Stable expression and pharmacological properties of the human alpha 7 nicotinic acetylcholine receptor

The double-edged nature of nicotine: toxicities and therapeutic potentials

Nicotine is the primary addictive component of cigarette smoke and is associated with various smoking-related diseases. However, recent research has revealed its broader cognitive-enhancing and anti-inflammatory properties, suggesting its potential therapeutic applications in several conditions. This review aims to examine the double-edged nature of nicotine, encompassing its positive and negative effects. We provide a concise overview of the physiochemical properties and pharmacology of nicotine, including insights into nicotine receptors. Therefore, the article is divided into two main sections: toxicity and therapeutic potential. We comprehensively explored nicotine-related diseases, focusing on specific signaling pathways and the underlying mechanisms that contribute to its effects. Furthermore, we addressed the current research challenges and future development perspectives. This review aims to inspire future researchers to explore the full medical potential of nicotine, which holds significant promise for the clinical management of specific diseases.

Molecular docking and dynamic simulation studies of α4β2 and α7 nicotinic acetylcholine receptors with tobacco smoke constituents nicotine, NNK and NNN

Smoking constitutes a major global health problem. As it triggers various health hazards including cancers, cardiac and pulmonary illness, it is imperative to understand the mechanism of action of various smoke constituents on our cellular processes. Various in vitro studies have compiled the affinity of cigarette smoke constituents on various nicotinic acetylcholine receptors (nAChRs). But the nature of the intermolecular interactions contributing to this affinity and the key amino acids in the receptor active sites involved in this are not investigated so far. Here, we are examining the interaction of α7nAChR and α4β2nAChR on nicotine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosornicotine (NNN), the physiologically significant constituents in smoke, through molecular docking and dynamics simulations study. The docking of α4β2nAChR structure with the ligands nicotine, NNK and NNN yielded docking scores of -41.45 kcal/mol, -59.28 kcal/mol and -54.60 kcal/mol, respectively, and that of α7nAChR receptor molecule with the ligands yielded docking scores of -59.54 kcal/mol, -71.06 kcal/mol and -70.86 kcal/mol, respectively. The study showed that NNK exhibited the highest affinity with the ligands which was confirmed by dynamics simulation. But higher stability of interactions as surmised from Molecular dynamics simulations was found for nicotine with α4β2nAChR and NNN with α7nAChR. The findings validate the in vitro studies comparing the affinities of these compounds. The study will be useful in formulating effective nAChR agonists to treat neurological disorders and antagonists for smoke deaddiction and improve health standards.Communicated by Ramaswamy H. Sarma.

Probing function in ligand-gated ion channels without measuring ion transport

Although the functional properties of ion channels are most accurately assessed using electrophysiological approaches, a number of experimental situations call for alternative methods. Here, working on members of the pentameric ligand-gated ion channel (pLGIC) superfamily, we focused on the practical implementation of, and the interpretation of results from, equilibrium-type ligand-binding assays. Ligand-binding studies of pLGICs are by no means new, but the lack of uniformity in published protocols, large disparities between the results obtained for a given parameter by different groups, and a general disregard for constraints placed on the experimental observations by simple theoretical considerations suggested that a thorough analysis of this classic technique was in order. To this end, we present a detailed practical and theoretical study of this type of assay using radiolabeled α-bungarotoxin, unlabeled small-molecule cholinergic ligands, the human homomeric α7-AChR, and extensive calculations in the framework of a realistic five-binding-site reaction scheme. Furthermore, we show examples of the practical application of this method to tackle two longstanding questions in the field: our results suggest that ligand-binding affinities are insensitive to binding-site occupancy and that mutations to amino-acid residues in the transmembrane domain are unlikely to affect the channel's affinities for ligands that bind to the extracellular domain.

A novel effect of PDLIM5 in α7 nicotinic acetylcholine receptor upregulation and surface expression

Nicotinic acetylcholine receptors (nAChRs) are widespread throughout the central nervous system. Signaling through nAChRs contributes to numerous higher-order functions, including memory and cognition, as well as abnormalities such as nicotine addiction and neurodegenerative disorders. Although recent studies indicate that the PDZ-containing proteins comprising PSD-95 family co-localize with nicotinic acetylcholine receptors and mediate downstream signaling in the neurons, the mechanisms by which α7nAChRs are regulated remain unclear. Here, we show that the PDZ-LIM domain family protein PDLIM5 binds to α7nAChRs and plays a role in nicotine-induced α7nAChRs upregulation and surface expression. We find that chronic exposure to 1 μM nicotine upregulated α7, β2-contained nAChRs and PDLIM5 in cultured hippocampal neurons, and the upregulation of α7nAChRs and PDLIM5 is increased more on the cell membrane than the cytoplasm. Interestingly, in primary hippocampal neurons, α7nAChRs and β2nAChRs display distinct patterns of expression, with α7nAChRs colocalized more with PDLIM5. Furthermore, PDLIM5 interacts with α7nAChRs, but not β2nAChRs in native brain neurons. Knocking down of PDLIM5 in SH-SY5Y abolishes nicotine-induced upregulation of α7nAChRs. In primary hippocampal neurons, using shRNA against PDLIM5 decreased both surface clustering of α7nAChRs and α7nAChRs-mediated currents. Proteomics analysis and isothermal titration calorimetry (ITC) results show that PDLIM5 interacts with α7nAChRs through the PDZ domain, and the interaction between PDLIM5 and α7nAChRs can be promoted by nicotine. Collectively, our data suggest a novel cellular role of PDLIM5 in the regulation of α7nAChRs, which may be relevant to plastic changes in the nervous system.

Acute effects of the imidacloprid metabolite desnitro-imidacloprid on human nACh receptors relevant for neuronal signaling

Several neonicotinoids have recently been shown to activate the nicotinic acetylcholine receptor (nAChR) on human neurons. Moreover, imidacloprid (IMI) and other members of this pesticide family form a set of diverse metabolites within crops. Among these, desnitro-imidacloprid (DN-IMI) is of special toxicological interest, as there is evidence (i) for human dietary exposure to this metabolite, (ii) and that DN-IMI is a strong trigger of mammalian nicotinic responses. We set out here to quantify responses of human nAChRs to DN-IMI and an alternative metabolite, IMI-olefin. To evaluate toxicological hazards, these data were then compared to those of IMI and nicotine. Ca2+-imaging experiments on human neurons showed that DN-IMI exhibits an agonistic effect on nAChRs at sub-micromolar concentrations (equipotent with nicotine) while IMI-olefin activated the receptors less potently (in a similar range as IMI). Direct experimental data on the interaction with defined receptor subtypes were obtained by heterologous expression of various human nAChR subtypes in Xenopus laevis oocytes and measurement of the transmembrane currents evoked by exposure to putative ligands. DN-IMI acted on the physiologically important human nAChR subtypes α7, α3β4, and α4β2 (high-sensitivity variant) with similar potency as nicotine. IMI and IMI-olefin were confirmed as nAChR agonists, although with 2-3 orders of magnitude lower potency. Molecular docking studies, using receptor models for the α7 and α4β2 nAChR subtypes supported an activity of DN-IMI similar to that of nicotine. In summary, these data suggest that DN-IMI functionally affects human neurons similar to the well-established neurotoxicant nicotine by triggering α7 and several non-α7 nAChRs.

