Induction and activation of cGAS DNA sensor by Sindbis virus

Abstract

All living organisms have a need to cope with exogenous pathogen invasion when they contact with infectious microorganisms in the environment. Immune responses have been developed to antagonize pathogen invasion. During viral infection, local antiviral response is triggered through type I interferon (IFN) production and signaling. The initiation of antiviral response requires recognition of pathogen associated molecular patterns (PAMPs) by host pattern recognition receptors. Among all PAMPs, cytosolic nucleic acids are primarily sensed during viral infection by cytosolic DNA and RNA sensors. Cyclic GMP-AMP synthase (cGAS) is generally accepted as a primary DNA sensor in the cytoplasm, which synthesizes cyclic GMP-AMP (cGAMP) upon activation. cGAMP then binds to stimulator of IFN genes (STING) to elicit type I IFN response. From in silico cGAS modeling, it is expected that RNA cannot activate cGAS mediated type I IFN response. However, cGAS confers protection against certain RNA viruses, which indicates that activation of cGAS is possible during the infection of at least some RNA viruses. The recent finding that DENV activates cGAS by inducing the release of mitochondrial DNA to the cytosol lends support to this notion. However, why cGAS has antiviral activity against some alphaviruses in the family of Togaviridae remains to be elucidated. Particularly, it is not understood whether these viruses may also be sensed by cGAS. In this thesis I sought to address these questions using the prototypic alphavirus named Sindbis virus (SINV) as a model virus of the Togaviridae family. cGAS mRNA and protein expression was found to be induced by SINV at a very early stage of infection and in an IFN-independent manner. Ectopic expression of cGAS and STING restricted SINV replication in human cells that are deficient of both cGAS and STING. On the contrary, depletion of cGAS impaired type I IFN production and facilitated SINV replication. Both gain-of-function and loss-of-function assays suggested that cGAS is required for type I IFN-dependent restriction of SINV replication. Mechanistically, SINV infection resulted in reactivation of endogenous retroelements, leading to the accumulation of cytosolic DNA reverse transcribed from endogenous retroelement mRNAs but not from RNA genome of SINV. Taken together, this study revealed a role for DNA sensor cGAS in the innate antiviral immune response against SINV and a new mechanism by which SINV activates cGAS by elevating the level of complementary DNA of endogenous retroelements in the cytoplasm of host cells. Our work refines and expands the existing model for the role of cGAS DNA sensor in the infection of RNA viruses. Our findings have broad implications in the study and treatment of viral and pro-inflammatory diseases.