中科院水生所在鱼类抗病毒天然免疫稳态调控研究取得进展

首页标题 ꄲ 生命科学 ꄲ 中国科学 ꄲ 中科院水生所在鱼类抗病毒天然免疫稳态调控研究取得进展

脊椎动物LGP2调控机体免疫稳态的分子机制和免疫稳态在机体抗病毒感染中的重要意义

来源：中国科学院水生生物研究所 2022-8

　　快速识别病毒感染、启动机体的抗病毒免疫功能以清除病毒，在完成病毒清除后，及时终止免疫反应避免对组织细胞的过度损伤，是机体在应对病毒感染时成功存活的关键。在与病毒感染的长期博弈中，脊椎动物为了更胜一筹，演化出了复杂精细的免疫稳态调控机制。

　　2003年，具有识别病毒感染信号功能的胞浆RLR受体被成功鉴定，为深刻理解脊椎动物如何快速启动抗病毒免疫反应打开了一扇大门。RLR受体家族有3个成员，包括RIG-I、MDA5和LGP2。哺乳类的研究表明，RIG-I和MDA5分别识别不同病毒的感染信号，以激活机体的干扰素抗病毒免疫反应清除病毒。但是，LGP2发挥何种功能，至今仍充满谜团和争议。目前流行的观点认为，LGP2能协助MDA5识别病毒感染，从而促进机体的抗病毒免疫反应，但又抑制RIG-I介导的抗病毒免疫反应。这个观点有体外数据支持。但是，应对病毒感染时，LGP2为何既要促进又要抑制机体的免疫反应令人费解。此外，3个实验室对小鼠LGP2基因的敲除研究出现了截然相反的结果。

　　鱼类也有3个保守的RLR受体。鱼类研究同样出现了LGP2或发挥正调控、或发挥负调控的矛盾结果。针对这种认知混淆的现状，中国科学院水生生物研究所研究员张义兵团队在2018年提供体外证据揭示，斑马鱼LGP2在病毒感染的早期能激活细胞的干扰素抗病毒免疫反应，发挥正调控功能；在病毒感染的晚期，则抑制细胞的抗病毒免疫反应，发挥负调控功能。近日，该团队又提供了体内证据：通过比较野生型和敲除LGP2基因的斑马鱼在感染病毒后的免疫反应，证明斑马鱼在缺失LGP2后死亡率显著增加。其机制是，缺失LGP2严重破坏了鱼体在应对病毒感染时的免疫稳态。这种严重破坏，导致在病毒感染的早期，鱼体的干扰素抗病毒免疫反应不能尽快启动到有效清除病毒感染的适度水平，而在病毒感染的晚期，又不能使鱼体的抗病毒免疫反应及时回归到正常的本底水平。该团队同时揭示了LGP2发生功能转换的分子机制。相关研究成果近期发表在iScience上。

　　该团队进一步证实了LGP2的功能在斑马鱼和人中的保守性，提供了在病毒感染的不同时期，斑马鱼和人的LGP2行使相同的双重调控功能，精确调控抗病毒免疫反应的分子证据。poly(I:C)是人工合成的双链RNA。通过用poly(I:C)模拟病毒感染细胞时产生的病毒核酸信号，研究采用滴定poly(I:C)转染细胞时的剂量来模拟病毒感染细胞的不同时期。研究发现，在模拟病毒感染的早期，LGP2通过促进MDA5快速启动细胞的干扰素抗病毒免疫反应以清除病毒；在模拟病毒感染的晚期，LGP2则反过来抑制RIG-I以及MDA5触发的信号通路，负调控细胞的抗病毒免疫反应，以使机体免疫反应恢复到正常的本底水平，最终维持机体的免疫稳态。该研究结果统一了对脊椎动物LGP2的功能认知，揭示了脊椎动物LGP2调控机体免疫稳态的分子机制和免疫稳态在机体抗病毒感染中的重要意义。相关成果于近期发表在Frontiers in Immunology上。

