The third component of Xenopus complement: cDNA cloning, structural and functional analysis, and evidence for an alternate C3 transcript

Complement component C3 - The "Swiss Army Knife" of innate immunity and host defense

As a preformed defense system, complement faces a delicate challenge in providing an immediate, forceful response to pathogens even at first encounter, while sparing host cells in the process. For this purpose, it engages a tightly regulated network of plasma proteins, cell surface receptors, and regulators. Complement component C3 plays a particularly versatile role in this process by keeping the cascade alert, acting as a point of convergence of activation pathways, fueling the amplification of the complement response, exerting direct effector functions, and helping to coordinate downstream immune responses. In recent years, it has become evident that nature engages the power of C3 not only to clear pathogens but also for a variety of homeostatic processes ranging from tissue regeneration and synapse pruning to clearing debris and controlling tumor cell progression. At the same time, its central position in immune surveillance makes C3 a target for microbial immune evasion and, if improperly engaged, a trigger point for various clinical conditions. In our review, we look at the versatile roles and evolutionary journey of C3, discuss new insights into the molecular basis for C3 function, provide examples of disease involvement, and summarize the emerging potential of C3 as a therapeutic target.

Molecular cloning and expression analysis of Pleurodeles waltl complement component C3 under normal physiological conditions and environmental stresses

C3 is a component of the complement system that plays a central role in immunity, development and tissue regeneration. In this study, we isolated the C3 cDNA of the Iberian ribbed newt Pleurodeles waltl. This cDNA encodes a 1637 amino acid protein with an estimated molecular mass of 212.5 kDa. The deduced amino acid sequence showed that P. waltl C3 contains all the conserved domains known to be critical for C3 function. Quantitative real-time PCR (qRT-PCR) demonstrated that under normal physiological conditions, P. waltl C3 mRNA is expressed early during development because it is likely required for neurulation. Then, its expression increased as the immune system developed. In adults, the liver is the richest source of C3, though other tissues can also contribute. Further analysis of C3 expression demonstrated that C3 transcription increased when P. waltl larvae were exposed to pH or temperature stress, suggesting that environmental modifications might affect this animal's defenses against pathogens.

Isolation and characterization of complement component C3 from Atlantic cod (Gadus morhua L.) and Atlantic halibut (Hippoglossus hippoglossus L.)

Complement component C3 was isolated from the plasma of cod (Gadus morhua L.) and halibut (Hippoglossus hippoglossus L.). Fast protein liquid chromatography (FPLC) techniques, involving ion exchange and gel filtration columns, were used. The purified proteins were analysed by SDS-PAGE which showed a two-chain structure, alpha- and beta-chains, as seen in higher vertebrates. Both proteins had intra-chain thioesters located within their alpha-chains and N-terminal amino acid sequencing confirmed their identity with reference to known C3 amino acid sequences from other species. Specific antibodies were prepared against cod and halibut C3 and tested in Western blotting on sera and purified C3. The proteolytic fragmentation of C3 was tested with trypsin, pepsin, papain and the extracellular product (ECP) from the bacterium Aeromonas salmonicida ssp. achromogenes (Asa). Both trypsin and papain were successful in cleaving C3 whereas pepsin and ECP had no effect. Carbohydrate moieties were detected in the alpha- and beta-chains of cod and halibut C3 and N-linked oligosaccharides were removed from the C3 with PNGase treatment, revealing a difference in C3 glycosylation between the two species.

