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. 2014 Feb 5;81(3):603-615.
doi: 10.1016/j.neuron.2013.12.010.

A hard-wired glutamatergic circuit pools and relays UV signals to mediate spectral preference in Drosophila

Thangavel Karuppudurai #  1 , Tzu-Yang Lin #  1 , Chun-Yuan Ting  1 , Randall Pursley  2 , Krishna V Melnattur  1 , Fengqiu Diao  3 , Benjamin H White  3 , Lindsey J Macpherson  4 , Marco Gallio  4 , Thomas Pohida  2 , Chi-Hon Lee  1
Affiliations

A hard-wired glutamatergic circuit pools and relays UV signals to mediate spectral preference in Drosophila

Thangavel Karuppudurai et al. Neuron. .

Abstract

Many visual animals have innate preferences for particular wavelengths of light, which can be modified by learning. Drosophila's preference for UV over visible light requires UV-sensing R7 photoreceptors and specific wide-field amacrine neurons called Dm8. Here we identify three types of medulla projection neurons downstream of R7 and Dm8 and show that selectively inactivating one of them (Tm5c) abolishes UV preference. Using a modified GRASP method to probe synaptic connections at the single-cell level, we reveal that each Dm8 neuron forms multiple synaptic contacts with Tm5c in the center of Dm8's dendritic field but sparse connections in the periphery. By single-cell transcript profiling and RNAi-mediated knockdown, we determine that Tm5c uses the kainate receptor Clumsy to receive excitatory glutamate input from Dm8. We conclude that R7s→Dm8→Tm5c form a hard-wired glutamatergic circuit that mediates UV preference by pooling ∼16 R7 signals for transfer to the lobula, a higher visual center.

