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. 2006 Sep;4(9):e274.
doi: 10.1371/journal.pbio.0040274.

A distributed chemosensory circuit for oxygen preference in C. elegans

Andy J Chang  1 , Nikolas Chronis, David S Karow, Michael A Marletta, Cornelia I Bargmann
Affiliations

A distributed chemosensory circuit for oxygen preference in C. elegans

Andy J Chang et al. PLoS Biol. 2006 Sep.

Abstract

The nematode Caenorhabditis elegans has complex, naturally variable behavioral responses to environmental oxygen, food, and other animals. C. elegans detects oxygen through soluble guanylate cyclase homologs (sGCs) and responds to it differently depending on the activity of the neuropeptide receptor NPR-1: npr-1(lf) and naturally isolated npr-1(215F) animals avoid high oxygen and aggregate in the presence of food; npr-1(215V) animals do not. We show here that hyperoxia avoidance integrates food with npr-1 activity through neuromodulation of a distributed oxygen-sensing network. Hyperoxia avoidance is stimulated by sGC-expressing oxygen-sensing neurons, nociceptive neurons, and ADF sensory neurons. In npr-1(215V) animals, the switch from weak aerotaxis on food to strong aerotaxis in its absence requires close regulation of the neurotransmitter serotonin in the ADF neurons; high levels of ADF serotonin promote hyperoxia avoidance. In npr-1(lf) animals, food regulation is masked by increased activity of the oxygen-sensing neurons. Hyperoxia avoidance is also regulated by the neuronal TGF-beta homolog DAF-7, a secreted mediator of crowding and stress responses. DAF-7 inhibits serotonin synthesis in ADF, suggesting that ADF serotonin is a convergence point for regulation of hyperoxia avoidance. Coalitions of neurons that promote and repress hyperoxia avoidance generate a subtle and flexible response to environmental oxygen.

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Conflict of interest statement

