Stable expression in HEK-293 cells of the rat alpha3/beta4 subtype of neuronal nicotinic acetylcholine receptor

Cellular, Synaptic and Network Effects of Acetylcholine in the Neocortex

The neocortex is densely innervated by basal forebrain (BF) cholinergic neurons. Long-range axons of cholinergic neurons regulate higher-order cognitive function and dysfunction in the neocortex by releasing acetylcholine (ACh). ACh release dynamically reconfigures neocortical microcircuitry through differential spatiotemporal actions on cell-types and their synaptic connections. At the cellular level, ACh release controls neuronal excitability and firing rate, by hyperpolarizing or depolarizing target neurons. At the synaptic level, ACh impacts transmission dynamics not only by altering the presynaptic probability of release, but also the magnitude of the postsynaptic response. Despite the crucial role of ACh release in physiology and pathophysiology, a comprehensive understanding of the way it regulates the activity of diverse neocortical cell-types and synaptic connections has remained elusive. This review aims to summarize the state-of-the-art anatomical and physiological data to develop a functional map of the cellular, synaptic and microcircuit effects of ACh in the neocortex of rodents and non-human primates, and to serve as a quantitative reference for those intending to build data-driven computational models on the role of ACh in governing brain states.

Bimodal concentration-response of nicotine involves the nicotinic acetylcholine receptor, transient receptor potential vanilloid type 1, and transient receptor potential ankyrin 1 channels in mouse trachea and sensory neurons

High concentrations of nicotine, as in the saliva of oral tobacco consumers or in smoking cessation aids, have been shown to sensitize/activate recombinant transient receptor potential vanilloid type 1 (rTRPV1) and mouse TRPA1 (mTRPA1) channels. By measuring stimulated calcitonin gene-related peptide (CGRP) release from the isolated mouse trachea, we established a bimodal concentration-response relationship with a threshold below 10 µM (-)-nicotine, a maximum at 100 µM, an apparent nadir between 0.5 and 10 mM, and a renewed increase at 20 mM. The first peak was unchanged in TRPV1/A1 double-null mutants as compared with wild-types and was abolished by specific nicotinic acetylcholine receptor (nAChR) inhibitors and by camphor, discovered to act as nicotinic antagonist. The nicotine response at 20 mM was strongly pHe-dependent, - five times greater at pH 9.0 than 7.4, indicating that intracellular permeation of the (uncharged) alkaloid was required to reach the TRPV1/A1 binding sites. The response was strongly reduced in both null mutants, and more so in double-null mutants. Upon measuring calcium transients in nodose/jugular and dorsal root ganglion neurons in response to 100 µM nicotine, 48% of the vagal (but only 14% of the somatic) sensory neurons were activated, the latter very weakly. However, nicotine 20 mM at pH 9.0 repeatedly activated almost every single cultured neuron, partly by releasing intracellular calcium and independent of TRPV1/A1 and nAChRs. In conclusion, in mouse tracheal sensory nerves nAChRs are 200-fold more sensitive to nicotine than TRPV1/A1; they are widely coexpressed with the capsaicin receptor among vagal sensory neurons and twice as abundant as TRPA1. Nicotine is the major stimulant in tobacco, and its sensory impact through nAChRs should not be disregarded.

HEK293 cell line: A vehicle for the expression of recombinant proteins

The HEK cell line has been extensively used as an expression tool for recombinant proteins since it was generated over 25 years ago. Although of epithelial origin, its biochemical machinery is capable of carrying out most of the post-translational folding and processing required to generate functional, mature protein from a wide spectrum of both mammalian and non-mammalian nucleic acids. Though popular as a transient expression system, this cell type has also seen wide use in stably transfected forms (i.e. transformed cells) to study a variety of cell-biological questions in neurobiology. The principal attributes which have made the HEK cell a popular choice among electrophysiologists to study isolated receptor channels include; its quick and easy reproduction and maintenance; amenability to transfection using a wide variety of methods; high efficiency of transfection and protein production; faithful translation and processing of proteins; and small cell size with minimal processes appropriate for voltage-clamp experimentation. These, and other attributes, also mean that complementary biochemical/cell biological evaluations of expressed proteins can be performed in concert with functional analyses to establish detailed pharmacological and biophysical profiles for the action of new drugs and their targets. The increased amount of sequence information available from the human genome has placed greater emphasis upon heterologous cell expression systems as targets for high throughput structure-function evaluation of novel drug targets and disease markers. Here we have highlighted some of the innate characteristics of the HEK cell in order that its suitability as a vehicle for the expression of a gene product can be assessed for particular needs. We have also detailed some of the standard methods used for transfection and obtaining functional data from electrophysiological recording techniques.

