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The role of formylpeptide receptors, C5a receptors, and cytosolic-free calcium in neutrophil priming.

Polymorphonuclear leukocytes (PMNL) exposed to chemoattractants or cytokines change their functional capacity. The effect of endotoxin-activated serum as a priming agent on human PMNL was tested. Pretreatment of PMNL with endotoxin-activated serum increased their oxidative burst in response to formylpeptide (FMLP) (P less than .02) and C5a (P less than .05). Priming for membrane depolarization was observed in PMNL preincubated with either endotoxin-activated serum, low concentrations of purified C5a, or endotoxin but not with decomplemented plasma. Primed PMNL had an increased number of FMLP but not C5a receptors as compared with control PMNL. The "resting" cytosolic free calcium was increased in primed PMNL (P less than .02). Intracellular calcium buffering abolished the priming effect of endotoxin-activated serum. Thus, endotoxin-activated serum can prime cellular responsiveness for membrane depolarization and superoxide production in response to FMLP and to C5a. Priming may be due to an increased resting cytosolic-free calcium.

Calcium↗

Independent regulation of human neutrophil chemotactic receptors after activation.

The fluoresceinated chemotactic factors, C5a, formyl-methionyl-leucyl-phenylalanyl-lysine (FMLPL), and casein were used in conjunction with flow cytometry to examine chemotactic factor receptor expression on polymorphonuclear leukocytes (PMN) activated with phorbol myristate acetate (PMA), C5a, or formyl-methionyl-leucyl-phenylalanine. Activation with PMA resulted in a dose-dependent increase in binding of fluorescein-labeled (FL)-casein and (FL-FMLPL) over the range of PMA concentrations from 0.5 to 50 ng/ml. In contrast, activation of PMN with PMA resulted in a dose-dependent decrease in FL-C5a binding, and activation with concentrations above 5 ng/ml resulted in a complete loss of binding. This loss of binding was not caused by inactivation of the ligand or prevented by the addition of superoxide dismutase and catalase or protease inhibitors. Furthermore, incubation of PMN with supernatants from PMN stimulated to degranulate did not reduce the availability of C5a receptors. This pattern of increased FMLPL and casein binding with decreased C5a binding was also observed with cytochalasin B-pretreated PMN that were stimulated with chemotactic factors. Parallel studies of superoxide anion generation demonstrated that PMA-treated PMN were still responsive to formyl-methionyl-leucyl-phenylalanine, but not to C5a. These data demonstrate that the activation of PMN up-regulates formyl peptide and casein receptors whereas C5a receptors are down-regulated under similar conditions.

Caseins↗

Comparative structural anatomy of the complement anaphylatoxin proteins C3a, C4a and C5a.

The anaphylatoxins are a family of proteins produced during the course of complement activation as the result of cleavage by specific serine proteases. These proteins are involved in a variety of biological functions, including inflammation. Comparative modeling techniques have been used to produce structures for C4a and C5a from the crystal structure of C3a. All three structures have conserved interior residues but very different external side chains and surface shapes and properties. Comparison of the anaphylatoxin structures and of the sequence conservation among different species suggests possible locations for their receptor binding sites and for their specificity residues which permit regulated proteolytic cleavage from precursor.

Amino Acid Sequence↗

Inhibition of C5a-induced neutrophil chemotaxis and macrophage cytokine production in vitro by a new C5a receptor antagonist.

A cyclic peptide, Phe-[Orn-Pro-D-Cyclohexylalanine-Trp-Arg] (F-[OPdChaWR]), was recently shown in vitro to antagonise the binding of C5a to its receptor (CD88) on human polymorphonuclear leukocytes (PMNs) and in vivo to inhibit the neutropenia associated with septic shock induced by lipopolysaccharide (LPS) in rats. The aim of this study was to investigate whether F-[OPdChaWR] inhibits C5a-mediated chemotaxis of human PMNs using a modified Boyden chamber and C5a-stimulated release of cytokines from human monocytes in vitro. Approximately 50% of the chemotactic activity induced by 10 nM C5a was inhibited by 76 nM F-[OPdChaWR]. This correlated with inhibition of C5a-induced polarisation of PMNs by F-[OPdChaWR]. C5a alone failed to induce release of the inflammatory cytokines interleukin(IL)-1beta, tumour necrosis factor (TNF)-alpha, and IL-6 from human monocytes at concentrations up to 100 nM. However, in the presence of low concentrations of LPS (50 ng/mL), both IL-1beta and TNF-alpha were stimulated by 1 nM C5a. This co-stimulation was inhibited by F-[OPdChaWR] with IC(50)s of 0.8 and 6.9 nM for release of TNF-alpha and IL-1beta, respectively. No agonist activity was detected for F-[OPdChaWR] in either the chemotaxis or cytokine release assays at concentrations up to 50 microM. These results show that F-[OPdChaWR] inhibits several important inflammatory activities of C5a and suggest that C5a receptor antagonists may be effective in the treatment of inflammatory diseases mediated by C5a.

