Combinatorial expression of TRPV channel proteins defines their sensory functions and subcellular localization in C. elegans neurons

Transient receptor potential vanilloid 4 is essential in chemotherapy-induced neuropathic pain in the rat

The development of treatments for neuropathic pain has been hindered by our limited understanding of the basic mechanisms underlying abnormalities in nociceptor hyperexcitability. We recently showed that the polymodal receptor transient receptor potential vanilloid 4 (TRPV4), a member of the transient receptor potential (TRP) family of ion channels, may play a role in inflammatory pain (Alessandri-Haber et al., 2003). The present study tested whether TRVP4 also contributes to neuropathic pain, using a rat model of Taxol-induced painful peripheral neuropathy. Taxol is the most widely used drug for the treatment of a variety of tumor types, but the dose of Taxol that can be tolerated is limited by the development of a small-fiber painful peripheral neuropathy. We found that Taxol treatment enhanced the nociceptive behavioral responses to both mechanical and hypotonic stimulation of the hind paw. Spinal administration of antisense oligodeoxynucleotides to TRPV4, which reduced the expression of TRPV4 in sensory nerve, abolished Taxol-induced mechanical hyperalgesia and attenuated hypotonic hyperalgesia by 42%. The enhancement of osmotic nociception involves sensitization of osmotransduction in primary afferents because osmotransduction was enhanced in cultured sensory neurons isolated from Taxol-treated rats. Taxol-induced TRPV4-mediated hyperalgesia and the enhanced osmotransduction in cultured nociceptors were dependent on integrin/Src tyrosine kinase signaling. These results suggest that TRPV4 plays a crucial role in a painful peripheral neuropathy, making it a very promising target for the development of a novel class of analgesics.

Natural variation in the npr-1 gene modifies ethanol responses of wild strains of C. elegans

Variation in the acute response to ethanol between individuals has a significant impact on determining susceptibility to alcoholism. The degree to which genetics contributes to this variation is of great interest. Here we show that allelic variation that alters the functional level of NPR-1, a neuropeptide Y (NPY) receptor-like protein, can account for natural variation in the acute response to ethanol in wild strains of Caenorhabditis elegans. NPR-1 negatively regulates the development of acute tolerance to ethanol, a neuroadaptive process that compensates for effects of ethanol. Furthermore, dynamic changes in the NPR-1 pathway provide a mechanism for ethanol tolerance in C. elegans. This suggests an explanation for the conserved function of NPY-related pathways in ethanol responses across diverse species. Moreover, these data indicate that genetic variation in the level of NPR-1 function determines much of the phenotypic variation in adaptive behavioral responses to ethanol that are observed in natural populations.

G protein-coupled receptor kinase function is essential for chemosensation in C. elegans

G protein-coupled receptors (GPCRs) mediate diverse signaling processes, including olfaction. G protein-coupled receptor kinases (GRKs) are important regulators of G protein signal transduction that specifically phosphorylate activated GPCRs to terminate signaling. Despite previously described roles for GRKs in GPCR signal downregulation, animals lacking C. elegans G protein-coupled receptor kinase-2 (Ce-grk-2) function are not hypersensitive to odorants. Instead, decreased Ce-grk-2 function in adult sensory neurons profoundly disrupts chemosensation, based on both behavioral analysis and Ca(2+) imaging. Although mammalian arrestin proteins cooperate with GRKs in receptor desensitization, loss of C. elegans arrestin-1 (arr-1) does not disrupt chemosensation. Either overexpression of the C. elegans Galpha subunit odr-3 or loss of eat-16, which encodes a regulator of G protein signaling (RGS) protein, restores chemosensation in Ce-grk-2 mutants. These results demonstrate that loss of GRK function can lead to reduced GPCR signal transduction and suggest an important role for RGS proteins in the regulation of chemosensation.

