Department of Medical Microbiology, Radboud University Nijmegen Medical Centre, Nijmegen Centre for Molecular Life Sciences & Nijmegen Institute for Infection, Inflammation and Immunology, Nijmegen, The Netherlands.
J Virol. 2013 Jun;87(11):6314-25. doi: 10.1128/JVI.03213-12. Epub 2013 Mar 27.
Virus infection can initiate a type I interferon (IFN-α/β) response via activation of the cytosolic RNA sensors retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5). Furthermore, it can activate kinases that phosphorylate eukaryotic translation initiation factor 2α (eIF2α), which leads to inhibition of (viral) protein translation and formation of stress granules (SG). Most viruses have evolved mechanisms to suppress these cellular responses. Here, we show that a mutant mengovirus expressing an inactive leader (L) protein, which we have previously shown to be unable to suppress IFN-α/β, triggered SG formation in a protein kinase R (PKR)-dependent manner. Furthermore, we show that infection of cells that are defective in SG formation yielded higher viral RNA levels, suggesting that SG formation acts as an antiviral defense mechanism. Since the induction of both IFN-α/β and SG is suppressed by mengovirus L, we set out to investigate a potential link between these pathways. We observed that MDA5, the intracellular RNA sensor that recognizes picornaviruses, localized to SG. However, activation of the MDA5 signaling pathway did not trigger and was not required for SG formation. Moreover, cells that were unable to form SG-by protein kinase R (PKR) depletion, using cells expressing a nonphosphorylatable eIF2α protein, or by drug treatment that inhibits SG formation-displayed a normal IFN-α/β response. Thus, although MDA5 localizes to SG, this localization seems to be dispensable for induction of the IFN-α/β pathway.
病毒感染可以通过激活细胞质 RNA 传感器视黄酸诱导基因-I（RIG-I）和黑色素瘤分化相关基因 5（MDA5）来引发 I 型干扰素（IFN-α/β）反应。此外，它还可以激活磷酸化真核翻译起始因子 2α（eIF2α）的激酶，从而导致（病毒）蛋白翻译抑制和应激颗粒（SG）的形成。大多数病毒已经进化出抑制这些细胞反应的机制。在这里，我们表明，一种表达无活性衣壳（L）蛋白的突变型肠道病毒，我们之前已经表明该蛋白无法抑制 IFN-α/β，以依赖蛋白激酶 R（PKR）的方式触发 SG 的形成。此外，我们还表明，在形成 SG 有缺陷的细胞中感染会产生更高水平的病毒 RNA，这表明 SG 的形成是一种抗病毒防御机制。由于肠道病毒 L 抑制了 IFN-α/β 和 SG 的诱导，我们着手研究这两条途径之间的潜在联系。我们观察到，MDA5，即识别小核糖核酸病毒的细胞内 RNA 传感器，定位于 SG。然而，MDA5 信号通路的激活并没有触发 SG 的形成，也不是其形成所必需的。此外，通过使用表达非磷酸化 eIF2α 蛋白的细胞或通过抑制 SG 形成的药物处理来耗尽 PKR，从而无法形成 SG 的细胞显示出正常的 IFN-α/β 反应。因此，尽管 MDA5 定位于 SG，但这种定位似乎对 IFN-α/β 途径的诱导是可有可无的。
