Amino acid determinants of alpha 7 nicotinic acetylcholine receptor surface expression

Probing the molecular basis for signal transduction through the Zinc-Activated Channel (ZAC)

The molecular basis for the signal transduction through the classical Cys-loop receptors (CLRs) has been delineated in great detail. The Zinc-Activated Channel (ZAC) constitutes a so far poorly elucidated fifth branch of the CLR superfamily, and in this study we explore the molecular mechanisms underlying ZAC signaling in Xenopus oocytes by two-electrode voltage clamp electrophysiology. In studies of chimeric receptors fusing either the extracellular domain (ECD) or the transmembrane/intracellular domain (TMD-ICD) of ZAC with the complementary domains of 5-HT3A serotonin or α1 glycine receptors, serotonin and Zn2+/H+ evoked robust concentration-dependent currents in 5-HT3A/ZAC- and ZAC/α1-Gly-expressing oocytes, respectively, suggesting that Zn2+ and protons activate ZAC predominantly through its ECD. The molecular basis for Zn2+-mediated ZAC signaling was probed further by introduction of mutations of His, Cys, Glu and Asp residues in this domain, but as none of the mutants tested displayed substantially impaired Zn2+ functionality compared to wild-type ZAC, the location of the putative Zn2+ binding site(s) in the ECD was not identified. Finally, the functional importance of Leu246 (Leu9') in the transmembrane M2 α-helix of ZAC was investigated by Ala, Val, Ile and Thr substitutions. In concordance with findings for this highly conserved residue in classical CLRs, the ZACL9'X mutants exhibited left-shifted agonist concentration-response relationships, markedly higher degrees of spontaneous activity and slower desensitization kinetics compared to wild-type ZAC. In conclusion, while ZAC is an atypical CLR in terms of its (identified) agonists and channel characteristics, its signal transduction seems to undergo similar conformational transitions as those in the classical CLR.

Farnesyl Transferase Inhibitor Lonafarnib Enhances α7nAChR Expression Through Inhibiting DNA Methylation of CHRNA7 and Increases α7nAChR Membrane Trafficking

Inhibition of Ras farnesylation in acute has been found to upregulate the α7 nicotinic acetylcholine receptor (α7nAChR) activity. This study was carried out to investigate the effect of chronic administration for 7 days of farnesyl transferase inhibitor lonafarnib (50 mg/kg, intraperitoneally injected) to male mice on the expression and activity of α7nAChR in hippocampal CA1 pyramidal cells. Herein, we show that lonafarnib dose dependently enhances the amplitude of ACh-evoked inward currents (I_ACh), owning to the increased α7nAChR expression and membrane trafficking. Lonafarnib inhibited phosphorylation of c-Jun and JNK, which was related to DNA methylation. In addition, reduced DNA methyltransferase 1 (DNMT1) expression was observed in lonafarnib-treated mice, which was reversed by JNK activator. Lonafarnib-upregulated expression of α7nAChR was mimicked by DNMT inhibitor, and repressed by JNK activator. However, only inhibited DNA methylation did not affect I_ACh, and the JNK activator partially decreased the lonafarnib-upregulated I_ACh. On the other hand, lonafarnib also increased the membrane expression of α7nAChR, which was partially inhibited by JNK activator or CaMKII inhibitor, without changes in the α7nAChR phosphorylation. CaMKII inhibitor had no effect on the expression of α7nAChR. Lonafarnib-enhanced spatial memory of mice was also partially blocked by JNK activator or CaMKII inhibitor. These results suggest that Ras inhibition increases α7nAChR expression through depressed DNA methylation of CHRNA7 via Ras-c-Jun-JNK pathway, increases the membrane expression of α7nAChR resulting in part from the enhanced CaMKII pathway and total expression of this receptor, and consequently enhances the spatial memory.

