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

KIN-29 SIK regulates chemoreceptor gene expression via an MEF2 transcription factor and a class II HDAC

The expression of individual chemoreceptor (CR) genes in Caenorhabditis elegans is regulated by multiple environmental and developmental cues, possibly enabling C. elegans to modulate its sensory responses. We had previously shown that KIN-29, a member of the salt-inducible kinase family, acts in a subset of chemosensory neurons to regulate the expression of CR genes, body size and entry into the alternate dauer developmental stage. Here, we show that KIN-29 regulates these processes by phosphorylating the HDA-4 class II histone deacetylase (HDAC) and inhibiting the gene repression functions of HDA-4 and an MEF-2 MADS domain transcription factor. MEF-2 binds directly to the CR gene regulatory sequences, and is required only to repress but not activate CR gene expression. A calcineurin phosphatase antagonizes the KIN-29/MEF-2-regulated pathway to modulate levels of CR gene expression. Our results identify KIN-29 as a new regulator of MEF2/HDAC functions in the nervous system, reveal cell-specific mechanisms of action of this pathway in vivo and demonstrate remarkable complexity in the regulation of CR gene expression in C. elegans.

Serotonin targets the DAF-16/FOXO signaling pathway to modulate stress responses

Stress response is a fundamental form of behavioral and physiological plasticity. Here we describe how serotonin (5HT) governs stress behavior by regulating DAF-2 insulin/IGF-1 receptor signaling to the DAF-16/FOXO transcription factor at the nexus of development, metabolism, immunity, and stress responses in C. elegans. Serotonin-deficient tph-1 mutants, like daf-2 mutants, exhibit DAF-16 nuclear accumulation and constitutive physiological stress states. Exogenous 5HT and fluoxetine (Prozac) prevented DAF-16 nuclear accumulation in wild-type animals under stresses. Genetic analyses imply that DAF-2 is a downstream target of 5HT signaling and that distinct serotonergic neurons act through distinct 5HT receptors to influence distinct DAF-16-mediated stress responses. We suggest that modulation of FOXO by 5HT represents an ancient feature of stress physiology and that the C. elegans is a genetically tractable model that can be used to delineate the molecular mechanisms and drug actions linking 5HT, neuroendocrine signaling, immunity, and mitochondrial function.

Noncell- and cell-autonomous G-protein-signaling converges with Ca2+/mitogen-activated protein kinase signaling to regulate str-2 receptor gene expression in Caenorhabditis elegans

In the sensory system of C. elegans, the candidate odorant receptor gene str-2 is strongly expressed in one of the two AWC neurons and weakly in both ASI neurons. Asymmetric AWC expression results from suppression of str-2 expression by a Ca2+/MAPK signaling pathway in one of the AWC neurons early in development. Here we show that the same Ca2+/MAPK pathway promotes str-2 expression in the AWC and ASI neurons together with multiple cell-autonomous and noncell-autonomous G-protein-signaling pathways. In first-stage larvae and adult animals, signals mediated by the Galpha subunits ODR-3, GPA-2, GPA-5, and GPA-6 and a Ca2+/MAPK pathway involving the Ca2+ channel subunit UNC-36, the CaMKII UNC-43, and the MAPKK kinase NSY-1 induce strong str-2 expression. Cell-specific rescue experiments suggest that ODR-3 and the Ca2+/MAPK genes function in the AWC neurons, but that GPA-5 and GPA-6 function in the AWA and ADL neurons, respectively. In Dauer larvae, the same network of genes promotes strong str-2 expression in the ASI neurons, but ODR-3 functions in AWB and ASH and GPA-6 in AWB. Our results reveal a complex signaling network, encompassing signals from multiple cells, that controls the level of receptor gene expression at different developmental stages.

Light activation of channelrhodopsin-2 in excitable cells of Caenorhabditis elegans triggers rapid behavioral responses

For studying the function of specific neurons in their native circuitry, it is desired to precisely control their activity. This often requires dissection to allow accurate electrical stimulation or neurotransmitter application , and it is thus inherently difficult in live animals, especially in small model organisms. Here, we employed channelrhodopsin-2 (ChR2), a directly light-gated cation channel from the green alga Chlamydomonas reinhardtii, in excitable cells of the nematode Caenorhabditis elegans, to trigger specific behaviors, simply by illumination. Channelrhodopsins are 7-transmembrane-helix proteins that resemble the light-driven proton pump bacteriorhodopsin , and they also utilize the chromophore all-trans retinal, but to open an intrinsic cation pore. In muscle cells, light-activated ChR2 evoked strong, simultaneous contractions, which were reduced in the background of mutated L-type, voltage-gated Ca2+-channels (VGCCs) and ryanodine receptors (RyRs). Electrophysiological analysis demonstrated rapid inward currents that persisted as long as the illumination. When ChR2 was expressed in mechanosensory neurons, light evoked withdrawal behaviors that are normally elicited by mechanical stimulation. Furthermore, ChR2 enabled activity of these neurons in mutants lacking the MEC-4/MEC-10 mechanosensory ion channel . Thus, specific neurons or muscles expressing ChR2 can be quickly and reversibly activated by light in live and behaving, as well as dissected, animals.

A specific subset of transient receptor potential vanilloid-type channel subunits in Caenorhabditis elegans endocrine cells function as mixed heteromers to promote neurotransmitter release

Transient receptor potential (TRP) channel subunits form homotetramers that function in sensory transduction. Heteromeric channels also form, but their physiological subunit compositions and functions are largely unknown. We found a dominant-negative mutant of the C. elegans TRPV (vanilloid-type) subunit OCR-2 that apparently incorporates into and inactivates OCR-2 homomers as well as heteromers with the TRPV subunits OCR-1 and -4, resulting in a premature egg-laying defect. This defect is reproduced by knocking out all three OCR genes, but not by any single knockout. Thus a mixture of redundant heteromeric channels prevents premature egg laying. These channels, as well as the G-protein G alpha(o), function in neuroendocrine cells to promote release of neurotransmitters that block egg laying until eggs filling the uterus deform the neuroendocrine cells. The TRPV channel OSM-9, previously suggested to be an obligate heteromeric partner of OCR-2 in sensory neurons, is expressed in the neuroendocrine cells but has no detectable role in egg laying. Our results identify a specific set of heteromeric TRPV channels that redundantly regulate neuroendocrine function and show that a subunit combination that functions in sensory neurons is also present in neuroendocrine cells but has no detectable function in these cells.

