Tethered ligand agonist peptides. Structural requirements for thrombin receptor activation reveal mechanism of proteolytic unmasking of agonist function

From peptide to non-peptide. 3. Atropisomeric GPIIbIIIa antagonists containing the 3,4-dihydro-1H-1,4-benzodiazepine-2,5-dione nucleus

The benzodiazepinedione class of non-peptidal GPIIbIIIa antagonists has been modified to allow the isolation of noninterconverting rotational isomers, or atropisomers, with the aim of examining their structure-activity relationships as compared to active RGD-containing peptides and other non-peptidal antagonists. Resolution of these antagonists was accomplished by the introduction of a tert-butyl group at N1 and a chlorine at C9 on the 3,4-dihydro-1H-1,4-benzodiazepine-2,5-dione nucleus and enantiospecific substitution on the beta-alanine side chain attached to N4. The relative configuration was determined by single-crystal X-ray analysis. Further, conformational analyses using ab initio calculations were performed to assess the conformational preferences about the beta-alanine side chain. The data support a good topographical correlation between the benzodiazepinedione class of antagonists and the "cupped" presentation of the RGD tripeptide sequence found in the cyclic peptide G4120. The relationship between these compounds with other peptidal and non-peptidal antagonists is discussed.

Thrombin receptors on human platelets. Initial localization and subsequent redistribution during platelet activation

Platelet responses to thrombin are at least partly mediated by a G-protein-coupled receptor whose NH2 terminus is a substrate for thrombin. In the present studies we have examined the location of thrombin receptors in resting platelets and followed their redistribution during platelet activation. The results reveal several new aspects of thrombin receptor biology. 1) On resting platelets, approximately two-thirds of the receptors were located in the plasma membrane. The remainder were present in the membranes of the surface connecting system. 2) When platelets were activated by ADP or a thromboxane analog, thrombin receptors that were initially in the surface connecting system were exposed on the platelet surface, increasing the number of detectable receptors by 40% and presumably making them available for subsequent activation by thrombin. 3) Platelet activation by thrombin rapidly abolished the binding of the antibodies whose epitopes are sensitive to receptor cleavage and left the platelets in a state refractory to both thrombin and the agonist peptide, SFLLRN. This was accompanied by a 60% decrease in the binding of receptor antibodies directed COOH-terminal to the cleavage site irrespective of whether the receptors were activated proteolytically by thrombin or nonproteolytically by SFLLRN. 4) The loss of antibody binding sites caused by thrombin was due in part to receptor internalization and in part to the shedding of thrombin receptors into membrane microparticles, especially under conditions in which aggregation was allowed to occur. However, at least 40% of the cleaved receptors remained on the platelet surface. 5) Lacking the ability to synthesize new receptors and lacking an intracellular reserve of preformed receptors comparable to that found in endothelial cells, platelets were unable to repopulate their surface with intact receptors following exposure to thrombin. This difference underlies the ability of endothelial cells to recover responsiveness to thrombin rapidly while platelets do not, despite the presence on both of the same receptor for thrombin.

Binding of a thrombin receptor tethered ligand analogue to human platelet thrombin receptor

A thrombin receptor-radioligand binding assay was developed using [3H]A(pF-F)R(ChA)(hR)Y-NH2 ([3H]haTRAP), a high affinity thrombin receptor-activating peptide (TRAP), and human platelet membranes. Scatchard analysis of saturation binding data indicated that [3H]haTRAP bound to platelet membranes with a Kd of 15 nM and a Bmax of 5.2 pmol/mg of protein. The binding was reduced by GPPNHP, a nonmetabolizable GTP analogue. Various TRAPs and a TRAP antagonist, but not other receptor agonists, displaced [3H]haTRAP from the binding sites. SFLLRN-NH2, a thrombin receptor-tethered ligand analogue, and [3H]haTRAP exhibited competitive binding for the same binding sites. The relative affinity of these peptides for the binding site paralleled their EC50 or IC50 values for platelet aggregation. These data indicate that [3H]haTRAP binds specifically and saturably to the functioning G protein-linked thrombin (tethered ligand) receptor in human platelet membranes.

Role of the thrombin receptor's cytoplasmic tail in intracellular trafficking. Distinct determinants for agonist-triggered versus tonic internalization and intracellular localization

The G protein-coupled thrombin receptor is activated by an irreversible proteolytic mechanism and, perhaps as a result, exhibits an unusual trafficking pattern in the cell. Naive receptors tonically cycle between the cell surface and a protected intracellular pool, whereas receptors cleaved and activated at the cell surface internalize and move to lysosomes. Toward understanding how these trafficking events are regulated, we examined a series of receptor mutants. A receptor with alanine substitutions at all potential phosphorylation sites in the cytoplasmic tail failed to display agonist-triggered internalization but, like wild type receptor, displayed robust signaling, tonic cycling, and localization to both the cell surface and an intracellular pool. A truncation mutant that lacked most of the cytoplasmic tail also signaled robustly, lacked phosphorylation, and was defective in agonist-triggered internalization. However, in contrast to the specific phosphorylation site mutant, the truncation mutant did not display tonic cycling and localized exclusively to the cell surface. An analysis of a series of truncation mutants localized residues important for receptor trafficking to a 10-amino acid stretch in its cytoplasmic tail. These data suggest that phosphorylation may trigger internalization of activated thrombin receptors but that a second phosphorylation-independent signal mediates tonic internalization of naive receptors. They further suggest that maintenance of the intracellular pool of naive thrombin receptors requires tonic receptor internalization.

