Dual Role of the RIC-3 Protein in Trafficking of Serotonin and Nicotinic Acetylcholine Receptors

N-Glycosylation Deficiency in Transgene α7 nAChR and RIC3 Expressing CHO Cells Without NACHO

The human neuronal nicotinic acetylcholine receptor α7 (nAChR) is an important target implicated in diseases like Alzheimer's or Parkinson's, as well as a validated target for drug discovery. For α7 nAChR model systems, correct folding and ion influx functions are essential. Two chaperones, resistance to inhibitors of cholinesterase 3 (RIC3) and novel nAChR regulator (NACHO), enhance the assembly and function of α7 nAChR. This study investigates the consequence of NACHO absence on α7 nAChR expression and function. Therefore, the sequences of human α7 nAChR and human RIC3 were transduced in Chinese hamster ovary (CHO) cells. Protein expression and function of α7 nAChR were confirmed by Western blot and voltage clamp, respectively. Cellular viability was assessed by cell proliferation and lactate dehydrogenase assays. Intracellular and extracellular expression were determined by in/on-cell Western, compared with another nAChR subtype by novel cluster fluorescence-linked immunosorbent assay, and N-glycosylation efficiency was assessed by glycosylation digest. The transgene CHO cell line showed expected protein expression and function for α7 nAChR and cell viability was barely influenced by overexpression. While intracellular levels of α7 nAChR were as anticipated, plasma membrane insertion was low. The glycosylation digest revealed no appreciable N-glycosylation product. This study demonstrates a stable and functional cell line expressing α7 nAChR, whose protein expression, function, and viability are not affected by the absence of NACHO. The reduced plasma membrane insertion of α7 nAChR, combined with incorrect matured N-glycosylation at the Golgi apparatus, suggests a loss of recognition signal for lectin sorting.

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.

Binding motif for RIC-3 chaperon protein in serotonin type 3A receptors

Serotonin or 5-hydroxytryptamine type 3 (5-HT3) receptors belong to the family of pentameric ligand-gated ion channels (pLGICs) that are therapeutic targets for psychiatric disorders and neurological diseases. Due to structural conservation and significant sequence similarities of pLGICs' extracellular and transmembrane domains, clinical trials for drug candidates targeting these two domains have been hampered by off-subunit modulation. With the present study, we explore the interaction interface of the 5-HT3A subunit intracellular domain (ICD) with the resistance to inhibitors of choline esterase (RIC-3) protein. Previously, we have shown that RIC-3 interacts with the L1-MX segment of the ICD fused to maltose-binding protein. In the present study, synthetic L1-MX-based peptides and Ala-scanning identify positions W347, R349, and L353 as critical for binding to RIC-3. Complementary studies using full-length 5-HT3A subunits confirm that the identified Ala substitutions reduce the RIC-3-mediated modulation of functional surface expression. Additionally, we find and characterize a duplication of the binding motif, DWLR…VLDR, present in both the MX-helix and the transition between the ICD MA-helix and transmembrane segment M4. Analogous Ala substitutions at W447, R449, and L454 disrupt MAM4-peptide RIC-3 interactions and reduce modulation of functional surface expression. In summary, we identify the binding motif for RIC-3 in 5-HT3A subunits at two locations in the ICD, one in the MX-helix and one at the MAM4-helix transition.

Deletion induced splicing in RIC3 drives nicotinic acetylcholine receptor regulation with implications for endoplasmic reticulum stress in human astrocytes

Nicotinic acetylcholine receptor (nAChR) dysregulation in astrocytes is reported in neurodegenerative disorders. Modulation of nAChRs through agonists confers protection to astrocytes from stress but regulation of chaperones involved in proteostasis with pathological implications is unclear. Resistance to inhibitors of cholinesterase 3 (RIC3), a potential chaperone of nAChRs is poorly studied in humans. We characterized RIC3 in astrocytes derived from an isogenic wild-type and Cas9 edited "del" human iPSC line harboring a 25 bp homozygous deletion in exon2. Altered RIC3 transcript ratio due to deletion induced splicing and an unexpected gain of α7nAChR expression were observed in "del" astrocytes. Transcriptome analysis showed higher expression of neurotransmitter/G-protein coupled receptors mediated by cAMP and calcium/calmodulin-dependent kinase signaling with increased cytokines/glutamate secretion. Functional implications examined using tunicamycin induced ER stress in wild-type astrocyte stress model showed cell cycle arrest, RIC3 upregulation, reduction in α7nAChR membrane levels but increased α4nAChR membrane expression. Conversely, tunicamycin-treated "del" astrocytes showed a comparatively higher α4nAChR membrane expression and upsurged cAMP signaling. Furthermore, reduced expression of stress markers CHOP, phospho-PERK and lowered XBP1 splicing in western blot and qPCR, validated by proteome-based pathway analysis indicated lowered disease severity. Findings indicate (i) a complex RNA regulatory mechanism via exonic deletion induced splicing; (ii) RIC-3 as a disordered protein having contrasting effects on co-expressed nAChR subtypes under basal/stress conditions; and (iii) RIC3 as a potential drug target against ER stress in astrocytes for neurodegenerative/nicotine-related brain disorders. Cellular rescue mechanism through deletion induced exon skipping may encourage ASO-based therapies for tauopathies.

SARS-CoV-2 Spike Protein Downregulates Cell Surface α7nAChR through a Helical Motif in the Spike Neck

A deficiency of the functional α7 nicotinic acetylcholine receptor (α7nAChR) impairs neuronal and immune systems. The SARS-CoV-2 spike protein (S12) facilitates virus cell entry during COVID-19 infection and can also independently disrupt cellular functions. Here, we found that S12 expression significantly downregulated surface expression of α7nAChR in mammalian cells. A helical segment of S12 (L1145-L1152) in the spike neck was identified to be responsible for the downregulation of α7nAChR, as the mutant S12AAA (L1145A-F1148A-L1152A) had minimal effects on surface α7nAChR expression. This S12 segment is homologous to the α7nAChR intracellular helical motif known for binding chaperone proteins RIC3 and Bcl-2 to promote α7nAChR surface expression. Competition from S12 for binding these proteins likely underlies suppression of surface α7nAChR. Considering the critical roles of α7nAChR in cellular functions, these findings provide a new perspective for improving mRNA vaccines and developing treatment options for certain symptoms related to long COVID.

