During the development of the mammalian neocortex, neural progenitors generate successive waves of neurons that give rise to its characteristic six-layered structure. Newly generated neurons first assume a multipolar morphology. These multipolar neurons extend several dynamic processes that display stochastic cycles of extension and retraction. After several hours, they become bipolar by extending an axon and a leading process while the remaining processes are retracted. After this transition from a multi- to a bipolar morphology neurons initiate their radial migration. We have shown that the small GTPases Rap1a and Rap1b act as master regulators of cell polarity in the developing cortex. In newborn neurons, Rap1 GTPases are required for the multi-to-bipolar transition in morphology. Currently, our aim is to understand how Rap1 GTPases direct this transition and the initiation of neuronal migration by life cell imaging in dissociated neurons and organotypic slice cultures from the embryonic mouse cortex. To dissect the signaling pathways regulated by Rap1 GTPases, we utilize different fluorescent reporter constructs to investigate intracellular trafficking after the inactivation of key components in these signaling pathways by knockdown or knockout. The life cell imaging experiments are combined with mathematical modeling and the development of new tools for high-resolution imaging and optogenetic manipulation of signaling pathways in neurons.
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