Bases moleculares de la especificación del patrón dorso-ventral en Drosophila

Resumen

RTK/Ras/MAPK signaling is one of the most common pathways for intercellular communication during development and in the adult organism. In addition, abnormal RTK signaling is associated with many pathological conditions, including cancer. Taking advantage of the available genetic tools of Drosophila, we use the DV axis specification as a model to study the molecular mechanisms by which RTK signaling regulates gene expression and how the same signaling pathway is interpreted differently in distinct tissues. Specifically, we have studied the mechanisms by which the localized activation of the RTK EGFR signaling pathway in the dorsal region of the follicular epithelium restricts pipe gene expression to ventral positions. pipe encodes a sulfotransferase enzyme which is essential for transfering DV polarity from the egg chamber to the embryo. Our results have shown that EGFR activity on pipe is mediated by Mirror, a homeodomain transcription factor induced by the pathway in dorsal-anterior cells. Mirror acts as a direct repressor of pipe by binding to a conserved motif (r1) in the pipe regulatory region. We also have characterized an additional aspect of pipe regulation that depends on Capicua (Cic), a HMG-box transcription factor post-transcriptionally downregulated by RTK/Ras/MAPK signaling. We have shown that the role of Cic is to support pipe expression in ventral follicle cells by repressing mirror in this region. On the other hand, we have analyzed the relevance of the negative post-transcriptional regulation of Cic by EGFR/Ras/MAPK signaling in the establishment of the initial DV asymmetries during oogenesis. Our results suggest a competition mechanism between Cic-mediated repression and EGFR-dependent and –independent activation of mirr, which leads to graded expression of mirr in dorsal and lateral follicle cells. We propose a model where the EGFR-dependent downregulation of Cic modulates the spatial distribution of Mirror protein in the lateral and dorsal-posterior follicle cells, where low, but functional Mirr activity defines the precise position at which the pipe expression border is formed. Finally, we have studied in collaboration with S. Y. Shvartsman group, the function of Cic in response to the RTK Torso signaling pathway in the early embryo. Together with previous results, our work has shown that Cic also participates in the DV subdivision of the embryo acting as a repressor of dorsal zygotic genes, as zerknüllt (zen), and that this activity is inhibited at the poles by Torso signaling. Taking together this result and other previous studies in collaboration with S. Y. Shvartsman group, we have proposed in a new model of gene regulation based in MAPK substrate competition. Molecular competition among MAPK substrates affects the expression of genes such as zen, reveals a new mechanism of integrating anterior, dorso-ventral and terminal systems. To summarize, in this thesis we show that different RTK/Ras/MAPK pathways with key roles in Drosophila development operate through common mechanisms that involve the post-transcriptional downregulation of Cic. By downregulating Cic activity, RTK signals create gradients or boundaries of Cic repressor activity that are then translated into complementary patterns of target gene expression. Since different Cic targets are regulated in different contexts, our results support the view that RTK specificity arises mainly at the level of downstream enhancers responding to general RTK effectors (including Cic) and additional ubiquitous and tissue-specific factors.