Functional alterations by a subgroup of neonicotinoid pesticides in human dopaminergic neurons

Neonicotinoid pesticides, originally developed to target the insect nervous system, have been reported to interact with human receptors and to activate rodent neurons. Therefore, we evaluated in how far these compounds may trigger signaling in human neurons, and thus, affect the human adult or developing nervous system. We used SH-SY5Y neuroblastoma cells as established model of nicotinic acetylcholine receptor (nAChR) signaling. In parallel, we profiled dopaminergic neurons, generated from LUHMES neuronal precursor cells, as novel system to study nAChR activation in human post-mitotic neurons. Changes of the free intracellular Ca²⁺ concentration ([Ca²⁺]_i) were used as readout, and key findings were confirmed by patch clamp recordings. Nicotine triggered typical neuronal signaling responses that were blocked by antagonists, such as tubocurarine and mecamylamine. Pharmacological approaches suggested a functional expression of α7 and non-α7 nAChRs on LUHMES cells. In this novel test system, the neonicotinoids acetamiprid, imidacloprid, clothianidin and thiacloprid, but not thiamethoxam and dinotefuran, triggered [Ca²⁺]_i signaling at 10-100 µM. Strong synergy of the active neonicotinoids (at low micromolar concentrations) with the α7 nAChR-positive allosteric modulator PNU-120596 was observed in LUHMES and SH-SY5Y cells, and specific antagonists fully inhibited such signaling. To provide a third line of evidence for neonicotinoid signaling via nAChR, we studied cross-desensitization: pretreatment of LUHMES and SH-SY5Y cells with active neonicotinoids (at 1-10 µM) blunted the signaling response of nicotine. The pesticides (at 3-30 µM) also blunted the response to the non-α7 agonist ABT 594 in LUHMES cells. These data show that human neuronal cells are functionally affected by low micromolar concentrations of several neonicotinoids. An effect of such signals on nervous system development is a toxicological concern.

Nicotine Evoked Currents in Human Primary Sensory Neurons

Sensory neuron nicotinic acetylcholine receptors (nAChRs) contribute to pain associated with tissue injury. However, there are marked differences between rats and mice with respect to both the properties and distribution of nAChR currents in sensory neurons. Because both species are used to understand pain signaling in humans, we sought to determine whether the currents present in either species was reflective of those present in human sensory neurons. Neurons from the L4/L5 dorsal root ganglia were obtained from adult male and female organ donors. Nicotine evoked currents were detected in 40 of 47 neurons (85%). In contrast with the naïve mouse, in which almost all nAChR currents are transient, or the rat, in which both mouse-like transient and more slowly activating and inactivating currents are detected, all the currents in human DRG neurons were slow, but slower than those in the rat. Currents were blocked by the nAChR antagonists mecamylamine (30 µmol/L), but not by the TRPA1 selective antagonist HC-030031 (10 µmol/L). Single cell polymerase chain reaction analysis of nicotinic receptor subunit expression in human DRG neurons are consistent with functional data indicating that receptor expression is detected 85 ± 2.1% of neurons assessed (n = 48, from 4 donors). The most prevalent coexpression pattern was α3/β2 (95 ± 4% of neurons with subunits), but α7 subunits were detected in 70 ± 3.4% of neurons. These results suggest that there are not only species differences in the sensory neuron distribution of nAChR currents between rodent and human, but that the subunit composition of the channel underlying human nAChR currents may be different from those in the mouse or rat. PERSPECTIVE: The properties and distribution of nicotine evoked currents in human sensory neurons were markedly different from those previously observed in mice and rats. These observations add additional support to the suggestion that human sensory neurons may be an essential screening tool for those considering moving novel therapeutics targeting primary afferents into clinical trials.

Endogenous CHRNA7-ligand SLURP1 as a potential tumor suppressor and anti-nicotinic factor in pancreatic cancer

Smoking is associated with increased risk and poorer prognosis of pancreatic ductal adenocarcinoma (PDAC). Nicotine acts through cholinergic nicotinic receptors, preferentially α7 (CHRNA7) that also binds the endogenous ligand SLURP1 (Secreted Ly-6/uPAR-Related Protein 1). The clinical significance of SLURP1 and its interaction with nicotine in PDAC are unclear. We detected similar levels of SLURP1 in sera from healthy donors and patients with chronic pancreatitis or PDAC; higher preoperative values were associated with significantly better survival in patients with resected tumors. Pancreatic tissue was not a source of circulating SLURP1 but contained diverse CHRNA7-expressing cells, preferentially epithelial and immune, whereas stromal stellate cells and a quarter of the tumor cells lacked CHRNA7. The CHRNA7 mRNA levels were decreased in PDAC, and CHRNA7^high-PDAC patients lived longer. In CHRNA7^high COLO357 and PANC-1 cultures, opposing activities of SLURP1 (anti-malignant/CHRNA7-dependent) and nicotine (pro-malignant/CHRNA7-infidel) were exerted without reciprocally interfering with receptor binding or downstream signaling. These data suggested that the ligands act independently and abolish each other’s effects through a mechanism resembling functional antagonism. Thus, SLURP1 might represent an inborn anti-PDAC defense being sensitive to and counteracting nicotine. Boosting SLURP1-CHRNA7 interaction might represent a novel strategy for treatment in high-risk individuals, i.e., smokers with pancreatic cancer.

