RESUMEN
Nicotinic acetylcholine receptors (nAChRs) are widely expressed in or on various cell types and have diverse functions. In immune cells nAChRs regulate proliferation, differentiation and cytokine release. Specifically, activation of the α7 nAChR reduces inflammation as part of the cholinergic anti-inflammatory pathway. Here we review numerous effects of α7 nAChR activation on immune cell function and differentiation. Further, we also describe evidence implicating this receptor and its chaperone RIC-3 in diseases of the central nervous system and in neuroinflammation, focusing on multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). Deregulated neuroinflammation due to dysfunction of α7 nAChR provides one explanation for involvement of this receptor and of RIC-3 in neurodegenerative diseases. In this review, we also provide evidence implicating α7 nAChRs and RIC-3 in neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD) involving neuroinflammation. Besides, we will describe the therapeutic implications of activating the cholinergic anti-inflammatory pathway for diseases involving neuroinflammation.
Asunto(s)
Péptidos y Proteínas de Señalización Intracelular/metabolismo , Enfermedades del Sistema Nervioso/metabolismo , Neuroinmunomodulación/fisiología , Receptor Nicotínico de Acetilcolina alfa 7/metabolismo , Animales , Humanos , Inflamación/metabolismo , Chaperonas Moleculares/metabolismo , Neuroinmunomodulación/inmunología , Transducción de SeñalRESUMEN
BACKGROUND: The α7 nicotinic acetylcholine receptor (α7 nAChR) negatively regulates the synthesis and release of pro-inflammatory cytokines by immune cells. Our previous studies showed that in encephalitogenic T cells, α7 nAChR expression is upregulated and that activation of the cholinergic system can attenuate experimental autoimmune encephalomyelitis (EAE). GAT107 is an allosteric agonist and positive allosteric modulator (ago-PAM) of α7 nAChR that can produce persistent activation of this receptor. Therefore, in the present study, we investigated the effect of GAT107 on neuroinflammation in EAE, the animal model used for the study of multiple sclerosis (MS) via α7 nAChR, and the inflammatory pathways involved. METHODS: EAE was induced by administration of myelin oligodendrocyte glycoprotein (MOG35-55) in C57BL/6 mice. EAE mice were treated with the ago-PAM GAT107 or a placebo for 9 days, starting from the day of EAE induction. Clinical assessment and immunological evaluation of immune cells and cytokine production was performed. RESULTS: Following activation of the α7 nAChR by GAT107 during EAE, disease severity was significantly reduced by 70% and was correlated with a reduction in the extent of neuroinflammation in the CNS. The treatment reduced encephalitogenic T cell proliferation and the production of pro-inflammatory cytokines, as well as increased the production of the anti-inflammatory cytokine IL-10. Furthermore, the expression of immune cell markers was altered by GAT107 treatment, which induced a significant reduction in macrophages, dendritic cells, and B cells, as well as a reduction in anti-MOG35-55 antibodies. Additionally, GAT107 was found to directly activate α7 nAChR in murine macrophage RAW264.7 cells and in human PBMCs derived from MS patients and healthy donors. CONCLUSIONS: Our results show that GAT107 can be a useful molecule for harnessing the cholinergic anti-inflammatory pathway for long-lasting and wide-ranging modulation and downregulation of neuroinflammation in EAE.
