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PubMed2003

Obtustatin: a potent selective inhibitor of alpha1beta1 integrin in vitro and angiogenesis in vivo.

A novel disintegrin, obtustatin, was purified from the venom of the Vipera lebetina obtusa viper. Obtustatin is the shortest disintegrin yet described, containing only 41 amino acids. It contains a similar pattern of cysteines to the short disintegrins echistatin and eristostatin but contains the sequence KTS rather than RGD in its active site loop. Obtustatin is a potent and selective inhibitor of alpha1beta1 integrin. It does not inhibit the closely related integrin alpha2beta1, nor a panel of other integrins tested. It does not inhibit ligand binding to the recombinant alpha1 I-domain. Importantly, obtustatin potently inhibited angiogenesis in vivo in the chicken chorioallantoic membrane assay, and in the Lewis lung syngeneic mouse model, it reduced tumor development by half, confirming and extending previous results on the relevance of alpha1beta1 integrin to angiogenesis and suggesting novel approaches to the generation of angiogenesis inhibitors.

Allantois↗

PubMed1998

Role of the I-domain in collagen binding specificity and activation of the integrins alpha1beta1 and alpha2beta1.

Adhesion to collagens by most cell types is mediated by the integrins alpha1beta1 and alpha2beta1. Both integrin alpha subunits belong to a group which is characterized by the presence of an I domain in the N-terminal half of the molecule, and this domain has been implicated in the ligand recognition. Since purified alpha1beta1 and alpha2beta1 differ in their binding to collagens I and IV and recognize different sites within the major cell binding domain of collagen IV, we investigated the potential role of the alpha1 and alpha2 I domains in specific collagen adhesion. We find that introducing the alpha2 I domain into alpha1 results in surface expression of a functional collagen receptor. The adhesion mediated by this chimeric receptor (alpha1-2-1beta1) is similar to the adhesion profile conferred by alpha2beta1, not alpha1beta1. The presence of alpha2 or alpha1-2-1 results in preferential binding to collagen I, whereas alpha1 expressing cells bind better to collagen IV. In addition, alpha1 containing cells bind to low amounts of a tryptic fragment of collagen IV, whereas alpha2 or alpha1-2-1 bearing cells adhere only to high concentrations of this substrate. We also find that collagen adhesion of NIH-3T3 mediated by alpha2beta1 or alpha1-2-1beta1, but not by alpha1, requires the presence of Mn2+ ions. This ion requirement was not found in CHO cells, implicating the I domain in cell type-specific activation of integrins.

3T3 Cells↗

PubMed2004

Transforming growth factor-beta1 stimulates collagen matrix remodeling through increased adhesive and contractive potential by human renal fibroblasts.

Renal tubulointerstitial fibrosis is the common final pathway leading to end-stage renal failure. Tubulointerstitial fibrosis is characterized by fibroblast proliferation and excessive matrix accumulation. Transforming growth factor-beta1 (TGF-beta1) has been implicated in the development of renal fibrosis accompanied by alpha-smooth muscle actin (alpha-SMA) expression in renal fibroblasts. To investigate the molecular and cellular mechanisms involved in tubulointerstitial fibrosis, we examined the effect of TGF-beta1 on collagen type I (collagen) gel contraction, an in vitro model of scar collagen remodeling. TGF-beta1 enhanced collagen gel contraction by human renal fibroblasts in a dose- and time-dependent manner. Function-blocking anti-alpha1 or anti-alpha2 integrin subunit antibodies significantly suppressed TGF-beta1-stimulated collagen gel contraction. Scanning electron microscopy showed that TGF-beta1 enhanced the formation of the collagen fibrils by cell attachment to collagen via alpha1beta1 and alpha2beta1 integrins. Flow cytometry and cell adhesion analyses revealed that the stimulation of renal fibroblasts with TGF-beta1 enhanced cell adhesion to collagen via the increased expression of alpha1 and alpha2 integrin subunits within collagen gels. Fibroblast migration to collagen was not up-regulated by TGF-beta1. Furthermore, TGF-beta1 increased the expression of a putative contractile protein, alpha-SMA, by human renal fibroblasts in collagen gels. These results suggest that TGF-beta1 stimulates fibroblast-collagen matrix remodeling by increasing both integrin-mediated cell attachment to collagen and alpha-SMA expression, thereby contributing to pathological tubulointerstitial collagen matrix reorganization in renal fibrosis.

