Reevaluation of the C3a active site using short synthetic C3a analogues

Functional Relevance of the Anaphylatoxin Receptor C3aR for Platelet Function and Arterial Thrombus Formation Marks an Intersection Point Between Innate Immunity and Thrombosis

BACKGROUND

Platelets have distinct roles in the vascular system in that they are the major mediator of thrombosis, critical for restoration of tissue integrity, and players in vascular inflammatory conditions. In close spatiotemporal proximity, the complement system acts as the first line of defense against invading microorganisms and is a key mediator of inflammation. Whereas the fluid phase cross-talk between the complement and coagulation systems is well appreciated, the understanding of the pathophysiological implications of such interactions is still scant.

METHODS

We analyzed coexpression of the anaphylatoxin receptor C3aR with activated glycoprotein IIb/IIIa on platelets of 501 patients with coronary artery disease using flow cytometry; detected C3aR expression in human or murine specimen by polymerase chain reaction, immunofluorescence, Western blotting, or flow cytometry; and examined the importance of platelet C3aR by various in vitro platelet function tests, in vivo bleeding time, and intravital microscopy. The pathophysiological relevance of C3aR was scrutinized with the use of disease models of myocardial infarction and stroke. To approach underlying molecular mechanisms, we identified the platelet small GTPase Rap1b using nanoscale liquid chromatography coupled to tandem mass spectrometry.

RESULTS

We found a strong positive correlation of platelet complement C3aR expression with activated glycoprotein IIb/IIIa in patients with coronary artery disease and coexpression of C3aR with glycoprotein IIb/IIIa in thrombi obtained from patients with myocardial infarction. Our results demonstrate that the C3a/C3aR axis on platelets regulates distinct steps of thrombus formation such as platelet adhesion, spreading, and Ca²⁺ influx. Using C3aR^-/- mice or C3^-/- mice with reinjection of C3a, we uncovered that the complement activation fragment C3a regulates bleeding time after tail injury and thrombosis. Notably, C3aR^-/- mice were less prone to experimental stroke and myocardial infarction. Furthermore, reconstitution of C3aR^-/- mice with C3aR^+/+ platelets and platelet depletion experiments demonstrated that the observed effects on thrombosis, myocardial infarction, and stroke were specifically caused by platelet C3aR. Mechanistically, C3aR-mediated signaling regulates the activation of Rap1b and thereby bleeding arrest after injury and in vivo thrombus formation.

CONCLUSIONS

Overall, our findings uncover a novel function of the anaphylatoxin C3a for platelet function and thrombus formation, highlighting a detrimental role of imbalanced complement activation in cardiovascular diseases.

C3a modulates IL-1β secretion in human monocytes by regulating ATP efflux and subsequent NLRP3 inflammasome activation

Interleukin-1β (IL-1β) is a proinflammatory cytokine and a therapeutic target in several chronic autoimmune states. Monocytes and macrophages are the major sources of IL-1β. IL-1β production by these cells requires Toll-like receptor (TLR) and adenosine triphosphate (ATP)-mediated P2X purinoceptor 7 (P2X7) signals, which together activate the inflammasome. However, how TLR signals and ATP availability are regulated during monocyte activation is unclear and the involvement of another danger signal system has been proposed. Here, we demonstrate that both lipopolysaccharide (LPS) and the anaphylatoxin C3a are needed for IL-1β production in human macrophages and dendritic cells, while in monocytes, C3a enhanced the secretion of LPS-induced IL-1β. C3a and LPS-stimulated monocytes increased T helper 17 (Th17) cell induction in vitro, and human rejecting, but not nonrejecting, kidney transplant biopsies were characterized by local generation of C3a and monocyte and Th17 cell infiltration. Mechanistically, C3a drives IL-1β production in monocytes by controlling the release of intracellular ATP into the extracellular space via regulation of as-yet unidentified ATP-releasing channels in an extracellular signal-regulated kinase 1/2-dependent fashion. These data define a novel function for complement in inflammasome activation in monocytes and suggest that C3aR-mediated signaling is a vital component of the IL-1β-Th17 axis.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors

The activation of the complement cascade, a cornerstone of the innate immune response, produces a number of small (74-77 amino acid) fragments, originally termed anaphylatoxins, that are potent chemoattractants and secretagogues that act on a wide variety of cell types. These fragments, C5a, C4a, and C3a, participate at all levels of the immune response and are also involved in other processes such as neural development and organ regeneration. Their primary function, however, is in inflammation, so they are important targets for the development of antiinflammatory therapies. Only three receptors for complement peptides have been found, but there are no satisfactory antagonists as yet, despite intensive investigation. In humans, there is a single receptor for C3a (C3a receptor), no known receptor for C4a, and two receptors for C5a (C5a₁ receptor and C5a₂ receptor). The most recently characterized receptor, the C5a₂ receptor (previously known as C5L2 or GPR77), has been regarded as a passive binding protein, but signaling activities are now ascribed to it, so we propose that it be formally identified as a receptor and be given a name to reflect this. Here, we describe the complex biology of the complement peptides, introduce a new suggested nomenclature, and review our current knowledge of receptor pharmacology.

