Light and hydrogen peroxide inhibit C. elegans Feeding through gustatory receptor orthologs and pharyngeal neurons

Abstract

While gustatory sensing of the five primary flavors (sweet, salty, sour, bitter, and savory) has been extensively studied, pathways that detect non-canonical taste stimuli remain relatively unexplored. In particular, while reactive oxygen species cause generalized damage to biological systems, no gustatory mechanism to prevent ingestion of such material has been identified in any organism. We observed that light inhibits C. elegans feeding and used light as a tool to uncover molecular and neural mechanisms for gustation. Light can generate hydrogen peroxide, and we discovered that hydrogen peroxide similarly inhibits feeding. The gustatory receptor family members LITE-1 and GUR-3 are required for the inhibition of feeding by light and hydrogen peroxide. The I2 pharyngeal neurons increase calcium in response to light and hydrogen peroxide, and these responses require GUR-3 and a conserved antioxidant enzyme peroxiredoxin PRDX-2. Our results demonstrate a gustatory mechanism that mediates the detection and blocks ingestion of a non-canonical taste stimulus, hydrogen peroxide.

Figure 1. Light inhibits C. elegans feeding

(A) Adult C. elegans head showing one pump of the grinder. d = dorsal, a = anterior. Scale bar = 20 μm. (B) Raster plot of pumps (each tick represents a pump), one trial per row, before, during and after light exposure (436 nm, 13 mW/mm²). (C) Backward moving average of (B), with the acute, burst, sustained and recovery responses labeled. (D) Pumping responses to 350 (UVA), 400 (violet), 450 (blue) and 500 nm (green) light of 0.6-0.9 mW/mm². 350 nm light effectively induces the burst response. (E) Pumping responses to 400, 450, 500 and 550 nm (green) light of 8-11 mW/mm². (F) Spectral sensitivity of the acute response latency, defined as the time from light onset to first missed pump (see Experimental Procedures). (G) Spectral sensitivity of the acute response amplitude, defined as the pumping rate after the first missed pump normalized to the pre-light pumping rate (see Experimental Procedures). (D-G) Shorter wavelength light is more potent than longer wavelength light for eliciting the pumping response to light. Colors of lines in (D-G) indicate wavelengths of light, as labeled in (G). n = 10-20 worms. Shading around traces and error bars indicate s.e.m. See also Figures S1-S3 and Movie S1.

Figure 2. The gustatory receptor orthologs lite-1 and gur-3 are required for the feeding response to light

(A) lite-1(ce314) mutants were defective in the acute and recovery responses to light. (B) gur-3(ok2245) mutants were defective in the acute response to light. (C) lite-1 gur-3 double mutants were severely defective in the acute and recovery responses to light. (D) Quantification of acute response latency. (E) Quantification of acute response amplitude. (F) Quantification of the recovery response amplitude. n = 60 worms. Shading around traces and error bars indicate s.e.m. ** p < 0.01, *** p < 0.001, t-test, compared to wild-type or indicated strain. See also Figures S4 and S5.

Figure 3. gur-3 functions in the I2 pharyngeal neurons for light-induced feeding inhibition

(A) gur-3 was expressed in I2, I4 and AVD, as indicated by a gur-3_prom::gfp transgene. Inset: mCherry-tagged gur-3 was localized throughout I2. Scale bar = 10 μm. (B) I2 ablation caused a defect in the acute response to light. n ≥ 32 worms, 72 trials. (C) I4 ablation did not cause a defect in the pumping response to light. n ≥ 12 worms, 30 trials. (D) Quantification of acute response latency. (E) Genetic ablation of I2 in lite-1 mutants nearly completely abolished the pumping response to light. n = 20 worms. (F) Genetic ablation of I2 in gur-3 mutants did not worsen the defect of the pumping response to light of gur-3 mutants. n = 20 worms. (G) Quantification of acute response latency. (H) I2-specific gur-3 expression restored a normal acute response latency in gur-3 mutants. n = 60 worms. (I) Quantification of acute response latency. Shading around traces and error bars indicate s.e.m. *** p < 0.001, ns = not significant, t-test, compared to mock control or wild-type. See also Figures S1 and S6.

