The chaperone protein HSP47: a platelet collagen binding protein that contributes to thrombosis and hemostasis

Essentials Heat shock protein 47 (HSP47), a collagen specific chaperone is present on the platelet surface. Collagen mediated platelet function was reduced following blockade or deletion of HSP47. GPVI receptor regulated signalling was reduced in HSP47 deficient platelets. Platelet HSP47 tethers to exposed collagen thus modulating thrombosis and hemostasis.

SUMMARY

Objective Heat shock protein 47 (HSP47) is an intracellular chaperone protein that is vital for collagen biosynthesis in collagen secreting cells. This protein has also been shown to be present on the surface of platelets. Given the importance of collagen and its interactions with platelets in triggering hemostasis and thrombosis, in this study we sought to characterize the role of HSP47 in these cells. Methods and Results The deletion of HSP47 in mouse platelets or its inhibition in human platelets reduced their function in response to collagen and the GPVI agonist (CRP-XL), but responses to thrombin were unaltered. In the absence of functional HSP47, the interaction of collagen with platelets was reduced, and this was associated with reduced GPVI-collagen binding, signalling and platelet activation. Thrombus formation on collagen, under arterial flow conditions, was also decreased following the inhibition or deletion of HSP47, in the presence or absence of eptifibatide, consistent with a role for HSP47 in enhancing platelet adhesion to collagen. Platelet adhesion under flow to von Willebrand factor was unaltered following HSP47 inhibition. Laser-induced thrombosis in cremaster muscle arterioles was reduced and bleeding time was prolonged in HSP47-deficient mice or following inhibition of HSP47. Conclusions Our study demonstrates the presence of HSP47 on the platelet surface, where it interacts with collagen, stabilizes platelet adhesion and increases collagen-mediated signalling and therefore thrombus formation and hemostasis.

Platelet collagen receptor Glycoprotein VI-dimer recognizes fibrinogen and fibrin through their D-domains, contributing to platelet adhesion and activation during thrombus formation

Essentials Glycoprotein VI (GPVI) binds collagen, starting thrombogenesis, and fibrin, stabilizing thrombi. GPVI-dimers, not monomers, recognize immobilized fibrinogen and fibrin through their D-domains. Collagen, D-fragment and D-dimer may share a common or proximate binding site(s) on GPVI-dimer. GPVI-dimer-fibrin interaction supports spreading, activation and adhesion involving αIIbβ3.

SUMMARY

Background Platelet collagen receptor Glycoprotein VI (GPVI) binds collagen, initiating thrombogenesis, and stabilizes thrombi by binding fibrin. Objectives To determine if GPVI-dimer, GPVI-monomer, or both bind to fibrinogen substrates, and which region common to these substrates contains the interaction site. Methods Recombinant GPVI monomeric extracellular domain (GPVIex ) or dimeric Fc-fusion protein (GPVI-Fc2 ) binding to immobilized fibrinogen derivatives was measured by ELISA, including competition assays involving collagenous substrates and fibrinogen derivatives. Flow adhesion was performed with normal or Glanzmann thrombasthenic (GT) platelets over immobilized fibrinogen, with or without anti-GPVI-dimer or anti-αIIbβ3. Results Under static conditions, GPVIex did not bind to any fibrinogen substrate. GPVI-Fc2 exhibited specific, saturable binding to both D-fragment and D-dimer, which was inhibited by mFab-F (anti-GPVI-dimer), but showed low binding to fibrinogen and fibrin under our conditions. GPVI-Fc2 binding to D-fragment or D-dimer was abrogated by collagen type III, Horm collagen or CRP-XL (crosslinked collagen-related peptide), suggesting proximity between the D-domain and collagen binding sites on GPVI-dimer. Under low shear, adhesion of normal platelets to D-fragment, D-dimer, fibrinogen and fibrin was inhibited by mFab-F (inhibitor of GPVI-dimer) and abolished by Eptifibatide (inhibitor of αIIbβ3), suggesting that both receptors contribute to thrombus formation on these substrates, but αIIbβ3 makes a greater contribution. Notably, thrombasthenic platelets showed limited adhesion to fibrinogen substrates under flow, which was further reduced by mFab-F, supporting some independent GPVI-dimer involvement in this interaction. Conclusion Only dimeric GPVI interacts with fibrinogen D-domain, at a site proximate to its collagen binding site, to support platelet adhesion/activation/aggregate formation on immobilized fibrinogen and polymerized fibrin.

Clustering of glycoprotein VI (GPVI) dimers upon adhesion to collagen as a mechanism to regulate GPVI signaling in platelets

Essentials Dimeric high-affinity collagen receptor glycoprotein VI (GPVI) is present on resting platelets. Spatio-temporal organization of platelet GPVI-dimers was evaluated using advanced microscopy. Upon platelet adhesion to collagenous substrates, GPVI-dimers coalesce to form clusters. Clustering of GPVI-dimers may increase avidity and facilitate platelet activation SUMMARY: Background Platelet glycoprotein VI (GPVI) binding to subendothelial collagen exposed upon blood vessel injury initiates thrombus formation. Dimeric GPVI has high affinity for collagen, and occurs constitutively on resting platelets. Objective To identify higher-order oligomerization (clustering) of pre-existing GPVI dimers upon interaction with collagen as a mechanism to initiate GPVI-mediated signaling. Methods GPVI was located by use of fluorophore-conjugated GPVI dimer-specific Fab (antigen-binding fragment). The tested substrates include Horm collagen I fibers, soluble collagen III, GPVI-specific collagen peptides, and fibrinogen. GPVI dimer clusters on the platelet surface interacting with these substrates were visualized with complementary imaging techniques: total internal reflection fluorescence microscopy to monitor real-time interactions, and direct stochastic optical reconstruction microscopy (dSTORM), providing relative quantification of GPVI cluster size and density. Confocal microscopy was used to locate GPVI dimer clusters, glycoprotein Ib, integrin α2 β1 , and phosphotyrosine. Results Upon platelet adhesion to all collagenous substrates, GPVI dimers coalesced to form clusters; notably clusters formed along the fibers of Horm collagen. dSTORM revealed that GPVI density within clusters depended on the substrate, collagen III being the most effective. Clusters on fibrinogen-adhered platelets were much smaller and more numerous; whether these are pre-existing oligomers of GPVI dimers or fibrinogen-induced is not clear. Some GPVI dimer clusters colocalized with areas of phosphotyrosine, indicative of signaling activity. Integrin α2 β1 was localized to collagen fibers close to GPVI dimer clusters. GPVI clustering depends on a dynamic actin cytoskeleton. Conclusions Platelet adhesion to collagen induces GPVI dimer clustering. GPVI clustering increases both avidity for collagen and the proximity of GPVI-associated signaling molecules, which may be crucial for the initiation and persistence of signaling.

Modified platelet deposition on matrix metalloproteinase 13 digested collagen I

BACKGROUND

Atherothrombosis underlies acute coronary syndromes, including unstable angina and acute myocardial infarction. Within the unstable plaque, monocytes express collagenolytic matrix metalloproteinases (MMPs), including MMP-13, which degrades fibrous collagen. Following rupture, vessel wall components including degraded collagen are exposed to circulating platelets. Platelet receptors then mediate the recruitment and activation of platelets to form a thrombus, blocking blood flow and resulting in myocardial infarction and sudden death.

OBJECTIVES

Here we aim to provide information on the effects of collagen degradation on platelet adhesion and thrombus formation.

METHODS

Using increasing concentrations of MMP-13, we induced progressive degradation of fibrous and monomeric collagen I, visualized by electrophoresis, and then investigated the capacity of the resulting fragments to support static platelet adhesion and thrombus formation in whole flowing blood.

RESULTS

Both integrin and glycoprotein VI-dependent interactions with fibrous collagen underpin high levels of platelet adhesion under both conditions, with little obvious effect of MMP-13 treatment. Static platelet adhesion to monomeric collagen was strongly α2β1-dependent regardless of degradation status. Under flow conditions, partially degraded monomeric collagen supported increased thrombus deposition at 10 μg mL(-1) MMP-13, falling close to background when collagen degradation was complete (100 μg mL(-1) MMP-13).

CONCLUSIONS

New binding activities come into play after partial digestion of collagen monomers, and net platelet-reactivity through all axes is abolished as degradation becomes more complete.

