Localized netrins act as positional cues to control layer-specific targeting of photoreceptor axons in Drosophila

Abstract

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.

Figure 1

Fra Is Expressed in R Cells and the Optic Lobe (A) Schematic of the Drosophila adult visual system highlighting the organization of the medulla neuropil into ten layers (M1–M10), and the neuron subtypes relevant for this study. sg, satellite glia; meg, medulla glia. (B) Schematic illustrates of R8 and R7 axon-targeting steps to temporary and final M3 and M6 layers during metamorphosis. (C–F′) In the retina, Fra (blue) is expressed along the cell body membranes of R8 cells at 24 hr (arrowheads in C′) and in rhabdomeres at 42 and 55 hr APF (arrowheads in E′ and F′). Fra is expressed in R1–R6 rhabdomeres at 42 hr (arrows in D′). R8 cells are labeled with caps-Gal4 and UAS-cd8GFP (green). (G–J′) In the medulla (Me), Fra (red) is enriched in the temporary layer of R8 axons at 24 and 42 hr (arrowheads in G′ and H′) and in the emerging and final M3 layer at 42 and 55 hr and in adults (arrows in H′, I′, and J′). In the lamina (La), Fra is enriched in R1–R6 axons at 42 hr (asterisk in H′). sg and meg glial subtypes are Fra positive (double arrowheads in H′, I′, and J′). Lo, lobula; Lop, lobula plate. (K–L′) Target area-specific knockdown reveals Fra expression in R8 axons at 24 and 42 hr (arrowheads in K′ and L′). (M and M′) Knockdown of fra using repo-Gal4 reduces expression in glia (double arrowheads in M′), but not in the M3 layer (arrow in M′), and does not affect R cell axon targeting. R cells are labeled with mAb24B10 (red in C–F′ and K–L′; green in G–J′, M, and M′). Scale bars, 5 μm (C–F) and 20 μm (G–M). See also Figure S1.

Figure 2

fra Is Required in R Cells to Regulate R8 Axon Targeting to the M3 Layer (A and A′) In controls, Rh6-lacZ-positive R8 axons (blue) target to the M3 layer in the medulla (Me) (double arrowhead in A′). La, lamina. (B and B′) In ey^3.5-FLP mosaic animals lacking fra in R cells, many R8 axons stall at the medulla neuropil border (arrows) or terminate incorrectly in the M1/M2 layers (arrowheads). (C) Quantification of phenotypes is shown. (D–E″′) A fra mutant, GFP-positive R8 axon generated by MARCM terminates incorrectly in M1/M2 (arrowheads). The area outlined in (D) is shown at higher magnification in (E)–(E″′). (F–G′) At 24 and 42 hr APF, all R8 axons expressing ato-τ-myc (blue) terminate in their temporary layer at the medulla neuropil border in controls (double arrowheads in F′ and G′). (H and H′) At 55 hr, all R8 axons proceed to the M3 layer in the medulla neuropil in controls (double arrowheads). (I–J′) In animals lacking fra in R cells, the majority of R8 axons pause at the medulla neuropil border, whereas a small percentage prematurely projects deeper (arrowheads). (K and K′) In fra ey^3.5-FLP mosaics, many R8 axons fail to enter the medulla neuropil (arrow) or terminate prematurely in the M1/M2 layers (arrowhead). (L–N) Quantification of phenotypes is presented. (O–P′) Overexpression of fra in R cells causes stalling of Rh6-eGFP-positive R8 axons (green) at the medulla neuropil border (arrows in P and P′), when compared to controls (double arrowheads in O and O′). R7 axons (red) are not retargeted to the M3 layer but occasionally show small extensions into deeper layers (arrowheads in P and P′). (Q) Quantification of phenotypes is shown. R cells are labeled with mAb24B10 (red). Scale bars, 20 μm. See also Figures S2 and S3.