Antigens, CD↗

Pathophysiology of cardiopulmonary bypass: current issues.

Much of the research related to cardiopulmonary bypass in recent years has been directed toward defining the changes in plasma and blood cells during bypass. In this review, recent information is reexamined for six areas of current interest. These areas are complement activation, immune response, anaphylactic reactions, coagulation, and cerebral dysfunction. Complement may be activated by either the classical or alternate pathway during cardiopulmonary bypass and protamine administration. Membrane oxygenators appear to diminish the degree of complement activation. Complement is a major factor in the whole body inflammatory response; which often accompanies cardiopulmonary bypass. A product of complement activation, C5a- desArg, causes activation and aggregation of granulocytes. Other products of complement activation lead to lysis of blood cells including granulocytes and red cells. Bubble oxygenators appear to have a distinct disadvantage compared to membrane oxygenators regarding infection. Airborne microorganisms are more likely to be entrained into circulating blood with bubble oxygenators than with membrane oxygenators. Bubble oxygenators cause a greater decrease in leukocyte number and function than membrane oxygenators. Anaphylactic reactions have been associated with use of antibiotics, blood products, protamine, and volume expanders during cardiopulmonary bypass. Protamine reactions may be on an immunological basis or due to direct toxicity of the drug. Free radicals including superoxide, hydrogen peroxide, and the hydroxyl radical may be generated during cardiopulmonary bypass and reperfusion. Free radical scavengers including; vitamin E, coenzyme Q, vitamin C, mannitol, and glutathione have been studied. The avoidance of blood transfusion because of risk of transmitted infection including AIDS has become a major goal in cardiac surgery. Factors that correlate with increased transfusion requirement include low hematocrit, female gender, increased age, small body size, low ejection fraction, reoperation, and emergency operation. Heparin resistance due to antithrombin III deficiency is being recognized more commonly. Antithrombin III deficiency may be corrected with fresh frozen plasma. Patients with heparin induced thrombocytopenia may be difficult to manage. Several management protocols are suggested. The most straightforward appears to be the use of aspirin preoperatively and platelet transfusions postoperatively. The incidence of cerebral dysfunction after cardiopulmonary bypass depends on the sensitivity of the test or indicator used. Perioperative stroke is associated with intrinsic cerebrovascular disease and atherosclerosis of the ascending aorta. Retinal angiograms during cardiopulmonary bypass show that microemboli are very common. Cerebroplegia has been shown to extend the period of safe circulatory arrest in animals. Much of the new knowledge concerning cardiopulmonary bypass is the result of close collaboration between cardiac surgeons and nonsurgical scientists.

Anaphylaxis↗

Solution structure of a unique C5a semi-synthetic antagonist: implications in receptor binding.

The tertiary structure of a unique C5a receptor antagonist was determined by two-dimensional NMR spectroscopy. The core domain of this 8-kDa antagonist exists as an antiparallel helical bundle, similar to recombinant human (rh)-C5a. However, unlike C5a, the antagonist's C terminus was found to be conformationally restricted along a groove between helices one and four in the core domain. This conformational restriction situates C-terminal D-Arg 75 in a wedge between core residues Arg 46 and His 15. Correlation of the antagonist's tertiary structure with point mutation analysis revealed the formation of a positively charged contiguous contact surface comprised of D-Arg 75, Arg 46, Lys 49, and His 15. The significance of this surface in generating antagonist properties implies a single binding site with the C5a receptor and provides a structural template for drug design.

