Flow cytometric discrimination of mitotic nuclei by right-angle light scatter

Metaphase Cells Enrichment for Efficient Use in the Dicentric Chromosome Assay

The dicentric chromosome assay (DCA), is considered the 'gold standard' for radiation biodosimetry. Yet, DCA, as currently implemented, may be impractical for emergency response applications, especially when time is of the essence, owing to its labor-intensive and time-consuming nature. The growth of a primary lymphocyte culture for 48 h in vitro is required for DCA, and manual scoring of dicentric chromosomes (DCs) requires an additional 24-48 h, resulting in an overall processing time of 72-96 h for dose estimation. In order to improve this timing. we introduce a protocol that will detect the metaphase cells in a population of cells, and then will harvest only those metaphase cells. Our metaphase enrichment approach is based on fixed human lymphocytes incubated with monoclonal, anti-phosphorylated H3 histone (ser 10). Antibodies against this histone have been shown to be specific for mitotic cells. Colcemid is used to arrest the mitotic cells in metaphase. Following that, a flow-cytometric sorting apparatus isolates the mitotic fraction from a large population of cells, in a few minutes. These mitotic cells are then spread onto a slide and treated with our C-Banding procedure [Gonen et al. 2022], to visualize the centromeres with DAPI. This reduces the chemical processing time to ~2 h. This reduces the time required for the DCA and makes it practical for a much wider set of applications, such as emergency response following exposure of a large population to ionizing radiation.

Detection of Silver and TiO2 Nanoparticles in Cells by Flow Cytometry

Evaluation of the potential hazard of man-made nanomaterials has been hampered by a limited ability to observe and measure nanoparticles in cells. A FACSCalibur™ flow cytometer and a Stratedigm S-1000 flow cytometer were used to measure changes in light scatter from cells after incubation with either silver nanoparticles (AgNP) or TiO₂ nanoparticles. Within the range of between 0.1 μg/mL and 30 μg/mL the nanoparticles caused a proportional increase of the side scatter and decrease of the forward scatter intensity signals. At the lowest concentrations of TiO₂ (ranging between 0.1 μg/mL and 0.3 μg/mL), the flow cytometer can detect as few as 5-10 nanoparticles per cell. The influence of nanoparticles on the cell cycle was detected by nonionic detergent lysis of nanoparticle incubated cells that were stained with DAPI or propidium iodide (PI). Viability of nanoparticle treated cells was determined by PI exclusion. Surface plasmonic resonance (SPR) was detected primarily in the far-red fluorescence detection channels after excitation with a 488 nm laser.Our results suggest that the uptake of nanoparticles within cells can be monitored using flow cytometry. This uptake of nanoparticle data was confirmed by viewing the nanoparticles in the cells using dark-field microscopy. The flow cytometry detection of nanoparticles approach may help fill a critical need to assess the relationship between nanoparticle dose and cellular toxicity. Such experiments using nanoparticles could potentially be performed quickly and easily using the flow cytometer to measure both nanoparticle uptake and cellular health.

Biophysical comparison of four silver nanoparticles coatings using microscopy, hyperspectral imaging and flow cytometry

This study compared the relative cellular uptake of 80 nm silver nanoparticles (AgNP) with four different coatings including: branched polyethyleneimine (bPEI), citrate (CIT), polyvinylpyrrolidone (PVP), and polyethylene glycol (PEG). A gold nanoparticle PVP was also compared to the silver nanoparticles. Biophysical parameters of cellular uptake and effects included flow cytometry side scatter (SSC) intensity, nuclear light scatter, cell cycle distributions, surface plasmonic resonance (SPR), fluorescence microscopy of mitochondrial gross structure, and darkfield hyperspectral imaging. The AgNP-bPEI were positively charged and entered cells at a higher rate than the negatively or neutrally charged particles. The AgNP-bPEI were toxic to the cells at lower doses than the other coatings which resulted in mitochondria being transformed from a normal string-like appearance to small round beaded structures. Hyperspectral imaging showed that AgNP-bPEI and AgNP-CIT agglomerated in the cells and on the slides, which was evident by longer spectral wavelengths of scattered light compared to AgNP-PEG and AgNP-PVP particles. In unfixed cells, AgNP-CIT and AgNP-bPEI had higher SPR than either AgNP-PEG or AgNP-PVP particles, presumably due to greater intracellular agglomeration. After 24 hr. incubation with AgNP-bPEI, there was a dose-dependent decrease in the G1 phase and an increase in the G2/M and S phases of the cell cycle suggestive of cell cycle inhibition. The nuclei of all the AgNP treated cells showed a dose-dependent increase in nanoparticles following non-ionic detergent treatment in which the nuclei retained extra-nuclear AgNP, suggesting that nanoparticles were attached to the nuclei or cytoplasm and not removed by detergent lysis. In summary, positively charged AgNP-bPEI increased particle cellular uptake. Particles agglomerated in the peri-nuclear region, increased mitochondrial toxicity, disturbed the cell cycle, and caused abnormal adherence of extranuclear material to the nucleus after detergent lysis of cells. These results illustrate the importance of nanoparticle surface coatings and charge in determining potentially toxic cellular interactions.

