Abstract
Terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay is a long-established assay used to detect cell death-associated DNA fragmentation (3’-OH DNA termini) by endonucleases. Because these enzymes are particularly active in the kidney, TUNEL is widely used to identify and quantify DNA fragmentation and cell death in cultured kidney cells and animal and human kidneys resulting from toxic or hypoxic injury. The early characterization of TUNEL as an apoptotic assay has led to numerous misinterpretations of the mechanisms of kidney cell injury. Nevertheless, TUNEL is becoming increasingly popular for kidney injury assessment because it can be used universally in cultured and tissue cells and for all mechanisms of cell death. Furthermore, it is sensitive, accurate, quantitative, easily linked to particular cells or tissue compartments, and can be combined with immunohistochemistry to allow reliable identification of cell types or likely mechanisms of cell death. Traditionally, TUNEL analysis has been limited to the presence or absence of a TUNEL signal. However, additional information on the mechanism of cell death can be obtained from the analysis of TUNEL patterns.
Highlights
DNA is the only molecule in the cell that can be repaired, but it cannot be completely resynthesized after damage
For models in which varying transferase dUTP nick-end labeling (TUNEL) signal intensity is observed, quantification by both the number of TUNEL-positive objects and the mean intensity of the staining is recommended
Due to the high activity of apoptotic endonucleases in the kidney, DNase I and endonuclease G (EndoG), the use of TUNEL for kidney research and diagnosis is very informative. It may become even more useful if TUNEL is applied beyond its initial use as a method to identify apoptosis
Summary
DNA is the only molecule in the cell that can be repaired, but it cannot be completely resynthesized after damage. Of the three main methods, TUNEL is the most sensitive, least time consuming, and most universally applicable It can be used in cultured cells and tissues and for all cell death mechanisms. DNase I is secreted by tubular epithelial cells, presumably to destroy viruses and bacteria in urine This function likely takes place in conjunction with proteinases, in particular, meprin, which is the main proteinase in urine [13]. High DNase I activity in kidneys makes kidney cells very sensitive to injury from toxic compounds and their metabolic products. This makes TUNEL the most appropriate and applicable method to measure injury to the kidney. The points raised in this review are broadly applicable for TUNEL use in organs other than the kidney
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