Actinin↗

PubMed2004

Glycoprotein VI/Fc receptor gamma chain-independent tyrosine phosphorylation and activation of murine platelets by collagen.

We have investigated the ability of collagen to induce signalling and functional responses in suspensions of murine platelets deficient in the FcRgamma (Fc receptor gamma) chain, which lack the collagen receptor GPVI (glycoprotein VI). In the absence of the FcRgamma chain, collagen induced a unique pattern of tyrosine phosphorylation which was potentiated by the thromboxane analogue U46619. Immunoprecipitation studies indicated that neither collagen alone nor the combination of collagen plus U46619 induced phosphorylation of the GPVI-regulated proteins Syk and SLP-76 (Src homology 2-containing leucocyte protein of 76 kDa). A low level of tyrosine phosphorylation of phospholipase Cgamma2 was observed, which was increased in the presence of U46619, although the degree of phosphorylation remained well below that observed in wild-type platelets (approximately 10%). By contrast, collagen-induced phosphorylation of the adapter ADAP (adhesion- and degranulation-promoting adapter protein) was substantially potentiated by U46619 to levels equivalent to those observed in wild-type platelets. Collagen plus U46619 also induced significant phosphorylation of FAK (focal adhesion kinase). The functional significance of collagen-induced non-GPVI signals was highlighted by the ability of U46619 and collagen to induce the secretion of ATP in FcRgamma chain-deficient platelets, even though neither agonist was effective alone. Protein tyrosine phosphorylation and the release of ATP were abolished by the anti-(alpha2 integrin) antibodies Ha1/29 and HMalpha2, but not by blockade of alphaIIbbeta3. These results illustrate a novel mechanism of platelet activation by collagen which is independent of the GPVI-FcRgamma chain complex, and is facilitated by binding of collagen to integrin alpha2beta1.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗

PubMed1996

Integrin VLA-2 (alpha2beta1) function in postextravasation movement of human rhabdomyosarcoma RD cells in the liver.

It is now known that members of the selectin and integrin families are critical in the initial interaction of cells in circulation with endothelial surfaces. Also, platelet/endothelial cell adhesion molecule-1 has been shown to be involved in transendothelial migration of extravasating cells. Little is known about adhesion molecules involved in subsequent postextravasation events. In this study, the significance of VLA-2 (alpha2beta1) integrin in the movement of human rhabdomyosarcoma RD cells in the liver was characterized by in vivo videomicroscopy. Results show that after extravasation, the mock-transfected RDpF cells were able to migrate to the subcapsular region of the liver. Although the RDX2C2 transfectant expressing VLA-2 integrin extravasated equally well, a majority of RDX2C2 cells remained in close proximity to blood vessels and failed to reach the subcapsular region. The functional involvement of VLA-2 in affecting the ability of RD cells to reach the subcapsular region was verified by the preparation of an RD transfectant [RDX2C2(I-)] expressing a nonfunctional variant of VLA-2 lacking the inserted (I)-domain of alpha2 subunit. In vivo microscopy showed that RDX2C2(I-) cells migrated in a manner similar to control RDpF cells. To demonstrate that RDX2C2 cells that remained in dose proximity to blood vessels were due to VLA-2 function, a blocking monoclonal antibody against VLA-2 (BHA2.1) was prepared. Mice were injected with BHA2.1 or control monoclonal antibody P3 at the time when RDX2C2 cells completed their extravasation. Treatment with BHA2.1 increased the number of RDX2C2 cells that reached the subcapsular region and subsequently formed tumor foci. Therefore, VLA-2 integrin expression has major roles in postextravasation movement and affects tumor foci formation at the liver surface.

