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

The potent, conformationally biased C5a agonist peptide YSFKPMPLaR (C5a(65-74), 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 Gin (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.

Differential activities of decapeptide agonists of human C5a: the conformational effects of backbone N-methylation

Analogues of the potent, conformationally biased, decapeptide agonist of human C5a anaphylatoxin, C5a(65-74)Y65,F67,P69,P71,D-Ala73 (YSFKPMPLaR, peptide 54), were synthesized with methyl groups occupying specific C5a,, amide nitrogen atoms along the peptide backbone. This N-methylation induced crucial extended backbone conformations in a manner similar to the two Pro residues, but without eliminating the contributions made by the side-chain of the residue for which Pro was substituted. The presence of backbone N-methyl groups on peptide 54 analogues had pronounced detrimental effects on the ability to bind and activate C5aRs expressed on human PMNs, but not on the ability to contract smooth muscle of human umbilical artery. Several N-methylated analogues of peptide 54 (peptides 56, 67, 124, 125, and 137) were significantly more selective for smooth muscle contraction, which is mediated by tissue resident macrophages, than for enzyme release from PMNs. Indeed, peptide 67, YSFKDMP(MeL)aR was almost 3000-fold more selective for smooth muscle contraction than for PMN enzyme release. Consistent with these differential activities was the observation that peptide 67 expressed a significantly greater binding affinity to C5aRs expressed on rat macrophages than on rat PMNs. This differential activity was also observed in vivo in the rat where peptide 67 induced a hypotensive response similar to peptide 54 and rhuC5a, but without accompanying neutropenia. (C) 2001 Elsevier Science B.V. All rights reserved.

Development of response-selective agonists of human C5a anaphylatoxin: Conformational, biological, and therapeutic considerations

Numerous studies on the relationship between the structure and function of peptide agonists derived from the biologically active, C-terminal region of human C5a anaphylatoxin have been reported over the past decade. These studies have been performed with the objective of parlaying this structure-function information into the design of peptide/peptidomimetic modulators of C5a receptor (C5aR)-mediated function. In this review, we describe a rational approach for the development of conformationally biased, decapeptide agonists of C5a and described how these stabilized and specific conformational features relate to the expression of specific C5a-like activities in vitro and in vivo. The therapeutic potential of such response-selective C5a agonists is discussed and underscored by the results of one such response-selective C5a agonist that was used in vivo as an effective molecular adjuvant capable of generating antigen-specific humoral and cellular immune responses. Finally, we describe the synthesis of a new generation of highly response-selective, conformationally biased C5a agonist and discuss the in vitro and in vivo biologic results that so indicate this biologic selectivity.

Small molecular probes for G-protein-coupled C5a receptors: Conformationally constrained antagonists derived from the C terminus of the human plasma protein C5a

Activation of the human complement system of plasma proteins in response to infection or injury produces a 4-helix bundle glycoprotein (74 amino acids) known as C5a. C5a binds to G-protein-coupled receptors on cell surfaces triggering receptor-ligand internalization, signal transduction, and powerful inflammatory responses. Since excessive levels of C5a are associated with autoimmune and chronic inflammatory disorders, inhibitors of receptor activation may have therapeutic potential. We now report solution structures and receptor-binding and antagonist activities for some of the first small molecule antagonists of C5a derived from its hexapeptide C terminus. The antagonist NMe-Phe-Lys-Pro-D-Cha-Trp-D-Arg-CO2H (1) surprisingly shows an unusually well-defined solution structure as determined by H-1 NMR spectroscopy. This is one of the smallest acyclic peptides found to possess a defined solution conformation, which can be explained by the constraining role of intramolecular hydrogen bonding. NOE and coupling constant data, slow deuterium exchange, and a low dependence on temperature for the chemical shift of the D-Cha-NH strongly indicate an inverse gamma turn stabilized by a D-Cha-NH ... OC-Lys hydrogen bond. Smaller conformational populations are associated with a hydrogen bond between Trp-NH ... OC-Lys, defining a type II beta turn distorted by the inverse gamma turn incorporated within it. An excellent correlation between receptor-affinity and antagonist activity is indicated for a limited set of synthetic peptides. Conversion of the C-terminal carboxylate of 1 to an amide decreases antagonist potency 5-fold, but potency is increased up to 10-fold over 1 if the amide bond is made between the C-terminal carboxylate and a Lys/Orn side chain to form a cyclic analogue. The solution structure of cycle 6 also shows gamma and beta turns; however, the latter occurs in a different position, and there are clear conformational changes in 6 vs 1 that result in enhanced activity. These results indicate that potent C5a antagonists can be developed by targeting site 2 alone of the C5a receptor and define a novel pharmacophore for developing powerful receptor probes or drug candidates.

