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Advances in Marine Sciences
Vol.04 No.02(2017), Article ID:21091,7 pages
10.12677/AMS.2017.42009

Advances of cGAS

Jiajing Xin, Xiaomin Guo, Li Meng, Guangdong Ji*

Institute of Evolution & Marine Biodiversity, College of Marine Life Sciences, Ocean University of China, Qingdao Shandong

Received: May 22nd, 2017; accepted: May 20th, 2017; published: Jun. 23rd, 2017

ABSTRACT

cGAS (cyclic GMP-AMP synthase), a kind of nucleic acid transferase and one of the latest DNA sensors being found in mammals, could identify DNA in cytoplasm and produce cGAMP (cyclic GMP-AMP) to activate interferon stimulated gene (STING), then activate the type I interferon and other cytokines to execute immune function. It was found that cGAS could not only be involved in antiviral response, but also in antibacteria response. cGAS is composed of DNA binding site in N-terminal, a central catalytic domain and a conserved Mab-21 (male abnormal 21) domain in C-terminus, which belongs to MAB21 family protein. Phylogenetic analysis showed that vertebrate cGAS is derived from ancestral Mab-21 domain, which also produced cGAS-like gene through genome duplication in fishes and some mammals. In this review, we discussed the recent research progress regarding the role of cGAS in immune response and its evolution scenario.

Keywords:cGAS, DNA Sensor, STING, Immune, Evolution

cGAS的研究进展

辛佳静，郭晓敏，孟丽，汲广东*

中国海洋大学海洋生命学院，海洋生物多样性与进化研究所，山东 青岛

收稿日期：2017年5月22日；录用日期：2017年5月20日；发布日期：2017年6月23日

摘 要

环鸟苷酸-腺苷酸合成酶(cyclic GMP-AMP synthase, cGAS)是一种核酸转移酶，在哺乳动物中具有DNA 感受器的功能，能识别胞质DNA并产生cGAMP (cyclic GMP-AMP)，激活干扰素刺激蛋白(stimulator of interferon genes, STING)，调控下游的I型干扰素(interferon, IFN)和其他细胞因子的分泌，启动机体的免疫反应。cGAS不仅能够抗病毒，也能抵抗细菌的感染。结构上，cGAS由氨基端的DNA结合位点，中间的催化结构域以及羧基端保守的Mab-21 (male abnormal 21)结构域组成，属于MAB21家族蛋白。进化分析发现，脊椎动物的cGAS来源于祖先Mab-21结构域，并且在鱼类和某些哺乳动物中通过基因组复制产生了cGAS和cGASL基因。本文就cGAS结构和免疫功能的研究进展以及演化情况进行了综述。

关键词 :cGAS，DNA感受器，STING，免疫，进化

Copyright © 2017 by authors and Hans Publishers Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY).

http://creativecommons.org/licenses/by/4.0/

1. 引言

生物体内的固有免疫系统利用一些病原受体来监测并发现一些非自身细胞的病原微生物以及自身细胞受损伤的成分，这些病原体受体通常被称为模式识别受体(Pattern Recognition Receptors, PRRs)，PRRs发现非自身的病原微生物以及自身受损细胞的分子后，机体会产生相应免疫应答来调节。最早发现的经典模式识别受体为Toll-like受体(TLRs)，它出现在细胞表面或胞质内涵体中，通过激活MyD88途径或TRIF途径诱导IRF3或NF-κB(nuclear factor KB)活化，从而使机体产生I型干扰素，激活免疫反应 [1] 。其他发现的PRRs还包括DNA识别受体家族、RNA识别受体家族、核苷酸结合寡聚化结构域样受体家族(nucleotide-binding oligomerization domain (NOD)-like receptors, NLRs)以及C型凝集素识别受体(C-type lectin receptors, CLRs)等。其中，DNA识别受体主要包括：DNA结合蛋白(DAI)、干扰素诱导蛋白16(IFI16)、RNA聚合酶III(RNA pol III)、DNA依赖的蛋白激酶亚基(Ku70)和解旋酶41、36、9(DDX41、DHX36、DHX9)等 [2] 。这些蛋白分子都依赖干扰素刺激蛋白STNG来刺激机体产生干扰素，从而引发机体的免疫反应 [3] [4] 。如果细胞质中的DNA不能被及时地清除掉，则会在细胞中积累，引起一系列的炎症和自身免疫病，如系统性红斑狼疮(systemic lupus erythematosus, SLE)和Aicardi-Goutieres综合征等 [2] [5] 。