Development of potent thrombin receptor antagonist peptides

A peptide-based structure-activity study is reported leading to the discovery of novel potent thrombin receptor antagonists. Systematic substitution of nonproteogenic amino acids for the second and third residues of the human thrombin receptor "tethered ligand" sequence (SFLLR) led to a series of agonists with enhanced potency. The most potent pentapeptide agonist identified was Ser-p-fluoroPhe-p-guanidinoPhe-Leu-Arg-NH2, 9 (EC50 approximately 0.04 microM for stimulation of human platelet aggregation, approximately 10-fold more potent than the natural pentapeptide). Systematic substitution of the NH2-terminal Ser in 9 with neutral hydrophobic NH2-acyl groups led to partial agonists and eventually antagonists with unprecedented potency (greater than 1000-fold increase over the previously reported antagonist 3-mercaptopropionyl-Phe-Cha-Cha-Arg-Lys-Pro-Asn-Asp-Lys-NH2). In the series of NH2-acyl tetrapeptide antagonists, N-transcinnamoyl-p-fluoroPhe-p-guanidinoPhe-Leu-Arg-NH 2, 41 (BMS-197525), was identified as the tightest binding (IC50 approximately 8 nM) and most potent with an IC50 approximately 0.20 microM for inhibition of SFLLRNP-NH2-stimulated platelet aggregation. Systematic single substitutions in 41 indicated that, in addition to the NH2-terminal acyl group, the side chains at the second and third positions were also responsible for important and specific receptor interactions. The p-fluoroPhe and p-guanidinoPhe residues in the second and third positions of 41 were observed to be optimal in both the agonist and antagonist series. In the case of antagonists, however, an appropriately positioned positively charged group (i.e., protonated base) at the third residue was required. In contrast, such a substitution was not required for potent agonist activity. An even more potent antagonist resulted when 41 was extended at the C-terminus by a single Arg residue giving rise to analog 90 (BMS-200261) which had an IC50 approximately 20 nM for inhibition of SFLLRNP-NH2-stimulated platelet aggregation. When the C-terminal Arg of 90 was replaced by an Orn-(Ndelta-propionyl) residue, the resulting antagonist 91 (BMS-200661) was suitable for use in radioligand binding assays (Kd = 10-30 nM). Antagonist activity observed for selected compounds was verified through secondary assays in that these analogs prevented SFLLRNP-NH2-stimulated GTPase activity in platelet membranes and Ca2+ mobilization in cultured human smooth muscle cells and mouse fibroblasts. Furthermore, this inhibition occurred at concentrations that had no effect on thrombin catalytic activity indicating a specific activity attributable to receptor binding and not enzyme inhibition.

The cloned thrombin receptor is necessary and sufficient for activation of mitogen-activated protein kinase and mitogenesis in mouse lung fibroblasts. Loss of responses in fibroblasts from receptor knockout mice

The mitogenic activity of thrombin on fibroblasts and smooth muscle cells may contribute to embryonic development and normal wound healing, and it may also play a role in pathological responses to vascular injury. To examine the importance of thrombin signaling in vivo and to define the cloned thrombin receptor's role, we disrupted the thrombin receptor gene (tr) in mice. Platelets from tr-/- mice responded normally to thrombin, but tr-/- fibroblasts showed no thrombin-induced calcium mobilization or phosphoinositide hydrolysis. Thus distinct thrombin receptors act in different tissues. This study focuses on the role of the thrombin receptor in thrombin-induced mitogenesis and mitogen-activated protein (MAP) kinase activation in mesenchymal cells. Thrombin and thrombin receptor agonist peptide both stimulated DNA synthesis and MAP kinase activation in fibroblasts derived from wild-type mice. These responses were selectively lost in fibroblasts from tr-/- mice. Activation of the cloned thrombin receptor is therefore necessary and sufficient for thrombin-induced mitogenesis and MAP kinase activation in mouse lung fibroblasts. The tr-/- mouse thus provides a valuable model for defining the role of thrombin-induced proliferative events in vivo. Because thrombin-induced MAP kinase activation was attributable to a single receptor expressed at natural levels, mouse lung fibroblasts presented an opportunity to define the pathways that normally mediate activation of MAP kinase by the thrombin receptor. Elimination of phorbol-sensitive protein kinase C by prolonged exposure to phorbol ester only partially inhibited MAP kinase activation by thrombin but completely blocked c-Raf kinase activation. Pertussis toxin partially inhibited MAP kinase activation by thrombin but had no significant effect on c-Raf kinase activation. Thus in mouse lung fibroblasts, one thrombin receptor utilizes two pathways for MAP kinase activation: one is protein kinase C- and c-Raf dependent, and a second is Gi-dependent and c-Raf-independent.

Ligand cross-reactivity within the protease-activated receptor family

Recently, a second member of the protease-activated receptor (PAR) family, named PAR-2, has been identified. Similar to the thrombin receptor, PAR-2 appears to be activated by proteolytic-mediated exposure of a "tethered ligand" sequence and can also be activated by the corresponding synthetic peptides. Similarities in the amino acid sequence of the receptors' tethered ligand sequences suggest that their respective agonist peptides might not be absolutely specific for their particular receptors. To test this, the receptor specificity of each agonist has been determined by measuring the responses of Xenopus oocytes expressing the thrombin receptor or PAR-2 to agonist peptides or enzymes. Thrombin receptors responded to thrombin, the human thrombin receptor-activating peptide SFLLRNP-NH2 (TRAP) (EC50 = 0.1 microM), and Xenopus TRAP, TFRIFD-NH2 (EC50 = 1 microM), but did not show any increase in calcium efflux over control levels with trypsin (50 nM) or PAR-2 agonist peptides (100 microM). Human and murine PAR-2 receptors responded comparably to human and murine PAR-2 agonist peptides (SLIGKVD and SLIGRL, respectively) (EC50 = 0.5-2.0 microM) and trypsin, but not to thrombin. PAR-2 was also found to be responsive to TRAP (EC50 = 1 microM) but was unresponsive to Xenopus TRAP (50 microM). Responses to additional peptide agonist analogs suggest that an amino-terminal serine is critical for PAR-2 agonist activity.

A binding site expressed on the surface of activated human platelets is shared by factor X and prothrombin

We have demonstrated the presence of a saturable, reversible, and Ca(2+)-dependent binding site for 125I-labeled factor X ([125I]factor X) on human platelets (16000 +/- 2000 sites per platelet, Kd = 320 +/- 40 nM, n = 12) activated with either thrombin or the thrombin receptor agonist peptide, SFLLRN-amide, but not with ADP. Bound [125I]factor X could be completely removed by the addition of a Ca2+ chelator or an excess of unlabeled factor X. Antibodies that inhibit binding of factor X to the MAC-1 integrin receptor of monocytes and those directed against human factor V, failed to disrupt [125I]factor X binding to platelets. Prothrombin, but neither factor VII, factor IX, protein C, nor protein S, was an effective competitor of [125I]factor X binding with a K1 approximately Kd. [125I]Prothrombin also binds to activated (but not unactivated) platelets in a saturable, reversible, and Ca(2+)-dependent manner (20500 +/- 1500 sites, Kd = 470 +/- 110 nM, n = 3). Annexin V potently inhibited the binding of both [125I]factor X and [125I]prothrombin (IC50 approximately 3 nM). Factor X, prothrombin, and prothrombin fragment 1 (residues 1-155) were equipotent inhibitors of [125I]prothrombin and [125I]factor X binding, whereas Gla-domain-less factor X was unable to compete with [125I]factor X for platelet binding sites. Thus, it is the Gla-domains of factor X and prothrombin that appear to contain the regions necessary for platelet binding. The results of studies utilizing artificial phospholipid surfaces have led to the hypothesis that the substrates (FX and prothrombin) for the intrinsic pathway FXase and prothrombinase complexes are bound to the phospholipid surface. The factor X/prothrombin binding site we have described on the surface of activated platelets permits the utilization of surface-bound substrates by these complexes when they are assembled on a physiologic surface.

