Epigenetic regulation of Cyprinus carpio ribosomal cistron during the acclimatization process

Ribosomal RNA synthesis is the major transcriptional activity in a cell, required for ribosome biogenesis, and it is critical to ensure the supply of ribosomal RNA in order to meet the cellular needs for protein synthesis. Considerable evidence indicates that epigenetics is an important regulatory mechanism for this intricate process. In addition, cells must also have the ability to maintain their functions by constantly sensing and adapting to environmental variations (homeostasis). Seasonal acclimatization of the fish Cyprinus carpio (common carp) requires the implementation of complex molecular and cellular mechanisms to coordinate “phenotypic plasticity. ” This process involves a reprogramming of gene expression, which in turn integrates the homeostatic response. Our laboratory has reported that in different cell types of the carp, the nucleolus undergoes a dramatic rearrangement of its molecular structure. During winter, fibrillar and granular components of the nucleolus are segregated and surrounded by a thick layer of heterochromatin, which is an ultrastructural feature accompanied by a transient repression of ribosomal RNA transcription. In contrast, this condition is reversed during the summer, and is associated with a recovery of active ribosomal biosynthesis. Thus, the process of seasonal adaptation in the carp certainly involves a fine modulation of the transcriptional activity of ribosomal genes (rDNA). Recent studies on the regulation of ribosomal genes indicate that the numerous copies of rDNA are comprised of at least two distinct transcriptional states: active or silenced. The two states can be differentiated by their chromatin configuration and may require specific triggers to be interconverted. Thus, the use of epigenetic mechanisms to control chromatin architecture may represent an important strategy to modulate and switch the transcriptional activity of rDNA during seasonal adaptation. In this context, the protein TTF-I has been described as playing a central role in the transcriptional modulation of ribosomal genes through its interaction with epigenetic modifiers, such as the nucleolar remodeling complex (NoRC), thus playing an active role in the silencing of rDNA transcription. Most of the conclusions regarding regulatory mechanisms that control rRNA transcription have been proposed based on in vitro approaches. Thus, the natural cyclic modulation of rRNA transcriptional activity observed during carp acclimatization emerges as a valuable model towards studying the overall phenomenon in a living organism. Therefore, the general purpose of this thesis consists in studying the epigenetic regulation of the ribosomal cistron during the acclimatization process of the Cyprinus carpio. Consequently, our hypothesis proposes that "The factor TTF-I and the chromatin remodeling complex NoRC contribute significantly to the negative regulation of transcriptional expression of rRNAs during the seasonal adaptation of Cyprinus carpio. ” Thus our results represent a first approach in epigenetic control involved in the expression of ribosomal genes in a natural context, where the factor TTF-I and NoRC complex play a fundamental role during the carp acclimatization process.