Signaling pathways in the activation and proliferation of Drosophila melanogaster blood cells

Abstract: The larva of the fruit fly Drosophila melanogaster is an excellent model to study the molecular control of innate cellular immune responses. Cellular responses take place, and can be studied, following infestation of the wasp Leptopilina boulardi. This response includes proliferation and activation (differentiation) of the blood cells (hemocytes). In a successful anti-parasitic response, an immune-induced lineage of hemocytes, the lamellocytes, forms a cellular capsule covering and killing the foreign intruder. I will in this thesis present data about the finding and characterization of a novel marker that is expressed specifically in the hemocytes, the Hemese gene. I furthermore describe the construction of a useful tool, the transgenic Hemese-Gal4 fly, which enables blood cell specific expression of any gene of interest. By using the Hemese-Gal4 fly in a directed screen, I have found that a surprisingly large number of genes, that in turn are members of seemingly diverse signaling pathways, are able to induce a cellular response. In many cases their expression is also associated with a blood cell tumor phenotype. Overexpression of certain genes, such as hopscotch (a Drosophila Jak homologue) and hemipterous (a c-jun kinase kinase) lead to the formation of lamellocytes. Other genes may control the cell number, such as Egfr and Ras, as their expression produced a massive in increase the numbers of hemocytes. A third group of genes, including, e.g. Alk, Rac1 and Pvr give a mixed response, promoting both hemocyte proliferation and activation. Surprisingly, the suppression of WNT signaling in hemocytes lead to hemocyte activation. In one case, with a UAS-Pvr dominant negative construct, we observe a reduction of the circulating blood cells in uninfested larva. The expression of DN-Pvr additionally contributes to reduce encapsulation rates in larvae subjected to Leptopilina infestation. In conclusion: the control of blood cells in larval hematopoiesis, and during parasitic wasp attacks, is complex and may involve multiple pathways. In a broader sense, the gene functions found in the directed screen may have implications also for understanding the molecular control of mammalian myeloid lineage blood cells.