Date Published: April 3, 2019
Publisher: Public Library of Science
Author(s): Liya Ye, Qiang Zhang, Tianzi Liuyu, Zhigao Xu, Meng-Xin Zhang, Min-Hua Luo, Wen-Bo Zeng, Qiyun Zhu, Dandan Lin, Bo Zhong, Pinghui Feng.
Mediator of IRF3 activation (MITA, also known as STING and ERIS) is an essential adaptor protein for cytoplasmic DNA-triggered signaling and involved in innate immune responses, autoimmunity and tumorigenesis. The activity of MITA is critically regulated by ubiquitination and deubiquitination. Here, we report that USP49 interacts with and deubiquitinates MITA after HSV-1 infection, thereby turning down cellular antiviral responses. Knockdown or knockout of USP49 potentiated HSV-1-, cytoplasmic DNA- or cGAMP-induced production of type I interferons (IFNs) and proinflammatory cytokines and impairs HSV-1 replication. Consistently, Usp49-/- mice exhibit resistance to lethal HSV-1 infection and attenuated HSV-1 replication compared to Usp49+/+ mice. Mechanistically, USP49 removes K63-linked ubiquitin chains from MITA after HSV-1 infection which inhibits the aggregation of MITA and the subsequent recruitment of TBK1 to the signaling complex. These findings suggest a critical role of USP49 in terminating innate antiviral responses and provide insights into the complex regulatory mechanisms of MITA activation.
The innate immune system is the first line of defense against invading pathogens and innate immune response to microbial species is initiated by the recognition of pathogen-associated molecular patterns (PAMPs) by germline-encoded pattern-recognition receptors (PRRs). Viral nucleic acid including RNA, DNA and RNA-DNA hybrid constitutes classical PMAPs that are detected by Toll-like receptors (TLRs), retinoic acid–inducible gene I protein (RIG-I)-like receptors (RLRs) and a variety of cytoplasmic DNA sensors [2–5]. Upon binding to the PAMPs, these PRRs recruit adaptor proteins or catalyze second messenger molecules for adaptor proteins activation, trigger a series of signaling cascades and eventually induce expression of an array of downstream genes such as type I interferons (IFNs) and proinflammatory cytokines to elicit antiviral immune responses.
Ubiquitination and deubiquitination are reversible posttranslational modifications involved in various biological or pathological processes [62, 63]. MITA is an adaptor protein critically mediating innate immune signaling in response to cytoplasmic DNA challenge. The ubiquitination of MITA is essential for its function and stability which should be properly controlled to elicit protective immunity and avoid excessive harmful immunity . AMFR promotes K27-linked ubiquitination and activation of MITA , whereas RNF5, TRIM30α and TRIM29 induce K48-linked ubiquitination and proteasomal degradation of MITA [39–42], which is counteracted by USP13 and USP18, USP21 or EIF3S5, respectively [23, 44–46]. Multiple E3 ligases including TRIM56, TRIM32 and MUL1 have been reported to promote K63-linked ubiquitination and aggregation of MITA [47–49]. However, the counteracting deubiquitination process is unclear. In this study, we demonstrated that USP49 interacted with MITA and deconjugated K63-linked polyubiquitin chains from MITA after HSV-1 infection, thereby downregulating MITA activation and turning down immune responses (Fig 6). Consistent with this notion, we found that HSV-1-induced K63-linked ubiquitination and aggregation of MITA and subsequent recruitment of TBK1 to MITA was substantially enhanced in USP49 deficient cells. USP49 deficiency also resulted in resistance to lethal HSV-1 infection, enhanced phosphorylation of TBK1, IRF3 and IκBα, increased production of type I IFNs and proinflammatory cytokines and compromised HSV-1 replication after HSV-1 infection. These findings together suggest an essential role of USP49 in the regulation of innate antiviral signaling by modulating K63-linked ubiquitination of MITA.