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Figures

Figure 1
Figure 1. Three Distinct Subtypes of Tm5 Projection Neuron Relay Chromatic Information to the Lobula, a Higher Visual Center
(A) A schematic illustration of the Drosophila visual system, including the eye (Eye), and four optic neuropils (lamina [La], medulla [Me], lobula [Lo], and lobula plate [Lop]). The inner photoreceptors, R7 (purple) and R8 (blue), project axons to the medulla layers M6 and M3, respectively, while the outer photoreceptors, R1-R6 (grey), terminate in the lamina. The amacrine neuron Dm8 (orange) extends dendrites in the M6 layer where they receive multiple R7 inputs. Three morphologically distinct types of Tm5 neurons, Tm5a (dark blue), Tm5b (cyan), and Tm5c (green), extend dendrites in multiple medulla layers, including M3 and M6, and project axons to the lobula layers Lo4 and Lo5. (B-B″) Single Dm8 neurons were labeled using the single-cell flip-out technique. The dendritic arbors were labeled by a membrane-tethered marker (spGFP11::CD4::HA) and visualized by anti-HA antibody while their presynaptic terminals were labeled by the presynaptic marker Brp::mCherry (red). Photoreceptors were visualized using 24B10 antibody (cyan). (B) a dorsal-ventral view. (B′-B″) proximal-distal views. Dm8 dendrites span approximately 16 medulla columns in the M6 layer. The presynaptic sites of Dm8 cluster at the center of its dendritic field in the M6 layer. For clarity, B″ shows the presynaptic sites (red) of Dm8 with R7 terminals marked with asterisks. (C-H′) The axonal and dendritic morphologies (C,C′,E,E′,G,G′) and dendritic arborization in the yellow/pale columns (D,D′,F,F′,H,H′) of Tm5a (C-D′), Tm5b (E-F′) and Tm5c (G-H′). Single Tm5a, Tm5b, or Tm5c clones were generated using the single-cell flip-out technique in flies carrying hs-Flp GMR-RFP; ortC1a-Gal4 UAS>CD2,y+>mCD8-GFP transgenes. R7 and R8 photoreceptors, visualized by GMR-RFP (red), served as landmarks for medulla columns. The medulla and lobula layers were identified using R7 and R8 terminals and anti-Connectin antibody (cyan), which labeled the Lo3 layer. (D, D′, F, F′, H, H′) The yellow-subtype R7 terminals (R7y), which occupied yellow-type medulla columns, were labeled by a Rh4-LacZ reporter and visualized by anti-β-galactosidase antibody (cyan). Left panels of (C, E, G): low magnification views showing the entire optic lobe; right panels of (C, E, G): high magnification views of the medulla; (C′, E′, G′): high magnification views of the lobula. (D, F, H): high magnification views of the medulla; (D′, F′, H′): close up of (D, F, H) showing skeletonized dendrites (green lines) and axons (white lines). (C, C′) The Tm5a neuron extends a single primary dendrite along the photoreceptor axon and arborizes into many fine processes in the M3, M6, and M8 layers. Its axon makes a sharp turn and branches out in the Lo5 layer. (D, D′) The main dendritic branch of Tm5a exclusively associates with R7y axons (cyan). (E-E′) The Tm5b neuron extends two or three main dendritic branches along the photoreceptor axons and arborizes into many fine processes in the M3, M6, and M8 layers. (E′) Its axonal terminal forms from multiple short branches in the Lo5 layer. (F, F′) The main dendritic branches of Tm5b associate with either R7y (arrows) or R7p (arrow head) axons without apparent discrimination. (G, G′) The Tm5c neuron extends multiple fine dendritic processes that span about eight medulla columns in the M1, M3 and M6 layers and the axon often bifurcates in the Lo4 layer and terminates in the Lo5/6 layers. The dendritic arborization in the M1 layer differentiates Tm5c from Tm5a and Tm5b. (H, H′) The dendrites of Tm5c associate with either R7y (arrows) or R7p (arrowhead) axons. Scale bar: 30 μm in C (left panel); 5 μm in B′ and D′; 10 μm for the remaining panels.
Figure 2
Figure 2. The Projection Neuron Tm5c Is Required for Normal UV Preference
(A) A schematic diagram of the T-maze apparatus used in the spectral preference assay. (B) The expression of the HA-tagged TNT (tetanus toxin light chain) in Tm5c neurons was assessed in the flies carrying Tm5c-Gal4 UAS-TNT-HA. TNT-HA was visualized using anti-HA antibody (green). Photoreceptors were visualized by 24B10 antibody (magenta), and medulla and lobula layers were visualized by anti-FasIII antibody (cyan), serving as landmarks. TNT-HA was detected as punctate staining in axonal and dendritic terminals as well as cell bodies of Tm5c. Scale bar: 30 μm. (C-D) Wild-type and various mutant flies were tested for phototactic preference to UV over green light. The intensity-response curves were measured at various UV/green intensity ratios (shown as a logarithmic scale). The performance index (P.I.) for each genotype was calculated from the numbers of flies choosing UV (NUV) or green (NG) light by the following formula: P.I. = [NUV - NG] / [NUV + NG]. The data were presented as mean ± standard deviation (C). Relative attractiveness (AttrUV/G) was calculated from the UV/green intensity ratio at which flies exhibited UV and green phototactic responses with equal frequency (i.e. P.I.