Different efficiency of auxiliary/chaperone proteins to promote the functional reconstitution of honeybee glutamate and acetylcholine receptors in Xenopus laevis oocytes

Heterologous expression systems (e.g., Xenopus laevis oocytes) are useful to study the biophysical properties and pharmacology of ionotropic receptors such as ionotropic glutamate (iGLuRs) and nicotinic acetylcholine (nAChRs) receptors. However, insect receptors often require the co-expression of chaperone proteins to be functional. Only few iGluRs and nAChRs have been successfully expressed in such systems. Here, we compared the efficiency of chaperone proteins to promote the functional expression of one Apis mellifera iGluR and several nAChR subunit combinations (α1α8β1, α7, α2α8β1 and α2α7α8β1) in Xenopus oocytes. To this end, we cloned a new iGluR (GluR-1) and potential chaperone proteins (e.g., SOL-1, Neto, NACHO) and tested more than 40 combinations of human, nematode and honeybee proteins. We obtained robust expression of GluR-1 and α1α8β1 when co-expressed with honeybee chaperone proteins and found that nAChR expression critically depended on the α1 subunit N-terminal sequence. We recorded small ACh-gated currents in few oocytes when the α7 subunit was co-expressed with Caenorhabditis elegans RIC-3, but none of the chaperone proteins allowed efficient expression of α2α8β1 or α2α7α8β1. Our results show that only some protein combinations can reconstitute functional receptors in Xenopus oocytes and that protein combination efficient in one species is not always efficient in another species.

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.

Nicotinic acetylcholine receptor redux: Discovery of accessories opens therapeutic vistas

The neurotransmitter acetylcholine (ACh) acts in part through a family of nicotinic ACh receptors (nAChRs), which mediate diverse physiological processes including muscle contraction, neurotransmission, and sensory transduction. Pharmacologically, nAChRs are responsible for tobacco addiction and are targeted by medicines for hypertension and dementia. Nicotinic AChRs were the first ion channels to be isolated. Recent studies have identified molecules that control nAChR biogenesis, trafficking, and function. These nAChR accessories include protein and chemical chaperones as well as auxiliary subunits. Whereas some factors act on many nAChRs, others are receptor specific. Discovery of these regulatory mechanisms is transforming nAChR research in cells and tissues ranging from central neurons to spinal ganglia to cochlear hair cells. Nicotinic AChR-specific accessories also enable drug discovery on high-confidence targets for psychiatric, neurological, and auditory disorders.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Cholinergic transmission in C. elegans: Functions, diversity, and maturation of ACh-activated ion channels

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.

RIC3, the cholinergic anti-inflammatory pathway, and neuroinflammation

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.

Delineating the Site of Interaction of the 5-HT3A Receptor with the Chaperone Protein RIC-3

The serotonin type 3A (5-HT_3A) receptor is a homopentameric cation-selective member of the pentameric ligand-gated ion channel (pLGIC) superfamily. Members of this superfamily assemble from five subunits, each of which consists of three domains: extracellular (ECD), transmembrane (TMD), and intracellular domain (ICD). Previously, we have demonstrated that the 5-HT_3A-ICD is required for the interaction between 5-HT_3A and the chaperone protein resistance to inhibitors of choline esterase (RIC-3). Additionally, we have shown that 5-HT_3A-ICD fused to maltose-binding protein (MBP) directly interacts with RIC-3, without the involvement of other protein(s). To elucidate the molecular determinants of this interaction, we developed different MBP-fused 5-HT_3A-ICD constructs by deleting large segments of its amino acid sequence. We expressed seven engineered ICDs in Escherichia coli and purified them to homogeneity. Using a RIC-3 affinity pull-down assay, the interaction between MBP-5HT_3A-ICD constructs and RIC-3 was investigated. In summary, we identify a 24-amino-acid-long segment of the 5-HT_3A-ICD as a molecular determinant for the interaction between the 5-HT_3A-ICD and RIC-3.

α7 nicotinic acetylcholine receptor upregulation by anti-apoptotic Bcl-2 proteins

Nicotinic acetylcholine receptors (nAChRs) mediate and modulate synaptic transmission throughout the brain, and contribute to learning, memory, and behavior. Dysregulation of α7-type nAChRs in neuropsychiatric as well as immunological and oncological diseases makes them attractive targets for pharmaceutical development. Recently, we identified NACHO as an essential chaperone for α7 nAChRs. Leveraging the robust recombinant expression of α7 nAChRs with NACHO, we utilized genome-wide cDNA library screening and discovered that several anti-apoptotic Bcl-2 family proteins further upregulate receptor assembly and cell surface expression. These effects are mediated by an intracellular motif on α7 that resembles the BH3 binding domain of pro-apoptotic Bcl-2 proteins, and can be blocked by BH3 mimetic Bcl-2 inhibitors. Overexpression of Bcl-2 member Mcl-1 in neurons enhanced surface expression of endogenous α7 nAChRs, while a combination of chemotherapeutic Bcl2-inhibitors suppressed neuronal α7 receptor assembly. These results demonstrate that Bcl-2 proteins link α7 nAChR assembly to cell survival pathways.

The α7 nicotinic acetylcholine receptor (nAChR) plays a major role in shaping the activity of neuronal circuits and contributes to the pathophysiology of several neurological disorders. Following cDNA library screening, the authors identify anti-apoptotic, Bcl-2 family proteins as enhancers of α7 nAChR assembly, acting through an intracellular BH3-like domain during receptor biogenesis in the endoplasmic reticulum.

