Physical and Functional Interaction Between Cell-Surface Calreticulin and the Collagen Receptors Integrin α2β1 and Glycoprotein VI in Human Platelets

Summary Calreticulin is an abundant protein in the endoplasmic reticulum of most cells. In this study, flow cytometry and immunoprecipitation from surface-biotinylated platelets each provided direct evidence that calreticulin is also expressed on the surface of human platelets. Anti-calreticulin antibodies caused platelet activation, inducing Fc RIIa-independent platelet aggregation. In addition, these antibodies inhibited platelet adhesion to the integrin α2β1-specific ligands, GFOGER-GPP and monomeric collagen I, and to the glycoprotein VI-specific ligand, CRP. Inhibition of platelet adhesion to these ligands was independent of integrin αIIbβ3. In resting platelets, calreticulin was shown to interact with integrin α2β1 and glycoprotein VI. Together, these data demonstrate that surface calreticulin is associated with collagen receptors on the platelet surface, where it may play a role in the modulation of the platelet-collagen interaction. Abbreviations: BSA: bovine serum albumin; cC1qR: complement component 1q receptor; CRP: collagen-related peptide; DTSSP: 3,3’-dithiobis (sulfosuccinimidyl propionate); ER: endoplasmic reticulum; GFOGER-GPP: GPC[GPP]5GFOGER[GPP]5GPC; GpVI: glycoprotein VI; HRP: horseradish peroxidase; PDI: protein disulfide isomerase.