Contributions of N-linked glycosylation to the expression of a functional alpha7-nicotinic receptor in Xenopus oocytes

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.

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.

Regulation of nicotinic acetylcholine receptors by post-translational modifications

Nicotinic acetylcholine receptors (nAChRs) comprise a family of pentameric ligand-gated ion channels widely distributed in the central and peripheric nervous system and in non-neuronal cells. nAChRs are involved in chemical synapses and are key actors in vital physiological processes throughout the animal kingdom. They mediate skeletal muscle contraction, autonomic responses, contribute to cognitive processes, and regulate behaviors. Dysregulation of nAChRs is associated with neurological, neurodegenerative, inflammatory and motor disorders. In spite of the great advances in the elucidation of nAChR structure and function, our knowledge about the impact of post-translational modifications (PTMs) on nAChR functional activity and cholinergic signaling has lagged behind. PTMs occur at different steps of protein life cycle, modulating in time and space protein folding, localization, function, and protein-protein interactions, and allow fine-tuned responses to changes in the environment. A large body of evidence demonstrates that PTMs regulate all levels of nAChR life cycle, with key roles in receptor expression, membrane stability and function. However, our knowledge is still limited, restricted to a few PTMs, and many important aspects remain largely unknown. There is thus a long way to go to decipher the association of aberrant PTMs with disorders of cholinergic signaling and to target PTM regulation for novel therapeutic interventions. In this review we provide a comprehensive overview of what is known about how different PTMs regulate nAChR.

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.

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.

Functional expression of human α7 nicotinic acetylcholine receptor in human embryonic kidney 293 cells

The functional expression of recombinant α7 nicotinic acetylcholine receptors in human embryonic kidney (HEK) 293 cells has presented a challenge. Resistance to inhibitors of cholinesterase 3 (RIC‑3) has been confirmed to act as a molecular chaperone of nicotinic acetylcholine receptors. The primary objectives of the present study were to investigate whether the co‑expression of human (h)RIC‑3 with human α7 nicotinic acetylcholine receptor in HEK 293 cells facilitates functional expression of the α7 nicotinic acetylcholine receptor. Subsequent to transfection, western blotting and polymerase chain reaction were used to test the expression of α7 nicotinic acetylcholine receptor and RIC-3. The α7 nicotinic acetylcholine receptor was expressed alone or co‑expressed with hRIC‑3 in the HEK 293 cells. Drug‑containing solution was then applied to the cells via a gravity‑driven perfusion system. Calcium influx in the cells was analyzed using calcium imaging. Nicotine did not induce calcium influx in the HEK 293 cells expressing human α7 nicotinic acetylcholine receptor only. However, in the cells co‑expressing human RIC‑3 and α7 nicotinic acetylcholine receptor, nicotine induced calcium influx via the α7 nicotinic acetylcholine receptor in a concentration‑dependent manner (concentration required to elicit 50% of the maximal effect=29.21 µM). Taken together, the results of the present study suggested that the co‑expression of RIC‑3 in HEK 293 cells facilitated the functional expression of the α7 nicotinic acetylcholine receptor.

The role of protein N-glycosylation in neural transmission

Recent studies have explored the function of N-linked glycosylation in the nervous system, demonstrating essential roles of carbohydrate structures in neural development. The function of N-glycans in neural physiology remains less understood; however, increasing evidence indicates that N-glycans can play specific modulatory roles controlling neural transmission and excitability of neural circuits. These roles are mediated via effects on synaptic proteins involved in neurotransmitter release, transporters that regulate nerotransmitter concentrations, neurotransmitter receptors, as well as via regulation of proteins that control excitability and response to milieu stimuli, such as voltage-gated ion channels and transient receptor potential channels, respectively. Sialylated N-glycan structures are among the most potent modulators of cell excitability, exerting prominent effects on voltage gated Na(+) and K(+) channels. This modulation appears to be underlain by complex molecular mechanisms involving electrostatic effects, as well as interaction modes based on more specific steric effects and interactions with lectins and other molecules. Data also indicate that particular features of N-glycans, such as their location on a protein and structural characteristics, can be specifically associated with the effect of glycosylation. These features and their functional implications can vary between different cell types, which highlight the importance of in vivo analyses of glycan functions. Experimental challenges are associated with the overwhelming complexity of the nervous system and glycosylation pathways in vertebrates, and thus model organisms like Drosophila should help elucidate evolutionarily conserved mechanisms underlying glycan functions. Recent studies supported this notion and shed light on functions of several glycosylation genes involved in the regulation of the nervous system.

N-glycosylation in regulation of the nervous system

Protein N-glycosylation can influence the nervous system in a variety of ways by affecting functions of glycoproteins involved in nervous system development and physiology. The importance of N-glycans for different aspects of neural development has been well documented. For example, some N-linked carbohydrate structures were found to play key roles in neural cell adhesion and axonal targeting during development. At the same time, the involvement of glycosylation in the regulation of neural physiology remains less understood. Recent studies have implicated N-glycosylation in the regulation of neural transmission, revealing novel roles of glycans in synaptic processes and the control of neural excitability. N-Glycans were found to markedly affect the function of several types of synaptic proteins involved in key steps of synaptic transmission, including neurotransmitter release, reception, and uptake. Glycosylation also regulates a number of channel proteins, such as TRP channels that control responses to environmental stimuli and voltage-gated ion channels, the principal determinants of neuronal excitability. Sialylated carbohydrate structures play a particularly prominent part in the modulation of voltage-gated ion channels. Sialic acids appear to affect channel functions via several mechanisms, including charge interactions, as well as other interactions that probably engage steric effects and interactions with other molecules. Experiments also indicated that some structural features of glycans can be particularly important for their function. Since glycan structures can vary significantly between different cell types and depend on the metabolic state of the cell, it is important to analyze glycan functions using in vivo approaches. While the complexity of the nervous system and intricacies of glycosylation pathways can create serious obstacles for in vivo experiments in vertebrates, recent studies have indicated that more simple and experimentally tractable model organisms like Drosophila should provide important advantages for elucidating evolutionarily conserved functions of N-glycosylation in the nervous system.

