The proteinase activated receptor-2 (PAR-2) mediates mitogenic responses in human vascular endothelial cells.

Proteolytically cleaved receptors, typified by the functional thrombin receptor (TR), represent a novel class of receptors that mediate signaling events by functional coupling to G proteins. Northern blot analysis completed with a human proteinase activated receptor-2 (PAR-2) cDNA as probe demonstrated the approximately 3.5kb PAR-2 transcript in total cellular RNA from human umbilical vein endothelial cells (HUVEC). Microspectrofluorimetry using Fura2-loaded HUVEC demonstrated a dose-dependent elevation in intracellular calcium transients ([Ca2+]i) to murine PAR39-44 (SLIGRL, putative neoligand after cleavage), with an approximate EC50 of 30 microM, and evidence for homologous desensitization with complete recovery at 45 min. Xenopus oocytes microinjected with TR cRNA failed to respond to 200 microM PAR39-44, and TR-targeted antisense oligonucleotides specifically abrogated thrombin-induced but not PAR39-44-mediated [Ca2+]i, excluding the possibility that TR/PAR-2 cell-surface coexpression was structurally linked. HUVEC incubated with PAR39-44 demonstrated a dose- and time-dependent mitogenic response similar to that seen with thrombin or TR42-47 (TR-activating peptide, SFLLRN). Preactivation of HUVEC with either PAR39-44 or thrombin resulted in heterologous desensitization to the corresponding agonist, an effect that was mediated primarily by TR internalization as evaluated by immunofluorescence and quantitative ELISA. These results ascribe a previously unrecognized function to the PAR-2 receptor, imply that a natural enzyme agonist may circulate in plasma, and suggest the presence of an additional regulatory mechanism controlling receptor activation events in vascular endothelial cells.

A monoclonal antibody to von Willebrand factor (vWF) inhibits factor VIII binding. Localization of its antigenic determinant to a nonadecapeptide at the amino terminus of the mature vWF polypeptide.

vWF is a multimeric glycoprotein that serves as the major carrier in plasma of Factor VIII (FVIII). We have used an anti-human vWF MAb W5-6A to investigate the FVIII binding site on vWF. W5-6A inhibited FVIII binding to vWF-coated polystyrene tubes in a concentration-dependent manner with 90% inhibition of FVIII binding at a concentration of 10 micrograms/ml. The W5-6A epitope was identified by screening a vWF fragment library using the bacteriophage expression vector lambda gt11. DNA sequence analysis of 29 immunoreactive phage clones localized the W5-6A epitope to a nonadecapeptide spanning amino acid residues threonine 78 to threonine 96 at the amino-terminus of the mature vWF polypeptide. Purified beta-galactosidase/vWF fusion protein from one of these clones, vWF9, was incubated with radiolabeled W5-6A and caused near complete inhibition of W5-6A binding to vWF. Inhibitory activity was lost after vWF9 trypsinization or reduction and alkylation. These data indicate that (a) the antigenic determinant recognized by W5-6A localizes to a nonadecapeptide at the NH2 terminus of the mature vWF polypeptide, (b) disulfide bonds within vWF9 may be necessary to maintain the structure required for immunoreactivity with W5-6A, and (c) W5-6A recognizes an immunogenic region on vWF that may be at (or near) the major FVIII binding domain.

The thrombin receptor extracellular domain contains sites crucial for peptide ligand-induced activation.

