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血小板整合素αIIbβ3在生理模拟巨型单层囊泡(GUVs)中的横向扩散及纤维蛋白原诱导的聚集

The lateral diffusion and fibrinogen induced clustering of platelet integrin αIIbβ3 reconstituted into physiologically mimetic GUVs.

作者信息

Gaul Vinnie, Lopez Sergio G, Lentz Barry R, Moran Niamh, Forster Robert J, Keyes Tia E

机构信息

School of Chemical Sciences and National Biophotonics and Imaging Platform, Dublin City University, Dublin 9, Ireland.

出版信息

Integr Biol (Camb). 2015 Apr;7(4):402-11. doi: 10.1039/c5ib00003c.

Abstract

Platelet integrin αIIbβ3 is a key mediator of platelet activation and thrombosis. Upon activation αIIbβ3 undergoes significant conformational rearrangement, inducing complex bidirectional signalling and protein recruitment leading to platelet activation. Reconstituted lipid models of the integrin can enhance our understanding of the structural and mechanistic details of αIIbβ3 behaviour away from the complexity of the platelet machinery. Here, a novel method of αIIbβ3 insertion into Giant Unilamellar Vesicles (GUVs) is described that allows for effective integrin reconstitution unrestricted by lipid composition. αIIbβ3 was inserted into two GUV lipid compositions that seek to better mimic the platelet membrane. First, "nature's own", comprising 32% DOPC, 25% DOPE, 20% CH, 15% SM and 8% DOPS, intended to mimic the platelet cell membrane. Fluorescence Lifetime Correlation Spectroscopy (FLCS) reveals that exposure of the integrin to the activators Mn(2+) or DTT does not influence the diffusion coefficient of αIIbβ3. Similarly, exposure to αIIbβ3's primary ligand fibrinogen (Fg) alone does not affect αIIbβ3's diffusion coefficient. However, addition of Fg with either activator reduces the integrin diffusion coefficient from 2.52 ± 0.29 to μm(2) s(-1) to 1.56 ± 0.26 (Mn(2+)) or 1.49 ± 0.41 μm(2) s(-1) (DTT) which is consistent with aggregation of activated αIIbβ3 induced by fibrinogen binding. The Multichannel Scaler (MCS) trace shows that the integrin-Fg complex diffuses through the confocal volume in clusters. Using the Saffman-Delbrück model as a first approximation, the diffusion coefficient of the complex suggests at least a 20-fold increase in the radius of membrane bound protein, consistent with integrin clustering. Second, αIIbβ3 was also reconstituted into a "raft forming" GUV with well defined liquid disordered (Ld) and liquid ordered (Lo) phases. Using confocal microscopy and lipid partitioning dyes, αIIbβ3 showed an affinity for the DOPC rich Ld phase of the raft forming GUVs, and was effectively excluded from the cholesterol and sphingomyelin rich Lo phase. Activation and Fg binding of the integrin did not alter the distribution of αIIbβ3 between the lipid phases. This observation suggests partitioning of the activated fibrinogen bound αIIbβ3 into cholesterol rich domains is not responsible for the integrin clustering observed.

摘要

血小板整合素αIIbβ3是血小板活化和血栓形成的关键介质。激活后,αIIbβ3会发生显著的构象重排,引发复杂的双向信号传导和蛋白质募集,从而导致血小板活化。整合素的重组脂质模型能够增进我们对αIIbβ3行为的结构和机制细节的理解,而无需考虑血小板机制的复杂性。在此,描述了一种将αIIbβ3插入巨型单层囊泡(GUV)的新方法,该方法可实现不受脂质组成限制的有效整合素重组。αIIbβ3被插入两种旨在更好模拟血小板膜的GUV脂质组合物中。第一种是“天然自身”组合物,包含32%的二油酰磷脂酰胆碱(DOPC)、25%的二油酰磷脂酰乙醇胺(DOPE)、20%的胆固醇(CH)、15%的鞘磷脂(SM)和8%的二油酰磷脂酰丝氨酸(DOPS),旨在模拟血小板细胞膜。荧光寿命相关光谱法(FLCS)显示,整合素暴露于激活剂锰离子(Mn(2+))或二硫苏糖醇(DTT)不会影响αIIbβ3的扩散系数。同样,单独暴露于αIIbβ3的主要配体纤维蛋白原(Fg)也不会影响αIIbβ3的扩散系数。然而,将Fg与任何一种激活剂一起添加会使整合素扩散系数从2.52±0.29μm² s⁻¹降低至1.56±0.26μm² s⁻¹(Mn(2+))或1.49±0.41μm² s⁻¹(DTT),这与纤维蛋白原结合诱导的活化αIIbβ3聚集一致。多通道标量器(MCS)轨迹显示,整合素 - Fg复合物以簇的形式扩散通过共聚焦体积。使用萨夫曼 - 德尔布吕克模型作为初步近似,复合物的扩散系数表明膜结合蛋白的半径至少增加了20倍,这与整合素聚集一致。第二种,αIIbβ3也被重组到具有明确的液相无序(Ld)和液相有序(Lo)相的“筏形成”GUV中。使用共聚焦显微镜和脂质分配染料,αIIbβ3对筏形成GUV中富含DOPC的Ld相表现出亲和力,并被有效地排除在富含胆固醇和鞘磷脂的Lo相中。整合素的激活和Fg结合并未改变αIIbβ3在脂质相之间的分布。这一观察结果表明,活化的纤维蛋白原结合的αIIbβ3分配到富含胆固醇的结构域中并非导致所观察到的整合素聚集的原因。

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