Trp ion channels and temperature sensation

The abilities to sense environmental and internal temperatures are required for survival, both for maintenance of homeostasis and for avoidance of tissue-damaging noxious temperatures. Vertebrates can sense external physical stimuli via specialized classes of neurons in the peripheral nervous system that project to the skin. Temperature-sensitive neurons can be divided into two classes: innocuous thermosensors (warm or cool) and noxious thermonociceptors (hot or cold). ThermoTRPs, a subset of the transient receptor potential family of ion channels, which are expressed in sensory nerve endings and in skin, respond to distinct thermal thresholds. In this review, we examine the extent to which thermoTRPs are responsible for providing a molecular basis for thermal sensation.

General and cell-type specific mechanisms target TRPP2/PKD-2 to cilia

Ciliary localization of the transient receptor potential polycystin 2 channel (TRPP2/PKD-2) is evolutionarily conserved, but how TRPP2 is targeted to cilia is not known. In this study, we characterize the motility and localization of PKD-2, a TRPP2 homolog, in C. elegans sensory neurons. We demonstrate that GFP-tagged PKD-2 moves bidirectionally in the dendritic compartment. Furthermore, we show a requirement for different molecules in regulating the ciliary localization of PKD-2. PKD-2 is directed to moving dendritic particles by the UNC-101/adaptor protein 1 (AP-1) complex. When expressed in non-native neurons, PKD-2 remains in cell bodies and is not observed in dendrites or cilia, indicating that cell-type specific factors are required for directing PKD-2 to the dendrite. PKD-2 stabilization in cilia and cell bodies requires LOV-1, a functional partner and a TRPP1 homolog. In lov-1 mutants, PKD-2 is greatly reduced in cilia and forms abnormal aggregates in neuronal cell bodies. Intraflagellar transport (IFT) is not essential for PKD-2 dendritic motility or access to the cilium, but may regulate PKD-2 ciliary abundance. We propose that both general and cell-type-specific factors govern TRPP2/PKD-2 subcellular distribution by forming at least two steps involving somatodendritic and ciliary sorting decisions.

The primary cilium as the cell's antenna: signaling at a sensory organelle

Almost every vertebrate cell has a specialized cell surface projection called a primary cilium. Although these structures were first described more than a century ago, the full scope of their functions remains poorly understood. Here, we review emerging evidence that in addition to their well-established roles in sight, smell, and mechanosensation, primary cilia are key participants in intercellular signaling. This new appreciation of primary cilia as cellular antennae that sense a wide variety of signals could help explain why ciliary defects underlie such a wide range of human disorders, including retinal degeneration, polycystic kidney disease, Bardet-Biedl syndrome, and neural tube defects.

Expression of mammalian GPCRs in C. elegans generates novel behavioural responses to human ligands

Background

G-protein-coupled receptors (GPCRs) play a crucial role in many biological processes and represent a major class of drug targets. However, purification of GPCRs for biochemical study is difficult and current methods of studying receptor-ligand interactions involve in vitro systems. Caenorhabditis elegans is a soil-dwelling, bacteria-feeding nematode that uses GPCRs expressed in chemosensory neurons to detect bacteria and environmental compounds, making this an ideal system for studying in vivo GPCR-ligand interactions. We sought to test this by functionally expressing two medically important mammalian GPCRs, somatostatin receptor 2 (Sstr2) and chemokine receptor 5 (CCR5) in the gustatory neurons of C. elegans.

Results

Expression of Sstr2 and CCR5 in gustatory neurons allow C. elegans to specifically detect and respond to somatostatin and MIP-1α respectively in a robust avoidance assay. We demonstrate that mammalian heterologous GPCRs can signal via different endogenous G_α subunits in C. elegans, depending on which cells it is expressed in. Furthermore, pre-exposure of GPCR transgenic animals to its ligand leads to receptor desensitisation and behavioural adaptation to subsequent ligand exposure, providing further evidence of integration of the mammalian GPCRs into the C. elegans sensory signalling machinery. In structure-function studies using a panel of somatostatin-14 analogues, we identified key residues involved in the interaction of somatostatin-14 with Sstr2.

Conclusion

Our results illustrate a remarkable evolutionary plasticity in interactions between mammalian GPCRs and C. elegans signalling machinery, spanning 800 million years of evolution. This in vivo system, which imparts novel avoidance behaviour on C. elegans, thus provides a simple means of studying and screening interaction of GPCRs with extracellular agonists, antagonists and intracellular binding partners.

The molecular phylogeny of a nematode-specific clade of heterotrimeric G-protein alpha-subunit genes

In animal olfactory systems, odorant molecules are detected by olfactory receptors (ORs). ORs are part of the G-protein-coupled receptor (GPCR) superfamily. Heterotrimeric guanine nucleotide binding G-proteins (G-proteins) relay signals from GPCRs to intracellular effectors. G-proteins are comprised of three peptides. The G-protein alpha subunit confers functional specificity to G-proteins. Vertebrate and insect Galpha-subunit genes are divided into four subfamilies based on functional and sequence attributes. The nematode Caenorhabditis elegans contains 21 Galpha genes, 14 of which are exclusively expressed in sensory neurons. Most individual mammalian cells express multiple distinct GPCR gene products, however, individual mammalian and insect olfactory neurons express only one functional odorant OR. By contrast C. elegans expresses multiple ORs and multiple Galpha subunits within each olfactory neuron. Here we show that, in addition to having at least one member of each of the four mammalian Galpha gene classes, C. elegans and other nematodes also possess two lineage-specific Galpha gene expansions, homologues of which are not found in any other organisms examined. We hypothesize that these novel nematode-specific Galpha genes increase the functional complexity of individual chemosensory neurons, enabling them to integrate odor signals from the multiple distinct ORs expressed on their membranes. This neuronal gene expansion most likely occurred in nematodes to enable them to compensate for the small number of chemosensory cells and the limited emphasis on cephalization during nematode evolution.

