Molecular characterization and localization of the RIC-3 protein, an effector of nicotinic acetylcholine receptor expression

Unraveling the molecular interactions between α7 nicotinic receptor and a RIC3 variant associated with backward speech

Recent work putatively linked a rare genetic variant of the chaperone Resistant to Inhibitors of acetylcholinesterase (RIC3) (NM_024557.4:c.262G > A, NP_078833.3:p.G88R) to a unique ability to speak backwards, a language skill that is associated with exceptional working memory capacity. RIC3 is important for the folding, maturation, and functional expression of α7 nicotinic acetylcholine receptors (nAChR). We compared and contrasted the effects of RIC3G88R on assembly, cell surface expression, and function of human α7 receptors using fluorescent protein tagged α7 nAChR and Förster resonance energy transfer (FRET) microscopy imaging in combination with functional assays and 125I-α-bungarotoxin binding. As expected, the wild-type RIC3 protein was found to increase both cell surface and functional expression of α7 receptors. In contrast, the variant form of RIC3 decreased both. FRET analysis showed that RICG88R increased the interactions between RIC3 and α7 protein in the endoplasmic reticulum. These results provide interesting and novel data to show that a RIC3 variant alters the interaction of RIC3 and α7, which translates to decreased cell surface and functional expression of α7 nAChR.

Two residues determine nicotinic acetylcholine receptor requirement for RIC-3

Nicotinic acetylcholine receptors (N-AChRs) mediate fast synaptic signaling and are members of the pentameric ligand-gated ion channel (pLGIC) family. They rely on a network of accessory proteins in vivo for correct formation and transport to the cell surface. Resistance to cholinesterase 3 (RIC-3) is an endoplasmic reticulum protein that physically interacts with nascent pLGIC subunits and promotes their oligomerization. It is not known why some N-AChRs require RIC-3 in heterologous expression systems, whereas others do not. Previously we reported that the ACR-16 N-AChR from the parasitic nematode Dracunculus medinensis does not require RIC-3 in Xenopus laevis oocytes. This is unusual because all other nematode ACR-16, like the closely related Ascaris suum ACR-16, require RIC-3. Their high sequence similarity limits the number of amino acids that may be responsible, and the goal of this study was to identify them. A series of chimeras and point mutations between A. suum and D. medinensis ACR-16, followed by functional characterization with electrophysiology, identified two residues that account for a majority of the receptor requirement for RIC-3. ACR-16 with R/K159 in the cys-loop and I504 in the C-terminal tail did not require RIC-3 for functional expression. Mutating either of these to R/K159E or I504T, residues found in other nematode ACR-16, conferred a RIC-3 requirement. Our results agree with previous studies showing that these regions interact and are involved in receptor synthesis. Although it is currently unclear what precise mechanism they regulate, these residues may be critical during specific subunit folding and/or assembly cascades that RIC-3 may promote.

Unanswered questions in the regulation and function of the duplicated α7 nicotinic receptor gene CHRFAM7A

The α7 nicotinic receptor (α7 nAChR) is an important entry point for Ca²⁺ into the cell, which has broad and important effects on gene expression and function. The gene (CHRNA7), mapping to chromosome (15q14), has been genetically linked to a large number of diseases, many of which involve defects in cognition. While numerous mutations in CHRNA7 are associated with mental illness and inflammation, an important control point may be the function of a recently discovered partial duplication CHRNA7, CHRFAM7A, that negatively regulates the function of the α7 receptor, through the formation of heteropentamers; other functions cannot be excluded. The deregulation of this human specific gene (CHRFAM7A) has been linked to neurodevelopmental, neurodegenerative, and inflammatory disorders and has important copy number variations. Much effort is being made to understand its function and regulation both in healthy and pathological conditions. However, many questions remain to be answered regarding its functional role, its regulation, and its role in the etiogenesis of neurological and inflammatory disorders. Missing knowledge on the pharmacology of the heteroreceptor has limited the discovery of new molecules capable of modulating its activity. Here we review the state of the art on the role of CHRFAM7A, highlighting unanswered questions to be addressed. A possible therapeutic approach based on genome editing protocols is also discussed.