Control of crosslinking for tailoring collagen-based scaffolds stability and mechanics

We provide evidence to show that the standard reactant concentrations used in tissue engineering to cross-link collagen-based scaffolds are up to 100 times higher than required for mechanical integrity in service, and stability against degradation in an aqueous environment. We demonstrate this with a detailed and systematic study by comparing scaffolds made from (a) collagen from two different suppliers, (b) gelatin (a partially denatured collagen) and (c) 50% collagen-50% gelatin mixtures. The materials were processed, using lyophilisation, to produce homogeneous, highly porous scaffolds with isotropic architectures and pore diameters ranging from 130 to 260 μm. Scaffolds were cross-linked using a carbodiimide treatment, to establish the effect of the variations in crosslinking conditions (down to very low concentrations) on the morphology, swelling, degradation and mechanical properties of the scaffolds. Carbodiimide concentration of 11.5mg/ml was defined as the standard (100%) and was progressively diluted down to 0.1%. It was found that 10-fold reduction in the carbodiimide content led to the significant increase (almost 4-fold) in the amount of free amine groups (primarily on collagen lysine residues) without compromising mechanics and stability in water of all resultant scaffolds. The importance of this finding is that, by reducing cross-linking, the corresponding cell-reactive carboxylate anions (collagen glutamate or aspartate residues) that are essential for integrin-mediated binding remain intact. Indeed, a 10-fold reduction in carbodiimide crosslinking resulted in near native-like cell attachment to collagen scaffolds. We have demonstrated that controlling the degree of cross-linking, and hence retaining native scaffold chemistry, offers a major step forward in the biological performance of collagen- and gelatin-based tissue engineering scaffolds.

STATEMENT OF SIGNIFICANCE

This work developed collagen and gelatine-based scaffolds with structural, material and biological properties suitable for use in myocardial tissue regeneration. The novelty and significance of this research consist in elucidating the effect of the composition, origin of collagen and crosslinking concentration on the scaffold physical and cell-binding characteristics. We demonstrate that the standard carbodiimide concentrations used to crosslink collagenous scaffolds are up to 100 times higher than required for mechanical integrity in service, and stability against dissolution. The importance of this finding is that, by reducing crosslinking, the corresponding cell-reactive carboxylate anions (essential for integrin-mediated binding) remain intact and the native scaffold chemistry is retained. This offers a major step forward in the biological performance of tissue engineered scaffolds.

The synthetic hydroxyproline-containing collagen analogue (Gly-Pro-Hyp)10 ameliorates acute DSS colitis

In experimental models of and humans with intestinal inflammation, increased levels of the matrix-degrading gelatinases MMP-2 and -9 in inflamed tissues can be detected. The synthetic collagen analogue (Gly-Pro-Hyp)10, (GPO)10, has been identified as a relevant binding structure for proMMP-2/-9 and promotes enzymatic activity of proMMP-2. Since targeted MMP strategies might offer promising anti-inflammatory treatment options, we for the first time studied in vivo actions exerted by (GPO)10 applying an acute dextrane sulfate sodium (DSS) induced colitis model. Seven-day intraperitoneal (GPO)10 treatment ameliorated clinical symptoms and histopathological colonic changes as compared to placebo controls with severe colitis. (GPO)10-treated mice displayed a diminished influx of neutrophils, and T- and B-lymphocytes into their colonic mucosa whereas numbers of regulatory T-cells and regenerative cells were higher as compared to placebo controls. Furthermore, IL-6 secretion was down-regulated in ex vivo colonic biopsies derived from (GPO)10-treated mice whereas higher concentrations of the anti-inflammatory cytokine IL-10 in extra-intestinal compartments such as MLN and spleen could be detected. Strikingly, influx of inflammatory cells into lungs was abolished following (GPO)10 application. We therefore propose (GPO)10 as a promising effective and safe treatment option of intestinal and extra-intestinal inflammatory conditions in humans.

The synthetic hydroxyproline-containing collagen analogue (Gly-Pro-Hyp)10 promotes enzymatic activity of matrixmetalloproteinase-2 in vitro

Diseases such as liver fibrosis and intestinal inflammation are characterized by accumulated components of the extracellular matrix (ECM). Given that fibrillar collagen structures were shown to serve as storage site for inactive proforms of matrixmetalloproteinases (MMPs), modulating this MMP-collagen interaction might offer a rational interventional (therapeutic) approach to enhance degradation of accumulated ECM. The synthetic triple helical collagen analogue (Gly-Pro-Hyp)10 - (GPO)10 - was shown to trigger release and enzymatic activation of collagen sequestered proMMP-2. In the presented study, we, for the first time, investigated how MMP-(GPO)10 interaction impacts cellular responses in vitro. We found that recombinant proMMP-2 induced proliferation of hepatic stellate cells (HSC), which was enhanced after addition of (GPO)10 reaching comparable levels following incubation with fully activated MMP-2. In addition, (GPO)10 induced HSC migration similar to the platelet-derived growth factor subunit-B. Further, the MMP-2-dependent invasion of HT1080 fibrosarcoma cells through an ECM membrane was enhanced after addition of (GPO)10. Since cellular proliferation and migration concomitant with matrix degradation is stimulated, we conclude that the MMP-(GPO)10 interaction also functions in a physiological environment. Thus, a potential therapeutic effect of (GPO)10 should be further tested in animal models for MMP-associated diseases such as colitis or fibrosis.

A role for adhesion and degranulation-promoting adapter protein in collagen-induced platelet activation mediated via integrin α(2) β(1)

BACKGROUND

Collagen-induced platelet activation is a key step in the development of arterial thrombosis via its interaction with the receptors glycoprotein (GP)VI and integrin α(2) β(1) . Adhesion and degranulation-promoting adapter protein (ADAP) regulates α(IIb) β(3) in platelets and α(L) β(2) in T cells, and is phosphorylated in GPVI-deficient platelets activated by collagen.

OBJECTIVES

To determine whether ADAP plays a role in collagen-induced platelet activation and in the regulation and function of α(2) β(1).

METHODS

Using ADAP(-/-) mice and synthetic collagen peptides, we investigated the role of ADAP in platelet aggregation, adhesion, spreading, thromboxane synthesis, and tyrosine phosphorylation.

RESULTS AND CONCLUSIONS

Platelet aggregation and phosphorylation of phospholipase Cγ2 induced by collagen were attenuated in ADAP(-/-) platelets. However, aggregation and signaling induced by collagen-related peptide (CRP), a GPVI-selective agonist, were largely unaffected. Platelet adhesion to CRP was also unaffected by ADAP deficiency. Adhesion to the α(2) β(1) -selective ligand GFOGER and to a peptide (III-04), which supports adhesion that is dependent on both GPVI and α(2) β(1), was reduced in ADAP(-/-) platelets. An impedance-based label-free detection technique, which measures adhesion and spreading of platelets, indicated that, in the absence of ADAP, spreading on GFOGER was also reduced. This was confirmed with non-fluorescent differential-interference contrast microscopy, which revealed reduced filpodia formation in ADAP(-/-) platelets adherent to GFOGER. This indicates that ADAP plays a role in mediating platelet activation via the collagen-binding integrin α(2) β(1). In addition, we found that ADAP(-/-) mice, which are mildly thrombocytopenic, have enlarged spleens as compared with wild-type animals. This may reflect increased removal of platelets from the circulation.

Nitric oxide specifically inhibits integrin-mediated platelet adhesion and spreading on collagen

BACKGROUND

Nitric oxide (NO) inhibits platelet adhesion to collagen, although the precise molecular mechanisms underlying this process are unclear.

OBJECTIVES

Collagen-mediated adhesion is a multifaceted event requiring multiple receptors and platelet-derived soluble agonists. We investigated the influence of NO on these processes.

RESULTS

S-nitrosoglutathione (GSNO) induced a concentration-dependent inhibition of platelet adhesion to immobilized collagen. Maximal adhesion to collagen required platelet-derived ADP and TxA(2). GSNO-mediated inhibition was lost in the presence of apyrase and indomethacin, suggesting that NO reduced the availability of, or signaling by, ADP and TxA(2). Exogenous ADP, but not the TxA(2) analogue U46619, reversed the inhibitory actions of GSNO on adhesion. Under adhesive conditions NO inhibited dense granule secretion but did not influence TxA(2) generation. These data indicated that NO may block signaling by TxA(2) required for dense granule secretion, thereby reducing the availability of ADP. Indeed, we found TxA(2)-mediated activation of PKC was required to drive dense granule secretion, a pathway that was inhibited by NO. Because our data demonstrated that NO only inhibited the activation-dependent component of adhesion, we investigated the effects of NO on individual collagen receptors. GSNO inhibited platelet adhesion and spreading on alpha(2)beta(1) specific peptide ligand GFOGER. In contrast, GSNO did not inhibit GPVI-mediated adhesion to collagen, or adhesion to the GPVI specific ligand, collagen related peptide (CRP).

