DISTRIBUTION OF DEOXYRIBONUCLEASE-I IN RAT-TISSUES AND ITS CORRELATION TO CELLULAR-TURNOVER AND APOPTOSIS (PROGRAMMED CELL-DEATH)

The expression of deoxyribonuclease I (DNase T) in various rat tissues was screened by use of a cDNA-probe of rat parotid DNase I and monosp ecific polyclonal antibodies. High amounts of DNase I-specific mRNA we re found in the parotid gland, kidney and small instestine. Homogenate s of these organs also contain elevated levels of DNase I-specific DNA -degrading activity as verified by the zymogram technique and immunobl ots. Affinity-purified polyclonal antibodies against rat parotid DNase I were employed in an immunohistochemical study of the cellular distr ibution of DNase I antigen in rat parotid gland, kidney, small intesti ne, and a number of stratified epithelia. In the parotid gland the DNa se I antigenicity was found to be confined to the secretory cells. Wit hin these cells the secretory granules exhibit the highest immunoreact ivity. In contrast, within the small intestine and stratified epitheli a we found a preferential localization and concentration of DNase I in cells prone to undergo apoptosis (programmed cell death), i.e., withi n the migrating enterocytes present at the villar tips and the keratin ocytes above the basal cell layer. Within the kidney, the cells lining the convoluted distal tubules and collecting ducts exhibit strong DNa se I immunoreactivity which was found to often localize perinuclearly. The cells exhibiting chromatin fragmentation were identified on paraf fin-embedded sections by in situ end-labeling of free 3'-OH-ends of cl eaved DNA using fluorescent dATP or dUTP and terminal transferase. It was found that only a small fraction of the DNase I positive cells sho wed signs of apoptotic chromatin degradation. Thus only a few enterocy tes at the uppermost villar tips and very few keratinocytes underneath the keratinized layer were in situ end-labeled, i.e., exhibited a hig h concentration of fragmented DNA. This result is taken as evidence th at these cells express DNase I in advance of their apoptotic death and furthermore that the actual apoptosis is a rapid process only detecta ble in a few cells. In contrast, no in situ end-labeled apoptotic nucl ei were detected in rat kidney provided that care was taken to rapidly excise and flu this organ.