The Platelet Receptor GPVI Mediates Both Adhesion and Signaling Responses to Collagen in a Receptor Density-dependent Fashion

The platelet response to collagen is a primary event in hemostasis and thrombosis, but the precise roles of the numerous identified platelet collagen receptors remain incompletely defined. Attention has recently focused on glycoprotein VI (GPVI), a receptor that is expressed on platelets in association with a signaling adapter, the Fc receptor gamma chain (Fc Rγ). Genetic and pharmacologic loss of GPVI function results in loss of collagen signaling in platelets, but studies to date have failed to demonstrate that GPVI-Fc Rγ expression is sufficient to confer collagen signaling responses. These results have led to the hypothesis that collagen responses mediated by GPVI-Fc Rγ may require the collagen-binding integrin α2β1 as a co-receptor, but this model has not been supported by a recent study of mouse platelets lacking α2β1. In the present study we have used a novel anti-GPVI monoclonal antibody to measure the level of GPVI on human platelets and to guide the development of GPVI-expressing cell lines to assess the role of GPVI in mediating platelet collagen responses. GPVI receptor density on human platelets appears tightly regulated, is independent from the level of α2β1 expression, and significantly exceeds that on previously characterized GPVI-expressing RBL-2H3 cells. Using newly generated GPVI-expressing RBL-2H3 cells with receptor densities equivalent to that on human platelets, we demonstrate that GPVI expression confers both adhesive and signaling responses to collagen in a graded fashion that is proportional to the GPVI receptor density. These results resolve some of the conflicting data regarding GPVI-collagen interactions and demonstrate that 1) GPVI-Fc Rγ expression is sufficient to confer both adhesion and signaling responses to collagen, and 2) GPVI-mediated collagen responses are receptor density-dependent at the receptor levels expressed on human platelets. The platelet response to collagen is a primary event in hemostasis and thrombosis, but the precise roles of the numerous identified platelet collagen receptors remain incompletely defined. Attention has recently focused on glycoprotein VI (GPVI), a receptor that is expressed on platelets in association with a signaling adapter, the Fc receptor gamma chain (Fc Rγ). Genetic and pharmacologic loss of GPVI function results in loss of collagen signaling in platelets, but studies to date have failed to demonstrate that GPVI-Fc Rγ expression is sufficient to confer collagen signaling responses. These results have led to the hypothesis that collagen responses mediated by GPVI-Fc Rγ may require the collagen-binding integrin α2β1 as a co-receptor, but this model has not been supported by a recent study of mouse platelets lacking α2β1. In the present study we have used a novel anti-GPVI monoclonal antibody to measure the level of GPVI on human platelets and to guide the development of GPVI-expressing cell lines to assess the role of GPVI in mediating platelet collagen responses. GPVI receptor density on human platelets appears tightly regulated, is independent from the level of α2β1 expression, and significantly exceeds that on previously characterized GPVI-expressing RBL-2H3 cells. Using newly generated GPVI-expressing RBL-2H3 cells with receptor densities equivalent to that on human platelets, we demonstrate that GPVI expression confers both adhesive and signaling responses to collagen in a graded fashion that is proportional to the GPVI receptor density. These results resolve some of the conflicting data regarding GPVI-collagen interactions and demonstrate that 1) GPVI-Fc Rγ expression is sufficient to confer both adhesion and signaling responses to collagen, and 2) GPVI-mediated collagen responses are receptor density-dependent at the receptor levels expressed on human platelets. von Willebrand factor glycoprotein VI Fc receptor gamma chain convulxin fluorescein isothiocyanate phosphate-buffered saline The response of platelets to vessel wall injury is a primary event in arterial thrombosis (1Lusis A.J. Nature. 2000; 407: 233-241Crossref PubMed Scopus (4566) Google Scholar). Platelets respond to vessel wall injury via surface receptors that recognize exposed subendothelial matrix proteins, the most abundant of which is collagen. Collagen is unusual among platelet ligands because it mediates both platelet adhesion and platelet activation (2Wilner G.D. Nossel H.L. LeRoy E.C. J. Clin. Invest. 