Characterization of synthetic C3a analog peptides on human eosinophils in comparison to the native complement component C3a

The C3a anaphylatoxin is a potent proinflammatory mediator derived from the complement system inducing biologic effects of human eosinophils like Ca2+ transients and the activation of the respiratory burst. These findings support an important role for C3a in diseases typically associated with a peripheral blood or tissue eosinophilia. Synthetic human C3a analogue peptides with variations at the C-terminal effector domain have been evaluated with respect to their binding affinity and signaling potency on human eosinophils. Flow cytometrical analysis and RT-PCR revealed that the C3a receptor is constitutively expressed on human eosinophils. Peptides bearing an N-terminal 9-fluorenylmethoxycarbonyl and the 6-aminohexanoyl motif were the most powerful peptides tested. Amino acid replacements in the conserved C-terminal pentapeptide decreased binding affinity and functional potency substantially. In addition, synthetic C3a analogue peptides induced C3aR internalization, led to transient changes of intracellular Ca2+ concentration, and did release reactive oxygen species in human eosinophils indicating the in vivo relevance of C3a-related sequences. The tripeptide LAR was found to be essential for C3a receptor binding on human eosinophils. Moreover, the putative binding motif of C3a anaphylatoxin is also crucial for the induction of biologic effects in the human system such as changes of intracellular Ca2+ concentration and the release of reactive oxygen species. This study demonstrates that the carboxyl terminus is important for the interaction with the C3aR and the biologic potency of C3a anaphylatoxin in the human system and plays a key role in the activation process of human eosinophils.

Complement factors and their receptors

In summary, recent advances in molecular cloning of anaphylatoxins and the anaphylatoxin receptors add new dimensions to our investigations and understanding of the molecular mechanisms involved in anaphylatoxin action. Combining knowledge accumulated from peptide modeling of the ligands with mutagenesis studies of these ligands and their receptors makes it possible to more accurately model interactive sites and understand the sequence of molecular interactions required for cellular activation. In addition, these new developments provide valuable tools for investigating, yet unknown, activities and cellular targets of the anaphylatoxin molecules.

Identification of casoxin C, an ileum-contracting peptide derived from bovine kappa-casein, as an agonist for C3a receptors

Casoxin C (Tyr-Ile-Pro-Ile-Gln-Tyr-Val-Leu-Ser-Arg) is a bioactive peptide that was isolated from a tryptic digest of bovine kappa-casein as an anti-opioid peptide in longitudinal strips of guinea pig ileum. Casoxin C also evokes contraction of the ileal strips, and we found that this process was biphasic with rapid and slow components. The contractile profile was very similar to that of human complement C3a(70-77), which is the COOH-terminal octapeptide of C3a and has, although less potent, qualitatively the same biological activities as C3a. Casoxin C also has homology with C3a(70-77). The rapid contraction was mediated by histamine release and the slow contraction was mediated by a prostaglandin E2-like substance, judging from the effects of various pharmacological inhibitors and antagonists on the ileal contraction. Casoxin C had affinity for C3a receptors (IC50 = 40 microM) in the radioreceptor assay. In addition, casoxin C showed phagocyte-stimulating activities. Casoxin C is therefore the first milk-derived peptide identified, that acts through complement C3a receptors.

Expression cloning of the human C3a anaphylatoxin receptor (C3aR) from differentiated U-937 cells

A cDNA clone encoding the human C3a anaphylatoxin receptor (C3aR) was isolated from a pcDNAI/Amp expression library prepared from U-937 cells which had been differentiated with dibutyryl cAMP to a macrophage-like phenotype. The cDNA clone contained an insert of 4.3 kbp and was able to confer to transfected human HEK-293 cells the capacity to bind specifically iodinated human C3a. Chinese hamster ovary cells co-transfected with this cDNA clone and a G-protein alpha subunit (G alpha-16) became functionally responsive to C3a and a C3a analog synthetic peptide, as measured by increased phosphoinositide hydrolysis. As inferred from the cDNA sequence, the clone encodes a 482-residue polypeptide with seven hydrophobic membrane-spanning helices and a high homology to the human C5a and formyl-Met-Leu-Phe receptors. Uniquely among the family of G-protein coupled receptors, the C3aR contains an exceptionally large second extracellular loop of approximately 175 residues. Northern hybridizations revealed an approximately 2.3-kb transcript as the major and an additional approximately 3.9 kb-transcript as a minor transcription product of the C3aR. The C3aR appears to be widely expressed in different lymphoid tissues, as shown by Northern hybridizations, providing evidence for a central role of the C3a anaphylatoxin in inflammatory processes.

