Transcriptional gene silencing by short interfering RNAs

Double-stranded RNA (dsRNA)-induced sequence-specific gene silencing in animals and plants is known as RNA interference. In mammalian cells, although long dsRNAs induce an interferon-mediated, non-specific gene silencing, short interfering RNAs (siRNAs) that are processed from long dsRNAs by Dicer can induce sequence-specific gene silencing. Since siRNAs can be designed for genes of interest, these molecules have great potential to be used as effective gene therapies. In plants, siRNAs targeted to CpG islands within a promoter can also induce RNA-directed DNA methylation. In addition, siRNAs have a role in heterochromatic gene silencing in fission yeasts, plants and animals. Recently, siRNAs were demonstrated to induce transcriptional gene silencing (TGS) via DNA methylation in human cells. This review will focus on TGS by siRNAs, such as DNA methylation and histone methylation.

Mitomycin C-induced pairing of heterochromatin reflects initiation of DNA repair and chromatid exchange formation

Chromatid interchanges induced by the DNA cross-linking agent mitomycin C (MMC) are over-represented in human chromosomes containing large heterochromatic regions. We found that nearly all exchange breakpoints of chromosome 9 are located within the paracentromeric heterochromatin and over 70% of exchanges involving chromosome 9 are between its homologues. We provide evidence that the required pairing of chromosome 9 heterochromatic regions occurs in G(0)/G(1) and S-phase cells as a result of an active cellular process initiated upon MMC treatment. By contrast, no pairing was observed for a euchromatic paracentromeric region of the equal-sized chromosome 8. The MMC-induced pairing of chromosome 9 heterochromatin is observed in a subset of cells; its percentage closely mimics the frequency of homologous interchanges found at metaphase. Moreover, the absence of pairing in cells derived from XPF patients correlates with an altered spectrum of MMC-induced exchanges. Together, the data suggest that the heterochromatin-specific pairing following MMC treatment reflects the initiation of DNA cross-link repair and the formation of exchanges.

Rumex acetosa Y chromosomes: constitutive or facultative heterochromatin?

Condensed Y chromosomes in Rumex acetosa L. root-tip nuclei were studied using 5-azaC treatment and immunohistochemical detection of methylated histones. Although Y chromosomes were decondensed within root meristem in vivo, they became condensed and heteropycnotic in roots cultured in vitro. 5-azacytidine (5-azaC) treatment of cultured roots caused transitional dispersion of their Y chromosome bodies, but 7 days after removal of the drug from the culture medium, Y heterochromatin recondensed and again became visible. The response of Rumex sex chromatin to 5-azaC was compared with that of condensed segments of pericentromeric heterochromatin in Rhoeo spathacea (Sw.) Steam roots. It was shown that Rhoeo chromocentres, composed of AT-rich constitutive heterochromatin, did not undergo decondensation after 5-azaC treatment. The Y-bodies observed within male nuclei of R. acetosa were globally enriched with H3 histone, demethylated at lysine 4 and methylated at lysine 9. This is the first report of histone tail-modification in condensed sex chromatin in plants. Our results suggest that the interphase condensation of Y chromosomes in Rumex is facultative rather than constitutive. Furthermore, the observed response of Y-bodies to 5-azaC may result indirectly from demethylation and the subsequent altered expression of unknown genes controlling tissue-specific Y-inactivation as opposed to the global demethylation of Y-chromosome DNA.

Region-specific chromatin decondensation and micronucleus formation induced by 5-azacytidine in human TIG-7 cells

A human diploid lung fibroblast cell strain, TIG-7, has a heteromorphic chromosome 15 with an extra short arm carrying a homogeneously staining region (15p+hsr). We demonstrated previously that the 15p+hsr consists of an inactive and G+C-rich rDNA cluster characterized by fluorescence in situ hybridization (FISH) and various chromosome banding techniques. Thus, it was suggested that the region could contain highly methylated DNA. To observe methylation status on the target region directly under the microscope, we used a demethylating agent, 5-azacytidine (5-azaC), to induce decondensation of the chromatin containing methylated DNA. At 24 h after treatment with 0.5 microM 5-azaC, marked decondensation of the 15p+hsr was observed in almost all of the metaphases. Furthermore, we observed micronuclei, which were equivalent to the rDNA of the 15p+hsr demonstrated by FISH in the same preparation. In contrast, the DNA cross-linking agent mitomycin C (MMC) preferentially induced 15p+hsr-negative micronuclei. These findings indicated that chromatin decondensation and subsequent DNA strand breakage induced by the demethylating effect of 5-azaC led specifically to 15p+hsr-positive micronuclei.

