Biotechnology Advances 32 (2014) 290–295
Contents lists available at ScienceDirect
Biotechnology Advances journal homepage: www.elsevier.com/locate/biotechadv
Research review paper
The olfactory signal transduction for attractive odorants in Caenorhabditis elegans Chunmei Zhang, Jinyuan Yan, Yao Chen, Chunyan Chen, Keqin Zhang ⁎, Xiaowei Huang ⁎ Laboratory for Conservation and Utilization of Bio-Resources, Yunnan University, Kunming, Yunnan 650091, PR China Key Laboratory for Microbial Resources of the Ministry of Education, Yunnan University, Kunming, Yunnan 650091, PR China
a r t i c l e
i n f o
Article history: Received 25 July 2013 Received in revised form 14 October 2013 Accepted 28 October 2013 Available online 2 November 2013 Keywords: C. elegans Olfaction Chemotaxis Signal pathways
a b s t r a c t Olfaction in Caenorhabditis elegans is a versatile and sensitive strategy to seek food and avoid danger by sensing volatile chemicals emitted by the targets. The ability to sense attractive odor is mainly accomplished by the AWA and AWC neurons. Previous studies have shown the components of the olfaction signal pathway in these two amphid chemosensory neurons, but integration of the individual signaling components requires further elucidation. Here we review the progresses in our understanding of signal pathways for attractive olfaction involving AWA and AWC neurons, and discuss how the different signal molecules might employ the common molecular cascades to transduce the olfactory system and guide behavior in each neuron. © 2014 Elsevier Inc. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . Signal transduction in olfactory neurons . . . . . . . . . . . . . 2.1. G protein-coupled olfactory receptors (GPCRs) . . . . . . 2.2. Gα subunits in olfactory signal cascade . . . . . . . . . . 2.3. Ion channel in the olfaction . . . . . . . . . . . . . . . 2.1.3.1. cGMP signal and CNG channel in AWC neurons . . 2.1.3.2. Receptor guanylate cyclases in G protein signaling 2.1.3.3. The TRPV channels OSM-9/OCR-2 . . . . . . . . 3. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . .
1. Introduction The free-living nematode Caenorhabditis elegans has not only been a good model to investigate the innate immune system, signal transduction, and development, but it has also attracted much attention for studying the nervous system and the related behaviors for its accessibility to genetics and the detailed knowledge of its nervous system. C. elegans has a simple nervous system consisting of 302 neurons (Bargmann and Mori, 1997; White et al., 1986). However, it exhibits a rich repertoire of behavioral responses to a variety of environmental changes, including mechanosensation (Chalfie and White, 1988), ⁎ Corresponding authors. Tel.: +86 871 5034878; fax: +86 871 5034838. E-mail addresses: [email protected] (K. Zhang), [email protected] (X. Huang). 0734-9750/$ – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.biotechadv.2013.10.010
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
290 291 291 292 292 292 293 293 294 294 294
olfactory sensation (Bargmann et al., 1993; Wood and the community of C. elegans researchers, 1988), salt chemotaxis (Bargmann and Horvitz, 1991; Iino and Yoshida, 2009), thermotaxis (Mori and Ohshima, 1995), and navigation (Gray et al., 2005; Tsalik and Hobert, 2003). Specifically, C. elegans naturally inhabits the soil, a complex environment that contains both food bacteria and dangerous pathogens (Brenner, 1974). Therefore, having a sophisticated chemosensory system would allow C. elegans to distinguish their potential food sources from pathogens. Indeed, previous studies have shown that C. elegans can discriminate the pathogens from their food bacteria (Bargmann, 2006; Beale et al., 2006). This ability mainly depends on the functions of its 12 pairs of amphid chemosensory neurons (Mori and Ohshima, 1997; Starich et al., 1995). According to the detailed cellular analysis
C. Zhang et al. / Biotechnology Advances 32 (2014) 290–295
of amphid chemosensory neurons, the 12 pairs can detect a wide variety of chemicals in the environment, including both volatile and watersoluble compounds (Bargmann, 1993; Bargmann and Horvitz, 1991; Mori and Ohshima,1997). In the chemosensory system, olfaction is one of the most sensitive and effective strategies because it specialize in detecting volatile chemicals. Five pairs of neurons, AWA, AWB, AWC, ASH and ADL, are involved in olfaction in C. elegans. Two of them, AWA and AWC, respond to attractive odorants. For example, the AWA neurons can respond to at least three attractive odors: diacetyl, pyrazine and 2, 4, 5-trimethylthiazolez (Bargmann, 2006). AWCs recognize at least five attractive odors: benzaldehyde, butanone, isoamyl alcohol, 2, 3-pentanedione and 2, 4, 5-trimethyl thiazole (Mori, 1999; Lans et al., 2004). The other three pairs of neurons, AWB, ASH and ADL, are responsible for the behavior of active avoidance from undesirable volatile chemicals and even microbial pathogens. However among these three neuron pairs, the AWB neurons' only function is to sense volatile repellents (Bargmann et al., 1990; Wood and the community of C. elegans researchers, 1988). In contrast, detecting repulsive volatile chemicals is only a minor function for ASH and ADL neurons. The ASH neurons mediate rapid withdrawal that is different from chemotaxis behaviors in gradients. This type of neurons mainly responds to high osmolarity (Culotti and Russell, 1978), heavy metals (Dusenbery, 1974), detergents (Hilliard et al., 2002), bitter alkaloids (Hilliard et al., 2004), acid pH (Dusenbery, 1974), and light touch to the tip of the nose (Colbert et al., 1997; Kaplan and Horvitz, 1993). On the other hand, the ADL neurons mainly deal with responses to social feeding (Chang et al., 2006; de Bono et al., 2002). Therefore, the typical olfactory neurons generally include three types, AWA, AWB and AWC. The worms chase food or escape dangers through discriminating attractants from repellants by the three pairs of olfactory neurons. Though all the 12 pairs of amphid neurons have cilium for sensing chemicals and mechanical cues, the olfactory neurons also have subtle differences in their structure profiles compared to the other sensory neurons (Bargmann and Horvitz, 1991). Most of the amphid neurons detecting water soluble chemicals are directly exposed to the environment and have no complex branched cilia (Mori and Ohshima, 1997; Schackwitz et al., 1996). However, three types of the olfactory neurons AWA, AWC, and AWB have complex and flattened cilia (Bargmann et al., 1993; Mori and Ohshima, 1997). Furthermore, their cilia are surrounded by a membrane formed by a sheath cell, and therefore the neurons' endings are not exposed to the external environment (Roayaie et al., 1998). Since the olfactory responses for the attractants are associated with the most powerful behaviors to pursue food, this mini-review will focus on the attractant signal transduction pathway in AWA and AWC. 2. Signal transduction in olfactory neurons As described above, C. elegans mainly employs the four cells (left and right AWA or AWC cells) to sense and discriminate a variety of volatile attractant. Part of the odorant specificity is due to the odorant differences detected by AWA and AWC, or the functional asymmetry between the left and the right AWC (Bargmann et al., 1993; Wes and Bargmann, 2001). However, these two factors above still cannot account for all the odorant specificity, suggesting that additional intracellular mechanisms exist to establish odorant specificity. Intracellular components participating in olfaction have been identified through the genetic analysis of mutations that affects olfactory sensing of specific chemicals. Generally, these components include olfactory receptors (especially G protein-couple receptors, GPCRs), G protein and cation channels, which are shown essential in the signal transduction of olfaction (Bargmann and Mori, 1997; Mori, 1999; Reed, 1990). Once the volatile odorants in the environment are recognized by specialized receptors located on the cilia, the olfactory signal cascades will be activated and consequently lead to behavioral responses of worms.