=0) using the following formula: AttrUV/G= −log(UV/green ratio at P.I.=0). AttrUV/G values for all genotypes were plotted together as bar graphs (D). (C) Inactivating ortC1a(+), Tm5c or Dm8 neurons, but not Tm5a/b, significantly reduced phototactic preference to UV light over green light. Wild-type (wt) flies exhibited phototactic preference to UV in an intensity-dependent fashion, resulting in a sigmoidal intensity-response curve. TNT expressed in ortC1a(+) neurons (ortC1a-G4>TNT) blocked the synaptic transmission of Tm5a/b/c and Tm20, and reduced flies’ phototactic preference to UV, resulting in a right-shifted intensity-response curve. Inactivating Tm5c alone using one of the three Tm5c-specific drivers to drive TNT expression (Tm5c-G4(1)>TNT, Tm5c-G4(2)>TNT, and Tm5c-LexA>TNT) caused reduced UV preference, comparable to that caused by inactivating Dm8 (Dm8-G4>TNT). In contrast, inactivating Tm5a/b (Tm5a/b(18)-G4>TNT and Tm5a/b(24)-G4>TNT) did not affect UV preference significantly. Inactivating Tm5a/b or Dm8 in addition to Tm5c (Tm5a/b/c(18)-G4>TNT, Tm5a/b/c(24)-G4>TNT, and Tm5c+Dm8-G4>TNT) did not enhance UV preference defects. Flies carrying Gal4 drivers or UAS-TNT alone had normal UV preference. (D) Bar graph of the relative attractiveness of UV over green light (AttrUV/G) calculated from (C). n.s.: not significant (p>0.05); * and **: significant (p<0.05 and p<0.01, respectively).
Figure 3
Figure 3. Tm5c Neurons Receive Direct Inputs from Photoreceptor R8s
(A) The expression of the HA-tagged Ort in Tm5c neurons was assessed in the ort mutant flies carrying Tm5c-Gal4 UAS-2xHA-ORT transgenes. HA-Ort was concentrated in Tm5c dendritic arbors and cell bodies. (B-C′) GRASP revealed functional R8->Tm5c synapses (B), but not R7-> Tm5c (C). Flies expressing syb::spGFP1-10 in R8s/ R7s and CD4::spGFP11 in Tm5c were examined for native fluorescence of reconstituted GFP (green). Strong GRASP signal (green) was observed along with the R8 axons and their terminals (B-B′). On the contrary, no detectable GRASP signal was found in R7s (C-C′). (B′, C′) shows the green channel of (B, C), respectively. R8/R7 photoreceptors were labeled with αGFP antibody (pseudocolored in cyan for R8s; magenta for R7s). Anti-hCD4 immunolabeling (Grey) outlines the morphology of Tm5c neurons. Scale bar: 30 μm in A and B′. (D) Restoring Ort expression in Tm5c neurons in ort, HisCl1 ort double or HisCl1 ort ninaE triple mutant flies rendered a stronger green preference. ort mutants had a reduced UV preference and their intensity-response curve was shifted to the right, as compared to wild-type. HisCl1 ort double mutants exhibit some residual green phototactic responses, resulting in a flattened curve, as compared to ort mutants. HisCl1 ort ninaE triple mutants are near blind, choosing UV and green light indiscriminately. Ort expression in Tm5c in ort single, HisCl1 ort double or HisCl1 ort ninaE triple mutants resulted in a stronger green preference. (E) Bar graph of the relative attractiveness (AttrUV/G) calculated from (D). HisCl1 ort ninaE triple mutant flies are near blind and its AttrUV/G cannot be ascertained (#). (F) Blockage of the photoreceptors’ inputs to Tm5c neurons caused no detectable UV preference defect. RNAi-mediated knockdown of ort in Dm8, but not Tm5c, neurons significantly decreased UV preference. (G) Bar graph indicating the relative attractiveness of UV over green light (AttrUV/G) calculated from (F). n.s.: not significant (p>0.05); *: significant (p<0.05).
Figure 4
Figure 4. GRASP Visualization of Dm8->Tm5c Synapses
(A, A′) Membrane contacts between Dm8 and Tm5c were assessed by the GRASP method. The split-GFP1-10 component was expressed in the Dm8 neurons and the split-GFP11 component was expressed in the Tm5c neurons. Strong green fluorescence signal from reconstituted GFP (green) was observed at the apparent contacts between Dm8 dendrites and Tm5c dendrites in the M6 layer. Top panel shows Dm8 (blue) and Tm5c (grey) neurons visualized by anti-GFP and anti-HRP antibodies, respectively. A′ shows the green channel (GRASP) alone. (B) Schematic illustration of the single-cell GRASP method. Dm8-LexA flies carrying LexAop>FRT-Stop-FRT>spGFP11::CD4::HA::T2A::Brp::mCherry and hs-Flp transgenes allow dual-labeling with the split-GFP (spGFP11) and the active zone marker, Brp::mCherry (separated by the ribosomal skipping sequence T2A) in single Dm8 neurons. A mild heat shock was applied to 3rd instar larvae to remove the FRT-Stop-FRT cassette and place both markers under LexAop control. The same flies also carried Tm5c-Gal4 UAS-spGFP1-10::CD4 transgenes to express the other split-GFP (spGFP1-10) in all Tm5c neurons. Functional GFP molecules were reconstituted at the membrane contacts between Tm5c and single Dm8. The active zone marker labels the presynaptic sites of Dm8, differentiating synaptic contacts (labeled by both GRASP and Brp::mCherry) from mere membrane contacts (GRASP alone). (C) A single Dm8 neuron