Functional Human α7 Nicotinic Acetylcholine Receptor (nAChR) Generated from Escherichia coli

Human Cys-loop receptors are important therapeutic targets. High-resolution structures are essential for rational drug design, but only a few are available due to difficulties in obtaining sufficient quantities of protein suitable for structural studies. Although expression of proteins in E. coli offers advantages of high yield, low cost, and fast turnover, this approach has not been thoroughly explored for full-length human Cys-loop receptors because of the conventional wisdom that E. coli lacks the specific chaperones and post-translational modifications potentially required for expression of human Cys-loop receptors. Here we report the successful production of full-length wild type human α7nAChR from E. coli Chemically induced chaperones promote high expression levels of well-folded proteins. The choice of detergents, lipids, and ligands during purification determines the final protein quality. The purified α7nAChR not only forms pentamers as imaged by negative-stain electron microscopy, but also retains pharmacological characteristics of native α7nAChR, including binding to bungarotoxin and positive allosteric modulators specific to α7nAChR. Moreover, the purified α7nAChR injected into Xenopus oocytes can be activated by acetylcholine, choline, and nicotine, inhibited by the channel blockers QX-222 and phencyclidine, and potentiated by the α7nAChR specific modulators PNU-120596 and TQS. The successful generation of functional human α7nAChR from E. coli opens a new avenue for producing mammalian Cys-loop receptors to facilitate structure-based rational drug design.

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 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.

Investigating the role of protein folding and assembly in cell-type dependent expression of alpha7 nicotinic receptors using a green fluorescent protein chimera

To test the hypothesis that cell-dependent expression of alpha7 receptors is due to differences in protein folding or assembly, we constructed a chimeric rat alpha7 subunit with green fluorescent protein (GFP) at the receptor C-terminal. Expression of alpha7-GFP in Xenopus oocytes resulted in currents that were indistinguishable from wild type receptors but were only 33% of control. (125)I-alpha-bungarotoxin (alphaBGT) binding at the oocyte surface was reduced to 23% of wild type. Transfection of alpha7-GFP into GH4C1 cells produced fluorescence that was less intense than GFP alone, but showed significant alpha-BGT binding compared to transfection with GFP. In contrast, alpha7-GFP transfection in SH-EP1, HEK293 and CHO-CAR cells produced fluorescence without alphaBGT binding. Flow cytometry of cells transfected with alpha7-GFP indicated fluorescence in both SH-EP1 and GH4C1 cells, but surface toxin binding sites and sites immunoprecipitated using anti-GFP antibodies were undetectable in SH-EP1 cells, suggesting a problem in folding/assembly rather than trafficking. Surprisingly, integrated fluorescence intensities in GH4C1 cells transfected with alpha7-GFP did not correlate with amounts of cell surface or immunoprecipitable alphaBGT binding. Therefore, GFP folding at the C-terminal of the alpha7-GFP chimera is cell-line independent, but toxin binding is highly cell-line dependent, suggesting that if altered protein folding is involved in the cell-type dependence of alpha7 receptor expression, the phenomenon is restricted to specific protein domains. Further, C-terminal GFP-labeled alpha7 receptors decreased the efficiency of folding/assembly not only of chimeric subunits, but also wild-type subunits, suggesting that the C-terminal is an important domain for alpha7 receptor assembly.

Cellular events in nicotine addiction

In the 25 years since the observation that chronic exposure to nicotine could regulate the number and function of high affinity nicotine binding sites in the brain there has been a major effort to link alterations in nicotinic acetylcholine receptors (nAChRs) to nicotine-induced behaviors that drive the addiction to tobacco products. Here we review the proposed roles of various nAChR subtypes in the addiction process, with emphasis on how they are regulated by nicotine and the implications for understanding the cellular neurobiology of addiction to this drug.

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.

Identification of domains influencing assembly and ion channel properties in alpha 7 nicotinic receptor and 5-HT3 receptor subunit chimaeras

BACKGROUND AND PURPOSE

Nicotinic acetylcholine receptors (nAChRs) and 5-hydroxytryptamine type 3 receptors (5-HT(3)Rs) are members of the superfamily of neurotransmitter-gated ion channels. Both contain five subunits which assemble to form either homomeric or heteromeric subunit complexes. With the aim of identifying the influence of subunit domains upon receptor assembly and function, a series of chimaeras have been constructed containing regions of the neuronal nAChR alpha 7 subunit and the 5-HT(3) receptor (3A) subunit.

