Differential decondensation of mitotic chromosomes during the hypotonic treatment of cells as a possible cause of G-banding

Chinese hamster metaphase chromosomes were studied after treating the cells with a hypotonic 0.075 M KCl solution, routinely used for differential staining of chromosomes. After the incubation of cells in KCl for 5 seconds-40 minutes and fixation with glutaraldehyde, the chromosomes displayed DNP fibrils about 20 nm in diameter. These fibrils were unevenly packed along the chromosomal arms and formed sites of differential density. In sister chromatids, the even density sites were located symmetrically. The use of serial ultrathin sections of marker chromosomes (e.g., chromosomes with a telomeric disposition of nucleolar organizing regions) made it possible to establish a good correlation between the sites with the light packing of DNP-fibrils and G-bands, identified in the same chromosomes by the standard Giemsa-staining technique. Fixation of the KCl-incubated cells with the methanol: glacial acetic acid (3:1) solution did not change the DNP packing heterogeneity. The chromosomal banding state is reversible, for with the transfer of cells from KCl to the normal cultural medium the chromosomes become homogeneous in length. Thus, the data obtained allow to propose that the capacity of chromosomes for differential staining may be based on an uneven sensitivity of G- and R-bands to the decondensing effect of hypotonic treatment.     

Differential decondensation of mitotic chromosomes in SPEV tissue cultured cells induced by repeated hypotonic shock

A new method of differential decondensation of mitotic chromosomes has been proposed by means of repeated treatment of live cells with 15% Hanks' balanced salt solution. The procedure of cell treatment includes three stages: the first hypotonic shock, cultivation in isotonic medium, and the second hypotonic shock. As a result, after a standard methanol-acetic acid fixation and Giemsa staining some discrete Giemsa-positive globules are revealed in mitotic chromosomes. Such globules are symmetrically arranged in axial regions of sister chromatids. The comparative analysis of marker chromosomes has revealed a topological conformity of these globules to G-bands of chromosomes. It has been shown that it is the first hypotonic shock that triggers induction of structural modification of chromatin in interphase nuclei and in mitotic chromosomes. Of interest is the fact that the effect of the first shock is prolonged in time and is realized during at least one cell cycle, with the normal structure of mitotic chromosomes being restored after S-phase of the successive cell cycle.     

Ultrastructure of the mitotic chromosomes in pig embryonic kidney cells during their reversible artificial decondensation in vivo

Using methods of in vivo observation and ultrathin sectioning, it is shown that chromosomes of metaphase PE cells, previously treated with diluted Henk's solutions (70, 30 and 15%), undergo some structural transitions resulting in the formation of micronuclei. At the early stages of hypotonic treatment chromosomes are seen considerably swollen and losing the higher levels of organization, including the chromonema and chromomeres. The chromosomal bodies are formed by DNP fibers 10-25 nm in diameter making loops radiating from the central part of the chromatids. Chromosomes are capable of recondensing from this state by consecutive reconstitution of G-bands, chromomeres and the chromonema. The subsequent secondary decondensation of chromosomes is analogous to telophase decondensation at the normal mitosis, but it results in the formation of a great number of small nuclei (micronuclei). The chromatin structure in micronuclei as well as their ability to synthesize RNA and to replicate DNA show these effects to be reversible. It has been suggested that the loop organization of DNP may be essential for sustaining the structural integrity of the mitotic chromosome.     