A thrombin receptor (TR) demonstrating a unique activation mechanism has recently been isolated from a megakaryocytic (Dami) cell line. To further study determinants of peptide ligand-mediated activation phenomenon, we have isolated, cloned, and stably expressed the identical receptor from a human umbilical vein endothelial cell (HUVEC) library. Chinese hamster ovary (CHO) cells expressing a functional TR (CHO-TR), platelets, and HUVECs were then used to specifically characterize alpha-thrombin- and peptide ligand-induced activation responses using two different antibodies: anti-TR34-52 directed against a 20-amino acid peptide spanning the thrombin cleavage site, and anti-TR1-160 generated against the NH2-terminal 160 amino acids of the TR expressed as a chimeric protein in Escherichia coli. Activation-dependent responses to both alpha-thrombin (10 nM) and peptide ligand (20 microM) were studied using fura 2-loaded cells and microspectrofluorimetry. Whereas preincubation of CHO-TR with anti-TR34-52 abolished only alpha-thrombin-induced [Ca2+]i transients, preincubation with anti-TR1-160 abrogated both alpha-thrombin- and peptide ligand-induced responses. This latter inhibitory effect was dose dependent and similar for both agonists, with an EC50 of approximately 90 micrograms/ml. Anti-TR1-160 similarly abolished peptide ligand-induced [Ca2+]i transients in platelets and HUVECs, whereas qualitatively different responses characterized by delayed but sustained elevations in [Ca2+]i transients were evident using alpha-thrombin. Platelet aggregation to low concentrations of both ligands was nearly abolished by anti-TR1-160, although some shape change remained; anti-TR34-52 only inhibited alpha-thrombin-induced aggregation. These data establish that a critical recognition sequence for peptide ligand-mediated receptor activation is contained on the NH2-terminal portion of the receptor, upstream from the first transmembrane domain. Furthermore, alpha-thrombin-induced activation of HUVECs and platelets may be partially mediated by an alternative mechanism(s) or receptor(s).

The molecular defect in type IIB von Willebrand disease. Identification of four potential missense mutations within the putative GpIb binding domain.

Type IIB von Willebrand Disease (vWD) is characterized by the selective loss of large von Willebrand Factor (vWF) multimers from plasma, presumably due to their increased reactivity with platelets and subsequent clearance from the circulation. Using the PCR, one of a panel of four potential missense mutations was identified in each of the 14 patients studied from 11 unrelated families. None of these substitutions was encountered in a large panel of normal DNAs. These changes all represent C----T transitions at CpG dinucleotides, proposed "hot spots" for mutation in the human genome. The four resulting amino acid substitutions, Arg543----Trp, Arg545----Cys, Val553----Met, and Arg578----Gln, are all clustered within the GpIb binding domain of vWF. Disruption of this latter functional domain may explain the pathogenesis of Type IIB vWD. By sequence polymorphism analysis, the Arg543----Trp substitution was shown to have occurred as at least two independent mutational events. This latter observation, along with the identification of mutations in all 14 patients studied and their localization to the GpIb binding domain, all strongly suggest that these substitutions represent the authentic defects responsible for Type IIB vWD. This panel of mutations may provide a useful diagnostic tool for the majority of patients with Type IIB vWD.

Mapping the platelet proteome: a report of the ISTH Platelet Physiology Subcommittee.

Proteomic technology has the potential to transform the way we analyze platelet biology, through the determination of platelet protein composition and its modification upon stimulation and with disease. We are a considerable way from achieving these goals, however, because of significant limitations in current methodology. It is therefore important to consider the extent to which these aims can be met and the way that proteomic data should be presented and used. These issues are discussed in the present paper by the Platelet Physiology Subcommittee of the ISTH Scientific Standardisation Committee (SSC). It is recommended that proteomic information be combined with data from other experimental approaches to establish a database on protein expression and function in platelets.

Proteolytically activated receptor-3. A member of an emerging gene family of protease receptors expressed on vascular endothelial cells and platelets.

Macromolecular assembly and generation of serine proteases on cellular surfaces is critically involved in regulation of hemostatic, inflammatory, or fibrinolytic pathways. The concept that a number of these serine proteases may effect cellular activation and proliferative responses has engendered an emerging paradigm focusing on the molecular mechanisms regulating cellular/protease interactions. Previous data suggest that some of these cellular responses are mediated by a novel class of G protein-coupled proteolytically activated receptors. Proteolytically activated receptor-3 (PAR-3) is the third member of this rapidly emerging gene family, all three of which (PAR-1, PAR-2, PAR-3) are known to co-cluster in the human genome, and are expressed on vascular endothelial cells, cells which critically regulate the hemostatic repertoire. This review will focus on the genetics of these receptors (emphasizing recent advances in the identification and characterization of PAR-3), review known structure/function similarities, and outline potential links in regulation of the hemostatic response by protease generation on the endothelial cell surface.

Fine mapping of monoclonal antibody epitopes on human von Willebrand factor using a recombinant peptide library.

