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Thrombin is the most potent agonist of platelet activation, and its effects are predominantly mediated by platelet thrombin receptors. Therefore, antagonists of the thrombin receptor have potential utility for the treatment of thrombotic disorders. Screening of combinatorial libraries revealed 2 to be a potent antagonist of the thrombin receptor. Modifications of this structure produced 11k, which inhibits thrombin receptor stimulated secretion and aggregation of platelets.

The N-terminal thrombin receptor peptide H-Ser-Phe-Leu-Leu-Arg-Asn-Pro-Asn-Asp-Lys-Tyr-Glu-Pro-Phe-OH (1) fully activates the thrombin receptor with an EC50 of 10 microM. Structural features in the tetradecapeptide which are responsible for receptor activation have been elucidated. Agonist potency has been enhanced 1000-fold with the design of the shortened peptide H-Ala-Phe(p-F)-Arg-Cha-HArg-Tyr-NH2 (56). This analog exhibits an EC50 of 0.01 microM and is the most potent agonist for receptor activation reported to date. The monoiodinated derivative H-Ala-Phe(p-F)-Arg-Cha-HArg-Tyr(3-I)-NH2 (59) exhibits an EC50 of 0.03 microM, a level sufficient for development of a radioligand.

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.

The aggregation of platelets from a variety of animal species in response to thrombin receptor-derived activating peptides was evaluated. A series of 14-(SFLLRNPNDKYEPF), 7-(SFLLRNP-NH2), 6-(SFLLRN-HN2) or 5-(SFLLR-NH2) residue peptides, the structures of which were based on the deduced amino acid sequence of the human thrombin receptor, promoted full aggregation of platelets in plasma from humans, African Green and Rhesus monkeys, baboons and guinea pigs at 4-50 microM depending on the peptide used. Platelets in plasma from rabbit, dog, pig, and hamster underwent a shape change but failed to aggregate in response to these peptides over 3 log units of peptide up to 800 microM, despite being fully responsive to human thrombin. However, because the receptor peptides induced shape change in the platelets from these non-aggregating species, they apparently can activate some of the intracellular signaling system(s) usually initiated by thrombin in these platelets. In contrast, platelets from rats did not undergo shape change or aggregate in response to the peptides. A 7-residue receptor-derived peptide based on the deduced amino acid sequence of the clone of the hamster thrombin receptor (SFFLRNP-N2) was nearly as efficacious as the corresponding human receptor-derived 7-residue peptide to promote aggregation of human platelets. However, the hamster peptide could not promote aggregation of hamster platelets in plasma at up to 800 microM peptide, while a shape change response was elicited. Platelets from rats, rabbits and pigs also did not aggregate in response to this peptide derived from the hamster thrombin receptor, but all species except the rat underwent a shape change.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro transcripts of vesicular stomatitis virus (VSV) were either 5'-terminally labeled by incorporation of [beta-(32)P]GTP or were selected on Hg-agarose after incorporation of gamma-thio-GTP. Capped RNAs ranged in size from 23 nucleotides, the shortest capped RNA detected, to full-length message size. The 5'-terminal sequences corresponded to those of N message and to a small amount of NS message. Approximately 14% of the capped N gene transcripts were terminated at positions 86 to 90 of the VSV genome, giving rise to specific, 36 to 40-nucleotide-long, capped RNA species. The GTP-initiated RNAs were short with a predominant 28-nucleotide-long RNA species. A minor portion was as large as mRNAs. Nucleotide sequence analyses of the short RNA revealed that it was specifically initiated at positon 91 of the VSV genome, 41 nucleotides within the N cistron. This corresponds exactly to the site where transcription of the 40-nucleotide-long, capped RNA terminated. Initiation with GTP at position 91 occurred at approximately the same frequency as termination of the capped RNA at position 90, suggesting that intracistronic initiation at position 91 may depend upon termination of transcription of the 5'-proximal region and therefore may be sequential. This unique RNA represents the first transcript of VSV which was initiated at an intracistronic site with GTP, and may also represent the first example of a transcript derived from a stop/start mechanism of VSV transcription in vitro. Although initiation occurred frequently at the beginning of the N cistron yielding 11 to 14-nucleotide-long, [beta-(32)P]ATP-labeled transcripts (D. F. Pinney and S. U. Emerson, J. Virol. 42:889-896, 1982), capping of these short RNAs was not detected. This suggests that transcripts may have to be 15 to 23 nucleotides long to be accepted as substrates by the guanyltransferase.

The present work reports the discovery and characterization of an anticoagulant protein in the salivary gland of the giant bloodsucking leech, H. ghilianii, which is a specific and potent inhibitor of coagulation factor Xa. The inhibitor, purified to homogeneity, displayed subnanomolar inhibition of bovine factor Xa and had a molecular weight of approximately 15,000 as deduced by denaturing SDS-PAGE. The amino acid sequence of the first 43 residues of the H. ghilianii derived inhibitor displayed a striking homology to antistasin, the recently described subnanomolar inhibitor of factor Xa isolated from the Mexican leech, H. officinalis. Antisera prepared to antistasin cross-reacted with the H. ghilianii protein in Western Blot analysis. These data indicate that the giant Amazonian leech, H. ghilianii, and the smaller Mexican leech, H. officinalis, have similar proteins which disrupt the normal hemostatic clotting mechanisms in their mammalian host's blood.