Activation and inhibition of the IRE1-mediated unfolded protein response by cytomegalovirus

在病毒感染过程中，对病毒糖蛋白的大量需求可能会压倒蛋白质折叠和质量控制机制的能力，导致内质网（ER）中未折叠蛋白质的积累。为了恢复内质网稳态，细胞通过激活三条内质网到细胞核的信号通路来启动未折叠蛋白反应（UPR），其中依赖于肌醇要求酶1 （IRE1）的通路是进化上最保守的。为了减少内质网应激，UPR减少蛋白质合成，增加未折叠蛋白质的降解，并上调伴侣蛋白表达以增强蛋白质折叠。巨细胞病毒和其他病毒病原体一样，调节普遍定期循环，使之对自己有利。然而，分子机制和负责UPR调节的病毒蛋白仍然不明确。在之前的研究中，我们证明了小鼠和人类巨细胞病毒（MCMV和HCMV）在感染后的后期抑制ire1介导的mRNA剪接和X-box结合蛋白1s （XBP1s）的表达。我们发现MCMV M50蛋白是一种诱导IRE1蛋白酶体降解的IRE1相互作用蛋白。我们进一步发现病毒感染会短暂激活IRE1-XBP1信号通路。这种激活似乎很重要，因为IRE1基因失活导致病毒复制大量减少。然而，令人惊讶的是，XBP1失活只导致病毒生产力的轻微下降。基于我们的初步数据，我们提出巨细胞病毒首先激活IRE1依赖通路，然后通过靶向IRE1降解来阻断IRE1信号通路。因此，本项目的目的是确定病毒高效复制所需的IRE1依赖性信号通路，确定病毒诱导IRE1降解的机制和生物学影响，并研究IRE1信号的激活和抑制在β -和γ -疱疹病毒中是否保守。这一结果将有助于更好地理解病毒如何操纵普遍定期审查以达到自己的目的，并可能揭示治疗干预的新靶点。

During viral infection, a massive demand for viral glycoproteins can overwhelm the capacity of the protein folding and quality control machinery, leading to an accumulation of unfolded proteins in the endoplasmic reticulum (ER). To restore ER homeostasis, cells initiate the unfolded protein response (UPR) by activating three ER-to-nucleus signaling pathways, of which the inositol-requiring enzyme 1 (IRE1)-dependent pathway is the evolutionary most conserved. To reduce ER stress, the UPR decreases protein synthesis, increases degradation of unfolded proteins, and upregulates chaperone expression to enhance protein folding. Cytomegaloviruses, as other viral pathogens, modulate the UPR to their own advantage. However, the molecular mechanisms and the viral proteins responsible for UPR modulation remain poorly defined. In previous work we demonstrated that murine and human cytomegalovirus (MCMV and HCMV) repress IRE1-mediated mRNA splicing and expression of the X-box binding protein 1s (XBP1s) at late times post infection. We identified the MCMV M50 protein as an IRE1-interacting protein that induces proteasomal degradation of IRE1. We further showed that viral infection briefly activates the IRE1-XBP1 signaling pathway. This activation appears to be important as genetic inactivation of IRE1 resulted in a massive reduction of viral replication. Surprisingly, however, XBP1 inactivation caused only a minor decrease in viral productivity. Based on our preliminary data we propose that cytomegaloviruses first activate an IRE-dependent pathway and later block IRE1 signaling by targeting it for degradation. Therefore, the aims of this project is to identify the IRE1-dependent signaling pathway required for efficient viral replication, to determine the mechanism and biological impact of viral induced IRE1 degradation, and to investigate whether the activation and inhibition of IRE1 signaling is conserved among beta- and gamma-herpesviruses. The results will provide a better understanding how viruses manipulate the UPR to their own advantage and might reveal new targets for therapeutic intervention.