CONCLUSIONS

NO targets activation-dependent adhesion mediated by alpha(2)beta(1), possibly by reducing bioavailability of platelet-derived ADP, but has no effect on activation-independent adhesion mediated by GPVI. Thus, NO regulates platelet spreading and stable adhesion to collagen.

Collagen-mimetic peptides mediate flow-dependent thrombus formation by high- or low-affinity binding of integrin alpha2beta1 and glycoprotein VI

BACKGROUND

Collagen acts as a potent surface for platelet adhesion and thrombus formation under conditions of blood flow. Studies using collagen-derived triple-helical peptides have identified the GXX'GER motif as an adhesive ligand for platelet integrin alpha2beta1, and (GPO)(n) as a binding sequence for the signaling collagen receptor, glycoprotein VI (GPVI).

OBJECTIVE

The potency was investigated of triple-helical peptides, consisting of GXX'GER sequences within (GPO)(n) or (GPP)(n) motifs, to support flow-dependent thrombus formation.

RESULTS

At a high-shear rate, immobilized peptides containing both the high-affinity alpha2beta1-binding motif GFOGER and the (GPO)(n) motif supported platelet aggregation and procoagulant activity, even in the absence of von Willebrand factor (VWF). With peptides containing only one of these motifs, co-immobilized VWF was needed for thrombus formation. The (GPO)(n) but not the (GPP)(n) sequence induced GPVI-dependent platelet aggregation and procoagulant activity. Peptides with intermediate affinity (GLSGER, GMOGER) or low-affinity (GASGER, GAOGER) alpha2beta1-binding motifs formed procoagulant thrombi only if both (GPO)(n) and VWF were present. At a low-shear rate, immobilized peptides with high- or low-affinity alpha2beta1-binding motifs mediated formation of thrombi with procoagulant platelets only in combination with (GPO)(n).

CONCLUSIONS

Triple-helical peptides with specific receptor-binding motifs mimic the properties of native collagen I in thrombus formation by binding to both platelet collagen receptors. At a high-shear rate, either GPIb or high-affinity (but not low-affinity) GXX'GER mediates GPVI-dependent formation of procoagulant thrombi. By extension, high-affinity binding for alpha2beta1 can control the overall platelet-adhesive activity of native collagens.

Mapping the platelet profile for functional genomic studies and demonstration of the effect size of the GP6 locus

BACKGROUND

Evidence suggests the wide variation in platelet response within the population is genetically controlled. Unraveling the complex relationship between sequence variation and platelet phenotype requires accurate and reproducible measurement of platelet response.

OBJECTIVE

To develop a methodology suitable for measuring signaling pathway-specific platelet phenotype, to use this to measure platelet response in a large cohort, and to demonstrate the effect size of sequence variation in a relevant model gene.

METHODS

Three established platelet assays were evaluated: mobilization of [Ca(2+)](i), aggregometry and flow cytometry, each in response to adenosine 5'-diphosphate (ADP) or the glycoprotein (GP) VI-specific crosslinked collagen-related peptide (CRP). Flow cytometric measurement of fibrinogen binding and P-selectin expression in response to a single, intermediate dose of each agonist gave the best combination of reproducibility and inter-individual variability and was used to measure the platelet response in 506 healthy volunteers. Pathway specificity was ensured by blocking the main subsidiary signaling pathways.

RESULTS

Individuals were identified who were hypo- or hyper-responders for both pathways, or who had differential responses to the two agonists, or between outcomes. 89 individuals, retested three months later using the same methodology, showed high concordance between the two visits in all four assays (r(2) = 0.872, 0.868, 0.766 and 0.549); all subjects retaining their phenotype at recall. The effect of sequence variation at the GP6 locus accounted for approximately 35% of the variation in the CRP-XL response.

CONCLUSION

Genotyping-phenotype association studies in a well-characterized, large cohort provides a powerful strategy to measure the effect of sequence variation in genes regulating the platelet response.

Platelet receptor recognition and cross-talk in collagen-induced activation of platelets

Comprehensive mapping of protein-binding sites within human collagen III has allowed the recognition motifs for integrin alpha(2)beta(1) and VWF A3 domain to be identified. Glycoprotein VI-binding sites are understood, although less well defined. This information, together with recent developments in understanding collagen fiber architecture, and crystal structures of the receptor collagen-binding domains, allows a coherent model for the interaction of collagen with the platelet surface to be developed. This complements our understanding of the orchestration of receptor presentation by membrane microdomains, such that the polyvalent collagen surface may stabilize signaling complexes within the heterogeneous receptor composition of the lipid raft. The ensuing interactions lead to the convergence of signals from each of the adhesive receptors, mediated by FcR gamma-chain and/or FcgammaRIIa, leading to concerted and co-operative platelet activation. Each receptor has a shear-dependent role, VWF/GpIb essential at high shear, and alpha(2)beta(1) at low and intermediate shear, whilst GpVI provides core signals that contribute to enhanced integrin affinity, tighter binding to collagen and consequent platelet activation.

Primary and secondary agonists can use P2X(1) receptors as a major pathway to increase intracellular Ca(2+) in the human platelet

In the platelet, it is well established that many G-protein- and tyrosine kinase-coupled receptors stimulate phospholipase-C-dependent Ca(2+) mobilization; however, the extent to which secondary activation of adenosine 5'-triphosphate (ATP)-gated P2X(1) receptors contributes to intracellular Ca(2+) responses remains unclear. We now show that selective inhibition of P2X(1) receptors substantially reduces the [Ca(2+)](i) increase evoked by several important agonists in human platelets; for collagen, thromboxane A(2), thrombin, and adenosine 5'-diphoshate (ADP) the maximal effect was a reduction to 18%, 34%, 52%, and 69% of control, respectively. The direct contribution of P2X(1) to the secondary Ca(2+) response was far greater than that of either P2Y receptors activated by co-released ADP, or via synergistic P2X(1):P2Y interactions. The relative contribution of P2X(1) to the peak Ca(2+) increase varied with the strength of the initial stimulus, being greater at low compared to high levels of stimulation for both glycoprotein VI and PAR-1, whereas P2X(1) contributed equally at both low and high levels of stimulation of thromboxane A(2) receptors. In contrast, only strong stimulation of P2Y receptors resulted in significant P2X(1) receptor activation. ATP release was detected by soluble luciferin:luciferase in response to all agonists that stimulated secondary P2X(1) receptor activation. However, P2X(1) receptors were stimulated earlier and to a greater extent than predicted from the average ATP release, which can be accounted for by a predominantly autocrine mechanism of activation. Given the central role of [Ca(2+)](i) increases in platelet activation, these studies indicate that ATP should be considered alongside ADP and thromboxane A(2) as a significant secondary platelet agonist.

Definition of novel GP6 polymorphisms and major difference in haplotype frequencies between populations by a combination of in-depth exon resequencing and genotyping with tag single nucleotide polymorphisms

BACKGROUND

Common genetic variants of cell surface receptors contribute to differences in functional responses and disease susceptibility. We have previously shown that single nucleotide polymorphisms (SNPs) in platelet glycoprotein VI (GP6) determine the extent of response to agonist. In addition, SNPs in the GP6 gene have been proposed as risk factors for coronary artery disease.

METHODS

To completely characterize genetic variation in the GP6 gene we generated a high-resolution SNP map by sequencing the promoter, exons and consensus splice sequences in 94 non-related Caucasoids. In addition, we sequenced DNA encoding the ligand-binding domains of GP6 from non-human primates to determine the level of evolutionary conservation.

RESULTS

Eighteen SNPs were identified, six of which encoded amino acid substitutions in the mature form of the protein. The single non-synonymous SNP identified in the exons encoding the ligand-binding domains, encoding for a 103Leu > Val substitution, resulted in reduced ligand binding. Two common protein isoforms were confirmed in Caucasoid with frequencies of 0.82 and 0.15. Variation at the GP6 locus was characterized further by determining SNP frequency in over 2000 individuals from different ethnic backgrounds.

