Characterization of drosophila nemo kinase, a conserved modulator of Wnt and TGF? signal transduction

The development of muticellular organisms requires precisely regulated cell-cell communication mediated by numerous signal transduction pathways. Regulatory "crosstalk" is essential in integrating the many inputs and stimuli that each cell receives, and ensuring that a cell responds appropriately. Drosophila nemo (nmo) is the founding member of an evolutionarily conserved family of serirdthreonine protein kinases kinases that are involved in several Wnt signal transduction pathways. Consistent with these findings, the detailed genetic analyses of the role of Nemo in Drosophila wing development support the proposed antagonistic role for Nemo in Drosophila Wingless (Wg) signaling pathway. In addition, I provide evidence that transcription of nmo is induced by high levels of Wg signaling in the developing wing disc. Our results indicate that Nemo acts as an intracellular feedback inhibitor of Wg during wing development and that it is a novel Wg target gene. In this study, a novel function for Nemo in inhibition of Drosophila Bone Morphogenetic Protein (BMP) signalling is also revealed. Genetic interaction studies demonstrate that nmo can antagonize BMP signaling and can inhibit the expression of BMP dependent target genes during wing development. Nemo can bind to and phosphorylate the BMP effectors Mad and Medea. In cell culture, phosphorylation by Nemo blocks the nuclear translocation of Mad. Mutation of a single Nemo phosphorylation site in Mad relieves the inhibition of nuclear translocation, and causes ligand-independent nuclear translocation. This is the first example of inhibition of Drosophila BMP signaling by a MAPK and also represents an original mechanism of Smad inhibition through phosphorylation of a conserved Serine residue within the MHI domain of Mad. In Drosophila wing imaginal disc development, Wg signalling pathway organizes the dorsal-ventral (DV) axis, while BMP signaling pathway is required to pattern the anterior-posterior (AP) axis. In the analyses of the roles of Nemo in Wg and BMP signaling pathways, a novel crosstalk between Wg and Dpp signaling is unveiled, in which Wg-dependent gene expression is suppressed by ectopic Dpp signaling. In addition, Arm and Mad compete for the binding of dTCF in cell culture. Consistently, in vivo, supplement of dTCF is able to rescue the suppression of Wg-dependent gene expression caused by ectopic Mad. Our results suggest a novel mechanism that Dpp represses Wg target gene by influencing the binding of Arm and dTCF.