EXPERIMENTAL APPROACH

A series of subunit chimaeras containing alpha 7 and 5-HT(3A) subunit domains have been constructed and expressed in cultured mammalian cells. Properties of the expressed receptors have been examined by means of radioligand binding, agonist-induced changes in intracellular calcium and patch-clamp electrophysiology.

KEY RESULTS

Subunit domains which influence properties such as rectification, desensitization and conductance have been identified. In addition, the influence of subunit domains upon subunit folding, receptor assembly and cell-surface expression has been identified. Co-expression studies with the nAChR-associated protein RIC-3 revealed that, in contrast to the potentiating effect of RIC-3 on alpha 7 nAChRs, RIC-3 caused reduced levels of cell-surface expression of some alpha 7/5-HT(3A) chimaeras.

CONCLUSIONS AND IMPLICATIONS

Evidence has been obtained which demonstrates that subunit transmembrane domains are critical for efficient subunit folding and assembly. In addition, functional characterization of subunit chimaeras revealed that both extracellular and cytoplasmic domains exert a dramatic and significant influence upon single-channel conductance. These data support a role for regions other than hydrophobic transmembrane domains in determining ion channel properties.

Pharmacological characteristics and binding modes of caracurine V analogues and related compounds at the neuronal alpha7 nicotinic acetylcholine receptor

The pharmacological properties of bisquaternary caracurine V, iso-caracurine V, and pyrazino[1,2-a;4,5-a']diindole analogues and of the neuromuscular blocking agents alcuronium and toxiferine I have been characterized at numerous ligand-gated ion channels. Several of the analogues are potent antagonists of the homomeric alpha7 nicotinic acetylcholine receptor (nAChR), displaying nanomolar binding affinities and inhibiting acetylcholine-evoked signaling through the receptor in a competitive manner. In contrast, they do not display activities at heteromeric neuronal nAChRs and only exhibit weak antagonistic activities at the related 5-HT3A serotonin receptor. In a mutagenesis study, five selected analogues have been demonstrated to bind to the orthosteric site of the alpha7 nAChR. The binding site of the compounds overlaps with that of the standard alpha7 antagonist methyllycaconitine, the binding of them being centered in a cation-pi interaction between the quaternary nitrogen atom of the ligand and the Trp149 residue in the receptor, with additional key contributions from other aromatic receptor residues such as Tyr188, Tyr195, and Trp55.

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.

Charged amino acid motifs flanking each extreme of the alphaM4 transmembrane domain are involved in assembly and cell-surface targeting of the muscle nicotinic acetylcholine receptor

The alphaM4 transmembrane domain of the nicotinic acetylcholine receptor (AChR) is flanked by two basic amino acids (His(408) and Arg(429)) located at its cytoplasmic- and extracellular-facing extremes, respectively, at the level of the phospholipid polar head regions of the postsynaptic membrane. A series of single and double alphaM4 mutants (His(408)Ala, Arg(429)Ala, Arg(429)Glu, His(408)Ala/Arg(429)Ala, and His(408)Ala/Arg(429)Glu) of the adult muscle-type AChR were produced and coexpressed with wild-type beta, delta, and epsilon subunits as stable clones in a mammalian heterologous expression system (CHO-K1 cells). The mutants were studied by alpha-bungarotoxin ([(125)I]alpha-BTX) binding, fluorescence microscopy, and equilibrium sucrose gradient centrifugation. Cell-surface [(125)I]alpha-BTX binding diminished approximately 40% in His(408)Ala and as much as 95% in the Arg(429)Ala mutant. Reversing the amino acid charge (e.g., Arg(429)Glu) abolished cell-surface expression of AChR. Fluorescence microscopy disclosed that AChR was retained at the endoplasmic reticulum, with an enhanced occurrence of unassembled AChR species in the mutant clones. Centrifugation analysis confirmed the lack of fully assembled AChR pentamers in all mutants with the exception of His(408)Ala. We conclude that His(408) and Arg(429) in alphaM4 are involved in assembly and cell-surface targeting of muscle AChR. Arg(429) plays a more decisive role in these two processes, suggesting an asymmetric weight of the charged motifs at each extreme of the alpha subunit M4 transmembrane segment. (c) 2006 Wiley-Liss, Inc.

