Regulation of N-cadherin trafficking during cortical development

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During the development of the mammalian neocortex neural progenitor cells located in the ventricular zone generate successive waves of neurons that give rise to the cortical plate. Newly generated neurons first assume a multipolar morphology in the subventricular and intermediate zone. After several hours, they become bipolar by extending an axon and a leading process. This transition from multipolar cells with several processes to a bipolar morphology (the multi-to-bipolar transition) precedes the radial migration of bipolar neurons along the basal processes of radial glia cells into the cortical plate. Defects in this polarization of neurons during the multi-to-bipolar transition or in their radial migration are a major cause of neurodevelopmental disorders. While many signaling pathways have been identified that direct neuronal polarization in primary cultures much less is known about the factors that regulate the multi-to-bipolar transition in vivo. The analysis of mutants that show defects in neuronal development indicates that the formation of axon and leading process is controlled by different, genetically separable pathways. A central pathway that directs the multi-to-bipolar transition in the cortex acts through the small GTPases Rap1A and Rap1B. Our analysis of conditional double knockout mice for Rap1a and Rap1b showed that they are essential for cortical development. They act redundantly to maintain the polarity of radial glia cells and direct the multi-to-bipolar transition. Live cell imaging of slice cultures showed that Rap1 is required cell-autonomously in multipolar neurons for polarization. An important function of Rap1 is the maintenance of N-cadherin localization at the cell surface. The polarization of neurons depends on a reorganization of intracellular trafficking that determines the cell surface expression of N-cadherin, which is essential for the interaction with radial glia cells and the transition to a bipolar mophology. In this project we will investigate the role of Ral GEFs and Phosphatidylinositol-4-phosphate 5-kinases for the regulation of N-cadherin using live cell imaging of slice cultures to understand how exocytosis and endocytosis are coordinately regulated and how Rap1 impinges on this regulation.