Activation of the first component of complement

The Role of Anti-Inflammatory Adipokines in Cardiometabolic Disorders: Moving beyond Adiponectin

The global burden of obesity has multiplied owing to its rapidly growing prevalence and obesity-related morbidity and mortality. In addition to the classic role of depositing extra energy, adipose tissue actively interferes with the metabolic balance by means of secreting bioactive compounds called adipokines. While most adipokines give rise to inflammatory conditions, the others with anti-inflammatory properties have been the novel focus of attention for the amelioration of cardiometabolic complications. This review compiles the current evidence on the roles of anti-inflammatory adipokines, namely, adiponectin, vaspin, the C1q/TNF-related protein (CTRP) family, secreted frizzled-related protein 5 (SFRP5), and omentin-1 on cardiometabolic health. Further investigations on the mechanism of action and prospective human trials may pave the way to their clinical application as innovative biomarkers and therapeutic targets for cardiovascular and metabolic disorders.

Pleiotropic genomic variants at 17q21.31 associated with bone mineral density and body fat mass: a bivariate genome-wide association analysis

Osteoporosis and obesity are two severe complex diseases threatening public health worldwide. Both diseases are under strong genetic determinants as well as genetically correlated. Aiming to identify pleiotropic genes underlying obesity and osteoporosis, we performed a bivariate genome-wide association (GWA) meta-analysis of hip bone mineral density (BMD) and total body fat mass (TBFM) in 12,981 participants from seven samples, and followed by in silico replication in the UK biobank (UKB) cohort sample (N = 217,822). Combining the results from discovery meta-analysis and replication sample, we identified one novel locus, 17q21.31 (lead SNP rs12150327, NC_000017.11:g.44956910G > A, discovery bivariate P = 4.83 × 10^-9, replication P = 5.75 × 10^-5) at the genome-wide significance level (ɑ = 5.0 × 10^-8), which may have pleiotropic effects to both hip BMD and TBFM. Functional annotations highlighted several candidate genes, including KIF18B, C1QL1, and PRPF19 that may exert pleiotropic effects to the development of both body mass and bone mass. Our findings can improve our understanding of the etiology of osteoporosis and obesity, as well as shed light on potential new therapies.

Discovery of plasma biomarkers for predicting the severity of coronary artery atherosclerosis by quantitative proteomics

INTRODUCTION

Cardiovascular disease (CVD) in patients with diabetes is the leading cause of death. Finding early biomarkers for detecting asymptomatic patients with CVD can improve survival. Recently, plasma proteomics-targeted selected reaction monitoring/multiple reaction monitoring analyses (MRM)-has emerged as highly specific and sensitive tools compared with classic ELISA methods. The objective was to identify differentially regulated proteins according to the severity of the coronary artery atherosclerosis.

RESEARCH DESIGN AND METHODS

A discovery cohort, a verification cohort and a validation cohort consisted of 18, 53, and 228 subjects, respectively. The grade of coronary artery stenosis was defined as a percentage of luminal stenosis of the major coronary arteries. Participants were divided into six groups, depending on the presence of diabetes and the grade of coronary artery stenosis. Two mass spectrometric approaches were employed: (1) conventional shotgun liquid chromatography tandem mass spectrometry for a discovery and (2) quantitative MRM for verification and validation. An analysis of the covariance was used to examine the biomarkers' predictivity beyond conventional cardiovascular risks.

RESULTS

A total of 1349 different proteins were identified from a discovery cohort. We selected 52 proteins based on the tandem mass tag quantitative analysis then summarized as follows: chemokine (C-X-C motif) ligand 7 (CXCL7), apolipoprotein C-II (APOC2), human lipopolysaccharide-binding protein (LBP) and dedicator of cytokinesis 2 (DOCK2) in diabetes; CXCL7, APOC2, LBP, complement 4A (C4A), vitamin D-binding protein (VTDB) and laminin β1 subunit in non-diabetes. Analysis of covariance showed that APOC2, DOCK2, CXCL7 and VTDB were upregulated and C4A was downregulated in patients with diabetes showing severe coronary artery stenosis. LBP and VTDB were downregulated in patients without diabetes, showing severe coronary artery stenosis.

CONCLUSION

We identified significant associations between circulating APOC2, C4A, CXCL7, DOCK2, LBP and VTDB levels and the degree of coronary artery stenosis using the MRM technique.

Structural insight into proteolytic activation and regulation of the complement system

The complement system is a highly complex and carefully regulated proteolytic cascade activated through three different pathways depending on the activator recognized. The structural knowledge regarding the intricate proteolytic enzymes that activate and control complement has increased dramatically over the last decade. This development has been pivotal for understanding how mutations within complement proteins might contribute to pathogenesis and has spurred new strategies for development of complement therapeutics. Here we describe and discuss the complement system from a structural perspective and integrate the most recent findings obtained by crystallography, small-angle X-ray scattering, and electron microscopy. In particular, we focus on the proteolytic enzymes governing activation and their products carrying the biological effector functions. Additionally, we present the structural basis for some of the best known complement inhibitors. The large number of accumulated molecular structures enables us to visualize the relative size, position, and overall orientation of many of the most interesting complement proteins and assembled complexes on activator surfaces and in membranes.

Fc microparticles can modulate the physical extent and magnitude of complement activity

The complement system is an integral component of the humoral immune system, and describes a cascade of interacting proteins responsible for the opsonization and lysis of foreign pathogens, in addition to the recruitment of immune cells. However, complement activation is also implicated in the progression and complication of immune dysfunctions such as sepsis. Microparticle (MP) biomaterials capable of tuning the local magnitude of serum complement activation could improve complement-mediated cytotoxicity to serum-resistant bacteria or calm an overactive immune response during sepsis. We demonstrate that model Fc-functionalized microparticles can be designed to either enhance or diminish the local cytotoxic effect of complement activation in human serum. The particles were formed with either the antibody Fc domains oriented outward from the particle surface or randomly adsorbed in a non-oriented fashion. In the oriented Fc form, complement products were directly sequestered to the particle surface, including C5a, a potent anaphylatoxin that, when elevated, is associated with poor sepsis prognosis. The oriented particle also lowered the cytotoxicity of serum and thus decreased the antibiotic effect when compared to serum alone. Conversely, the non-oriented microparticles were found to sequester similar levels of C5a, but much lower levels of iC3b and TCC on the microparticle surface, thereby increasing the amount of the soluble terminal complement complex. In addition, the non-oriented microparticles extend the distance over which TCC forms and enhance serum cytotoxicity to bacteria. Together, these two types of complement-modulating particles provide the first biomaterial that can functionally modify the range of complement activation at sites distant from the particle surface. Thus, biomaterials that exploit Fc presentation provide new possibilities to functionally modulate complement activation to achieve a desired clinical result.