291
2.1. G protein-coupled olfactory receptors (GPCRs) C. elegans has only two pairs of olfactory neurons AWA and AWC for the attractants. However, each neuron expresses multiple receptors to recognize odorants and thus detect several different chemicals, similar to those in mammalian (Troemel et al., 1995). The differences of both mammalian and C.elegans include: (Alcedo and Kenyon, 2004) the olfactory neurons express only one functional receptor gene for each neuron in mammals, but in C. elegans one neuron expresses several functional receptor genes at the same time (Axel, 2005; Buck and Axel, 1991); (Altun-Gultekin et al., 2001) Though the olfactory systems in these two groups of organisms are potentially able to sense a comparable diversity of odors, the smaller number of neurons in C. elegans limits its discriminatory power (Bargmann, 2006). GPCRs have been reported as the most common olfactory receptors in either C.elegans or mammalian. The genome of C. elegans encodes more than 500 putative GPCRs, among which are some classical, well-conserved GPCRs that recognize serotonin, acetylcholine, glutamate, and neuropeptides, and these canonical receptors are involved in regular neurotransmission (Bargmann, 1998, 2006; Krieger and Breer 1999). However, the GPCRs that only express or express primarily in single left–right pair of chemosensory neurons are highly divergent between C. elegans and other species (Chen et al., 2005; Colosimo et al., 2004; McCarroll et al., 2005; Troemel et al., 1995). Furthermore, these GPCRs that are specifically expressed in one or more amphid neurons at variable expression levels usually localize to the cilia (Bargmann, 2006, 1998; Clyne et al., 1999; Ngai et al., 1993). It is reasonable to hypothesize that multiple GPCRs are expressed by each neuron, enabling C. elegans to respond to multiple, sometimes unrelated chemicals, using only one or few cells. The AWA neurons have been reported to express two GPCRs, ODR-10 and SRA-13. Gene odr-10 encodes a seven-transmembrane domain protein that is distantly related to the G protein-coupled receptor superfamily. It only expresses in the AWA neurons, and the ODR-10GPF fused protein localizes to the distal cilia of the dendrites where odorant detection is thought to occur (Sengupta et al., 1996; Zhang et al., 1997). ODR-10 is also the only chemosensory receptor whose ligand is known in C. elegans. Forward genetic analysis for olfactory mutation has suggested that ODR-10 is the high affinity GPCR for the odorant diacetyl (Sengupta et al., 1996). Worms with the odr-10 mutation have a 100-fold reduced sensitivity to the volatile attractant diacetyl, and failed to sense low concentrations of diacetyl. However, the odr-10 mutant animal had a normal response to other odorants detected by AWA neuron, and the chemotaxis to the odorants detected by AWC was not affected, including the odor chemicals with similar chemical structures (Sengupta et al., 1996; Zhang et al., 1997). The gene sra-13 also encodes a GPCR, a member of the SRA family of chemosensory receptors. SRA family was classified as a group of the chemoreceptor gene family based on similar sequence (Troemel et al., 1995), and named starting with sr for serpentine receptor (Troemel et al., 1995; Robertson and Thomas, 2006). The gene family was annotated as part of the comparison with Caenorhabditis briggsae (Robertson and Thomas, 2006; Stein et al., 2003). Gene sra-13 was expressed in AWA and AWC neurons, as well as non-neuron cells such as muscle and hypodermal cells. It is localized to the cell body, axons, and dendrites in AWA and AWC neurons. The SRA-13 protein affects vulval development as well as olfactory plasticity in AWA and AWC chemosensory neurons by modulating the MAPK signal transduction pathway, but its ligand has not been identified (Battu et al., 2003; Hirotsu et al., 2000). Another GPCR STR-2 that expresses at the top of cilia in AWC neurons is related to longevity of C. elegans (Alcedo and Kenyon, 2004; Bargmann, 2006). This candidate odorant receptor is asymmetrically expressed in one of the two AWC neurons and weakly expressed in the ASI neurons (Pecko et al., 2001; Troemel et al., 1999). During embryonic development, its expression is controlled by Ca2+, MAPK and axon
292
C. Zhang et al. / Biotechnology Advances 32 (2014) 290–295
guidance genes. During later stages, sensory signaling genes such as the guanylyl cyclases odr-1, daf-11 as well as the cGMP gated cation channel subunits tax-2 and tax-4 are necessary to maintain str-2 expression (Coburn et al., 1998; Peckol et al., 1999). Though all these genes seem to encompass an olfactory signaling network, the exact function of str2 still remains unknown in sensing attractive odorant. The gene sra-11 encodes an orphan GPCR that belongs to a large family of putative chemoreceptor (Troemel et al., 1995). Expression of this gene can be observed through all larval and adult stages (AltunGultekin et al., 2001). Unlike other chemoreceptor family members, which are expressed in sensory neurons, sra-11 is exclusively expressed in three interneuron classes, AIY, AIA, and AVB (Tsalik and Hobert, 2003). However, both the odorant attraction assays and the odorantinduced egg-laying responses have illustrated that the SRA-11 protein is specifically required for olfactory imprinting (Remy and Hobert, 2005). 2.2. Gα subunits in olfactory signal cascade Heterotrimeric G protein-coupled signaling cascades are used to transduce diverse signals, varying from intercellular mediators to environmental stimuli, including the olfaction in C. elegans (Fire et al., 1998; Hamm 1998; Mori, 1999; Mori and Ohshima 1995; Reed, 1990; Yau and Baylor, 1989). The genome of C. elegans encodes the subunits such as 21 Gα, 2 Gβ and 3 Gγ (Cuppen et al., 2003; Roayaie et al., 1998; Zwaal et al., 1997). According to the analysis of sequence similarity, four Gα subunits share high homologies to mammals, and they are GSA-1, GOA-1, EGL-30 and GPA-12, respectively. Three of these four conserved G protein α subunits are expressed broadly except for the GPA-12. Both gene GSA-1 and GOA-1 are expressed in all the amphid neurons, but which amphid neurons express EGL-30 is still unknown. Additionally, several of the C. elegans specific subunits (GPAs) are involved either positively or negatively in chemosensation, including odr-3, gpa-3, gpa-5 and gpa-6 in AWA neuron, as well as odr-3, gpa-2, gpa-3, and gpa-13 in AWC neurons (Jansen et al., 1999; Roayaie et al., 1998; Troemel et al., 1995; Zwaal et al., 1997). Odr-3, one of the Gi-like genes, strongly expresses in cilia of AWA and AWC neurons and functions in olfactory response in both type of neurons (Bargmann, 2006; Troemel et al., 1995; Zwaal et al., 1997). Screens of olfactory mutants demonstrated that odr-3 mutation generated highly defective olfaction responses mediated by AWA and AWC neurons (Troemel et al., 1995). However, there were no observable effects in osmotic avoidance sensed by AWA and AWC in these mutants. Thus, the results suggest that ODR-3 is a strong candidate for the G protein that acts downstream of the olfactory receptor (Bargmann, 2006). Meanwhile, the odr-3 gene is also involved in cilium morphogenesis of olfactory neurons. When AWA and AWC cilia with abnormal odr-3 expression were closely examined by light microscopy, it was found that the absence of odr-3 resulted in branched filament-like cilia reminiscent of AWA cilia, rather than the normal large membranous, fan-like AWC cilia. The amount of ODR-3 can thus determine the extent of cilium outgrowth in AWA and AWC (Roayaie et al., 1998). Though ODR-3 is the main Gα protein to activate the olfactory signal transduction in both AWA and AWC neurons, some other Gα proteins can perform the overlapped or complementary roles. For example, the gpa-3 gene is also expressed in cilia, cell bodies and axons of AWA and AWC neurons. In olfaction signals, GPA-3 functions redundantly to ODR-3. For many odorant chemicals detected by AWA and AWC neurons, GPA-3 is sufficient except for butanone. Contrarily, loss of gpa-3 activity enhances the chemotaxis defect of odr-3(lf) mutants to all odorants, indicating that GPA-3 is redundant to ODR-3, and that it plays a positive role in odorant detection (Lans et al., 2004; Troemel et al., 1995; Zwaal et al., 1997). Mutations at gpa-13 cause mild defects for responses to 2, 3-pentanedione, and this phenotype is weakly enhanced by mutations of odr-3, suggesting that gpa-13 plays a minor stimulatory role in olfaction (Mori and Ohshima, 1995). Though gpa-6 has been shown to
be expressed in olfactory neurons, a gpa-6(lf) mutant showed a similar activity as wild type strains for chemotaxis to other soluble and volatile attractants and repellents. This mutant showed only a slightly increased preference for NaCl when placed between NaCl and NaAc (Jansen et al., 1999). Besides the stimulatory signals that the Gα proteins recognize, several Gα subunits also negatively regulate the olfactory signal network. For example, the gpa-5 gene is expressed in cilia, cell bodies and axons of AWA neurons and, is supposed to execute an inhibitory effect on AWA function, probably via GPCR SRA-13(Battu et al., 2003; Troemel et al., 1995). Though gpa-5(lf) mutants responded similarly to soluble attractants and repellents as wild type worms, the gpa-5(lf) mutants rescued the chemotaxis defect of odr-3 mutants. These mutants showed an increased sensitivity to volatile attractants sensed by AWA, and to one attractant, isoamyl alcohol, sensed by AWC. Therefore, these data suggest that gpa-5 is a negative regulator of chemosensation in worms (Battu et al., 2003; Jansen et al., 1999; Lans et al., 2004). The other Gα that performs the similarly negative function in olfaction is GPA-2. But the cell type is different because gpa-2 expresses in cilia, cell bodies and axons of AWC neurons, and thus it plays an inhibitory role in AWC for odorants (Lans et al., 2004; Troemel et al., 1995; Zwaal et al., 1997). For example, the experimental evidences from Lans revealed the improved odorant responses in gpa-2(lf)::odr-3(lf) double mutants for all odorants except isoamyl alcohol and butanone, indicating a negative regulatory role for GPA-2 in detecting most odorants (Lans et al., 2004). However, it was also reported that gpa-2(lf) mutations enhanced the mild defect in chemotaxis to butanone observed in odr-3(lf), suggesting that GPA-2 and ODR-3 might act redundantly in response to this odorant (Troemel et al., 1995). In conclusion, AWA, which mediates the response to volatile odorants including diacetyl, pyrazine and 2, 4, 5-trimethyl- thiazole, expresses four Gα genes odr-3, gpa-3, gpa-5, and gpa-6. Among them, ODR-3 and GPA-3 provide the main stimulatory signal and a redundantly stimulatory signal, respectively. However, GPA-5 is a negative regulator in AWA. To the neurons of AWC, which mediate the response to volatile attractants including benzaldehyde, butanone, and isoamyl alcohol, the expressed Gα genes include odr-3, gpa-2, gpa-3, and gpa-13. Among them, GPA-13 provides a mild stimulatory effect besides the similarly positive function performed by ODR-3 and GPA-3 as they do in AWA. GPA-2 plays an inhibitory role in AWC. Although expression of GPA-5 has not been detected in AWC, gpa-5(lf) mutants are hypersensitive to isoamyl alcohol and suppress the reduced response to this odorant caused by overexpression of SRA-13, a GPCR expressed in both AWA and AWC (Battu et al., 2003). 2.3. Ion channel in the olfaction When recognizing external stimuli, sensory neurons generate electrical signals that in turn lead to the regulation of neurotransmitter releases (Mori and Ohshima, 1997). In the olfactory signal cascade of C.elegans, ion channel is the key component in the last step to sense and transduce receptor activity into electrical activity in olfactory cells (Zagotta and Siegelbaum, 1996). Genetic evidences suggest that the AWC-sensed odorants elicit chemotaxis through a cyclic nucleotidegated channel (CNG) consisting of TAX-4 and TAX-2 subunits (Coburn and Bargmann, 1996; Komatsu et al., 1996). However, a transient receptor potential vanilloid (TRPV) channel is required for chemotaxis to odors detected by AWA (Colbert and Bargmann, 1997; Hilliard et al., 2002, 2005). 2.1.3.1. cGMP signal and CNG channel in AWC neurons Stimulation of a G protein-coupled receptor and its downstream G protein initiates a signaling cascade that leads to an increase in the intracellular concentration of a cyclic nucleotide in AWC. This alters the opening of a CNG channel, changes the membrane potential of the cell, and resultantly produces the signaling processes that allow us to
C. Zhang et al. / Biotechnology Advances 32 (2014) 290–295
smell (Nakamura and Gold, 1987). CNG channels are nonselective cation channels that belong to a family that includes voltage-gated channels (Jan and Jan, 1990; Komatsu et al., 1996). A CNG channel encoded by tax-4 and tax-2 genes is essential for the function of many sensory neurons in C. elegans (Coburn and Bargmann, 1996; Komatsu et al., 1996; Zagotta and Siegelbaum, 1996). tax-4 encodes an alpha subunit that forms a channel on its own, while tax-2 encodes a beta subunit that enhances tax-4 activity. Both the tax-4 and tax-2 are expressed in the sensory cilia of AWC (Coburn and Bargmann, 1996; Komatsu et al., 1996), and coordinately form a cGMP-gated channel, a type of CNG channels. In other words, heterologous expression of TAX-4 and TAX-2 generates a channel that is highly sensitive to cGMP, like the mammalian phototransduction channel (Coburn and Bargmann, 1996; Komatsu et al., 1999). Thus, a plausible pathway for olfaction transduction in the AWC neurons is that G protein signaling regulates the function of either a cGMP phosphodiesterase (as it occurs in mammalian vision) or a guanylate cyclase, thereby closing or opening the cGMP-gated channel. Worms with dysfunctional tax-4 or tax-2 are defective in AWC chemotaxis to volatile odors, and also in sensory axon structure (Bargmann, 2006). 2.1.3.2. Receptor guanylate cyclases in G protein signaling The CNG channel encoded by tax-4 and tax-2 is controlled by the intracellular messenger cGMP. By analogy with vertebrate phototransduction, the intervening steps between the Gα proteins and the cGMP-gated channel presumably involve a guanylate cyclase or a cGMP phosphodiesterase in AWC. However, such a cGMP phosphodiesterase affecting the olfactory has not been identified till now. The C. elegans genome encodes 34 guanulate cyclases that could be responsible for cGMP production in TAX4/TAX-2 neurons. Among these 34, 27 are transmembrane proteins, or receptor-like guanylate cyclases (RGCs), and 7 are cytosolic soluble guanylate cyclases (sGCs) (Ortiz et al., 2006). The daf-11 gene encodes a receptor guanylate cyclase in AWC olfactory neurons but its function is non-autonomous (Birnby et al., 2000; Fulle et al., 1995; Vowels and Thomas, 1994; Yu et al., 1997). odr-1, a second receptor guanylate cyclase, is expressed in the same cells that express daf-11. Because ODR-1 and DAF-11 each lack the key residues required for catalysis, these two proteins likely function as heterodimers that provide cGMP for the opening
293
of the TAX-4/TAX-2 channel (Morton, 2004). It has also been shown that odr-1 olfactory defects can be rescued by mutated ODR-1 proteins lacking an extracellular domain, suggesting that the cyclase domains are probably the primary mediators of ODR-1 function in AWC (L'Etoile and Bargmann, 2000). These results suggest that the olfaction system in AWC appears to use guanylate cyclase DAF-11 and ODR-1 as ligandindependent RGCs downstream of G protein signaling (Fig. 1).