Nicotinic cholinergic intercellular communication: implications for the developing auditory system

In this paper, research on the temporal and spatial distribution of cholinergic-related molecules in the lower auditory brainstem, with an emphasis on nicotinic acetylcholine receptors (nAChRs), is reviewed. The possible functions of acetylcholine (ACh) in driving selective auditory neurons before the onset of hearing, inducing glutamate receptor gene expression, synaptogenesis, differentiation, and cell survival are discussed. Experiments conducted in other neuronal and non-neuronal systems are drawn on extensively to discuss putative functions of ACh and nAChRs. Data from other systems may provide insight into the functions of ACh and nAChRs in auditory processing. The mismatch of presynaptic and postsynaptic markers and novel endogenous agonists of nAChRs are discussed in the context of non-classical interneuronal communication. The molecular mechanism that may underlie the many functions of ACh and its agonists is the regulation of intracellular calcium through nAChRs. The possible reorganization that may take place in the auditory system by the exposure to nicotine during critical developmental periods is also briefly considered.

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

The ric-3 gene is required for maturation of nicotinic acetylcholine receptors in Caenorhabditis elegans. The human homolog of RIC-3, hRIC-3, enhances expression of alpha7 nicotinic receptors in Xenopus laevis oocytes, whereas it totally abolishes expression of alpha4beta2 nicotinic and 5-HT3 serotonergic receptors. Both the N-terminal region of hRIC-3, which contains two transmembrane segments, and the C-terminal region are needed for these differential effects. hRIC-3 inhibits receptor expression by hindering export of mature receptors to the cell membrane. By using chimeric proteins made of alpha7 and 5-HT3 receptors, we have shown that the presence of an extracellular isoleucine close to the first transmembrane receptor fragment is responsible for the transport arrest induced by hRIC-3. Enhancement of alpha7 receptor expression occurs, at least, at two levels: by increasing the number of mature receptors and facilitating its transport to the membrane. Certain amino acids of a putative amphipathic helix present at the large cytoplasmic region of the alpha7 subunit are required for these actions. Therefore, hRIC-3 can act as a specific regulator of receptor expression at different levels.

Stable expression and characterisation of a human alpha 7 nicotinic subunit chimera: a tool for functional high-throughput screening

A chimera comprising the N-terminal region of the human alpha7 nicotinic acetylcholine receptor, fused to the transmembrane/C-terminal domains of the mouse serotonin 5-HT3 receptor, was constructed. Injection of the chimera cDNA into Xenopus oocytes, or transient transfection in human embryonic kidney (HEK-293) cells, resulted in the expression of functional channels that were sensitive to nicotinic acetylcholine, but not serotonin receptor ligands. In both systems, the responses obtained from chimeric receptors inactivated more slowly than those recorded following activation of wild-type alpha7 receptors. A stable HEK-293 cell line expressing the human alpha7/mouse 5-HT3 chimera was established, which showed that the chimera displayed a similar pharmacological profile to wild-type alpha7 receptors. Use of this chimera in high-throughput screening may enable the identification of novel pharmacological agents that will help to define further the role of alpha7 nicotinic receptors in physiology and disease.