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Encefalomielitis Autoinmune Experimental/tratamiento farmacológico , Quinolinas/farmacología , Quinolinas/uso terapéutico , Sulfonamidas/farmacología , Sulfonamidas/uso terapéutico , Receptor Nicotínico de Acetilcolina alfa 7 , Animales , Técnicas de Cultivo de Célula , Citocinas/metabolismo , Modelos Animales de Enfermedad , Encefalomielitis Autoinmune Experimental/inmunología , Encefalomielitis Autoinmune Experimental/metabolismo , Femenino , Humanos , Inflamación/tratamiento farmacológico , Inflamación/inmunología , Inflamación/metabolismo , Ratones , Ratones Endogámicos C57BL , Esclerosis Múltiple , Quinolinas/química , Médula Espinal/efectos de los fármacos , Médula Espinal/inmunología , Médula Espinal/patología , Sulfonamidas/química , Receptor Nicotínico de Acetilcolina alfa 7/agonistas , Receptor Nicotínico de Acetilcolina alfa 7/efectos de los fármacos , Receptor Nicotínico de Acetilcolina alfa 7/inmunología , Receptor Nicotínico de Acetilcolina alfa 7/metabolismoRESUMEN
Tobacco smoking is a risk factor for several human diseases. Conversely, smoking also reduces the prevalence of Parkinson's disease, whose hallmark is degeneration of substantia nigra dopaminergic neurons (DNs). We use C. elegans as a model to investigate whether tobacco-derived nicotine activates nicotinic acetylcholine receptors (nAChRs) to selectively protect DNs. Using this model, we demonstrate conserved functions of DN-expressed nAChRs. We find that DOP-2, a D3-receptor homolog; MCU-1, a mitochondrial calcium uniporter; PINK-1 (PTEN-induced kinase 1); and PDR-1 (Parkin) are required for nicotine-mediated protection of DNs. Together, our results support involvement of a calcium-modulated, mitochondrial stress-activated PINK1/Parkin-dependent pathway in nicotine-induced neuroprotection. This suggests that nicotine-selective protection of substantia nigra DNs is due to the confluence of two factors: first, their unique vulnerability to mitochondrial stress, which is mitigated by increased mitochondrial quality control due to PINK1 activation, and second, their specific expression of D3-receptors.
RESUMEN
Acetylcholine is an abundant neurotransmitter in all animals. Effects of acetylcholine are excitatory, inhibitory, or modulatory depending on the receptor and cell type. Research using the nematode C. elegans has made ground-breaking contributions to the mechanistic understanding of cholinergic transmission. Powerful genetic screens for behavioral mutants or for responses to pharmacological reagents identified the core cellular machinery for synaptic transmission. Pharmacological reagents that perturb acetylcholine-mediated processes led to the discovery and also uncovered the composition and regulators of acetylcholine-activated channels and receptors. From a combination of electrophysiological and molecular cellular studies, we have gained a profound understanding of cholinergic signaling at the levels of synapses, neural circuits, and animal behaviors. This review will begin with a historical overview, then cover in-depth current knowledge on acetylcholine-activated ionotropic receptors, mechanisms regulating their functional expression and their functions in regulating locomotion.
Asunto(s)
Acetilcolina/metabolismo , Proteínas de Caenorhabditis elegans/metabolismo , Activación del Canal Iónico/fisiología , Locomoción/fisiología , Receptores Colinérgicos/metabolismo , Transmisión Sináptica/fisiología , Acetilcolina/genética , Acetilcolina/farmacología , Animales , Caenorhabditis elegans , Proteínas de Caenorhabditis elegans/genética , Proteínas de Caenorhabditis elegans/fisiología , Colinérgicos/metabolismo , Colinérgicos/farmacología , Humanos , Activación del Canal Iónico/efectos de los fármacos , Contracción Muscular/efectos de los fármacos , Contracción Muscular/fisiología , Receptores Colinérgicos/genética , Transmisión Sináptica/efectos de los fármacosRESUMEN
Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels having many functions including inflammation control, as part of the cholinergic anti-inflammatory pathway. Genome wide association studies implicated RIC3, a chaperone of nAChRs, in multiple sclerosis (MS), a neuroinflammatory disease. To understand the involvement of RIC3 in inflammatory diseases we examined its expression, regulation, and function in activated immune cells. Our results show that immune activation leads to dynamic changes in RIC3 expression, in a mouse model of MS and in human lymphocytes and macrophages. We also show similarities in the expression dynamics of RIC3 and CHRNA7, encoding for the α7 nAChR subunit. Homomeric α7 nAChRs were shown to mediate the anti-inflammatory effects of cholinergic agonists. Thus, similarity in expression dynamics between RIC3 and CHRNA7 is suggestive of functional concordance. Indeed, siRNA mediated silencing of RIC3 in a mouse macrophage cell line eliminates the anti-inflammatory effects of cholinergic agonists. Furthermore, we show increased average expression of RIC3 and CHRNA7 in lymphocytes from MS patients, and a strong correlation between expression levels of these two genes in MS patients but not in healthy donors. Together, our results are consistent with a role for RIC3 and for the mechanisms regulating its expression in inflammatory processes and in neuroinflammatory diseases.
