Implications of the Progressive Self-association of Wild-type Human Factor H for Complement Regulation and Disease

The SCR-17 and SCR-18 glycans in human complement factor H enhance its regulatory function

Human complement factor H (CFH) plays a central role in regulating activated C3b to protect host cells. CFH contain 20 short complement regulator (SCR) domains and eight N-glycosylation sites. The N-terminal SCR domains mediate C3b degradation while the C-terminal CFH domains bind to host cell surfaces to protect these. Our earlier study of Pichia-generated CFH fragments indicated a self-association site at SCR-17/18 that comprises a dimerization site for human factor H. Two N-linked glycans are located on SCR-17 and SCR-18. Here, when we expressed SCR-17/18 without glycans in an Escherichia coli system, analytical ultracentrifugation showed that no dimers were now formed. To investigate this novel finding, full-length CFH and its C-terminal fragments were purified from human plasma and Pichia pastoris respectively, and their glycans were enzymatically removed using PNGase F. Using size-exclusion chromatography, mass spectrometry, and analytical ultracentrifugation, SCR-17/18 from Pichia showed notably less dimer formation without its glycans, confirming that the glycans are necessary for the formation of SCR-17/18 dimers. By surface plasmon resonance, affinity analyses interaction showed decreased binding of deglycosylated full-length CFH to immobilized C3b, showing that CFH glycosylation enhances the key CFH regulation of C3b. We conclude that our study revealed a significant new aspect of CFH regulation based on its glycosylation and its resulting dimerization.

Proof of concept of a new plasma complement Factor H from waste plasma fraction

Introduction

Complement factor H (FH) is a major regulator of the complement alternative pathway, its mutations predispose to an uncontrolled activation in the kidney and on blood cells and to secondary C3 deficiency. Plasma exchange has been used to correct for FH deficiency and although the therapeutic potential of purified FH has been suggested by in vivo experiments in animal models, a clinical approved FH concentrate is not yet available. We aimed to develop a purification process of FH from a waste fraction rather than whole plasma allowing a more efficient and ethical use of blood and plasma donations.

Methods

Waste fractions from industrial plasma fractionation (pooled human plasma) were analyzed for FH content by ELISA. FH was purified from unused fraction III and its decay acceleration, cofactor, and C3 binding capacity were characterized in vitro. Biodistribution was assessed by high-resolution dynamic PET imaging. Finally, the efficacy of the purified FH preparation was tested in the mouse model of C3 glomerulopathy (Cfh-/- mice).

Results

Our purification method resulted in a high yield of highly purified (92,07%), pathogen-safe FH. FH concentrate is intact and fully functional as demonstrated by in vitro functional assays. The biodistribution revealed lower renal and liver clearance of human FH in Cfh-/- mice than in wt mice. Treatment of Cfh-/- mice documented its efficacy in limiting C3 activation and promoting the clearance of C3 glomerular deposits.

Conclusion

We developed an efficient and economical system for purifying intact and functional FH, starting from waste material of industrial plasma fractionation. The FH concentrate could therefore constitute possible treatments options of patients with C3 glomerulopathy, particularly for those with FH deficiency, but also for patients with other diseases associated with alternative pathway activation.

A Dimerization Site at SCR-17/18 in Factor H Clarifies a New Mechanism for Complement Regulatory Control