Rapid upregulation of alpha7 nicotinic acetylcholine receptors by tyrosine dephosphorylation

Alpha7 nicotinic acetylcholine receptors (nAChRs) modulate network activity in the CNS. Thus, functional regulation of alpha7 nAChRs could influence the flow of information through various brain nuclei. It is hypothesized here that these receptors are amenable to modulation by tyrosine phosphorylation. In both Xenopus oocytes and rat hippocampal interneurons, brief exposure to a broad-spectrum protein tyrosine kinase inhibitor, genistein, specifically and reversibly potentiated alpha7 nAChR-mediated responses, whereas a protein tyrosine phosphatase inhibitor, pervanadate, caused depression. Potentiation was associated with an increased expression of surface alpha7 subunits and was not accompanied by detectable changes in receptor open probability, implying that the increased function results from an increased number of alpha7 nAChRs. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor-mediated exocytosis was shown to be a plausible mechanism for the rapid delivery of additional alpha7 nAChRs to the plasma membrane. Direct phosphorylation/dephosphorylation of alpha7 subunits was unlikely because mutation of all three cytoplasmic tyrosine residues did not prevent the genistein-mediated facilitation. Overall, these data are consistent with the hypothesis that the number of functional cell surface alpha7 nAChRs is controlled indirectly via processes involving tyrosine phosphorylation.

Ric-3 promotes functional expression of the nicotinic acetylcholine receptor alpha7 subunit in mammalian cells

Expression of functional, recombinant alpha7 nicotinic acetylcholine receptors in several mammalian cell types, including HEK293 cells, has been problematic. We have isolated the recently described human ric-3 cDNA and co-expressed it in Xenopus oocytes and HEK293 cells with the human nicotinic acetylcholine receptor alpha7 subunit. In addition to confirming the previously reported effect on alpha7 receptor expression in Xenopus oocytes we demonstrate that ric-3 promotes the formation of functional alpha7 receptors in mammalian cells, as determined by whole cell patch clamp recording and surface alpha-bungarotoxin binding. Upon application of 1 mm nicotine, currents were undetectable in HEK293 cells expressing only the alpha7 subunit. In contrast, co-expression of alpha7 and ric-3 cDNAs resulted in currents that averaged 42 pA/pF with kinetics similar to those observed in cells expressing endogenous alpha7 receptors. Immunoprecipitation studies demonstrate that alpha7 and ric-3 proteins co-associate. Additionally, cell surface labeling with biotin revealed the presence of alpha7 protein on the plasma membrane of cells lacking ric-3, but surface alpha-bungarotoxin staining was only observed in cells co-expressing ric-3. Thus, ric-3 appears to be necessary for proper folding and/or assembly of alpha7 receptors in HEK293 cells.

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.

Functional nicotinic acetylcholine receptors on subplate neurons in neonatal rat somatosensory cortex

The establishment of cortical synaptic circuits during early development requires the presence of subplate neurons (SPn's), a heterogeneous population of neurons capable to integrate and process synaptic information from the thalamus, cortical plate, and neighboring SPn's. An accumulation of cholinergic afferents and nicotinic acetylcholine receptors (nAChRs) has been documentated in the subplate around birth. To assess the developmental role of the cholinergic innervation onto SPn's, we used whole cell patch-clamp recordings of visually identified and biocytin-labeled SPn's in neonatal rat somatosensory cortex. Functional nAChRs were present in 92% of the investigated SPn's. Activation of postsynaptic nAChRs by local application of agonists elicited a brief membrane depolarization associated with a barrage of action potentials and large inward currents reversing around 0 mV. According to our pharmacological data, excitation of SPn's is mediated by alpha4beta2 receptors. In contrast, functional alpha7 nAChRs could not be identified on SPn's. Activation of nAChRs affected neither the spontaneous synaptic activity of SPn's nor the synaptic connections between thalamus and SPn's and within subplate. Nicotine, at concentrations reaching the developing brain by maternal smoking, induced a severe desensitization of nAChRs and an increase in the baseline noise. These results indicate that nAChR-mediated excitation of SPn's may stabilize the developing synaptic circuits and suggest the involvement of nAChRs located on SPn's in the fetal tobacco syndrome.