1968; 47: 2616-2621Crossref PubMed Scopus (100) Google Scholar). Platelet adhesion to collagen initiates the thrombotic response, and platelet activation in response to collagen accelerates the thrombotic response through release of granule contents and activation of platelet integrins. The ability of collagen to mediate both of these steps suggests that the platelet response to collagen may be a key step in the regulation of arterial thrombosis. A molecular understanding of the platelet receptors that mediate collagen responses is therefore likely to shed light on the pathogenesis of common vascular diseases such as myocardial infarction and stroke. The receptors for collagen on the surface of platelets may be divided into those which interact indirectly through collagen-bound von Willebrand factor (vWF),1including GPIbα and the integrin αIIbβ3, and those which interact directly with collagen, including glycoprotein VI (GPVI), the integrin α2β1, CD36, and perhaps others (recently reviewed in Ref. 3Clemetson K.J. Clemetson J.M. Thromb. Haemostasis. 2001; 86: 189-197Crossref PubMed Scopus (236) Google Scholar). The present model of platelet responses to collagen is one of successive receptor interactions mediating rolling, firm adhesion to collagen and platelet activation (4Sixma J.J. van Zanten G.H. Huizinga E.G. van der Plas R.M. Verkley M. Wu Y.P. Gros P. de Groot P.G. Thromb. Haemostasis. 1997; 78: 434-438Crossref PubMed Scopus (155) Google Scholar, 5Barnes M.J. Knight C.G. Farndale R.W. Curr. Opin. Hematol. 1998; 5: 314-320Crossref PubMed Scopus (74) Google Scholar). Although the receptor actions responsible for collagen-vWF interaction have been clearly identified (6Savage B. Saldivar E. Ruggeri Z.M. Cell. 1996; 84: 289-297Abstract Full Text Full Text PDF PubMed Scopus (994) Google Scholar, 7Savage B. Almus-Jacobs F. Ruggeri Z.M. Cell. 1998; 94: 657-666Abstract Full Text Full Text PDF PubMed Scopus (672) Google Scholar), those responsible for direct collagen interactions remain unclear. A recently proposed two-receptor, two-step model in which adhesion to collagen is first mediated by high affinity interaction with α2β1 and platelet activation is subsequently initiated by low affinity interaction with GPVI (5Barnes M.J. Knight C.G. Farndale R.W. Curr. Opin. Hematol. 1998; 5: 314-320Crossref PubMed Scopus (74) Google Scholar) is attractive because it assigns unique roles to each receptor and presents a stepwise sequence of events that resemble those described for leukocyte-endothelial interactions during inflammatory responses (8Hynes R.O. Wagner D.D. J. Clin. Invest. 1997; 100: S11-13Crossref PubMed Scopus (75) Google Scholar). Unfortunately, the experimental data reported to date have been conflicting and have failed to support this model or to demonstrate clear and sufficient roles for α2β1 and GPVI. Human α2β1 deficiency states reportedly result in bleeding disorders and platelets with severely reduced collagen responses (9Nieuwenhuis H.K. Akkerman J.W. Houdijk W.P. Sixma J.J. Nature. 1985; 318: 470-472Crossref PubMed Scopus (388) Google Scholar, 10Kehrel B. Balleisen L. Kokott R. Mesters R. Stenzinger W. Clemetson K.J. van de Loo J. Blood. 1988; 71: 1074-1078Crossref PubMed Google Scholar), but a recent analysis of mice lacking platelet α2β1 demonstrated normal bleeding times, normal platelet adhesion to collagen, and almost no loss of platelet signaling responses to collagen (11Nieswandt B. Brakebusch C. Bergmeier W. Schulte V. Bouvard D. Mokhtari-Nejad R. Lindhout T. Heemskerk J.W. Zirngibl H. Fassler R. EMBO J. 2001; 20: 2120-2130Crossref PubMed Scopus (435) Google Scholar). Deficiency of GPVI and its signaling partner Fc Rγ, however, results in loss of platelet collagen responses in both human and mouse platelets (11Nieswandt B. Brakebusch C. Bergmeier W. Schulte V. Bouvard D. Mokhtari-Nejad R. Lindhout T. Heemskerk J.W. Zirngibl H. Fassler R. EMBO J. 2001; 20: 2120-2130Crossref PubMed Scopus (435) Google Scholar, 12Sugiyama T. Okuma M. Ushikubi F. Sensaki S. Kanaji K. Uchino H. Blood. 