Intracellular Eukaryotic Parasites Have a Distinct Unfolded Protein Response

Insult to the endoplasmic reticulum (ER) activates the Unfolded Protein Response (UPR), a set of signaling pathways that protect the cell from the potential damage caused by improperly folded proteins. Accumulation of misfolded proteins in the ER lumen initiates a series of signal transduction events via activation of three transmembrane ER proteins: Ire1, Atf6 and PERK. Activation of these proteins results in the transcriptional up-regulation of the components of the folding, trafficking and degradation machinery in the ER. PERK further reduces the load on the ER via the phosphorylation of eIF2α, attenuating general protein translation. It is believed that the UPR evolved as a transcriptional response that up-regulates protein folding machinery in the ER and later gained the ability to decrease ER load by attenuating general protein translation in metazoa. However, our in silico analyses of protozoan parasites revealed an absence of proteins involved in the transcriptionally mediated UPR and the presence of both PERK and its target eIF2α. Consistent with these observations, stimulation of the UPR in Leishmania donovani identified an absence of up-regulation of the ER chaperone BiP, the canonical ER chaperone modulated by the UPR in higher eukaryotes, while exhibiting increased phosphorylation of eIF2α which has been shown to attenuate protein translation. We further observed that L. donovani is more sensitive to UPR inducing agents than host macrophages, suggesting that the less evolved stress response could provide a new avenue for therapeutic treatment of parasitic infections.内质网（endoplasmic reticulum, ER）受到损伤时会激活未折叠蛋白反应（Unfolded Protein Response, UPR），这是一组可保护细胞免受错误折叠蛋白潜在损伤的信号通路。内质网腔内错误折叠蛋白的积累，可通过激活Ire1、Atf6与PERK这三种跨膜内质网蛋白，启动一系列信号转导事件。这些蛋白的激活会使内质网内与蛋白质折叠、转运及降解相关的组分发生转录上调。PERK还可通过磷酸化真核翻译起始因子2α（eukaryotic translation initiation factor 2α, eIF2α），减弱整体蛋白质翻译，进一步减轻内质网的负荷。学界普遍认为，UPR最初作为一种转录应答机制进化而来，仅用于上调内质网内的蛋白质折叠相关组分；后续在后生动物（metazoa）中，UPR才获得了通过减弱整体蛋白质翻译来降低内质网负荷的能力。然而，我们对原生动物寄生虫开展的计算机模拟分析（in silico）显示，这类寄生虫中不存在参与转录介导型UPR的蛋白，但同时存在PERK及其靶蛋白eIF2α。与上述观测结果一致的是，在杜氏利什曼原虫（Leishmania donovani）中诱导UPR后，并未检测到内质网伴侣蛋白BiP的表达上调——而BiP正是高等真核生物中受UPR调控的经典内质网伴侣蛋白；但此时eIF2α的磷酸化水平显著升高，该修饰已被证实可减弱蛋白质翻译。我们进一步发现，杜氏利什曼原虫对UPR诱导剂的敏感性高于宿主巨噬细胞，这提示这种进化程度较低的应激应答机制，可为寄生虫感染的治疗提供全新的策略方向。