Ploidy and Proliferation Evaluated by Flow Cytometry. An Overview of Techniques and Impact in Oncology

Flow cytometric methods for the assessment of nuclear and chromosomal DNA content and of cell proliferation (including methods based on pulse-chase of bromodeoxyuridine and on monoclonal antibodies against nuclear oncoproteins and proliferation-associated antigens) are illustrated by examples and analyzed critically. The impact of most of these techniques for the study of human solid tumors, with exception of nuclear DNA content evaluation, appears still limited. In particular, new studies of cell lines and clinical material from human tumors using new proliferation markers and multiparameter flow cytometry are necessary to solve a considerable number of methodologic and scientific problems.

Evaluation of two distinct methods to quantify the uptake of crocidolite fibers by mesothelial cells

Exposure to asbestos fibers increases the risk of mesothelioma in humans. One hypothetical carcinogenic mechanism is that asbestos fibers may directly induce mutations in mesothelial cells. Although the uptake of asbestos fibers by mesothelial cells is recognized, methods for the quantification of the uptake have not been well established. In the present study, we evaluated two distinct methods, using crocidolite fibers and MeT5A mesothelial cells. One method is histological evaluation using the cell-block technique, which allows for the direct cross-sectional observation of cells and fibers. We found the bright field observation with ×1000 magnification (oil-immersion) of the sample with Kernechtrot staining was most suitable for this purpose. The other method is flow cytometric analysis, which permits the evaluation of a much larger number of cells. We observed that the side scatter (SSC) increased with the intracellular fibers, and that the “mean SSC ratio (treated/control)” was useful for quantification. We could collect the cells with abundant internalized crocidolite fibers by sorting. Results of the two methodologies were correlated well in the experiments. The quantities of internalized fibers increased with incubation time and loaded dosage, but they were inversely associated with cellular density in culture.

Transcellular neuroligin-2 interactions enhance insulin secretion and are integral to pancreatic β cell function

Normal glucose-stimulated insulin secretion is dependent on interactions between neighboring β cells. Elucidation of the reasons why this cell-to-cell contact is essential will probably yield critical insights into β cell maturation and function. In the central nervous system, transcellular protein interactions (i.e. interactions between proteins on the surfaces of different cells) involving neuroligins are key mediators of synaptic functional development. We previously demonstrated that β cells express neuroligin-2 and that insulin secretion is affected by changes in neuroligin-2 expression. Here we show that the effect of neuroligin-2 on insulin secretion is mediated by transcellular interactions. Neuroligin-2 binds with nanomolar affinity to a partner on the β cell surface and contributes to the increased insulin secretion brought about by β cell-to-β cell contact. It does so in a manner seemingly independent of interactions with neurexin, a known binding partner. As in the synapse, transcellular neuroligin-2 interactions enhance the functioning of the submembrane exocytic machinery. Also, as in the synapse, neuroligin-2 clustering is important. Neuroligin-2 in soluble form, rather than presented on a cell surface, decreases insulin secretion by rat islets and MIN-6 cells, most likely by interfering with endogenous neuroligin interactions. Prolonged contact with neuroligin-2-expressing cells increases INS-1 β cell proliferation and insulin content. These results extend the known parallels between the synaptic and β cell secretory machineries to extracellular interactions. Neuroligin-2 interactions are one of the few transcellular protein interactions thus far identified that directly enhance insulin secretion. Together, these results indicate a significant role for transcellular neuroligin-2 interactions in the establishment of β cell function.