Low-molecular-weight peptidic and cyclic antagonists of the receptor for the complement factor C5a

Activation of the human complement system of plasma proteins during immunological host defense can result in overproduction of potent proinflammatory peptides such as the anaphylatoxin C5a. Excessive levels of C5a are associated with numerous immunoinflammatory diseases, but there is as yet no clinically available antagonist to regulate the effects of C5a. We now describe a series of small molecules derived from the C-terminus of C5a, some of which are the most potent low-molecular-weight C5a receptor antagonists reported to date for the human polymorphonuclear leukocyte (PMN) C5a receptor. H-1 NMR spectroscopy was used to determine solution structures for two cyclic antagonists and to indicate that antagonism is related to a turn conformation, which can be stabilized in cyclic molecules that are preorganized for receptor binding. While several cyclic derivatives were of similar antagonistic potency, the most potent antagonist was a hexapeptide-derived macrocycle AcF[OPdChaWR] with an IC50 = 20 nM against a maximal concentration of C5a (100 nM) on intact human PMNs. Such potent C5a antagonists may be useful probes to investigate the role of C5a in host defenses and to develop therapeutic agents for the treatment of many currently intractable inflammatory conditions.

Induction of an antigen-specific CTL response by a conformationally biased agonist of human C5a anaphylatoxin as a molecular adjuvant

A conformationally biased decapeptide agonist of human C5a anaphylatoxin (YSPKPMPLaR) was used as a molecular adjuvant in stimulating an Ag-specific CTL response against murine P815S target cells expressing an Ld-restricted CTL epitope of the hepatitis B surface Ag (HBsAg), Groups of BALB/c mice (H-2(d)) were immunized with aqueous solutions of the HBsAg CTL epitopes (IPQSLDSWWTSL and IPQSLDSTaVTSLRR); the C5a agonist (YSFKPMPLaR); the C5a agonist and HBsAg CTL epitopes admired (IPQSLDSWWTSL and IPQSLDSWWTSLRR + YSFKPMPLaR); the C5a-active, HBsAg CTL epitope-C5a agonist constructs (IPQSLDSWWTSLYSFKPMPLaR, IPQSLDSWWTSLRRYSFKPMPLaR, and IPQSLDSWWTSLRVRRYSFPMPLaR); a C5a-inactive, reverse-moiety construct (YSFKPMPLaRRRIPQSLDSWWTSL); and a C5a-attenuated, carboxyl-terminal-blocked construct (IPQSLDSWWTSLRRYSFKPMPLaRG). Ag-specific CD8(+) CTL responses were observed after the secondary boost in the absence of any added adjuvant only in mice that were immunized with C5a-active contructs, IPQSLDSWWTSLRRYSFKPMPLaR and IPQSLDSWWTSLRVRRYSFKPMPLaR. These two C5a-active immunogens contained potential subtilisin-sensitive linker sequences between the HBsAg CTL epitope and the C5a agonist; i.e., a double-Arg (RR) and a furin protease sensitive sequence (RVRR), The introduction of these potentially cleavable sequences may be a method of increasing the likelihood of liberating the CTL epitope from the C5a agonist by intracellular proteases, thereby facilitating entry of the epitope into Ag-processing pathways via an exogenous route.

