Search PubMed⌕ Search

Biomedical subjects

S Dimitrov

Publications and source records attributed to S Dimitrov.

At least 19 recordsLinked to original sources
PubMed1999

Elasticity measurements show the existence of thin rigid cores inside mitotic chromosomes.

Chromosome condensation is one of the most critical steps during cell division. However, the structure of condensed mitotic chromosomes is poorly understood. In this paper we describe a new approach based on elasticity measurements for studying the structure of in vitro assembled mitotic chromosomes in Xenopus egg extract. The approach is based on a unique combination of measurements of both longitudinal deformability and bending rigidity of whole chromosomes. By using specially designed micropipettes, the chromosome force-extension curve was determined. Analysis of the curvature fluctuation spectrum allowed for the measurement of chromosome bending ridigity. The relationship between the values of these two parameters is very specific: the measured chromosome flexibility was found to be 2,000 times lower than the flexibility calculated from the experimentally determined Young modulus. This requires the chromosome structure to be formed of one or a few thin rigid elastic axes surrounded by a soft envelope. The properties of these axes are well-described by models developed for the elasticity of titin-like molecules. Additionally, the deformability of in vitro assembled chromosomes was found to be very similar to that of native somatic chromosomes, thus demonstrating the existence of an essentially identical structure.

Animals↗
PubMed1998

The globular domain of histone H1 is sufficient to direct specific gene repression in early Xenopus embryos.

One molecule of a linker histone such as histone H1 is incorporated into every metazoan nucleosome [1]. Histone H1 has three distinct structural domains: the positively charged amino-terminal and carboxy-terminal tails are separated by a globular domain that is similar to the winged-helix motif found in sequence-specific DNA-binding proteins [2]. The globular domain interacts with DNA immediately contiguous to that wrapped around the core histones [3,4], whereas the tail domains are important for the compaction of nucleosomal arrays [5]. Experiments in vivo indicate that histone H1 does not function as a global transcriptional repressor, but instead has more specific regulatory roles [6-9]. In Xenopus, maternal stores of the B4 linker histone that are assembled into chromatin during the early cleavage divisions are replaced by somatic histone H1 during gastrulation [10]. This transition in chromatin composition causes the repression of genes encoding oocyte-type 5S rRNAs, and restricts the competence of ectodermal cells to differentiate into mesoderm [6,9-11]. Here, we demonstrate that the globular domain of histone H1 is sufficient for directing gene-specific transcriptional repression and for restricting the mesodermal competence of embryonic ectoderm. We discuss our results in the context of specific structural roles for this domain in the nucleosome.

Animals↗
PubMed1998

Persistent interactions of core histone tails with nucleosomal DNA following acetylation and transcription factor binding.

In this study, we examined the effect of acetylation of the NH2 tails of core histones on their binding to nucleosomal DNA in the absence or presence of bound transcription factors. To do this, we used a novel UV laser-induced protein-DNA cross-linking technique, combined with immunochemical and molecular biology approaches. Nucleosomes containing one or five GAL4 binding sites were reconstituted with hypoacetylated or hyperacetylated core histones. Within these reconstituted particles, UV laser-induced histone-DNA cross-linking was found to occur only via the nonstructured histone tails and thus presented a unique tool for studying histone tail interactions with nucleosomal DNA. Importantly, these studies demonstrated that the NH2 tails were not released from nucleosomal DNA upon histone acetylation, although some weakening of their interactions was observed at elevated ionic strengths. Moreover, the binding of up to five GAL4-AH dimers to nucleosomes occupying the central 90 bp occurred without displacement of the histone NH2 tails from DNA. GAL4-AH binding perturbed the interaction of each histone tail with nucleosomal DNA to different degrees. However, in all cases, greater than 50% of the interactions between the histone tails and DNA was retained upon GAL4-AH binding, even if the tails were highly acetylated. These data illustrate an interaction of acetylated or nonacetylated histone tails with DNA that persists in the presence of simultaneously bound transcription factors.

Acetylation↗
PubMed1997

Histone acetylation: influence on transcription, nucleosome mobility and positioning, and linker histone-dependent transcriptional repression.

We demonstrate using a dinucleosome template that acetylation of the core histones enhances transcription by RNA polymerase III. This effect is not dependent on an increased mobility of the core histone octamer with respect to DNA sequence. When linker histone is subsequently bound, we find both a reduction in nucleosome mobility and a repression of transcription. These effects of linker histone binding are independent of core histone acetylation, indicating that core histone acetylation does not prevent linker histone binding and the concomitant transcriptional repression. These studies are complemented by the use of a Xenopus egg extract competent both for chromatin assembly on replicating DNA and for RNA polymerase III transcription. Incorporation of acetylated histones and lack of linker histones together facilitate transcription by >10-fold in this system; however, they have little independent effect on transcription. Thus core histone acetylation significantly facilitates transcription, but this effect is inhibited by the assembly of linker histones into chromatin.