Glycosylation of {beta}2 subunits regulates GABAA receptor biogenesis and channel gating

γ-aminobutyric acid type A (GABA(A)) receptors are heteropentameric glycoproteins. Based on consensus sequences, the GABA(A) receptor β2 subunit contains three potential N-linked glycosylation sites, Asn-32, Asn-104, and Asn-173. Homology modeling indicates that Asn-32 and Asn-104 are located before the α1 helix and in loop L3, respectively, near the top of the subunit-subunit interface on the minus side, and that Asn-173 is located in the Cys-loop near the bottom of the subunit N-terminal domain. Using site-directed mutagenesis, we demonstrated that all predicted β2 subunit glycosylation sites were glycosylated in transfected HEK293T cells. Glycosylation of each site, however, produced specific changes in α1β2 receptor surface expression and function. Although glycosylation of Asn-173 in the Cys-loop was important for stability of β2 subunits when expressed alone, results obtained with flow cytometry, brefeldin A treatment, and endo-β-N-acetylglucosaminidase H digestion suggested that glycosylation of Asn-104 was required for efficient α1β2 receptor assembly and/or stability in the endoplasmic reticulum. Patch clamp recording revealed that mutation of each site to prevent glycosylation decreased peak α1β2 receptor current amplitudes and altered the gating properties of α1β2 receptor channels by reducing mean open time due to a reduction in the proportion of long open states. In addition to functional heterogeneity, endo-β-N-acetylglucosaminidase H digestion and glycomic profiling revealed that surface β2 subunit N-glycans at Asn-173 were high mannose forms that were different from those of Asn-32 and N104. Using a homology model of the pentameric extracellular domain of α1β2 channel, we propose mechanisms for regulation of GABA(A) receptors by glycosylation.

Campylobacter jejuni PglH is a single active site processive polymerase that utilizes product inhibition to limit sequential glycosyl transfer reactions

Asparagine-linked protein glycosylation is essential for the virulence of the human gut mucosal pathogen Campylobacter jejuni . The heptasaccharide that is transferred to proteins is biosynthesized via the glycosyltransferase-catalyzed addition of sugar units to an undecaprenyl diphosphate-linked carrier. Genetic studies on the heptasaccharide assembly enzymes have shown that PglH, which transfers three terminal N-acetyl-galactosamine (GalNAc) residues to the carrier polyisoprene, is essential for chick colonization by C. jejuni . While it is now clear that PglH catalyzes multiple transfer reactions, the mechanism whereby the reactions cease after the addition of just three GalNAc residues has yet to be understood. To address this issue, a series of mechanistic biochemical studies was conducted with purified native PglH. This enzyme was found to follow a processive mechanism under initial rate conditions; however, product inhibition and product accumulation led to PglH release of intermediate products prior to complete conversion to the native ultimate product. Point mutations of an essential EX(7)E sequence motif were used to demonstrate that a single active site was responsible for all three transferase reactions, and a homology model with the mannosyltransferase PimA, from Mycobacteria smegmatis , establishes the requirement of the EX(7)E motif in catalysis. Finally, increased binding affinity with increasing glycan size is proposed to provide PglH with a counting mechanism that does not allow the transfer of more than three GalNAc residues. These results provide important mechanistic insights into the function of the glycosyl transfer polymerase that is related to the virulence of C. jejuni .

Cellular distribution of the nicotinic acetylcholine receptor alpha7 subunit in rat hippocampus

The hippocampus is a region of the mammalian brain that has been extensively studied due to its role in many forms of memory. To better understand hippocampal function, significant attention has focused upon the cellular distribution of ligand-gated ion channels. Despite strong cholinergic innervation from the basal forebrain and a dense expression of nicotinic acetylchoine receptors (nAChRs), the cellular distribution of subunits forming these receptors has received little attention. We used organotypic hippocampal slice cultures (OHSCs) to study native alpha7 subunits, which, unlike other nAChR subunits, form a homomeric receptor. Cell-surface biotinylation, cross-linking of surface proteins, and sub-cellular fractionation all revealed a very limited presence of the subunit at the plasma membrane. In contrast, subunits of other receptors displayed significant surface expression. Notably, subunits in adult hippocampal tissue were distributed in a fashion similar to that observed in OHSCs. To monitor alpha7 subunits contained in functional nAChRs, a colourimetric assay using alpha-bungarotoxin (a specific alpha7 nAChR antagonist) was developed, and revealed a majority of binding at the cell surface. To change alpha7 subunit distribution, OHSCs were treated with compounds known to affect other ionotropic receptors-insulin, genistein, and elevated external K(+); however, neither subunit surface expression nor antagonist binding was affected. Our data reveal that hippocampal neurons possess a large internal population of alpha7 subunits under basal conditions, which persists during stimuli affecting tyrosine phosphorylation or neuronal activity. The nature of the internal pool of alpha7 subunits remains to be determined, but should have important implications for hippocampal activity.

A 2-base pair deletion polymorphism in the partial duplication of the alpha7 nicotinic acetylcholine gene (CHRFAM7A) on chromosome 15q14 is associated with schizophrenia

Multiple genetic linkage studies support the hypothesis that the 15q13-14 chromosomal region contributes to the etiology of schizophrenia. Among the putative candidate genes in this area are the alpha7 nicotinic acetylcholine receptor gene (CHRNA7) and its partial duplication, CHRFAM7A. A large chromosomal segment including the CHRFAM7A gene locus, but not the CHRNA7 locus, is deleted in some individuals. The CHRFAM7A gene contains a polymorphism consisting of a 2 base pair (2 bp) deletion at position 497-498 bp of exon 6. We employed PCR-based methods to quantify the copy number of CHRFAM7A and the presence of the 2 bp polymorphism in a large, multi-ethnic population. The 2 bp polymorphism was associated with schizophrenia in African Americans (genotype p=0.005, allele p=0.015), and in Caucasians (genotype p=0.015, allele p=0.009). We conclude that the presence of the 2 bp polymorphism at the CHRFAM7A locus may have a functional significance in schizophrenia.

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.

A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.

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.

