Abstract 2765: Detection and quantification of cellular GAMP using homogenous HTS formatted bioluminescent assay
Authors: Goueli Said A. , Hsiao Kevin , Wang Hui , Larsen Matt 
Published: April 2023

Abstract
The STimulator of INterferon Genes (STING) is a transmembrane protein located at the endoplasmic reticulum, which serves as a molecular hub in response to cytoplasmic cyclic dinucleotide (CDN) second messengers such as cyclic GMP-AMP (cGAMP). Upon stimulation of cyclic GMP-AMP synthase (cGAS) by a wide array of pathogens that provide extracellular microbial DNA and by intracellular (nuclear and mitochondrial) DNA, the second messenger cGAMP is generated, which then binds directly to the STING protein resulting in the activation of the STING pathway. This activation by cytosolic DNA initiates a cascade of events, cGAS-STING-TBK1-IRF3 signaling, that induces type 1 IFN response, and eventually the signaling is terminated by degradation of pathway components and clearance of stimulatory DNA. Given the significant roles of STING as an adaptor for DNA sensors to pathogens and its involvement in immune sensing, tumor growth control, and control of autoinflammatory and autoimmune disorders, the STING pathway has become a very important drug target. cGAS is well-positioned to act as a DNA sensor that triggers innate immune responses and initiates host immune responses against pathogens and cancer, but inappropriate activation of STING signaling causes severe and often fatal autoimmune or autoinflammatory diseases. Credible evidence suggests that the cGAS-cGAMP-STING pathway makes fundamental contributions to at least three major cancer therapies: radiation therapy, chemotherapy, and immunotherapy, and it thus represents a promising drug target. Therefore, there is a need to identify lead compounds that effectively modulate human STING for further drug development. Towards this goal, we have developed a bioluminescent assay to monitor the concentration of cGAMP in intracellular as well as extracellular compartments. The assay relies on the new concept of nanoluciferase (nanoluc) complementation where a small peptide (SmBiT) linked to cGAMP through a linker (cGAMP tracer) can be recognized by an anti cGAMP antibody that is linked to a nanoluc complementary fragment (LgBiT) resulting in bioluminescence upon the addition of nanoluc substrate. The presence of free cGAMP that is generated in the reaction competes with cGAMP tracer and decreases bioluminescence. The assay is sensitive and can detect less than 10 nM cGAMP and less than one ng of cGAS and thus, it is suitable for testing compounds that modulate cGAS activity. We will show data that correlate the activation of cGAS using less than 10 ng of dsDNA stimulation, and the activation of IFN response element reporter assays to detect cytokine release upon cGAS-STING activation. The assay is homogenous, HTS formatted and can be completed in less than one hour. Thus, this assay provides valuable contribution for investigating modulators of the STING pathway and facilitates the search for novel therapeutics.
Citation Format: Said A. Goueli, Kevin Hsiao, Hui Wang, Matt Larsen. Detection and quantification of cellular GAMP using homogenous HTS formatted bioluminescent assay [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2765.