A recombinant human von Willebrand factor (vWF) cDNA fragment library was constructed in lambda gt11 for the localization of anti-vWF monoclonal antibody epitopes. Twelve of 21 monoclonal antibodies screened identified epitopes expressed in lambda gt11 as beta-galactosidase fusion proteins. By sequence analysis, these antigenic determinants were localized to segments ranging from 17 to 105 amino acids in length. Four epitopes apparently shared by more than one antibody were identified, suggesting the presence of immuno-dominant epitopes within vWF. Monoclonal antibody C3, which blocks factor VIII (FVIII) binding to vWF, bound to the same epitope previously identified by a second monoclonal antibody which also blocks this function, suggesting that this region may be at or near the vWF/FVIII binding domain. Three antibodies recognize the same region within the vWF A2 repeat. Mutations near this region appear to be responsible for Type IIA von Willebrand's disease. The co-localization of these antibodies suggests that this domain might be exposed on the surface of vWF, consistent with its apparent increased sensitivity to plasma proteases.

The human thrombin receptor gene and the 5q-syndrome.

The human thrombin receptor gene has been localized to band q13 of chromosome 5, a site that is at or contiguous to the common proximal breakpoint found in the majority of patients with interstitial deletions involving 5q (5q- syndrome; refractory anemia with dysmegakaryocytopoiesis). Recent evidence suggests that the thrombin receptor may represent the prototype of an emerging family of proteolytically activated receptors that may be clustered within this region of the human genome. The phenotypic heterogeneity evident in patients with the 5q- syndrome may be explained by two (or more) distinct molecular defects-one associated with megakaryocytic dyspoiesis and the other dysregulated myeloid growth potentially related to development of leukemogenesis. Because the thrombin receptor is known to mediate proliferative effects on diverse cells including vascular smooth muscle cells, endothelial cells and megakaryocytes, we have studied the role of this receptor in the pathogenesis of this syndrome using fluorescent in situ hybridization (FISH) analysis. Dual-label FISH using a q12-specific genomic fragment and the TR gene was completed using interphase and metaphase analysis from seven patients with a del(5)(q13q33). These data unequivocally demonstrate that the thrombin receptor gene is located centromeric to the common proximal breakpoint, and is grossly present in the seven patients containing this specific interstitial deletion. Additionally, one patient demonstrated a small proximal rearrangement, most likely representing a paracentric inversion, which has not previously been described within the intact region centromeric to the breakpoint. The biological properties of proteolytically activated receptors are reviewed in more detail, with a focus on the molecular genetics of the thrombin receptor and its potential role in megakaryoctyopoiesis.

Characterization of recombinant adeno-associated virus-2 as a vehicle for gene delivery and expression into vascular cells.

BACKGROUND: We have used wild-type and recombinant adeno-associated virus-2 (AAV) to study transduction, replication efficiencies, functional protein expression, and gene delivery to vascular cells in vitro and in vivo. METHODS: Recombinant adeno-associated virus-2 (rAAV) plasmids (ranging in size to 110% of wild-type AAV) driven by 6 distinct promoters upstream of a beta-galactosidase cassette were effectively used for generation of replication-deficient virus, with titers consistently ranging from 2.5 x 10(5) IU/mL. AAV infectivity and replication in human umbilical vein endothelial cells (HUVEC) were unrelated to cellular proliferative index establishing the potential utility of the virus for transduction of quiescent vascular cells. Long-term cultures of AAV-infected HUVEC established the presence of episomal forms at 18 days, although chromosome 19-specific integration was not evident. Functional beta-galactosidase activity approximately 400% above control was evident in HUVEC using either a murine collagen alpha 1(I) promoter (pTRCol alpha 1(I) beta) or CMV promoter (pTRCMV beta). RESULTS: Based on these initial data, in vivo studies were completed using a rat carotid artery model. Both wild-type AAV (titers -1X10(9) IU/mL) and rAAV (pTRCol alpha 1(I) beta or pTRCMV beta) efficiently infected vascular cells in vivo with endothelial and vascular smooth muscle cell transduction frequencies approaching 90% as judged by DNA in situ polymerase chain reaction, with no evidence for disrupted vessel architecture. Protein expression using total vessel extracts at 48 hours postinfection demonstrated 20-fold increase in functional beta-galactosidase activity using pTRCol alpha 1(I) beta compared to saline-injected controls vessels (799 +/- 236 microU/mg protein vs 40.7 +/- 17 microU/mg protein). CONCLUSIONS: These data provide the first evidence that rAAV may be adapted for directed high-level transgene delivery and expression into normally quiescent vascular endothelial and smooth muscle cells both in vitro and in vivo.