Amino Acid Sequence↗

Residues 21-30 within the extracellular N-terminal region of the C5a receptor represent a binding domain for the C5a anaphylatoxin.

The functions of the C5a anaphylatoxin are expressed through its interaction with a cell-surface receptor with seven transmembrane helices. The interaction of C5a with the receptor has been explained by a two-site model whereby recognition and effector sites on C5a bind, respectively, to recognition and effector domains on the receptor, leading to receptor activation (Chenoweth, D. E., and Hugli, T. E. (1980) Mol. Immunol. 17, 151-161. In addition, the extracellular N-terminal region of the C5a receptor has been implicated as the recognition domain for C5a, responsible for approximately 50% of the binding energy of the C5a-receptor complex (Mery, L., and Boulay, F. (1994) J. Biol. Chem. 269, 3457-3463; DeMartino, J. A., Van Riper, G., Siciliano, S. J., Molineaux, C. J., Konteatis, Z. D., Rosen, H., and Springer, M. S. (1994) J. Biol. Chem. 269, 14446-14450). In this work, the interactions of C5a with the N-terminal domain of the C5a receptor were examined by use of recombinant human C5a molecules and peptide fragments M1NSFN5YTTPD10YGHYD15DKDTL20DLNTP25VDKTS30NTLR(hC5aRF-1-34), acetyl-HYD15DKDTL20DLNTP25VDKTS30NTLR (hC5aRF-13-34), and acetyl-TL20DLNTP25VDKTS30N-amide (hC5aRF-19-31) derived from human C5a receptor. Binding induced resonance perturbations in the NMR spectra of the receptor fragments and the C5a molecules indicated that the isolated Nterminal domain or residues 1-34 of the C5a receptor retain specific binding to C5a and to a C5a analog devoid of the agonistic C-terminal tail in the intact C5a. Residues of C5a perturbed by the binding of the receptor peptides are localized within the helical core of the C5a structure, in agreement with the results from functional studies employing mutated C5a and intact receptor molecules. All three receptor peptides, hC5aRF-1-34, hC5aRF-13-34, and hC5aRF-19-31, responded to the binding of C5a through the 21-30 region containing either hydrophobic, polar, or positively charged residues such as Thr24, Pro25, Val26, Lys28, Thr29, and Ser30. The 21-30 segment of all three receptor fragments was found to have a partially folded conformation in solution, independent of residues 1-18. These results indicate that a short peptide sequence, or residues 21-30, of the C5a receptor N terminus may constitute the binding domain for the recognition site on C5a.

Amino Acid Sequence↗

Interference by anti-C5a monoclonal antibodies with the binding of C5a to cellular receptors and the induction of enzyme release from granulocytes.

A sandwich ELISA based on monoclonal antibodies (MAb) for the quantification of rabbit C5a is only modestly influenced by the presence of MAb 4A3A9, which reacts with an epitope on C5a different from those of MAbs used in the sandwich ELISA. Incubation of rabbit or human granulocytes with C5a in the presence of MAb 4A3A9 results in a less pronounced decrease of the C5a concentration as compared to the concentration measured in the absence of MAb. We interpret the results to indicate an interference of MAb 4A3A9 with the binding of C5a to and/or the dissociation of C5a from its cellular receptor. MAb 4A3A9 is the least effective (25.4% inhibition) among anti-C5a MAbs in inhibiting C5a-induced beta-glucuronidase release from neutrophils. Together, these findings may indicate that a C5a-MAb complex is still able to induce a C5a effect even if MAb does interfere with the binding of ligand to the receptor.

Animals↗

Urokinase-induced activation of the gp130/Tyk2/Stat3 pathway mediates a pro-inflammatory effect in human mesangial cells via expression of the anaphylatoxin C5a receptor.

