The Signaling Pathway of Caenorhabditis elegans Mediates Chemotaxis Response to the Attractant 2-Heptanone in a Trojan Horse-like Pathogenesis

Abstract

The nematode Caenorhabditis elegans exhibits behavioral responses to a wide range of odorants associated with food and pathogens. A previous study described a Trojan Horse-like strategy of pathogenesis whereby the bacterium Bacillus nematocida B16 emits the volatile organic compound 2-heptanone to trap C. elegans for successful infection. Here, we further explored the receptor for 2-heptanone as well as the pathway involved in signal transduction in C. elegans. Our experiments showed that 2-heptanone sensing depended on the function of AWC neurons and a GPCR encoded by str-2. Consistent with the above observation, the HEK293 cells expressing STR-2 on their surfaces showed a transient elevation in intracellular Ca 2؉ levels after 2-heptanone applications. After combining the assays of RNA interference and gene mutants, we also identified the G␣ subunits and their downstream components in the olfactory signal cascade that are necessary for responding to 2-heptanone, including G␣ subunits of egl-30 and gpa-3, phospholipase C of plc-1and egl-8, and the calcium channel of cmk-1 and cal-1. Our work demonstrates for the first time that an integrated signaling pathway for 2-heptanone response in C. elegans involves recognition by GPCR STR-2, activation by G␣ subunits of egl-30/gpa-3 and transfer to the PLC pathway, indicating that a potentially novel olfactory pathway exists in AWC neurons. Meanwhile, since 2-heptanone, a metabolite from the pathogenic bacterium B. nematocida B16, can be sensed by C. elegans and thus strongly attract its host, our current work also suggested coevolution between the pathogenic microorganism and the chemosensory system in C. elegans.

Genetic and functional diversification of chemosensory pathway receptors in mosquito-borne filarial nematodes

“…Historically, it has been difficult to identify endogenous ligands for nematode chemoreceptors. In C. elegans , only a small fraction of chemoreceptors have been linked to activating molecules, some of which are not likely to be the natural ligand (46,(69)(70)(71)(72)(73)(74)(75)(76)(77) . This is partly a function of the large number of chemoreceptors (> 1,400) and the large space of potentially relevant terrestrial cues in free-living clade V nematodes.…”

The Natural Biotic Environment ofCaenorhabditis elegans

“…For example, C. elegans is initially attracted to pathogenic S. marcescens , which it subsequently avoids (Pradel et al 2007) (see also below S. marcescens as a likely natural pathogen ). The nematode also responds positively to specific volatile organic compounds, such as 2-heptanone, of B. nematocida (Niu et al 2010; C. Zhang et al 2016).…”

“…The animals can behaviorally avoid this bacterium, a behavior mediated at least to some extent through the insulin-like signaling cascade and the neuropeptide receptor gene npr-1 (Hasshoff et al 2007; Nakad et al 2016). Glycolipids on intestinal cells act as receptors for the crystal toxin Cry5B (Griffitts et al 2005; Barrows et al 2007), while ASP-1-mediated necrosis enhances susceptibility to the toxin Cry6Aa (F. Zhang et al 2016).…”

An atlas ofCaenorhabditis eleganschemoreceptor expression

“…The majority of these 76 reporters revealed expression in chemosensory neurons [2]. Functional studies have linked a small subset of these receptors to the sensation of specific environmental or 5 pheromonal cues [12][13][14][15][16][17][18][19][20][21], but in the absence of concerted de-orphanization efforts like those seen in other organisms [22,23], the number of receptors with assigned ligands is still remarkably low.…”

“…WormBase ASK, unidentifed cells in tail sra- 9 WormBase ASK sra- 10 WormBase URX, ALA, some sensory neurons, interneurons, pharyngeal neurons and muscle / tail neurons, intestine sra-11 BC15147 URX, AVB, RIF, AIY, 1 pair in ventral ganglion, 1 pharyngeal neuron, PVT (in Wormbase another sra-11 reporter is annotated as also being expressed in AIA and VC) sra-13 BC13517 ADL, ASI, ASJ (in Wormbase another sra-11 reporter is annotated as being expressed in AWC, AWA, hypodermis, body wall muscle, vulval muscle, seam cells) sra-14 BC14819 ADL, ASJ, 1 pair anterior to AWB, RID, PHA, PHB, 1 pair a bit posterior to PHB, 1 more neuron in the tail, ray expression in males sra-17 BC13708 ASI (in Wormbase another sra-17 reporter is annotated as being expressed in AWA and several other unidentified neurons) sra- 20 WormBase head neurons sra- 21 WormBase head neurons sra- 22 WormBase amphid neurons sra-23 BC13491 ASI, AWB, 2 single midline neurons -one dorsal (on top of ASKs) and one ventral (anterior to ASJ), PQR sra-25 BC13401 ASI, ASH, BAG (dim) sra-28 BC14650 ASI, PQR, other dim pairs, sometimes PVT sra- 29 WormBase amphids, intestine sra-36 BC11976 ASI, some worms show dim expression in a couple more head neuron pairs, DA8, DA9, VA11 sra-35 BC14646 OLQ, ADL, ASI, HSN sra- 38 WormBase pharynx, head neurons, unidentifed cells in head sra- 39 WormBase ASJ, ADL, PHA, PHB, intestine, hypodermis srab-1 WormBase AWB srab-3 BC14730 ASK, RID, AIY, RIF, 1 pair in ventral ganglion srab- 4 WormBase pm6, AWB, RID, AIA, AFD, PVT, NSM, I4, I2, DVC, RME, VA, VB srab-6 BC14733 Hypodermis srab-7 BC14732 ASK, ASH, ASI (dim & not so consistent), (AWA dim), vulva muscle?, anal depressor muscle srab-8 BC14734 ASK, ASI, ASH, ASJ, one pair posterior to AWB (some space in btw), PHA, PHB, one pair just posterior to PHB (sometimes dimmer), HSN, AVM, BDU, Amso, ray expression in males srab-9 BC12175 ASH, ASJ, (AWA), 1 WormBase AWA Table S2: List of all identified sensory neurons with GPCR expression Cell ADF ADL AFD ALA ALM ALN AQR ASE ASG ASH ASI ASJ ASK AVM AWA AWB AWC BAG CEP FLP IL1 IL2 OLL OLQ PDE PHA PHB PHC PLM PLN PQR PVD PVM URB URX # GPCR 9 72 3 1 5 2 3 5 4 51 99 39 41 4 12 19 22 8 2 2 1 5 3 3 1 51 49…”