2.1.3.3. The TRPV channels OSM-9/OCR-2 Similar to that in AWC neurons, the amphid sensory neurons of AWA also require a channel for odorant chemotaxis, and this channel is encoded by the osm-9 and ocr-2 genes. OSM-9 and OCR-2 proteins are localized to the AWA cilia and are mutually required for each others' cilia localization. That is to say, OCR-2 is not localized to the cilia in osm-9 mutants, and vice versa. Therefore these two channel proteins are suggested to assemble into a single channel complex (Tobin et al., 2002). The channel with OSM-9 and OCR-2 subunits belongs to the transient receptor potential (TRP) channel superfamily, a large group of channels that functions in insect phototransduction, in vertebrate pain sensation, non-neuronal pressure sensation and osmosensation. There are seven families of TRP channels, and OSM-9 and OCR-2 belong to the transient receptor potential vanilloid (TRPV) family (Ramsey et al., 2006). The OSM-9/OCR-2 channel functions the downstream of GPCRs and ODR-3 and participates in the primary signal transduction of olfaction in AWA neurons, which is similar to the TAX-2/TAX-4 channel in AWC neurons (Bargmann, 2006). Mutants of osm-9 or ocr-2 have mild to severe defects in AWA chemotaxis to volatile odors (Colbert et al., 1997; Hilliard et al., 2002, 2005; Tobin et al., 2002). In summary, the olfactory pathway in AWA neurons also begins with the binding of an odorant to a GPCR in the AWA cell membrane, which leads to the activation of the Gα subunits (eg. ODR-3 or/and GPA-3) as well as an unknown PLC, leading to the production of important intracellular second messengers diacyl glycerol (DAG) and inositol 1, 4, 5-trisphosphate (IP3). DAG can then activate the nPKCs TTX-4 and TPA-1 and produce PUFAs, which subsequently regulate the opening of the OSM-9/OCR-2 TRP channels. Meanwhile, the binding between IP3 and its receptor IP3R/ITR-1 leads to a Ca2+ influx (Fig. 2) (Battu et al., 2003).
Fig. 1. Summary of the potential olfactory signal transduction pathway in the AWC neuron. Genetic analysis supports that GPCRs initiate the olfaction system. The activated GPCRs transduce the signals to the G proteins and (ODR-3/GPA-3/GPA-13), and they in turn regulate receptor guanylate cyclase (ODR-1/DAF-11) to produce cGMP. The intracellular signal molecule cGMP finally opens the cyclic nucleotide-gated channel composed of the subunits TAX-2/TAX-4. Meantime, the genes odr-1, daf-11, tax-2 and tax-4 are necessary for maintaining the expressions of GPCRs in adult animals. This figure is modified from that of Bargmann (2006).
294
C. Zhang et al. / Biotechnology Advances 32 (2014) 290–295
Fig. 2. Summary of the potential olfactory signal transduction pathway in the AWA neuron. In AWA neurons, the attractive odors are detected by GPCRs, which activate the Gα subunits (ODR-3/GPA-3) in heterotrimeric G proteins. The G protein then regulates the production of DAG via the PLC pathway, and produces the intracellular signal molecules of IP3 and DAG. Between these two molecules, DAG activates the nPKCs and produces PUFAs; IP3 binds to its receptor IP3R/ITR-1. The results from the two intracellular signal molecule finally lead to the opening of OSM-9/OCR-2 channel and Ca2+ influx.
3. Conclusions
References
C. elegans has a simple but elegant olfactory system to detect volatile chemicals, and the neurons AWA and AWC are required for detecting attractive odors. Previous studies have revealed that two distinct pathways of olfactory signal transduction are employed in these two type of neurons based on the analyses of gene mutants of C. elegans. In spite of the different molecules involved, a common pathway for olfaction transduction in either AWA or AWC consists of recognition by the specialized GPCRs, activation of the G protein, and transduction of the odorant signal to channel proteins. Different GPCRs located on the same set of neurons are involved in sensing different attractive odorants, with many using the same signal transduction pathways. However, other yet to be discovered independent signal transduction pathways might also be involved in olfactory responses.
Alcedo J, Kenyon C. Regulation of C. elegans longevity by specific gustatory and olfactory neurons. Neuron 2004;41(1):45–55. Altun-Gultekin Z, Andachi Y, Tsalik EL, Pilgrim D, Kohara Y, Hobert O. A regulatory cascade of three homeobox genes, ceh-10, ttx-3 and ceh-23, controls cell fate specification of a defined interneuron class in C. elegans. Development 2001;128(11):1951–69. Axel R. Scents and sensibility: a molecular logic of olfactory perception (Nobel lecture). Angew Chem Int Ed Engl 2005;44(38):6110–27. Bargmann CI. Genetic and cellular analysis of behavior in C. elegans. Annu Rev Neurosci 1993;16:47–71. Bargmann CI. Neurobiology of the Caenorhabditis elegans genome. Science 1998;282(5396): 2028–33. Bargmann CI. In: The C. elegans Research Community, editor. Wormbook, 1. WormBook; 2006. http://dx.doi.org/10.1895/wormbook.1.123. [http://www. wormbook.orga]. Bargmann CI, Horvitz HR. Chemosensory neurons with overlapping functions direct chemotaxis to multiple chemicals in C. elegans. Neuron 1991;7(5):729–42. Bargmann CI, Mori I. Chemotaxis and thermotaxis. In: Riddle DL, Blumenthal T, Meyer BJ, Priess JR, editors. C. elegans II. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997. Bargmann CI, Hartwieg E, Horvitz HR. Odorant-selective genes and neurons mediate olfaction in C. elegans. Cell 1993;74(3):515–27. Battu G, Hoier EF, Hajnal A. The C. elegans G-protein-coupled receptor SRA-13 inhibits RAS/MAPK signalling during olfaction and vulval development. Development 2003;130(12):2567–77. Beale E, Li G, Tan MW, Rumbaugh KP. Caenorhabditis elegans senses bacterial autoinducers. Appl Environ Microbiol 2006;72(7):5135–7. Birnby DA, Link EM, Vowels JJ, Tian H, Colacurcio PL, Thomas JH. A transmembrane guanylyl cyclase (DAF-11) and Hsp90 (DAF-21) regulate a common set of chemosensory behaviors in Ccaenorhabditis elegans. Genetics 2000;155(1):85–104. Brenner S. The genetics of Caenorhabditis elegans. Genetics 1974;77(1):71–94.
Acknowledgments We sincerely thank Prof. Jianping Xu, from the McMaster University in Canada, for his suggestive criticism for this manuscript. This work is supported by the National Basic Research Program of China (grant no. 2011AA10A203 and no. 2013CB127500), the National Natural Science Foundation Program of China (grant no. 31370162 and no. U1036602), and the Department of Science and Technology of Yunnan Province, China (grant no. 2010GA012 as well as the grant 2010YN17).