Assembly and trafficking of nicotinic acetylcholine receptors (Review)

Nicotinic acetylcholine receptors (nAChRs) are members of an extensive super-family of neurotransmitter-gated ion channels. In humans, nAChRs are expressed within the nervous system and at the neuromuscular junction and are important targets for pharmaceutical drug discovery. They are also the site of action for neuroactive pesticides in insects and other invertebrates. Nicotinic receptors are complex pentameric transmembrane proteins which are assembled from a large family of subunits; seventeen nAChR subunits (alpha1-alpha10, beta1-beta4, gamma, delta and epsilon) have been identified in vertebrate species. This review will discuss nAChR subunit diversity and factors influencing receptor assembly and trafficking.

The nicotinic acetylcholine receptor subunits Mdalpha5 and Mdbeta3 on autosome 1 of Musca domestica are not involved in spinosad resistance

Spinosad is a relatively new insecticide that exerts its toxic action via nicotinic acetylcholine receptors (nAChRs). Spinosad resistance in house flies appears to be due to an altered target site. To evaluate the molecular basis of spinosad resistance, two nAChR subunit genes, Mdalpha5 and Mdbeta3 were cloned and characterized from an insecticide-susceptible (aabys) and spinosad resistant (rspin) strain of the house fly, Musca domestica. The Mdalpha5 and Mdbeta3 cDNAs encode proteins of 781 and 432 amino acid residues, respectively. Phylogenetic analysis with insect nAChR subunits suggested that Mdalpha5 and Mdbeta3 are most closely related to Dalpha5 and Dbeta3 of Drosophila melanogaster, respectively. Mdbeta3 is intronless, which is unique among all previously described nAChR genes. A-to-I RNA editing was found at 13 sites in Mdalpha5, eleven of which resulted in amino acid substitutions. No evidence for A-to-I RNA editing was found in Mdbeta3. Mdalpha5 expression, quantified by real time PCR, was 340- and 23-fold higher in the head and thorax than in the abdomen. Mdbeta3 expression was more uniform, being only 2.4-fold higher in the head and 1.4-fold lower in the thorax, compared to the abdomen. There was no difference in the expression of Mdalpha5 and Mdbeta3 between the aabys and rspin strains. Although Mdalpha5 and Mdbeta3 both map to the same chromosome as spinosad resistance, there were no unique features of either gene in rspin, relative to the aabys strain. This suggests neither Mdalpha5 nor Mdbeta3 is responsible for spinosad resistance in house flies.

Crystal structure of the extracellular domain of nAChR alpha1 bound to alpha-bungarotoxin at 1.94 A resolution

We determined the crystal structure of the extracellular domain of the mouse nicotinic acetylcholine receptor (nAChR) alpha1 subunit bound to alpha-bungarotoxin at 1.94 A resolution. This structure is the first atomic-resolution view of a nAChR subunit extracellular domain, revealing receptor-specific features such as the main immunogenic region (MIR), the signature Cys-loop and the N-linked carbohydrate chain. The toxin binds to the receptor through extensive protein-protein and protein-sugar interactions. To our surprise, the structure showed a well-ordered water molecule and two hydrophilic residues deep in the core of the alpha1 subunit. The two hydrophilic core residues are highly conserved in nAChRs, but correspond to hydrophobic residues in the nonchannel homolog acetylcholine-binding proteins. We carried out site-directed mutagenesis and electrophysiology analyses to assess the functional role of the glycosylation and the hydrophilic core residues. Our structural and functional studies show essential features of the nAChR and provide new insights into the gating mechanism.

The nicotinic acetylcholine receptor subunit Mdalpha6 from Musca domestica is diversified via post-transcriptional modification

Recent studies showed that deletion of a nicotinic acetylcholine receptor (nAChR) subunit gene, Dalpha6 in Drosophila melanogaster results in a strain that is resistant to spinosad, indicating that Dalpha6 is important for the toxic action of this insecticide. To determine if spinosad resistance in house flies was due to a mutation(s) of Mdalpha6 (the orthologue of Dalpha6 from house flies), cDNAs were cloned and characterized from an insecticide-susceptible and a spinosad-resistant strain of the house fly, Musca domestica. The cDNAs contain a 1470-bp open reading frame encoding 490 amino acid residues, 415-bp 5' untranslated region (UTR) and a polymorphic 3'-UTR of approximately 371 bp. The predicted mature protein possesses 468 amino acid residues, has the typical features of a nAChR alpha subunit and is 97% identical to Dalpha6. Quantitative real-time PCR analysis revealed that Mdalpha6 was expressed in the head and the thorax at 1300- and 26-fold higher levels, respectively, than in the abdomen. There was no difference in the expression level of Mdalpha6 between spinosad-resistant and susceptible strains. Ten isoforms arising from alternative splicing were characterized, with isoform II being most common. A-to-I RNA editing was examined and found at 12 sites: editing at 11 of these sites resulted in an amino acid substitution. Mdalpha6 is linked to autosome 1 (spinosad resistance was previously shown to be linked to autosome 1). Single nucleotide polymorphisms, alternative splicing, mRNA levels and A-to-I RNA editing were compared between head and thorax and between insecticide-susceptible and spinosad-resistant strains. These comparisons indicate that Mdalpha6 is not responsible for spinosad resistance in house flies.

Characterization of the nicotinic acetylcholine receptor subunit gene Mdalpha2 from the house fly, Musca domestica

A nicotinic acetylcholine receptor (nAChR) subunit gene, Mdalpha2, was isolated and characterized from the house fly, Musca domestica. This is the first nAChR family member cloned from house flies. Mdalpha2 had a cDNA of 2,607 bp, which included a 696 bp 5'-untranslated region (UTR), an open reading frame of 1,692 bp, and a 219 bp 3'-UTR. Its deduced amino acid sequence possesses the typical characteristics of nAChRs. Mdalpha2 genomic sequence was 11.2 kb in length in the aabys strain and 10.9 kb in the OCR strain, including eight exons and seven introns. Based on the deduced amino acid sequence, Mdalpha2 had the closest phylogenetic relationship to the Drosophila melanogaster Dalpha2 and Anopheles gambiae Agamalpha2, and a similar genomic structure to Dalpha2. Quantitative real-time PCR analysis showed that Mdalpha2 is expressed in the head and the thorax at 150- and 8.5-fold higher levels than in the abdomen. Linkage analysis of a Mdalpha2 polymorphism indicates this gene is on autosome 2. The importance of these results in understanding the diversity and phylogenetic relationships of insect nAChRs, the physiology of nAChRs in the house fly, and the utility of nAChR sequences in resistance detection/monitoring is discussed.