Cathepsin L inhibitor I blocks mitotic chromosomes decondensation during cleavage cell cycles of sea urchin embryos

We have previously reported that sperm histones (SpH) degradation after fertilization is catalyzed by a cystein-protease (SpH-protease). Its inhibition blocks the degradation of SpH in vivo and also aborts sea urchin development at the initial embryonic cell cycles. It remains unknown if this effect is a consequence of the persistence of SpH on zygotic chromatin, or if this protease is involved per-se in the progression of the embryonic cell cycles. To discriminate among these two options we have inhibited this protease at a time when male chromatin remodeling was completed and the embryos were engaged in the second cell cycle of the cleavage divisions. The role of this enzyme in cell cycle was initially analyzed by immuno-inhibiting its SpH degrading activity in one of the two blastomeres after the initial cleavage division, while the other blastomere was used as a control. We found that in the blastomere injected with the anti-SpH-protease antibodies the cytokinesis was arrested, the chromatin failed to decondense after mitosis and BrdU incorporation into DNA was blocked. Since the N-terminal sequence and the SpH protease was homologous to the cathepsin L (Cat L) family of proteases, we subsequently investigated if the deleterious effect of the inhibition of this protease is related to its Cat L activity. In this context we analyzed the effect of Cat L inhibitor I (Z-Phe-Phe-CH(2)F) on embryonic development. We found that the addition of 100 uM of this inhibitor to the embryos harvested at the time of the initial cleavage division (80 min p.i.) mimics perfectly the effects of the immuno-inhibition of this enzyme obtained by microinjecting the anti-SpH-protease antibodies. Taken together these results indicate that the activity of this protease is required for embryonic cell cycle progression. Interestingly, we observed that when this protease was inhibited the chromatin decondensation after mitosis was abolished indicating that the inhibition of this enzyme affects chromosomes decondensation after mitosis.     

The role of Ca ions in restoration of the structure of interphase and mitotic chromosomes in PK living cells after hypotonic stress.

The dynamics of mitotic chromosome and interphase chromatin recondensation in living PK cells during their adaptation to hypotonic medium was studied. The recondensation process was found to be slowed down by the modification of plasma membrane with low concentrations of glutaraldehyde, while osmotic reactions of glutaraldehyde-treated cells remain unchanged. The effect of glutaraldehyde can be rapidly reversed by the addition of Ca(2+)-ionophore A23187. Intracellular Ca(2+)measurements show that the adaptation to hypotonic shock is accompanied by restoration of free Ca concentration, whereas the delay of chromatin condensation in glutaraldehyde-treated cells is paralleled by the decrease of Ca level. The mechanisms implying the role of low concentration of Ca(2+)in chromatin compactization in vivo are discussed.     

An ultrastructural study of mitotic division in differentiated gastric smooth muscle cells

Summary An ultrastructural examination of tissue from the gizzards of chicks just before and just after hatching showed numerous mitotic divisions in the well differentiated and functional smooth muscle. The nuclei in the very elongate, dividing cells were located centrally. The cytoplasm immediately adjacent to the nuclei contained the normal fully differentiated complement of myofilaments. During the active stages of division, after the breakdown of the nuclear membrane, myofilaments were shown to lie between the individual chromosomes. The process of division only occupied a small portion of the long muscle cells; the ultrastructural changes seen appeared similar to those described in other cell types.This work was supported by grants from the National Heart Foundation of Australia and the Australian Research Grants Committee. Part of this study was completed while J.L.S.C. was in receipt of a Queen Elizabeth II Research Fellowship. T. B. was supported by a Commonwealth Postgraduate Award.     

Proteolysis of mitotic chromosomes induces gradual and anisotropic decondensation correlated with a reduction of elastic modulus and structural sensitivity to rarely cutting restriction enzymes

The effect of nonspecific proteolysis on the structure of single isolated mitotic newt chromosomes was studied using chromosome elastic response as an assay. Exposure to either trypsin or proteinase K gradually decondensed and softened chromosomes but without entirely eliminating their elastic response. Analysis of chromosome morphology revealed anisotropic decondensation upon digestion, with length increasing more than width. Prolonged protease treatment resulted only in further swelling of the chromosome without complete dissolution. Mild trypsinization induced sensitivity of chromosome elasticity to five- and six-base-specific restriction enzymes. These results, combined with previous studies of effects of nucleases on mitotic chromosome structure, indicate that mild proteolysis gradually reduces the density of chromatin-constraining elements in the mitotic chromosome, providing evidence consistent with an anisotropically folded "chromatin network" model of mitotic chromosome architecture.     