Speculation on How RIC-3 and Other Chaperones Facilitate α7 Nicotinic Receptor Folding and Assembly

The process of how multimeric transmembrane proteins fold and assemble in the endoplasmic reticulum is not well understood. The alpha7 nicotinic receptor (α7 nAChR) is a good model for multimeric protein assembly since it has at least two independent and specialized chaperones: Resistance to Inhibitors of Cholinesterase 3 (RIC-3) and Nicotinic Acetylcholine Receptor Regulator (NACHO). Recent cryo-EM and NMR data revealed structural features of α7 nAChRs. A ser-ala-pro (SAP) motif precedes a structurally important but unique "latch" helix in α7 nAChRs. A sampling of α7 sequences suggests the SAP motif is conserved from C. elegans to humans, but the latch sequence is only conserved in vertebrates. How RIC-3 and NACHO facilitate receptor subunits folding into their final pentameric configuration is not known. The artificial intelligence program AlphaFold2 recently predicted structures for NACHO and RIC-3. NACHO is highly conserved in sequence and structure across species, but RIC-3 is not. This review ponders how different intrinsically disordered RIC-3 isoforms from C. elegans to humans interact with α7 nAChR subunits despite having little sequence homology across RIC-3 species. Two models from the literature about how RIC-3 assists α7 nAChR assembly are evaluated considering recent structural information about the receptor and its chaperones.

Functional evidence for the inflammatory reflex in teleosts: A novel α7 nicotinic acetylcholine receptor modulates the macrophage response to dsRNA

The inflammatory reflex modulates the innate immune system, keeping in check the detrimental consequences of overstimulation. A key player controlling the inflammatory reflex is the alpha 7 acetylcholine receptor (α7nAChR). This receptor is one of the signalling molecules regulating cytokine expression in macrophages. In this study, we characterize a novel teleost α7nAChR. Protein sequence analysis shows a high degree of conservation with mammalian orthologs and trout α7nAChR has all the features and essential amino acids to form a fully functional receptor. We demonstrate that trout macrophages can bind α-bungarotoxin (α-BTX), a competitive antagonist for α7nAChRs. Moreover, nicotine stimulation produces a decrease in pro-inflammatory cytokine expression after stimulation with poly(I:C). These results suggest the presence of a functional α7nAChR in the macrophage plasma membrane. Further, in vivo injection of poly(I:C) induced an increase in serum ACh levels in rainbow trout. Our results manifest for the first time the functional conservation of the inflammatory reflex in teleosts.

Modulatory effects of α7 nAChRs on the immune system and its relevance for CNS disorders

The clinical development of selective alpha-7 nicotinic acetylcholine receptor (α7 nAChR) agonists has hitherto been focused on disorders characterized by cognitive deficits (e.g., Alzheimer's disease, schizophrenia). However, α7 nAChRs are also widely expressed by cells of the immune system and by cells with a secondary role in pathogen defense. Activation of α7 nAChRs leads to an anti-inflammatory effect. Since sterile inflammation is a frequently observed phenomenon in both psychiatric disorders (e.g., schizophrenia, melancholic and bipolar depression) and neurological disorders (e.g., Alzheimer's disease, Parkinson's disease, and multiple sclerosis), α7 nAChR agonists might show beneficial effects in these central nervous system disorders. In the current review, we summarize information on receptor expression, the intracellular signaling pathways they modulate and reasons for receptor dysfunction. Information from tobacco smoking, vagus nerve stimulation, and cholinesterase inhibition is used to evaluate the therapeutic potential of selective α7 nAChR agonists in these inflammation-related disorders.

Direct interaction of the resistance to inhibitors of cholinesterase type 3 protein with the serotonin receptor type 3A intracellular domain