Norchloro-fluoro-homoepibatidine: specificity to neuronal nicotinic acetylcholine receptor subtypes in vitro

The subtype-specificity of newly synthesised epibatidine-related compounds, norchloro-fluoro-homoepibatidine (NCFHEB) and derivatives, to neuronal nicotinic acetylcholine receptors (nAChR) has been investigated. NCFHEBs were assayed in competitive binding assays to (+/-)-[(3)H]epibatidine-labelled rat thalamic nAChRs and human alpha4beta2, alpha3beta4, and alpha7 nAChRs, expressed in stably transfected HEK-293 and SH-SY5Y cells. The binding affinity of (+)-NCFHEB (K(i): 0.064 nM) and (-)-NCFHEB (K(i): 0.112 nM) to human alpha4beta2 nAChR is in the same order of magnitude as that of epibatidine (K(i): 0.014 nM). However, because the affinity of both NCFHEB-enantiomers to human alpha3beta4 nAChR is up to 65 times lower than that of epibatidine, the alpha4beta2 subtype-specificity of NCFHEB is increased up to 1,400% compared to epibatidine.

Nicotinic AChR in subclassified capsaicin-sensitive and -insensitive nociceptors of the rat DRG

Nociceptive cells of the dorsal root ganglion (DRG) were subclassified, in vitro, according to patterns of voltage-activated currents. The distribution and form of nicotinic ACh receptors (nAChRs) were determined. nAChRs were present on both capsaicin-sensitive and -insensitive nociceptors but were not universally present in unmyelinated nociceptors. In contrast, all A delta nociceptors (types 4, 6, and 9) expressed slowly decaying nAChR. Three major forms of nicotinic currents were identified. Specific agonists and antagonists were used to demonstrate the presence of alpha7 in two classes of capsaicin-sensitive, unmyelinated nociceptors (types 2 and 8). In type 2 cells, alpha7-mediated currents were found in isolation. Whereas alpha7 was co-expressed with other nAChR in type 8 cells. These were the only classes in which alpha7 was identified. Other nociceptive classes expressed slowly decaying currents with beta4 pharmacology. Based on concentration response curves formed by nicotinic agonists [ACh, nicotine, dimethyl phenyl piperazinium (DMPP), cytisine] evidence emerged of two distinct nAChR differentially expressed in type 4 (alpha3beta4) and types 5 and 8 (alpha3beta4 alpha5). Although identification could not be made with absolute certainty, patterns of potency (type 4: DMPP > cytisine > nicotine = ACh; type 5 and type 8: DMPP = cytisine > nicotine = ACh) and efficacy provided strong support for the presence of two distinct channels based on an alpha3beta4 platform. Studies conducted on one nonnociceptive class (type 3) failed to reveal any nAChR. After multiple injections of Di-I (1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) into the hairy skin of the hindlimb, we identified cell types 2, 4, 6, 8, and 9 as skin nociceptors that expressed nicotinic receptors. We conclude that at least three nicotinic AChR are diversely distributed into discrete subclasses of nociceptors that innervate hairy skin.

Distinct subcellular targeting of fluorescent nicotinic alpha 3 beta 4 and serotoninergic 5-HT3A receptors in hippocampal neurons