DRD4 methylation as a potential biomarker for physical aggression: An epigenome-wide, cross-tissue investigation

Epigenetic processes that regulate gene expression, such as DNA methylation (DNAm), have been linked to individual differences in physical aggression. Yet, it is currently unclear whether: (a) DNAm patterns in humans associate with physical aggression independently of other co-occurring psychiatric and behavioral symptoms; (b) whether these patterns are observable across multiple tissues; and (c) whether they may function as a causal versus noncausal biomarker of physical aggression. Here, we used a multisample, cross-tissue design to address these questions. First, we examined genome-wide DNAm patterns (buccal swabs; Illumina 450k) associated with engagement in physical fights in a sample of high-risk youth (n = 119; age = 16-24 years; 53% female). We identified one differentially methylated region in DRD4, which survived genome-wide correction, associated with physical aggression above and beyond co-occurring symptomatology (e.g., ADHD, substance use), and showed strong cross-tissue concordance with both blood and brain. Second, we found that DNAm sites within this region were also differentially methylated in an independent sample of young adults, between individuals with a history of chronic-high versus low physical aggression (peripheral T cells; ages 26-28). Finally, we ran a Mendelian randomization analysis using GWAS data from the EAGLE consortium to test for a causal association of DRD4 methylation with physical aggression. Only one genetic instrument was eligible for the analysis, and results provided no evidence for a causal association. Overall, our findings lend support for peripheral DRD4 methylation as a potential biomarker of physically aggressive behavior, with no evidence yet of a causal relationship.

A human-specific, truncated α7 nicotinic receptor subunit assembles with full-length α7 and forms functional receptors with different stoichiometries

The cholinergic α7 nicotinic receptor gene, CHRNA7, encodes a subunit that forms the homopentameric α7 receptor, involved in learning and memory. In humans, exons 5-10 in CHRNA7 are duplicated and fused to the FAM7A genetic element, giving rise to the hybrid gene CHRFAM7A Its product, dupα7, is a truncated subunit lacking part of the N-terminal extracellular ligand-binding domain and is associated with neurological disorders, including schizophrenia, and immunomodulation. We combined dupα7 expression on mammalian cells with patch clamp recordings to understand its functional role. Transfected cells expressed dupα7 protein, but they exhibited neither surface binding of the α7 antagonist α-bungarotoxin nor responses to acetylcholine (ACh) or to an allosteric agonist that binds to the conserved transmembrane region. To determine whether dupα7 assembles with α7, we generated receptors comprising α7 and dupα7 subunits, one of which was tagged with conductance substitutions that report subunit stoichiometry and monitored ACh-elicited channel openings in the presence of a positive allosteric α7 modulator. We found that α7 and dupα7 subunits co-assemble into functional heteromeric receptors, which require at least two α7 subunits for channel opening, and that dupα7's presence in the pentameric arrangement does not affect the duration of the potentiated events compared with that of α7. Using an α7 subunit mutant, we found that activation of (α7)₂(dupα7)₃ receptors occurs through ACh binding at the α7/α7 interfacial binding site. Our study contributes to the understanding of the modulation of α7 function by the human specific, duplicated subunit, associated with human disorders.

The wonderland of neuronal nicotinic acetylcholine receptors

Nearly 30 years of experimental evidence supports the argument that ligands of nicotinic acetylcholine receptors (nAChRs) have potential as therapeutic agents. However, as in the famous Lewis Carroll novel "Alice in Wonderland", there have been many unexpected adventures along the pathway of development, and few nAChR ligands have been approved for any clinical condition to date with the exception of nicotine dependence. The recent failures of nAChR ligands in AD and schizophrenia clinical trials have reduced enthusiasm for this therapeutic strategy and many pharmaceutical companies have now abandoned this field of research. As with other clinical failures, multiple questions arise as to the basis for the failure. More generic questions focus on a potential translational gap between the animal models used and the human clinical condition they are meant to simulate, or the clinical trial mindset that large Ns have to be achieved for statistical power (often requiring multiple trial sites) as opposed to smaller patient cohorts at limited sites where conditions can be better controlled and replicated. More specific to the nAChR field are questions about subtype selectivity, dose selection, whether an agonist, antagonist, or allosteric modulator strategy is best, etc. The purpose of this review is to discuss each of these questions, but also to provide a brief overview of the remarkable progress that has been made over the last three decades in our understanding of this unique ligand-gated ion channel and how this new knowledge may help us improve drug development successes in the future.

Dual effects of insect nAChR chaperone RIC-3 on hybrid receptor: Promoting assembly on endoplasmic reticulum but suppressing transport to plasma membrane on Xenopus oocytes

Resistance to inhibitors of cholinesterase (RIC) -3 promotes the maturation (folding and assembly) of neuronal nicotinic acetylcholine receptors (nAChRs) as a molecular chaperone. The modulation effects of RIC-3 on homomeric α7 nAChRs are always positive, but its effects on heteromeric subtypes are inconsistent among reports. In this study, five RIC-3 isoforms were identified from Locusta migratoria. Four isoforms showed obvious effects on hybrid receptor Locα1/rβ2 expressed in Xenopus oocytes. As a representative, the co-expression of RIC-3v4 exhibited the decreased agonist responses (I_max) on oocytes, lower specific [³H]epibatidine binding (B_max) on plasma membrane protein (PMP), and reduced subunit levels in PMP, which showed that the mature Locα1/rβ2 on the plasma membrane was decreased by the co-expression of RIC-3. In contrast, the [³H]epibatidine binding and mature Locα1/rβ2 levels in the endoplasmic reticulum membrane protein (ERMP) were much increased when co-expressing with RIC-3v4. The [³H]epibatidine binding and mature Locα1/rβ2 levels in total membrane protein (TMP) gave the similar results as that in ERMP. Taking data together, the results showed that the co-expression of RIC-3 increased the mature Locα1/rβ2 receptor levels on ER of Xenopus oocytes, but these mature receptors were mostly kept on ER and suppressed to transport to plasma membrane.

