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 α2β1 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 α2β1. Therefore, it is possible that collagen still induces some signaling via α2β1, 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 α2β1 to arrest platelets under flow and subsequent recognition of another specific collagen sequence by an activatory receptor, namely GPVI.

COLLAGENS OF THE subendothelium are major determinants of the thrombogenicity of the blood vessel wall.1 In particular, the fibrous collagens I and III, exposed as a consequence of injury to the vessel wall, are strong platelet agonists that induce shape change, release reaction, and aggregation. The mechanism of collagen-platelet interaction involves both the direct recognition of collagen receptors and indirect binding of collagen to the platelet surface via intermediary proteins.2,3 As an example of the latter, von Willebrand factor (vWF) plays an essential role in hemostasis by complexing to collagen(s) in the vessel wall and concomitantly binding to specific receptors on the platelet surface,4,5 the glycoprotein (GP) Ib/V/IX complex, and the activated GPIIb/IIIa complex,6thus forming a bridge between collagen(s) and platelets.

A number of platelet membrane proteins have been proposed as collagen receptors, including integrin α2β1(GPIa/IIa; as reviewed by Santoro and Zutter7), CD36 (GPIIIb, GPIV), and GPVI (p62). Their precise role in the overall process of interaction and the relationship between them remains unclear.

Nieuwenhuis et al8 and Kehrel et al9 have described patients with mild bleeding disorders attributable to deficient expression of platelet α2β1. The platelets from these patients had impaired collagen-induced aggregation but responded normally to all other platelet agonists. Further evidence for the involvement of α2β1 is that monoclonal antibodies (MoAbs) directed against the receptor are able to inhibit adhesion of platelets to collagen under either static or flow conditions.7,10-12 

CD36 binds to collagen type I fibers,13 and Fab fragments of polyclonal antibodies against CD36 have been reported to inhibit collagen-induced platelet aggregation.13,14 However, the role of CD36 as a platelet collagen receptor is not clear, because a significant proportion (1% to 4%) of Japanese and other East Asian populations lack this receptor on platelets but have no obvious bleeding disorder. Platelet adhesion to collagens I and III15,16 and platelet aggregation and secretion induced by these collagens is normal in CD36-deficient platelets.17,18 

Other Japanese patients with mild bleeding disorders have been described whose platelets show reduced responses to collagen associated with functional deficiency in GPVI.19-21 One of these patients developed antibodies to GPVI, which, when added to normal platelets, caused their aggregation and whose Fab fragments were able to reduce platelet aggregation induced by collagen.21,22GPVI-deficient platelets showed defective second phase adhesion in flowing blood, a reaction that is attributable mainly to the platelet-platelet interaction.23 

The interaction of platelets with collagen involves firstly adhesion and, subsequently, activation leading to second phase adhesion, secretion, and ultimately aggregation. Different mechanisms and receptor populations may be involved in these two processes.24,25 Collagen-induced platelet activation requires the expression by collagen of both tertiary (triple-helical) and quaternary (polymeric) structure.26-28 Recently, Morton et al29 have described simple collagen-related peptides (CRPs), comprising a repeating Gly-Pro-Hyp motif, that mimic the collagen tertiary (triple-helical) structure. They have shown that, when cross-linked via either lysyl or cysteine residues to impart quaternary (polymeric) structure, these peptides (CRP-XL) are extremely active platelet agonists, being more reactive than collagen fibers. In contrast, non–cross-linked (nonpolymeric) CRP antagonizes platelet activation stimulated by either native collagen or CRP-XL.29 CRPs possess the same tertiary structure as collagen and, like collagen, this conformation is essential for activity. Like collagen, CRP molecules spontaneously assemble into an organized quaternary structure and, again like collagen, the polymeric structure is essential for activity. It is therefore most likely that the potent platelet reactivity of CRP-XL reflects the platelet reactivity to collagen.

The interaction of CRP-XL with platelets does not involve the integrin α2β1; thus, CRP supports static platelet adhesion that is not blocked by the α2β1-directed MoAbs, 6F1 and MoAb13.29 These same antibodies fully block α2β1-mediated adhesion to immobilized monomeric collagen. The MoAbs are also without effect on aggregation stimulated by CRP-XL.29 The recognition of α2β1 by collagen is essential for adhesion of platelets under flow. However, CRP-XL is unable to support adhesion under flow conditions.30 Furthermore, the purified I-domain of the α2 subunit, which mediates the adhesion of native collagens to α2β1, does not recognize CRP-XL,31 and affinity chromatography using CRP-XL does not extract α2β1 from platelet lysates (Barnes et al, unpublished data). All of this establishes CRP as an agonist that operates entirely independently from α2β1.

