Selective Binding of Heparin/Heparan Sulfate Oligosaccharides to Factor H and Factor H-Related Proteins: Therapeutic Potential for C3 Glomerulopathies

Abstract

Complement dysregulation is characteristic of the renal diseases atypical hemolytic uremic syndrome (aHUS) and complement component 3 glomerulopathy (C3G). Complement regulatory protein Factor H (FH) inhibits complement activity, whereas FH-related proteins (FHRs) lack a complement regulatory domain. FH and FHRs compete for binding to host cell glycans, in particular heparan sulfates (HS). HS is a glycosaminoglycan with an immense structural variability, where distinct sulfation patterns mediate specific binding of proteins. Mutations in FH, FHRs, or an altered glomerular HS structure may disturb the FH : FHRs balance on glomerular endothelial cells, thereby leading to complement activation and the subsequent development of aHUS/C3G. In this study, we aimed to identify specific HS structures that could specifically compete off FHRs from HS glycocalyx (HSGlx), without interfering with FH binding. FH/FHR binding to human conditionally immortalized glomerular endothelial cells (ciGEnCs) and HSGlx purified from ciGEnC glycocalyx was assessed. HS modifications important for FH/FHR binding to HSGlx were analyzed using selectively desulfated heparins in competition with purified HSGlx. We further assessed effects of heparinoids on FHR1- and FHR5-mediated C3b deposition on ciGEnCs. In the presence of C3b, binding of FH, FHR1 and FHR5 to ciGEnCs was significantly increased, whereas binding of FHR2 was minimal. FHR1 and 5 competitively inhibited FH binding to HSGlx, leading to alternative pathway dysregulation. FHR1 and FHR5 binding was primarily mediated by N-sulfation while FH binding depended on N-, 2-O- and 6-O-sulfation. Addition of 2-O-desulfated heparin significantly reduced FHR1- and FHR5-mediated C3b deposition on ciGEnCs. We identify 2-O-desulfated heparin derivatives as potential therapeutics for C3G and other diseases with dysregulated complement.

Factor H–Related Protein 1 Drives Disease Susceptibility and Prognosis in C3 Glomerulopathy

“…This is supported by the identification of heterozygous duplications of CFHR-1 and high plasma FHR-1 levels in patients with C3G from our and other registries. 35,36 Following this reasoning, we postulated that the deficiency of FHR-1 should be a protective factor against the development of the disease, a hypothesis that is consistent with the observation, reported here, that the allele frequency of D CFHR3-CFHR1 is significantly reduced in our Spanish C3G population. This finding corroborates a previous report that variation in CFHR3-CFHR1 copy number is associated with the C3G phenotype.…”

“…This mechanistic understanding of the C3G-associated mutants implies that increased FHR-1 levels could also contribute to the development of C3G. This is supported by the identification of heterozygous duplications of CFHR-1 and high plasma FHR-1 levels in patients with C3G from our and other registries 35 , 36 . Following this reasoning, we postulated that the deficiency of FHR-1 should be a protective factor against the development of the disease, a hypothesis that is consistent with the observation, reported here, that the allele frequency of Δ CFHR3-CFHR1 is significantly reduced in our Spanish C3G population.…”

C3 glomerulopathy: Understanding an ultra‐rare complement‐mediated renal disease

“…FH binding regulates formation of C3 convertase while FHRs promote C3 convertase activation. The local balance between FH and FHRs impacts complement control and when altered can potentiate glomerular disease (Csincsi et al, 2017; Goodship et al, 2017; Loeven et al, 2021; Tortajada et al, 2013). (a) The balance between FH and FHRs favors complement control.…”

Defining Nephritic Factors as Diverse Drivers of Systemic Complement Dysregulation in C3 Glomerulopathy

“… 68 Possibly, these agents may be beneficial in some patients with C3G by sequestering FH-related proteins that perpetuate C3 activation. 69 …”