Plasmin-platelet interaction involves cleavage of functional thrombin receptor

We tested the hypothesis that the inhibition of thrombin-induced platelet activation by plasmin is mediated via the enzymatic action of plasmin on the functional thrombin receptor. We monitored the binding of the anti-thrombin receptor antibody [anti-TR-(34-46)] to platelets; this binding is sensitive to the cleavage of the thrombin receptor at amino acid residues Arg-41 to Ser-42. Plasmin inhibited anti-TR-(34-46) binding in dose- and time-dependent manners. The inactive synthetic peptide with the amino acid sequence 40-55 of the thrombin receptor (D-FPRSFLLRNPNDKYEPF) was similarly cleaved by thrombin and plasmin to an active peptide (SFLLRNPNDKYEPF) that produced robust cytosolic Ca2+ responses. At high concentrations, plasmin itself can activate platelets. We explored this effect with the use of anti-TR-(1-160). This antibody abolished the cytosolic Ca2+ responses to thrombin and to the thrombin receptor-activating peptide SFLLRN but did not attenuate the plasmin-induced cytosolic Ca2+ response. Thus plasmin inhibits thrombin-evoked platelet activation by cleaving the thrombin receptor, but the plasmin-induced cytosolic Ca2+ response is not due to the generation of the tethered peptide of the thrombin receptor.

Endothelin and nitric oxide release modulate aortic contraction to selected thrombin receptor agonists

The contribution of endothelin-1 (ET-1) and nitric oxide (NO) release to vascular contraction induced by synthetic peptide agonists of the alpha-thrombin receptor, TRAP-14 and TRAP-6, was evaluated with the use of rings of rat aorta. TRAP-6 induced fourfold greater contraction than that induced by addition of TRAP-14. TRAP-14, but not TRAP-6, stimulation of aortic rings resulted in a rapid (in seconds) and dose-dependent increase in ET-1 levels detected in the vessel perfusate. Release of ET-1 in vessels denuded of endothelium was indistinguishable from that of intact vessels, suggesting that endothelial cells are not required for the observed ET-1 release. The contractile potency of TRAP-14 was reduced in the presence of BQ-123, a specific antagonist of the endothelin A subtype of ET receptors, whereas TRAP-14 potency was increased significantly by pretreatment with the NO synthetase inhibitor NG-nitro-L-arginine (L-NNA). The contractile potency of TRAP-6 was not altered in the presence of BQ-123 or L-NNA, suggesting that TRAP-14 but not TRAP-6 potency is modulated by ET-1 and NO release. These data indicate that TRAP-6 has limited function relative to TRAP-14 and that thrombin receptor activation is not sufficient to induce ET-1 and NO release from rat aorta.

Rat proteinase-activated receptor-2 (PAR-2): cDNA sequence and activity of receptor-derived peptides in gastric and vascular tissue

1. The biological activities of the proteinase-activated receptor number 2 (PAR-2)-derived peptides, SLIGRL (PP6) SLIGRL-NH2 (PP6-NH2) and SLIGR-NH2 (PP5-NH2) were measured in mouse and rat gastric longitudinal muscle (LM) tissue and in a rat aortic ring preparation and the actions of the PAR-2-derived peptides were compared with trypsin and with the actions of the thrombin receptor activating peptide, SFLLR-NH2 (TP5-NH2). 2. From a neonatal rat intestinal cDNA library, and from intestinal and kidney-derived cDNA, the coding region of the rat PAR-2 receptor was cloned and sequenced, thereby establishing its close sequence identity with the previously described mouse PAR-2 receptor; and this information, along with a reverse-transcriptase (RT) polymerase chain reaction (PCR) analysis of cDNA derived from gastric and aortic tissue was used to establish the concurrent presence of PAR-2 and thrombin receptor mRNA in both tissues. 3. In the mouse and rat gastric preparations, the PAR-2-derived polypeptides, PP6, PP6-HN2 and PP5-NH2 caused contractile responses that mimicked the contractile actions of low concentrations of trypsin (5 u/ml-1; 10 nM) and that were equivalent to contractions caused by TP5-NH2. 4. The cumulative exposure of the rat LM tissue to PP6-NH2 led to a desensitization of the contractile response to this polypeptide, but not to TP5-NH2 and vice versa, so as to indicate a lack of cross-desensitization between the receptors responsive to the PAR-2 and thrombin receptor-derived peptides. 5. In the rat gastric preparation, the potencies of the PAR-2-activating peptides were lower than the potency of TP5-NH2 (potency order: TP5-NH2 > > PP6-NH2 > or = PP6 > PP5-NH2); PP6 was a partial agonist in this preparation. 6. The contractile actions of PP6 and PP6-NH2 in the rat gastric preparation required the presence of extracellular calcium, were inhibited by nifedipine and were blocked by the cyclo-oxygenase inhibitor, indomethacin and by the tyrosine kinase inhibitor, genistein, but not by the kinase C inhibitor, GF109203X. The contractile responses were not blocked by atropine, chlorpheniramine, phenoxybenzamine, propranolol, ritanserin or tetrodotoxin. 7. In a precontracted rat aortic ring preparation, with an intact endothelium, all of the PAR-2-derived peptides caused a prompt relaxation response that was blocked by the nitric oxide synthase inhibitor, N omega-nitro-L-arginine-methyl ester (L-NAME) but not by D-NAME; in an endothelium-free preparation, which possessed mRNA for both the PAR-2 and thrombin receptors, the PAR-2-activating peptides caused neither a relaxation nor a contraction, in contrast with the contractile action of TP5-NH2. The relaxation response to PP6-NH2 was not blocked by atropine, chlorpheniramine, genistein, indomethacin, propranolol or ritanserin. 8. In the rat aortic preparation, the potencies of PP6, PP6-NH2 and PP5-NH2 were greater than those of the thrombin receptor activating peptide, TP5-NH2 (potency order: PP6-NH2 > or = PP6 > PP5-NH2 > TP5-NH2). 9. In the rat aortic preparation, the relaxant actions of the PAR-2-derived peptides were mimicked by trypsin, at concentrations (0.5-1 u ml-1; 1-2 nM) lower than those that can activate the thrombin receptor. 10. The bioassay data obtained with the PAR-2 peptides and with trypsin, along with the molecular cloning/RT-PCR analysis, point to the presence of functional PAR-2 receptors that can activate distinct responses in the gastric and vascular smooth muscle preparations. These responses were comparable to those resulting from thrombin receptor activation in the same tissues, so as to suggest that the receptor for the PAR-2-activating peptides may play a physiological role as far reaching as the one proposed for the thrombin receptor.

