Titolo della tesi: The critical role of circadian clock in the interaction between environmental stress and genomic instability
The biological clock is an important regulatory mechanism permeating every aspect of physiology. Its main role is harmonizing endogenous physiological functions (from metabolism to complex behaviors) and environmental cycles, bringing ‘temporal homeostasis’. Our mature understanding of the circadian clock has been made possible by the study of the fruit fly Drosophila melanogaster, a powerful model system that has informed our knowledge beyond the boundaries of species. However, even in Drosophila, we know little about the role of the clock in development; in particular, whether its machinery contributes to cellular specification and differentiation and if so, how. A long-standing interest of the laboratory is the study of ‘stress’ as an evolutionary force. Stress may induce chromatin remodeling leading to the activation of transposable elements (TEs) and to a dramatic change in mutation rate. In populations, this may occasionally result in the emergence of genotypes better adapted to challenging conditions and better equipped to compete. Populations may consist of individuals or cells. In both cases, suboptimal genotypes are eliminated (during evolution or during ontogeny) to the benefit of those with higher fitness. The motivation of this study was to investigate whether an insult to the circadian clock (in the form of a null mutation of a key circadian gene, per0) may cause chromatin modifications in somatic cells, which then may shape cell competition during development.
I have identified the presence of dsDNA (double-strand DNA) breaks and chromosome aberrations in progenitor neuronal cells in 3rd instar per0 larvae. Experiments measuring activation of TEs in the adult and their pharmacological inhibition in larvae, suggest that activation of TEs is a cause of such a genotoxic effect. Moreover, the effect is not cell-autonomous. Dividing neuronal progenitors are protected and supported by glia, which provides a “niche” microenvironment. I have observed that PER expression in glia is necessary and sufficient to prevent chromosome aberrations in neuronal progenitor cells. Finally, treatment with an antioxidant and respirometry measures overall suggest that mitochondria dysfunction and reactive oxygen species (ROS) may be the primary cause of chromosome aberrations in per0 larvae.
In summary, this work shows for the first time the presence of a functional clock in larval glia and its requirement to orchestrate normal neuronal development in Drosophila. Indirectly, it also shows that PER expression in the larval brain is more widespread than currently accepted, i.e. limited to nine canonical clock neurons. This work reports an oxidative burden in per0 larvae, previously known for mutant adults only, and its translation into genotoxic effects through cell-to-cell interactions. Finally, it provides the basis for future work aimed at investigating cell competition during development as a response to circadian insult.