Enzymatically catalyzed disulfide exchange is required for platelet adhesion to collagen via integrin α2β1

Integrin alpha(2)beta(1) is the principal adhesive receptor for collagen but platelets also adhere through glycoprotein VI (GPVI). Integrin alpha(IIb)beta(3) may augment platelet adhesion. We have shown that disulfide exchange is necessary for platelet adhesion to fibrinogen, fibronectin, and collagen. However 2 questions remained: (1) Can activated alpha(IIb)beta(3) explain the observed role of disulfide exchange in adhesion to collagen, or is this role common to other integrins? (2) Is disulfide dependence specific to the integrin receptors or shared with GPVI? To discriminate adhesive functions of alpha(2)beta(1) from those of alpha(IIb)beta(3) we used Glanzmann platelets and alpha(IIb)beta(3)-specific antibodies applied to normal platelets. To resolve adhesive events mediated by alpha(2)beta(1) from those of GPVI we used synthetic peptides specific to each receptor. We addressed direct integrin ligation using purified alpha(2)beta(1), and recombinant I domain. We observed the following: adhesion to the alpha(2)beta(1)-specific peptide was disulfide-exchange dependent and protein disulfide isomerase (PDI) mediated; membrane-impermeant thiol blockers inhibited alpha(2)beta(1) but not GPVI mediated, adhesion; direct blockade of PDI revealed that it is involved in adhesion through alpha(2)beta(1) but not GPVI; and purified alpha(2)beta(1), but not recombinant I domain, depended on free thiols for ligation. These data suggest that the enzymatically catalyzed adhesion-associated reorganization of disulfide bonds is common to members of the integrin family and specific to this family.