Professor Pärt Peterson, PhD, University of Tartu
Senior Lecturer Hanna Jarva, MD, PhD, University of Helsinki
AIRE is a transcription regulator protein for tissue-specific autoantigens. In humans, the mutations in AIRE gene cause autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED), a rare autosomal recessive disease, characterized by defective immune tolerance to many self-antigens. Based on its monogenic etiology, APECED can be thought of as a model for organ-specific autoimmune diseases and thus can provide insights into the pathogenesis of autoimmunity. The expression of AIRE is found mostly in the thymus but also in the peripheral immune organs, spleen, fetal liver, and the lymph nodes. In the thymus, AIRE is expressed in medullary epithelial cells as the antigen-presenting cells that are important in the negative selection of self-reactive T cells, which in turn is required for induction of central immune tolerance. In this study we investigated the functions of AIRE protein at molecular level using biochemical methods, cell culture and flourescence microscopy. We found new protein interaction partners of AIRE: the DNA-dependent protein kinase (DNA-PK), Ku70 and Ku80. We showed that the biological importance of the interaction of DNA-PK is to phosphorylate AIRE at threonine 68 and serine 156 positions and that phosphorylation of these residues are important for AIRE transcriptional activity. We also analyzed promoter activations of the AIRE-dependent tissue-specific genes, involucrin and S100A8, and also of an interferon beta promoter, which has previously been used as a model promoter for different APECED-causing AIRE mutants. We found that most but not all of the AIRE homogeneously staining region (HSR) domain mutations inactivated AIRE transcriptional activity. In addition, a general transcriptional activator CBP did not enchance transcriptionally inactive AIRE mutants. We confirmed the correlation between transcriptional inactivity/activity of AIRE mutants and the protein fold. We discovered that HSR domain in AIRE has a caspase recruitment domain (CARD) structure, which is common among apoptotic and inflammation related proteins. We proved that this domain represents also AIRE's apoptotic function. We described AIRE association with the cell stress and apoptosis indicator, GAPDH protein, and showed that AIRE-induced apoptosis caused GAPDH protein translocation to the nucleus. In conclusion, our studies broaden the knowledge on AIRE protein functions and help to understand the basic mechanisms behind autoimmune diseases.