Circadian oscillators are endogenous processes which cycle with a period of 24 hours and allow synchronization of biological functions with temporal changes in the environment. Aim of the first part of this PhD thesis was to analyze proteins and biological processes circadianly regulated in human keratinocytes. The results show that temperature entrainment is associated with the enrichment in molecular components regulating cell proliferation, energy metabolism and redox balance. In particular, our analysis revealed the presence of the antioxidant enzyme Peroxiredoxin 2 in the nuclear fraction of synchronized cells suggesting that clock synchronization regulates nuclear levels of Peroxiredoxin 2. These results open new important insights in the role of peroxiredoxins within the circadian clock. In the second part of this work we showed for the first time that a second splicing variant of the core clock gene Period 2 (Per2), Per2S, is expressed both at mRNA and protein levels in human keratinocytes and that it displays circadian oscillation in its expression. Moreover, we found that PER2S preferentially localizes in the nucleoli of human keratinocytes and that a reversible perturbation of nucleolar structure induces reversible nucleolar disruption and PER2S de-localization, acting as a resetting stimulus for circadian synchronization of the clock. These results give the first evidence that Per2S splicing isoform is a clock component expressed in human cells localizing in the nucleolus and suggest a critical role for the nucleolus in the process of circadian synchronization in human keratinocytes.
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