Studies on the ileum-contracting mechanisms and identification as a complement C3a receptor agonist of oryzatensin, a bioactive peptide derived from rice albumin

Oryzatensin (Gly-Tyr-Pro-Met-Tyr-Pro-Leu-Pro-Arg) is an ileum-contracting and immunostimulating peptide derived from rice albumin. The mechanisms for the ileal contraction that it induces, consisting of rapid and slow components, were examined. The rapid contraction was mediated by histamine release and the slow contraction by a prostaglandin E2-like substance, judging from the effects of various pharmacological inhibitors and antagonists on ileal contraction and titration of histamine release. The contractile profile was very similar to that of human complement C3a(70-77), which is the COOH-terminal octapeptide of C3a and has, although less potent, qualitatively the same biological activities as C3a. Oryzatensin showed homology with C3a(70-77) and affinity for C3a receptors (IC50 = 44 microM) by radioreceptor assay. This is the first report of a food-derived bioactive peptide acting through complement C3a receptors.

Cyclic disulfide analogues of the complement component C3a. Synthesis and conformational investigations

The flexible C-terminal region of the anaphylatoxic peptide C3a was reported to contain the receptor binding site. To elucidate the receptor binding conformation of the C-terminus, as well as to examine a synthetic approach to potential C3a-antagonists, 26 cyclic disulfide bridged C3a analogues were synthesized. Solid phase peptide synthesis was performed on different polymeric supports by individual peptide synthesis, with Fmoc strategy, and simultaneous multiple peptide synthesis, using Boc and Fmoc strategies. Both strategies gave open-chain peptides in comparable yields. Syntheses using the Boc strategy employed the HF-labile 4(methoxy)benzyl group (Mob) for beta-thiol protection of cysteine; in contrast, the TFA-stable protecting groups, acetamidomethyl (Acm) and trityl (Trt), were chosen for syntheses employing Fmoc strategy. Ring closure reactions by iodine oxidation were carried out starting from protected (Acm/Acm, Trt/Acm) or unprotected dithiols. The resulting cyclic C3a analogues were characterized by HPLC, amino acid analysis, and FAB-MS. Conformational investigations using CD spectroscopy and theoretical structural investigations by means of molecular dynamics calculations revealed that slight variations in sequence result in pronounced conformational consequences. The potential of cyclic C3a analogues to activate or to desensitize guinea pig platelets, a standard test system for biological activities of anaphylatoxic peptides like C3a, revealed relatively low activities for cyclic peptides (< 0.1% C3a activity). N-terminal acylation with cationic, arginine-rich sequences like YRRGR- led to amplified biological effects. Three of the synthesized peptides, namely CAALCLAR (P1), YRRGRCGGLCLAR (P5) and YRRGRAhxCGGLCLAR (P8), point in the direction of C3a antagonists.

Evaluation of the C-terminal C5a effector site with short synthetic C5a analog peptides

Biological activities have been determined for a series of 18 peptides based on the C-terminal sequence of human or rat C5a. Lysosomal enzyme release was tested in two cell types, the promyelotic leukemia cell line U937 and polymorphonuclear leukocytes. In addition, an ATP-release assay with guinea pig platelets was performed. It was demonstrated that the C-terminal octapeptide 67-74 of human C5a represents the minimal sequence required to induce a measurable biological signal in all assays. Extending this peptide to a length of 21 amino acids produced at best only a slight enhancement of potency. Amino acid replacements with either tryptophanyl or phenylalanyl residues in positions between 65-69 either increased potency (at position 67), or abrogated potency (at position 66) in the two lysosomal enzyme assays. N-terminal acylation with the fluorenylmethoxy-carbonyl-aminohexanoyl group slightly enhanced C5a potency. In desensitization experiments with guinea pig platelets all peptides with a C5a activity were able to desensitize not only the C5a but also the C3a responses.