Pentameric ligand-gated ion channels (pLGIC) are expressed in both excitable and non-excitable cells that are targeted by numerous clinically used drugs. Assembly from five identical or homologous subunits yields homo- or heteromeric pentamers, respectively. The protein known as Resistance to Inhibitors of Cholinesterase (RIC-3) was identified to interfere with assembly and functional maturation of pLGICs. We have shown previously for serotonin type 3A homopentamers (5-HT3A ) that the interaction with RIC-3 requires the intracellular domain (ICD) of this pLGIC. After expression in Xenopus laevis oocytes RIC-3 attenuated serotonin-induced currents in 5-HT3A wild-type channels, but not in functional 5-HT3A glvM3M4 channels that have the 115-amino acid ICD replaced by a heptapeptide. In complementary experiments we have shown that engineering the Gloeobacter violaceus ligand-gated ion channel (GLIC) to contain the 5-HT3A -ICD confers sensitivity to RIC-3 in oocytes to otherwise insensitive GLIC. In this study, we identify endogenous RIC-3 protein expression in X. laevis oocytes. We purified RIC-3 to homogeneity after expression in Echericia coli. By using heterologously over-expressed and purified RIC-3 and the chimera consisting of the 5-HT3A -ICD and the extracellular and transmembrane domains of GLIC in pull-down experiments, we demonstrate a direct and specific interaction between the two proteins. This result further underlines that the domain within 5-HT3 A R that mediates the interaction with RIC-3 is the ICD. Importantly, this is the first experimental evidence that the interaction between 5-HT3 A R-ICD and RIC-3 does not require other proteins. In addition, we demonstrate that the pentameric assembly of the GLIC-5-HT3A -ICD chimera interacts with RIC-3. We hypothesized that pentameric ligand-gated ion channels (pLGICs) associate directly with the chaperone protein RIC-3 (resistance to inhibitors of cholinesterase type 3), and that the interaction does not require other protein factors. We found that the two proteins indeed interact directly, that the pLGIC intracellular domain is required for the effect, and that pLGICs in their pentameric form associate with RIC-3. These results provide the basis for future studies aimed at investigating which motifs provide the interaction surfaces, and at delineating the mechanism(s) of RIC-3 modulation of functional pLGIC surface expression.

Drug Development in Alzheimer's Disease: The Contribution of PET and SPECT

Clinical trials aiming to develop disease-altering drugs for Alzheimer’s disease (AD), a neurodegenerative disorder with devastating consequences, are failing at an alarming rate. Poorly defined inclusion-and outcome criteria, due to a limited amount of objective biomarkers, is one of the major concerns. Non-invasive molecular imaging techniques, positron emission tomography and single photon emission (computed) tomography (PET and SPE(C)T), allow visualization and quantification of a wide variety of (patho)physiological processes and allow early (differential) diagnosis in many disorders. PET and SPECT have the ability to provide biomarkers that permit spatial assessment of pathophysiological molecular changes and therefore objectively evaluate and follow up therapeutic response, especially in the brain. A number of specific PET/SPECT biomarkers used in support of emerging clinical therapies in AD are discussed in this review.

Multidisciplinary investigation links backward-speech trait and working memory through genetic mutation

Case studies of unusual traits can provide unique snapshots of the effects of modified systems. In this study, we report on an individual from a Serbian family with the ability to rapidly, accurately and voluntarily speak backwards. We consider psychological, neural and genetic correlates of this trait to identify specific relevant neural mechanisms and new molecular pathways for working memory and speech-related tasks. EEG data suggest that the effect of word reversal precedes semantic integration of visually presented backward-words, and that event-related potentials above the frontal lobe are affected by both word reversal and the maintenance of backward-words in working memory. fMRI revealed that the left fusiform gyrus may facilitate the production of backward-speech. Exome sequencing identified three novel coding variants of potential significance in the RIC3, RIPK1 and ZBED5 genes. Taken together, our data suggest that, in this individual, the ability to speak backwards is afforded by an extraordinary working memory capacity. We hypothesise that this is served by cholinergic projections from the basal forebrain to the frontal cortex and supported by visual semantic loops within the left fusiform gyrus and that these neural processes may be mediated by a genetic mutation in RIC3; a chaperone for nicotinic acetylcholine receptors.

