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Premortem enzymatic DNA damage in kidney injury

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The occurrence of acute kidney injury (AKI) in humans is associated with a poor long-term prognosis including higher morbidity and increased mortality. Our recent studies showed that the inactivation of apoptotic DNases/endonucleases provided protection against AKI induced by ischemic or toxic insults. These observations led to the conclusion that endonucleases are involved in premortem DNA fragmentation, which precedes and causes cell death. This proposal is a continuation of the previous accomplished study, which was focused on the role of DNase I in ischemic acute renal failure. This study resulted in two important conclusions. One of them is that the inactivation of DNase I has much broader implications than it was initially thought, and it can be applied to the toxic kidney injury induced by cisplatin. Another observation was that DNase I may act in concert with other endonucleases. Our preliminary studies showed that another DNase, endonuclease G (EndoG), is induced in the tubular epithelium by cisplatin in vitro and in vivo. These studies showed that DNase I is necessary for EndoG induction in several in vitro and in vivo models. The regulation of EndoG expression by DNase I is a previously unknown pathway. The hypothesis of the current proposal is that (a) during cisplatin kidney injury, premortem enzymatic DNA damage is induced by EndoG which acts along the pathway initiated by DNase I, and (b) the inactivation of EndoG may protect the kidney against injury induced by cisplatin. This hypothesis is supported by the preliminary data, which showed that the genetic inactivation of EndoG in mice or primary tubular epithelial cells, or the silencing of EndoG in tubular epithelial cells provided protection against DNA damage and cell death induced by cisplatin. In Specific Aim 1, we plan to determine the role of DNase I in the regulation of EndoG expression, DNA damage and apoptosis. Specific Aim 2 will be focused on examining the effects of EndoG activation or inactivation on tubular epithelial cell injury. In Specific Aim 3, we are going to define the intermediate mechanisms of EndoG regulation by apoptosis, DNA damage, membrane permeability and others during cisplatin injury. DNase I and EndoG null mice, RNA interference, overexpression of DNase I and EndoG, dominant-negative mutant and inhibitors will be used to interrupt specific pathways in vivo or in vitro and thus address the goals in a mechanistic and cause-effect relationship manner. Our endpoints will include: expression of endonucleases quantified by real-time RT-PCR, Western blotting and activity, immunolocalization of EndoG, the assessment of DNA fragmentation by TUNEL or Comet assays, and the analysis of cell viability. It is very likely that the identification of the role and regulation of EndoG in the mechanisms of premortem DNA damage and kidney cell death pathways will provide new modalities for the prevention and treatment of AKI in humans. PUBLIC HEALTH RELEVANCE The proposal is focused on the DNA damage mechanisms induced during toxic kidney injury, and the role of two enzymes which are responsible for this DNA damage: DNase I and endonuclease G. We propose that these two enzymes are linked in a pathway, in which DNase I activates endonuclease G, and that inactivation of any of these endonuclease can protect kidney from toxic injury.

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