Cadmium induced apoptotic changes in chromatin structure and subphases of nuclear growth during the cell cycle in CHO cells

Cell organelles as targets of mammalian cadmium toxicity

Ever increasing environmental presence of cadmium as a consequence of industrial activities is considered a health hazard and is closely linked to deteriorating global health status. General animal and human cadmium exposure ranges from ingestion of foodstuffs sourced from heavily polluted hotspots and cigarette smoke to widespread contamination of air and water, including cadmium-containing microplastics found in household water. Cadmium is promiscuous in its effects and exerts numerous cellular perturbations based on direct interactions with macromolecules and its capacity to mimic or displace essential physiological ions, such as iron and zinc. Cell organelles use lipid membranes to form complex tightly-regulated, compartmentalized networks with specialized functions, which are fundamental to life. Interorganellar communication is crucial for orchestrating correct cell behavior, such as adaptive stress responses, and can be mediated by the release of signaling molecules, exchange of organelle contents, mechanical force generated through organelle shape changes or direct membrane contact sites. In this review, cadmium effects on organellar structure and function will be critically discussed with particular consideration to disruption of organelle physiology in vertebrates.

Micronucleus formation during chromatin condensation and under apoptotic conditions

In early S phase the newly replicated DNA is folded back to increasingly compact structures. The process of chromatin condensation inside the nucleus starts with the formation of a micronucleus observed in five established cell lines (K562, CHO, Indian muntjac, murine preB and SCC). Supercoiling of chromatin generates a polarized end-plate region extruded from the nucleus. The extruded chromatin is turned around itself forming the head portion (micronucleus) visible by fluorescence microscopy until the middle of S phase when chromatin structures are succeeded by distinguishable early forms of chromosomes. The generation of micronuclei upon apoptotic treatment was achieved by the methotrexate (MTX) treatment of cells. A close correlation was found between the frequency of micronucleus and MTX concentration, with low frequency at low (0.1 µM) and increasingly higher frequency between 1 and 100 µM concentrations. Characteristic deformation and shrinkage of nuclei indicated apoptosis. High MTX concentration (100 µM) caused the enlargement and necrotic disruption of nuclei. Inhibition of DNA synthesis during replicative DNA synthesis by biotinylated nucleotide prevented the formation of metaphase chromosomes and elevated the frequency of early intermediates of chromosome condensation including micronucleus formation. Based on these observations the micronucleus is regarded as: (a) a regularly occuring element of early chromatin condensation and (b) a typical sign of nuclear membrane damage under toxic conditions. Explanation is given why the micronucleus is hidden in nuclei under normal chromatin condensation and why chromatin motifs including micronuclei become visible upon cellular damage.

Caffeine-Induced Premature Chromosome Condensation Results in the Apoptosis-Like Programmed Cell Death in Root Meristems of Vicia faba

We have demonstrated that the activation of apoptosis-like programmed cell death (AL-PCD) was a secondary result of caffeine (CF) induced premature chromosome condensation (PCC) in hydroxyurea-synchronized Vicia faba root meristem cells. Initiation of the apoptotic-like cell degradation pathway seemed to be the result of DNA damage generated by treatment with hydroxyurea (HU) [double-stranded breaks (DSBs) mostly] and co-treatment with HU/CF [single-stranded breaks (SSBs) mainly]. A single chromosome comet assay was successfully used to study different types of DNA damage (neutral variant–DSBs versus alkaline–DSBs or SSBs). The immunocytochemical detection of H2AXS139Ph and PARP-2 were used as markers for DSBs and SSBs, respectively. Acridine orange and ethidium bromide (AO/EB) were applied for quantitative immunofluorescence measurements of dead, dying and living cells. Apoptotic-type DNA fragmentation and positive TUNEL reaction finally proved that CF triggers AL-PCD in stressed V. faba root meristem cells. In addition, the results obtained under transmission electron microscopy (TEM) further revealed apoptotic-like features at the ultrastructural level of PCC-type cells: (i) extensive vacuolization; (ii) abnormal chromatin condensation, its marginalization and concomitant degradation; (iii) formation of autophagy-like vesicles (iv) protoplast shrinkage (v) fragmentation of cell nuclei and (vi) extensive degeneration of the cells. The results obtained have been discussed with respect to the vacuolar/autolytic type of plant-specific AL-PCD.