Probing the Allosteric Role of the α5 Subunit of α3β4α5 Nicotinic Acetylcholine Receptors by Functionally Selective Modulators and Ligands

Nicotinic acetylcholine receptors regulate the nicotine dependence encountered with cigarette smoking, and this has stimulated a search for drugs binding the responsible receptor subtypes. Studies link a gene cluster encoding for α3β4α5-D398N nicotinic acetylcholine receptors to lung cancer risk as well as link a second mutation in this cluster to an increased risk for nicotine dependence. However, there are currently no recognized drugs for discriminating α3β4α5 signaling. In this study, we describe the development of homogeneous HEK-293 cell clones of α3β4 and α3β4α5 receptors appropriate for drug screening and characterizing biochemical and pharmacological properties of incorporated α5 subunits. Clones were assessed for plasma membrane expression of the individual receptor subunits by mass spectrometry and immunochemistry, and their calcium flux was measured in the presence of a library of kinase inhibitors and a focused library of acetylcholine receptor ligands. We demonstrated an incorporation of two α3 subunits in approximately 98% of plasma membrane receptor pentamers, indicating a 2/3 subunit expression ratio of α3 to β4 alone or to coexpressed β4 and α5. With prolonged nicotine exposure, the plasma membrane expression of receptors with and without incorporated α5 increased. Whereas α5 subunit expression decreased the cell calcium response to nicotine and reduced plasma membrane receptor number, it partially protected receptors from nicotine mediated desensitization. Hit compounds from both libraries suggest the α5 and α5-D398N subunits allosterically modify the behavior of nicotine at the parent α3β4 nicotinic acetylcholine receptor. These studies identify pharmacological tools from two distinct classes of drugs, antagonists and modifiers that are α5 and α5-D398N subtype selective that provide a means to characterize the role of the CHRNA5/A3/B4 gene cluster in smoking and cancer.

Inhibition of pancreatic acinar mitochondrial thiamin pyrophosphate uptake by the cigarette smoke component 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

Thiamin is essential for normal metabolism in pancreatic acinar cells (PAC) and is obtained from their microenvironment through specific plasma-membrane transporters, converted to thiamin pyrophosphate (TPP) in the cytoplasm, followed by uptake of TPP by mitochondria through the mitochondrial TPP (MTPP) transporter (MTPPT; product of SLC25A19 gene). TPP is essential for normal mitochondrial function. We examined the effect of long-term/chronic exposure of PAC in vitro (pancreatic acinar 266-6 cells) and in vivo (wild-type or transgenic mice carrying the SLC25A19 promoter) of the cigarette smoke toxin, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), on the MTPP uptake process. Our in vitro and in vivo findings demonstrate that NNK negatively affects MTPP uptake and reduced expression of MTPPT protein, MTPPT mRNA, and heterogenous nuclear RNA, as well as SLC25A19 promoter activity. The effect of NNK on Slc25a19 transcription was neither mediated by changes in expression of transcriptional factor NFY-1 (known to drive SLC25A19 transcription), nor due to changes in methylation profile of the Slc25a19 promoter. Rather, it appears to be due to changes in histone modifications that involve significant decreases in histone H3K4-trimethylation and H3K9-acetylation (activation markers). The effect of NNK on MTPPT function is mediated through the nonneuronal α7-nicotinic acetylcholine receptor (α7-nAChR), as indicated by both in vitro (using the nAChR antagonist mecamylamine) and in vivo (using an α7-nAchR(-/-) mouse model) studies. These findings demonstrate that chronic exposure of PAC to NNK negatively impacts PAC MTPP uptake. This effect appears to be exerted at the level of Slc25a19 transcription, involve epigenetic mechanism(s), and is mediated through the α7-nAchR.

The Oncogenic Functions of Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs) are ion channels that are expressed in the cell membrane of all mammalian cells, including cancer cells. Recent findings suggest that nAChRs not only mediate nicotine addiction in the brain but also contribute to the development and progression of cancers directly induced by nicotine and its derived carcinogenic nitrosamines whereas deregulation of the nAChRs is observed in many cancers, and genome-wide association studies (GWAS) indicate that SNPs nAChRs associate with risks of lung cancers and nicotine addiction. Emerging evidences suggest nAChRs are posited at the central regulatory loops of numerous cell growth and prosurvival signal pathways and also mediate the synthesis and release of stimulatory and inhibitory neurotransmitters induced by their agonists. Thus nAChRs mediated cell signaling plays an important role in stimulating the growth and angiogenic and neurogenic factors and mediating oncogenic signal transduction during cancer development in a cell type specific manner. In this review, we provide an integrated view of nAChRs signaling in cancer, heightening on the oncogenic properties of nAChRs that may be targeted for cancer treatment.

Neuronal α7 Nicotinic Receptors as a Target for the Treatment of Schizophrenia

Schizophrenia is a lifelong disease, the burden of which is often underestimated. Characterized by positive (e.g., hallucinations) and negative (e.g., avolition, amotivation) symptoms, schizophrenia is also accompanied with profound impairments in cognitive function that progress throughout the development of the disease. Although treatment with antipsychotic medications can effectively dampen some of the positive symptoms, these medications largely fail to reverse cognitive deficits or to mitigate negative symptoms. With a worldwide prevalence of approximately 1%, schizophrenia remains a large unmet medical need that stands to benefit greatly from (1) continued research to better understand the biological underpinnings of the disease and (2) the targeted development of novel therapeutics to improve the lives of those affected individuals. Improvements in our understanding of the neuronal networks associated with schizophrenia as well as progress in identifying genetic risk factors and environmental conditions that may predispose individuals to developing the disease are advancing new strategies to study and treat it. Herein, we review the evidence that supports the role of α7 nicotinic acetylcholine receptors in the central nervous system and why these receptors constitute a promising target to treat some of the prominent symptoms of schizophrenia.

Development of Automated Patch Clamp Assay for Evaluation of α7 Nicotinic Acetylcholine Receptor Agonists in Automated QPatch-16

The α7 nicotinic acetylcholine receptor (α7 nAChR) is an important and challenging target for drug discovery in the area of neuropsychiatric disorders. The current screening for chemicals targeting α7 nAChRs is primarily achieved by the use of low-throughput assay two-electrode voltage clamp (TEVC) in nonmammalian Xenopus oocytes. Automated patch clamp system has emerged as an attractive approach compared to conventional electrophysiology. To develop a mammalian cell-based functional assay in an automated electrophysiology system, we in this study generated a stable expression of α7 nAChRs in GH3 cells that originated from a rat pituitary tumor cell line and utilized automated QPatch-16 to test a set of tool compounds and chemicals identified as α7 agonists by TEVC. For the improvement of evaluating weak or partial α7 nAChRs agonists, we achieved enhancement of the signal-to-noise ratio by the addition of a positive allosteric modulator PNU-120596, which only activates α7 current in the presence of agonist. This improved assay was further validated by using known α7 partial agonists, such as RG3487, EVP-6124, and A-P90. Using this validated assay, we were able to identify a novel agonist 140507C that partially activates α7 nAChRs. Taken together, our results validate the use of QPatch-16 for evaluation α7 partial agonists, demonstrating its utility as an effective tool for α7 ion channel drug discovery.

