Motor control in a Drosophila taste circuit

Abstract

Tastes elicit innate behaviors critical for directing animals to ingest nutritious substances and reject toxic compounds, but the neural basis of these behaviors is not understood. Here, we use a neural silencing screen to identify neurons required for a simple Drosophila taste behavior and characterize a neural population that controls a specific subprogram of this behavior. By silencing and activating subsets of the defined cell population, we identify the neurons involved in the taste behavior as a pair of motor neurons located in the subesophageal ganglion (SOG). The motor neurons are activated by sugar stimulation of gustatory neurons and inhibited by bitter compounds; however, experiments utilizing split-GFP detect no direct connections between the motor neurons and primary sensory neurons, indicating that further study will be necessary to elucidate the circuitry bridging these populations. Combined, these results provide a general strategy and a valuable starting point for future taste circuit analysis.

Figure 1. Summary of PER screen

(A) Average response frequencies for different classes of mutants observed in screen. Each class represents the listed genotype crossed to UAS-KIR2.1, tub-Gal80^ts. Mutants were designated based on a response frequency more than three standard deviations away from Wild type for one of the three conditions: 100mM sucrose for “unresponsive” mutants, 50mM sucrose for “sugar mutants”, and a mixture of 100mM sucrose and 75mM caffeine for “bitter mutants”. (B) Expression analysis of Gal4 enhancer traps from different phenotypic classes. Gal4 expression was assayed by crossing to flies carrying UAS-CD8::GFP. Pie charts indicate the number of lines in each class showing expression in taste sensory neurons (blue), other cells in the SOG (pink), or only cells outside the SOG (gray).

Figure 2. E49 neurons are necessary and sufficient for proboscis extension subprogram

(A) Diagram of fly head with proboscis extended. The three segments of the proboscis are labeled, with arrows indicating their direction of movement during PER. The rostrum is extended and lifted upwards; the haustellum is extended by flipping downwards, away from its resting position against the rostrum; and the two lobes of the labellum are spread to allow food intake. (B) Quantification (mean +/− s.e.m.) of PER phenotypes upon silencing of E49-Gal4 neurons. “Full extension” indicates movement of the rostrum, haustellum and labella. Flies expressing tetanus toxin driven by E49-Gal4 exhibit a specific deficit during extension. Frequencies are not significantly different (ns) between the three genotypes at either concentration. Values reflect 3 trials with 20–25 flies per trial per genotype. (C) Time lapse photographs of wild-type (top row) and E49-Gal4, UAS-TNT (bottom row) flies following 100mM sucrose stimulation. Wild-type flies show full proboscis extension. E49-Gal4, UAS-TNT flies spread labella (arrows) and tilt head forward in the absence of other PER subprograms. Weak extension of the haustellum is observed after 5 seconds of stimulation. For movies, see supplemental materials (movies S1 and S2) (D) Flies expressing ChR2 in Gr5a-expressing cells show robust full extension (black bars) in response to blue light (480 nm). Flies expressing ChR2 in E49-Gal4 neurons exhibit rostrum extension (gray bars) in response to blue light. n=45 flies for each genotype. Student t-test,*** = p<10⁻⁷. (E) Time lapse photographs showing response of flies expressing ChR2 in Gr5a cells (top row) or E49-Gal4 neurons (bottom row) to intermittent exposure to red light (690 nm; 0s and 3s frames) and blue light (480 nm; 1s and 5s frames). For movies, see supplemental materials (movies S3 and S4).

Figure 3. Expression pattern of E49-Gal4 in the central nervous system

(A–E) Immunofluorescent detection of mCD8::GFP driven by E49-Gal4. Expression is seen in approximately 50 cells in the brain and numerous projections in the thoracic ganglia (A). Full (B), anterior (C), medial (D) and posterior (E) projections through the brain reveal E49-Gal4 expressing cells in these areas. Scale bars are 100μm.

Figure 4. Mosaic analysis reveals neurons in E49-Gal4 population responsible for behavior

