Human C5a anaphylatoxin: gene synthesis, expression, and recovery of biologically active material from Escherichia coli

Comparative analysis of different biofactories for the production of a major diabetes autoantigen

The 65-kDa isoform of human glutamic acid decarboxylase (hGAD65) is a major diabetes autoantigen that can be used for the diagnosis and (more recently) the treatment of autoimmune diabetes. We previously reported that a catalytically-inactive version (hGAD65mut) accumulated to tenfold higher levels than its active counterpart in transgenic tobacco plants, providing a safe and less expensive source of the protein compared to mammalian production platforms. Here we show that hGAD65mut is also produced at higher levels than hGAD65 by transient expression in Nicotiana benthamiana (using either the pK7WG2 or MagnICON vectors), in insect cells using baculovirus vectors, and in bacterial cells using an inducible-expression system, although the latter system is unsuitable because hGAD65mut accumulates within inclusion bodies. The most productive of these platforms was the MagnICON system, which achieved yields of 78.8 μg/g fresh leaf weight (FLW) but this was substantially less than the best-performing elite transgenic tobacco plants, which reached 114.3 μg/g FLW after six generations of self-crossing. The transgenic system was found to be the most productive and cost-effective although the breeding process took 3 years to complete. The MagnICON system was less productive overall, but generated large amounts of protein in a few days. Both plant-based systems were therefore advantageous over the baculovirus-based production platform in our hands.

Expression and subcellular targeting of human complement factor C5a in Nicotiana species

We evaluated transgenic tobacco plants as an alternative to Escherichia coli for the production of recombinant human complement factor 5a (C5a). C5a has not been expressed in plants before and is highly unstable in vivo in its native form, so it was necessary to establish the most suitable subcellular targeting strategy. We used the strong and constitutive CaMV 35S promoter to drive transgene expression and compared three different subcellular compartments. The yields of C5a in the T(0) transgenic plants were low in terms of the proportion of total soluble protein (TSP) when targeted to the apoplast (0.0002% TSP) or endoplasmic reticulum (0.0003% TSP) but was one order of magnitude higher when targeted to the vacuole (0.001% TSP). The yields could be increased by conventional breeding (up to 0.014% TSP in the T₂ generation). C5a accumulated to the same level in seeds and leaves when targeted to the apoplast but was up to 1.7-fold more abundant in the seeds when targeted to the ER or vacuole, although this difference was less striking in the better-performing lines. When yields were calculated as an amount per gram fresh weight of transgenic plant tissue, the vacuole targeting strategy was clearly more efficient in seeds, reaching 35.8 µg C5a per gram of fresh seed weight compared to 10.62 µg C5a per gram fresh weight of leaves. Transient expression of C5aER and C5aVac in N. benthamiana, using MagnICON vectors, reached up to 0.2% and 0.7% of TSP, respectively, but was accompanied by cytotoxic effects and induced leaf senescence. Western blot of the plant extracts revealed a band matching the corresponding glycosylated native protein and the bioassay demonstrated that recombinant C5a was biologically active.

A selection system to study C5a-C5a-receptor interactions: phage display of a novel C5a anaphylatoxin, Fos-C5aAla27

Binding and effector domains of the human anaphylatoxin C5a have been determined by either site directed mutagenesis or synthetic peptide studies. However, the lack of specific selection methods, which allow direct investigation of C5a-C5a-receptor interaction made these studies laborious. To overcome these limitations we have constructed a novel Fos-C5a expressed on the tip of a filamentous phage. To guarantee for a free C-terminus which is required for C5a activity C5a cDNA was cloned into the phagemid vector pJuFo. Helper phage infection of pJuFc-C5a transformed cells resulted in a mutant phage displaying Fos-C5a on its surface. However studies with Bt2cAMP differentiated U937 cells revealed that phage displayed Fos-C5a is functional inactive. Subsequently we replaced a nonconserved cysteine residue at position 27 by alanine and obtained Fos-C5aAla27. Both the purified and the phage displayed Fos-C5aAla27 proteins were functional active and induced enzyme release from differentiated U937 cells. In addition, purified Fos-C5aAla27 exhibited the same binding profile as compared to rhC5a. Fos-C5aAla27 displaying phages were mixed with phage harboring only the pJuFo plasmid at a ratio of 10(6). After four successive rounds of panning on differentiated U937 cells Fos-C5aAla27 phages were enriched to 100% as shown by C5a-specific ELISA. We expect this approach to prove helpful for studying C5a-C5a-receptor interactions. i.e. to screen C5a libraries for high affinity binders with agonistic or antagonistic properties directly on cells.