TRP channels in C. elegans

The TRP (transient receptor potential) superfamily of cation channels is present in all eukaryotes, from yeast to mammals. Many TRP channels have been studied in the nematode Caenorhabditis elegans, revealing novel biological functions, regulatory modes, and mechanisms of localization. C. elegans TRPV channels function in olfaction, mechanosensation, osmosensation, and activity-dependent gene regulation. Their activity is regulated by G protein signaling and polyunsaturated fatty acids. C. elegans TRPPs related to human polycystic kidney disease genes are expressed in male-specific neurons. The KLP-6 kinesin directs TRPP channels to cilia, where they may interact with F0/F1 ATPases. A sperm-specific TRPC channel, TRP-3, is required for fertilization. Upon sperm activation, TRP-3 translocates from an intracellular compartment to the plasma membrane to allow store-operated Ca2+ entry. The TRPM channels GON-2 and GTL-2 regulate Mg2+ homeostasis and Mg2+ uptake by intestinal cells; GON-2 is also required for gonad development. The TRPML CUP-5 promotes normal lysosome biogenesis and prevents apoptosis. Dynamic, precise expression of TRP proteins generates a remarkable range of cellular functions.

Sensory innervation of the brain (primary interoceptor neurons of the brain and their asynaptic dendrites)

Published data and our own studies on the sensory innervation of the brain are summarized. Primary interoceptive sensory neurons were analyzed: brain neurons bearing cilia; supraependymal plexuses and intraependymal neurons in contact with the cerebrospinal fluid; Cajal-Retzius neurons in the boundary layer of the cerebral cortex; Dolgo-Saburov paravasal neurons in the brain and spinal cord; Lugaro cells in the cerebellum; and various synaptically NO-positive neurons in the cerebral cortex, whose asynaptic dendrites innervate the precapillary space. Consideration of the lack of pain sensitivity of the brain and the parenchymatous tissues of the internal organs, which contain local primary sensory neurons similar to intramural metasympathetic sensory neurons of Dogel type II, led to the hypothesis that brain and other intraorgan tissue receptors are involved in short "autonomic" reflex arcs controlling only local metabolism but not pain reactions.

Intraflagellar transport is required for the vectorial movement of TRPV channels in the ciliary membrane

The membranes of all eukaryotic motile (9 + 2) and immotile primary (9 + 0) cilia harbor channels and receptors involved in sensory transduction (reviewed by). These membrane proteins are transported from the cytoplasm onto the ciliary membrane by vesicles targeted for exocytosis at a point adjacent to the ciliary basal body. Here, we use time-lapse fluorescence microscopy to demonstrate that select GFP-tagged sensory receptors undergo rapid vectorial transport along the entire length of the cilia of Caenorhabditis elegans sensory neurons. Transient receptor potential vanilloid (TRPV) channels OSM-9 and OCR-2 move in ciliary membranes at rates comparable to the intraflagellar transport (IFT) machinery located between the membrane and the underlying axonemal microtubules. OSM-9 motility is disrupted in certain IFT mutant backgrounds. Surprisingly, motility of transient receptor potential polycystin (TRPP) channel PKD-2 (polycystic kidney disease-2), a mechano-receptor, was not detected. Our study demonstrates that IFT, previously shown to be necessary for transport of axonemal components, is also involved in the motility of TRPV membrane protein movement along cilia of C. elegans sensory cells.

Deciphering the neural and molecular mechanisms of C. elegans behavior

Because of its small and well-characterized nervous system and amenability to genetic manipulation, the nematode Caenorhabditis elegans offers the promise of understanding the mechanisms underlying a whole animal's behavior at the molecular and cellular levels. In fact, this goal was a primary motivation behind the development of C. elegans as an experimental organism 40 years ago. Yet it has proven surprisingly difficult to obtain a mechanistic understanding of how the C. elegans nervous system generates behavior, despite the existence of a 'wiring diagram' that contains a degree of information about neural connectivity unparalleled in any organism. This review describes three types of information--molecular data on cellular neurochemistry, temporal information about neural activity patterns, and behavioral data on the consequences of neural ablation and manipulation--that, along with genetic analysis, may ultimately lead to a complete functional map of the C. elegans nervous system.

Imaging taste responses in the fly brain reveals a functional map of taste category and behavior

The sense of taste allows animals to distinguish nutritious and toxic substances and elicits food acceptance or avoidance behaviors. In Drosophila, taste cells that contain the Gr5a receptor are necessary for acceptance behavior, and cells with the Gr66a receptor are necessary for avoidance. To determine the cellular substrates of taste behaviors, we monitored taste cell activity in vivo with the genetically encoded calcium indicator G-CaMP. These studies reveal that Gr5a cells selectively respond to sugars and Gr66a cells to bitter compounds. Flies are attracted to sugars and avoid bitter substances, suggesting that Gr5a cell activity is sufficient to mediate acceptance behavior and that Gr66a cell activation mediates avoidance. As a direct test of this hypothesis, we inducibly activated different taste neurons by expression of an exogenous ligand-gated ion channel and found that cellular activity is sufficient to drive taste behaviors. These studies demonstrate that taste cells are tuned by taste category and are hardwired to taste behaviors.

Evolutionary origins of the endocannabinoid system

Endocannabinoid system evolution was estimated by searching for functional orthologs in the genomes of twelve phylogenetically diverse organisms: Homo sapiens, Mus musculus, Takifugu rubripes, Ciona intestinalis, Caenorhabditis elegans, Drosophila melanogaster, Saccharomyces cerevisiae, Arabidopsis thaliana, Plasmodium falciparum, Tetrahymena thermophila, Archaeoglobus fulgidus, and Mycobacterium tuberculosis. Sequences similar to human endocannabinoid exon sequences were derived from filtered BLAST searches, and subjected to phylogenetic testing with ClustalX and tree building programs. Monophyletic clades that agreed with broader phylogenetic evidence (i.e., gene trees displaying topographical congruence with species trees) were considered orthologs. The capacity of orthologs to function as endocannabinoid proteins was predicted with pattern profilers (Pfam, Prosite, TMHMM, and pSORT), and by examining queried sequences for amino acid motifs known to serve critical roles in endocannabinoid protein function (obtained from a database of site-directed mutagenesis studies). This novel transfer of functional information onto gene trees enabled us to better predict the functional origins of the endocannabinoid system. Within this limited number of twelve organisms, the endocannabinoid genes exhibited heterogeneous evolutionary trajectories, with functional orthologs limited to mammals (TRPV1 and GPR55), or vertebrates (CB2 and DAGLbeta), or chordates (MAGL and COX2), or animals (DAGLalpha and CB1-like receptors), or opisthokonta (animals and fungi, NAPE-PLD), or eukaryotes (FAAH). Our methods identified fewer orthologs than did automated annotation systems, such as HomoloGene. Phylogenetic profiles, nonorthologous gene displacement, functional convergence, and coevolution are discussed.

