Structure of the N-terminal region of complement factor H and conformational implications of disease-linked sequence variations

The role of complement in kidney disease: conclusions from a Kidney Disease: Improving Global Outcomes (KDIGO) Controversies Conference

Uncontrolled complement activation can cause or contribute to glomerular injury in multiple kidney diseases. Although complement activation plays a causal role in atypical hemolytic uremic syndrome and C3 glomerulopathy, over the past decade, a rapidly accumulating body of evidence has shown a role for complement activation in multiple other kidney diseases, including diabetic nephropathy and several glomerulonephritides. The number of available complement inhibitor therapies has also increased during the same period. In 2022, Kidney Diseases: Improving Global Outcomes (KDIGO) convened a Controversies Conference, "The Role of Complement in Kidney Disease," to address the expanding role of complement dysregulation in the pathophysiology, diagnosis, and management of various glomerular diseases, diabetic nephropathy, and other forms of hemolytic uremic syndrome. Conference participants reviewed the evidence for complement playing a primary causal or secondary role in progression for several disease states and considered how evidence of complement involvement might inform management. Participating patients with various complement-mediated diseases and caregivers described concerns related to life planning, implications surrounding genetic testing, and the need for inclusive implementation of effective novel therapies into clinical practice. The value of biomarkers in monitoring disease course and the role of the glomerular microenvironment in complement response were examined, and key gaps in knowledge and research priorities were identified.

The hypothetical molecular mechanism of the ethnic variations in the manifestation of age-related macular degeneration; focuses on the functions of the most significant susceptibility genes

Age-related macular degeneration (AMD) is the leading sight-threatening disease in developed countries. On the other hand, recent studies indicated an ethnic variation in the phenotype of AMD. For example, several reports demonstrated that the incidence of drusen in AMD patients is less in Asians compared to Caucasians though the reason has not been clarified yet. In the last decades, several genome association studies have disclosed many susceptible genes of AMD and revealed that the association strength of some genes was different among races and AMD phenotypes. In this review article, the essential findings of the clinical studies and genome association studies for the most significant genes CFH and ARMS2/HTRA1 in AMD of different races are summarized, and theoretical hypotheses about the molecular mechanisms underlying the ethnic variation in the AMD manifestation mainly focused on those genes between Caucasians and Asians are discussed.

Polygenic Risk Score Impact on Susceptibility to Age-Related Macular Degeneration in Polish Patients

Age-related macular degeneration (AMD) is a common retina degenerative disease with a complex genetic and environmental background. This study aimed to determine the polygenic risk score (PRS) stratification between the AMD case and control patients. The PRS model was established on the targeted sequencing data of a cohort of 471 patients diagnosed with AMD and 167 healthy controls without symptoms of retinal degeneration. The highest predictive value to the target dataset was achieved for a 22-variant model with a p-value lower than threshold PT = 0.0123. The median PRS for cases was higher by 1.1 than for control samples (95% CI: (−1.19; −0.85)). The patients in the highest quantile had a significantly higher relative risk of developing AMD than those in the lowest reference quantile (OR = 35.13, 95% CI: (7.9; 156.1), p < 0.001). The diagnostic ability was investigated using ROC analysis with AUC = 0.76 (95% CI: (0.72; 0.80)). The polygenic susceptibility to AMD may be the starting point to expand AMD diagnostics based on rare highly penetrant variants and investigate associations with disease progression and treatment response in Polish patients in future studies.

A synthetic protein as efficient multitarget regulator against complement over-activation

The complement system constitutes the innate defense against pathogens. Its dysregulation leads to diseases and is a critical determinant in many viral infections, e.g., COVID-19. Factor H (FH) is the main regulator of the alternative pathway of complement activation and could be a therapy to restore homeostasis. However, recombinant FH is not available. Engineered FH versions may be alternative therapeutics. Here, we designed a synthetic protein, MFHR13, as a multitarget complement regulator. It combines the dimerization and C5-regulatory domains of human FH-related protein 1 (FHR1) with the C3-regulatory and cell surface recognition domains of human FH, including SCR 13. In summary, the fusion protein MFHR13 comprises SCRs FHR11-2:FH1-4:FH13:FH19-20. It protects sheep erythrocytes from complement attack exhibiting 26 and 4-fold the regulatory activity of eculizumab and human FH, respectively. Furthermore, we demonstrate that MFHR13 and FHR1 bind to all proteins forming the membrane attack complex, which contributes to the mechanistic understanding of FHR1. We consider MFHR13 a promising candidate as therapeutic for complement-associated diseases.

Streptococcus Pneumoniae-Associated Hemolytic Uremic Syndrome in the Era of Pneumococcal Vaccine

Streptococcus pneumoniae-associated hemolytic uremic syndrome (Sp-HUS) is a serious complication of invasive pneumococcal disease that is associated with increased mortality in the acute phase and morbidity in the long term. Recently, Sp-HUS definition has undergone revision and cases are categorized as definite, probable, and possible, based on less invasive serological investigations that evaluate Thomsen-Friedenreich crypt antigen (T-antigen) activation. In comparison to the pre-vaccine era, Sp-HUS incidence seems to be decreasing after the introduction of 7-serotype valence and 13-serotype valence pneumococcal vaccines in 2000 and 2010, respectively. However, Sp-HUS cases continue to occur secondary to vaccine failure and emergence of non-vaccine/replacement serotypes. No single hypothesis elucidates the molecular basis for Sp-HUS occurrence, although pneumococcal neuraminidase production and formation of T-antigen antibody complexes on susceptible endothelial and red blood cells continues to remain the most acceptable explanation. Management of Sp-HUS patients remains supportive in nature and better outcomes are being reported secondary to earlier recognition, better diagnostic tools and improved medical care. Recently, the addition of eculizumab therapy in the management of Sp-HUS for control of dysregulated complement activity has demonstrated good outcomes, although randomized clinical trials are awaited. A sustained pneumococcal vaccination program and vigilance for replacement serotypes will be the key for persistent reduction in Sp-HUS cases worldwide.

Functional Characterization of Rare Genetic Variants in the N-Terminus of Complement Factor H in aHUS, C3G, and AMD

Membranoproliferative glomerulonephritis (MPGN), C3 glomerulopathy (C3G), atypical haemolytic uraemic syndrome (aHUS) and age-related macular degeneration (AMD) have all been strongly linked with dysfunction of the alternative pathway (AP) of complement. A significant proportion of individuals with MPGN, C3G, aHUS and AMD carry rare genetic variants in the CFH gene that cause functional or quantitative deficiencies in the factor H (FH) protein, an important regulator of the AP. In silico analysis of the deleteriousness of rare genetic variants in CFH is not reliable and careful biochemical assessment remains the gold standard. Six N-terminal variants of uncertain significance in CFH were identified in patients with these diseases of the AP and selected for analysis. The variants were produced in Pichia Pastoris in the setting of FH CCPs 1-4, purified by nickel affinity chromatography and size exclusion and characterized by surface plasmon resonance and haemolytic assays as well as by cofactor assays in the fluid phase. A single variant, Q81P demonstrated a profound loss of binding to C3b with consequent loss of cofactor and decay accelerating activity. A further 2 variants, G69E and D130N, demonstrated only subtle defects which could conceivably over time lead to disease progression of more chronic AP diseases such as C3G and AMD. In the variants S159N, A161S, and M162V any functional defect was below the capacity of the experimental assays to reliably detect. This study further underlines the importance of careful biochemical assessment when assigning functional consequences to rare genetic variants that may alter clinical decisions for patients.