labeled with both spGFP11::CD4::HA and Brp::mCherry was visualized by anti-HA antibody (cyan). Tm5c neurons co-expressing spGFP1-10 and HRP::CD2 were visualized by anti-HRP antibody (grey). Photoreceptor axons visualized by 24B10 antibody were used as landmarks (magenta). (D-D″) Presynaptic sites of a single Dm8 neuron and its membrane contacts with Tm5c were visualized by native fluorescence of Brp::mCherry (red) and reconstituted GFP (GRASP, green), respectively. Most GRASP puncta co-localized with Brp::mCherry signal, suggesting they are synaptic contacts. Dm8 dendrites, which span about 14 medulla columns, were visualized by anti-HA antibody (cyan). GRASP/Brp::mCherry puncta cluster in the center of the Dm8 dendritic field. D′ and D″ show the green (GRASP) and red (Brp::mCherry) channels of D, respectively. Arrows mark the puncta with GRASP signal but not Brp::mCherry and arrow heads mark the puncta with Brp::mCherry alone. The dotted line circles the area corresponding to one medullar column in the center of the Dm8 dendritic field. Photoreceptor terminals (marked by asterisks), visualized by 24B10 antibody, were used as landmarks for medulla columns. Scale bar: 30 μm for A and C; 5 μm in D for D-D″.
Figure 5
Figure 5. Glutamatergic Output of Dm8 and Tm5c Is Required for Normal UV Preference
(A-B″) Dm8 and Tm5c express the vesicular glutamate transporter (VGlut). The expression of VGlut and choline acetyl transferase (ChAT) in Dm8 (A-A″) and Tm5c (B-B″) neurons was assessed using anti-VGlut (red) and anti-ChAT (cyan) antibodies in adult flies. Dm8 and Tm5c (asterisks) neurons were labeled with the mCD8GFP membrane marker (green) using the Dm8-Gal4 and Tm5c-Gal4 drivers, respectively. For both Dm8 and Tm5c, anti-GFP staining co-localized with anti-VGlut, but not anti-ChAT, staining. (A′, B′) are high magnification views of the medulla cortex in (A, B). (A″, B″), GFP staining was omitted from (A′, B′) for clarity. Scale bar: 50 μm for A and B; 5 μm for A′ and B′. (C, D) RNAi-mediated knockdown of VGlut in Dm8 or Tm5c neurons significantly reduced UV preference, as compared with wild-type and UAS-RNAi controls. Wild-type and various VGlut RNAi-knockdown flies were tested for spectral preference to UV light over green light as described in Figure 2. Dm8-G4 and Tm5c-G4 drivers were used to drive two different UAS-VGlut RNAi transgenes (VGlut RNAi-1 [Bloomington] and VGlut RNAi-2 [VDRC]) in Dm8 and Tm5c, respectively. (D) Bar graph indicating the relative attractiveness of UV over green light (AttrUV/G) calculated from (C). n.s.: not significant (p>0.05); *: significant (p<0.05).
Figure 6
Figure 6. The Kainate iGluR Clumsy Is Required in Tm5c Neurons for Normal UV Preference
(A) Domain structures of the subunits of kainate ionotropic glutamate receptors in vertebrates (GluK1-5) and Drosophila (Clumsy, CG9935, CG11155, DKaiRIC, DKaiRID). The Drosophila kainate iGluR subunits share the same domain organization as those of vertebrates. Clumsy has a long cytoplasmic terminal domain (CTD), which shares limited sequence homology with that of the vertebrate receptor subunit GluK5. ATD: amino-terminal domain; S1 and S2: two parts of ligand binding domain; TMD: transmembrane domain; CTD: cytoplasmic terminal domain. (B) Wild-type and various iGluR RNAi-knockdown flies were tested for spectral preference to UV over green light as described in Figure 2. Tm5c-Gal4 was used to drive RNAi expression of four kainate iGluRs subunits that are normally expressed in Tm5c neurons. RNAi-mediated knockdown of the Clumsy iGluR subunit (Tm5c-G4>Clumsy-RNAi [1X]), but not DKaiRIC, DKaiRID, or CG11155, caused a significant reduction of UV preference. The performance of wild-type flies and matched controls is shown for comparison. Increasing the copy number of UAS-Clumsy RNAi transgenes (Tm5c-G4>Clumsy-RNAi [2X]) further reduced flies’ UV preference. n.s.: not significant (p>0.05); *: significant (p<0.05). (C) RNAi-knockdown any of the three subunits (i.e. DKaiRIC, DKaiRID and CG11155), in addition to Clumsy, caused no further reduction of UV preference, as compared with Clumsy RNAi knockdown. n.s.: not significant (p>0.05); *: significant (p<0.05). RNAi-knockdown of the three subunits, DKaiRIC, DKaiRID and CG11155, reduced UV preference close to the level caused by RNAi clumsy alone, suggesting these subunits are functionally redundant. n.s.: not significant (p>0.05); *: significant (p<0.05).
Figure 7
Figure 7. A Hard-wired Glutamatergic Pooling Circuit Mediates UV Preference
Summary diagram of R7->Dm8->Tm5c pathway. The amacrine neuron Dm8 receives inhibitory histaminergic inputs from approximately 16 R7 photoreceptors and provides excitatory glutamatergic inputs to one (or at most a few) Tm5c projection neurons. Kainate iGluR receptors expressed in Tm5c are required for receiving glutamatergic inputs from Dm8 and for flies’ UV preference. In addition to the indirect pathway via Dm8, Tm5c neurons also receive direct photoreceptor inputs from R8.
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