除了上述DNA感受器之外，2011年，首次发现含有Mab-21结构域(由于它和雄性线虫生殖发育异常(male abnormal 21)有关，故此命名 [6] )的核酸转移酶C6orf150(chromosome 6 open reading frame 150，后来称之为cGAS)在病毒复制过程中参与形成I型干扰素 [7] 。2013年，Wu等通过基因沉默和液相色谱分析法第一次在真核生物中提取到一种能够激活STING的物质：环状核苷酸cGAMP [8] ，更新了人们对于真核细胞内第二信使的分子作用的认识。随后，研究人员通过质谱分离和蛋白纯化等技术鉴定出了催化cGAMP合成的酶，命名为环鸟苷酸-腺苷酸合成酶，即cGAS [9] 。这为理清cGAS的作用机制打下了良好的基础。

2. cGAS的结构

cGAS含有一个与寡腺苷酸合成酶(oligoadenylate synthase, OAS)催化结构域同源的结构域，属于核酸转移酶家族 [10] ，在其氨基端和羧基端分别含有DNA结合位点和Mab-21结构域，后者又含有一个高度保守的锌囊结构(图1)，通过结合Zn2+维持其高级结构和正常生理功能。cGAS依靠其DNA结合位点和锌囊结构通过识别磷酸-核糖骨架结构来识别dsDNA，这种识别没有序列特异性，几乎能够识别所有类型的dsDNA [11] [12] 。在这个过程中，cGAS的R150和R192氨基酸残基都能插入到DNA双螺旋结构的

Figure 1. Human cGAS domain composition

图1. 人的cGAS结构域组成

空隙中，使cGAS-DNA复合体更加牢固 [13] [14] 。实际上，最近在体外的研究证实，cGAS除了能够识别dsDNA之外，对dsRNA、ssDNA和ssRNA都能结合，且呈现出浓度依赖性 [15] ，但还不清楚它们的结合机制是否与dsDNA的相同。

3. cGAS的免疫功能及其作用机制

细胞内游离的DNA被DNA感受器识别后将引发多种信号通路 [16] ，如STNG、NF-κB、Caspase1、Caspase9、RIP3、P62和NDPS2等因子介导的通路，其中由STING介导的通路较为典型。STING主要定位于线粒体和内质网上 [6] [17] ，可以激活并启动I型干扰素的产生 [18] [19] 。cGAS主要依赖STING通路发挥免疫功能。研究表明，敲除STING后，在STING不表达或者不存在的情况下，无论是在细胞中表达cGAS还是向细胞质中导入cGAMP，都不能使IFN-β激活 [9] ，同其它DNA感受器中一样，STING在cGAS发挥免疫作用的过程中起着关键作用。

cGAS识别dsDNA后，催化ATP和GTP生成cGAMP，释放出焦磷酸。这个过程需要cGAS蛋白相互交联形成一个二聚体，然后与dsDNA结合，形成一种cGAS和dsDNA二者比例等于2:2的复合体，催化ATP和GTP生成cGAMP，cGAMP进一步结合并激活STING [20] [21] ，处于激活态的STING能进一步激活TANK-结合激酶1 (TBKl)，促进干扰素调节因子3(IRF3)磷酸化，IRF3磷酸化后发生聚合作用形成二聚体 [9] [22] ；除此之外，cGAMP与STING结合后也可以激活IKK (IKB kinase)从而促使NF-κB磷酸化 [4] [16] ；磷酸化的NF-κB和IRF3进入细胞核后可诱导干扰素和相关细胞因子的表达，从而启动先天性免疫反应 [4] [10] (图2)。

cGAS通过识别细胞质DNA激活STING通路既能抗病毒，也能抗细菌。敲除小鼠cGAS的实验组相对于正常对照组更易感染西尼罗河病毒(West Nile virus，单链RNA病毒)，说明cGAS参与了正常机体抵抗RNA病毒的过程 [23] 。另外，cGAS对腺病毒(adenovirus) [24] 以及获得性免疫缺陷病毒(human immunodeficiency virus, HIV)等也有一定的抵抗作用 [25] 。除此之外，cGAS在抗菌方面也有探索。例如，弗朗西斯氏菌(Francisellanovicida)、沙眼衣原体(Chlamydia trachomatis)、淋球菌(Neisseria gonorrhoeae)以及结核分歧杆菌(Mycobacterium tuberculosis)都能诱导I型干扰素的产生 [26] - [33] 。