Structure and activation of C1, the complex initiating the classical pathway of the complement cascade

The complement system is an important antimicrobial and inflammation-generating component of the innate immune system. The classical pathway of complement is activated upon binding of the 774-kDa C1 complex, consisting of the recognition molecule C1q and the tetrameric protease complex C1r₂s₂, to a variety of activators presenting specific molecular patterns such as IgG- and IgM-containing immune complexes. A canonical model entails a C1r₂s₂ with its serine protease domains tightly packed together in the center of C1 and an intricate intramolecular reaction mechanism for activation of C1r and C1s, induced upon C1 binding to the activator. Here, we show that the serine protease domains of C1r and C1s are located at the periphery of the C1r₂s₂ tetramer both when alone or within the nonactivated C1 complex. Our structural studies indicate that the C1 complex adopts a conformation incompatible with intramolecular activation of C1, suggesting instead that intermolecular proteolytic activation between neighboring C1 complexes bound to a complement activating surface occurs. Our results rationalize how a multitude of structurally unrelated molecular patterns can activate C1 and suggests a conserved mechanism for complement activation through the classical and the related lectin pathway.

Chemical Synthesis of GM2 Glycans, Bioconjugation with Bacteriophage Qβ, and the Induction of Anticancer Antibodies

The development of carbohydrate-based antitumor vaccines is an attractive approach towards tumor prevention and treatment. Herein, we focused on the ganglioside GM2 tumor-associated carbohydrate antigen (TACA), which is overexpressed in a wide range of tumor cells. GM2 was synthesized chemically and conjugated with a virus-like particle derived from bacteriophage Qβ. Although the copper-catalyzed azide-alkyne cycloaddition reaction efficiently introduced 237 copies of GM2 per Qβ, this construct failed to induce significant amounts of anti-GM2 antibodies compared to the Qβ control. In contrast, GM2 immobilized on Qβ through a thiourea linker elicited high titers of IgG antibodies that recognized GM2-positive tumor cells and effectively induced cell lysis through complement-mediated cytotoxicity. Thus, bacteriophage Qβ is a suitable platform to boost antibody responses towards GM2, a representative member of an important class of TACA: the ganglioside.

The amount of C1q-adiponectin complex is higher in the serum and the complex localizes to perivascular areas of fat tissues and the intimal-medial layer of blood vessels of coronary artery disease patients

Background

The complement component C1q triggers activation of the classical immune pathway and can bind to adiponectin (APN). Recently, some studies have been reported that serum C1q-APN/total APN ratio correlates with atherosclerosis and coronary artery disease (CAD). We assessed the relationships between C1q related variables and the severity of CAD, and investigated the localization of the C1q–APN complex.

Methods

The sample included 153 subjects comprising healthy controls and patients with subclinical or overt CAD. We measured the serum concentrations of C1q, total APN, and high-molecular weight (HMW)-APN, and the amount of C1q–APN complex. We identified the sites of C1q–APN complex deposition in various adipose tissues and blood vessels.

Results

Serum concentrations of C1q and HMW-APN and the C1q/HMW-APN ratio were independently associated with the severity of coronary stenosis. The amount of C1q–APN complex was significantly higher in patients with CAD compared with controls. C1q and APN co-localized in perivascular areas of subcutaneous, visceral, and pericardial fat tissues, and the internal mammary artery of patients with severe CAD.

Conclusions

Serum C1q concentration and the C1q/HMW-APN ratio were independent markers of coronary artery stenosis. The amount of C1q–APN complex was significantly greater in serum from CAD patients. C1q and APN co-localized to perivascular areas in adipose tissue and blood vessels. The association between the increased amount of the C1q–APN complex and CAD should be investigated further.

Deciphering the fine details of c1 assembly and activation mechanisms: "mission impossible"?

The classical complement pathway is initiated by the large (~800 kDa) and flexible multimeric C1 complex. Its catalytic function is triggered by the proteases hetero-tetramer C1r2s2, which is associated to the C1q sensing unit, a complex assembly of 18 chains built as a hexamer of heterotrimers. Initial pioneering studies gained insights into the main architectural principles of the C1 complex. A dissection strategy then provided the high-resolution structures of its main functional and/or structural building blocks, as well as structural details on some key protein–protein interactions. These past and current discoveries will be briefly summed up in order to address the question of what is still ill-defined. On a functional point of view, the main molecular determinants of C1 activation and its tight control will be delineated. The current perspective remains to decipher how C1 really works and is controlled in vivo, both in normal and pathological settings.

In silico and in vitro studies on the protein-protein interactions between Brugia malayi immunomodulatory protein calreticulin and human C1q

Filarial parasites modulate effective immune response of their host by releasing a variety of immunomodulatory molecules, which help in the long persistence of the parasite within the host. The present study was aimed to characterize an immunomodulatory protein of Brugia malayi and its interaction with the host immune component at the structural and functional level. Our findings showed that Brugia malayi Calreticulin (BmCRT) is responsible for the prevention of classical complement pathway activation via its interaction with the first component C1q of the human host. This was confirmed by inhibition of C1q dependent lysis of immunoglobulin-sensitized Red Blood Cells (S-RBCs). This is possibly the first report which predicts CRT-C1q interaction on the structural content of proteins to explain how BmCRT inhibits this pathway. The molecular docking of BmCRT-C1q complex indicated that C1qB chain (IgG/M and CRP binding sites on C1q) played a major role in the interaction with conserved and non-conserved regions of N and P domain of BmCRT. Out of 37 amino acids of BmCRT involved in the interaction, nine amino acids (Pro(126), Glu(132), His(147), Arg(151), His(153), Met(154), Lys(156), Ala(196) and Lys(212)) are absent in human CRT. Both ELISA and in silico analysis showed the significant role of Ca(+2) in BmCRT-HuC1q complex formation and deactivation of C1r2-C1s2. Molecular dynamics studies of BmCRT-HuC1q complex showed a deviation from ∼ 0.4 nm to ∼ 1.0 nm. CD analyses indicated that BmCRT is composed of 49.6% α helix, 9.6% β sheet and 43.6% random coil. These findings provided valuable information on the architecture and chemistry of BmCRT-C1q interaction and supported the hypothesis that BmCRT binds with huC1q at their targets (IgG/M, CRP) binding sites. This interaction enables the parasite to interfere with the initial stage of host complement activation, which might be helpful in parasites establishment. These results might be utilized for help in blocking the C1q/CRT interaction and preventing parasite infection.

cC1qR/CR and gC1qR/p33: observations in cancer

The survival and growth of a primary tumor depends, by and large, on three major events: immune evasion, angiogenesis and metastasis. Tumor cells are "modified self", and as such express a plethora of modified surface antigens capable of inducing antibody production. Anti-tumor cell antibodies should, in theory, activate complement resulting in cell destruction. But this is not the case. Akin to many pathogenic microorganisms whose survival depends on evading the immune system, cancer cells have also evolved diverse mechanisms to prevent host mediated cell destruction by either retaining critical regulatory molecules or by hijacking host proteins to ensure their survival. Although immune evasion, angiogenesis and metastasis are complex biological processes involving a myriad of tumor associated proteins, enzymes, and cytokines, C1qRs can, nonetheless play an important role in all or part of these processes. Although both cC1qR/CR and gC1qR are expressed by all somatic cells, with the exception of red blood cells, both are highly upregulated on almost all types of tumors. It is not surprising therefore that blockade of C1qR on tumor cells inhibits their proliferation suggesting the significance of C1qRs in tumor growth and progression. Interestingly, the two C1q receptors: cC1qR/CR and gC1qR play a differential role in carcinogenesis. While gC1qR promotes tumor cell survival by enhancing angiogenesis and metastasis and also by contributing to the hypercoagulable and prothrombotic microenvironment, cC1qR/CR expression represents a pro-phagocytic "eat-me" signal through which cC1qR/CR expressing tumor cells are tagged for destruction by macrophages. The data accumulated to date therefore identify gC1qR and cC1qR/CR as potential targets for the design of either protein-based, antibody-based or chemical based therapeutic intervention that could be used to enhance conventional anti-cancer therapy. The inhibition of tumor cell proliferation by monoclonal antibody recognizing the C1q site on gC1qR, as well as the identification of agents such as anthracyclin that enhance cC1qR/CR expression on tumor cells, are indeed steps in the right direction.