Protease activated receptors modulate aortic vascular tone

The effect of agonists of the known protease activated receptors (PAR), the thrombin and the PAR-2 receptors, on vasoactive mediator release and vascular tone were studied using rings of rat aorta. Stimulation of aortic rings with the thrombin receptor agonist, Trap-14, or the PAR-2 agonist, SLIGRL, resulted in a rapid release of nitric oxide. Trap-14 and SLIGRL-induced nitric oxide release was reduced by pre-treatment with BQ-788, an ETB endothelin receptor-specific antagonist. Consistent with a role for endothelin-1 receptor activation in Trap-14 and SLIGRL-induced nitric oxide release, endothelin-1 levels were increased significantly following 5 min treatment of aortic rings with Trap-14 or SLIGRL. Cumulative addition of Trap-14 to aortic rings denuded of endothelium resulted in dose-dependent contraction with an EC50 value of 23 +/- 5 microM, whereas SLIGRL addition failed to induce aortic contraction. These data suggest that the known protease activated receptors are functionally coupled to nitric oxide release. In addition, the thrombin receptor appears to modulate both vasodilator and contractile responses, whereas the PAR-2 receptor is linked only to vasodilation.

Thrombin receptor actions in neonatal rat ventricular myocytes

Previous studies established that thrombin stimulates phosphoinositide hydrolysis and modulates contractile function in neonatal rat ventricular myocytes. The present study further defines the signaling pathways activated by the thrombin receptor and their role in thrombin's actions in cardiac myocytes. The thrombin receptor-derived agonist peptide (TRAP, a portion of the tethered ligand created by thrombin's proteolytic activity) stimulates the rapid and transient accumulation of inositol bis- and tris-phosphates (IP2 and IP3, respectively), which is followed by the more gradual and sustained accumulation of inositol monophosphate (IP1). TRAP elicits a larger and more sustained accumulation of IP1 than does thrombin. Thrombin and TRAP also activate mitogen-activated protein kinase (MAPK) in cultured neonatal rat ventricular myocytes. Differences in the kinetics and magnitude of thrombin- and TRAP-dependent inositol phosphate (IP) accumulation are paralleled by differences in the kinetics and magnitude of thrombin- and TRAP-dependent activation of MAPK. Pretreatment with phorbol 12-myristate 13-acetate (PMA) to downregulate protein kinase C (PKC) attenuates thrombin- and TRAP-dependent activation of MAPK, although small and equivalent effects of thrombin and TRAP to stimulate MAPK persist in PMA-pretreated cells. These results support the notion that the thrombin receptor activates MAPK through PKC-dependent pathways and that the incremental activation of MAPK by TRAP over that induced by thrombin is the consequence of enhanced activation through the PKC limb of the phosphoinositide lipid pathway. TRAP also increases the beating rate of spontaneously contracting ventricular myocytes and elevates cytosolic calcium in myocytes electrically driven at a constant basic cycle length. The effects of TRAP to modulate contractile function and elevate intracellular calcium are not inhibited by tricyclodecan-9-yl-xanthogenate (D609, to block TRAP-dependent IP accumulation) or pretreatment with PMA (to downregulate PKC). The TRAP-dependent rise in intracellular calcium also is not inhibited by verapamil or removal of extracellular calcium but is markedly attenuated by depletion of sarcoplasmic reticular calcium stores by caffeine. Patch-clamp experiments demonstrate that TRAP elevates intracellular calcium in cells held at a membrane potential of -70 mV. Taken together, these results support the conclusion that the thrombin receptor modulates contractile function by mobilizing intracellular calcium through an IP3-independent mechanism and that this response does not require activation of voltage-gated ion channels.

Tethered ligand library for discovery of peptide agonists

We exploited the mechanism underlying thrombin receptor activation to develop a novel screening method to identify peptide agonists. The thrombin receptor is activated by limited proteolysis of its amino-terminal exodomain. Thrombin cleaves this domain to unmask a new amino terminus, which then functions as a tethered peptide agonist, binding intramolecularly to the body of the receptor to trigger signaling. The thrombin receptor's amino-terminal exodomain can also donate the tethered agonist intermolecularly to activate nearby thrombin receptors. We utilized this ability by co-expressing a "tethered ligand library," which displayed the thrombin receptor's amino-terminal exodomain bearing random pentapeptides in place of the native tethered ligand together with target receptors in Xenopus oocytes. Clones that conferred thrombin-dependent signaling by intermolecular ligation of the target receptor were isolated by sib selection. Agonists for the thrombin receptor itself (GFIYF) and for the formyl peptide receptor (MMWLL) were identified. Surprisingly, the latter agonist was quite active at the formyl peptide receptor even without N-formylation, and its formylated form, fMMWLL, was more potent than the classical formyl peptide receptor agonist fMLF. In addition to identifying novel peptide agonists for targets of pharmacological interest, this method might be used to discover agonists for orphan receptors. It also suggests a possible evolutionary path from peptide to protease-activated receptors.

Mechanisms of thrombin receptor agonist specificity. Chimeric receptors and complementary mutations identify an agonist recognition site

Identification of the docking interactions by which peptide agonists activate their receptors is critical for understanding signal transduction at the molecular level. The human and Xenopus thrombin receptors respond selectively to their respective hexapeptide agonists, SFLLRN and TFRIFD. A systematic analysis of human/Xenopus thrombin receptor chimeras revealed that just two human-for-Xenopus amino acid substitutions, Phe for Asn87 in the Xenopus receptor's amino-terminal exodomain and Glu for Leu260 in the second extracellular loop, conferred human receptor-like specificity to the Xenopus receptor. This observation prompted complementation studies to test the possibility that Arg5a in the human agonist peptide might normally interact with Glu260 in the human receptor. The mutant agonist peptide SFLLEN was a poor agonist at the wild type human receptor but an effective agonist at a mutant human receptor in which Glu260 was converted to Arg. An "arginine scan" of the receptor's extracellular surface revealed additional complementary mutations in the vicinity of position 260 and weak complementation at position 87 but not elsewhere in the receptor. Strikingly, a double alanine substitution that removed negative charge from the Glu260 region of the human receptor also effectively complemented the SFLLEN agonist. The functional complementation achieved with single Arg substitutions was thus due at least in part to neutralization of a negatively charged surface on the receptor and not necessarily to introduction of a new salt bridge. By contrast, charge neutralization did not account for the gain of responsiveness to SFLLRN seen in the human/Xenopus receptor chimeras. Thus two independent approaches, chimeric receptors and arginine scanning for complementary mutations, identified the Glu260 region and to a lesser degree Phe87 as important determinants of agonist specificity. These extracellular sites promote receptor responsiveness to the "correct" agonist and inhibit responsiveness to an "incorrect" agonist. They may participate directly in agonist binding or regulate agonist access to a nearby docking site.