Figure 4. GUR-3 is required for the I2 response to light and is sufficient to confer light-sensitivity to normally light-insensitive neurons and muscle

(A) Representative example of the adult I2 calcium response (GCaMP3) to light (485 nm, 26 mW/mm²). Scale bar = 10 μm. (B) Light caused a calcium increase in all three compartments of I2. n = 20 cells. (C) I2 was most sensitive to shorter wavelengths of light. n = 10 cells per data point. (D) gur-3 mutants were completely defective in the I2 response to light. n ≥ 22 cells. (E) I2-specific gur-3 expression restored the I2 response to light in gur-3 mutants. n = 24 cells. (F) lite-1 mutants exhibited a normal I2 response to light. n ≥ 20 cells. (G) Quantification of the peak calcium response in I2 in response to light. (C-G) The posterior neurite of I2 was analyzed. (H) AWC-specific gur-3 expression caused an increase in AWC calcium in response to light, as measured by GCaMP3. n ≥ 16 cells. (I) AWB-specific gur-3 expression caused an increase in AWB calcium in response to light, as measured by GCaMP3. n ≥ 11 cells. (J) URX-specific gur-3 expression caused a calcium increase in five of 12 URX neurons, as measured by GCaMP. An average of the five responsive neurons is shown. (K) Quantification of the effect of ectopic gur-3 expression on somatic neuronal calcium in response to light. (L) Quantification of the effect of gur-3 expression in the HSN neurons as measured by egg-laying induced by 20 s of light. (M) Quantification of the effect of gur-3 expression in body wall muscle as measured by a reduction in light avoidance during 20 s of light. Shading around traces and error bars indicate s.e.m. *** p < 0.001, t-test or Mann-Whitney test, compared to corresponding wild-type or indicated strain. Transgenes containing “I2” refer to the flp-15 promoter, “AWC” to the odr-1 promoter, “AWB” to the odr-1 promoter, “HSN” to the egl-6a promoter, and “MUS” to the myo-3 promoter. Light for (A, B, D-G) was 485 nm, 26 mW/mm²; light for (H-K) was 485 nm, 10 mW/mm²; and light for (L, M) was 436 nm, 13 mW/mm². See also Figures S7 and S8 and Movie S2.

Figure 5. Light can generate hydrogen peroxide and worms respond to hydrogen peroxide.

(A) Light can generate hydrogen peroxide (H₂O₂) in M9 solution containing riboflavin (50 nM) as measured using a hydrogen peroxide sensor electrode. Three pulses of light, identical in power used to expose worms (436 nm, 13 mW/mm²), are shown. (B) The generation of H₂O₂ by light is reduced in a solution containing catalase (1000 U/ml), indicating that the signal measured by the electrode is in fact hydrogen peroxide. Three pulses of light are shown. (C) RNAi against the riboflavin transporter rft-1 caused a defect in the pumping response to light. n = 60 worms. (D) RNAi against the riboflavin transporter rft-2 caused a defect in the pumping response to light. n = 60 worms. (E) Quantification of acute response latency. (F) Vapor emitted by H₂O₂ (8.82 M) inhibited feeding by wild-type worms. n = 150 worms. (G) Fraction of worms that inhibit pumping in response to various concentrations of liquid H₂O₂. n ≥ 80 worms per data point. (H) Number of body bends in a reversal in response to various concentrations of liquid H₂O₂. n ≥ 40 worms per data point. (I) All three compartments of the I2 neurons responded to H₂O₂ vapor (8.82 M). n = 10 cells. (J) Quantification of the I2 calcium response to a panel of compounds dissolved in water. Hydrogen peroxide (10 mM), paraquat (1 M) and sodium hypochlorite (750 mM) activated the I2 neuron, while all other compounds failed to elicit an effect. n = 3 cells. Shading around traces and error bars indicate s.e.m. * p < 0.05, ** p < 0.01, *** p < 0.001, t-test compared to mock control or water control. See also Figure S9.