Novel gene expression domains reveal early patterning of the Xenopus endoderm

The endoderm gives rise the respiratory and digestive tract epithelia as well as associated organs such as the liver, lungs and pancreas. Investigations examining the molecular basis of embryonic endodermal patterning and organogenesis have been hampered by the lack of regionally expressed molecular markers in the early endoderm. By differentially screening an arrayed cDNA library, combined with an in situ hybridization screen we identified 13 new genes regionally expressed in the early tailbud endoderm of the Xenopus embryo. The putative proteins encoded by these cDNAs include a cell surface transporter, secreted proteins, a protease, a protease inhibitor, an RNA-binding protein, a phosphatase inhibitor and several enzymes. We find that the expression of these genes falls into one of three re-occurring domains in the tailbud embryo; (1). a ventral midgut, (2). posterior to the midgut and (3). in the dorsal endoderm beneath the notochord. Several of these genes are also regionally expressed at gastrula and neurula stages and appear to mark territories that were previously only predicted by the endoderm fate map. This indicates that there is significant positional identity in the early endoderm long before stages 28-32 when regional specification of the endoderm is thought to occur. These new genes provide valuable tools for studying endodermal patterning and organogenesis in Xenopus.

Linkage relationships of genes coding for alpha2-macroglobulin, C3 and C4 in the zebrafish: implications for the evolution of the complement and Mhc systems

The alpha2-macroglobulin (A2M) and the complement components C3 and C4 are related proteins derived from a common ancestor. Theoretically, this derivation could have occurred either by tandem duplications of their encoding genes or by polyploidization involving chromosomal segments, a chromosome or the whole genome. In tetrapods the A2M-, C3- and C4-encoding genes are generally each located on a different chromosome. This observation has been interpreted as supporting their origin by polyploidization. We identified and mapped (with the help of a radiation hybrid panel of cell lines) the A2M, C3 and C4 loci in the zebrafish, Danio rerio. Each of the three types of loci is present in the zebrafish in multiple copies, but all of the identified copies of a given type map to the same region in linkage groups 1 (C3) and 15 (A2M, C4). The A2M and C4 loci are mapped in the same region not linked to any of the class I or class II major histocompatibility complex (Mhc) loci. These observations are interpreted as supporting the origin of the A2M family of genes by tandem duplications, followed by the dispersal of the copies to different chromosomes. It is also argued that the association of C4 with the class I/II loci in tetrapods is accidental and without functional significance.

A simple procedure for isolation of Bufo arenarum C3 complement fraction and preparation of antiserum

Because of the need for antibodies in our studies involving the third component of complement in Bufo arenarum, we performed a simple procedure to purify C3 from B. arenarum serum to use as antigen in the preparation of the antiserum. The strategy was based on the well-known ability of C3 to bind to zymosan (Zy), a yeast cell wall extract comprised of polysaccharides. The Zy-bound fraction showed cross reactivity with a commercial antibody to human C3 as well as a similar electrophoretic profile (SDS-PAGE) to C3 from other species. The Zy-C3 complex resulting from binding Zy to B. arenarum serum was injected into rabbits and the antiserum against this C3-like fraction was purified by protein A-Sepharose chromatography. The purified C3 antibody was found to be suitable for immunochemical studies.

cDNA sequence coding for the alpha'-chain of the third complement component in the African lungfish

cDNA clones coding for almost the entire C3 alpha-chain of the African lungfish (Protopterus aethiopicus), a representative of the Sarcopterygii (lobe-finned fishes), were sequenced and characterized. From the sequence it is deduced that the lungfish C3 molecule is probably a disulphide-bonded alpha:beta dimer similar to that of the C3 components of other jawed vertebrates. The deduced sequence contains conserved sites presumably recognized by proteolytic enzymes (e.g. factor I) involved in the activation and inactivation of the component. It also contains the conserved thioester region and the putative site for binding properdin. However, the site for the interaction with complement receptor 2 and factor H are poorly conserved. Either complement receptor 2 and factor H are not present in the lungfish or they bind to different residues at the same or a different site than mammalian complement receptor 2 and factor H. The C3 alpha-chain sequences faithfully reflect the phylogenetic relationships among vertebrate classes and can therefore be used to help to resolve the long-standing controversy concerning the origin of the tetrapods.