Carbamoylcholine homologs: novel and potent agonists at neuronal nicotinic acetylcholine receptors

The classic muscarinic acetylcholine receptor (mAChR) agonist carbamoylcholine (carbachol) does not seem to be the most obvious lead for the development of selective ligands at nicotinic acetylcholine receptors (nAChRs). In the past, however, N-methylations of carbachol have provided N-methylcarbamoylcholine and N,N-dimethylcarbamoylcholine (DMCC), which predominantly display nicotinic activity. In this study, 12 homologous analogs of DMCC and its corresponding tertiary amine, N,N-dimethylcarbamoyl-N,N-dimethylaminoethanol, were synthesized and their binding affinities to native mAChR and nAChR sites estimated. One of the compounds in the series, 3-N,N-dimethylaminobutyl-N,N-dimethylcarbamate (7), displayed low nanomolar binding affinity to nAChRs and a 400-fold selectivity for nAChRs over mAChRs. Hence, a new series of compounds was synthesized in which alkyl and aryl groups and different ring systems were introduced in the carbamate moiety of 7. In a [3H]epibatidine binding assay, the Ki values of 7 and its analogs at rat alpha2beta2, alpha4beta2, alpha2beta4, alpha3beta4, and alpha4beta4 nAChRs, stably expressed in mammalian cell lines, ranged from low nanomolar to midmicromolar concentrations, whereas all of the compounds displayed weak binding to an alpha7/5-HT3 chimera and to native mAChRs. Compound 7 and its analogs were determined to be agonists at the alpha3beta4 nAChR subtype. This series includes the most potent and selective nicotinic agonists structurally derived from ACh to date. Furthermore, the compounds are tertiary amines, implying some advantages in terms of bioavailability pertinent to future in vivo pharmacological studies. Finally, observations made in the study hold promising perspectives for future development of ligands selective for specific nAChR subtypes.

SSR591813, a novel selective and partial alpha4beta2 nicotinic receptor agonist with potential as an aid to smoking cessation

(5aS,8S,10aR)-5a,6,9,10-Tetrahydro,7H,11H-8,10a-methanopyrido[2',3':5,6]pyrano[2,3-d]azepine (SSR591813) is a novel compound that binds with high affinity to the rat and human alpha4beta2 nicotinic acetylcholine receptor (nAChR) subtypes (Ki = 107 and 36 nM, respectively) and displays selectivity for the alpha4beta2 nAChR (Ki, human alpha3beta4 > 1000, alpha3beta2 = 116; alpha1beta1deltagamma > 6000 nM and rat alpha7 > 6000 nM). Electrophysiological experiments indicate that SSR591813 is a partial agonist at the human alpha4beta2 nAChR subtype (EC50 = 1.3 micro M, IA =19% compared with the full agonist 1,1-dimethyl-4-phenyl-piperazinium). In vivo findings from microdialysis and drug discrimination studies confirm the partial intrinsic activity of SSR591813. The drug increases dopamine release in the nucleus accumbens shell (30 mg/kg i.p.) and generalizes to nicotine or amphetamine (10-20 mg/kg i.p.) in rats, with an efficacy approximately 2-fold lower than that of nicotine. Pretreatment with SSR591813 (10 mg/kg i.p.) reduces the dopamine-releasing and discriminative effects of nicotine. SSR591813 shows activity in animal models of nicotine dependence at doses devoid of unwanted side effects typically observed with nicotine (hypothermia and cardiovascular effects). The compound (10 mg/kg i.p.) also prevents withdrawal signs precipitated by mecamylamine in nicotine-dependent rats and partially blocks the discriminative cue of an acute precipitated withdrawal. SSR591813 (20 mg/kg i.p.) reduces i.v. nicotine self-administration and antagonizes nicotine-induced behavioral sensitization in rats. The present results confirm important role for alpha4beta2 nAChRs in mediating nicotine dependence and suggest that SSR591813, a partial agonist at this particular nAChR subtype, may have therapeutic potential in the clinical management of smoking cessation.

