Platelet—von Willebrand Factor Interactions

Abstract

Clinical observations indicate that the interaction between von Willebrand factor (VWF)* and platelets plays a central role in platelet physiology. The most convincing evidence is that patients with quantitative and/or qualitative abnormalities of their VWF protein have a hemorrhagic diathesis characterized by abnormal platelet function (Zimmerman and Ruggeri, 1983; Coller, 1984a). Additional support for the importance of this interaction comes from observations of patients whose platelets cannot bind VWF to their surface as a result of abnormalities in the platelet membrane receptor mechanisms; however, conclusions from these patients must be tempered by an appreciation of the complexity of the disorders. Thus, patients with the Bernard-Soulier syndrome, whose platelets do not bind human VWF in the presence of ristocetin (Zucker et al., 1977; Moake et al., 1980) and cannot be aggregated by bovine VWF (Bithell et al., 1972; Howard et al., 1973), have a serious bleeding disorder (George et al., 1984; Coller, 1984b). As discussed in Chapter 16, the platelets from these patients lack normal glycoprotein (GP) lb (Nurden and Caen, 1975), the glycoprotein that several lines of evidence indicate is the receptor for both human VWF (when platelets are incubated with ristocetin) and bovine VWF (Kirby, 1977; Phillips, 1980; Coller et al., 1983). In addition to the GP Ib abnormality, platelets from these patients also appear to lack GP V and a glycoprotein of M _r 17-22,000 (Clemetson et al., 1982; Berndt et al., 1983; Nurden et al., 1983) that may be noncovalently associated with GP Ib (Coller et al, 1983; Bemdt et al., 1983; Chong et al., 1983; see Chapter 4). Patients with Glanzmann’s thrombasthenia, whose washed platelets fail to bind VWF when stimulated with either thrombin (Ruggeri et al., 1982a) or ADP (Gralnick et al., 1984), also suffer from a severe bleeding disorder (George et al., 1984; Coller, 1984b).