Rearrangement of nicotinic receptor alpha subunits during formation of the ligand binding sites

Protein networking: nicotinic acetylcholine receptors and their protein-protein-associations

Nicotinic acetylcholine receptors (nAChR) are complex transmembrane proteins involved in neurotransmission in the nervous system and at the neuromuscular junction. nAChR disorders may lead to severe, potentially fatal pathophysiological states. To date, the receptor has been the focus of basic and applied research to provide novel therapeutic interventions. Since most studies have investigated only the nAChR itself, it is necessary to consider the receptor as part of its protein network to understand or elucidate-specific pathways. On its way through the secretory pathway, the receptor interacts with several chaperones and proteins. This review takes a closer look at these molecular interactions and focuses especially on endoplasmic reticulum biogenesis, secretory pathway sorting, Golgi maturation, plasma membrane presentation, retrograde internalization, and recycling. Additional knowledge regarding the nAChR protein network may lead to a more detailed comprehension of the fundamental pathomechanisms of diseases or may lead to the discovery of novel therapeutic drug targets.

A Stable Cell Line Expressing Clustered AChR: A Novel Cell-Based Assay for Anti-AChR Antibody Detection in Myasthenia Gravis

Cell-based assays (CBAs) and radioimmunoprecipitation assay (RIPA) are the most sensitive methods for identifying anti-acetylcholine receptor (AChR) antibody in myasthenia gravis (MG). But CBAs are limited in clinical practice by transient transfection. We established a stable cell line (KL525) expressing clustered AChR by infecting HEK 293T cells with dual lentiviral vectors expressing the genes encoding the human AChR α1, β1, δ, ϵ and the clustering protein rapsyn. We verified the stable expression of human clustered AChR by immunofluorescence, immunoblotting, and real-time PCR. Fluorescence-activated cell sorting (FACS) was used to detect anti-AChR antibodies in 103 MG patients and 58 healthy individuals. The positive results of MG patients reported by the KL525 was 80.6% (83/103), 29.1% higher than the 51.4% (53/103) of RIPA. 58 healthy individuals tested by both the KL525 CBA and RIPA were all negative. In summary, the stable expression of clustered AChR in our cell line makes it highly sensitive and advantageous for broad clinical application in CBAs.

Evidence for the involvement of adenomatous polyposis coli (APC) protein in maintaining cellular distributions of α3β4 nicotinic receptors

Evidence exists supporting the involvement of adenomatous polyposis coli (APC) protein in the assembly of neuronal nicotinic acetylcholine receptors (nAChRs) in the postsynaptic complex. In the following studies, the effects of APC protein on cellular distribution of recombinant α3β4 nAChRs was investigated. RT-PCR and Western blotting techniques established the expression of APC protein both in bovine adrenal chromaffin cells, which express native α3β4* nAChRs, and in a HEK293 cell line expressing recombinant bovine adrenal α3β4 nAChRs (BMα3β4 cells). Transfection of BMα3β4 cells with siRNA to APC, reduced APC protein levels to 52.4% and 61.9% of control values at 24 and 48 h after transfection. To investigate the effects of APC on the cellular distribution of α3β4 nAChRs, [(3)H]epibatidine binding approaches, coupled with APC siRNA treatment, were used. Twenty-four and 48 h after APC siRNA transfection, intracellular nAChRs were significantly reduced to 71% and 68% of control, respectively, while the total population of nAChRs were not significantly changed. Given that total cellular nAChRs represent the sum of surface and intracellular nAChRs, these studies support a re-distribution of nAChRs to the plasma membrane with APC siRNA treatment. Treatment of the cells with the protein synthesis inhibitor, puromycin, also caused a significant reduction (55%) in APC protein levels, and produced a similar re-distribution of cellular nAChRs. These studies support the involvement of APC protein in the maintenance of normal cellular distribution of α3β4 nAChRs.