Competing interests. The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Two Distinct Groups of sGC-Expressing Neurons Promote Hyperoxia Avoidance
(A) A cartoon of the typical distribution of 100 wild-type N2 animals (red dots) in a 0%–21% oxygen gradient. Adult animals are placed on an agar surface under a 3-cm × 1.5-cm PDMS chamber; laminar flow of gases at either end of the chamber generates an oxygen gradient within the chamber. The positions of animals are scored after 25 min. For counting, animals are binned in nine equally spaced regions along the device. (B) In a 0%–21% oxygen gradient, wild-type N2 animals avoid both hyperoxia (14%–21% O2) and hypoxia (0%–7% O2), preferring the center of the gradient (7%–14% O2) (n = 28 assays, 80–100 animals/assay). (C) Neurons (top row) and genes that appear in this study. npr-1 is expressed in SDQ and ASH, but not known to affect their function [13]. (D–E) Aerotaxis of gcy-35 and gcy-36 mutants (D) and gcy-32 and gcy-34 mutants (E). (F) Aerotaxis of qaIs2241 animals, which bear a transgene that kills the URX, AQR, and PQR neurons. (G) Aerotaxis of gcy-35; qaIs2241 mutants. (H) Aerotaxis of gcy-35 mutants in which SDQ, ALN, and PLN were rescued with a lad-2::gcy-35 transgene. In (D–H), asterisks denote distributions different by chi-square analysis at p < 0.01 from the first distribution in the panel, unless otherwise noted. n ≥ 3 assays per genotype, 80–100 animals/assay. Error bars are standard error of the mean (SEM). Aerotaxis assays in all figures follow a standard color code: red and blue colors are used for mutants, green is used for transgenic rescue strains, and gray is used for results repeated from an earlier figure. (I) The hyperoxia avoidance index is defined as [(fraction of animals in 7%–14% O2) − (fraction of animals in 14%–21% O2)] / (fraction of animals in 7%–21% O2). In (I), asterisks denote values different from N2 controls at p < 0.01 by Dunnett test. Cross, value different from the gcy-35 control at p < 0.05 by Bonferroni t test with N2 and gcy-35 controls. Error bars denote SEM. (J) Hyperoxia avoidance is promoted by two sets of sGC-expressing neurons: (1) some or all of URX, PQR, and PQR; and (2) some or all of SDQ, ALN, and PLN.
Figure 2
Figure 2. Multiple TRPV-Expressing Neurons Contribute to Hyperoxia Avoidance
(A) Aerotaxis of TRPV single mutants. The median preferred oxygen concentration of ocr-2 mutants was significantly higher than N2 (p < 0.05 by Dunnett test). (B) Aerotaxis of osm-9 mutants in which ASH, PHA, and PHB were rescued with an osm-10::osm-9 transgene [26]. Animals rescued for ASH function were preselected for avoidance of high osmolarity, an ASH behavior (see Materials and Methods). (C) Aerotaxis of osm-9 mutants in which ADF was rescued by expression from a cat-1::osm-9 transgene [26]. (D) Aerotaxis of tph-1 mutants. (E) Aerotaxis of tph-1 mutants rescued in ADF neurons using a srh-142::tph-1::gfp transgene [28]. (F) Aerotaxis of tph-1 mutants rescued in NSM neurons using a ceh-2::tph-1::gfp transgene [28]. (G) Aerotaxis of TRPV; qaIs2241 double mutants. (H) Aerotaxis of tph-1; qaIs2241 double mutants. For (A–H), asterisks denote distributions different by chi-square analysis at p < 0.01 from the first distribution in the panel, unless otherwise noted. n ≥ 3 assays per genotype, 80–100 animals/assay. Error bars denote SEM. (I) Hyperoxia avoidance index as defined in Figure 1. Asterisks, values different from N2 controls at p < 0.01 by Dunnett test. Single crosses, values different from the osm-9 or tph-1 control at p < 0.05 by Bonferroni t test with N2 and mutant controls. Double crosses, values different from qaIs2241 controls at p < 0.01 by Dunnett test. NS, not significant. Error bars denote SEM. (J) ASH and ADF sensory neurons promote hyperoxia avoidance through the activity of TRPV channels osm-9 and ocr-2. Serotonin production in ADF by tph-1, which is regulated by TRPV channels, also drives this behavior.
Figure 3
Figure 3. Hyperoxia Avoidance by npr-1 Mutants Requires sGC and TRPV Activity, but not Serotonin