Asunto(s)
Encefalomielitis Autoinmune Experimental/metabolismo , Péptidos y Proteínas de Señalización Intracelular/metabolismo , Linfocitos/inmunología , Macrófagos/inmunología , Esclerosis Múltiple/metabolismo , Inflamación Neurogénica/metabolismo , Receptor Nicotínico de Acetilcolina alfa 7/metabolismo , Animales , Antiinflamatorios , Células Cultivadas , Colinérgicos , Modelos Animales de Enfermedad , Femenino , Regulación de la Expresión Génica , Humanos , Ratones , Ratones Endogámicos C57BL , Transducción de SeñalRESUMEN
C. elegans PVD neurons are conserved for morphology, function and molecular determinants with mammalian polymodal nociceptors. Functions of polymodal nociceptors require activities of multiple ion channels and receptors including members of the TRP family. GTL-1, a member of the TRPM subclass of TRP channels, was previously shown to amplify PVD-mediated responses to optogenetic stimuli. Here we characterize effects of GTL-1 on PVD-mediated behavioral responses to noxious stimuli. We show that GTL-1 is required within PVD for the immediate and enduring response to thermal (cold) stimuli. But, find no significant reduction in percent animals responding to single or to repeated noxious mechanical stimuli. Nevertheless, PVD specific knockdown of gtl-1expression reduces the magnitude of responses to noxious mechanical stimuli. To understand GTL-1's mechanism of action we expressed it in HEK293 cells. Our results show GTL-1-dependent currents induced by activation of a Gαq-coupled Designer Receptor Exclusively Activated by Designer Drugs (DREADD). In addition, using excised patches we show that GTL-1 can be activated by internal calcium. Our results are consistent with indirect, calcium dependent, activation of GTL-1 by noxious stimuli. This mechanism explains the GTL-1-dependent amplification of responses to multiple stimuli optogenetic and sensory in PVD.
RESUMEN
BACKGROUND AND PURPOSE: GAT107 ((3aR,4S,9bS)-4-(4-bromo-phenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta-[c]quinoline-8-sulfonamide) is a positive allosteric modulator (PAM) and agonist of α7 nicotinic acetylcholine receptors (nAChRs)that can cause a prolonged period of primed potentiation of acetylcholine responses after drug washout. NS6740 is a silent agonist of α7 nAChRs that has little or no efficacy for activating the ion channel but induces stable desensitization states, some of which can be converted into channel-active states by PAMs. Although GAT107 and NS6740 appear to stably induce different non-conducting states, both agents are effective treatment for inflammation and inflammatory pain models. We sought to better understand how both of these drugs that have opposite effects on channel activation could regulate signal transduction. EXPERIMENTAL APPROACH: Voltage-clamp experiments were conducted with α7 nAChRs expressed in Xenopus oocytes. KEY RESULTS: Long-lived sensitivity to a PAM or to an agonist was produced by NS6740 or GAT107 respectively. With sequential applications, these two drugs induced varying levels of persistent activation, which is a unique condition for a receptor that is known for rapid desensitization. The non-conducting states induced by NS6740 or GAT107 differ in their sensitivity to an α7 nAChR-selective antagonist and in how effectively they promote current. CONCLUSIONS & IMPLICATIONS: Our data suggest that the persistent currents represent a dynamic interconversion between different stable desensitized states and the PAM-inducible conducting states. However, the similarity of NS6740 and GAT107 effects on inflammation and pain suggests that the different stable non-conducting states have common activity on signal transduction. LINKED ARTICLES: This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.