Expression of the Third Component of Complement, C3, in Regenerating Limb Blastema Cells of Urodeles

In this study we have shown that complement component C3 is expressed in the regenerating tissue during urodele limb regeneration. C3 was expressed in the dedifferentiated regeneration blastema and in the redifferentiated limb tissues in the axolotl, Amblystoma mexicanum, and in Notophthalmus viridescens. This expression was verified by immunofluorescent staining using an Ab against axolotl C3 and by in situ hybridization with an axolotl C3 cDNA probe. In the early stages of regeneration C3 appeared to be equally present in all mesenchymal cells and in the wound epithelium, whereas in the later stages it was mainly expressed in the differentiating muscle cells. Since no expression was seen in the developing limb, it appears that the C3 expression was specific to the regeneration process. We then demonstrated by hybridization experiments that a blastema cell line of myogenic origin expresses C3. All these findings implicate C3 in the dedifferentiation process and may indicate a new role for this molecule in muscle differentiation.

Molecular genetics of the complement C3 convertases in lower vertebrates

Evolution of the two gene families of the complement system involved in the formation of the C3 convertases, B/C2 and C3/C4/C5, was studied at the cDNA level in lower vertebrates. Cyclostomes, the most primitive extant vertebrates, seem to possess only one member each of these families, indicating that gene duplication between B and C2 or among C3, C4 and C5 occurred in the lineage of jawed vertebrates. Typical C3 and C4 cDNAs were identified in both amphibian (Xenopus) and teleost (medaka fish), locating the C3/C4 gene duplication before the divergence of ray-finned fish and lobe-finned fish. On the other hand, typical B cDNA was identified in Xenopus, whereas teleost counterparts from three species all showed intermediate character between B and C2, suggesting the possibility that the B/C2 gene duplication occurred in the tetrapod lineage. Genetic linkage between these two family genes within the MHC was observed in Xenopus but not in medaka fish.

The phylogeny and evolution of the thioester bond-containing proteins C3, C4 and alpha 2-macroglobulin

The complement system is an effector of both the acquired and innate immune systems of the higher vertebrates. It has been traced back at least as far as the echinoderms and so predates the appearance of the antibodies, T-cell receptors and MHC molecules of adaptive immunity. Central to the function of complement is the reaction of the thioester bond located within the structure of complement components C3 and C4. The structural thioester first appeared in a protease inhibitor, alpha 2-macroglobulin, in which it is involved in the immobilisation and entrapment of proteases. An important development in the C3 molecule has been the acquisition of a catalytic His residue which greatly increases the rate of reaction of the thioester with hydroxyl groups and with water.

Evolution and diversity of the complement system of poikilothermic vertebrates

In mammals the complement system plays an important role in innate and acquired host defense mechanisms against infection and in various immunoregulatory processes. The complement system is an ancient defense mechanism that is already present in the invertebrate deuterostomes. In these species as well as in agnathans (the most primitive vertebrate species), both the alternative and lectin pathway of complement activation are already present, and the complement system appears to be involved mainly in opsonization of foreign material. With the emergence of immunoglobulins in cartilaginous fish, the classical and lytic pathways first appear. The rest of the poikilothermic species, from teleosts to reptilians, appear to contain a well-developed complement system resembling that of homeothermic vertebrates. However, important differences remain. Unlike homeotherms, several species of poikilotherms have recently been shown to possess multiple forms of complement components (C3 and factor B) that are structurally and functionally more diverse than those of higher vertebrates. It is noteworthy that the multiple forms of C3 that have been characterized in several teleost fish are able to bind with varying efficiencies to various complement-activating surfaces. We hypothesize that this diversity has allowed these animals to expand their innate capacity for immune recognition.