Inflammation-induced increase in nicotinic acetylcholine receptor current in cutaneous nociceptive DRG neurons from the adult rat

The goals of the present study were to determine (1) the properties of the nicotinic acetylcholine receptor (nAChR) currents in rat cutaneous dorsal root ganglion (DRG) neurons; (2) the impact of nAChR activation on the excitability of cutaneous DRG neurons; and (3) the impact of inflammation on the density and distribution of nAChR currents among cutaneous DRG neurons. Whole-cell patch-clamp techniques were used to study retrogradely labeled DRG neurons from naïve and complete Freund's adjuvant inflamed rats. Nicotine-evoked currents were detectable in ∼70% of the cutaneous DRG neurons, where only one of two current types, fast or slow currents based on rates of activation and inactivation, was present in each neuron. The biophysical and pharmacological properties of the fast current were consistent with nAChRs containing an α7 subunit while those of the slow current were consistent with nAChRs containing α3/β4 subunits. The majority of small diameter neurons with fast current were IB4- while the majority of small diameter neurons with slow current were IB4+. Preincubation with nicotine (1 μM) produced a transient (1 min) depolarization and increase in the excitability of neurons with fast current and a decrease in the amplitude of capsaicin-evoked current in neurons with slow current. Inflammation increased the current density of both slow and fast currents in small diameter neurons and increased the percentage of neurons with the fast current. With the relatively selective distribution of nAChR currents in putative nociceptive cutaneous DRG neurons, our results suggest that the role of these receptors in inflammatory hyperalgesia is likely to be complex and dependent on the concentration and timing of acetylcholine release in the periphery.

Methyllycaconitine- and scopolamine-induced cognitive dysfunction: differential reversal effect by cognition-enhancing drugs

There is a growing body of evidence pointing to the pivotal role of alpha-7 nicotinic acetylcholine receptor (α7 nAchR) dysfunction in cognitive disorders such as Alzheimer’s disease or schizophrenia. This study was undertaken to establish and characterize an in vivo model for cognitive disorder secondary to the blockade of α7 nAChR by its specific antagonist, methyllycaconitine (MLA). The results show that MLA elicited cognitive dysfunction as assessed by reduced spontaneous alternation of mice in the T-maze. The maximal effect of MLA produced 25–30% reduction in the spontaneous alternation of mice, a level comparable with that induced by the muscarinic antagonism of scopolamine. Donepezil and galantamine fully reversed both MLA and scopolamine-induced cognitive dysfunction. However, the ED₅₀ of donepezil and galantamine was significantly shifted to the left in the MLA- compared to scopolamine-treated mice (0.0005 and 0.002 mg/kg for donepezil; 0.0003 and 0.7 mg/kg for galantamine). Moreover, memantine elicited marked reversion of cognitive dysfunction (up to 70%) in MLA-treated mice while only a weak reversal effect at high dose of memantine (less than 20%) was observed in scopolamine-treated mice. The above findings indicate that MLA-induced cognitive dysfunction in the mouse is highly sensitive and more responsive to the current procognitive drugs than the traditional scopolamine-based assay. Thus, it can be of value for the preclinical screening and profiling of cognition-enhancing drugs.

Kinetic properties and open probability of α7 nicotinic acetylcholine receptors

The alpha7 nicotinic acetylcholine receptor (nAChR) has some peculiar kinetic properties. From the literature of α7 nAChR-mediated currents we concluded that experimentally measured kinetic properties reflected properties of the solution exchange system, rather than genuine kinetic properties of the receptors. We also concluded that all experimentally measured EC50 values for agonists must inherently be inaccurate. The aim of this study was to assess the undistorted kinetic properties of α7 nAChRs, and to construct an improved kinetic model, which can also serve as a basis of modeling the effect of the positive allosteric modulator PNU-120596, as it is described in the accompanying paper. Agonist-evoked currents were recorded from GH4C1 cells stably transfected with pCEP4/rat α7 nAChR using patch-clamp and fast solution exchange. We used two approaches to circumvent the problem of insufficient solution exchange rate: extrapolation and kinetic modeling. First, using different solution exchange rates we recorded evoked currents, and extrapolated their amplitude and kinetics to instantaneous solution exchange. Second, we constructed a kinetic model that reproduced concentration-dependence and solution exchange rate-dependence of receptors, and then we simulated receptor behavior at experimentally unattainably fast solution exchange. We also determined open probabilities during choline-evoked unmodulated and modulated currents using nonstationary fluctuation analysis. The peak open probability of 10 mM choline-evoked currents was 0.033 ± 0.006, while in the presence of choline (10 mM) and PNU-120596 (10 μM), it was increased to 0.599 ± 0.058. Our kinetic model could adequately reproduce low open probability, fast kinetics, fast recovery and solution exchange rate-dependent kinetics.

Nicotine signaling and progression of chronic kidney disease in smokers

The deleterious health effects of cigarette smoking are far reaching, and it remains the most important modifiable risk factor for improving overall morbidity and mortality. In addition to being a risk factor for cancer, cardiovascular disease and lung disease, there is strong evidence, both from human and animal studies, demonstrating a role for cigarette smoking in the progression of chronic kidney disease (CKD). Clinical studies have shown a strong correlation between cigarette smoking and worsening CKD in patients with diabetes, hypertension, polycystic kidney disease, and post kidney transplant. Nicotine, in addition to its role in the addictive properties of cigarette smoking, has other biological effects via activation of non-neuronal nicotinic acetylcholine receptors (nAChRs). Several nAChR subunits are expressed in the normal kidney and blockade of the α7-nAChR subunit ameliorates the effects of nicotine in animal models of CKD. Nicotine increases the severity of renal injury in animal models including acute kidney injury, diabetes, acute nephritis and subtotal nephrectomy. The renal effects of nicotine are also linked to increased generation of reactive oxygen species and activation of pro-fibrotic pathways. In humans, nicotine induces transitory increases in blood pressure accompanied by reductions in glomerular filtration rate and effective renal plasma flow. In summary, clinical and experimental evidence indicate that nicotine is at least in part responsible for the deleterious effects of cigarette smoking in the progression of CKD. The mechanisms involved are the subject of active investigation and may result in novel strategies to ameliorate the effects of cigarette smoking in CKD.

Chemical chaperones exceed the chaperone effects of RIC-3 in promoting assembly of functional α7 AChRs

Functional α7 nicotinic acetylcholine receptors (AChRs) do not assemble efficiently in cells transfected with α7 subunits unless the cells are also transfected with the chaperone protein RIC-3. Despite the presence of RIC-3, large amounts of these subunits remain improperly assembled. Thus, additional chaperone proteins are probably required for efficient assembly of α7 AChRs. Cholinergic ligands can act as pharmacological chaperones to promote assembly of mature AChRs and upregulate the amount of functional AChRs. In addition, we have found that the chemical chaperones 4-phenylbutyric acid (PBA) and valproic acid (VPA) greatly increase the amount of functional α7 AChRs produced in a cell line expressing both α7 and RIC-3. Increased α7 AChR expression allows assay of drug action using a membrane potential-sensitive fluorescent indicator. Both PBA and VPA also increase α7 expression in the SH-SY5Y neuroblastoma cell line that endogenously expresses α7 AChRs. VPA increases expression of endogenous α7 AChRs in hippocampal neurons but PBA does not. RIC-3 is insufficient for optimal assembly of α7 AChRs, but provides assay conditions for detecting additional chaperones. Chemical chaperones are a useful pragmatic approach to express high levels of human α7 AChRs for drug selection and characterization and possibly to increase α7 expression in vivo.