Glomerular mesangial cells (MCs) are central to the pathogenesis of progressive glomeruli-associated renal diseases. However, molecular mechanisms underlying changes in MC functions still remain poorly understood. Here, we show that in MCs, the urokinase-type plasminogen activator (uPA) induces, via its specific receptor (uPAR, CD87), upregulated expression of the complement anaphylatoxin C5a receptor (C5aR, CD88), and modulates C5a-dependent functional responses. This effect is mediated via the interaction of the uPA-specific receptor (uPAR, CD87) and gp130, a signal transducing subunit of the receptor complexes for the IL-6 cytokine family. The Janus kinase Tyk2 and the transcription factor Stat3 serve as downstream components in the signaling cascade resulting in upregulation of C5aR expression. In vivo, expression of C5aR and uPAR was increased in the mesangium of wild-type mice in a lipopolysaccharide (LPS)-induced model of inflammation, whereas in uPAR(-/-) animals C5aR expression remained unchanged. This is the first demonstration in vitro and in vivo that uPA acts in MCs as a modulator of immune responses via control of immune-competent receptors. The data suggest a novel role for uPA/uPAR in glomeruli-associated renal failure via a signaling cross-talk between the fibrinolytic and immune systems.

Animals↗

The influence of Lys68 in decepeptide agonists of C5a on C5a receptor binding, activation and selectivity.

The potent, conformationally biased C5a agonist peptide YSFKPMPLaR (C5a65-74, Y65, F67, P69, P71, D-Ala73) was used as a template to gain insight into the nature and importance of lysine at position 68 in the peptide-receptor interaction. A panel of YSFKPMPLaR analogs with systematic substitutions for Lys68 was evaluated for C5a receptor (C5aR) binding affinity and activation in two well-characterized assay systems: human polymorphonuclear leukocytes (PMNs) and human fetal artery. In addition, we determined the activity of these new analogs in transfected rat basophilic leukemia (RBL) cells in which the Glu at position 199 of the C5aR (wtGlu199) was replaced by a Gln (C5aR-Gln199) or a Lys (C5aR-Lys199). Our results indicated that Lys68 in YSFKPMPLaR plays an important role in binding the C5aR expressed on PMNs and RBL cells. Furthermore, the data indicated that Lys68 interacted with Glu199 of the C5aR in PMNs and RBL cells. In human fetal artery, however, Lys68 substitutions had little or no effect on activity, which suggested that the receptor conformation may be different in this tissue. Thus, the interaction between Lys68 of the decapeptide agonist and Glu199 of the C5aR may be cell type-specific and may form the molecular basis for tissue-specific responses to C5a agonists.

Antigens, CD↗

The NH2-terminal region of C5aR but not that of FPR is critical for both protein transport and ligand binding.

The N-formylated tripeptide, formylmethionylleucyl-phenylalanine (fMLP), and the 74-amino-acid long human C5a anaphylatoxin activate phagocytic cells via two structurally related G protein-coupled receptors (FPR and C5aR), which are 34% identical in amino acid sequence. C5aR chimeras were constructed in which the entire NH2-terminal extracellular sequence or part of it was replaced by the counterparts from FPR or FPRH. Although the NH2-terminal region of C5aR presents an extremely high interspecies variability, substitution of the entire NH2-terminal sequence of C5aR by that of FPR or FPRH surprisingly resulted in chimeras that were apparently retained in the endoplasmic reticulum. In contrast, when the NH2-terminal domain of FPR was replaced by the corresponding region from C5aR or FPRH normal expression to the plasma membrane and high affinity binding of N-formylated peptides were observed. Thus, the NH2-terminal region of C5aR, in contradistinction to that of FPR, seems to be required either for the translocation of C5aR through the ER membrane or for correct folding and sorting of C5aR to the plasma membrane. Replacement of the first 8 residues of C5aR by the corresponding region of FPR did not alter the cellular transport and the C5a binding capacity, whereas the exchange of the first 13 residues resulted in a chimera that was readily transported to the plasma membrane but showed no capability to bind C5a. Mutations of Asp into Asn in the NH2-terminal segment of C5aR further indicated that negative charges are required to endow the receptor with a C5a binding capacity. The residues critical for binding are either involved directly by interacting with cationic residues of C5a, or indirectly by influencing the overall structure of the ligand-binding site.

Amino Acid Sequence↗

Circulating complement proteins in patients with sepsis or systemic inflammatory response syndrome.

