Regulation of Dlg-containing adhesion complexes during epithelial and synaptic plasticity in Drosophila

Discs large (Dlg) is a multi-PDZ domain-containing protein belonging to the MAGUK superfamily of scaffolding proteins. In epithelia, Dlg serves as an apicobasal polarity determinant, where its disruption leads to tumor formation. Dlg also plays roles in maintaining synaptic structure and function, and has been implicated in neurodegeneration. Thus, understanding Dlg regulation can provide insights into human diseases including cancers and neurological disorders. In this thesis, we characterize novel Dlg regulators during epithelial and synaptic plasticity events in Drosophila development through two separate but overlapping projects. Project 1: Dorsal closure, which is a wound healing model, occurs when a hole in the embryonic epidermis is closed due to surrounding epidermal flanks that migrate towards each other over the amnioserosa occupying the hole. During migration, the leading edges of the epidermal cells abutting the hole exhibit a breakdown in apicobasal polarity as adhesions are severed with the amnioserosa. At the end of migration, however, apicobasal polarity is re-established as adhesions must form between the opposing epidermal flanks in order to seal the hole shut. Mammalian studies indicate that the Dlg-containing Scribble complex recruits p21-activated kinases (Paks), effectors for Rho GTPase signalling, to the leading edge during cell migration. We show that this interaction can act in the opposite direction as Paks can recruit the Scribble complex back to the leading edge upon dorsal closure completion. We propose that the bidirectional relationship between Paks and Dlg may allow epithelia to toggle between migratory and adhered states. Project 2: Previous studies have shown that adducin, a membrane cytoskeletal protein that regulates actin, is hyperphosphorylated in spinal cord tissue taken from patients who died with the motor neuron disease, Amyotrophic Lateral Sclerosis. To further explore the roles of adducin in the nervous system, we decided to study the Drosophila orthologue encoded by hu-li tai shao (hts). We show that Hts regulates larval neuromuscular junction morphogenesis by controlling Dlg postsynaptic targeting via indirect phosphorylation. This process is partially supressed when Hts phosphorylation in the MARCKS domain is blocked. We propose that Hts is a signalling-responsive cytoskeletal protein that contributes to synaptic growth through Dlg-mediated adhesion.