Cell-specific effects on surface α7 nicotinic receptor expression revealed by over-expression and knockdown of rat RIC3 protein

We tested whether surface α7 nicotinic acetylcholine receptor expression is dependent on an endogenous chaperone named Resistance to Inhibitors of Cholinesterase 3 (RIC3) by comparing RIC3 protein in rat GH4C1 and human SH-EP1 cells, which express strikingly different surface receptor levels following α7 transfection. Cloned rat RIC3 exists in at least two isoforms because of an ambiguous splice site between exons 4 and 5. Both rat isoforms permit surface α7 expression in SH-EP1 and human embryonic kidney (HEK) cells measured by α-bungarotoxin binding. Contrary to expectations, endogenous RIC3 protein expression determined by immunoblots did not differ between untransfected GH4C1 or SH-EP1 cells. siRNA against rat RIC3 exon 4 and shRNA against exons 2, 5 and 6 knocked down transfected rat RIC3 expression in SH-EP1 cells and simultaneously blocked toxin binding. However, no RNAi construct blocked binding when co-transfected with α7 into GH4C1 cells. shRNA against rat exons 2 and 5 knocked down rat RIC3 protein transfected into GH4C1 cells with a time course suggesting a protein half-life of a few days. These results suggest GH4C1 cells may possess unknown chaperone(s) allowing high surface α7 expression in the absence of known RIC3 splice variants.

The nicotinic α7 receptor agonist GTS-21 improves cognitive performance in ketamine impaired rhesus monkeys

The cognitive deficits associated with schizophrenia are recognized as a core component of the disorder, yet there remain no available therapeutics to treat these symptoms of the disease. As a result, there is a need for establishing predictive preclinical models to identify the therapeutic potential of novel compounds. In the present study, rhesus monkeys were trained in the object retrieval-detour task, which is dependent on the prefrontal cortex, a brain region implicated in the cognitive deficits associated with schizophrenia. The NMDA receptor antagonist ketamine significantly impaired performance without affecting measures of motor or visuospatial abilities. Pre-treatment with the nicotinic α7 agonist GTS-21 (0.03 mg/kg) significantly attenuated the ketamine-induced impairment, consistent with reports from clinical trials suggesting that nicotinic α7 receptor agonism has pro-cognitive potential in clinical populations. In contrast, pretreatment with the acetylcholinesterase inhibitor donepezil failed to reverse the ketamine-induced impairment, consistent with studies showing a lack of pro-cognitive effects in patients with schizophrenia. These data suggest that the ketamine-impaired object retrieval-detour task could provide a model with improved predictive validity for drug development, and confirm the need for additional efforts in back-translation. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Combined α7 nicotinic acetylcholine receptor agonism and partial serotonin transporter inhibition produce antidepressant-like effects in the mouse forced swim and tail suspension tests: a comparison of SSR180711 and PNU-282987

Emerging evidence points to an involvement of nicotinic acetylcholine receptors (nAChRs) in major depression. Nicotine improves symptoms of depression in humans and shows antidepressant-like effects in rodents. Monoamine release is facilitated by nAChR stimulation, and nicotine-evoked serotonin (5-HT) release has been shown to depend on α7 nAChR activation. The α7 nAChR agonist PNU-282987 shows no antidepressant-like activity when tested alone in the mouse forced swim (mFST) or tail suspension tests (mTST). However, in combination with a sub-active dose of the selective 5-HT reuptake inhibitor citalopram, inducing ~50% 5-HT reuptake inhibition, PNU-282987 has shown marked antidepressant-like effects in the mFST. SSR180711 is a recently described α7 nAChR agonist that has shown antidepressant-like activity in the rat forced swim test. To address the possibility that 5-HT reuptake inhibition contributes to the antidepressant-like profile of SSR180711, we compared the behavioural and biochemical profiles of PNU-282987 and SSR180711. In the mFST and mTST, SSR180711 (3-30 mg/kg, s.c.) showed dose-dependent antidepressant-like activity, while PNU-282987 (3-30 mg/kg, s.c.) showed no significant effect. The ED(50) to displace [³H]α-bungarotoxin binding was 1.7 and 5.5 mg/kg for SSR180711 and PNU-282987, respectively, suggesting that both compounds produce near-maximal α7 nAChR occupancy at the highest dose. While PNU-282987 did not affect ex vivo [³H]5-HT uptake, SSR180711 inhibited [³H]5-HT uptake with an ED₅₀ of 30 mg/kg. This degree of inhibition is similar to that observed with a citalopram dose of ~2.4 mg/kg, a dose that is normally not active in the mFST or mTST. This suggests that the antidepressant-like activity of SSR180711 may involve partial 5-HT reuptake inhibition. SSR180711 therefore represents a compound displaying the synergistic effect of α7 nAChR agonism combined with partial 5-HT reuptake inhibition previously described. The addition of α7 nAChR agonism to classical monoamine-based mechanisms may represent a novel option for the improved treatment of major depression.

Creating an α7 nicotinic acetylcholine recognition domain from the acetylcholine-binding protein: crystallographic and ligand selectivity analyses

Determining the structure of the ligand-binding domain of the nicotinic acetylcholine receptor (nAChR) has been a long standing goal in the design of selective drugs useful in implicated diseases for this prevalent receptor family. Acetylcholine-binding proteins have proven to be valuable surrogates with structural similarity and sequence identity to the extracellular domain of the nicotinic receptor, yet these soluble proteins have their unique features and do not serve as exact replicates of the nAChRs of interest. Here we systematically modify the sequence of these proteins toward the homomeric human α7 nAChR. These chimeric proteins exhibit a shift in affinities to reflect α7 binding characteristics yet maintain expression levels and stability conducive for crystallization. We also present a pentameric humanoid nAChR extracellular domain with the structural determination of the α7 nAChR glycosylation site.

Naturally-expressed nicotinic acetylcholine receptor subtypes

Nicotinic acetylcholine receptors (nAChRs) warrant attention, as they play many critical roles in brain and body function and have been implicated in a number of neurological and psychiatric disorders, including nicotine dependence. nAChRs are composed as diverse subtypes containing specific combinations of genetically-distinct subunits and that have different functional properties, distributions, and pharmacological profiles. There had been confidence that the rules that define ranges of assembly partners for specific subunits were well-established, especially for the more prominent nAChR subtypes. However, we review here some newer findings indicating that nAChRs having largely the same, major subunits exist as isoforms with unexpectedly different properties. Moreover, we also summarize our own studies indicating that novel nAChR subtypes exist and/or have distributions not heretofore described. Importantly, the nAChRs that exist as new isoforms or subtypes or have interesting distributions require alteration in thinking about their roles in health and disease.