Bioregulator Vilon-Induced Reactivation of Chromatin in Cultured Lymphocytes from Old People

The effect of the synthetic peptide bioregulator Vilon on structural and facultative heterochromatin of cultured lymphocytes from old people has been studied. The data obtained indicate that Vilon (a) induces unrolling (deheterochromatinization) of total heterochromatin; (b) activates synthetic processes caused by the reactivation of ribosomal genes as a result of deheterochromatinization of nucleolus organizer regions; (c) releases the genes repressed due to the condensation of euchromatic regions forming facultative heterochromatin; (d) does not induce decondensation of pericentromeric structural heterochromatin. Our results indicate that Vilon causes progressive activation (deheterochromatinization) of the facultative heterochromatin with increased aging.

Peptide Epitalon activates chromatin at the old age

OBJECTIVES and design. We have studied the effect of synthetic peptide Epitalon on the activity of ribosomal genes, denaturation parameters of total heterochromatin, polymorphism of structural C-heterochromatin and the variability of facultative heterochromatin in cultured lymphocytes of persons aged 76-80 years.

RESULTS

The obtained data demonstrate that Epitalon induces the activation of ribosomal genes, decondensation of pericentromeric structural heterochromatin and the release of genes repressed due to the age-related condensation of euchromatic chromosome regions.

CONCLUSIONS

Epitalon has shown its ability to activate chromatin by modifying heterochromatin and heterochromatinized chromosome regions in the cells of older persons.

Overexpression of the SuUR gene induces reversible modifications at pericentric, telomeric and intercalary heterochromatin of Drosophila melanogaster polytene chromosomes

The SuUR (suppressor of underreplication) gene controls late replication and underreplication of DNA in Drosophila melanogaster polytene chromosomes: its mutation suppresses DNA underreplication whereas additional doses of the normal allele strongly enhances underreplication. The SuUR protein is localized in late replicating and underreplicating regions. The N-terminal part of the SuUR protein shares modest similarity with the ATPase/helicase domain of SWI2/SNF2 chromatin remodeling factors, suggesting a role in modification of chromatin structure. Here we describe novel structural modifications of polytene chromosomes (swellings) and show that SuUR controls chromatin organization in polytene chromosomes. The swellings develop as the result of SuUR ectopic expression in the transgene system Sgs3-GAL4; UAS-SuUR(+). They are reminiscent of chromosome puffs and appear in approximately 190 regions of intercalary, pericentric and telomeric heterochromatin; some of them attain tremendous size. The swellings are temperature sensitive: they are maximal at 29 degrees C and are barely visible at 18 degrees C. Shifting from 29 degrees C to 18 degrees C results in the complete recovery of the normal structure of chromosomes. The swellings are transcriptionally inactive, since they do not incorporate [(3)H]uridine. The SuUR protein is not visualized in regions of maximally developed swellings. Regular ecdysone-inducible puffs are not induced in cells where these swellings are apparent.

Effects of Livagen peptide on chromatin activation in lymphocytes from old people

We studied the effects of the synthetic peptide Livagen on activity of ribosomal genes, denaturation parameters of heterochromatin, polymorphism of structural C-heterochromatin, and variability of facultative heterochromatin in lymphocytes from old people. Livagen induced activation of ribosomal genes, decondensation of pericentromeric structural heterochromatin, and release of genes repressed due to age-related condensation of euchromatic regions in chromosomes. Our results indicate that Livagen causes de-heterochromatinization (activation) of chromatin, which is realized via modification of heterochromatin and heterochromatinized regions in chromosomes from old people.

