Decoy strategies: The structural and biochemical characterization of TL1A, DcR3 and their interactions
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The Tumor Necrosis Factor (TNF) and TNF receptor (TNFR) superfamilies represent major classes of co-stimulatory molecules and play critical roles in immunity. Decoy Receptor 3 (DcR3) is a secreted TNFR that is highly expressed in human cancer and autoimmune patients. The interpretation of DcR3's functions is complicated by its "promiscuity", as it neutralizes three TNF ligands: FasL, LIGHT and TL1A. Each of these ligands binds distinct signaling TNF receptors, and subsequently direct diverse immune responses. The objective of my thesis was to utilize structural and biochemical approaches to elucidate the mechanisms of DcR3 that underlie its ability to neutralize multiple distinct TNF ligands.;TL1A is a newly described member of the TNF superfamily and its interaction with the signaling receptor death receptor 3 (DR3) is essential to the pathogenesis of diverse inflammatory diseases. Our crystal structure of the human TL1A ectodomain demonstrates that the TL1A protein adopts a typical TNF family "jelly-roll" fold and exists as a stable trimer in both solution and the crystalline state. Mutagenesis of TL1A revealed residues critical to binding the decoy receptor, DcR3, and predicted both common and unique receptor binding features compared with previously characterized TNF ligand-receptor complexes.;The determinants that support TL1A:DcR3 recognition were further defined by our crystal structures of the human DcR3 cysteine-rich domain and its complex with the human TL1A ectodomain in two different crystal forms. DcR3 is an elongated molecule composed of 4 tandemly arranged cysteine-rich domains (CRD). Each DcR3 protein binds the groove between two adjacent TL1A subunits, yielding a receptor:ligand assembly with a 3:3 stoichiometry. The biochemical and structural analysis of the TL1A:DcR3 interface reveals that DcR3 interacts predominately with the invariant atoms located at the membrane-proximal half of TL1A, while the contacts with the specificity-determining membrane-distal half are modest. These results are consistent with the TL1A mutagenesis study, and provide the mechanistic basis that allows DcR3 to serve as a generic decoy receptor that neutralizes multiple TNF ligands. The information derived from this study provides insight into the regulation of cross-reactivities in TNF superfamily and suggests new potential therapeutic opportunities to treat inflammatory diseases.
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