CONCLUSIONS

The SNPs were polymorphic in all populations studied although significant differences in allele frequencies were observed. Twelve additional GP6 protein isoforms were identified from the genotyping results and, despite extensive variation in GP6, the sequence of the ligand-binding domains is conserved. Sequences from non-human primates confirmed this observation. These data provide valuable information for the optimal selection of genetic variants for use in future association studies.

Gain- and loss-of-function mutants confirm the importance of apical residues to the primary interaction of human glycoprotein VI with collagen

BACKGROUND

By site-directed mutagenesis of recombinant receptor fragments, we have previously identified residue lysine59 of the platelet collagen receptor glycoprotein VI (GPVI) as being critical for its interaction with the synthetic ligand collagen-related peptide (CRP) and the inhibitory phage antibody 10B12. Lysine59 is proposed to lie on the apical surface of the receptor near the linker joining the two immunoglobulin (Ig)-like extracellular domains. Recently, others have postulated the involvement of a portion of the first domain distant from the interdomain hinge as being involved in an extended collagen-binding site.

AIM AND METHODS

To extend our knowledge of the primary collagen-binding site of GPVI, a number of neighboring residues on the apical surface of recombinant soluble GPVI were mutated to alanine and binding of these mutants, as well as the lysine59 mutant, to fibrillar collagen was measured.

RESULTS

Binding of recombinant GPVI to collagen, like CRP, was dramatically reduced by the mutation of residue lysine59 to glutamate. Remarkably, the mutation of residues arginine60 in domain one and arginine166 in domain two, individually to alanine, which had no significant affect on CRP binding, reduced binding of recombinant GPVI to collagen. Mutation of the residue lysine41 to alanine dramatically increased binding to both CRP and collagen. This mutation abolished 10B12 binding, confirming its position in the epitope of our inhibitory phage antibody.

CONCLUSIONS

Residues lysine59, arginine60, and arginine166, from both Ig-like domains of GPVI, are critical for collagen binding by the receptor. This provides additional evidence for a basic patch on the apical surface of the receptor as the primary collagen-binding site of GPVI.

Adhesive surface determines raft composition in platelets adhered under flow

Adhesion to von Willebrand factor (VWF) induces platelet spreading, whereas adhesion to collagen induces aggregation. Here we report that cholesterol-rich domains (CRDs) or rafts play a critical role in clustering of receptors that control these responses. Platelets adhered to VWF and collagen show CRDs concentrated in filopodia which contain both the VWF receptor glycoprotein (GP) Ibalpha and the collagen receptor GPVI. Biochemical analysis of CRDs shows a threefold enrichment of GPIbalpha (but not GPVI) in VWF-adhered platelets and a fourfold enrichment of GPVI (but not GPIbalpha) in collagen-adhered platelets. Depletion of cholesterol (i) leaves the initial adhesion unchanged, (ii) inhibits spreading on VWF and aggregate formation on collagen, (iii) leaves filopodia formation intact, and (iv) reduces the localization in filopodia of GPIbalpha but not of GPVI. These data show that the adhesive substrate determines the composition of CRDs, and that cholesterol is crucial for redistribution of GPIbalpha but not of GPVI.

Platelet endothelial cell adhesion molecule-1 is a negative regulator of platelet-collagen interactions

The functional importance of platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) in platelets is unclear. Because PECAM-1 represents a newly assigned immunoglobulin-ITIM superfamily member expressed on the surface of platelets, it was hypothesized that it may play an important regulatory role in modulating ITAM-bearing receptors such as collagen (GP)VI receptor and FcgammaRIIA. To examine the functional role of PECAM-1 in regulating platelet-collagen interactions, 2 different approaches were applied using recombinant human PECAM-1-immunoglobulin chimeras and platelets derived from PECAM-1-deficient mice. Stimulation of platelets by collagen-, (GP)VI-selective agonist, collagen-related peptide (CRP)-, and PECAM-1-immunoglobulin chimera induced tyrosine phosphorylation of PECAM-1 in a time- and dose-dependent manner. Activation of PECAM-1 directly through the addition of soluble wild-type PECAM-1-immunoglobulin chimera, but not mutant K89A PECAM-1-immunoglobulin chimera that prevents homophilic binding, was found to inhibit collagen- and CRP-induced platelet aggregation. PECAM-1-deficient platelets displayed enhanced platelet aggregation and secretion responses on stimulation with collagen and CRP, though the response to thrombin was unaffected. Under conditions of flow, human platelet thrombus formation on a collagen matrix was reduced in a dose-dependent manner by human PECAM-1-immunoglobulin chimera. Platelets derived from PECAM-1-deficient mice form larger thrombi when perfused over a collagen matrix under flow at a shear rate of 1800 seconds(-1) compared to wild-type mice. Collectively, these results indicate that PECAM-1 serves as a physiological negative regulator of platelet-collagen interactions that may function to negatively limit growth of platelet thrombi on collagen surfaces.

Platelet adhesion enhances the glycoprotein VI-dependent procoagulant response: Involvement of p38 MAP kinase and calpain

In the final stages of activation, platelets express coagulation-promoting activity by 2 simultaneous processes: exposure of aminophospholipids, eg, phosphatidylserine (PS), at the platelet surface, and formation of membrane blebs, which may be shed as microvesicles. Contact with collagen triggers both processes via platelet glycoprotein VI (GPVI). Here, we studied the capacity of 2 GPVI ligands, collagen-related peptide (CRP) and the snake venom protein convulxin (CVX), to elicit the procoagulant platelet response. In platelets in suspension, either ligand induced full aggregation and high Ca(2+) signals but little microvesiculation or PS exposure. However, most of the platelets adhering to immobilized CRP or CVX had exposed PS and formed membrane blebs after a prolonged increase in cytosolic [Ca(2+)](i). Platelets adhering to fibrinogen responded similarly but only when exposed to soluble CRP or CVX. By scanning electron microscopic analysis, the bleb-forming platelets were detected as either round, spongelike structures with associated microparticles or as arrays of vesicular cell fragments. The phosphorylation of p38 mitogen-activated protein kinase (MAPK) elicited by CRP and CVX was enhanced in fibrinogen-adherent platelets compared with that in platelets in suspension. The p38 inhibitor SB203580 and the calpain protease inhibitor calpeptin reduced only the procoagulant bleb formation, having no effect on PS exposure. Inhibition of p38 also downregulated calpain activity. We conclude that the procoagulant response evoked by GPVI stimulation is potentiated by platelet adhesion. The sequential activation of p38 MAPK and calpain appears to regulate procoagulant membrane blebbing but not PS exposure.

Integrin-independent tyrosine phosphorylation of p125(fak) in human platelets stimulated by collagen

Collagen fibers or a glycoprotein VI-specific collagen-related peptide (CRP-XL) stimulated tyrosine phosphorylation of the focal adhesion kinase, p125(fak) (FAK), in human platelets. An integrin alpha(2)beta(1)-specific triple-helical peptide ligand, containing the sequence GFOGER (single-letter nomenclature, O = Hyp) was without effect. Antibodies to the alpha(2) and beta(1) integrin subunits did not inhibit platelet FAK tyrosine phosphorylation caused by either collagen fibers or CRP-XL. Tyrosine phosphorylation of FAK caused by CRP-XL or thrombin, but not that caused by collagen fibers, was partially inhibited by GR144053F, an antagonist of integrin alpha(IIb)beta(3). The intracellular Ca(2+) chelator, BAPTA, and the protein kinase C inhibitor, Ro31-8220, were each highly effective inhibitors of the FAK tyrosine phosphorylation caused by collagen or CRP-XL. These data suggest that, in human platelets, 1) occupation or clustering of the integrin alpha(2)beta(1) is neither sufficient nor necessary for activation of FAK, 2) the fibrinogen receptor alpha(IIb)beta(3) is not required for activation of FAK by collagen fibers, and 3) both intracellular Ca(2+) and protein kinase C activity are essential intermediaries of FAK activation.