The response to thrombin of human neutrophils: evidence for two novel receptors

Human alpha-thrombin was a chemoattractant for human neutrophils yielding a maximal response of similar magnitude to that observed with formyl-Met-Leu-Phe. The observed chemotaxis was not due to stimulation of the proteolytically activated thrombin receptor since: (1) this receptor was not detected by flow cytometry; (2) the inactive thrombin mutant Ser195-->Ala elicited a chemotactic response indistinguishable from that caused by wild-type thrombin; (3) antibodies to the cleavage site of the proteolytically activated receptor did not affect thrombin-induced chemotaxis; (4) a thrombin receptor activating peptide (TRAP) failed to stimulate chemotaxis. These data indicate the existence of a thrombin receptor for neutrophil chemotaxis which is not activated by proteolysis. In addition, although wild-type and ser195-->Ala thrombin did not cause an increase in intracellular Ca2+, a Ca2+ response to TRAP was observed with neutrophils from some donors. The TRAP-induced increase in Ca2+ was reproducible, dose dependent and specific. The use of alanine-substituted peptides demonstrated that the Ca2+ response was due to TRAP stimulation of a receptor other than the proteolytically activated thrombin receptor. Thus, it is necessary to re-evaluate the assumption made in previous studies that responses to TRAP are mediated by the proteolytically activated thrombin receptor.

Molecular cloning and functional expression of the gene encoding the human proteinase-activated receptor 2

We previously reported the molecular cloning of a mouse guanosine-nucleotide-binding-protein-coupled receptor similar to the thrombin receptor. Since the physiological agonist was unknown, the receptor was named proteinase-activated receptor 2. We describe here the cloning and functional expression of the gene encoding the corresponding human receptor. The gene is divided into two exons separated by about 14 kb intronic DNA. The deduced protein sequence is 397 amino acids long and 83% identical to the mouse receptor sequence. Within the extracellular amino terminus, the residues predicted to form the tethered agonist ligand differ between the two receptors; of the first six residues only four are conserved. At positions five and six, a lysine residue and a valine residue, respectively, have replaced arginine and leucine residues found in the mouse sequence. When the human receptor is expressed in Chinese hamster ovary cells, it can be activated by low nanomolar concentrations of the serine proteinase trypsin and by peptides made from the receptor sequence. Northern-blot analysis of receptor expression showed that the receptor transcript is widely expressed in human tissues with especially high levels in pancreas, liver, kidney, small intestine and colon. Moderate expression was detected in many organs but none in brain or skeletal muscle. By fluorescence in situ hybridization, the human proteinase-activated receptor 2 gene was mapped to chromosomal region 5q13, where, previously, the related thrombin receptor gene has been located.

Determinants of thrombin receptor cleavage. Receptor domains involved, specificity, and role of the P3 aspartate

Thrombin receptor cleavage at the Arg41- decreases -Ser42 peptide bond in the receptor's amino-terminal exodomain is necessary and sufficient for receptor activation. The rate of receptor cleavage at this site is a critical determinant of the magnitude of the cellular response to thrombin. These observations underscore the importance of defining the molecular basis for thrombin-receptor interaction and cleavage. We report that chimeric proteins bearing only thrombin receptor amino-terminal exodomain residues 36-60 are cleaved at rates similar to the wild-type thrombin receptor when expressed on the cell surface. A soluble amino-terminal exodomain protein was also cleaved efficiently by thrombin with a Km of 15-30 microM and k(cat) of approximately 50 s-1, with cleavage occurring only at the Arg41- decreases -Ser42 peptide bond. In the context of previous studies, these data suggest that the receptor's LDPR cleavage recognition sequence and DKYEPF hirudin-like domain account for thrombin-receptor interaction. Because a P3 aspartate in protein C's cleavage site inhibits cleavage by free thrombin, we investigated the role of the P3 aspartate in the receptor's LDPR sequence. Studies with mutant receptors revealed an inhibitory role for this residue only in the absence of the receptor's hirudin-like domain. These and other data suggest that the receptor's hirudin-like domain causes a conformational change in thombin's active center to accommodate the LDPR sequence and promote efficient receptor cleavage. Taken together, these studies imply that the thrombin receptor's amino-terminal exodomain contains all the machinery needed for efficient recognition and cleavage by thrombin. Thrombin appears to bind and cleave this domain independently of the rest of the receptor, with one thrombin molecule probably activating multiple receptors.

Differential activation of cytosolic phospholipase A2 (cPLA2) by thrombin and thrombin receptor agonist peptide in human platelets. Evidence for activation of cPLA2 independent of the mitogen-activated protein kinases ERK1/2

The thrombin receptor agonist peptide SFLLRN was less effective than thrombin in eliciting the liberation of arachidonic acid and the generation of thromboxane A2 by human platelets. We found that while SFLLRN evokes an initial transient increase in cystolic free calcium concentration ([Ca2+]i) of similar magnitude as that caused by thrombin, the SFLLRN-induced elevation of [Ca2+]i declines more rapidly to near resting levels than that evoked by thrombin, suggesting that disparate levels of [Ca2+]i may contribute to the attenuated arachidonic acid release. Furthermore, we observed that SFLLRN is less effective than thrombin in mediating the "activating" phosphorylation of cytolic phospholipase A2 (cPLA2). Both thrombin and SFLLRN rapidly and transiently activated kinases that phosphorylate the 21-residue synthetic peptide Thr669 derived from the epidermal growth factor receptor, but the maximal activation of proline-directed kinases by SFLLRN was less pronounced than that by thrombin. MonoQ chromatography and immunoblot analysis of extracts from stimulated platelets revealed that while thrombin induced a prominent activation of the mitogen-activated protein kinases ERK1 and ERK2, SFLLRN completely failed to do so. On the other hand, SFLLRN, like thrombin, stimulated the activity of a proline-directed kinase distinct from ERK1/2, but the activation of this kinase was less pronounced following stimulation of platelets with SFLLRN compared with thrombin. We conclude 1) that the partial activation of cPLA2 and the subsequent attenuated mobilization of arachidonic acid in response to SFLLRN may be the consequence of a less prolonged elevation of [Ca2+]i and insufficient activation of proline-directed kinase(s) by SFLLRN and 2) that the ability of SFLLRN to mediate the activating phosphorylation of cPLA2 in the absence of ERK1/2 stimulation suggest that, at least in human platelets, proline-directed kinases other than ERK1/2 may phosphorylate and activate cPLA2.

An antibody against the exosite of the cloned thrombin receptor inhibits experimental arterial thrombosis in the African green monkey

BACKGROUND

Thrombin inhibitors have been shown to be efficacious in animal models of thrombosis and in initial human clinical trials. It is unknown if their efficacy is due to their prevention of thrombin-mediated fibrin formation or to an inhibitory effect on thrombin-stimulated platelet activation. Appropriate tools to address this question have not been available. Therefore, to evaluate the role of the platelet thrombin receptor in intravascular thrombus formation, a polyclonal antibody was raised against a peptide derived from the thrombin-binding exosite region of the cloned human thrombin receptor. This antibody serves as a selective inhibitor of the thrombin receptor for in vivo evaluation.

