Mechanisms and functions of host organelle usurpation by intravacuolar Toxoplasma

总结 脂质通过囊泡和非囊泡途径在膜之间转移。许多微生物， 感染哺乳动物细胞破坏这些宿主细胞脂质运输途径的功能以获得脂质。 刚地弓形虫是一种专性细胞内寄生虫， 一个自制的膜结合隔室--寄生液泡（PV）。T.弓形虫不 与宿主细胞器融合然而，我们发现寄生虫的细胞内生存依赖于脂质回收， 各种哺乳动物细胞器中提取的。例如T.弓形虫清除宿主体内的胆固醇和鞘脂 内吞细胞器和高尔基体囊泡，这提出了一个令人困惑的问题，T。弓形虫病 在不融合的情况下获取这些细胞器的脂质含量。为了解决这个问题，我们的第一个策略是分析 感染的哺乳动物细胞中的囊泡运输途径。我们发现弓形虫拦截哺乳动物 Rab囊泡与回收，内吞和分泌途径，并螯合这些囊泡到一个 附在PV膜上的膜小管网络。我们的第二种方法是分析非- 脂质转移的囊泡途径，特别是膜接触位点（MCS）。通过检查 哺乳动物宿主细胞器与PV膜的连接，我们表明弓形虫吸引宿主 ER小管和脂滴到PV，在PV处它们与PV膜紧密贴壁， 让人想起细胞器间的接触。哺乳动物ER驻留囊泡相关膜蛋白（VAP）， MCS的组分与PV膜相关，表明脂质的潜在开发 弓形虫用于脂质获取的转移蛋白。 基于这些初步的观察，我们提出了两个模型，由弓形虫清除脂质 哺乳动物囊泡或非囊泡脂质转运途径。我们将评估这些模型的步骤， 通过以下方式定义参与宿主囊泡途径拦截的分子机器和机制： T.弓形虫（目的1），在PV中膜小管网络的形成及其在哺乳动物中的作用 细胞器隔离（目的2）和通过非囊泡转移从哺乳动物获得脂质 与PVM密切相关的细胞器，可能通过MCS（目的3）。 完成这些目标将揭示弓形虫介导的脂质补救过程的复杂性， 提供机械细节，并确定未来的干预目标。是的，T。弓形虫会导致致命的 免疫功能低下的个体中的脑炎，并且目前弓形虫病的治疗选择是有限的。 此外，研究弓形虫利用机制篡夺Rab-mediated囊泡运输可能会产生 对Rab GTP酶如何协调哺乳动物细胞的膜转运有价值的见解。检查 弓形虫开发的利用MCS的潜在策略也可能提供关于如何利用MCS的重要信息。 MCS的丧失影响哺乳动物细胞生理学和生物体功能。

SUMMARY Lipids are transferred between membranes by vesicular and non-vesicular routes. Many microorganisms that infect mammalian cells subvert the function of these host cellular lipid trafficking pathways to acquire lipids. Toxoplasma gondii is an obligate intracellular parasite that multiplies in the cytoplasm of mammalian cells within a self-made membrane-bound compartment – the parasitophorous vacuole (PV). The PV of T. gondii does not fuse with host organelles. However, we showed that the parasite’s intracellular survival relies on lipids retrieved from various mammalian organelles. For example, T. gondii scavenges cholesterol and sphingolipids from host endocytic organelles and Golgi vesicles, respectively, which raises the perplexing question of how T. gondii can access the lipid content of these organelles without fusion. To address this issue, our first strategy was to analyze vesicular trafficking pathways in infected mammalian cells. We showed that Toxoplasma intercepts mammalian Rab vesicles associated with recycling, endocytic and secretory pathways, and sequesters these vesicles into a network of membranous tubules appended to the PV membrane. Our second approach was to analyze non- vesicular routes of lipid transfer, specifically Membrane Contact Sites (MCS). By examining the physical connectivity of mammalian host organelles with the PV membrane, we showed that Toxoplasma attracts host ER tubules and lipid droplets to the PV, where they are closely apposed to the PV membrane at distances reminiscent of inter-organelle contacts. Mammalian ER-resident Vesicle-Associated Membrane Proteins (VAP), components of MCS, are associated with the PV membrane, suggesting the potential exploitation of Lipid Transfer Proteins by Toxoplasma for lipid acquisition. Based on these preliminary observations, we propose two models for lipid scavenging by Toxoplasma either mammalian vesicular or non-vesicular lipid transport pathways. We will assess the steps of these models by defining the molecular machineries and mechanisms involved in the interception of host vesicular pathways by T. gondii (Aim 1), the formation of a network of membranous tubules in the PV and its role in mammalian organelle sequestration (Aim 2) and the acquisition of lipids via non-vesicular transfer from mammalian organelles closely associated with the PVM, possibly through MCS (Aim 3). Completing these aims would unravel the complexity of lipid salvage processes mediated by Toxoplasma, providing mechanistic details and identifying future targets for intervention. Indeed, T. gondii can cause fatal encephalitis in immunocompromised individuals, and current treatment options for toxoplasmosis are limited. Furthermore, studying the mechanisms used by Toxoplasma to usurp Rab-mediated vesicle trafficking may yield valuable insights into how Rab GTPases coordinate membrane transport in mammalian cells. Examining the potential strategies developed by Toxoplasma to exploit MCS may also provide important information on how the loss of MCS affect mammalian cellular physiology and organismal function.