Axon Guidance Mechanisms in Drosophila

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To find and synapse with their appropriate target cells, the growth cones of developing neurons recognize guidance cues in their environment and transduce them into changes in direction of growth. In Drosophila, the binary choice of anterior vs. posterior commissure (AC vs. PC, respectively) made by the growth cones of all neurons that project across the midline, is controlled by Derailed (Drl), an atypical receptor tyrosine kinase, and its ligand Wnt5, a member of the Wnt family of secreted signaling molecules. Wnt5 is secreted by PC neurons and acts as a chemorepellent to keep the Drl-expressing AC growth cones out of the PC. Our goal in this project is to understand how the Wnt5/Drl guidance mechanism functions. We will develop a novel in vitro growth cone turning assay for Drosophila neurons to test whether Wnt5 acts directly on the growth cones of Drl-expressing neurons. Wnt5 is proteolytically cleaved in vivo, but the role this event plays in Wnt5 function is unknown. Using epitope tags, we will determine where and when Wnt5 is cleaved and test the role of cleavage in Wnt5 function. The signaling pathway downstream of Drl that transduces the repulsive signal within the growth cone is unknown. Since Drl is a novel Wnt receptor, we will genetically test whether known components of Wnt signaling, such as Frizzled receptors and Disheveled, function with Drl in axon guidance. To further identify signaling components downstream of Drl, we will determine the identity of two genes that we have shown to strongly suppress Drl function when deleted. In addition, we will express a functional Flag-tagged version of Drl to immunoprecipitate proteins that specifically bind to the Drl cytoplasmic domain and test whether these proteins bind to regions of the Drl cytoplasmic domain required for signaling in vivo. Finally, our work in Drosophila raises the question of how universal the Wnt/Drl axon guidance mechanism might be. We will test the guidance role of Ryk, the mammalian homologue of Drl, by examining motor axon projections in Ryk knockout mice. Relevance: A normal functioning brain relies on the generation of specific connections between nerve cells, as does functional recovery after spinal cord injury. The studies in this project will provide insight into how nerve cells become properly wired.