METHODS AND RESULTS

The immune IgG (IgG 9600) inhibited thrombin-stimulated aggregation and secretion of human platelets. In contrast, it had no effect on platelet activation induced by other agonists including ADP, collagen, or the thrombin receptor-derived peptide SFLLR-NH2. IgG 9600 also inhibited thrombin-induced aggregation of African Green monkey (AGM) platelets. By Western blot analysis, the IgG identified a protein of approximately 64 kD in homogenates of both human and AGM platelets. The effect of thrombin receptor blockade by this antibody on arterial thrombosis was evaluated in an in vivo model of platelet-dependent cyclic flow reductions (CFRs) in the carotid artery of the AGM. The intravenous administration of IgG 9600 (10 mg/kg) abolished CFRs in three monkeys and reduced CFR frequency by 50% in a fourth monkey. Ex vivo platelet aggregation in response to up to 100 nmol/L thrombin was completely inhibited during the 120-minute postbolus observation period in all four animals. There was a twofold increase in bleeding time, which was not statistically different from baseline, and ex vivo clotting time (APTT) was not changed. The glycoprotein IIb/IIIa receptor antagonist MK-0852 and the thrombin inhibitor recombinant hirudin also demonstrated inhibitory effects on CFRs at doses that did not significantly prolong template bleeding time. Control IgG had no effect on CFRs, ex vivo platelet aggregation, bleeding time, or APTT.

CONCLUSIONS

These results demonstrate that blockade of the platelet thrombin receptor can prevent arterial thrombosis in this animal model without significantly altering hemostatic parameters and suggest that the thrombin receptor is an attractive antithrombotic target.

Proteolysis of the human platelet and endothelial cell thrombin receptor by neutrophil-derived cathepsin G

Cathepsin G is a neutrophil-derived protease that has been shown to inhibit the effects of thrombin on some cells expressing thrombin receptors while acting as an agonist on others. The present studies examine whether cleavage of the thrombin receptor by cathepsin G can account for these diverse effects. When added to cells that normally respond to thrombin, cathepsin G prevented a subsequent cytosolic Ca2+ increase caused by thrombin, but had no effect on responses to the thrombin receptor agonist peptide, SFLLRN. These effects were observed in cells in which cathepsin G had little or no agonist effect (human umbilical vein endothelial cells and HEL cells), as well as in cells in which cathepsin G acted as an agonist (platelets and CHRF-288 cells). Binding studies using monoclonal antibodies with defined epitopes within the first 60 residues of the thrombin receptor N-terminus showed that incubation of platelets and endothelial cells with cathepsin G abolished the binding of all of the antibodies, while thrombin abolished only the binding of antibodies whose epitopes were N-terminal to the known thrombin cleavage site between Arg41 and Ser42. Analysis of peptide proteolytic fragments identified three potential cleavage sites for cathepsin G: Arg41-Ser42, Phe43-Leu44, and Phe55-Trp56. Cleavage at Phe55-Trp56 would account for both the observed loss of all of the antibody binding sites and the inhibition of receptor activation by thrombin. Two approaches were used to determine whether a solitary cleavage at Arg41-Ser42 could result in receptor activation. In the first, HEL cells were exposed to cathepsin G or thrombin in the presence of an antibody whose epitope includes Phe55. The antibody inhibited responses to thrombin, but augmented the response to cathepsin G. In the second, COS-1 cells were transfected with variant thrombin receptors in which Phe55 and Trp56 were mutated to alanine. Transfected wild-type receptors responded to thrombin, but not cathepsin G, while the variant receptors responded to both proteases. These results 1) suggest that the ability of cathepsin G to inhibit responses to thrombin, but not SFLLRN, is due to cleavage of the receptor at Phe55-Trp56, deleting the tethered ligand domain, and 2) show that cathepsin G can activate thrombin receptors, but only if the cleavage site at Phe55-Trp56 is mutated or otherwise protected.(ABSTRACT TRUNCATED AT 400 WORDS)

The mouse proteinase-activated receptor-2 cDNA and gene. Molecular cloning and functional expression

We have reported the cloning from mouse genomic DNA of a fragment encoding a G-protein-coupled receptor related to the receptor for the blood clotting enzyme thrombin. Like the thrombin receptor this receptor is activated by proteolytic cleavage of its extracellular amino terminus. Because the physiological agonist at the receptor was unknown, we provisionally named it proteinase-activated receptor 2 (PAR-2). Here we present a PAR-2 cDNA of 2729 nucleotides that differs from the published genomic sequence at the 5' end, including a part of the protein coding region. The differences do not affect the peptide sequence of the activating proteinase cleavage site proper, but may include amino acid residues important for enzyme-substrate recognition. Analysis of the PAR-2 gene structure showed that the cDNA 5' end is derived from a separate exon located about 10 kilobases away from the 3' exon. Results from a primer extension experiment indicate that transcription starts at a unique site around nucleotide -203 respective to the translation initiation ATG. Chinese hamster ovary cells transfected with either the PAR-2 cDNA or a construct made from the published PAR-2 genomic sequence responded with intracellular calcium mobilization to stimulation with 1 nM trypsin, 10 microM PAR-2-activating peptide (SLIGRL), or 1 microM thrombin receptor-activating peptide (SFLLRN). Untransfected cells responded only to stimulation with thrombin receptor activating peptide. Only transcripts corresponding to the PAR-2 cDNA could be detected in three mouse tissues examined.

The functional thrombin receptor is associated with the plasmalemma and a large endosomal network in cultured human umbilical vein endothelial cells

The functional thrombin receptor, normally expressed by endothelial cells and platelets, is a member of the G protein-coupled, seven membrane-spanning-domain receptor family and is thought to be responsible for most, if not all, the cell stimulatory effects of thrombin. Upon binding, thrombin cleaves the receptor's N-terminal ectodomain, unmasking a new N terminus, which by itself activates the receptor. Using antibodies to different domains of the human thrombin receptor, we have localized the receptor in cultured human umbilical vein endothelial cells by indirect immunofluorescence and immunoelectron microscopy. We found the receptor expressed on the plasmalemma of cultured endothelial cells in individual units rather than in clusters, at lower concentration than, and at different sites from, thrombomodulin. We also found the receptor associated with a distinct, intracellular, transferrin receptor-containing, tubulovesicular network. The thrombin receptor-positive structure spread from the perinuclear region to the periphery of the cells, exhibiting a number of varicosities interconnected by branching tubular elements, strikingly similar to an image recently described for a continuous endosomal reticulum. Our results provide morphological evidence for the presence of the functional thrombin receptor at relative low density on the surface of cultured endothelial cells (compared to thrombomodulin) and in relatively large quantities inside the cells, associated with an endosomal compartment.

