerular basement membrane, and to mesangial laminin -211, fostering complement activation at damaged sites. 11 It is thus conceivable that a pathogenic variant of this protein could potentiate complement activation in an endothelial cell initially damaged by an immune complex process, leading to the subsequent development of complement -mediated TMA.…”

Self-Damage Caused by Dysregulation of the Complement Alternative Pathway: Relevance of the Factor H Protein Family

“…Factor H-related protein 1 can also bind to host surfaces via its FH-homologous C-terminus ( 22 , 29 ), and FHR-3 binds heparin through CCP2, which is homologous to CCP7 of FH ( 23 , 87 ). In addition, FHR-5 has a heparin-binding site in CCPs 5–7 ( 24 , 94 ). The functional relevance of these interactions needs to be investigated further, but they could anchor these proteins on certain cells and surfaces.…”

“…As noted above, differences in ECMs and in domain composition of the FH family proteins may target FH and FHRs toward distinct sites, such as FH to the glomerular basement membrane ( via CCPs 19–20) and FHL-1 to the Bruch’s membrane ( via CCP7) ( 95 ). FHR-5 was shown to bind to MaxGel, an ECM extract, and de-regulate complement on this surface ( 32 ); a recent study identified laminin as an ECM ligand of FHR-5 ( 94 ).…”