The VLA-2 (alpha 2 beta 1) I domain functions as a ligand-specific recognition sequence for endothelial cell attachment and spreading: molecular and functional characterization.

The integrin VLA-2 (alpha 2 beta 1), generally considered to represent the specific collagen receptor on human endothelial cells, contains an alpha 2-subunit inserted I domain with structural similarity to the type A domains found within the recently described superfamily of receptor-ligand recognition proteins. This region of the cDNA has now been isolated and used for molecular and functional characterization of this heterodimeric receptor complex. Comparative sequence analysis with the porcine homologue revealed 93% amino acid sequence identity, suggestive of a developmentally conserved function. To complete structure/function studies, this region of the human cDNA was expressed as a chimeric protein in Escherichia coli, and a rabbit polyclonal antibody (anti-I domain) was used to study determinants of endothelial cell attachment and spreading in vitro. Quantifiable and visual disruption of endothelial cell attachment to gelatin, type I collagen, and laminin was evident using the specific anti-I domain antibody, with minimal inhibitory effects demonstrable using fibronectin or fibrinogen matrices. Therefore, these data would suggest that the alpha 2 beta 1 I domain confers ligand-binding specificity for both known alpha 2 beta 1 substrates (laminin and collagen), and that this region subserves a regulatory function in the molecular processes controlling endothelial cell attachment and spreading in vitro.

Genomic cloning and characterization of the human thrombin receptor gene. Structural similarity to the proteinase activated receptor-2 gene.

The seven-transmembrane segment thrombin receptor (TR) represents the prototype of a putative family of proteolytically cleaved receptors that may include the proteinase activated receptor-2. A panel of somatic cell hybrids retaining distinct portions of human chromosome 5 were used to establish that the human TR gene is present as a single-copy locus within the region 5q11.2 -->q13.3, confirming our previous localization using fluorescent in situ hybridization analysis. To further characterize the TR gene, overlapping clones from a human genomic library were isolated. Genomic analysis confirmed that the TR gene is of limited complexity, spanning approximately 27 kilobases and containing two exons separated by a large approximately 22-kilobase intron. The larger second exon contains the majority of the coding sequence and the thrombin cleavage site, remarkably similar to the organization of the proteinase activated receptor-2 gene in which the putative cleavage site is also contained within the large second exon. Primer extension analysis using two 30-mer oligonucleotide primers known to be contained within the first exon identified the predominant transcription initiation site 351 base pairs upstream from the initiator methionine in both human umbilical vein endothelial and human erythroleukemia cells. Sequence analysis of the 5'-flanking region revealed the TR promoter to be TATA-less, although nucleic acid motifspotentially involved in transcriptional gene regulation were evident and include a GATA motif, octamer enhancer sequences, AP-2-like sites, and Sp1 sites. These data provide evidence for remarkable similarity at the gene level between both proteolytically cleaved receptors described to date.

In vivo gene transfer into rat arterial walls with novel adeno-associated virus vectors.

PURPOSE: We studied the ability of recombinant adeno-associated virus (rAAV) vectors to achieve gene transfer in vivo to intact rat carotid arteries. METHODS: Isolated segments of uninjured rat carotid arteries were incubated with (1) rAAV vectors that expressed a beta-galactosidase gene, (2) a related vector with no promoter, or (3) a normal saline solution. Gene transfer was evaluated with in situ polymerase chain reaction (PCR). Transgene expression was assessed at intervals that ranged from 24 hours to 2 months by measurement of beta-galactosidase activity and protein mass in tissue extracts with fluorometric and enzyme-linked immunosorbent assays, respectively. Dose dependence of expression was determined for virus concentrations that ranged from 5 x 10(4) to 5 x 10(5) infectious units (iu)/ml. RESULTS: Light microscopic analysis of in situ PCR-stained histologic sections of transduced vessel walls showed approximately 90% of intimal and medial cell nuclei contained the beta-galactosidase gene, compared with none in control arteries. In vivo beta-galactosidase expression was (1) highest 24 hours after gene transfer, (2) elevated for 1 month, and (3) dose responsive. CONCLUSIONS: rAAV vectors can mediate focal gene transfer into the intact rat carotid artery with detectable levels of transgene expression for 1 month and are potentially useful agents for in vivo gene transfer into intact arteries.

cDNA cloning and molecular characterization of MSE55, a novel human serum constituent protein that displays bone marrow stromal/endothelial cell-specific expression.