Asunto(s)
Receptor Nicotínico de Acetilcolina alfa 7/metabolismo , Regulación Alostérica/efectos de los fármacos , Animales , Compuestos de Azabiciclo/farmacología , Femenino , Furanos/farmacología , Quinolinas/farmacología , Sulfonamidas/farmacología , Xenopus laevis , Receptor Nicotínico de Acetilcolina alfa 7/agonistasRESUMEN
BACKGROUND: The nicotinic acetylcholine receptor (nAChR) gene family encodes for subunits of acetylcholine gated ion channels. These receptors are expressed widely and have many functions: They mediate excitation at neuro-muscular junctions. Nicotinic Acetylcholine Receptor: In the central nervous system nAChRs have been implicated in memory, cognition, and addiction. And in non-excitatory cells they regulate differentiation, proliferation and inflammatory responses. The CHRNA7 gene encodes for the α7 nAChR subunit that assembles into a homomeric receptor having unusual properties. It is expressed widely and has many functions atypical for nAChRs; specifically, in immune cells α7 is required for the anti-inflammatory effects of acetylcholine and has been implicated in inflammatory autoimmune diseases including Multiple Sclerosis (MS). Interestingly, although, α7 receptors are found at the outer membranes of immune cells, acetylcholine-dependent currents have not been recorded from these cells. Therefore, its mechanism of action in immune cells needs further evaluation. Maturation of α7 into functional ligand-gated channels in the plasma membrane is a complex process shown to depend on the ER-resident chaperone, RIC-3. Therefore, RIC-3 regulates functional expression of α7. RIC-3 like α7 is expressed in immune cells and has been implicated in MS. Thus, RIC-3 may regulate functional expression of α7 in immune cells. CONCLUSION: In this review we describe effects and mechanism of action of α7 nAChR and RIC-3 in the immune cholinergic system. Elucidating these mechanisms and the regulation of α7 and RIC-3 in the immune cholinergic system can pave the way for novel immunomodulatory agents, or towards extending the application of cholinergic agents.
Asunto(s)
Antiinflamatorios/uso terapéutico , Colinérgicos/uso terapéutico , Mediadores de Inflamación/inmunología , Péptidos y Proteínas de Señalización Intracelular/fisiología , Transducción de Señal/fisiología , Receptor Nicotínico de Acetilcolina alfa 7/fisiología , Animales , Antiinflamatorios/farmacología , Colinérgicos/farmacología , Humanos , Factores Inmunológicos/farmacología , Factores Inmunológicos/uso terapéutico , Inflamación/tratamiento farmacológico , Inflamación/inmunología , Mediadores de Inflamación/antagonistas & inhibidores , Transducción de Señal/efectos de los fármacosRESUMEN
Brain function depends on a delicate balance between excitation and inhibition. Similarly, Caenorhabditis elegans motor system function depends on a precise balance between excitation and inhibition, as C. elegans muscles receive both inhibitory, GABAergic and excitatory, cholinergic inputs from motor neurons. Here we show that phosphorylation of the ER-resident chaperone of nicotinic acetylcholine receptors, RIC-3, leads to increased muscle excitability. RIC-3 phosphorylation at Ser-164 depends on opposing functions of the phosphatase calcineurin (TAX-6), and of the casein kinase II homologue KIN-10. Effects of calcineurin down-regulation and of phosphorylated RIC-3 on muscle excitability are mediated by GABAA receptor inhibition. Thus RIC-3 phosphorylation enables effects of this chaperone on GABAA receptors in addition to nAChRs. This dual effect provides coordinated regulation of excitation and inhibition and enables fine-tuning of the excitation-inhibition balance. Moreover, regulation of inhibitory GABAA signaling by calcineurin, a calcium- and calmodulin-dependent phosphatase, enables homeostatic balancing of excitation and inhibition.
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Proteínas de Caenorhabditis elegans/genética , Proteínas de Caenorhabditis elegans/metabolismo , Calcineurina/metabolismo , Secuencia de Aminoácidos , Animales , Caenorhabditis elegans/genética , Caenorhabditis elegans/metabolismo , Proteínas de Caenorhabditis elegans/fisiología , Quinasa de la Caseína II/metabolismo , Acoplamiento Excitación-Contracción/fisiología , Chaperonas Moleculares/metabolismo , Neuronas Motoras/metabolismo , Músculos/metabolismo , Fosforilación , Receptores Nicotínicos/metabolismoRESUMEN
BACKGROUND: The nicotinic acetylcholine receptors form a large and diverse family of acetylcholine gated ion channels having diverse roles in the central nervous system. Maturation of nicotinic acetylcholine receptors is a complex and inefficient process requiring assistance from multiple cellular factors including RIC-3, a functionally conserved endoplasmic reticulum-resident protein and nicotinic acetylcholine receptor-specific chaperone. In mammals and in Drosophila melanogaster RIC-3 is alternatively spliced to produce multiple isoforms. RESULTS: We used electrophysiological analysis in Xenopus laevis oocytes, in situ hybridization, and quantitative real-time polymerase chain reaction assays to investigate regulation of RIC-3's expression and splicing and its effects on the expression of three major neuronal nicotinic acetylcholine receptors. We found that RIC-3 expression level and splicing affect nicotinic acetylcholine receptor functional expression and that two conserved RIC-3 isoforms express in the brain differentially. Moreover, in immune cells RIC-3 expression and splicing are regulated by inflammatory signals. CONCLUSIONS: Regulation of expression level and splicing of RIC-3 in brain and in immune cells following inflammation enables regulation of nicotinic acetylcholine receptor functional expression. Specifically, in immune cells such regulation via effects on α7 nicotinic acetylcholine receptor, known to function in the cholinergic anti-inflammatory pathway, may have a role in neuroinflammatory diseases.