Detection of TiO2 nanoparticles in cells by flow cytometry

Evaluation of the potential hazard of man-made nanomaterials has been hampered by a limited ability to observe and measure nanoparticles in cells. A FACSCalibur™ flow cytometer was used to measure changes in light scatter from cells after incubation with TiO(2) nanoparticle. Both the side scatter and forward scatter changed substantially in response to the TiO(2). Between 0.1 and 30 μg/mL TiO(2), the side scatter increased sequentially while the forward scatter decreased, presumably due to substantial light reflection by the TiO(2) particles. At the lowest concentrations of TiO(2) (0.1-0.3 μg/mL), the flow cytometer apparently could detect as few as 5-10 nanoparticles per cell as shown using dark field microscopy. The influence of nanoparticles on the cell cycle was detected by nonionic detergent lysis of nanoparticle-incubated cells. Viability of nanoparticle-treated cells was determined by PI exclusion.These data suggest that the uptake of nanoparticles within cells can be monitored using flow cytometry and confirmed by dark field microscopy. This approach may help fill a critical need to assess the relationship between nanoparticle dose and cellular toxicity. Such experiments could potentially be performed quickly and easily using the flow cytometer to measure both nanoparticle uptake and cellular health.

Detection of TiO2 nanoparticles in cells by flow cytometry

Evaluation of the potential hazard of man-made nanomaterials has been hampered by a limited ability to observe and measure nanoparticles in cells. In this study, different concentrations of TiO(2) nanoparticles were suspended in cell culture medium. The suspension was then sonicated and characterized by dynamic light scattering and microscopy. Cultured human-derived retinal pigment epithelial cells (ARPE-19) were incubated with TiO(2) nanoparticles at 0, 0.1, 0.3, 1, 3, 10, and 30 microg/ml for 24 hours. Cellular reactions to nanoparticles were evaluated using flow cytometry and dark field microscopy. A FACSCalibur flow cytometer was used to measure changes in light scatter after nanoparticle incubation. Both the side scatter and forward scatter changed substantially in response to the TiO(2). From 0.1 to 30 microg/ml TiO(2), the side scatter increased sequentially while the forward scatter decreased, presumably due to substantial light reflection by the TiO(2) particles. Based on the parameters of morphology and the calcein-AM/propidium iodide viability assay, TiO(2) concentrations below 30 microg/ml TiO(2) caused minimal cytotoxicity. Microscopic analysis was done on the same cells using an E-800 Nikon microscope containing a xenon light source and special dark field objectives. At the lowest concentrations of TiO(2) (0.1-0.3 microg/ml), the flow cytometer could detect as few as 5-10 nanoparticles per cell due to intense light scattering by TiO(2). Rings of concentrated nanoparticles were observed around the nuclei in the vicinity of the endoplasmic reticulum at higher concentrations. These data suggest that the uptake of nanoparticles within cells can be monitored with flow cytometry and confirmed by dark field microscopy. This approach may help fulfill a critical need for the scientific community to assess the relationship between nanoparticle dose and cellular toxicity Such experiments could potentially be performed more quickly and easily using the flow cytometer to measure both nanoparticle uptake and cellular health.

Ki-67 staining for determination of rhesus macaque T cell proliferative responses ex vivo

The capacity for robust proliferation upon re-infection is a hallmark of adaptive immunity and the basis of vaccination. A widely used animal model for the study of human disease is the rhesus macaque (RM), where capacity for proliferation can be assessed ex vivo using carboxyfluorescein succinimidyl ester (CFSE)-based dilution assays. However, we show over the course of the standard ex vivo proliferation assay that CFSE-labeling at commonly used dye concentrations induces significant cell death, but that this phenomenon is dose-dependent. Here, we describe an alternative semiquantitative method for estimating T cell proliferative responses that avoids the putative biases associated with chemical modification. RM peripheral blood mononuclear cells were stimulated ex vivo with cognate peptides for 5 days, immunostained for intracellular Ki-67, and then analyzed by flow cytometry. We describe a gating strategy using Ki-67 and side light scatter, also a marker of blastogenesis, which correlates strongly with data from CFSE dilution. We show that this method is a valid tool for measuring RM antigen-specific cellular proliferation ex vivo and can be used as an alternative to CFSE dilution assays.

