Single Ionic Channels of Two Caenorhabditis elegans Chemosensory Neurons in Native Membrane

The genome of Caenorhabditis elegans contains representatives of the channel families found in both vertebrate and invertebrate nervous systems. However, it lacks the ubiquitous Hodgkin-Huxley Na+ channel that is integral to long-distance signaling in other animals. Nematode neurons are presumed to communicate by electrotonic conduction and graded depolarizations. This fundamental difference in operating principle may require different channel populations to regulate transmission and transmitter release. We have sampled ionic channels from the somata of two chemosensory neurons (AWA and AWC) of C. elegans. A Ca2+-activated, outwardly rectifying channel has a conductance of 67 pS and a reversal potential indicating selectivity for K+. An inwardly rectifying channel is active at potentials more negative than -50 mV. The inward channel is notably flickery even in the absence of divalent cations; this prevented determination of its conductance and reversal potential. Both of these channels were inactive over a range of membrane potentials near the likely cell resting potential; this would account for the region of very high membrane resistance observed in whole-cell recordings. A very-large-conductance (> 100 pS), inwardly rectifying channel may account for channel-like fluctuations seen in whole-cell recordings.     

Chemosensory signal transduction in Caenorhabditis elegans

Chemosensory neurons translate perception of external chemical cues, including odorants, tastants, and pheromones, into information that drives attraction or avoidance motor programs. In the laboratory, robust behavioral assays, coupled with powerful genetic, molecular and optical tools, have made Caenorhabditis elegans an ideal experimental system in which to dissect the contributions of individual genes and neurons to ethologically relevant chemosensory behaviors. Here, we review current knowledge of the neurons, signal transduction molecules and regulatory mechanisms that underlie the response of C. elegans to chemicals, including pheromones. The majority of identified molecules and pathways share remarkable homology with sensory mechanisms in other organisms. With the development of new tools and technologies, we anticipate that continued study of chemosensory signal transduction and processing in C. elegans will yield additional new insights into the mechanisms by which this animal is able to detect and discriminate among thousands of chemical cues with a limited sensory neuron repertoire.     

Chemosensory behavior of semi-restrained Caenorhabditis elegans

A new behavioral assay is described for studying chemosensation in the nematode Caenorhabditis elegans. This assay presents three main characteristics: (1) the worm is restrained by gluing, preserving correlates of identifiable behaviors; (2) the amplitude and time course of the stimulus are controlled by the experimenter; and (3) the behavior is recorded quantitatively. We show that restrained C. elegans display behaviors comparable to those of freely moving worms. Moreover, the chemosensory response of wild-type glued animals to changes in salt concentration is similar to that of freely moving animals. This glued-worm assay was used to reveal new chemosensory deficits of the potassium channel mutant egl-2. We conclude that the glued worm assay can be used to study the chemosensory regulation of C. elegans behavior and how it is affected by neuronal or genetic manipulations.     

Diverse Cell Type-Specific Mechanisms Localize G Protein-Coupled Receptors to Caenorhabditis elegans Sensory Cilia

The localization of signaling molecules such as G protein-coupled receptors (GPCRs) to primary cilia is essential for correct signal transduction. Detailed studies over the past decade have begun to elucidate the diverse sequences and trafficking mechanisms that sort and transport GPCRs to the ciliary compartment. However, a systematic analysis of the pathways required for ciliary targeting of multiple GPCRs in different cell types in vivo has not been reported. Here we describe the sequences and proteins required to localize GPCRs to the cilia of the AWB and ASK sensory neuron types in Caenorhabditis elegans. We find that GPCRs expressed in AWB or ASK utilize conserved and novel sequences for ciliary localization, and that the requirement for a ciliary targeting sequence in a given GPCR is different in different neuron types. Consistent with the presence of multiple ciliary targeting sequences, we identify diverse proteins required for ciliary localization of individual GPCRs in AWB and ASK. In particular, we show that the TUB-1 Tubby protein is required for ciliary localization of a subset of GPCRs, implying that defects in GPCR localization may be causal to the metabolic phenotypes of tub-1 mutants. Together, our results describe a remarkable complexity of mechanisms that act in a protein- and cell-specific manner to localize GPCRs to cilia, and suggest that this diversity allows for precise regulation of GPCR-mediated signaling as a function of external and internal context.     