Despite the absence of recognition of the integrin α2β1, CRPs signal in platelets in precisely the same way as native collagen fibers.32,33 Specifically, they stimulate tyrosine phosphorylation of the Fc receptor γ chain,34 of p72syk, and of phospholipase Cγ2 (PLCγ2)35—all responses characteristic of platelet activation by collagen. CRPs are therefore valuable tools for the identification of the collagen receptor that recognizes the quaternary structure of collagen and is responsible for signaling leading to activation. To this end, we have evaluated the response to CRP-XL of platelets deficient in GPIIb/IIIa, CD36, vWF, and GPVI.

Materials.

Ristocetin was obtained from Paesel (Frankfurt, Germany).

Human vWF was purified from Haemate HS (a kind gift of Centeon, Marburg, Germany), as previously described.36 

The vWF-directed MoAb, 4F9 (Immunotech, Marseilles, France), was conjugated to fluorescein isothiocyanate (FITC) using the antibody conjugation kit from Sigma (Deisenhofen, Germany) according to the manufacturer's instructions. The F/P ratio, calculated from the FITC absorbance at 495 nm and the IgG content, was 1.4. FITC-coupled MoAbs recognizing CD62 (P-selectin, GMP140, and PADGEM) or CD63 (GP53 and granulophysin), clone CLB-thromb/6 and CLB-gran/12, respectively, were obtained from Immunotech (Marseilles, France).

Highly purified human fibrinogen (Enzyme Research Labs, South Bend, IN) was conjugated with FITC using the FITC-celite (Calbiochem, Bad Soden, Germany) method according to Xia et al37 but using pH 7.8 buffer for 48 hours at 4°C, resulting in an F/P ratio of 5.0 to 5.2.

Collagen type I for use as a reference reagent has been described previously.18 

The triple-helical peptide, Gly-Lys-Hyp-[Gly-Pro-Hyp]10-Gly-Lys-Hyp-Gly (CRP), was synthesized and cross-linked via its lysyl residues to yield CRP-XL as described in detail by Morton et al29 and Barnes et al.32 

Patients.

All studies were performed with the patients or other blood donors giving informed consent. Samples from healthy controls were processed in parallel with all patient samples. None of the subjects had taken any medication affecting platelet functions for at least 2 weeks before the study.

The CD36-deficient platelets of a Japanese male blood donor Y.A. have been described before.18,38 Platelets from patient A.M. with Glanzmann's thrombasthenia type I were shown to contain less than 1% of normal amounts of GPIIb/IIIa.39 Patient P.R. had von Willebrand disease type 3 (according to the Sadler and Gralnick classification40). In addition, the patient suffered from an intestinal angiodysplasia, which is associated with life-threatening bleeding episodes, requiring frequent replacement therapy with vWF. The blood for the experiments was taken 2 weeks after the last treatment and directly before new medication. At that time the vWF:RCO was less than 1% of normal and vWF antigen was undetectable (Asserachrom vWF; Stago-Diagnostica, Asniers, France) in both the plasma and the platelets. The GPVI-deficient platelets of the female Japanese patient Y.A. have been described in detail.21,22,41 

Platelet aggregation.

Platelet aggregation studies were performed in an aggregometer (Chrono-Log, Haverton, PA) within 3 hours of venipuncture using platelet-rich plasma (PRP) containing 2 × 108platelets/mL, as described by Born and Cross.42 Blood was anticoagulated with 0.1 vol of 3.8% Na-citrate and PRP was prepared by centrifugation at 200g for 10 minutes at room temperature.

Flow cytometric analysis of ristocetin-induced vWF binding to platelets.

PRP was diluted to 5 × 107 platelets/mL with Tyrode's buffer, pH 7.4, to minimize the formation of platelet aggregates. Platelet suspension (200 μL) was added to 20 μL of ristocetin (at a range of concentrations in Tyrode's buffer, pH 7.4) without stirring to avoid aggregate formation. After 180 seconds, the platelets were fixed for 30 minutes with 1% formaldehyde (final concentration) in phosphate-buffered saline (PBS). Platelets were washed and resuspended in 200 μL of PBS, and FITC-conjugated anti-vWF MoAb was added at a saturating concentration (determined beforehand). After 1 hour of incubation at room temperature, 104 single platelets were analyzed in a flow cytometer (FACScan; Becton Dickinson, Heidelberg, Germany). Background binding obtained from parallel samples with FITC-conjugated isotype-specific mouse IgG was subtracted from each test sample. Excitation was at a wavelength of 488 nm. The FACScan was used in a standard configuration with a 530 nm bandpass filter. Data were obtained from fluorescence channels in a logarithmic mode.

Flow cytometric analysis of fibrinogen-FITC binding to CRP-activated platelets.