Molecular adjuvant effects of a conformationally biased agonist elf human C5a anaphylatoxin

A conformationally biased decapeptide agonist of human C5a anaphylatoxin (YSFKPMPLaR) was used as a molecular adjuvant in stimulating Ab responses against peptide epitopes derived from human MUC1 glycoprotein and the human mu and kappa opioid receptors. C57BL6 mice were immunized with the MUC1 epitope (YKQGGFLGL); the C5a agonist (YSFKPMPLaR); YSFKPMPLaR and YKQGGFLGL together, but unconjugated; a C5a-active, MUC1 epitope construct (YKQGGFLGLYSFKPMPLaR); and a C5a-inactive, reversed moiety construct (YSFKPMPLaRYKQGGFLGL). High Ab titers specific for the MUC1 epitope were observed Only in mice immunized with the C5a-active epitope construct. Similar results were obtained in BALB/c mice immunized with the C5a-active, MUC1 epitope construct, Abs from the sera of the C57BL6 mice were predominately of the IgG2a, IgC2b, and IgM isotypes and were reactive against human recombinant MUC1 and MUC1 expressed by the Panc-1 M1F.15 pancreatic cell line, When compared with the corresponding KLH-epitope conjugates in C57BL6 mice, the epitope-C5a agonist constructs produced titers of specific IgG Abs of isotypes distinct from those generated by the keyhole limpet hemocyanin-epitope conjugates, Rabbits immunized with a mu opioid receptor epitope-C5a agonist construct (GDLSDPCGNRTNLGGRDSLYSFKPMPLaR) or a kappa opioid receptor epitope-C5a agonist construct (FPGWAEPDSNGSEDAQLYSFKPMPLaR) generated high titer, epitope-specific Ab responses, Ab titers generated in response to the opioid epitope-C5a agonist constructs were comparable to those generated by the opioid KLH-epitope conjugates, The results of this study are discussed in terms of possible mechanisms by which the conformationally biased C5a agonist serves as a molecular adjuvant.

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 (C5a(65-74)Y65,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, C5a(12-20)-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.

Potent cyclic antagonists of the complement C5a receptor on human polymorphonulcear leukocytes. Relationships between structures and activity

Human C5a is a plasma protein with potent chemoattractant and pro-inflammatory properties, and its overexpression correlates with severity of inflammatory diseases. C5a binds to its G protein-coupled receptor (C5aR) on polymorphonuclear leukocytes (PMNLs) through a high-affinity helical bundle and a low-affinity C terminus, the latter being solely responsible for receptor activation. Potent and selective C5a antagonists are predicted to be effective anti-inflammatory drugs, but no pharmacophore for small molecule antagonists has yet been developed, and it would significantly aid drug design. We have hypothesized that a turn conformation is important for activity of the C terminus of C5a and herein report small cyclic peptides that are stable turn mimics with potent antagonism at C5aR on human PMNLs. A comparison of solution structures for the C terminus of C5a, small acyclic peptide ligands, and cyclic antagonists supports the importance of a turn for receptor binding. Competition between a cyclic antagonist and either C5a or an acyclic agonist for C5aR on PMNLs supports a common or overlapping binding site on the C5aR. Structure-activity relationships for 60 cyclic analogs were evaluated by competitive radioligand binding with C5a (affinity) and myeloperoxidase release (antagonist potency) from human PMNLs, with 20 compounds having high antagonist potencies (IC50, 20 nM(-1) muM). Computer modeling comparisons reveal that potent antagonists share a common cyclic backbone shape, with affinity-determining side chains of defined volume projecting from the cyclic scaffold. These results define a new pharmacophore for C5a antagonist development and advance our understanding of ligand recognition and receptor activation of this G protein-coupled receptor.

