| The innate immune system functions as a protective system to the host against microbes,toxins,allergen or damaging signals,essentially through self-nonself discrimination.Acting at the molecular level of the innate immune system,pattern recognition receptors(PRRs)are a group of innate immune sensors that engage with these threats and initiate a cascade of host defence system through the induction of various inflammatory cytokines,chemokines and interferons.Double-stranded DNA(dsDNA)serves primarily as the genetic code in many organisms and so is vital to both the host and many invading microbes.Exposure of dsDNA thus points to danger signals.Low structural complexity and near omnipresence of dsDNA drives the evolution of its compartmentalization,typically as nucleus and mitochondrion in eukaryotes,to exempt self from nonself that could be targeted by host surveillance.Cytosolic dsDNA can be sensed by several proteins,but the most important dsDNA-sensing PRR in metazoa is the cyclic GMP-AMP synthase(cGAS,also known as MB21D1).Upon dsDNA recognition,cGAS catalyses the synthesis of cyclic GMP-AMP(cGAMP),which stimulates the induction of type I interferons through the STING-TBK1-IRF3 signalling axis(STING,stimulator of interferon genes,also known as STING1,TMEM173,MITA,MPYS,and ERIS;TBK1,TANK-binding kinase 1;IRF3 that activates interferon regulatory factor 3).STING also induces autophagy upon cGAMP binding,but this process is independent of its interferon activation capability.Toll-like receptor(TLRs)are a class of single transmembranous innate immune receptors for a quantity of molecules found in various microbes and eukaryotes.Among them,nucleic acid(NA)-sensing TLRs localize at the endolysosomal compartment and their translocation from endoplamic reticulum(ER)to Golgi apparatus and subsequently endolysosome needs UNC93B1,an ER-resident multi-transmembranous protein.Moreover,mutant H412 R or knockout of UNC93B1 abolishes the signal transduction of NA-sensing TLRs and affects their protein level,but the mechanisms of which are still elusive.Autophagy is highly orderly regulated self-degradation and-recycling of cellular components to maintain macromolecular synthesis and energy homeostasis during starvation and other stressful conditions.It has been implicated in many cellular processes as preventing accumulation of damaged organelles or long-lived,aggregate-prone proteins,removing incoming threats such as intracellular pathogens,regulating the activity of specific signaling proteins,including those taking part in innate immunity,and so on.In this study,we aim to address whether the cGAS-STING sensing of cytosolic dsDNA is regulated by UNC93B1 and how.What we found was that,in contrast to the decreased responses to TLR agonist treatment,attenuation of UNC93B1 expression potentiates innate immune responses initiated by cytosolic dsDNA recognition.Furthermore,the augmentative effect of UNC93B1 deficiency on innate immune response was found to be dependent on STING,but not on other parts in the cGAS-STING-TBK1 signaling pathway.Mechanistically,we found UNC93B1 interacts with STING in ER membrane at rest condition or in lysosomes late after dsDNA stimulation.However,UNC93B1 seemed not to have affected the ER-exit of STING,but rather,it affected the protein level of STING and thus the signaling responses it transduces.Furthermore,we found UNC93B1 affects lysosomal degradation of STING through autophagy.And more interestingly,the histidine at position 412 of UNC93B1 was also found to be important for its action on STING as that on NA-sensing TLRs,making the question what exact physiological function of UNC93B1 is more prominent.In all,we demonstrated that UNC93B1 negatively regulated the innate immune responses initiated by cytosolic DNA sensing by the cGAS-STING pathway through harnessing the autophagic turnover of STING.So far,studies on UNC93B1 had only focused on the pathways of NA-sensing TLRs,but not on other pathways in innate immunity.Our study sheds more light on the physiological function of UNC93B1 and may be implicatitive to its role in some other cellular processes regarding autophagy,endolysosomal degradation,vesicle trafficking and innate immune signal transduction.Moreover,considering the important role of STING in fighting against invading pathogens and autoimmune diseases,down-regulation of the signaling pathway by UNC93B1 is also of significant importance to control a whole vast cascade of signaling events. |