The nicotinic acetylcholine receptors (nAChRs) and the 5-HT3 serotonin receptor subtype belong to a superfamily of neurotransmitter-gated ion channels involved in fast synaptic communication throughout the nervous system. Their trafficking to the neuron plasmalemma, as well as their targeting to specific subcellular compartments, is critical for understanding their physiological role. In order to investigate the cellular distribution of these receptors, we tagged the N-termini of alpha3beta4-nAChR subunits and the 5-HT3AR subunit with cyan and yellow fluorescent proteins (CFP, YFP). The fusion subunits were coexpressed in human embryonic kidney (HEK-293) cells, where they assemble into functional receptor channels, as well as in primary cultures of hippocampal neurons. Fluorescence microscopy of living cells revealed that the heteropentameric alpha3CFP-beta4 and YFP-alpha3beta4 receptors are mainly distributed in the endoplasmic reticulum, while the homopentameric YFP-5-HT3A receptor was localized both to the plasma membrane and within intracellular compartments. Moreover, the YFP-5-HT3A receptor was found to be targeted to the micropodia in HEK-293 cells and to the dendritic spines in hippocampal neurons, where it could be accessed by extracellularly applied specific fluorescent probes. The efficient targeting of the YFP-5-HT3A to the cytoplasmic membrane is in line with the large serotonin-elicited currents (nA range) measured by whole-cell voltage-clamp recordings in transfected HEK-293 cells. In contrast, alpha3beta4-nAChRs expressed in the same cells yielded weaker ACh-evoked responses. Taken together, the fluorescent and electrophysiological studies presented here demonstrate the predominant intracellular location of alpha3beta4-nACh receptors and the predominant expression of the 5-HT3AR in dendritic surface loci.

Galantamine is an allosterically potentiating ligand of neuronal nicotinic but not of muscarinic acetylcholine receptors

Galantamine (Reminyl), an approved treatment for Alzheimer's disease (AD), is a potent allosteric potentiating ligand (APL) of human alpha 3 beta 4, alpha 4 beta 2, and alpha 6 beta 4 nicotinic receptors (nAChRs), and of the chicken/mouse chimeric alpha 7/5-hydroxytryptamine3 receptor, as was shown by whole-cell patch-clamp studies of human embryonic kidney-293 cells stably expressing a single nAChR subtype. Galantamine potentiates agonist responses of the four nAChR subtypes studied in the same window of concentrations (i.e., 0.1-1 microM), which correlates with the cerebrospinal fluid concentration of the drug at the recommended daily dosage of 16 to 24 mg. At concentrations >10 microM, galantamine acts as an nAChR inhibitor. The other presently approved AD drugs, donepezil and rivastigmine, are devoid of the nicotinic APL action; at micromolar concentrations they also block nAChR activity. Using five CHO-SRE-Luci cell lines, each of them expressing a different human muscarinic receptor, and a reporter gene assay, we show that galantamine does not alter the activity of M1-M5 receptors, thereby confirming that galantamine modulates selectively the activity of nAChRs. These studies support our previous proposal that the therapeutic action of galantamine is mainly produced by its sensitizing action on nAChRs rather than by general cholinergic enhancement due to cholinesterase inhibition. Galantamine's APL action directly addresses the nicotinic deficit in AD.

Galantamine is an allosterically potentiating ligand of the human alpha4/beta2 nAChR

Galantamine (Reminyl) is a novel drug treatment for mild to moderate Alzheimer's disease (AD). Originally established as a reversible inhibitor of the acetylcholine-degrading enzyme acetylcholinesterase (AChE), galantamine also acts as an allosterically potentiating ligand (APL) on nicotinic acetylcholine receptors (nAChR). Having previously established this second mode of action on nAChRs from murine brain, we demonstrate here the same action of galantamine on the most abundant nAChR in the human brain, the alpha4/beta2 subtype. This nAChR-sensitizing action is not a common property of all, or most, AChE inhibitors, as is shown by the absence of this effect for other therapeutically applied AChE inhibitors including tacrine, metrifonate, rivastigmine and donepezil. The possible benefits for therapy of AD of an APL action on nicotinic receptors is discussed.

Allosteric sensitization of nicotinic receptors by galantamine, a new treatment strategy for Alzheimer's disease

Cholinesterase inhibitors are the only approved drug treatment for patients with mild to moderately severe Alzheimer's disease. Interestingly, the clinical potency of these drugs does not correlate well with their activity as cholinesterase inhibitors, nor is their action as short lived as would be expected from purely symptomatic treatment. A few cholinesterase inhibitors, including galantamine, produce beneficial effects even after drug treatment has been terminated. These effects assume modes of action other than mere esterase inhibition and are capable of inducing systemic changes. We have recently discovered a mechanism that could account, at least in part, for the above-mentioned unexpected properties of some cholinesterase inhibitors. We have found that a subgroup of cholinesterase inhibitors, including galantamine but excluding tacrine, directly interacts with nicotinic acetylcholine receptors. These compounds, named allosterically potentiating ligands, sensitize nicotinic receptors by increasing the probability of channel opening induced by acetylcholine and nicotinic agonists and by slowing down receptor desensitization. The allosterically potentiating ligand action, which is not necessarily associated with cholinesterase inhibition, has been demonstrated by whole-cell patch-clamp recordings to occur in natural murine and human neurons and in murine and human cell lines expressing various subtypes of neuronal nicotinic acetylcholine receptors.

