A higher order chromatin structure that is lost during differentiation of mouse neuroblastoma cells

Differential scanning calorimetric study of antibiotic distamycin A binding with chromatin within isolated rat liver nuclei

CONTEXT

Natural oligopeptide antibiotic distamycin A (Dst) biosynthesized by Streptomyces distallicus is traditionally used in medical practice as an anti-inflammatory and antitumour drug.

OBJECTIVE

Dst was investigated for its effect on the structural components of native chromatin directly within isolated rat liver nuclei in the presence of physiologically significant cations (magnesium or spermine and spermidine).

MATERIALS AND METHODS

Differential scanning calorimetry (DSC) was used to study the Dst action at molar ratio Dst/DNA = 0.1 and 0.15 mM Dst on the melting profile of nuclei suspension in different conditions.

RESULTS

Results showed that the thermodynamic parameters of control nuclei in the presence of polyamines or Mg²⁺were different. The incubation of nuclei with Dst raised transition temperatures of relaxed (peak II) and topologically constrained DNA (peak III) by 6-8 °C and decreased by 2-4 °C that of core-histones (peak I). The total excess transition enthalpy (ΔH_exc) in buffer with polyamines (24.7 kJ/mol DNA nucleotides) increased by1.5 times versus control but in buffer with Mg²⁺, the value of ΔH_exc (35.8 kJ/mol DNA nucleotides) remained unchanged.

CONCLUSIONS

The association of Dst with chromatin in the nucleus weakens histone-DNA contacts and causes additional strengthening of interaction between two complementary DNA chains. Our results contribute towards validation of DSC to test drug ability to modulate chromatin structure in the physiological environment and to clarify the mechanism of these modulations.

Hho1p, the linker histone of Saccharomyces cerevisiae, is important for the proper chromatin organization in vivo

Despite the existence of certain differences between yeast and higher eukaryotic cells a considerable part of our knowledge on chromatin structure and function has been obtained by experimenting on Saccharomyces cerevisiae. One of the peculiarities of S. cerevisiae cells is the unusual and less abundant linker histone, Hho1p. Sparse is the information about Hho1p involvement in yeast higher-order chromatin organization. In an attempt to search for possible effects of Hho1p on the global organization of chromatin, we have applied Chromatin Comet Assay (ChCA) on HHO1 knock-out yeast cells. The results showed that the mutant cells exhibited highly distorted higher-order chromatin organization. Characteristically, linker histone depleted chromatin generally exhibited longer chromatin loops than the wild-type. According to the Atomic force microscopy data the wild-type chromatin appeared well organized in structures resembling quite a lot the "30-nm" fiber in contrast to HHO1 knock-out yeast.

Effects of tacrine upon murine neuroblastoma cells

Tacrine [9-amino-1,2,3,4-tetrahydroacridine] (THA), a potent acetylcholinesterasic inhibitor, is utilized in the pharmacological treatment of Alzheimer's disease (Birne and Arie, 1994). Cytopharmacology of THA is still largely to be discovered. In the present paper we report some effects produced by THA on murine neuroblastoma cells (N2A) used as an experimental model. N2A cells treated with THA at low concentration (1 mu M) showed a reduced cell's mitosis and a remarkable reduction of protein synthesis. Eventually, a marked reduction on the phosphorylation of proteins associated to neurofilaments 200 kD, is observed using specific antibody.

Differential scanning calorimetry of chicken erythrocyte nuclei

Investigation of structural features of native chromatin requires the use of intact nuclei, a turbid material which cannot be analyzed by optical methods. Differential scanning calorimetry does not require optically clear samples and has been proved by a number of authors to be a powerful tool in this field of study. By this technique, chicken erythrocyte nuclei were found to undergo at least four thermal transitions, centered at 59, 74, 88 and 98 degrees C. The highest temperature transition is strongly dependent on age and storage conditions of the nuclei. Adequate storage conditions overcame this problem and reproducible scans were obtained over a period of several months. This technical improvement has permitted the reconsideration of the occurrence of the fourth calorimetric transition, previously believed to be displayed only in replicating nuclei. Evidence gathered in the presence of perturbants and possible ligands allows the assignment of the four transitions to a nuclear protein scaffold, histones, nucleosomal DNA and a superstructured form of DNA. Moreover, it suggests that the higher-order structure is stabilized by fibronectin-like proteins.