Diluted PRP (5 × 107 platelets/mL) was preincubated for 3 minutes at room temperature with 150 μg/mL fibrinogen-FITC (saturating concentration). Two hundred microliters of platelet suspension was added to 20 μL of a solution containing a range of concentrations of CRP-XL in 0.01 mol/L acetic acid at room temperature. The reaction was stopped after 120 seconds by fixation with 1% formaldehyde as described above. Platelets were washed, resuspended in 200 μL of PBS, and analyzed in a flow cytometer as described above. The nonspecific background labeling was determined using platelets from different patients with thrombasthenia type I and control platelets treated with 10 mmol/L of the inhibitory peptide GRGDSP (Novabiochem, Bad Soden, Germany) to prevent specific fibrinogen binding.

Flow cytometric analysis of CD62 and CD63 expression on CRP-activated platelets.

Two hundred microliters of diluted PRP (5 × 107platelets/mL) was added at room temperature to 20 μL 0.01 mol/L acetic acid containing CRP-XL at a range of concentrations. The reaction was stopped after 120 seconds with 1% formaldehyde, as described above. Platelets were washed and resuspended in PBS, and FITC-coupled MoAbs recognizing CD62 or CD63 were added and incubated for 1 hour at room temperature. The concentrations required for saturated binding of both antibodies were determined beforehand. After incubation, platelets were washed, resuspended in 200 μL of PBS, and analyzed as described above.

The interaction of CD36-deficient platelets with CRP.

Platelets from 8 control donors were aggregated by CRP-XL at concentrations from 20 to 100 ng/mL. Aggregation curves from 2 such donors are shown in Fig 1A. The CD36-deficient platelets of donor Y.A. reacted as normal control platelets when treated with CRP-XL, which induced full aggregation at a concentration of 37 ng/mL (Fig 1B). Fibrinogen binding to CRP-XL-activated control and CD36-deficient platelets did not differ significantly (Fig 2). CD36-deficient platelets activated by CRP-XL expressed CD62 and CD63 to the same extent as control platelets (data not shown).

Fig. 1.

Normal platelet aggregation induced by CRP-XL in platelet-rich plasma (PRP). (A) Control platelets; (B) CD36-deficient platelets from donor Y.A. The platelet counts were adjusted to 2 × 108/mL. The experiment shown is representative of three similar experiments.

Fig. 1.

Normal platelet aggregation induced by CRP-XL in platelet-rich plasma (PRP). (A) Control platelets; (B) CD36-deficient platelets from donor Y.A. The platelet counts were adjusted to 2 × 108/mL. The experiment shown is representative of three similar experiments.

Close modal
Fig. 2.

Normal binding of fibrinogen to CD36-deficient platelets and control platelets activated with CRP-XL measured by flow cytometry using FITC-labeled fibrinogen. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Fig. 2.

Normal binding of fibrinogen to CD36-deficient platelets and control platelets activated with CRP-XL measured by flow cytometry using FITC-labeled fibrinogen. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Close modal
The reaction of CRP with thrombasthenic platelets.

Because, as expected, specific fibrinogen binding after exposure to agonist was not observed in these GPIIb/IIIa-deficient platelets, activation was determined on the basis of the surface expression of CD62 (Fig 3A) and of CD63 (Fig 3B). Expression of both in GPIIb/IIIa-deficient platelets treated with CRP-XL was not lower than in controls.

Fig. 3.

Normal expression of CD62 (A) and of CD63 (B) on GPIIb/IIIa-deficient platelets and control platelets induced by CRP-XL, measured by flow cytometry after labeling with anti-CD62-FITC or anti-CD63-FITC, respectively. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Fig. 3.

Normal expression of CD62 (A) and of CD63 (B) on GPIIb/IIIa-deficient platelets and control platelets induced by CRP-XL, measured by flow cytometry after labeling with anti-CD62-FITC or anti-CD63-FITC, respectively. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Close modal
The reaction of CRP with platelets in von Willebrand disease.

Ristocetin, at concentrations up to 4 mg/mL, did not induce vWF-mediated platelet agglutination in PRP from patient P.R. (data not shown). The addition of human vWF (10 μg/mL) corrected the defect. No vWF binding to the surface of the patient's platelets was observed when these platelets were treated with ristocetin at concentrations ranging from 0.2 to 2 mg/mL, whereas 1.0 mg/mL induced maximum vWF binding to control platelets (data not shown). No vWF was detectable in samples of the patient's platelets and plasma taken at the time these platelets were activated with CRP-XL. CRP-XL induced normal aggregation of vWF-deficient platelets in vWF-deficient plasma (Fig 4A) and induced fibrinogen binding to a somewhat greater extent than in controls (Fig 4B). The surface expression of CD62 (Fig 5A) and of CD63 (Fig 5B) induced by CRP-XL in PRP of patient P.R. were identical in terms of both extent and concentration dependence to that occurring in control platelets.

Fig. 4.

