Structure of the major oligosaccharide of cobra venom factor

Structures of N-Glycans of Bothrops Venoms Revealed as Molecular Signatures that Contribute to Venom Phenotype in Viperid Snakes

The complexity of snake venoms has long been investigated to explore a myriad of biologically active proteins and peptides that are used for immobilizing or killing prey, and are responsible for the pathological effects observed on envenomation. Glycosylation is the main post-translational modification (PTM) of viperid venoms but currently there is little understanding of how protein glycosylation impacts the variation of venom proteomes. We have previously reported that Bothrops venom glycoproteomes contain a core of components that markedly define their composition and parallel their phylogenetic classification. Here we extend those observations to eight Bothrops species evaluating the N-glycomes by LC-MS as assigned cartoon structures and detailing those structures separately as methylated analogs using ion-trap mass spectrometry (MSⁿ). Following ion disassembly through multiple steps provided sequence and linkage isomeric details that characterized 52 unique compositions in Bothrops venoms. These occurred as 60 structures, of which 26 were identified in the venoms of the Jararaca Complex (B. alcatraz, B. insularis, and B. jararaca), 20 in B. erythromelas, B. jararacussu, B. moojeni and B. neuwiedi venoms, and 22 in B. cotiara venom. Further, quantitative analysis of these N-glycans showed variable relative abundances in the venoms. For the first time a comprehensive set of N-glycan structures present in snake venoms are defined. Despite the fact that glycosylation is not template-defined, the N-glycomes of these venoms mirror the phylogeny cladograms of South American bothropoid snakes reported in studies on morphological, molecular data and feeding habits, exhibiting distinct molecular signatures for each venom. Considering the complexity of N-glycan moieties generally found in glycoproteins, characterized by different degrees of branching, isomer structures, and variable abundances, our findings point to these factors as another level of complexity in Bothrops venoms, features that could dramatically contribute to their distinct biological activities.

Cobra venom factor: Structure, function, and humanization for therapeutic complement depletion

Cobra venom factor (CVF) is the complement-activating protein in cobra venom. This manuscript reviews the structure and function of CVF, how it interacts with the complement system, the structural and functional homology to complement component C3, and the use of CVF as an experimental tool to decomplement laboratory animals to study the functions of complement in host defense and immune response as well as in the pathogenesis of diseases. This manuscript also reviews the recent progress in using the homology between CVF and C3 to study C3 structure and function, and to develop human C3 derivatives with the complement-depleting function of CVF. These human C3 derivatives represent humanized CVF, and are a conceptually different concept for pharmacological intervention of the complement system, therapeutic complement depletion. The use of humanized CVF for therapeutic complement depletion in several pre-clinical models of human diseases is also reviewed.

The crystal structure of cobra venom factor, a cofactor for C3- and C5-convertase CVFBb

Cobra venom factor (CVF) is a functional analog of human complement component C3b, the active fragment of C3. Similar to C3b, in human and mammalian serum, CVF binds factor B, which is then cleaved by factor D, giving rise to the CVFBb complex that targets the same scissile bond in C3 as the authentic complement convertases C4bC2a and C3bBb. Unlike the latter, CVFBb is a stable complex and an efficient C5 convertase. We solved the crystal structure of CVF, isolated from Naja naja kouthia venom, at 2.6 A resolution. The CVF crystal structure, an intermediate between C3b and C3c, lacks the TED domain and has the CUB domain in an identical position to that seen in C3b. The similarly positioned CUB and slightly displaced C345c domains of CVF could play a vital role in the formation of C3 convertases by providing important primary binding sites for factor B.

Proteome and immunome of the venom of the Thai cobra, Naja kaouthia

The proteome of the Thai cobra, Naja kaouthia, venom, revealed by two-dimensional liquid chromatography/tandem mass spectrometry, was found to consist of peptides which could be matched with 61 proteins in the database. These proteins were classified into 12 groups according to the differences in their biological activities: cardiotoxins, cobra venom factors, a cysteine-rich toxin, cytotoxins, kaouthiagin, mocarhagin, muscarinic toxin-like proteins, neurotoxins, an oxoglutarate dehydrogenase, phospholipases, serum albumin, and a weak toxin. Horse derived- anti-N. kaouthia venom hyperimmune serum currently used for the treatment of cobra ophitoxaemia reacted only to the cobra venom factors and phospholipases in the cobra holovenom by two-dimensional gel electrophoresis based-immunoblotting. The venom proteomic insight of this study should pave the way for preparing a therapeutic anti-venom of improved quality, i.e. also containing antibodies to the newly revealed toxic, but poorly immunogenic, minor venom components. It is expected that such a preparation should have a higher effectiveness than the currently used anti-venom in resuscitating cobra-bite victims.

