Arrestin-mediated desensitization enables intraneuronal olfactory discrimination in Caenorhabditis elegans

Abstract

In the mammalian olfactory system, cross-talk between olfactory signals is minimized through physical isolation: individual neurons express one or few olfactory receptors among those encoded in the genome. Physical isolation allows for segregation of stimuli during signal transduction; however, in the nematode worm Caenorhabditis elegans, ∼1,300 olfactory receptors are primarily expressed in only 32 neurons, precluding this strategy. Here, we report genetic and behavioral evidence that β-arrestin-mediated desensitization of olfactory receptors, working downstream of the kinase GRK-1, enables discrimination between intraneuronal olfactory stimuli. Our findings suggest that C. elegans exploits β-arrestin desensitization to maximize responsiveness to novel odors, allowing for behaviorally appropriate responses to olfactory stimuli despite the large number of olfactory receptors signaling in single cells. This represents a fundamentally different solution to the problem of olfactory discrimination than that which evolved in mammals, allowing for economical use of a limited number of sensory neurons.

Keywords: GRK-1; arrestin; discrimination; olfaction; signaling.

Fig. 1.

Olfactory discrimination depends on ARR-1. (A) Chemotaxis of wild-type N2 animals and arr-1(ok401) animals to a point of the AWC-sensed odorant isoamyl alcohol on unsaturated plates and plates containing a saturating concentration of the AWC-sensed odorant benzaldehyde. A two-way ANOVA revealed a significant effect of the interaction of strain and saturation condition (F = 7.606, P < 0.01), and a t test revealed a significant difference between N2 and arr-1(ok401) in the benzaldehyde saturated conditions (t = 4.997, P < 0.01) and a significant difference between arr-1(ok401) in the unsaturated and saturated conditions (t = 9.900, P < 0.01). (B) Chemotaxis of wild-type N2 animals, arr-1(ok401) animals and animals in which arr-1 has been selectively rescued primarily in AWC to a point of isoamyl alcohol on plates containing a saturating concentration of benzaldehyde. A t test revealed a significant difference between arr-1(ok401) and odr-3p::arr-1 (t = 2.56, P < 0.05). (C) Chemotaxis of wild-type N2 animals and arr-1(ok401) animals to a point of the AWA-sensed odorant pyrazine on unsaturated plates and plates containing a saturating concentration of diacetyl. A two-way ANOVA revealed a significant effect of the interaction of strain and saturation condition (F = 25.89, P < 0.01), while a t test revealed a significant difference between N2 and arr-1(ok401) in the diacetyl saturated conditions (t = 7.96, P < 0.01) and a significant difference between arr-1(ok401) in the unsaturated and saturated conditions (t = 5.824, P < 0.01). (D) Chemotaxis of wild-type N2 animals to a point of the AWC-sensed odorant isoamyl alcohol on plates saturated with benzaldehyde, in the absence (−Barb) and presence (+Barb) of the β-arrestin inhibitor Barbadin. A t test revealed a significant difference between the –Barb and +Barb groups (t = −2.27, P < 0.05). (E) Chemotaxis of wild-type N2 animals and arr-1(ok401) animals to a point of the AWA-sensed odorant diacetyl on unsaturated plates and plates containing a saturating concentration of the AWC-sensed odorant benzaldehyde. A two-way ANOVA revealed no significant interaction between strain and saturation condition (F = 0.171, P > 0.05), *P < 0.05.

Fig. 2.

Experimental support for model predictions. (A) Chemotaxis of wild-type N2 animals and arr-1(ok401) animals to a point of benzaldehyde on both unsaturated agar plates, and plates containing a saturating concentration of benzaldehyde. A two-way ANOVA revealed a significant interaction between strain and saturation condition (F = 16.60, P < 0.01), and a t test indicated a significant difference between N2 and arr-1(ok401) in the benzaldehyde saturated condition (t = −2.58, P < 0.05). (B) Chemotaxis of arr-1(ok401), odr-10(ky225), and arr-1(ok401);odr-10(ky225) animals to a point of pyrazine on plates containing a saturating concentration of diacetyl. A t test revealed a significant difference between arr-1(ok401) and arr-1(ok401);odr-10(ky225) animals (t = −5.07, P < 0.01), *P < 0.05.

Fig. 3.

GRK-1 activity is required for olfactory discrimination. Chemotaxis of wild-type N2 animals, arr-1(ok401) and grk-1(ok1239) animals to a point of isoamyl alcohol on unsaturated agar plates and on plates containing a saturating concentration of benzaldehyde. A two-way ANOVA revealed a significant interaction between strain and saturation condition (F = 5.695, P < 0.01), and a t test indicated a significant difference between N2 and grk-1(ok1239) in the benzaldehyde saturated condition (t = 3.40, P < 0.01), and between grk-1(ok1230) and arr-1(ok401) in the unsaturated condition (t = 4.158, P < 0.01), but no significant difference between N2 and grk-1(ok1239) in the unsaturated condition (t = −0.76, P > 0.05) or between grk-1(ok1329) and arr-1(ok401) in the saturated condition (t = 1.333, P > 0.05), P < 0.05.

Fig. 4.

Diagram of discrimination model. In olfactory neurons of wild-type N2 animals (Upper), ARR-1 acts to desensitize the receptors for saturating odorants, here shown as the AWC-sensed odorant benzaldehyde, leaving only signaling from the point odorant, here shown as the AWC-sensed odorant isoamyl alcohol, to determine chemotactic behavior. In arr-1 mutant animals (Lower), ubiquitous signaling from the receptor for the saturating odorant overwhelms signaling from the point odorant, preventing chemotaxis toward it.