Functional characterization and high-throughput screening of positive allosteric modulators of α7 nicotinic acetylcholine receptors in IMR-32 neuroblastoma cells

α7 nicotinic acetylcholine receptors (nAChRs) are characterized by relatively low ACh sensitivity, rapid activation, and fast desensitization kinetics. ACh/agonist evoked currents at the α7 nAChR are transient, and, typically, calcium flux responses are difficult to detect using conventional fluorometric assay techniques. One approach to study interactions of agonists with the α7 nAChR is by utilizing positive allosteric modulators (PAMs). In this study, we demonstrate that inclusion of type II PAMs such as PNU-120596, but not type I, can enable detection of endogenous α7 nAChR-mediated calcium responses in human neuroblastoma (IMR-32) cells. Using this approach, we characterized the pharmacological profile of nicotine, epibatidine, choline, and other nAChR agonists such as PNU-282987, SSR-180711, GTS-21, OH-GTS21, tropisetron, NS6784, and A-582941. The rank order potency of agonists well correlated with α7 nAChR binding affinities measured in brain membranes. Inhibition of calcium response by methyllycaconitine in the presence of increasing concentrations of PNU-282987 or PNU-120596 revealed that the IC(50) value of methyllycaconitine was sensitive to varying concentrations of the agonist, but not that of the PAM. This format demonstrated the feasibility of this approach for high-throughput screening to identify small molecule, PAMs, which were further confirmed in electrophysiological assays of human α7 nAChR expressed in oocytes.

Neurocircuitry of the nicotinic cholinergic system

Continuing to discover how the brain works is one of the great challenges ahead of us. Although understanding the brain anatomy and its functional organization provided a first and indispensable foundation, it became clear that a static view was insufficient. To understand the complexity of neuronal communication, it is necessary to examine the chemical nature of the neurotransmission and, using the example of the acetylcholine receptors, follow the different layers of networks that can be distinguished. The natural alkaloid nicotine contained in tobacco leaves acts as an agonist with a subclass of acetylcholine receptors, and provides an interesting tool to approach brain functions. Analysis of the nicotinic acetylcholine receptors, which are ligand gated channels, revealed that these receptors are expressed at different critical locations on the neurons including the synaptic boutons, neurites, cell bodies, and even on the axons. These receptors can modulate the activity at the microcircuit synaptic level, in the cell processing of information, and, by acting on the velocity of action potential, the synchrony of communication between brain areas. These actions at multiple levels of brain organization provide an example of the complexity of brain neurocircuitry and an illustration of the relevance of this knowledge for psychiatry.

Nicotinic activation of laterodorsal tegmental neurons: implications for addiction to nicotine

Identifying the neurological mechanisms underlying nicotine reinforcement is a healthcare imperative, if society is to effectively combat tobacco addiction. The majority of studies of the neurobiology of addiction have focused on dopamine (DA)-containing neurons of the ventral tegmental area (VTA). However, recent data suggest that neurons of the laterodorsal tegmental (LDT) nucleus, which sends cholinergic, GABAergic, and glutamatergic-containing projections to DA-containing neurons of the VTA, are critical to gating normal functioning of this nucleus. The actions of nicotine on LDT neurons are unknown. We addressed this issue by examining the effects of nicotine on identified cholinergic and non-cholinergic LDT neurons using whole-cell patch clamp and Ca(2+)-imaging methods in brain slices from mice (P12-P45). Nicotine applied by puffer pipette or bath superfusion elicited membrane depolarization that often induced firing and TTX-resistant inward currents. Nicotine also enhanced sensitivity to injected current; and, baseline changes in intracellular calcium were elicited in the dendrites of some cholinergic LDT cells. In addition, activity-dependent calcium transients were increased, suggesting that nicotine exposure sufficient to induce firing may lead to enhancement of levels of intracellular calcium. Nicotine also had strong actions on glutamate and GABA-releasing presynaptic terminals, as it greatly increased the frequency of miniature EPSCs and IPSCs to both cholinergic and non-cholinergic neurons. Utilization of nicotinic acetylcholine receptors (nAChR) subunit antagonists revealed that presynaptic, inhibitory terminals on cholinergic neurons were activated by receptors containing alpha 7, beta2, and non-alpha 7 subunits, whereas, presynaptic glutamatergic terminals were activated by nAChRs that comprised non-alpha 7 subunits. We also found that direct nicotinic actions on cholinergic LDT neurons were mediated by receptors containing alpha 7, beta2, and non-alpha 7 subunits. These findings led us to suggest that nicotine exposure from smoking will enhance both the excitability and synaptic modulation of cholinergic and non-cholinergic LDT neurons, and increase their signature neurotransmitter outflow to target regions, including the VTA. This may reinforce the direct actions of this drug within reward circuitry and contribute to encoding stimulus saliency.

Chronic oral nicotine increases brain [3H]epibatidine binding and responsiveness to antidepressant drugs, but not nicotine, in the mouse forced swim test

INTRODUCTION

Smoking rates among depressed individuals is higher than among healthy subjects, and nicotine alleviates depressive symptoms. Nicotine increases serotonergic and noradrenergic neuronal activity and facilitates serotonin and noradrenaline release. In mice, acute nicotine administration enhances the activity of antidepressants in the mouse forced swim (mFST) and tail suspension tests. Here, we investigated if this action of nicotine is also reflected in a chronic treatment regimen.

MATERIALS AND METHODS

After chronic treatment with nicotine in the drinking water, mice were challenged with nicotine, duloxetine, citalopram, and reboxetine in the mFST. Additionally, 8-OH-DPAT- and clonidine-induced hypothermia was tested in vehicle- and nicotine-pretreated mice, as a measure of 5-HT(1A) and alpha(2)-adrenoceptor function, respectively. Finally, the effects on the brain expression levels of high- and low-affinity nicotinic acetylcholine receptors (nAChRs) and the transporters for serotonin (SERT) and noradrenaline (NET) were assessed using [(3)H]epibatidine, [(3)H]alpha-bungarotoxin, [(3)H]citalopram, and [(3)H]nisoxetine binding, respectively.