Pathway analysis of cervical cancer genome-wide association study highlights the MHC region and pathways involved in response to infection

Cervical cancer is caused by infection with human papillomavirus (HPV). A genome-wide association study (GWAS) has identified several susceptibility loci for cervical cancer, but they explain only a small fraction of cervical cancer heritability. Other variants with weaker effect may be missed due to the stringent significance threshold. To identify important pathways in cervical carcinogenesis, we performed a two-stage pathway analysis in two independent GWASs in the Swedish population, using the single-nucleotide polymorphism (SNP) ratio test. The 565 predefined pathways from Kyoto Encyclopedia of Genes and Genomes and BioCarta databases were systematically evaluated in the discovery stage (1034 cases and 3948 controls with 632,668 SNPs) and the suggestive pathways were further validated in the replication stage (616 cases and 506 controls with 341,358 SNPs). We found 12 pathways that were significant in both stages, and these were further validated using set-based analysis. For 10 of these pathways, the effect was mainly due to genetic variation within the major histocompatibility complex (MHC) region. In addition, we identified a set of novel candidate genes outside the MHC region in the pathways denoted 'Staphylococcus aureus infection' and 'herpes simplex infection' that influenced susceptibility to cervical cancer (empirical P = 4.99 × 10(-5) and 4.99 × 10(-5) in the discovery study; empirical P = 8.98 × 10(-5) and 0.009 in the replication study, respectively). Staphylococcus aureus infection may evoke an inflammatory response that inadvertently enhances malignant progression caused by HPV infection, and Herpes simplex virus-2 infection may act in conjunction with HPV infection to increase the risk of cervical carcinoma development. These findings provide new insights into the etiology of cervical cancer.

Purification of C1q receptors and functional analysis

The recognition subunit of C1, C1q, has emerged as an important player in various pathophysiologic conditions largely in part due to its ability to interact with pathogen-associated or cell surface expressed ligands and receptors. Identification and purification of these molecules is therefore of paramount importance if we are to procure valuable information with regards to the structure, function, and cell surface distribution. Since the interaction of C1q is better served when the receptors are purified from homologous species, we discuss here a simple guideline for the purification and characterization of the two C1q receptors, cC1qR (calreticulin) and gC1qR, from human cell lines.

TUNABLE COMPLEMENT ACTIVATION BY PARTICLES WITH VARIABLE SIZE AND Fc DENSITY

The complement system is an integral innate immune component that is made up of a cascade of enzymatic proteins that, once activated, results in lysis of invading pathogens, opsonization or recruitment of other innate and/or acquired immune responders, or some combination of the three. Due to the importance of the signal amplification and control points present in the cascade, complement is highly sensitive to subtle variations in initiation conditions, including nanoscale changes to molecular spacing. Using Fc-functionalized microparticles and nanoparticles, we find that activation requires a minimum threshold surface concentration of Fc of at least 20% surface coverage. This result indicates that a high surface density Fc is necessary for micro/nanoparticle complement activation through the classical pathway. In addition, the magnitude of the response was dependent on the size of the particle, with larger particles causing decreased activation. We hypothesize that a high density of Fc is needed to efficiently bind and closely appose molecular initiators of the complement cascade, from initiation to terminal complement complex formation. These fundamental studies of the interaction of microparticles and nanoparticles with the immune system suggest design rules for particle size and molecular density that impact immunostimulation through the complement system. Providing a therapeutic agent to modulate the complement response could aid a variety of treatment strategies. Engineered nanoparticles with controlled gaps between molecular activators could lead to new types of immunomodulatory agents.

The role of complement regulatory proteins in peripheral blood cells of patients with systemic lupus erythematosus: review

CD55, CD59, CD35 and CD46 are cell membrane proteins that have regulatory properties on the activation of the complement cascade. Deficiency in the expression of these proteins may be associated with lower protection of healthy cells against complement mediated lysis and also with the accumulation of immune complexes in tissues. Few studies assess the expression of these proteins in patients with SLE and the mechanisms that regulate reduction in cellular expression, whereas its impact on manifestations of systemic lupus erythematosus is still unknown.

The C1q family of proteins: insights into the emerging non-traditional functions

Research conducted over the past 20 years have helped us unravel not only the hidden structural and functional subtleties of human C1q, but also has catapulted the molecule from a mere recognition unit of the classical pathway to a well-recognized molecular sensor of damage-modified self or non-self antigens. Thus, C1q is involved in a rapidly expanding list of pathological disorders – including autoimmunity, trophoblast migration, preeclampsia, and cancer. The results of two recent reports are provided to underscore the critical role C1q plays in health and disease. First is the observation by Singh et al. (2011) showing that pregnant C1q−/− mice recapitulate the key features of human preeclampsia that correlate with increased fetal death. Treatment of the C1q−/− mice with pravastatin restored trophoblast invasiveness, placental blood flow, and angiogenic balance and, thus, prevented the onset of preeclampsia. Second is the report by Hong et al. (2009) which showed that C1q can induce apoptosis of prostate cancer cells by activating the tumor suppressor molecule WW-domain containing oxydoreductase (WWOX or WOX1) and destabilizing cell adhesion. Downregulation of C1q on the other hand, enhanced prostate hyperplasia and cancer formation due to failure of WOX1 activation. C1q belongs to a family of structurally and functionally related TNF-α-like family of proteins that may have arisen from a common ancestral gene. Therefore C1q not only shares the diverse functions with the tumor necrosis factor family of proteins, but also explains why C1q has retained some of its ancestral “cytokine-like” activities. This review is intended to highlight some of the structural and functional aspects of C1q by underscoring the growing list of its non-traditional functions.

Kikuchi-Fujimoto disease (histiocytic necrotizing lymphadenitis): report of a case with other autoimmune manifestations

Kikuchi-Fujimoto disease (KFD), or histiocytic necrotizing lymphadenitis, is a benign and self-limited disease that mainly affects young women. Patients present with localized lymphadenopathy, fever, and leukopenia in up to half of the cases. KFD can occur in association with systemic lupus erythematosus. We present the case of a patient with KFD and systemic lupus erythematosus, as well as relapsing polychondritis. This patient had persistently low C4 complement levels, so she was evaluated for a genetic defect in complement production and was found to have two "null" C4 alleles. We believe that this may have contributed to the development of her diseases.