The human CHRNA7 and CHRFAM7A genes: A review of the genetics, regulation, and function

The human α7 neuronal nicotinic acetylcholine receptor gene (CHRNA7) is ubiquitously expressed in both the central nervous system and in the periphery. CHRNA7 is genetically linked to multiple disorders with cognitive deficits, including schizophrenia, bipolar disorder, ADHD, epilepsy, Alzheimer's disease, and Rett syndrome. The regulation of CHRNA7 is complex; more than a dozen mechanisms are known, one of which is a partial duplication of the parent gene. Exons 5-10 of CHRNA7 on chromosome 15 were duplicated and inserted 1.6 Mb upstream of CHRNA7, interrupting an earlier partial duplication of two other genes. The chimeric CHRFAM7A gene product, dupα7, assembles with α7 subunits, resulting in a dominant negative regulation of function. The duplication is human specific, occurring neither in primates nor in rodents. The duplicated α7 sequence in exons 5-10 of CHRFAM7A is almost identical to CHRNA7, and thus is not completely queried in high throughput genetic studies (GWAS). Further, pre-clinical animal models of the α7nAChR utilized in drug development research do not have CHRFAM7A (dupα7) and cannot fully model human drug responses. The wide expression of CHRNA7, its multiple functions and modes of regulation present challenges for study of this gene in disease. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Resistance to Inhibitors of Cholinesterase 3 (Ric-3) Expression Promotes Selective Protein Associations with the Human α7-Nicotinic Acetylcholine Receptor Interactome

The α7-nicotinic acetylcholine receptor (α7-nAChR) is a ligand-gated ion channel widely expressed in vertebrates and is associated with numerous physiological functions. As transmembrane ion channels, α7-nAChRs need to be expressed on the surface of the plasma membrane to function. The receptor has been reported to associate with proteins involved with receptor biogenesis, modulation of receptor properties, as well as intracellular signaling cascades and some of these associated proteins may affect surface expression of α7-nAChRs. The putative chaperone resistance to inhibitors of cholinesterase 3 (Ric-3) has been reported to interact with, and enhance the surface expression of, α7-nAChRs. In this study, we identified proteins that associate with α7-nAChRs when Ric-3 is expressed. Using α-bungarotoxin (α-bgtx), we isolated and compared α7-nAChR-associated proteins from two stably transfected, human tumor-derived cell lines: SH-EP1-hα7 expressing human α7-nAChRs and the same cell line further transfected to express Ric-3, SH-EP1-hα7-Ric-3. Mass spectrometric analysis of peptides identified thirty-nine proteins that are associated with α7-nAChRs only when Ric-3 was expressed. Significantly, and consistent with reports of Ric-3 function in the literature, several of the identified proteins are involved in biological processes that may affect nAChR surface expression such as post-translational processing of proteins, protein trafficking, and protein transport. Additionally, proteins affecting the cell cycle, the cytoskeleton, stress responses, as well as cyclic AMP- and inositol triphosphate-dependent signaling cascades were identified. These results illuminate how α-bgtx may be used to isolate and identify α7-nAChRs as well as how the expression of chaperones such as Ric-3 can influence proteins associating with α7-nAChRs. These associating proteins may alter activities of α7-nAChRs to expand their functionally-relevant repertoire as well as to affect biogenesis and membrane trafficking of α7-nAChRs.

Regulation of nicotinic acetylcholine receptors in Alzheimer׳s disease: a possible role of chaperones

Nicotinic acetylcholine receptors (nAChRs) seem to play an integral role in the progress and/or prevention of Alzheimer׳s diseases (AD). Functional abnormalities and problems in biogenesis and trafficking of nAChRs are two major culprits in AD; on the other hand, chaperones modulate post-translational changes in nAChRs. Moreover, they indirectly regulate nAChRs by controlling AD-related proteins such as tau and amyloid beta (Aβ). In this review, we go through recent studies which are showing that chaperones modulate the expression of nAChRs in a subtype-specific manner and explain how AD progress is affected by nAChRs chaperoning.

RIC-3 differentially modulates α4β2 and α7 nicotinic receptor assembly, expression, and nicotine-induced receptor upregulation

Background

Recent work has shown that the chaperone resistant to inhibitors of acetylcholinesterase (RIC-3) is critical for the folding, maturation and functional expression of a variety of neuronal nicotinic acetylcholine receptors. α7 nicotinic receptors can only assemble and functionally express in select lines of cells, provided that RIC-3 is present. In contrast, α4β2 nicotinic receptors can functionally express in many cell lines even without the presence of RIC-3. Depending on the cell line, RIC-3 has differential effects on α4β2 receptor function – enhancement in mammalian cells but inhibition in Xenopus oocytes. Other differences between the two receptor types include nicotine-induced upregulation. When expressed in cell lines, α4β2 receptors readily and robustly upregulate with chronic nicotine exposure. However, α7 nicotinic receptors appear more resistant and require higher concentrations of nicotine to induce upregulation. Could the coexpression of RIC-3 modulate the extent of nicotine-induced upregulation not only for α7 receptors but also α4β2 receptors? We compared and contrasted the effects of RIC-3 on assembly, trafficking, protein expression and nicotine-induced upregulation on both α7 and α4β2 receptors using fluorescent protein tagged nicotinic receptors and Förster resonance energy transfer (FRET) microscopy imaging.

