Regulation Of Antiviral Innate Immune Response By ZCCHC3

The innate immune system is the first line of host defense against virus.Upon infection,the conserved microbial components called pathogen-associated molecular patterns(PAMPs)are sensed by cellular pattern recognition receptors(PRRs),which trigger signaling leading to the induction of type I interferons(IFNs),pro-inflammatory cytokines and other downstream effector genes.These downstream effector proteins mediate innate immune responses by inhibiting virus replication and promoting clearance of infected cells,and facilitate the host adaptive immune response to eliminate infected pathogens.Upon infection,viral RNA is detected by the retinoic acid-inducible gene-I(RIG-I)-like receptors(RLRs),including RIG-I and MDA5.RIG-I and MDA5 detect distinct viral RNAs.It has been demonstrated that RIG-I preferably recognizes viral 5'-pppRNA and short double-strand RNA(dsRNA),such as Sendai virus(SeV),vesicular stomatitis virus(VSV),Newcastle disease virus(NDV)and influenza virus.While MDA5 has higher affinity to long dsRNA,such as encephalomyocarditis virus(EMCV).After binding to viral RNA,RIG-I and MDA5 are activated and translocate to mitochondria,and then RIG-I and MDA5 interact with mitochondrion-located adaptor protein VISA.VISA then acts as a central platform for recruitment of downstream signaling components such as TRAF2/3/6,TBK1 and IKKs,leading to activation of the kinases TBK1 and IKKs.These kinases further phosphorylate the transcription factor IRF3 and the inhibitor of NF-?B(I?B?),leading to activation of IRF3 and NF-?B and eventual induction of downstream antiviral genes.Activation of RIG-I and MDA5 is regulated by a series of post-translational modifications,such as polyubiquitination,sumoylation and phosphorylation.The polyubiquitination of RIG-I and MDA5 plays an essential role in the regulation of their activities.Upon infection,viral DNA is detected by several DNA sensors,such as cGAS,IFI16,DDX41,AIM2 and LSm14A.Among these sensors,cGAS(cyclic GMP-AMP synthase)has been defined as a key DNA sensor.Afte binding to viral DNA,cGAS undergoes oligomerization and catalyzes the synthesis of the second messenger molecule cGAMP(2'-3' cyclic AMP-GMP),which in turn binds to and activates the adaptor MITA located in the endoplasmic reticulum(ER).MITA recruits the kinase TBK1 and the transcription factor IRF3,leading to their phosphorylation and activation as well as induction of downstream antiviral genes.After RNA and DNA virus infection,the recognition of receptors to viral RNA and DNA is the important event to initiate innate antiviral response.Whether and how the binding of viral RNA and DNA to RIG-I,MDA5 and cGAS is regulated remains elusive respectively.In our study,we identified the CCHC-type zinc finger(ZF)protein ZCCHC3 as a candidate.The results indicated that ZCCHC3 positively regulate RNA and DNA virus-triggered signaling.In the first part of this study,ZCCHC3-deficiency inhibited RNA virus-induced transcription of downstream antiviral genes.Zcchc3-/-mice were more susceptible to lethal infection by VSV or EMCV.Mechanistically,ZCCHC3 bound to dsRNA and promoted the binding of RIG-I and MDA5 to dsRNA.ZCCHC3 also promoted the interactions of TRIM25 with RIG-I and MDA5,which is important for TRIM25-mediated K63-linked polyubiquitination and activation of RIG-I and MDA5.These results suggest that ZCCHC3 acts as an important regulator of RIG-I and MDA5-mediated signaling pathways.ZCCHC3 acts as a co-receptor for RIG-I and MDA5,which promotes their activation by two distinct mechanisms.In the second part of this study,ZCCHC3-deficiency inhibited DNA virus-triggered induction of downstream antiviral genes.Zcchc3-/-mice were more susceptible to lethal infection by DNA virus.Mechanistically,ZCCHC3 bound to dsDNA,facilitated the binding of cGAS to dsDNA,and promoted the oligomerization and enzymatic activity of cGAS.Our findings suggest that ZCCHC3 acts as a co-sensor of cGAS and plays an essencial role in innate immune response to DNA virus.In conclusion,we investigated the molecular mechanisms of ZCCHC3 acts as a co-receptor involved in innate immune response to RNA and DNA virus by using in vitro cell model and in vivo mouse model.Our studies provide new molecular mechanisms and new therapeutic targets for virus-induced diseases.