SAMNet: a network-based approach to integrate multi-dimensional high throughput datasets

The rapid development of high throughput biotechnologies has led to an onslaught of data describing genetic perturbations and changes in mRNA and protein levels in the cell. Because each assay provides a one-dimensional snapshot of active signaling pathways, it has become desirable to perform multiple assays (e.g. mRNA expression and phospho-proteomics) to measure a single condition. However, as experiments expand to accommodate various cellular conditions, proper analysis and interpretation of these data have become more challenging. Here we introduce a novel approach called SAMNet, for Simultaneous Analysis of Multiple Networks, that is able to interpret diverse assays over multiple perturbations. The algorithm uses a constrained optimization approach to integrate mRNA expression data with upstream genes, selecting edges in the protein-protein interaction network that best explain the changes across all perturbations. The result is a putative set of protein interactions that succinctly summarizes the results from all experiments, highlighting the network elements unique to each perturbation. We evaluated SAMNet in both yeast and human datasets. The yeast dataset measured the cellular response to seven different transition metals, and the human dataset measured cellular changes in four different lung cancer models of Epithelial-Mesenchymal Transition (EMT), a crucial process in tumor metastasis. SAMNet was able to identify canonical yeast metal-processing genes unique to each commodity in the yeast dataset, as well as human genes such as β-catenin and TCF7L2/TCF4 that are required for EMT signaling but escaped detection in the mRNA and phospho-proteomic data. Moreover, SAMNet also highlighted drugs likely to modulate EMT, identifying a series of less canonical genes known to be affected by the BCR-ABL inhibitor imatinib (Gleevec), suggesting a possible influence of this drug on EMT.

Cell separation by countercurrent centrifugal elutriation: recent developments

Countercurrent centrifugal elutriation (CCE) is a cell separation technique that separates particles predominantly according to their size, and to some degree according to their specific density, without a need for antibodies or ligands tagging cell surfaces. The principles of this technique have been known for half a century. Still, numerous recent publications confirmed that CCE is a valuable supplement to current cell separation technology. It is mainly applied when homogeneous populations of cells, which mirror an in vivo situation, are required for answering scientific questions or for clinical transplantation, while antibodies or ligands suitable for cell isolation are not available. Currently, new technical developments are expanding its application toward fractionation of healthy and malignant tissue cells and the preparation of dendritic cells for immunotherapy.

Incomplete chromatin condensation in enlarged rat myelocytic leukemia cells

The distinguishable morphologic features of nuclei of acute myelogenous leukemia cells with enlarged size and finely distributed nuclear chromatin indicate incomplete chromosome condensation that can be related to elevated gene expression. To confirm this, interphase chromosome structures were studied in exponentially growing rat myelomonocytic leukemia 1 cells isolated at the University of Debrecen (My1/De cells). This cell line was established from primary rat leukemia chemically induced by 7,12-dimethylbenz[a]anthracene treatment. The enlarged nuclei of My1/De cells allowed improved fluorescent visualization of chromosomal structures. Increased resolution revealed major interphase intermediates consisting of (1) veil-like chromatin, (2) chromatin ribbon, (3) chromatin funnel, (4) chromatin bodies, (5) elongated prechromosomes, (6) seal-ring, spiral shaped, and circular chromosomal subunits, (7) elongated, bent, u- and v-shaped prechromosomes, and (8) metaphase chromosomes. Results confirmed the existence of the chromatin funnel, the first visible interphase chromosome generated by the supercoiling of the chromatin ribbon. Other intermediates not seen previously included the spiral subunits that are involved in the chromonemic folding of metaphase chromosomes. The existence of spiral subunits favors the helical coil model of chromosome condensation. Incomplete chromatin condensation in leukemia cells throughout the cell cycle is an indication of euchromatization contributing to enhanced gene expression and is regarded as a leukemic factor.

Time-lapse analysis of cell death in mammalian and fungal cells

Time-lapse video microscopy was designed to follow the movement of single cells for an unlimited period of time under physiological conditions. The system is based on two inverted microscopes located in a CO(2) incubator and equipped with charge-coupled device cameras connected to the computer. Frames were recorded every minute and the subsequent video sequence was converted to database form. The system was applied to describe the movements of normal HaCaT cells and Pb-treated cells causing the so-called apoptotic dance during cell death. The apoptotic movement was also followed in high-osmolarity glycerol-type mitogen-activated protein kinase (MAPK) null mutant of Fusarium proliferatum, a filamentous fungus, during osmotic stress. The shortest (20 min) and most vigorous death movements were observed in apoptotic fungal cells subjected to salt stress. The necrotic process at higher Pb concentration (50 microM) took 2-3 h, whereas the apoptotic process at lower Pb concentrations lasted from minutes to days.