Phylogenomic and chemotaxonomic analysis of the endocannabinoid system

The endocannabinoid system consists of two cannabinoid (CB) receptors, seven ligands, and ligand-catabolizing enzymes such as fatty acid amid hydrolase (FAAH) and monoglyceride lipase (MGL). The system's phylogenetic distribution is poorly known. The ligands cannot be molecularly investigated because they are not polypeptides and their specific synthetic enzymes have not been identified, so no sequences are available. Ligand phylogenetics can be inferred, nonetheless, by their presence in a range of extant organisms. Thus a meta-analysis of ligand extraction studies was performed (chemotaxonomy), and compared to a molecular search for homologs of CB receptors, vanilloid receptors (VR1), FAAH, and MGL in the genomes of sequenced organisms (phylogenomics). Putative homologs underwent functional mapping to ascertain the presence of critical amino acid motifs known to impart protein functionality. From an evolutionary perspective it appears that (1) endocannabinoid ligands evolved before CB receptors; (2) the ligands evolved independently multiple times; (3) CB receptors evolved prior to the metazoan-bilaterian divergence (ie, between extant Hydra and leech), but were secondarily lost in the Ecdysozoa; (4) VR1 may predate CB receptors but its affinity for endocannabinoids is a recent acquisition, appearing after the lower vertebrate-mammal divergence; (5) MGL may be as old as the ligands, whereas FAAH evolved recently, after the appearance of vertebrates. FAAH's emergence correlates with VR1's newly-found affinity for anandamide; this overlap in evolutionary time is recapitulated by complementary distribution patterns of FAAH, VR1, and anandamide in the brain. Linking FAAH, VR1, and anandamide implies a coupling among the remaining "older" parts of the endocannabinoid system, MGL, CB receptors, and 2-AG.

Activation and sensitisation of the vanilloid receptor: role in gastrointestinal inflammation and function

The exquisite specific excitatory and desensitising actions of capsaicin on a subpopulation of primary sensory neurons have been instrumental in identifying the roles of these neurons in nociception, reflex responses and neurogenic inflammation. Structure activity studies with capsaicin-like molecules have suggested that a "receptor" should mediate the effects of capsaicin on sensory neurons. The cloning of the vanilloid receptor-1 (VR1) has confirmed this hypothesis. VR1 (TRPV1) belongs to the transient receptor potential (TRP) family of channels, and its activation by various xenobiotics, noxious temperature, extracellular low pH and high concentration of certain lipid derivatives results in cation influx and sensory nerve terminal excitation. TRPV1 may dimerise or form tetramers or heteromers with PLC-gamma and TrkA or even with other TRPs. TRPV1 is markedly upregulated and/or "sensitised" under inflammatory conditions via protein kinase C-epsilon-, cAMP-dependent PK- and PLC-gamma-dependent pathways or by exposure to dietary agents as ethanol. TRPV1 is expressed on sensory neurons distributed in all the regions of the gastrointestinal tract in myenteric ganglia, muscle layer and mucosa. There is evidence of TRPV1 expression also in epithelial cells of the gastrointestinal tract. High expression of TRPV1 has been detected in several inflammatory diseases of the colon and ileum, whereas neuropeptides released upon sensory nerve stimulation triggered by TRPV1 activation seem to play a role in intestinal motility disorders. TRPV1 antagonists, which will soon be available for clinical testing, may undergo scrutiny for the treatment of inflammatory diseases of the gut.

Caenorhabditis elegans TRPV ion channel regulates 5HT biosynthesis in chemosensory neurons

Serotonin (5HT) is a pivotal signaling molecule that modulates behavioral and endocrine responses to diverse chemical and physical stimuli. We report cell-specific regulation of 5HT biosynthesis by transient receptor potential V (TRPV) ion channels in C. elegans. Mutations in the TRPV genes osm-9 or ocr-2 dramatically downregulate the expression of the gene encoding the 5HT synthesis enzyme tryptophan hydroxylase (tph-1) in the serotonergic chemosensory neurons ADF, but neither the mutation nor the double mutation of both channel genes affects other types of serotonergic neurons. The TRPV genes are expressed in the ADF neurons but not in other serotonergic neurons, and act cell-autonomously to regulate a neuron-specific transcription program. Whereas in olfactory neurons OSM-9 and OCR-2 function is dependent on ODR-3 Galpha, the activity of ODR-3 or two other Galpha proteins expressed in the ADF neurons is not required for upregulating tph-1 expression, thus the TRPV ion channels in different neurons may be regulated by different mechanisms. A gain-of-function mutation in CaMKII UNC-43 partially suppresses the downregulation of tph-1 in the TRPV mutants, thus CaMKII may be an effector of the TRPV signaling. Mutations in the TRPV genes cause worms developmentally arrest at the Dauer stage. This developmental defect is due in part to reduced 5HT inputs into daf-2/insulin neuroendocrine signaling.