The systemic inflammatory response of the body to invading microorganisms, termed sepsis, leads to profound activation of the complement system. Pathophysiological concepts suggest that complement activation occurs very early in this syndrome. Thus, we discuss whether the determination of concentrations of the complement components C3a, C5a, and C3 in plasma as well as of the C3a/C3 ratio might be helpful to diagnose sepsis early. For this purpose, 33 patients from an intensive care unit were monitored for 10 days. In comparison with healthy donors, C3a levels and the C3a/C3 ratio of intensive-care-unit patients were significantly elevated (P < 0.0001) on admission. In contrast, C3 levels were significantly reduced (P < 0.0001) but increased during the study. C5a levels in the plasma of healthy donors and patients were identical. Twenty-two of 33 patients fulfilled microbiological and clinical criteria of sepsis. Eleven patients had signs of systemic inflammatory response syndrome but no microbiological evidence of sepsis. The groups could be differentiated from each other by their C3a levels or their C3a/C3 ratios during the first 24 h after the clinical onset of sepsis (P < 0.05). Septic patients in shock had higher C3a levels than normotensive septic patients, although the differences were not significant. Nonsurvivors had significantly higher C3a levels on admission than survivors (P = 0.0185). No differences were found between septic patients who developed adult respiratory distress syndrome and those who did not. Thus, determination of C3a concentrations in plasma may prove useful (i) to diagnose sepsis early, (ii) to differentiate between patients with sepsis and those with systemic inflammatory response syndrome, and (iii) to assess prognosis.

Adolescent↗

Direct binding of a fragment of the Wiskott-Aldrich syndrome protein to the C-terminal end of the anaphylatoxin C5a receptor.

Migration of myeloid cells towards a source of chemoattractant, such as the C5a anaphylatoxin, is triggered by the activation of a G-protein-coupled receptor. In the present study, we have used a yeast two-hybrid approach to find unknown partners of the C5a receptor (C5aR). Using the cytosolic C-terminal region of C5aR as bait to screen a human leucocyte cDNA library, we identified the Wiskott-Aldrich syndrome protein (WASP) as a potential partner of C5aR. WASP is known to have an essential function in regulating actin dynamics at the cell leading edge. The interaction was detected with both the fragment of WASP containing amino acids 1-321 (WASP.321) and WASP with its actin-nucleation-promoting domain [verprolin-like, central and acidic (VCA) domain] deleted. The interaction between C5aR and the WASP.321 was supported further by an in vitro binding assay between a radiolabelled WASP.321 fragment and a receptor C-terminus glutathione S-transferase (GST) fusion protein, as well as by GST pull-down, co-immunoprecipitation and immunofluorescence experiments. In the yeast two-hybrid assay, full-length WASP showed no ability to interact with the C-terminal domain of C5aR. This is most probably due to an auto-inhibited conformation imposed by the VCA domain. In HEK-293T cells co-transfected with full-length WASP and C5aR, only a small amount of WASP was co-precipitated with the receptor. However, in the presence of the active form of the GTPase Cdc42 (Cdc42V12), which is thought to switch WASP to an active 'open conformation', the amount of WASP associated with the receptor was markedly increased. We hypothesize that a transient interaction between C5aR and WASP occurs following the stimulation of C5aR and Cdc42 activation. This might be one mechanism by which WASP is targeted to the plasma membrane and by which actin assembly is spatially controlled in cells moving in a gradient of C5a.

Actins↗

Involvement of complement in psoriasis and atopic dermatitis--measurement of C3a and C5a, C3, C4 and C1 inactivator.

Normal complement components and activation products were determined in the peripheral blood of 35 patients with atopic dermatitis (AD) and 24 patients with psoriasis at a mild to intermediate stage. None of the patients had received systemic or local steroid therapy 6 weeks prior to blood collection. Levels of C3, C4 and C1 inactivator (C1 INA) were determined in serum by radial immunodiffusion, whereas C3a and C5a levels were measured by radioimmunoassay. In comparison to healthy non-atopic controls, the levels of C3, C4 and C1 INA were found to be significantly increased in both diseases. No substantial differences were detected between patients with psoriasis vulgaris and psoriasis guttata, which suggests that the dissimilarities found were not due to preceding or concomitant infections. In AD, there was a tendency towards increased C3a levels, whereas in psoriasis, C3a levels were significantly increased. In both diseases, no measurable amounts of C5a could be detected. The results indicate that, in both AD and psoriasis, the complement participates in the inflammatory process. Elevated levels of C3a suggest that there is a continuous activation of the complement system leading to the generation of inflammatory mediators.

