Chromatin supraorganization and extensibility, and nuclear composition in mouse cells / Supraorganização e extensibilidade da cromatina, e composição nuclear em celulas de camundongo

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

Aging may be defined as the changes that take place in an organism with time. This process, in biology, is called senescence. Cellular senescence is observed in isolated cells, and has been studied typically in cultured cells, but its occurrence in vivo has been shown only in some mammalian tissues. Chromatin changes that take place with cellular senescence include increase in the resistance of chromatin to nuclease digestion and accumulation of histone modifications and non-histone proteins associated with heterochromatin. Although not all cells in an aged organism are subjected to cellular senescence, it is expected that changes in the chromatin structure and organization still occur. Caloric restriction is the only intervention known to extend life span in mammals. It has been shown that many genes whose expression pattern is altered in aged animals can be reverted to the levels observed in young animals after a caloric restriction diet or complete food withdrawal. Changes in chromatin structure may occur during the starvation period to induce changes in the expression level of several genes. With the aim of screening for alterations in the chromatin organization in mouse hepatocyte nuclei with aging or following starvation, we observed an increase in the viscoelastic properties of chromatin with aging, in terms of changes in the ability of this chromatin to form extended chromatin fibers after a lysis treatment in liver imprints on histological slides. These differences were accompanied by chromatin unpackage. Most of the viscoelasticity of the chromatin were dependent on its interactions with the nuclear matrix, and copies of genes whose transcription are no longer required in aged animals, tended to detach from the nuclear matrix. Changes in the viscoelastic properties and packing degree of chromatin had been shown previously in starved animals. However, no differences regarding this feature were seen in the present work. Nevertheless, regardless the physiological condition, DNA attached to the nuclear matrix seems to be gene-rich, while heterochromatic gene-poor regions were found both attached and detached from the nuclear matrix. We observed accumulation of heterochromatic marks (histone modifications) and non-histone proteins (heterochromatin proteins and glycoproteins present mainly in the chromocenters), as well as decreased histone modifications associated with transcription in hepatocyte nuclei of aged mice. All these changes are related to altered RNA synthesis observed in aged animals and are an evidence of the strong relationship between chromatin organization, composition, and control of gene expression. In another cell type, mouse sperm cells, its nuclear organization lead to different chromatin properties regarding its viscoelastic properties (increased). These differences are possibly related to a modified pattern of gene expression since gene transcription is almost or completely absent in sperm cells.

ASSUNTO(S)

envelhecimento aging cromatina starvation jejum genetic matriz nuclear epigenesis epigenesia genetica nuclear matrix chromatin

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