Worms taste bitter: ASH neurons, QUI-1, GPA-3 and ODR-3 mediate quinine avoidance in Caenorhabditis elegans

An animal's ability to detect and avoid toxic compounds in the environment is crucial for survival. We show that the nematode Caenorhabditis elegans avoids many water-soluble substances that are toxic and that taste bitter to humans. We have used laser ablation and a genetic cell rescue strategy to identify sensory neurons involved in the avoidance of the bitter substance quinine, and found that ASH, a polymodal nociceptive neuron that senses many aversive stimuli, is the principal player in this response. Two G protein alpha subunits GPA-3 and ODR-3, expressed in ASH and in different, nonoverlapping sets of sensory neurons, are necessary for the response to quinine, although the effect of odr-3 can only be appreciated in the absence of gpa-3. We identified and cloned a new gene, qui-1, necessary for quinine and SDS avoidance. qui-1 codes for a novel protein with WD-40 domains and which is expressed in the avoidance sensory neurons ASH and ADL.

TRPV4 calcium entry channel: a paradigm for gating diversity

The vanilloid receptor-1 (VR1, now TRPV1) was the founding member of a subgroup of cation channels within the TRP family. The TRPV subgroup contains six mammalian members, which all function as Ca2+ entry channels gated by a variety of physical and chemical stimuli. TRPV4, which displays 45% sequence identity with TRPV1, is characterized by a surprising gating promiscuity: it is activated by hypotonic cell swelling, heat, synthetic 4alpha-phorbols, and several endogenous substances including arachidonic acid (AA), the endocannabinoids anandamide and 2-AG, and cytochrome P-450 metabolites of AA, such as epoxyeicosatrienoic acids. This review summarizes data on TRPV4 as a paradigm of gating diversity in this subfamily of Ca2+ entry channels.

Mechanosensitive channels: multiplicity of families and gating paradigms

Mechanosensitive ion channels are the primary transducers that convert mechanical force into an electrical or chemical signal in hearing, touch, and other mechanical senses. Unlike vision, olfaction, and some types of taste, which all use similar kinds of primary heterotrimeric GTP-binding protein-coupled receptors, mechanosensation relies on diverse types of transducer molecules. Unrelated types of channels can be used for the perception of various mechanical stimuli, not only in distant groups of organisms, but also in separate locations of the same organism. The extreme sensitivity of the transduction mechanism in the auditory system, which relies on an elaborate structure of rigid cilia, filamentous links, and molecular motors to focus force on transduction channels, contrasts with that of the bacterial channel MscL, which is opened by high lateral tension in the membrane and fulfills a safety-valve rather than a sensory function. The spatial scales of conformational movement and force in these two systems are described, and are shown to be consistent with a general physical description of mechanical channel gating. We outline the characteristics of several types of mechanosensitive channels and the functional contexts in which they participate in signaling and cellular regulation in sensory and nonsensory cells.

The vanilloid receptor family of calcium-permeable channels: molecular integrators of microenvironmental stimuli

The TRPV subfamily of calcium-permeable channels is widely distributed in sensory and nonsensory cells from nematodes to mammals. These channels can be variably activated by a diverse range of stimuli (osmotic/mechanical stress, noxious chemicals and heat, endogenous mediators) that often converge on the same channel. Evidence is presented that TRPV channels function as novel "molecular integrators" of diverse microenvironmental stimuli.

The worm's sense of smell

Animals sense their chemical environment using multiple chemosensory neuron types, each of which exhibits characteristic response properties. The chemosensory neurons of the nematode Caenorhabditis elegans provide an excellent system in which to explore the developmental mechanisms giving rise to this functional diversity. In this review, we discuss the principles underlying the patterning, generation, differentiation, and diversification of chemosensory neuron subtypes in C. elegans. Current knowledge of the molecular mechanisms underlying each of these individual steps is derived from work in different model organisms. It is essential to describe the complete developmental pathways in each organism to determine whether functional diversification in chemosensory systems is achieved via conserved or novel mechanisms. Such a complete description may be possible in C. elegans.

Invertebrate nociception: Behaviors, neurons and molecules

Genetic analysis of nociceptive behaviors in the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster has led to the discovery of conserved sensory transduction channels and signaling molecules. These are embedded in neurons and circuits that generate responses to noxious signals. This article reviews the neurons and molecular mechanisms that underlie invertebrate nociception. We begin with the neurobiology of invertebrate nociception, and then focus on molecules with conserved functions in vertebrate nociception and sensory biology.

Sensation is painless

Emily Dickinson declared: 'After great pain, a formal feeling comes'. This formal feeling begins when sensory neurons are activated by noxious stimuli, such as stepping on a tack. Recently, Seymour Benzer's group identified sensory neurons in Drosophila larvae that mediate aversive responses to noxious heat and mechanical stimuli. Thresholds for behavioral and nerve responses are elevated by mutations in the painless gene, which encodes a TRP ion channel protein. Painless thus joins an elite group of TRPs implicated in sensory transduction in insects, nematodes, mammals and fish.