Animals↗

PubMed2005

Density of platelet collagen receptors glycoprotein VI and alpha2beta1 and prior myocardial infarction in human subjects, a pilot study.

BACKGROUND: Prior studies evaluating whether platelet collagen receptors affect risk of myocardial infarction (MI) have used common polymorphisms thought to affect receptor density, and have yielded conflicting results. Our objective in the current pilot study was to obtain preliminary information on the relationship between platelet collagen receptor density and prior MI. MATERIAL/METHODS: We measured glycoprotein VI (GPVI) and integrin alpha2beta1 density by flow cytometry using FITC-conjugated monoclonal antibodies in consecutive subjects from an outpatient cardiology practice. We used a case-control design to match 77 patients with prior MI to 77 control patients. Matching was performed on the basis of traditional risk factors for MI. RESULTS: The mean GPVI density was 14% higher in those with prior MI (P = 0.098), and the alpha2beta1 density was 6% higher in those with prior MI (P = 0.49). A GPVI or alpha2beta1 density within the lowest decile was associated with a lower MI prevalence (OR = 6.3, 95% CI: 1.3-29.2 P = 0.009). CONCLUSIONS: Our preliminary data suggests that platelet collagen receptor density, when measured directly, may be related to prior myocardial infarction. An appropriately sized clinical study using this methodology is warranted to further investigate whether collagen receptor density serves as a novel risk factor for myocardial infarction.

Aged↗

PubMed2005

Bifunctional peptides derived from homologous loop regions in the laminin alpha chain LG4 modules interact with both alpha 2 beta 1 integrin and syndecan-2.

Laminin alpha chains show diverse biological functions in a chain-specific fashion. The laminin G-like modules (LG modules) of the laminin alpha chains consist of a 14-stranded beta-sheet sandwich structure with biologically active sequences found in the connecting loops. Previously, we reported that connecting loop regions between beta-strands E and F in the mouse laminin alpha chain LG4 modules exhibited chain-specific activities. In this study, we focus on the homologous loop regions in human laminin alpha chain LG4 modules using five synthetic peptides (hEF-1-hEF-5). These homologous peptides induced chain-specific cellular responses in various cell types. Next, to examine the dual-receptor recognition model, we synthesized chimeras (cEF13A-cEF13E) derived from peptides hEF-1 and hEF-3. All of the chimeric peptides promoted fibroblast attachment as well as the parental peptides. Attachment of fibroblasts to cEF13A and cEF13B was inhibited by anti-integrin alpha2 and beta1 antibodies and by heparin, while cell adhesion to cEF13C, cEF13D, and cEF13E was blocked only by heparin. Actin organization of fibroblasts on cEF13C was not different from that on hEF-3, but cEF13B induced membrane ruffling at the tips of the actin stress fibers. These results suggest that cEF13B had bifunctional effects on cellular behaviors through alpha2beta1 integrin and heparin/heparan sulfate proteoglycan. We conclude that the approach utilizing chimeric peptides is useful for examining cellular mechanisms in dual-receptor systems.

Actins↗

PubMed2005

The intermediate filament protein vimentin binds specifically to a recombinant integrin alpha2/beta1 cytoplasmic tail complex and co-localizes with native alpha2/beta1 in endothelial cell focal adhesions.