Naja melanoleuca cobra venom contains two forms of complement-depleting factor (CVF)

Two forms of complement-depleting cobra venom factor (CVFm1 and CVFm2), possessing molecular masses of 142.6 kDa (CVFm1) and 143.1 kDa (CVFm2), according to MALDI mass-spectrometry, were isolated from the Naja melanoleuca cobra venom. As shown by polyacrylamide gel electrophoresis in the presence of SDS, both forms similarly to factor from the Naja kaouthia cobra venom (CVFk) consist of three polypeptide chains with molecular masses of about 70, 50, and 30 kDa, the two large subunits being glycosylated. As determined by MALDI mass-spectrometry, 30 kDa subunits of CVFm1 and CVFm2 have considerably different finger-prints of tryptic digests that suggests differences in their amino acid sequences. A study of activity in vivo has shown no significant differences in C3 consumption by CVFm1, CVFm2 and CVFk in mouse blood. However, as shown by an immunoassay method, they differ in their ability to activate the complement system via C3 conversion, the ratio of these activities for CVFm1:CVFm2:CVFk being 2.5:1.6:1. Kinetic studies using a hemolytic test showed that complement depletion by CVFm1 is faster than that by CVFm2. Thus, for the first time the presence in a single venom of two forms of CVF differing by both amino acid sequence and biological activity has been shown.

Structure and function of recombinant cobra venom factor

Cobra venom factor (CVF) is the complement-activating protein from cobra venom. It is a structural and functional analog of complement component C3. CVF functionally resembles C3b, the activated form of C3. Like C3b, CVF binds factor B, which is subsequently cleaved by factor D to form the bimolecular complex CVF,Bb. CVF,Bb is a C3/C5 convertase that cleaves both complement components C3 and C5. CVF is a three-chain protein that structurally resembles the C3b degradation product C3c, which is unable to form a C3/C5 convertase. Both C3 and CVF are synthesized as single-chain prepro-proteins. This study reports the recombinant expression of pro-CVF in two insect cell expression systems (baculovirus-infected Sf9 Spodoptera frugiperda cells and stably transfected S2 Drosophila melanogaster cells). In both expression systems pro-CVF is synthesized initially as a single-chain pro-CVF molecule that is subsequently proteolytically processed into a two-chain form of pro-CVF that structurally resembles C3. The C3-like form of pro-CVF can be further proteolytically processed into another two-chain form of pro-CVF that structurally resembles C3b. Unexpectedly, all three forms of pro-CVF exhibit functional activity of mature, natural CVF. Recombinant pro-CVF supports the activation of factor B in the presence of factor D and Mg2+ and depletes serum complement activity like natural CVF. The bimolecular convertase pro-CVF,Bb exhibits both C3 cleaving and C5 cleaving activity. The activity of pro-CVF and the resulting C3/C5 convertase is indistinguishable from CVF and the CVF,Bb convertase. The ability to produce active forms of pro-CVF recombinantly ensures the continued availability of an important research reagent for complement depletion because cobra venom as the source for natural CVF will be increasingly difficult to obtain as the Indian cobra is on the list of endangered species. Experimental systems to express pro-CVF recombinantly will also be invaluable for studies to delineate the structure and function relationship of CVF and its differences from C3 as well as to generate human C3 derivatives with CVF-like function for therapeutic complement depletion ("humanized CVF").

The first representative of glycosylated three-fingered toxins. Cytotoxin from the Naja kaouthia cobra venom

There are different glycosylated proteins in snake venoms, but no glycosylated representatives of a large family of three-fingered toxins have previously been detected. A new glycoprotein was isolated from the venom of the Thai cobra Naja kaouthia. MALDI MS of the glycoprotein contained an array of peaks in the range from approximately 8900 to approximately 9400 Da indicating its microheterogeneity. Carbohydrate analysis showed the presence of mannose, galactose, N-acetylglucosamine, fucose and neuraminic acid. The N-terminal sequence of the glycoprotein was identical to that of cytotoxin 3 (CX3) from N. kaouthia, and CD spectra of the glycoprotein and CX3 were almost the same. Cleavage of a glycan moiety by N-glycosidase F gave a protein of molecular mass practically coinciding with that of CX3. MALDI MS of the tryptic digest of reduced glycoprotein S-pyridylethylated at cysteine residues, contained peaks corresponding to all tryptic fragments of CX3, with the exception of fragment 24-30. The peak corresponding to this peptide appeared in the mass-spectrum of similarly treated deglycosylated glycoprotein. These data show that the potential N-glycosylation site at Asn29 in CX3 is utilized for glycan attachment and that the glycoprotein is glycosylated CX3. In vivo toxicity of the glycoprotein to the cricket Gryllus assimilis was twofold lower than that of CX3. The cytotoxic activity of the glycoprotein towards HL60 cells was about two orders of magnitude lower than that of CX3, but could be made equal to the CX3 cytotoxicity by deglycosylation. Thus for the first time we have isolated a glycosylated three-fingered snake venom toxin wherein glycosylation appears to modulate its biological activity.