RESULTS

In the mFST, nicotine-pretreated mice did not show altered response to the nicotine challenge, but increased responses to all three antidepressants tested were observed when compared to mice that had been administered drinking water without nicotine. There was no change in hypothermic responses to 8-OH-DPAT or clonidine. [(3)H]epibatidine binding was significantly increased in all brain regions investigated; whereas, [(3)H]alpha-bungarotoxin, [(3)H]citalopram, and [(3)H]nisoxetine binding were not altered, indicating that chronic oral nicotine increases the expression and/or affinity of high-affinity nAChRs, but not low-affinity nAChRs, SERT, or NET.

DISCUSSION

It is suggested that the increased sensitivity to antidepressants after chronic nicotine exposure involves increased high-affinity nAChR-mediated neurotransmission.

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.

Ric-3 chaperone-mediated stable cell-surface expression of the neuronal alpha7 nicotinic acetylcholine receptor in mammalian cells

AIM

Studies of the alpha7-type neuronal nicotinic acetylcholine receptor (AChR), one of the receptor forms involved in many physiologically relevant processes in the central nervous system, have been hampered by the inability of this homomeric protein to assemble in most heterologous expression systems. In a recent study, it was shown that the chaperone Ric-3 is necessary for the maturation and functional expression of alpha7-type AChRs(1). The current work aims at obtaining and characterizing a cell line with high functional expression of the human alpha7 AChR.

METHODS

Ric-3 cDNA was incorporated into SHE-P1-halpha7 cells expressing the alpha7-type AChR. Functional studies were undertaken using single-channel patch-clamp recordings. Equilibrium and kinetic [(125)I]alpha-bungarotoxin binding assays, as well as fluorescence microscopy using fluorescent alpha-bungarotoxin, anti-alpha7 antibody, and GFP-alpha7 were performed on the new clone.

RESULTS

The human alpha7-type AChR was stably expressed in a new cell line, which we coined SHE-P1-halpha7-Ric-3, by co-expression of the chaperone Ric-3. Cell-surface AChRs exhibited [(125)I]alphaBTX saturable binding with an apparent K(D) of about 55 nmol/L. Fluorescence microscopy revealed dispersed and micro-clustered AChR aggregates at the surface of SHE-P1-halpha7-Ric-3 cells. Larger micron-sized clusters were observed in the absence of receptor-clustering proteins or upon aggregation with anti-alpha7 antibodies. In contrast, chaperone-less SHE-P1-halpha7 cells expressed only intracellular alpha7 AChRs and failed to produce detectable single-channel currents.

CONCLUSION

The production of a stable and functional cell line of neuroepithelial lineage with robust cell-surface expression of neuronal alpha7-type AChR, as reported here, constitutes an important advance in the study of homomeric receptors in mammalian cells.

Rat neuronal nicotinic acetylcholine receptors containing alpha7 subunit: pharmacological properties of ligand binding and function

AIM

To compare pharmacological properties of heterologously expressed homomeric alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs) with those of native nAChRs containing alpha7 subunit (alpha7* nAChRs) in rat hippocampus and cerebral cortex.

METHODS

We established a stably transfected HEK-293 cell line that expresses homomeric rat alpha7 nAChRs. We studies ligand binding profiles and functional properties of nAChRs expressed in this cell line and native rat alpha7* nAChRs in rat hippocampus and cerebral cortex. We used [(125)I]-alpha-bungarotoxin to compare ligand binding profiles in these cells with those in rat hippocampus and cerebral cortex. The functional properties of the alpha7 nAChRs expressed in this cell line were studied using whole-cell current recording.

RESULTS

The newly established cell line, KXalpha7R1, expresses homomeric alpha7 nAChRs that bind [(125)I]-alpha-bungarotoxin with a K(d) value of 0.38+/-0.06 nmol/L, similar to K(d) values of native rat alpha7* nAChRs from hippocampus (K(d)=0.28+/-0.03 nmol/L) and cerebral cortex (K(d)=0.33+/-0.05 nmol/L). Using whole-cell current recording, the homomeric alpha7 nAChRs expressed in the cells were activated by acetylcholine and (-)-nicotine with EC(50) values of 280+/-19 micromol/L and 180+/-40 micromol/L, respectively. The acetylcholine activated currents were potently blocked by two selective antagonists of alpha7 nAChRs, alpha-bungarotoxin (IC(50)=19+/-2 nmol/L) and methyllycaconitine (IC(50)=100+/-10 pmol/L). A comparative study of ligand binding profiles, using 13 nicotinic ligands, showed many similarities between the homomeric alpha7 nAChRs and native alpha7* receptors in rat brain, but it also revealed several notable differences.

CONCLUSION

This newly established stable cell line should be very useful for studying the properties of homomeric alpha7 nAChRs and comparing these properties to native alpha7* nAChRs.

Is cancer triggered by altered signalling of nicotinic acetylcholine receptors?

Nicotinic acetylcholine receptors (nAChRs) are the central regulators of stimulatory and inhibitory neurotransmitters that control the synthesis and release of growth, angiogenic and neurotrophic factors in cancer cells, the cancer microenvironment and distant organs. Data discussed in this Review suggests that smoking and possibly other environmental and lifestyle factors increase the function of nAChRs that stimulate cancer cells and reduce the function of nAChRs that inhibit cancer cells. This novel paradigm necessitates the development of marker-guided cancer intervention strategies that aim to restore the balance between nAChR-mediated stimulatory and inhibitory neurotransmitters and their downstream effectors.

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.

The interface between extracellular and transmembrane domains of homomeric Cys-loop receptors governs open-channel lifetime and rate of desensitization

The lifetimes of activated postsynaptic receptor channels contribute to the efficiency of synaptic transmission. Here we show that structural differences within the interface dividing extracellular and transmembrane domains of homomeric alpha7 and 5-HT(3A) receptors account for the large differences in open-channel lifetime and time of desensitization onset between these contrasting members of the Cys-loop receptor superfamily. For alpha7 receptors, agonist-evoked single-channel currents appear mainly as isolated brief openings (tau(o) = 0.35 ms), whereas macroscopic currents after a step pulse of agonist desensitize rapidly (tau(d) = 0.4 ms). In contrast for 5-HT(3A) receptors, agonist-evoked single-channel currents appear as clusters of many long openings in quick succession (tau(cluster) = 1.2 s), whereas macroscopic currents desensitize slowly (tau(d) = 1.1 s). A chimeric alpha7-5HT(3A) receptor exhibits functional properties intermediate between those of the parent receptors, but the functional signatures of each parent are reconstituted after substituting the major loops within the interface of the extracellular and transmembrane domains from the corresponding parent receptor. Furthermore, these structural loops contribute to open-channel lifetime and time of desensitization onset in a nonadditive manner. The results suggest that desensitization is the major determinant of the lifetimes of activated alpha7 and 5-HT(3A) receptors and that functional differences between the two receptors arise primarily through structural differences at the interface between extracellular and transmembrane domains.