Normal platelet aggregation induced by CRP-XL in PRP from a von Willebrand disease type 3 patient and from a control (A). Normal binding of fibrinogen to vWF-deficient platelets and to control platelets in vWF-deficient plasma activated with CRP-XL, measured by flow cytometry using FITC-labeled fibrinogen (B). Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Fig. 4.

Normal platelet aggregation induced by CRP-XL in PRP from a von Willebrand disease type 3 patient and from a control (A). Normal binding of fibrinogen to vWF-deficient platelets and to control platelets in vWF-deficient plasma activated with CRP-XL, measured by flow cytometry using FITC-labeled fibrinogen (B). Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Close modal
Fig. 5.

Normal expression of (A) CD62 and (B) CD63 on vWF-deficient platelets in vWF-deficient plasma and control platelets in control plasma, induced by CRP-XL, measured by flow cytometry using FITC-labeled anti-CD62 antibody or anti-CD63 antibody, respectively. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Fig. 5.

Normal expression of (A) CD62 and (B) CD63 on vWF-deficient platelets in vWF-deficient plasma and control platelets in control plasma, induced by CRP-XL, measured by flow cytometry using FITC-labeled anti-CD62 antibody or anti-CD63 antibody, respectively. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Close modal
The reaction of CRP with GPVI-deficient platelets.

Aggregation of GPVI-deficient platelets was totally absent even in the presence of a concentration of either collagen or CRP-XL 20-fold higher than was needed to aggregate normal platelets (Fig 6A and B). In contrast, aggregation of these platelets by ADP was normal (Fig 6C). Neither collagen nor CRP-XL induced any surface expression of CD62 (Fig7A and B) or CD63 (Fig 8A and B). There was no expression of fibrinogen binding in response to CRP-XL (Fig 9B), although collagen-induced fibrinogen binding was not absent but expressed to a decreased extent compared with normal platelets, especially at low concentrations of collagen (Fig 9A).

Fig. 6.

Aggregation of GPVI-deficient and control platelets induced by (A) collagen type I, (B) CRP-XL, and (C) ADP. The platelet count had to be adjusted to 1.8 × 108/mL because the patient was slightly thrombocytopenic. Collagen and CRP-XL did not induce platelet aggregation in GPVI-deficient platelets, although these platelets were able to aggregate in response to ADP. Data shown are representative of three similar experiments.

Fig. 6.

Aggregation of GPVI-deficient and control platelets induced by (A) collagen type I, (B) CRP-XL, and (C) ADP. The platelet count had to be adjusted to 1.8 × 108/mL because the patient was slightly thrombocytopenic. Collagen and CRP-XL did not induce platelet aggregation in GPVI-deficient platelets, although these platelets were able to aggregate in response to ADP. Data shown are representative of three similar experiments.

Close modal
Fig. 7.

Expression of P-selectin (CD62) on GPVI-deficient platelets and control platelets induced by (A) collagen type I and (B) CRP-XL, measured by flow cytometry using FITC-labeled anti-CD62 antibody. Neither collagen nor CRP-XL induced CD62 expression on GPVI-deficient platelets. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Fig. 7.

Expression of P-selectin (CD62) on GPVI-deficient platelets and control platelets induced by (A) collagen type I and (B) CRP-XL, measured by flow cytometry using FITC-labeled anti-CD62 antibody. Neither collagen nor CRP-XL induced CD62 expression on GPVI-deficient platelets. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Close modal
Fig. 8.

Expression of CD63 on GPVI-deficient platelets and control platelets induced by (A) collagen type I and (B) CRP-XL, measured by flow cytometry using FITC-labeled anti-CD63 antibody. Neither collagen nor CRP-XL induced CD63 expression on GPVI-deficient platelets. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Fig. 8.

Expression of CD63 on GPVI-deficient platelets and control platelets induced by (A) collagen type I and (B) CRP-XL, measured by flow cytometry using FITC-labeled anti-CD63 antibody. Neither collagen nor CRP-XL induced CD63 expression on GPVI-deficient platelets. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Close modal
Fig. 9.

Binding of fibrinogen to GPVI-deficient platelets and control platelets activated by (A) collagen type I and (B) CRP-XL, measured by flow cytometry using FITC-labeled fibrinogen. In GPVI-deficient platelets, there was no expression of fibrinogen binding in response to CRP-XL; collagen-induced fibrinogen binding was expressed to a decreased extent, especially at low concentrations of collagen. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Fig. 9.

Binding of fibrinogen to GPVI-deficient platelets and control platelets activated by (A) collagen type I and (B) CRP-XL, measured by flow cytometry using FITC-labeled fibrinogen. In GPVI-deficient platelets, there was no expression of fibrinogen binding in response to CRP-XL; collagen-induced fibrinogen binding was expressed to a decreased extent, especially at low concentrations of collagen. Fluorescence of unstimulated control platelets is defined as 1. Data shown are representative of two similar experiments.

