DNA denaturation in situ. Effect of divalent cations and alcohols

Discontinuous fragmentation of nuclear DNA during apoptosis revealed by discrete “sub-G1” peaks on DNA content histograms

BACKGROUND

Internucleosomal DNA fragmentation is one of the hallmarks of apoptosis. Because the low molecular weight DNA fragments are extracted during cell staining in aqueous solutions, apoptotic cells can be identified on DNA content frequency histograms as cells with fractional ("sub-G(1)") DNA content. The aim of the present study was to explore whether in situ DNA fragmentation during apoptosis is discontinuous or progresses incessantly and if it is discontinuous, to define the resistant to cleavage fraction of DNA that remains stainable with the fluorochrome.

MATERIALS AND METHODS

The model of activation-induced apoptosis of human lymphocytes was chosen as it provides uniform cell population with identical DNA content (DI = 1.00) that undergo apoptosis. Their apoptosis was induced by multivalent mitogen phytohemagglutinin (PHA) in the absence and presence of geldanamycin (GA), the benzoquinone ansamycin antibiotic which binds to Hsp90 (Heat Shock Protein 90) and alters its function. The cells were stained with acridine orange, the metachromatic fluorochrome that differentially stains cellular DNA and RNA.

RESULTS

A sharp, discrete peak representing the subpopulation of "sub-G(1)" cells with highly reproducible DI = 0.42 +/- 0.02 (CV = 5.5 +/- 1.2) was observed on DNA content histograms of lymphocytes whose apoptosis was induced by PHA alone. Two distinct peaks, one representing cell subpopulations with DI = 0.42 (as above) and another, with DI = 0.79 +/- 0.04 (CV = 5.8 +/- 0.4), respectively, were seen in apoptotic cells from cultures stimulated with PHA in the presence of GA. The frequency of cells represented by the sub-G(1) peaks varied depending on time of induction of apoptosis and GA concentration.

CONCLUSIONS

Apoptosis-induced DNA fragmentation is discontinuous; approximately 42% of DNA is relatively stable and remains within the cell. The data suggest that the stable DNA is associated with nuclear matrix while the degradable fraction represents DNA in loop domains. A transient DNA stabilization is apparent in the presence of GA as evidenced by the presence of cell subpopulations with 79% of DNA retained in the cell. The observed discontinuity of DNA fragmentation appears to reflect sequential involvement of different nucleases and may also be modulated by chromatin structure.

Interplay between folding/unfolding and helix/coil transitions in giant DNA

It has been well established that double-stranded DNA undergoes a melting, or helix/coil, transition into a single-stranded coil state with an increase in temperature. On the other hand, it has recently been found that, at a fixed temperature, long DNA, larger than several kilobase pairs, exhibits a discrete transition, or switching, between elongated and folded states, preserving its double-stranded structure, with the addition of various condensation agents, such as alcohol, hydrophilic polymer, multivalent cation, and cationic surfactant. In the present study, we examined the interplay between the folding/unfolding transition and the helix/coil transition in individual giant DNA molecules, by observing the conformation of single molecular chains with fluorescence microscopy. The results indicate that the helix-to-coil transition tightly cooperates with the unfolding transition in DNA.

Ethanol-enhanced permeation of phosphatidylcholine/ phosphatidylethanolamine mixed liposomal membranes due to ethanol-induced lateral phase separation

Effects of ethanol on permeability of large unilamellar vesicles (ca. 160 nm in diameter), composed of dipalmitoyl phosphatidylcholine/dilauroyl phosphatidylethanolamine (DLPE) mixture, were studied by monitoring leakage of the fluorescent dye, calcein, entrapped in the inner aqueous phase of the vesicles. In the presence of 2.1 M ethanol, permeabilities of membranes in various phases were G (bilayer gel) phase > L (bilayer liquid-crystalline) phase with a high mole fraction of DLPE and (I (ethanol-induced interdigitated gel phase) + G) phase > (I + L) at 20 mol % DLPE. Arrhenius plots of the leakage rate constants demonstrated that the permeability was greater with 2.1 M ethanol than without ethanol, especially in the temperature above 33 degrees C, suggesting that the presence of ethanol can induce lateral phase separation of liposomal membranes and cause them to have a high permeability even if they are stable and have low permeability in its absence.

Decreased stability of DNA in cells treated with alkylating agents

A modified highly sensitive procedure for the evaluation of DNA damage in individual cells treated with alkylating agents is reported. The new methodology is based on the amplification of single-strandedness in alkylated DNA by heating in the presence of Mg2+. Human ovarian carcinoma cells A2780 were treated with nitrogen mustard (HN2), fixed in methanol, and stained with monoclonal antibody (MOAB) F7-26 generated against HN2-treated DNA. Binding of MOAB was measured by flow cytometry with indirect immunofluorescence. The maximal difference in fluorescence between untreated and HN2-treated cells was observed after heating at 100 degrees C for 5 min in PBS containing 1.25 mM MgCl2. Higher concentrations of MgCl2 inhibited MOAB binding to HN2-treated cells and heating at lower concentrations induced binding to control cells. Intensive binding of MOAB to control and drug-treated cells was observed after heating in Tris buffer supplemented with MgCl2. Thus, the presence of phosphates and MgCl2 during heating was necessary for the detection of HN2-induced changes in DNA stability. Fluorescence of HN2-treated cells decreased to background levels after treatment with single-strand-specific S1 nuclease. MOAB F7-26 interacted with single-stranded regions in DNA and did not bind to dsDNA or other cellular antigens. Specific reactivity of MOAB F7-26 with deoxycytidine was established by avidin-biotin ELISA. Single-stranded conformation was necessary for the binding of MOAB to deoxycytidine on the DNA molecule. It is suggested that alkylation of guanines decreased the stability of the DNA molecule and increased the access of MOAB F7-26 to deoxycytidines on the opposite DNA strand.

