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.
The nematode C. elegans expresses a large and diverse family of acetylcholine‐activated ion channels. These receptors express in multiple neuron and muscle types (A). In this review, we describe acetylcholine‐activated channels that have been characterized for their function in neurons or muscle or for their unique properties. We also describe the many proteins affecting maturation and function of these receptors, starting at the endoplasmic reticulum (ER) and ending at the plasma membrane (PM) (B). Finally, we describe novel findings on a motor neuron expressed acetylcholine receptor (ACR‐2R) and its effects on the C. elegans motor circuit (C).
Highlights • Intracellular domains of Cys -loop receptors are variable and unique to each subunit. • Intracellular subdomains may contribute to ion-conducting submembrane portals. • The α7 ...intracellular domains contain well-conserved sites for protein interactions.
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.
The mechanisms controlling the formation and maintenance of neuronal trees are poorly understood. We examined the dynamic development of two arborized mechanoreceptor neurons (PVDs) required for ...reception of strong mechanical stimuli in Caenorhabditis elegans. The PVDs elaborated dendritic trees comprising structural units we call "menorahs." We studied how the number, structure, and function of menorahs were maintained. EFF-1, an essential protein mediating cell fusion, acted autonomously in the PVDs to trim developing menorahs. eff-1 mutants displayed hyperbranched, disorganized menorahs. Overexpression of EFF-1 in the PVD reduced branching. Neuronal pruning appeared to involve EFF-1-dependent branch retraction and neurite-neurite autofusion. Thus, EFF-1 activities may act as a quality control mechanism during the sculpting of dendritic trees.
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.
EAE was induced by administration of myelin oligodendrocyte glycoprotein (MOG
) 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.
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-MOG
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.
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.
Nociceptive neurons innervate the skin with complex dendritic arbors that respond to pain-evoking stimuli such as harsh mechanical force or extreme temperatures. Here we describe the structure and ...development of a model nociceptor, the PVD neuron of
C. elegans, and identify transcription factors that control morphogenesis of the PVD dendritic arbor. The two PVD neuron cell bodies occupy positions on either the right (PVDR) or left (PVDL) sides of the animal in posterior–lateral locations. Imaging with a GFP reporter revealed a single axon projecting from the PVD soma to the ventral cord and an elaborate, highly branched arbor of dendritic processes that envelop the animal with a web-like array directly beneath the skin. Dendritic branches emerge in a step-wise fashion during larval development and may use an existing network of peripheral nerve cords as guideposts for key branching decisions. Time-lapse imaging revealed that branching is highly dynamic with active extension and withdrawal and that PVD branch overlap is prevented by a contact-dependent self-avoidance, a mechanism that is also employed by sensory neurons in other organisms. With the goal of identifying genes that regulate dendritic morphogenesis, we used the mRNA-tagging method to produce a gene expression profile of PVD during late larval development. This microarray experiment identified
>
2,000 genes that are 1.5X elevated relative to all larval cells. The enriched transcripts encode a wide range of proteins with potential roles in PVD function (e.g., DEG/ENaC and Trp channels) or development (e.g., UNC-5 and LIN-17/frizzled receptors). We used RNAi and genetic tests to screen 86 transcription factors from this list and identified eleven genes that specify PVD dendritic structure. These transcription factors appear to control discrete steps in PVD morphogenesis and may either promote or limit PVD branching at specific developmental stages. For example, time-lapse imaging revealed that MEC-3 (LIM homeodomain) is required for branch initiation in early larval development whereas EGL-44 (TEAD domain) prevents ectopic PVD branching in the adult. A comparison of PVD-enriched transcripts to a microarray profile of mammalian nociceptors revealed homologous genes with potentially shared nociceptive functions. We conclude that PVD neurons display striking structural, functional and molecular similarities to nociceptive neurons from more complex organisms and can thus provide a useful model system in which to identify evolutionarily conserved determinants of nociceptor fate.
Background and Purpose
GAT107 ((3aR,4S,9bS)‐4‐(4‐bromo‐phenyl)‐3a,4,5,9b‐tetrahydro‐3H‐cyclopenta‐cquinoline‐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
Polymodal nociceptors detect noxious stimuli, including harsh touch, toxic chemicals and extremes of heat and cold. The molecular mechanisms by which nociceptors are able to sense multiple ...qualitatively distinct stimuli are not well understood. We found that the C. elegans PVD neurons are mulitidendritic nociceptors that respond to harsh touch and cold temperatures. The harsh touch modality specifically required the DEG/ENaC proteins MEC-10 and DEGT-1, which represent putative components of a harsh touch mechanotransduction complex. In contrast, responses to cold required the TRPA-1 channel and were MEC-10 and DEGT-1 independent. Heterologous expression of C. elegans TRPA-1 conferred cold responsiveness to other C. elegans neurons and to mammalian cells, indicating that TRPA-1 is a cold sensor. Our results suggest that C. elegans nociceptors respond to thermal and mechanical stimuli using distinct sets of molecules and identify DEG/ENaC channels as potential receptors for mechanical pain.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Mutations in ric‐3 (resistant to inhibitors of cholinesterase) suppress the neuronal degenerations caused by a gain of function mutation in the Caenorhabditis elegans DEG‐3 acetylcholine receptor. ...RIC‐3 is a novel protein with two transmembrane domains and extensive coiled‐coil domains. It is expressed in both muscles and neurons, and the protein is concentrated within the cell bodies. We demonstrate that RIC‐3 is required for the function of at least four nicotinic acetylcholine receptors. However, GABA and glutamate receptors expressed in the same cells are unaffected. In ric‐3 mutants, the DEG‐3 receptor accumulates in the cell body instead of in the cell processes. Moreover, co‐expression of ric‐3 in Xenopus laevis oocytes enhances the activity of the C.elegans DEG‐3/DES‐2 and of the rat α‐7 acetylcholine receptors. Together, these data suggest that RIC‐3 is specifically required for the maturation of acetylcholine receptors.
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
expression 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.
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