Abnormal osmotic regulation in trpv4-/- mice

Osmotic homeostasis is one of the most aggressively defended physiological parameters in vertebrates. However, the molecular mechanisms underlying osmotic regulation are poorly understood. The transient receptor potential channel, vanilloid subfamily (TRPV4), is an osmotically activated ion channel that is expressed in circumventricular organs in the mammalian CNS, which is an important site of osmotic sensing. We have generated trpv4-null mice and observed abnormalities of their osmotic regulation. trpv4-/- mice drank less water and became more hyperosmolar than did wild-type littermates, a finding that was seen with and without administration of hypertonic saline. In addition, plasma levels of antidiuretic hormone were significantly lower in trpv4-/- mice than in wild-type littermates after a hyperosmotic challenge. Continuous s.c. infusion of the antidiuretic hormone analogue, dDAVP, resulted in systemic hypotonicity in trpv4-/- mice, despite the fact that their renal water reabsorption capacity was normal. Thus, the response to both hyper- and hypoosmolar stimuli is impaired in trpv4-/- mice. After a hyperosmolar challenge, there was markedly reduced expression of c-FOS in the circumventricular organ, the organum vasculosum of the lamina terminalis, of trpv4-/- mice compared with wild-type mice. This finding suggests that there is an impairment of osmotic sensing in the CNS of trpv4-/- mice. These data indicate that TRPV4 is necessary for the normal response to changes in osmotic pressure and functions as an osmotic sensor in the CNS.

TRPV2 Is a Component of Osmotically Sensitive Cation Channels in Murine Aortic Myocytes

Changes in membrane tension resulting from membrane stretch represent one of the key elements in blood flow regulation in vascular smooth muscle. However, the molecular mechanisms involved in the regulation of membrane stretch remain unclear. In this study, we provide evidence that a vanilloid receptor (TRPV) homologue, TRPV2 is expressed in vascular smooth muscle cells, and demonstrate that it can be activated by membrane stretch. Cell swelling caused by hypotonic solutions activated a nonselective cation channel current (NSCC) and elevated intracellular Ca 2+ ([Ca 2+ ] i ) in freshly isolated cells from mouse aorta. Both of these signals were blocked by ruthenium red, an effective blocker of TRPVs. The absence of external Ca 2+ abolished this increase in [Ca 2+ ] i caused by the hypotonic stimulation and reduced the activation of NSCC. Significant immunoreactivity to mouse TRPV2 protein was detected in single mouse aortic myocytes. Moreover, the expression of TRPV2 was found in mesenteric and basilar arterial myocytes. Treatment of mouse aorta with TRPV2 antisense oligonucleotides resulted in suppression of hypotonic stimulation-induced activation of NSCC and elevation of [Ca 2+ ] i as well as marked inhibition of TRPV2 protein expression. In Chinese hamster ovary K1 (CHO) cells transfected with TRPV2 cDNA (TRPV2-CHO), application of membrane stretch through the recording pipette and hypotonic stimulation consistently activated single NSCC. Moreover, stretch of TRPV2-CHO cells cultured on an elastic silicon membrane significantly elevated [Ca 2+ ] i . These results provide a strong basis for our purpose that endogenous TRPV2 in mouse vascular myocytes functions as a novel and important stretch sensor in vascular smooth muscles.

Mammalian TRPV4 (VR-OAC) directs behavioral responses to osmotic and mechanical stimuli in Caenorhabditis elegans

All animals detect osmotic and mechanical stimuli, but the molecular basis for these responses is incompletely understood. The vertebrate transient receptor potential channel vanilloid subfamily 4 (TRPV4) (VR-OAC) cation channel has been suggested to be an osmo/mechanosensory channel. To assess its function in vivo, we expressed TRPV4 in Caenorhabditis elegans sensory neurons and examined its ability to generate behavioral responses to sensory stimuli. C. elegans ASH neurons function as polymodal sensory neurons that generate a characteristic escape behavior in response to mechanical, osmotic, or olfactory stimuli. These behaviors require the TRPV channel OSM-9 because osm-9 mutants do not avoid nose touch, high osmolarity, or noxious odors. Expression of mammalian TRPV4 in ASH neurons of osm-9 worms restored avoidance responses to hypertonicity and nose touch, but not the response to odorant repellents. Mutations known to reduce TRPV4 channel activity also reduced its ability to direct nematode avoidance behavior. TRPV4 function in ASH required the endogenous C. elegans osmotic and nose touch avoidance genes ocr-2, odr-3, osm-10, and glr-1, indicating that TRPV4 is integrated into the normal ASH sensory apparatus. The osmotic and mechanical avoidance responses of TRPV4-expressing animals were different in their sensitivity and temperature dependence from the responses of wild-type animals, suggesting that the TRPV4 channel confers its characteristic properties on the transgenic animals' behavior. These results provide evidence that TRPV4 can function as a component of an osmotic/mechanical sensor in vivo.