Shark complement: an assessment

The classical (CCP) and alternative (ACP) pathways of complement activation have been established for the nurse shark (Ginglymostoma cirratum). The isolation of a cDNA clone encoding a mannan-binding protein-associated serine protease (MASP)-1-like protein from the Japanese dogfish (Triakis scyllia) suggests the presence of a lectin pathway. The CCP consists of six functionally distinct components: C1n, C2n, C3n, C4n, C8n and C9n, and is activated by immune complexes in the presence of Ca++ and Mg++ ions. The ACP is antibody independent, requiring Mg++ ions and a heat-labile 90 kDa factor B-like protein for activity. Proteins considered homologues of C1q, C3 and C4 (C2n) of the mammalian complement system have been isolated from nurse shark serum. Shark C1q is composed of at least two chain types each showing 50% identity to human C1q chains A and B. Partial sequence of the globular domain of one of the chains shows it to be C1q-like rather than like mannan-binding protein. N-terminal amino acid sequences of the alpha and beta chain of shark C3 and C4 molecules show significant identity with corresponding human C3 and C4 chains. A sequence representing shark C4 gamma chain, shows little similarity to human C4 gamma chain. The terminal shark components C8n and C9n are functional analogues of mammalian C8 and C9. Anaphylatoxin activity has been demonstrated in activated shark serum, and porcine C5a desArg induces shark leucocyte chemotaxis. The deduced amino acid sequence of a partial C3 cDNA clone from the nurse shark shows 50%, 30% and 24% homology with the corresponding region of mammalian C3, C4 and alpha 2-macroglobulin. Deduced amino acid sequence data from partial Bf/C2 cDNA clones, two from the nurse shark and one from the Japanese dogfish, suggest that at least one species of elasmobranch has two distinct Bf/C2 genes.

The Covalent Binding Reaction of Complement Component C3

The covalent binding of C3 to target molecules on the surfaces of pathogens is crucial in most complement-mediated activities. When C3 is activated, the acyl group is transferred from the sulfhydryl of the internal thioester to the hydroxyl group of the acceptor molecule; consequently, C3 is bound to the acceptor surface by an ester bond. It has been determined that the binding reaction of the B isotype of human C4 uses a two-step mechanism. Upon activation, a His residue first attacks the internal thioester to form an acyl-imidazole bond. The freed thiolate anion of the Cys residue of the thioester then acts as a base to catalyze the transfer of the acyl group from the imidazole to the hydroxyl group of the acceptor molecule. In this article, we present results which indicate that this two-step reaction mechanism also occurs in C3.

Isolation and initial characterisation of complement components C3 and C4 of the nurse shark and the channel catfish

Complement components C3 and C4 have been isolated from the serum of the nurse shark (Ginglymostoma cirratum) and of the channel catfish (Ictalurus punctatus). As in the higher vertebrates, the fish C4 proteins have three-chain structures while the C3 proteins have two-chain structures. All four proteins have intra-chain thioesters located within their highest molecular mass polypeptides. N-terminal sequence analysis of the polypeptides has confirmed the identity of the proteins. In all cases except the catfish C3 alpha-chain, which appears to have a blocked N-terminus, sequence similarities are apparent in comparisons with the chains of C3 and C4 from higher vertebrates. We have confirmed that the activity/protein previously designated C2n is the nurse shark analogue of mammalian C4. This is the first report of structural evidence for C4 in both the bony and cartilaginous fish.

Structure, functions, and evolution of the third complement component and viral molecular mimicry

The third component of the complement system, C3, is a common denominator in the activation of the classical, alternative, and lectin pathways. The ability of C3 molecule to interact with at least 20 different proteins makes it the most versatile component of this system. Since these interactions are important for phagocytic, immunoregulatory, and immune evasion mechanisms, the analysis of its structure and functions has been a subject of intense research. Here we review our current work on the C3-ligand interactions, C3-related viral molecular mimicry, evolution of the complement system, and identification of C3-based complement inhibitors.