Complement Factor H (CFH), with 20 short complement regulator (SCR) domains, regulates the alternative pathway of complement in part through the interaction of its C-terminal SCR-19 and SCR-20 domains with host cell-bound C3b and anionic oligosaccharides. In solution, CFH forms small amounts of oligomers, with one of its self-association sites being in the SCR-16/20 domains. In order to correlate CFH function with dimer formation and the occurrence of rare disease-associated variants in SCR-16/20, we identified the dimerization site in SCR-16/20. For this, we expressed, in Pichia pastoris, the five domains in SCR-16/20 and six fragments of this with one-three domains (SCR-19/20, SCR-18/20, SCR-17/18, SCR-16/18, SCR-17 and SCR-18). Size-exclusion chromatography suggested that SCR dimer formation occurred in several fragments. Dimer formation was clarified using analytical ultracentrifugation, where quantitative c(s) size distribution analyses showed that SCR-19/20 was monomeric, SCR-18/20 was slightly dimeric, SCR-16/20, SCR-16/18 and SCR-18 showed more dimer formation, and SCR-17 and SCR-17/18 were primarily dimeric with dissociation constants of ~5 µM. The combination of these results located the SCR-16/20 dimerization site at SCR-17 and SCR-18. X-ray solution scattering experiments and molecular modelling fits confirmed the dimer site to be at SCR-17/18, this dimer being a side-by-side association of the two domains. We propose that the self-association of CFH at SCR-17/18 enables higher concentrations of CFH to be achieved when SCR-19/20 are bound to host cell surfaces in order to protect these better during inflammation. Dimer formation at SCR-17/18 clarified the association of genetic variants throughout SCR-16/20 with renal disease.

Characterization of Binding Properties of Individual Functional Sites of Human Complement Factor H

Factor H exists as a 155,000 dalton, extended protein composed of twenty small domains which is flexible enough that it folds back on itself. Factor H regulates complement activation through its interactions with C3b and polyanions. Three binding sites for C3b and multiple polyanion binding sites have been identified on Factor H. In intact Factor H these sites appear to act synergistically making their individual contributions difficult to distinguish. Recombinantly expressed fragments of human Factor H were examined using surface plasmon resonance (SPR) for interactions with C3, C3b, iC3b, C3c, and C3d. Eleven recombinant proteins of lengths from one to twenty domains were used to show that the three C3b-binding sites exhibit 100-fold different affinities for C3b. The N-terminal site [complement control protein (CCP) domains 1-6] bound C3b with a K_d of 0.08 μM and this interaction was not influenced by the presence or absence of domains 7 and 8. Full length Factor H similarly exhibited a K_d for C3b of 0.1 μM. Unexpectedly, the N-terminal site (CCP 1-6) bound native C3 with a K_d of 0.4 μM. The C-terminal domains (CCP 19-20) exhibited a K_d of 1.7 μM for C3b. We localized a weak third C3b binding site in the CCP 13-15 region with a K_d estimated to be ~15 μM. The C-terminal site (CCP 19-20) bound C3b, iC3b, and C3d equally well with a K_d of 1 to 2 μM. In order to identify and compare regions of Factor H that interact with polyanions a family of 18 overlapping three domain recombinant proteins spanning the entire length of Factor H were expressed and purified. Immobilized heparin was used as a model polyanion and SPR confirmed the presence of heparin binding sites in CCP 6-8 (K_d 1.2 μM) and in CCP 19-20 (4.9 μM) and suggested the existence of a weak third polyanion binding site in the center of Factor H (CCP 11-13). Our results unveil the relative contributions of different regions of Factor H to its regulation of complement, and may contribute to the understanding of how defects in certain Factor H domains lead to disease.

Regulation of regulators: Role of the complement factor H-related proteins

The complement system, while being an essential and very efficient effector component of innate immunity, may cause damage to the host and result in various inflammatory, autoimmune and infectious diseases or cancer, when it is improperly activated or regulated. Factor H is a serum glycoprotein and the main regulator of the activity of the alternative complement pathway. Factor H, together with its splice variant factor H-like protein 1 (FHL-1), inhibits complement activation at the level of the central complement component C3 and beyond. In humans, there are also five factor H-related (FHR) proteins, whose function is poorly characterized. While data indicate complement inhibiting activity for some of the FHRs, there is increasing evidence that FHRs have an opposite role compared with factor H and FHL-1, namely, they enhance complement activation directly and also by competing with the regulators FH and FHL-1. This review summarizes the current stand and recent data on the roles of factor H family proteins in health and disease, with focus on the function of FHR proteins.