The pharmacophore of the human C5a anaphylatoxin

We have determined which amino acids contribute to the pharmacophore of human C5a, a potent inflammatory mediator. A systematic mutational analysis of this 74-amino acid protein was performed and the effects on the potency of receptor binding and of C5a-induced intracellular calcium ion mobilization were measured. This analysis included the construction of hybrids between C5a and the homologous but unreactive C3a protein and site-directed mutagenesis. Ten noncontiguous amino acids from the structurally well-defined 4-helix core domain (amino acids 1-63) and the C-terminal arginine-containing tripeptide were found to contribute to the pharmacophore of human C5a. The 10 mostly charged amino acids from the core domain generally made small incremental contributions toward binding affinity, some of which were independent. Substitutions of the C-terminal amino acid Arg 74 produced the largest single effect. We also found the connection between these 2 important regions to be unconstrained.

Site-specific mutations in the N-terminal region of human C5a that affect interactions of C5a with the neutrophil C5a receptor

C5a is an inflammatory mediator that evokes a variety of immune effector functions including chemotaxis, cell activation, spasmogenesis, and immune modulation. It is well established that the effector site in C5a is located in the C-terminal region, although other regions in C5a also contribute to receptor interaction. We have examined the N-terminal region (NTR) of human C5a by replacing selected residues in the NTR with glycine via site-directed mutagenesis. Mutants of rC5a were expressed as fusion proteins, and rC5a was isolated after factor Xa cleavage. The potency of the mutants was evaluated by measuring both neutrophil chemotaxis and degranulation (beta-glucuronidase release). Mutants that contained the single residue substitutions Ile-6-->Gly or Tyr-13-->Gly were reduced in potency to 4-30% compared with wild-type rC5a. Other single-site glycine substitutions at positions Leu-2, Ala-10, Lys-4, Lys-5, Glu-7, Glu-8, and Lys-14 showed little effect on C5a potency. The double mutant, Ile-6-->Gly/Tyr-13-->Gly, was reduced in potency to < 0.2%, which correlated with a correspondingly low binding affinity for neutrophil C5a receptors. Circular dichroism studies revealed a 40% reduction in alpha-helical content for the double mutant, suggesting that the NTR contributes stabilizing interactions that maintain local secondary or tertiary structure of C5a important for receptor interaction. We conclude that the N-terminal region in C5a is involved in receptor binding either through direct interaction with the receptor or by stabilizing a binding site elsewhere in the intact C5a molecule.

Specific inhibition of leukotriene B4 (LTB4)-induced neutrophil emigration by 20-hydroxy LTB4: implications for the regulation of inflammatory responses