Hemonectin is a lineage-specific cytoadhesive protein that may be involved in the developmentally regulated adhesion of granulocytic cells to bone marrow stroma. Immunoblot analysis using an anti-hemonectin antibody recognizes two distinct immunoreactive species in endothelial cell lysates (approximately M(r) 65,000) and human serum (approximately M(r) 55,000). Initial characterization of the 55-kDa protein has now been completed by isolating the cDNA from a human endothelial cell expression library. Sequence analysis of overlapping clones identifies a composite sequence spanning 2030 nucleotides with an open reading frame of 1173 base pairs. No significant sequence similarity was observed on analysis of current GenBank databases. The open reading frame was expressed as a recombinant protein in Escherichia coli and used as an immunogen for the production of a specific polyclonal antibody. Immunoblotting with this antibody identifies a single immunoreactive species of apparent M(r) 55,000 in HUVEC lysates and human serum, confirming that a secreted form normally circulates as a serum constituent protein. This antibody fails to recognize purified hemonectin, suggesting that the M(r) 55,000 protein is not hemonectin. Cross-species Southern blot analysis reveals persistent hybridizing fragments in all species tested, suggestive of a developmentally conserved function. Northern blot analysis demonstrates expression limited to endothelial and bone marrow stromal cells, but not poly(A) RNA from monkey liver, spleen, brain, lung, and kidney. On this basis, we have designated this novel protein MSE55, for marrow stromal/endothelial cell protein with a molecular mass of 55,000 daltons. Its tissue-specific expression may suggest a functional role in hematopoiesis.

Identification of a novel thrombin receptor sequence required for activation-dependent responses.

Thrombin receptor (TR) activation by alpha-thrombin requires proteolytic cleavage, although synthetic peptides modeled after the new N-terminus directly effect receptor activation without cleavage, presumably by interacting with an unidentified region of the receptor. To further define critical residues responsible for receptor activation, we performed epitope mapping of anti-TR1-160, a previously described polyclonal antibody that inhibits peptide ligand-induced receptor activation in various cell types expressing a functional TR. An enzyme-linked immunosorbent assay (ELISA) using overlapping decapeptides derived from the TR extracellular domains identified four immunodominant peaks within the long N-terminal extension centered between amino acids 34-44, 48-67, 65-79, and 87-94. Soluble peptides derived from regions 83-94, but not those from other regions of the receptor, neutralized the ability of anti-TR1-160 to inhibit peptide ligand-induced platelet aggregation, suggesting that antibodies directed against this region of the TR are important in ligand-mediated activation. Thrombin receptor mutants lacking discrete regions of the TR were subsequently evaluated using microinjected Xenopus oocytes. Whereas a TR mutant lacking amino acid residues Thr67-Lys82 (TR delta 67-82) showed normal to exaggerated responses to either alpha-thrombin or synthetic peptide ligands, only TR mutants with limited deletions spanning the residues Gln83-Ser93 exhibited dysfunctional responses to either agonist (200 nmol/L alpha-thrombin or 200 mumol/L TR42-47). These data provide a model for receptor activation that implicates a discrete and previously uncharacterized sequence within the TR N-terminal extension that is necessary for initiation of signal transduction events independent of the initiating agonist.

Platelet glycoprotein Ib: a zinc-dependent binding protein for the heavy chain of high-molecular-weight kininogen.