Flow cytometry of mitotic cells

We have developed a procedure using flow cytometric measurement of a mitosis-specific antigen that may be used to count mitotic cells and sort them from non-mitotic cells. The procedure may also be used in conjunction with measurement of cellular DNA content and of bromodeoxyuridine incorporation into cellular DNA to assign cells to the G1/G0, S, G2, or M phase of the cell cycle.

Nucleoside-mediated mitigation of 5-fluorouracil-induced toxicity in synchronized murine erythroleukemic cells

5-Fluorouracil (5-FU) is a chemotherapeutic agent known to retard embryonic growth and induce cleft palate and limb deformities. The predominant mechanism underlying its toxic action is thought to be inhibition of thymidylate synthetase (TS), and hence thymidine triphosphate (dTTP) synthesis, resulting in alteration of the balance of deoxynucleotide (dNTP) pools and disruption of DNA synthesis. Indeed, previously we demonstrated retarded cell-cycle progression concurrent with a 60% decrease in TS activity in rat whole embryos following maternal exposure to 40 mg/kg 5-FU on Gestational Day 14 and in the murine erythroleukemic cell (MELC) suspension culture following exposure to 5-25 microM 5-FU for 2 hr. In the study described herein, we used high-performance liquid chromatography (HPLC) to demonstrate in both of these model systems that 5-FU exposure results in similar patterns of dNTP perturbations: a prolonged decrease in dTTP and dGTP levels and an increase in dCTP and dATP. In addition, we used centrifugal elutriation to synchronize MELC in the phases of the cell cycle (G0/G1 and early S) most sensitive to 5-FU to investigate the ability of nucleoside supplementation to mitigate 5-FU-induced toxicity. Our data indicate that following a 2-hr exposure to 5-25 microM 5-FU, supplementation with 1-10 microM thymidine (TdR) for 24 hr partially reverses 5-FU-induced toxicity as evidenced by increased cellular proliferation and cell-cycle progression and amelioration of 5-FU-induced perturbations of protein synthesis and cellular membrane permeability compared to unsupplemented 5-FU-exposed cells. However, TdR concentrations >/=100 microM inhibited growth or were cytotoxic. In comparison, supplementation with 10 microM-10 mM of deoxycytidine (CdR) was not toxic, but effected a dose-dependent recovery from 5-FU-induced toxicity. At 1-100 microM, neither deoxyadenosine nor deoxyguanosine supplementation reduced 5-FU-induced toxicity; at higher concentrations, both purine nucleotides inhibited cell growth. Although these results support the hypothesis that 5-FU disrupts the MELC cell cycle by depleting dTTP (a perturbation that is reversible by TdR supplementation), they also indicate that CdR supplementation offers an additional recovery pathway.

Increase in total protein following infection of CV-1 cells with SV40 virus as assayed by flow cytometry

The changes in cell size and total protein were determined for G1-arrested, contact-inhibited CV-1 cells infected with Simian virus 40 (SV40). The assays used were the Biorad total protein assays (Bradford and DC protein assays) on a standard number of cells, total protein as assayed by fluorescein isothiocyanate (FITC) and SR101 by flow cytometry, orthoganol (90 degrees) light scatter by flow cytometry, and direct microscopic measurement with an ocular micrometer. Uninfected CV-1 cells and two cell lines with variations in DNA content (diploid vs. tetraploid) were used as controls for the studies presented. The results demonstrated a 40-60% increase in total protein at 32 to 42 h postinfection. These increases were similar to values obtained due to cellular changes resulting from viral replication and cell death.