Genetic analysis of the RIC3 gene in Han Chinese patients with Parkinson's disease

Parkinson's disease (PD) is the second-most common etiologically complex neurodegenerative disease. Genetic abnormalities are thought to play an important role in the development of PD. Recently, mutations in the resistance to inhibitors of cholinesterase 3 gene (RIC3) have been reported to cause autosomal-dominant PD in Indian population. To determine whether RIC3 gene coding variant(s) are associated with PD in Han Chinese population, the RIC3 gene coding region in 218 mainland Han Chinese patients with PD and the identified variants in 242 normal controls were examined using direct sequencing analysis. Four known single nucleotide variants (c.354C>A, p.L118L, rs10839976; c.389G>A, p.C130Y, rs55990541; c.403C>T, p.P135S, rs73411617; and c.1054G>A, p.D352N, rs11826236) were identified in the RIC3 gene coding region. No significant differences were observed in either genotypic or allelic distributions between the PD patients and the normal controls (all P>0.05) for these four variants. Haplotype analysis showed that the presence of haplotype A-G-C-G (rs10839976-rs55990541-rs73411617-rs11826236) was associated with a 0.764-fold decreased risk (P=0.049, OR=0.764, 95% CI=0.585-0.999) for PD, whereas the presence of haplotype C-A-C-A was associated with a 2.143-fold increased risk (P=0.039, OR=2.143, 95% CI=1.023-4.488) for PD. The findings indicate that four variants: rs10839976, rs55990541, rs73411617 and rs11826236 in the RIC3 gene coding region may play little or no role in the development of PD. Two RIC3 gene haplotypes of four variants: A-G-C-G, and C-A-C-A might relate to either protection against or increased susceptibility to PD in the Han Chinese population, respectively.

Proteins and chemical chaperones involved in neuronal nicotinic receptor expression and function: an update

Neuronal nicotinic ACh receptors (nAChRs) are a family of ACh-gated cation channels, and their homeostasis or proteostasis is essential for the correct physiology of the central and peripheral nervous systems. The proteostasis network regulates the folding, assembly, degradation and trafficking of nAChRs in order to ensure their efficient and functional expression at the cell surface. However, as nAChRs are multi-subunit, multi-span, integral membrane proteins, the folding and assembly is a very inefficient process, and only a small proportion of subunits can form functional pentamers. Moreover, the efficiency of assembly and trafficking varies widely depending on the nAChR subtypes and the cell type in which they are expressed. A detailed understanding of the mechanisms that regulate the functional expression of nAChRs in neurons and non-neuronal cells is therefore important. The purpose of this short review is to describe more recent findings concerning the chaperone proteins and target-specific and target-nonspecific pharmacological chaperones that modulate the expression of nAChR subtypes, and the possible mechanisms that underlie the dynamic changes of cell surface nAChRs.

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.

Understanding the Bases of Function and Modulation of α7 Nicotinic Receptors: Implications for Drug Discovery

The nicotinic acetylcholine receptor (nAChR) belongs to a superfamily of pentameric ligand-gated ion channels involved in many physiologic and pathologic processes. Among nAChRs, receptors comprising the α7 subunit are unique because of their high Ca(2+) permeability and fast desensitization. nAChR agonists elicit a transient ion flux response that is further sustained by the release of calcium from intracellular sources. Owing to the dual ionotropic/metabotropic nature of α7 receptors, signaling pathways are activated. The α7 subunit is highly expressed in the nervous system, mostly in regions implicated in cognition and memory and has therefore attracted attention as a novel drug target. Additionally, its dysfunction is associated with several neuropsychiatric and neurologic disorders, such as schizophrenia and Alzheimer's disease. α7 is also expressed in non-neuronal cells, particularly immune cells, where it plays a role in immunity, inflammation, and neuroprotection. Thus, α7 potentiation has emerged as a therapeutic strategy for several neurologic and inflammatory disorders. With unique activation properties, the receptor is a sensitive drug target carrying different potential binding sites for chemical modulators, particularly agonists and positive allosteric modulators. Although macroscopic and single-channel recordings have provided significant information about the underlying molecular mechanisms and binding sites of modulatory compounds, we know just the tip of the iceberg. Further concerted efforts are necessary to effectively exploit α7 as a drug target for each pathologic situation. In this article, we focus mainly on the molecular basis of activation and drug modulation of α7, key pillars for rational drug design.

A New Mechanism of Receptor Targeting by Interaction between Two Classes of Ligand-Gated Ion Channels

The 5-HT3 receptors are serotonin-gated ion channels that physically couple with purinergic P2X2 receptors to trigger a functional cross-inhibition leading to reciprocal channel occlusion. Although this functional receptor-receptor coupling seems to serve a modulatory role on both channels, this might not be its main physiological purpose. Using primary cultures of rat hippocampal neurons as a quantitative model of polarized targeting, we show here a novel function for this interaction. In this model, 5-HT3A receptors did not exhibit by themselves the capability of distal targeting in dendrites and axons but required the presence of P2X2R for their proper subcellular localization. 5-HT3AR distal targeting occurred with a delayed time course and exhibited a neuron phenotype dependency. In the subpopulation of neurons expressing endogenous P2X2R, 5-HT3AR distal neuritic localization correlated with P2X2R expression and could be selectively inhibited by P2X2R RNA interference. Cotransfection of both receptors revealed a specific colocalization, cotrafficking in common surface clusters, and the axonal rerouting of 5-HT3AR. The physical association between the two receptors was dependent on the second intracellular loop of the 5-HT3A subunit, but not on the P2X2R C-terminal tail that triggers the functional cross-inhibition with the 5-HT3AR. Together, these data establish that 5-HT3AR distal targeting in axons and dendrites primarily depends on P2X2R expression. Because several P2XR have now been shown to functionally interact with several other members of the 4-TMD family of receptor channels, we propose to reconsider the real functional role for this receptor family, as trafficking partner proteins dynamically involved in other receptors targeting.

