Mechanism Studies Of DSTING In Drosophila Antiviral Immune Response

Innate immunity is the first line of host defense against invading pathogens.As the key mediator of mammalian antiviral innate immune response,STING aggregates and recruits downstream signal moleculars,such as TAK1 and IRF3,to regulate the production of antiviral factors upon binding c GAMP.Mechanism of STING activation and its interaction with other signaling moleculars has been intensively studied in recent years.In contrast,although insect STING homolog plays a similar role as mammalian STING in mediating anti-viral or anti-bacterial immune response through activating the NF-?B transcription factor Relish,how it is activated and mediates Relish activation is still unclear.To study how Drosophila STING(dSTING)activates Relish to mediate antiviral immune response,we performed amino acid sequence analysis,Native-PAGE assay,fluorescence confocal microscopy and Thioflavin T-binding analysis on dSTING,and demonstrated that through its RHIM motify,dSTING forms amyloid-like fiber polymers.Further more,virus-infected Drosophila S2 cells were tested for the induction level of downstream antiviral factors and the replication level of the virus,confirming that the formation of dSTING polymer is necessary to mediate antiviral immune response as well as inhibit viral replication.In addition,immunocoprecipitation and fluorescence co-localization assay elucidated the interaction between dSTING and IMD,which underlies Relish activation.The main results are as follows:1.dSTING is activated through forming an amyloid-like fiber polymerTwo conserved sequences were identified in dSTING through RHIM sequence homology analysis,named RHIM1 and RHIM2 respectively.In addition,both human STING and silkworm STING contain a hypothetical RHIM motif.Then stable cell lines expressing dSTING-WT or RHIM-deleted mutants,dSTING-?RHIM1 and dSTING-?RHIM2 were constructed and subjected to Native-PAGE analysis.Results showed that both dSTING-WT and dSTING-?RHIM2 formed dimer,tetramer,octamer,hexamer,and even-larger oligomers,while significantly less dSTING-?RHIM1oligomers was observed.Distribution of dSTING-mcherry in the cell examined by confocal microscopy also indicated that dSTING formed aggregates through RHIM1 motify in perinuclear region,and the oligomerization was further enhanced under the induction of c GAMP,as puncta dSTING aggregates increased.Staining with Th T which is a specific fluorescent dye for amyloid-like fiber polymers showed that the oligomers formed by dSTING specifically binds to Th T,and c GAMP stimulation further ebhanced Th T fluorescence.Besides,transcriptional level of antiviral factors downstream of the antiviral signaling pathway regulated by dSTING detected by q PCR showed that both dSTING-WT and dSTING-?RHIM2 induced the production of antiviral factors,on the contrary,dSTING-?RHIM1 lost its ability to induce antiviral factors.Moreover,the replication of DCV and Cr PV in cells expressing dSTING-WT or dSTING-?RHIM2was significantly lower than cells expressing dSTING-?RHIM1.All the above results confirmed that dSTING activates and mediates the insect antiviral innate immune response through oligomerization via its RHIM1 motify upon immune stimulation.2.dSTING interacts with IMD to regulate antiviral immune responseIMD is an upstream signaling molecular of Relish in IMD signaling pathways and contains c RHIM(Cryptic RHIM)motif.The interaction between HA-dSTING and Myc-IMD was examined by co-immunoprecipitation,and the results showed that IMD was co-immunoprecipitated with dSTING under DCV infection.In additon,the fluorescence co-localization of dSTING-m Cherry and IMD-EGFP analyzed by confocal microscopy showed that in the absence of c GAMP,dSTING and IMD aggregated separately and were distributed in different cytoplasmic regions,while in the presence of c GAMP,dSTING and IMD were co-localized in perinuclear region.Interestingly,when the RHIM of dSTING or the c RHIM of IMD was deleted,co-immunoprecipitation assay showed that dSTING and IMD no longer interacted with each other,which proved that the interaction between dSTING and IMD was mediated by RHIM motifs.All the above results demonstrated that dSTING interacts with IMD through their RHIM motifs,and potentially recruits other related molecules to form immune signaling complex to activate Relish and regulate the antiviral innate immune response.