Integrin receptors are crucial players in cell adhesion and migration. Identification and characterization of cellular proteins that interact with their short alpha and beta cytoplasmic tails will help to elucidate the molecular mechanisms by which integrins mediate bi-directional signaling across the plasma membrane. Integrin alpha2beta1 is a major collagen receptor but to date, only few proteins have been shown to interact with the alpha2 cytoplasmic tail or with the alpha2beta1 complex. In order to identify novel binding partners of a alpha2beta1cytoplasmic domain complex, we have generated recombinant GST-fusion proteins, incorporating the leucine zipper heterodimerization cassettes of Jun and Fos. To ascertain proper functionality of the recombinant proteins, interaction with natural binding partners was tested. GST-alpha2 and GST-Jun alpha2 bound His-tagged calreticulin while GST-beta1 and GST-Fos beta1 proteins bound talin. In screening assays for novel binding partners, the immobilized GST-Jun alpha2/GST-Fos beta1 heterodimeric complex, but not the single subunits, interacted specifically with endothelial cell-derived vimentin. Vimentin, an abundant intermediate filament protein, has previously been shown to co-localize with alphavbeta3-positive focal contacts. Here, we provide evidence that this interaction also occurs with alpha2beta1-enriched focal adhesions and we further show that this association is lost after prolonged adhesion of endothelial cells to collagen.

Amino Acid Sequence↗

PubMed2005

Two functional active conformations of the integrin {alpha}2{beta}1, depending on activation condition and cell type.

For several integrins, the existence of multiple conformational states has been studied intensively. For the integrin alpha2beta1, a major collagen receptor on platelets and other cell types, however, no such experimental data were available thus far. Recently, our group has developed a monoclonal antibody IAC-1 sensitive to the molecular conformation of alpha2beta1 because it only binds to the activated state of alpha2beta1 on platelets, induced upon inside-out signaling. By investigating IAC-1 binding in combination with collagen binding after inside-out stimulation and outside manipulation, we demonstrated the existence of three different conformations of alpha2beta1 on platelets and Chinese hamster ovary cells as follows: (i) a nonactivated, resting state with no collagen nor IAC-1 binding; (ii) an intermediate state, induced by outside manipulation, with collagen but no IAC-1 binding; and (iii) a fully activated state, induced after inside-out stimulation, with both collagen and IAC-1 binding. Moreover, these different conformational states of alpha2beta1 are dependent on the cell type where alpha2beta1 is expressed, as IAC-1 binding to peripheral blood mononuclear cells and Jurkat cells could also be induced by outside manipulation, in contrast to platelets and alpha2beta1-expressing Chinese hamster ovary cells. Finally, we revealed a functional relevance for these different conformational states because the conformation of alpha2beta1, induced after outside manipulation, resulted in significantly more cell spreading on coated collagen compared with nonactivated or inside-out stimulated cells.

Actins↗

PubMed2006

The influence of N- and O-glycosylation inhibitors on the glycosylation profile of cellular membrane proteins and adhesive properties of carcinoma cell lines.

The effects of N- and O-glycosylation inhibitors on the expression of membrane proteins (MUC1 and some integrins) were evaluated in human endometrial (Ishikawa) and breast (MCF-7) cancer cells. Subconfluent cells were treated with 1-3 mg% concentration of tunicamycin and 2-10 mM of benzyl-N-acetyl-alpha-galactosaminide for 1-2 days, and used for flow cytometry, immunohistochemical staining, adhesion test and Western blotting. Benzyl-N-acetyl-alpha-galactosaminide inhibits MUC1 expression on the surface of breast more than endometrial cancer cells. Tunicamycin reduces MUC1 concentration on the cellular surface more than benzylglycoside, and greatly reduces glycosylation of glycoproteins, causing an increase in cell adhesion in both types of cancer cells. The expression of alpha2beta1 integrins on the surface of these cells was weak and decreased after treatment with inhibitors. Two different glycoforms of MUC1 proteins in endometrial cells and three in breast cancer cells were expressed and their molecular weights were reduced after treatment with glycosylation inhibitors. It was confirmed with lectin detection of carbohydrate epitopes (Tn and T) in MUC1 proteins. These observations show that glycosylation inhibitors altered the N- and O-glycan patterns in a sufficient manner, and positively modified the biological features of cancer cells.

Antigens, Neoplasm↗

PubMed2003

Signaling events underlying thrombus formation.