New TRP channels in hearing and mechanosensation

Despite extensive biophysical characterization and the superb example of the bacterial MscL channel, molecular identification of eukaryotic mechanosensitive channels has been slow. New members of the TRP superfamily have emerged as candidate channels to mediate touch, hearing, fluid flow, and osmosensation in sensory and nonsensory cells. Distinguishing between direct mechanical activation and indirect second messenger activation is still a challenge.

A C. elegans sperm TRP protein required for sperm-egg interactions during fertilization

Fertilization, a critical step in animal reproduction, is triggered by a series of specialized sperm-egg interactions. However, the molecular mechanisms underlying fertilization are not well understood. Here, we identify a sperm-enriched C. elegans TRPC homolog, TRP-3. Mutations in trp-3 lead to sterility in both hermaphrodites and males due to a defect in their sperm. trp-3 mutant sperm are motile, but fail to fertilize oocytes after gamete contact. TRP-3 is initially localized in intracellular vesicles, and then translocates to the plasma membrane during sperm activation. This translocation coincides with a marked increase in store-operated calcium entry, providing an in vivo mechanism for the regulation of TRP-3 activity. As C. elegans oocytes lack egg coats, our data suggest that some TRPC family channels might function to mediate calcium influx during sperm-egg plasma membrane interactions leading to fertilization.

Temperature-modulated diversity of TRPV4 channel gating: activation by physical stresses and phorbol ester derivatives through protein kinase C-dependent and -independent pathways

The TRPV4 calcium-permeable channel was cloned from mouse kidney M-1 cells, and the effect of temperature modulation on channel gating/activation by physical and chemical signals was evaluated. A TRPV4 cDNA construct with a C-terminal V5 epitope was stably transfected into human embryonic kidney (HEK) 293 and Chinese hamster ovary cells resulting in high levels of expression at the plasma membrane. Channel activation was assessed from changes in calcium influx (fura-2 fluorescence measurements) or whole cell currents (patch clamp analysis). At room temperature (22-24 degrees C), exposure of TRPV4-transfected cells to hypotonic medium (225 mOsm/liter) or a non-protein kinase C (PKC)-activating phorbol ester derivative, 4alpha-phorbol 12,13-decanoate (100 nm), induces modest channel activation, whereas phorbol 12-myristate 13-acetate (100 nm), a PKC-activating phorbol ester, and shear stress (3-20 dyne/cm2) had minimal or no effect on channel activation. In contrast, at elevated temperatures (37 degrees C) the channel was rapidly activated by all stimuli. Inhibition of PKC by calphostin C (50 nm) or staurosporine (500 nm) abolished phorbol 12-myristate 13-acetate-induced activation of the channel without affecting the response to other stimuli. Ruthenium red (1 microm) effectively blocked the channel activity by all stimuli. It is concluded that temperature is a critical modulator of TRPV4 channel gating, leading to activation of the channel by a diverse range of microenvironmental chemical and physical signals utilizing a least two transduction pathways, one PKC-dependent and one PKC-independent. The convergence of multiple signals and transduction pathways on the same channel indicate that the channel functions as a molecular integrator of microenvironmental chemical and physical signals.

Genetics of sensory mechanotransduction

The molecular mechanisms for the transduction of light and chemical signals in animals are fairly well understood. In contrast, the processes by which the senses of touch, balance, hearing, and proprioception are transduced are still largely unknown. Biochemical approaches to identify transduction components are difficult to use with mechanosensory systems, but genetic approaches are proving more successful. Genetic research in several organisms has demonstrated the importance of cytoskeletal, extracellular, and membrane components for sensory mechanotransduction. In particular, researchers have identified channel proteins in the DEG/ENaC and TRP families that are necessary for signaling in a variety of mechanosensory cells. Proof that these proteins are components of the transduction channel, however, is incomplete.

