Virology Division, Department of Infectious Diseases and Immunology, Faculty of Veterinary Medicine, Utrecht University, 3584 CL Utrecht, The Netherlands.
Howard Hughes Medical Institute, University of California, San Francisco, CA 94143.
Proc Natl Acad Sci U S A. 2019 Feb 5;116(6):2097-2102. doi: 10.1073/pnas.1815767116. Epub 2019 Jan 23.
Activation of the integrated stress response (ISR) by a variety of stresses triggers phosphorylation of the α-subunit of translation initiation factor eIF2. P-eIF2α inhibits eIF2B, the guanine nucleotide exchange factor that recycles inactive eIF2•GDP to active eIF2•GTP. eIF2 phosphorylation thereby represses translation. Persistent activation of the ISR has been linked to the development of several neurological disorders, and modulation of the ISR promises new therapeutic strategies. Recently, a small-molecule ISR inhibitor (ISRIB) was identified that rescues translation in the presence of P-eIF2α by facilitating the assembly of more active eIF2B. ISRIB enhances cognitive memory processes and has therapeutic effects in brain-injured mice without displaying overt side effects. While using ISRIB to investigate the ISR in picornavirus-infected cells, we observed that ISRIB rescued translation early in infection when P-eIF2α levels were low, but not late in infection when P-eIF2α levels were high. By treating cells with varying concentrations of poly(I:C) or arsenite to induce the ISR, we provide additional proof that ISRIB is unable to inhibit the ISR when intracellular P-eIF2α concentrations exceed a critical threshold level. Together, our data demonstrate that the effects of pharmacological activation of eIF2B are tuned by P-eIF2α concentration. Thus, ISRIB can mitigate undesirable outcomes of low-level ISR activation that may manifest neurological disease but leaves the cytoprotective effects of acute ISR activation intact. The insensitivity of cells to ISRIB during acute ISR may explain why ISRIB does not cause overt toxic side effects in vivo.
整合应激反应 (ISR) 的激活会引发起始因子 eIF2 的 α 亚基的磷酸化。磷酸化的 eIF2α 抑制 eIF2B，eIF2B 是一种鸟嘌呤核苷酸交换因子，可将无活性的 eIF2•GDP 循环回有活性的 eIF2•GTP。因此，eIF2 的磷酸化抑制了翻译。ISR 的持续激活与几种神经退行性疾病的发生有关，而对 ISR 的调节有望为新的治疗策略提供帮助。最近，鉴定出一种小分子 ISR 抑制剂 (ISRIB)，它通过促进更活跃的 eIF2B 的组装，在存在 P-eIF2α 的情况下恢复翻译。ISRIB 增强了认知记忆过程，并在脑损伤的小鼠中具有治疗效果，而没有表现出明显的副作用。在使用 ISRIB 研究正粘病毒感染细胞中的 ISR 时，我们观察到，在 P-eIF2α 水平较低时，ISRIB 在感染早期恢复了翻译，但在 P-eIF2α 水平较高时，在感染晚期则无法恢复翻译。通过用不同浓度的 poly(I:C) 或亚砷酸盐处理细胞来诱导 ISR，我们进一步证明，当细胞内 P-eIF2α 浓度超过临界阈值水平时，ISRIB 无法抑制 ISR。总之，我们的数据表明，eIF2B 的药理学激活效果受 P-eIF2α 浓度的调节。因此，ISRIB 可以减轻低水平 ISR 激活的不良后果，这些后果可能表现为神经退行性疾病，但保留急性 ISR 激活的细胞保护作用。在急性 ISR 期间，细胞对 ISRIB 的不敏感性可能解释了为什么 ISRIB 在体内不会引起明显的毒性副作用。