Results

RIC-3 increases assembly and cell surface trafficking of α7 receptors but does not alter α7 protein expression in transfected HEK293T cells. In contrast, RIC-3 does not affect assembly of α4β2 receptors but increases α4 and β2 subunit protein expression. Acute nicotine (30 min exposure) was sufficient to upregulate FRET between α4 and β2 subunits. Surprisingly, when RIC-3 was coexpressed with α4β2 receptors nicotine-induced upregulation was prevented. α7 receptors did not upregulate with acute nicotine in the presence or absence of RIC-3.

Conclusions

These results provide interesting novel data that RIC-3 differentially regulates assembly and expression of different nicotinic receptor subunits. These results also show that nicotine-mediated upregulation of α4β2 receptors can be dynamically regulated by the presence of the chaperone, RIC-3. This could explain a novel mechanism why high affinity α4β2 receptors are upregulated in specific neuronal subtypes in the brain and not others.

Cell-specific effects on surface α7 nicotinic receptor expression revealed by over-expression and knockdown of rat RIC3 protein

We tested whether surface α7 nicotinic acetylcholine receptor expression is dependent on an endogenous chaperone named Resistance to Inhibitors of Cholinesterase 3 (RIC3) by comparing RIC3 protein in rat GH4C1 and human SH-EP1 cells, which express strikingly different surface receptor levels following α7 transfection. Cloned rat RIC3 exists in at least two isoforms because of an ambiguous splice site between exons 4 and 5. Both rat isoforms permit surface α7 expression in SH-EP1 and human embryonic kidney (HEK) cells measured by α-bungarotoxin binding. Contrary to expectations, endogenous RIC3 protein expression determined by immunoblots did not differ between untransfected GH4C1 or SH-EP1 cells. siRNA against rat RIC3 exon 4 and shRNA against exons 2, 5 and 6 knocked down transfected rat RIC3 expression in SH-EP1 cells and simultaneously blocked toxin binding. However, no RNAi construct blocked binding when co-transfected with α7 into GH4C1 cells. shRNA against rat exons 2 and 5 knocked down rat RIC3 protein transfected into GH4C1 cells with a time course suggesting a protein half-life of a few days. These results suggest GH4C1 cells may possess unknown chaperone(s) allowing high surface α7 expression in the absence of known RIC3 splice variants.

Xenopus laevis RIC-3 enhances the functional expression of the C. elegans homomeric nicotinic receptor, ACR-16, in Xenopus oocytes

RIC-3 enhances the functional expression of certain nicotinic acetylcholine receptors (nAChRs) in vertebrates and invertebrates and increases the availability of functional receptors in cultured cells and Xenopus laevis oocytes. Maximal activity of RIC-3 may be cell-type dependent, so neither mammalian nor invertebrate proteins is optimal in amphibian oocytes. We cloned the X. laevis ric-3 cDNA and tested the frog protein in oocyte expression studies. X. laevis RIC-3 shares 52% amino acid identity with human RIC-3 and only 17% with that of Caenorhabditis elegans. We used the C. elegans nicotinic receptor, ACR-16, to compare the ability of RIC-3 from three species to enhance receptor expression. In the absence of RIC-3, the proportion of oocytes expressing detectable nAChRs was greatly reduced. Varying the ratio of acr-16 to X. laevis ric-3 cRNAs injected into oocytes had little impact on the total cell current. When X. laevis, human or C. elegans ric-3 cRNAs were co-injected with acr-16 cRNA (1 : 1 ratio), 100 μM acetylcholine induced larger currents in oocytes expressing X. laevis RIC-3 compared with its orthologues. This provides further evidence for a species-specific component of RIC-3 activity, and suggests that X. laevis RIC-3 is useful for enhancing the expression of invertebrate nAChRs in X. laevis oocytes.

