脂筏--细胞信号转导发生的平台

在脂筏和胞膜窖中存在有多种参与细胞信号转导的跨膜蛋白质,在细胞内或／和细胞外信号的刺激下。脂筏能改变蛋白质的大小和组成,有助于特异的蛋白质与蛋白质之间的相互作用,从而导致了信号级联反应的激活。脂筏在细胞信号转导事件中的重要作用已越来越受到人们的关注。     

脂筏与精子膜信号传导

脂筏是细胞膜内由特殊脂质与蛋白质构成的微域。小窝是脂筏的一种形式,小窝标记蛋白有小窝蛋白和小窝舟蛋白。脂筏或小窝与生物信号传导、细胞蛋白转运和胆固醇平衡有关。最近实验证实哺乳动物精子膜具有脂筏结构,脂筏与膜胆固醇外逸对于启动受精的信号传导具有重要作用。     

脂筏介导的病毒内吞

近几年的研究表明,病毒内吞进入细胞的途径是多样化的。除了经典的网格蛋白介导的病毒内吞,还有小窝（caveolae）或脂筏（lipid raft）介导的病毒内吞。在研究过程中还发现了新的细胞器小窝体（caveosome）。小窝体甚至还与网格蛋白介导的内吞相关的细胞器（如内体）存在着联系。这些研究加深了我们对病毒的认识,为我们发现新的抗病毒药物打下基础。同时病毒可以作为一个有用的工具来研究细胞内吞的路径和与之相关的细胞器。使人类更加了解细胞本身的奥秘。     

脂筏在病毒感染中的作用

脂筏是细胞膜上富含鞘脂和胆固醇的微区结构,广泛分布于细胞的膜系统.脂筏中含有诸多信号分子和免疫受体,在细胞的生命活动中扮演非常重要的角色.更为重要的是,脂筏为细胞表面发生的蛋白质-蛋白质和蛋白质-脂类分子间的相互作用提供了平台.研究表明,很多病毒可以利用细胞膜表面的脂筏结构介导其侵入宿主细胞,一些病毒可以借助脂筏结构完成病毒颗粒的组装和出芽.本文将综述不同类型的病毒如SV40、HIV等借助脂筏完成入侵以及流感病毒等利用脂筏完成组装和出芽的证据及机理,并概述目前研究病毒与脂筏相互作用的方法及存在的问题.深入研究脂筏在病毒感染中的作用,将有助于对病毒与宿主细胞的相互作用的理解,从而可能发现新的、有效的对抗病毒的方法。     

脂筏与T细胞信号转导

抗原提呈细胞将抗原加工处理后通过MHCⅠ/MHCⅡ类分子提呈供T细胞识别。TCR对抗原的识别引起一系列下游信号事件的发生,最终使T细胞激活,但对TCR复合物结合抗原后引起胞内区磷酸化的早期事件机制还不是很清楚。最近的研究揭示脂筏参与了这一早期信号事件的发生。脂筏是一种膜脂双层内含有的特殊微区,T细胞膜表面参与T细胞激活的各种关键信号分子都定位于脂筏。T细胞激活过程中脂筏通过聚集和重分配形成一个信号转导的平台。     

去垢剂法提取脂筏的量化

目的:将去垢剂法提取脂筏的操作方法量化.方法:依据脂质筏在4℃时不溶于去垢剂的特性提取脂筏,再用蔗糖密度梯度离心法将去垢剂不溶组分分离出来.用胆固醇吸光度及浮舰蛋白1(flotillin-1)作为脂质筏的特异性标记,验证提取物的特性.结果:在离心管5％蔗糖与30％蔗糖分界处看到一层连成片状乳黄色脂质物质,光散射法显示该提取物在620 nm处有最大吸光值,Western blot结果显示在相对分子质量48 kDa处可见条带.结论:提取物符合脂筏的多种特性,操作方法量化的去垢剂法是一种简单、稳定提取脂质筏的方法.     

