[Possible involvement of DNA breaks in epigenetic regulation of cell differentiation].

The review summarizes the authors' and literature data on accumulation of DNA breaks in differentiating cells. Large 50-kb free DNA fragments were observed by several research teams in non-apoptotic insect, mammal, and plant cells. More intense DNA breakage was observed during maturation of spermatides, embryo development, and differentiation of myotubes, epidermal cells, lymphocytes, and neutrophils. In general, accumulation of DNA breaks in differentiating cells cannot be attributed to a decrease in the DNA repair efficiency. Poly(ADP)ribose synthesis often follows the DNA breakage in differentiating cells. We hypothesize that DNA fragmentation is an epigenetic tool for regulating the differentiation process. Scarce data on localization of the differentiation-associated DNA breaks indicate their preferable accumulation in specific DNA sequences including the nuclear matrix attachment sites. he same sites are degraded at early stages of apoptosis. Recent data on non-apoptotic function of caspases provide more evidence for possible existence of a DNA breakage mechanism in differentiating cells, resembling the initial stage of apoptosis. Excision of methylated cytosine and recombination are other possible explanations of the phenomenon. Elucidation of mechanisms of differentiation-induced DNA breaks appears to be a prospective research direction.

[The effect of mildronate and related substances on levels of thyroid hormones and some intermediates of lipid and carbohidrate metabolism in the hyperthyroid and hypothyroid rats].

We have investigated effects of Mildronate, gamma-butyrobetaine (GBB) and their combination ("Neomildronate") on the plasma levels of thyroid gland hormones and some intermediates of basal metabolism (free fatty acids, triglycerides, glucose) in rats with different dysfunctions of thyroid gland, including idiopathic hyperfunction and hypofunction induced by propylthiouracil or L-carnitin administration. Histological investigation of the thyroid gland was also performed. Intraperitoneal injections of Mildronate (150 mg/kg) during 20 days to Wistar male rats with elevated level of thyroid hormones and basal metabolism normalized thyroxine level and parameters of lipid metabolism. Mildronate, GBB and their combination did not affect the natural resurgence of rats with experimental hypofunction induced by propylthiouracil or L-carnitin administration. The possible biochemical role of given treatment in regulation of thyroid gland function is discussed.

[Structure of genomic domains of mammalian and avian globin genes]. / Struktura genomnykh domenov genov globinov mlekopitaiushchikh i ptits.

The data on the genomic domain structure of both mammalian and avian alpha- and beta-globin genes are reviewed. The specific features of chromatin, DNA binding to the nuclear matrix, and domain-specific transcripts are discussed. In humans, the beta-globin gene domain is located in the GC-depleted isochore and contains multiple nuclear matrix attachment regions. The locus is controlled by six chromatin regions hypersensitive to DNase located far upstream of the first structural gene. Some of these regions display enhancer activity to support normal transcription level in the domain. Other mammalian beta-globin domains are similarly organized. The avian beta-globin genes are specifically arranged and their expression is less dependent from the locus control region. The human alpha-globin gene is located in the GC-rich isochore. The nuclear matrix attachment sites are not identified in this gene. An analog of the locus control region is located 40 kb upstream of the zeta-globin gene. The avian alpha-globin gene domains contain numerous nuclear matrix attachment regions. In these domains, an element located far upstream the genes regulates positive rather than negative transcription. An unidentified housekeeping gene as well as some other transcripts not encoding the structural globin genes is transcribed in the direction opposite to that of the globin genes in both mammalian and avian domains.

[Changes in the macromolecular composition and organization of the cell nucleus during gameto-, embryo- and histogenesis]. / Izmeneniia makromolekuliarnogo sostava i organizatsii kletochnogo iadra pri gameto-, émbrio- i histogeneze.

Data about changes in the molecular organization of the cell nucleus during the organism development and tissue differentiation are summarized. The peculiarities of the nucleus organization of gametes, its changes in the course of fertilization and early embryogenesis are discussed. Data concerning different objects of developmental biology are reviewed separately. The data about tissue specificity of the chromatin structure and characteristic features of nuclei of nervous, muscle, epithelial, and connective tissue cells are also presented. Different levels of chromatin organization, i.e., the primary and secondary DNA structure, protein composition, nucleosomal and supranucleosomal structures, DNA supercoiling in chromatin domains, nuclear skeleton structures, are specifically concerned in each particular case.

[The structural changes in chromatin at different levels of its organization during aging]. / Strukturnye izmeneniia khromatina na razlichnykh urovniakh ego organizatsii pri starenii.

The data about changes in the molecular organization of the cell nucleus during aging are reviewed. Changes in DNA primary and secondary structure include deletions of some sequences, changes in base methylation pattern and the increasing number of DNA breaks. Conditions underlying poor reproducibility of the results obtained in corresponding experiments are discussed. Changes observed in the nucleosomal and supranucleosomal chromatin structure reflect either its increasing compactization or the loss the nucleosomal structure during aging. The data about the increased DNA superhelicity and topoisomerase activity in aging organisms contradict the accepted views about age-related decrease in chromatin activity. It is suggested that the gene activity in aging organisms is specifically altered rather than generally decreased. The data are presented about the ways of preventing or reverting the age-related changes in chromatin structure.

Differential dissociation of chromatin digests: a novel approach revealing a hierarchy of DNA-protein interactions within chromatin domains.

We describe here a novel approach to the dissection of chromatin structure by extracting DNA fragments from digested nuclei irreversibly immobilized (via proteins) on Celite columns. Three successive gradients (NaCl, LiCl-urea, temperature) are used to release three families of DNA fragments: namely, the 'DNA adherence' classes DNA-0, DNA-I and DNA-II, respectively. This 'protein image' DNA chromatography separates DNA fragments in accordance with the tightness of their bonds with proteins in situ. There are at least two DNA-skeleton attachment sites differing from each other by their resistance to the dissociating agents used as well as their susceptibility to DNAase I and S1 nuclease treatments, DNA cross-linking and single-stranded breaks. Several lines of evidence show a specific, topological rather than chemical, DNA-protein linkage at the tight attachment site. A hierarchy of chromatin loops demarcated by these attachment sites was determined. The technique described is generally applicable and can be used both to probe DNA-protein interactions and to map specific DNA sequences within the chromatin domain.

Rearrangements of DNA-protein interactions in animal cells coupled with cellular growth-quiescence transitions.

Overall DNA-protein interactions in animal cells undergo drastic changes coupled with cellular transitions from quiescence to growth and reversely as revealed by nucleoprotein-Celite chromatography. DNA of chromatin was found to exist in one of the two sharply distinct alternative forms, namely, either tightly or weakly bound to protein moiety. These forms are specific for cycling and quiescent cells, respectively. The tight DNA-protein interactions characterize all cycling cells independent of the cell cycle phase. Transition of DNA of cycling cells from one form to another was observed as a result of treatment of isolated nuclei with DNase I.