The Importance of cGMP Signaling in Sensory Cilia for Body Size Regulation in Caenorhabditis elegans

The body size of Caenorhabditis elegans is thought to be controlled by sensory inputs because many mutants with sensory cilium structure defects exhibit small body size. The EGL-4 cGMP-dependent protein kinase acts in sensory neurons to reduce body size when animals fail to perceive sensory signals. In addition to body size control, EGL-4 regulates various other behavioral and developmental pathways, including those involved in the regulation of egg laying and chemotaxis behavior. Here we have identified gcy-12, which encodes a receptor-type guanylyl cyclase, as a gene involved in the sensory regulation of body size. Analyses with GFP fusion constructs showed that gcy-12 is expressed in several sensory neurons and localizes to sensory cilia. Genetic analyses indicated that GCY-12 acts upstream of EGL-4 in body size control but does not affect other EGL-4 functions. Our studies indicate that the function of the GCY-12 guanylyl cyclase is to provide cGMP to the EGL-4 cGMP-dependent kinase only for limited tasks including body size regulation. We also found that the PDE-2 cyclic nucleotide phosphodiesterase negatively regulates EGL-4 in controlling body size. Thus, the cGMP level is precisely controlled by GCY-12 and PDE-2 to determine body size through EGL-4, and the defects in the sensory cilium structure may disturb the balanced control of the cGMP level. The large number of guanylyl cyclases encoded in the C. elegans genome suggests that EGL-4 exerts pleiotropic effects by partnering with different guanylyl cyclases for different downstream functions.     

Mutations Affecting the Chemosensory Neurons of Caenorhabditis Elegans

We have identified and characterized 95 mutations that reduce or abolish dye filling of amphid and phasmid neurons and that have little effect on viability, fertility or movement. Twenty-seven mutations occurred spontaneously in strains with a high frequency of transposon insertion. Sixty-eight were isolated after treatment with EMS. All of the mutations result in defects in one or more chemosensory responses, such as chemotaxis to ammonium chloride or formation of dauer larvae under conditions of starvation and overcrowding. Seventy-five of the mutations are alleles of 12 previously defined genes, mutations which were previously shown to lead to defects in amphid ultrastructure. We have assigned 20 mutations to 13 new genes, called dyf-1 through dyf-13. We expect that the genes represented by dye-filling defective mutants are important for the differentiation of amphid and phasmid chemosensilla.     

Chemosensory behavior of semi‐restrained Caenorhabditis elegans

A new behavioral assay is described for studying chemosensation in the nematode Caenorhabditis elegans. This assay presents three main characteristics: (1) the worm is restrained by gluing, preserving correlates of identifiable behaviors; (2) the amplitude and time course of the stimulus are controlled by the experimenter; and (3) the behavior is recorded quantitatively. We show that restrained C. elegans display behaviors comparable to those of freely moving worms. Moreover, the chemosensory response of wild‐type glued animals to changes in salt concentration is similar to that of freely moving animals. This glued‐worm assay was used to reveal new chemosensory deficits of the potassium channel mutant egl‐2. We conclude that the glued worm assay can be used to study the chemosensory regulation of C. elegans behavior and how it is affected by neuronal or genetic manipulations. © 2005 Wiley Periodicals, Inc. J Neurobiol, 2005     

Chemosensory control of surface antigen switching in the nematode Caenorhabditis elegans

Nematodes change their surface compositions in response to environmental signals, which may allow them to survive attacks from microbial pathogens or host immune systems. In the free-living species Caenorhabditis elegans, wild-type worms are induced to display an L1 (first larval stage) surface epitope at later larval stages when grown on an extract of spent culture medium (Inducible Larval Display or ILD). Before this study, it was not known whether ILD was regulated by the well-characterized, neurologically based chemical senses of C. elegans, which mediate other behavioural and developmental responses to environmental signals such as chemotaxis and formation of the facultatively arrested dauer larva stage. We show here that ILD requires the activities of three genes that are essential for the function of the C. elegans chemosensory neurons. ILD was abolished in chemotaxis-defective che-3, osm-3 and tax-4 mutants. In contrast, chemotaxis-defective mutants altered in a different gene, srf-6, show constitutive display of the L1 epitope on all four larval stages. The ILD-defective che-3, osm-3 and tax-4 mutations blocked the constitutive larval display of an srf-6 mutant. Combining srf-6 and certain dauer-constitutive mutations in double mutants enhanced constitutive dauer formation, consistent with the idea that srf-6 acts in parallel with specific components of the dauer formation pathway. These results taken together are consistent with the hypothesis that ILD is triggered by environmental signals detected by the nematode's chemosensory neurons.     

Sensory processing by neural circuits in Caenorhabditis elegans

The anatomical and developmental constancy of Caenorhabditis elegans belies the complexity of its numerically small nervous system. Indeed, there is an increased appreciation of C. elegans as an organism to study systems level questions. Many recent studies focus on the circuits that control locomotion, egg-laying, and male mating behaviors and their modulation by multiple sensory stimuli.     

