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

Abstract

SummaryA 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.

Sequential Axon-Derived Signals Couple Target Survival and Layer Specificity in the Drosophila Visual System

“…Previous studies established that L3 neurons target to M3 prior to R8 (Nern et al, 2008; Pecot et al, 2013; Timofeev et al, 2012). R8 initially terminates in a temporary region at the outer surface of the medulla and extends to M3 between 45 and 55 APF (Ting et al, 2005).…”

“…Salecker and colleagues reported that Netrin is required for R8 targeting to the M3 layer, provided evidence that L3 neurons express Netrin, and demonstrated that expression of a Netrin transgene in L3 neurons is sufficient to rescue R8 mis-targeting in animals lacking Netrin (Timofeev et al, 2012). As shown in the previous section, we established that L3 neurons are required for R8 targeting (Figures 4G–K).…”

Synaptic vesicle clustering requires a distinct MIG-10/Lamellipodin isoform and ABI-1 downstream from Netrin

“…Chemotrophic factors such as Netrin instruct diverse neurodevelopmental outcomes (Hedgecock et al 1990 Cold Chang et al 2006;Quinn et al 2006Colon-Ramos et al 2007;Poon et al 2008;Manitt et al 2009;Xu et al 2010;Park et al 2011;Smith et al 2012;Stavoe and Colon-Ramos 2012;Timofeev et al 2012). Signaling modules, such as the Rac GTPase pathway and the adaptor protein MIG-10, are required downstream from Netrin for cell migration, axon guidance, axon arborization, and synaptic vesicle clustering Chang et al 2006;Quinn et al 2006Manitt et al 2009;Stavoe and Colon-Ramos 2012).…”

“…Netrin is required for additional neurodevelopmental processes other than axon outgrowth; Netrin can also instruct cell migration, dendritic branching, axon arborization, or synapse formation (Hedgecock et al 1990;Ishii et al 1992;Colon-Ramos et al 2007;Poon et al 2008;Manitt et al 2009;Xu et al 2010;Park et al 2011;Smith et al 2012;Stavoe and Colon-Ramos 2012;Timofeev et al 2012). The ability of a neuron to respond cellspecifically to Netrin in guidance or synapse formation is also critical for orchestrated circuit assembly in vivo (Salie et al 2005;Colon-Ramos 2009;Sanes and Yamagata 2009;Shen and Cowan 2010;Williams et al 2010).…”

Loss of Na+/K+-ATPase in Drosophila photoreceptors leads to blindness and age-dependent neurodegeneration

“…The drivers lGMR-Gal4 (Chen et al, 2014; Timofeev et al, 2012; Wernet et al, 2003) and GMR-Gal4 (Velentzas et al, 2013) have a photoreceptor-specific expression pattern, and repo- (Awasaki and Ito, 2004) and elav- Gal4 (Zhan et al, 2004) have glial and neuronal expression patterns, respectively. The Gal80ts/TARGET system was used for temporal control of UAS-RNAi expression (McGuire et al, 2004).…”