Age-related changes of the H1 and H1(0) histone variants in murine tissues

Gene Expression, Epigenetics and Ageing

As the popular adage goes, all diseases run into old age and almost all physiological changes are associated with alterations in gene expression, irrespective of whether they are causal or consequential. Therefore, the quest for mechanisms that delay ageing and decrease age-associated diseases has propelled researchers to unravel regulatory factors that lead to changes in chromatin structure and function, which ultimately results in deregulated gene expression. It is therefore essential to bring together literature, which until recently has investigated gene expression and chromatin independently. With advances in biomedical research and the emergence of epigenetic regulators as potential therapeutic targets, enhancing our understanding of mechanisms that 'derail' transcription and identification of causal genes/pathways during ageing will have a significant impact. In this context, this chapter aims to not only summarize the key features of age-associated changes in epigenetics and transcription, but also identifies gaps in the field and proposes aspects that need to be investigated in the future.

Glucocorticoid-induced changes in liver: effect of dexamethasone administration on DNA topoisomerase I and II activities and distribution of histone H1 subtypes

The activities of DNA topoisomerase I and II and the relative proportions of the histone H1 subtypes were investigated in rat liver which was undergoing hypertrophy and exhibiting increased transcriptional activity following the administration of dexamethasone. There was a rise in the level of activity of DNA topoisomerase I and a slight fall in that of DNA topoisomerase II. The relative proportions of the H1 subtypes were altered due to a preferential increase in H1.1. The results are discussed in relation to the effect of glucocorticoids on the transcription and replication of hepatic DNA.

Aging and radiation induced alterations of histones in regenerating rat liver

We studied the age-related and radiation-induced changes in histones in regenerating liver of rats between 1 and 28 months of age. The examinations were carried out 30 h after a two-third hepatectomy, i.e. after the first wave of DNA and histone synthesis. During induced regeneration age-related changes in liver manifested themselves in decreasing the cellularity per gram wet weight, in slowing down the increase in DNA and histone contents per organ, in changing the mutual proportions of histone fractions H1, H2A + H2B, H4 and in increasing the H1 zero variant within histone H1. Radiation-induced latent injury (a dose of 5.7 Gy gamma radiation 30 min before partial hepatectomy) was manifested during induced regeneration in similar but much more profound changes with their earlier onset. Since the changes in regenerating liver were found not be milder than in intact liver it means, that induced proliferation did not lead to elimination of altered cells or to their rejuvenation.

Age-related change in the amount of ubiquitinated histones in the mouse brain

We investigated age-related changes in the ubiquitination of histones in mouse brains. By Western blot analysis using rabbit anti-ubiquitin antibodies, histone H2A and H2B were found to be the only proteins that are ubiquitinated in the acid-extractable nuclear proteins of both young (4-month-old) and senescent (29-month-old) mouse brains. The proportion of ubiquitinated histones was about 30% higher in senescent mice than in young ones. When total brain homogenates were analyzed similarly using somewhat different age groups of the animals, the proportion in the old animals was about 2-fold higher than that in the young.

The biochemistry of mammalian senescence

Senescence is a process which, until quite recently, has been the subject of little scientific investigation. Even the word "senescence" is difficult to define, and complex methodological pitfalls have impeded progress. In the past few years, there have been exciting advances in understanding the physiological, cell biological, biochemical, and molecular biological nature of senescence. Changes in membrane function, protein synthesis, DNA structure (including glycosylation, altered tertiary structure, free-radical effects, and loss of telomeric DNA), and changes in gene regulation with age are reviewed. Recent work on changes in responses to transcriptional regulatory proteins and cellular senescence factors, some of which have been identified, is particularly promising and leads to the conclusion that senescence, at least in part, is a programmed process. Despite these advances, the fundamental cause of senescence remains elusive but might, as in the case of other biological processes which are phylogenetically widespread, turn out to be quite simple, and perhaps, even modifiable.