生物膜的生物物理观——从微区到脂筏

大量的实验表明,在细胞质膜中,由于不同成分具有不同的生物化学特性,发生相分离而局部形成微区.不同的微区可行使不同的功能.近年来一种富含胆固醇、鞘脂类以及大量的受体和信号分子的液态有序相的微区,即脂筏(lipid rafts),由于被发现参与信号转导和一些物质的生理循环过程而备受关注.随着实验手段的提高,人们对生物膜在分子水平上认识的不断深化,脂筏结构和功能的物理、化学基础研究方面也取得了初步的进展.     

脂筏在真菌极性生长中的作用

脂筏（lipid raft）是细胞膜上富含胆固醇和鞘磷脂的微结构域（microdomain）,参与细胞的多种生物学行为.随着研究的进一步深入,发现脂筏在真菌的极性生长方面也起着重要的作用.该文通过介绍真菌极性生长、脂筏的结构与功能等,阐述脂筏在真菌细胞极性生长中的作用,为阻断真菌极性生长并开发新的抗真菌药物靶点提供理论依据.     

脂筏在阿尔茨海默症中的作用

脂筏(lipid raft)是细胞膜中富含胆固醇的功能性微区,在信号转导、物质运输等方面发挥着重要作用。大量证据显示脂筏与阿尔茨海默症(Alzheimer’s disease,AD)的致病机理密切相关。β-淀粉样肽(amyloidβ-peptide,Aβ)的异常代谢和聚集可能是AD的致病主因,而脂筏不但是Aβ产生的主要场所,还能调节Aβ的聚集行为及神经毒性,因而在AD的病理过程中扮演着关键角色。     

用鞘磷脂作为标志分子鉴定脂筏

脂筏是质膜双层中富含鞘脂、胆固醇及特殊蛋白质的质膜微区.对其功能的研究,首先要对其进行分离和鉴定.常利用密度梯度超速离心将其分离,然后以脂筏中富含的神经节苷脂GM₁作为标志分子,利用荧光或生物素标记的霍乱毒素-B亚基进行亲和标记来鉴定脂筏.但这一鉴定方法操作复杂、费时、易对环境造成污染,所用关键试剂霍乱毒素不易获得,再加上一些组织GM₁含量甚微或不含GM₁,使其应用受到局限.为建立一个特异性高又对各种组织广泛适应的脂筏鉴定方法.对两种细胞系脂筏的脂类组分进行了分析.结果发现,可用鞘磷脂作为脂筏的特异性标志分子,采用高效薄层层析技术对脂筏进行鉴定.     

Occludin 蛋白与细胞间紧密连接关系及其临床意义

细胞间紧密连接(tight junctions)广泛存在于上皮细胞及内皮细胞之间,其作用是保持细胞间结构的完整性,确保其功能的正常发挥,紧密连接上有很多种蛋白,occludin蛋白是其中主要蛋白之一,occludin蛋白的结构发生变化会导致紧密连接结构及功能的改变,而紧密连接结构与功能的紊乱是很多临床疾病共同的病理生理学特点,如肿瘤、中风及炎症性肺疾病。Occludin蛋白的结构及功能的改变受很多机制的调控,本文主要对occludin蛋白的结构、功能、调控机制及其与紧密连接之间的关系进行叙述。     

Lysophosphatidic Acid Increases Tight Junction Permeability in Cultured Brain Endothelial Cells

Abstract: Brain capillary endothelial cells are coupled by a continuous belt of complex high-electrical-resistance tight junctions that are largely responsible for the blood-brain barrier. We have investigated mechanisms regulating tight junction permeability in brain endothelial cells cultured to maintain high-resistance junctions. The phospholipid lysophosphatidic acid (LPA) was found to cause a rapid, reversible, and dose-dependent decrease in transcellular electrical resistance in brain endothelial cells. LPA also increased the paracellular flux of sucrose, which, together with the resistance decrease, indicated increased tight junction permeability. Activation of protein kinase C attenuated the effect of LPA, suggesting that it was mediated by activation of a signalling pathway. LPA did not cause any obvious relocalization of adherens junction- or tight junction-associated proteins. However, it did stimulate the formation of stress fibres, the recruitment of focal adhesion components, and the appearance of tyrosine phosphorylated protein at focal contacts. Our study shows that LPA is a modulator of tight junction permeability in brain endothelial cells in culture and raises the possibility that it triggers blood-brain barrier permeability changes under (patho)physiological conditions.     