Molecular Mechanisms of Complement System Proteins and Matrix Metalloproteinases in the Pathogenesis of Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is an eye disorder affecting predominantly the older people above the age of 50 years in which the macular region of the retina deteriorates, resulting in the loss of central vision. The key factors associated with the pathogenesis of AMD are age, smoking, dietary, and genetic risk factors. There are few associated and plausible genes involved in AMD pathogenesis. Common genetic variants (with a minor allele frequency of >5% in the population) near the complement genes explain 40-60% of the heritability of AMD. The complement system is a group of proteins that work together to destroy foreign invaders, trigger inflammation, and remove debris from cells and tissues. Genetic changes in and around several complement system genes, including the CFH, contribute to the formation of drusen and progression of AMD. Similarly, Matrix metalloproteinases (MMPs) that are normally involved in tissue remodeling also play a critical role in the pathogenesis of AMD. MMPs are involved in the degradation of cell debris and lipid deposits beneath retina but with age their functions get affected and result in the drusen formation, succeeding to macular degeneration. In this review, AMD pathology, existing knowledge about the normal and pathological role of complement system proteins and MMPs in the eye is reviewed. The scattered data of complement system proteins, MMPs, drusenogenesis, and lipofusogenesis have been gathered and discussed in detail. This might add new dimensions to the understanding of molecular mechanisms of AMD pathophysiology and might help in finding new therapeutic options for AMD.

Two distinct conformations of factor H regulate discrete complement-binding functions in the fluid phase and at cell surfaces

Factor H (FH) is the major regulator of C3b in the alternative pathway of the complement system in immunity. FH comprises 20 short complement regulator (SCR) domains, including eight glycans, and its Y402H polymorphism predisposes those who carry it to age-related macular degeneration. To better understand FH complement binding and self-association, we have studied the solution structures of both the His-402 and Tyr-402 FH allotypes. Analytical ultracentrifugation revealed that up to 12% of both FH allotypes self-associate, and this was confirmed by small-angle X-ray scattering (SAXS), MS, and surface plasmon resonance analyses. SAXS showed that monomeric FH has a radius of gyration (Rg ) of 7.2-7.8 nm and a length of 25 nm. Starting from known structures for the SCR domains and glycans, the SAXS data were fitted using Monte Carlo methods to determine atomistic structures of monomeric FH. The analysis of 29,715 physically realistic but randomized FH conformations resulted in 100 similar best-fit FH structures for each allotype. Two distinct molecular structures resulted that showed either an extended N-terminal domain arrangement with a folded-back C terminus or an extended C terminus and a folded-back N terminus. These two structures are the most accurate to date for glycosylated full-length FH. To clarify FH functional roles in host protection, crystal structures for the FH complexes with C3b and C3dg revealed that the extended N-terminal conformation accounted for C3b fluid-phase regulation, the extended C-terminal conformation accounted for C3d binding, and both conformations accounted for bivalent FH binding to glycosaminoglycans on the target cell surface.

Regulator-dependent mechanisms of C3b processing by factor I allow differentiation of immune responses

The complement system labels microbes and host debris for clearance. Degradation of surface-bound C3b is pivotal to direct immune responses and protect host cells. How the serine protease factor I (FI), assisted by regulators, cleaves either two or three distant peptide bonds in the CUB domain of C3b remains unclear. We present a crystal structure of C3b in complex with FI and regulator factor H (FH; domains 1-4 with 19-20). FI binds C3b-FH between FH domains 2 and 3 and a reoriented C3b C-terminal domain and docks onto the first scissile bond, while stabilizing its catalytic domain for proteolytic activity. One cleavage in C3b does not affect its overall structure, whereas two cleavages unfold CUB and dislodge the thioester-containing domain (TED), affecting binding of regulators and thereby determining the number of cleavages. These data explain how FI generates late-stage opsonins iC3b or C3dg in a context-dependent manner, to react to foreign, danger or healthy self signals.

Disease-linked mutations in factor H reveal pivotal role of cofactor activity in self-surface-selective regulation of complement activation

Spontaneous activation enables the complement system to respond very rapidly to diverse threats. This activation is efficiently suppressed by complement factor H (CFH) on self-surfaces but not on foreign surfaces. The surface selectivity of CFH, a soluble protein containing 20 complement-control protein modules (CCPs 1-20), may be compromised by disease-linked mutations. However, which of the several functions of CFH drives this self-surface selectivity remains unknown. To address this, we expressed human CFH mutants in Pichia pastoris We found that recombinant I62-CFH (protective against age-related macular degeneration) and V62-CFH functioned equivalently, matching or outperforming plasma-derived CFH, whereas R53H-CFH, linked to atypical hemolytic uremic syndrome (aHUS), was defective in C3bBb decay-accelerating activity (DAA) and factor I cofactor activity (CA). The aHUS-linked CCP 19 mutant D1119G-CFH had virtually no CA on (self-like) sheep erythrocytes (E_S) but retained DAA. The aHUS-linked CCP 20 mutant S1191L/V1197A-CFH (LA-CFH) had dramatically reduced CA on E_S but was less compromised in DAA. D1119G-CFH and LA-CFH both performed poorly at preventing complement-mediated hemolysis of E_S PspCN, a CFH-binding Streptococcus pneumoniae protein domain, binds CFH tightly and increases accessibility of CCPs 19 and 20. PspCN did not improve the DAA of any CFH variant on E_S Conversely, PspCN boosted the CA, on E_S, of I62-CFH, R53H-CFH, and LA-CFH and also enhanced hemolysis protection by I62-CFH and LA-CFH. We conclude that CCPs 19 and 20 are critical for efficient CA on self-surfaces but less important for DAA. Exposing CCPs 19 and 20 with PspCN and thus enhancing CA on self-surfaces may reverse deficiencies of some CFH variants.

A novel, multiplexed targeted mass spectrometry assay for quantification of complement factor H (CFH) variants and CFH-related proteins 1-5 in human plasma

Age-related macular degeneration (AMD) is a leading cause of visual loss among older adults. Two variants in the complement factor H (CFH) gene, Y402H and I62V, are strongly associated with risk of AMD. CFH is encoded in regulator of complement activation gene cluster in chromosome 1q32, which includes complement factor related (CFHR) proteins, CFHR1 to CFHR5, with high amino acid sequence homology to CFH. Our goal was to build a SRM assay to measure plasma concentrations of CFH variants Y402, H402, I62, and V62, and CFHR1-5. The final assay consisted of 24 peptides and 72 interference-free SRM transition ion pairs. Most peptides showed good linearity over 0.3-200 fmol/μL concentration range. Plasma concentrations of CFH variants and CFHR1-5 were measured using the SRM assay in 344 adults. Plasma CFH concentrations (mean, SE in μg/mL) by inferred genotype were: YY402, II62 (170.1, 31.4), YY402, VV62 (188.8, 38.5), HH402, VV62 (144.0, 37.0), HY402, VV62 (164.2, 42.3), YY402, IV62 (194.8, 36.8), HY402, IV62 (181.3, 44.7). Mean (SE) plasma concentrations of CFHR1-5 were 1.63 (0.04), 3.64 (1.20), 0.020 (0.001), 2.42 (0.18), and 5.49 (1.55) μg/mL, respectively. This SRM assay should facilitate the study of the role of systemic complement and risk of AMD.