Sgbeta1, a novel locust (Schistocerca gregaria) non-alpha nicotinic acetylcholine receptor-like subunit with homology to the Drosophila melanogaster Dbeta1 subunit

The cloning, sequencing and functional expression of Sgbeta1, a novel locust (Schistocerca gregaria) non-alpha nicotinic acetylcholine receptor (nAChR) subunit is described. This subunit shows 80% identity with the Drosophila melanogaster Dbeta1 and 92% identity with the Locusta migratoria beta1, non-alpha subunits but only 38% identity to Sgalpha1 (also referred to as alphaL1), a previously cloned S. gregaria nAChR alpha-subunit. When expressed in Xenopus laevis oocytes, Sgbeta1 does not respond to nicotine. Responses to nicotine are observed, however, in oocytes co-expressing Sgalpha1 and Sgbeta1, but the pharmacology is indistinguishable from that of currents produced by expressing Sgalpha1 alone. We conclude that either Sgbeta1 does not co-assemble with Sgalpha1, or that it is unable to contribute to the functional properties of the receptor, in the Xenopus oocyte expression system.

Edit, cut and paste in the nicotinic acetylcholine receptor gene family ofDrosophila melanogaster

Nicotinic acetylcholine receptors (nAChRs) are important for fast synaptic cholinergic transmission. They are targets of drugs/chemicals for human and animal health as well as for pest control. With the advent of genome sequencing, entire nAChR gene families have now been described for vertebrates and invertebrates. Mostly, these are extensive with a large number of distinct subunits, making possible many nAChR subtypes differing in transmitter affinity, channel conductance, ion selectivity, desensitization, modulation and pharmacology. The smallest nAChR gene family to date is that of the fruit fly, Drosophila melanogaster, with only 10 members. This apparently compact family belies its true diversity as 4 of the 10 subunits show alternative splicing. Also, using Drosophila, A-to-I pre-mRNA editing has been demonstrated for the first time in nAChRs. Such is the extent of this variation, that one subunit alone (Dalpha6) can potentially generate far more isoforms than seen in entire gene families from other species. We present here three-dimensional models constructed for insect nAChRs, which show that many variations introduced by alternative splicing and RNA editing may influence receptor function.

Three putative N-glycosylation sites within the murine 5-HT3A receptor sequence affect plasma membrane targeting, ligand binding, and calcium influx in heterologous mammalian cells

The serotonin type 3(A) receptor (5-HT3(A)R) is a ligand-gated ion channel (LGIC) that modulates a diverse set of cognitive and physiological functions. The 5-HT3(A)R, as with other LGICs, is a pentameric ion channel comprising five glycoprotein subunits. Although the N-terminal of the 5-HT3(A)R contains three putative N-linked glycosylation sites, the importance of each glycosylation site has not yet been established. To address this question, we used tunicamycin treatment and site-directed mutagenesis to inhibit selectively N-linked glycosylation at each site and then examined the effects of these treatments on receptor expression and function in transiently transfected heterologous cells. We show that the murine 5-HT3(A)R is glycosylated and that each N-linked glycosylation site plays a role in receptor regulation. Our findings suggest that N109 is necessary for receptor assembly, whereas N174 and N190 are important for plasma membrane targeting and ligand binding. Furthermore, we demonstrate that each site is necessary for 5-HT3(A)R-mediated Ca(2+) influx. We conclude that N-glycosylation is a critical step in the maturation, trafficking, and function of the murine 5-HT3(A)R.

Assembly of nicotinic α7 subunits inXenopusoocytes is partially blocked at the tetramer level

The assembly of nicotinic alpha1beta1gammadelta, alpha3beta4, and alpha7 receptors and 5-hydroxytryptamine 3A (5HT3A) receptors was comparatively evaluated in Xenopus oocytes by blue native PAGE analysis. While alpha1betagammadelta subunits, alpha3beta4 subunits, and 5HT3A subunits combined efficiently to pentamers, alpha7 subunits existed in various assembly states including trimers, tetramers, pentamers, and aggregates. Only alpha7 subunits that completed the assembly process to homopentamers acquired complex-type carbohydrates and appeared at the cell surface. We conclude that Xenopus oocytes have a limited capacity to guide the assembly of alpha7 subunits, but not 5HT3A subunits to homopentamers. Accordingly, ER retention of imperfectly assembled alpha7 subunits rather than inefficient routing of fully assembled alpha7 receptors to the cell surface limits surface expression levels of alpha7 nicotinic acetylcholine receptors.

Identification of an alternatively processed nicotinic receptor alpha7 subunit RNA in mouse brain

The data in this report describe the discovery and characterization of a previously unidentified alternatively processed RNA for the neuronal nicotinic receptor alpha7 subunit. The unique transcript contains an extra exon that arises from alternative splicing of intron nine of the alpha7 subunit RNA. The alpha7 subunit protein resulting from this alternatively processed RNA is truncated shortly after transmembrane domain three. The variant protein also has a predicted amino acid substitution in the large N-terminal domain as a consequence of a non-templated nucleotide substitution present in the variant alpha7 subunit RNA. The mechanism responsible for the nucleotide substitution is not known. Initial characterization of the variant alpha7 subunit suggests that it is expressed in mouse brain in a pattern similar to the standard alpha7 subunit although at reduced levels. The variant alpha7 subunit was also found to act as a dominant-negative effecter of normal alpha7 subunit function.