Antagonistic sensory cues generate gustatory plasticity in Caenorhabditis elegans

Caenorhabditis elegans shows chemoattraction to 0.1-200 mM NaCl, avoidance of higher NaCl concentrations, and avoidance of otherwise attractive NaCl concentrations after prolonged exposure to NaCl (gustatory plasticity). Previous studies have shown that the ASE and ASH sensory neurons primarily mediate attraction and avoidance of NaCl, respectively. Here we show that balances between at least four sensory cell types, ASE, ASI, ASH, ADF and perhaps ADL, modulate the response to NaCl. Our results suggest that two NaCl-attraction signalling pathways exist, one of which uses Ca(2+)/cGMP signalling. In addition, we provide evidence that attraction to NaCl is antagonised by G-protein signalling in the ASH neurons, which is desensitised by the G-protein-coupled receptor kinase GRK-2. Finally, the response to NaCl is modulated by G-protein signalling in the ASI and ADF neurons, a second G-protein pathway in ASH and cGMP signalling in neurons exposed to the body fluid.

An N-terminal variant of Trpv1 channel is required for osmosensory transduction

Body fluid homeostasis requires the release of arginine-vasopressin (AVP, an antidiuretic hormone) from the neurohypophysis. This release is controlled by specific and highly sensitive 'osmoreceptors' in the hypothalamus. Indeed, AVP-releasing neurons in the supraoptic nucleus (SON) are directly osmosensitive, and this osmosensitivity is mediated by stretch-inhibited cation channels. However, the molecular nature of these channels remains unknown. Here we show that SON neurons express an N-terminal splice variant of the transient receptor potential vanilloid type-1 (Trpv1), also known as the capsaicin receptor, but not full-length Trpv1. Unlike their wild-type counterparts, SON neurons in Trpv1 knockout (Trpv1(-/-)) mice could not generate ruthenium red-sensitive increases in membrane conductance and depolarizing potentials in response to hyperosmotic stimulation. Moreover, Trpv1(-/-) mice showed a pronounced serum hyperosmolality under basal conditions and severely compromised AVP responses to osmotic stimulation in vivo. These results suggest that the Trpv1 gene may encode a central component of the osmoreceptor.

Emerging roles of TRPM6/TRPM7 channel kinase signal transduction complexes

Investigations into Drosophila mutants with impaired vision due to mutations in the transient receptor potential gene (trp) initiated a systematic search for TRP homologs in other species, finally leading to the discovery of a whole new family of plasma membrane cation channels involved in multiple physiological processes. Among the recently discovered TRP cation channels two homologous proteins, TRPM6 and TRPM7, display unique domain compositions and biophysical properties. These remarkable genes are vital for Mg(2+) homeostasis in vertebrates and, if disrupted, lead to cell death or human disease.

Structure-function analysis of TRPV channels

In recent years many new members of the family of TRP ion channels have been identified. These channels are classified into several subgroups and participate in many sensory and physiological functions. TRPV channels are important for the perception of pain, temperature sensing, osmotic regulation, and maintenance of calcium homeostasis, and much recent research concerns the identification of protein domains involved in mediating specific channel functions. Recent literature on TRPV channel subunit composition, protein domains required for subunit assembly, trafficking, and regulation will be reviewed and discussed.

The UNC-3 Olf/EBF protein represses alternate neuronal programs to specify chemosensory neuron identity

Neuronal identities are specified by the combinatorial functions of activators and repressors of gene expression. Members of the well-conserved Olf/EBF (O/E) transcription factor family have been shown to play important roles in neuronal and non-neuronal development and differentiation. O/E proteins are highly expressed in the olfactory epithelium, and O/E binding sites have been identified upstream of olfactory genes. However, the roles of O/E proteins in sensory neuron development are unclear. Here we show that the O/E protein UNC-3 is required for subtype specification of the ASI chemosensory neurons in Caenorhabditis elegans. UNC-3 promotes an ASI identity by directly repressing the expression of alternate neuronal programs and by activating expression of ASI-specific genes including the daf-7 TGF-beta gene. Our results indicate that UNC-3 is a critical component of the transcription factor code that integrates cell-intrinsic developmental programs with external signals to specify sensory neuronal identity and suggest models for O/E protein functions in other systems.

Neuronal substrates of complex behaviors in C. elegans

A current challenge in neuroscience is to bridge the gaps between genes, proteins, neurons, neural circuits, and behavior in a single animal model. The nematode Caenorhabditis elegans has unique features that facilitate this synthesis. Its nervous system includes exactly 302 neurons, and their pattern of synaptic connectivity is known. With only five olfactory neurons, C. elegans can dynamically respond to dozens of attractive and repellent odors. Thermosensory neurons enable the nematode to remember its cultivation temperature and to track narrow isotherms. Polymodal sensory neurons detect a wide range of nociceptive cues and signal robust escape responses. Pairing of sensory stimuli leads to long-lived changes in behavior consistent with associative learning. Worms exhibit social behaviors and complex ultradian rhythms driven by Ca(2+) oscillators with clock-like properties. Genetic analysis has identified gene products required for nervous system function and elucidated the molecular and neural bases of behaviors.