Inhibition of thrombin and SFLLR-peptide stimulation of platelet aggregation, phospholipase A2 and Na+/H+ exchange by a thrombin receptor antagonist

A thrombin receptor has been described that is activated by thrombin cleavage generating a new N-terminus. The newly exposed SFLLR-containing "tethered-ligand" then activates the receptor. In these studies, we used 3-mercapto-propionyl-Phe-Cha-Cha-Arg-Lys-Pro-Asn- Asp-Lys-amide (Mpapeptide) as a thrombin receptor antagonist. This compound was capable of preventing both thrombin- and SFLLR-peptide-induced platelet aggregation with little effect on collagen-induced platelet aggregation. It also prevented thrombin- and SFLLRNP-induced calcium mobilization with little effect on thromboxane receptor-activated platelet Ca2+ mobilization. Platelet membrane GTPase could be activated by peptides that activated the thrombin receptor, and the thrombin receptor antagonist also prevented receptor-stimulated GTPase activity. Platelet phospholipase A2 (PLA2) activity (measured as the release of radiolabeled arachidonic acid) and Na+/H+ exchange activation were stimulated by alpha-thrombin and by SFLLR-containing peptides. Activation of both processes with low concentrations of thrombin required thrombin's anion-binding exosite, as they were not activated by similar concentrations of gamma-thrombin, and the alpha- and zeta-thrombin activation was blocked by peptides mimicking the C-terminal region of hirudin. Stimulation of PLA2 and Na+/H+ exchange by both thrombin and SFLLR-containing peptides was inhibited by the thrombin receptor antagonist Mpa-peptide. These results support the hypothesis that thrombin stimulation of PLA2 activity and Na+/H+ exchange occurs via activation of the thrombin tethered-ligand receptor. Moreover, these data are consistent with the tethered-ligand receptor mediating most actions elicited by low concentrations of alpha-thrombin involved in human platelet activation.

'Tethered ligand' derived pentapeptide agonists of thrombin receptor: a study of side chain requirements for human platelet activation and GTPase stimulation

Proteolytic action of alpha-thrombin on human thrombin receptor results in cleavage of a portion of the N-terminus, thereby generating a 'tethered ligand' at the newly exposed N-terminus, which then activates the receptor in an intramolecular fashion. Agonist peptides incorporating the amino acid sequence of the newly exposed N-terminal portion of the cleaved receptor cause receptor activation without requiring prior cleavage of the receptor by thrombin. The pentapeptide amide Ser-Phe-Leu-Leu-Arg-NH2, which retains the N-terminal sequence of the 'tethered ligand' of the receptor, has been shown to be the minimum sequence to cause receptor activation. To understand the importance of the side chains of various residues within the pentapeptide amide, we carried out an extensive structure-activity study of the ability of peptides to stimulate gel-filtered platelet aggregation. In this study 106 pentapeptide amides were synthesized, utilizing naturally occurring L-amino acids, unnatural amino acids, D-amino acids and N-methyl amino acids for replacements. At position-1, charged residues (acidic or basic) were not tolerated, and the size and shape of the residue were important. Position-2 tolerated only aromatic residues. Position-3 accommodated various residues. A significant finding of this study was that two very different residues, [3-(2-naphthyl)]-L-alanine and L-arginine, when substituted for leucine residue at position-3, resulted in more active agonists. At position-4 aromatic and aliphatic residues were well tolerated, whereas basic and acidic residues were less tolerated. Position-5 mimicked position-3 in its ability to tolerate a wide range of residues.(ABSTRACT TRUNCATED AT 250 WORDS)

Thrombin, phorbol ester, and cAMP regulate thrombin receptor protein and mRNA expression by different pathways

Human mesangial cells have been used to study the regulation of thrombin receptor protein and mRNA expression during cross-talk between different signal transduction pathways. Persistent activation of thrombin receptor by thrombin led to homologous down-regulation of thrombin receptor protein. However, thrombin receptor mRNA expression was not affected, suggesting that increased receptor degradation is responsible for homologous down-regulation. Chronic activation of protein kinase C by phorbol 12-myristate 13-acetate (PMA) and of adenylylcyclase by prostaglandin E1 (PGE1) resulted in heterologous down-regulation of thrombin receptor protein. In contrast to thrombin, PMA and PGE1 reduced in parallel thrombin receptor mRNA levels to 51% and 24% of control, respectively, indicating that heterologous down-regulation of thrombin receptor protein is, at least in part, due to inhibition of receptor mRNA expression. The mechanisms of heterologous down-regulation of thrombin receptor protein have been studied in detail and compared to homologous down-regulation. PMA-induced down-regulation was completely blocked by GF 109 203 X, an inhibitor of protein kinase C. However, the loss of thrombin receptor induced by thrombin was not prevented by GF 109 203 X, indicating that homologous regulation is not dependent on protein kinase C activation. The heterologous effect of PGE1 was mimicked by 8-bromo-cAMP, isobutylmethylxanthine, and forskolin, suggesting that an increase in intracellular cAMP level is involved in heterologous regulation. Interestingly, heterologous down-regulation induced by PGE1 seems not to require previous internalization of thrombin receptor. These data indicate that thrombin receptor protein and mRNA expression can be regulated in homologous and heterologous ways by different mechanisms.

Analogues of the thrombin receptor tethered-ligand enhance mesangial cell proliferation

Thrombin stimulates cytosolic calcium mobilization and tritiated thymidine incorporation in rat glomerular mesangial cells. This effect may be mediated by a thrombin receptor similar to the receptor found in human platelets. In order to test this possibility, a series of analogues of the thrombin receptor peptide, SFLL-RNPNDKYEPF, was evaluated for their effects on mesangial cells. Analogues of the thrombin receptor peptide containing five, six, seven and 14 amino acids were as efficacious as thrombin with respect to calcium mobilization and thymidine incorporation, although they were significantly less potent. The dissimilarity in potency between thrombin and the thrombin receptor peptides is consistent with the kinetics of the proposed mechanism of action of the enzyme, since the cleavage by thrombin of its receptor results in a tethered ligand which is at a relatively high concentration compared to the free peptides in solution. Those thrombin receptor peptide analogues which showed decreased activity in platelets were tested in mesangial cells. Removal of serine at position one, N-acetylation, or replacement of the phenylalanine at position two with alanine resulted in analogues which were inactive in stimulating mesangial cell proliferation or calcium mobilization. In addition, those analogues which had no stimulatory effects in mesangial cells were not antagonists of SFLLRN-mediated calcium mobilization and thymidine incorporation in mesangial cells.