Recent in vivo studies have highlighted the dynamic and complex nature of platelet thrombus growth and the requirement for multiple adhesive receptor-ligand interactions in this process. In particular, the importance of von Willebrand factor (VWF) in promoting both primary adhesion and aggregation under high shear conditions is now well established. In general, the efficiency with which platelets adhere and aggregate at sites of vessel wall injury is dependent on the synergistic action of various adhesive and soluble agonist receptors, with the contribution of each of the individual receptors dependent on the prevailing blood flow conditions. In this review, we will discuss the major platelet adhesive interactions regulating platelet thrombus formation under high shear, with specific focus on the VWF (GPIb and integrin alphaIIbbeta3) and collagen receptors (GPVI and integrin alpha2beta1). We will also discuss the signaling mechanisms utilized by these receptors to induce platelet activation with specific emphasis on the role of cytosolic calcium flux in regulating platelet adhesion dynamics. The role of soluble agonists in promoting thrombus growth will be highlighted and a model to explain the synergistic requirement for adhesive and soluble stimuli for efficient platelet aggregation will be discussed.

Animals↗

PubMed2005

The subendothelial extracellular matrix modulates NF-kappaB activation by flow: a potential role in atherosclerosis.

Atherosclerotic plaque forms in regions of the vasculature exposed to disturbed flow. NF-kappaB activation by fluid flow, leading to expression of target genes such as E-selectin, ICAM-1, and VCAM-1, may regulate early monocyte recruitment and fatty streak formation. Flow-induced NF-kappaB activation is downstream of conformational activation of integrins, resulting in new integrin binding to the subendothelial extracellular matrix and signaling. Therefore, we examined the involvement of the extracellular matrix in this process. Whereas endothelial cells plated on fibronectin or fibrinogen activate NF-kappaB in response to flow, cells on collagen or laminin do not. In vivo, fibronectin and fibrinogen are deposited at atherosclerosis-prone sites before other signs of atherosclerosis. Ligation of integrin alpha2beta1 on collagen prevents flow-induced NF-kappaB activation through a p38-dependent pathway that is activated locally at adhesion sites. Furthermore, altering the extracellular matrix to promote p38 activation in cells on fibronectin suppresses NF-kappaB activation, suggesting a novel therapeutic strategy for treating atherosclerosis.

Animals↗

PubMed2005

Human atheromatous plaques stimulate thrombus formation by activating platelet glycoprotein VI.

Lipid-rich atherosclerotic plaques are vulnerable, and their rupture can cause the formation of a platelet- and fibrin-rich thrombus leading to myocardial infarction and ischemic stroke. Although the role of plaque-based tissue factor as stimulator of blood coagulation has been recognized, it is not known whether plaques can cause thrombus formation through direct activation of platelets. We isolated lipid-rich atheromatous plaques from 60 patients with carotid stenosis and identified morphologically diverse collagen type I- and type III-positive structures in the plaques that directly stimulated adhesion, dense granule secretion, and aggregation of platelets in buffer, plasma, and blood. This material also elicited platelet-monocyte aggregation and platelet-dependent blood coagulation. Plaques exposed to flowing blood at arterial wall shear rate induced platelets to adhere to and spread on the collagenous structures, triggering subsequent thrombus formation. Plaque-induced platelet thrombus formation was observed in fully anticoagulated blood (i.e., in the absence of tissue factor-mediated coagulation). Mice platelets lacking glycoprotein VI (GPVI) were unable to adhere to atheromatous plaque or form thrombi. Human platelet thrombus formation onto plaques in flowing blood was completely blocked by GPVI inhibition with the antibody 10B12 but not affected by integrin alpha2beta1 inhibition with 6F1 mAb. Moreover, the initial platelet response, shape change, induced by plaque was blocked by GPVI inhibition but not with alpha2beta1 antagonists (6F1 mAb or GFOGER-GPP peptide). Pretreatment of plaques with collagenase or anti-collagen type I and anti-collagen type III antibodies abolished plaque-induced platelet activation. Our results indicate that morphologically diverse collagen type I- and collagen type III-containing structures in lipid-rich atherosclerotic plaques stimulate thrombus formation by activating platelet GPVI. This platelet collagen receptor, essential for plaque-induced thrombus formation, presents a promising new anti-thrombotic target for the prevention of ischemic cardiovascular diseases.