Close modal

All collagens share a common tertiary structure, the collagen triple helix. The collagen molecule contains three α chains, each with a repeating Gly-X-Y sequence in which X and Y are frequently the amino acids, Pro and Hyp, respectively. The Gly-Pro-Hyp triplet represents approximately 12% of the primary sequence of type I collagen. Within the molecule each of these chains forms a left-handed polyproline II helix and the three helices are intertwined to form a right-handed superhelix, the collagen triple helix. Molecules then associate very specifically to yield the highly ordered quaternary structure of the collagen fiber.43 

The native triple helical structure of collagen is required for platelet secretion and aggregation. Furthermore, collagen molecules must also assemble into fibrous form for collagen to be an effective agonist.26-28 In contrast, monomeric collagen immobilized on plastic can serve as a very effective substrate for adhesion without any activation.44 This adhesion is mediated by the integrin α2β1 and can be blocked by antibodies to α2β1.7,10-12 

Although evidence has been presented suggesting that signaling in platelets by collagen is subsequent to recognition of α2β1,35,45-47 it is not clear how far α2β1 is directly responsible for signaling leading to platelet activation. Although collagen-induced phosphorylation events may under some circumstances be attenuated by blockade of α2β1 by antibodies, persistence of signaling has been reported in response to collagen or CRP-XL.29,34,35,47 Indeed, in platelets lacking GPVI, but in which α2β1 is expressed normally, collagen is able to activate the tyrosine kinase c-src, but not p72syk, PLCγ2, or p125fak.41 All of these are regarded as important signaling molecules in the activation of normal platelets by collagen. This indicates the possibility that GPVI (rather than α2β1) may be crucial as an activatory receptor for collagen in platelets.

A triple-helical synthetic analogue of collagen, composed simply of a repeating Gly-Pro-Hyp sequence, has been shown in polymeric form (CRP-XL) to elicit a full aggregatory response from human platelets without the involvement of the integrin α2β1.29 The triple-helical non–cross-linked peptide (CRP) is not activatory but inhibits platelet activation by collagen and by CRP-XL.29 This corroborates the existence of a signaling receptor on platelets other than, or in addition to, α2β1, which requires the tertiary and quaternary structures of collagen for platelet activation.

As shown here, CD36-deficient platelets bind fibrinogen and exhibit normal secretion and aggregation in response to CRP-XL (as is true for collagen15-18). CD36 cannot, therefore, be this collagen receptor.

Platelets bind indirectly to collagen, via vWF, through GPIb and GPIIb/IIIa complexes.48 This binding confers resistance to the shear forces generated by blood flowing along the vessel wall.49,50 However, platelets from a Glanzmann's thrombasthenia type I patient, totally deficient in GPIIb/IIIa, responded normally to CRP-XL, which induced normal secretion, measured as expression on the plasma membrane of CD62 (from the α-granules) and CD63 (from the dense bodies). These data exclude GPIIb/IIIa as the primary signaling receptor that recognizes the quaternary structure of collagen.

The same can be concluded for vWF. Despite the absence of vWF, both in the platelets and plasma of patient P.R., CRP-XL induced normal fibrinogen binding, normal secretion (CD62 and CD63 expression), and normal aggregation.

These results confirm the view that CD36, GPIIb/IIIa, and vWF are not essential for platelet activation by collagen.

In contrast, GPVI-deficient platelets were unresponsive to CRP-XL, showing that GPVI is a signaling receptor for the polymeric cross-linked triple-helical Gly-Pro-Hyp motif. Our findings are in line with the observation that antibody-induced cross-linking of GPVI in normal platelets results in activation of src and p72syk in parallel with activation of PLCγ222 and that CRP-XL stimulates tyrosine phosphorylation of PLCγ2 and p72sykin normal platelets independent of the integrin α2β1.35 

In GPVI-deficient platelets CRP-XL did not induce the activation-dependent conformational changes in GPIIb/IIIa that are necessary for fibrinogen binding. However, it is interesting that fibrinogen binding was induced in these platelets by increasing collagen levels, although to a lower degree than in equivalent controls (Fig 9A). Collagen (but not CRP-XL) contains binding sites for α2β1, and GPVI-deficient platelets express α2β1, which may possibly be involved in signal transduction by collagen, sufficient for partial activation of GPIIb/IIIa. Nevertheless, collagen fibers provided insufficient signals to cause platelet aggregation in GPVI-deficient platelets, even at collagen levels 20-fold higher than are needed to aggregate normal platelets. Granule release was completely absent, as with CRP-XL (Figs7 and 8). Recently, the activation of c-src, but not of p72syk or PLCγ2, has been shown in GPVI-deficient platelets stimulated with collagen,41 providing firm evidence for signaling, but not sufficient to cause aggregation, arising from a second population of collagen receptors as well as GPVI. It is plausible that GPVI may be linked more to positive feedback events, such as thromboxane production, a process essential for the aggregation of platelets by collagen, or granule release, providing further activation mediated by secreted ADP, so leading to full activation of GPIIb/IIIa. Results from these and other studies suggest that, in platelets, α2β1 has a major role in adhesion to collagen and a less-well defined role in platelet activation in support of GPVI. These ideas are consistent with the two-site two-step model of platelet activation by collagen.24,25,51 The major role for α2β1 may be to provide a stable interaction, especially under nonstatic conditions, to allow subsequent interaction of the Gly-Pro-Hyp motif with other, activatory receptors. This study establishes GPVI as an activatory receptor crucial for the full activation of platelets by collagen.

