Apoptotic DNA fragmentation can be revealed in situ: an ultrastructural approach

A common pattern of apoptotic death is DNA cleavage, initially producing large fragments (50 kbp), followed by the production of nucleosomic/oligonucleosomic fragments. Nevertheless, apoptosis without DNA fragmentation, at least of the nucleosomic type, has been reported. To investigate the spatial relationship between DNA cleavage and chromatin condensation, we applied the TUNEL technique to the ultrastructural analysis of apoptotic cells. A modified method, utilizing a gold-conjugated antidigoxigenin antibody, was carried out on U937 versus Molt-4 cells, both exposed to UVB radiation or staurosporine treatment. Gold particle density in the different domains of apoptotic cells was evaluated by a four-way ANOVA test. Gold labelling was more strongly localised in condensed chromatin than in the diffuse chromatin. U937 cells, which evidenced in vitro oligonucleosomic fragmentation after both UVB and staurosporine treatments, revealed a significantly higher gold particle density, when compared with Molt-4, which did not show, on the other hand, oligonucleosomic cleavage even in the presence of < or = 50 kbp cleavage. Thus, a correlation between DNA fragment sizes and gold particle density appears. TUNEL applied to electron microscopy is an effective approach to study the relationship between apoptotic chromatin condensation and DNA cleavage. Both these events indeed appear in the apoptotic nucleus, but their reciprocal correlation is still greatly unknown. Microsc. Res. Tech. 2009. (c) 2009 Wiley-Liss, Inc.

Method of specific detection of apoptosis using formamide-induced DNA denaturation assay

We compared the reliability between apoptosis detection methods, namely, the terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end labeling (TUNEL) method and formamide-induced DNA denaturation assay using a monoclonal antibody (MAb) to single-stranded DNA (ssDNA) (formamide-MAb assay). Reaction targets in these methods are different: the TUNEL method recognizes free 3'-OH DNA ends, whereas the formamide-MAb assay detects ssDNA itself (25-30 bp). We found that the formamide-MAb assay immunohistochemically detected apoptotic cells, whereas the TUNEL method detected apoptotic cells as well as mitotic and necrotic cells. The TUNEL method recognized not only 3'-OH DNA ends cleaved by DNase during apoptosis but also constitutive physiological nicking that occurs in DNA duplication and histone posttranslational modifications during mitosis and random DNA breaks during necrotic execution. By electron microscopy, the mean labeling density (the number of 3'-OH DNA ends/nuclear area) obtained by the TUNEL method was determined to be consistently higher than that (the number of ssDNAs/nuclear area) obtained by the formamide-MAb assay. On the basis of these findings, we conclude that the formamide-MAb assay was more specific than the TUNEL method for the detection of apoptotic cells using electron microscopy; however, the labeling intensity of the formamide-MAb assay was slightly weaker than that of the TUNEL method.

Evaluation of cell death and proliferation in psoriatic epidermis

BACKGROUND

Previous studies of psoriatic epidermis using the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick-end labeling (TUNEL) method, a type of apoptotic detection method, showed that TUNEL-positive keratinocytes were abundantly distributed in all layers of the psoriatic epidermis, although psoriasis is a hyperproliferative disorder.

OBJECTIVE

We sought to clarify the nature of cell kinetics in a psoriatic epidermis on the basis of differences in the reactivities in TUNEL and formamide-induced DNA denaturation assay combined with the detection of denatured DNA with a monoclonal antibody (MAb) against single-stranded DNA (formamide-MAb assay) between the normal and psoriatic epidermides.

METHODS

The kinetics of keratinocytes was evaluated by the immunohistochemistry of Ki-67 for proliferation activity and by TUNEL, TUNEL combined with transmission electron microscopy (TUNEL/TEM), and formamide-MAb assay for apoptosis.

RESULTS

The number of Ki-67-positive cells in the psoriatic epidermis was significantly higher than that in the normal epidermis. In the normal epidermis, both TUNEL and formamide-MAb assay showed a similar distribution pattern, that is, both TUNEL and formamide-MAb assay-positive keratinocytes were present only in the upper granular layer. In the psoriatic epidermis, most keratinocytes were negative for the formamide-MAb assay, while TUNEL-positive cells were abundantly distributed in all layers of the psoriatic epidermis. TUNEL/TEM method clearly demonstrated that many immunogold particles that stain the sites of 3'-OH DNA ends were evenly distributed on the euchromatin in psoriatic keratinocyte nuclei, in contrast to their presence on the peripheral condensed chromatin in normal keratinocyte nuclei.

CONCLUSION

The increased TUNEL reactivity in psoriatic lesions is due to the increase in the number of DNA nicks resulting from active DNA replication but not due to DNA double-strand breaks produced during the apoptotic process, and the formamide-MAb assay is a reliable method for the detection of apoptosis, particularly in the epidermis.