Lo stress cellulare e l'invecchiamento influenzano l'organizzazione del nucleolo e dell'eterocromatina negli epatociti murini

Ageing is characterized by progressive physiological decline of tissues functions and performances. It is a multifactorial process, influenced by various environmental, genetic and epigenetic factors, in which the organism continuously adapts the progressive accumulation of damages to the system maintenance over time. This prompted researchers to find possible strategies to delay ageing in order to extend lifespan and improve human’s quality of life. Liver carries out many different metabolic processes that are fundamental for our organism. The low rate of cell renewal and the high metabolism of hepatocytes probably makes them particularly prone to the effects of ageing. Additionally, the involvement in such different metabolic processes and the frequent exposure to different exogenous molecules due to their prominent role in detoxification, make hepatocyte ageing complex to analyze and subjected to great variations depending on individual lifestyle. In this study, we investigated the effects of ageing and of various stressful conditions that could affect ageing process by accelerating or slowing it down in hepatocytes, on the murine model, both on mouse hepatic tissue and the mouse hepatocyte cell line AML12. We focused on the nucleus and specifically on the changes induced in the heterochromatin organization and the nucleolar activity. More in detail, we compared liver tissue from young (3 months old) and old (24 months old) mice. Moreover, we analyzed the effects of exposure to the toxic agent mercury chloride, because of the primary liver involvement in xenobiotics detoxification; the depletion of serum to mimic the reduction of calorie intake, given the critical role of the liver in sugars and fats metabolism and being this a strategy known to slow down ageing. We investigated whether the use of a high dose of dexamethasone, a corticosteroid with anti-inflammatory properties, could exert a protective effect and finally, we analyzed senescent hepatocytes as a model of precocious ageing. Studying the effect of ageing in liver tissue on mouse model in a controlled environment did not highlight major variations in the heterochromatin organization and more generally in the nuclear architecture. We only detected changes in some histone post-translational modifications associated to heterochromatin. Concerning the hepatocytes cell model of senescence instead, as generally happens, they have larger dimensions, consequently, the nuclei were also enlarged compared to control hepatocytes. Heterochromatin domains increased their extension and in many cases occupies the largest part of the nucleoplasm. The nucleoli were very large with many fibrillar centers. Mercury Chloride treatment induced changes in morphology and activity of the nucleoli making them similar to that observed in senescent cells.However, this treatment, differently from senescence, causes extensive heterochromatin decondensation. Serum-depleted cells show a similar degree of chromatin condensation as in control cells grown in complete medium, although the total amount of heterochromatin resulted higher, while nucleoli have a reduced size and activity compared to the control. Finally, Dexamethasone treatment seemed to induce changes that are different from the other cell stress analyzed and opposite to those observed in senescent cells, as chromatin decondensation, but at the same time a reduction of the activity and size of the nucleolus. Based on these results we could uncover a trend according to which smaller and less active nucleoli slow down the ageing process. Conversely, a general correlation between the organization of heterochromatin and ageing or the effects of cell stress was more difficult to establish, probably because of the subtle dynamics that come into play in the different conditions that are hardly attributable to a single trend at the level of the total fraction of repressed chromatin and not of the specific gene locus.