Presynaptic type III neuregulin1-ErbB signaling targets {alpha}7 nicotinic acetylcholine receptors to axons

Type III Neuregulin1 (Nrg1) isoforms are membrane-tethered proteins capable of participating in bidirectional juxtacrine signaling. Neuronal nicotinic acetylcholine receptors (nAChRs), which can modulate the release of a rich array of neurotransmitters, are differentially targeted to presynaptic sites. We demonstrate that Type III Nrg1 back signaling regulates the surface expression of α7 nAChRs along axons of sensory neurons. Stimulation of Type III Nrg1 back signaling induces an increase in axonal surface α7 nAChRs, which results from a redistribution of preexisting intracellular pools of α7 rather than from increased protein synthesis. We also demonstrate that Type III Nrg1 back signaling activates a phosphatidylinositol 3-kinase signaling pathway and that activation of this pathway is required for the insertion of preexisting α7 nAChRs into the axonal plasma membrane. These findings, in conjunction with prior results establishing that Type III Nrg1 back signaling controls gene transcription, demonstrate that Type III Nrg1 back signaling can regulate both short-and long-term changes in neuronal function.

Endoplasmic reticulum chaperones stabilize nicotinic receptor subunits and regulate receptor assembly

We examined interactions between the endoplasmic reticulum (ER) chaperones calnexin (CN), ERp57, and immunological heavy chain-binding protein (BiP) and nicotinic acetylcholine receptor (nAChR) subunits. The three chaperones rapidly associate with newly synthesized nAChR subunits. Interactions between nAChR subunits and ERp57 occur via transient intermolecular disulfide bonds and do not require subunit N-linked glycosylation. The associations of ERp57 or CN with AChR subunits are long lived and prolong subunit lifetime approximately 10-fold. Coexpression of CN or ERp57 alone does not affect nAChR assembly or trafficking, but together they cause a significant decrease in nAChR expression and assembly. In contrast, associations with BiP are shorter lived and do not alter nAChR expression and assembly. However, a mutated BiP that slows its dissociation significantly increases its associations and decreases nAChR expression and assembly. Our results suggest that interactions with the chaperones regulate the levels of nAChRs assembled in the ER by stabilizing and sequestering subunits during assembly.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

Nicotine enhances intracellular nicotinic receptor maturation: A novel mechanism of neural plasticity?

Nicotine addiction, the primary cause of tobacco consumption, is mediated through nicotine binding to brain nicotinic acetylcholine receptor (nAChRs). Upon chronic exposure, nicotine elicits a cascade of events, starting with nAChR activation and desensitization, followed by a long term up-regulation that corresponds to an increase in the number of the high affinity nAChRs, a paradoxical process that occurs in the brain of smokers. Recent investigation of the maturation and trafficking of the major brain alpha4beta2 nAChR demonstrates that up-regulation is initiated in the endoplasmic reticulum soon after protein translation. The data thus far accumulated provide evidence that nicotine elicits up-regulation by promoting maturation of nAChR precursors that would otherwise be degraded. This "maturational enhancer" action of nicotine probably contributes to the long term effect of chronic nicotine, and suggests a novel mechanism of neuronal plasticity through an yet unknown endogenous substance which would modulate the receptor expression under physiological conditions.

Expression of native alpha3beta4* neuronal nicotinic receptors: binding and functional studies investigating turnover of surface and intracellular receptor populations

Several pathological conditions involve alterations in expression of neuronal nicotinic acetylcholine receptors (nAChRs). Although some studies have addressed processes involved with muscle nAChR expression, knowledge of the regulation of neuronal nAChRs is particularly sparse. The following studies were designed to investigate cellular mechanisms involved with expression of neuronal alpha3beta4* nAChRs. Catecholamine secretion assays and receptor binding studies coupled with receptor alkylation were used to study the nAChR regulation and turnover. Alkylation of adrenal nAChRs results in a rapid and complete loss of receptor-mediated neurosecretion and surface [(3)H]epibatidine binding sites. After alkylation, both neurosecretory function and nAChR binding slowly (24-48 h) return to prealkylation levels. When cells are treated with the protein synthesis inhibitor puromycin, after alkylation, receptor-mediated neurosecretion does not recover. Long-term treatment (24-48-h) with puromycin, in the absence of alkylation, results in a slow, time-dependent shift to the right, followed by a downward shift, in the nicotine concentration-response curve, documenting a disappearance of surface nAChRs. Puromycin treatment alone also results in a loss to both surface and intracellular [(3)H]epibatidine binding sites. nAChR beta4 subunit levels are significantly decreased after treatment with puromycin. These data support a constitutive turnover of adrenal alpha3beta4* nAChRs, requiring continual de novo synthesis of new receptor protein.