Reorganization and polarization of the meiotic bouquet-stage cell can be uncoupled from telomere clustering

Striking cellular reorganizations mark homologous pairing during meiotic prophase. We address the interdependence of chromosomal and cellular polarization during meiotic telomere clustering, the defining feature of the bouquet stage, by examining nuclear positioning and microtubule and nuclear pore reorganization. Polarization of meiotic cellular architecture was coincident with telomere clustering: microtubules were focused on the nuclear surface opposite the telomere cluster, the nucleus was positioned eccentrically in the cell such that the telomeres faced the direction of nuclear displacement and nuclear pores were clustered in a single region of the nuclear surface opposite the telomeres. Treatment of pre-bouquet stage cells with colchicine inhibited telomere clustering. Asymmetric nuclear positioning and nuclear pore clustering were normal in the presence of unclustered telomeres resulting from colchicine treatment. Nuclear pores were positioned normally with respect to the cell cortex in the absence of telomere clustering, indicating that telomere positioning is not required for polarization. This work provides evidence of meiotic cell polarization and suggests that telomeres may be positioned relative to an asymmetry present in the cell at the time of bouquet formation.

5-Aza-2'-deoxycytidine induces histone hyperacetylation of mouse centromeric heterochromatin by a mechanism independent of DNA demethylation

5-Aza-2'-deoxycytidine (5-azadC) is widely used as a potent inhibitor of DNA methyltransferase. Cells treated with this drug show various phenomena such as the reactivation of repressed genes, change in replication timing, and decondensation of heterochromatin. A number of studies using this drug have been reported so far but it is still controversial whether such changes are due to 5-azadC-induced demethylation itself or the side effects of the drug. Here we report that 5-azadC treatment induces histone hyperacetylation in mouse centromeric heterochromatin which normally contains methylated DNA and hypoacetylated histones. Treatment also affects the intranuclear distribution of histone deacetylase 2 (HDAC2). However, histone hyperacetylation was not observed in DNA methyltransferase 1-deficient cells with a reduced level of genomic DNA methylation. Our results suggest that 5-azadC-induced histone hyperacetylation is independent of DNA demethylation and that DNA methylation is not essential for the maintenance of the histone hypoacetylated state in centromeric heterochromatin.

Inhibition of mitogen-activated protein kinase kinase induces apoptosis of human chondrocytes

We previously have reported that the mitogen-activated protein kinase (MAPK) pathway is stimulated by adhesion of human chondrocytes to anti-beta(1)-integrin antibodies or collagen type II in vitro. These mechanisms most likely prevent chondrocyte dedifferentiation to fibroblast-like cells and chondrocyte death. To investigate whether this pathway plays an essential role for the differentiation, phenotype, and survival of chondrocytes, we blocked mitogen-activated protein kinase/extracellular signal-regulated kinase (Erk) (MEK), a kinase upstream of the kinase Erk by using U0126. Exposure of chondrocytes to U0126 caused activation of caspase-3 in a dose-dependent manner. Western blot analysis with an antibody specific for dually phosphorylated Erk shows that collagen type II induced phosphorylation of Erk1/2 was specifically blocked by U0126 in a dose-dependent manner. Immunohistochemical analysis showed that treated chondrocytes were caspase-3 positive. In treated chondrocytes, the cleavage of 116-kDa poly(ADP-ribose)polymerase resulted in the 85-kDa apoptosis-related cleavage fragment and was associated with caspase-3 activity. Analysis by electron microscopy showed typical morphological signs of apoptosis, such as crescent-shaped clumps of heterochromatin, and a degraded pericellular matrix. Thus, these results indicate that the MEK/Erk signal transduction pathway is involved in the maintenance of chondrocytes differentiation and survival. These data stimulate further investigations on the role of mitogen-activated protein kinase pathways in human chondrocytes.