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.

Mapping the Complement Factor H-Related Protein 1 (CFHR1):C3b/C3d Interactions

Complement factor H-related protein 1 (CFHR1) is a complement regulator which has been reported to regulate complement by blocking C5 convertase activity and interfering with C5b surface association. CFHR1 also competes with complement factor H (CFH) for binding to C3b, and may act as an antagonist of CFH-directed regulation on cell surfaces. We have employed site-directed mutagenesis in conjunction with ELISA-based and functional assays to isolate the binding interaction that CFHR1 undertakes with complement components C3b and C3d to a single shared interface. The C3b/C3d:CFHR1 interface is identical to that which occurs between the two C-terminal domains (SCR19-20) of CFH and C3b. Moreover, we have been able to corroborate that dimerization of CFHR1 is necessary for this molecule to bind effectively to C3b and C3d, or compete with CFH. Finally, we have established that CFHR1 competes with complement factor H-like protein 1 (CFHL-1) for binding to C3b. CFHL-1 is a CFH gene splice variant, which is almost identical to the N-terminal 7 domains of CFH (SCR1-7). CFHR1, therefore, not only competes with the C-terminus of CFH for binding to C3b, but also sterically blocks the interaction that the N-terminus of CFH undertakes with C3b, and which is required for CFH-regulation.

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.

Molecular Interactions between Complement Factor H and Its Heparin and Heparan Sulfate Ligands

Complement factor H (CFH) is the major regulator of the central complement protein C3b in the alternative pathway of complement activation. A molecular view of the CFH interaction with native heparan sulfate (HS) is central for understanding the mechanism of how surface-bound CFH interacts with C3b bound to host cell surfaces. HS is composed of sulfated heparin-like S-regions that alternate with desulfated NA-regions. Solution structural studies of heparin (equivalent to the S-regions) and desulfated HS (the NA-regions) by scattering and ultracentrifugation showed that each structure was mostly extended and partially bent, but with greater bending and flexibility in the NA-regions compared to the S-regions. Their solution structures have been deposited in the Protein Data Bank. The largest HS oligosaccharides showed more bent and flexible structures than those for heparin. A folded-back domain structure for the solution structure of the 20 domains in CFH was determined likewise. CFH binds to the S-regions but less so to the NA-regions of HS. The bivalent interaction of CFH–heparin was observed by ultracentrifugation, and binding studies of CFH fragments with heparin-coated sensor chips. In common with other CFH interactions with its physiological and pathophysiological ligands, the CFH–heparin and CFH–C3b interactions have moderate micromolar dissociation constants K_D, meaning that these complexes do not fully form in vivo. The combination of the solution structures and binding studies indicated a two-site interaction model of CFH with heparin at cell surfaces. By this, the bivalent binding of CFH to a cell surface is co-operative. Defective interactions at either of the two independent CFH–heparin sites reduce the CFH interaction with surface-bound C3b and lead to immune disorders.

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 TT30, a novel complement therapeutic agent, provides insight into its joint binding to complement C3b and C3d

A novel therapeutic reagent TT30 was designed to be effective in diseases of the alternative pathway of complement such as paroxysmal nocturnal hemoglobinuria and other diseases. TT30 is constructed from the first four short complement regulator (SCR) domains of complement receptor type 2 (CR2) that bind to complement C3d, followed by the first five SCR domains of complement factor H that bind to complement C3b. In order to assess how TT30 binds to C3d and C3b, we determined the TT30 solution structure by a combination of analytical ultracentrifugation, X-ray scattering and constrained modeling. The sedimentation coefficients and radius of gyration of TT30 were unaffected by citrate or phosphate-buffered saline buffers and indicate an elongated monomeric structure with a sedimentation coefficient of 3.1 S and a radius of gyration R(G) of 6.9 nm. Molecular modeling starting from 3000 randomized TT30 conformations showed that high-quality X-ray curve fits were obtained with extended SCR arrangements, showing that TT30 has a limited degree of inter-SCR flexibility in its solution structure. The best-fit TT30 structural models are readily merged with the crystal structure of C3b to show that the four CR2 domains extend freely into solution when the five complement factor H domains are bound within C3b. We reevaluated the solution structure of the CR2-C3d complex that confirmed its recent crystal structure. This recent CR2-C3d crystal structure showed that TT30 is able to interact readily with C3d ligands in many orientations when TT30 is bound to C3b.