Changes in conformation and subcellular distribution of alpha4beta2 nicotinic acetylcholine receptors revealed by chronic nicotine treatment and expression of subunit chimeras

Chronic exposure to nicotine, as occurs during tobacco smoking, is one of several factors that have been reported to cause an upregulation of neuronal nicotinic acetylcholine receptors (nAChRs). Here, the influence of both chronic exposure to nicotine (10 microm, 24 hr) and the coexpression of subunit chimeras has been examined in cultured cell lines expressing recombinant alpha4beta2 nAChRs, a major nicotinic receptor subtype expressed in the mammalian brain. Evidence is presented which demonstrates that both chronic exposure to nicotine and the coexpression of subunit chimeras upregulates levels of receptor expressed on the cell surface. Immunoblotting data indicate that neither chronic nicotine treatment nor coexpressed subunit partners greatly affect the level of total subunit protein. This finding, together with radioligand and antibody binding studies conducted on both intact and permeabilized cells, reveals that receptor upregulation corresponds to an increase in the proportion of total receptor expressed on the cell surface. It is also apparent that nicotine-induced nAChR upregulation is very strongly dependent on subunit composition and subunit domains. An important aspect of this study is that direct evidence has been obtained indicating that both chronic exposure to nicotine and coexpressed subunit partners can influence subunit conformation. The influence of chronic nicotine treatment on subunit folding may help to explain the phenomenon of nicotine-induced receptor upregulation. The finding that subunit conformation can be influenced by coassembled subunit partners is in agreement with models of receptor assembly which propose that subunit folding continues after initial subunit-subunit interactions.

Rat nicotinic ACh receptor alpha7 and beta2 subunits co-assemble to form functional heteromeric nicotinic receptor channels

Rat hippocampal interneurons express diverse subtypes of functional nicotinic acetylcholine receptors (nAChRs), including alpha7-containing receptors that have properties unlike those expected for homomeric alpha7 nAChRs. We previously reported a strong correlation between expression of the alpha7 and of the beta2 subunits in individual neurons. To explore whether co-assembly of the alpha7 and beta2 subunits might occur, these subunits were co-expressed in Xenopus oocytes and the functional properties of heterologously expressed nAChRs were characterized by two-electrode voltage clamp. Co-expression of the beta2 subunit, both wild-type and mutant forms, with the alpha7 subunit significantly slowed the rate of nAChR desensitization and altered the pharmacological properties. Whereas ACh, carbachol and choline were full or near-full agonists for homomeric alpha7 receptor channels, both carbachol and choline were only partial agonists in oocytes expressing both alpha7 and beta2 subunits. In addition the EC(50) values for all three agonists significantly increased when the beta2 subunit was co-expressed with the alpha7 subunit. Co-expression with the beta2 subunit did not result in any significant change in the current-voltage curve. Biochemical evidence for the co-assembly of the alpha7 and beta2 subunits was obtained by co-immunoprecipitation of these subunits from transiently transfected human embryonic kidney (TSA201) cells. These data provide direct biophysical and molecular evidence that the nAChR alpha7 and beta2 subunits co-assemble to form a functional heteromeric nAChR with functional and pharmacological properties different from those of homomeric alpha7 channels. This co-assembly may help to explain nAChR channel diversity in rat hippocampal interneurons, and perhaps in other areas of the nervous system.

Neurobiology of the nicotine withdrawal syndrome

The aversive aspects of withdrawal from chronic nicotine exposure are thought to be an important motivational factor contributing to the maintenance of the tobacco habit in human smokers. Much emphasis has been placed on delineating the underlying neurobiological mechanisms mediating different components of the nicotine withdrawal syndrome. Recent studies have shown that both central and peripheral populations of nicotinic acetylcholine receptors (nAChRs) are involved in mediating somatic signs of nicotine withdrawal as measured by the rodent nicotine abstinence scale. However, only central populations of nAChRs are involved in mediating affective aspects of nicotine withdrawal, as measured by elevations in brain-stimulation reward thresholds and conditioned place aversion. Nicotine interacts with several neurotransmitter systems, including acetylcholine, dopamine, opioid peptides, serotonin, and glutamate systems. Evidence so far suggests that these neurotransmitters play a role in nicotine dependence and withdrawal processes. The available evidence also suggests that different underlying neurochemical deficits mediate somatic and affective components of nicotine withdrawal. The aim of the present review is to discuss preclinical findings concerning the neuroanatomical and neurochemical substrates involved in these different aspects of nicotine withdrawal.

Pharmacological differences between immunoisolated native brain and heterologously expressed rat alpha4beta2 nicotinic receptors

Native brain and heterologously expressed rat alpha4beta2 nicotinic receptors (in Xenopus oocytes and CV-1 cells) were immunoisolated with the anti-alpha4 antibody mAb 299 and their pharmacological properties were compared using [3H](+/-)epibatidine, the novel N-alkylnicotinium analog N-n-octylnicotinium iodide (NONI), and the ganglionic antagonist trimethaphan (TRM). The equilibrium dissociation constant (K(d)) for [3H](+/-)epibatidine binding to the native and heterologously expressed receptors ranged from 13 to 21 pM. The Hill coefficients for [3H](+/-)epibatidine binding to the native and expressed receptors ranged from 0.8 to 1.1 and were consistent with a single high-affinity site. NONI inhibited 30 pM [3H](+/-)epibatidine binding to the native and expressed receptors with similar potency (IC(50) values of 6-7 microM). However, [3H](+/-)epibatidine dissociated 2-3 times more slowly from the native, than from the expressed receptors and TRM inhibited 30 pM [3H](+/-)epibatidine binding to the native receptors (IC(50) value of 330 microM) less potently than it did to the receptors expressed in oocytes (IC(50) value of 16 microM) or CV-1 cells (IC(50) value of 55 microM). The differences between the native and expressed [3H](+/-)epibatidine dissociation rate constants and IC(50) values for TRM were significant for both host cell types, although the values for the CV-1-expressed receptors were closer to the native ones than were those for the oocyte-expressed receptors. Thus, the epibatidine and trimethaphan binding sites in native and expressed alpha4beta2 receptors appear to have significantly different structural or chemical properties.

