DEFECTIVE THROMBIN-INDUCED CALCIUM CHANGES AND AGGREGATION OF BERNARD-SOULIER PLATELETS ARE NOT ASSOCIATED WITH DEFICIENT MODERATE-AFFINITYRECEPTORS

Cloning of the moderate-affinity, serpentine thrombin receptor has hel ped clarify the mechanism of thrombin-induced platelet activation. Pro teolytic cleavage by thrombin generates a new amino terminal that auto stimulates the receptor, leading to activation of multiple signaling p athways and the platelet response. Tile function of other thrombin rec eptors, 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 l ow amounts of GPIb. Platelets from four BSS subjects displayed normal aggregation profiles and cytosolic calcium changes in response to mode rate or high concentrations of thrombin. In contrast, the BSS platelet aggregation response was delayed and calcium changes were absent in r esponse 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 rece ptor by a synthetic peptide caused similar aggregation in platelets fr om all individuals. The synthetic peptide also elicited calcium respon ses in BSS platelets. Platelets from the BSS subjects and from an indi vidual with the May-Hegglin anomaly showed increased expression of the moderate-affinity thrombin receptor by flow-cytometric analyses. Thes e results suggest that BSS platelets possess high levels of a function al moderate-affinity thrombin receptor, probably due to large platelet size, and provide indirect evidence that a high-affinity thrombin rec eptor is associated with GPIb.