Multivalent rituximab lipid nanoparticles as improved lymphoma therapies: indirect mechanisms of action and in vivo activity

AIMS

The activity of therapeutic antibodies can be enhanced by creating multivalent constructs, such as antibody lipid nanoparticles (LNPs). Here, we examine differences between rituximab (Ritux) and Ritux-LNPs in terms of their indirect mechanisms of action: complement-dependent cytotoxicity (CDC) and antibody-dependent cell-mediated cytotoxicity (ADCC).

MATERIALS & METHODS

We employed two mantle-cell lymphoma cell lines, Z138 and JVM2, which exhibit different in vivo sensitivities to Ritux along with variable expression levels of cell-surface proteins that regulate ADCC and CDC.

RESULTS

In both cell lines, CDC and ADCC were found to be significantly enhanced after treatment with Ritux-LNPs compared with Ritux. In vivo efficacy studies, however, suggested that the therapeutic activities of Ritux and Ritux-LNPs were equivalent, which was subsequently explained in part by pharmacokinetic studies indicating rapid elimination of Ritux-LNP.

CONCLUSION

Although indirect and direct mechanisms of multivalent Ritux are enhanced, its further development requires methods to improve its circulation lifetime.

Distinct in vitro binding properties of the anti-CD20 small modular immunopharmaceutical 2LM20-4 result in profound and sustained in vivo potency in cynomolgus monkeys

Objectives. To characterize the in vitro binding and effector function properties of CD20-directed small modular immunopharmaceutical (SMIP) 2LM20-4, and to compare its in vivo B-cell depletion activity with the mutated 2LM20-4 P331S [no in vitro complement-dependent cytotoxicity (CDC)] and rituximab in cynomolgus monkeys.

Methods. Direct binding is examined in flow cytometry, confocal microscopy, scatchard and lipid raft assays. Effector function assays include CDC and Fc-mediated cellular toxicity. In the 6-month-long in vivo B-cell depletion study, single i.v. dosages of 1 or 10 mg/kg of anti-CD20 proteins were administered to monkeys and B-cell counts were monitored in peripheral blood, bone marrow and lymph nodes.

Results. 2LM20-4 has lower saturation binding to human primary B cells and recruits fewer CD20 molecules into lipid rafts compared with rituximab; however, it induces higher in vitro CDC. In competitive binding, 2LM20-4 only partially displaces rituximab, suggesting that it binds to a fraction of CD20 molecules within certain locations of the plasma membrane as compared with rituximab. In monkeys, 2LM20-4 had more sustained B-cell depletion activity than rituximab in peripheral blood and had significantly more profound and sustained activity than 2LM20-4 P331S and rituximab in the lymph nodes.

Conclusions. SMIP 2LM20-4, which binds to a fraction of CD20 molecules as compared with rituximab, has more potent in vitro CDC, and more potent and sustained B-cell depletion activity in cynomolgus monkeys. Our work has considerable clinical relevance since it provides novel insights related to the emerging B-cell depletion therapies in autoimmune diseases.

Early complement proteases: C1r, C1s and MASPs. A structural insight into activation and functions

C1r, C1s and the mannose-binding lectin-associated serine proteases (MASPs) are responsible for the initiation of the classical- and lectin pathway activation of the complement system. These enzymes do not act alone, but form supramolecular complexes with pattern recognition molecules such as C1q, MBL, and ficolins. They share the same domain organization but have different substrate specificities and fulfill different physiological functions. In the recent years the rapid progress of structural biology facilitated the understanding of the molecular mechanism of complement activation at atomic level. In this review we summarize our current knowledge about the structure and function of the early complement proteases, delineate the latest models of the multimolecular complexes and present the functional consequences inferred from the structural studies. We also discuss some open questions and debated issues that need to be resolved in the future.

Target pattern recognition by complement proteins of the classical and alternative pathways

The complement system is a major component of the innate defence of animals against invading microorganisms, and is also essential for the recognition and clearance of damaged or structurally-altered host cells or macromolecules. The system is activated by three different pathways, each of which responds, using different recognition molecules, to a very wide range of activators. The recognition protein of the complement classical pathway, C1q is described in detail here, with comparisons to the alternative pathway.

Activation of complement component C5: comparison of C5 convertases of the lectin pathway and the classical pathway of complement

Although the initiating complex of lectin pathway (called M1 in this study) generates C3/C5 convertases similar to those assembled by the initiating complex (C1) of the classical pathway, activation of complement component C5 via the lectin pathway has not been examined. In the present study kinetic analysis of lectin pathway C3/C5 convertases assembled on two surfaces (zymosan and sheep erythrocytes coated with mannan (E(Man))) revealed that the convertases (ZymM1,C4b,C2a and E(Man)M1,C4b,C2a) exhibited a similar but weak affinity for the substrate, C5 indicated by a high K(m) (2.73-6.88 microm). Very high affinity C5 convertases were generated when the low affinity C3/C5 convertases were allowed to deposit C3b by cleaving native C3. These C3b-containing convertases exhibited K(m) (0.0086-0.0075 microm) well below the normal concentration of C5 in blood (0.37 microm). Although kinetic parameters, K(m) and k(cat), of the lectin pathway C3/C5 convertases were similar to those reported for classical pathway C3/C5 convertases, studies on the ability of C4b to bind C2 indicated that every C4b deposited on zymosan or E(Man) was capable of forming a convertase. These findings differ from those reported for the classical pathway C3/C5 convertase, where only one of four C4b molecules deposited formed a convertase. The potential for four times more amplification via the lectin pathway than the classical pathway in the generation of C3/C5 convertases and production of pro-inflammatory products, such as C3a, C4a, and C5a, implies that activation of complement via the lectin pathway might be a more prominent contributor to the pathology of inflammatory reactions.

Revisiting the mechanism of the autoactivation of the complement protease C1r in the C1 complex: structure of the active catalytic region of C1r

C1r is a modular serine protease which is the autoactivating component of the C1 complex of the classical pathway of the complement system. We have determined the first crystal structure of the entire active catalytic region of human C1r. This fragment contains the C-terminal serine protease (SP) domain and the preceding two complement control protein (CCP) modules. The activated CCP1-CCP2-SP fragment makes up a dimer in a head-to-tail fashion similarly to the previously characterized zymogen. The present structure shows an increased number of stabilizing interactions. Moreover, in the crystal lattice there is an enzyme-product relationship between the C1r molecules of neighboring dimers. This enzyme-product complex exhibits the crucial S1-P1 salt bridge between Asp631 and Arg446 residues, and intermolecular interaction between the CCP2 module and the SP domain. Based on these novel structural information we propose a new split-and-reassembly model for the autoactivation of the C1r. This model is consistent with experimental results that have not been explained adequately by previous models. It allows autoactivation of C1r without large-scale, directed movement of C1q arms. The model is concordant with the stability of the C1 complex during activation of the next complement components.