An amphibian CD3 homologue of the mammalian CD3 gamma and delta genes

T cell receptor (TCR) genes have been identified in representatives of both cartilaginous and bony vertebrates. The CD3 chains that serve as signal transducing elements of the TCR complex in mammals have been defined to a limited extent in birds. In these studies a CD3 homologue was identified in an amphibian representative, Xenopus laevis, using degenerate oligomer primers designed from conserved regions of avian and mammalian CD3 gamma/delta subunits. The reverse transcriptase polymerase chain reaction amplified product of Xenopus splenocyte RNA was then used to isolate full-length cDNA clones from a splenic library. When employed as probes, the cDNA clones hybridized with a 1-kb mRNA transcript in Xenopus T cells, but not in other cell types. Comparison of the deduced amino acid sequence indicated a similar degree of homology with mammalian and avian CD3 gamma and delta chains. Genomic analysis indicated that the Xenopus CD3 molecule is encoded by five exons, a structure resembling the mammalian CD3 delta gene rather than the seven exon CD3 gamma gene. Southern blot analysis and sequencing of the 5' flanking region failed to yield evidence of a related Xenopus gene. This amphibian CD3 gene thus appears to represent an ancestral form of the mammalian CD3 gamma and delta genes.

Molecular cloning and linkage analysis of the Japanese medaka fish complement Bf/C2 gene

Evolutionary studies of complement factor B (Bf) and C2 in lower vertebrates have revealed the presence of the Bf/C2 common ancestor-like molecule in lamprey (cyclostome) and the Bf molecule encoded by the duplicated genes closely linked to the major histocompatibility complex (MHC) in Xenopus (amphibian). To further define when Bf/C2 gene duplication occurred and when linkage between the Bf/C2 gene and the MHC was established, we amplified the Bf/C2 sequences in teleost, the Japanese medaka (Oryzias latipes), by reverse transcription - polymerase chain reaction with primers corresponding to the common amino acid sequences shared by mammalian Bf and C2. Only a single molecular species has been amplified, and the corresponding cDNA clones were isolated from the liver cDNA library. The longest insert contained 2384 nucleotides with an open reading frame of 754 residues. The deduced amino acid sequence showed 33.6% and 34.1% overall identity with the human Bf and C2 sequences, respectively, hence this clone was named medaka Bf/C2. The single-copy medaka Bf/C2 gene had exactly the same exon-intron organization as the mammalian Bf and C2 genes, and spanned about 8 kilobases. The Bf/C2 locus was mapped to the close proximity (2.9 cM) of the superoxide dismutase locus on the linkage group XX by the use of a restriction site polymorphism between two inbred strains of the medaka.

Fourth component of Xenopus laevis complement: cDNA cloning and linkage analysis of the frog MHC

Complement C4 shows extensive structural and functional similarity to complement C3, hence these components are believed to have originated by gene duplication from a common ancestor. Although to date C3 cDNA clones have been isolated from all major classes of extant vertebrates including Xenopus, C4 cDNA clones have been isolated from mammalian species only. We describe here the molecular cloning and structural analysis of Xenopus C4 cDNA. The cDNA sequence encoding the thioester region of Xenopus C4 was amplified by reverse transcriptase-polymerase chain reaction using Xenopus liver mRNA as a template, and then used to screen a liver cDNA library. The amino acid sequence of Xenopus C4 deduced from a clone containing the entire protein-coding sequence showed 39%, 30%, 25%, and 20% overall identity with those of human C4, C3, C5, and alpha2-macroglobulin, respectively. The predicted amino acid sequence consisted of a 22-residue putative signal peptide, a 634-residue beta chain, a 732-residue alpha chain, and a 287-residue gamma chain. Of 30 cysteine residues, 27 were found in exactly the same positions as in human C4. Genomic Southern blotting analysis indicated that C4 is a single copy gene in Xenopus and is part of the frog MHC cluster. These results clearly demonstrate that C3/C4 gene duplication and linkage between the C4 gene and the major histocompatibility complex predate mammalian/amphibian divergence.