Acetylation↗
PubMed1997

What do linker histones do in chromatin?

Knockout experiments in Tetrahymena show that linker histone H1 is not essential for nuclear assembly or cell viability. These results, together with a series of biochemical and cell biological observations, challenge the existing paradigm that requires linker histones to be a key organizing component of higher-order chromatin structure. The H1 knockouts also reveal a much more subtle role for H1. Instead of acting as a general transcriptional repressor, H1 is found to regulate a limited number of specific genes. Surprisingly, H1 can both activate and repress transcription. We discuss how this architectural protein might accomplish this important regulatory role.

Amino Acid Sequence↗
PubMed1997

[Problems in the diagnosis and treatment of solitary and multiple kidney cysts].

Over the period 1989-1994, thirty-three patients presenting solitary cysts, and 19--polycystosis undergo clinical study. A variety of examination and differential diagnostic methods are used, namely: echography, venous urography, gamma chamber scintigraphy, CAT and biochemical analysis of the cystic fluid. The listed methods contribute greatly to make exact diagnosis and undertake adequate treatment of the patients.

Biopsy, Needle↗
PubMed1996

Remodeling somatic nuclei in Xenopus laevis egg extracts: molecular mechanisms for the selective release of histones H1 and H1(0) from chromatin and the acquisition of transcriptional competence.

The molecular mechanisms responsible for the remodeling of entire somatic erythrocyte nuclei in Xenopus laevis egg cytoplasm have been examined. These transitions in chromosomal composition are associated with the capacity to activate new patterns of gene expression and the re-acquisition of replication competence. Somatic linker histone variants H1 and H1 (0) are released from chromatin in egg cytoplasm, whereas the oocyte-specific linker histone B4 and HMG1 are efficiently incorporated into remodeled chromatin. Histone H1 (0) is released from chromatin preferentially in comparison with histone H1. Core histones H2A and H4 in the somatic nucleus are phosphorylated during this remodeling process. These transitions recapitulate the chromosomal environment found within the nuclei of the early Xenopus embryo. Phosphorylation of somatic linker histone variants is demonstrated not to direct their release from chromatin, nor does direct competition with cytoplasmic stores of linker histone B4 determine their release. However, the molecular chaperone nucleoplasmin does have an important role in the selective removal of linker histones from somatic nuclei. For Xenopus erythrocyte nuclei, this disruption of chromatin structure leads to activation of the 5S rRNA genes. These results provide a molecular explanation for the remodeling of chromatin in Xenopus egg cytoplasm and indicate the capacity of molecular chaperones to disrupt a natural chromosomal environment, thereby facilitating transcription.

Animals↗
PubMed1996

Evidence for a shared structural role for HMG1 and linker histones B4 and H1 in organizing chromatin.

The high mobility group proteins 1 and 2 (HMG1/2) and histone B4 are major components of chromatin within the nuclei assembled during the incubation of Xenopus sperm chromatin in Xenopus egg extract. To investigate their potential structural and functional roles, we have cloned and expressed Xenopus HMG1 and histone B4. Purified histone B4 and HMG1 form stable complexes with nucleosomes including Xenopus 5S DNA. Both proteins associate with linker DNA and stabilize it against digestion with micrococcal nuclease, in a similar manner to histone H1. However, neither histone B4 nor HMG1 influence the DNase I or hydroxyl radical digestion of DNA within the nucleosome core. We suggest that HMG1/2 and histone B4 have a shared structural role in organizing linker DNA in the nucleosome.

Amino Acid Sequence↗
PubMed1994

Nuclear assembly is independent of linker histones.

The role of linker histones in the assembly of functional nuclei was examined with the use of a cell-free extract of Xenopus eggs that transforms condensed sperm chromatin into DNA-replication-competent pronuclei. When linker histones were removed from the extract, the resultant pronuclei were indistinguishable from those formed in the complete extract. The assembly of functional nuclear membrane, nuclear lamina, and prereplication centers allowed identical DNA replication efficiencies. Thus, linker histones are not required for the assembly of morphologically normal nuclei capable of DNA replication.

Animals↗
PubMed1994

Specific regulation of Xenopus chromosomal 5S rRNA gene transcription in vivo by histone H1.

The incorporation of histone H1 into chromatin during embryogenesis directs the specific repression of the Xenopus oocyte 5S rRNA genes. An increase in histone H1 content specifically restricts TFIIIA-activated transcription, and a decrease in histone H1 within chromatin facilitates the activation of the oocyte 5S rRNA genes by TFIIIA. Variation in the amount of histone H1 in chromatin does not significantly influence somatic 5S rRNA gene transcription. Thus, the regulated expression of histone H1 during Xenopus development has a specific and dominant role in mediating the differential expression of the oocyte and somatic 5S rRNA genes. This example demonstrates that histones can exert dominant repressive effects on the transcription of a gene in vivo in spite of an abundance of transcription factors for that gene.