Un–cross-linked CRP, although not causing platelet activation, does inhibit aggregation by both CRP-XL and collagen fibers, which may suggest that GPVI can recognize the monomeric molecule. Lack of activation by monomeric CRP suggests, in turn, that clustering of GPVI molecules may be necessary for activation.

In view of the potency of the repeating Gly-Pro-Hyp sequence in triple-helical, polymeric form, we believe that Gly-Pro-Hyp in collagen may be a highly specific recognition sequence for GPVI. However, we cannot rule out the possibility, as we have recently conjectured,29 that GPVI recognizes the triple helix per se and that any assembly of Gly-X-Y amino acid triplets generating a triple-helical structure may be active. Further studies are in progress to affirm the crucial importance of the Gly-Pro-Hyp motif in the recognition of collagen by GPVI.

The authors thank all blood donors and patients involved in this study for their kind cooperation and Joachim Kardoeus for preparation of the figures and editorial assistance.

Supported in part by the Deutsche Forschungsgemeinschaft (Grant No. Ke397/1-3) and the Gesellschaft für Thrombose- und Hämostaseforschung (travel support). K.J.C. was supported by Grant No. 31-42336.94 from the Swiss National Science Foundation. R.W.F., C.G.K., and M.J.B. were supported by the Medical Research Council, UK, to whose External Scientific Staff M.J.B. belongs.

Address reprint requests to Beate Kehrel, PhD, Experimentelle Haemostaseforschung, Medizinische Klinik und Poliklinik, Innere Medizin A, Universität Münster, Domagkstrasse 3, D-48129 Münster, Germany.

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. section 1734 solely to indicate this fact.