　　相关研究工作得到国家重点研发计划、中科院战略性先导科技专项和国家自然科学基金等项目的支持。

Graphical abstract

斑马鱼LGP2在病毒感染的不同时期维持机体的免疫稳态以促进鱼体存活

Front. Immunol., 17 August 2022

Sec. Comparative Immunology

https://doi.org/10.3389/fimmu.2022.985792

Function conservation and disparities of zebrafish and human LGP2 genes in fish and mammalian cells responsive to poly(I:C)

Xiu-Ying Gong1,2, Zi-Ling Qu1,2, Yi-Lin Li1,2, Hao-Yu Sun1,2, Xiang Zhao1,2, Cheng Dan1,2, Jian-Fang Gui1,2,3 and Yi-Bing Zhang1,2,3*

1State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, China

2College of Advanced Agricultural Science, University of Chinese Academy of Sciences, Beijing, China

3The Innovation Academy of Seed Design, Chinese Academy of Sciences, Wuhan, China

Retinoic acid inducible gene-I (RIG-I)-like receptors (RLRs) are viral RNA sensors that regulate host interferon (IFN)-mediated antiviral signaling. LGP2 (laboratory genetics and physiology 2) lacks the N-terminal caspase activation and recruitment domains (CARDs) responsible for signaling transduction in the other two RLR proteins, RIG-I and melanoma differentiation associated gene-5 (MDA5). How LGP2 regulates IFN signaling is controversial, and inconsistent results have often been obtained in overexpression assays when performed in fish cells and mammalian cells. Here we report that the differential sensitivity of fish cells and mammalian cells to poly(I:C) transfection conceals the function conservation of zebrafish and human LGP2. In fish cells, overexpression of zebrafish or human LGP2 initially activates IFN signaling in a dose-dependent manner, followed by inhibition at a critical threshold of LGP2 expression. A similar trend exists for LGP2-dependent IFN induction in response to stimulation by low and high concentrations of poly(I:C). In contrast, overexpression of zebrafish or human LGP2 alone in mammalian cells does not activate IFN signaling, but co-stimulation with very low or very high concentrations of poly(I:C) shows LGP2-dependent enhancement or inhibition of IFN signaling, respectively. Titration assays show that LGP2 promotes MDA5 signaling in mammalian cells mainly under low concentration of poly(I:C) and inhibits RIG-I/MDA5 signaling mainly under high concentration of poly(I:C). Our results suggest that fish and human LGP2s switch regulatory roles from a positive one to a negative one in increasing concentrations of poly(I:C)-triggered IFN response.

iScience

Volume 25, Issue 8, 19 August 2022, 104821

Journal home page for iScience

LGP2 is essential for zebrafish survival through dual regulation of IFN antiviral response

Author links open overlay panelXiu-YingGong12Qi-MinZhang124XiangZhao12Yi-LinLi12Zi-LingQu12ZhiLi1ChengDan12Jian-FangGui123Yi-BingZhang1235

https://doi.org/10.1016/j.isci.2022.104821Get rights and content

Under a Creative Commons licenseOpen access

Highlights

Zebrafish LGP2 is crucial for host survival through initiating IFN response

Zebrafish LGP2 exerts dual regulation of IFN response during SVCV infection

The function switch of zebrafish LGP2 is related to cellular IFN production

Summary

In mammals, LGP2 is the enigmatic RLR family member, being initially believed as an inhibitor of RLR-triggered IFN response but subsequently as an activator of MDA5 signaling and an inhibitor of RIG-I signaling. The contradiction happens to fish LGP2. Here, we generate a lgp2 loss-of-function (lgp2lof/lof) zebrafish mutant, which is highly susceptible to SVCV infection, displaying an initially decreased activation of IFN response and a following increased one. Mechanistically, zebrafish LGP2 functions as the essential activator of IFN response dependent on MDA5 at the early stage of viral infection but as a negative regulator by impairing mRNA levels of tbk1 and ikki at the late stage of viral infection. The function switch of LGP2 is related to cellular IFN production during viral infection. Our data demonstrate that zebrafish LGP2 is a key homeostatic regulator of IFN response and thus essential for zebrafish survival against SVCV infection.