Creating functional sophistication from simple protein building blocks, exemplified by factor H and the regulators of complement activation

Complement control protein modules (CCPs) occur in numerous functionally diverse extracellular proteins. Also known as short consensus repeats (SCRs) or sushi domains each CCP contains approximately 60 amino acid residues, including four consensus cysteines participating in two disulfide bonds. Varying in length and sequence, CCPs adopt a β-sandwich type fold and have an overall prolate spheroidal shape with N- and C-termini lying close to opposite poles of the long axis. CCP-containing proteins are important as cytokine receptors and in neurotransmission, cell adhesion, blood clotting, extracellular matrix formation, haemoglobin metabolism and development, but CCPs are particularly well represented in the vertebrate complement system. For example, factor H (FH), a key soluble regulator of the alternative pathway of complement activation, is made up entirely from a chain of 20 CCPs joined by short linkers. Collectively, therefore, the 20 CCPs of FH must mediate all its functional capabilities. This is achieved via collaboration and division of labour among these modules. Structural studies have illuminated the dynamic architectures that allow FH and other CCP-rich proteins to perform their biological functions. These are largely the products of a highly varied set of intramolecular interactions between CCPs. The CCP can act as building block, spacer, highly versatile recognition site or dimerization mediator. Tandem CCPs may form composite binding sites or contribute to flexible, rigid or conformationally 'switchable' segments of the parent proteins.

Probing the solution structure of Factor H using hydroxyl radical protein footprinting and cross-linking

The control protein Factor H (FH) is a crucial regulator of the innate immune complement system, where it is active on host cell membranes and in the fluid phase. Mutations impairing the binding capacity of FH lead to severe autoimmune diseases. Here, we studied the solution structure of full-length FH, in its free state and bound to the C3b complement protein. To do so, we used two powerful techniques, hydroxyl radical protein footprinting (HRPF) and chemical cross-linking coupled with mass spectrometry (MS), to probe the structural rearrangements and to identify protein interfaces. The footprint of C3b on the FH surface matches existing crystal structures of C3b complexed with the N- and C-terminal fragments of FH. In addition, we revealed the position of the central portion of FH in the protein complex. Moreover, cross-linking studies confirmed the involvement of the C-terminus in the dimerization of FH.

Membrane-proximal TRAIL species are incapable of inducing short circuit apoptosis signaling: Implications for drug development and basic cytokine biology

TRAIL continues to garner substantial interest as a recombinant cancer therapeutic while the native cytokine itself serves important tumor surveillance functions when expressed in membrane-anchored form on activated immune effector cells. We have recently developed the genetically stabilized TRAIL platform TR3 in efforts to improve the limitations associated with currently available drug variants. While in the process of characterizing mesothelin-targeted TR3 variants using a single chain antibody (scFv) delivery format (SS-TR3), we discovered that the membrane-tethered cytokine had a substantially increased activity profile compared to non-targeted TR3. However, cell death proceeded exclusively via a bystander mechanism and protected the mesothelin-positive targets from apoptosis rather than leading to their elimination. Incorporation of a spacer-into the mesothelin surface antigen or the cancer drug itself-converted SS-TR3 into a cis-acting phenotype. Further experiments with membrane-anchored TR3 variants and the native cytokine confirmed our hypothesis that membrane-proximal TRAIL species lack the capacity to physically engage their cognate receptors coexpressed on the same cell membrane. Our findings not only provide an explanation for the “peaceful” coexistence of ligand and receptor of a representative member of the TNF superfamily but give us vital clues for the design of activity-enhanced TR3-based cancer therapeutics.

Mutational analysis of Kaposica reveals that bridging of MG2 and CUB domains of target protein is crucial for the cofactor activity of RCA proteins

The complement system has evolved to annul pathogens, but its improper regulation is linked with diseases. Efficient regulation of the system is primarily provided by a family of proteins termed regulators of complement activation (RCA). The knowledge of precise structural determinants of RCA proteins critical for imparting the regulatory activities and the molecular events underlying the regulatory processes, nonetheless, is still limited. Here, we have dissected the structural requirements of RCA proteins that are crucial for one of their two regulatory activities, the cofactor activity (CFA), by using the Kaposi's sarcoma-associated herpesvirus RCA homolog Kaposica as a model protein. We have scanned the entire Kaposica molecule by sequential mutagenesis using swapping and site-directed mutagenesis, which identified residues critical for its interaction with C3b and factor I. Mapping of these residues onto the modeled structure of C3b-Kaposica-factor I complex supported the mutagenesis data. Furthermore, the model suggested that the C3b-interacting residues bridge the CUB (complement C1r-C1s, Uegf, Bmp1) and MG2 (macroglobulin-2) domains of C3b. Thus, it seems that stabilization of the CUB domain with respect to the core of the C3b molecule is central for its CFA. Identification of CFA-critical regions in Kaposica guided experiments in which the equivalent regions of membrane cofactor protein were swapped into decay-accelerating factor. This strategy allowed CFA to be introduced into decay-accelerating factor, suggesting that viral and human regulators use a common mechanism for CFA.

Characterization of a factor H mutation that perturbs the alternative pathway of complement in a family with membranoproliferative GN

Complement C3 activation is a characteristic finding in membranoproliferative GN (MPGN). This activation can be caused by immune complex deposition or an acquired or inherited defect in complement regulation. Deficiency of complement factor H has long been associated with MPGN. More recently, heterozygous genetic variants have been reported in sporadic cases of MPGN, although their functional significance has not been assessed. We describe a family with MPGN and acquired partial lipodystrophy. Although C3 nephritic factor was shown in family members with acquired partial lipodystrophy, it did not segregate with the renal phenotype. Genetic analysis revealed a novel heterozygous mutation in complement factor H (R83S) in addition to known risk polymorphisms carried by individuals with MPGN. Patients with MPGN had normal levels of factor H, and structural analysis of the mutant revealed only subtle alterations. However, functional analysis revealed profoundly reduced C3b binding, cofactor activity, and decay accelerating activity leading to loss of regulation of the alternative pathway. In summary, this family showed a confluence of common and rare functionally significant genetic risk factors causing disease. Data from our analysis of these factors highlight the role of the alternative pathway of complement in MPGN.

Atypical hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is a triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. The atypical form of HUS is a disease characterized by complement overactivation. Inherited defects in complement genes and acquired autoantibodies against complement regulatory proteins have been described. Incomplete penetrance of mutations in all predisposing genes is reported, suggesting that a precipitating event or trigger is required to unmask the complement regulatory deficiency. The underlying genetic defect predicts the prognosis both in native kidneys and after renal transplantation. The successful trials of the complement inhibitor eculizumab in the treatment of atypical HUS will revolutionize disease management.

