Sacks D L, Pimenta P F, McConville M J, Schneider P, Turco S J
Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892.
J Exp Med. 1995 Feb 1;181(2):685-97. doi: 10.1084/jem.181.2.685.
The life cycle of Leishmania parasites within the sand fly vector includes the development of extracellular promastigotes from a noninfective, procyclic stage into an infective, metacyclic stage that is uniquely adapted for transmission by the fly and survival in the vertebrate host. These adaptations were explored in the context of the structure and function of the abundant surface lipophosphoglycan (LPG) on Leishmania donovani promastigotes. During metacyclogenesis, the salient structural feature of L. donovani LPG is conserved, involving expression of a phosphoglycan chain made up of unsubstituted disaccharide-phosphate repeats. Two important developmental modifications were also observed. First, the size of the molecule is substantially increased because of a twofold increase in the number of phosphorylated disaccharide repeat units expressed. Second, there is a concomitant decrease in the presentation of terminally exposed sugars. This later property was indicated by the reduced accessibility of terminal galactose residues to galactose oxidase and the loss of binding by the lectins, peanut agglutinin, and concanavalin A, to metacyclic LPG in vivo and in vitro. The loss of lectin binding was not due to downregulation of the capping oligosaccharides as the same beta-linked galactose or alpha-linked mannose-terminating oligosaccharides were present in both procyclic and metacyclic promastigotes. The capping sugars on procyclic LPG were found to mediate procyclic attachment to the sand fly midgut, whereas these same sugars on metacyclic LPG failed to mediate metacyclic binding. And whereas intact metacyclic LPG did not inhibit procyclic attachment, depolymerized LPG inhibited as well as procyclic LPG, demonstrating that the ligands are normally buried. The masking of the terminal sugars is attributed to folding and clustering of the extended phosphoglycan chains, which form densely distributed particulate structures visible on fracture-flip preparations of the metacyclic surface. The exposure and subsequent masking of the terminal capping sugars explains the stage specificity of promastigote attachment to and release from the vector midgut, which are key events in the development of transmissible infections in the fly.
利什曼原虫寄生虫在白蛉媒介体内的生命周期包括将细胞外前鞭毛体从无感染性的前循环阶段发育为有感染性的后循环阶段，后循环阶段特别适合由白蛉传播并在脊椎动物宿主中存活。这些适应性变化是在杜氏利什曼原虫前鞭毛体上丰富的表面脂磷壁酸（LPG）的结构和功能背景下进行研究的。在后循环发育过程中，杜氏利什曼原虫LPG的显著结构特征得以保留，包括由未取代的二糖 - 磷酸重复序列组成的磷酸聚糖链的表达。还观察到两个重要的发育变化。首先，由于表达的磷酸化二糖重复单元数量增加了两倍，分子大小显著增加。其次，末端暴露糖的呈现随之减少。这一特性通过末端半乳糖残基对半乳糖氧化酶的可及性降低以及凝集素（花生凝集素和伴刀豆球蛋白A）在体内和体外对后循环LPG的结合丧失得以体现。凝集素结合的丧失并非由于封端寡糖的下调，因为前循环和后循环前鞭毛体中都存在相同的β - 连接半乳糖或α - 连接甘露糖末端寡糖。发现前循环LPG上的封端糖介导前循环体附着于白蛉中肠，而后循环LPG上的相同糖未能介导后循环体结合。并且完整的后循环LPG不抑制前循环体附着，解聚的LPG与前循环LPG一样具有抑制作用，表明配体通常被掩埋。末端糖的掩盖归因于延伸的磷酸聚糖链的折叠和聚集，这些链形成了在后循环表面的断裂 - 翻转制剂上可见的密集分布的颗粒结构。末端封端糖的暴露及随后的掩盖解释了前鞭毛体附着于媒介中肠并从中释放的阶段特异性，这是白蛉中可传播感染发展中的关键事件。