Matching accessories

Heterodimerization enhances the complexity of ligand recognition and diversity of signaling responses of heterotrimeric guanine nucleotide-binding protein-coupled receptors (GPCRs). Many accessory proteins (for ion channels or GPCRs) appear to associate with their partners relatively early in the process whereby proteins are transported to the cell surface; their roles in modulating function may have evolved out of simple proximity to a protein that once upon a time they either facilitated or accompanied through the maturation process. The receptor activity-modifying proteins (RAMPs) are a family of single-transmembrane accessory proteins that heterodimerize with GPCRs and, thereby, allow individual GPCRs to recognize multiple ligands and to activate various signaling pathways in response to ligand binding. The M10 family of major histocompatibility complex (MHC) class 1b proteins has recently been shown to associate with murine vomeronasal V2R receptors, as well as to escort them to the cell surface. The exact role of M10 in modulating V2R function (or vice versa) remains to be determined.

A TRPV family ion channel required for hearing in Drosophila

The many types of insect ear share a common sensory element, the chordotonal organ, in which sound-induced antennal or tympanal vibrations are transmitted to ciliated sensory neurons and transduced to receptor potentials. However, the molecular identity of the transducing ion channels in chordotonal neurons, or in any auditory system, is still unknown. Drosophila that are mutant for NOMPC, a transient receptor potential (TRP) superfamily ion channel, lack receptor potentials and currents in tactile bristles but retain most of the antennal sound-evoked response, suggesting that a different channel is the primary transducer in chordotonal organs. Here we describe the Drosophila Nanchung (Nan) protein, an ion channel subunit similar to vanilloid-receptor-related (TRPV) channels of the TRP superfamily. Nan mediates hypo-osmotically activated calcium influx and cation currents in cultured cells. It is expressed in vivo exclusively in chordotonal neurons and is localized to their sensory cilia. Antennal sound-evoked potentials are completely absent in mutants lacking Nan, showing that it is an essential component of the chordotonal mechanotransducer.

TRPM8 mRNA is expressed in a subset of cold-responsive trigeminal neurons from rat

Recent electrophysiological studies of cultured dorsal root and trigeminal ganglion neurons have suggested that multiple ionic mechanisms underlie the peripheral detection of cold temperatures. Several candidate "cold receptors," all of them ion channel proteins, have been implicated in this process. One of the most promising candidates is TRPM8, a nonselective cationic channel expressed in a subpopulation of sensory neurons that is activated both by decreases in temperature and the cooling compound menthol. However, evidence for the expression of TRPM8 in functionally defined cold-sensitive neurons has been lacking. Here, we combine fluorometric calcium imaging of cultured rat trigeminal neurons with single-cell RT-PCR to demonstrate that there are distinct subpopulations of cold responsive neurons and that TRPM8 likely contributes to cold transduction in one of them. TRPM8 is preferentially expressed within a subset of rapidly responsive, low-threshold (approximately 30 degrees C), cold-sensitive neurons. A distinct class of slowly responsive cold-sensitive neurons that is activated at lower temperatures (approximately 20 degrees C) generally lacks detectable TRPM8 mRNA. Together with previous findings, our data support the notion that cold responsive neurons are functionally heterogeneous.

Transducing touch in Caenorhabditis elegans

Mechanosensation has been studied for decades, but understanding of its molecular mechanism is only now emerging from studies in Caenorhabditis elegans and Drosophila melanogaster. In both cases, the entry point proved to be genetic screens that allowed molecules needed for mechanosensation to be identified without any prior understanding of the likely components. In C. elegans, genetic screens revealed molecules needed for touch sensation along the body wall and other regions of force sensitivity. Members of two extensive membrane protein families have emerged as candidate sensory mechanotransduction channels: mec-4 and mec-10, which encode amiloride-sensitive channels (ASCs or DEG/ENaCs), and osm-9, which encodes a TRP ion channel. There are roughly 50 other members of these families whose functions in C. elegans are unknown. This article classifies these channels in C. elegans, with an emphasis on insights into their function derived from mutation. We also review the neuronal cell types in which these channels might be expressed and mediate mechanotransduction.

The transient receptor potential channel on the yeast vacuole is mechanosensitive

Ca2+ is released from the vacuole into the yeast cytoplasm on an osmotic upshock, but how this upshock is perceived was unknown. We found the vacuolar channel, Yvc1p, to be mechanosensitive, showing that the Ca2+ conduit is also the sensing molecule. Although fragile, the yeast vacuole allows limited direct mechanical examination. Pressures at tens of millimeters of Hg (1 mmHg = 133 Pa) activate the 400-pS Yvc1p conductance in whole-vacuole recording mode as well as in the excised cytoplasmic-side-out mode. Raising the bath osmolarity activates this channel and causes vacuolar shrinkage and deformation. It appears that, on upshock, a transient osmotic force activates Yvc1p to release Ca2+ from the vacuole. Mechanical activation of Yvc1p occurs regardless of Ca2+ concentration and is apparently independent of its known Ca2+ activation, which we now propose to be an amplification mechanism (Ca2+-induced Ca2+ release). Yvc1p is a member of the transient receptor potential-family channels, several of which have been associated with mechanosensation in animals. The possible use of Yvc1p as a molecular model to study mechanosensation in general is discussed.