Bovine Brain Endothelial Cells Express Tight Junctions and Monoamine Oxidase Activity in Long-Term Culture

The passage of substances across the blood-brain barrier is regulated by cerebral capillaries which possess certain distinctly different morphological and enzymatic properties compared to capillaries of other organs. Investigations of the functional characteristics of brain capillaries have been facilitated by the use of cultured brain endothelial cells, but in most studies a number of characteristics of the in vivo system are lost. To provide an in vitro system for studies of brain capillary functions, we developed a method of isolating and producing a large number of bovine brain capillary endothelial cells. These cells, absolutely free of pericyte contamination, are subcultured, at the split ratio of 1:20 (20-fold increase of the cultured surface), with no apparent changes in cell morphology up to the fiftieth generation (10 passages). Retention of endothelial-specific characteristics (factor VIII-related antigen, angiotensin-converting enzyme, and nonthrombogenic surface) is shown for brain capillary-derived endothelial cells up to passage 10, even after frozen storage at passage 3. Furthermore, we showed that bovine brain capillary endothelial cells retain, up to the fiftieth generation, some of the characteristics of the blood-brain barrier: occurrence of tight junctions, paucity of pinocytotic vesicles, and monoamine oxidase activity.     

Serum Opens Tight Junctions and Reduces ZO-1 Protein in Retinal Epithelial Cells

Abstract: We have shown previously that serum inhibits tight junction formation in a retinal epithelial cell culture model for the blood-brain barrier. We have now examined in detail the effects of serum on the tight junctions. Our data show that serum induces a breakdown in tight junction function as indicated by decreased transepithelial electrical resistance and increased permeability. Rat serum had effects similar to those of bovine serum, indicating that the activity is species-independent. The effect is concentration-dependent, reversible, and specific for the apical surface, suggesting the involvement of a specific receptor-ligand interaction. Differences in the time course, response magnitude, and structural manifestations between the serum-induced breakdown and that induced by switching the cultures to a low-calcium medium suggest fundamental differences in their mechanisms. The calcium switch results in an immediate and complete junctional breakdown with cell retraction and perinuclear translocation of both actin and the tight junction protein zonula occludens-1. The serum-induced breakdown occurs slowly, is incomplete, and is manifested structurally by decreases in zonula occludens-1 protein, whereas actin organization is unchanged. Thus, serum induces a specific breakdown in retinal epithelial cell tight junctions that may be mediated by effects on the expression of zonula occludens-1.     

Intracerebral Dialysis and the Blood-Brain Barrier

Abstract: The aim of the study was to evaluate how implantation of a dialysis probe influences the blood-brain barrier. Leakage of endogenous serum albumin was evaluated by Evans blue/albumin staining and by immunohistochemistry. The passage from blood to dialysate of two substances that normally do not pass into the brain, [³H]inulin and glutamate, was studied 3 and 24 h after insertion of a dialysis probe. Evans blue, given 20 min before rats were killed, was observed around the probe and surrounding brain tissue. Albumin immunoreactivity was seen at considerable distance from the probe with larger spread at 24 h than at 3 h after probe insertion. Glutamate and [³H]inulin were detected in the dialysate with no significant further increase of radioactivity after intracarotid infusion of protamine sulfate that enhances the permeability over the blood-brain barrier. When protamine was followed by infusion of glutamate, the concentrations of taurine increased in the dialysate in four of eight rats. That plasma constituents have access to the brain around the dialysis probe is essential to consider, particularly in studies using substances and drugs that do not pass an intact blood-brain barrier.     