1. The interaction between leukotriene B4 (LTB4) and its metabolite, 20-hydroxy LTB4 in the control of neutrophil emigration was examined in guinea-pig skin. 2. Leukotriene B4 (10-300 ng) elicited a dose-dependent increase in neutrophil infiltration (as measured by myeloperoxidase activity) 4 h after injection into guinea-pig skin. In contrast, 20-hydroxy LTB4 (30-1000 ng) displayed only weak inflammatory activity in this assay. 3. Although 20-hydroxy LTB4 had low agonist activity, this metabolite caused a potent dose-dependent inhibition of responses to LTB4 (100 ng), when administered systemically (ED50 = 1.3 micrograms kg-1, s.c.) without significantly affecting neutrophil infiltration in response to C5a (2 micrograms). Systemic administration of 20-carboxy LTB4 (10 micrograms) did not affect neutrophil accumulation in response to LTB4 or C5a. In addition, neither 15(S)-hydroxy 5(S)-HPETE(10 micrograms) nor lipoxin A4 (10 micrograms) inhibited responses to LTB4. 4. Addition of 20-hydroxy LTB4 (10(-11)-10(-8) M) to human blood prior to isolation of the neutrophils led to concentration-dependent decrease in the number of LTB4 receptors and decreased chemotactic responsiveness to LTB4 without affecting responses to C5a. Incubation of blood with 20-carboxy LTB4 (10(-8) M) did not reduce LTB4 receptor number of chemotactic responsiveness to LTB4. 5. These data indicate that although 20-hydroxy LTB4 is a weak agonist at LTB4 receptors, it can desensitize neutrophils to the effects of LTB4 via down-regulation of the high affinity receptor and thus provides evidence for a mechanism whereby inflammatory responses may be regulated.

A recombinant hybrid anaphylatoxin with dual C3a/C5a activity

By site-directed mutagenesis of a human complement factor C5a cDNA clone, we have designed a hybrid anaphylatoxin in which three amino acid residues in the C-terminal sequence of human C5a were exchanged to create the native C-terminal human C3a (hC3a) sequence Leu-Gly-Leu-Ala-Arg. This hybrid anaphylatoxin rC5a-(1-69)-LGLAR exhibited true C3a and C5a activity when tested in the guinea pig ileum contraction assay. Quantitative measurements of ATP release from guinea pig platelets revealed about 1% intrinsic C3a activity for this hybrid, while the C5a activity was essentially unchanged. Competitive binding assays confirmed that the rC5a-(1-69)-LGLAR mutant was able to displace radioiodinated rhC5a with a KI of approx. 40 nM and hC3a with a KI of approx. 3.7 microM from guinea pig platelets. Since the C-termini of both human C3a and C5a anaphylatoxins are known to interact with their respective receptors, we conclude that the same peptidic sequence, LGLAR, is able to bind to and activate two different receptors, the C3a receptor as well as the C5a receptor. This clone provides a novel tool for the identification of further receptor-binding residues in both anaphylatoxins, since any mutants may be tested for altered C3a and C5a activity simultaneously.

Human C5a anaphylatoxin: gene cloning and expression in Escherichia coli

A gene coding for the human anaphylatoxin C5a was cloned and expressed in Escherichia coli. A combination of reverse transcription of mRNA of the U937 cell line with subsequent preparative polymerase chain reaction was employed to obtain the gene. The sequence was cloned into the plasmid vector pKK 233-2 behind an ATG initiation codon under the control of a trc promotor. After purification by ion exchange chromatography and reversed phase FPLC a mixture of predominantly non-glycosylated recombinant human C5a with a beta-mercaptoethanol adduct at cysteine 27 and the N-methionyl derivative was obtained which was homogeneous on silver-stained gels, immunoreactive with C5a-specific monoclonal antibodies and functionally active in releasing myeloperoxidase from human granulocytes and ATP from guinea pig platelets. The final yield was about 0.4-0.8 mg purified recombinant C5a per liter bacterial culture.

Tyrosine phosphorylation is an early signaling event common to Fc receptor crosslinking in human neutrophils and rat basophilic leukemia cells (RBL-2H3)

Phosphotyrosine-containing proteins were detected by western blotting of whole cell lysates of purified human neutrophils or rat basophilic leukemia cells (RBL-2H3) using a polyclonal anti-phosphotyrosine antibody. When either cell type was stimulated with the appropriate Fc crosslinking agent, heat-aggregated IgG for the neutrophil or DNP-HSA for the IgE-sensitized RBL-2H3, a rapid increase in the phosphotyrosine content of several proteins was observed. The kinetics and specificity of both responses suggest that Fc receptor crosslinking activates a receptor-associated tyrosine kinase, probably a member of the src family of tyrosine protein kinases. The subsequent tyrosine phosphorylation events are likely to be important in Fc receptor-mediated stimulus-response coupling in inflammatory cells.