Synthetic peptides as antagonists of the anaphylatoxin C3a

Peptide compounds resembling the receptor-binding C-terminal domain of the anaphylatoxic peptide C3a were synthesized to examine two kinds of C3a antagonism: (a) specific desensitization of C3a-sensitive cells and (b) competitive binding to the C3a receptor. We used guinea-pig platelets, which express a C3a receptor and specifically release ATP upon stimulation, to evaluate the actions of the C3a analogues. The ATP liberation can be inhibited by pretreatment (i.e. desensitization) of the guinea-pig platelets with substimulatory concentrations of C3a or its analogues. Compared to C3a, several peptides were found with at least a tenfold greater difference between the required concentrations for C3a-specific half-maximal desensitization (DD50) and half-maximal platelet activation (ED50). The most potent compounds were YAAALKLAR and Fmoc-EAALKLAR (Fmoc: 9-fluorenylmethoxycarbonyl) with an ED50/DD50 of 140 +/- 28 and 80 +/- 17, respectively (mean +/- standard deviation). The ED50/DD50 of human C3a was found to be only 6 +/- 2. Some C3a derivatives were also tested in competitive binding studies for their ability to compete with C3a for receptor sites on guinea-pig platelets. Three of them were considered partial antagonists [YRRGRCGGLCLAR, YRRGRXCGGLCLAR and YRRGRXCGALCLAR (X = 6-aminohexanoyl)] because their Ki were smaller than their ED50 (Ki/ED50 = 0.6 +/- 0.3, 0.5 +/- 0.1 and 0.4 +/- 0.2, respectively). Interestingly, the last two compounds also had ED50/DD50 values greater than 60. Common to all three peptides are N-terminal arginine-rich sequences and intramolecular disulfide bridges which introduce conformational constraint.

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.

Enhanced biopotency of synthetic C3a analogues by membrane binding. A fluorescence anisotropy decay study

The biological activity of oligopeptide analogues of C3a is markedly increased by N-terminal attachment of a hydrophobic group as, for instance, 9-fluorenylmethoxycarbonyl (Fmoc), either direct or via a flexible 6-aminohexanoyl (Ahx) spacer. This study presents evidence from fluorescence anisotropy decay measurements that the hydrophobic appendix mediates non-specific binding of the synthetic peptide analogues to phospholipid vesicles. According to quantitative considerations no alternative or additional rate-enhancing mechanisms other than surface diffusion are required to account for the gain in biopotency.

Functional activities of synthetic anaphylatoxic peptides in widely used biological assays

A comparison study was carried out between the modern ATP release assay (ARA) with guinea-pig platelets and the traditional guinea-pig ileum contraction assay (ICA). The biological activities of the anaphylatoxin C3a and synthetic C3a analogue peptides were determined in both assays. In dose-response curves with C3a, a human C3a peptide with the last 21 amino acids of the C terminus (C3a 56-77) and a peptide with 13 amino acids which was acylated N-terminal with the aromatic fluorenylmethoxycarbonyl group and an aminohexanoyl group (Fmoc-Ahx YRRGRAAALGLAR) were tested. The ARA turned out to be 100 times more sensitive than the ICA. In contrast to previous reports the 21 amino acid long C3a analogue peptide did not exhibit full C3a activity but only 7% (ARA) or 12% (ICA). The potentiation of biological activity in the ARA by coupling non-peptide acyl-residues N terminal to peptide C3a analogues could be confirmed with Fmoc-Ahx-YRRGRAAALGLAR in the ICA. In addition, the tri-peptide Fmoc-Ahx-LAR displayed C3a specific activity in the ICA demonstrated by desensitization experiments.

Molecular epitope identification by limited proteolysis of an immobilized antigen-antibody complex and mass spectrometric peptide mapping

Sequences of antigenic determinants were identified by limited proteolysis of peptide antigens bound to an immobilized monoclonal antibody and direct molecular weight determination of the monoclonal antibody-bound peptide fragments by 252Cf plasma desorption mass spectrometry. The epitope peptides to the monoclonal antibody h453 [Burger, R., Zilow, G., Bader, A., Friedlein, A. & Naser, W. (1988) J. Immunol. 141, 553-558] were isolated from immobilized antigen-antibody complexes by partial trypsin digestion. A synthetic eicosapeptide comprised of the C-terminal sequence of the human complement component polypeptide des-Arg77-C3a as well as guinea pig des-Arg78-C3a was used as an antigen. Conditions were developed under which trypsin specifically degraded the antigens without inactivation of the immobilized antibody. After proteolysis, epitope peptides were dissociated from the antibody with 4 M MgCl2. The antigenic peptides were purified by HPLC and identified by 252Cf plasma desorption mass spectrometry. The epitope recognized by h453 resides on the C-terminal tryptic peptides of human (residues 70-76) and guinea pig (residues 70-77) C3a. As an estimation of accuracy this method is able to provide, trypsin digestion of immune complexes caused cleavage of the antigen within a distance of two amino acid residues upstream from the epitope.