Using mutagenesis and structural biology to map the binding site for the Plasmodium falciparum merozoite protein PfRh4 on the human immune adherence receptor

To survive and replicate within the human host, malaria parasites must invade erythrocytes. Invasion can be mediated by the P. falciparum reticulocyte-binding homologue protein 4 (PfRh4) on the merozoite surface interacting with complement receptor type 1 (CR1, CD35) on the erythrocyte membrane. The PfRh4 attachment site lies within the three N-terminal complement control protein modules (CCPs 1-3) of CR1, which intriguingly also accommodate binding and regulatory sites for the key complement activation-specific proteolytic products, C3b and C4b. One of these regulatory activities is decay-accelerating activity. Although PfRh4 does not impact C3b/C4b binding, it does inhibit this convertase disassociating capability. Here, we have employed ELISA, co-immunoprecipitation, and surface plasmon resonance to demonstrate that CCP 1 contains all the critical residues for PfRh4 interaction. We fine mapped by homologous substitution mutagenesis the PfRh4-binding site on CCP 1 and visualized it with a solution structure of CCPs 1-3 derived by NMR and small angle x-ray scattering. We cross-validated these results by creating an artificial PfRh4-binding site through substitution of putative PfRh4-interacting residues from CCP 1 into their homologous positions within CCP 8; strikingly, this engineered binding site had an ∼30-fold higher affinity for PfRh4 than the native one in CCP 1. These experiments define a candidate site on CR1 by which P. falciparum merozoites gain access to human erythrocytes in a non-sialic acid-dependent pathway of merozoite invasion.

Does dysregulated complement activation contribute to haemolytic uraemic syndrome secondary to Streptococcus pneumoniae?

We describe two patients with haemolytic uraemic syndrome (HUS) associated with invasive Streptococcus pneumoniae infection. Both patients had transiently reduced serum concentrations of complement C3. One had reduced expression of CD46 and never recovered renal function. No constitutive defect in regulation of the alternative pathway of complement activation was demonstrated in the second patient but there was an apparent improvement in her condition after administration of eculizumab. The most widely accepted mechanism for pneumococcal HUS is endothelial cell damage by pre-formed antibodies against the Thomsen-Friedenreich antigen. This explanation does not bear rigorous scrutiny. We postulate that transiently dysregulated complement activation may play a role in the pathogenesis of pneumococcal disease. We further postulate that the mechanism could be enhanced binding of factor H to the neuraminidase-altered surface of endothelial cells or reduced binding of factor H to the endothelial cell surface mediated by competitive binding of factor H by pneumococcal surface protein C (pspC).

Functional anatomy of complement factor H

Factor H (FH) is a soluble regulator of the proteolytic cascade at the core of the evolutionarily ancient vertebrate complement system. Although FH consists of a single chain of similar protein modules, it has a demanding job description. Its chief role is to prevent complement-mediated injury to healthy host cells and tissues. This entails recognition of molecular patterns on host surfaces combined with control of one of nature's most dangerous examples of a positive-feedback loop. In this way, FH modulates, where and when needed, an amplification process that otherwise exponentially escalates the production of the pro-inflammatory, pro-phagocytic, and pro-cytolytic cleavage products of complement proteins C3 and C5. Mutations and single-nucleotide polymorphisms in the FH gene and autoantibodies against FH predispose individuals to diseases, including age-related macular degeneration, dense-deposit disease, and atypical hemolytic uremic syndrome. Moreover, deletions or variations of genes for FH-related proteins also influence the risk of disease. Numerous pathogens hijack FH and use it for self-defense. As reviewed herein, a molecular understanding of FH function is emerging. While its functional oligomeric status remains uncertain, progress has been achieved in characterizing its three-dimensional architecture and, to a lesser extent, its intermodular flexibility. Models are proposed, based on the reconciliation of older data with a wealth of recent evidence, in which a latent circulating form of FH is activated by its principal target, C3b tethered to a self-surface. Such models suggest hypotheses linking sequence variations to pathophysiology, but improved, more quantitative, functional assays and rigorous data analysis are required to test these ideas.

Solution structure of CCP modules 10-12 illuminates functional architecture of the complement regulator, factor H

The 155-kDa plasma glycoprotein factor H (FH), which consists of 20 complement control protein (CCP) modules, protects self-tissue but not foreign organisms from damage by the complement cascade. Protection is achieved by selective engagement of FH, via CCPs 1-4, CCPs 6-8 and CCPs 19-20, with polyanion-rich host surfaces that bear covalently attached, activation-specific, fragments of complement component C3. The role of intervening CCPs 9-18 in this process is obscured by lack of structural knowledge. We have concatenated new high-resolution solution structures of overlapping recombinant CCP pairs, 10-11 and 11-12, to form a three-dimensional structure of CCPs 10-12 and validated it by small-angle X-ray scattering of the recombinant triple-module fragment. Superimposing CCP 12 of this 10-12 structure with CCP 12 from the previously solved CCP 12-13 structure yielded an S-shaped structure for CCPs 10-13 in which modules are tilted by 80-110° with respect to immediate neighbors, but the bend between CCPs 10 and 11 is counter to the arc traced by CCPs 11-13. Including this four-CCP structure in interpretation of scattering data for the longer recombinant segments, CCPs 10-15 and 8-15, implied flexible attachment of CCPs 8 and 9 to CCP 10 but compact and intimate arrangements of CCP 14 with CCPs 12, 13 and 15. Taken together with difficulties in recombinant production of module pairs 13-14 and 14-15, the aberrant structure of CCP 13 and the variability of 13-14 linker sequences among orthologues, a structural dependency of CCP 14 on its neighbors is suggested; this has implications for the FH mechanism.

Use of time-resolved FRET to validate crystal structure of complement regulatory complex between C3b and factor H (N terminus)

Structural knowledge of interactions amongst the ~ 40 proteins of the human complement system, which is central to immune surveillance and homeostasis, is expanding due primarily to X-ray diffraction of co-crystallized proteins. Orthogonal evidence, in solution, for the physiological relevance of such co-crystal structures is valuable since intermolecular affinities are generally weak-to-medium and inter-domain mobility may be important. In this current work, Förster resonance energy transfer (FRET) was used to investigate the 10 μM K(D) (210 kD) complex between the N-terminal region of the soluble complement regulator, factor H (FH1-4), and the key activation-specific complement fragment, C3b. Using site-directed mutagenesis, seven cysteines were introduced individually at potentially informative positions within the four CCP modules comprising FH1-4, then used for fluorophore attachment. C3b possesses a thioester domain featuring an internal cycloglutamyl cysteine thioester; upon hydrolysis this yields a free thiol (Cys988) that was also fluorescently tagged. Labeled proteins were functionally active as cofactors for cleavage of C3b to iC3b except for FH1-4(Q40C) where conjugation with the fluorophore likely abrogated interaction with the protease, factor I. Time-resolved FRET measurements were undertaken to explore interactions between FH1-4 and C3b in fluid phase and under near-physiological conditions. These experiments confirmed that, as in the cocrystal structure, FH1-4 binds to C3b with CCP module 1 furthest from, and CCP module 4 closest to, the thioester domain, placing subsequent modules of FH near to any surface to which C3b is attached. The data do not rule out flexibility of the thioester domain relative to the remainder of the complex.