Stochastic resonance as a possible mechanism of amplification of weak electric signals in living cells

The most important but still unresolved problem in bioelectromagnetics is the interaction of weak electromagnetic fields (EMFs) with living cells. Thermal and other types of noise pose restrictions in cell detection of weak signals. As a consequence, some extant experimental results that indicate low-intensity field effects cannot be accounted for, and this renders the results themselves questionable. One way out of this dead end is to search for possible mechanisms of signal amplification. In this paper, we discuss a general mechanism in which a weak signal is amplified by system noise itself. This mechanism was discovered several years ago in physics and is known, in its simplest form, as a stochastic resonance. It was shown that signal amplification may exceed a factor of 1000, which renders existing estimations of EMF thresholds highly speculative. The applicability of the stochastic resonance concept to cells is discussed particularly with respect to the possible role of the cell membrane in the amplification process. © 1994 Wiley-Liss, Inc.     

Lipid peroxidation as a possible cause of TCDD toxicity

The target tissues of TCDD, the dysfunctions that result in death in experimental animals, and the ultimate biochemical lesion(s) caused by TCDD are not known despite numerous studies. We have shown by the thiobarbituric acid and conjugated diene methods that TCDD induces hepatic lipid peroxidation in rats. The lipid peroxidation produced by TCDD is both dose and time dependent. A 5-6 fold increase in lipid peroxidation occurs within 6 days following the administration of 40 micrograms TCDD/kg body weight/day for 3 days. Thus, the toxicity of TCDD may be caused in part by free radical-mediated lipid peroxidation that leads to general cell membrane damage which can ultimately produce death in experimental animals at acutely toxic doses.     

Mycoplasma infection as a possible cause of hybridoma instability

Cytogenetic analysis of mouse hybridoma producing monoclonal antibodies to diphtheria toxin and of its derivative, that lost secretory activity at the third passage in vivo, has been carried out. 58% cells of antibody secreting cell lines belonged to a modal class (76-79 chromosomes per cll). The modal chromosomal number of the subline that has stopped producing antibodies decreased to 63-66 per cell and the stem line of this derivative consisted of 30% of cell population only. Chromosome aberrations were much more frequent in hybridoma cells, that ceased to secrete antibodies, than in cells of original hybridoma: 32.3% of aberrant metaphases (1.38 break per cell) and 6.3% of aberrant metaphases (0.1 break per cell), respectively. Mycoplasma infection was found in the hybridoma subline that stopped producing antibodies as defined by the microbiological and cytochemical techniques. Mice might be the possible source of infection. By means of cloning of hybridoma variant, that did not secrete immunoglobulins, several sublines with the recovered secretory function were obtained.     

Differential tyrosine phosphorylation of leukemic cells during apoptosis as a result of treatment with imatinib mesylate

Bcr-Abl fusion tyrosine kinase contributes to leukemic transformation. Imatinib mesylate inhibits Bcr-Abl tyrosine kinase, resulting in a blockage of tyrosine phosphorylation in its downstream pathways. We analyzed the alteration of tyrosine phosphorylation, on BCR/ABL+ chronic myelogenous leukemia cells, after treatment with imatinib mesylate. Data were collected using a two-dimensional gel electrophoresis followed by Western blot and mass spectrometry. The inhibition of Bcr-Abl tyrosine kinase by 2.5 microM imatinib mesylate caused both cell cycle arrest in the G0/G1 phase and increased the portion of apoptotic cells. As a result, the population of leukemic cells decreased by 30% and 70% compared to controls at 24 and 72 h, respectively. Furthermore, treatment with imatinib mesylate altered tyrosine phosphorylation of 24 protein spots as the incubation time proceeded from 0 to 24 and 72 h. Ten of the 24 protein spots are visible at all three times. Four are detectable at both the 0 and 24 h points in time. Eight were detectable only at time 0.