Chemosensory and Mechanosensory Functions of Olfactory Cilia

Biophysics - This review presents the modern understanding of olfactory cilia as organelles that possess both chemosensory and mechanosensory properties. The molecular mechanisms that underlie...     

Cyclic AMP-Dependent Protein Phosphorylation in Chemosensory Neurons: Identification of Cyclic Nucleotide-Regulated Phosphoproteins in Olfactory Cilia

Chemosensory dendritic membranes (olfactory cilia) contain protein kinase activity that is stimulated by cyclic AMP and more efficiently by the nonhydrolyzable GTP analog guanosine-5'-O-(3-thio)triphosphate (GTP gamma S). In control nonsensory (respiratory) cilia, the cyclic AMP-dependent protein kinase is practically GTP gamma S-insensitive. GTP gamma S activation of the olfactory enzyme appears to be mediated by a stimulatory GTP-binding protein (G-protein) and adenylate cyclase previously shown to be enriched in the sensory membranes. Protein kinase C activity cannot be detected in the chemosensory cilia preparation under the conditions tested. Incubation of olfactory cilia with [gamma-32P]ATP leads to the incorporation of [32P]phosphate into many polypeptides, four of which undergo covalent modification in a cyclic nucleotide-dependent manner. The phosphorylation of one polypeptide, pp24, is strongly and specifically enhanced by cyclic AMP at concentrations lower than 1 microM. This phosphoprotein is not present in respiratory cilia, but is seen also in membranes prepared from olfactory neuroepithelium after cilia removal. Cyclic AMP-dependent protein kinase and phosphoprotein pp24 may be candidate components of the molecular machinery that transduces odor signals.     

Specific Expression of Channelrhodopsin-2 in Single Neurons of Caenorhabditis elegans

Optogenetic approaches using light-activated proteins like Channelrhodopsin-2 (ChR2) enable investigating the function of populations of neurons in live Caenorhabditis elegans (and other) animals, as ChR2 expression can be targeted to these cells using specific promoters. Sub-populations of these neurons, or even single cells, can be further addressed by restricting the illumination to the cell of interest. However, this is technically demanding, particularly in free moving animals. Thus, it would be helpful if expression of ChR2 could be restricted to single neurons or neuron pairs, as even wide-field illumination would photostimulate only this particular cell. To this end we adopted the use of Cre or FLP recombinases and conditional ChR2 expression at the intersection of two promoter expression domains, i.e. in the cell of interest only. Success of this method depends on precise knowledge of the individual promoters' expression patterns and on relative expression levels of recombinase and ChR2. A bicistronic expression cassette with GFP helps to identify the correct expression pattern. Here we show specific expression in the AVA reverse command neurons and the aversive polymodal sensory ASH neurons. This approach shall enable to generate strains for optogenetic manipulation of each of the 302 C. elegans neurons. This may eventually allow to model the C. elegans nervous system in its entirety, based on functional data for each neuron.     

Defining Specificity Determinants of cGMP Mediated Gustatory Sensory Transduction in Caenorhabditis elegans

Cyclic guanosine monophosphate (cGMP) is a key secondary messenger used in signal transduction in various types of sensory neurons. The importance of cGMP in the ASE gustatory receptor neurons of the nematode Caenorhabditis elegans was deduced by the observation that multiple receptor-type guanylyl cyclases (rGCs), encoded by the gcy genes, and two presently known cyclic nucleotide-gated ion channel subunits, encoded by the tax-2 and tax-4 genes, are essential for ASE-mediated gustatory behavior. We describe here specific mechanistic features of cGMP-mediated signal transduction in the ASE neurons. First, we assess the specificity of the sensory functions of individual rGC proteins. We have previously shown that multiple rGC proteins are expressed in a left/right asymmetric manner in the functionally lateralized ASE neurons and are required to sense distinct salt cues. Through domain swap experiments among three different rGC proteins, we show here that the specificity of individual rGC proteins lies in their extracellular domains and not in their intracellular, signal-transducing domains. Furthermore, we find that rGC proteins are also sufficient to confer salt sensory responses to other neurons. Both findings support the hypothesis that rGC proteins are salt receptor proteins. Second, we identify a novel, likely downstream effector of the rGC proteins in gustatory signal transduction, a previously uncharacterized cyclic nucleotide-gated (CNG) ion channel, encoded by the che-6 locus. che-6 mutants show defects in gustatory sensory transduction that are similar to defects observed in animals lacking the tax-2 and tax-4 CNG channels. In contrast, thermosensory signal transduction, which also requires tax-2 and tax-4, does not require che-6, but requires another CNG, cng-3. We propose that CHE-6 may form together with two other CNG subunits, TAX-2 and TAX-4, a gustatory neuron-specific heteromeric CNG channel complex.     