C. Zhang et al. / Biotechnology Advances 32 (2014) 290–295 Buck L, Axel R. A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 1991;65(1):175–87. Chalfie M, White J. The nervous system. In: Wood WB, editor. The nematode Caenorhabditis elegans. Cold Spring Harbor, New York: Cold Spring Harbor Laboratory; 1988. p. 337–91. Chang AJ, Chronis N, Karow DS, Marletta MA, Bargmann CI. A distributed chemosensory circuit for oxygen preference in C. elegans. PLoS Biol 2006;4(9):e274. Chen N, Pai S, Zhao Z, Mah A, Newbury R, Johnsen RC, et al. Identification of a nematode chemosensory gene family. Proc Natl Acad Sci U S A 2005;102(1):146–51. Clyne PJ, Warr CG, Freeman MR, Lessing D, Kim J, Carlson JR. A novel family of divergent seven-transmembrane proteins: candidate odorant receptors in Drosophila. Neuron 1999;22(2):327–38. Coburn CM, Bargmann CI. A putative cyclic nucleotide-gated channel is required for sensory development and function in C. elegans. Neuron 1996;17(4):695–706. Coburn CM, Mori I, Ohshima Y, Bargmann CI. A cyclic nucleotide-gated channel inhibits sensory axon outgrowth in larval and adult Caenorhabditis elegans: a distinct pathway for maintenance of sensory axon structure. Development 1998;125(2):249–58. Colbert HA, Bargmann CI. Environmental signals modulate olfactory acuity, discrimination, and memory in Caenorhabditis elegans. Learn Mem 1997;4(2):179–91. Colbert HA, Smith TL, Bargmann CI. OSM-9, a novel protein with structural similarity to channels, is required for olfaction, mechanosensation, and olfactory adaptation in Caenorhabditis elegans. J Neurosci 1997;17(21):8259–69. Colosimo ME, Brown A, Mukhopadhyay S, Gabel C, Lanjuin AE, Samuel AD, et al. Identification of thermosensory and olfactory neuron-specific genes via expression profiling of single neuron types. Curr Biol 2004;14(24):2245–51. Culotti JG, Russell RL. Osmotic avoidance defective mutants of the nematode Caenorhabditis elegans. Genetics 1978;90(2):243–56. Cuppen E, van der Linden AM, Jansen G, Plasterk RH. Proteins interacting with Caenorhabditis elegans Galpha subunits. Comp Funct Genomics 2003;4(5):479–91. de Bono M, Tobin DM, Davis MW, Avery L, Bargmann CI. Social feeding in Caenorhabditis elegans is induced by neurons that detect aversive stimuli. Nature 2002;419(6910): 899–903. Dusenbery DB. Analysis of chemotaxis in the nematode Caenorhabditis elegans by countercurrent separation. J Exp Zool 1974;188(1):41–7. Fire A, Xu S, Montgomery MK, Kostas SA, Driver SE, Mello CC. Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans. Nature 1998;391(6669): 806–11. Fulle HJ, Vassar R, Foster DC, Yang RB, Axel R, Garbers DL. A receptor guanylyl cyclase expressed specifically in olfactory sensory neurons. Proc Natl Acad Sci U S A 1995;92(8):3571–5. Gray JM, Hill JJ, Bargmann CI. A circuit for navigation in Caenorhabditis elegans. Proc Natl Acad Sci U S A 2005;102(9):3184–91. Hamm HE. The many faces of G protein signaling. J Biol Chem 1998;273(2):669–72. Hilliard MA, Bargmann CI, Bazzicalupo P. C. elegans responds to chemical repellents by integrating sensory inputs from the head and the tail. Curr Biol 2002;12(9):730–4. Hilliard MA, Bergamasco C, Arbucci S, Plasterk RH, Bazzicalupo P. Worms taste bitter: ASH neurons, QUI-1, GPA-3 and ODR-3 mediate quinine avoidance in Caenorhabditis elegans. EMBO J 2004;23(5):1101–11. Hilliard MA, Apicella AJ, Kerr R, Suzuki H, Bazzicalupo P, Schafer WR. In vivo imaging of C. elegans ASH neurons: cellular response and adaptation to chemical repellents. EMBO J 2005;24(1):63–72. Hirotsu T, Saeki S, Yamamoto M, Iino Y. The Ras-MAPK pathway is important for olfaction in Caenorhabditis elegans. Nature 2000;404(6775):289–93. Iino Y, Yoshida K. Parallel use of two behavioral mechanisms for chemotaxis in Caenorhabditis elegans. J Neurosci 2009;29(17):5370–80. Jan LY, Jan YN. A superfamily of ion channels. Nature 1990;345(6277):672. Jansen G, Thijssen KL, Werner P, van der Horst M, Hazendonk E, Plasterk RH. The complete family of genes encoding G proteins of Caenorhabditis elegans. Nat Genet 1999;21(4):414–9. Kaplan JM, Horvitz HR. A dual mechanosensory and chemosensory neuron in Caenorhabditis elegans. Proc Natl Acad Sci U S A 1993;90(6):2227–31. Komatsu H, Mori I, Rhee JS, Akaike N, Ohshima Y. Mutations in a cyclic nucleotide-gated channel lead to abnormal thermosensation and chemosensation in C. elegans. Neuron 1996;17(4):707–18. Komatsu H, Jin YH, L'Etoile N, Mori I, Bargmann CI, Akaike N, et al. Functional reconstitution of a heteromeric cyclic nucleotide-gated channel of Caenorhabditis elegans in cultured cells. Brain Res 1999;821(1):160–8. Krieger J, Breer H. Olfactory reception in invertebrates. Science 1999;286(5440):720–3. Lans H, Rademakers S, Jansen G. A network of stimulatory and inhibitory Galpha-subunits regulates olfaction in Caenorhabditis elegans. Genetics 2004;167(4):1677–87. L'Etoile ND, Bargmann CI. Olfaction and odor discrimination are mediated by the C. elegans guanylyl cyclase ODR-1. Neuron 2000;25(3):575–86. McCarroll SA, Li H, Bargmann CI. Identification of transcriptional regulatory elements in chemosensory receptor genes by probabilistic segmentation. Curr Biol 2005;15(4): 347–52.
295
Mori I. Genetics of chemotaxis and thermotaxis in the nematode Caenorhabditis elegans. Annu Rev Genet 1999;33:399–422. Mori I, Ohshima Y. Neural regulation of thermotaxis in Caenorhabditis elegans. Nature 1995;376(6538):344–8. Mori I, Ohshima Y. Molecular neurogenetics of chemotaxis and thermotaxis in the nematode Caenorhabditis elegans. Bioessays 1997;19(12):1055–64. Morton DB. Invertebrates yield a plethora of atypical guanylyl cyclases. Mol Neurobiol 2004;29(2):97–116. Nakamura T, Gold GH. A cyclic nucleotide-gated conductance in olfactory receptor cilia. Nature 1987;325(6103):442–4. Ngai J, Dowling MM, Buck L, Axel R, Chess A. The family of genes encoding odorant receptors in the channel catfish. Cell 1993;72(5):657–66. Ortiz CO, Etchberger JF, Posy SL, Frøkjaer-Jensen C, Lockery S, Honig B, et al. Searching for neuronal left/right asymmetry: genomewide analysis of nematode receptor-type guanylyl cyclases. Genetics 2006;173(1):131–49. Peckol EL, Zallen JA, Yarrow JC, Bargmann CI. Sensory activity affects sensory axon development in C. elegans. Development 1999;126(9):1891–902. Peckol EL, Troemel ER, Bargmann CI. Sensory experience and sensory activity regulate chemosensory receptor gene expression in Caenorhabditis elegans. Proc Natl Acad Sci U S A 2001;98(20):11032–8. Ramsey IS, Delling M, Clapham DE. An introduction to TRP channels. Annu Rev Physiol 2006;68:619–47. Reed RR. G protein diversity and the regulation of signaling pathways. New Biol 1990;2(11):957–60. Remy JJ, Hobert O. An interneuronal chemoreceptor required for olfactory imprinting in C. elegans. Science 2005;309(5735):787–90. Roayaie K, Crump JG, Sagasti A, Bargmann CI. The G alpha protein ODR-3 mediates olfactory and nociceptive function and controls cilium morphogenesis in C. elegans olfactory neurons. Neuron 1998;20(1):55–67. Robertson HM, Thomas JH. The putative chemoreceptor families of C.elegans. WormBook; 20061–12. Schackwitz WS, Inoue T, Thomas JH. Chemosensory neurons function in parallel to mediate a pheromone response in C. elegans. Neuron 1996;17(4):719–28. Sengupta P, Chou JH, Bargmann CI. odr-10 encodes a seven transmembrane domain olfactory receptor required for responses to the odorant diacetyl. Cell 1996;84(6): 899–909. Starich TA, Herman RK, Kari CK, Yeh WH, Schackwitz WS, Schuyler MW, et al. Mutations affecting the chemosensory neurons of Caenorhabditis elegans. Genetics 1995;139(1): 171–88. Stein LD, Bao Z, Blasiar D, Blumenthal T, Brent MR, Chen N, et al. The genome sequence of Caenorhabditis briggsae: a platform for comparative genomics. PLoS Biol 2003;1(2): E45. Tobin D, Madsen D, Kahn-Kirby A, Peckol E, Moulder G, Barstead R, et al. Combinatorial expression of TRPV channel proteins defines their sensory functions and subcellular localization in C. elegans neurons. Neuron 2002;35(2):307–18. Troemel ER, Chou JH, Dwyer ND, Colbert HA, Bargmann CI. Divergent seven transmembrane receptors are candidate chemosensory receptors in C. elegans. Cell 1995;83(2):207–18. Troemel ER, Sagasti A, Bargmann CI. Lateral signaling mediated by axon contact and calcium entry regulates asymmetric odorant receptor expression in C. elegans. Cell 1999;99(4):387–98. Tsalik EL, Hobert O. Functional mapping of neurons that control locomotory behavior in Caenorhabditis elegans. J Neurobiol 2003;56(2):178–97. Vowels JJ, Thomas JH. Multiple chemosensory defects in daf-11 and daf-21 mutants of Caenorhabditis elegans. Genetics 1994;138(2):303–16. Wes PD, Bargmann CI. C. elegans odour discrimination requires asymmetric diversity in olfactory neurons. Nature 2001;410(6829):698–701. White JG, Southgate E, Thomson JN, Brenner S. The structure of the nervous system of the nematode Caenorhabditis elegans. Philos Trans R Soc Lond B Biol Sci 1986;314(1165): 1–340. Wood WB, The community of C. elegans researchers, editors. The nematode Caenorhabditis elegans. Cold Spnng Harbor, New York: Cold Spring Harbor Laboratory; 1988. Yau KW, Baylor DA. Cyclic GMP-activated conductance of retinal photoreceptor cells. Annu Rev Neurosci 1989;12:289–327. Yu S, Avery L, Baude E, Garbers DL. Guanylyl cyclase expression in specific sensory neurons: a new family of chemosensory receptors. Proc Natl Acad Sci U S A 1997;94(7):3384–7. Zagotta WN, Siegelbaum SA. Structure and function of cyclic nucleotide-gated channels. Annu Rev Neurosci 1996;19:235–63. Zhang Y, Chou JH, Bradley J, Bargmann CI, Zinn K. The Caenorhabditis elegans seven-transmembrane protein ODR-10 functions as an odorant receptor in mammalian cells. Proc Natl Acad Sci U S A 1997;94(22):12162–7. Zwaal RR, Mendel JE, Sternberg PW, Plasterk RH. Two neuronal G proteins are involved in chemosensation of the Caenorhabditis elegans Dauer-inducing pheromone. Genetics 1997;145(3):715–27.