Serine proteases of the classical and lectin pathways: Similarities and differences

C1r, C1s, MBL-associated serine protease (MASP)-1, MASP-2 and MASP-3 are mosaic serine proteases of the classical and lectin pathways of complement. They form a family of enzymes with identical domain organization and similar overall structure, but with different enzymatic properties. MASP-2 of the lectin pathway can autoactivate and cleave C4 and C2 components. In the classical pathway two enzymes mediate these functions: C1r autoactivates and activates C1s, while C1s cleaves C4 and C2. The substrate specificity and the biological function of MASP-1 and MASP-3 have not yet been completely resolved. MASP-1 can autoactivate and the activated MASP-1 has more relaxed substrate specificity than the other members of the family. It was demonstrated that MASP-1 can specifically cleave C2, C3 and fibrinogen, but the physiological relevance of these findings has to be proved. We do not know how MASP-3 becomes activated and its biological function is also not clear. In this review, we will summarize current knowledge about the structure and function of these proteases. Special emphasis will be laid on the specificity, autoactivation and evolution of these enzymes.

Functional anthology of intrinsic disorder. 2. Cellular components, domains, technical terms, developmental processes, and coding sequence diversities correlated with long disordered regions

Biologically active proteins without stable ordered structure (i.e., intrinsically disordered proteins) are attracting increased attention. Functional repertoires of ordered and disordered proteins are very different, and the ability to differentiate whether a given function is associated with intrinsic disorder or with a well-folded protein is crucial for modern protein science. However, there is a large gap between the number of proteins experimentally confirmed to be disordered and their actual number in nature. As a result, studies of functional properties of confirmed disordered proteins, while helpful in revealing the functional diversity of protein disorder, provide only a limited view. To overcome this problem, a bioinformatics approach for comprehensive study of functional roles of protein disorder was proposed in the first paper of this series (Xie, H.; Vucetic, S.; Iakoucheva, L. M.; Oldfield, C. J.; Dunker, A. K.; Obradovic, Z.; Uversky, V. N. Functional anthology of intrinsic disorder. 1. Biological processes and functions of proteins with long disordered regions. J. Proteome Res. 2007, 5, 1882-1898). Applying this novel approach to Swiss-Prot sequences and functional keywords, we found over 238 and 302 keywords to be strongly positively or negatively correlated, respectively, with long intrinsically disordered regions. This paper describes approximately 90 Swiss-Prot keywords attributed to the cellular components, domains, technical terms, developmental processes, and coding sequence diversities possessing strong positive and negative correlation with long disordered regions.

IFN-gamma upregulates expression of the mouse complement C1rA gene in keratinocytes via IFN-regulatory factor-1

We examined the expression of the mouse complement component C1rA (mC1rA) in IFN-gamma-stimulated mouse keratinocytes (Pam 212) and found that it was upregulated. To analyze the mechanism involved, we cloned the 2,150 bp 5'-flanking region of mC1rA by the vectorette-PCR technique, and identified the transcription start site of mC1rA by rapid amplification of complementary DNA ends. Analysis of the 5' sequence revealed putative binding sites for activator protein 1, CCAAT/enhancer binding protein (C/EBP), signal transducer and activator of transcription 1 (STAT-1), IFN-regulatory factor-1 (IRF-1), and others. We detected transcriptional activation dependent on this upstream region in reporter gene assays and Northern blots. To identify the cis-acting regulatory elements involved, we analyzed serial deletion constructs of the promoter using luciferase reporters. The -80 to -19 bp region, which contains a putative IRF-1 binding site, was required for both basal promoter activity and responses to IFN-gamma. The use of site-directed point mutations, electrophoresis mobility shift assays, and supershift assays indicated that the putative IRF-1 binding site was essential for both IFN-gamma-dependent and -independent transcriptional activity of the mC1rA promoter. We conclude that IFN-gamma stimulates mC1rA gene expression via IRF-1 in mouse keratinocytes.

Evidence That Complement Protein C1q Interacts with C-Reactive Protein through Its Globular Head Region

C1q acts as the recognition unit of the first complement component, C1, and binds to immunoglobulins IgG and IgM, as well as to non-Ig ligands, such as C-reactive protein (CRP). IgG and IgM are recognized via the globular head regions of C1q (C1qGR), whereas CRP has been postulated to interact with the collagen-like region (C1qCLR). In the present study, we used a series of nine mAbs to C1q, five directed against C1qGR and four against C1qCLR, to inhibit the interaction of C1q with CRP. The F(ab')(2) of each of the five mAbs directed against C1qGR inhibited binding of C1q to polymerized IgG. These five mAbs also successfully inhibited the interaction of C1q with CRP. Moreover, these five mAbs inhibited C1 activation by CRP as well as by polymerized IgG in vitro. In contrast, none of the four mAbs against C1qCLR inhibited C1q interaction with CRP or IgG, or could reduce activation of complement by CRP or polymerized IgG. These results provide the first evidence that the interaction of C1q with CRP or IgG involves sites located in the C1qGR, whereas sites in the CLR do not seem to be involved in the physiological interaction of C1q with CRP.

Functional role of the linker between the complement control protein modules of complement protease C1s

C1s is the modular serine protease responsible for cleavage of C4 and C2, the protein substrates of the first component of C (C1). Its catalytic domain comprises two complement control protein (CCP) modules connected by a four-residue linker Gln340-Pro-Val-Asp343 and a serine protease domain. To assess the functional role of the linker, a series of mutations were performed at positions 340-343 of human C1s, and the resulting mutants were produced using a baculovirus-mediated expression system and characterized functionally. All mutants were secreted in a proenzyme form and had a mass of 77,203-77,716 Da comparable to that of wild-type C1s, except Q340E, which had a mass of 82,008 Da, due to overglycosylation at Asn391. None of the mutations significantly altered C1s ability to assemble with C1r and C1q within C1. Whereas the other mutations had no effect on C1s activation, the Q340E mutant was totally resistant to C1r-mediated activation, both in the fluid phase and within the C1 complex. Once activated, all mutants cleaved C2 with an efficiency comparable to that of wild-type C1s. In contrast, most of the mutations resulted in a decreased C4-cleaving activity, with particularly pronounced inhibitory effects for point mutants Q340K, P341I, V342K, and D343N. Comparable effects were observed when the C4-cleaving activity of the mutants was measured inside C1. Thus, flexibility of the C1s CCP1-CCP2 linker plays no significant role in C1 assembly or C1s activation by C1r inside C1 but plays a critical role in C4 cleavage by adjusting positioning of this substrate for optimal cleavage by the C1s active site.

Elucidation of the Substrate Specificity of the C1s Protease of the Classical Complement Pathway

The complement system is a central component of host defense but can also contribute to the inflammation seen in pathological conditions. The C1s protease of the first complement component, the C1 complex, initiates the pathway. In this study we have elucidated the full specificity of the enzyme for the first time using a randomized phage display library. It was found that, aside from the crucial P(1) position, the S(3) and S(2) subsites (in that order) played the greatest role in determining specificity. C1s prefers Leu or Val at P(3) and Gly or Ala residues at P(2). Apart from the S(2)' position, which showed specificity for Leu, prime subsites did not greatly affect specificity. It was evident, however, that together they significantly contributed to the efficiency of cleavage of a peptide. A peptide substrate based on the top sequence obtained in the phage display validated these results and produced the best kinetics of any C1s substrate to date. The results allow an understanding of the active site specificity of the C1s protease for the first time and provide a basis for the development of specific inhibitors aimed at controlling inflammation associated with complement activation in adverse pathological situations.

