TREX1 C-Terminus Regulates Oligosaccharyltransfererase to Prevent the Accumulation of an Endogenous Bioactive Disaccharide Associated with Autoimmune Disorders
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The innate immune system is the first line of defense against infectious pathogens and serves a vital role in activating the adaptive immune system. Detection of viral and bacterial intracellular DNA by cytosolic DNA sensors (CDS) is crucial for the activation of the innate immune system and for eliciting a proper immune response. However, self-DNA species that originate from retro-elements, genomic DNA replication, and damaged mitochondria can inappropriately activate the innate immune system through recognition by CDS. The cell employs strategically placed DNases to remove these self-DNA ligands that would otherwise cause chronic activation of the innate immune system leading toautoimmunity. DNase III, also known as TREX1, is one of negative regulators of CDS-mediated innate immune response. TREX1 is a 314 amino acid endoplasmic reticulum (ER) tail-anchored 3' exonuclease where the N-terminal region contains the DNase domain and the C-terminal end controls TREX1 localization to the surface of the ER. Disease mutations that abrogate the N-terminal DNase function of TREX1 lead to chronic activation of CDS-mediated innate immune response to self-DNA ligands leading to autoimmunity.In contrast, disease mutations in the C-terminal region are mostly frame-shift (fs) mutations that alter TREX1 localization to the ER, without affecting the DNase function. This difference invoked the hypothesis that TREX1 C-terminus engages in the interaction with ER-residing proteins and such interaction could contribute to the C-terminus associated diseases. Through investigating of this possibility, I identified that TREX1 interacts with the oligosaccharyltransferase subunits Ribophorin 1 (RPN1) and DDOST. This interaction was dependent solely on TREX1 C-terminus. Furthermore, TREX1 C-terminus modulates OST's preference to hydrolyze lipid-linked oligosaccharides (LLOs) into bioactive free oligosaccharides (fOS) in a switch-like manner. Introduction of TREX1 disease fs mutations stably switched OST to promote rapid release of bioactive free oligosaccharidesthat activates immunesignaling (Chapter 2).Structural analysis of thebioactive fOS revealed that they differ from typical N-glycans structures with high accumulation of mannosyl tetra, tri and disaccharides species. Among these, the structure responsible for the bioactivity is a mannose (Man) 1-4 N-acetylglucosamine (GlcNAc) disaccharide. The bioactive disaccharide is produced from OST's hydrolyzed LLOsin the cytoplasm and activates a TBK1 dependent immune signal that leads to the upregulation of interferon-stimulated genes (ISGs) and chemokine genes (Chapter 3). The TREX1 C-terminal region is composed of 72 amino acids from residues 242 to 314. I examined what portion of TREX1 C-terminus was required for interaction with RPN1 and DDOST. The TREX1 and OST's interaction interface was mapped to residues 235 to 290 where a regulatory phosphorylation site was identified at serine 261. The phosphorylation event takes place during mitosis. Phosphomimetic mutant ofTREX1 S261 disrupted the interaction between TREX1 and OST, implying that the phosphorylation event regulates the interaction between TREX1 and OST as well as potentially OSThydrolysis of LLOs into free oligosaccharides (Chapter 4). Altogether, these results provide mechanistic insights into how TREX1 C-terminus fs mutations cause accumulation of a bioactive disaccharide which triggers immune activation and suggests potential therapeutic options for TREX1-fs mutant-associated diseases.