Comparative study of cyanotoxins affecting cytoskeletal and chromatin structures in CHO-K1 cells

In this study we compared the effects of the two frequently occuring and most dangerous cyanobacterial toxins on the cellular organization of microfilaments, microtubules and on the chromatin structure in Chinese hamster ovary (CHO-K1) cells. These compounds are the widely known microcystin-LR (MC-LR) and cylindrospermopsin (CYN) classified as the highest-priority cyanotoxin. Toxic effects were tested in a concentration and time dependent manner. The hepatotoxic MC-LR did not cause significant cytotoxicity on CHO-K1 cells under 20 microM, but caused apoptotic changes at higher concentrations. Apoptotic shrinkage was associated with the shortening and loss of actin filaments and with a concentration dependent depolymerization of microtubules. No necrosis was observed over the concentration range (1-50 microM MC-LR) tested. Cylindrospermopsin did cause apoptosis at low concentrations (1-2 microM) and over short exposure periods (12h). Necrosis was observed at higher concentrations (5-10 microM) and following longer exposure periods (24 or 48h). Cyanotoxins also affected the chromatin structure. The condensation process was inhibited by MC-LR at a later stage and manifested as broken elongated prechromosomes. CYN inhibited chromatin condensation at the early fibrillary stage leading to blurred fluorescent images of apoptotic bodies and preventing the formation of metaphase chromosomes. Cylindrospermopsin exhibited a more pronounced toxic effect causing cytoskeletal and nuclear changes as well as apoptotic and necrotic alterations.

Cadmium- and lead-induced apoptosis in mallard erythrocytes (Anas platyrhynchos)

Cadmium, lead and cadmium-lead (1:10) induced apoptosis were studied using mallard blood cells. The allowable range in concentrations were: 0.01-0.5, 0.1-5.0, and 0.01:0.10-0.50:5.00 mM, for cadmium, lead and cadmium-lead, respectively. The lowest EC(50) achieved was for cadmium (0.22+/-0.04 mM). Two doses from each treatment group were chosen to study apoptosis and the presence of metals in cells. The percentage of apoptotic cells increased as the concentration of metals increased. The percentage of cells with intracellular metals was high for both exposure levels and the quantity of intracellular metal was greater for exposure to high concentrations. Morphological alterations for all types of exposure were related to the diverse range of effects that these metals have on membranes. We suggest that the decrease in the number of erythrocytes observed in specimens suffering from lead and cadmium poisoning is related to the induction of apoptosis.

Chemically Induced Carcinogenesis Affecting Chromatin Structure in Rat Hepatocarcinoma Cells

A new, chemically induced animal tumor cell line (HeDe) was established and characterized by its property of causing aggressively growing tumors in specific strain of rats and changes in the chromatin structure. Results show that (1) the nuclear material in nuclei of normal resting (G0) hepatocytes consists mainly of decondensed veil-like chromatin, chromosomes being clustered in six lobular domains; (2) nuclei of HeDe cells contain primarily supercoiled chromatin; or (3) the nuclear material of tumor cells undergoes apoptosis seen as apoptotic bodies. Heterogeneity of chromatin structures was expressed as contour/area ratio and was nine times higher in apoptotic cells and two times higher in tumor cells compared to resting cells.

Preapoptotic chromatin changes induced by ultraviolet B irradiation in human erythroleukemia K562 cells

Exponentially growing human erythroleukemia K562 cells were permeabilized and the dose dependent decrease of DNA synthesis rate was measured after ultraviolet (UV B, 290 nm) irradiation. Cells were able to overcome 2 and 5 J/m2 UV doses, partial recovery was observed at 15 J/m2, while at high (25 J/m2) UV dose replicative DNA synthesis remained suppressed. K562 cells were subjected to synchronization prior to and after UV irradiation (24 J/m2) and 18 fractions were collected by centrifugal elutriation. Cell cycle analysis by flow cytometry did not show early apoptotic cells after UV irradiation. The gradual increase in DNA content typical for non-irradiated cells was contrasted by an early S phase block between 2.2 and 2.4 C-values after UV irradiation. Cell cycle dependent chromatin changes after ultraviolet irradiation were seen as a fine fibrillary network covering the mainly fibrous chromatin structures and incompletely folded primitive chromosomes. Based on observations after UV irradiation and on earlier results with cadmium treatment and gamma irradiation, we confirm that typical chromatin changes characteristic to genotoxic agents can be recognized and classified.