Structural and functional characterization of the product of disease-related factor H gene conversion

Numerous complement factor H (FH) mutations predispose patients to atypical hemolytic uremic syndrome (aHUS) and other disorders arising from inadequately regulated complement activation. No unifying structural or mechanistic consequences have been ascribed to these mutants beyond impaired self-cell protection. The S1191L and V1197A mutations toward the C-terminus of FH, which occur in patients singly or together, arose from gene conversion between CFH encoding FH and CFHR1 encoding FH-related 1. We show that neither single nor double mutations structurally perturbed recombinant proteins consisting of the FH C-terminal modules, 19 and 20 (FH19-20), although all three FH19-20 mutants were poor, compared to wild-type FH19-20, at promoting hemolysis of C3b-coated erythrocytes through competition with full-length FH. Indeed, our new crystal structure of the S1191L mutant of FH19-20 complexed with an activation-specific complement fragment, C3d, was nearly identical to that of the wild-type FH19-20:C3d complex, consistent with mutants binding to C3b with wild-type-like affinity. The S1191L mutation enhanced thermal stability of module 20, whereas the V1197A mutation dramatically decreased it. Thus, although mutant proteins were folded at 37 °C, they differ in conformational rigidity. Neither single substitutions nor double substitutions increased measurably the extent of FH19-20 self-association, nor did these mutations significantly affect the affinity of FH19-20 for three glycosaminoglycans, despite critical roles of module 20 in recognizing polyanionic self-surface markers. Unexpectedly, FH19-20 mutants containing Leu1191 self-associated on a heparin-coated surface to a higher degree than on surfaces coated with dermatan or chondroitin sulfates. Thus, potentially disease-related functional distinctions between mutants, and between FH and FH-related 1, may manifest in the presence of specific glycosaminoglycans.

Complement factor H-ligand interactions: self-association, multivalency and dissociation constants

Factor H (FH) is the major plasma regulator of the central complement protein C3b in the alternative pathway of complement activation. The elucidation of the FH interactions with five major ligands (below) is complicated by their weak μM dissociation constants K(D) and FH multivalency. We present the first survey of all the K(D) values for the major FH-ligand interactions and critically review their physiological significance. (i) FH self-association is presently well-established. We review multiple data sets that show that 5-14% of FH is self-associated in physiological conditions. FH self-association is significant for both laboratory investigations and physiological function.(ii) The FH-C3b complex shows low M affinity, meaning that the complex is not fully formed in plasma. In addition, C3, its hydrolysed form C3u, and its cleaved forms C3b and C3d show multimerisation. Current data favour a model when two C3b molecules bind independently to one FH molecule, as opposed to a1:1 stoichiometry where FH wraps itself around C3b.(iii) Heparin is often used as an analogue of the polyanionic host cell surface. The FH-heparin complex also shows a low M affinity, again meaning that complexes are not fully formed in vivo. The oligomeric FH-heparin complexes clarify a two-site interaction model of FH with host-cell surfaces.(iv) Reinvestigation of the FH and C-reactive protein (CRP) interaction revealed that this can only occur in plasma when CRP levels are elevated during acute-phase conditions. Given that CRP binds more weakly to the His402 allotype of FH than the Tyr402 allotype, this suggested a link with age-related macular degeneration (AMD).(v) FH activity is inhibited by zinc, which causes FH to aggregate strongly. High levels of bioavailable zinc occur in sub-retinal pigment epithelial deposits which lead to AMD. Excess zinc binds weakly to a central region of FH, explaining how zinc inhibits FH regulation of C3b.

Estimation of interdomain flexibility of N-terminus of factor H using residual dipolar couplings

Characterization of segmental flexibility is needed to understand the biological mechanisms of the very large category of functionally diverse proteins, exemplified by the regulators of complement activation, that consist of numerous compact modules or domains linked by short, potentially flexible, sequences of amino acid residues. The use of NMR-derived residual dipolar couplings (RDCs), in magnetically aligned media, to evaluate interdomain motion is established but only for two-domain proteins. We focused on the three N-terminal domains (called CCPs or SCRs) of the important complement regulator, human factor H (i.e., FH1-3). These domains cooperate to facilitate cleavage of the key complement activation-specific protein fragment, C3b, forming iC3b that no longer participates in the complement cascade. We refined a three-dimensional solution structure of recombinant FH1-3 based on nuclear Overhauser effects and RDCs. We then employed a rudimentary series of RDC data sets, collected in media containing magnetically aligned bicelles (disklike particles formed from phospholipids) under three different conditions, to estimate interdomain motions. This circumvents a requirement of previous approaches for technically difficult collection of five independent RDC data sets. More than 80% of conformers of this predominantly extended three-domain molecule exhibit flexions of <40°. Such segmental flexibility (together with the local dynamics of the hypervariable loop within domain 3) could facilitate recognition of C3b via initial anchoring and eventual reorganization of modules to the conformation captured in the previously solved crystal structure of a C3b:FH1-4 complex.

Structural basis for complement factor I control and its disease-associated sequence polymorphisms

The complement system is a key component of innate and adaptive immune responses. Complement regulation is critical for prevention and control of disease. We have determined the crystal structure of the complement regulatory enzyme human factor I (fI). FI is in a proteolytically inactive form, demonstrating that it circulates in a zymogen-like state despite being fully processed to the mature sequence. Mapping of functional data from mutants of fI onto the structure suggests that this inactive form is maintained by the noncatalytic heavy-chain allosterically modulating activity of the light chain. Once the ternary complex of fI, a cofactor and a substrate is formed, the allosteric inhibition is released, and fI is oriented for cleavage. In addition to explaining how circulating fI is limited to cleaving only C3b/C4b, our model explains the molecular basis of disease-associated polymorphisms in fI and its cofactors.

Age-related macular degeneration and the complement system

Age-related macular degeneration (AMD) is the leading cause of blindness in the developed world. It is a complex multifactorial disease, and despite new advances in treatment, many patients still succumb to visual impairment. The complement pathway has been implicated in the pathogenesis of many diseases, and recently variants in several genes encoding complement pathway proteins have been associated with AMD. Complement proteins have been found in histological specimens of eyes with AMD. Altered levels of both intrinsic complement proteins and activated products have been found in the circulation of patients with AMD. Complement activation may be triggered by oxidative stress, resulting from retinal exposure to incoming light; indeed an inter-play between these two pathological processes seems to exist. Finally, complement inhibitors are currently being evaluated in clinical trials. This article reviews the role of the complement system in AMD, and the potential of complement inhibition in preventing the devastating blindness resulting from this disease.

Structures of the rat complement regulator CrrY

Complement receptor 1-related protein Y (CrrY) is an important cell-surface regulator of complement that is unique to rodent species. The structure of rat CrrY domains 1-4 has been determined in two distinct crystal forms and reveals a 70° bend between domains 3 and 4. Comparisons of this structure with those of other complement regulators suggests that rearrangement of this interface may occur on forming the regulatory complex with C3b.