Contents lists available at ScienceDirect
Biotechnology Advances journal homepage: www.elsevier.com/locate/biotechadv
Research review paper
The olfactory signal transduction for attractive odorants in Caenorhabditis elegans Chunmei Zhang, Jinyuan Yan, Yao Chen, Chunyan Chen, Keqin Zhang ⁎, Xiaowei Huang ⁎ Laboratory for Conservation and Utilization of Bio-Resources, Yunnan University, Kunming, Yunnan 650091, PR China Key Laboratory for Microbial Resources of the Ministry of Education, Yunnan University, Kunming, Yunnan 650091, PR China
a r t i c l e
i n f o
Article history: Received 25 July 2013 Received in revised form 14 October 2013 Accepted 28 October 2013 Available online 2 November 2013 Keywords: C. elegans Olfaction Chemotaxis Signal pathways
a b s t r a c t Olfaction in Caenorhabditis elegans is a versatile and sensitive strategy to seek food and avoid danger by sensing volatile chemicals emitted by the targets. The ability to sense attractive odor is mainly accomplished by the AWA and AWC neurons. Previous studies have shown the components of the olfaction signal pathway in these two amphid chemosensory neurons, but integration of the individual signaling components requires further elucidation. Here we review the progresses in our understanding of signal pathways for attractive olfaction involving AWA and AWC neurons, and discuss how the different signal molecules might employ the common molecular cascades to transduce the olfactory system and guide behavior in each neuron. © 2014 Elsevier Inc. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . Signal transduction in olfactory neurons . . . . . . . . . . . . . 2.1. G protein-coupled olfactory receptors (GPCRs) . . . . . . 2.2. Gα subunits in olfactory signal cascade . . . . . . . . . . 2.3. Ion channel in the olfaction . . . . . . . . . . . . . . . 2.1.3.1. cGMP signal and CNG channel in AWC neurons . . 2.1.3.2. Receptor guanylate cyclases in G protein signaling 2.1.3.3. The TRPV channels OSM-9/OCR-2 . . . . . . . . 3. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . .
1. Introduction The free-living nematode Caenorhabditis elegans has not only been a good model to investigate the innate immune system, signal transduction, and development, but it has also attracted much attention for studying the nervous system and the related behaviors for its accessibility to genetics and the detailed knowledge of its nervous system. C. elegans has a simple nervous system consisting of 302 neurons (Bargmann and Mori, 1997; White et al., 1986). However, it exhibits a rich repertoire of behavioral responses to a variety of environmental changes, including mechanosensation (Chalfie and White, 1988), ⁎ Corresponding authors. Tel.: +86 871 5034878; fax: +86 871 5034838. E-mail addresses: [email protected] (K. Zhang), [email protected] (X. Huang). 0734-9750/$ – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.biotechadv.2013.10.010
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
. . . . . . . . . . .
290 291 291 292 292 292 293 293 294 294 294
olfactory sensation (Bargmann et al., 1993; Wood and the community of C. elegans researchers, 1988), salt chemotaxis (Bargmann and Horvitz, 1991; Iino and Yoshida, 2009), thermotaxis (Mori and Ohshima, 1995), and navigation (Gray et al., 2005; Tsalik and Hobert, 2003). Specifically, C. elegans naturally inhabits the soil, a complex environment that contains both food bacteria and dangerous pathogens (Brenner, 1974). Therefore, having a sophisticated chemosensory system would allow C. elegans to distinguish their potential food sources from pathogens. Indeed, previous studies have shown that C. elegans can discriminate the pathogens from their food bacteria (Bargmann, 2006; Beale et al., 2006). This ability mainly depends on the functions of its 12 pairs of amphid chemosensory neurons (Mori and Ohshima, 1997; Starich et al., 1995). According to the detailed cellular analysis
C. Zhang et al. / Biotechnology Advances 32 (2014) 290–295
of amphid chemosensory neurons, the 12 pairs can detect a wide variety of chemicals in the environment, including both volatile and watersoluble compounds (Bargmann, 1993; Bargmann and Horvitz, 1991; Mori and Ohshima,1997). In the chemosensory system, olfaction is one of the most sensitive and effective strategies because it specialize in detecting volatile chemicals. Five pairs of neurons, AWA, AWB, AWC, ASH and ADL, are involved in olfaction in C. elegans. Two of them, AWA and AWC, respond to attractive odorants. For example, the AWA neurons can respond to at least three attractive odors: diacetyl, pyrazine and 2, 4, 5-trimethylthiazolez (Bargmann, 2006). AWCs recognize at least five attractive odors: benzaldehyde, butanone, isoamyl alcohol, 2, 3-pentanedione and 2, 4, 5-trimethyl thiazole (Mori, 1999; Lans et al., 2004). The other three pairs of neurons, AWB, ASH and ADL, are responsible for the behavior of active avoidance from undesirable volatile chemicals and even microbial pathogens. However among these three neuron pairs, the AWB neurons' only function is to sense volatile repellents (Bargmann et al., 1990; Wood and the community of C. elegans researchers, 1988). In contrast, detecting repulsive volatile chemicals is only a minor function for ASH and ADL neurons. The ASH neurons mediate rapid withdrawal that is different from chemotaxis behaviors in gradients. This type of neurons mainly responds to high osmolarity (Culotti and Russell, 1978), heavy metals (Dusenbery, 1974), detergents (Hilliard et al., 2002), bitter alkaloids (Hilliard et al., 2004), acid pH (Dusenbery, 1974), and light touch to the tip of the nose (Colbert et al., 1997; Kaplan and Horvitz, 1993). On the other hand, the ADL neurons mainly deal with responses to social feeding (Chang et al., 2006; de Bono et al., 2002). Therefore, the typical olfactory neurons generally include three types, AWA, AWB and AWC. The worms chase food or escape dangers through discriminating attractants from repellants by the three pairs of olfactory neurons. Though all the 12 pairs of amphid neurons have cilium for sensing chemicals and mechanical cues, the olfactory neurons also have subtle differences in their structure profiles compared to the other sensory neurons (Bargmann and Horvitz, 1991). Most of the amphid neurons detecting water soluble chemicals are directly exposed to the environment and have no complex branched cilia (Mori and Ohshima, 1997; Schackwitz et al., 1996). However, three types of the olfactory neurons AWA, AWC, and AWB have complex and flattened cilia (Bargmann et al., 1993; Mori and Ohshima, 1997). Furthermore, their cilia are surrounded by a membrane formed by a sheath cell, and therefore the neurons' endings are not exposed to the external environment (Roayaie et al., 1998). Since the olfactory responses for the attractants are associated with the most powerful behaviors to pursue food, this mini-review will focus on the attractant signal transduction pathway in AWA and AWC. 2. Signal transduction in olfactory neurons As described above, C. elegans mainly employs the four cells (left and right AWA or AWC cells) to sense and discriminate a variety of volatile attractant. Part of the odorant specificity is due to the odorant differences detected by AWA and AWC, or the functional asymmetry between the left and the right AWC (Bargmann et al., 1993; Wes and Bargmann, 2001). However, these two factors above still cannot account for all the odorant specificity, suggesting that additional intracellular mechanisms exist to establish odorant specificity. Intracellular components participating in olfaction have been identified through the genetic analysis of mutations that affects olfactory sensing of specific chemicals. Generally, these components include olfactory receptors (especially G protein-couple receptors, GPCRs), G protein and cation channels, which are shown essential in the signal transduction of olfaction (Bargmann and Mori, 1997; Mori, 1999; Reed, 1990). Once the volatile odorants in the environment are recognized by specialized receptors located on the cilia, the olfactory signal cascades will be activated and consequently lead to behavioral responses of worms.