Nicotinic agonists stimulate acetylcholine release from mouse interpeduncular nucleus: a function mediated by a different nAChR than dopamine release from striatum

Acetylcholine release stimulated by nicotinic agonists was measured as radioactivity released from perfused synaptosomes prepared from mouse interpeduncular nucleus (IPN) that had been loaded with [(3)H]choline. Agonist-stimulated release was dependent upon external calcium and over 90% of released radioactivity was acetylcholine. The release process was characterized by dose response curves for 13 agonists and inhibition curves for six antagonists. alpha-Conotoxin MII did not inhibit this release, while alpha-conotoxin AuIB inhibited 50% of agonist-stimulated release. Comparison of this process with [(3)H]dopamine release from mouse striatal synaptosomes indicated that different forms of nicotinic acetylcholine receptors (nAChRs) may mediate these processes. This was confirmed by assays using mice homozygous for the beta 2 subunit null mutation. The deletion of the beta 2 subunit had no effect on agonist-stimulated acetylcholine release, but abolished agonist-stimulated release of dopamine from striatal synaptosomes. Mice heterozygous for the beta 2 subunit null mutation showed decreased dopamine release evoked by L-nicotine with no apparent change in EC(50) value, as well as similar decreases in both transient and persistent phases of release with no changes in desensitization rates.

Chronic nicotine treatment up-regulates human alpha3 beta2 but not alpha3 beta4 acetylcholine receptors stably transfected in human embryonic kidney cells

Human nicotinic acetylcholine receptor (AChR) subtypes alpha3 beta2, alpha3 beta2 alpha5, alpha3 beta4, and alpha3 beta4 alpha5 were stably expressed in cells derived from the human embryonic kidney cell line 293. alpha3 beta4 AChRs were found in prominent 2-micrometer patches on the cell surface, whereas most alpha3 beta2 AChRs were more diffusely distributed. The functional properties of the alpha3 AChRs in tsA201 cells were characterized by whole cell patch clamp using both acetylcholine and nicotine as agonists. Nicotine was a partial agonist on alpha3 beta4 AChRs and nearly a full agonist on alpha3 beta2 alpha5 AChRs. Chronic exposure of cells expressing alpha3 beta2 AChRs or alpha3 beta2 alpha5 AChRs to nicotine or carbamylcholine increased their amount up to 24-fold but had no effect on the amount of alpha3beta4 or alpha3 beta4 alpha5 AChRs, i.e. the up-regulation of alpha3 AChRs depended on the presence of beta2 but not beta4 subunits in the AChRs. This was also found to be true of alpha3 AChRs in the human neuroblastoma SH-SY5Y. In the absence of nicotine, alpha3 beta2 AChRs were expressed at much lower levels than alpha3 beta4 AChRs, but in the presence of nicotine, the amount of alpha3 beta2 AChRs exceeded that of alpha3 beta4 AChRs. Up-regulation was seen for both total AChRs and surface AChRs. Up-regulated alpha3beta2 AChRs were functional. The nicotinic antagonists curare and dihydro-beta-erythroidine also up-regulated alpha3 beta2 AChRs, but only by 3-5-fold. The channel blocker mecamylamine did not cause up-regulation of alpha3 beta2 AChRs and inhibited up-regulation by nicotine. Our data suggest that up-regulation of alpha3 beta2 AChRs in these lines by nicotine results from both increased subunit assembly and decreased AChR turnover.

