Análisis experimental de la apoptosis en el disco imaginal de ala de drosophila melanogaster

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Biología Molecular. Fecha de lectura 15-07-2005Certain stress situations in Dmsophila cause a great amount of apoptosis, which can elirninate even more than 50% of the cells. Yet, the organism is able to cornpensate for the loss of cells and to form organs of normal size and shape. To study this cornpensatory phenornenon in detail we have used a rnethod to keep apoptotic cells alive by inhibiting the activity of effector caspases with the baculovirus protein P35. This allows to uncouple the initiation and the execution of apoptosis. Under these conditions we obseive a great deal of aberrations and abnorrnalities such as increased proliferation, ectopic activation of wingless (wg) and decapentaplegic (dpp) signalling genes, and alteration of cornpartrnent borders. The production by the apoptotic cells of the Wg and Dpp signals is likely reponsible for the induction of the cornpensatory proliferation. We also show the production of these signals is dependant upon the activity of the JNK pathway. We have also analyzed the individual roles of Wg and Dpp in this phenornenon by generating p35 expressing clones that are, at the same time, mutant for either wg or for dpp. Subsequently these clones are given an apoptotic stirnulus (1500R). P35 dpp- clones of irradiated l a ~ a edo not show increased proliferation and do not cause overgrowths, clearly indicating that the Dpp signal provides a proliferative stirnulus. In contrast, irradiated discs with P35 wg- clones develop huge overgrowths. These clones can even fullfil the entire disc by elirninating sunounding non-apoptotic cells (without p35), rnost likely by cell cornpetition. We propose that in response to the activation of the apoptotic pathway, the cells send two antagonistic but slightly unbalanced signals: one triggers a mitogenic response and prornotes proliferation (Dpp) and the other (Wg) has an inhibitory effect that rnodulates the response to Dpp. We speculate that tumor formation in vertebrates could be caused in certain cases by a process similar to what we describe here: the uncoupling of apoptosis and cells death (caused by a virus infection) rnay lead to the appearence of cells that continously produce and ernit rnitogenic signals, causing uncontrolled growth. If, in addition, the apoptotic cells fail to send suficient arnount of the inhibitory signal the arnount of overgrowth is much increased and lead to the elirnination of the surrounding non apoptotic tissue