291
2.1. G protein-coupled olfactory receptors (GPCRs) C. elegans has only two pairs of olfactory neurons AWA and AWC for the attractants. However, each neuron expresses multiple receptors to recognize odorants and thus detect several different chemicals, similar to those in mammalian (Troemel et al., 1995). The differences of both mammalian and C.elegans include: (Alcedo and Kenyon, 2004) the olfactory neurons express only one functional receptor gene for each neuron in mammals, but in C. elegans one neuron expresses several functional receptor genes at the same time (Axel, 2005; Buck and Axel, 1991); (Altun-Gultekin et al., 2001) Though the olfactory systems in these two groups of organisms are potentially able to sense a comparable diversity of odors, the smaller number of neurons in C. elegans limits its discriminatory power (Bargmann, 2006). GPCRs have been reported as the most common olfactory receptors in either C.elegans or mammalian. The genome of C. elegans encodes more than 500 putative GPCRs, among which are some classical, well-conserved GPCRs that recognize serotonin, acetylcholine, glutamate, and neuropeptides, and these canonical receptors are involved in regular neurotransmission (Bargmann, 1998, 2006; Krieger and Breer 1999). However, the GPCRs that only express or express primarily in single left–right pair of chemosensory neurons are highly divergent between C. elegans and other species (Chen et al., 2005; Colosimo et al., 2004; McCarroll et al., 2005; Troemel et al., 1995). Furthermore, these GPCRs that are specifically expressed in one or more amphid neurons at variable expression levels usually localize to the cilia (Bargmann, 2006, 1998; Clyne et al., 1999; Ngai et al., 1993). It is reasonable to hypothesize that multiple GPCRs are expressed by each neuron, enabling C. elegans to respond to multiple, sometimes unrelated chemicals, using only one or few cells. The AWA neurons have been reported to express two GPCRs, ODR-10 and SRA-13. Gene odr-10 encodes a seven-transmembrane domain protein that is distantly related to the G protein-coupled receptor superfamily. It only expresses in the AWA neurons, and the ODR-10GPF fused protein localizes to the distal cilia of the dendrites where odorant detection is thought to occur (Sengupta et al., 1996; Zhang et al., 1997). ODR-10 is also the only chemosensory receptor whose ligand is known in C. elegans. Forward genetic analysis for olfactory mutation has suggested that ODR-10 is the high affinity GPCR for the odorant diacetyl (Sengupta et al., 1996). Worms with the odr-10 mutation have a 100-fold reduced sensitivity to the volatile attractant diacetyl, and failed to sense low concentrations of diacetyl. However, the odr-10 mutant animal had a normal response to other odorants detected by AWA neuron, and the chemotaxis to the odorants detected by AWC was not affected, including the odor chemicals with similar chemical structures (Sengupta et al., 1996; Zhang et al., 1997). The gene sra-13 also encodes a GPCR, a member of the SRA family of chemosensory receptors. SRA family was classified as a group of the chemoreceptor gene family based on similar sequence (Troemel et al., 1995), and named starting with sr for serpentine receptor (Troemel et al., 1995; Robertson and Thomas, 2006). The gene family was annotated as part of the comparison with Caenorhabditis briggsae (Robertson and Thomas, 2006; Stein et al., 2003). Gene sra-13 was expressed in AWA and AWC neurons, as well as non-neuron cells such as muscle and hypodermal cells. It is localized to the cell body, axons, and dendrites in AWA and AWC neurons. The SRA-13 protein affects vulval development as well as olfactory plasticity in AWA and AWC chemosensory neurons by modulating the MAPK signal transduction pathway, but its ligand has not been identified (Battu et al., 2003; Hirotsu et al., 2000). Another GPCR STR-2 that expresses at the top of cilia in AWC neurons is related to longevity of C. elegans (Alcedo and Kenyon, 2004; Bargmann, 2006). This candidate odorant receptor is asymmetrically expressed in one of the two AWC neurons and weakly expressed in the ASI neurons (Pecko et al., 2001; Troemel et al., 1999). During embryonic development, its expression is controlled by Ca2+, MAPK and axon
292
C. Zhang et al. / Biotechnology Advances 32 (2014) 290–295
guidance genes. During later stages, sensory signaling genes such as the guanylyl cyclases odr-1, daf-11 as well as the cGMP gated cation channel subunits tax-2 and tax-4 are necessary to maintain str-2 expression (Coburn et al., 1998; Peckol et al., 1999). Though all these genes seem to encompass an olfactory signaling network, the exact function of str2 still remains unknown in sensing attractive odorant. The gene sra-11 encodes an orphan GPCR that belongs to a large family of putative chemoreceptor (Troemel et al., 1995). Expression of this gene can be observed through all larval and adult stages (AltunGultekin et al., 2001). Unlike other chemoreceptor family members, which are expressed in sensory neurons, sra-11 is exclusively expressed in three interneuron classes, AIY, AIA, and AVB (Tsalik and Hobert, 2003). However, both the odorant attraction assays and the odorantinduced egg-laying responses have illustrated that the SRA-11 protein is specifically required for olfactory imprinting (Remy and Hobert, 2005). 2.2. Gα subunits in olfactory signal cascade Heterotrimeric G protein-coupled signaling cascades are used to transduce diverse signals, varying from intercellular mediators to environmental stimuli, including the olfaction in C. elegans (Fire et al., 1998; Hamm 1998; Mori, 1999; Mori and Ohshima 1995; Reed, 1990; Yau and Baylor, 1989). The genome of C. elegans encodes the subunits such as 21 Gα, 2 Gβ and 3 Gγ (Cuppen et al., 2003; Roayaie et al., 1998; Zwaal et al., 1997). According to the analysis of sequence similarity, four Gα subunits share high homologies to mammals, and they are GSA-1, GOA-1, EGL-30 and GPA-12, respectively. Three of these four conserved G protein α subunits are expressed broadly except for the GPA-12. Both gene GSA-1 and GOA-1 are expressed in all the amphid neurons, but which amphid neurons express EGL-30 is still unknown. Additionally, several of the C. elegans specific subunits (GPAs) are involved either positively or negatively in chemosensation, including odr-3, gpa-3, gpa-5 and gpa-6 in AWA neuron, as well as odr-3, gpa-2, gpa-3, and gpa-13 in AWC neurons (Jansen et al., 1999; Roayaie et al., 1998; Troemel et al., 1995; Zwaal et al., 1997). Odr-3, one of the Gi-like genes, strongly expresses in cilia of AWA and AWC neurons and functions in olfactory response in both type of neurons (Bargmann, 2006; Troemel et al., 1995; Zwaal et al., 1997). Screens of olfactory mutants demonstrated that odr-3 mutation generated highly defective olfaction responses mediated by AWA and AWC neurons (Troemel et al., 1995). However, there were no observable effects in osmotic avoidance sensed by AWA and AWC in these mutants. Thus, the results suggest that ODR-3 is a strong candidate for the G protein that acts downstream of the olfactory receptor (Bargmann, 2006). Meanwhile, the odr-3 gene is also involved in cilium morphogenesis of olfactory neurons. When AWA and AWC cilia with abnormal odr-3 expression were closely examined by light microscopy, it was found that the absence of odr-3 resulted in branched filament-like cilia reminiscent of AWA cilia, rather than the normal large membranous, fan-like AWC cilia. The amount of ODR-3 can thus determine the extent of cilium outgrowth in AWA and AWC (Roayaie et al., 1998). Though ODR-3 is the main Gα protein to activate the olfactory signal transduction in both AWA and AWC neurons, some other Gα proteins can perform the overlapped or complementary roles. For example, the gpa-3 gene is also expressed in cilia, cell bodies and axons of AWA and AWC neurons. In olfaction signals, GPA-3 functions redundantly to ODR-3. For many odorant chemicals detected by AWA and AWC neurons, GPA-3 is sufficient except for butanone. Contrarily, loss of gpa-3 activity enhances the chemotaxis defect of odr-3(lf) mutants to all odorants, indicating that GPA-3 is redundant to ODR-3, and that it plays a positive role in odorant detection (Lans et al., 2004; Troemel et al., 1995; Zwaal et al., 1997). Mutations at gpa-13 cause mild defects for responses to 2, 3-pentanedione, and this phenotype is weakly enhanced by mutations of odr-3, suggesting that gpa-13 plays a minor stimulatory role in olfaction (Mori and Ohshima, 1995). Though gpa-6 has been shown to
be expressed in olfactory neurons, a gpa-6(lf) mutant showed a similar activity as wild type strains for chemotaxis to other soluble and volatile attractants and repellents. This mutant showed only a slightly increased preference for NaCl when placed between NaCl and NaAc (Jansen et al., 1999). Besides the stimulatory signals that the Gα proteins recognize, several Gα subunits also negatively regulate the olfactory signal network. For example, the gpa-5 gene is expressed in cilia, cell bodies and axons of AWA neurons and, is supposed to execute an inhibitory effect on AWA function, probably via GPCR SRA-13(Battu et al., 2003; Troemel et al., 1995). Though gpa-5(lf) mutants responded similarly to soluble attractants and repellents as wild type worms, the gpa-5(lf) mutants rescued the chemotaxis defect of odr-3 mutants. These mutants showed an increased sensitivity to volatile attractants sensed by AWA, and to one attractant, isoamyl alcohol, sensed by AWC. Therefore, these data suggest that gpa-5 is a negative regulator of chemosensation in worms (Battu et al., 2003; Jansen et al., 1999; Lans et al., 2004). The other Gα that performs the similarly negative function in olfaction is GPA-2. But the cell type is different because gpa-2 expresses in cilia, cell bodies and axons of AWC neurons, and thus it plays an inhibitory role in AWC for odorants (Lans et al., 2004; Troemel et al., 1995; Zwaal et al., 1997). For example, the experimental evidences from Lans revealed the improved odorant responses in gpa-2(lf)::odr-3(lf) double mutants for all odorants except isoamyl alcohol and butanone, indicating a negative regulatory role for GPA-2 in detecting most odorants (Lans et al., 2004). However, it was also reported that gpa-2(lf) mutations enhanced the mild defect in chemotaxis to butanone observed in odr-3(lf), suggesting that GPA-2 and ODR-3 might act redundantly in response to this odorant (Troemel et al., 1995). In conclusion, AWA, which mediates the response to volatile odorants including diacetyl, pyrazine and 2, 4, 5-trimethyl- thiazole, expresses four Gα genes odr-3, gpa-3, gpa-5, and gpa-6. Among them, ODR-3 and GPA-3 provide the main stimulatory signal and a redundantly stimulatory signal, respectively. However, GPA-5 is a negative regulator in AWA. To the neurons of AWC, which mediate the response to volatile attractants including benzaldehyde, butanone, and isoamyl alcohol, the expressed Gα genes include odr-3, gpa-2, gpa-3, and gpa-13. Among them, GPA-13 provides a mild stimulatory effect besides the similarly positive function performed by ODR-3 and GPA-3 as they do in AWA. GPA-2 plays an inhibitory role in AWC. Although expression of GPA-5 has not been detected in AWC, gpa-5(lf) mutants are hypersensitive to isoamyl alcohol and suppress the reduced response to this odorant caused by overexpression of SRA-13, a GPCR expressed in both AWA and AWC (Battu et al., 2003). 2.3. Ion channel in the olfaction When recognizing external stimuli, sensory neurons generate electrical signals that in turn lead to the regulation of neurotransmitter releases (Mori and Ohshima, 1997). In the olfactory signal cascade of C.elegans, ion channel is the key component in the last step to sense and transduce receptor activity into electrical activity in olfactory cells (Zagotta and Siegelbaum, 1996). Genetic evidences suggest that the AWC-sensed odorants elicit chemotaxis through a cyclic nucleotidegated channel (CNG) consisting of TAX-4 and TAX-2 subunits (Coburn and Bargmann, 1996; Komatsu et al., 1996). However, a transient receptor potential vanilloid (TRPV) channel is required for chemotaxis to odors detected by AWA (Colbert and Bargmann, 1997; Hilliard et al., 2002, 2005). 2.1.3.1. cGMP signal and CNG channel in AWC neurons Stimulation of a G protein-coupled receptor and its downstream G protein initiates a signaling cascade that leads to an increase in the intracellular concentration of a cyclic nucleotide in AWC. This alters the opening of a CNG channel, changes the membrane potential of the cell, and resultantly produces the signaling processes that allow us to
C. Zhang et al. / Biotechnology Advances 32 (2014) 290–295
smell (Nakamura and Gold, 1987). CNG channels are nonselective cation channels that belong to a family that includes voltage-gated channels (Jan and Jan, 1990; Komatsu et al., 1996). A CNG channel encoded by tax-4 and tax-2 genes is essential for the function of many sensory neurons in C. elegans (Coburn and Bargmann, 1996; Komatsu et al., 1996; Zagotta and Siegelbaum, 1996). tax-4 encodes an alpha subunit that forms a channel on its own, while tax-2 encodes a beta subunit that enhances tax-4 activity. Both the tax-4 and tax-2 are expressed in the sensory cilia of AWC (Coburn and Bargmann, 1996; Komatsu et al., 1996), and coordinately form a cGMP-gated channel, a type of CNG channels. In other words, heterologous expression of TAX-4 and TAX-2 generates a channel that is highly sensitive to cGMP, like the mammalian phototransduction channel (Coburn and Bargmann, 1996; Komatsu et al., 1999). Thus, a plausible pathway for olfaction transduction in the AWC neurons is that G protein signaling regulates the function of either a cGMP phosphodiesterase (as it occurs in mammalian vision) or a guanylate cyclase, thereby closing or opening the cGMP-gated channel. Worms with dysfunctional tax-4 or tax-2 are defective in AWC chemotaxis to volatile odors, and also in sensory axon structure (Bargmann, 2006). 2.1.3.2. Receptor guanylate cyclases in G protein signaling The CNG channel encoded by tax-4 and tax-2 is controlled by the intracellular messenger cGMP. By analogy with vertebrate phototransduction, the intervening steps between the Gα proteins and the cGMP-gated channel presumably involve a guanylate cyclase or a cGMP phosphodiesterase in AWC. However, such a cGMP phosphodiesterase affecting the olfactory has not been identified till now. The C. elegans genome encodes 34 guanulate cyclases that could be responsible for cGMP production in TAX4/TAX-2 neurons. Among these 34, 27 are transmembrane proteins, or receptor-like guanylate cyclases (RGCs), and 7 are cytosolic soluble guanylate cyclases (sGCs) (Ortiz et al., 2006). The daf-11 gene encodes a receptor guanylate cyclase in AWC olfactory neurons but its function is non-autonomous (Birnby et al., 2000; Fulle et al., 1995; Vowels and Thomas, 1994; Yu et al., 1997). odr-1, a second receptor guanylate cyclase, is expressed in the same cells that express daf-11. Because ODR-1 and DAF-11 each lack the key residues required for catalysis, these two proteins likely function as heterodimers that provide cGMP for the opening
293
of the TAX-4/TAX-2 channel (Morton, 2004). It has also been shown that odr-1 olfactory defects can be rescued by mutated ODR-1 proteins lacking an extracellular domain, suggesting that the cyclase domains are probably the primary mediators of ODR-1 function in AWC (L'Etoile and Bargmann, 2000). These results suggest that the olfaction system in AWC appears to use guanylate cyclase DAF-11 and ODR-1 as ligandindependent RGCs downstream of G protein signaling (Fig. 1).