(A) Mosaic silencing strategy using FLP-out Gal80. Prior to FLP expression, E49-Gal4 activity is repressed by ubiquitous expression of Gal80 repressor. Heat shock-driven FLP expression induces excision of the Gal80 cassette in a subset of cells, thereby allowing expression of tetanus toxin (UAS-TNT) and a visible marker (UAS-dsRed) (not illustrated) in those cells. (B) Illustration of 11 cell types observed in FLP-out labeling during behavioral experiments. (C) Frequencies of labeled (and therefore silenced) cell types observed in flies showing normal extension after low FLP expression (light blue bars; n=100 sampled from 1549 animals), impaired extension following low FLP expression (dark blue bars; n=68), normal extension following high FLP expression (pink bars; n=50 sampled from 220 animals), or impaired extension following high FLP expression (red bars; n=10), (genotypes: high FLP: hs-FLP; E49-Gal4/UAS-TNT; tub>Gal80>/UAS-dsRed; low FLP: tub>Gal80>; E49-Gal4/UAS-TNT; MKRS, hsFLP/UAS-dsRed). Frequencies represent the proportion of flies in each class where one or more cells of the given type have been silenced. Fisher’s exact test: *** = p<10⁻¹². Cutoff for significance was set at 0.005 to produce an expected false positive rate of 5% of data sets (each including 11 cell types). (D,E) Immunofluorescent detection of dsRed in example brains exposed to high FLP expression. The brain shown in (D) is from a fly showing normal extension; the brain in (E) is from a fly showing impaired extension. Numbers identify neuron types according to scheme shown in (B). (F–H) Brains (F,G) or the complete CNS (H) of flies showing impaired PER following low FLP expression. Scale bars are 100μm. (I,J) Representative images of brain (I) or full CNS (J) from flies showing rostrum extension in response to blue light following expression of dsRed (and hence ChR2) in neuron #1. Genotype: tub>Gal80>; E49-Gal4/UAS-ChR2; MKRS, hsFLP/UAS-dsRed. (K) Frequency distributions of labeled (and therefore activated) cell types observed in flies showing no rostrum extension following exposure to blue light (light blue bars; n=50 sampled from 314 total) or extension of the rostrum (dark blue bars; n=12). Fisher’s exact test: *** = p<10⁻⁴. Cutoff for significance was set at 0.005 to produce an expected false positive rate of 5% of data sets (each including 11 cell types). From 314 animals screened, 12 showed extension of the rostrum. The false positives observed can be explained by the low level of background extension seen in the absence of Gal4 expression (figure 2).

Figure 5. E49 motor neurons synapse on proboscis muscle

(A) Single E49 motor neuron labeled with mCD8::GFP. Pupae of the genotype yw,hs-flp; E49-Gal4; UAS> CD2,y+>mCD8::GFP were heat shocked for 10 minutes to remove the CD2 FLP-out cassette, and adult brains were stained with antibodies against GFP (green) and nc82 (magenta) to visualize neuropil. Scale bars are 100μm in (A) and (B). (B) Synapse between E49 motor neuron and protractor of rostrum. Dissected proboscises from E49-Gal4, UAS-dsRed flies were fixed and stained with antibodies against RFP (green). Proboscis and muscles are visualized with autofluorescence (magenta). (C) Diagram illustrating position of protractor of rostrum muscle.

Figure 6. Taste cell stimulation activates E49 motor neurons

(A) Images of G-CaMP fluorescence in E49 motor neurons. The top image is initial G-CaMP fluorescence, bottom is change in fluorescence (ΔF/F) to 100 mM sucrose. Fluorescence changes are color-coded differences (poststimulation minus prestimulation/prestimulation). Scale bar is 50 μm. (B) Representative traces showing fluorescence changes of E49 motor neurons in response to stimulation with 100mM sucrose, water, or 100mM sucrose plus 50mM caffeine. (C) Fluorescence changes in E49 motor neurons following stimulation with water, 100mM sucrose, 200mM maltose, 100mM sucrose plus 50mM caffeine, or 100mM sucrose plus 1mM denatonium. 6–8 flies were used for each condition. Values are mean +/− s.e.m. in this and subsequent panel. student t-test versus water: *** = p<10⁻⁴, ns = not significant (D) Fluorescence changes in E49 motor neurons following stimulation with 100mM sucrose or 1M sucrose. 10 flies were used for each condition. Asterisks indicate significance by student t-test: ** p<0.01. (E) Scatter plot of fluorescence changes seen in E49 neurons and the concomitant behavioral response following stimulation with 100mM sucrose mixed with varying concentrations of caffeine (n=10 trials for each condition). Red dots indicate trials in which rostrum extension was observed, black dots indicate no observed rostrum extension. Red lines indicate average fluorescence change among trials where a behavioral response was observed. Response frequency was significantly lowered by addition of 10mM and 100mM caffeine (p<0.01, Fisher’s exact test).