Optical imaging and control of genetically designated neurons in functioning circuits

Proteins with engineered sensitivities to light are infiltrating the biological mechanisms by which neurons generate and detect electrochemical signals. Encoded in DNA and active only in genetically specified target cells, these proteins provide selective optical interfaces for observing and controlling signaling by defined groups of neurons in functioning circuits, in vitro and in vivo. Light-emitting sensors of neuronal activity (reporting calcium increase, neurotransmitter release, or membrane depolarization) have begun to reveal how information is represented by neuronal assemblies, and how these representations are transformed during the computations that inform behavior. Light-driven actuators control the electrical activities of central neurons in freely moving animals and establish causal connections between the activation of specific neurons and the expression of particular behaviors. Anchored within mathematical systems and control theory, the combination of finely resolved optical field sensing and finely resolved optical field actuation will open new dimensions for the analysis of the connectivity, dynamics, and plasticity of neuronal circuits, and perhaps even for replacing lost--or designing novel--functionalities.

Polymodal sensory function of the Caenorhabditis elegans OCR-2 channel arises from distinct intrinsic determinants within the protein and is selectively conserved in mammalian TRPV proteins

Caenorhabditis elegans OCR-2 (OSM-9 and capsaicin receptor-related) is a TRPV (vanilloid subfamily of transient receptor potential channel) protein that regulates serotonin (5-HT) biosynthesis in chemosensory neurons and also mediates olfactory and osmotic sensation. Here, we identify the molecular basis for the polymodal function of OCR-2 in its native cellular environment. We show that OCR-2 function in 5-HT production and osmotic sensing is governed by its N-terminal region upstream of the ankyrin repeats domain, but the diacetyl sensitivity is mediated by independent mechanisms. The ocr-2(yz5) mutation results in a glycine-to-glutamate substitution (G36E) within the N-terminal region. The G36E substitution causes dramatic downregulation of 5-HT synthesis in the ADF neurons, eliminates osmosensation mediated by the ASH neurons, but does not affect the response to the odorant diacetyl mediated by the AWA neurons. Conversely, wild-type sequence of the N-terminal segment confers osmotic sensitivity and upregulation of 5-HT production to a normally insensitive C. elegans homolog, OCR-4, but this chimeric channel does not respond to diacetyl stimuli. Furthermore, expression of either the mouse or human TRPV2 gene under the ocr-2 promoter can substantially restore 5-HT biosynthesis in ocr-2-null mutants but cannot improve the deficits in osmotic or olfactory sensation, suggesting that TRPV2 can substitute for the role of OCR-2 only in serotonergic neurons. Thus, different sensory functions of OCR-2 arise from separable intrinsic determinants, and specific functional properties of TRPV channel proteins may be selectively conserved across phyla.

A possible unifying principle for mechanosensation

Of Aristotle's five senses, we know that sight, smell and much of taste are initiated by ligands binding to G-protein-coupled receptors; however, the mechanical sensations of touch and hearing remain without a clear understanding of their molecular basis. Recently, the relevant force-transducing molecules--the mechanosensitive ion channels--have been identified. Such channel proteins purified from bacteria sense forces from the lipid bilayer in the absence of other proteins. Recent evidence has shown that lipids are also intimately involved in opening and closing the mechanosensitive channels of fungal, plant and animal species.

Hearing in Drosophila: development of Johnston's organ and emerging parallels to vertebrate ear development

In this review, I describe recent progress toward understanding the developmental genetics governing formation of the Drosophila auditory apparatus. The Drosophila auditory organ, Johnston's organ, is housed in the antenna. Intriguingly, key genes needed for specification or function of auditory cell types in the Drosophila antenna also are required for normal development or function of the vertebrate ear. These genes include distal-less, spalt and spalt-related, atonal, crinkled, nanchung and inactive, and prestin, and their vertebrate counterparts Dlx, spalt-like (sall), atonal homolog (ath), myosin VIIA, TRPV, and prestin, respectively. In addition, Drosophila auditory neurons recently were shown to serve actuating as well as transducing roles, much like their hair cell counterparts of the vertebrate cochlea. The emerging genetic and physiologic parallels have come as something of a surprise, because conventional wisdom holds that vertebrate and invertebrate hearing organs have separate evolutionary origins. The new findings raise the possibility that auditory organs are more ancient than previously thought and indicate that Drosophila is likely to be a powerful model system in which to gain insights regarding the etiologies of human deafness disorders.

Feeling the pressure in mammalian somatosensation

Mechanoreceptor cells of the somatosensory system initiate the perception of touch and pain. Molecules required for mechanosensation have been identified from invertebrate neurons, and recent functional studies indicate that ion channels of the transient receptor potential and degenerin/epithelial Na+ channel families are likely to be transduction channels. The expression of related channels in mammalian somatosensory neurons has fueled the notion that these channels mediate mechanotransduction in vertebrates; however, genetic disruption and heterologous expression have not yet revealed a direct role for any of these candidates in somatosensory mechanotransduction. Thus, new systems are needed to define the function of these ion channels in somatosensation and to pinpoint molecules or signaling pathways that underlie mechanotransduction in vertebrates.

The contribution of TRPM8 channels to cold sensing in mammalian neurones

Different classes of ion channels have been implicated in sensing cold temperatures at mammalian thermoreceptor nerve endings. A major candidate is TRPM8, a non-selective cation channel of the transient receptor potential family, activated by menthol and low temperatures. We investigated the role of TRPM8 in cold sensing during transient expression in mouse cultured hippocampal neurones, a tissue that lacks endogenous expression of thermosensitive TRPs. In the absence of synaptic input, control hippocampal neurones were not excited by cooling. In contrast, all TRPM8-transfected hippocampal neurones were excited by cooling and menthol. However, in comparison to cold-sensitive trigeminal sensory neurones, hippocampal neurones exhibited much lower threshold temperatures, requiring temperatures below 27 degrees C to fire action potentials. These results directly demonstrate that expression of TRPM8 in mammalian neurones induces cold sensing, albeit at lower temperatures than native TRPM8-expressing neurones, suggesting the presence of additional modulatory mechanisms in the cold response of sensory neurones.