Functional characterization of complement proteases C1s/mannan-binding lectin-associated serine protease-2 (MASP-2) chimeras reveals the higher C4 recognition efficacy of the MASP-2 complement control protein modules

C1s and mannan-binding lectin-associated serine protease-2 (MASP-2) are the proteases that trigger the classical and lectin pathways of complement, respectively. They have identical modular architectures and cleave the same substrates, C2 and C4, but show markedly different efficiencies toward C4. Multisite-directed mutagenesis was used to engineer hybrid C1s/MASP-2 molecules where either the complement control protein (CCP) modules or the serine protease (SP) domain of C1s were swapped for their MASP-2 counterparts. The resulting chimeras (C1s(MASP-2 CCP1/2) and C1s(MASP-2 SP), respectively) were expressed and characterized chemically and functionally. Whereas C1s(MASP-2 SP) was recovered as an active enzyme, C1s(MASP-2 CCP1/2) was produced in a proenzyme form and was susceptible to activation by C1r, indicating that the activation properties of the chimeras were dictated by the nature of their SP domain. Similarly, each activated chimera had an esterolytic activity characteristic of its own SP domain and cleaved C2 with an efficiency comparable with that of their parent C1s and MASP-2 proteases. Both chimeras cleaved C4, but whereas C1s(MASP-2 SP) and C1s had Km values in the micromolar range, C1s(MASP-2 CCP1/2) and MASP-2 had Km values in the nanomolar range, resulting in 21-27-fold higher kcat/Km ratios. Thus, the higher C4 cleavage efficiency of MASP-2 arises from a higher substrate recognition efficacy of its CCP modules. Remarkably, C1s(MASP-2 CCP1/2) retained C1s ability to associate with C1r and C1q to form a pseudo-C1 complex and to undergo activation within this complex, indicating that the C1s-CCP modules have no direct implication in either function.

A novel human complement-related protein, C1r-like protease (C1r-LP), specifically cleaves pro-C1s

The availability of the human genome sequence allowed us to identify a human complement-related, C1r-like protease gene (c1r-LP) located 2 kb centromeric of the C1r gene (c1r). Compared with c1r, c1r-LP carries a large deletion corresponding to exons 4-8 of c1r. The open reading frame of the C1r-LP cDNA predicts a 50 kDa modular protein displaying 52% amino acid residue identity with the corresponding regions of C1r and 75% identity with a previously described murine C1r-LP. The serine protease domain of C1r-LP, despite an overall similarity with the AGY group of complement serine proteases, has certain structural features characteristic of C2 and factor B, thus raising interesting evolutionary questions. Northern blotting demonstrated the expression of C1r-LP mRNA mainly in the liver and ELISA demonstrated the presence of the protein in human serum at a concentration of 5.5+/-0.9 microg/ml. Immunoprecipitation experiments failed to demonstrate an association of C1r-LP with the C1 complex in serum. Recombinant C1r-LP exhibits esterolytic activity against peptide thioesters with arginine at the P1 position, but its catalytic efficiency (kcat/K(m)) is lower than that of C1r and C1s. The enzymic activity of C1r-LP is inhibited by di-isopropyl fluorophosphate and also by C1 inhibitor, which forms stable complexes with the protease. Most importantly, C1r-LP also expresses proteolytic activity, cleaving pro-C1s into two fragments of sizes identical with those of the two chains of active C1s. Thus C1r-LP may provide a novel means for the formation of the classical pathway C3/C5 convertase.

The Structure of MBL-associated Serine Protease-2 Reveals that Identical Substrate Specificities of C1s and MASP-2 are Realized Through Different Sets of Enzyme–Substrate Interactions

A family of serine proteases mediates the proteolytic cascades of several defense mechanisms in vertebrates, such as the complement system, blood coagulation and fibrinolysis. These proteases usually form large complexes with other glycoproteins. Their common features are their modular structures and restricted substrate specificities. The lectin pathway of complement, where mannose-binding lectin (MBL) recognizes the carbohydrate structures on pathogens, is activated by mannose-binding lectin-associated serine protease-2 (MASP-2). We present the 2.25A resolution structure of the catalytic fragment of MASP-2 encompassing the second complement control protein module (CCP2) and the serine protease (SP) domain. The CCP2 module stabilizes the structure of the SP domain as demonstrated by differential scanning calorimetry measurements. The asymmetric unit contains two molecules with different CCP-SP domain orientations, reflecting increased modular flexibility at the CCP2/SP joint. This flexibility may partly explain the ability of the MASP-2 dimer to perform all of its functions alone, whereas the same functions are mediated by the much larger C1r2-C1s2 tetramer in the C1 complex of the classical pathway. The main scaffold of the MASP-2 SP domain is chymotrypsin-like. Eight surface loops determine the S1 and other subsite specificities. Surprisingly, some surface loops of MASP-2, e.g. loop 1 and loop 2, which form the S1 pocket are similar to those of trypsin, and show significant differences if compared with those of C1s, indicating that the nearly identical substrate specificities of C1s and MASP-2 are realized through different sets of enzyme-substrate interactions.

Importance of the Prime Subsites of the C1s Protease of the Classical Complement Pathway for Recognition of Substrates

The classical complement pathway, which plays a vital role in preventing infection, is initiated by the action of the serine proteases C1r and C1s. We have examined the hydrolysis of substrates representing cleavage sequences in the physiological substrates for C1s, C2 and C4. These studies showed that the P(1)'-P(4)' substrate residues of C2 and C4 conferred greater affinity of substrate for enzyme and also induced a sigmoidal dependence of enzyme velocity on substrate concentration. This indicates that the substrate gave rise to homotropic positive cooperative behavior in the enzyme. When C1s was in complex with C1q and C1r, as would occur under physiological conditions, the same behavior was observed, indicating that this mechanism is relevant in the complement pathway in vivo. We further investigated the requirements of C1s for prime side amino acids by examining a substrate library in which each of the P(1)'-P(4)' positions had been substituted by different classes of amino acids. This revealed that the P(1)' position was a major determinant of the selectivity of the enzyme, while certain substitutions at the P(1)'-P(4)' positions abolished the allosteric behavior, indicating that contact residues at these positions in the C1s enzyme must mediate the cooperativity. The studies reported here highlight the importance of prime subsites in C1s for interaction with its cognate substrates in the complement pathway and therefore yield greater understanding of the mechanism of interaction between this vital protease and its physiological substrates.

