Localized Netrins Act as Positional Cues to Control Layer-Specific Targeting of Photoreceptor Axons in Drosophila

A shared feature of many neural circuits is their organization into synaptic layers. However, the mechanisms that direct neurites to distinct layers remain poorly understood. We identified a central role for Netrins and their receptor Frazzled in mediating layer-specific axon targeting in the Drosophila visual system. Frazzled is expressed and cell autonomously required in R8 photoreceptors for directing their axons to the medulla-neuropil layer M3. Netrin-B is specifically localized in this layer owing to axonal release by lamina neurons L3 and capture by target neuron-associated Frazzled. Ligand expression in L3 is sufficient to rescue R8 axon-targeting defects of Netrin mutants. R8 axons target normally despite replacement of diffusible Netrin-B by membrane-tethered ligands. Finally, Netrin localization is instructive because expression in ectopic layers can retarget R8 axons. We propose that provision of localized chemoattractants by intermediate target neurons represents a highly precise strategy to direct axons to a positionally defined layer. ► Netrins selectively target Frazzled-expressing R8 photoreceptor axons to layer M3 ► Precise positional information is generated by Netrin localization in a single layer ► Local axonal release and receptor-mediated capture restrict ligand distribution ► Layer-specific targeting relies on ligand release by intermediate target neurons Many neural circuits share a remarkable organization into synaptic layers. Timofeev et al. find that in the Drosophila visual system, Netrins act as short-range chemoattractants to target photoreceptor axons, specifically expressing the Frazzled/DCC/Unc-40 receptor, to a single positionally defined layer.