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Empalme Alternativo/genética , Proteínas de la Membrana/genética , Chaperonas Moleculares/genética , Receptor Nicotínico de Acetilcolina alfa 7/metabolismo , Acetilcolina/farmacología , Empalme Alternativo/efectos de los fármacos , Animales , Encéfalo/efectos de los fármacos , Encéfalo/metabolismo , Hibridación in Situ , Inflamación/patología , Activación del Canal Iónico/efectos de los fármacos , Proteínas de la Membrana/metabolismo , Ratones , Chaperonas Moleculares/metabolismo , Dominios Proteicos , Isoformas de Proteínas/metabolismo , Subunidades de Proteína/metabolismo , Células RAW 264.7 , Bazo/metabolismo , Xenopus laevisRESUMEN
Sensory loss induces cross-modal plasticity, often resulting in altered performance in remaining sensory modalities. Whereas much is known about the macroscopic mechanisms underlying cross-modal plasticity, only scant information exists about its cellular and molecular underpinnings. We found that Caenorhabditis elegans nematodes deprived of a sense of body touch exhibit various changes in behavior, associated with other unimpaired senses. We focused on one such behavioral alteration, enhanced odor sensation, and sought to reveal the neuronal and molecular mechanisms that translate mechanosensory loss into improved olfactory acuity. To this end, we analyzed in mechanosensory mutants food-dependent locomotion patterns that are associated with olfactory responses and found changes that are consistent with enhanced olfaction. The altered locomotion could be reversed in adults by optogenetic stimulation of the touch receptor (mechanosensory) neurons. Furthermore, we revealed that the enhanced odor response is related to a strengthening of inhibitory AWCâAIY synaptic transmission in the olfactory circuit. Consistently, inserting in this circuit an engineered electrical synapse that diminishes AWC inhibition of AIY counteracted the locomotion changes in touch-deficient mutants. We found that this cross-modal signaling between the mechanosensory and olfactory circuits is mediated by neuropeptides, one of which we identified as FLP-20. Our results indicate that under normal function, ongoing touch receptor neuron activation evokes FLP-20 release, suppressing synaptic communication and thus dampening odor sensation. In contrast, in the absence of mechanosensory input, FLP-20 signaling is reduced, synaptic suppression is released, and this enables enhanced olfactory acuity; these changes are long lasting and do not represent ongoing modulation, as revealed by optogenetic experiments. Our work adds to a growing literature on the roles of neuropeptides in cross-modal signaling, by showing how activity-dependent neuropeptide signaling leads to specific cross-modal plastic changes in neural circuit connectivity, enhancing sensory performance.
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Caenorhabditis elegans/fisiología , Células Quimiorreceptoras/fisiología , Mecanorreceptores/metabolismo , Neuropéptidos/fisiología , Olfato , Animales , Locomoción , Plasticidad Neuronal , Transmisión SinápticaRESUMEN
Optogenetics is a powerful tool for manipulating neuronal activity with high temporal and spatial precision. In the nematode C. elegans optogentics is especially useful and easy to apply. This is because C. elegans is translucent, so its neurons are highly accessible to optic stimulation. In addition, many of its neurons can be exclusively targeted using cell-specific promoters. We have recently taken advantage of optogentics to deliver artificial patterns of prolonged activation to a class of mechanosensory neurons, called touch receptor neurons (TRNs) in worms that lack touch sensation due to a genetic mutation. Our aim was to examine whether we can counteract the effects of sensory loss by artificially activating the sensory neurons. Here we describe in detail the various components of the protocol that we used. This consists of exposing worms expressing the light-sensitive ion channel Channelrohdopsin 2 (ChR2) in TRNs to long-term random flashes of light.