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.

The detection and quantitation of protein oligomerization

There are many different techniques available to biologists and biochemists that can be used to detect and characterize the self-association of proteins. Each technique has strengths and weaknesses and it is often useful to combine several approaches to maximize the former and minimize the latter. Here we review a range of methodologies that identify protein self-association and/or allow the stoichiometry and affinity of the interaction to be determined, placing an emphasis on what type of information can be obtained and outlining the advantages and disadvantages involved. In general, in vitro biophysical techniques, such as size exclusion chromatography, analytical ultracentrifugation, scattering techniques, NMR spectroscopy, isothermal titration calorimetry, fluorescence anisotropy and mass spectrometry, provide information on stoichiometry and/or binding affinities. Other approaches such as cross-linking, fluorescence methods (e.g., fluorescence correlation spectroscopy, FCS; Förster resonance energy transfer, FRET; fluorescence recovery after photobleaching, FRAP; and proximity imaging, PRIM) and complementation approaches (e.g., yeast two hybrid assays and bimolecular fluorescence complementation, BiFC) can be used to detect protein self-association in a cellular context.

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.

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.

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.

Analytical ultracentrifugation combined with X-ray and neutron scattering: Experiment and modelling

Analytical ultracentrifugation and solution scattering provide different multi-parameter structural and compositional information on proteins. The joint application of the two methods supplements high resolution structural studies by crystallography and NMR. We summarise the procedures required to obtain equivalent ultracentrifugation and X-ray and neutron scattering data. The constrained modelling of ultracentrifugation and scattering data is important to confirm the experimental data analysis and yields families of best-fit molecular models for comparison with crystallography and NMR structures. This modelling of ultracentrifugation and scattering data is described in terms of starting models, their conformational randomisation in trial-and-error fits, and the identification of the final best-fit models. Seven applications of these methods are described to illustrate the current state-of-the-art. These include the determination of antibody solution structures (the human IgG4 subclass, and oligomeric forms of human IgA and its secretory component), the solution structures of the complement proteins of innate immunity (Factor H and C3/C3u) and their interactions with macromolecular ligands (C-reactive protein), and anionic polysaccharides (heparin). Complementary features of joint ultracentrifugation and scattering experiments facilitate an improved understanding of crystal structures (illustrated for C3/C3u, C-reactive protein and heparin). If a large protein or its complex cannot be crystallised, the joint ultracentrifugation-scattering approach provides a means to obtain an overall macromolecular structure.

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.

Complement control protein factor H: the good, the bad, and the inadequate

The complement system is an essential component of the innate immune system that participates in elimination of pathogens and altered host cells and comprises an essential link between the innate and adaptive immune system. Soluble and membrane-bound complement regulators protect cells and tissues from unintended complement-mediated injury. Complement factor H is a soluble complement regulator essential for controlling the alternative pathway in blood and on cell surfaces. Normal recognition of self-cell markers (i.e. polyanions) and C3b/C3d fragments is necessary for factor H function. Inadequate recognition of host cell surfaces by factor H due to mutations and polymorphisms have been associated with complement-mediated tissue damage and disease. On the other hand, unwanted recognition of pathogens and altered self-cells (i.e. cancer) by factor H is used as an immune evasion strategy. This review will focus on the current knowledge related to these versatile recognition properties of factor H.

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.