Remote control of behavior through genetically targeted photostimulation of neurons

Optically gated ion channels were expressed in circumscribed groups of neurons in the Drosophila CNS so that broad illumination of flies evoked action potentials only in genetically designated target cells. Flies harboring the "phototriggers" in different sets of neurons responded to laser light with behaviors specific to the sites of phototrigger expression. Photostimulation of neurons in the giant fiber system elicited the characteristic escape behaviors of jumping, wing beating, and flight; photostimulation of dopaminergic neurons caused changes in locomotor activity and locomotor patterns. These responses reflected the direct optical activation of central neuronal targets rather than confounding visual input, as they persisted unabated in carriers of a mutation that eliminates phototransduction. Encodable phototriggers provide noninvasive control interfaces for studying the connectivity and dynamics of neural circuits, for assigning behavioral content to neurons and their activity patterns, and, potentially, for restoring information corrupted by injury or disease.

Drosophila TRP channels

The transient receptor potential (TRP) superfamily comprises a large group of related cation channels that display surprising diversity in the specific modes of activation and cation selectivities. However, a unifying theme is that many TRP channels play important roles in sensory physiology. The superfamily includes 28 mammalian members, which are subdivided into multiple subfamilies. Each of these subfamilies is represented by at least one of the 13 members in Drosophila, suggesting common evolutionary relationships. In recent years it has become clear that TRP channels in flies and mammals participate in similar sensory modalities. These include, but are not limited to, hearing, thermosensation, and certain specialized types of vision. With the recent flurry of new studies, 9 out of the 13 TRPs have been addressed in various contexts. As a result, the repertoire of biological roles attributed to Drosophila TRPs has increased considerably and is likely to lead to many additional surprises over the next few years.

TRP channels in mechanosensation

Channels of the TRP superfamily have sensory roles in a wide variety of receptor cells, especially in mechanosensation. In some cases, the channels appear to be directly activated by mechanical force; in others, they appear to be downstream of a messenger pathway initiated by force on a non-channel sensor. A remaining challenge for most of these mechanosensory TRPs is to clarify the specific mechanism of activation.

Diverse regulation of sensory signaling by C. elegans nPKC-epsilon/eta TTX-4

Molecular and pharmacological studies in vitro suggest that protein kinase C (PKC) family members play important roles in intracellular signal transduction. Nevertheless, the in vivo roles of PKC are poorly understood. We show here that nPKC-epsilon/eta TTX-4 in the nematode Caenorhabditis elegans is required for the regulation of signal transduction in various sensory neurons for temperature, odor, taste, and high osmolality. Interestingly, the requirement for TTX-4 differs in different sensory neurons. In AFD thermosensory neurons, gain or loss of TTX-4 function inactivates or hyperactivates the neural activity, respectively, suggesting negative regulation of temperature sensation by TTX-4. In contrast, TTX-4 positively regulates the signal sensation of ASH nociceptive neurons. Moreover, in AWA and AWC olfactory neurons, TTX-4 plays a partially redundant role with another nPKC, TPA-1, to regulate olfactory signaling. These results suggest that C. elegans nPKCs regulate different sensory signaling in various sensory neurons. Thus, C. elegans provides an ideal model to reveal genetically novel components of nPKC-mediated molecular pathways in sensory signaling.

The mechanosensitive nature of TRPV channels

Transient receptor potential vanilloid (TRPV) channels are widely expressed in both sensory and nonsensory cells. Whereas the channels display a broad diversity to activation by chemical and physical stimuli, activation by mechanical stimuli is common to many members of this group in both lower and higher organisms. Genetic screening in Caenorhabditis elegans has demonstrated an essential role for two TRPV channels in sensory neurons. OSM-9 and OCR-2, for example, are essential for both osmosensory and mechanosensory (nose-touch) behaviors. Likewise, two Drosophila TRPV channels, NAN and IAV, have been shown to be critical for hearing by the mechanosensitive chordotonal organs located in the fly's antennae. The mechanosensitive nature of the channels appears to be conserved in higher organisms for some TRPV channels. Two vertebrate channels, TRPV2 and TRPV4, are sensitive to hypotonic cell swelling, shear stress/fluid flow (TRPV4), and membrane stretch (TRPV2). In the osmosensing neurons of the hypothalamus (circumventricular organs), TRPV4 appears to function as an osmoreceptor, or part of an osmoreceptor complex, in control of vasopressin release, whereas in inner ear hair cells and vascular baroreceptors a mechanosensory role is suggestive, but not demonstrated. Finally, in many nonsensory cells expressing TRPV4, such as vascular endothelial cells and renal tubular epithelial cells, the channel exhibits well-developed local mechanosensory transduction processes where both cell swelling and shear stress/fluid flow lead to channel activation. Hence, many TRPV channels, or combinations of TRPV channels, display a mechanosensitive nature that underlies multiple mechanosensitive processes from worms to mammals.

Invertebrate TRP proteins as functional models for mammalian channels

Transient receptor potential (TRP) channels constitute a large and diverse family of channel proteins that are expressed in many tissues and cell types in both vertebrates and invertebrates. While the biophysical features of many of the mammalian TRP channels have been described, relatively little is known about their biological roles. Invertebrate TRPs offer valuable genetic handles for characterizing the functions of these cation channels in vivo. Importantly, studies in model organisms can help to identify fundamental mechanisms involved in normal cellular functions and human disease. In this review, we give an overview of the different TRP channels known in the two most utilized invertebrate models, the nematode Caenorhabditis elegans and the fruit-fly Drosophila melanogaster, and discuss briefly the heuristic impact of these invertebrate channels with respect to TRP function in mammals.

Dissecting neural circuitry by combining genetics and pharmacology

In systems neuroscience, advances often come from lesioning and reversible inhibition of brain regions. Dissecting the circuitry of regions involves conceptually the same approach - stop a class of cell from firing action potentials, or make the cells fire more, then deduce how these components influence the performance of the circuit and animal behaviour. To perform such cell-type-specific and reversible fine-scale analysis of circuitry, and to do so on the fast signalling timescale of the brain (milliseconds to seconds), is challenging in mammals. Ingenious and diverse methods are being developed towards this goal. These new tools will encourage further synergy between molecular biologists, systems neuroscientists and electrophysiologists.

