DNA damage-induced inflammation and its brain-specific consequences

摘要

Abstract Neurodegenerative diseases such as Alzheimer's and Parkinson's disease represent a diverse group of conditions. While the loss of brain cells is the most prominent phenotype, each disease also presents with evidence of a chronic inflammatory process. The proposed work will explore a newly recognized trigger of this toxic neuroinflammatory process, cytoplasmic DNA (cytoDNA). The cells of the innate immune system, using cGAS/STING and TLR9, interpret the presence of cytoDNA as an invading viral or bacterial pathogen and respond vigorously. In aging and neurodegenerative diseases, cellular stresses lead to the release of DNA fragments from the cell's own mitochondrial or nuclear genome into the cytoplasm, triggering a "sterile" inflammatory response. Interferon related genes and the NFκB system are mobilized and both cause great harm if their responses become chronic. Studies of sterile inflammation traditionally focus on the cells of the innate immune system (e.g., microglia), or accessory cells such as astrocytes. We propose to take a much broader approach, by separately testing each cell type in the brain for its role in contributing to the chronic inflammation found in Alzheimer's and Parkinson's. We will track the relative contributions of nuclear and mitochondrial fragments to the total amount of cytoDNA. As stressors, we will use ATM inhibition, to block DNA repair, TFAM inhibition to damage mitochondria, as well as more disease-relevant stimuli such as Aβ for Alzheimer's disease and α-synuclein for Parkinson's disease. We predict that the contributions of mitochondrial and nuclear cytoDNA fragments will differ in different situations, resulting in unique TLR9 and cGAS/STING responses that result in a signature secretome that contributes to the diversity of clinical symptoms. To test the pathological potential of each cytoDNA-stimulated immune response, we will use different stimuli to challenge cultures of astrocytes, neurons, oligodendrocytes and microglia. We will then collect their conditioned medium and test it on naïve cultures of the same cell types to isolate the full range of toxic and trophic elements released from different cells. The transcriptomes of the stimulated cells will be defined by RNAseq; the protein composition of the secretome will be determined by LC/MS. While our initial studies will be in cultured cells, we will validate the in vitro findings in vivo using mouse models of three distinct neurodegenerative diseases: Alzheimer's, Parkinson's and ataxia-telangiectasia. Finally, we propose to develop a multi-pronged strategy to block the impact of cytoDNA-induced inflammation. We will search for compounds that block the export of cytoDNA from the nucleus or stimulate its elimination from the cytoplasm. The significance of the findings derives from the fact that understanding the process of sterile inflammation and how to block it will offer fresh strategies to improve our approach to many neurodegenerative diseases.