Age-dependent increase of thermal stability of in situ chromatin of rat liver and its reversal after hepatectomy

Age changes of chromatin. A review

The age-related studies of chromatin and DNA has attracted significant interest in recent years. However, individual works describe only some and a few of the many changes of chromatin. It is often difficult to decide whether these changes have secondary or primary nature. The overview of these studies makes it possible to realize how many very complex and interdependent changes occur in chromatin during ageing. Chromatin is the most complex among self-reproducible parts of the cell. A very sophisticated structure of chromatin makes possible the differential transcription of a genetic programme which supports the accurate specialized functions of each cell in interphase and also provides a mechanism for perfect reproduction of this complex machinery of genetic information during cell division. It is known that chromatin proteins, more than chromatin DNA show tissue specificity and developmental changes. There are many theories of cellular ageing which select some special types of DNA, RNA or protein changes and to promote them as the main or primary causes of cellular senescence. However, if these changes are considered within the more comprehensive picture of functional structure of chromatin the results show the interdependence of individual alterations and their proper place in the complex, multichannel, species and tissue-specific character of actual ageing. An attempt to summarize the basic facts and theories about age changes of the two main parts of chromatin structure, proteins and DNA is being made in this review. At the same time the author tried to develop a concept of non-random distribution of the age changes in chromatin and a possible higher rate of accumulation of different alteration and lesions in the transcribed and functionally active parts of chromatin.

Disulfide bonds and the structure of the chromatin complex in relation to aging

Triton X-100 washed nuclei from livers of young-adult and old mice were digested with micrococcal nuclease and pelleted. Supernatants (1SF) were saved and the pellets lysed in a hypotonic EDTA buffer. A second supernatant (2SF) and a final pellet (P) were obtained by recentrifugation (7000 g, 7 minutes). The 1SF/2SF ratio, which has been shown to be an index of the transcriptionally active to inactive chromatin ratio, was lower in older mice. The fraction relatively resistant to solubilization by the nuclease (P) was found by isopycnic sucrose gradient centrifugation to be in a more compatc, condensed state when prepared from older mice. Higher amounts of heavy density chromatin were obtained from nuclei of old than young mice by hypotonic lysis plus minimal mechanical shearing. 2-Mercaptoethanol (2ME) treatment brought the density of the material of P from old mice back to the levels of young mice. In both age groups 2ME decreased the densities of mechanically sheared chromatin as well as of the whole Triton X-100 washed nuclei. In nuclease digestion experiments treatment of the nuclei from both age groups with S-S reducing agents increased the release of DNA from P into the supernatants. The results are consistent with S-S bonds being involved in the condensed structure of chromatin in young and old mice and in the shift of the chromatin complex towards a more compact, condensed state in old age.

On the aging of organisms and their cells

An explanation of organismic ageing based on a limited division capacity of dividing cells is difficult to reconcile with much of the available data. The physiology of cells in ageing organisms tends, on the contrary, to suggest that organisms age as a result of degeneration in their non-dividing cell populations. Senescence in these non-mitotic cells resmebles the ageing of the non-dividing fraction of cell cultures clonally senescing, or maintained in long-term quiescence in vitro. As cultures of diploid human fibroblasts senesce there is an accumulation of non-dividing cells. Alterations in these post-mitotic cells can explain the senescent properties of late passage cultures. It is proposed that during the in vitro senescence of fibroblast cultures, cell ageing results from, as opposed to causes, the absence of mitosis. Cell ageing may primarily result from changes in the chromatin induced by the non-mitotic state.