1987; 69: 1712-1720Crossref PubMed Google Scholar, 13Poole A. Gibbins J.M. Turner M. van Vugt M.J. van de Winkel J.G. Saito T. Tybulewicz V.L. Watson S.P. EMBO J. 1997; 16: PubMed Scopus Google Scholar). loss of function studies have a in which the roles of collagen receptors may be but are In to the roles of collagen receptors we have a model with which to collagen receptor function by a of function to cells. the role of GPVI we have expressed the receptor in RBL-2H3 a cell that GPVI and α2β1 but the GPVI signaling Fc Rγ C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). expression of GPVI in RBL-2H3 cells the role of Fc Rγ for GPVI signaling and demonstrated that both the GPVI and are for Fc Rγ C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). GPVI expression both signaling and adhesive responses to the high affinity GPVI convulxin a used to the receptor J.M. J. E. Clemetson K.J. J. Full Text Full Text PDF PubMed Scopus Google Scholar), but failed to confer adhesive or signaling responses to collagen C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). These results that GPVI one or of the platelet collagen receptors as a for collagen A of this however, the to directly GPVI receptor density on RBL-2H3 cells with that on human platelets because of the of anti-GPVI In the present study we have used a novel anti-GPVI monoclonal antibody to directly measure GPVI receptor density on human platelets and on both previously described and newly generated GPVI-expressing RBL-2H3 cells. results demonstrate that GPVI expression is sufficient to confer both adhesive and signaling responses to collagen independent of collagen receptors but that GPVI-collagen responses are on receptor density. A in GPVI receptor density RBL-2H3 cells that no collagen responses C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar) from those that both adhesion and signaling responses. of the GPVI receptor density to confer collagen responses in RBL-2H3 cells with that on the surface of human platelets that platelets have a GPVI receptor density equivalent to that to confer a collagen response in RBL-2H3 cells. the GPVI expression level on human platelets collagen responses we GPVI levels on the platelets of a but of and with those of α2β1. are with those mouse platelets and support a model of platelet collagen responses in which GPVI a role for both adhesion and the role of GPVI in the pathogenesis of arterial thrombotic diseases such as and myocardial infarction is of collagen and convulxin used in the studies are from the as previously described C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). isothiocyanate from GPVI monoclonal antibody mouse in antibody from RBL-2H3 cells in with and Fc receptor gamma chain (Fc mice to mice to Fc Rγ Fc of the mice used for study in the of the of RBL-2H3 cells generated as previously A of for expression antibody and for collagen signaling and adhesion as described for monoclonal the human GPVI receptor we expressed C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar) on the surface of from mice that as The GPVI which has a expressed to that surface GPVI expression not by of Fc Rγ C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). generated by the monoclonal antibody at the of or and on GPVI-expressing RBL-2H3 cells for to GPVI. human GPVI on human platelets cell antibody by affinity on from in the as previously described C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). cells to to the with of or collagen for and The with The of cells to the to as previously described C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar) the RBL-2H3 cell to measure cell K. B. J. T. D. B. J. Google Scholar). into from the of mice by from the of the cells by of with at for at platelets by through a in and The platelets in a and to in In each of platelets with or collagen from to Platelet in a cells and GPVI-expressing cells in and as previously described C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). cells with of for at with The cells with and in of into each of and at for with The with times, with of with and for The of in and as described that the primary antibody antibody of the with the to the with antibody of to the Human and GPVI-expressing cells with for at The on of a of in and in and for at for platelets and at for cells. The and of cell for of the in the of of and from the for the to the of GPVI receptors cell on the of and the that one antibody to one GPVI the platelet GPVI receptor density of GPVI-expressing RBL-2H3 we used platelet of R. J. V. E. and and Scholar) and RBL-2H3 cell of The RBL-2H3 cell the with a for the and of the cell by a of the and the used to the The to be by the The cell a with and These results with data direct A. D. S. A. PubMed Scopus Google Scholar, D. A. J. Full Text PDF PubMed Scopus Google Scholar). used to a of for surface on the surface as a function of The platelet receptor density therefore as RBL-2H3 measure GPVI receptor density on RBL-2H3 cells and human platelets we monoclonal mouse which the GPVI receptor on the surface as described a of on the cell surface because the GPVI receptor expression from that of Fc Rγ, a not expressed in but for GPVI expression C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar, B. Bergmeier W. Schulte V. K. Zirngibl H. J. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). has a and a molecular from GPVI C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar, data not antibody which to the surface of RBL-2H3 cells and cells in a to the antibody but not RBL-2H3 or cells with of of cell from and GPVI-expressing RBL-2H3 cells with identified a in the from GPVI-expressing cells but not cells studies identified the by as antibody The ability of to recognize GPVI on the surface of human platelets demonstrated by cell analysis and is a novel monoclonal antibody that the of human GPVI. of to human platelets or GPVI-expressing cells not with collagen or signaling not that it a of the of GPVI from the of both of GPVI receptor density on human platelets and surface expression of the integrin to human platelets. The receptor platelet as described are the and of and are of on with of GPVI receptor levels on The with platelets from and one with of α2β1 receptor levels on The are the as in and lines of the a with the in platelet levels but in platelet GPVI the of GPVI receptors on human platelets we used both and to human platelets is of platelets from a as in in for each GPVI receptor density of platelet and data not receptors and analysis of from and of both and data not by the GPVI receptor levels by in this but of In analysis of α2β1 levels in the platelets a Although these results are by the of the the level of GPVI receptors on human platelets not to as as or in with that of α2β1. have described previously GPVI-expressing RBL-2H3 cells that respond to the GPVI but not to collagen C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). The of a GPVI antibody direct of GPVI receptor density on these cells with that on platelets and the that the cell GPVI receptor densities on the of responses C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). The development of the anti-GPVI antibody to directly measure the GPVI receptor on the GPVI-expressing RBL-2H3 cells we characterized and to the receptor density on the model cell with that on human platelets. results in that responses to the GPVI receptor density on is of that on human platelets. These results that the to confer collagen responses in those cells may have been because of receptor and to GPVI-expressing RBL-2H3 cell lines with GPVI receptor densities to that of human platelets. the GPVI-expressing cell lines for analysis of collagen adhesive and signaling responses. GPVI at the receptor density of platelets, and GPVI at the density as platelets and receptor density on GPVI-expressing RBL-2H3 cell lines and with that on human RBL-2H3 platelet GPVI receptor and level of GPVI receptor expression on GPVI-expressing RBL-2H3 cell lines as described The for each cell is from a that of The receptor is the of and is of The of platelet GPVI receptor density on platelet and RBL-2H3 cell and as described in a The and level of GPVI receptor expression on GPVI-expressing RBL-2H3 cell lines as described The for each cell is from a that of The receptor is the of and is of The of platelet GPVI receptor density on platelet and RBL-2H3 cell and as described the role of GPVI in platelet adhesion to collagen, adhesion and previously GPVI-expressing cells with the density of GPVI receptors but not collagen and however, both to but and to a to to adhesion to however, GPVI-mediated adhesion to collagen not require or not These results demonstrate that GPVI expression is sufficient to confer adhesion to collagen but that GPVI-collagen interaction a receptor density that is of that expressed on human platelets. The role of receptor density for GPVI-collagen adhesive interaction is by the of adhesion cells that the receptor of The ability to confer collagen adhesion through expression of GPVI at receptor density direct GPVI-collagen interaction and suggests that collagen signaling may be mediated by GPVI in a density-dependent the role of GPVI receptor density for collagen signaling we the ability of and to mediate signaling to collagen, a that initiates platelet activation and data not previously to but not to collagen demonstrated no signaling responses to collagen to to collagen and GPVI at a receptor density that is that in human platelets however, a in response to collagen as low as and data not signaling responses in GPVI-expressing cell lines but and in in the GPVI-expressing cells with receptor These results that GPVI is of mediating collagen signaling of platelet collagen receptors and that GPVI signaling in response to collagen, to GPVI adhesion to collagen, is