Structural biology of the C1 complex of complement unveils the mechanisms of its activation and proteolytic activity

C1 is the multimolecular protease that triggers activation of the classical pathway of complement, a major element of antimicrobial host defense also involved in immune tolerance and various pathologies. This 790,000 Da complex is formed from the association of a recognition protein, C1q, and a catalytic subunit, the Ca2+-dependent tetramer C1s-C1r-C1r-C1s comprising two copies of each of the modular proteases C1r and C1s. Early studies mainly based on biochemical analysis and electron microscopy of C1 and its isolated components have allowed for characterization of their domain structure and led to a low-resolution model of the C1 complex in which the elongated C1s-C1r-C1r-C1s tetramer folds into a more compact, "8-shaped" conformation upon interaction with C1q. A major strategy used over the past years has been to dissect the C1 proteins into modular segments to characterize their function and solve their structure by either X-ray crystallography or nuclear magnetic resonance spectroscopy (NMR). The purpose of this review is to focus on this information, with particular emphasis on the architecture of the C1 complex and the mechanisms underlying its activation and proteolytic activity.

Sphingosine 1-phosphate, present in serum-derived lipoproteins, activates matriptase

We describe here a novel biological function of sphingosine 1-phosphate (S1P): the activation of a serine protease, matriptase. Matriptase is a type II integral membrane serine protease, expressed on the surface of a variety of epithelial cells; it may play an important role in tissue remodeling. We have previously reported that the activation of matriptase is regulated by serum. We have now identified the bioactive component from serum. First, the activity was observed to co-purify with lipoproteins by conventional liquid chromatography and immunoaffinity chromatography. The ability of lipoproteins to induce the activation of matriptase was further confirmed with commercial preparations of low density lipoprotein (LDL) and very low density lipoprotein (VLDL). Next, we observed that the bioactive component of LDL is associated with the phospholipid components of LDL. Fractionation of lipid components of LDL by thin layer chromatography (TLC) revealed that the bioactive component of LDL comigrates with S1P. Nanomolar concentrations of commercially obtained S1P were then observed to induce the rapid activation of matriptase on the surfaces of nontransformed human mammary epithelial cells. Other structurally related sphingolipids, including dihydro-S1P, ceramide 1-phosphates, and sphingosine phosphocholine as well as lysophosphatidic acid, can also induce the activation of matriptase, but at significantly higher concentrations than S1P. Furthermore, S1P-dependent matriptase activation is dependent on Ca(2+) but not via G(i) protein-coupled receptors. Our results demonstrate that bioactive phospholipids can function as nonprotein activators of a cell surface protease, suggesting a possible mechanistic link between S1P and normal and possibly pathologic tissue remodeling.

The role of the individual domains in the structure and function of the catalytic region of a modular serine protease, C1r

The first enzymatic event in the classical pathway of complement activation is autoactivation of the C1r subcomponent of the C1 complex. Activated C1r then cleaves and activates zymogen C1s. C1r is a multidomain serine protease consisting of N-terminal alpha region interacting with other subcomponents and C-terminal gammaB region mediating proteolytic activity. The gammaB region consists of two complement control protein modules (CCP1, CCP2) and a serine protease domain (SP). To clarify the role of the individual domains in the structural and functional properties of the gammaB region we produced the CCP1-CCP2-SP (gammaB), the CCP2-SP, and the SP fragments in recombinant form in Escherichia coli. We successfully renatured the inclusion body proteins. After renaturation all three fragments were obtained in activated form and showed esterolytic activity on synthetic substrates similar to each other. To study the self-activation process in detail zymogen mutant forms of the three fragments were constructed and expressed. Our major statement is that the ability of autoactivation and C1s cleavage is an inherent property of the SP domain. We observed that the CCP2 module significantly increases proteolytic activity of the SP domain on natural substrate, C1s. Therefore, we propose that CCP2 module provides accessory binding sites. Differential scanning calorimetric measurements demonstrated that CCP2 domain greatly stabilizes the structure of SP domain. Deletion of CCP1 domain from the CCP1-CCP2-SP fragment results in the loss of the dimeric structure. Our experiments also provided evidence that dimerization of C1r is not a prerequisite for autoactivation.

Assembly and enzymatic properties of the catalytic domain of human complement protease C1r

The catalytic properties of C1r, the protease that mediates activation of the C1 complex of complement, are mediated by its C-terminal region, comprising two complement control protein (CCP) modules followed by a serine protease (SP) domain. Baculovirus-mediated expression was used to produce fragments containing the SP domain and either 2 CCP modules (CCP1/2-SP) or only the second CCP module (CCP2-SP). In each case, the wild-type species and two mutants stabilized in the proenzyme form by mutations at the cleavage site (R446Q) or at the active site serine residue (S637A), were produced. Both wild-type fragments were recovered as two-chain, activated proteases, whereas all mutants retained a single-chain, proenzyme structure, providing the first experimental evidence that C1r activation is an autolytic process. As shown by sedimentation velocity analysis, all CCP1/2-SP fragments were dimers (5.5-5.6 S), and all CCP2-SP fragments were monomers (3.2-3.4 S). Thus, CCP1 is essential to the assembly of the dimer, but formation of a stable dimer is not a prerequisite for self-activation. Activation of the R446Q mutants could be achieved by extrinsic cleavage by thermolysin, which cleaved the CCP2-SP species more efficiently than the CCP1/2-SP species and yielded enzymes with C1s-cleaving activities similar to their active wild-type counterparts. C1r and its activated fragments all cleaved C1s, with relative efficiencies in the order C1r < CCP1/2-SP < CCP2-SP, indicating that CCP1 is not involved in C1s recognition.

The potentiation of human C1-inhibitor by dextran sulphate is transient in vivo: studies in a rat model

C1-inhibitor (C1-Inh) is an important regulator of inflammatory reactions because it is a potent inhibitor of the contact and complement system. C1-Inh application in inflammatory disease is, however, restricted because of the high doses required. The glycosaminoglycan-like molecule dextran sulphate (DXS) enhances C1-Inh function in vitro. Hence, we investigated whether co-administration with dextran sulphate reduces the amount of C1-Inh required, through enhancement in vivo. C1-Inh potentiation was measured in a newly developed C1s-inactivation assay that is based on activation of C4 by purified C1s. Activated C4 in rat plasma was quantified with a newly developed ELISA. Human C1-Inh (2.5 microM) inhibited C1s in rat plasma 55-fold faster in the presence of dextran sulphate (15 kDa, 5 microM). To study the stability of the complex in vivo, rats were given a mixture of C1-Inh (10 mg/kg) and dextran sulphate (3 mg/kg). C1-Inh activity during 5 h was analyzed ex vivo with the C1s inactivation assay. The noncovalent C1-Inh-dextran sulphate complex resulted in a transient enhancement of the inhibitory capacity of C1-Inh, lasting for 60-90 min. Dextran sulphate did not affect plasma clearance of C1-Inh. We conclude that the enhanced inhibitory capacity of C1-Inh complexed to dextran sulphate is transient in vivo. Hence, co-administration of these compounds seems a feasible approach to achieve short-term inhibition of complement in vivo.