A flow cytometric study of cell death: failure of some models to correlate with morphological assessment

The balance between cell death and cell proliferation is a significant factor in the growth kinetics of normal and neoplastic tissues. Distinction between the two major forms of cell death, necrosis and apoptosis, is now recognized as important in understanding mechanisms regulating cell survival. A recent approach in the study of apoptosis has been the use of flow cytometry, with some reports indicating that, when stained with propidium iodide (PI), the DNA of apoptotic cells has decreased fluorescence compared with that of viable cells. In this study, we investigated a flow cytometric procedure which used the simultaneous analysis of DNA content and 90 degrees light scatter (90LS). Significant differences in the PI staining pattern and a shift in 90LS were observed when apoptotic death occurred at different stages of the cell cycle. Importantly, such differences only allowed accurate quantification of apoptosis when it occurred in G1. While necrosis could be distinguished from apoptosis when examined during its early stages, a similar staining pattern to that found with apoptosis was observed when necrosis was examined during its latter stages. The results indicate that the measurement of DNA staining cannot be exclusively relied upon to detect apoptosis occurring in all models. However it is useful in the investigation of this process when the death occurs in G1, in that the method offers a rapid means for quantification.

Discrimination of bromodeoxyuridine labelled and unlabelled mitotic cells in flow cytometric bromodeoxyuridine/DNA analysis

Bromodeoxyuridine (BrdUrd) labelled and unlabelled mitotic cells, respectively, can be discriminated from interphase cells using a new method, based on immunocytochemical staining of BrdUrd and flow cytometric four-parameter analysis of DNA content, BrdUrd incorporation, and forward and orthogonal light scatter. The method was optimized using the human leukemia cell lines HL-60 and K-562. Samples of 10(5) ethanol-fixed cells were treated with pepsin/HCl and stained as a nuclear suspension with anti-BrdUrd antibody, FITC-conjugated secondary antibody, and propidium iodide. Labelled mitoses could be discriminated from unlabelled mitoses, and from labelled and unlabelled G2 cells, by their intermediate log FITC fluorescence intensity. In addition, mitoses and G2 nuclei differed in forward and orthogonal light scattering, but had equal intensity of propidium iodide fluorescence. This method for discrimination of labelled mitoses was also tested on cultured normal adult human keratinocytes labelled with iododeoxyuridine (IdUrd). In keratinocytes, where the cell structure was preserved after pepsin/HCl, IdUrd labelled mitotic cells were similarly discriminated in the log FITC/propidium iodide fluorescence distribution. This interpretation was supported by experiments using mitotic arrest, fluorescence activated cell sorting and microscopy, and comparison with an alternative flow cytometric method for discrimination of mitoses.

Quantitative analysis of mitotic and early-G1 cells using monoclonal antibodies against the AF-2 protein

We have recently described a novel protein (AF-2), conserved between fission yeast and man, and we have shown by flow cytometry (FCM) that AF-2 is highly accessible to specific monoclonal antibodies (MoAbs) in mitotic and postmitotic early-G1 phase cells. The aim of the present study was to optimize the FCM methodology using MoAbs against AF-2 and to show that the evaluation of the mitotic cells, using different cell lines, was quantitative and reproducible. We found that a method based on fixation with ethanol, instead of formalin, resulted in improved DNA histogram coefficients of variation and implemented separation of early-G1 cells from late-G1 cells. In addition, by eliminating several cell permeabilization and protein salt extraction steps, the method became straightforward, conserved a clear-cut separation of the green fluorescence of M- with respect to G2-phase cells, and did not significantly affect cellular integrity. The coefficient of correlation among the mitotic index values evaluated by this FCM method using MoAbs against AF-2 and by microscopic visual counting was R = 0.94. When the FCM/AF-2 method was tested against an independent FCM method, which allows clear separation of M- and G2-phase cells according to 90 degrees scattering, we found R = 0.93. We conclude that MoAbs against the AF-2 protein may be used in FCM for quantitative analysis and for isolation of M-phase cells, providing as well, the identification of the early-G1 cell subcompartment. The method may, in addition, be useful for the simultaneous detection of cytoplasmic cytokeratin and nuclear AF-2 antigen.