on receptor signaling responses to collagen are receptor GPVI-expressing RBL-2H3 cell lines exposed to convulxin or collagen and signaling the that signaling in GPVI-expressing RBL-2H3 cells collagen signaling in those the levels of GPVI The that collagen signaling be in RBL-2H3 cells that GPVI at the receptor density of human platelets but not in cells that that receptor density suggests that platelet collagen responses may be to a in the level of GPVI this hypothesis we mice for a in the Fc Rγ T. M. D. R. Cell. Full Text PDF PubMed Scopus Google Scholar), expression is for GPVI C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). loss of Fc Rγ (Fc Rγ is to the of Fc Rγ and of GPVI levels in Fc Rγ platelets the GPVI antibody a in receptor and platelets from Fc mice in responses to collagen the responses to Fc and Fc platelets. a in GPVI receptor density collagen signaling in GPVI-expressing RBL-2H3 a has in mouse platelets. this of signaling in platelets or the of platelet collagen receptors is and is Collagen adhesion and signaling are to roles in the platelet response to vessel but the molecular events that these responses remain is that the first platelet response to exposed collagen is the successive and of platelet at high (6Savage B. Saldivar E. Ruggeri Z.M. Cell. 1996; 84: 289-297Abstract Full Text Full Text PDF PubMed Scopus (994) Google Scholar, 7Savage B. Almus-Jacobs F. Ruggeri Z.M. Cell. 1998; 94: 657-666Abstract Full Text Full Text PDF PubMed Scopus (672) Google Scholar), interaction that platelets to on exposed subendothelial matrix and The and sequence of these platelet interactions with collagen, however, remain model of interaction that firm adhesion is in mediated by a high affinity interaction that collagen signaling via a low affinity GPVI-collagen interaction (5Barnes M.J. Knight C.G. Farndale R.W. Curr. Opin. Hematol. 1998; 5: 314-320Crossref PubMed Scopus (74) Google Scholar). date this model has been loss of function with conflicting and the for a model is S.P. B. D. R. S. D. M. D. J. P. J. Thromb. Haemostasis. 2001; 86: PubMed Scopus Google Scholar). Human and mouse GPVI deficiency states support a role for GPVI-Fc Rγ in collagen signaling in platelets B. Bergmeier W. Schulte V. K. Zirngibl H. J. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar, M. Okuma M. K. J. Clin. Invest. 84: PubMed Scopus Google Scholar), but studies of human and mouse α2β1 deficiency states have results regarding the for α2β1 for both collagen adhesion and collagen signaling in platelets (9Nieuwenhuis H.K. Akkerman J.W. Houdijk W.P. Sixma J.J. Nature. 1985; 318: 470-472Crossref PubMed Scopus (388) Google Scholar, 10Kehrel B. Balleisen L. Kokott R. Mesters R. Stenzinger W. Clemetson K.J. van de Loo J. Blood. 1988; 71: 1074-1078Crossref PubMed Google Scholar, B. Brakebusch C. Bergmeier W. Schulte V. Bouvard D. Mokhtari-Nejad R. Lindhout T. Heemskerk J.W. Zirngibl H. Fassler R. EMBO J. 2001; 20: 2120-2130Crossref PubMed Scopus (435) Google Scholar). are in these loss of function is the to loss of function to of the of a receptor of interactions loss of collagen adhesion with loss of GPVI may result from the loss of direct GPVI-collagen interaction or from the loss of α2β1 integrin activation (11Nieswandt B. Brakebusch C. Bergmeier W. Schulte V. Bouvard D. Mokhtari-Nejad R. Lindhout T. Heemskerk J.W. Zirngibl H. Fassler R. EMBO J. 2001; 20: 2120-2130Crossref PubMed Scopus (435) Google Scholar, M. 2000; PubMed Scopus Google is the to loss of function to the of a receptor that to the of independent receptors GPVI and α2β1 may but the actions of both are for platelet collagen the of platelet collagen receptors and signaling it is therefore not that loss of function have not a clear of the events in platelet collagen adhesion and to the function of collagen receptors is to sufficient roles by of expression in collagen cells. the role of GPVI we have expressed GPVI in RBL-2H3 a cell that Fc Rγ chain and is a model cell for receptor signaling but not GPVI or α2β1 not in platelets, Fc Rγ chain signaling in RBL-2H3 cells results in of and C. J. PubMed Scopus Google Scholar). have previously that expression of GPVI in RBL-2H3 cells confer adhesion and signaling to the but not to collagen C. H. D. J. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). a result the that GPVI is for collagen signaling in platelets, and it that GPVI be but not sufficient for platelet collagen responses perhaps to a of collagen such as α2β1 for or that model not GPVI expression or function on platelets responses. the we have a monoclonal antibody that human GPVI and directly GPVI receptor levels on human platelets and on previously characterized GPVI-expressing cell The that GPVI-expressing cell lines have a significantly GPVI receptor density platelets that the in GPVI receptor density may be responsible for the to confer collagen responses. In the present study we have directly this and results demonstrate that expression of GPVI in the of platelet collagen receptors is sufficient to confer both collagen adhesion as as signaling and that GPVI-collagen responses are both proportional to and on receptor density. has that are 1) the experimental for this GPVI is sufficient for some or platelet collagen responses. 2) in platelet GPVI receptor expression or may significantly platelet collagen and or the for arterial thrombotic that GPVI is sufficient for both collagen adhesion and signaling is supported by the recent that mouse platelets in α2β1 normal adhesion to collagen and as as normal responses to collagen, GPVI function is by antibody (11Nieswandt B. Brakebusch C. Bergmeier W. Schulte V. Bouvard D. Mokhtari-Nejad R. Lindhout T. Heemskerk J.W. Zirngibl H. Fassler R. EMBO J. 2001; 20: 2120-2130Crossref PubMed Scopus (435) Google Scholar). these studies GPVI-Fc Rγ at the of interactions and to collagen receptors such as α2β1 to is to these however, with the of bleeding and platelet collagen responses (9Nieuwenhuis H.K. Akkerman J.W. Houdijk W.P. Sixma J.J. Nature. 1985; 318: 470-472Crossref PubMed Scopus (388) Google Scholar, 10Kehrel B. Balleisen L. Kokott R. Mesters R. Stenzinger W. Clemetson K.J. van de Loo J. Blood. 1988; 71: 1074-1078Crossref PubMed Google Scholar). has been that this may be by platelet in the or may a in the of collagen receptors (11Nieswandt B. Brakebusch C. Bergmeier W. Schulte V. Bouvard D. Mokhtari-Nejad R. Lindhout T. Heemskerk J.W. Zirngibl H. Fassler R. EMBO J. 2001; 20: 2120-2130Crossref PubMed Scopus (435) Google Scholar). bleeding in mice to platelet are likely to and are to human bleeding as by the of through analysis of a bleeding T. Okuma M. Ushikubi F. Sensaki S. Kanaji K. Uchino H. Blood. 1987; 69: 1712-1720Crossref PubMed Google Scholar, M. Okuma M. K. J. Clin. Invest. 84: PubMed Scopus Google Scholar), the loss of collagen responses in GPVI-Fc mice T. M. D. R. Cell. Full Text PDF PubMed Scopus Google Scholar). The responses to collagen in human and mouse platelets lacking α2β1, however, are to In the of platelet in this regarding the of studies in mice to human platelet collagen responses. for this is by that GPVI-collagen responses with receptor expression is that in the of levels of GPVI α2β1 a but in the of levels of GPVI α2β1 is for normal platelet collagen adhesion and platelets from in α2β1 normal levels of GPVI may present platelet collagen responses. in receptor expression level is to be in of The ability of α2β1 to to platelet collagen responses it is not for is supported by studies of with with high levels of platelet α2β1 platelet collagen responses R. D. Blood. PubMed Google Scholar) and to be at for S. C. C. A. Blood. PubMed Google Scholar). In to the role of α2β1 in the of and levels of GPVI we the expression of these collagen receptors on a of and a in the level of α2β1 GPVI receptor levels by in GPVI receptor levels is likely to be in a it appears that GPVI receptor levels significantly those of α2β1 and that with low or high levels of GPVI may be In to the of a in GPVI levels in platelets in the of levels of α2β1, we platelets from mice in which one of Fc Rγ by (Fc GPVI-expressing RBL-2H3 however, Fc in responses. for the in GPVI receptor levels by in RBL-2H3 cells 1) direct signaling by platelet collagen 2) of signaling responses in platelets of and that are not present in RBL-2H3 a of the GPVI receptor density on mouse platelets RBL-2H3 cells to the used or to a in GPVI expression levels on mouse and human platelets. the hypothesis that in GPVI levels in human platelets the role of α2β1 and perhaps platelet collagen receptors incompletely These studies are the first to clearly confer platelet collagen responses through expression of GPVI and resolve some of the conflicting that have been because the receptor and it is clear that GPVI is of mediating of the described platelet responses to collagen, but it in the of platelet collagen receptors and signaling responses to be the roles of collagen receptors such as α2β1 and the to GPVI is likely to require the of in and the study of collagen receptor expression and function in human of GPVI as a receptor that mediates both platelet adhesion and activation vessel injury for to common vascular and in the by and for and