Isolation and characterization of biologically active murine interleukin-6 produced in Escherichia coli

Interleukin-6 (IL-6) is a multi-functional cytokine produced and secreted by several different cell types, including those of the immune system. A cDNA coding for the mature murine IL-6 (mIL-6), which extends from amino acid (aa) 25 through 211, was cloned into a prokaryotic vector and then expressed in Escherichia coli. The recombinant mIL-6 (remIL-6) was isolated from bacterial inclusion bodies by solubilization in 4 M guanidine hydrochloride followed by gel-filtration chromatography. The protein was refolded to an active conformation by dialysis against 25 mM Na. acetate pH 5.5. A final step of purification and concentration on a cation exchange resin yielded pure and biologically active remIL-6. The purified preparation had the expected aa composition, as confirmed by aa analysis and pI of 7.0-7.1. The biological activity of the recombinant protein was measured in two systems; a proliferation assay employing 7TD1 cells, and a fibrinogen biosynthesis assay employing primary rat hepatocytes. Both assay systems demonstrated that the remIL-6 was active in the range of 10(8) units/mg, which is similar to that estimated for native cytokine. Antibodies raised in rabbits against remIL-6 neutralized the biological activity of both recombinant and native IL-6.

Active recombinant C3a of human anaphylatoxin produced in Escherichia coli

DNA sequence coding for the complete human C3a with 77 amino acids was divided into three portions, synthesized separately and constructed for expression in Escherichia coli. High expression of the recombinant C3a was achieved by an expression system using T7 polymerase. Purified recombinant C3a showed the same activities of ileum contraction and platelet aggregation of guinea pig as C3a purified from human serum.

Neutrophil chemotactic factors

Polymorphonuclear leukocytes (neutrophils) are recruited to inflammatory sites by a variety of soluble mediators (chemoattractants) that stimulate neutrophil directed migration (chemotaxis). Many neutrophil chemoattractants such as neutrophil activating proteins, leukotriene B4 (LTB4), platelet activating factor, and complement-derived C5a, are generated endogenously by host cells or enzymatic cleavage of host proteins. Other chemoattractants such as N-formyl peptides are generated exogenously by bacteria that invade the host. Oxidative modification of methionine residues or changes in the amino acid sequence of peptide chemoattractants dramatically alter their chemoattractive properties. Many of the well-defined neutrophil chemotactic factors and studies of their structure-function relationships will be reviewed.

Construction and expression of a novel recombinant anaphylatoxin, C5a-N19, as a probe for the human C5a receptor

We have constructed a novel recombinant C5a anaphylatoxin (C5a-N19) containing a 19-residue amino-terminal extension peptide, using a plasmid vector which secretes the nascent polypeptide to the Escherichia coli periplasmic space. C5a-N19 was purified from cell lysates by immunoaffinity chromatography using a monoclonal antibody which recognizes a portion of the amino-terminal extension peptide. C5a-N19 was characterized as biologically indistinguishable from the unmodified recombinant anaphylatoxin for release of lysosomal enzymes from dibutyryl-cAMP-differentiated U937 cells. In contrast to unmodified C5a, which is not recognized by anti-C5a antibodies following binding to its cellular receptor, receptor-bound C5a-N19 is recognized by the monoclonal antibody directed against the amino-terminal extension sequence. Because the monoclonal antibody recognizes the C5a-receptor complex on cells, this methodology is useful in fluorescence sorting of C5a receptor-positive cells. A C5a receptor affinity column was constructed by saturating monoclonal antibody bound to agarose with C5a-N19. Digitonin-solubilized C5a receptor from dibutyryl-cAMP-induced U937 cells was adsorbed to the matrix and eluted by dissociation of the ligand-receptor complex from the antibody. Analysis by SDS-polyacrylamide gel electrophoresis revealed a unique protein band at 41K, consistent with the molecular weight predicted from cross-linking experiments when the contribution of C5a is subtracted. Development of this recombinant C5a derivative provides a useful probe previously unavailable for the C5a receptor molecule.