Rat alpha3/beta4 subtype of neuronal nicotinic acetylcholine receptor stably expressed in a transfected cell line: pharmacology of ligand binding and function

We stably transfected human kidney embryonic 293 cells with the rat neuronal nicotinic acetylcholine receptor (nAChR) alpha3 and beta4 subunit genes. This new cell line, KXalpha3 beta4R2, expresses a high level of the alpha3/beta4 receptor subtype, which binds (+/-)- [3H]epibatidine with a Kd value of 304+/-16 pM and a Bmax value of 8942 +/- 115 fmol/mg protein. Comparison of nicotinic drugs in competing for alpha3/beta4 receptor binding sites in this cell line and the binding sites in rat forebrain (predominantly alpha4/beta2 receptors) revealed marked differences in their Ki values, but similar rank orders of potency for agonists were observed, with the exception of anatoxin-A. The affinity of the competitive antagonist dihydro-beta-erythroidine is >7000 times higher at alpha4/beta2 receptors in rat forebrain than at the alpha3/beta4 receptors in these cells. The alpha3/beta4 nAChRs expressed in this cell line are functional, and in response to nicotinic agonists, 86Rb+ efflux was increased to levels 8-10 times the basal levels. Acetylcholine, (-)-nicotine, cytisine, carbachol, and (+/-)-epibatidine all stimulated 86Rb+ efflux, which was blocked by mecamylamine. The EC50 values for acetylcholine and (-)-nicotine to stimulate 86Rb+ effluxes were 114 +/- 24 and 28 +/- 4 microM, respectively. The rank order of potency of nicotinic antagonists in blocking the function of this alpha3/beta4 receptor was mecamylamine > d-tubocurarine > dihydro-beta-erythroidine > hexamethonium. Mecamylamine, d-tubocurarine, and hexamethonium blocked the function by a noncompetitive mechanism, whereas dihydro-beta-erythroidine blocked the function competitively. The KXalpha3 beta4R2 cell line should prove to be a very useful model for studying this subtype of nAChRs.

Neuronal nicotinic receptors in the locust Locusta migratoria. Cloning and expression

We have identified five cDNA clones that encode nicotinic acetylcholine receptor (nAChR) subunits expressed in the nervous system of the locust Locusta migratoria. Four of the subunits are ligand-binding alpha subunits, and the other is a structural beta subunit. The existence of at least one more nAChR gene, probably encoding a beta subunit, is indicated. Based on Northern analysis and in situ hybridization, the five subunit genes are expressed. localpha1, localpha3, and locbeta1 are the most abundant subunits and are expressed in similar areas of the head ganglia and retina of the adult locust. Because Loc<alpha3 binds alpha-bungarotoxin with high affinity, it may form a homomeric nAChR subtype such as the mammalian alpha7 nAChR. Localpha1 and Locbeta1 may then form the predominant heteromeric nAChR in the locust brain. localpha4 is mainly expressed in optic lobe ganglionic cells and localpha2 in peripherally located somata of mushroom body neurons. localpha3 mRNA was additionally detected in cells interspersed in the somatogastric epithelium of the locust embryo, suggesting that this isoform may also be involved in functions other than neuronal excitability. Transcription of all nAChR subunit genes begins approximately 3 days before hatching and continues throughout adult life. Electrophysiological recordings from head ganglionic neurons also indicate the existence of more than one functionally distinct nAChR subtype. Our results suggest the existence of several nAChR subtypes, at least some of them heteromeric, in this insect species.

The ion channel properties of a rat recombinant neuronal nicotinic receptor are dependent on the host cell type