Regulation by cycloheximide and lowered temperature of cell-surface alpha7-nicotinic acetylcholine receptor expression on transfected SH-EP1 cells

Heterologous expression of functional, nicotinic acetylcholine receptors (nAChR) in mammalian cells has been difficult to achieve or optimize, even for nAChR containing only one kind of subunit. In this study, we determined effects of lowered temperature or of exposure to the protein synthesis inhibitor cycloheximide (CHX) on cell surface expression of homomeric alpha7-nAChR in transfected SH-EP1 human epithelial cells. We found that incubation of cells for 2 days at 25 degrees C or in the presence of 0.5-2 microg/mL of CHX caused approximately four- or approximately eight-fold increases, respectively, in surface binding sites for 125I-labeled alpha-bungarotoxin (I-Bgt). These increases were accompanied by increases in peak whole-cell current responses to nicotinic agonists. Either treatment lowered protein synthesis and cell proliferation, but experiments using puromycin indicated that a reduction in protein synthesis or cell proliferation per se was not sufficient to increase surface binding. I-Bgt binding to whole-cell membrane pools increased in response to either treatment, suggesting that the increase in surface binding was due, at least in part, to an increase in intracellular receptor levels. The cyclophilin inhibitor cyclosporin A reduced surface expression in untreated as well as CHX- or 25 degrees C-treated cells. The results suggest practical means for increasing cell surface and functional expression of alpha7-nAChR. Although these effects are not simply due to protein synthesis inhibition or reduced cell proliferation, they do involve an increase in intracellular receptor pool size.

A transmembrane motif governs the surface trafficking of nicotinic acetylcholine receptors

Surface expression of the nicotinic acetylcholine receptor (AChR) requires the assembly of multiple subunits in the endoplasmic reticulum (ER). Little is known, however, about the mechanism by which assembled receptor pentamers are transported to the cell membrane while unassembled subunits are retained in the ER. Here we report that a motif conserved in the transmembrane domain of AChR subunits is critically involved in this process. In COS cells, mutation within this signal allowed surface expression of unassembled subunits. Conversely, insertion of the sequence to unrelated proteins that are normally transported to the surface resulted in ER retention. The signal is buried in AChR pentamers, but is exposed on unassembled subunits in the ER, where it promotes protein degradation. We therefore conclude that this signal ensures surface trafficking of only functional AChRs.

Role of the large cytoplasmic loop of the alpha 7 neuronal nicotinic acetylcholine receptor subunit in receptor expression and function

The role of the large intracellular loop of the nicotinic acetylcholine receptor (nAChR) alpha7 subunit in the expression of functional channels was studied. For this purpose, systematic deletions and substitutions were made throughout the loop and the ability of the mutated alpha7 subunits to support expression of functional nAChRs at the Xenopus oocyte membrane was tested. Surface nAChR expression was abolished upon removal of sequences at two regions, a 29-amino acid segment close to the N-terminus of the loop (amino acids 297-325) and adjacent to the third transmembrane region and an 11-amino acid segment near the fourth transmembrane region. Some residues (amino acids 317-322) within the 29 amino acids N-terminal segment could be substituted by others but not deleted without loss of expression, suggesting that a certain structure, determined by the number of amino acids rather than by their identity, has to be maintained in this region. The contiguous sequence M323 K324 R325 did not tolerate deletions and substitutions. Removal of the rest of the cytoplasmic loop was not deleterious; even higher expression levels (2-4-fold) were obtained upon large deletions of the loop (Delta399-432 and Delta339-370). High expression levels were observed provided that a minimal sequence of three amino acids (E371, G372, and M373) was present. In addition, some electrophysiological properties of mutant nAChRs were modified. Substitution of the EGM sequence by other protein segments produced a variety of effects, but, in general, insertions were not well tolerated, suggesting the existence of tight structural restrictions in the large cytoplasmic region of the rat alpha7 subunit.

The C. elegans ric-3 gene is required for maturation of nicotinic acetylcholine receptors

Mutations in ric-3 (resistant to inhibitors of cholinesterase) suppress the neuronal degenerations caused by a gain of function mutation in the Caenorhabditis elegans DEG-3 acetylcholine receptor. RIC-3 is a novel protein with two transmembrane domains and extensive coiled-coil domains. It is expressed in both muscles and neurons, and the protein is concentrated within the cell bodies. We demonstrate that RIC-3 is required for the function of at least four nicotinic acetylcholine receptors. However, GABA and glutamate receptors expressed in the same cells are unaffected. In ric-3 mutants, the DEG-3 receptor accumulates in the cell body instead of in the cell processes. Moreover, co-expression of ric-3 in Xenopus laevis oocytes enhances the activity of the C.elegans DEG-3/DES-2 and of the rat alpha-7 acetylcholine receptors. Together, these data suggest that RIC-3 is specifically required for the maturation of acetylcholine receptors.