Noninvasive Imaging of 5-HT3 Receptor Trafficking in Live Cells

Sequential stages in the life cycle of the ionotropic 5-HT(3) receptor (5-HT(3)R) were resolved temporally and spatially in live cells by multicolor fluorescence confocal microscopy. The insertion of the enhanced cyan fluorescent protein into the large intracellular loop delivered a fluorescent 5-HT(3)R fully functional in terms of ligand binding specificity and channel activity, which allowed for the first time a complete real-time visualization and documentation of intracellular biogenesis, membrane targeting, and ligand-mediated internalization of a receptor belonging to the ligand-gated ion channel superfamily. Fluorescence signals of newly expressed receptors were detectable in the endoplasmic reticulum about 3 h after transfection onset. At this stage receptor subunits assembled to form active ligand binding sites as demonstrated in situ by binding of a fluorescent 5-HT(3)R-specific antagonist. After novel protein synthesis was chemically blocked, the 5-HT(3) R populations in the endoplasmic reticulum and Golgi cisternae moved virtually quantitatively to the cell surface, indicating efficient receptor folding and assembly. Intracellular 5-HT(3) receptors were trafficking in vesicle-like structures along microtubules to the cell surface at a velocity generally below 1 mum/s and were inserted into the plasma membrane in a characteristic cluster distribution overlapping with actin-rich domains. Internalization of cell surface 5-HT(3) receptors was observed within minutes after exposure to an extracellular agonist. Our orchestrated use of spectrally distinguishable fluorescent labels for the receptor, its cognate ligand, and specific organelle markers can be regarded as a general approach allowing subcellular insights into dynamic processes of membrane receptor trafficking.

Critical role of the C-terminal segment in the maturation and export to the cell surface of the homopentameric alpha7-5HT3A receptor

Many neurological pathologies are related to misfolded proteins. During folding and assembly in the endoplasmic reticulum, the nicotinic acetylcholine receptor (nAChR) subunits undergo several conformational changes to acquire the ability to bind ligands. After folding and maturation, by mechanisms largely unknown, receptors are exported to the cell surface. We investigated the maturational role of the extracellular C-terminal segment located at the boundary between the extracellular and the transmembrane domains. In the functional chimeric alpha7-5HT3A receptor used as a model system, amino acids from the C-terminal segment were successively deleted or mutated. Upon progressive shortening of the peptide we observed less and less alpha-bungarotoxin binding sites until no sites could be detected when the entire peptide had been deleted (chimera Del 5). Protein synthesis and pentameric assembly were not altered. In Del 5 transfected cells, pentameric receptors present in the endoplasmic reticulum were not detected on the cell surface where Del 5 proteins appeared as patches. With the Del 5 chimera, export of proteins to the cell surface diminished to about half that of wild-type. We propose that the C-terminal segment plays a double role: (i) through an interaction between the penultimate tyrosine residue of the C-terminal segment and the Cys loop of the N-terminal domain, it locks the receptor in a mature alpha-bungarotoxin binding conformation; (ii) this mature conformation, in turn, masks a retention signal present in the first transmembrane segment allowing properly assembled and matured receptors to escape to the cell surface.

