The role of eIF2alpha phosphorylation in controlling intracellular development of Leishmania

寄生原生动物利什曼原虫是世界范围内人类利什曼病的病原。利什曼原虫存在于两个主要的发育阶段：白蛉载体中的自由生活的promastigotes和哺乳动物巨噬细胞吞噬溶酶体中的胞内无尾线虫。利什曼原虫在从promastigote形态向amastigote形态分化的过程中，经历了剧烈的环境变化。为了使寄生虫在吞噬溶酶体的恶劣环境中分化和复制，它必须发展几种适应性反应/策略，包括重要的形态和生化变化以及基因表达调控的动态改变。在真核生物中，控制翻译起始的最保守的应激反应途径之一是真核生物起始因子-2的α亚基被应激反应性eif2 α激酶磷酸化，导致全局翻译减少。我们最近的研究表明，eIF2alpha磷酸化是巨噬细胞无尾体分化的先决条件。缓慢复制的无尾线虫必须保持较低的翻译率，以适应吞噬溶酶体中的应激情况，如酸性pH值、营养和氧化应激。在无纺锤体中，翻译水平的降低与eIF2alpha磷酸化一致。有趣的是，我们发现eIF2alpha的磷酸化受到一种新的协同翻译/翻译后机制的发育调节。这种调节使昆虫promastigote形式增殖更快，而细胞内的amastigotes分裂更慢，作为一种适应高压力和动态环境并逃避宿主巨噬细胞免疫反应的策略。我们提出了一系列的实验，以更好地了解细胞内寄生虫应对应激和确保其生存的分子机制。这些研究将为应激和eIF2alpha磷酸化在细胞内病原体的发展中所起的作用提供重要的见解。

The parasitic protozoan Leishmania is the etiological agent of human leishmaniasis worldwide. Leishmania exists in two major developmental stages-free-living promastigotes in the sandfly vector and intracellular amastigotes in the phagolysosome of mammalian macrophages. During its differentiation from promastigote to amastigote forms, Leishmania experiences drastic environmental changes. In order for the parasite to differentiate and replicate within the harsh environment of the phagolysosome, it has to develop several adaptive responses/strategies involving important morphological and biochemical changes and dynamic alterations in the regulation of gene expression. In eukaryotes, one of the most conserved stress response pathways to control translation initiation is the phosphorylation of the alpha-subunit of eukaryotic initiation factor-2 by stress-responsive eIF2alpha kinases, resulting in the reduction of global translation. We have shown recently that eIF2alpha phosphorylation is a prerequisite for amastigote differentiation in macrophages. Slowly replicating amastigotes have to maintain translation rates low in order to adapt to stressful situations in the phagolysosome, such as acidic pH, nutritional and oxidative stresses. Decreased levels of translation in amastigotes coincide with eIF2alpha phosphorylation. Interestingly, we showed that phosphorylation of eIF2alpha is developmentally regulated by a novel cotranslational/ posttranslational mechanism. Such a regulation allows the insect promastigote form to proliferate faster but intracellular amastigotes to divide more slowly, as a strategy to adapt to a highly stressful and dynamic environment and to evade the immune response of the host macrophage. We propose a series of experiments to better understand the molecular mechanism(s) by which intracellular parasites respond to stress and ensure their survival. These studies will provide significant insights in the role that stress and eIF2alpha phosphorylation play in the development of intracellular pathogens.