RESUMEN
The amino acid sequences of nicotinic acetylcholine receptors (nAChRs) from diverse species can be compared across extracellular, transmembrane, and intracellular domains. The intracellular domains are most divergent among subtypes, yet relatively consistent among species. The diversity indicates that each nAChR subtype has a unique language for communication with its host cell. The conservation across species also suggests that the intracellular domains have defining functional roles for each subtype. Secondary structure prediction indicates two relatively conserved alpha helices within the intracellular domains of all nAChRs. Among all subtypes, the intracellular domain of α7 nAChR is one of the most well conserved, and α7 nAChRs have effects in non-neuronal cells independent of generating ion currents, making it likely that the α7 intracellular domain directly mediates signal transduction. There are potential phosphorylation and protein-binding sites in the α7 intracellular domain, which are conserved and may be the basis for α7-mediated signal transduction.
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Receptores Nicotínicos/química , Secuencia de Aminoácidos , Animales , Secuencia Conservada , Humanos , Datos de Secuencia Molecular , Estructura Terciaria de Proteína , Receptores Nicotínicos/metabolismoRESUMEN
Neural circuits are functional ensembles of neurons that are selectively interconnected by chemical or electrical synapses. Here we describe a synthetic biology approach to the study of neural circuits, whereby new electrical synapses can be introduced in novel sites in the neuronal circuitry to reprogram behaviour. We added electrical synapses composed of the vertebrate gap junction protein Cx36 between Caenorhabditis elegans chemosensory neurons with opposite intrinsic responses to salt. Connecting these neurons by an ectopic electrical synapse led to a loss of lateral asymmetry and altered chemotaxis behaviour. In a second example, introducing Cx36 into an inhibitory chemical synapse between an olfactory receptor neuron and an interneuron changed the sign of the connection from negative to positive, and abolished the animal's behavioural response to benzaldehyde. These data demonstrate a synthetic strategy to rewire behavioural circuits by engineering synaptic connectivity in C. elegans.
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Caenorhabditis elegans/metabolismo , Conexinas/metabolismo , Sinapsis Eléctricas/metabolismo , Animales , Animales Modificados GenéticamenteRESUMEN
Polymodal nociceptors sense and integrate information on injurious mechanical, thermal, and chemical stimuli. Chemical signals either activate nociceptors or modulate their responses to other stimuli. One chemical known to activate or modulate responses of nociceptors is acetylcholine (ACh). Across evolution nociceptors express subunits of the nicotinic acetylcholine receptor (nAChR) family, a family of ACh-gated ion channels. The roles of ACh and nAChRs in nociceptor function are, however, poorly understood. Caenorhabditis elegans polymodal nociceptors, PVD, express nAChR subunits on their sensory arbor. Here we show that mutations reducing ACh synthesis and mutations in nAChR subunits lead to defects in PVD function and morphology. A likely cause for these defects is a reduction in cytosolic calcium measured in ACh and nAChR mutants. Indeed, overexpression of a calcium pump in PVD mimics defects in PVD function and morphology found in nAChR mutants. Our results demonstrate, for the first time, a central role for nAChRs and ACh in nociceptor function and suggest that calcium permeating via nAChRs facilitates activity of several signaling pathways within this neuron.
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Proteínas de Caenorhabditis elegans/metabolismo , Caenorhabditis elegans/metabolismo , Nocicepción , Receptores Nicotínicos/metabolismo , Acetilcolina/metabolismo , Animales , Caenorhabditis elegans/fisiología , Proteínas de Caenorhabditis elegans/genética , Mutación , Nociceptores/metabolismo , Receptores Nicotínicos/genéticaRESUMEN
BACKGROUND: To survive dynamic environments, it is essential for all animals to appropriately modulate their behavior in response to various stimulus intensities. For instance, the nematode Caenorhabditis elegans suppresses the rate of egg-laying in response to intense mechanical stimuli, in a manner dependent on the mechanosensory neurons FLP and PVD. We have found that the unilaterally placed single interneuron ALA acted as a high-threshold mechanosensor, and that it was required for this protective behavioral response. RESULTS: ALA was required for the inhibition of egg-laying in response to a strong (picking-like) mechanical stimulus, characteristic of routine handling of the animals. Moreover, ALA did not respond physiologically to less intense touch stimuli, but exhibited distinct physiological responses to anterior and posterior picking-like touch, suggesting that it could distinguish between spatially separated stimuli. These responses required neither neurotransmitter nor neuropeptide release from potential upstream neurons. In contrast, the long, bilaterally symmetric processes of ALA itself were required for producing its physiological responses; when they were severed, responses to stimuli administered between the cut and the cell body were unaffected, while responses to stimuli administered posterior to the cut were abolished. CONCLUSION: C. elegans neurons are typically classified into three major groups: sensory neurons with specialized sensory dendrites, interneurons, and motoneurons with neuromuscular junctions. Our findings suggest that ALA can autonomously sense intense touch and is thus a dual-function neuron, i.e., an interneuron as well as a novel high-threshold mechanosensor.