Molecular cloning and mapping of the human nicotinic acetylcholine receptor alpha10 (CHRNA10)

We report the isolation and initial characterization of a new member of the human nicotinic acetylcholine receptor (nAChR) subunit family, alpha10 (CHRNA10), from both inner-ear neuroepithelium and lymphoid tissue. The cDNA is 1959 nucleotides in length, with a coding region predicting a protein of 451 amino acids that is 90% identical to rat alpha10. The alpha10 gene was localized to chromosome 11p15.5. Human alpha10 was detected in human inner-ear tissue, tonsil, immortalized B-cells, cultured T-cells and peripheral blood lymphocytes using reverse transcriptase-polymerase chain reaction, Northern blot hybridization, and immunohistochemistry. We also detected the expression of the human nAChR alpha9 (CHRNA9) mRNA in these same tissues using RT-PCR and Northern blot hybridization.

Specific activation of the alpha 7 nicotinic acetylcholine receptor by a quaternary analog of cocaine

Effects of cocaine and cocaine methiodide were evaluated on the homomeric alpha 7 neuronal nicotinic receptor (nAChR). Whereas cocaine itself is a general nAChR noncompetitive antagonist, we report here the characterization of cocaine methiodide, a novel selective agonist for the alpha 7 subtype of nAChR. Data from (125)I-alpha-bungarotoxin binding assays indicate that cocaine methiodide binds to alpha 7 nAChR with a K(i) value of approximately 200 nM while electrophysiology studies indicate that the addition of a methyl group at the amine moiety of cocaine changes the drug's activity profile from inhibitor to agonist. Cocaine methiodide activates alpha 7 nAChR with an EC(50) value of approximately 50 microM and shows comparable efficacy to ACh in oocyte experiments. While agonist effects are specific for the alpha 7 neuronal nAChR and are not observed with heteromeric neuronal or skeletal muscle nAChR, antagonist effects are present for heteromeric nAChR combinations. Studies of PC12 cells transiently transfected with human alpha 7 cDNA and expressing a variety of functional nicotinic receptor subtypes confirm the specificity of cocaine methiodide agonist effects. Our results indicate that a quaternary structural derivative of cocaine can be used as a specific agonist for the alpha 7 subtype of neuronal nicotinic receptor.

The A-type potassium channel Kv4.2 is a substrate for the mitogen-activated protein kinase ERK

The mitogen-activated protein kinase ERK has recently become a focus of studies of synaptic plasticity and learning and memory. Due to the prominent role of potassium channels in regulating the electrical properties of membranes, modulation of these channels by ERK could play an important role in mediating learning-related synaptic plasticity in the CNS. Kv4.2 is a Shal-type potassium channel that passes an A-type current and is localized to dendrites and cell bodies in the hippocampus. The sequence of Kv4.2 contains several consensus sites for ERK phosphorylation. In the present studies, we tested the hypothesis that Kv4.2 is an ERK substrate. We determined that the Kv4.2 C-terminal cytoplasmic domain is an effective ERK2 substrate, and that it is phosphorylated at three sites: Thr(602), Thr(607), and Ser(616). We used this information to develop antibodies that recognize Kv4.2 phosphorylated by ERK2. One of our phospho-site-selective antibodies was generated using a triply phosphorylated peptide as the antigen. We determined that this antibody recognizes ERK-phosphorylated Kv4.2 in COS-7 cells transfected with Kv4.2 and native ERK-phosphorylated Kv4.2 in the rat hippocampus. These observations indicate that Kv4.2 is a substrate for ERK in vitro and in vivo, and suggest that ERK may regulate potassium-channel function by direct phosphorylation of the pore-forming alpha subunit.