(A) Aerotaxis of npr-1 mutants. (B) Aerotaxis of npr-1; qaIs2241 strain. (C) Aerotaxis of gcy-35; npr-1 double mutants. (D) Aerotaxis of osm-9; npr-1 double mutants. (E) Aerotaxis of ocr-2; npr-1 double mutants. (F) Aerotaxis of tph-1; npr-1 double mutants. For (A–F), asterisks denote distributions different by chi-square analysis at p < 0.01 from the first distribution in the panel. n ≥ 3 assays per genotype, 80–100 animals/assay. Error bars denote SEM. For (B–E), all double mutants are significantly different from npr-1 controls, but not significantly different from animals carrying the other mutation (p < 0.01 by chi-square analysis of the complete distribution). (G) Hyperoxia avoidance index as defined in Figure 1. Asterisks and double asterisks, values different from npr-1 controls at p < 0.05 and p < 0.01, respectively, by Dunnett test. Error bars denote SEM.
Figure 4
Figure 4. sGC and ocr-2 TRPV Mutations Restore Food Regulation of Hyperoxia Avoidance to npr-1
(A–J) In all panels, dotted lines indicate aerotaxis in the presence of a small amount of bacterial food. (A) Aerotaxis of wild-type N2 animals. (B) Aerotaxis of npr-1 mutants. (C) Aerotaxis of qaIs2241 strain (URX, AQR, PQR killed). (D) Aerotaxis of npr-1 qaIs2241 strain. (E) Aerotaxis of gcy-35 mutants. (F) Aerotaxis of gcy-35; npr-1 double mutants. (G) Aerotaxis of osm-9 mutants. (H) Aerotaxis of osm-9; npr-1 double mutants. (I) Aerotaxis of ocr-2 mutants. (J) Aerotaxis of ocr-2; npr-1 double mutants. For (A–J), asterisks denote distributions different by chi-square analysis at p < 0.01 from the same genotype without food. n ≥ 3 assays per genotype and condition, 80–100 animals/assay. Error bars denote SEM. (K) Hyperoxia avoidance index as defined in Figure 1. Asterisks denote values significantly different from the same genotype without food at p < 0.05 by t test. Error bars denote SEM. Double crosses indicate that ocr-2 mutants are significantly regulated by food using chi-square analysis of the entire distribution (p < 0.01), and that ocr-2; npr-1 on food is significantly different from ocr-2; npr-1 off food and npr-1 on food (p < 0.01 by chi-square analysis) and not significantly different from ocr-2 mutants on food.
Figure 5
Figure 5. Levels of Serotonin in ADF Neurons Affect Food Regulation of Hyperoxia Avoidance
(A–E) In all panels, dotted lines indicate aerotaxis in the presence of a small amount of bacterial food. (A) Aerotaxis of wild-type N2 animals. (B) Aerotaxis of tph-1 mutants. (C) Aerotaxis of tph-1; npr-1 double mutants. (D) Aerotaxis of tph-1 animals rescued in ADF with a srh-142::tph-1::gfp transgene [28]. (E) Aerotaxis of wild-type animals expressing tph-1 in ADF from a srh-142::tph-1::gfp transgene. For (A–E), asterisks denote distributions different by chi-square analysis at p < 0.01 from the same genotype without food. n ≥ 3 assays per genotype and condition, 80–100 animals/assay. Error bars denote SEM. (F) Hyperoxia avoidance index as defined in Figure 1. Asterisks, values significantly different from the same genotype without food at p < 0.05 by t test. Crosses, values significantly different from N2 on food at p < 0.01 by Dunnett test. NS, not significant. Error bars denote SEM.
Figure 6
Figure 6. TGF-β Signaling Mediates Food Suppression of Hyperoxia Avoidance
(A–E), (H), (I) In all panels, dotted lines indicate aerotaxis in the presence of a small amount of bacterial food. (A) Aerotaxis of wild-type N2 animals. (B) Aerotaxis of daf-7 mutants. (C) Aerotaxis of daf-3 mutants. (D) Aerotaxis of daf-7; daf-3 double mutants. (E) Aerotaxis of daf-3 npr-1 double mutants. (F–G) daf-7::GFP expression in ASI was reduced in a tax-4 mutant. Anterior is to the left. (H) Aerotaxis of tax-4; kyIs342 animals, which bear a transgene that rescues tax-4 in URX, AQR, and PQR, but not in ASI or other neurons. (I) Aerotaxis of tph-1; daf-3 double mutants. For (A–E), (H), and (I), asterisks denote distributions different by chi-square analysis at p < 0.01 from the same genotype without food. n ≥ 3 assays per genotype and condition, 80–100 animals/assay. Error bars denote SEM. (J) Hyperoxia avoidance index as defined in Figure 1. Asterisks, values significantly different from the same genotype without food at p < 0.05 by t test. In the absence of food, no strain is different from N2 controls by Dunnett test. In the presence of food, daf-7, daf-3 npr-1, and tph-1; daf-3 are different from N2 at p < 0.01 and tax-4; kyIs342 different from N2 at p < 0.05 by Dunnett test. Error bars denote SEM.