Regulation of nicotinic acetylcholine receptor assembly

The four muscle-type nicotinic acetylcholine receptor (AChR) subunits, alpha, beta, gamma, and delta, assemble into functional alpha(2)betagammadelta pentamers in the endoplasmic reticulum (ER) through a series of interdependent folding and oligomerization events. The first stable assembly intermediate is a trimer composed of alpha, beta, and gamma subunits. The formation of alphabetagamma trimers initiates a series of subunit folding and processing events that allow addition of delta subunits to form alphabetagammadelta tetramers. Subunit folding and processing continue with formation of the ligand-binding sites on the alpha subunit of alphabetagammadelta tetramers and the second alpha subunit added to assemble alpha(2)betagammadelta pentamers. AChR assembly is inefficient. Only 20-30% of synthesized subunits assemble into mature receptors in the ER, while the remaining unassembled subunits are degraded. However, the efficiency of subunit assembly can be regulated under certain conditions leading to higher AChR expression. Increased intracellular cAMP levels cause a 2- to 3-fold increase in AChR assembly efficiency and a comparable increase in surface expression. Additionally, block of ubiquitin-proteasome degradation appears to enhance AChR assembly and expression. Thus, the regulation of AChR assembly through posttranslational mechanisms is a potential therapeutic target for increasing AChR expression in diseases in which expression is compromised.

Antisense inhibition of neuronal nicotinic receptors in the tobacco-feeding insect, Manduca sexta

Acetylcholine is the predominant excitatory transmitter in the insect central nervous system with many of its effects mediated by nicotinic acetylcholine receptors. These receptors are present at very high density and are structurally heterogeneous, although little is known about functional distinctions between them. An interesting system for examining these receptors is the larval stage of Manduca sexta, a nicotine-resistant tobacco-feeding insect. The nicotinic responses of cultured neurons were found to be blocked by mecamylamine and curare but highly resistant to alpha-bungarotoxin. The responses were also unaffected by the reducing agent dithiothreitol and the alkylating agent bromoacetylcholine suggesting that the alpha-subunit dicysteine agonist binding site is protected. To begin determining the functional roles of different subunits in these receptors, cultured neurons were treated with oligonucleotides based on the gene sequence of the alpha subunit, MARA1. Antisense DNA caused a significant downward shift in the amplitude distribution of nicotinic responses compared to sense or reverse antisense treatments. These treatments did not affect currents mediated by the application of GABA. The reduction in the nicotinic depolarization and inward currents did not affect the rate of current onset or recovery, suggesting that antisense MARA1 causes a partial block of all nicotinic responses in these neurons. These results demonstrate that receptor gene expression in insect neurons can be manipulated in a sequence-specific manner by antisense treatment and they provide evidence that MARA1 is important for normal nicotinic responses in Manduca.

A role for presenilin 1 in regulating the delivery of amyloid precursor protein to the cell surface

Presenilin 1 (PS1) and presenilin 2 play a critical role in the gamma-secretase processing of amyloid precursor protein (APP) and Notch1. Here, we investigate maturation and intracellular trafficking of APP and other membrane proteins in cells expressing an experimental PS1 deletion mutant (deltaM1,2). Stable expression of deltaM1,2 impairs gamma-secretase processing of Notch1 and delays Abeta secretion. Kinetic studies show enhanced O-glycosylation and sialylation of holo-APP and marked accumulation of APP COOH-terminal fragments (CTFs). Surface biotinylation, live staining, and trafficking studies show increased surface accumulation of holo-APP and CTFs in deltaM1,2 cells resulting from enhanced surface delivery of newly synthesized APP. Expression of a loss-of-function PS1 mutant (D385A) or incubation of cells with gamma-secretase inhibitors also increases surface levels of holo-APP and CTFs. In contrast to APP, glycosylation and surface accumulation of another type I membrane protein, nicastrin, are markedly reduced in deltaM1,2 cells. Finally, expression of deltaM1,2 results in the increased assembly and surface expression of nicotinic acetylcholine receptors, illustrating that PS1's influence on protein trafficking extends beyond APP and other type I membrane protein substrates of gamma-secretase. Collectively, our findings provide evidence that PS1 regulates the glycosylation and intracellular trafficking of APP and select membrane proteins.