Brain nitric oxide production by a proline-rich decapeptide from Bothrops jararaca venom improves baroreflex sensitivity of spontaneously hypertensive rats

Baroreflex sensitivity is disturbed in many people with cardiovascular diseases such as hypertension. Brain deficiency of nitric oxide (NO), which is synthesized by NO synthase (NOS) in the citrulline-NO cycle (with argininosuccinate synthase (ASS) activity being the rate-limiting step), contributes to impaired baroreflex. We recently showed that a decapeptide isolated from Bothrops jararaca snake venom, denoted Bj-PRO-10c, exerts powerful and sustained antihypertensive activity. Bj-PRO-10c promoted vasodilatation dependent on the positive modulation of ASS activity and NO production in the endothelium, and also acted on the central nervous system, inducing the release of GABA and glutamate, two important neurotransmitters in the regulation of autonomic systems. We evaluated baroreflex function using the regression line obtained by the best-fit points of measured heart rate (HR) and mean arterial pressure (MAP) data from spontaneously hypertensive rats (SHRs) treated with Bj-PRO-10c. We also investigated molecular mechanisms involved in this effect, both in vitro and in vivo. Bj-PRO-10c mediated an increase in baroreflex sensitivity and a decrease in MAP and HR. The effects exerted by the peptide include an increase in the gene expression of endothelial NOS and ASS. Bj-PRO-10c-induced NO production depended on intracellular calcium fluxes and the activation of a G(i/o)-protein-coupled metabotropic receptor. Bj-PRO-10c induced NO production and the gene expression of ASS and endothelial NOS in the brains of SHRs, thereby improving baroreflex sensitivity. Bj-PRO-10c may reveal novel approaches for treating diseases with impaired baroreflex function. Hypertension Research (2010) 33, 1283-1288; doi: 10.1038/hr.2010.208

The contribution of classical (beta(1/2)-) and atypical beta-adrenoceptors to the stimulation of human white adipocyte lipolysis and right atrial appendage contraction by novel beta(3)-adrenoceptor agonists of differing selectivities

The role of beta(3)- and other putative atypical beta-adrenaceptors in human white adipocytes and right atrial appendage has been investigated using CGP 12177 and novel phenylethanolamine and aryloxypropanolamine beta(3)-adrenoceptor (beta(3)AR) agonists with varying intrinsic activities and selectivities for human cloned PAR subtypes. The ability to demonstrate beta(1/2)AR antagonist-insensitive (beta(3) or other atypical beta AR-mediated) responses to CGP 12177 was critically dependent on the albumin batch used to prepare and incubate the adipocytes. Four aryloxypropanolamine selective beta(3)AR agonists (SB-226552, SB-229432, SB-236923, SB-246982) consistently elicited beta(1/2)AR antagonist-insensitive lipolysis. However, a phenylethanolamine (SB-220646) that was a selective full beta(3)AR agonist elicited full lipolytic and inotropic responses that were sensitive to beta(1/2)AR antagonism, despite it having very low efficacies at cloned beta(1)- and beta(2)ARs. A component of the response to another phenylethanolamine selective beta(3)AR agonist (SB-215691) was insensitive to beta(1/2)AR antagonism in some experiments. Because novel aryloxypropanolamine had a beta(1/2)AR antagonist-insensitive inotropic effect, these results establish more firmly that beta(3)ARs mediate lipolysis in human white adipocytes, and suggest that putative 'beta(4)ARs' mediate inotropic responses to CGP 12177. The results also illustrate the difficulty of predicting from studies on cloned beta ARs which beta ARs will mediate responses to agonists in tissues that have a high number of beta(1)- and beta(2)ARs or a low number of beta(3)ARs.