SIGNIFICANCE STATEMENT

So far, receptor targeting mechanisms were found to involve intracellular partner proteins or supramolecular complexes that couple receptors to cytoskeletal elements and recruit them into cargo vesicles. In this paper, we describe a new trafficking mechanism for the neuronal serotonin 5-HT3A ionotropic channel receptor, in which the role of routing partner is endowed by a functionally interacting purinergic receptor: the P2X2 receptor. This work not only unveils the mechanism by which 5-HT3 receptors can reach their axonal localization required for the control of neurotransmitter release, but also suggests that, in addition to their modulatory role, the family of P2X receptors could have a previously undescribed functional role of trafficking partner proteins dynamically involved in the targeting of other receptors.

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.

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.

An Unaltered Orthosteric Site and a Network of Long-Range Allosteric Interactions for PNU-120596 in α7 Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs) are vital to neuronal signaling, are implicated in important processes such as learning and memory, and are therapeutic targets for neural diseases. The α7 nAChR has been implicated in Alzheimer's disease and schizophrenia, and allosteric modulators have become one focus of drug development efforts. We investigate the mode of action of the α7-selective positive allosteric modulator, PNU-120596, and show that the higher potency of acetylcholine in the presence of PNU-120596 is not due to an altered agonist binding site. In addition, we propose several residues in the gating interface and transmembrane region that are functionally important to transduction of allosteric properties, and link PNU-120596, the acetylcholine binding region, and the receptor gate. These results suggest global protein stabilization from a communication network through several key residues that alter the gating equilibrium of the receptor while leaving the agonist binding properties unperturbed.

Ly6h regulates trafficking of alpha7 nicotinic acetylcholine receptors and nicotine-induced potentiation of glutamatergic signaling

α7 nAChRs are expressed widely throughout the brain, where they are important for synaptic signaling, gene transcription, and plastic changes that regulate sensory processing, cognition, and neural responses to chronic nicotine exposure. However, the mechanisms by which α7 nAChRs are regulated are poorly understood. Here we show that trafficking of α7-subunits is controlled by endogenous membrane-associated prototoxins in the Ly6 family. In particular, we find that Ly6h reduces cell-surface expression and calcium signaling by α7 nAChRs. We detect Ly6h in several rat brain regions, including the hippocampus, where we find it is both necessary and sufficient to limit the magnitude of α7-mediated currents. Consistent with such a regulatory function, knockdown of Ly6h in rat hippocampal pyramidal neurons enhances nicotine-induced potentiation of glutamatergic mEPSC amplitude, which is known to be mediated by α7 signaling. Collectively our data suggest a novel cellular role for Ly6 proteins in regulating nAChRs, which may be relevant to plastic changes in the nervous system including rewiring of glutamatergic circuitry during nicotine addiction.

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.

Drug-dependent behaviors and nicotinic acetylcholine receptor expressions in Caenorhabditis elegans following chronic nicotine exposure

Nicotine, the major psychoactive compound in tobacco, targets nicotinic acetylcholine receptors (nAChRs) and results in drug dependence. The nematode Caenorhabditis elegans' (C. elegans) genome encodes conserved and extensive nicotinic receptor subunits, representing a useful system to investigate nicotine-induced nAChR expressions in the context of drug dependence. However, the in vivo expression pattern of nAChR genes under chronic nicotine exposure has not been fully investigated. To define the role of nAChR genes involved in nicotine-induced locomotion changes and the development of tolerance to these effects, we characterized the locomotion behavior combining the use of two systems: the Worm Tracker hardware and the WormLab software. Our results indicate that the combined system is an advantageous alternative to define drug-dependent locomotion behavior in C. elegans. Chronic (24-h dosing) nicotine exposure at 6.17 and 61.7μM induced nicotine-dependent behaviors, including drug stimulation, tolerance/adaption, and withdrawal responses. Specifically, the movement speed of naïve worms on nicotine-containing environments was significantly higher than on nicotine-free environments, suggesting locomotion stimulation by nicotine. In contrast, the 24-h 6.17μM nicotine-treated worms exhibited significantly higher speeds on nicotine-free plates than on nicotine-containing plates. Furthermore significantly increased locomotion behavior during nicotine cessation was observed in worms treated with a higher nicotine concentration of 61.7μM. The relatively low locomotion speed of nicotine-treated worms on nicotine-containing environments also indicates adaption/tolerance of worms to nicotine following chronic nicotine exposure. In addition, this study provides useful information regarding the comprehensive in vivo expression profile of the 28 "core" nAChRs following different dosages of chronic nicotine treatments. Eleven genes (lev-1, acr-6, acr-7, acr-11, lev-8, acr-14, acr-16, acr-20, acr-21, ric-3, and unc-29) were significantly up-regulated following 61.7μM nicotine treatment, in which worms showed significantly increased locomotion behavior. This study provides insights into the linkage between nicotine-induced locomotion behavior and the regulation of nicotinic acetylcholine receptors.