Experimental condensation inhibition in constitutive and facultative heterochromatin of mammalian chromosomes

What drives the dramatic changes in chromosome structure during the cell cycle is one of the oldest questions in genetics. During mitosis, all chromosomes become highly condensed and, as the cell completes mitosis, most of the chromatin decondenses again. Only chromosome regions containing constitutive or facultative heterochromatin remain in a more condensed state throughout interphase. One approach to understanding chromosome condensation is to experimentally induce condensation defects. 5-Azacytidine (5-aza-C) and 5-azadeoxycytidine (5-aza-dC) drastically inhibit condensation in mammalian constitutive heterochromatin, in particular in human chromosomes 1, 9, 15, 16, and Y, as well as in facultative heterochromatin (inactive X chromosome), when incorporated into late-replicating DNA during the last hours of cell culture. The decondensing effects of 5-aza-C analogs, which do not interfere with normal base pairing in substituted duplex DNA, have been correlated with global DNA hypomethylation. In contrast, decondensation of constitutive heterochromatin by incorporation of 5-iododeoxyuridine (IdU) or other non-demethylating base analogs, or binding of AT-specific DNA ligands, such as berenil and Hoechst 33258, may reflect an altered steric configuration of substituted or minor-groove-bound duplex DNA. Consequently, these compounds exert relatively specific effects on certain subsets of AT-rich constitutive heterochromatin, i.e. IdU on human chromosome 9, berenil on human Y, and Hoechst 33258 on mouse chromosomes, which provide high local concentrations of IdU incorporation sites or DNA-ligand-binding sites. None of these non-demethylating compounds affect the inactive X chromosome condensation. Structural features of chromosomes are largely determined by chromosome-associated proteins. In this light, we propose that both DNA hypomethylation and steric alterations in chromosomal DNA may interfere with the binding of specific proteins or multi-protein complexes that are required for chromosome condensation. The association between chromosome condensation defects, genomic instability, and epigenetic reprogramming is discussed. Chromosome condensation may represent a key ancestral mechanism for modulating chromatin structure that has since been realloted to other nuclear processes.

Synergistic neurotoxicity of opioids and human immunodeficiency virus-1 Tat protein in striatal neurons in vitro

Human immunodeficiency virus (HIV) infection selectively targets the striatum, a region rich in opioid receptor-expressing neural cells, resulting in gliosis and neuronal losses. Opioids can be neuroprotective or can promote neurodegeneration. To determine whether opioids modify the response of neurons to human immunodeficiency virus type 1 (HIV-1) Tat protein-induced neurotoxicity, neural cell cultures from mouse striatum were initially characterized for mu and/or kappa opioid receptor immunoreactivity. These cultures were continuously treated with morphine, the opioid antagonist naloxone, and/or HIV-1 Tat (1-72) protein, a non-neurotoxic HIV-1 Tat deletion mutant (TatDelta31-61) protein, or immunoneutralized HIV-1 Tat (1-72) protein. Neuronal and astrocyte viability was examined by ethidium monoazide exclusion, and by apoptotic changes in nuclear heterochromatin using Hoechst 33342. Morphine (10nM, 100nM or 1microM) significantly increased Tat-induced (100 or 200nM) neuronal losses by about two-fold at 24h following exposure. The synergistic effects of morphine and Tat were prevented by naloxone (3microM), indicating the involvement of opioid receptors. Furthermore, morphine was not toxic when combined with mutant Tat or immunoneutralized Tat. Neuronal losses were accompanied by chromatin condensation and pyknosis. Astrocyte viability was unaffected. These findings demonstrate that acute opioid exposure can exacerbate the neurodegenerative effect of HIV-1 Tat protein in striatal neurons, and infer a means by which opioids may hasten the progression of HIV-associated dementia.

Differential chromosome sensitivity to 5-azacytidine in Alzheimer's disease

BACKGROUND

The methylation process in the DNA has been considered a control mechanism of gene activity, connected with genetic imprinting. 5-Azacytidine (5-AZC) is known to be a demethylation agent.

OBJECTIVE

We studied the cytogenetic effect of 5-AZC in Alzheimer's disease patients and in two control groups.

METHODS

Peripheral lymphocyte cultures derived from 8 patients with Alzheimer's disease and 8 elderly and 8 healthy young individuals, all female, were studied. The parameters investigated were: the undercondensation of constitutive heterochromatin of chromosomes 1, 9, and 16: the number of lesions in fragile sites 1q42 and 19q13; heterochromatin association, and the total number of induced lesions.

RESULTS

Our results showed a significantly increased frequency of undercondensation of chromosomes 1, 9, and 16 in Alzheimer's disease patients when compared with elderly and young healthy groups.