Figure 3

NetB Expression in the Optic Lobe (A–C) NP4151-, NP0831-, and NP4012-Gal4 insertions adjacent to NetB and NetA loci drive GFP expression (green) in lamina (La) neurons L3, which extend axon terminals into layer M3, and in medulla (Me) neuron subtypes (mn). Inset in (A) presents cross sections through lamina cartridges showing the asymmetric dendritic arbors of lamina neurons L3. Lo, lobula; Lop, lobula plate. (D) R30D09-Gal4 identifies T1 neurons among NetB-expressing medulla neurons. (E–G′) Mapping of Netrin-producing neuron subtypes using the FB2.0 approach and NP4151-Gal4 identifies lamina neurons L3 (yellow, arrowheads in E and E′), and the transmedullary neurons Tm3 (n = 8, red in F and F′) and Tm20 (n = 11, yellow in G and G′) based on their characteristic arborizations within the medulla and lobula (arrowheads). Reconstructions of neurons expressing mCitrine or mCherry are shown in (E′), (F′), and (G′). (H and I) Protein trap insertions into NetB (H) and NetA (I) show GFP expression in lamina neurons L3 and some medulla neurons at 48 hr APF. (J–M′) Schematic of NetB^myc shows the laminin N-terminal domain (Lam^NT), EGF repeats, Netrin-like domain (NTR), and myc tag. Animals with NetB^myc (green) knocked into its endogenous locus in NetA mutants show NetB protein in lamina neuron (ln) and medulla neuron (mn) cell bodies and an enrichment in the emerging M3 layer at 42 and 55 hr (arrows in K–L′) and in the lobula neuropil (asterisks in J′, K′, and L′). Expression decreases in adults (arrows in M and M′). R cells are labeled with mAb24B10 (red in A–D and H–M; blue in E, F, and G). Scale bars, 20 μm. See also Figure S4.

Figure 4

NetB Is Required in Target Neurons for R8 Axon Targeting to the M3 Layer (A–B′) In single mutants for NetA^Δ and NetB^Δ, Rh6-lacZ-positive R8 axons (green) target correctly to the M3 layer in the medulla (Me) neuropil (double arrowheads). (C and C′) In hemizygous NetAB^Δ males, R8 axons stall at the medulla neuropil border (arrowheads) or terminate prematurely in layers M1/M2 (arrows) instead of M3 (double arrowheads). (D) Quantification of phenotypes (including the small percentage of axons mistargeting to the M6 layer) is presented. (E and E′) Knockdown (KD) of NetA and NetB in the eye using UAS-RNAi (UAS-NetA^IR and UAS-NetB^IR) and ey^3.5-FLP, act≫Gal4 FLPout transgenes: Rh6-lacZ-positive R8 axons (blue) correctly target to the M3 layer (double arrowheads). (F and F′) In flies, in which NetA and NetB have been knocked down in the target area using ey-FLP, act≫Gal4 FLPout and ey^3.5-Gal80 and lGMR-Gal80 transgenes, R8 axons stall at the medulla neuropil border (arrowheads) instead of terminating in M3 (double arrowheads). The inset shows R8 and R7 projections in the medulla at higher magnification. GFP (green) labels the cells in (E) and (F), in which RNAi transgenes are expressed. La, lamina; R cell axons are labeled with mAb24B10 (red). Scale bars, 20 μm.

Figure 5

Ligand Capture and Local Release Contribute to Layer-Specific NetB Expression (A and A′) In controls at 55 hr APF, NetB (blue) is enriched in the M3 layer; R8 axons terminate in this layer (double arrowhead in inset in A). La, lamina; Me, medulla. (B and B′) fra knockdown (KD) in the eye using ey^3.5-FLP, act≫Gal4 FLPout transgenes: NetB localization in M3 is unaffected, despite the failure of R8 axons to extend into this layer (arrow in inset in B), suggesting that innervation by R8 axons is not required for maintenance of this layer. (C and C′) fra eye and target area knockdown using ey-FLP, act≫Gal4 FLPout transgenes: NetB expression in M3 is strongly decreased, but not abolished; R8 axons show targeting defects (arrow in inset in C). (D and D′) fra target area knockdown using ey-FLP, act≫Gal4 FLPout and ey^3.5-Gal80 and lGMR-Gal80 transgenes: NetB expression in M3 is decreased; R8 axons target correctly (double arrowhead in inset in D). (E–H) Graphs present average NetB fluorescence intensities within white boxes (A–D′). (I and I′) In controls, Fra (blue) is widely expressed in the medulla including the M3 layer (arrowheads). (J and J′) fra target area knockdown using ey-FLP, act≫Gal4 FLPout and ey^3.5-Gal80 and lGMR-Gal80 transgenes: Fra expression is strongly decreased in the medulla neuropil including the M3 layer (arrowheads). (K–L′) NP4151- and NP0831-Gal4 drive expression of the presynaptic marker UAS-syt^HA (blue) and GFP (green). Primary release sites of NetB-expressing neurons include the axon terminals of lamina neurons L3 in layer M3 (arrowheads) and of medulla neurons in the lobula (arrow, Lo). Lop, lobula plate. R cell axons are labeled with mAb24B10 (red). Fra or NetB protein expression is shown in pseudocolor in (A′), (B′), (C′), (D′), (I′), and (J′) (see scale for values). GFP expression (green) marks the cells in (B), (C), (D), and (J), in which UAS-fra^IR is expressed. Scale bars, 20 μm. See also Figure S5.