Monoclonal antibodies identify residues 199-216 of the integrin alpha2 vWFA domain as a functionally important region within alpha2beta1

Integrin alpha2beta1 is the major receptor for collagens in the human body, and the collagen-binding site on the alpha2 subunit von Willebrand factor A-type domain (vWFA domain) is now well defined. However, the biologically important conformational changes that are associated with collagen binding, and the means by which the vWFA domain is integrated into the whole integrin are not completely understood. We have raised monoclonal antibodies against recombinant alpha2 vWFA domain for use as probes of function. Three antibodies, JA202, JA215 and JA218, inhibited binding to collagen, collagen I C-propeptide and E-cadherin, demonstrating that their function is important for structurally diverse alpha2beta1 ligands. Cross-blocking studies grouped the epitopes into two clusters: (I) JA202, the inhibitory antibody, Gi9, and a non-inhibitory antibody, JA208; (II) JA215 and JA218. Both clusters were sensitive to events at the collagen binding site, as binding of Gi9, JA202, JA215 and JA218 were inhibited by collagen peptide, JA208 binding was enhanced by collagen peptide, and binding of JA202 was decreased after mutagenesis of the cation-binding residue Thr(221) to alanine. Binding of cluster I antibodies was inhibited by the anti-functional anti-beta1 antibody Mab13, and binding of Gi9 and JA218 to alpha2beta1 was inhibited by substituting Mn(2+) for Mg(2+), demonstrating that these antibodies were sensitive to changes initiated outside the vWFA domain. Mapping of epitopes showed that JA202 and Gi9 bound between residues 212-216, while JA208 bound between residues 199-216. We have therefore identified two epitope clusters with novel properties; i.e. they are intimately associated with the collagen-binding site, responsive to conformational changes at the collagen-binding site and sensitive to events initiated outside the vWFA domain.

Receptors and signalling mechanisms in the procoagulant response of platelets

Platelets in an advanced stage of activation change from coagulation-inactive to coagulation-promoting cells. This procoagulant response is characterised by exposure of aminophospholipids, such as phosphatidylserine, to the platelet surface and by formation of microvesicles. Under specific conditions, when both signalling and adhesive platelet receptors are occupied, collagen and also thrombin are able to trigger this response. Thus, platelets express high coagulation-promoting activity only after interacting with multiple receptors.

Micromolar Ca2+ concentrations are essential for Mg2+-dependent binding of collagen by the integrin alpha 2beta 1 in human platelets

Integrin receptor alpha(2)beta(1) requires micromolar Ca(2+) to bind to collagen and to the peptide GPC(GPP)(5)GFOGER(GPP)(5)GPC (denoted GFOGER-GPP, where O represents hydroxyproline), which contains the minimum recognition sequence for the collagen-binding alpha(2) I-domain (Knight, C. G., Morton, L. F., Peachey, A. R., Tuckwell, D. S., Farndale, R. W., and Barnes, M. J. (2000) J. Biol. Chem. 275, 35-40). Platelet adhesion to these ligands is completely dependent on alpha(2)beta(1) in the presence of 2 mm Mg(2+). However, we show here that this interaction was abolished in the presence of 25 microm EGTA. Adhesion of Glanzmann's thrombasthenic platelets, which lack the fibrinogen receptor alpha(IIb)beta(3), was also inhibited by micromolar EGTA. Mg(2+)-dependent adhesion of platelets was restored by the addition of 10 microm Ca(2+), but millimolar Ca(2+) was inhibitory. Binding of isolated alpha(2)beta(1) to GFOGER-GPP was 70% inhibited by 50 microm EGTA but, as with intact platelets, was fully restored by the addition of micromolar Ca(2+). 2 mm Ca(2+) did not inhibit binding of isolated alpha(2)beta(1) to collagen or to GFOGER-GPP. Binding of recombinant alpha(2) I-domain was not inhibited by EGTA, nor did millimolar Ca(2+) inhibit binding. Our data suggest that high affinity Ca(2+) binding to alpha(2)beta(1), outside the I-domain, is essential for adhesion to collagen. This is the first demonstration of a Ca(2+) requirement in alpha(2)beta(1) function.

Convulxin binding to platelet receptor GPVI: competition with collagen related peptides

Convulxin (CVX), a potent platelet aggregating protein from the venom of the snake Crotalus durissus terrificus, is known to bind to the platelet collagen receptor, glycoprotein VI (GPVI). CVX binding to human platelets was investigated by flow cytometry, using fluorescein labeled convulxin (FITC-CVX). Scatchard analysis indicated high and low affinity binding sites with Kd values of 0.6 and 4 nM and Bmax values of 1200 and 2000 binding sites per platelet. FITC-CVX binding was inhibited by collagen related peptides (CRPs) comprising a repeated GPO sequence, namely GCO(GPO)(10)GCOGNH(2) and GKO(GPO)(10)GKOGNH(2), which also bind to receptor GPVI. These peptides (monomeric or cross-linked forms) gave a high affinity inhibition of 10-20% for concentrations between 10 ng/ml and 5 microg/ml, followed by a second phase of inhibition at concentrations greater than 5 microg/ml. It was shown also that the inhibition of FITC-CVX binding by CRPs was independent on the time of preincubation of platelets with CRPs, and the same percentage of inhibition was seen with various concentrations of convulxin. Confocal microscopy of the distribution of FITC-CVX binding sites on platelets showed an homogeneous distribution of FITC-CVX bound to GPVI, although some limited clustering may exist.

Structural basis of collagen recognition by integrin alpha2beta1

We have determined the crystal structure of a complex between the I domain of integrin alpha2beta1 and a triple helical collagen peptide containing a critical GFOGER motif. Three loops on the upper surface of the I domain that coordinate a metal ion also engage the collagen, with a collagen glutamate completing the coordination sphere of the metal. Comparison with the unliganded I domain reveals a change in metal coordination linked to a reorganization of the upper surface that together create a complementary surface for binding collagen. Conformational changes propagate from the upper surface to the opposite pole of the domain, suggesting both a basis for affinity regulation and a pathway for signal transduction. The structural features observed here may represent a general mechanism for integrin-ligand recognition.

The collagen-binding A-domains of integrins alpha(1)beta(1) and alpha(2)beta(1) recognize the same specific amino acid sequence, GFOGER, in native (triple-helical) collagens

We have previously assigned an integrin alpha(2)beta(1)-recognition site in collagen I to the sequence, GFOGERGVEGPOGPA (O = Hyp), corresponding to residues 502-516 of the alpha(1)(I) chain and located in the fragment alpha(1)(I)CB3 (Knight, C. G., Morton, L. F., Onley, D. J., Peachey, A. R., Messent, A. J., Smethurst, P. A., Tuckwell, D. S., Farndale, R. W., and Barnes, M. J. (1998) J. Biol. Chem. 273, 33287-33294). In this study, we show that recognition is entirely contained within the six-residue sequence GFOGER. This sequence, when in triple-helical conformation, readily supports alpha(2)beta(1)-dependent cell adhesion and exhibits divalent cation-dependent binding of isolated alpha(2)beta(1) and recombinant alpha(2) A-domain, being at least as active as the parent collagen. Replacement of E by D causes loss of recognition. The same sequence binds integrin alpha(1) A-domain and supports integrin alpha(1)beta(1)-mediated cell adhesion. Triple-helical GFOGER completely inhibits alpha(2) A-domain binding to collagens I and IV and alpha(2)beta(1)-dependent adhesion of platelets and HT 1080 cells to these collagens. It also fully inhibits alpha(1) A-domain binding to collagen I and strongly inhibits alpha(1)beta(1)-mediated adhesion of Rugli cells to this collagen but has little effect on either alpha1 A-domain binding or adhesion of Rugli cells to collagen IV. We conclude that the sequence GFOGER represents a high-affinity binding site in collagens I and IV for alpha(2)beta(1) and in collagen I for alpha(1)beta(1). Other high-affinity sites in collagen IV mediate its recognition of alpha(1)beta(1).