2.1.3.3. The TRPV channels OSM-9/OCR-2 Similar to that in AWC neurons, the amphid sensory neurons of AWA also require a channel for odorant chemotaxis, and this channel is encoded by the osm-9 and ocr-2 genes. OSM-9 and OCR-2 proteins are localized to the AWA cilia and are mutually required for each others' cilia localization. That is to say, OCR-2 is not localized to the cilia in osm-9 mutants, and vice versa. Therefore these two channel proteins are suggested to assemble into a single channel complex (Tobin et al., 2002). The channel with OSM-9 and OCR-2 subunits belongs to the transient receptor potential (TRP) channel superfamily, a large group of channels that functions in insect phototransduction, in vertebrate pain sensation, non-neuronal pressure sensation and osmosensation. There are seven families of TRP channels, and OSM-9 and OCR-2 belong to the transient receptor potential vanilloid (TRPV) family (Ramsey et al., 2006). The OSM-9/OCR-2 channel functions the downstream of GPCRs and ODR-3 and participates in the primary signal transduction of olfaction in AWA neurons, which is similar to the TAX-2/TAX-4 channel in AWC neurons (Bargmann, 2006). Mutants of osm-9 or ocr-2 have mild to severe defects in AWA chemotaxis to volatile odors (Colbert et al., 1997; Hilliard et al., 2002, 2005; Tobin et al., 2002). In summary, the olfactory pathway in AWA neurons also begins with the binding of an odorant to a GPCR in the AWA cell membrane, which leads to the activation of the Gα subunits (eg. ODR-3 or/and GPA-3) as well as an unknown PLC, leading to the production of important intracellular second messengers diacyl glycerol (DAG) and inositol 1, 4, 5-trisphosphate (IP3). DAG can then activate the nPKCs TTX-4 and TPA-1 and produce PUFAs, which subsequently regulate the opening of the OSM-9/OCR-2 TRP channels. Meanwhile, the binding between IP3 and its receptor IP3R/ITR-1 leads to a Ca2+ influx (Fig. 2) (Battu et al., 2003).
Fig. 1. Summary of the potential olfactory signal transduction pathway in the AWC neuron. Genetic analysis supports that GPCRs initiate the olfaction system. The activated GPCRs transduce the signals to the G proteins and (ODR-3/GPA-3/GPA-13), and they in turn regulate receptor guanylate cyclase (ODR-1/DAF-11) to produce cGMP. The intracellular signal molecule cGMP finally opens the cyclic nucleotide-gated channel composed of the subunits TAX-2/TAX-4. Meantime, the genes odr-1, daf-11, tax-2 and tax-4 are necessary for maintaining the expressions of GPCRs in adult animals. This figure is modified from that of Bargmann (2006).
294
C. Zhang et al. / Biotechnology Advances 32 (2014) 290–295
Fig. 2. Summary of the potential olfactory signal transduction pathway in the AWA neuron. In AWA neurons, the attractive odors are detected by GPCRs, which activate the Gα subunits (ODR-3/GPA-3) in heterotrimeric G proteins. The G protein then regulates the production of DAG via the PLC pathway, and produces the intracellular signal molecules of IP3 and DAG. Between these two molecules, DAG activates the nPKCs and produces PUFAs; IP3 binds to its receptor IP3R/ITR-1. The results from the two intracellular signal molecule finally lead to the opening of OSM-9/OCR-2 channel and Ca2+ influx.
3. Conclusions
References
C. elegans has a simple but elegant olfactory system to detect volatile chemicals, and the neurons AWA and AWC are required for detecting attractive odors. Previous studies have revealed that two distinct pathways of olfactory signal transduction are employed in these two type of neurons based on the analyses of gene mutants of C. elegans. In spite of the different molecules involved, a common pathway for olfaction transduction in either AWA or AWC consists of recognition by the specialized GPCRs, activation of the G protein, and transduction of the odorant signal to channel proteins. Different GPCRs located on the same set of neurons are involved in sensing different attractive odorants, with many using the same signal transduction pathways. However, other yet to be discovered independent signal transduction pathways might also be involved in olfactory responses.
Alcedo J, Kenyon C. Regulation of C. elegans longevity by specific gustatory and olfactory neurons. Neuron 2004;41(1):45–55. Altun-Gultekin Z, Andachi Y, Tsalik EL, Pilgrim D, Kohara Y, Hobert O. A regulatory cascade of three homeobox genes, ceh-10, ttx-3 and ceh-23, controls cell fate specification of a defined interneuron class in C. elegans. Development 2001;128(11):1951–69. Axel R. Scents and sensibility: a molecular logic of olfactory perception (Nobel lecture). Angew Chem Int Ed Engl 2005;44(38):6110–27. Bargmann CI. Genetic and cellular analysis of behavior in C. elegans. Annu Rev Neurosci 1993;16:47–71. Bargmann CI. Neurobiology of the Caenorhabditis elegans genome. Science 1998;282(5396): 2028–33. Bargmann CI. In: The C. elegans Research Community, editor. Wormbook, 1. WormBook; 2006. http://dx.doi.org/10.1895/wormbook.1.123. [http://www. wormbook.orga]. Bargmann CI, Horvitz HR. Chemosensory neurons with overlapping functions direct chemotaxis to multiple chemicals in C. elegans. Neuron 1991;7(5):729–42. Bargmann CI, Mori I. Chemotaxis and thermotaxis. In: Riddle DL, Blumenthal T, Meyer BJ, Priess JR, editors. C. elegans II. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997. Bargmann CI, Hartwieg E, Horvitz HR. Odorant-selective genes and neurons mediate olfaction in C. elegans. Cell 1993;74(3):515–27. Battu G, Hoier EF, Hajnal A. The C. elegans G-protein-coupled receptor SRA-13 inhibits RAS/MAPK signalling during olfaction and vulval development. Development 2003;130(12):2567–77. Beale E, Li G, Tan MW, Rumbaugh KP. Caenorhabditis elegans senses bacterial autoinducers. Appl Environ Microbiol 2006;72(7):5135–7. Birnby DA, Link EM, Vowels JJ, Tian H, Colacurcio PL, Thomas JH. A transmembrane guanylyl cyclase (DAF-11) and Hsp90 (DAF-21) regulate a common set of chemosensory behaviors in Ccaenorhabditis elegans. Genetics 2000;155(1):85–104. Brenner S. The genetics of Caenorhabditis elegans. Genetics 1974;77(1):71–94.
Acknowledgments We sincerely thank Prof. Jianping Xu, from the McMaster University in Canada, for his suggestive criticism for this manuscript. This work is supported by the National Basic Research Program of China (grant no. 2011AA10A203 and no. 2013CB127500), the National Natural Science Foundation Program of China (grant no. 31370162 and no. U1036602), and the Department of Science and Technology of Yunnan Province, China (grant no. 2010GA012 as well as the grant 2010YN17).