1
van Zanten
GH
de Graaf
S
Slootweg
PJ
Heijnen
HF
Connolly
TM
de Groot
PG
Sixma
JJ
Increased platelet deposition on atherosclerotic coronary arteries.
J Clin Invest
93
1994
615
2
Kehrel
B
Platelet receptors for collagens.
Platelets
6
1995
11
3
Kehrel
B
Platelet-collagen interactions.
Semin Thromb Hemost
21
1995
123
4
Sakariassen
KS
Fressinaud
E
Girma
JP
Meyer
D
Baumgartner
HR
Role of platelet membrane glycoproteins and von Willebrand factor in adhesion of platelets to subendothelium and collagen.
Ann NY Acad Sci
516
1987
52
5
Badimon
L
Badimon
JJ
Turitto
VT
Vallabhajosula
S
Fuster
V
Platelet thrombus formation on collagen type I—A model of deep vessel injury—Influence of blood rheology, von Willebrand factor and blood coagulation.
Circulation
78
1988
1431
6
Savage
B
Saldivar
E
Ruggeri
ZM
Initiation of platelet adhesion by arrest onto fibrinogen or translocation on von Willebrand factor.
Cell
84
1996
289
7
Santoro
SA
Zutter
MM
The α2β1 integrin: A collagen receptor on platelets and other cells.
Thromb Haemost
74
1995
813
8
Nieuwenhuis
HK
Akkermann
JWN
Houdijk
WPM
Sixma
JJ
Human blood platelets showing no response to collagen fail to express surface glycoprotein Ia.
Nature
318
1985
470
9
Kehrel
B
Balleisen
L
Kokott
R
Mesters
R
Stenzinger
W
Clemetson
KJ
van de Loo
J
Deficiency of thrombospondin and membrane glycoprotein Ia in platelets with defective collagen-induced aggregation and spontaneous loss of disorder.
Blood
71
1988
1074
10
Coller
BS
Beer
JH
Scudder
LE
Steinberg
MH
Collagen-platelet interactions: Evidence for a direct interaction of collagen with platelet GPIa/IIa and indirect interaction with platelet GPIIb/IIIa mediated by adhesive proteins.
Blood
74
1989
182
11
Morton
LF
Peachey
AR
Zijenah
LS
Goodall
AH
Humphries
MJ
Barnes
MJ
Conformation-dependent platelet adhesion to collagen involving integrin α2β1-mediated and other mechanisms: Multiple α2β1-recognition sites in collagen type I.
Biochem J
299
1994
791
12
Saelman
EUM
Nieuwenhuis
HK
Hese
KM
de Groot
PG
Heijnen
HFG
Sage
EH
Williams
S
McKeown
L
Gralnick
HR
Sixma
JJ
Platelet adhesion to collagen types I through VIII under conditions of stasis and flow is mediated by GPIa/IIa (α2β1-integrin).
Blood
83
1994
1244
13
Tandon
NN
Kralisz
U
Jamieson
GA
Identification of glycoprotein IV (CD36) as a primary receptor for platelet-collagen adhesion.
J Biol Chem
264
1989
7576
14
McGregor
JL
Catimel
B
Parmentier
S
Clezardin
P
Dechavanne
M
Leung
LLK
Rapid purification and partial characterization of human platelet glycoprotein IIIb—Interaction with thrombospondin and its role in platelet aggregation.
J Biol Chem
264
1989
501
15
Saelman
EUM
Kehrel
B
Hese
KM
de Groot
PG
Sixma
JJ
Nieuwenhuis
HK
Platelet adhesion to collagen and endothelial cell matrix under flow conditions is not dependent on platelet glycoprotein IV.
Blood
83
1994
3240
16
McKeown
L
Vail
M
Williams
S
Kramer
W
Hansmann
K
Gralnick
H
Platelet adhesion to collagen in individuals lacking glycoprotein IV.
Blood
83
1994
2866
17
Yamamoto
N
Akamatsu
N
Yamazaki
H
Tanoue
K
Normal aggregations of glycoprotein IV (CD36)-deficient platelets from seven healthy Japanese donors.
Br J Haematol
81
1992
86
18
Kehrel
B
Kronenberg
A
Rauterberg
J
Niesing-Bresch
D
Niehues
U
Kardoeus
J
Schwippert
B
Tschöpe
D
van de Loo
J
Clemetson
KJ
Platelets deficient in glycoprotein IIIb aggregate normally to collagens type I and III but not to collagen type V.
Blood
82
1993
3364
19
Moroi
M
Jung
SM
Okuma
M
Shinmyozu
K
A patient with platelets deficient in glycoprotein VI that lack both collagen-induced aggregation and adhesion.
J Clin Invest
84
1989
1440
20
Arai
M
Yamamoto
N
Moroi
M
Akamatsu
N
Fukutake
K
Tanoue
K
Platelets with 10% of the normal amount of glycoprotein VI have an impaired response to collagen that results in a mild bleeding tendency.
Br J Haematol
89
1995
124
21
Sugiyama
T
Okuma
M
Ushikubi
F
Sensaki
S
Kanaji
K
Uchino
H
A novel platelet aggregating factor found in a patient with defective collagen-induced platelet aggregation and auto-immune thrombocytopenia.
Blood
69
1987
1712
22
Ichinohe
T
Takayama
H
Ezumi
Y
Yanagi
S
Yamamura
H
Okuma
M
Cyclic AMP-insensitive activation of c-Src and Syk protein-tyrosine kinases through platelet membrane glycoprotein VI.
J Biol Chem
270
1995
28029
23
Moroi
M
Jung
SM
Shinmyozu
K
Tomiyama
Y
Ordinas
A
Diaz-Ricard
M
Analysis of platelets adhesion to a collagen-coated surface under flow conditions: The involvement of glycoprotein VI in the platelet adhesion.
Blood
88
1996
2081
24
Morton
LF
Peachey
AR
Barnes
MJ
Platelet-reactive sites in collagens type I and type III. Evidence for separate adhesion and aggregatory sites.
Biochem J
258
1989
157
25
Santoro
SA
Walsh
J
Staatz
W
Baranski
K
Distinct determinants on collagen support α2β1 integrin-mediated platelet adhesion and platelet activation.
Cell Regul
2