Figure 6. Responses to hydrogen peroxide require gur-3 and lite-1

(A) lite-1 mutants inhibited pumping in response to H₂O₂ vapor like wild-type worms. n ≥ 64 worms. (B) gur-3 mutants exhibited a defect in the pumping response to H₂O₂ vapor. n ≥ 78 worms. (C) lite-1 gur-3 double mutants were nearly completely defective in the pumping response to H₂O₂ vapor. n ≥ 72 worms. (D) Genetic ablation of the I2 neurons resulted in a defect in response to H₂O₂ vapor. n = 60 worms. (E) Quantification of the acute response latency to H₂O₂ vapor. (F) Fraction of worms that inhibit pumping in response to various concentrations of liquid H₂O₂. lite-1 mutants responded normally across a range of H₂O₂ concentrations. gur-3 mutants showed a reduced sensitivity to H₂O₂, and lite-1 gur-3 double mutants were completely defective in the pumping response to H₂O₂. lite-1 gur-3 data points do not have error bars because only one experiment was done. n ≥ 20 worms per data point. (G) Number of body bends in a reversal in response to various concentrations of liquid H₂O₂. lite-1 mutants were completely defective in avoiding H₂O₂, while gur-3 mutants were not defective. n ≥ 20 worms per data point. (H) The I2 neurons failed to respond to H₂O₂ vapor in gur-3 mutants. The response in the I2 soma is shown; all three compartments failed to respond in gur-3 mutants (other compartments not shown). n = 10 cells. (I) Quantification of the I2 calcium response to H₂O₂ vapor. Shading around traces and error bars indicate s.e.m. *** p < 0.001, ns = not significant, t-test compared to corresponding wild-type or indicated strain. See also Figure S10.

Figure 7. The peroxiredoxin prdx-2 functions in the I2 neurons for feeding inhibition by light

(A) prdx-2(gk169) mutants had a defective acute response to light. n = 60 worms. (B) prdx-2; lite-1 double mutants had a worse defect than prdx-2 single mutants. n = 60 worms. (C) prdx-2 showed a broad expression pattern, as indicated by a functional prdx-2_prom::prdx-2::mCherry transgene with mosaic transmission. (D) Inset: mCherry-tagged prdx-2 was localized throughout I2. Arrowheads point to two parts of the anterior neurite, the soma, and the posterior neurite. Scale bar = 50 μm (top), 10 μm (middle), 20 μm (inset). (E) I2-specific prdx-2 expression restored a normal pumping response in prdx-2 mutants. n = 60 worms. (F) Quantification of the acute response latency to light. (G) prdx-2 mutants were completely defective in the I2 response to light. n ≥ 20 cells. (H) I2-specific prdx-2 expression restored the I2 response to light in prdx-2 mutants. n ≥ 20 cells. (I) Quantification of the peak response of the I2 neurons to light. (G-I) The posterior neurite of I2 was analyzed. Shading around traces and error bars indicate s.e.m. *** p < 0.001, t-test compared to the corresponding wild-type or indicated strain. See also Figures S7 and S11.

Figure 8. prdx-2 is required for feeding inhibition and the I2 calcium response caused by hydrogen peroxide

(A) prdx-2 mutants were defective in the acute response to H₂O₂ vapor. n = 60 worms. (B) prdx-2; lite-1 double mutants were more defective in the pumping response to H₂O₂ than prdx-2 mutants. n ≥ 86 worms. (C) Quantification of the acute response latency to H₂O₂ vapor. (D) prdx-2 mutants were defective in the pumping response to H₂O₂ liquid. (E) prdx-2 mutants were defective in the I2 soma response to H₂O₂ vapor. (F) Quantification of the peak response of the I2 neurons to H₂O₂ vapor. Shading around traces and error bars indicate s.e.m. *** p < 0.001, t-test.