Zinc binding to the Tyr402 and His402 allotypes of complement factor H: possible implications for age-related macular degeneration

The Tyr402His polymorphism of complement factor H (FH) with 20 short complement regulator (SCR) domains is associated with age-related macular degeneration (AMD). How FH contributes to disease pathology is not clear. Both FH and high concentrations of zinc are found in drusen deposits, the key feature of AMD. Heterozygous FH is inhibited by zinc, which causes FH to aggregate. Here, zinc binding to homozygous FH was studied. By analytical ultracentrifugation, large amounts of oligomers were observed with both the native Tyr402 and the AMD-risk His402 homozygous allotypes of FH and both the recombinant SCR-6/8 allotypes with Tyr/His402. X-ray scattering also showed that both FH and SCR-6/8 allotypes strongly aggregated at >10 μM zinc. The SCR-1/5 and SCR-16/20 fragments were less likely to bind zinc. These observations were supported by bioinformatics predictions. Starting from known zinc binding sites in crystal structures, we predicted 202 putative partial surface zinc binding sites in FH, most of which were in SCR-6. Metal site prediction web servers also suggested that SCR-6 and other domains bind zinc. Predicted SCR-6/8 dimer structures showed that zinc binding sites could be formed at the protein-protein interface that would lead to daisy-chained oligomers. It was concluded that zinc binds weakly to FH at multiple surface locations, most probably within the functionally important SCR-6/8 domains, and this explains why zinc inhibits FH activity. Given the high pathophysiological levels of bioavailable zinc present in subretinal deposits, we discuss how zinc binding to FH may contribute to deposit formation and inflammation associated with AMD.

Production of biologically active complement factor H in therapeutically useful quantities

Human complement factor H (FH), an abundant 155-kDa plasma glycoprotein with 40 disulphide bonds, regulates the alternative-pathway complement cascade. Mutations and single nucleotide polymorphisms in the FH gene predispose to development of age-related macular degeneration, atypical haemolytic uraemic syndrome and dense deposit disease. Supplementation with FH variants protective against disease is an enticing therapeutic prospect. Current sources of therapeutic FH are restricted to human blood plasma highlighting a need for recombinant material. Previously FH expression in cultured plant, mammalian or insect cells yielded protein amounts inadequate for full characterisation, and orders of magnitude below therapeutic usefulness. Here, the V62,Y402 variant of FH has been produced recombinantly (rFH) in Pichia pastoris cells. Codon-optimisation proved essential whilst exploitation of the yeast mating α-factor peptide ensured secretion. We thereby produced multiple 10s-of-milligram of rFH. Following endoglycosidase H digestion of N-linked glycans, rFH (with eight residual N-acetylglucosamine moieties) was purified on heparin-affinity resin and anion-exchange chromatography. Full-length rFH was verified by mass spectrometry and Western blot using monoclonal antibodies to the C-terminus. Recombinant FH is a single non-aggregated species (by dynamic light scattering) and fully functional in biochemical and biological assays. An additional version of rFH was produced in which eight N-glycosylation sequons were ablated by Asn-Gln substitutions resulting in a glycan-devoid product. Successful production of rFH in this potentially very highly expressing system makes production of therapeutically useful quantities economically viable. Furthermore, ease of genetic manipulation in P. pastoris would allow production of engineered FH versions with enhanced pharmacokinetic and pharmacodynamic properties.

Complement factor H deficiency and endocapillary glomerulonephritis due to paternal isodisomy and a novel factor H mutation

Complement factor H (CFH) is a regulator of the alternative complement activation pathway. Mutations in the CFH gene are associated with atypical hemolytic uremic syndrome, membranoproliferative glomerulonephritis type II and C3 glomerulonephritis. Here, we report a 6-month-old CFH-deficient child presenting with endocapillary glomerulonephritis rather than membranoproliferative glomerulonephritis (MPGN) or C3 glomerulonephritis. Sequence analyses showed homozygosity for a novel CFH missense mutation (Pro139Ser) associated with severely decreased CFH plasma concentration (<6%) but normal mRNA splicing and expression. The father was heterozygous carrier of the mutation, but the mother was a non-carrier. Thus, a large deletion in the maternal CFH locus or uniparental isodisomy was suspected. Polymorphic markers across chromosome 1 showed homozygosity for the paternal allele in all markers and a lack of the maternal allele in six informative markers. This combined with a comparative genomic hybridization assay demonstrated paternal isodisomy. Uniparental isodisomy increases the risk of homozygous variations in other genes on the affected chromosome. Therefore, we analyzed other susceptibility genes on chromosome 1 and found no sequence variation in membrane cofactor protein, but homozygosity for the common deletion of CFH-related proteins 1 and 3, which may contribute to the early onset of disease.

Disease-associated N-terminal complement factor H mutations perturb cofactor and decay-accelerating activities

Many mutations associated with atypical hemolytic uremic syndrome (aHUS) lie within complement control protein modules 19-20 at the C terminus of the complement regulator factor H (FH). This region mediates preferential action of FH on self, as opposed to foreign, membranes and surfaces. Hence, speculation on disease mechanisms has focused on deficiencies in regulation of complement activation on glomerular capillary beds. Here, we investigate the consequences of aHUS-linked mutations (R53H and R78G) within the FH N-terminal complement control protein module that also carries the I62V variation linked to dense-deposit disease and age-related macular degeneration. This module contributes to a four-module C3b-binding site (FH1-4) needed for complement regulation and sufficient for fluid-phase regulatory activity. Recombinant FH1-4(V62) and FH1-4(I62) bind immobilized C3b with similar affinities (K(D) = 10-14 μM), whereas FH1-4(I62) is slightly more effective than FH1-4(V62) as cofactor for factor I-mediated cleavage of C3b. The mutant (R53H)FH1-4(V62) binds to C3b with comparable affinity (K(D) ∼12 μM) yet has decreased cofactor activities both in fluid phase and on surface-bound C3b, and exhibits only weak decay-accelerating activity for C3 convertase (C3bBb). The other mutant, (R78G)FH1-4(V62), binds poorly to immobilized C3b (K(D) >35 μM) and is severely functionally compromised, having decreased cofactor and decay-accelerating activities. Our data support causal links between these mutations and disease; they demonstrate that mutations affecting the N-terminal activities of FH, not just those in the C terminus, can predispose to aHUS. These observations reinforce the notion that deficiency in any one of several FH functional properties can contribute to the pathogenesis of this disease.

Alternative complement pathway assessment in patients with atypical HUS

The atypical Hemolytic Uremic Syndrome (aHUS) is a rare thrombotic microangiopathy leading to end stage renal disease in approximately 60% of patients. Over the last decade, a clear link has been demonstrated between this disease and defective complement regulation. The hallmark of the aHUS is the association with mutations in complement alternative pathway genes. Endothelial damage is related to complement dysregulation, but the exact mechanism is just starting to be elucidated. Screening for and characterization of mutations in the components of the C3 convertase (C3 and FB) or its regulators (FH, FI, MCP, and Thrombomodulin) or anti-FH antibodies has become an indispensable part of the disease's diagnostic. This review will initially summarize current knowledge on the understanding of complement activation and regulation, followed by a description on the genetic analysis as well as the methods used for complement protein quantification. Another part of this review will focus on the mechanisms of action of aHUS-associated mutations. We will emphasize on when and why some mutations lead to protein deficiency, while others result in - to dysfunctional but normally expressed proteins. Finally, we will discuss how the therapy of aHUS patients can be modified according to the functional consequences of each particular genetic defect.

