cGas-Like Receptors in Animal Innate Immunity

• Main Author: Slavik, Kailey Madison
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2023
• Subjects: Immunology; Virology
• ISBN: 9798379603243

The molecular detection of pathogen infection and subsequent initiation of an immune response is an essential function of cellular life. Animals encode innate immune proteins known as PRRs (patternrecognition receptors) dedicated to the direct detection of PAMPs (pathogen-associated molecular patterns). In mammalian cells, the PRR cGAS (cyclic GMP-AMP synthase) controls an immune response to the PAMP cytosolic DNA by synthesizing the nucleotide second messenger 2′3′-cGAMP. 2′3′-GAMP is specifically recognized by the adaptor protein STING (stimulator of interferon genes), which upon binding cyclic dinucleotide signals initiates a potent antiviral gene program through NF-κB signaling and the induction of type I interferon. Most animal species encode multiple predicted homologs of cGAS, known as cGLRs (cGAS-like receptors); however, it is unknown if these proteins synthesize nucleotide second messenger signals or control an immune response. Using a structural biology approach, we determine that animal cGLRs share close structural homology with mammalian cGAS and that remodeling of the shared ligand binding surface enables cGLRs to detect alternative PAMPs. We then focus on the model organism Drosophila and use a forward biochemical screen to discover Drosophila cGLR1 as a direct sensor of the PAMP dsRNA. cGLR1 synthesizes a distinct nucleotide second messenger, 3′2′-cGAMP, that is specifically recognized by Drosophila STING to control antiviral immunity through NF-κB. We further characterize Drosophila cGLR2 as an additional nucleic acid sensor with preference for dsRNA ligands. cGLR2 synthesizes the nucleotide second messenger 2′3′-c-di-GMP, supporting that animals encode multiple cGLRs to control distinct immune signaling pathways. Finally, using a bioinformatic approach we identify >3,000 cGLRs encoded in nearly all metazoan phyla. A broad biochemical screen of animal cGLRs reveals that dsRNA and dsDNA are commonly recognized PAMPs and that cGLRs synthesize diverse nucleotide second messengers to control downstream immunity. Structural analysis of animal STING proteins further demonstrates that synthesis of distinct nucleotide signals enables cGLRs to control discrete signaling pathways. Together, my work expands our understanding of how animal cells detect and control pathogen infection by establishing cGLRs as a broadly conserved family of PRRs that respond to diverse PAMPs and use distinct nucleotide second messengers to control a cellular response.