Fluid secretion by isolated Malpighian tubules of Drosophila melanogaster Meig.: effects of organic anions, quinacrine and a diuretic factor found in the secreted fluid

Para-aminohippuric acid (PAH, 0.2 and 1 mmol l(−)(1)) had no effect on the basal fluid secretion rate (FSR) of isolated Malpighian tubules of Drosophila melanogaster Meig. and did not affect stimulation of the FSR induced by adenosine 3′,5′-monophosphate (cAMP). Phenol Red (phenolsulphonphthalein, PSP; 0.5 and 1 mmol l(−)(1)) slowed the FSR and abolished stimulation of the FSR by cAMP. Diodrast (1 mmol l(−)(1)) slightly, but significantly, reduced the FSR and greatly reduced the stimulation of the FSR normally provoked by cAMP and by the 3′,5′-monophosphates of guanosine (cGMP), inosine (cIMP) and uridine (cUMP). However, stimulation of the FSR by the 3′, 5′-monophosphate of cytidine (cCMP) was little affected by diodrast. Probenecid (0.2 or 1 mmol l(−)(1)) consistently stimulated the FSR, on average by approximately 25 %, but did not markedly inhibit the subsequent stimulation of the FSR by cAMP, cGMP or cIMP. However, the FSR of tubules stimulated by cGMP was temporarily lowered by probenecid. Quinacrine (0.1 mmol l(−)(1)) slowed basal FSR by an average of approximately 30 %, but subsequent stimulation of the FSR by cAMP was not noticeably affected. Both 0.1 mmol l(−)(1) cAMP and 1 mmol l(−)(1) probenecid stimulated adenylate cyclase activity in extracts of Malpighian tubules, but cIMP, cGMP, cUMP and diodrast were without effect in this regard. Uptake of radioactivity from a solution containing 500 nmol l(−)(1) [(3)H]cAMP and 9.5 μmol l(−)(1) cAMP was reduced by more than 90 % by 1 mmol l(−)(1) PSP, by approximately 40 % by 0.2 mmol l(−)(1) probenecid, by 36 % by 1 mmol l(−)(1) diodrast and by 30 % by 1 mmol l(−)(1) PAH. Neither 0.01 mmol l(−)(1) ouabain nor 0.1 mmol l(−)(1) quinacrine affected the uptake of [(3)H]cAMP by the Malpighian tubules. Fluid secreted by isolated Malpighian tubules of Drosophila melanogaster contains a factor that stimulated the FSR on average by approximately 50 %. The presence in the secreted fluid of cGMP at a concentration of 8.3 μmol l(−)(1) did not explain the stimulatory effect on FSR. These results support the existence of a carrier-mediated uptake of cyclic nucleotides into the Malpighian tubules of Drosophila melanogaster, possibly involving a multispecific transporter.

Collagens and atherosclerosis

Smooth muscle cells in the atherosclerotic lesions of diseased arteries produce new extracellular matrix, largely collagenous in nature, which is responsible in part for the occlusion of the vessel lumen by the atherosclerotic plaque. These smooth muscle cells express a different phenotype, responsive to growth factors, to that of the differentiated, nondividing contractile cell in the media. Specific collagens may be involved in the regulation of phenotype and in the migration of the cells to the site of lesion growth. Collagens may also be involved in the calcification of lesions, in the retention of low-density lipoprotein in the vessel wall and in smooth muscle cell survival. Glycation of collagen may promote atherogenesis. Effects as summarized in this short review, are not always, at first sight, consistent. The following points should be kept in mind, though, when considering the response of a cell to collagen. Any effect may be governed not just by the identity of the collagen type as such but by its state of polymerization: monomeric collagen, for instance, whether in solution or immobilized on plastic, may express different effects to the same collagen type when presented in its native polymerized state, e.g., as fibers. The precise identity of the cell and its location may be important: SMCs in secondary culture may not necessarily respond to any given collagen exactly as SMCs within the lesion or possess precisely the same properties, albeit both types are regarded as expressing the same (synthetic) phenotype. Effects may not necessarily be directly attributable to collagen, but to some other matrix constituent bound to collagen.

Inhibition of human platelet adenylate cyclase activity by adrenaline, thrombin and collagen: analysis and reinterpretation of experimental data

Mathematical models based on the current understanding of stimulation and inhibition of adenylate cyclase (AC) activity have been developed and used to analyse experimental data [Farndale, Winkler, Martin and Barnes (1992) Biochem. J. 282, 2532] describing the inhibition of human platelet AC by collagen, thrombin and adrenaline. Here it has been demonstrated that neither affinities of receptors specific for adrenaline or thrombin nor the activity of cAMP phosphodiesterase are affected by collagen. Both collagen and thrombin at high doses act as effective inhibitors of AC activity. Inhibition of AC activity by collagen proceeds via two parallel pathways; the same is true for thrombin at moderate concentrations, and the two ligands act independently. The G-protein-dependence of these pathways is distinct from that mediating inhibition of AC activity by adrenaline, i.e. Gi2. Convergence of the inhibitory pathways takes place at the catalytic subunit of AC.

Function of glycoprotein VI and integrin alpha2beta1 in the procoagulant response of single, collagen-adherent platelets

Various collagen-based materials were used to assess the structural requirements of collagen for inducing the procoagulant response of adhering platelets, as well as the collagen receptors involved. Cross-linked or monomeric collagen-related peptide (CRP), Gly-Cys-Hyp-(Gly-Pro-Hyp)10-Gly-Cys-Hyp-Gly was highly adhesive for platelets in a glycoprotein VI-(GpVI-)dependent manner. Adhesion was followed by a prolonged increase in cytosolic [Ca2+]i, formation of membrane blebs, exposure of phosphatidylserine (PS) and generation of prothrombinase-stimulating activity. Fibrils of type-I collagen were less adhesive but, once adhered, many of the platelets presented a full procoagulant response. Monomeric type-I collagen was unable to support adhesion, unless Mg(2+)-dependent integrin alpha2beta1 interactions were facilitated by omission of Ca2+ ions. With all surfaces, however, post-addition of CaCl2 to adhering platelets resulted in a potent Ca(2+)-influx signal, followed by PS exposure and bleb formation. The procoagulant response elicited by binding to CRP was inhibited by anti-GpVI Fab fragments, but not by impeding integrin alpha2beta1-mediated events. With fibrillar collagen, it was inhibited by blocking either the GpVI- or integrin alpha2beta1-mediated interactions. This suggests that the triple-helical Gly-Pro-Hyp repeat in CRP and analogous sequences in fibrillar collagen stimulate the procoagulant response of adherent platelets by acting as ligands for GpVI. Influx of Ca2+ is required for this response, and adhesion via integrin alpha2beta1 serves to potentiate the signaling effects of GpVI.

Monomeric (glycine-proline-hydroxyproline)10 repeat sequence is a partial agonist of the platelet collagen receptor glycoprotein VI

We have previously reported that a triple-helical, collagen-related peptide (CRP; also known as CRP-XL) containing a glycine-proline-hydroxyproline (GPP*) repeat motif and cross-linked through cysteine residues at its N-terminus and C-terminus is a powerful stimulus of platelet aggregation and secretion through the surface receptor glycoprotein VI (GPVI). The activation of platelets is associated with tyrosine phosphorylation of the tyrosine kinase Syk and phospholipase C gamma2 (PLCgamma2). We now report that the non-cross-linked backbone of CRP, monomeric CRP (mCRP), stimulates the tyrosine phosphorylation of Syk and PLCgamma2 in platelets and induces the weak secretion of [3H]5-hydroxytryptamine ([3H]5-HT) and aggregation. The action of mCRP does not seem to be due to spontaneous cross-linking, because alkylation of the cysteine residues leads to an increase in activity. The tripeptide backbone of CRP, GPP*10 (in which P* represents hydroxyproline) also stimulates platelet shape change and the weak tyrosine phosphorylation of Syk and PLCgamma2, but is unable to induce aggregation or secretion. The monomeric peptides partly inhibit the release of [3H]5-HT by CRP, suggesting that they are partial agonists of the collagen receptor GPVI. These results demonstrate that GPP* present as a repeat motif is sufficient to activate the platelet collagen receptor GPVI but that the cross-linking of monomers brings about an increase in activity.

Collagen-platelet interaction: Gly-Pro-Hyp is uniquely specific for platelet Gp VI and mediates platelet activation by collagen

OBJECTIVE

Peptides consisting of a repeat Gly-Pro-Hyp sequence are potent platelet agonists. The aim of this study was: (1) to examine the specificity of this sequence for platelet activation; (2) to confirm its recognition by platelet glycoprotein VI; and (3) to assess with suitable peptides the relative importance of glycoprotein VI and integrin alpha 2 beta 1 in platelet activation by collagen.

METHODS

Peptides were synthesized by standard Fmoc chemistry and tested for their ability to support adhesion of human platelets and HT 1080 cells, induce platelet aggregation, bind integrin alpha 2 subunit A-domain and to cause tyrosine phosphorylation of platelet proteins.