1991
905
26
Muggli
R
Baumgartner
HR
Collagen-induced platelet aggregation: Requirement for tropocollagen multimers.
Thromb Res
3
1973
715
27
Jaffe
R
Deykin
D
Evidence for a structural requirement for the aggregation of platelets by collagen.
J Clin Invest
53
1974
875
28
Brass
LF
Bensusan
HB
The role of quaternary structure in platelet-collagen interaction.
J Clin Invest
54
1974
1480
29
Morton
LF
Hargreaves
PG
Farndale
RW
Young
RD
Barnes
MJ
Integrin α2β1-independent activation of platelets by collagen: Collagen tertiary (triple helical) and quaternary (polymeric) structures are sufficient alone for activity.
Biochem J
306
1995
337
30
(abstr, suppl)
Verkleij
MW
Morton
F
Barnes
MJ
Gralnick
HR
de Groot
PG
Sixma
JJ
Simple collagen-like peptides support platelet adhesion under static but not under flow conditions: Interaction via α2β1 with specific collagen sequences is a requirement to withstand shear forces.
Thromb Haemost
78
1997
11
31
Tuckwell
DS
Reid
KBM
Barnes
MJ
Humphries
MJ
Integrin α2 A-domain binds specifically to a range of collagens but is not a general receptor for the collagenous motif.
Eur J Biochem
241
1996
732
32
Barnes
MJ
Knight
CG
Farndale
RW
Model collagen peptides.
Biopolymers
40
1996
383
33
Achison
M
Joel
C
Hargreaves
PG
Sage
SO
Barnes
MJ
Farndale
RW
Signals elicited from human platelets by synthetic, triple-helical, collagen-like peptides.
Blood Coag Fibrinolysis
7
1996
149
34
Gibbins
J
Asselin
J
Law
CL
Farndale
RW
Barnes
MJ
Watson
SP
Collagen stimulates tyrosine phosphorylation of the Fc receptor γ chain in platelets.
J Biol Chem
271
1996
18095
35
Asselin
J
Gibbins
JM
Achison
M
Lee
YH
Morton
LF
Farndale
RW
Barnes
MJ
Watson
SP
A collagen-like peptide stimulates tyrosine phosphorylation of syk and phospholipase Cγ2 in platelets independent of the integrin α2β1.
Blood
89
1997
1235
36
Herrmann M, Hartleib J, Kehrel B, Montgomery RR, Sixma JJ, Peters G: Interaction of vWf with Staphylococcus aureus. J Infect Dis 176:984, 1997
37
Xia
Z
Wong
T
Liu
Q
Kasirer-Friede
A
Brown
E
Frojmovic
MM
Optimally functional fluorescein isothiocyanate (FITC)-labelled fibrinogen for quantitative studies of binding to activated platelets and platelet aggregation.
Br J Haematol
93
1996
204
38
Kehrel
B
Kronenberg
A
Schwippert
B
Niesing-Bresch
D
Niehues
U
Tschöpe
D
van de Loo
J
Clemetson
KJ
Thrombospondin binds normally to glycoprotein IIIb deficient platelets.
Biochem Biophys Res Commun
179
1991
985
39
Nofer
JR
Walter
M
Kehrel
B
Seedorf
U
Assmann
G
HDL3 activates phospholipase D in normal but not in glycoprotein IIb/IIIa-deficient platelets.
Biochem Biophys Res Commun
207
1995
148
40
Sadler
JE
Gralnick
HR
Commentary: A new classification for von Willebrand disease.
Blood
84
1994
676
41
Ichinohe
T
Takayama
H
Ezumi
Y
Arai
M
Yamamoto
N
Takahashi
H
Okuma
M
Collagen-stimulated activation of Syk but not c-Src is severely compromised in human platelets lacking membrane glycoprotein VI.
J Biol Chem
272
1997
63
42
Born GVR, Cross MJ: The aggregation of blood platelets. J Physiol (Lond) 168:178, 1963
43
Kielty CM, Hopkinson I, Grant ME. Collagen: The collagen family: Structure, assembly, and organization in the extracellular matrix, in Royce PM, Steinman B (eds): Connective Tissue and Its Heritable Disorders. New York, NY, Wiley-Liss, 1993, p 103
44
Santoro
SA
Identification of a 160 000 Dalton platelet membrane protein that mediates the initial divalent cation-dependent adhesion to collagen.
Cell
46
1986
913
45
Daniel
JL
Dangelmaier
C
Smith
JB
Evidence that adhesion of electrically permeabilized platelets to collagen is mediated by guanine nucleotide regulatory proteins.
Biochem J
286
1992
701
46
Haimovich
B
Lipfert
L
Brugge
JS
Shattil
SJ
Tyrosine phosphorylation and cytoskeletal reorganization in platelets are triggered by interaction of integrin receptors with their immobilized ligands.
J Biol Chem
268
1993
15868
47
Keely
PJ
Parise
LV
The α2β1 integrin is a necessary co-receptor for collagen-induced activation of syk and the subsequent phosphorylation of phospholipase Cγ2 in platelets.
J Biol Chem
271
1996
26668
48
Savage
B
Shattil
SJ
Ruggeri
ZM
Modulation of platelet function through adhesion receptors.
J Biol Chem
267
1992
11300
49
Santoro SA: Molecular basis of platelet adhesion to collagen, in Jamieson GA (ed): Platelet Membrane Receptors: Molecular Biology, Immunology, Biochemistry and Pathology. New York, NY, Liss, 1988, p 291
50
Houdijk
WP
Sakariassen
KS
Nievelstein
PF
Sixma
JJ
Role of factor VIII-von Willebrand factor and fibronectin in the interaction of platelets in flowing blood with monomeric and fibrillar human collagen types I and III.
J Clin Invest
75
1985
531
51
Clemetson
KJ
Platelet activation: Signal transduction via membrane receptors.
Thromb Haemost
74
1995
111
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