Specific polyunsaturated fatty acids drive TRPV-dependent sensory signaling in vivo

A variety of lipid and lipid-derived molecules can modulate TRP cation channel activity, but the identity of the lipids that affect TRP channel function in vivo is unknown. Here, we use genetic and behavioral analysis in the nematode C. elegans to implicate a subset of 20-carbon polyunsaturated fatty acids (PUFAs) in TRPV channel-dependent olfactory and nociceptive behaviors. Olfactory and nociceptive TRPV signaling are sustained by overlapping but nonidentical sets of 20-carbon PUFAs including eicosapentaenoic acid (EPA) and arachidonic acid (AA). PUFAs act upstream of TRPV family channels in sensory transduction. Short-term dietary supplementation with PUFAs can rescue PUFA biosynthetic mutants, and exogenous PUFAs elicit rapid TRPV-dependent calcium transients in sensory neurons, bypassing the normal requirement for PUFA synthesis. These results suggest that a subset of PUFAs with omega-3 and omega-6 acyl groups act as endogenous modulators of TRPV signal transduction.

A network of stimulatory and inhibitory Galpha-subunits regulates olfaction in Caenorhabditis elegans

The two pairs of sensory neurons of C. elegans, AWA and AWC, that mediate odorant attraction, express six Galpha-subunits, suggesting that olfaction is regulated by a complex signaling network. Here, we describe the cellular localization and functions of the six olfactory Galpha-subunits: GPA-2, GPA-3, GPA-5, GPA-6, GPA-13, and ODR-3. All except GPA-6 localize to sensory cilia, suggesting a direct role in sensory transduction. GPA-2, GPA-3, GPA-5, and GPA-6 are also present in cell bodies and axons and GPA-5 specifically localizes to synaptic sites. Analysis of animals with single- to sixfold loss-of-function mutations shows that olfaction involves a balance between multiple stimulatory and inhibitory signals. ODR-3 constitutes the main stimulatory signal and is sufficient for the detection of odorants. GPA-3 forms a second stimulatory signal in the AWA and AWC neurons, also sufficient for odorant detection. In AWA, signaling is suppressed by GPA-5. In AWC, GPA-2 and GPA-13 negatively and positively regulate signaling, respectively. Finally, we show that only ODR-3 plays a role in cilia morphogenesis. Defects in this process are, however, independent of olfactory behavior. Our findings reveal the existence of a complex signaling network that controls odorant detection by C. elegans.

The TRP superfamily of cation channels

The transient receptor potential (TRP) protein superfamily consists of a diverse group of cation channels that bear structural similarities to Drosophila TRP. TRP channels play important roles in nonexcitable cells; however, an emerging theme is that many TRP-related proteins are expressed predominantly in the nervous system and function in sensory physiology. The TRP superfamily is divided into seven subfamilies, the first of which is composed of the "classical" TRPs" (TRPC subfamily). Some TRPCs may be store-operated channels, whereas others appear to be activated by production of diacylglycerol or regulated through an exocytotic mechanism. Many members of a second subfamily (TRPV) function in sensory physiology and respond to heat, changes in osmolarity, odorants, and mechanical stimuli. Two members of the TRPM family function in sensory perception and three TRPM proteins are chanzymes, which contain C-terminal enzyme domains. The fourth and fifth subfamilies, TRPN and TRPA, include proteins with many ankyrin repeats. TRPN proteins function in mechanotransduction, whereas TRPA1 is activated by noxious cold and is also required for the auditory response. In addition to these five closely related TRP subfamilies, which comprise the Group 1 TRPs, members of the two Group 2 TRP subfamilies, TRPP and TRPML, are distantly related to the group 1 TRPs. Mutations in the founding members of these latter subfamilies are responsible for human diseases. Each of the TRP subfamilies are represented by members in worms and flies, providing the potential for using genetic approaches to characterize the normal functions and activation mechanisms of these channels.

Heterologously expressed fungal transient receptor potential channels retain mechanosensitivity in vitro and osmotic response in vivo

The budding yeast Saccharomyces cerevisiae has a mechanosensitive channel, TrpY1, a member of the Trp superfamily of channels associated with various sensations. Upon a hyperosmotic shift, a yeast cell releases Ca(2+) from the vacuole to the cytoplasm through this channel. The TRPY1 gene has orthologs in other fungal genomes, including TRPY2 of Kluyveromyces lactis and TRPY3 of Candida albicans. We subcloned TRPY2 and TRPY3 and expressed them in the vacuole of S. cerevisiae deleted of TRPY1. The osmotically induced Ca(2+) transient was restored in vivo as reported by transgenic aequorin. Patch-clamp examination showed that the TrpY2 or the TrpY3 channel was similar to TrpY1 in unitary conductance, rectification properties, Ca(2+) sensitivity, and mechanosensitivity. The retention of mechanosensitivity of transient receptor potential channels in a foreign setting, shown here both in vitro and in vivo, implies that these mechanosensitive channels, like voltage-gated or ligand-gated channels, do not discriminate their settings. We discuss various mechanisms, including the possibility that stress from the lipid bilayer by osmotic force transmits forces to the transmembrane domains of these channels.

Caenorhabditis elegans OSR-1 regulates behavioral and physiological responses to hyperosmotic environments

The molecular mechanisms that enable multicellular organisms to sense and modulate their responses to hyperosmotic environments are poorly understood. Here, we employ Caenorhabditis elegans to characterize the response of a multicellular organism to osmotic stress and establish a genetic screen to isolate mutants that are osmotic stress resistant (OSR). In this study, we describe the cloning of a novel gene, osr-1, and demonstrate that it regulates osmosensation, adaptation, and survival in hyperosmotic environments. Whereas wild-type animals exposed to hyperosmotic conditions rapidly lose body volume, motility, and viability, osr-1(rm1) mutant animals maintain normal body volume, motility, and viability even upon chronic exposures to high osmolarity environments. In addition, osr-1(rm1) animals are specifically resistant to osmotic stress and are distinct from previously characterized osmotic avoidance defective (OSM) and general stress resistance age-1(hx546) mutants. OSR-1 is expressed in the hypodermis and intestine, and expression of OSR-1 in hypodermal cells rescues the osr-1(rm1) phenotypes. Genetic epistasis analysis indicates that OSR-1 regulates survival under osmotic stress via CaMKII and a conserved p38 MAP kinase signaling cascade and regulates osmotic avoidance and resistance to acute dehydration likely by distinct mechanisms. We suggest that OSR-1 plays a central role in integrating stress detection and adaptation responses by invoking multiple signaling pathways to promote survival under hyperosmotic environments.

