I am interested in the molecular mechanisms directing axons to their correct targets. Insights into axon target selection mechanisms may help us protect or reestablish neuronal connections compromised by illness or injury. My lab uses the Drosophila melanogaster eye's photoreceptors (R-cells) to study how axons choose their correct targets. The axons of different R-cell subtypes project to targets in distinct layers of the brain. The signals that specify R-cell axon targeting are unknown, and how these signals stop R-cell axons at the correct target layer is not understood. Through genetic screens, we have found that mutations in two genes, slit and Ptp69, disrupt target selection in a related fashion. Slit encodes an extracellular protein that can act as a signal to guide axons, and Ptp69d encodes a receptor protein tyrosine phosphatase. We will use slit and Ptp69d to investigate the mechanisms determining R-cell axon target choice. We will: 1. Examine slit's role in R-cell axon target selection. Slit loss-of-function causes one subtype of R-cell axons to grow through their normal target layer and into other target layers. We will test the hypothesis that Slit is a signal controlling layer-specific termination of R-cell axons. We will identify the location of the Slit signal and the cells in which it functions using genetic and molecular experiments. We will test Slit's effect on R-cell axon behavior both in vivo, through ectopic expression, and in vitro, using cultured R-cells. 2. Identify substrates of Ptp69d. Ptp69d loss-of-function causes R-cell axons to terminate in inappropriate target layers. Ptp69d acts in a phosphatase-dependent fashion, but its substrates are unknown. We will identify candidate substrates from the fly nervous system biochemically using a Ptp69d substrate-trap mutant that can bind but not dephosphorylate its targets. We will further study those candidate substrates that disrupt axon targeting when inhibited with RNAi. These candidates will be tested for genetic and biochemical interactions with Ptp69d and roles in R-cell axon targeting. 3. Examine interactions between slit, Ptp69d, and robo-family members in R-cell axon target selection. Overexpression of the Slit receptors Robo and Robo2 causes R-cell axon target layer selection defects similar to slit loss-of-function. We will examine loss-of-function mutations in robo-family members for R-cell axon targeting defects and examine potential interactions between slit, Ptp69d, and robo-family members in R-cell axon target selection. We will also examine potential downstream signaling pathways by examining interactions between these genes and potential downstream effectors.