Expression of cloned α6* nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (AChRs) are ACh-gated ion channels formed from five homologous subunits in subtypes defined by their subunit composition and stoichiometry. Some subtypes readily produce functional AChRs in Xenopus oocytes and transfected cell lines. α6β2β3* AChRs (subtypes formed from these subunits and perhaps others) are not easily expressed. This may be because the types of neurons in which they are expressed (typically dopaminergic neurons) have unique chaperones for assembling α6β2β3* AChRs, especially in the presence of the other AChR subtypes. Because these relatively minor brain AChR subtypes are of major importance in addiction to nicotine, it is important for drug development as well as investigation of their functional properties to be able to efficiently express human α6β2β3* AChRs. We review the issues and progress in expressing α6* AChRs. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

The duplicated α7 subunits assemble and form functional nicotinic receptors with the full-length α7

The α7 nicotinic acetylcholine receptor gene (CHRNA7) is linked to schizophrenia. A partial duplication of CHRNA7 (CHRFAM7A) is found in humans on 15q13-14. Exon 6 of CHRFAM7A harbors a 2-bp deletion polymorphism, CHRFAM7AΔ2bp, which is also associated with schizophrenia. To understand the effects of the duplicated subunits on α7 receptors, we fused α7, dupα7, and dupΔα7 subunits with various fluorescent proteins. The duplicated subunits co-localized with full-length α7 subunits in mouse neuroblastoma cells (Neuro2a) as well as rat hippocampal neurons. We investigated the interaction between the duplicated subunits and full-length α7 by measuring Förster resonance energy transfer using donor recovery after photobleaching and fluorescence lifetime imaging microscopy. The results revealed that the duplicated proteins co-assemble with α7. In electrophysiological studies, Leu at the 9'-position in the M2 membrane-spanning segment was replaced with Cys in dupα7 or dupΔα7, and constructs were co-transfected with full-length α7 in Neuro2a cells. Exposure to ethylammonium methanethiosulfonate inhibited acetylcholine-induced currents, showing that the assembled functional nicotinic acetylcholine receptors (nAChRs) included the duplicated subunit. Incorporation of dupα7 and dupΔα7 subunits modestly changes the sensitivity of receptors to choline and varenicline. Thus, the duplicated proteins are assembled and transported to the cell membrane together with full-length α7 subunits and alter the function of the nAChRs. The characterization of dupα7 and dupΔα7 as well as their influence on α7 nAChRs may help explain the pathophysiology of schizophrenia and may suggest therapeutic strategies.

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.

Functional screening of α7 nicotinic receptor ligands

BACKGROUND

The α7 nicotinic acetylcholine receptor, a ligand-gated ion channel, is an attractive drug discovery target in schizophrenia and Alzheimer's disease.

OBJECTIVE

We have evaluated the various approaches to discovering ligands targeting the α7 nicotinic receptor to define the current paradigm driving drug discovery efforts in this area.

METHODS

Assays using functional read-outs as a consequence of α7 nicotinic receptor activation have been reviewed.

CONCLUSION

Functional assays using fluorescence-based optical methods in combination with direct electrophysiological recordings of channel function currently provide an integrated approach to the discovery of α7 nicotinic receptor targeted ligands.

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.

Molecular dissection of Cl--selective Cys-loop receptor points to components that are dispensable or essential for channel activity

Cys-loop receptors are pentameric ligand-gated ion channels (pLGICs) that bind neurotransmitters to open an intrinsic transmembrane ion channel pore. The recent crystal structure of a prokaryotic pLGIC from the cyanobacterium Gloeobacter violaceus (GLIC) revealed that it naturally lacks an N-terminal extracellular α helix and an intracellular domain that are typical of eukaryotic pLGICs. GLIC does not respond to neurotransmitters acting at eukaryotic pLGICs but is activated by protons. To determine whether the structural differences account for functional differences, we used a eukaryotic chimeric acetylcholine-glutamate pLGIC that was modified to carry deletions corresponding to the sequences missing in the prokaryotic homolog GLIC. Deletions made in the N-terminal extracellular α helix did not prevent the expression of receptor subunits and the appearance of receptor assemblies on the cell surface but abolished the capability of the receptor to bind α-bungarotoxin (a competitive antagonist) and to respond to the neurotransmitter. Other truncated chimeric receptors that lacked the intracellular domain did bind ligands; displayed robust acetylcholine-elicited responses; and shared with the full-length chimeric receptor similar anionic selectivity, effective open pore diameter, and unitary conductance. We suggest that the integrity of the N-terminal α helix is crucial for ligand accommodation because it stabilizes the intersubunit interfaces adjacent to the neurotransmitter-binding pocket(s). We also conclude that the intracellular domain of the chimeric acetylcholine-glutamate receptor does not modulate the ion channel conductance and is not involved in positioning of the pore-lining helices in the conformation necessary for coordinating a Cl- ion within the intracellular vestibule of the ion channel pore.

Engineering a prokaryotic Cys-loop receptor with a third functional domain

Prokaryotic members of the Cys-loop receptor ligand-gated ion channel superfamily were recently identified. Previously, Cys-loop receptors were only known from multicellular organisms (metazoans). Contrary to the metazoan Cys-loop receptors, the prokaryotic ones consist of an extracellular (ECD) and a transmembrane domain (TMD), lacking the large intracellular domain (ICD) present in metazoa (between transmembrane segments M3 and M4). Using a chimera approach, we added the 115-amino acid ICD from mammalian serotonin type 3A receptors (5-HT(3A)) to the prokaryotic proton-activated Gloeobacter violaceus ligand-gated ion channel (GLIC). We created 12 GLIC-5-HT(3A)-ICD chimeras by replacing a variable number of amino acids in the short GLIC M3M4 linker with the entire 5-HT(3A)-ICD. Two-electrode voltage clamp recordings after expression in Xenopus laevis oocytes showed that only two chimeras were functional and produced currents upon acidification. The pH(50) was comparable with wild-type GLIC. 5-HT(3A) receptor expression can be inhibited by the chaperone protein RIC-3. We have shown previously that the 5-HT(3A)-ICD is required for the attenuation of 5-HT-induced currents when RIC-3 is co-expressed with 5-HT(3A) receptors in X. laevis oocytes. Expression of both functional 5-HT(3A) chimeras was inhibited by RIC-3 co-expression, indicating appropriate folding of the 5-HT(3A)-ICD in the chimeras. Our results indicate that the ICD can be considered a separate domain that can be removed from or added to the ECD and TMD while maintaining the overall structure and function of the ECD and TMD.