The venerable inveterate invertebrate TRP channels

The transient receptor potential (TRP) superfamily is subdivided into four main classes of cation channels, TRPC, TRPV, TRPM and TRPN, each of which includes members in worms, flies, mice and humans. While the biophysical features of many of the mammalian channels have been described, relatively little is known concerning the biological roles of these channels. Forward genetic screens in Drosophila melanogaster and Caenorhabditis elegans have led to the identification of the founding members of each of these four subfamilies. Moreover, phenotypic analyses of invertebrate mutants have contributed greatly to our understanding of the roles of TRP proteins. A recurring theme is that many of these proteins function in sensory signaling processes ranging from vision to olfaction, osmosensation, light touch, social feeding, and temperature- and mechanically-induced nociception. In addition, at least one invertebrate TRP protein is required for cell division. As many of these functions may be conserved among the mammalian TRPs, the invertebrate TRPs offer valuable genetic handles for characterizing the functions of these cation channels in vivo.

TRPV channels as temperature sensors

The past year has seen a doubling in the number of heat-sensitive ion channels to six, and four of these channels are from the TRPV family. These channels characteristically have Q(10) values of >10 above the thermal threshold, very different from the Q(10) values of 1.5-2.0 seen in most ion channels. Cells expressing TRPV1 show similar temperature sensitivity to small capsaicin-sensitive nociceptor neurons, consistent with these neurons expressing homomers of TRPV1. A-delta fibres exhibit properties that may be explained by TRPV2 containing channels which is present in large diameter sensory neurons that do not express TRPV1. TRPV3 has a lower temperature threshold and may contribute to warm-sensitive channels together with TRPV1. Warm sensation may also be transduced by TRPV4 expressing sensory neurons and hypothalamic neurons. We can now look forward to further work defining the properties of the recombinant channels in more detail and a re-analysis of endogenous i(heat) currents in thermosensitive neurons and other cells. Data from the study of mice in which TRPV2, TRPV3 or TRPV4 have been deleted are also eagerly awaited.

Vertebrate and invertebrate TRPV-like mechanoreceptors

Our senses of touch, hearing, and balance are mediated by mechanosensitive ion channels. In vertebrates, little is known about the molecular composition of these mechanoreceptors, an example of which is the transduction channel of the inner ear's receptor cells, hair cells. Members of the TRP family of ion channels are considered candidates for the vertebrate hair cell's mechanosensitive transduction channel and here we review the evidence for this candidacy. We start by examining the results of genetic screens in invertebrates that identified members of the TRP gene family as core components of mechanoreceptors. In particular, we discuss the Caenorhabditis elegans OSM-9 channel, an invertebrate TRPV channel, and the Drosophila melanogaster TRP channel NOMPC. We then evaluate basic features of TRPV4, a vertebrate member of the TRPV subfamily, which is gated by a variety of physical and chemical stimuli including temperature, osmotic pressure, and ligands. Finally, we compare the characteristics of all discussed mechanoreceptive TRP channels with the biophysical characteristics of hair cell mechanotransduction, speculating about the possible make-up of the elusive inner ear mechanoreceptor.

painless, a Drosophila gene essential for nociception

We describe a paradigm for nociception in Drosophila. In response to the touch of a probe heated above 38 degrees C, Drosophila larvae produce a stereotypical rolling behavior, unlike the response to an unheated probe. In a genetic screen for mutants defective in this noxious heat response, we identified the painless gene. Recordings from wild-type larval nerves identified neurons that initiated strong spiking above 38 degrees C, and this activity was absent in the painless mutant. The painless mRNA encodes a protein of the transient receptor potential ion channel family. Painless is required for both thermal and mechanical nociception, but not for sensing light touch. painless is expressed in peripheral neurons that extend multiple branched dendrites beneath the larval epidermis, similar to vertebrate pain receptors. An antibody to Painless binds to localized dendritic structures that we hypothesize are involved in nociceptive signaling.