Domains 3 and 5 of high-molecular-weight kininogen (HK) have been shown to bind to platelets in a zinc-dependent reaction. However, the platelet-binding proteins responsible for this interaction have not been identified. We have focused on the platelet-binding site for the heavy chain (domain 3), which we approached using a domain 3-derived peptide ligand and isolated binding proteins by affinity chromatography. The domain 3-derived peptide, thrombin, HK, factor XII, as well as antibody to glycocalicin (the N-terminal portion of the alpha chain of GPIb) recognized a protein at 74 kD. We also isolated the thrombin receptor (PAR 1) at 45 kD, however, none of the above-mentioned ligands bound to this protein. Isolation of platelet membrane proteins using a monoclonal anti-glycocalicin antibody column revealed the same HK binding protein at 74 kD, which was reactive with anti-GPIb and represents a GPIb fragment. By photoaffinity labeling, HK interacted with membrane GPIb, which was then isolated in native form (135 kD) along with gC1qR, a ligand for the HK light chain. Finally, (125)I-HK binding to platelets was significantly inhibited by the anti-GPIb antibody. These results suggest that the GPIb alpha chain, a known thrombin binding protein, is also one of the zinc-dependent platelet membrane binding sites for HK domain 3.

The propeptide domain of membrane type 1 matrix metalloproteinase is required for binding of tissue inhibitor of metalloproteinases and for activation of pro-gelatinase A.

Activation of secreted latent matrix metalloproteinases (MMPs) is accompanied by cleavage of the N-terminal propeptide, thereby liberating the active zinc from binding to the conserved cysteine in the pro-domain. It has been assumed that an analogous mechanism is responsible for the activation of membrane type 1 MMP (MT1-MMP). Using recombinant wild-type MT1-MMP cDNA and mutant cDNAs transfected into COS-1 cells lacking endogenous MT1-MMP, we have examined the function of the propeptide domain of MT1-MMP. MT1-MMP was characterized by immunoblotting, surface biotinylation, gelatin substrate zymography, and 125I-tissue inhibitor of metalloproteinases 2 (TIMP-2) binding. In contrast to wild-type MT1-MMP-transfected COS-1 cells, transfected COS-1 cells containing a deletion of the N-terminal propeptide domain of MT1-MMP or a chimeric construction (substitution of the pro-domain of MT1-MMP with that of collagenase 3) were functionally inactive in terms of binding of 125I-labeled TIMP-2 to the cell surface and initiating the activation of pro-gelatinase A. These results support the concept that in its native plasma membrane-inserted form, the pro-domain of MT1-MMP plays an essential role in TIMP-2 binding and subsequent activation of pro-gelatinase A.

Molecular and functional characterization of the non-isopeptide-selective ETB receptor in endothelial cells. Receptor coupling to nitric oxide synthase.

There is accumulating evidence that endothelial cells express a non-isopeptide-selective endothelin (ET) receptor, ETB, which may be responsible for ET-1-induced transient vasorelaxation. The purpose of the present study was to seek direct evidence for ETB receptor expression in human umbilical vein endothelial cells (HUVEC) and to characterize its functional role in HUVEC and in Chinese hamster ovary cells stably transfected with ETB receptor cDNA (CHO-ETB). Reverse polymerase chain reaction using HUVEC total RNA and ETB receptor-specific oligonucleotide primers firmly demonstrated the presence of an endogenous transcript of the appropriate molecular size. Next, a biotinylated ligand specifically recognizing the ETB receptor, IRL-1620, was synthesized, and immunocytochemical mapping of binding sites was performed in CHO-ETB cells. Specific binding of biotinylated IRL-1620 was evident in CHO-ETB cells, confirming appropriate cell surface receptor expression. Continuous nitric oxide (NO) monitoring with NO-selective electrode revealed a dose-dependent ET-1 stimulation of NO production by HUVEC. Stable transfection of CHO-ETB cells with endothelial nitric oxide synthase (NOS), but not mock-transfection, imparted responsiveness to ET-1 similar to that for HUVEC and was characterized by the immediate release of NO. Protein tyrosine kinase-dependent and calcium-calmodulin-dependent pathways were involved in ET-1-induced activation of the constitutive NOS in CHO-ETB/NOS cells, but coupling of the receptor to the enzyme in HUVEC appeared to be predominantly protein tyrosine kinase-dependent. Although sufficient, calcium/calmodulin system was not an obligatory prerequisite for the ET-1-induced activation of NOS in HUVEC. In conclusion, using two cell systems, we demonstrated that the ETB receptor is functionally coupled to NOS and coordinates the generation of NO via a tyrosine kinase-dependent and a calcium/calmodulin-dependent pathway.

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.