Novel and conserved protein macoilin is required for diverse neuronal functions in Caenorhabditis elegans

Neural signals are processed in nervous systems of animals responding to variable environmental stimuli. This study shows that a novel and highly conserved protein, macoilin (MACO-1), plays an essential role in diverse neural functions in Caenorhabditis elegans. maco-1 mutants showed abnormal behaviors, including defective locomotion, thermotaxis, and chemotaxis. Expression of human macoilin in the C. elegans nervous system weakly rescued the abnormal thermotactic phenotype of the maco-1 mutants, suggesting that macoilin is functionally conserved across species. Abnormal thermotaxis may have been caused by impaired locomotion of maco-1 mutants. However, calcium imaging of AFD thermosensory neurons and AIY postsynaptic interneurons of maco-1 mutants suggest that macoilin is required for appropriate responses of AFD and AIY neurons to thermal stimuli. Studies on localization of MACO-1 showed that C. elegans and human macoilins are localized mainly to the rough endoplasmic reticulum. Our results suggest that macoilin is required for various neural events, such as the regulation of neuronal activity.

Any animals, including humans, have to be capable of properly sensing and responding to various environmental stimuli for survival and reproduction. Environmental stimuli are evaluated and, based on past experiences, converted to produce appropriate adaptive behaviors. Even the small, free-living soil nematode Caenorhabditis elegans can sense diverse environment stimuli using a nervous system that consists of only 302 neurons. C. elegans exhibit thermotaxis that allows them to remember ambient temperatures and use this information as a cue to seek and remain near food sources. We show here that a novel and conserved protein, MACO-1, is important for proper execution of thermotaxis by C. elegans. The maco-1 gene was originally identified in a novel thermotaxis-defective mutant. Abnormal thermotaxis of maco-1 mutants was weakly but apparently rescued by expressing human MACO-1, suggesting that MACO-1 is functionally conserved among species. MACO-1 protein is required for appropriate regulation of neuronal activity, and the activity of neurons required for thermotaxis is impaired in maco-1 mutants. This analysis provides an important basis for the function of the macoilin family.

Two neuronal nicotinic acetylcholine receptors, alpha4beta4 and alpha7, show differential agonist binding modes

Nicotinic acetylcholine receptors (nAChRs) are pentameric, neurotransmitter-gated ion channels responsible for rapid excitatory neurotransmission in the central and peripheral nervous systems, resulting in skeletal muscle tone and various cognitive effects in the brain. These complex proteins are activated by the endogenous neurotransmitter ACh as well as by nicotine and structurally related agonists. Activation and modulation of nAChRs has been implicated in the pathology of multiple neurological disorders, and as such, these proteins are established therapeutic targets. Here we use unnatural amino acid mutagenesis to examine the ligand binding mechanisms of two homologous neuronal nAChRs: the α4β4 and α7 receptors. Despite sequence identity among the residues that form the core of the agonist-binding site, we find that the α4β4 and α7 nAChRs employ different agonist-receptor binding interactions in this region. The α4β4 receptor utilizes a strong cation-π interaction to a conserved tryptophan (TrpB) of the receptor for both ACh and nicotine, and nicotine participates in a strong hydrogen bond with a backbone carbonyl contributed by TrpB. Interestingly, we find that the α7 receptor also employs a cation-π interaction for ligand recognition, but the site has moved to a different aromatic amino acid of the agonist-binding site depending on the agonist. ACh participates in a cation-π interaction with TyrA, whereas epibatidine participates in a cation-π interaction with TyrC2.

The use of Xenopus oocytes in drug screening

INTRODUCTION

The physiological roles of ion channels are receiving increased interest in both basic research and drug discovery, and a demand for pharmacological approaches that can characterize or screen ion channels and their ligands with higher throughput has emerged. Traditionally, screening of compound libraries at ion channel targets has been performed using assays such as binding assays, fluorescence-based assays and flux assays that allow high-throughput, but sacrifice high data quality. The use of these assays with ion channel targets can also be problematic, emphasizing the usefulness of automated Xenopus oocyte electrophysiological assays in drug screening.

AREAS COVERED

This review summarizes the use of Xenopus oocytes in drug screening, presents the advantages and disadvantages of the use of Xenopus oocytes as expression system, and addresses the options available for automated two-electrode voltage-clamp recordings from Xenopus oocytes.

EXPERT OPINION

Automated and manual Xenopus oocyte two-electrode voltage-clamp recordings are useful and important techniques in drug screening. Although they are not compatible with high-throughput experimentation, these techniques are excellent in combination or as alternatives to fluorescence-based assays for hit validation, screening of focused compound libraries and safety screening on ion channels with their high flexibility for the choice of molecular targets, quality of data and reproducibility.

Insights into the biophysical properties of GABA(A) ion channels: modulation of ion permeation by drugs and protein interactions

The fundamental properties of ion channels assure their selectivity for a particular ion, its rapid permeation through a central pore and that such electrical activity is modulated by factors that control the opening and closing (gating) of the channel. All cell types possess ion channels and their regulated flux of ions across the membrane play critical roles in all steps of life. An ion channel does not act alone to control cell excitability but rather forms part of larger protein complexes. The identification of protein interaction partners of ion channels and their influence on both the fundamental biophysical properties of the channel and its expression in the membrane are revealing the many ways in which electrical activity may be regulated. Highlighted here is the novel use of the patch clamp method to dissect out the influence of protein interactions on the activity of individual GABA(A) receptors. The studies demonstrate that ion conduction is a dynamic property of a channel and that protein interactions in a cytoplasmic domain underlie the channel's ability to alter ion permeation. A structural model describing a reorganisation of the conserved cytoplasmic gondola domain and the influence of drugs on this process are presented.

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.