Animals↗

PubMed2005

Association of polymorphisms of platelet membrane integrins alpha IIb(beta)3 (HPA-1b/Pl) and alpha2(beta)1 (alpha807TT) with premature myocardial infarction.

Conflicting results of an association of the human platelet antigen 1b (HPA-1b/PlA2), localized on the beta-subunit of the integrin alpha(IIb)beta3, and the alpha(2)807TT genotype of the integrin alpha2beta1 with coronary atherosclerosis and myocardial infarction have been reported. Both platelet receptor polymorphisms were genotyped in 3261 patients who had undergone coronary angiography, including 1175 survivors of a myocardial infarction, 1211 individuals with coronary artery disease but no history of myocardial infarction, and 571 control patients without angiographic coronary artery disease, and in 793 blood donors. In a case-control design, the prevalence of HPA-1b and alpha(2)807TT genotypes did not differ significantly between the patient groups with coronary artery disease or myocardial infarction and patient controls or blood donors. By contrast, using a multivariate case-only design, it was found that the median age of onset of myocardial infarction was 5.2 years earlier (P = 0.006) in carriers of the HPA-1b allele and 6.3 years earlier (P = 0.006) in carriers of the alpha(2)807TT genotype in the 264 survivors of myocardial infarction of recent onset with one- or two-vessel coronary artery disease. A significant interaction with the conventional risk factors hypercholesterolemia, smoking, diabetes, hypertension, and hyperfibrinogenemia was excluded. Human platelet antigen 1b and alpha(2)807TT are associated with premature myocardial infarction but not with coronary artery disease, suggesting a role of distinct integrin genotypes for increased platelet thrombogenicity. This association requires confirmation in follow-up studies.

Age of Onset↗

PubMed2006

A single high-affinity binding site for von Willebrand factor in collagen III, identified using synthetic triple-helical peptides.

The essential event in platelet adhesion to the injured blood vessel wall is the binding to subendothelial collagen of plasma von Willebrand factor (VWF), a protein that interacts transiently with platelet glycoprotein Ibalpha (GPIbalpha), slowing circulating platelets to facilitate firm adhesion through collagen receptors, including integrin alpha2beta1 and GpVI. To locate the site in collagen that binds VWF, we synthesized 57 overlapping triple-helical peptides comprising the whole triple-helical domain of collagen III. Peptide no. 23 alone bound VWF, with similar affinity to that of native collagen III. Immobilized peptide no. 23 supported platelet adhesion under static and flow conditions, processes blocked by an antibody that prevents collagen from binding the VWF A3 domain. Truncated and alanine-substituted peptides derived from no. 23 either strongly interacted with both VWF and platelets or lacked both VWF and platelet binding. Thus, we identified the sequence RGQOGVMGF (O is hydroxyproline) as the minimal VWF-binding sequence in collagen III.

Amino Acid Motifs↗

PubMed2002

Iron deficiency anaemia in young women.

OBJECTIVES: The expression density of GPIaIIa, the primary platelet collagen receptor (integrin alpha2beta1), is linked to two polymorphisms (GPIa-807C/T and HPA-5a/b). During evolution a gene shift from the genotypes GPIa-807CC-HPA-5bb to the genotypes GPIa-807CT-HPA-5aa has taken place. The aim of the study was to assess whether iron deficiency anaemia (e.g. increased blood loss) in young women could be associated with a specific genotype, indicating a role as potential evolutionary selection factor. STUDY DESIGN: Women between 18 and 40 yr of age presenting for their first blood donation were asked about alimentary habits and use of oral contraception. Haemoglobin and serum ferritin were measured and the GPIa-C807T and HPA-5 genotypes were determined. RESULTS: Two hundred women were included and grouped according to the WHO definition for iron deficiency anaemia (haemoglobin <121 g L(-1) and ferritin <15 microg L(-1)). Eight women fulfilled both WHO-criteria for iron deficiency anaemia, 145 women fulfilled none. No differences regarding age, use of oral contraceptives, alimentary habits, and HPA-5 were found between the groups. The GPIa-807CC genotype was strongly over-represented in the WHO-anaemic women as compared to the non-WHO-anaemic women (87.5% vs. 33.1%, P=0.003). CONCLUSION: Iron deficiency anaemia in young women might have been the evolutionary disadvantage causing the gene shift from GPIa-807CC to 807CT.

