Role of mirnas in early brain development

Resumen

The brain is the most complex biological structure, from which our conscience emerges. During evolution, the increase in size and complexity of brains (especially the cortex) paved the way to a spectacular development of cognitive and mental abilities. This expansion facilitated the addition of microcircuits with a similar basic structure, which increased brain complexity and contributed to its uniqueness. Given that the final size and shape of an adult brain are determined by the behaviour of progenitor cells during early development, understanding the cellular and molecular mechanisms regulating these actions is fundamental to unravel of how the brain behaves in healthy and pathological conditions. Cellular mechanisms regulating brain formation have been widely studied; however, many of the molecules that control these mechanisms remain unknown. One of the most important mechanisms controlling the molecular program of progenitor cells involves miRNAs. These are molecules that bind specifically messenger RNA molecules, controlling their expression and abundance, and consequently controlling cellular fate. In this thesis we have investigated how microRNAs control brain development at early embryonic stages. Firstly, we studied how the absence of Dicer (enzyme required for miRNA maturation) impacts brain size and organization. We found that loss of miRNAs produces changes in cell proliferation and cell death, inducing the formation of proliferative rosettes. Next, we investigated which miRNAs were responsible for the defects observed after loss of Dicer, finding that let-7 is the family of miRNAs most affected. To understand which protein coding genes are responsible for the formation of rosettes and the other phenotypes found in Dicer mutants, we searched for differentially expressed genes in this condition that may be influenced by let-7 miRNAs. We found an increase in the expression of genes involved in signalling pathways regulating proliferation and cell death including Irs-2 and p53 respectively. Previous studies demonstrated that Irs-2 and p53 are regulated by let-7 and that stress signals upregulate Irs-2. We then found that in Dicer mutants an increase p53 activation (presumably increasing cell death) is necessary for producing rosettes. However, in wild-type animals rosettes can be generated overexpressing Irs-2 (which increases proliferation but not cell death) and this is rescued just by overexpressing let-7. These results suggest that high Irs-2 (direct or indirectly- through an increase in stress signals or a decrease in let-7) is sufficient to produce rosettes. We have identified a signalling pathway through which two antagonistic processes (cell death and proliferation) are linked by let-7 and whose deregulation leads to the disruption of brain germinal layers and the appearance of rosettes.