Leung L L
J Clin Invest. 1984 Nov;74(5):1764-72. doi: 10.1172/JCI111595.
Thrombospondin (TSP), the major alpha-granule protein of human platelets, binds to the activated platelet surface upon platelet stimulation. TSP has hemagglutinating (lectin-like) activity and forms a specific complex with fibrinogen. Based on these observations, it was postulated that the interaction of TSP and fibrinogen on the activated platelet surface may be an important step in the platelet aggregation process. To test this hypothesis, monospecific, affinity-purified anti-TSP Fab fragments were prepared and their effects on platelet aggregation and platelet fibrinogen binding were studied. Anti-TSP Fab caused significant interference with thrombin- and collagen-induced platelet aggregation, as monitored by both turbidometric aggregometry and particle counting measuring the disappearance of single platelets. Phase-contrast microscopy revealed that anti-TSP Fab caused a marked decrease in platelet macroaggregates and an increase in microaggregates and nonaggregated single platelets. Anti-TSP Fab did not affect the initial phase of ADP-induced platelet aggregation but caused rapid platelet disaggregation with the abolition of the secondary phase of aggregation. The effect of anti-TSP Fab was not mediated by a direct inhibition of platelet secretion. The effect of anti-TSP Fab on specific binding of labeled fibrinogen to thrombin-stimulated platelets was also studied. Anti-TSP Fab caused a marked decrease in the affinity of fibrinogen binding to the receptors on the activated platelet surface. Kinetic analyses revealed significant displacement of labeled fibrinogen by unlabeled fibrinogen in the presence of anti-TSP Fab, suggesting that TSP serves to stabilize fibrinogen binding to the activated platelet surface and reinforces the strength of interplatelet interactions. It is proposed that platelet aggregation is a dynamic, multistep process, governed initially by the platelet membrane glycoprotein IIb/IIIa-fibrinogen interaction, with the TSP-fibrinogen interaction playing an important role in determining the size and reversibility of platelet aggregates.
血小板反应蛋白（TSP）是人类血小板主要的α-颗粒蛋白，在血小板受到刺激时会与活化的血小板表面结合。TSP具有血凝（类凝集素）活性，并与纤维蛋白原形成特定复合物。基于这些观察结果，推测TSP与纤维蛋白原在活化血小板表面的相互作用可能是血小板聚集过程中的一个重要步骤。为了验证这一假设，制备了单特异性、亲和纯化的抗TSP Fab片段，并研究了它们对血小板聚集和血小板纤维蛋白原结合的影响。通过比浊法血小板聚集测定法和测量单个血小板消失的颗粒计数法监测发现，抗TSP Fab对凝血酶和胶原诱导的血小板聚集有显著干扰。相差显微镜显示，抗TSP Fab使血小板大聚集体显著减少，微聚集体和未聚集的单个血小板增加。抗TSP Fab不影响ADP诱导的血小板聚集的初始阶段，但会导致血小板迅速解聚，并消除聚集的第二阶段。抗TSP Fab的作用不是通过直接抑制血小板分泌介导的。还研究了抗TSP Fab对标记纤维蛋白原与凝血酶刺激的血小板特异性结合的影响。抗TSP Fab使纤维蛋白原与活化血小板表面受体结合的亲和力显著降低。动力学分析表明，在存在抗TSP Fab的情况下，未标记的纤维蛋白原会显著取代标记的纤维蛋白原，这表明TSP有助于稳定纤维蛋白原与活化血小板表面的结合，并增强血小板间相互作用的强度。有人提出，血小板聚集是一个动态的多步骤过程，最初由血小板膜糖蛋白IIb/IIIa-纤维蛋白原相互作用控制，而TSP-纤维蛋白原相互作用在决定血小板聚集体的大小和可逆性方面起重要作用。
