Danielsen E Michael, Hansen Gert H
Department of Medical Biochemistry and Genetics, The Panum Institute, University of Copenhagen, Blegdamsvej 3, N DK-2200 Copenhagen, Denmark.
Biochim Biophys Acta. 2003 Oct 31;1617(1-2):1-9. doi: 10.1016/j.bbamem.2003.09.005.
Epithelial cells that fulfil high-throughput digestive/absorptive functions, such as small intestinal enterocytes and kidney proximal tubule cells, are endowed with a dense apical brush border. It has long been recognized that the microvillar surface of the brush border is organized in cholesterol/sphingolipid-enriched membrane microdomains commonly known as lipid rafts. More recent studies indicate that microvillar rafts, in particular those of enterocytes, have some unusual properties in comparison with rafts present on the surface of other cell types. Thus, microvillar rafts are stable rather than transient/dynamic, and their core components include glycolipids and the divalent lectin galectin-4, which together can be isolated as "superrafts", i.e., membrane microdomains resisting solubilization with Triton X-100 at physiological temperature. These glycolipid/lectin-based rafts serve as platforms for recruitment of GPI-linked and transmembrane digestive enzymes, most likely as an economizing effort to secure and prolong their digestive capability at the microvillar surface. However, in addition to microvilli, the brush border surface also consists of membrane invaginations between adjacent microvilli, which are the only part of the apical surface sterically accessible for membrane fusion/budding events. Many of these invaginations appear as pleiomorphic, deep apical tubules that extend up to 0.5-1 microm into the underlying terminal web region. Their sensitivity to methyl-beta-cyclodextrin suggests them to contain cholesterol-dependent lipid rafts of a different type from the glycolipid-based rafts at the microvillar surface. The brush border is thus an example of a complex membrane system that harbours at least two different types of lipid raft microdomains, each suited to fulfil specialized functions. This conclusion is in line with an emerging, more varied view of lipid rafts being pluripotent microdomains capable of adapting in size, shape, and content to specific cellular functions.
执行高通量消化/吸收功能的上皮细胞，如小肠肠上皮细胞和肾近端小管细胞，具有密集的顶端刷状缘。长期以来人们一直认识到，刷状缘的微绒毛表面是由富含胆固醇/鞘脂的膜微区组成，通常称为脂筏。最近的研究表明，微绒毛脂筏，尤其是肠上皮细胞的微绒毛脂筏，与其他细胞类型表面的脂筏相比具有一些不寻常的特性。因此，微绒毛脂筏是稳定的，而不是短暂/动态的，其核心成分包括糖脂和二价凝集素半乳糖凝集素-4，它们可以一起作为“超级脂筏”分离出来，即膜微区在生理温度下能抵抗 Triton X-100 的溶解作用。这些基于糖脂/凝集素的脂筏作为募集糖基磷脂酰肌醇（GPI）连接和跨膜消化酶的平台，很可能是一种节约的努力，以确保并延长它们在微绒毛表面的消化能力。然而，除了微绒毛外，刷状缘表面还由相邻微绒毛之间的膜内陷组成，这是顶端表面在空间上唯一可用于膜融合/出芽事件的部分。许多这些内陷表现为多形性的、深的顶端小管，延伸到下方的终末网区域达 0.5 - 1 微米。它们对甲基-β-环糊精的敏感性表明它们含有与微绒毛表面基于糖脂脂筏不同类型的胆固醇依赖性脂筏。因此，刷状缘是一个复杂膜系统的例子，它包含至少两种不同类型的脂筏微区，每种微区都适合履行特定的功能。这一结论与脂筏作为能够在大小、形状和成分上适应特定细胞功能的多能微区这一新兴的、更多样化的观点一致。