Age-related macular degeneration: genetic and clinical findings

Age-related macular degeneration (AMD) is a sight threatening eye disease that affects millions of humans over the age of 65 years. It is considered to be the major cause of irreversible blindness in the elderly population in the developed world. The disease is prevalent in Europe and the United States, which has a large number of individuals of European descent. AMD is characterized by a progressive loss of central vision attributable to degenerative and neovascular changes that occur in the interface between the neural retina and the underlying choroid. This location contains the retinal photoreceptors, the retinal pigmented epithelium, a basement membrane complex known as Bruch's membrane and a network of choroidal capillaries. AMD is increasingly recognized as a complex genetic disorder where one or more genes contribute to an individual's susceptibility to development of the condition, while the prevailing view is that the disease stems from the interaction of multiple genetic and environmental factors. Although it has been proposed that a threshold event occurs during normal aging, the sequelae of biochemical, cellular, and molecular events leading to AMD are not fully understood. Here, we review the clinical aspects of AMD and summarize the genes which have been reported to have a positive association with the disease.

Genetics and complement in atypical HUS

Central to the pathogenesis of atypical hemolytic uremic syndrome (aHUS) is over-activation of the alternative pathway of complement. Following the initial discovery of mutations in the complement regulatory protein, factor H, mutations have been described in factor I, membrane cofactor protein and thrombomodulin, which also result in decreased complement regulation. Autoantibodies to factor H have also been reported to impair complement regulation in aHUS. More recently, gain of function mutations in the complement components C3 and Factor B have been seen. This review focuses on the genetic causes of aHUS, their functional consequences, and clinical effect.

Unravelling protein-protein interactions between complement factor H and C-reactive protein using a multidisciplinary strategy

Experimental studies of protein-protein interactions are very much affected by whether the complexes are fully formed (strong, with nanomolar dissociation constants) or partially dissociated (weak, with micromolar dissociation constants). The functions of the complement proteins of innate immunity are governed by the weak interactions between the activated proteins and their regulators. Complement is effective in attacking pathogens, but not the human host, and imbalances in this process can lead to disease conditions. The inherent complexity in analysing complement interactions is augmented by the multivalency of its main regulator, CFH (complement factor H), for its physiological or pathophysiological ligands. The unravelling of such weak protein-protein or protein-ligand interactions requires a multidisciplinary approach. Synchrotron X-ray solution scattering and constrained modelling resulted in the determination of the solution structure of CFH and its self-associative properties, whereas AUC (analytical ultracentrifugation) identified the formation of much larger CFH multimers through the addition of metals such as zinc. The ligands of CFH, such as CRP (C-reactive protein), also undergo self-association. The combination of X-rays and AUC with SPR (surface plasmon resonance) proved to be essential to identify CRP self-association and revealed how CFH interacts with CRP. We show that CRP unexpectedly binds to CFH at two non-contiguous sites and explain its relevance to age-related macular degeneration.

Atypical hemolytic uremic syndrome and mutation analysis of factor H gene in two Tunisian families

We carried out a protein and genetic investigation of the factor H gene mutations within two families presenting with a diagnostic suspicion of atypical hemolytic uremic syndrome (aHUS). The results within the patients of the first family revealed a factor H-deficiency. Direct sequencing allowed the detection of a 4-nucleotide deletion in the factor H gene. This deletion was found as the homozygote form in the proband and as the heterozygote form in the parents. Protein and functional analyses of the complement system were normal in all members of the second family. However, the molecular investigation for the father showed the presence of an amino acid substitution in the FH gene. Unfortunately, his two affected children died without being investigated for mutations. The functional consequences of these abnormal proteins are still to be demonstrated.

The role of complement Factor H in age-related macular degeneration: a review

Factor H is a 155kDa sialic acid containing glycoprotein that plays an integral role in the regulation of the complement-mediated immune system that is involved in microbial defense, immune complex processing, and programmed cell death. These events take place primarily in fluid phase and on the cell surface and are particularly important in the context of distinguishing self from non-self. Activation of the complement system occurs within seconds and results in a proteolytic cascade eventually forming the membrane attack complex leading to cell lysis. Factor H protects host cells from injury resulting from unrestrained complement activation. Mutations and SNPs (single nucleotide polymorphisms) in Factor H have been implicated in a variety of human conditions including age-related macular degeneration (AMD), atypical hemolytic uremic syndrome, and membranoproliferative glomuleronephritis type II or dense deposit disease. It should not be surprising that these seemingly unrelated diseases involving mutations in Factor H may share common features. Because the immune process involves, in part, an inflammatory response and common or similar surface antigens, it is also not unexpected to observe features of inflammation, including deposition of bioactive complement fragments such as C3a and C5a, a cellular influx of immune related cells such as lymphocytes, and the potential for multiple organ involvement. We review recent developments in molecular genetics; SNPs, including Y402H; the three-dimensional structure; and mass spectroscopy of Factor H as it relates to the pathogenesis of eye disease. In addition, we discuss the concepts of molecular mimicry, sequestered or hidden antigens, and antigenic cross reactivity, and propose that AMD should not simply be considered to be an eye disease, but rather a systemic vascular disease where the eye has the ability to self regulate a local immune response. Identification of the initial event or inciting antigen has yet to be determined and will significantly advance the understanding of the pathogenesis of AMD.

Annexin-II, DNA, and histones serve as factor H ligands on the surface of apoptotic cells

Apoptotic cells are opsonized by complement components such as C1q and C3b, which increases their susceptibility to phagocytosis. Soluble complement inhibitors such as factor H (fH) also recognize apoptotic cells to minimize the pro-inflammatory effects of downstream complement activation. We used four radiolabeled protein constructs that span different regions of the 20 complement control protein (CCP) modules that make up fH and found that fragments comprising CCPs 6-8, CCPs 8-15, and CCPs 19-20 but not CCPs 1-4, bound to apoptotic Jurkat T cells. There are four possible ligand types on apoptotic cells that could recruit fH: proteins, carbohydrates, lipids, and DNA. We found that CCPs 6-8 of fH bind to annexin-II, a trypsin-insensitive protein that becomes exposed on surfaces of apoptotic cells. The second ligand of fH, which interacts with CCPs 6-8 and 19-20, is DNA. Confocal microscopy showed co-localization of fH with antibodies specific for DNA. fH also binds to histones devoid of DNA, and CCPs 1-4, 6-8, and 8-15 mediate this interaction. Treatment of apoptotic cells with neuraminidase, chondroitinase, heparitinase, and heparinase did not change fH binding. Treatment of apoptotic cells with phospholipase A(2) dramatically increased both binding of fH and cell-surface DNA. We also excluded the possibility that fH interacts with lysophospholipids using surface plasmon resonance and flow cytometry with lipid-coated beads. Identification of annexin-II as one of the fH ligands on apoptotic cells together with the fact that autoantibodies against annexin-II are found in systemic lupus erythematosus provides further insight into understanding the pathogenesis of this disease.

Multiple interactions of complement Factor H with its ligands in solution: a progress report

Factor H (FH) is the major regulator of the central complement protein C3b in the alternative pathway of complement activation, and is comprised of 20 SCR domains. A FH Tyr402His polymorphism in SCR-7 is associated with age-related macular degeneration (AMD) and leads to deposition of complement in drusen. The unravelling of how FH interacts with five major physiological and patho-physiological ligands is complicated by the weak nature of these interactions, coupled with the multivalency of FH. Using multiple biophysical methods, we summarise our recent results for these five FH ligands: (1) FH by itself shows a folded-back SCR domain structure in solution, and self-associates in a manner dependent on electrostatic forces. (2) FH activity is inhibited by zinc, which causes FH to aggregate. The onset of FH-zinc aggregation for zinc concentrations above 20 muM appears to be enhanced with the His402 allotype, and may be relevant to AMD. (3) The FH and C-reactive protein (CRP) interaction has been controversial; however our new work resolves earlier discrepancies. The FH-CRP interaction is only observed when native CRP is at high acute-phase concentration levels, and CRP binds weakly to the His402 FH allotype to suggest a molecular mechanism that leads to AMD. (4) Heparin is an analogue of the polyanionic host cell surface, and FH forms higher oligomers with larger heparin fragments, suggesting a mechanism for more effective FH regulation. (5) The interaction of C3b with FH also depends on buffer, and FH forms multimers with the C3d fragment of C3b. This FH-C3d interaction at high FH concentration may also facilitate complement regulation. Overall, our results to date suggest that the FH interactions involving zinc and native CRP have the closest relevance for explaining the onset of AMD.