GDNF的生物学效应对脂筏中胞膜蛋白定位的重要性研究

目的：探讨GDNF的生物学效应对胞膜蛋白在脂筏的定位的影响。方法：首先以PBS或GDNF预处理体外培养的真核细胞,提取脂筏,以免疫印迹方法检测三种胞膜蛋白（RET,NCAMl40及integrinβ1）在脂筏的含量变化。结果：GDNF预处理组RET和NCAMl40蛋白在脂筏的含量增加,而integrinlM蛋白的含量无显著性变化。在脂筏中也可检测到integrinβ1蛋白。结论：GDNF可影响某些胞膜蛋白在细胞膜上的定位,使其招募到脂筏,这可能是GDNF的一种重要生物学效应。     

GDNF的生物学效应对脂筏中胞膜蛋白定位的重要性研究*

摘要目的：探讨GDNF的生物学效应对胞膜蛋白在脂筏的定位的影响。方法：首先以PBS 或GDNF 预处理体外培养的真核细 胞,提取脂筏,以免疫印迹方法检测三种胞膜蛋白（RET,NCAM140 及integrinβ1）在脂筏的含量变化。结果：GDNF预处理组RET 和NCAM140蛋白在脂筏的含量增加,而integrinβ1 蛋白的含量无显著性变化。在脂筏中也可检测到integrinβ1 蛋白。结论： GDNF 可影响某些胞膜蛋白在细胞膜上的定位,使其招募到脂筏,这可能是GDNF的一种重要生物学效应。     

Tight Junctions and the Experimental Modifications of Lipid Content

Tight junctions (TJs) are cell-to-cell contacts made of strands, which appear as ridges on P faces and complementary furrows on E faces on freeze fracture replicas. Evidences and opinions on whether these strands are composed of either membrane-bound proteins or lipid micelles are somewhat varied. In the present work we alter the lipid composition of Madin-Darby canine kidney monolayers using a novel approach, while studying (i) their transepithelial electrical resistance, a parameter that depends on the degree of sealing of the TJs; (ii) the apical-to-basolateral flux of 4 kD fluorescent dextran (J_DEX), that reflects the permeability of the intercellular spaces; (iii) the ability of TJs to restrict apical-to-basolateral diffusion of membrane lipids; and (iv) the pattern of distribution of endogenous and transfected occludin, the sole membrane protein presently known to form part of the TJs. We show that changing the total composition of phospholipids, sphingolipids, cholesterol and the content of fatty acids, does not alter TER nor the structure of the strands. Interestingly, enrichment with linoleic acid increases the J_DEX by 631%. The fact that this increase is not reflected in a decrease of TER, suggests that junctional strands do not act as simple resistive elements but may contain mobile translocating mechanisms. Received: 7 November 1997/Revised: 20 March 1998     

Connexins Induce and Maintain Tight Junctions in Epithelial Cells

Connexins (Cx) are considered to play a crucial role in the differentiation of epithelial cells and to be associated with adherens and tight junctions. This review describes how connexins contribute to the induction and maintenance of tight junctions in epithelial cells, hepatic cells and airway epithelial cells. Endogenous Cx32 expression and mediated intercellular communication are associated with the expression of tight junction proteins of primary cultured rat hepatocytes. We introduced the human Cx32 gene into immortalized mouse hepatic cells derived from Cx32-deficient mice. Exogenous Cx32 expression and the mediated intercellular communication by transfection could induce the expression and function of tight junctions. Transfection also induced expression of MAGI-1, which localized at adherens and tight junction areas in a gap junctional intercellular communication (GJIC)–independent manner. Furthermore, expression of Cx32 was related to the formation of single epithelial cell polarity of the hepatic cells. On the other hand, Cx26 expression, but not mediated intercellular communication, contributed to the expression and function of tight junctions in human airway epithelial cells. We introduced the human Cx26 gene into the human airway epithelial cell line Calu-3 and used a model of tight junction disruption by the Na⁺/K⁺-ATPase inhibitor ouabain. Transfection with Cx26 prevented disruption of both tight junction functions, the fence and barrier, and the changes of tight junction proteins by treatment with ouabain in a GJIC–independent manner. These results suggest that connexins can induce and maintain tight junctions in both GJIC-dependent and –independent manners in epithelial cells.