Figure 7
Figure 7. Serotonin Production in ADF Neurons is Regulated by TGF-β Signaling
(A–B) tph-1::GFP expression in wild-type (A) and daf-7 (B) adults. Anterior is to the left. Two NSM neurons and two ADF neurons are visible in each animal. (C) Quantitation of tph-1::GFP fluorescence in ADF neurons. Asterisks, values different from N2 controls at p < 0.01 by Dunnett test. daf-7 mutants were different from N2, daf-3, and daf-7; daf-3 at p < 0.05 by Bonferroni t test. n ≥ 18 animals per genotype. Error bars denote SEM. (D) Model for food regulation of aerotaxis, combining genetic results from Figure 6 with molecular results from this Figure.
Figure 8
Figure 8. A Distributed Network of Oxygen-Sensing Neurons
(A) Aerotaxis-promoting neurons. In the absence of food, parallel networks of sensory neurons generate hyperoxia avoidance. Triangles denote sensory neurons; hexagons denote interneurons. URX, AQR, and PQR, and SDQ, ALN, and BDU neurons (abbreviated as “SDQ” for simplicity) express sGCs; these neurons are likely oxygen sensors. ASH and ADF neurons express the TRPV channels osm-9 and ocr-2; they might be modulatory neurons, or might respond to oxygen directly. ADF promotes aerotaxis by producing serotonin. Our genetic results suggest that robust aerotaxis requires at least one class of sGC neuron and at least one class of TRPV neuron. Synaptic connections to the AUA interneurons and AVA backward command neurons are shown; additional synapses are omitted [34]. (B) Aerotaxis-suppressing pathways. In the presence of food, the aerotaxis neurons are inhibited by the TGF-β homolog DAF-7 and the neuropeptide receptor NPR-1. Food and tax-4 activity in ASI neurons stimulate the synthesis of DAF-7, which acts through DAF-3 to inhibit serotonin production in ADF and suppress aerotaxis. Increased or unregulated serotonin expression in ADF allows aerotaxis in the presence of food. Food inhibition may involve additional food-sensing pathways in ADF (perhaps via osm-9 and ocr-2) or serotonin signaling from other food-sensing neurons (such as NSM). NPR-1 is expressed in URX, AQR, PQR, ASH, SDQ, and AUA neurons [14] and could inhibit their function in a food-dependent or food-independent fashion. Many of the signaling pathways and neurons described here have the potential to regulate each other's activity. tph-1 mutants have decreased daf-7 expression [30], and daf-7 affects gene expression in the ASH neurons [47]. A food-related change in serotonin levels affects nociceptive signaling in ASH [38]. The NPR-1 ligand FLP-21 is expressed by ASH and ADL [15]. The regulatory relationships in (B) are supported by the genetics, molecular biology, and behavioral assays in this paper, but these relationships could be reconfigured under different conditions.
Figure 9
Figure 9. Second and Third Principal Components of Aerotaxis Data (PC2, PC3)
(A) Assays with the highest (blue) and lowest (red) values for the second principal component (PC2), out of 36 sets of assays examined. Wild-type N2 animals off food are included for comparison (black, thick lines). (B) Assays with the highest (blue) and lowest (red) values for the third principal component (PC3), out of 36 sets of assays examined. Wild-type N2 animals off food are included for comparison (black, thick lines).
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References

    1. Kossl M, Vater M. The cochlear frequency map of the mustache bat, Pteronotus parnellii . J Comp Physiol [A] 1985;157:687–697. - PubMed
    1. Sigman M, Cecchi GA, Gilbert CD, Magnasco MO. On a common circle: Natural scenes and Gestalt rules. Proc Natl Acad Sci U S A. 2001;98:1935–1940. - PMC - PubMed
    1. Gu Q. Neuromodulatory transmitter systems in the cortex and their role in cortical plasticity. Neuroscience. 2002;111:815–835. - PubMed
    1. Wu RS. Hypoxia: From molecular responses to ecosystem responses. Mar Pollut Bull. 2002;45:35–45. - PubMed
    1. Wannamaker CM, Rice JA. Effects of hypoxia on movements and behavior of selected estuarine organisms from the southeastern United States. J Exp Mar Biol Ecol. 2000;249:145–163. - PubMed

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