The innate immune system senses non-self nucleic acids through pattern recognition receptors. Little is known about sensors for cytoplasmic DNA. Recently, Aim2 (absent in melanoma 2) and p202 proteins (encoded by the Aim2 and Ifi202 genes, respectively), members of the interferon (IFN)-inducible p200-protein family, have been identified as novel sensors of double-stranded DNA (dsDNA) in the cytoplasm. The HIN-200 domain of the Aim2 protein binds to dsDNA, while the pyrin (PYD) domain associates with the adapter molecule ASC to form an Aim2 inflammasome to activate caspase-1 and NF-κB and induce cell death by pyroptosis. Interestingly, p202, a candidate lupus susceptibility protein, lacks the PYD domain. Consequently, the p202 protein is unable to form an inflammasome to activate caspase-1 in response to cytosolic DNA. Generation of Aim2-deficient mice has revealed that the Aim2-deficient mice are more susceptible to certain bacterial and viral infections. Moreover, the studies also indicated that the Aim2 protein acts to negatively regulate the IFN responses. The major long-term objective of our proposed studies is to understand the role of the Aim2 protein in the regulation of chronic stimulation of the innate immune system by endogenous DNA. Based on our preliminary observations, we hypothesize that expression levels of the Aim2 protein at certain thresholds are necessary for the suppression of the type I IFN response, the expression of the IFN-inducible proteins (including the p202 protein), and the transcriptional activation by NF-κB. Additionally, we postulate that increased levels of the p202 protein in immune cells with sub-threshold levels of the Aim2 protein inhibit the formation of the Aim2 inflammasome in response to cytosolic DNA and activation of caspase-1, thus, contributing to chronic stimulation of the innate immune responses.
Aim #1 : Identify the molecular mechanisms that regulate constitutive and inducible expression of the Aim2 gene in immune cells. Both in vivo and in vitro approaches will be used, including STAT1-null and AR-null mice (and isogenic wild type mice), mutational analyses, and chromatin immunoprecipitation assays.
Aim #2 : Investigate the molecular mechanisms by which the Aim2 protein stimulates the transcriptional activity of NF-κB in immune cells and suppresses transcription of the Ifnb gene. Genetic, biochemical, and molecular cell biology approaches will be used.
Aim #3 : Elucidate the role of the Aim2 protein in inflammation and autoimmunity. We propose to use various strains of mice (including the Aim2 knockout mice) that differ in expression of the Aim2 protein to understand the role of Aim2 in anti-DNA antibody response and collagen antibody-induced arthritis (CAIA). Public Health Relevance: Proving our hypotheses is important to understand the role of the Aim2 protein as a sensor of microbial as well as host DNA. Moreover, our studies will help in the rational design of new therapies and treatments for chronic inflammatory diseases, including systemic lupus erythematosus (SLE).
Our proposed studies will identify the molecular mechanisms by which the Aim2 protein, a newly identified cytosolic DNA sensor, plays a role in gender bias in chronic stimulation of the innate immune system by endogenous DNA. Our studies will serve basis to understand the role of Aim2 in microbial infections, inflammation, and autoimmune diseases, such as systemic lupus erythematosus (SLE).