Figure 7. Fly GRASP is detectable by GFP fluorescence and immunofluorescence

Full brains (A–C) or one antennal lobe (D–I) were imaged from flies of the following genotypes: +/+; GH146-Gal4/LexAop-CD4:spGFP11; Or83b-LexA::VP16/UAS-CD4::spGFP1-10 (A,D,G), +/+; GH146-Gal4/LexAop-CD4::spGFP11; +/UAS-CD4::spGFP1-10 (B,E,H) and +/+; +/LexAop-CD4::spGFP11; Or83b-LexA::VP16/UAS-CD4::spGFP1-10 (C,F,I). Or83b-LexA::VP16 drives expression in 80% of olfactory sensory neurons, and GH146-Gal4 drives expression in second order projection neurons that project dendrites to about 30 glomeruli (Jefferis et al., 2007; Lai et al., 2008; Marin et al., 2002). (A–C) Unfixed brains were imaged for GFP fluorescence. Long dashes outline brain and short dashes outline antennal lobes. (D–F) The antennal lobes of fixed brains were imaged for immunofluorescence against GFP to detect GRASP (green; Invitrogen rabbit polyclonal antibody) and the neuropil (magenta; nc82). Arrows indicate weak detection of CD4::spGFP1-10 in PN cell bodies; this polyclonal antibody weakly recognized CD4::spGFP1-10 expressed alone. (G–H) The antennal lobes of fixed brains were imaged for immunofluorescence against GFP to detect GRASP (green; mouse monoclonal antibody). No signal was observed in the absence of GFP reconstitution. Dashes outline antennal lobe. Scale bars are 50 μm.

Figure 8. E49 motor neurons do not synapse on gustatory sensory neurons

(A) Expression of CD2::GFP under the control of Gr5a-LexA::VP16 detected by immunofluorescence. (B,C) Single labeled E49 motor neurons (magenta) and projections from Gr5a (B) or Gr66a (C) gustatory neurons (green). Genotypes are tub>Gal80>/Gr5a-LexA::VP16; E49-Gal4, UAS-CD8::dsRed; lexAop-CD2::GFP/MKRS, hsFLP and tub>Gal80>/Gr66a-Ires-GFP; E49-Gal4/UAS-CD8::dsRed; MKRS, hsFLP. Dashed box in (B) indicates size and approximate position of images presented in H and I. (D) GRASP between Gr5a neurons expressing CD4::spGFP11 and GAD1 neurons expressing CD4::spGFP1-10 and CD8:dsRed (magenta). GRASP detected by immunofluorescence using monoclonal GFP antibody (green; Sigma). Genotype is GAD1-Gal4/lexAop-CD4::spGFP11; Gr5a-LexA::VP16/UAS-CD4::spGFP1-10. Dashed box indicates approximate position of images in E and F. (E,F) GRASP between Gr5a neurons expressing CD4::spGFP11 and 2 GAD1 neurons expressing CD4:spGFP1-10. GRASP detected by immunofluorescence using monoclonal GFP antibody (green; Sigma). GAD1 cells expressing CD4::spGFP1-10 identified using rabbit polyclonal GFP antibody (magenta; Abcam). Genotype is Gr5a-LexA::VP16/tub>Gal80>; LexAop-CD4::spGFP11/GAD1-Gal4; UAS-CD4::spGFP1-10/MKRS, hsFLP. (G) Expression of CD4::spGFP1-10 under control of E49-Gal4 detected by immunofluorescence against GFP1-10 (Abcam polyclonal antibody). (H,I) Lack of GRASP observed between E49 neurons co-expressing CD4::spGFP1-10 and CD8::dsRed (magenta), and Gr5a neurons expressing CD4::spGFP11. Panels are representative images of E49 dendrites in Gr5a region. GRASP detected by immunofluorescence using polyclonal GFP antibody (green; Invitrogen). Arrow in (I) indicates infrequently observed GRASP signal. Dashes outline approximate position of Gr5a cells in each panel. Genotype is Gr5a-LexA::VP16/+; LexAop-CD4::spGFP11/E49-Gal4; UAS-CD4::spGFP1-10/UAS-CD8::dsRed. (J–L) Lack of GRASP between Gr5a neurons expressing CD4::spGFP11 and a single labeled E49 motor neuron co-expressing CD4::spGFP1-10 and CD8::dsRed (magenta). GRASP detected by immunofluorescence using monoclonal GFP antibody (green; Sigma). (K) and (L) are enlargements of indicated areas from (J). Arrows in (L) show infrequently observed GRASP signals. Dashes outline approximate position of Gr5a cells in (K). Genotype is Gr5a-LexA::VP16/tub>Gal80>; LexAop-CD4::spGFP11/E49-Gal4, UAS-CD8::dsRed; UAS-CD4::spGFP1-10/MKRS, hsFLP. Scale bar is 50 μm in (A) and applies to all panels not noted otherwise. Scale bars in (E.F) are 20 μm and (H,I,K,L) are 10 μm. “E49” indicates expression in all E49-Gal4 neurons, “E49 MN” indicates expression in a single E49 motor neuron.