Activation of human platelets by C5a-stimulated neutrophils: a role for cathepsin G

Human platelets can be stimulated by recombinant human fifth component of complement (rhC5a) in the presence of human neutrophils. After challenge with N-formyl-Met-Leu-Phe or rhC5a, concentrated neutrophils release cathepsin G into the supernatant. The concentrations of cathepsin G recovered by titration of the enzymatic activity correlate with the capability of these supernatants to induce platelet stimulation as measured by serotonin release. Cathepsin G purified from neutrophil granules triggered platelet aggregation and serotonin release independent of arachidonic acid metabolites and platelet-activating factor formation. A concentration of 100 nM of cathepsin G, which was reached in the surrounding space of activated neutrophils, induced a 50% platelet stimulation. Three distinct antiproteinases were tested against cathepsin G-induced platelet activation. Z-Gly-Leu-Phe-CH2Cl, a specific inhibitor of cathepsin G enzymatic activity, proved to be nonspecific in our biological system. By contrast, alpha 1-antichymotrypsin and alpha 1-antitrypsin displayed specific activities. The physiological specific inhibitor of cathepsin G, alpha 1-antichymotrypsin, was the most potent and was used in the rhC5a-induced neutrophils-mediated platelet activation. A complete inhibition was achieved, showing that release of cathepsin G from neutrophils accounts for platelet activation. Such a chain of events involving C5a, neutrophils, cathepsin G, and platelets may be of relevance in certain inflammatory states, particularly the adult respiratory distress syndrome.

Expression of human plasminogen activator inhibitor type-1 (PAI-1) in Escherichia coli as a soluble protein comprised of active and latent forms. Isolation and crystallization of latent PAI-1

Expression of human recombinant plasminogen activator inhibitor type-1 (PAI-1) in Escherichia coli has led to crystallization of 'latent' PAI-1. Cleavage with restriction endonucleases of a cDNA clone encoding PAI-1 yielded an 1127 base pair fragment encoding residues 2-376 of the 379 amino acid serpin. Synthetic DNA linkers were ligated to the 5' and 3' ends of the subclone to add an initiation codon and restore the full coding sequence, and the resulting semisynthetic gene was incorporated into an expression plasmid, pPAIST-7, under the control of the E. coli trp promoter. Transformation of E. coli GE81 with pPAIST-7 led to expression of unglycosylated PAI-1. Lysates of expression cultures contained PAI-1 activity and PAI-1 protein with the predicted Mr. Unglycosylated PAI-1 from E. coli exhibited characteristic properties of authentic PAI-1: (1) it was recovered in both active and inactive (latent) forms; (2) its activity declined during incubation at 37 degrees C; (3) latent PAI-1 was activated by treatment with 4 M guanidine hydrochloride; (4) reactivated PAI-1 formed a detergent-stable complex with tissue plasminogen activator. Latent PAI-1 accounted for more than 85% of PAI-1 in cell lysates and was purified by ammonium sulfate fractionation, anion-exchange chromatography and hydrophobic interaction chromatography. The purified latent PAI-1 was crystallized.