RESULTS

(1) Peptides consisting of a repeat Gly-Pro-Pro, Gly-Pro-Ala or Gly-Pro-Arg sequence exhibited little if any platelet-reactivity. (2) The platelet-reactive peptide consisting of a repeating Gly-Pro-Hyp sequence failed to induce tyrosine phosphorylation in glycoprotein VI-deficient platelets. Platelet adhesion to this peptide was inhibited by intact anti-glycoprotein VI antibody and its Fab fragment. The latter inhibited aggregation by the peptide and fibres of both collagens I and III. (3) A peptide containing a 15-mer alpha 2 beta 1-binding sequence in a repeat Gly-Pro-Pro structure supported alpha 2 beta 1-mediated platelet and HT 1080 cell adhesion and bound alpha 2 A-domain, but failed to activate platelets or to induce tyrosine phosphorylation. Conversely, a peptide containing this sequence but with an essential Glu replaced by Ala and inserted in a repeat Gly-Pro-Hyp structure did not recognize alpha 2 beta 1, but was highly platelet activatory.

CONCLUSIONS

Platelet activation by collagen involves the highly-specific recognition of the Gly-Pro-Hyp sequence by platelet glycoprotein VI. Recognition of alpha 2 beta 1 is insufficient to cause activation. Interaction between collagen and glycoprotein VI is unique since Gly-Pro-Hyp is common in collagens but occurs rarely in other proteins, and glycoprotein VI may be expressed solely by platelets. This sequence could provide a basis for a highly-specific anti-thrombotic reagent to control thrombosis associated with plaque rupture.

Stimulation of fluid secretion of malpighian tubules of drosophila melanogaster meig. by cyclic nucleotides of inosine, cytidine, thymidine and uridine

External application of the 3',5'-cyclic monophosphates of inosine,cytidine, uridine and thymidine stimulated the fluid secretion rate (FSR)of Malpighian tubules isolated from Drosophila melanogaster. The evidence suggested that the cyclic nucleotides acted intracellularly in some capacity. Receptors of the 'purinergic' type appeared not to be major contributors to fluid secretion; of three purinergic agonists tried,adenosine, adenosine 5'-monophosphate (AMP) and adenosine 5'-triphosphate(ATP), only adenosine had an effect, but this was not observed consistently. None of the purinergic agonists interfered with the stimulation of the FSR by adenosine 3',5'-cyclic monophosphate (cAMP). The maximum stimulation of the fluid-secretion rate by any cyclic nucleotide was approximately double the unstimulated (control) rate. Tubules stimulated to less than maximal FSR by one cyclic nucleotide could be stimulated maximally by an appropriate concentration of another cyclic nucleotide. Malpighian tubules bathed in solutions that contained either[3H]cAMP or [3H]cGMP accumulated radioactivity to a level many times that in the medium. Accumulation of radioactivity by tubules bathed in 430 nmol l-1 [3H]cAMP was suppressed by 1 mmol l-1 non-radioactive cyclic nucleotides in the order cAMP>>cGMP>cIMP>cCMP; neither cTMP nor cUMP suppressed the accumulation of [3H]cAMP. Approximately 35 % of the[3H]cAMP and 80 % of the [3H]cGMP that entered the Malpighian tubule cells was metabolised to compounds that were not identified. It was concluded that cyclic nucleotides enter the Malpighian tubule cells by at least one transport mechanism which is particularly sensitive to purine-based nucleotides.

Identification in collagen type I of an integrin alpha2 beta1-binding site containing an essential GER sequence

The collagen type I-derived fragment alpha1(I)CB3 is known to recognize the platelet collagen receptor integrin alpha2beta1 as effectively as the parent collagen, although it lacks platelet-aggregatory activity. We have synthesized the fragment as seven overlapping peptides that spontaneously assemble into triple helices. On the basis of their capacity to bind purified alpha2 beta1 and the recombinant alpha2 A-domain, and their ability to support alpha2 beta1-mediated cell adhesion, we identified two peptides, CB3(I)-5 and -6, which contain an alpha2 beta1 recognition site. Synthesis of the peptide CB3(I)-5/6, containing the overlap sequence between peptides 5 and 6, allowed us to locate the binding site within the 15-residue sequence, GFP*GERGVEGPP*GPA (where P* represents hydroxyproline), corresponding to residues 502-516 of the collagen type I alpha1 chain. The Glu and Arg residues in the GER triplet were found to be essential for recognition since substitution of either residue with Ala caused a loss of alpha2 A-domain binding. By contrast, substitution of the Glu in GVE did not reduce binding, but rather enhanced it slightly. We were unable to detect significant recognition of alpha2 beta1 by the peptide CB3(I)-2 containing the putative alpha2 beta1 recognition sequence DGEA. Peptides CB3(I)-1 to -6, together with peptide CB3(I)-5/6, exhibited good platelet-aggregatory activity, in some cases better than collagen. However, peptide CB3(I)-7 was inactive, suggesting the presence of an inhibitory element that might account for the lack of aggregatory activity of the parent alpha1(I)CB3 fragment.

The collagen-platelet interaction

Collagen-platelet interaction, occurring in hemostasis but also a cause of thrombosis, is a two-step process of adhesion and activation involving the sequential recognition of distinct receptors. Adhesion involves first the reversible recognition of collagen-bound von Willebrand factor by the platelet receptor complex Gp Ib/IX/V, followed by direct interaction between collagen and the platelet integrin receptor alpha2beta1, which binds to specific sequences in collagen in which the GER motif appears important. Platelet activation then follows from the recognition by the receptor Gp VI of GPP* sequences in collagen, culminating in signalling events unique to collagen as a platelet agonist: Gp VI leads via the novel platelet Fc receptor gamma-chain to p72syk and phospholipase Cgamma2.

Glycoprotein VI is a major collagen receptor for platelet activation: it recognizes the platelet-activating quaternary structure of collagen, whereas CD36, glycoprotein IIb/IIIa, and von Willebrand factor do not

Simple collagen-related peptides (CRPs) containing a repeat Gly-Pro-Hyp sequence are highly potent platelet agonists. Like collagen, they must exhibit tertiary (triple-helical) and quaternary (polymeric) structure to activate platelets. Platelet signaling events induced by the peptides are the same as most of those induced by collagen. The peptides do not recognize the alpha 2 beta 1 integrin. To identify the signaling receptor involved, we have evaluated the response to the CRP, Gly-Lys-Hyp(Gly-Pro-Hyp)10-Gly-Lys-Hyp-Gly of platelets with defined functional deficiencies. These studies exclude a primary recognition role for CD36, von Willebrand factor (vWF), or glycoprotein (GP) IIb/IIIa. Thus, both CD36 and vWF-deficient platelets exhibited normal aggregation, normal fibrinogen binding, and normal expression of CD62 and CD63, measured by flow cytometry, in response to the peptide, and there was normal expression of CD62 and CD63 on thrombasthenic platelets. In contrast, GPVI-deficient platelets were totally unresponsive to the peptide, indicating that this receptor recognizes the Gly-Pro-Hyp sequence in collagen. GPVI-deficient platelets showed some fibrinogen binding in response to collagen but failed to aggregate and to express CD62 and CD63. Collagen, but not CRP-XL, contains binding sites for alpha 2 beta 1. Therefore, it is possible that collagen still induces some signaling via alpha 2 beta 1, leading to activation of GPIIb/IIIa. Our findings are consistent with a two-site, two-step model of collagen interaction with platelets involving recognition of specific sequences in collagen by an adhesive receptor such as alpha 2 beta 1 to arrest platelets under flow and subsequent recognition of another specific collagen sequence by an activatory receptor, namely GPVI.

Human platelet activation is inhibited upstream of the activation of phospholipase A2 by U73343

U73122 is known as an inhibitor of phospholipase C (PLC; EC 3.1.4.11). Its close structural analogue, U73343, lacks this activity and is used as a control compound. We have found that both compounds interfere with platelet signal transduction. U73122 completely abolished aggregation evoked by thrombin, TG, and collagen. Aggregation evoked by TG and collagen was also blocked by U73343, an effect due to inhibition of TxA2 production. U73343 was a potent inhibitor of TG-evoked arachidonic acid release, but a weak inhibitor of cytosolic phospholipase A2 (cPLA2; EC 3.1.1.4) activity. Cytosolic PLA2 activation in platelets involves protein tyrosine phosphorylation. U73343 inhibited TG- and collagen-evoked protein tyrosine phosphorylation, which can thus explain its action against these agents. These data indicate that caution is needed when using U73343 along with U73122 in the study of intracellular signalling pathways.