Oligomeric assembly and interactions within the human RuvB-like RuvBL1 and RuvBL2 complexes

The two closely related eukaryotic AAA+ proteins (ATPases associated with various cellular activities), RuvBL1 (RuvB-like 1) and RuvBL2, are essential components of large multi-protein complexes involved in diverse cellular processes. Although the molecular mechanisms of RuvBL1 and RuvBL2 function remain unknown, oligomerization is likely to be important for their function together or individually, and different oligomeric forms might underpin different functions. Several experimental approaches were used to investigate the molecular architecture of the RuvBL1-RuvBL2 complex and the role of the ATPase-insert domain (domain II) for its assembly and stability. Analytical ultracentrifugation showed that RuvBL1 and RuvBL2 were mainly monomeric and each monomer co-existed with small proportions of dimers, trimers and hexamers. Adenine nucleotides induced hexamerization of RuvBL2, but not RuvBL1. In contrast, the RuvBL1-RuvBL2 complexes contained single- and double-hexamers together with smaller forms. The role of domain II in complex assembly was examined by size-exclusion chromatography using deletion mutants of RuvBL1 and RuvBL2. Significantly, catalytically competent dodecameric RuvBL1-RuvBL2, complexes lacking domain II in one or both proteins could be assembled but the loss of domain II in RuvBL1 destabilized the dodecamer. The composition of the RuvBL1-RuvBL2 complex was analysed by MS. Several species of mixed RuvBL1/2 hexamers with different stoichiometries were seen in the spectra of the RuvBL1-RuvBL2 complex. A number of our results indicate that the architecture of the human RuvBL1-RuvBL2 complex does not fit the recent structural model of the yeast Rvb1-Rvb2 complex.

Complement factor H binds at two independent sites to C-reactive protein in acute phase concentrations

Factor H (FH) regulates the activation of C3b in the alternative complement pathway, both in serum and at host cell surfaces. It is composed of 20 short complement regulator (SCR) domains. The Y402H polymorphism in FH is a risk factor for age-related macular degeneration. C-reactive protein (CRP) is an acute phase protein that binds Ca(2+). We established the FH-CRP interaction using improved analytical ultracentrifugation (AUC), surface plasmon resonance (SPR), and synchrotron x-ray scattering methods. Physiological FH and CRP concentrations were used in 137 mM NaCl and 2 mM Ca(2+), in which the occurrence of denatured CRP was avoided. In solution, AUC revealed FH-CRP binding. The FH-CRP interaction inhibited the formation of higher FH oligomers, indicating that CRP blocked FH dimerization sites at both SCR-6/8 and SCR-16/20. SPR confirmed the FH-CRP interaction and its NaCl concentration dependence upon using either immobilized FH or CRP. The SCR-1/5 fragment of FH did not bind to CRP. In order of increasing affinity, SCR-16/20, SCR-6/8 (His-402), and SCR-6/8 (Tyr-402) fragments bound to CRP. X-ray scattering showed that FH became more compact when binding to CRP, which is consistent with CRP binding at two different FH sites. We concluded that FH and CRP bind at elevated acute phase concentrations of CRP in physiological buffer. The SCR-16/20 site is novel and indicates the importance of the FH-CRP interaction for both age-related macular degeneration and atypical hemolytic uremic syndrome.

Constrained solution scattering modelling of human antibodies and complement proteins reveals novel biological insights

X-ray and neutron-scattering techniques characterize proteins in solution and complement high-resolution structural studies. They are useful when either a large protein cannot be crystallized, in which case scattering yields a solution structure, or a crystal structure has been determined and requires validation in solution. These solution structures are determined by the application of constrained modelling methods based on known subunit structures. First, an appropriate starting model is generated. Next, its conformation is randomized to generate thousands of models for trial-and-error fits. Comparison with the experimental data identifies a small family of best-fit models. Finally, their significance for biological function is assessed. We illustrate this in application to structure determinations for secretory immunoglobulin A, the most prevalent antibody in the human body and a first line of defence in mucosal immunity. We also discuss the applications to the large multi-domain proteins of the complement system, most notably its major regulator factor H, which is important in age-related macular degeneration and renal diseases. We discuss the importance of complementary data from analytical ultracentrifugation, and structural studies of protein-protein complexes. We conclude that constrained scattering modelling makes useful contributions to our understanding of antibody and complement structure and function.

