PROJECT SUMMARY/ABSTRACT Generation of neuronal connectivity is regulated during brain development, with the formation of synapses being dependent on the morphology of dendrites, branch-like structures extending out from neurons. Disruptions in dendrite formation can thus lead to atypical synaptic connectivity, associated with neurodevelopmental disorders such as autism, schizophrenia and Cri-du-chat syndrome. Our proposal seeks to better understand molecular processes underlying the morphology of dendrites. This will occur through examination of two novel protein complexes we recently revealed, each involving delta-catenin. delta-Catenin belongs the p120-subfamily of catenin proteins, whose members have are best known for their functions in association with cadherin cell-adhesion proteins and cytoskeletal-associated partners including small- GTPases. delta-Catenin localizes to neuronal dendrites and synapses where it functions in development and homeostasis. Delta-catenin possesses a central armadillo-repeat domain present in most catenins, but it additionally contains a PDZ-binding ligand at its extreme C-terminus. This PDZ-ligand binds to a number of PDZ-domain proteins crucial for synaptic and dendritic functions. Our preliminary findings have revealed two novel interactions between delta-catenin's PDZ-ligand and the PDZ-domain proteins Magi1 and Pdlim5. Magi1 is a member of the membrane-associated guanylate kinase (MAGUK) family that is expressed along dendrites with enrichment within the tips at early stages, potentially relevant to a role in dendrite extension. Pdlim5 is a PDZ-LIM protein also found in dendrites, but is believed to negatively regulate the neurite growth cone to halt the dendrite's lengthening. Our preliminary work using rat hippocampal neurons suggests that Magi1 expression promotes the lengthening of dendrites, while Pdlim5 appears instead to enhance branching. This presents us with the intriguing thought that two proteins with seemingly opposing roles can each bind delta- catenin. Analysis of these interactions has revealed a critical phosphorylation site in delta-catenin's PDZ-ligand that appears to determine which of the two interactions occurs, potentially contributing to the spatial and temporal control of dendritic morphologies. Using primary rat hippocampal neurons as well as HEK293 cells, the experiments proposed in this application examine the phospho-dependency (Aim 1) and the cellular and developmental significance of the delta-catenin:Magi1 and delta-catenin:Pdlim5 complexes (Aim 2). Experimental approaches include (among others) the selective disruption of these two delta-catenin complexes using knock-down/ add-back strategies. Given that the dysfunction of each of the three proteins under study are implicated in neurologic diseases, addressing their interactions and contributions to dendrite development could provide insight to the progression of neurodevelopmental disorders.