Transition from Chromatin Bodies to Linear Chromosomes in Nuclei of Murine PreB Cells Synchronized in S Phase

Chromatin structures and individual interphase chromosomes escaping nuclei of reversibly permeabilized cells were analyzed in a cell cycle-dependent manner. Cells were synchronized by counterflow centrifugal elutriation. Individual interphase chromosomes became visible as distinct fibrous chromatin bodies from mid-S-phase, turning to elongated chromosomes by the end of S phase. Major interphase chromosomal forms include (1) mid-S-phase chromatin bodies at 3.0 C-value, (2) elongated chromatin bodies later in mid-S-phase (3.25 C-value), (3) chromatin bodies with head and leg portions later in S phase (3.5 C-value), (4) supercoiled ribbons later in S phase seen as twisted prechromosomes (3.7 C-value), and (5) end-S-phase elongated, bent prechromosomal structures (3.9 C-value). The first karyotype analysis of the earliest forms of chromosomes referred to as chromatin bodies was performed.

Cell culture density dependent toxicity and chromatin changes upon cadmium treatment in murine pre-B-cells

Murine pre-B-cells grown in the presence of lower (1 microM) or higher (5 microM) concentration of cadmium chloride were separated into 13 fractions by centrifugal elutriation. The rate of DNA synthesis after cadmium treatment determined in permeable cells was dependent on cell culture density during cadmium treatment. Cell cycle analysis revealed a shift in the profile of DNA synthesis from replicative to repair DNA synthesis upon cadmium treatment. The study of the relationship between cell culture density and cell diameter at lower and higher cell densities in the presence of 1 microM cadmium chloride concentration showed that a. at 5 x 10(5) cell/ml or lower densities cells were shrinking indicating apoptotic changes, b. at higher cell culture densities the average cell size increased, c. the treatment of cells with low CdCl(2) concentration (1 microM) at higher cell culture density (>5 x 10(5) cell/ml) did not change significantly the average cell diameter. At 5 microM cadmium concentration and higher cell culture densities (>5 x 10(5) cell/ml) the average cell size decreased in each elutriated fraction. Most significant inhibition of cell growth took place in early S phase (2.0-2.5 C value). Apoptotic chromatin changes in chromatin structure after cadmium treatment were seen as large extensive disruptions, holes in the nuclear membrane and stickiness of incompletely folded chromosomes.

Supranucleosomal Organization of Chromatin Fibers in Nuclei ofDrosophilaS2 Cells

Earlier, the interphase chromatin structures could not be visualized due to the stickiness of the nuclear material. We have reduced stickiness by the reversal of permeabilization allowing the isolation and microscopic imaging of interphase chromatin structures. By using a high resolution of synchronization, collecting 36 elutriation fractions, we show that major intermediates of chromatin condensation include: (a) decondensed veillike chromatin at the unset of the S phase (2.0-2.2 C-value), (b) polarization of veiled chromatin (2.2-2.6 C), (c) fibrous chromatin (2.6-3.0 C), chromatin bodies (3.0-3.3 C), early precondensed chromosomes (3.3-3.6). The compaction of Drosophila chromosomes did not reach that of the mammalian cells in the final stage of condensation (3.6-4.0 C). Drosophila chromosomes consist of smaller units called rodlets. Results demonstrate that nucleosomal chromatin ("beads on string") does not form a solenoid structure; rather, the topological arrangement consists of meandering and plectonemic loops.

Condensation of Interphase Chromosomes in Nuclei of Synchronized CHO Cells

The escape of individual interphase chromosomes from nuclei of reversibly permeabilized Chinese hamster ovary (CHO) cells was utilized for the visualization of condensing interphase chromosomes in a cell cycle-dependent manner in synchronized cells. Major interphase chromosomal forms include: (a) mid-S-phase globular chromosomes at 3.0 C-value, (b) late mid-S-phase fibrous hemicircular forms (3.3 C), (c) late-S-phase supercoiled ribbons (3.7 C), and (d) end-S-phase elongated, bent prechromosomal structures (4.0 C).

Structure of Interphase Chromosomes in the Nuclei of Drosophila Cells

Fluorescent images of interphase chromatin structures and chromosome structures isolated from reversibly permeable Drosophila cells were analyzed. Decondensed chromatin in early S phase (2.0-2.5 C-value) consisted of a veil-like fibrillary network. Fibrillar chromatin formed rodlets later in the early S phase (2.5-2.75 C). Drosophila chromosomes contain several smaller subunits called rodlets. Fibrillar chromatin turned to chromatin ribbon and the early mid-S-phase globular chromosomes (2.75-3.0 C), then to opened fibrous globular forms later in the mid-S-phase (3.0-3.25 C), to late-S-phase supercoiled ribbons (3.25-3.5 C), end-S-phase elongated prechromosomes (3.5-3.75 C), bent and linear chromosomes (3.75-4.0 C). Early-S phase chromatin fibrils in the nuclei of Drosophila cells are thinner than the veil-like structures in mammalian cells. The connectivity of chromosomes shows linear arrangement (3, 1, 2, 4), with larger chromosomes (1 and 2) inside and smaller chromosomes (3, 4) at the two ends in the chromosomal chain.