Effects of salt concentration and H1 histone removal on the differential scanning calorimetry of nuclei

The effects of increasing NaCl concentrations on the melting profiles of chromatin in isolated nuclei contradicted published claims that structural transitions near 76 degrees C (Tn-7), near 89 degrees C (Tn-8), and near 105 degrees C (Tn-10) were respectively the melting of linker DNA, the melting of extended nucleosomal strands, and the collapse of nucleosomes in the 300-A fiber. Contrary to expectations of such an interpretation, decreases in salt concentration stabilized Tn-7 and failed to eliminate Tn-10. Moreover, nuclei depleted of H1 histone, which is known to be essential for the formation of the 300-A fiber, gave the same melting profile as intact nuclei with regard to the relative magnitudes of Tn-8 and Tn-10. The effect of salt concentration on the melting profiles and the insensitivity of Tn-8 and Tn-10 to H1 histone removal supports the notion that Tn-7 is the collapse of the nucleosome while Tn-8 and Tn-10 are respectively the unstacking of nucleotide bases in relaxed chromatin and supercoiled chromatin. The identification of Tn-8 as the unstacking of bases in relaxed DNA, and Tn-10 as unstacking in supercoiled DNA, shows that scanning calorimetry can be used to measure the state of repair of DNA in the nucleus. The gain in Tn-8 at the expense of Tn-10 that is seen as the mitotic index drops and differentiation occurs suggests that nicks accumulate in the DNA, perhaps because the gross aggregation of the inactive majority of the chromatin makes it inaccessible to repair enzymes.

The nucleoskeleton and the topology of transcription

Transcription is conventionally believed to occur by passage of a mobile polymerase along a fixed template. Evidence for this model is derived almost entirely from material prepared using hypotonic salt concentrations. Studies on subnuclear structures isolated using hypertonic conditions, and more recently using conditions closer to the physiological, suggest an alternative. Transcription occurs as the template moves past a polymerase attached to a nucleoskeleton; this skeleton is the active site of transcription. Evidence for the two models is summarised. Much of it is consistent with the polymerase being attached and not freely diffusible. Some consequences of such a model are discussed.

Competence growth factors can cause modification in higher-order chromatin structure in mouse embryo 3T3 fibroblasts

The authors compared sedimentation rates of nucleoids from mouse embryo 3T3 fibroblasts cultured in the presence or absence of different cell growth factors. The results clearly showed that rapidly sedimenting nucleoids are obtained only when cells are supplied with any of the following competence growth factors: platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), or the product of the oncogene v-sis (a peptide homologous to PDGF). The tumor promoter phorbol 12-myristate 13-acetate, an activator of protein kinase C and a partial mitogen, shares this property with the competence growth factors. Removal of these factors from the medium causes cells to enter Go and nucleoids to sediment at a slower rate. Protein synthesis is required for growth factor induction of change in nucleoid sedimentation, but inhibition of either DNA synthesis or DNA repair does not antagonize the effect of growth factors. Titration of nucleoids with ethidium bromide indicates that one possible mechanism for the nucleoid change is the unwinding of DNA in supercoiled loops. The results indicated that the nucleoid change constitutes a cell response to competence factors that might have an important role in cell proliferation.