The binding of factor H to a complex of physiological polyanions and C3b on cells is impaired in atypical hemolytic uremic syndrome

Factor H (fH) is essential for complement homeostasis in fluid-phase and on surfaces. Its two C-terminal domains (CCP 19-20) anchor fH to self-surfaces where it prevents C3b amplification in a process requiring its N-terminal four domains. In atypical hemolytic uremic syndrome (aHUS), mutations clustering toward the C terminus of fH may disrupt interactions with surface-associated C3b or polyanions and thereby diminish the ability of fH to regulate complement. To test this, we compared a recombinant protein encompassing CCP 19-20 with 16 mutants. The mutations had only very limited and localized effects on protein structure. Although we found four aHUS-linked fH mutations that decreased binding to C3b and/or to heparin (a model compound for cell surface polyanionic carbohydrates), we identified five aHUS-associated mutants with increased affinity for either or both ligands. Strikingly, these variable affinities for the individual ligands did not correlate with the extent to which all the aHUS-associated mutants were found to be impaired in a more physiological assay that measured their ability to inhibit cell surface complement functions of full-length fH. Taken together, our data suggest that disruption of a complex fH-self-surface recognition process, involving a balance of affinities for protein and physiological carbohydrate ligands, predisposes to aHUS.

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.

Functional and structural characterisation of human colostrum free secretory component

Secretory component (SC) in association with polymeric IgA (pIgA) forms secretory IgA (SIgA), the major antibody active at mucosal surfaces. SC also exists in a free form in secretions, with innate neutralizing properties against important pathogens. IgA-bound SC and free secretory component (FSC) are both produced by proteolytic cleavage of the polymeric Ig receptor whose function is to transport IgA and IgM across mucosal epithelia. Although the proteases have not been characterised and the site(s) of cleavage of the polymeric Ig receptor has been debated, it has been assumed that bound and free SC are produced by cleavage at the same site. Here we show by SDS-PAGE analyses that FSC is slightly smaller than SIgA1- or SIgA2-bound SC when purified simultaneously. The FSC preparation was functionally active, shown by binding to dimeric and polymeric IgA, and by its ability to trigger a respiratory burst by binding to 'SC receptors' on eosinophils. We also show that FSC from different human secretions have different molecular sizes. The solution structure of FSC from colostrum was studied by analytical ultracentrifugation and X-ray scattering. The sedimentation coefficient of 4.25S is close to that for recombinant FSC. The X-ray scattering curve showed that FSC adopts a compact structure in solution which corresponds well to the J-shaped domain arrangement determined previously for recombinant FSC which terminates at residue Arg585. The smaller sizes of the FSC forms are attributable to variable cleavages of the C-terminal linker region, and may result from the absence of dimeric IgA. The FSC modelling accounts for the lack of effect of the C-terminal linker on the known functions of FSC.

