The Pin1 orthologue Dodo regulates heterochromatin formation and activity of transposable elements in Drosophila germline and neural tissue.

All eukaryotic genomes contain a substantial proportion of repetitive DNA sequences known as transposable elements (TEs), some of which retain the ability to mobilize in the genome. In different organisms, some TEs have been shown to transpose in the germline and in somatic tissues, in particular in the Central Nervous System, leading to genome heterogeneity. Mechanisms restricting TE activity both in the germline and in somatic tissues have evolved in all organisms, because excessive transposition of TEs may cause mutagenesis and genomic instability. Indeed, unscheduled activation of TEs has been associated with ageing and age-related pathological conditions, including neurodegenerative diseases, in different animal models. Pin1 is an evolutionarily conserved enzyme with the unique feature of promoting phosphorylation-dependent isomerization of S/T-P motifs. Its activity is essential to link phosphorylation signalling to modulation of cellular processes. Pin1 widely impacts on chromatin state and transcription, cell proliferation, DNA repair and stress response pathways. Additionally, Pin1 activity appears to be required for healthy ageing and prevention of age-related diseases, such as neurodegeneration of Alzheimer’s Disease type. Drosophila is a widely used model organism that offers specific advantages for studying the regulation and the impact of TEs. Importantly, the Drosophila Pin1 orthologue Dodo protein shares a high degree of sequence similarity with mammalian Pin1 In this thesis, we provide evidence that Dodo acts as a negative regulator of TE expression and integration in both the germline and Central Nervous System. We showed that Dodo depletion licenses the expression of some TEs that are normally repressed by heterochromatinization through HP1a. Mechanistically, we observed that loss of Dodo leads to reduction of HP1a expression at the post-transcriptional level, with reduced formation of HP1a-containing heterochromatin foci and loss of HP1a occupancy at TE regulatory sequences. Moreover, we observed that loss of Dodo impaired the formation of HP1a/B-type Lamin complex at the intranuclear periphery. These observations suggest that Dodo negatively regulates TE expression through heterochromatin-mediated transcriptional gene silencing. Moreover, we observed that dodo mutant brains display increased de novo TE insertions especially in coding and regulatory sequences involved in neuronal function. Consistently, we report that both Dodo and HP1a exert a neuroprotective function in ageing flies and that, upon loss of either Dodo or HP1a, TE mobilisation impairs maintenance of neuronal survival. In conclusion, Dodo may exert its neuroprotective function by restricting TE activity through heterochromatin maintenance.