Animals↗
PubMed1994

Histone acetylation influences both gene expression and development of Xenopus laevis.

We examine the potential role of histone hyperacetylation in gene activation during Xenopus development using Trichostatin A, (TSA), a specific inhibitor of histone deacetylase. We find that TSA is very effective in inducing both core histone hyperacetylation and histone H1(0) gene expression in a Xenopus somatic cell line. In contrast, TSA does not induce histone hyperacetylation or histone H1(0) transcription in Xenopus oocytes. Histone hyperacetylation is developmentally regulated during Xenopus embryogenesis; hyperacetylated histones first accumulate early in gastrulation. The capacity of TSA to induce histone H1(0) gene expression correlates with the induction of histone hyperacetylation. Concentrations of TSA sufficient to induce histone hyperacetylation in Xenopus embryos delay gastrulation and cause diminished midtrunk and posterior formation, suggesting defects in mesoderm formation. Although the constitutive hyperacetylation of the histones does not prevent either the cell division or differentiation sufficient for early morphogenesis it has a role in establishing stable states of differential gene activity during gastrulation.

Acetylation↗
PubMed1994

Remodeling sperm chromatin in Xenopus laevis egg extracts: the role of core histone phosphorylation and linker histone B4 in chromatin assembly.

We find that the remodeling of the condensed Xenopus laevis sperm nucleus into the paternal pronucleus in egg extracts is associated with phosphorylation of the core histones H2A, H2A.X and H4, and uptake of a linker histone B4 and a HMG 2 protein. Histone B4 is required for the assembly of chromatosome structures in the pronucleus. However neither B4 nor core histone phosphorylation are required for the assembly of spaced nucleosomal arrays. We suggest that the spacing of nucleosomal arrays is determined by interaction between adjacent histone octamers under physiological assembly conditions.

Animals↗
PubMed1993

Chromatin transitions during early Xenopus embryogenesis: changes in histone H4 acetylation and in linker histone type.

We describe major transitions in the type and modification of chromatin-associated proteins during the early development of Xenopus laevis. Histone H4 is stored in the diacetylated form in the egg and is progressively deacetylated during normal development. If histone deacetylases are inhibited with sodium butyrate, hyperacetylated histone H4 only accumulates after the mid-blastula transition. The type of linker histone in chromatin also changes during embryogenesis, from predominantly the B4 protein at the mid-blastula transition to predominantly histone H1 at the end of gastrulation. These transitions in chromatin composition correlate with major changes in the replicative and transcriptional activity of embryonic nuclei.

Acetylation↗
PubMed1990

Histones H1 and H4 are present near the replication fork.

The presence of histones H1 and H4 at the sites of actual DNA synthesis has been studied with Ehrlich ascites tumour cells, pulse labeled for different times with 3H-thymidine and then treated with formaldehyde to crosslink histones to DNA. The fixed chromatin fragments were sonicated to reduce the size of DNA, purified in a CsCl gradient and immunoprecipitated with antibodies to histones H1 and H4. Determination of specific radioactivity in precipitated probes showed that both histones have been associated with nascent DNA even upon 1 min pulse with 3H-thymidine, thus indicating their presence near the replication fork.

Animals↗
PubMed1990

[The value of the electrocardiographic indices of left-ventricular hypertrophy for the approximation of the valvular systolic gradient in patients with aortic stenosis].

In 36 patients with mild, moderate and severe aortic stenosis, proved by cardiac catheterization, some electrocardiographic indices for left ventricular hypertrophy were compared with the value of the aortic valve peak systolic gradient. The correlative analysis showed a moderate correlation between the T wave amplitude in leads V5 and V6 of the ECG and the aortic valve gradient--r = 0.46 (p less than 0.005). On the basis of this linear correlation a regression equalization is proposed for the approximate electrocardiographic assessment of the degree of aortic stenosis.

Adult↗
PubMed1989

[CT tomography of normal parotid and mandibular salivary glands].

Computed tomography (CT) allows the differentiation of tissues by measurement of their roentgenologic densities. There are a few controversial literature data on the roentgenologic densities of the parotid and submandibular glands. These differences are, in our opinion, due to several factors: 1. The complex anatomic structure of the facial region of the skull, that demands an exact centering. In daily practice the respective imaging parameters are not always kept constant (including partial volume effects), thus introducing density errors. 2. Metallic prostheses and obturators as well as the high-density bones of the facial skull cause artefacts that impair image quality and density measurements. 3. The small density differences between the salivary glands and the adjacent soft tissues makes their differentiation difficult. 4. Different devices and parameters are used for scanning, resulting in different densities. Considering these facts, we have devised a complex method for the investigation of the parotid and mandibular salivary glands.

Adolescent↗