Two interdependent TRPV channel subunits, inactive and Nanchung, mediate hearing in Drosophila

Hearing in Drosophila depends on the transduction of antennal vibration into receptor potentials by ciliated sensory neurons in Johnston's organ, the antennal chordotonal organ. We previously found that a Drosophila protein in the vanilloid receptor subfamily (TRPV) channel subunit, Nanchung (NAN), is localized to the chordotonal cilia and required to generate sound-evoked potentials (Kim et al., 2003). Here, we show that the only other Drosophila TRPV protein is mutated in the behavioral mutant inactive (iav). The IAV protein forms a hypotonically activated channel when expressed in cultured cells; in flies, it is specifically expressed in the chordotonal neurons, localized to their cilia and required for hearing. IAV and NAN are each undetectable in cilia of mutants lacking the other protein, indicating that they both contribute to a heteromultimeric transduction channel in vivo. A functional green fluorescence protein-IAV fusion protein shows that the channel is restricted to the proximal cilium, constraining models for channel activation.

In vivo imaging of C. elegans ASH neurons: cellular response and adaptation to chemical repellents

ASH sensory neurons are required in Caenorhabditis elegans for a wide range of avoidance behaviors in response to chemical repellents, high osmotic solutions and nose touch. The ASH neurons are therefore hypothesized to be polymodal nociceptive neurons. To understand the nature of polymodal sensory response and adaptation at the cellular level, we expressed the calcium indicator protein cameleon in ASH and analyzed intracellular Ca(2+) responses following stimulation with chemical repellents, osmotic shock and nose touch. We found that a variety of noxious stimuli evoked strong responses in ASH including quinine, denatonium, detergents, heavy metals, both hyper- and hypo-osmotic shock and nose touch. We observed that repeated chemical stimulation led to a reversible reduction in the magnitude of the sensory response, indicating that adaptation occurs within the ASH sensory neuron. A key component of ASH adaptation is GPC-1, a G-protein gamma-subunit expressed specifically in chemosensory neurons. We hypothesize that G-protein gamma-subunit heterogeneity provides a mechanism for repellent-specific adaptation, which could facilitate discrimination of a variety of repellents by these polymodal sensory neurons.

The C. elegans thermosensory neuron AFD responds to warming

The mechanism of temperature sensation is far less understood than the sensory response to other environmental stimuli such as light, odor, and taste. Thermotaxis behavior in C. elegans requires the ability to discriminate temperature differences as small as approximately 0.05 degrees C and to memorize the previously cultivated temperature. The AFD neuron is the only major thermosensory neuron required for the thermotaxis behavior. Genetic analyses have revealed several signal transduction molecules that are required for the sensation and/or memory of temperature information in the AFD neuron, but its physiological properties, such as its ability to sense absolute temperature or temperature change, have been unclear. We show here that the AFD neuron responds to warming. Calcium concentration in the cell body of AFD neuron is increased transiently in response to warming, but not to absolute temperature or to cooling. The transient response requires the activity of the TAX-4 cGMP-gated cation channel, which plays an essential role in the function of the AFD neuron. Interestingly, the AFD neuron further responds to step-like warming above a threshold that is set by temperature memory. We suggest that C. elegans provides an ideal model to genetically and physiologically reveal the molecular mechanism for sensation and memory of temperature information.

Genetic Models of Mechanotransduction: The NematodeCaenorhabditis elegans

Mechanotransduction, the conversion of a mechanical stimulus into a biological response, constitutes the basis for a plethora of fundamental biological processes such as the senses of touch, balance, and hearing and contributes critically to development and homeostasis in all organisms. Despite this profound importance in biology, we know remarkably little about how mechanical input forces delivered to a cell are interpreted to an extensive repertoire of output physiological responses. Recent, elegant genetic and electrophysiological studies have shown that specialized macromolecular complexes, encompassing mechanically gated ion channels, play a central role in the transformation of mechanical forces into a cellular signal, which takes place in mechanosensory organs of diverse organisms. These complexes are highly efficient sensors, closely entangled with their surrounding environment. Such association appears essential for proper channel gating and provides proximity of the mechanosensory apparatus to the source of triggering mechanical energy. Genetic and molecular evidence collected in model organisms such as the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse highlight two distinct classes of mechanically gated ion channels: the degenerin (DEG)/epithelial Na+channel (ENaC) family and the transient receptor potential (TRP) family of ion channels. In addition to the core channel proteins, several other potentially interacting molecules have in some cases been identified, which are likely parts of the mechanotransducing apparatus. Based on cumulative data, a model of the sensory mechanotransducer has emerged that encompasses our current understanding of the process and fulfills the structural requirements dictated by its dedicated function. It remains to be seen how general this model is and whether it will withstand the impiteous test of time.

TRP ion channels in the nervous system

The transient receptor potential (TRP) superfamily comprises a group of non-selective cation channels that sense and respond to changes in their local environments. TRP channels are found in many eukaryotes, from yeast to mammals. They are a diverse group of proteins organized into six families: classical (TRPC), vanilloid (TRPV), melastatin (TRPM), muclopins (TRPML), polycystin (TRPP), and ANKTM1 (TRPA). In the peripheral nervous system, stimuli including temperature, pressure, inflammatory agents, and receptor activation effect TRP-mediated responses. In the central nervous system, TRPs participate in neurite outgrowth, receptor signalling and excitotoxic cell death resulting from anoxia. TRP channels are emerging as essential cellular switches that allow animals to respond to their environments.

Genetic methods for illuminating the function of neural circuits

Guided by the notion that biology itself offers some of the most incisive tools for studying biological systems, neurophysiologists rely increasingly on cell biological mechanisms and materials encoded in DNA to visualize and control the activity of neurons in functional circuits. Optical reporter proteins can broadcast the operational states of genetically designated cells and synapses; remote-controlled effectors can suppress or induce electrical activity. Many challenges, however, remain. These include the development of novel gene expression systems that target reporters and effectors to functionally relevant neuronal ensembles, the capacity to monitor and manipulate multiple populations of neurons in parallel, the ability to observe and elicit precisely timed action potentials, and the power to communicate with genetically designated target neurons through electromagnetic signals other than light.