Ultrastructural distribution of the alpha7 nicotinic acetylcholine receptor subunit in rat hippocampus

Acetylcholine (ACh) is an important neurotransmitter in the mammalian brain; it is implicated in arousal, learning, and other cognitive functions. Recent studies indicate that nicotinic receptors contribute to these cholinergic effects, in addition to the established role of muscarinic receptors. In the hippocampus, where cholinergic involvement in learning and memory is particularly well documented, alpha7 nicotinic acetylcholine receptor subunits (alpha7 nAChRs) are highly expressed, but their precise ultrastructural localization has not been determined. Here, we describe the results of immunogold labeling of serial ultrathin sections through stratum radiatum of area CA1 in the rat. Using both anti-alpha7 nAChR immunolabeling and alpha-bungarotoxin binding, we find that alpha7 nAChRs are present at nearly all synapses in CA1 stratum radiatum, with immunolabeling present at both presynaptic and postsynaptic elements. Morphological considerations and double immunolabeling indicate that GABAergic as well as glutamatergic synapses bear alpha7 nAChRs, at densities approaching those observed for glutamate receptors in CA1 stratum radiatum. Postsynaptically, alpha7 nAChRs often are distributed at dendritic spines in a perisynaptic annulus. In the postsynaptic cytoplasm, immunolabeling is associated with spine apparatus and other membranous structures, suggesting that alpha7 nAChRs may undergo dynamic regulation, with insertion into the synapse and subsequent internalization. The widespread and substantial expression of alpha7 nAChRs at synapses in the hippocampus is consistent with an important role in mediating and/or modulating synaptic transmission, plasticity, and neurodegeneration.

Glycosylated and phosphorylated proteins--expression in yeast and oocytes of Xenopus: prospects and challenges--relevance to expression of thermostable proteins

Phosphorylation and glycosylation are important posttranslational events in the biosynthesis of proteins. The different degrees of phosphorylation and glycosylation of proteins have been an intriguing phenomenon. Advances in genetic engineering have made it possible to control the degree of glycosylation and phosphorylation of proteins. Structural biology of phosphorylated and glycosylated proteins has been advancing at a much slower pace due to difficulties in using high-resolution NMR studies in solution phase. Major difficulties have arisen from the inherent mobilities of phosphorylated and glycosylated side chains. This paper reviews molecular and structural biology of phosphorylated and glycosylated proteins expressed in eukaryotic expression systems which are especially suited for large-scale production of these proteins. In our laboratory, we have observed that eukaryotic expression systems are particularly suited for the expression of thermostable light-activated proteins, e.g., bacteriorhodopsins and plastocyanins.

Inefficient cell-surface expression of hybrid complexes formed by the co-assembly of neuronal nicotinic acetylcholine receptor and serotonin receptor subunits

Previous studies have demonstrated that relatively low levels of alpha4beta2 neuronal nicotinic acetylcholine receptors (nAChRs) are expressed on the cell surface of transfected mammalian cell lines but that surface expression levels can be dramatically up-regulated by co-expression of these subunits with chimeric subunits containing the N-terminal portion of the neuronal nAChR alpha4 or beta2 subunits together with the C-terminal domain of the 5-HT(3A) subunit. Recent work has also suggested that the nAChR alpha4 subunit can co-assemble in a "promiscuous" manner with the serotonin receptor 5-HT(3A) subunit to form functional hybrid receptors. In this study we have examined whether co-assembly of either alpha4 or beta2 with 5-HT(3A) itself (rather than with the alpha4/5-HT(3A) or beta2/5-HT(3A) subunit chimeras) can also facilitate cell surface expression of alpha4 and beta2 subunits in transfected mammalian cells. Evidence has been obtained by immunoprecipitation, cell-surface antibody binding and radioligand binding which indicates that the 5-HT(3A) can co-assemble with both the alpha4 and beta2 nAChR subunits. We conclude, however, that co-assembly of 5-HT(3A) with either alpha4 or beta2 does not result in efficient cell surface expression of the nAChR subunits and that co-assembled hybrid (nAChR subunit + 5-HT(3)R subunit) receptor complexes are largely retained within the cell.

Structure-activity relationships in a peptidic alpha7 nicotinic acetylcholine receptor antagonist

alpha-Conotoxins are small disulfide-constrained peptide toxins which act as antagonists at specific subtypes of nicotinic acetylcholine receptors (nACh receptors). In this study, we analyzed the structures and activities of three mutants of alpha-conotoxin ImI, a 12 amino acid peptide active at alpha7 nACh receptors, in order to gain insight into the primary and tertiary structural requirements of neuronal alpha-conotoxin specificity. NMR solution structures were determined for mutants R11E, R7L, and D5N, resulting in representative ensembles of 20 conformers with average pairwise RMSD values of 0.46, 0.52, and 0.62 A from their mean structures, respectively, for the backbone atoms N, C(alpha), and C' of residues 2-11. The R11E mutant was found to have activity near that of wild-type ImI, while R7L and D5N demonstrated activities reduced by at least two orders of magnitude. Comparison of the structures reveals a common two-loop architecture, with variations observed in backbone and side-chain dihedral angles as well as surface electrostatic potentials upon mutation. Correlation of these structures and activities with those from previously published studies emphasizes that existing hypotheses regarding the molecular determinants of alpha-conotoxin specificity are not adequate for explaining peptide activity, and suggests that more subtle features, visualized here at the atomic level, are important for receptor binding. These data, in conjunction with reported characterizations of the acetylcholine binding site, support a model of toxin activity in which a single solvent-accessible toxin side-chain anchors the complex, with supporting weak interactions determining both the efficacy and the subtype specificity of the inhibitory activity.