Carbohydrate-directed conjugation of cobra venom factor to antibody by selective derivatization of the terminal galactose residues

Cobra venom factor (CVF) can cause cell death by complement-mediated bystander cell lysis. Several studies have investigated CVF for application in cancer therapy by conjugating CVF to antibodies against tumor cell surface-specific antigens via the side-chain amino acid residues. In most cases, the activity of CVF was markedly impaired, presumably by modification of the factor B binding domain due to random derivatization. Since CVF is a glycoprotein and its oligosaccharide chains are distal to the factor B binding domain, coupling of CVF to antibodies through its oligosaccharide chains is expected to yield immunoconjugates with retention of CVF activity and elimination of the immunoreactivity of the terminal alpha-galactosyl residues. In this study, we investigated the carbohydrate site-directed conjugation of CVF to a monoclonal IgG specific to a cell-surface antigen of human ovarian cancer cells. The terminal galactosyl residues of CVF were selectively modified at C-6 by treatment with galactose oxidase, and the generated aldehyde groups were derivatized in situ with hydrazides containing either protected thiol or maleimide functional groups. The CVF derivatives were allowed to react with thiol groups introduced to the antibody by derivatization with 2-iminothiolane to yield carbohydrate site-directed CVF-antibody conjugates. In both cases, 30-40% of the antibody cross-linked to CVF to yield predominantly monovalent CVF-antibody conjugates. The purified immunoconjugates retained 70-75% of CVF activity and significant level of antigen-binding capacity. This is the first study to exploit the oligosaccharide chains of CVF for the preparation of active immunoconjugates.

N-linked oligosaccharides of cobra venom factor contain novel alpha(1-3)galactosylated Le(x) structures

Cobra venom factor (CVF), a nontoxic, complement-activating glycoprotein in cobra venom, is a functional analog of mammalian complement component C3b. The carbohydrate moiety of CVF consists exclusively of N-linked oligosaccharides with terminal alpha1-3-linked galactosyl residues, which are antigenic in human. CVF has potential for several medical applications, including targeted cell killing and complement depletion. Here, we report a detailed structural analysis of the oligosaccharides of CVF. The structures of the oligosaccharides were determined by lectin affinity chromatography, antibody affinity blotting, compositional and methylation analyses, and high-resolution (1)H-NMR spectroscopy. Approximately 80% of the oligosaccharides are diantennary complex-type, approximately 12% are tri- and tetra-antennary complex-type, and approximately 8% are oligomannose type structures. The majority of the complex-type oligosaccharides terminate in Galalpha1-3Galbeta1-4(Fucalpha1-3)GlcNAcbeta1, a unique carbohydrate structural feature abundantly present in the glycoproteins of cobra venom.

Expression of factor I-resistant mutants of the human complement component C3 in heterologous systems

Complement plays a major role in hyperacute rejection of xenografts. In order to overcome this, we are developing, by minimal mutagenesis, a modified C3 molecule that, like cobra venom factor (CVF), escapes normal complement regulatory processes and inhibits complement-mediated responses by systemic depletion of C3. Unlike CVF, this protein should have little or no immunogenicity and be suitable for repeat administrations. As an initial step in this process, we have modified human C3 to make it resistant to inactivation by factor I. The factor I resistant C3 is capable of forming an active C3 convertase. Preincubation with normal human serum abrogated subsequent complement-mediated cytolysis by both the classical and alternative pathways, while wild-type (wt) C3 was inactive. The modified human C3 also blocked complement activity of guinea-pig serum. For economical and rapid production, we have developed expression of recombinant C3 wt and mutant proteins in the Baculovirus system. Large quantities are also being produced from stably transfected CHO cell lines. In addition, we have developed a fast C3 purification method by engineering a 6XHIS tag into the C3a portion of the molecule, thereby avoiding the need for subsequent separation of the tag from active C3b molecules.