CONCLUSION

These results suggest that the demethylating action of 5-AZC could reveal differential gene activity in the Alzheimer group at the level of cellular division.

'Glowing' chromosomes in cells undergoing rapid division

Previous investigations in which metaphase plates of cells in rapid division were incubated in phosphate buffer at high temperature revealed numerous heterochromatic dots in chromosomes after Giemsa staining. In contrast, chromosomes from cells with a reduced capacity for reproduction were devoid of such dots, or the dots were sloughed-off into rings and patches of heterochromatin. In two types of cells which were rapidly dividing, namely HeLa cells (cervical cancer) and cells from regenerating planaria, phosphate incubation followed by Giemsa staining revealed an 'aura' or 'glowing' effect on the chromosomes, consisting of a densely staining core surrounded by a lightly stained periphery. This finding might be developed into a diagnostic test for certain malignancies, for cells undergoing dedifferentiation, or for tissues undergoing regeneration.

Effects of steroid hormones on nuclear membrane and membrane-bound heterochromatin from breast cancer cells evaluated by fractal morphometry

OBJECTIVE

To evaluate the effect of steroid hormones on the ultrastructure of nuclear heterochromatin and perinuclear membranes in human MCF-7 breast cancer cells.

STUDY DESIGN

MCF-7 cells were cultured briefly (five minutes) in the presence of 10(-9) M estrogen 17 beta-estradiol, a stimulator of cell proliferation and/or 10(-9) M glucocorticoid dexamethasone. Changes in the morphologic complexity of nuclear membrane-bound heterochromatin (NMBHC) and nuclear membranes (ENM) were assessed by means of the fractal capacity dimension, D, a noneuclidean geometric descriptor of complex, irregular bodies.

RESULTS

17 beta-estradiol (10(-9) M) enhanced the ultrastructural irregularity of NMBHC, as documented by the increased value of D, whereas dexamethasone (10(-9) M) reduced it when compared to NMBHC from untreated MCF-7 control cells. In contrast, neither steroid modified ENM ultrastructure. Changes in the nuclear heterochromatin complexity induced by estrogen 17 beta-estradiol occurred concomitantly with functional changes at the cell periphery, such as activation of the phospholipase C, a cell membrane-associated enzyme involved in signal transduction. Dexamethasone reduced the ultrastructural complexity of NMBHC without affecting functional processes.

CONCLUSION

Fractal morphometry proved its usefulness in quantifying early ultrastructural changes in nuclear components induced in MCF-7 cells by steroid hormones, 17 beta-estradiol and dexamethasone.

Comparison of spontaneous and idoxuridine-induced micronuclei by chromosome painting

Fluorescence in situ hybridisation (FISH) technique with chromosome specific library (CSL) DNA probes for all human chromosomes were used to study about 9000 micronuclei (MN) in normal and idoxuridine (IUdR)-treated lymphocyte cultures of female and male donors. In addition, MN rates and structural chromosome aberrations were scored in Giemsa-stained chromosome spreads of these cultures. IUdR treatment (40 microg/ml) induced on the average a 12-fold increase of the MN rate. Metaphase analysis revealed no distinct increase of chromosome breaks but a preferential decondensation at chromosome 9q12 (28-79%) and to a lower extend at 1q12 (8-21%). Application of FISH technique with CSL probes to one male and one female untreated proband showed that all human chromosomes except chromosome 12 (and to a striking high frequency chromosomes 9, X and Y) occurred in spontaneous MN. In cultures containing IUdR, the chromosomal spectrum found in MN was reduced to 10 chromosomes in the male and 13 in the female proband. Eight chromosomes (2, 6, 12, 13, 14, 15, 17 and 18) did not occur in MN of both probands. On the contrary chromosomes 1 and especially 9 were found much more frequently in the MN of IUdR-treated cultures than in MN of control cultures. DAPI-staining revealed heterochromatin signals in most of the IUdR-induced MN. In an additional study, spontaneous and IUdR-induced MN were investigated in lymphocytes of another female donor using CSL probes only for chromosomes 1, 6, 9, 15, 16 and X. The results confirmed the previous finding that chromosomes 1 and 9 occur very often in MN after IUdR-treatment. The results indicate that decondensation of heterochromatic regions on chromosomes 1 and 9 caused by IUdR treatment strongly correlates with MN formation by these chromosomes.