From genes to integrative physiology: ion channel and transporter biology in Caenorhabditis elegans

The stunning progress in molecular biology that has occurred over the last 50 years drove a powerful reductionist approach to the study of physiology. That same progress now forms the foundation for the next revolution in physiological research. This revolution will be focused on integrative physiology, which seeks to understand multicomponent processes and the underlying pathways of information flow from an organism's "parts" to increasingly complex levels of organization. Genetically tractable and genomically defined nonmammalian model organisms such as the nematode Caenorhabditis elegans provide powerful experimental advantages for elucidating gene function and the molecular workings of complex systems. This review has two main goals. The first goal is to describe the experimental utility of C. elegans for investigating basic physiological problems. A detailed overview of C. elegans biology and the experimental tools, resources, and strategies available for its study is provided. The second goal of this review is to describe how forward and reverse genetic approaches and direct behavioral and physiological measurements in C. elegans have generated novel insights into the integrative physiology of ion channels and transporters. Where appropriate, I describe how insights from C. elegans have provided new understanding of the physiology of membrane transport processes in mammals.

ANKTM1, a TRP-like channel expressed in nociceptive neurons, is activated by cold temperatures

Mammals detect temperature with specialized neurons in the peripheral nervous system. Four TRPV-class channels have been implicated in sensing heat, and one TRPM-class channel in sensing cold. The combined range of temperatures that activate these channels covers a majority of the relevant physiological spectrum sensed by most mammals, with a significant gap in the noxious cold range. Here, we describe the characterization of ANKTM1, a cold-activated channel with a lower activation temperature compared to the cold and menthol receptor, TRPM8. ANKTM1 is a distant family member of TRP channels with very little amino acid similarity to TRPM8. It is found in a subset of nociceptive sensory neurons where it is coexpressed with TRPV1/VR1 (the capsaicin/heat receptor) but not TRPM8. Consistent with the expression of ANKTM1, we identify noxious cold-sensitive sensory neurons that also respond to capsaicin but not to menthol.

Photochemical gating of heterologous ion channels: remote control over genetically designated populations of neurons

Heterologous proteins capable of transducing physical or chemical stimuli into electrical signals can be used to control the function of excitable cells in intact tissues or organisms. Restricted genetically to circumscribed populations of cellular targets, these selectively addressable sources of depolarizing current can supply distributed inputs to neural circuits, stimulate secretion, or regulate force and motility. In an initial demonstration of this principle, we have used elements of a G protein coupled signaling system, the phototransduction cascade of the fruit fly, to sensitize generalist vertebrate neurons to light [Zemelman, B. V., Lee, G. A., Ng, M. & Miesenböck, G. (2002) Neuron 33, 15-22]. We now describe the use of ectopically expressed ligand-gated ion channels as transducers of optical or pharmacological stimuli. When either the capsaicin receptor, TRPV1, the menthol receptor, TRPM8, or the ionotropic purinergic receptor P2X(2) was introduced into hippocampal neurons, the cells responded to pulsed applications of agonist with characteristic sequences of depolarization, spiking, and repolarization. Responses required cognate matches between receptor and agonist, peaked at firing frequencies of approximately 40 Hz, initiated and terminated rapidly, and did not attenuate. Precise dose-response relationships allowed current amplitudes and firing frequencies to be tuned by varying the concentration of ligand. Agonist could be administered either pharmacologically or, in the cases of TRPV1 and P2X(2), optically, through photorelease of the active compounds from the respective "caged" precursors, 4,5-dimethoxy-2-nitrobenzyl-capsaicin and P(3)-[1-(4,5-dimethoxy-2-nitrophenyl)ethyl]-ATP.

Social feeding in Caenorhabditis elegans is induced by neurons that detect aversive stimuli

Natural Caenorhabditis elegans isolates exhibit either social or solitary feeding on bacteria. We show here that social feeding is induced by nociceptive neurons that detect adverse or stressful conditions. Ablation of the nociceptive neurons ASH and ADL transforms social animals into solitary feeders. Social feeding is probably due to the sensation of noxious chemicals by ASH and ADL neurons; it requires the genes ocr-2 and osm-9, which encode TRP-related transduction channels, and odr-4 and odr-8, which are required to localize sensory chemoreceptors to cilia. Other sensory neurons may suppress social feeding, as social feeding in ocr-2 and odr-4 mutants is restored by mutations in osm-3, a gene required for the development of 26 ciliated sensory neurons. Our data suggest a model for regulation of social feeding by opposing sensory inputs: aversive inputs to nociceptive neurons promote social feeding, whereas antagonistic inputs from neurons that express osm-3 inhibit aggregation.