Chaperoning α7 neuronal nicotinic acetylcholine receptors

The α7 subtype of nicotinic acetylcholine receptors (AChRs) is one of the most abundant members of the Cys-loop family of receptors present in the central nervous system. It participates in various physiological processes and has received much attention as a potential therapeutic target for a variety of pathologies. The importance of understanding the mechanisms controlling AChR assembly and cell-surface delivery lies in the fact that these two processes are key to determining the functional pool of receptors actively engaged in synaptic transmission. Here we review recent studies showing that RIC-3, a protein originally identified in the worm Caenorhabditis elegans, modulates the expression of α7 AChRs in a subtype-specific manner. Potentiation of AChR expression by post-transcriptional events is also critically assessed.

Age dependency of inhibition of alpha7 nicotinic receptors and tonically active N-methyl-D-aspartate receptors by endogenously produced kynurenic acid in the brain

In the mouse hippocampus normal levels of kynurenic acid (KYNA), a neuroactive metabolite synthesized in astrocytes primarily by kynurenine aminotransferase II (KAT II)-catalyzed transamination of L-kynurenine, maintain a degree of tonic inhibition of α7 nicotinic acetylcholine receptors (nAChRs). The present in vitro study was designed to test the hypothesis that α7 nAChR activity decreases when endogenous production of KYNA increases. Incubation (2-7 h) of rat hippocampal slices with kynurenine (200 μM) resulted in continuous de novo synthesis of KYNA. Kynurenine conversion to KYNA was significantly decreased by the KAT II inhibitor (S)-(-)-9-(4-aminopiperazine-1-yl)-8-fluoro-3-methyl-6-oxo-2,3,5,6-tetrahydro-4H-1-oxa-3a-azaphenalene-5carboxylic acid (BFF122) (100 μM) and was more effective in slices from postweaned than preweaned rats. Incubation of slices from postweaned rats with kynurenine inhibited α7 nAChRs and extrasynaptic N-methyl-D-aspartate receptors (NMDARs) on CA1 stratum radiatum interneurons. These effects were attenuated by BFF122 and mimicked by exogenously applied KYNA (200 μM). Exposure of human cerebral cortical slices to kynurenine also inhibited α7 nAChRs. The α7 nAChR sensitivity to KYNA is age-dependent, because neither endogenously produced nor exogenously applied KYNA inhibited α7 nAChRs in slices from preweaned rats. In these slices, kynurenine-derived KYNA also failed to inhibit extrasynaptic NMDARs, which could, however, be inhibited by exogenously applied KYNA. In slices from preweaned and postweaned rats, glutamatergic synaptic currents were not affected by endogenously produced KYNA, but were inhibited by exogenously applied KYNA. These results suggest that in the mature brain α7 nAChRs and extrasynaptic NMDARs are in close apposition to KYNA release sites and, thereby, readily accessible to inhibition by endogenously produced KYNA.

Ric-3 promotes alpha7 nicotinic receptor assembly and trafficking through the ER subcompartment of dendrites

The function of Ric-3, which is required for nicotinic acetylcholine receptor (nAChR) expression in C. elegans, is unclear. Here we found that Ric-3 can promote or inhibit cell-surface delivery of alpha-bungarotoxin-binding nAChRs (BgtRs) composed of alpha7 subunits. At low levels, Ric-3 promoted BgtR assembly, endoplasmic reticulum (ER) release, and cell-surface delivery without trafficking from the ER. At high Ric-3 levels, Ric-3 suppressed BgtR surface delivery, but not its assembly, and BgtRs were retained in the ER or in Ric-3-containing aggregates. In PC12 cells, native BgtRs trafficked to the cell surface from the ER where low levels of endogenous Ric-3 were observed. In cultured neurons, native Ric-3 levels were higher than in PC12 cells, and Ric-3 and alpha7 subunits were found in somata and dendrites, but not axons, of inhibitory interneurons. Ric-3 trafficked with alpha7 subunits in rapidly moving vesicles to dendrites, where it was restricted to the ER subcompartment. We conclude that Ric-3 has two potential functions. At low levels, Ric-3 interactions are short-lived and promote BgtR assembly and ER release. At higher levels, Ric-3 interactions are longer-lived and mediate ER retention. In neurons, Ric-3 ER retention appears to promote transport within the dendritic ER subcompartment, thereby restricting alpha7 trafficking to dendrites and preventing axonal transport.