Figure 6

NetB Acts at Short Range (A–E′) Membrane-tethered myc-tagged NetB (green indicates NetB^TM expression in A, B, C, and D) replaces endogenous NetB in a NetA^Δ mutant background. Schematic of NetB^TM shows the laminin N-terminal domain (Lam^NT), EGF repeats, Netrin-like domain (NTR), transmembrane domain (TM), and myc and V5 tags. NetB^TM is expressed by lamina neurons L3 and medulla neurons (mn) and enriched in the emerging and final M3 layer (asterisks in B, C, and D). At 24 and 42 hr APF, few R8 axons extend incorrectly past their temporary layer (arrows in A–B′). At 55 hr (C and C′) and in adults (D–E′), R8 axons labeled with mAb2B10 (red) and Rh6-lacZ (green in E and E′) correctly terminate in M3 (double arrowheads in C–E′). (F–I) Quantification of phenotypes is shown. (J–L) R8 axons were labeled with pGMR-Gal4 and FB1.1. At 42 hr, R8 growth cones spread within the temporary layer (J), at 48 hr, they extend a thin filopodium (arrowhead) along R7 axons toward the emerging M3 layer (asterisk in K), and at 53 hr, this process develops into a mature terminal (L). (M) Drawing shows changing R8 growth cone morphology during pupal development. La, lamina; Me, medulla; Lo, lobula; Lop, lobula plate. R cells are labeled with mAb24B10 (red in A–E). Scale bars, 20 μm (A–E′) and 10 μm (J–L).

Figure 7

NetB Expression in Lamina Neurons L3 Rescues R8 Axon-Targeting Defects (A) MH56-Gal4 drives GFP expression (green) in adult lamina (La) neurons L3. (B) NetB (blue) is detected in cell bodies of lamina neurons L3 and in their axon terminals in layer M3 of adults upon expression with MH56-Gal4. Schematic of NetB protein shows the laminin N-terminal domain (Lam^NT), EGF repeats, and Netrin-like domain (NTR). (C and C′) Overexpression of NetB with MH56-Gal4 does not interfere with targeting of Rh6-lacZ-positive R8 axons (blue, double arrowheads). (D and D′) In hemizygous NetAB^Δ males, many Rh6-lacZ-positive R8 axons stall at the medulla (Me) neuropil border (arrowheads) or prematurely terminate in layers M1/M2 (arrows). (E and E′) Expression of UAS-NetB with MH56-Gal4 rescues R8 axon-targeting defects of NetAB^Δ mutants. R8 axons target correctly to layer M3 layer (double arrowheads). (F) Quantification of phenotypes is shown. Lo, lobula; Lop, lobula plate. R cells are labeled with mAb24B10 (red in A–C, D, E, and F; white in C′, D′, and E′). Scale bars, 20 μm. See also Figure S6.

Figure 8

Localized NetB Is Sufficient to Target R8 Axons to Ectopic Layers (A) NetB^cd8 schematic showing laminin N-terminal (Lam^NT), EGF, Netrin-like (NTR), Cd8, and V5-epitope-tagged Cerulean domains. (B and B′) In controls, Rh6-lacZ-positive R8 axons (blue) terminate in layer M3 (double arrowheads). (C–F) Expression is under the control of ap-Gal4. At 55 hr APF, GFP (green in C) and NetB^cd8 (blue in D) are detected in cell bodies and processes of lamina (La) neurons L4 and medulla (Me) neurons (mn). In adults, many R8 axons stop at the medulla neuropil border (26.8%, arrowheads) and in distal layers (7.9%, arrows) (E and E′). Quantification of phenotypes is shown in (F). (G–J) Expression is under the control of MH502-Gal4. GFP labels lamina neurons L1/L2, T1 medulla, and C2/C3 neurons (G). NetB^cd8 localizes to layers M1/M2 (H). Of R8 axons, 18.9% terminated in M1/M2 (arrows); 32.2% stalled at the border (arrowheads) (I and I′). Quantification of phenotypes is shown in (J). In (F) and (J), green symbolizes endogenous and blue ectopic NetB expression. (K) This model illustrates that during early development, axon terminals of lamina neurons L3 locally release NetB in the emerging M3 layer. Fra expressed by unidentified medulla neurons captures NetB. Fra-positive R8 axons pause in their temporary layer. During midpupal development, R8 axons expressing Fra are guided to the NetB-positive M3 layer. Scale bars, 20 μm. See also Figure S7.