Dual C3a/C5a receptor agonism by a C5a C-terminal analogue decapeptide

No abstract

Mucociliary clearance in patients with chronic asthma: Effects of beta(2) agonists

Chronic asthma is characterized by airway inflammation, mucus hypersecretion and impaired mucociliary clearance (MCC). We investigated baseline MCC and the acute effect of terbutaline in chronic asthmatics with sputum production while on long-term treatment with salmeterol in combination with inhaled corticosteroids (ICS). MCC was measured at baseline and in response to 1 mg terbutaline (or placebo) on three visits over 80 min in 16 asthmatics (52 +/- 13 years of age). Subjects who had greater than 10% absolute increase in MCC above baseline and placebo, after terbutaline, were categorized in group A and subjects who had less than 10% in group B. In group A subjects (n = 6), MCC increased from 23.7 +/- 4.0% at baseline to 43.7 +/- 4.9% with terbutaline (P < 0.0001) and to 34.4 +/- 5.7% with placebo (P < 0.01). In group B subjects (n = 10), MCC remained similar: 11.3 +/- 3.2% at initial baseline, 12.0 +/- 3.2% with terbutaline and 7.3 +/- 3.0% with placebo (P > 0.05). Group B subjects withdrew from all beta(2) agonists for a week and MCC was remeasured. After withdrawal, baseline MCC (7.0 +/- 1.8%) was similar to the initial baseline value (P > 0.1) and MCC with terbutaline (15.8 +/- 4.9%) was greater than baseline (P < 0.005) but remained abnormal in most subjects. Baseline percentage predicted FEV1 and FEF25-75% were 77.3 +/- 7.2 and 41.7 +/- 5.6 in group A and 59.9 +/- 8.1 and 29.5 +/- 8.4 in group B subjects, respectively. MCC was impaired in most of these asthmatics with persistent airway obstruction and sputum production, despite regular treatment with ICS and salmeterol. In addition, there was little or no stimulation of MCC acutely after terbutaline in most of these asthmatics.

Proposal for the interaction of non-conventional partial agonists and catecholamines with the 'putative beta(4)-adrenoceptor' in mammalian heart

1. Evidence for a 'putative beta(4)-adrenoceptor' originated over 20 years ago when cardiostimulant effects were observed to nonconventional partial agonists, These agonists were originally described as beta(1)- and beta(2)-adrenoceptor antagonists; however, they cause cardiostimulant effects at much higher concentrations than those required to block beta(1)- and beta(2)-adrenoceptors. Cardiostimulant effects of non-conventional partial agonists have been observed in mouse, rat, guinea-pig, cat, ferret and human heart tissues, 2. The receptor is expressed in several heart regions, including the sinoatrial node, atrium and ventricle, 3. The receptor is resistant to blockade by most antagonists that possess high affinity for beta(1)- and beta(2)- adrenoceptors, but is blocked with moderate affinity by (-)-bupranolol and CGP 20712A. 4. The receptor is pharmacologically distinct from the beta(3)-adrenoceptor. Micromolar concentrations of beta(3)-adrenoceptor agonists have no agonist or blocking activity, The receptor is also resistant to blockade by a beta(3)-adrenoceptor-selective antagonist. 5. The receptor mediates increases in cAMP levels and cAMP-dependent protein kinase (PK) A activity in cardiac tissues. Phosphodiesterase inhibition potentiates the positive chronotropic and inotropic effects of non-conventional partial agonists. 6. The receptor mediates hastening of atrial and ventricular relaxation, which is consistent with involvement of a cAMP-dependent pathway. 7. The non-conventional partial agonist (-)-[H-3]-CGP 12177A labels the cardiac putative beta(4)-adrenoceptor, Non-conventional partial agonists compete for binding with affinities that are closely similar to their agonist potencies, Catecholamines compete for binding in a stereoselective manner with a rank order of affinity of (-)-R0363 > (-)-isoprenaline > (-)-noradrenaline greater than or equal to (-)-adrenaline much greater than (-)-isoprenaline, suggesting that catecholamines can interact with the receptor. 8. The putative beta(4)-adrenoceptor appears to be coupled to the G(s)-adenylyl cyclase system, which could serve as a guide to its future cloning, Activation of the receptor may plausibly improve diastolic function but could also mediate arrhythmias.