Mouse RIC-3, an endoplasmic reticulum chaperone, promotes assembly of the alpha7 acetylcholine receptor through a cytoplasmic coiled-coil domain

RIC-3 (resistant to inhibitor of cholinesterase) is a transmembrane protein, found in invertebrates and vertebrates, that modulates the surface expression of a variety of nicotinic acetylcholine receptors (nAChRs) in neurons and other cells. To understand its mechanism of action, we investigated the cellular location, transmembrane topology and cellular mechanism by which RIC-3 facilitates alpha7 assembly and surface expression in cultured mammalian cells. We show that the mouse protein is targeted to the ER by the first 31 aa which act as a cleavable signal sequence. The mature protein is a single-pass type I transmembrane protein whose N terminus resides in the lumen of the ER with the coiled-coil domain in the cytoplasm. RIC-3, which binds both unfolded and folded alpha7 subunits, facilitates the surface expression of receptor principally by promoting the folding and assembly of the alpha7 subunits in the ER into fully polymerized receptor. Functional analysis shows that facilitation of surface expression of alpha7 in mammalian cells is reduced in RIC-3 mutants lacking the signal peptide, the lumenal segment or the coiled-coil domain, but not in mutants lacking the long C-terminal region downstream of the coiled-coil domain. We show that the coiled-coil domain of mRIC-3 is not required for the interaction of mRIC-3 with alpha7, but does mediate a homotypic interaction between molecules of mRIC-3. We suggest that efficient assembly of the homomeric alpha7 nAChR may thus require mRIC-3 self-association through the cytoplasmic coiled-coil domain and suggest a model by which this may occur.

The ubiquitin-proteasome system regulates the stability of neuronal nicotinic acetylcholine receptors

Ubiquitination is a key event for protein degradation by the proteasome system, membrane protein internalization, and protein trafficking among cellular compartments. Few data are available on the role of the ubiquitin-proteasome system (UPS) in the trafficking of neuronal nicotinic acetylcholine receptors (nAChRs). Experiments conducted in neuron-like differentiated rat pheochromocytoma cells (PC12 cells) show that the alpha3, beta2, and beta4 nAChR subunits are ubiquitinated and that their ubiquitination is necessary for degradation. A 24-h treatment with the proteasome inhibitor PS-341 increased the total levels of alpha3 and the two beta subunits in both whole cell lysates and fractions enriched for the ER/Golgi compartment. nAChR subunit upregulation was also detected in plasma membrane-enriched fractions. Inhibition of the lysosomal degradation machinery by E-64 had a significantly smaller effect on nAChR turnover. The present data, together with previous results showing that the alpha7 nAChR subunit is a target of the UPS, point to a prominent role of the proteasome in nAChR trafficking.

Haemonchus contortus acetylcholine receptors of the DEG-3 subfamily and their role in sensitivity to monepantel

Gastro-intestinal nematodes in ruminants, especially Haemonchus contortus, are a global threat to sheep and cattle farming. The emergence of drug resistance, and even multi-drug resistance to the currently available classes of broad spectrum anthelmintics, further stresses the need for new drugs active against gastro-intestinal nematodes. A novel chemical class of synthetic anthelmintics, the Amino-Acetonitrile Derivatives (AADs), was recently discovered and the drug candidate AAD-1566 (monepantel) was chosen for further development. Studies with Caenorhabditis elegans suggested that the AADs act via nicotinic acetylcholine receptors (nAChR) of the nematode-specific DEG-3 subfamily. Here we identify nAChR genes of the DEG-3 subfamily from H. contortus and investigate their role in AAD sensitivity. Using a novel in vitro selection procedure, mutant H. contortus populations of reduced sensitivity to AAD-1566 were obtained. Sequencing of full-length nAChR coding sequences from AAD-susceptible H. contortus and their AAD-1566-mutant progeny revealed 2 genes to be affected. In the gene monepantel-1 (Hco-mptl-1, formerly named Hc-acr-23H), a panel of mutations was observed exclusively in the AAD-mutant nematodes, including deletions at intron-exon boundaries that result in mis-spliced transcripts and premature stop codons. In the gene Hco-des-2H, the same 135 bp insertion in the 5′ UTR created additional, out of frame start codons in 2 independent H. contortus AAD-mutants. Furthermore, the AAD mutants exhibited altered expression levels of the DEG-3 subfamily nAChR genes Hco-mptl-1, Hco-des-2H and Hco-deg-3H as quantified by real-time PCR. These results indicate that Hco-MPTL-1 and other nAChR subunits of the DEG-3 subfamily constitute a target for AAD action against H. contortus and that loss-of-function mutations in the corresponding genes may reduce the sensitivity to AADs.