Structural domains and conformational changes in nuclear chromatin: a quantitative thermodynamic approach by differential scanning calorimetry

A good deal of information on the thermodynamic properties of chromatin was derived in the last few years from optical melting experiments. The structural domains of the polynucleosomal chain, the linker, and the core particle denature as independent units. The differential scanning calorimetry profile of isolated chromatin is made up of three endotherms, at approximately 74, 90, and 107 degrees C, having an almost Gaussian shape. Previous work on this matter, however, was mainly concerned with the dependence of the transition enthalpy on external parameters, such as the ionic strength, or with the melting of nuclei from different sources. In this paper we report the structural assignment of the transitions of rat liver nuclei, observed at 58, 66, 75, 92, and 107 degrees C. They are representative of the quiescent state of the cell. The strategy adopted in this work builds on the method developed for the investigation of complex biological macromolecules. The heat absorption profile of the nucleus was related to the denaturation of isolated nuclear components; electron microscopy and electrophoretic techniques were used for their morphological and molecular characterization. The digestion of chromatin by endogenous nuclease mimics perfectly the decondensation of the higher order structure and represented the source of several misinterpretations. This point was carefully examined in order to define unambiguously the thermal profile of native nuclei. The low-temperature transitions, centered around 58 and 66 degrees C, arise from the melting of scaffolding structures and of the proteins associated with heterogeneous nuclear RNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of choline acetyltransferase activity by retinoic acid and sodium butyrate in a cultured human neuroblastoma

Choline acetyltransferase (Acetyl-CoA:choline O-acetyltransferase, EC 2.3.1.6, abbreviated ChAT), the biosynthetic enzyme for acetylcholine and acetylcholinesterase (EC 3.1.1.7, abbreviated AChE) are expressed in a human cholinergic neuroblastoma cell line, MC-IXC. We have shown that ChAT activity can be regulated in culture by retinoic acid, an active metabolite of vitamin A, and by sodium butyrate, an organic fatty acid. Optimal concentrations of these agents produce 4.3-fold and 1.6-fold increases in ChAT activity, respectively. The effects of retinoic acid are statistically significant after 24 h, whereas for sodium butyrate significant differences are seen only after 48 h. Since retinoic acid stimulation of ChAT activity was reversed only by trypsin treatment and not by removal of retinoic acid from the medium, this suggests that this agent may be acting at the level of the cell surface. Other differentiating conditions, such as culture in serum-free medium or addition of 1-2% dimethylsulfoxide did not increase ChAT activity. Acetylcholinesterase activity was shown to increase only in the presence of sodium butyrate, suggesting that retinoic acid and sodium butyrate may be acting via different pathways. Retinoic acid and sodium butyrate both seem to be permissive rather than instructive in regulating ChAT activity in that they are unable to induce ChAT expression de novo in cell lines which do not already express ChAT activity.

A high melting structure in DNA distinguishes phases of the cell cycle

Differential scanning microcalorimetry of the nuclei of dividing CHO cells revealed DNA structures that showed structural transitions at 60, 76, 88, and 105 degrees C (transitions I to IV, respectively). In cultures synchronized by isoleucine deprivation the enthalpies of transitions I and II were rather constant throughout the cell cycle. While the sum of the enthalpies of III and IV was nearly constant, the ratio of IV to III varied substantially from one phase of the cycle to another. A high IV:III ratio of 6 characterized G1 while S phase gave a IV:III ratio of about 2. Cells containing metaphase chromosomes also showed a IV:III ratio near 2. The IV:III ratio for CHO cells showed a progressive decrease as the cells were maintained in isoleucine-free medium from 0 to 6 days.

