Defective thrombin-induced calcium changes and aggregation of Bernard-Soulier platelets are not associated with deficient moderate-affinity receptors.

Cloning of the moderate-affinity, serpentine thrombin receptor has helped clarify the mechanism of thrombin-induced platelet activation. Proteolytic cleavage by thrombin generates a new amino terminal that autostimulates the receptor, leading to activation of multiple signaling pathways and the platelet response. The function of other thrombin receptors, such as high-affinity glycoprotein Ib (GPIb), on platelets and their relationships to the moderate-affinity receptor remain unclear. The present study examined the role of the moderate-affinity thrombin receptor in Bernard-Soulier syndrome (BSS) platelets, which contain low amounts of GPIb. Platelets from four BSS subjects displayed normal aggregation profiles and cytosolic calcium changes in response to moderate or high concentrations of thrombin. In contrast, the BSS platelet aggregation response was delayed and calcium changes were absent in response to low thrombin concentrations. Platelets from an asymptomatic BSS heterozygote displayed an activation profile similar to those of control individuals. Specific activation of the moderate-affinity receptor by a synthetic peptide caused similar aggregation in platelets from all individuals. The synthetic peptide also elicited calcium responses in BSS platelets. Platelets from the BSS subjects and from an individual with the May-Hegglin anomaly showed increased expression of the moderate-affinity thrombin receptor by flow-cytometric analyses. These results suggest that BSS platelets possess high levels of a functional moderate-affinity thrombin receptor, probably due to large platelet size, and provide indirect evidence that a high-affinity thrombin receptor is associated with GPIb.