The central portion of factor H (modules 10-15) is compact and contains a structurally deviant CCP module

The first eight and the last two of 20 complement control protein (CCP) modules within complement factor H (fH) encompass binding sites for C3b and polyanionic carbohydrates. These binding sites cooperate self-surface selectively to prevent C3b amplification, thus minimising complement-mediated damage to host. Intervening fH CCPs, apparently devoid of such recognition sites, are proposed to play a structural role. One suggestion is that the generally small CCPs 10–15, connected by longer-than-average linkers, act as a flexible tether between the two functional ends of fH; another is that the long linkers induce a 180° bend in the middle of fH. To test these hypotheses, we determined the NMR-derived structure of fH12–13 consisting of module 12, shown here to have an archetypal CCP structure, and module 13, which is uniquely short and features a laterally protruding helix-like insertion that contributes to a prominent electropositive patch. The unusually long fH12–13 linker is not flexible. It packs between the two CCPs that are not folded back on each other but form a shallow vee shape; analytical ultracentrifugation and X-ray scattering supported this finding. These two techniques additionally indicate that flanking modules (within fH11–14 and fH10–15) are at least as rigid and tilted relative to neighbours as are CCPs 12 and 13 with respect to one another. Tilts between successive modules are not unidirectional; their principal axes trace a zigzag path. In one of two arrangements for CCPs 10–15 that fit well with scattering data, CCP 14 is folded back onto CCP 13. In conclusion, fH10–15 forms neither a flexible tether nor a smooth bend. Rather, it is compact and has embedded within it a CCP module (CCP 13) that appears to be highly specialised given both its deviant structure and its striking surface charge distribution. A passive, purely structural role for this central portion of fH is unlikely.

The disease-protective complement factor H allotypic variant Ile62 shows increased binding affinity for C3b and enhanced cofactor activity

Mutations and polymorphisms in the gene encoding factor H (CFH) have been associated with atypical haemolytic uraemic syndrome, dense deposit disease and age-related macular degeneration. The disease-predisposing CFH variants show a differential association with pathology that has been very useful to unravel critical events in the pathogenesis of one or other disease. In contrast, the factor H (fH)-Ile(62) polymorphism confers strong protection to all three diseases. Using ELISA-based methods and surface plasmon resonance analyses, we show here that the protective fH-Ile(62) variant binds more efficiently to C3b than fH-Val(62) and competes better with factor B in proconvertase formation. Functional analyses demonstrate an increased cofactor activity for fH-Ile(62) in the factor I-mediated cleavage of fluid phase and surface-bound C3b; however, the two fH variants show no differences in decay accelerating activity. From these data, we conclude that the protective effect of the fH-Ile(62) variant is due to its better capacity to bind C3b, inhibit proconvertase formation and catalyze inactivation of fluid-phase and surface-bound C3b. This demonstration of the functional consequences of the fH-Ile(62) polymorphism provides relevant insights into the complement regulatory activities of fH that will be useful in disease prediction and future development of effective therapeutics for disorders caused by complement dysregulation.

Multimeric interactions between complement factor H and its C3d ligand provide new insight on complement regulation

Activation of C3 to C3b signals the start of the alternative complement pathway. The C-terminal short complement regulator (SCR)-20 domain of factor H (FH), the major serum regulator of C3b, possesses a binding site for C3d, a 35-kDa physiological fragment of C3b. Size distribution analyses of mixtures of SCR-16/20 or FH with C3d by analytical ultracentrifugation in 50 and 137 mM NaCl buffer revealed a range of discrete peaks, showing that multimeric complexes had formed at physiologically relevant concentrations. Surface plasmon resonance studies showed that native FH binds C3d in two stages. An equilibrium dissociation constant K(D)(1) of 2.6 microM in physiological buffer was determined for the first stage. Overlay experiments indicated that C3d formed multimeric complexes with FH. X-ray scattering showed that the maximum dimension of the C3d complexes with SCR-16/20 at 29 nm was not much longer than that of the unbound SCR-16/20 dimer. Molecular modelling suggested that the ultracentrifugation and scattering data are most simply explained in terms of associating dimers of each of SCR-16/20 and C3d. We conclude that the physiological interaction between FH and C3d is not a simple 1:1 binding stoichiometry between the two proteins that is often assumed. Because the multimers involve the C-terminus of FH, which is bound to host cell surfaces, our results provide new insight on FH regulation during excessive complement activation, both in the fluid phase and at host cell surfaces decorated by C3d.

Electrostatic interactions contribute to the folded-back conformation of wild type human factor H

Factor H (FH), a major serum regulator of C3b in the complement alternative pathway, is composed of 20 short complement regulator (SCR) domains. Earlier solution structures for FH showed that this has a folded-back domain arrangement and exists as oligomers. To clarify the molecular basis for this, analytical ultracentrifugation and X-ray scattering studies of native FH were performed as a function of NaCl concentration and pH. The sedimentation coefficient for the FH monomer decreased from 5.7 S to 5.3 S with increase in NaCl concentration, showing that weak electrostatic inter-domain interactions affect its folded-back structure. FH became more elongated at pH 9.4, showing the involvement of histidine residue(s) in its folded-back structure. Similar studies of partially deglycosylated FH suggested that oligosaccharides were not significant in determining the FH domain structure. The formation of FH oligomers decreased with increased NaCl concentration, indicating that electrostatic interactions also affect this. X-ray scattering showed that the maximum length of FH increased from 32 nm in low salt to 38 nm in high salt. Constrained X-ray scattering modelling was used to generate significantly improved FH molecular structures at medium resolution. In 50 mM NaCl, the modelled structures showed that inter-SCR domain contacts are likely, while these contacts are fewer in 250 mM NaCl. The results of this study show that the conformation of FH is affected by its local environment, and this may be important for its interactions with C3b and when bound to polyanionic cell surfaces.

Structure of complement fragment C3b-factor H and implications for host protection by complement regulators

Factor H (FH) is an abundant regulator of complement activation and protects host cells from self-attack by complement. Here we provide insight into the regulatory activity of FH by solving the crystal structure of the first four domains of FH in complex with its target, complement fragment C3b. FH interacted with multiple domains of C3b, covering a large, extended surface area. The structure indicated that FH destabilizes the C3 convertase by competition and electrostatic repulsion and that FH enables proteolytic degradation of C3b by providing a binding platform for protease factor I while stabilizing the overall domain arrangement of C3b. Our results offer general models for complement regulation and provide structural explanations for disease-related mutations in the genes encoding both FH and C3b.