Expression and channel properties of alpha-bungarotoxin-sensitive acetylcholine receptors on chick ciliary and choroid neurons

Cell-specific expression of nicotinic acetylcholine receptors (AChRs) was examined using ciliary and choroid neurons isolated from chick ciliary ganglia. At embryonic days 13 and 14 (E13,14) the neurons can be distinguished by size, with ciliary neuron soma diameters exceeding those of choroid neurons by about twofold. Both neuronal populations are known to express two major AChR types: alpha3*-AChRs recognized by mAb35, that contain alpha3, alpha5, beta4, and occasionally beta2 subunits, and alpha-bungarotoxin (alphaBgt)-AChRs recognized and blocked by alphaBgt, that contain alpha7 subunits. We found that maximal whole cell current densities (I/C(m)) mediated by alphaBgt-AChRs were threefold larger for choroid compared with ciliary neurons, while alpha3*-AChR current densities were similar in the two populations. Different densities of total cell-surface alphaBgt-AChRs could not explain the distinct alphaBgt-AChR response densities associated with ciliary and choroid neurons. Ciliary ganglion neurons display abundant [(125)I]-alphaBgt binding ( approximately 10(6) sites/neuron), but digital fluorescence measurements revealed equivalent site densities on both populations. AChR channel classes having single-channel conductances of approximately 30, 40, 60, and 80 pS were present in patches excised from both ciliary and choroid neurons. Treating the neurons with alphaBgt selectively abolished the 60- and 80-pS events, identifying them as arising from alphaBgt-AChRs. Kinetic measurements revealed brief open and long closed durations for alphaBgt-AChR channel currents, predicting a very low probability of being open (p(o)) when compared with 30- or 40-pS alpha3*-AChR channels. None of the channel parameters associated with the 60- and 80-pS alphaBgt-AChRs differed detectably, however, between choroid and ciliary neurons. Instead calculations based on the combined whole cell and single-channel results indicate that choroid neurons express approximately threefold larger numbers of functional alphaBgt-AChRs (N(F)) per unit area than do ciliary neurons. Comparison with total surface [(125)I]-alphaBgt-AChR sites (N(T)), reveals that N(F)/N(T) << 1 for both neuron populations, suggesting that "silent" alphaBgt-AChRs predominate. Choroid neurons may therefore express a higher density of functional alphaBgt-AChRs by recruiting a larger fraction of receptors from the silent pool than do ciliary neurons.

Amino acid determinants of alpha 7 nicotinic acetylcholine receptor surface expression

Transient transfection has not been a successful method to express the alpha7 nicotinic acetylcholine receptor such that these receptors are detected on the cell surface. This is not the case for all ligand-gated ion channels. Transient transfection with the 5-hydroxytryptamine type 3 subunit cDNA results in detectable surface receptor expression. Cell lines stably expressing the alpha7 nicotinic acetylcholine receptor produce detectable, albeit variable, levels of surface receptor expression. alpha7 nicotinic acetylcholine receptor surface expression is dependent, at least in part, on cell-specific factors. In addition to factors provided by the cells used for receptor expression, we hypothesize that the surface expression level in transfected cells is an intrinsic property of the receptor protein under study. Employing a set of alpha7-5-hydroxytryptamine type 3 chimeric receptor subunit cDNAs, we expressed these constructs in a transient transfection system and quantified surface receptor expression. We have identified amino acids that control receptor distribution between surface and intracellular pools; surface receptor expression can be manipulated without affecting the total number of receptors. These determinants function independently of the cell line used for expression and the transfection method employed. How these surface expression determinants in the alpha7 nicotinic acetylcholine receptor might influence synaptic efficacy is discussed.

Choline acetyltransferase, acetylcholinesterase, and nicotinic acetylcholine receptors of human gingival and esophageal epithelia

A non-neuronal cholinergic system that includes neuronal-like nicotinic acetylcholine receptors (nAChRs) has recently been described in epithelial cells that line the skin and the upper respiratory tract. Since the use of nicotine-containing products is associated with morbidity in the upper digestive tract, and since nicotine may alter cellular functions directly via nAChRs, we sought to identify and characterize a non-neuronal cholinergic system in the gingival and esophageal epithelia. mRNA transcripts for alpha3, alpha5, alpha7, and beta2 nAChR subunits, choline acetyltransferase, and the asymmetric and globular forms of acetylcholinesterase were amplified from gingival keratinocytes (KC) by means of polymerase chain-reactions. These proteins were visualized in the gingival and esophageal epithelia by means of specific antibodies. Variations in distribution and intensity of immunostaining were found, indicating that the repertoire of cholinergic enzymes and receptors expressed by the cells changes during epithelial maturation, and that an upward concentration gradient of free acetylcholine exists. Blocking of the nAChRs with mecamylamine resulted in reversible loss of cell-to-cell adhesion, and shrinking and rounding of cultured gingival KC. Activation of the receptors with acetylcholine or carbachol caused stretching and peripheral ruffling of the cytoplasmic aprons, and formation of new intercellular contacts. These results demonstrate that both the keratinizing epithelium of attached gingiva and the non-keratinizing epithelium lining the upper two-thirds of the esophageal mucosa possess a non-neuronal cholinergic system. The nAChRs expressed by these epithelia are coupled to regulation of cell adhesion and motility, and may provide a target for the deleterious effects of nicotine.

Individual variation in the expression profiles of nicotinic receptors in the olfactory bulb and trigeminal ganglion and identification of alpha2, alpha6, alpha9, and beta3 transcripts

Nicotine evokes dose-dependent and often variable chemosensory responses in animals and humans. Earlier observations that nicotine binds to some nicotinic acetylcholine receptor (nAChR) subtypes in the olfactory bulb (OB) and trigeminal ganglion (TG) led us to investigate the complete nAChR expression profile in each tissue and to determine whether inter-individual differences exist in male and female rats. Total RNA was extracted from individual samples of dissected OB and TG and analyzed by a sensitive reverse transcription-polymerase chain reaction (RT-PCR) assay to determine the messenger RNA profiles of ten transcripts encoded by the alpha2, alpha3, alpha4, alpha5, alpha6, alpha7, alpha9, beta2, beta3, and beta4 nAChR genes. We found that (a) in the OB, all animals expressed alpha2, alpha3, alpha4, alpha5, alpha7, beta2, and beta4 subunit mRNAs, whereas alpha6, beta3, and alpha9 transcripts were expressed in only 17, 28, and 33% of the animals, respectively, and (b) in the TG, all animals expressed alpha2, alpha3, alpha6, alpha7, beta2, and beta4 subunit mRNAs, whereas alpha9, beta3, alpha4, and alpha5 transcripts were expressed in 4, 38, 88, and 92% of the animals, respectively. These results also identified new subunits that are expressed in each tissue (alpha2, alpha6, alpha9, and beta3) and demonstrated that individual rats may have different tissue-specific expression profiles for alpha4, alpha5, alpha6, alpha9, and beta3 transcripts. Such variations are likely to be reflected in the composition of functional receptor subtypes in the rat OB and TG that have different activation and desensitization characteristics to acetylcholine and nicotine.

