Investigating the function of the Receptor Tyrosine Kinase ALK during Drosophila melanogaster development

Abstract: The Drosophila melanogaster gene Anaplastic Lymphoma Kinase (DAlk) is homologous to mammalian Alk, which is a member of the Alk/Ltk family of receptor tyrosine kinases (RTKs). In humans the t(2;5) translocation involving the Alk locus encodes an active form of Alk that is the causative agent in Non-Hodgkin’s Lymphoma (Morris et al., 1994). Alk has also been associated with other cancers such as inflammatory myofibroblastic tumours (IMTs). The physiological function of the Alk RTK has not been described in any system until very recently, and is still not defined in vertebrates. The molecular similarity between Drosophila Alk and mammalian Alk suggested that mutation of Alk in flies may affect similar functional and developmental processes, and thus lead to some understanding of Alk function in vivo.By employing an EMS mutagenesis screen we were able to obtain loss-of-function mutants in the Drosophila DAlk gene. Eleven independent DAlk mutants were identified and characterized. DAlk is normally expressed in the developing gut and in the CNS. DAlk mutant animals have a lethal phenotype and die at late embryonic stages or as 1st instar larva. In DAlk mutant embryos there is a complete failure in the development of the midgut whereas the CNS appears normal. The midgut consists of visceral musculature that is syncytial and is formed by fusion of multiple myoblasts. This is a dynamic process where two types of myoblasts, i.e. fusion-competent-myoblasts and founder-cells that function as seeds for muscle formation, fuse. In DAlk homozygous embryos there is no founder cell specification, which explains the failure of midgut formation in these embryos.Recently a novel secreted molecule Jelly Belly (Jeb) was identified. Jeb is expressed in the tissue neighbouring the DAlk expressing cells of the developing visceral mesoderm. Jeb mutant embryos show a phenotype that is similar to that of DAlk mutant embryos. We have been able to show that Jeb is the ligand for DAlk in the developing visceral mesoderm and that Jeb binding stimulates a DAlk driven ERK signaling pathway. This leads to the expression of Dumbfounded (duf)/kin of Irregular chiasm-C (kirre), a founder-cell specific immunoglobulin that has an important role in myoblast aggregation and fusion.The functional Drosophila midgut is made up of the visceral muscle that encircles the endodermal tube. This tube formation includes migration of cells originating in the anterior and posterior parts of the embryo, first along the anterior-posterior axis using the visceral mesoderm as a template, then dorsally and ventrally. In DAlk mutant embryos there is no visceral muscle fusion and both the visceral mesoderm and the endoderm fail to undergo dorsal-ventral migration.