Linear Connection of Condensing Chromosomes in Nuclei of Synchronized CHO Cells

Reversibly permeabilized cells allowed the analysis of intermediates of large-scale chomatin condensation in a cell cycle-dependent manner. This paper summarizes major intermediates of chromatin condensation and visualizes connectivity between different forms of chromosomes. At the early S phase the veil-like fibrillary chromatin is supercoiled to form chromatin bodies representing the earliest visible chromosomes. Supercoiling results in a chromatin fiber, which turns to rope (thick fiber) forming loops and chromatin ribbon. The elongated shape of prechromosomes indicates that they are arranged head to tail. Bent forms (loop, c-, and v-shaped structures) of interphase chromosomes open at the end of S phase. Linear arrangement of chromosomes was observed to the end of the condensation process, suggesting that connectivity of chromosomes is maintained throughout the cell cycle.

Common Pathway of Chromosome Condensation in Mammalian Cells

Chromatin folding in the interphase nucleus is not known. We compared the pattern of chromatin condensation in Indian muntjac, Chinese hamster ovary, murine pre B, and K562 human erythroleukemia cells during the cell cycle. Fluorescent microscopy showed that chromosome condensation follows a general pathway. Synchronized cells were reversibly permeabilized and used to isolate interphase chromatin structures. Based on their structures two major categories of intermediates were distinguished: (1) decondensed chromatin and (2) condensed chromosomal forms. (1) Chromatin forms were found between the G1 and mid-S phase involving veil-like, supercoiled, fibrous, ribboned structures; (2) condensing chromosomal forms appeared in the late-S, G2, and M phase, including strings, chromatin bodies, elongated pre-chromosomes, pre-condensed chromosomes, and metaphase chromosomes. Results demonstrate that interphase chromosomes are clustered in domains; condensing interphase chromosomes are linearly arranged. Our results raise questions related to telomer sequences and to the chemical nature of chromosome connectivity.

Why ribose was selected as the sugar component of nucleic acids

During evolution ribose was selected as the exclusive sugar component of nucleic acids. The selection is explained by using molecular models and by eliminating most of the other common sugars by looking at their chemical structure and envisioning how they would fit in a nucleic acid model. Comparisons of sugar pucker conformations and configurations of pentoses indicate that ribose was not randomly selected but the only choice, since beta-D-ribose fits best into the structure of physiological forms of nucleic acids. In other nucleotides containing arabinose, xylose, or lyxose, the C(2)'-OH and/or the C(3)'-OH are above the furanose ring, causing steric interference with the bulky base and the C(5)'-OH group.

Stage-dependency of apoptosis and the blood-testis barrier in the dogfish shark (Squalus acanthias): cadmium-induced changes as assessed by vital fluorescence techniques

Naturally occurring heavy metals and synthetic compounds are potentially harmful for testicular function but evidence linking heavy metal exposure to reduced semen parameters is inconclusive. Elucidation of the exact stage at which the toxicant interferes with spermatogenesis is difficult because the various germ cell stages may have different sensitivities to any given toxicant, germ cell development is influenced by supporting testicular somatic cells and the presence of inter-Sertoli cell tight junctions create a blood-testis barrier, sequestering meiotic and postmeiotic germ cells in a special microenvironment. Sharks such as Squalus acanthias provide a suitable model for studying aspects of vertebrate spermatogenosis because of their unique features: spermatogenesis takes place within spermatocysts and relies mainly on Sertoli cells for somatic cell support; spermatocysts are linearly arranged in a maturational order across the diameter of the elongated testis; spermatocysts containing germ cells at different stages of development are topographically separated, resulting in visible zonation in testicular cross sections. We have used the vital dye acridine orange and a novel fluorescence staining technique to study this model to determine (1) the efficacy of these methods in assays of apoptosis and blood-testis barrier function, (2) the sensitivity of the various spermatogonial generations in Squalus to cadmium (as an illustrative spermatotoxicant) and (3) the way that cadmium might affect more mature spermatogenic stages and other physiological processes in the testis. Our results show that cadmium targets early spermatogenic stages, where it specifically activates a cell death program in susceptible (mature) spermatogonial clones, and negatively affects blood-testis barrier function. Since other parameters are relatively unaffected by cadmium, the effects of this toxicant on apoptosis are presumably process-specific and not attributable to general toxicity.