Two homogeneous myosins in body-wall muscle of Caenorhabditis elegans

Myosin purified from the body-wall muscle-defective mutant E675 of the nematode. Caenorhabditis elegans, has heavy chain polypeptides which can be distinguished on the basis of molecular weight. On SDS-polyacrylamide gels, bands are found at 210,000 and 203,000 daltons. This is in contrast to myosin from the wild-type, N2, which has a single heavy chain band at 210,000 daltons. Both heavy chains of E675 are found in body-wall muscle (Epstein, Waterston and Brenner, 1974).When native myosin from E675 is fractionated on hydroxyapatite, it is separated into myosin containing predominantly one or the other molecular weight heavy chain and myosin containing a mixture of the heavy chains. Comparison of the CNBr fragments of myosin that contains predominantly 210,000 dalton heavy chains with those of myosin that contains predominantly 203,000 dalton heavy chains reveals multiple differences. These differences are not explained by the difference in molecular weight of the heavy chains, but may be explained if each type of heavy chain is the product of a different structural gene. Furthermore, because there are fractions which exhibit >80% 210,000 or >80% 203,000 dalton heavy chain, there is myosin which is homogeneous for each of the heavy chains.Although N2 myosin has only a single molecular weight heavy chain, it too is fractionated by hydroxyapatite. By comparing the CNBr fragments of different myosin fractions, we show that N2, like E675, has two kinds of heavy chains.E190, a body-wall muscle-defective mutant in the same complementation group as E675, is lacking the myosin heavy chain affected by the e675 mutation. This property has allowed us to determine by co-purification of labeled E190 myosin in the presence of excess, unlabeled E675 myosin that most, if not all, of the myosin that contains two different molecular weight heavy chains is due to the formation of complexes between homogeneous myosins and not to a heterogeneous myosin.     

Two distinct yolk lipoprotein complexes from Caenorhabditis elegans

The four yolk polypeptides of the nematode Caenorhabditis elegans are found in two types of lipoprotein particle: 12 S particles with Mr estimated at 450,000 and 8 S particles with Mr estimated at 250,000. Both types of particle contain approximately 8% phospholipids, 3% triglycerides, and 3% other lipids by mass. All four C. elegans yolk polypeptides can be found in either 12 or 8 S particles, depending upon the conditions of isolation. While the properties of the 12 and 8 S lipoprotein particles are consistent with a dimermonomer relationship, the asymmetric distribution of the yolk polypeptides between 12 and 8 S fractions suggests that at least two different oligomeric lipoprotein complexes are present in C. elegans embryos. In order to clarify the subunit composition of the C. elegans yolk lipoproteins, the patterns of polypeptides retained in immunoaffinity binding procedures by immunoglobulins of different antigenic specificities have been compared. When immunoaffinity binding is performed in the absence of sodium dodecyl sulfate, three C. elegans yolk proteins (yp170A, yp115, and yp88) are retained together by polyclonal immunoglobulins directed against either yp115 or yp88. A monoclonal immunoglobulin also retains these three proteins together. In contrast, a second monoclonal immunoglobulin retains only the fourth yolk protein (yp170B). Aggregate species, evidently reflecting the spontaneous formation of interchain disulfide bonds, indicate that yp170A and yp88 are physically associated, whereas yp170B self-associates in dimers. It is concluded that there are two distinct lipoprotein complexes in C. elegans: the A complex, which consists of yp170A, yp115, and yp88 and is essentially heterodimeric and the B dimer, a simple dimer of yp170B.     

Chemosensory signaling in C. elegans

The nematode Caenorhabditis elegans can sense and respond to hundreds of different chemicals with a simple nervous system, making it an excellent model for studies of chemosensation. The chemosensory neurons that mediate responses to different chemicals have been identified through laser ablation studies, providing a cellular context for chemosensory signaling. Genetic and molecular analyses indicate that chemosensation in nematodes involves G protein signaling pathways, as it does in vertebrates, but the receptors and G proteins involved belong to nematode-specific gene families. It is likely that about 500 different chemosensory receptors are used to detect the large spectrum of chemicals to which C. elegans responds, and one of these receptors has been matched with its odorant ligand. C. elegans olfactory responses are also subject to regulation based on experience, allowing the nematode to respond to a complex and changing chemical environment.