Reduced levels of poly(ADP-ribosyl)ation result in chromatin compaction and hypermethylation as shown by cell-by-cell computer-assisted quantitative analysis

The unmethylated status of the CpG islands is important for gene expression of correlated housekeeping genes since it is well known that their methylation inhibits transcription process. An interesting question that has been discussed but not solved is how the CpG islands maintain their characteristic unmethylated status even though they are rich in CpG dinucleotides. Our previous in vitro and in vivo research has shown that poly(ADP-ribosyl)ation is involved in protecting CpG dinucleotides from full methylation in genomic DNA and that a block of poly(ADP-ribosyl)ation is also involved in modifying the methylation pattern in the promoter region of Htf9 housekeeping gene. In this study we locked for cytological evidence that in the absence of an active poly(ADP-ribosyl)ation the DNA methylation pattern in L929 and NIH/3T3 mouse fibroblast cell lines is altered. For this purpose, differences in the methylation levels of interphase nuclei from control and treated cultures of two murine cell lines preincubated with 2 mM 3-aminobenzamide, an inhibitor of poly(ADP-ribosyl)ation, were measured in individual cells after indirect immunolabeling with anti-5MeC antibodies. The quantitative analysis allowed us to demonstrate that blocking of the poly(ADP-ribosyl)ation results in a higher number, size, and density of antibody binding regions in treated cells when compared to the controls. Analogously, sequential Giemsa staining and indirect immunolabeling of the same slides showed the heterochromatic regions colocalized with the extended methyl-rich domains.

Targeted breakage of paracentromeric heterochromatin induces chromosomal instability

Current models suggest that genomic instability is crucial in the accumulation of the multiple alterations required for tumorigenesis. However, the nature of the initial damage responsible for the origin of genomic instability remains poorly understood. In this investigation we demonstrate that the nucleotide analog 2,6-diaminopurine (DAP) can be used to induce highly focused damage to the large blocks of paracentromeric heterochromatin on chromosomes 1, 9 and 16. A large fraction of cells exposed to DAP exhibit undercondensation of alpha and classical heterochromatin which persists into metaphase. Subsequent chromosome breakage was observed for one of the target chromosomes by preferential exclusion of chromosome 16 fragments into micronuclei (P < 0.0001). The specificity of DAP-induced chromosomal breakage enabled us to utilize it as a reagent to demonstrate that paracentromeric heterochromatin is a sensitive target for the induction of persistent genomic instability. We observed a 100-fold increase in mutagenesis affecting a chromosome 16 marker (APRT) compared with marker loci on chromosomes 17 (TK) or X (HPRT). We previously reported that APRT- mutants were recovered at a high rate upon selection in DAP in a process involving recombinationally mediated loss of heterozygosity that extends from the telomere to the boundary region of the paracentromeric heterochromatin. Karyotypic analysis of DAP-resistant APRT- mutant clones demonstrated extensive genomic instability, particularly evidence of multiple and sequential events affecting chromosome 16. These data suggest that the heterochromatic breakage observed cytogenetically immediately following DAP exposure is also responsible for the initiation of persistent genomic instability.

Steroid hormones modify nuclear heterochromatin structure and plasma membrane enzyme of MCF-7 cells. A combined fractal, electron microscopical and enzymatic analysis

Ultrastructural features of the nuclear membrane envelope (ENM) and the nuclear membrane-bound heterochromatin (NMBHC) were investigated in MCF-7 human breast cancer cells by fractal morphometry. The fractal dimension D established by the box counting method proved to be effective for quantifying nuclear changes in MCF-7 cells treated with steroid hormones, namely the estrogen 17 beta-estradiol, which stimulates cell proliferation, and the glucocorticoid dexamethasone. When MCF-7 cells were briefly (5 min) cultured in the presence of 17 beta-estradiol (10(-9) M), the irregularity of the NMBHC outline was increased as documented by the increased fractal dimension D. Changes in the ultrastructural complexity of the nuclear heterochromatin were observed in concomitance with functional changes at the cell periphery, namely the modulation of the estrogen-induced activity of phospholipase C, a cell membrane-associated enzyme involved in the signal transduction pathway via phosphoinositides metabolism. Dexamethasone did not affect the in vitro proliferation, the phospholipase C activity nor the shape of the ENM of MCF-7 cells, but reduced the structural complexity of the nuclear membrane-bound heterochromatin.