RIC-3 exclusively enhances the surface expression of human homomeric 5-hydroxytryptamine type 3A (5-HT3A) receptors despite direct interactions with 5-HT3A, -C, -D, and -E subunits

Although five 5-hydroxytryptamine type 3 (5-HT3) subunits (A-E) have been cloned, knowledge on the regulation of their assembly is limited. RIC-3 has been identified as a chaperone specific for the pentameric ligand-gated nicotinic acetylcholine and 5-HT(3) receptors. Therefore, we examined the impact of RIC-3 on differently composed 5-HT(3) receptors with the focus on 5-HT3C, -D, and -E subunits. The influence of RIC-3 on these receptor subtypes is supported by the presence of RIC3 mRNA in tissues expressing at least one of the subunits 5-HT3C, -D, and -E. Furthermore, immunocytochemical studies on transfected mammalian cells revealed co-localization in the endoplasmic reticulum and direct interaction of RIC-3 with 5-HT3A, -C, -D, and -E. Functional and pharmacological characterization was performed using HEK293 cells expressing 5-HT3A or 5-HT3A + 5-HT3B (or -C, -D, or -E) in the presence or absence of RIC-3. Ca(2+) influx analyses revealed that RIC-3 does not influence the 5-HT concentration-response relationship on 5-HT(3)A receptors but leads to differential increases of 5-HT-induced maximum response (E(max)) on cells expressing different subunits. Increases of E(max) were due to analogously enhanced B(max) values for binding of the 5-HT(3) receptor antagonist [(3)H]GR65630. The observed enhanced cell surface expression of the tested 5-HT3 subunit combinations correlated with the increased surface expression of 5-HT3A as determined by flow cytometry. In conclusion, we showed that RIC-3 can interact with 5-HT3A, -C, -D, and -E subunits and predominantly enhances the surface expression of homomeric 5-HT(3)A receptors in HEK293 cells. These data implicate a possible role of RIC-3 in determining 5-HT(3) receptor composition in vivo.

Receptor and subunit specific interactions of RIC-3 with nicotinic acetylcholine receptors

RIC-3 belongs to a conserved family of proteins influencing maturation of nicotinic acetylcholine receptors (nAChRs). RIC-3 homologues were shown to differently affect different nAChRs. Here we show that coexpression with RIC-3 increases the level of surface expression of DEG-3 while slightly reducing the level of surface expression of DES-2, both subunits of the DEG-3/DES-2 nAChRs. Those different effects are a likely explanation for the previously demonstrated effects of RIC-3, an endoplasmic reticulum resident protein, on properties of this receptor. To understand how RIC-3 interacts with different nAChR subunits, we identified and characterized domains and residues enabling this interaction. This analysis shows that conserved residues in the second RIC-3 transmembrane domain are needed for its interactions with two different Caenorhabditis elegans nAChRs, DEG-3/DES-2 and ACR-16. These conserved residues do not, however, function alone; rather, we show that additional domains also enable RIC-3's interactions with these receptors. Interestingly, the relative importance of these residues or of other domains mediating interactions of RIC-3 with nAChRs differs for the two different receptors. Differences in the way that RIC-3, predicted to be an intrinsically disordered protein, interacts with different receptors and receptor subunits suggest that it may adopt different conformations to enable these interactions. Such differences may explain both the effects of RIC-3 on receptor properties and the differences in its effects on different receptors.

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.

Protein interactions involving the gamma2 large cytoplasmic loop of GABA(A) receptors modulate conductance

Native GABA(A) channels display a single-channel conductance ranging between approximately 10 and 90 pS. Diazepam increases the conductance of some of these native channels but never those of recombinant receptors, unless they are coexpressed with GABARAP. This trafficking protein clusters recombinant receptors in the membrane, suggesting that high-conductance channels arise from receptors that are at locally high concentrations. The amphipathic (MA) helix that is present in the large cytoplasmic loop of every subunit of all ligand-gated ion channels mediates protein-protein interactions. Here we report that when applied to inside-out patches, a peptide mimicking the MA helix of the gamma2 subunit (gamma(381-403)) of the GABA(A) receptor abrogates the potentiating effect of diazepam on both endogenous receptors and recombinant GABA(A) receptors coexpressed with GABARAP, by substantially reducing their conductance. The protein interaction disrupted by the peptide did not involve GABARAP, because a shorter peptide (gamma(386-403)) known to compete with the gamma2-GABARAP interaction did not affect the conductance of recombinant alphabetagamma receptors coexpressed with GABARAP. The requirement for receptor clustering and the fact that the gamma2 MA helix is able to self-associate support a mechanism whereby adjacent GABA(A) receptors interact via their gamma2-subunit MA helices, altering ion permeation through each channel. Alteration of ion-channel function arising from dynamic interactions between ion channels of the same family has not been reported previously and highlights a novel way in which inhibitory neurotransmission in the brain may be differentially modulated.

Mutations of cytosolic loop residues impair assembly and maturation of alpha7 nicotinic acetylcholine receptors

Mechanisms that regulate early events in the biogenesis of the alpha7 nicotinic acetylcholine receptor (alpha7 AChR) are not well understood. Data presented here show that single amino acid mutations in the cytoplasmic loop of the alpha7 AChR, between position 335 and 343, abolish or attenuate expression of mature pentameric alpha7 AChRs in both human embryonic kidney tsA201 (HEK) and neuronal SH-SY5Y cells. Although the number of mature alpha7 AChRs is increased significantly in the presence of the chaperone protein resistant to inhibitors of cholineesterase-3 in HEK cells, sucrose gradient sedimentation reveals that the vast majority of alpha7 subunits are aggregated or improperly assembled. Transfection of alpha7 AChRs in SH-SY5Y cells, which endogenously express the alpha7 AChR, results in a much larger fraction of subunits assembled into mature AChRs. Thus, efficient assembly of alpha7 AChRs is influenced by several regions of the large cytoplasmic domain, as well perhaps by other parts of its structure, and requires as yet unknown factors not required by other AChR subtypes.