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Alexander Ogilvy, De Fen Shen, Chi-Chao Chan, Mones S Abu-Asab; Is There a Relationship Between Extranuclear DNA and Degenerative Mitochondria?. Invest. Ophthalmol. Vis. Sci. 2014;55(13):6324.
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© ARVO (1962-2015); The Authors (2016-present)
Nuclear DNA leakage to the cytoplasm is a catastrophic event with consequences such as inflammation. However, little is known about the triggers of this phenomenon. In our previous work we have established the presence of extranuclear DNA (enDNA) in abnormal retinal pigmented epithelial (RPE) in our AMD mouse model of double deficiencies of Ccl2 and Cx3cr1 on rd8 background. Aberrant inflammatory responses are detected in this mouse model. During our investigation we noted that the mitochondria of enDNA cells had various degrees of degeneration. By examining mouse strains that were previously found to contain enDNA, our investigation sought to determine if there was a correlation between mitochondrial dysfunction and the presence of enDNA.
RPE of 4 mouse strains were studied using transmission electron microscopy (TEM): 1) wild type negative control of rd8 C57BL/6J, 2) C57BL/6N positive control of rd8; two mutant strains of C57/B6N, 3) Ccl2 -/-, and 4) double knockout Ccl2 -/- Cx3cr1 -/-. Mice were 1.5 to 6 months old. Eyes were inoculated and fixed in 2.5% gluteraldehyde for 24 hours, embedded in epoxy resin, sectioned and stained with uranyl acetate and lead citrate. Samples were analyzed at 25-50,000x magnification using a JOEL JM-1010 transmission electron microscope.
TEM examination of RPE confirmed the presence of higher percentage of degenerate mitochondria in cells with enDNA compared with the controls . The damaged mitochondria had lighter matrix and disintegrated inner membrane . Cells containing degenerative mitochondria also showed signs of autophagy, including vacuole-formation in the cytosol. Interestingly, enDNA was not found in cells with healthy mitochondria.
Our research supports a correlation between degenerate mitochondria and the presence of enDNA, however, it is still unknown which one triggers the other. A plausible, but unconfirmed, explanation might be that mitochondrial degradation robs the cell of the energy it needs to maintain homeostasis, including mechanisms of DNA packaging in the nucleus. Without sufficient energy to keep DNA organized inside the nucleus it begins to leak out into the cytoplasm. Cytosolic DNA receptors, like cGAS, could then detect the leaking DNA and trigger an inflammatory immune response, resulting in lesions.
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