, 2011). In their present work, Timofeev et al. (2012) uncover yet another INCB28060 mechanism to increase both the functional range and specificity of a well-characterized guidance molecule. They demonstrate that secreted Netrins can be localized to a specific layer in the Drosophila medulla by ligand capture and that this local concentration of Netrin is sensed by a specific

photoreceptor type that innervates this layer. The Drosophila visual system provides a powerful model for dissecting the molecular mechanisms of layer specificity. In this system, photoreceptors, designated R cells, project their axons directly into the brain, with subtypes of R cells targeting to different layers in different neuropils. While photoreceptors R1–R6 project their axons to one brain region, the lamina, a second subset of photoreceptors, designated R7 and R8, extend

their axons into a different brain region, the medulla. The medulla neuropil is organized into both columns and layers, comprising roughly 800 columnar elements, each divided into ten distinct layers (designated M1–M10; Figure 1A). Each layer contains a specific combination of processes from projection neurons originating in the lamina, ascending neurons from deeper brain centers, and many types of medulla neurons. In aggregate, this structure is arguably the most complex neuropil in the Drosophila brain, but incoming R7 and R8 axons manage to invariably terminate in two specific layers, M6 and M3, respectively. Targeting occurs HER2 inhibitor in two sequential steps. First, during larval development, R7 and R8 innervate specific, L-NAME HCl “temporary” layers. Second, during midpupal stages, R7 and R8 extend deeper into the medulla, innervating their “recipient” layers, after which they form synapses with their target neurons.

Several cell surface molecules, including Flamingo, Golden Goal and N-cadherin, are expressed in R7 and/or R8 and play critical roles in layer-specific targeting of these cells ( Senti et al., 2003, Tomasi et al., 2008, Ting et al., 2005 and Lee et al., 2001). However, exactly how these molecules catalyze assembly of a layer remains unclear. The study by Timofeev et al. (2012) in this issue of Neuron identifies a novel strategy to achieve layer-specific targeting in the fly visual system ( Figures 1B and 1C). They demonstrate that the guidance cue Netrin localizes to the R8 target layer and that R8 axons detect Netrin by expressing the attractive Netrin receptor Frazzled (Fra). R8 axons that have lost Fra stall at their temporary layer and fail to extend toward their final target. Conversely, removing Netrin from the R8 target area precisely phenocopies these defects, demonstrating that target-derived Netrin attracts R8 axons by activating Fra.