Detection of cells resistant to alkylating agents by flow cytometric analysis of DNA damage

DNA damage was measured by flow cytometric analysis of cells sensitive and resistant to alkylating agents. Human ovarian carcinoma cell line A2780 and a subline which is 7 times more resistant to L-phenylalanine mustard (L-PAM) were treated with the drug, fixed, and stained with monoclonal antibody (MOAB) F7-26 which detects single-stranded regions in alkylated DNA. Mean fluorescent intensity was measured on a flow cytometer. Cells were heated before staining to amplify single-strandedness in alkylated DNA. Significantly larger amount of MOAB was bound to DNA in sensitive than in resistant cells. Fluorescence increased by 80 channels per micrograms L-PAM insensitive cells and only by 17 channels in resistant cells. Sensitive and resistant cells were treated with L-PAM, mixed in different proportions, and stained with MOAB. Populations of sensitive and resistant cells were clearly separated on fluorescence histograms by more than a decade difference in fluorescence intensity. Presence of 2-5% resistant cells was detected among sensitive cells as a separate cell subset. We conclude that staining with MOAB F7-26 can be used as an indicator of cell sensitivity or resistance to alkylating agents. Detection of minor subsets of resistant cells in heterogeneous populations by FCM analysis may be useful for monitoring emerging drug resistance.

Ultrastructural localization of calcium in post-mortem bovine muscle: a cytochemical and X-ray microanalytical study

Calcium localization was demonstrated in bovine longissimus muscle using the antimonate precipitation technique in combination with electron probe X-ray microanalysis. Samples were taken each hour during the first 24 h post-mortem, and then after a storage period of 8 and 15 days. For all sampling times analysed, heavy precipitates were seen in dense parts of nuclei and on N-lines of myofibrils. Up to 18-20 h post-mortem, deposits were observed in sarcoplasmic reticulum at the level of triads. In comparison with the earlier post-mortem samples, myoplasmic precipitates were strongly increased at 4 h post-mortem, and just before rigor onset, at 19 h where intermyofibrillar spaces were completely blackened and triads were no more visible. These localizations of precipitates were still observed up to 15 days post-mortem. At these storage times, myofibril disruptions were seen at the level of N-lines. Wavelength-dispersive and energy-dispersive spectrometric analyses indicated that significant amounts of calcium occurred in the dense precipitates observed.

Mechanism of lysis of Escherichia coli by ethanol and other chaotropic agents

Ethanol has been shown to inhibit the assembly of cross-linked peptidoglycan and to induce cell lysis in Escherichia coli. These effects of ethanol appear to result from the weakening of hydrophobic interactions by ethanol rather than from the intercalation of ethanol into membranes. Other chaotropic agents also inhibited cross-linking and induced lysis. The potency of chaotropic anions with regard to this effect followed the expected chaotropic series. Antichaotropic agents, which strengthened hydrophobic interactions, antagonized ethanol-induced lysis. The weakening of hydrophobic interactions by ethanol is proposed as a general mechanism by which ethanol and other chaotropic agents could affect membrane-associated enzyme activities.

Cytochemical and ultrastructural studies on the synaptonemal complex of rat spermatocytes

The effects of several dehydration treatments on the synaptonemal complex (SC), histone solubility in 2.0 M NaCl, and histone-DNA interaction in unfixed rat spermatocytes were evaluated. Freeze substitution with ethanol or dehydration with polyethylene glygol resulted in loss of the SC, preservation of histone solubility and DNA-histone salt linkages. Dehydration with ethylene gylcol or hexylene glycol resulted in preservation of SC with a clear delineation of attachment of the chromatin fibrils to the lateral elements, but a loss of histone solubility and histone-DNA linkages. Dehydration to a fifty percent concentration with glycerol with completion of dehydration with ethylene glycol had the same effect but also resulted in an even distribution of chromatin fibrils. Dehydration with glycerol alone resulted in clumping of chromatin and loss of SC structure, histone solubility and histone-DNA linkages. Partial dehydration to a fifty percent concentration with these three solvents followed by freeze substitution with ethanol resulted in the loss of SC structure and histone solubility but the preservation of histone-DNA linkages. It is likely that these nonaqueous solvents affected the histone hydrophobic groups and thereby altered histone conformation and interactions. These alterations, depending on the treatment used, resulted in the loss or preservation of SC, histone solubility and histone-DNA interactions thereby indicating that the hydrophobic interactions of the histones are crucial for the preservation of these feature of meiotic chromosomes. These results also demonstrate that neither does the preservation of the histone-DNA salt linkages suffice for the preservation of the SC nor does their disruption necessarily result in its loss. The lysine-rich histones, particularly that one unique to meiotic cells, may through their interactions play a crucial role in SC structure.