An efficient multiple-exposure analysis of the toxicity of crisnatol, a DNA intercalator in phase II clinical trials

To investigate the toxicity and mechanism of action of crisnatol (CRS), a new DNA intercalator currently in phase II clinical trials, we analyzed cellular and nuclear flow cytometric (FCM) parameters of murine erythroleukemic cells (MELC) exposed to a range of CRS concentrations over three exposure conditions: short-term (4 h), long-term (24 h), and short-term with recovery (4 h+/19 h-). At 0.5-1.0 microM CRS, 4 h exposure results in a reversible G2-phase block, while 24 h exposure results in greater than G2 polyploidy. At 5-10 microM CRS concentrations, cells exhibit persistent retardation of S-phase progression or irreversible G2 and/or greater than G2 blocks, depending on duration of exposure. Cells terminally blocked in G2 exhibit increased nuclear/cellular volumes and increased nuclear fluorescein isothiocyanate (protein) staining, suggestive of unbalanced growth. At 25-50 microM CRS concentrations, MELC exhibit severe membrane perturbation (loss of viability) regardless of exposure. In contrast, following similar exposures to an inactive isomer of CRS, MELC exhibit minimal cell cycle effects, suggesting that cell cycle kinetics may be a useful criterion for assessing potential efficacy. Similar analyses with different classes of chemotherapeutic agents reveal that the range of induced cellular/nuclear perturbations varies with the class of compound used. Taken together, these results suggest that drug toxicity can vary with both concentration and duration of exposure and, as such, a selective multiple-exposure FCM analysis may better represent the spectrum of drug action for drug development and pharmacodynamic studies.

Polyploidy induction as a consequence of topoisomerase inhibition. A flow cytometric assessment

Following recovery from a 4-hr exposure to clinically achievable concentrations of the topoisomerase II inhibitors Adriamycin, teniposide, or amsacrine or the putative topoisomerase II inhibitor crisnatol, murine erythroleukemic cells remained viable for up to 48 hr, but did not proliferate. Cell cycle analysis after a 24-hr recovery revealed blocks in G2 (4N DNA) or greater than G2 (up to 8N DNA) polyploid stages. The relative percentages of cells in either stage was a function of drug concentration and cell cycle stage at time of exposure: typically, cells exposed during S phase became blocked in G2, whereas those exposed during G2/M progressed into greater than G2 polyploid stages. G2-blocked cells exhibited a 2- to 3-fold increase in nuclear protein content and cellular/nuclear volume (i.e. unbalanced growth) and approximately 5% more DNA stainability (as a consequence of nuclear conformational changes rather than redundant DNA synthesis). In all cases, at the drug concentrations studied, mitotic figures were absent and G2 and greater than G2 blocks were irreversible, indicating that the mechanism of polyploidy induction differs from that of microtubule inhibitors. These findings suggest that although topoisomerase inhibitors interfere with DNA synthesis in the S phase, their induction of greater than G2 polyploid blocks may involve direct or indirect inhibition of chromosome condensation.

Discrimination of mitotic cells using anti-p105 monoclonal antibody to analyze the mode of action of etoposide and podophyllotoxin in human gastric cancer cells

Anti-p105 monoclonal antibody was used to discriminate between M-phase and G2-phase of gastric cancer cells. p105 is a proliferation-associated nuclear antigen and its expression increases with cell cycle progression, especially in the mitotic phase. As an example of cell cycle analysis, the modes of action of etoposide and podophyllotoxin were examined by multiparameter flow cytometry. We found that etoposide caused G2 block and retarded S phase transit and podophyllotoxin caused potential M phase block in gastric cancer cells. This cell cycle analysis by using anti-p105 monoclonal antibody should be useful for analysis of the actions of anti-tumor agents, especially for M phase analysis, because of its convenience and reliability.

Flow cytometric analysis of G1- and G2/M-phase subpopulations in mammalian cell nuclei using side scatter and DNA content measurements

Several subcompartments of the cell cycle in addition to the G1-, S-, and G2-phases usually observed were identified by simultaneous flow cytometric measurements of ethidium bromide fluorescence and side scatter intensity of cell nuclei. Metaphase cells and very early G1-phase cells (G1A) with low side scatter intensities were discriminated from interphase cells with high side scatter intensities. The reason for the various side scatter intensities was found to be the different structure of metaphase cells and early G1-phase cells due to chromatin condensation as shown by sorting of the respective cell nuclei. The G1A-phase could further be subdivided into two compartments with very low side scatter (G1A1) and intermediate side scatter (G1A2) intensities. Using partially synchronized cells the duration of these subcompartments of the G1-phase could be estimated. The durations of G1A1- and G1A2-phases were found to be about 10 min and 20 min, respectively, compared to the total duration of the G1-phase of about 3 h. Additional flow cytometric measurements of side scatter intensities of cell nuclei provide therefore further information on subcompartments of the G1- and G2/M-phases.