The sigma receptor ligand, reduced haloperidol, induces apoptosis and increases intracellular-free calcium levels [Ca2+]i in colon and mammary adenocarcinoma cells

The sigma receptor ligand reduced haloperidol (50 and 100 microM), potently inhibited cell proliferation, and induced apoptosis in WIDr colon and MCF-7 adenocarcinoma cell lines. Apoptosis was confirmed after drug treatment of the cells by the presence of nuclear fragmentation after staining of the cells with Hoechst 33258 and cellular DNA fragmentation ELISA and by condensation of the heterochromatin using transmission electron microscopy. However, internucleosomal DNA cleavage was not detected using gel electrophoresis. Reduced haloperidol (100 microM) increased the intracellular free calcium levels [Ca2+]i in both cell lines, which was independent of extracellular calcium, suggesting that the rise in [Ca2+]i was from intracellular stores and that an increase in [Ca2+]i may act as a "trigger" for apoptosis in these cell lines.

Epileptic discharges induced by pentylenetetrazol: ultrastructural alterations in identified neurons and glial cells (Helix pomatia)

The effects of sustained epileptic activity induced by pentylenetetrazol on morphology of buccal ganglia of Helix pomatia were investigated. Neuronal somata and processes as well as glial cells were evaluated after 5 hours of epileptic activity and after 5 hours under control conditions. After epileptic activity neurons showed signs of degeneration consisting of condensation of nuclear chromatin, decreased activity of Golgi apparatus, increased numbers of lamellar bodies and multivesicular bodies, clusters of vesicles and vacuoles, loss of microtubuli, and scattered lamellar bodies. Neuronal somata and large neuronal processes appeared less affected than the smaller processes. Glial cells showed signs of phagocytotic activity as increased cell size, numerous degenerating neuronal processes within the cytoplasm as well as lysosome like bodies and vacuoles. The changes developing along with epileptic activity were interpreted to indicate degeneration and subsequent phagocytotic activity of neuronal processes in synaptic regions of the ganglia. Thus, evidence is presented for synaptically induced degenerative processes in an intact nervous tissue that is not affected by seizure-induced alterations of respiration or systemic circulation.

Cytogenetic and immunofluorescence analysis of benzo[a]pyrene-DNA adduct formation and chromosome damage in larval brain neuroblasts of Drosophila melanogaster

Recently we have evaluated the relationship between benzo[a]-pyrene(BaP)-DNA adducts, determined by 32P-postlabelling, and clone frequencies in the somatic mutation and recombination test (SMART) in Drosophila melanogaster. Following that study we proceeded to characterise further the mechanism of induction of genetic damage in vivo by BaP in Drosophila by cytogenetic analysis of larval brain neuroblasts. Third stage larvae were treated with 4 and 10 mM BaP for 24, 48 or 72 h. In all cases, the larvae were killed 72 h after the beginning of treatment, entailing 48, 24 or 0 h post-treatment recovery in BaP-free medium, respectively. At the end of the treatment the following data were collected: (i) the types and levels of chromosome aberrations in neuroblast metaphase and anaphase nuclei; (ii) the distribution and level of BaP-DNA adducts, revealed by indirect immunofluorescence in neuroblast nuclei using an anti-(BaP-DNA) antibody. The results indicate that BaP induces chromosome breaks, deletions and exchanges in this system. In particular, chromosome exchanges decrease as the post-treatment recovery time increases, and the dynamics of breaks and deletions appear to be inversely related to those of the exchanges. This suggests that exchanges may require few preconditions to occur and are thus expressed soon after treatment. Chromosome breaks and deletions could require multiple single events before the actual damage is expressed (even some cell divisions away from the end of treatment). The immunofluorescence analysis suggests that BaP-DNA adducts are more abundant in the heterochromatin of the neuroblast nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)