Identification of cDNA encoding a serine protease homologous to human complement C1r precursor from grafted mouse skin

We isolated a cDNA clone from grafted mouse skin that encodes a serine protease homologous to human C1r. The C1r protease is involved in the activation of the first component of the classical pathway in the complement system. In order to identify novel transcripts whose expression is regulated in grafted mouse skin, we first performed differential display reverse transcription polymerase chain reaction analysis and obtained 18 partial cDNA clones whose protein products are likely to play an important role in allograft rejection. One of these showed significant sequence homology with human complement C1r precursor. The other clones displayed no homology to any known sequences, however. Northern blot analysis demonstrated that the level of this transcript was upregulated in day 8 postgrafted skin. The full-length cDNA 2121 nucleotides in length obtained from screening a mouse skin cDNA library contained a single open reading frame encoding 707 amino acid residues with a calculated molecular weight of 80,732 Da. Its deduced amino acid sequence revealed an 81% identity and 89% similarity to the human C1r counterpart. In particular, mouse C1r contained His501, Asp559, and Ser656, which were conserved among this group of serine proteases. This protein was thus designated as mouse C1r. We have expressed a truncated fragment of C1r protein without the N-terminal hydrophobic sequence in Escherichia coli and generated a polyclonal antibody against it. Subsequent immunohistochemical analysis confirmed that mouse C1r was significantly expressed 8 d after the skin graft in both allografted and autografted skins, compared with normal skins. These collective data suggest that a component of the complement system, C1r, might contribute to the graft versus host immune responses in mice.

Anti-GM1 antibodies can block neuronal voltage-gated sodium channels

Anti-GM1 antibodies, frequently found in the serum of patients with Guillain-Barré syndrome (GBS), have been suggested to interfere with axonal function. We report that IgG anti-GM1 antibodies, raised in rabbits, can reversibly block the voltage-gated Na(+) channels of nerve cells, thus causing a reduction of the excitatory Na(+) current. The block was, however, only substantial when the antibodies were applied together with rabbit complement factors. A solution containing anti-GM1 sera (dilution 1:100) and complement (1:50) reduced the Na(+) current to 0.5 +/- 0.2 times control (mean value +/- SD). Applications of the antibody by itself, complement by itself, or anti-GM2 or anti-GM4 antibodies (1:100) plus complement had little effect. The complexes of anti-GM1 antibodies and complement factors block the ion-conducting pore of the channel directly. In addition, they increase the fraction of channels that are inactivated at the resting potential and alter channel function by changing the membrane surface charge. The described effects may be responsible for conduction slowing and reversible conduction failure in some GBS patients.

The cleavage of two C1s subunits by a single active C1r reveals substantial flexibility of the C1s-C1r-C1r-C1s tetramer in the C1 complex

The activation of the C1s-C1r-C1r-C1s tetramer in the C1 complex, which involves the cleavage of an Arg-Ile bond in the catalytic domains of the subcomponents, is a two-step process. First, the autolytic activation of C1r takes place, then activated C1r cleaves zymogen C1s. The Arg463Gln mutant of C1r (C1rQI) is stabilized in the zymogen form. This mutant was used to form a C1q-(C1s-C1rQI-C1r-C1s) heteropentamer to study the relative position of the C1r and C1s subunits in the C1 complex. After triggering the C1 by IgG-Sepharose, both C1s subunits are cleaved by the single proteolytically active C1r subunit in the C1s-C1rQI-C1r-C1s tetramer. This finding indicates that the tetramer is flexible enough to adopt different conformations within the C1 complex during the activation process, enabling the single active C1r to cleave both C1s, the neighboring and the sequentially distant one.

A novel PCR-based technique using expressed sequence tags and gene homology for murine genetic mapping: localization of the complement genes

The complement system is a cascade of serum proteins and receptors which forms a vital arm of innate immunity and enhances the adaptive immune response. This work establishes the chromosomal localization of four key genes of the murine complement system. Mapping was performed using a novel and rapid PCR restriction length polymorphism method which was developed to exploit the murine expressed sequence tag (EST) database. This technique circumvents the laborious cDNA or genomic cloning steps of other mapping methods by relying on EST data and the prediction of exon-intron boundaries. This method can be easily applied to the genes of other systems, ranging from the interests of the individual researcher to large-scale gene localization projects. Here the complement system, probably one of the most well-characterized areas of immunology, was used as a model system. It was shown that the C3a receptor C1r and C1s genes form an unexpected complement gene cluster towards the telomeric end of chromosome 6. The second mannose binding lectin-associated serine protease gene was mapped to the telomeric end of chromosome 4, which is distinct from other complement-activating serine proteases. These results provide new insights into the evolution of this group of proteins.

Structure and functions of the interaction domains of C1r and C1s: keystones of the architecture of the C1 complex

C1r and C1s, the proteases responsible for activation and proteolytic activity of the C1 complex of complement, share similar overall structural organizations featuring five nonenzymic protein modules (two CUB modules surrounding a single EGF module, and a pair of CCP modules) followed by a serine protease domain. Besides highly specific proteolytic activities, both proteases exhibit interaction properties associated with their N-terminal regions. These properties include the ability to bind Ca2+ ions with high affinity, to associate with each other within a Ca2+-dependent C1s-C1r-C1r-C1s tetramer, and to interact with C1q upon C1 assembly. Precise functional mapping of these regions has been achieved recently, allowing identification of the domains responsible for these interactions, and providing a comprehensive picture of their structure and function. The objective of this article is to provide a detailed and up-to-date overview of the information available on these domains, which are keystones of the assembly of C1, and appear to play an essential role at the interface between the recognition function of C1 and its proteolytic activity.

The N-terminal CUB-epidermal growth factor module pair of human complement protease C1r binds Ca2+ with high affinity and mediates Ca2+-dependent interaction with C1s

The Ca2+-dependent interaction between complement serine proteases C1r and C1s is mediated by their alpha regions, encompassing the major part of their N-terminal CUB-EGF-CUB (where EGF is epidermal growth factor) module array. In order to define the boundaries of the C1r domain(s) responsible for Ca2+ binding and Ca2+-dependent interaction with C1s and to assess the contribution of individual modules to these functions, the CUB, EGF, and CUB-EGF fragments were expressed in eucaryotic systems or synthesized chemically. Gel filtration studies, as well as measurements of intrinsic Tyr fluorescence, provided evidence that the CUB-EGF pair adopts a more compact conformation in the presence of Ca2+. Ca2+-dependent interaction of intact C1r with C1s was studied using surface plasmon resonance spectroscopy, yielding KD values of 10.9-29.7 nM. The C1r CUB-EGF pair bound immobilized C1s with a higher KD (1.5-1.8 microM), which decreased to 31.4 nM when CUB-EGF was used as the immobilized ligand and C1s was free. Half-maximal binding was obtained at comparable Ca2+ concentrations ranging from 5 microM with intact C1r to 10-16 microM for C1ralpha and CUB-EGF. The isolated CUB and EGF fragments or a CUB + EGF mixture did not bind C1s. These data demonstrate that the C1r CUB-EGF module pair (residues 1-175) is the minimal segment required for high affinity Ca2+ binding and Ca2+-dependent interaction with C1s and indicate that Ca2+ binding induces a more compact folding of the CUB-EGF pair.