Adolescent↗

PubMed1996

The recognition sites of the integrins alpha1beta1 and alpha2beta1 within collagen IV are protected against gelatinase A attack in the native protein.

The susceptibility of three different solubilized forms of type IV collagen to gelatinase A cleavage and the concomitant effects on cell and integrin binding have been assessed. Dithiothreitol-solubilized Engelbreth-Holm Swarm (EHS) type IV collagen with disrupted intramolecular disulfide bonds in the CB3[IV] region was cleaved N-terminally to the CB3[IV] region into the two characteristic 100-300-nm fragments at 30 degrees C and was totally degraded at 37 degrees C. This was reflected in the partial or total loss of the alpha1beta1 and alpha2beta1 integrin binding sites within this region. The ability of gelatinase A to cleave EHS type IV collagen preparations with intact interchain disulfide bonds in CB3[IV] only occurred at higher temperatures. Furthermore, no effect on binding of cells or isolated integrins to the gelatinase-treated collagen could be detected after treatment at 37 degrees C. Dimeric collagen IV of human placenta with intact disulfide bonds in the CB3[IV] region was not degraded at all by gelatinase A at 37 degrees C.

Amino Acid Sequence↗

PubMed2003

Reciprocal signaling by integrin and nonintegrin receptors during collagen activation of platelets.

Activation of platelets by exposed collagen after vessel wall injury is a primary event in the pathogenesis of stroke and myocardial infarction. Two collagen receptors, integrin alpha2beta1 and glycoprotein VI (GPVI), are expressed at similar levels on human and mouse platelets, but their individual roles during collagen activation remain poorly defined. Recent genetic and pharmacologic experiments have revealed an essential role for GPVI but have failed to define the role of alpha2beta1 or explain how two structurally distinct collagen receptors might function together to mediate platelet collagen responses. Discriminating the roles of these two collagen receptors is complicated by evidence suggesting that GPVI and platelet integrins may activate a common intracellular signaling pathway. To determine how alpha2beta1 and GPVI activate platelets in response to collagen, we have (i) examined collagen signaling conferred by expression of these receptors in hematopoietic cell lines; (ii) determined the effect of blocking each receptor on the activation of human platelets by collagen; (iii) generated low-GPVI mice in which the alpha2beta1/GPVI receptor ratio has been altered from 1:1 to 50:1 to expose alpha2beta1 function; (iv) studied the collagen responses of mouse platelets lacking LAT, an adaptor protein critical for GPVI but not integrin signaling; and (v) addressed the mechanism by which soluble collagens activate wild-type platelets. These studies demonstrate that alpha2beta1 requires inside-out signals to participate in collagen signaling and that alpha2beta1 is required for collagen activation of platelets when GPVI signals are reduced by blocking anti-GPVI antibody, low receptor number, specific disruption of the GPVI signaling pathway, or forms of collagen that bind weakly to GPVI relative to alpha2beta1. We propose a reciprocal two-receptor model of collagen signaling in platelets in which the nonintegrin receptor GPVI provides the primary collagen signal that activates and recruits the integrin receptor alpha2beta1 to further amplify collagen signals and fully activate platelets through a common intracellular signaling pathway. This model explains many of the genetic and pharmacologic observations regarding collagen signaling in platelets and demonstrates a novel mechanism by which hematopoietic cells integrate signaling by structurally distinct receptors that share a common ligand.

Adaptor Proteins, Signal Transducing↗