Regulation of layer-specific axon targeting in the developing visual system of Drosophila

The ability of a nervous system to correctly process sensory information depends on the precise wiring of axonal and dendritic projections. The visual system of vertebrates and invertebrates consists of many neuron subtypes, whose neurites are organized into columns and layers. Layered pathways are pivotal to enable parallel processing of several visual features within a network, such as motion and color detection. Despite their functional relevance, the molecular mechanisms controlling their formation are still poorly understood. In the visual system of Drosophila, photoreceptor neurons (R-cells, R1-R8) extend axons into the optic lobe. R1-R6 axons target to the lamina, while R8 and R7 axons terminate in two distinct neuropil layers in the medulla, called M3 and M6. This thesis investigated the roles of Netrin ligands and the attractive receptor Frazzled and repellent receptor Unc-5 in mediating layer-specific targeting of R8 axons during development. Immunohistological and genetic analyses showed that Frazzled is expressed and cell-autonomously required in R8 photoreceptors for targeting their axons to layer M3. Netrin-B is specifically localized in this layer due to axonal release primarily by lamina neurons L3 and ligand capture by Frazzled expressed in target neurons. Loss of Netrins in the target area causes similar defects as loss of Netrins in R-cells. Netrin-B expression in L3 is sufficient to substantially rescue these R8-axon targeting defects. R8 axons target normally despite replacement of diffusible Netrin-B by membrane-tethered ligands, indicating that they act at short-range. Netrin localization is instructive, as expression of a tethered ligand-variant in ectopic layers can retarget R8 axons. Finally, R8 axons also express and require Unc-5 for layer-specific targeting suggesting a modulatory role of repellent input in this process. Together, this provides evidence for a distinct mechanism that uses the release of localized chemotropic guidance cues to precisely direct axons to a positionally defined layer