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Caenorhabditis elegans/fisiología , Interneuronas/fisiología , Mecanorreceptores/fisiología , Animales , Conducta Animal/fisiología , Caenorhabditis elegans/citología , Interneuronas/citología , Mecanorreceptores/citología , Tacto/fisiologíaRESUMEN
The simple and well-characterized nervous system of C. elegans facilitates the analysis of mechanisms controlling behavior. Locomotion is a major behavioral output governed by multiple external and internal signals. Here, we examined the roles of low- and high-threshold mechanosensors in locomotion, using high-resolution and detailed analysis of locomotion and its dynamics. This analysis revealed a new role for touch receptor neurons in suppressing an intrinsic direction bias of locomotion. We also examined the response to noxious mechanical stimuli, which was found to involve several locomotion properties and to last several minutes. Effects on different locomotion properties have different half-lives and depend on different, partly overlapping sets of sensory neurons. PVD and FLP, high-threshold mechanosensors, play a major role in some of these responses. Overall, our results demonstrate the power of detailed, prolonged and high-resolution analysis of locomotion and locomotion dynamics in enabling better understanding of gene and neuron function.
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Caenorhabditis elegans/fisiología , Mecanotransducción Celular , Células Receptoras Sensoriales/fisiología , Animales , Conducta Animal/fisiología , LocomociónRESUMEN
BACKGROUND: Nociception generally evokes rapid withdrawal behavior in order to protect the tissue from harmful insults. Most nociceptive neurons responding to mechanical insults display highly branched dendrites, an anatomy shared by Caenorhabditis elegans FLP and PVD neurons, which mediate harsh touch responses. Although several primary molecular nociceptive sensors have been characterized, less is known about modulation and amplification of noxious signals within nociceptor neurons. First, we analyzed the FLP/PVD network by optogenetics and studied integration of signals from these cells in downstream interneurons. Second, we investigated which genes modulate PVD function, based on prior single-neuron mRNA profiling of PVD. RESULTS: Selectively photoactivating PVD, FLP, and downstream interneurons via Channelrhodopsin-2 (ChR2) enabled the functional dissection of this nociceptive network, without interfering signals by other mechanoreceptors. Forward or reverse escape behaviors were determined by PVD and FLP, via integration by command interneurons. To identify mediators of PVD function, acting downstream of primary nocisensor molecules, we knocked down PVD-specific transcripts by RNAi and quantified light-evoked PVD-dependent behavior. Cell-specific disruption of synaptobrevin or voltage-gated Ca(2+) channels (VGCCs) showed that PVD signals chemically to command interneurons. Knocking down the DEG/ENaC channel ASIC-1 and the TRPM channel GTL-1 indicated that ASIC-1 may extend PVD's dynamic range and that GTL-1 may amplify its signals. These channels act cell autonomously in PVD, downstream of primary mechanosensory molecules. CONCLUSIONS: Our work implicates TRPM channels in modifying excitability of and DEG/ENaCs in potentiating signal output from a mechano-nociceptor neuron. ASIC-1 and GTL-1 homologs, if functionally conserved, may denote valid targets for novel analgesics.
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Canales Iónicos/metabolismo , Neuronas/citología , Neuronas/metabolismoRESUMEN
Sensory dendrites fall into many different morphological and functional classes. Polymodal nociceptors are one subclass of sensory neurons, which are of particular note owing to their elaborate dendritic arbors. Complex developmental programs are required to form these arbors and there is striking conservation of morphology, function and molecular determinants between vertebrate and invertebrate polymodal nociceptors. Based on these studies, we argue that arbor morphology plays an important role in the function of polymodal nociceptors. Similar associations between form and function might explain the plethora of dendrite morphologies seen among all sensory neurons.