Dephosphorylation of the RNA Sensors RIG-I and MDA5 by the Phosphatase PP1 Is Essential for Innate Immune Signaling

Effi Wies, May K. Wang, Natalya P. Maharaj, Kan Chen, Shenghua Zhou, Robert W. Finberg, and Michaela U. Gack. Immunity. (2013) 38, 437-449

Speaker: Tao-Sheng Liu (劉道生)                                Time: 15:00~16:00, Nov. 6, 2013

Commentator: Dr. Chiou-Feng Lin (林秋烽老師)       Place: Room 601

Abstract:

RIG-I-Like receptors (RLRs), such as RIG-I and MDA5, played an important role in sensing RNA virus infection and then activating innate antiviral responses. Recent studies have discovered post-translational modifications of RIG-I, which are important for regulating RIG-I antiviral activity. However, the regulation of MDA5 signaling pathway remains unclear. The authors’ previous studies indicated that phosphorylation at S8 and T170 of RIG-I-CARD keeps RIG-I inactive and thus prevents downstream signaling transduction (1, 2). In contrast to RIG-I regulatory mechanism, the authors found that MDA5 signaling activity is regulated by phosphorylation and dephosphorylation of MDA5-CARD. The authors also analyzed possible phosphorylation sites of MDA5-CARD through mass spectrometry and found that S88 and S104 were potential sites affecting MDA5 signaling transduction. To determine the role of S88 and S104 phosphorylation in the regulation of MDA5 signaling activity, the authors proved that S88 was more important by measuring phosphorylation level of S88 and S104 at MDA5. Performing a phosphatase RNAi screen test, the authors identified PP1α and PP1γ as the potential phosphatases that are responsible for MDA5 and RIG-I dephosphorylation, and leading to their activation. Silencing PP1α and PP1γthroughRNAi enhanced RIG-I and MDA5 CARD phosphorylation and reduced IFN-β production. Besides, silencing PP1α and PP1γ inhibited the expression of IFN stimulated gene (ISG) during RNA virus infection, which resulted in enhanced RNA virus replication. Base on above findings, the authors identify PP1α and PP1γas regulators of RIG-I-like receptors to fight various RNA viruses, including influenza virus, picornavirus, and dengue virus.

References:

1.     Michaela U. Gack. et al. (2010) Phosphorylation-mediated negative regulation of RIG-I antiviral activity. J Virol. 84, 3220-3229.

2.     Natalya P. Maharaj. et al. (2012) Conventional protein kinase C-α (PKC-α) and PKC-β negatively regulate RIG-I antiviral signal transduction. J Virol. 86, 1358–1371.