4. cGAS的演化

在脊椎动物如人类中，含有MAB21结构域的蛋白除了包括cGAS (MB21D1，Mab-21 domain containg 1)之外，还包括MB21D2，MAB21-like protein 1 (MAB21L1)，MAB21L2和MAB21L3 [34] 。MAB21L1和MAB21L2在氨基酸水平上具有94%一致性，其表达模式相似，功能也类似 [35] [36] [37] ，MAB21L2在胚胎发育过程中能影响视网膜的发育 [38] 。与在线虫中的类似，MAB21L2的突变能够导致小鼠胚胎妊娠中期死亡 [39] ；而MAB21L3与前面两者相比仅具有25%的序列一致性，在Notch通路的下游发挥作用 [40] 。由于cGAS缺乏在胚胎发育方面的探索，同时其他蛋白也缺乏在成体免疫方面的研究，一定程度上阻碍了它们在功能进化方面的联系。不过最近的一项研究发现，与MAB21L1相比，cGAS对核苷酸具有更高的亲和性，虽然二者的蛋白结构比较相似，但是功能可能有很多不同 [15] 。由于cGAS与Mab-21like蛋白中的MB21D2、MAB21L1和MAB21L2的一致性较低(13%~18%)，故在构建cGAS的进化树时暂不予考虑。

Figure 2. Immune responses induced by cGAS

图2. cGAS引起的免疫应答反应

Figure 3. Phylogenetic tree of cGAS

图3. cGAS的系统进化树

利用人的cGAS(XP_016865721)作为查询序列，在NCBI或其他物种蛋白数据库中Blast，收集小于1 × e−5的蛋白序列，得到cGAS的同源基因。选取有代表性的模式生物或进化中关键节点的物种，利用邻接法(NJ)构建该蛋白的系统进化树(图3)。发现，在脊椎动物中既存在cGAS，也存在之前未发现的旁系同源基因cGASL，而在线虫等低等无脊椎动物中，仅存在原始的Mab21基因，说明脊椎动物的MAB21相关基因都来源于祖先Mab21基因，无论它们是否位于同一条染色体上，都有可能是因为基因组复制而产生的，特别是在脊椎动物中，基因组复制产生了cGAS和cGASL，由于在鱼类中额外的一次基因组的复制，某些鱼类(如罗非鱼，Oreochromisniloticus)出现了位于同一条染色体上的串联重复的基因如cGASa，cGASb。比较有意思的是并非所有的哺乳动物都存在cGASL，说明在向哺乳动物进化的过程中这个基因会发生丢失。

5. 总结与展望

自从cGAS作为新型的DNA感受器的免疫功能被发现以来，人们对cGAS的抗病毒功能有了新的认识。已有研究发现，cGAS能够对抗多种病毒如获得性免疫缺陷综合征病毒HIV的感染，这为治疗艾滋病及其他病毒方面的疾病提供了新思路。另外，在脊椎动物中，由于基因组的复制，存在cGASa，cGASb或者cGASL，在进化上它们都来源于祖先Mab-21结构域，它们之间的功能是否有差异还没有研究。而在海鞘、文昌鱼等低等物种中，该基因还没有功能研究的报道。因此推进这类研究或许能够为探索高等动物同源基因的功能和演化提供新的视角。

基金项目

山东省自然科学基金(ZR2012CM015)。

文章引用

辛佳静,郭晓敏,孟 丽,汲广东. cGAS的研究进展
Advances of cGAS[J]. 海洋科学前沿, 2017, 04(02): 61-67. http://dx.doi.org/10.12677/AMS.2017.42009

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