C. Zhang et al. / Biotechnology Advances 32 (2014) 290–295 Buck L, Axel R. A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 1991;65(1):175–87. Chalfie M, White J. The nervous system. In: Wood WB, editor. The nematode Caenorhabditis elegans. Cold Spring Harbor, New York: Cold Spring Harbor Laboratory; 1988. p. 337–91. Chang AJ, Chronis N, Karow DS, Marletta MA, Bargmann CI. A distributed chemosensory circuit for oxygen preference in C. elegans. PLoS Biol 2006;4(9):e274. Chen N, Pai S, Zhao Z, Mah A, Newbury R, Johnsen RC, et al. Identification of a nematode chemosensory gene family. Proc Natl Acad Sci U S A 2005;102(1):146–51. Clyne PJ, Warr CG, Freeman MR, Lessing D, Kim J, Carlson JR. A novel family of divergent seven-transmembrane proteins: candidate odorant receptors in Drosophila. Neuron 1999;22(2):327–38. Coburn CM, Bargmann CI. A putative cyclic nucleotide-gated channel is required for sensory development and function in C. elegans. Neuron 1996;17(4):695–706. Coburn CM, Mori I, Ohshima Y, Bargmann CI. A cyclic nucleotide-gated channel inhibits sensory axon outgrowth in larval and adult Caenorhabditis elegans: a distinct pathway for maintenance of sensory axon structure. Development 1998;125(2):249–58. Colbert HA, Bargmann CI. Environmental signals modulate olfactory acuity, discrimination, and memory in Caenorhabditis elegans. Learn Mem 1997;4(2):179–91. Colbert HA, Smith TL, Bargmann CI. OSM-9, a novel protein with structural similarity to channels, is required for olfaction, mechanosensation, and olfactory adaptation in Caenorhabditis elegans. J Neurosci 1997;17(21):8259–69. Colosimo ME, Brown A, Mukhopadhyay S, Gabel C, Lanjuin AE, Samuel AD, et al. Identification of thermosensory and olfactory neuron-specific genes via expression profiling of single neuron types. Curr Biol 2004;14(24):2245–51. Culotti JG, Russell RL. Osmotic avoidance defective mutants of the nematode Caenorhabditis elegans. Genetics 1978;90(2):243–56. Cuppen E, van der Linden AM, Jansen G, Plasterk RH. Proteins interacting with Caenorhabditis elegans Galpha subunits. Comp Funct Genomics 2003;4(5):479–91. de Bono M, Tobin DM, Davis MW, Avery L, Bargmann CI. Social feeding in Caenorhabditis elegans is induced by neurons that detect aversive stimuli. Nature 2002;419(6910): 899–903. Dusenbery DB. Analysis of chemotaxis in the nematode Caenorhabditis elegans by countercurrent separation. J Exp Zool 1974;188(1):41–7. Fire A, Xu S, Montgomery MK, Kostas SA, Driver SE, Mello CC. Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans. Nature 1998;391(6669): 806–11. Fulle HJ, Vassar R, Foster DC, Yang RB, Axel R, Garbers DL. A receptor guanylyl cyclase expressed specifically in olfactory sensory neurons. Proc Natl Acad Sci U S A 1995;92(8):3571–5. Gray JM, Hill JJ, Bargmann CI. A circuit for navigation in Caenorhabditis elegans. Proc Natl Acad Sci U S A 2005;102(9):3184–91. Hamm HE. The many faces of G protein signaling. J Biol Chem 1998;273(2):669–72. Hilliard MA, Bargmann CI, Bazzicalupo P. C. elegans responds to chemical repellents by integrating sensory inputs from the head and the tail. Curr Biol 2002;12(9):730–4. Hilliard MA, Bergamasco C, Arbucci S, Plasterk RH, Bazzicalupo P. Worms taste bitter: ASH neurons, QUI-1, GPA-3 and ODR-3 mediate quinine avoidance in Caenorhabditis elegans. EMBO J 2004;23(5):1101–11. Hilliard MA, Apicella AJ, Kerr R, Suzuki H, Bazzicalupo P, Schafer WR. In vivo imaging of C. elegans ASH neurons: cellular response and adaptation to chemical repellents. EMBO J 2005;24(1):63–72. Hirotsu T, Saeki S, Yamamoto M, Iino Y. The Ras-MAPK pathway is important for olfaction in Caenorhabditis elegans. Nature 2000;404(6775):289–93. Iino Y, Yoshida K. Parallel use of two behavioral mechanisms for chemotaxis in Caenorhabditis elegans. J Neurosci 2009;29(17):5370–80. Jan LY, Jan YN. A superfamily of ion channels. Nature 1990;345(6277):672. Jansen G, Thijssen KL, Werner P, van der Horst M, Hazendonk E, Plasterk RH. The complete family of genes encoding G proteins of Caenorhabditis elegans. Nat Genet 1999;21(4):414–9. Kaplan JM, Horvitz HR. A dual mechanosensory and chemosensory neuron in Caenorhabditis elegans. Proc Natl Acad Sci U S A 1993;90(6):2227–31. Komatsu H, Mori I, Rhee JS, Akaike N, Ohshima Y. Mutations in a cyclic nucleotide-gated channel lead to abnormal thermosensation and chemosensation in C. elegans. Neuron 1996;17(4):707–18. Komatsu H, Jin YH, L'Etoile N, Mori I, Bargmann CI, Akaike N, et al. Functional reconstitution of a heteromeric cyclic nucleotide-gated channel of Caenorhabditis elegans in cultured cells. Brain Res 1999;821(1):160–8. Krieger J, Breer H. Olfactory reception in invertebrates. Science 1999;286(5440):720–3. Lans H, Rademakers S, Jansen G. A network of stimulatory and inhibitory Galpha-subunits regulates olfaction in Caenorhabditis elegans. Genetics 2004;167(4):1677–87. L'Etoile ND, Bargmann CI. Olfaction and odor discrimination are mediated by the C. elegans guanylyl cyclase ODR-1. Neuron 2000;25(3):575–86. McCarroll SA, Li H, Bargmann CI. Identification of transcriptional regulatory elements in chemosensory receptor genes by probabilistic segmentation. Curr Biol 2005;15(4): 347–52.
295
Mori I. Genetics of chemotaxis and thermotaxis in the nematode Caenorhabditis elegans. Annu Rev Genet 1999;33:399–422. Mori I, Ohshima Y. Neural regulation of thermotaxis in Caenorhabditis elegans. Nature 1995;376(6538):344–8. Mori I, Ohshima Y. Molecular neurogenetics of chemotaxis and thermotaxis in the nematode Caenorhabditis elegans. Bioessays 1997;19(12):1055–64. Morton DB. Invertebrates yield a plethora of atypical guanylyl cyclases. Mol Neurobiol 2004;29(2):97–116. Nakamura T, Gold GH. A cyclic nucleotide-gated conductance in olfactory receptor cilia. Nature 1987;325(6103):442–4. Ngai J, Dowling MM, Buck L, Axel R, Chess A. The family of genes encoding odorant receptors in the channel catfish. Cell 1993;72(5):657–66. Ortiz CO, Etchberger JF, Posy SL, Frøkjaer-Jensen C, Lockery S, Honig B, et al. Searching for neuronal left/right asymmetry: genomewide analysis of nematode receptor-type guanylyl cyclases. Genetics 2006;173(1):131–49. Peckol EL, Zallen JA, Yarrow JC, Bargmann CI. Sensory activity affects sensory axon development in C. elegans. Development 1999;126(9):1891–902. Peckol EL, Troemel ER, Bargmann CI. Sensory experience and sensory activity regulate chemosensory receptor gene expression in Caenorhabditis elegans. Proc Natl Acad Sci U S A 2001;98(20):11032–8. Ramsey IS, Delling M, Clapham DE. An introduction to TRP channels. Annu Rev Physiol 2006;68:619–47. Reed RR. G protein diversity and the regulation of signaling pathways. New Biol 1990;2(11):957–60. Remy JJ, Hobert O. An interneuronal chemoreceptor required for olfactory imprinting in C. elegans. Science 2005;309(5735):787–90. Roayaie K, Crump JG, Sagasti A, Bargmann CI. The G alpha protein ODR-3 mediates olfactory and nociceptive function and controls cilium morphogenesis in C. elegans olfactory neurons. Neuron 1998;20(1):55–67. Robertson HM, Thomas JH. The putative chemoreceptor families of C.elegans. WormBook; 20061–12. Schackwitz WS, Inoue T, Thomas JH. Chemosensory neurons function in parallel to mediate a pheromone response in C. elegans. Neuron 1996;17(4):719–28. Sengupta P, Chou JH, Bargmann CI. odr-10 encodes a seven transmembrane domain olfactory receptor required for responses to the odorant diacetyl. Cell 1996;84(6): 899–909. Starich TA, Herman RK, Kari CK, Yeh WH, Schackwitz WS, Schuyler MW, et al. Mutations affecting the chemosensory neurons of Caenorhabditis elegans. Genetics 1995;139(1): 171–88. Stein LD, Bao Z, Blasiar D, Blumenthal T, Brent MR, Chen N, et al. The genome sequence of Caenorhabditis briggsae: a platform for comparative genomics. PLoS Biol 2003;1(2): E45. Tobin D, Madsen D, Kahn-Kirby A, Peckol E, Moulder G, Barstead R, et al. Combinatorial expression of TRPV channel proteins defines their sensory functions and subcellular localization in C. elegans neurons. Neuron 2002;35(2):307–18. Troemel ER, Chou JH, Dwyer ND, Colbert HA, Bargmann CI. Divergent seven transmembrane receptors are candidate chemosensory receptors in C. elegans. Cell 1995;83(2):207–18. Troemel ER, Sagasti A, Bargmann CI. Lateral signaling mediated by axon contact and calcium entry regulates asymmetric odorant receptor expression in C. elegans. Cell 1999;99(4):387–98. Tsalik EL, Hobert O. Functional mapping of neurons that control locomotory behavior in Caenorhabditis elegans. J Neurobiol 2003;56(2):178–97. Vowels JJ, Thomas JH. Multiple chemosensory defects in daf-11 and daf-21 mutants of Caenorhabditis elegans. Genetics 1994;138(2):303–16. Wes PD, Bargmann CI. C. elegans odour discrimination requires asymmetric diversity in olfactory neurons. Nature 2001;410(6829):698–701. White JG, Southgate E, Thomson JN, Brenner S. The structure of the nervous system of the nematode Caenorhabditis elegans. Philos Trans R Soc Lond B Biol Sci 1986;314(1165): 1–340. Wood WB, The community of C. elegans researchers, editors. The nematode Caenorhabditis elegans. Cold Spnng Harbor, New York: Cold Spring Harbor Laboratory; 1988. Yau KW, Baylor DA. Cyclic GMP-activated conductance of retinal photoreceptor cells. Annu Rev Neurosci 1989;12:289–327. Yu S, Avery L, Baude E, Garbers DL. Guanylyl cyclase expression in specific sensory neurons: a new family of chemosensory receptors. Proc Natl Acad Sci U S A 1997;94(7):3384–7. Zagotta WN, Siegelbaum SA. Structure and function of cyclic nucleotide-gated channels. Annu Rev Neurosci 1996;19:235–63. Zhang Y, Chou JH, Bradley J, Bargmann CI, Zinn K. The Caenorhabditis elegans seven-transmembrane protein ODR-10 functions as an odorant receptor in mammalian cells. Proc Natl Acad Sci U S A 1997;94(22):12162–7. Zwaal RR, Mendel JE, Sternberg PW, Plasterk RH. Two neuronal G proteins are involved in chemosensation of the Caenorhabditis elegans Dauer-inducing pheromone. Genetics 1997;145(3):715–27.
