Distribution of deoxyribonuclease I in rat tissues and its correlation to cellular turnover and apoptosis (programmed cell death).

The expression of deoxyribonuclease I (DNase I) in various rat tissues was screened by use of a cDNA-probe of rat parotid DNase I and monospecific polyclonal antibodies. High amounts of DNase I-specific mRNA were found in the parotid gland, kidney and small intestine. Homogenates of these organs also contain elevated levels of DNase I-specific DNA-degrading activity as verified by the zymogram technique and immunoblots. Affinity-purified polyclonal antibodies against rat parotid DNase I were employed in an immunohistochemical study of the cellular distribution of DNase I antigen in rat parotid gland, kidney, small intestine, and a number of stratified epithelia. In the parotid gland the DNase I antigenicity was found to be confined to the secretory cells. Within these cells the secretory granules exhibit the highest immunoreactivity. In contrast, within the small intestine and stratified epithelia we found a preferential localization and concentration of DNase I in cells prone to undergo apoptosis (programmed cell death), i.e., within the migrating enterocytes present at the villar tips and the keratinocytes above the basal cell layer. Within the kidney, the cells lining the convoluted distal tubules and collecting ducts exhibit strong DNase I immunoreactivity which was found to often localize perinuclearly. The cells exhibiting chromatin fragmentation were identified on paraffin-embedded sections by in situ end-labeling of free 3'-OH-ends of cleaved DNA using fluorescent dATP or dUTP and terminal transferase. It was found that only a small fraction of the DNase I positive cells showed signs of apoptotic chromatin degradation. Thus only a few enterocytes at the uppermost villar tips and very few keratinocytes underneath the keratinized layer were in situ end-labeled, i.e., exhibited a high concentration of fragmented DNA. This result is taken as evidence that these cells express DNase I in advance of their apoptotic death and furthermore that the actual apoptosis is a rapid process only detectable in a few cells. In contrast, no in situ end-labeled apoptotic nuclei were detected in rat kidney provided that care was taken to rapidly excise and fix this organ.