Polyanion-induced self-association of complement factor H

Factor H is the primary soluble regulator of activation of the alternative pathway of complement. It prevents activation of complement on host cells and tissues upon association with C3b and surface polyanions such as sialic acids, heparin, and other glycosaminoglycans. Here we show that interaction with polyanions causes self-association forming tetramers of the 155,000 Da glycosylated protein. Monomeric human factor H is an extended flexible protein that exhibits an apparent size of 330,000 Da, relative to globular standards, during gel filtration chromatography in the absence of polyanions. In the presence of dextran sulfate (5000 Da) or heparin an intermediate species of apparent m.w. 700,000 and a limit species of m.w. 1,400,000 were observed by gel filtration. Sedimentation equilibrium analysis by analytical ultracentrifugation indicated a monomer Mr of 163,000 in the absence of polyanions and a Mr of 607,000, corresponding to a tetramer, in the presence of less than a 2-fold molar excess of dextran sulfate. Increasing concentrations of dextran sulfate increased binding of factor H to zymosan-C3b 4.5-fold. This result was accompanied by an increase in both the decay accelerating and cofactor activity of factor H on these cells. An expressed fragment encompassing the C-terminal polyanion binding site (complement control protein domains 18-20) also exhibited polyanion-induced self-association, suggesting that the C-terminal ends of factor H mediate self-association. The results suggest that recognition of polyanionic markers on host cells and tissues by factor H, and the resulting regulation of complement activation, may involve formation of dimers and tetramers of factor H.

Uncontrolled zinc- and copper-induced oligomerisation of the human complement regulator factor H and its possible implications for function and disease

Polymorphisms in factor H (FH), a major regulator of complement activation, and the accumulation of high zinc concentrations in the outer retina are both associated with age-related macular degeneration. FH is inhibited by zinc, which causes FH to aggregate. To investigate this, we quantitatively studied zinc-induced FH self-association by X-ray scattering and analytical ultracentrifugation to demonstrate uncontrolled FH oligomerisation in conditions corresponding to physiological levels of FH and pathological levels of zinc in the outer retina. By scattering, FH at 2.8-7.0 microM was unaffected until [Zn] increased to 20 microM, whereupon the radius of gyration, RG, values increased from 9 to 15 nm at [Zn]=200 microM. The maximum dimension of FH increased from 32 to 50 nm, indicating that compact oligomers had formed. By ultracentrifugation, size-distribution analyses showed that monomeric FH at 5.57 S was the major species at [Zn] up to 60 microM. At [Zn] above 60 microM, a series of large oligomers were formed, ranging up to 100 S in size. Oligomerisation was reversed by ethylenediaminetetraacetic acid. Structurally distinct large oligomers were observed for Cu, while Ni, Cd and Fe showed low amounts of oligomers and Mg and Ca showed no change. Fluid-phase assays showed reduced FH activities that correlated with increased oligomer formation. The results were attributed to different degrees of stabilisation of weak self-dimerisation sites in FH by transition metals. The relevance of metal-induced FH oligomer formation to complement regulation and age-related macular degeneration is discussed.

Factor H dysfunction in patients with atypical hemolytic uremic syndrome contributes to complement deposition on platelets and their activation

Atypical hemolytic uremic syndrome (aHUS) may be associated with mutations in the C-terminal of factor H (FH). FH binds to platelets via the C-terminal as previously shown using a construct consisting of short consensus repeats (SCRs) 15 to 20. A total of 4 FH mutations, in SCR15 (C870R) and SCR20 (V1168E, E1198K, and E1198Stop) in patients with aHUS, were studied regarding their ability to allow complement activation on platelet surfaces. Purified FH-E1198Stop mutant exhibited reduced binding to normal washed platelets compared with normal FH, detected by flow cytometry. Washed platelets taken from the 4 patients with aHUS during remission exhibited C3 and C9 deposition, as well as CD40-ligand (CD40L) expression indicating platelet activation. Combining patient serum/plasma with normal washed platelets led to C3 and C9 deposition, CD40L and CD62P expression, aggregate formation, and generation of tissue factor-expressing microparticles. Complement deposition and platelet activation were reduced when normal FH was preincubated with platelets and were minimal when using normal serum. The purified FH-E1198Stop mutant added to FH-deficient plasma (complemented with C3) allowed considerable C3 deposition on washed platelets, in comparison to normal FH. In summary, mutated FH enables complement activation on the surface of platelets and their activation, which may contribute to the development of thrombocytopenia in aHUS.