Locating ligand-binding sites in 7TM receptors by protein engineering

Over the past year, mutational analysis of peptide receptors has started to change our understanding of the interaction between G protein coupled receptors and their ligands, an area previously almost totally dominated by results from studies of monoamine receptors. A picture is currently emerging, in which small ligands appear to bind in three (more or less) overlapping ligand-binding pockets in between the transmembrane segments. In contrast, contact residues for peptide and protein ligands have mainly been found in exterior regions of peptide and protein receptors. It is also becoming increasingly clear that agonists and antagonists may interact in vastly different manners, even though they are competitive ligands for a common receptor.

Synthesis and characterization of photoactivatable peptide agonists of the human thrombin receptor

Chemical synthesis and biochemical analysis of modified agonist peptides of the human thrombin receptor derived from the sequence SFLLRNP containing photoactivatable azido groups and biotin for sensitive detection is described. Substitution of leucine in position three with p-azidophenylalanine and extension of the C-terminus with a KGGK spacer containing biotin covalently linked to the side chain of the C-terminal lysine residue resulted in an active receptor agonist as determined by intracellular Ca2+ mobilization in human erythroleukemia (HEL) cells. In contrast, substitution of phenylalanine in position two with p-azidophenylalanine reduced agonist activity significantly.

Alpha-thrombin and trypsin use different receptors to stimulate arachidonic acid metabolism

Rat liver cells (the C-9 cell line) are stimulated to metabolize arachidonic acid by alpha-thrombin, its receptor polypeptide, gamma-thrombin, and trypsin. Prostaglandin (PG) I2 synthesis stimulated by alpha-thrombin is inhibited by dansylarginine N-(3-ethyl-1,5-pentanediyl) amide (DAPA), by hirudin, by the synthetic tyrosine-sulfated dodecapeptide corresponding to residues 53-64 of hirudin (hirugen), by the Tyr(SO3H)63-hirudin fragment 54-65 and by rabbit lung thrombomodulin. Stimulation of arachidonic acid metabolism by the receptor octapeptide, SFLLRNPN, is not affected by DAPA or hirudin. gamma-Thrombin stimulates arachidonic acid metabolism but at 300 to 400-fold higher concentrations. Trypsin stimulates arachidonic acid metabolism. Trypsin's proteolytic activity is required--its ability to stimulate is abolished if it is incubated with Na-p-tosyl-L-lysine chloromethyl ketone (TLCK) or bovine pancreatic trypsin inhibitor. Prior treatment of the rat liver cells with alpha-thrombin blocks subsequent stimulation by alpha-thrombin, but not by trypsin, whereas prior treatment with trypsin blocks subsequent stimulation by trypsin, but not the activity stimulated by alpha-thrombin. Prior treatment of the cells with the serine-proteases, chymotrypsin, pancreatic or neutrophil elastase and thrombocytin from Bothrups atrox venom, block alpha-thrombin's activation of PGI2 production, but not the activity stimulated by trypsin. These findings indicate that alpha-thrombin and trypsin stimulate PGI2 production via different receptors.

Structure around the cleavage site in the thrombin receptor determined by NMR spectroscopy

NMR spectroscopic experiments were performed to study the structure of synthetic peptides identical to two extracellular regions of the human thrombin receptor. The smaller molecule, comprising 14 amino acids, was biologically active and was equivalent to the "tethered ligand" exposed after cleavage of the receptor by thrombin. The principal structural elements were two overlapping turns (amino acids 5-8 and 6-9), the second of which was stabilized by a hydrogen bond, 6CO-9NH. The five N-terminal residues, considered to be responsible for biological activity, were essentially unstructured. A second version of this peptide, biologically inactive due to the reversal of the two N-terminal amino acids, had a very similar structure. A longer peptide (23 amino acids) covering the proposed thrombin cleavage site was found to be more highly structured. The seven residues from Pro-2 to Arg5 (the N-terminal amino acid exposed after cleavage is taken as residue 1) formed a 3(10) helix which is not present in the shorter tethered ligand peptide. The structure is partially stabilized by a charged hydrogen bond between the side chains of Arg-1 and Asp-3. The overlapping turns observed in the shorter peptides could also be distinguished in the longer molecule. On the basis of the structure determined for the peptide which encompasses the cleavage site and the determined structure of thrombin, a model is postulated for the interaction of the thrombin receptor and the protease during activation.

The thrombin receptor

1. The thrombin receptor has now been cloned and found to be a member of the G-protein-coupled seven-transmembrane domain receptor family. 2. The receptor has been detected directly in platelets, endothelial cells and smooth muscle cells and studies using receptor-derived peptides have demonstrated that this receptor may be the one responsible for many of the actions of thrombin in platelets, endothelial cells, smooth muscle cells, fibroblasts, mesangial cells and neural cells. 3. The receptor appears to be activated by the novel mechanism of cleavage by thrombin to yield a new N-terminus which then interacts with the receptor as a tethered ligand to initiate cell activation.

Specificity of the thrombin receptor for agonist peptide is defined by its extracellular surface

G-protein-coupled receptors for catecholamines and some other small ligands are activated when agonists bind to the transmembrane region of the receptor. The docking interactions through which peptide agonists activate their receptors are less well characterized. The thrombin receptor is a specialized peptide receptor. It is activated by binding its tethered ligand domain, which is unmasked upon receptor cleavage by thrombin. Human and Xenopus thrombin receptor homologues are each selectively activated by the agonist peptide representing their respective tethered ligand domains. Here we identify receptor domains that confer this agonist specificity by replacing the Xenopus receptor's aminoterminal exodomain and three extracellular loops with the corresponding human structures. This switches receptor specificity from Xenopus to human. The specificity of these thrombin receptors for their respective peptide agonists is thus determined by their extracellular surfaces. Our results indicate that agonist interaction with extracellular domains is important for thrombin receptor activation.

Role of Ca2+ in the vascular contraction caused by a thrombin receptor activating peptide

Thrombin receptor activating peptide (TRAP) caused a slowly developing, sustained contraction of endothelium denuded rat aortic rings. Both nifedipine (10 microM) and removal of Ca2+ from the physiological salt solution (PSS) caused significant (60-75%) reductions in the contractile response to TRAP. In Ca(2+)-free PSS the response to both phenylephrine and TRAP were markedly reduced. Readministration of Ca2+ quickly restored the full response to phenylephrine. In contrast, readministration of Ca2+ only partially restored the TRAP response. Depletion of TRAP-sensitive intracellular Ca2+ stores had no effect on the phenylephrine response in Ca(2+)-free PSS. A threshold contracting concentration of TRAP (10 microM) enhanced contractions to the activator of voltage regulated Ca2+ channels Bay K 8644. Similarly, Bay K 8644 enhanced responses to TRAP. It is concluded that the contractile response of rat aortic rings to TRAP is largely mediated by influx of extracellular Ca2+. Furthermore, the intracellular Ca2+ pool(s) activated appears to be different from the phenylephrine-sensitive pools, which cannot be depleted by TRAP.