Improved bromodeoxyuridine/DNA analysis by anti-BudR monoclonal antibody versus right angle light scatter

Dynamic cell cycle analysis is based on the incorporation of labelled precursors into DNA. Although antibodies to BrdU are very useful for analysing in flow cells which synthesize DNA, this approach has two main limitations. First, the detection of low incorporating cells is often difficult; second, four parameter flow cytometry is not able to correlate cell cycle to any other cellular marker. We have developed a methodology that, employing an IgGH + L as a second antibody and side scatter instead of propidium iodide fluorescence, allows a better discrimination of BudR+ cells. This approach allows the collection of an extra-fluorescent signal, and the analysis of specific cellular markers within the cell cycle.

Flow cytometric comparison of the effects of trialkyltins on the murine erythroleukemic cell

Cellular effects of exposure to tributyltin (TBT), triethyltin (TET), or trimethyltin (TMT) were investigated by flow cytometry employing the murine erythroleukemic cell (MELC) as a model cellular system. Cell viability was investigated by the carboxyfluorescein diacetate (CFDA) uptake/propidium iodide (PI) exclusion method: above a critical concentration (exposure for 4 h), which was specific for each of the trialkyltin compounds, the cell becomes permeable to PI, indicating loss of viability. Cellular CF fluorescence (derived from intracellular hydrolysis of CFDA) increased as a function of alkyltin concentration below the critical concentration and decreased as viability decreased above the critical concentration. Relative membrane potential, monitored with a cyanine dye (DiOC6), correlated with viability (PI exclusion), remaining essentially unaltered below the critical concentration and decreasing above it. At/above 1 microM TBT, 5 microM TET, or 100 microM TMT, the cell cycle was blocked in the G2/M phase. The 90 degrees light scatter (a measure of refractive index), axial light loss (a measure of volume), and fluorescein isothiocyanate (FITC) fluorescence (a measure of protein content) of nuclei isolated from trialkyltin-treated MELC by detergent treatment, increased as a function of organotin dose. Fluorescence and interference microscopy revealed increased quantities of residual cytoplasmic tags adherent to the nuclei as a function of organotin dose, apparently resulting from increased cytoplasmic resistance to detergent-mediated solubilization. The effects of the trialkyltins correlated with their lipophilicity (octanol/water coefficient). These data support the hypothesis that fixation (protein denaturation, cross-linking, etc.) is an important mode of organotin cytotoxicity.

A new method to discriminate G1, S, G2, M, and G1 postmitotic cells

A new flow cytometric method combining light scattering measurements, detection of bromodeoxyuridine (BrdU) incorporation via fluorescent antibody, and quantitation of cellular DNA content by propidium iodide (PI) allows identification of additional compartments in the cell cycle. Thus, while cell staining with BrdU-antibodies and PI reveals the G1, S, and G2 + M phases of the cell cycle, differences in light scattering allow separation of G2 phase cells from M phase cells and subdivision of G1 phase into two compartments, i.e., G1A representing postmitotic cells which mature to G1B cells ready to initiate DNA synthesis. The method involves fixation of cells in 70% ethanol, extraction of histones with HC1, and thermal denaturation of DNA. This treatment appears to enhance the differences in chromatin structure of cells in the various phases of the cell cycle to the extent that cells could be separated on the basis of the 90 degrees scatter. Mitotic cells show much lower scatter than G2 phase cells, and G1 postmitotic cells (G1A) show lower scatter than G1 cells about to enter the S phase (G1B). Light scattering is correlated with chromatin condensation, as judged by microscopic evaluation of cells sorted on the basis of light scatter. The method has the advantage over the parental BrdU/DNA bivariate analysis in allowing the G2 and M phases of the cell cycle to be separated and the G1 phase to be analyzed in more detail. The method may also allow separation of unlabeled S phase cells from mitotic cells and distinguish between labeled and unlabeled mitotic cells.