The platelet reactivity of synthetic peptides based on the collagen III fragment alpha1(III)CB4. Evidence for an integrin alpha2beta1 recognition site involving residues 522-528 of the alpha1(III) collagen chain

The platelet-reactive collagen III-derived fragment alpha1(III)CB4 has been synthesized as seven overlapping peptides, each as a homotrimeric triple-helical species covalently linked at the C terminus. Additional Gly-Pro-Hyp triplets were introduced at each end of the peptide sequence to ensure a stable triple-helical conformation at 20 degrees C, the temperature at which cell reactivity was measured. A Cys-containing triplet was included at each end to allow intermolecular cross-linking. All seven peptides in triple-helical, cross-linked form were able to cause platelet aggregation. Peptide 6, the most reactive species, was more aggregatory than collagen fibers. Platelet adhesion occurred to all peptides immobilized on plastic in monomeric form. Adhesion was integrin alpha2beta1-independent except in the case of peptide 6, adhesion to which was partially reduced by anti-integrin alpha2beta1 monoclonal antibodies. The presence of an alpha2beta1 recognition site in peptide 6 was confirmed using HT 1080 cells, which express alpha2beta1 as their major or sole collagen receptor. HT 1080 adhesion to both peptide 6 and collagen was strongly inhibited by anti-integrin alpha2beta1 monoclonal antibodies. These cells did not adhere to any of the other peptides. Comparison of the structure of peptide 6 with that of adjacent peptides indicates that the sequence Gly-Gly-Pro-Hyp-Gly-Pro-Arg, residues 522-528 of the collagen alpha1(III) chain, represents the minimum structure required for the recognition of alpha2beta1. Our findings support the view that the collagen triple helix possesses an intrinsic platelet reactivity that can be expressed independently of integrin alpha2beta1 and the precise level of which is governed by the exact nature of the primary sequence. Sequences such as those recognizing alpha2beta1 may potentiate the activity, whereas others may have the opposite effect.

A collagen-like peptide stimulates tyrosine phosphorylation of syk and phospholipase C gamma2 in platelets independent of the integrin alpha2beta1

Activation of platelets by collagen is mediated through a tyrosine kinase-dependent pathway that is associated with phosphorylation of the Fc receptor gamma chain, the tyrosine kinase syk, and phospholipase C gamma2 (PLC gamma2). We recently described a collagen-related triple-helical peptide (CRP) with the sequence GCP*(GPP*)GCP*G (single letter amino acid code: P* = hydroxyproline; Morton et al, Biochem J306:337, 1995). The cross-linked peptide is a potent stimulus of platelet activation but, unlike collagen, does not support alpha2beta1-mediated, Mg2+-dependent adhesion, suggesting that its action is independent of the integrin alpha2beta1. This finding suggests the existence of a platelet receptor other than alpha2beta1 that underlies activation. In the present study, we show that CRP stimulates tyrosine phosphorylation of the same pattern of proteins in platelets as collagen, including syk and PLC gamma2. Protein tyrosine phosphorylation induced by CRP is not altered in the absence of Mg2+ or the presence of monoclonal antibodies (MoAbs) to the integrin alpha2beta1 (MoAb 6F1 and MoAb 13), conditions that prevent the interaction of collagen with the integrin. In contrast, phosphorylation of syk and PLC gamma2 by collagen is partially reduced by MoAb 6F1 and MoAb 13 or by removal of Mg2+. This may reflect a direct role of alpha2beta1 in collagen-induced signaling events or an indirect role in which the integrin facilitates the binding of collagen to its signaling receptor. The results show an alpha2beta1-independent pathway of platelet activation by CRP that involves phosphorylation of syk and PLC gamma2. This pathway appears to contribute to platelet activation by collagen.

Effects of U73122 and U73343 on human platelet calcium signalling and protein tyrosine phosphorylation

We have investigated the actions of the PLC inhibitor, U73122, and its close analogue, U73343, which does not inhibit PLC, in Fura-2-loaded human platelets. Rises in [Ca2+]i evoked by thrombin and collagen, and the TxA2-dependent rise in [Ca2+]i evoked by thapsigargin, were abolished by U73122, indicating that it inhibits the activity of both beta and gamma isoforms of PLC. The supposed control compound U73343, was found to inhibit TxA2 formation; it therefore partially inhibited the rise in [Ca2+]i evoked by low concentrations of thrombin, by thapsigargin or by collagen. U73343 had a greater effect than aspirin on the action of collagen, indicating an action on the TxA2-independent component of the signal, via PLC gamma-U73343 lowered TxA2 production by inhibiting the activation of cPLA2, probably at a tyrosine phosphorylation step. U73343 seems to inhibit only the tyrosine kinases involved in the activation of PLC gamma and the generation of TxA2. In contrast, U73122 increased tyrosine phosphorylation of platelet proteins, perhaps by inhibiting receptor independent tyrosine phosphatases, but inhibited all further tyrosine phosphorylation on addition of thrombin or other agonists.

Ionomycin-stimulated arachidonic acid release in human platelets: a role for protein kinase C and tyrosine phosphorylation

Collagen (10-90 micrograms/ml) and ionomycin (1 microM; a calcium ionophore) each evoked rises in intracellular free calcium, protein kinase C activity and arachidonic acid release in human platelets, and as previously demonstrated for collagen, ionomycin (1 microM) stimulated protein tyrosine phosphorylation. However, at lower concentrations (60 and 250 nM) ionomycin selectively mobilised calcium. Ro31-8220 (a selective inhibitor of protein kinase C) inhibited (by 50%) ionomycin-stimulated arachidonic acid release. Genistein (an inhibitor of protein tyrosine kinases) also reduced by 50% ionomycin-stimulated arachidonic acid release. In combination, genistein and Ro31-8220 abolished ionomycin-stimulated arachidonic acid release. These findings show 1) that a rise in calcium is not sufficient, and 2) the activation of both protein kinase C and protein tyrosine phosphorylation is necessary, for full ionomycin-stimulated arachidonic acid release in human platelets.

Role for p38 mitogen-activated protein kinase in platelet aggregation caused by collagen or a thromboxane analogue

p38 mitogen-activated protein kinase (MAPK) was identified in platelets on the basis of (a) its reactivity with antibodies to C-terminal and N-terminal peptides, and (b) its ability to activate MAPK-activated protein kinase-2, which phosphorylates the small heat shock protein, hsp27. p38 MAPK was activated in platelets by collagen fibers, a collagen-related cross-linked peptide, thrombin, or the thromboxane analogue U46619. A highly specific inhibitor of p38 MAPK, a pyridinyl imidazole known as SB203580, inhibited the platelet enzyme in vitro (IC50 approximately 0.5 microM). At similar concentrations it also inhibited agonist-stimulated phosphorylation of hsp27 in platelets, and platelet aggregation and secretion induced by minimal aggregatory concentrations of collagen or U46619, but not thrombin. Inhibition of aggregation was overcome by increasing agonist dose. SB203580 might act by inhibiting thromboxane generation, but this was only inhibited by 10-20% at low agonist concentrations. p38 MAPK provides a crucial signal, which is necessary for aggregation caused by minimal concentrations of collagen fibers or U46619. Thrombin or high doses of these agonists generate signals that bypass the enzyme, or render the enzyme no longer rate-limiting.

Wortmannin inhibits store-mediated calcium entry and protein tyrosine phosphorylation in human platelets

The effects of the WT on store-mediated Ca2+ entry and protein tyrosine phosphorylation were investigated in fura-2-loaded human platelets. Wortmannin (2 microM) attenuated the rise in [Ca2+]i caused by Ca2+ entry while having no effect on the mobilisation of Ca2+ from internal stores. It also reduced store-depletion-evoked protein tyrosine phosphorylation. These findings demonstrate that WT is an inhibitor of tyrosine phosphorylation and store-mediated calcium entry and provide further evidence for the involvement of a tyrosine phosphorylation step in the link between Ca2+ store depletion and Ca2+ influx in human platelets.

Signals elicited from human platelets by synthetic, triple helical, collagen-like peptides

Synthetic collagen-like peptides, of general structure [Gly-Pro-HyP]n, adopt the triple-helical structure which is essential for the platelet-reactivity of native collagens. These peptides are potent activators of platelets, stimulating platelet aggregation at much lower dose than collagen fibres, but, unlike collagen fibres, they are not recognised by the integrin alpha 2 beta 1. We have examined the ability of the synthetic peptides to activate the various signalling pathways which regulate human platelet function. The peptides are potent activators of Ca2+ mobilisation and of protein kinase C, and they stimulate tyrosine phosphorylation of some substrates preferentially. However, in contrast with native type I collagen fibres, they are unable to inhibit platelet adenylate cyclase. This suggests a mode of action for the synthetic peptides which substantially overlaps, but which is not entirely identical with, that of native collagen.