1. A stable mammalian cell line (L-alpha 3 beta 4) has been established which expresses the cloned rat neuronal nicotinic acetylcholine receptor (nAChR) subunits alpha 3 and beta 4, which are the most abundant in autonomic ganglia. Ion channel properties of nAChRs expressed in L-alpha 3 beta 4 cells were investigated by single-channel and whole-cell recording techniques, and compared with both rat alpha 3 beta 4 nAChRs expressed in Xenopus oocytes, and endogenous nicotinic receptors in rat superior cervical ganglion (SCG) neurones, using identical solutions for all cell types. 2. Acetylcholine (ACh) caused activation of single ion channel currents with a range of amplitudes. Some channels had high conductances (30-40 pS), and relatively brief lifetimes; these resembled the predominant native channel from SCG. Other channels had low conductances (20-26 pS) and long bursts of openings which were quite unlike native channels, but which were similar to channels formed by alpha 3 beta 4 in oocytes. Both types often occurred in the same patch. 3. Cytisine was about 3 times more potent than ACh (low-concentration potency ratio) in L-alpha 3 beta 4 cells, which is not dissimilar to the 5-fold potency ratio found in both SCG and oocytes, whereas 1,1-dimethyl-4-phenylpiperazinium (DMPP) was less potent than ACh in some cells (as in the oocyte), but more potent in others (as in SCG). 4. While the channels expressed in L-alpha 3 beta 4 cells do not mimic exactly those expressed in rat SCG, they differ considerably from the same subunit combination expressed in oocytes. Larger conductance, SCG-like channels were detected frequently in L-alpha 3 beta 4, but were rarely, if ever, seen in oocytes injected with alpha 3 and beta 4 mRNA. Our results indicate that ion channel properties such as single-channel conductance can be influenced by the choice of heterologous expression system.

Human neuronal nicotinic receptors

Nicotine is a very widely used drug of abuse, which exerts a number of neurovegetative, behavioural and psychological effects by interacting with neuronal nicotinic acetylcholine receptors (NAChRs). These receptors are distributed widely in human brain and ganglia, and form a family of ACh-gated ion channels of different subtypes, each of which has a specific pharmacology and physiology. As human NAChRs have been implicated in a number of human central nervous system disorders (including the neurodegenerative Alzheimer's disease, schizophrenia and epilepsy), they are suitable potential targets for rational drug therapy. Much of our current knowledge about the structure and function of NAChRs comes from studies carried out in other species, such as rodents and chicks, and information concerning human nicotinic receptors is still incomplete and scattered in the literature. Nevertheless, it is already evident that there are a number of differences in the anatomical distribution, physiology, pharmacology, and expression regulation of certain subtypes between the nicotinic systems of humans and other species. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining the molecular basis of their functional diversity viewed mainly from pharmacological and biochemical perspectives. It will also summarize our current knowledge concerning the structure and function of the NAChRs expressed by other species, and the newly discovered drugs used to classify their numerous subtypes. Finally, the role of NAChRs in behaviour and pathology will be considered.

Recombinant nicotinic receptors, expressed in Xenopus oocytes, do not resemble native rat sympathetic ganglion receptors in single-channel behaviour

1. In order to establish the subunit composition of neuronal nicotinic receptors in rat superior cervical ganglia (SCG), their single-channel properties were compared with those of recombinant receptors expressed in Xenopus oocytes, using outside-out excised patch recording. 2. The mean main conductance of SCG channels from adult and 1-day-old rats was 34.8 and 36.6 pS, respectively. Less frequent openings to lower conductances occurred both as isolated bursts and as events connected to the main level by direct transitions. There was considerable interpatch variability in the values of the lower conductances. 3. Nicotinic receptors from oocytes expressing alpha3beta4 and alpha4beta4 subunits had chord conductances lower than that of SCG neurones (22 pS for alpha3beta4 and 29 pS for alpha4beta4). 4. Prolonged recording from both native and recombinant channels was precluded by 'run-down', i.e. channel activity could be elicited for only a few minutes after excision. Nevertheless, SCG channel openings were clearly seen to occur as short bursts (slowest component, 38 ms), whereas recombinant channels opened in very prolonged bursts of activity, the major component being the slowest (480 ms). 5. Addition of the alpha5 subunit to the alpha3beta4 pair produced channels with a higher conductance than those observed after injection of the pair alone (24.9 vs. 22 pS), suggesting incorporation of alpha5 into the channel. Addition of the beta2 subunit did not change alpha3beta4 single-channel properties. In one out of fourteen alpha3alpha5beta4 patches, both ganglion-like, high conductance, short burst openings and recombinant-type, low conductance, slow burst openings were observed. 6. Channels produced by expression in Xenopus oocytes of neuronal nicotinic subunits present in rat SCG as a rule differ from native ganglion receptors in single-channel conductance and gross kinetics. While it is possible that an essential nicotinic subunit remains to be cloned, it is perhaps more likely that oocytes either cannot assemble neuronal nicotinic subunits efficiently into channels with the correct composition and stoichiometry, or that they produce post-translational channel modifications which differ from those of mammalian neurones.