Additive cytotoxicity of different monoclonal antibody-cobra venom factor conjugates for human neuroblastoma cells

Insufficient numbers of antigen molecules and heterogeneity of antigen expression on tumor cells are major factors limiting the immunotherapeutic potential of the few clinically useful monoclonal antibodies capable of mediating complement cytotoxicity and antibody-dependent cellular cytotoxicity. To overcome this limitation, we converted two non-cytotoxic monoclonal anti-neuroblastoma antibodies, designated 3E7 (IgG2b) and 8H9 (IgG1), and the non-cytotoxic F(ab')2 fragment of the cytotoxic monoclonal anti-GD2 antibody 3F8 (IgG3) into cytotoxic antibody conjugates by covalent attachment of cobra venom factor (CVF), a structural and functional homologue of the activated third component of complement. Competitive binding experiments confirmed the different specificities of the three antibodies. In the presence of human complement, all three antibody-CVF conjugates mediated selective complement-dependent lysis of human neuroblastoma cells. Consistent with the kinetics of the alternative pathway of complement, approximately seven hours incubation were required to reach maximum cytotoxicity of up to 25% for the 3E7-CVF conjugate, up to 60% for the 8H9-CVF conjugate, and up to 95% for the 3F8 F(ab')2-CVF conjugate. The different extent of maximal cytotoxic activity of the three conjugates was reflected by corresponding differences in the extent of binding of both unconjugated antibodies and the respective conjugates. Any combination of the three antibody-CVF conjugates caused an additive effect in complement-mediated lysis. Using a cocktail of all three conjugates, the extent of complement-mediated killing could be increased up to 100%. These data demonstrate that by coupling of CVF the relative large number of non-cytotoxic monoclonal anti-tumor antibodies of interesting specificity can be used to design cocktails of cytotoxic conjugates and, thereby, to overcome the problem of insufficient and heterogeneous antigen expression on tumor cells for immunotherapy.

Tissue targeting and plasma clearance of cobra venom factor in mice

The tissue targeting and rate of clearance of cobra venom factor (CVF) from the circulation was studied in mice by intravenous or intraperitoneal injection of radioiodinated CVF. In both modes of administrations, CVF was targeted mainly to liver. CVF injected directly into the blood was cleared from the circulation with a plasma half life of about 10 h, whereas CVF injected into the peritoneal cavity was slowly absorbed into the blood stream reaching a maximum level at approximately 6 h, and it was then cleared from the circulation with a plasma half life of about 18 h. The rate of plasma clearance of CVF was markedly decreased upon removal of the terminal alpha-galactosyl residues of the oligosaccharide chains; the plasma half lives for intravenously and intraperitoneally administered de-alpha-galactosylated CVF were approximately 5 and approximately 10 h, respectively. However, the clearance rate was not affected by complete deglycosylation using N-glycanase or by chemical modification of the terminal galactosyl residues. Together, these data demonstrate that the terminal alpha-galactosyl residues of CVF mask the Lewis X-dependent uptake of CVF by liver.

Cobra venom factor stimulates anti-alpha-galactose antibody production in baboons. Implications for pig-to-human xenotransplantation

Cobra venom factor (CVF) depletes complement and may therefore be of use in preventing the hyperacute rejection that follows discordant organ xenotransplantation. In two baboons studied, the intramuscular injection of CVF (0.25 mg/kg) was followed by a marked reduction in serum C3 and CH50, and serum cytotoxicity to pig kidney (PK15) cells. There was, however, a very rapid rise in the level of anti-alpha-galactose (alpha Gal) antibody, and a slower rise in anti-CVF antibody. A second intramuscular injection of CVF on day 14 was ineffective in reducing C3, CH50, and serum cytotoxicity. The major oligosaccharide of CVF is known to contain alpha Gal residues, which we suggest stimulate the major increase in anti-alpha Gal antibody level seen in the present study. In the clinical situation, this might lead to an increased immune response to a concomitantly transplanted pig organ.

The emergence of xenotransplantation

The field of transplantation is faced with a growing shortage of human organs as the list of potential recipients continues to increase. Those currently listed can already expect long waits; some die waiting. Xenotransplantation is a potential solution to this widening donor-recipient disparity. Consequently, in recent years, there have been several clinical attempts using organs from nonhuman primates and pigs. The results with nonhuman primates as donors have been encouraging, but it is unlikely that these species will provide a long-term solution to the organ shortage. Most recent xenotransplantation research has therefore shifted to more phylogenetically disparate species, such as pigs, as potential donors. The major barrier to transplantation between members of disparate species combinations has been hyperacute rejection (HAR). The elements of humoral immunity involved in this rejection process include (1) naturally occurring antibodies directed against carbohydrate and other antigens expressed on pig endothelium, and (2) the complement system, which is activated by binding of natural antibodies to their targets. Several elegant strategies to prevent HAR are being developed. The creation of transgenic pigs, whose cells express human regulators of complement activation, is one such strategy. Another promising approach has been to remove antidonor antibodies from the recipient by absorption with some recently characterized carbohydrate epitopes of porcine endothelial xenoantigens. Recent experimental work indicates that HAR can successfully be prevented by inhibition or depletion of complement. A delayed type of xenograft rejection, characterized by endothelial cell antibody deposition and cellular infiltration, occurs over the next three to four days. The likely mechanisms involved in delayed xenograft rejection include antibody-dependent cell-mediated cytotoxicity and the phenomenon of endothelial cell activation.