Worldwide, sheep and cattle farming are threatened by anthelmintic-resistant gastro-intestinal nematodes. A novel chemical class of synthetic anthelmintics was recently discovered, the Amino-Acetonitrile Derivatives (AADs), which exhibit excellent efficacy against various species of livestock-pathogenic nematodes and, more importantly, overcome existing resistances to the currently available anthelmintics. Haemonchus contortus, the largest nematode found in the abomasum of sheep and cattle, is a blood-feeding parasite that causes severe anemia that can lead to the sudden death of the infected animal; H. contortus is highly susceptible to AADs. In order to elucidate the mode of action of the AADs, we have developed 2 independent H. contortus mutants with reduced sensitivity to monepantel (AAD-1566). Both mutants were affected in their acetylcholine receptor (ACR) genes of the DEG-3 subfamily. In particular, we discovered a panel of mutations in the gene monepantel-1 (Hco-mptl-1) including deletions leading to mis-splicing, insertions and point mutations leading to premature termination of translation of the protein. These findings support the notion that Hco-MPTL-1 and other nAChR subunits of the DEG-3 subfamily are targets of the AADs. The fact that the DEG-3 subfamily of acetylcholine receptors is nematode-specific may explain the good therapeutic index of AADs in mammals.

Molecular cloning and characterization of a novel human variant of RIC-3, a putative chaperone of nicotinic acetylcholine receptors

Recent reports demonstrate that the RIC-3 (resistant to inhibitors of cholinesterase-3) protein is important for the maturation of nAChRs (nicotinic acetylcholine receptors). In the present study RIC-3e, a novel variant of RIC-3, is described. This variant contains a deletion of exons 4 and 5 of RIC-3, resulting in a protein product lacking a conserved coiled-coil domain. Like RIC-3, the new variant is predominantly, but not exclusively, expressed in the brain. The analysis of expression of variant RIC-3 mRNA and of alpha7-nAChR mRNA in a set of human tissues shows a similar profile. The RIC-3e protein is functionally active and enables surface expression of mature alpha7-nAChRs in cell lines not otherwise permissive for the expression of this receptor.

RIC-3: a nicotinic acetylcholine receptor chaperone

RIC-3 is a transmembrane protein which acts as a molecular chaperone of nicotinic acetylcholine receptors (nAChRs). For some nAChR subtypes (such as homomeric alpha7 neuronal nAChRs), RIC-3 is required for efficient receptor folding, assembly and functional expression. In contrast, for other nAChR subtypes (such as heteromeric alpha4beta2 neuronal nAChRs) there have been reports that RIC-3 can both enhance and reduce levels of functional expression. There is also evidence that RIC-3 can modulate maturation of the closely related 5-hydroxytryptamine (5-HT) receptor (5-HT(3)R). As with heteromeric nAChRs, apparently contradictory results have been reported for the influence of RIC-3 on 5-HT(3)R maturation in different expression systems. Recent evidence indicates that these differences in RIC-3 chaperone activity may be influenced by the host cell, suggesting that other proteins may play an important role in modulating the effects of RIC-3 as a chaperone. RIC-3 was originally identified in the nematode Caenorhabditis elegans as the protein encoded by the gene ric-3 (resistance to inhibitors of cholinesterase) and has subsequently been cloned and characterized from mammalian and insect species. This review provides a brief history of RIC-3; from the identification of the ric-3 gene in C. elegans in 1995 to the more recent demonstration of its activity as a nAChR chaperone.