Characterization of the Role of Nemo and Homeodomain-interacting Protein Kinase, Two Conserved Regulators of Signalling, in Drosophila Development

Development of multicellular organisms requires regulated cell-cell communication mediated by a handful of conserved signalling pathways. Precise regulation of signalling is essential in integrating the many inputs that cells receive. In this study, I investigated several different aspects of cell communication and signal transduction. First, I describe a novel mode of cross-talk between the Wingless (Wg) and the Bone Morphogenic Protein (BMP) Pathway where Wg-dependent gene transcription is suppressed by ectopic BMP signaling, and the BMP effector Mad disrupts the interaction between Wg effectors Arm and dTcf. This represents a novel mode of regulation to integrate input from various pathways, and does not involve post-translational modification.Protein phosphorylation by kinases is a post-translational modification reiteratively used in many different pathways and processes to regulate signaling. My further studies have involved two kinases, Nemo (Nmo) and Homeodomain-interacting Protein Kinase (Hipk), and their roles in cell signaling and development. Nmo is an evolutionarily conserved kinase involved in the modulation of several pathways. It was originally implicated to regulate cell movements during Planar Cell Polarity (PCP) signaling. Our genetic and molecular studies show that Nmo binds the core PCP factors, and phosphorylates the E-cadherin-Arm complex, implicating Nmo as the molecular link between these two complexes to promote cell motility and rotation. Additionally, we found that nmo inhibits the BMP pathway and suppresses BMP-regulated gene transcription during wing development. Nemo binds to and phosphorylates Mad at a conserved serine, leading to its nuclear exclusion in a kinase dependent manner. This is the first known kinase to target the MH1 domain of Mad to alter its subcellular localization.Hipk represents another class of conserved kinases important in the regulation of signaling. Hipk was previously implicated in growth, apoptosis and patterning. I describe a novel role for Hipk in the regulation of cell junctions and apicobasal polarity in the adult midgut. Loss of hipk leads to a disruption in junctions, epithelial disorganization and cell death. Hipk genetically and physically interacts with the polarity protein Discs Large (Dlg) and is required for its membrane localization.