Adolescent↗

Structure-function studies in a series of carboxyl-terminal octapeptide analogues of anaphylatoxin C5a.

The synthesis and structure-activity relationships of C-terminal octapeptide analogues of anaphylatoxin C5a have been studied. The introduction of hydrophobic amino acids into the N-acetylated native octapeptide (N-Ac-His-Lys-Asp-Met-Gln-Leu-Gly-Arg-OH) (1) has led to an analogue with 100 times more activity than the native octapeptide in inhibiting the binding of 125I-labeled anaphylatoxin C5a to human neutrophil membrane receptors. The observed apparent binding Ki's for the compounds (8-10) are in the range of 1-3 microM, and they possess nearly full agonist activity, despite the fact that these analogues are one-eighth or -ninth the size of the natural ligand anaphylatoxin C5a.

Amino Acid Sequence↗

Biologically active conformer of the effector region of human C5a and modulatory effects of N-terminal receptor binding determinants on activity.

A conformationally biased decapeptide agonist of human C5a (C5a55-74Y65,F67,P69,P71,D-Ala73 or YSFKPMPLaR) was used as a functional probe of the C5a receptor (C5aR) in order to understand the conformational features in the C-terminal effector region of C5a that are important for C5aR binding and signal transduction. YSFKPMPLaR was a potent, full agonist of C5a, but at higher concentrations had a superefficacious effect compared to the natural factor. The maximal efficacy of this analogue was 216 +/- 56% that of C5a in stimulating the release of beta-glucuronidase from human neutrophils. C5aR activation and binding curves both occurred in the same concentration range with YSFKPMPLaR, characteristics not observed with natural C5a or more conformationally flexible C-terminal agonists. YSFKPMPLaR was then used as a C-terminal effector template onto which was synthesized various C5aR binding determinants from the N-terminal core domain of the natural factor. In general, the presence of N-terminal binding determinants had little effect on either potency or binding affinity when the C-terminal effector region was presented to the C5aR in this biologically active conformation. However, one peptide, C5a12-20-Ahx-YSFKPMPLaR, expressed a 100-fold increase in affinity for the neutrophil C5aR and a 6-fold increase in potency relative to YSFKPMPLaR. These analyses showed that the peptides used in this study have up to 25% of the potency of C5a in human fetal artery and up to 5% of the activity of C5a in the PMN enzyme release assay.

Antigens, CD↗

Structure and function of human C5a anaphylatoxin. Selective modification of tyrosine 23 alters biological activity but not antigenicity.

Reaction of either human C5a or its des-Arg74 derivative (des-Arg74-C5a) with tetranitromethane under nondenaturing conditions results in selective nitration of only 1 of the 2 tyrosine residues found in these glycopolypeptides. This reactive tyrosyl residue was identified as that found in position 23 of the sequence. Nitrotyrosyl23-C5a and -des-Arg74-C5a were separated from their respective unmodified precursors by cation-exchange fast protein liquid chromatography. These purified derivatives served as reagents for the subsequent preparation of both aminotyrosyl23-C5a and -des-Arg74-C5a as well as photoreactive analogs of C5a. Radioimmunoassays performed with C5a derivatives serving as competing ligands and a murine antihuman C5a monoclonal antibody employed as first antibody demonstrated that selective modification of tyrosine23 did not produce a detectible alteration in the antigenic properties of C5a. By contrast, either nitro- or aminotyrosyl23-C5a was approximately 3-fold less active than native C5a in both bioassays and competitive ligand-receptor binding assays. Additionally, photoreactive derivatives prepared by coupling either N-succinimidyl-6-(4'-azido-2'-nitrophenylamino)-hexanoate or p-nitrophenyl-2-diazo-3,3,3-trifluoropropionate to aminotyrosyl23-C5a at pH 5.0 failed to interact with the neutrophil C5a receptor. These observations suggest that the tyrosyl23 residue of C5a may participate importantly in the binding interactions of this chemotactic factor and its granulocyte receptor.

Affinity Labels↗