Flow cytometric detection of mitotic cells using the bromodeoxyuridine/DNA technique in combination with 90 degrees and forward scatter measurements

Mitotic cells could be well discriminated from the cells in the G1-, S- and G2-phases of the cell cycle using pulse labeling of S-phase cells with bromodeoxy-uridine (BrdUrd) and staining of the cells for incorporated BrdUrd and total DNA content. Unlabeled G2- and M-phase cells could be measured as two separate peaks according to propidium iodide fluorescence. M-phase cells showed lower propidium iodide fluorescence emission compared to G2-phase cells. The fluorescence difference of M- and G2-phase cells was caused by the different thermal denaturation of their DNA. Best separation of M- and G2-phase cells was obtained after 30-50 min heat treatment at 95 degrees C. Mitotic index could be measured if no unlabeled S-phase cells were present in the cell culture. With additional measurements of 90 degree scatter and/or forward scatter signals, mitotic cells could be clearly discriminated from both unlabeled G2- and S-phase cells. The correct discrimination (about 99%) of mitotic cells from interphase cells was verified by visual analysis of the nuclear morphology after selective sorting. Unlabeled and labeled mitotic cells could be observed as pulse-labeled cells progressed through the cell cycle. We conclude that this modified BrdUrd/DNA technique using prolonged thermal denaturation and the simultaneous measurement of scatter signals may offer additional information especially in the presence of BrdUrd-unlabeled S-phase cells.

Metal-induced alteration of the cell membrane/cytoplasm complex studied by flow cytometry and detergent lysis

Flow cytometric analysis of the cell cycle is most effectively accomplished with membrane-/cytoplasm-free ("clean") nuclei. Non-ionic detergents (e.g. NP40 or Triton X-100) commonly are employed to solubilize cell membranes/cytoplasm to produce "clean" nuclei. Treatment of murine erythroleukemic cells (MELC) with tri-n-butyltin methoxide, cadmium acetate, zinc sulfate, or lead acetate alters the properties of the cell membrane/cytoplasm complex making it resistant to NP40 dissolution. On a molar basis, the organotin compound was more effective in inducing resistance to detergent-mediated dissolution than the inorganic metal compounds. Resistance to NP40-mediated dissolution was manifested as an increase in the flow cytometric parameters 90 degrees scatter and fluorescein isothiocyanate (FITC) fluorescence and was confirmed by light microscopy.

Effects of tributyltin on biomembranes: alteration of flow cytometric parameters and inhibition of Na+, K+-ATPase two-dimensional crystallization

Carboxyfluorescein diacetate (CFDA) is a lipophilic nonfluorescent molecule that readily crosses the cell membrane. In the cytoplasm, it is hydrolyzed by nonspecific esterases to carboxyfluorescein (CF), a negatively charged fluorescent molecule, which is retained incompletely by cells with an intact plasma membrane. Exposure (4 hr) of the murine erythroleukemic cell (MELC) to micromolar quantities (0.1 to 5.0 microM) of tributyltin (TBT) results in increased cellular CF fluorescence. The increase occurs within a range below a critical value of the product (CPV) of the concentration (C) of TBT X duration (T) of exposure to TBT. Fluorescence increase is a sensitive indicator of the interaction of TBT with the cell: it is observed following exposure to 0.1 microM TBT for 4 hr at 37 degrees C. In the range above the CPV, cellular CF fluorescence is reduced apparently resulting from perturbation of membrane structure. For example, exposure of MELC to 2.5 microM TBT for 4 hr at 37 degrees C produces resistance to detergent-mediated cytolysis and inhibition of vanadate-mediated two-dimensional crystallization of Na+, K+-ATPase molecules in porcine renal microsomal membrane preparations, a process requiring molecular mobility within the membrane. Taken together, the increased cellular CF fluorescence and resistance of the MELC to cytolysis along with the inhibition of Na+, K+-ATPase crystallization in the microsomal membrane preparations suggest fixation (protein denaturation, cross-linking, etc.) at the level of the plasma membrane as a mode of toxic action of TBT.