Sodium butyrate preserves aspects of the differentiated phenotype of normal adult rat hepatocytes in culture

We have determined that sodium butyrate and, to a lesser extent, dimethylsulfoxide (DMSO) and 3-aminobenzamide (3-AB) preserve aspects of the differentiated phenotype of primary cultures of adult rat hepatocytes. The histone deacetylase inhibitor, butyrate, inhibits the increase in gamma-glutamyltranspeptidase (GGT) activity and the decrease in basal tyrosine aminotransferase (TAT) activity normally observed when hepatocytes are cultured under appropriate conditions. The effects of butyrate on GGT and TAT activities are accompanied by parallel changes in GGT and TAT mRNA levels. The poly(ADP)ribose-synthetase inhibitor, 3-aminobenzamide, has effects similar to butyrate on GGT activity and mRNA levels, while both 3-AB and DMSO increase basal TAT activity in cultured hepatocytes. Under appropriate conditions all three agents--butyrate, 3-AB, and DMSO--extend the length of time cultured hepatocytes can be maintained as confluent monolayers. However, under all the conditions studied, butyrate extended the length of time hepatocytes could be maintained as monolayers more than any other treatment used. Butyrate-treated hepatocytes maintained ultrastructural features that were more similar to those of hepatocytes in vivo than hepatocytes treated with any other of the agents tested. Histone acetylation levels of primary cultures of adult rat hepatocytes declined concomitant with the loss of the differentiated phenotype of the cells. These results suggest that histone acetylation may play a role in the changes in gene expression observed when hepatocytes are placed in culture.

A cell line with decreased sensitivity to the methyl mercury-induced stimulation of alpha-amanitin sensitive RNA synthesis in isolated nuclei

1. In nuclei isolated from cells of the B50 rat neuroblastoma line the stimulatory effect of methyl mercury on alpha-amanitin-sensitive RNA synthesis is very much reduced compared to the stimulatory effect in HeLa nuclei (see: Frenkel G. D. and Randles K. (1982) Specific stimulation of alpha-amanitin-sensitive RNA synthesis in isolated HeLa nuclei by methyl mercury. J. biol. Chem. 257, 6275-6279). 2. The stimulatory effect of another mercury compound, p-hydroxymercuribenzoate, was also much less pronounced in the B50 nuclei. 3. Similar results were obtained with nuclei isolated from B50 cells which had been induced to differentiate by exposure to dibutaryl cyclic AMP. 4. Nuclei isolated from cells of another rat neuroblastoma line (B35), and nuclei from cells of a human neuroblastoma line both exhibited levels of stimulation similar to that of HeLa nuclei. 5. The B50 and HeLa cells were also compared as to their sensitivity to other effects of methyl mercury.

In situ factors affecting stability of the DNA helix in interphase nuclei and metaphase chromosomes

The data from earlier cytochemical studies, in which the metachromatic fluorochrome acridine orange (AO) was used to differentially stain single vs double-stranded DNA, suggested that DNA in situ in intact metaphase chromosomes or in condensed chromatin of G0 cells is more sensitive to denaturation, induced by heat or acid, than DNA in decondensed chromatin of interphase nuclei. Present studies show that, indeed, DNA in permeabilized metaphase cells, in contrast to cells in interphase, when exposed to buffers of low pH (1.5-2.8) becomes digestible with the single-strand-specific S1 or mung bean nucleases. A variety of extraction procedures and enzymatic treatments provided evidence that the presence of histones, HMG proteins, and S-S bonds in chromatin, as well as phosphorylation or poly(ADP)ribosylation of chromatin proteins, can be excluded as a factor responsible for the differential sensitivity of metaphase vs interphase DNA to denaturation. Cell treatment with NaCl at a concentration of 1.2 N and above abolished the difference between interphase and mitotic cells, rendering DNA in mitotic cells less sensitive to denaturation; such treatment also resulted in decondensation of chromatin visible by microscopy. The present data indicate that structural proteins extractable with greater than or equal to 1.2 N NaCl may be involved in anchoring DNA to the nuclear matrix or chromosome scaffold and may be responsible for maintaining a high degree of chromatin compaction in situ, such as that observed in metaphase chromosomes or in G0 cells. Following dissociation of histones, the high spatial density of the charged DNA polymer may induce topological strain on the double helix, thus decreasing its local stability; this can be detected by metachromatic staining of DNA with AO or digestion with single-strand-specific nucleases.