Recombinant human C5a-induced bronchoconstriction in the guinea-pig: a histamine independent mechanism

Recombinant human C5a (rHuC5a), produced by a synthetic gene expressed in Escherichia coli, causes a decrease in dynamic lung compliance and an increase in pulmonary resistance when injected intravenously in anesthetized mechanically ventilated guinea pigs over a dose range of 5-20 micrograms/kg. Intravenous injection of rHuC5a also caused an immediate decrease in mean arterial blood pressure followed by a transient increase. The purpose of this study was to determine the mediators responsible for these effects. To assess the role of histamine, plasma levels of histamine were monitored and the effects of the H1 antagonist pyrilamine were assessed. rHuC5a caused a significant increase in plasma histamine. However, the H1 antagonist did not alter the maximum or the time course of the bronchoconstrictor response indicating that histamine did not play a major role. The LTD4 antagonist L-649,923 did not inhibit the rHuC5a-induced bronchoconstriction whereas the cyclo-oxygenase inhibitor indomethacin did. Thus, to assess the role of cyclo-oxygenase products, plasma levels of thromboxane (TX) B2, prostaglandin (PG) D2 and PGF2 alpha were monitored after injection of rHuC5a. In addition, guinea pigs were treated with either the TX synthetase inhibitor U-63557A or with the TX receptor antagonist SQ 29,548. rHuC5a challenge caused an increase in plasma concentrations of TXB2, PGD2 and PGF2 alpha which peaked before the maximum of the bronchoconstriction. SQ 29,548 significantly inhibited the maximum of the bronchoconstrictor response, whereas U-63557A did not inhibit the maximum but did inhibit the time course of the response.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and characterization of biologically active murine interleukin-1 alpha derived from expression of a synthetic gene in Escherichia coli

A murine interleukin-1 alpha (mIL-1 alpha) gene coding for amino acids 115 to 270 of the precursor protein (Lomedico, P.T., Gubler, U., Hellmann, C.P., Dukovich, M., Giri, J.G., Pan, Y.E., Collier, K., Semionow, R., Chua, A.O. and Mizel, S.B. (1984) Nature 312, 458-462) was chemically synthesized and expressed in Escherichia coli. mIL-1 alpha, in the form of insoluble inclusion bodies, accounted for approx. 30% of total cellular protein produced by the recombinant strain. A simple isolation protocol was developed in which inclusion body material was first solubilized in 3 M guanidine hydrochloride, and the mIL-1 alpha was then simultaneously purified and allowed to fold to its active conformation by dialysis against distilled water. This procedure yielded pure, biologically active mIL-1 alpha with 41% recovery of the mIL-1 alpha present in the guanidine hydrochloride extract. The purified preparation had the expected amino acid composition, a molar absorptivity of 28,200 M-1.cm-1 and a pI of 5.2. No methionyl-mIL-1 alpha was detected by N-terminal sequence analysis, and the endotoxin level was less than 10 pg per micrograms of mIL-1 alpha. The specific biological activity was 3.10(7) units/mg in a co-mitogenic thymocyte proliferation assay. In addition to full-length mIL-1 alpha, the preparation contained N-terminally truncated mIL-1 alpha species (mainly des-4 and des-6 amino acid forms). The truncated species were isolated and found to have the same biological activity as the complete polypeptide. Thus, the active fragment of mIL-1 alpha appears to consist of a proteinase-sensitive N-terminal region which is not essential for activity, and a proteinase-resistant core which harbors the essential determinants of its cytokine function.

The use of high field NMR to determine homogeneity in a preparation of human C5a produced by Escherichia coli

The polypeptide backbone of human C5a was prepared by recombinant DNA techniques. Standard biochemical analysis guided the protein separation to give a sample of C5a which was deemed homogeneous. However, nuclear magnetic resonance (NMR) studies showed the material to be significantly heterogeneous. Reanalysis by high performance liquid chromatography (HPLC) corroborated the NMR results. Further separation by HPLC and analysis by NMR spectroscopy guided the isolation of rC5a to greater than 92% purity. NMR analysis, immunochemical and biological evaluation of the impurities showed them to be C5a structural variants. These results indicate that conventional methods of protein chemistry can fail to reveal heterogeneity in recombinant proteins, and in some circumstances NMR spectroscopy can aid in their purification.