Altered sensitivity of DNA in situ to denaturation in apoptotic cells

Single-cell quantification of molecules and rates using open-source microscope-based cytometry

Microscope-based cytometry provides a powerful means to study cells in high throughput. Here we present a set of refined methods for making sensitive measurements of large numbers of individual Saccharomyces cerevisiae cells over time. The set consists of relatively simple 'wet' methods, microscope procedures, open-source software tools and statistical routines. This combination is very sensitive, allowing detection and measurement of fewer than 350 fluorescent protein molecules per living yeast cell. These methods enabled new protocols, including 'snapshot' protocols to calculate rates of maturation and degradation of molecular species, including a GFP derivative and a native mRNA, in unperturbed, exponentially growing yeast cells. Owing to their sensitivity, accuracy and ability to track changes in individual cells over time, these microscope methods may complement flow-cytometric measurements for studies of the quantitative physiology of cellular systems.

Major changes in chromatin condensation suggest the presence of an apoptotic pathway in plant cells

A large decrease in fluorescence intensity of propidium iodide (PI)-stained nuclei is observed during senescence of plant cells. The phenomenon reflects a decrease in accessibility of DNA to this fluorochrome and is a consequence of chromatin condensation. This decrease is substantially greater than usually found in animal nuclei whose chromatin undergoes condensation, e.g., during differentiation or quiescence. Chromatin condensation was confirmed by analyses of (i) DNA accessibility to DNase I, (ii) histone disassociation induced by HCl, (iii) saturation of binding sites by the PI fluorochrome (iv), and (v) visual inspection by fluorescence and confocal microscopy. The extent of changes revealed by these assays was used to map progressive changes in chromatin condensation which allowed us to identify different stages in an apoptosis-like pathway in plants. The initial step of chromatin condensation which occurred prior to endonucleolytic DNA degradation was detected by fluorescence and confocal microscopy and confirmed by a variety of assays employing flow cytometry. The initial chromatin condensation appears to be a reversible step in the early stage of apoptosis. The loss of reversibility of chromatin condensation observed subsequently may be a critical point in the cascade of apoptotic events, leading to further irreversible changes during apoptosis in plants.

Differential staining of apoptotic nuclei in living cells: application to macronuclear elimination in Tetrahymena

Acridine orange (AO) has been used as a vital fluorescent stain to identify apoptotic cells in Drosophila, but little is known about what structures are stained. We explored the specificity of AO staining while studying nuclear apoptosis in Tetrahymena. Using AO alone or together with the vital nuclear stain Hoechst 33342 (HO), we find that lysosomes are generally clustered around the degenerating nucleus and that such nuclei are stained an orange-red color, like lysosomes. Significantly, the combined dyes, more so than with AO alone, distinguish between apoptotic and normal (or necrotic) nuclei by a clear color difference. Moreover, these dyes differentially stain apoptotic and normal nuclei in avian chondrocytes. The differential staining results are nullified in fixed cells or in cytoskeletal preparations treated with RNAse. Similarly, lysosomotrophic agents eliminate the differential staining. Our results are consistent with acidification of the apoptotic nucleus, possibly by fusion with lysosomes. However, even under basic conditions, the macronucleus condenses and is eliminated, suggesting that, if the nucleus is becoming acidified, acidification by itself is not essential for nuclear elimination. The differential staining procedure may provide a useful method for specifically identifying apoptotic cells and separating them for further analysis.

Flow cytometric determination of apoptosis in heterogeneous cell populations

We have shown that apoptosis and surface antigen expression can be detected simultaneously using multicolor flow cytometry. Apoptosis was measured with an in situ assay that makes use of the ability of the enzyme terminal deoxyribonucleotidyl transferase to catalyze the addition of biotinylated nucleotides to the free 3'-OH groups produced during endonucleolytic cleavage of DNA. The incorporation of the biotinylated nucleotides was quantified using fluorochrome-coupled streptavidin. Immunofluorescence was used to identify the phenotype of apoptotic cells using three color flow cytometry. In model systems, apoptosis was detected in human PBMNC treated with the DNA topoisomerase I inhibitor CAM and in murine thymocytes treated with DEX. The simultaneous application of this method for detecting apoptosis and immunofluorescence offers several advantages. (1) Apoptotic and necrotic cells can be distinguished. (2) The phenotype of the cells undergoing apoptosis can be determined in heterogeneous systems. Thus, we could show that T cells are relatively resistant to CAM-induced apoptosis compared to other peripheral blood mononuclear cells and that both double negative and single positive thymocytes are resistant to steroid-induced apoptosis. (3) The flow cytometric TdT assay to detect apoptosis-associated DNA degradation method is extremely sensitive compared with gel electrophoresis. As few as 2-5 x 10(3) cells give an adequate signal and it is possible to detect DNA strand breaks even when only a small proportion of the cells are undergoing apoptosis. Neither the sensitivity nor specificity of this method can be matched by any electrophoretic method of detecting apoptosis.

Flow cytometric detection of apoptosis: comparison of the assays of in situ DNA degradation and chromatin changes

The aim of this study was to compare three methods of detection of apoptotic cells: (1) the method based on elution of low molecular weight DNA from the ethanol fixed cells followed by cell staining with DAPI (diamidino-2-phenylindole) or propidium iodide as the DNA fluorochromes, (2) the method of in situ labeling of DNA strand breaks with biotinylated dUTP, utilizing exogenous terminal deoxyribonucleotide transferase, and (3) the method of analysis of DNA denaturation in situ using acridine orange to differentially stain denatured and double-stranded DNA sections following cell exposure to 0.1 M HCl. Cells of the human promyelocytic HL-60 line, treated in vitro with the DNA topoisomerase I inhibitor camptothecin, which selectively triggers apoptosis of S-phase cells, were chosen as a model. The method based on analysis of changes in DNA denaturability was the most sensitive in terms of detection of the earliest changes in chromatin of cells undergoing apoptosis; the increased sensitivity of DNA to denaturation in S-phase cells was measured as early as 100 min after addition of camptothecin. DNA cleavage, assayed either by the univariate measurement of DNA content following extraction of low molecular weight DNA, or by labeling DNA strand breaks with biotinylated dUTP, was detected in S-phase cells after 120 min incubation with camptothecin. The percentage of apoptotic cells at the late stage of apoptosis, the kinetics of cell transition to apoptosis, and kinetics of the loss of S phase cells were all essentially similar when measured by any method.(ABSTRACT TRUNCATED AT 250 WORDS)