Assembly and cell surface expression of homomeric and heteromeric 5-HT3 receptors: the role of oligomerization and chaperone proteins

The ability of differing subunit combinations of 5-HT3 receptors to form functional cell surface receptors was analyzed by a variety of approaches. The results revealed that 5-HT3 receptor assembly occurred within the endoplasmic reticulum (ER) and involved the interaction with chaperone proteins. The 5-HT3A subunit could assemble into functional homomeric receptors that were expressed on the cell surface. In contrast, the 5-HT3B subunit did not exhibit 5-hydroxytryptamine binding or function, could not assemble, and was efficiently retained and degraded within the ER. However, upon the coexpression of the 5-HT3A subunit, 5-HT3B could be "rescued" from the ER and transported to the cell surface to form functional heteromeric receptors with distinct functional characteristics. In support of the existence of homomeric 5-HT3 receptors in vivo, recombinantly expressed 5-HT3A receptors were capable of clustered cell surface expression in cortical neurons.

Refolding of the Escherichia coli expressed extracellular domain of alpha 7 nicotinic acetylcholine receptor

Heterologous expression of the extracellular domains (ECDs) of the nicotinic acetylcholine receptor (AChR) subunits may give large amounts of proteins for studying the functional and spatial characteristics of their ligand-binding sites. The ECD of the alpha 7 subunit of the homo-oligomeric alpha 7 neuronal AChR appears to be a more suitable object than the ECDs of other heteromeric neuronal or muscle-type AChRs. The rat alpha 7 ECDs (amino-acid residues approximately 1-210) were recently expressed in Escherichia coli as fusion proteins with maltose-binding protein [Fischer, M., Corringer, P., Schott, K., Bacher, A. & Changeux, J. (2001) Proc. Natl Acad. Sci. USA 98, 3567-3570] and glutathione S-transferase (GST) [Utkin, Y., Kukhtina, V., Kryukova, E., Chiodini, F., Bertrand, D., Methfessel, C. & Tsetlin, V. (2001) J. Biol. Chem. 276, 15810-15815]. However, these proteins exist in solution mostly as high-molecular mass aggregates rather than monomers or oligomers. In the present work it is found that refolding of GST-alpha 7-(1-208) protein in the presence of 0.1% SDS considerably decreases the formation of high-molecular mass aggregates. The C116S mutation in the alpha 7 moiety was found to further decrease the aggregation and to increase the stability of protein solutions. This mutation slightly increased the affinity of the protein for alpha-bungarotoxin (from Kd approximately 300 to 150 nm). Gel-permeation HPLC was used to isolate the monomeric form of the GST-alpha 7-(1-208) protein and its mutant almost devoid of SDS. CD spectra revealed that the C116S mutation considerably increased the content of beta structure and made it more stable under different conditions. The monomeric C116S mutant appears promising both for further structural studies and as a starting material for preparing the alpha 7 ECD in an oligomeric form.

Alternate use of distinct intersubunit contacts controls GABAA receptor assembly and stoichiometry

GABA(A) receptors are the major inhibitory transmitter receptors in the CNS. Recombinant GABA(A) receptors composed of alpha(1)beta(3)gamma(2) subunits have been demonstrated to assemble as pentamers consisting of two alpha(1), two beta(3), and one gamma(2) subunit. Using truncated and chimeric alpha(1) subunits, we identified the alpha(1)(80-100) sequence as a major binding site for gamma(2) subunits. In addition, we demonstrated its direct interaction with gamma(2)(91-104), a sequence that previously has been identified to form the contact to alpha(1) subunits. The observation that the amino acid residues alpha(1)P96 and alpha(1)H101, which can be photolabeled by [(3)H]flunitrazepam, are located within or adjacent to the alpha(1)(80-100) sequence, indicates that the benzodiazepine binding site of GABA(A) receptors is located close to this intersubunit contact. The observation that alpha(1)(80-100) interacts with gamma(2) but not with beta(3) subunits indicates the existence of an additional beta(3) binding site on alpha(1) subunits. The preferred alternate use of the gamma(2) and beta(3) binding sites in two different alpha(1) subunits of the same receptor ensures the incorporation of only a single gamma(2) subunit and thus, determines subunit stoichiometry of alpha(1)beta(3)gamma(2) receptors. Distinct binding sites and their alternate use can therefore explain how subunits of hetero-oligomeric transmembrane proteins assemble into a defined protein complex.