Neuronal alpha-bungarotoxin receptors are alpha7 subunit homomers

Nicotinic acetylcholine receptors in the nervous system are heterogeneous with distinct pharmacological and functional properties resulting from differences in post-translational processing and subunit composition. Because of nicotinic receptor diversity, receptor purification and biochemical characterization have been difficult, and the precise subunit composition of each receptor subtype is poorly characterized. Evidence is presented that alpha-bungarotoxin (Bgt)-binding nicotinic receptors found in pheochromocytoma 12 (PC12) cells are pentamers composed solely of alpha7 subunits. Metabolically labeled, affinity-purified Bgt receptors (BgtRs) consisted of a single 55 kDa band on SDS gels, which was recognized by anti-alpha7 antibodies on immunoblots. Isoelectric focusing separated the 55 kDa band into multiple spots, all recognized by anti-alpha7 antibodies and, therefore, each a differentially processed alpha7 subunit. Cell-surface BgtR subunits, cross-linked to each other and (125)I-Bgt, migrated on gels as a ladder of five bands with each band a multiple of an alpha7 subunit monomer. Similar characteristics of BgtRs from rat brain suggest that they, like PC12 BgtRs, are alpha7 pentamers containing differentially processed alpha7 subunits.

Up-regulation of cell-surface alpha4beta2 neuronal nicotinic receptors by lower temperature and expression of chimeric subunits

The predominant nicotinic acetylcholine receptor (nAChR) expressed in vertebrate brain is a pentamer containing alpha4 and beta2 subunits. In this study we have examined how temperature and the expression of subunit chimeras can influence the efficiency of cell-surface expression of the rat alpha4beta2 nAChR. Functional recombinant alpha4beta2 nAChRs, showing high affinity binding of nicotinic radioligands (K(d) = 41 +/- 22 pM for [(3)H]epibatidine), are expressed in both stably and transiently transfected mammalian cell lines. Despite this, only very low levels of alpha4beta2 nAChRs can be detected on the cell surface of transfected mammalian cells maintained at 37 degrees C. At 30 degrees C, however, cells expressing alpha4beta2 nAChRs show a 12-fold increase in radioligand binding (with no change in affinity), and a 5-fold up-regulation in cell-surface receptors with no increase in total subunit protein. In contrast to "wild-type" alpha4 and beta2 subunits, chimeric nicotinic/serotonergic subunits ("alpha4chi" and "beta2chi") are expressed very efficiently on the cell surface (at 30 degrees C or 37 degrees C), either as hetero-oligomeric complexes (e.g. alpha4chi+beta2 or alpha4chi+beta2chi) or when expressed alone. Compared with alpha4beta2 nAChRs, expression of complexes containing chimeric subunits typically results in up to 20-fold increase in nicotinic radioligand binding sites (with no change in affinity) and a similar increase in cell-surface receptor, despite a similar level of total chimeric and wild-type protein.

Cell-free expression and functional reconstitution of homo-oligomeric alpha7 nicotinic acetylcholine receptors into planar lipid bilayers

The alpha7 nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel that modulates neurotransmitter release in the central nervous system. We show here that functional, homo-oligomeric alpha7 nAChRs can be synthesized in vitro with a rabbit reticulocyte lysate translation system supplemented with endoplasmic reticulum microsomes, reconstituted into planar lipid bilayers, and evaluated using single-channel recording techniques. Because wild-type alpha7 nAChRs desensitize rapidly, we used a nondesensitizing form of the alpha7 receptor with mutations in the second transmembrane domain (S2'T and L9'T) to record channel activity in the continuous presence of agonist. Endoglycosidase H treatment of microsomes containing nascent alpha7 S2'T/L9'T nAChRs indicated that the receptors were glycosylated. A proteinase K protection assay revealed a 36-kDa fragment in the ER lumen, consistent with a large extracellular domain predicted by most topological models, indicating that the protein was folded integrally through the ER membrane. alpha7 S2'T/L9'T receptors reconstituted into planar lipid bilayers had a unitary conductance of approximately 50 pS, were highly selective for monovalent cations over Cl(-), were nonselective between K(+) and Na(+), and were blocked by alpha-bungarotoxin. This is the first demonstration that a functional ligand-gated ion channel can be synthesized using an in vitro expression system.

alpha-bungarotoxin receptors contain alpha7 subunits in two different disulfide-bonded conformations

Neuronal nicotinic alpha7 subunits assemble into cell-surface complexes that neither function nor bind alpha-bungarotoxin when expressed in tsA201 cells. Functional alpha-bungarotoxin receptors are expressed if the membrane-spanning and cytoplasmic domains of the alpha7 subunit are replaced by the homologous regions of the serotonin-3 receptor subunit. Bgt-binding surface receptors assembled from chimeric alpha7/serotonin-3 subunits contain subunits in two different conformations as shown by differences in redox state and other features of the subunits. In contrast, alpha7 subunit complexes in the same cell line contain subunits in a single conformation. The appearance of a second alpha7/serotonin-3 subunit conformation coincides with the formation of alpha-bungarotoxin-binding sites and intrasubunit disulfide bonding, apparently within the alpha7 domain of the alpha7/serotonin-3 chimera. In cell lines of neuronal origin that produce functional alpha7 receptors, alpha7 subunits undergo a conformational change similar to alpha7/serotonin-3 subunits. alpha7 subunits, thus, can fold and assemble by two different pathways. Subunits in a single conformation assemble into nonfunctional receptors, or subunits expressed in specialized cells undergo additional processing to produce functional, alpha-bungarotoxin-binding receptors with two alpha7 conformations. Our results suggest that alpha7 subunit diversity can be achieved postranslationally and is required for functional homomeric receptors.