RIC-3: a nicotinic acetylcholine receptor chaperone

RIC-3 is a transmembrane protein which acts as a molecular chaperone of nicotinic acetylcholine receptors (nAChRs). For some nAChR subtypes (such as homomeric alpha7 neuronal nAChRs), RIC-3 is required for efficient receptor folding, assembly and functional expression. In contrast, for other nAChR subtypes (such as heteromeric alpha4beta2 neuronal nAChRs) there have been reports that RIC-3 can both enhance and reduce levels of functional expression. There is also evidence that RIC-3 can modulate maturation of the closely related 5-hydroxytryptamine (5-HT) receptor (5-HT(3)R). As with heteromeric nAChRs, apparently contradictory results have been reported for the influence of RIC-3 on 5-HT(3)R maturation in different expression systems. Recent evidence indicates that these differences in RIC-3 chaperone activity may be influenced by the host cell, suggesting that other proteins may play an important role in modulating the effects of RIC-3 as a chaperone. RIC-3 was originally identified in the nematode Caenorhabditis elegans as the protein encoded by the gene ric-3 (resistance to inhibitors of cholinesterase) and has subsequently been cloned and characterized from mammalian and insect species. This review provides a brief history of RIC-3; from the identification of the ric-3 gene in C. elegans in 1995 to the more recent demonstration of its activity as a nAChR chaperone.

Nicotinic Receptors: Role in Addiction and Other Disorders of the Brain

Nicotine, the addictive component of cigarette smoke has profound effects on the brain. Activation of its receptors by nicotine has complex consequences for network activity throughout the brain, potentially contributing to the addictive property of the drug. Nicotinic receptors have been implicated in psychiatric illnesses like schizophrenia and are also neuroprotective, potentially beneficial for neurodegenerative diseases. These effects of nicotine serve to emphasize the multifarious roles the drug, acting through multiple nicotinic acetylcholine receptor subtypes. The findings also remind us of the complexity of signaling mechanisms and stress the risks of unintended consequences of drugs designed to combat nicotine addiction.

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.

Differential nicotinic regulation of the nigrostriatal and mesolimbic dopaminergic pathways: implications for drug development

Neuronal nicotinic acetylcholine receptors (nAChRs) modulate dopaminergic function. Discovery of their multiplicity has lead to the search for subtype-selective nAChR agonists that might be therapeutically beneficial in diseases linked to brain dopaminergic pathways. The regulation and responses of the nigrostriatal and mesolimbic dopaminergic pathways are often similar, but some differences do exist. The cerebral distribution and characteristics of various nAChR subtypes differ between nigrostriatal and mesolimbic dopaminergic pathways. Comparison of nicotine and epibatidine, two nAChR agonists whose relative affinities for various nAChR subtypes differ, revealed differences in the nAChR-mediated regulation of dopaminergic activation between these dopamine systems. Nicotine preferentially stimulates the mesolimbic pathway, whereas epibatidine's stimulatory effect falls on the nigrostriatal pathway. Thus, it may be possible to stimulate the nigrostriatal pathway with selective nAChR agonists that do not significantly affect the mesolimbic pathway, and thus lack addictive properties. Furthermore, dopamine uptake inhibition revealed a novel inhibitory effect of epibatidine on accumbal dopamine release, which could form a basis for novel antipsychotics that could alleviate the elevated accumbal dopaminergic tone found in schizophrenia during the active psychotic state. Different regulation of nigrostriatal and mesolimbic dopaminergic pathways by nAChRs could be an important basis for developing novel drugs for treatment of Parkinson's disease and schizophrenia.

Potentials of mean force for acetylcholine unbinding from the alpha7 nicotinic acetylcholine receptor ligand-binding domain

The nicotinic acetylcholine receptor is a prototype ligand-gated ion channel that mediates signal transduction in the neuromuscular junctions and other cholinergic synapses. The molecular basis for the energetics of ligand binding and unbinding is critical to our understanding of the pharmacology of this class of receptors. Here, we used steered molecular dynamics to investigate the unbinding of acetylcholine from the ligand-binding domain of human alpha7 nicotinic acetylcholine receptor along four different predetermined pathways. Pulling forces were found to correlate well with interactions between acetylcholine and residues in the binding site during the unbinding process. From multiple trajectories along these unbinding pathways, we calculated the potentials of mean force for acetylcholine unbinding. Four available methods based on Jarzynski's equality were used and compared for their efficiencies. The most probable pathway was identified to be along a direction approximately parallel to the membrane. The derived binding energy for acetylcholine was in good agreement with that derived from the experimental binding constant for acetylcholine binding protein, but significantly higher than that for the complete human alpha7 nicotinic acetylcholine receptor. In addition, it is likely that several intermediate states exist along the unbinding pathways.

Nicotine and clozapine selectively reverse a PCP-induced deficit of PPI in BALB/cByJ but not NMRI mice: Comparison with risperidone

Schizophrenic patients have deficits in prepulse inhibition (PPI) that may be alleviated by smoking/nicotine. The effect of nicotinic agents on PPI in rodents is equivocal and few studies in mice have been reported. Thus, we assessed nicotine's (0.03-1mg/kg) effect on PPI in five mouse strains with no effects. We next determined if nicotine would reverse a phencyclidine (PCP)-induced deficit of PPI in BALB/cByJ and NMRI mice. BALB/cByJ mice have a low density of [(125)I]alpha-bungaratoxin binding in the hippocampus and poor inhibitory gating of auditory evoked potentials (AEPs), a model related to PPI. At 1mg/kg, nicotine selectively reversed the PCP-induced deficit of PPI in BALB/cByJ mice. The pharmacokinetic profile of nicotine (T(1/2), C(max), T(max) and AUC) was identical in both strains, obviating this as a factor for the strain-dependent effect observed. Moreover, 1mg/kg nicotine inhibited in vivo [(3)H]epibatidine binding with the same time-course in both strains, indicating no difference in brain "kinetics". Since high doses of nicotine were effective in BALB/cByJ mice a role for low-affinity nicotinic receptors, e.g. alpha(7) receptors, is plausible. Clozapine, but not risperidone, also only reversed the PCP deficit of PPI in BALB/cByJ. Clozapine and nicotine also enhance inhibitory gating of AEPs in DBA/2 mice, and clozapine's effect is antagonized by an alpha(7) antagonist. Our data and previous evidence possibly suggest a role for low-affinity nicotinic receptors in the effects of clozapine and nicotine. Furthermore, BALB/cByJ mice may represent a model to test the effects of nicotinic agents acting at low-affinity nicotinic receptors.

RIC-3 Enhances Functional Expression of Multiple Nicotinic Acetylcholine Receptor Subtypes in Mammalian Cells

Recent studies have shown that RIC-3, originally identified in Caenorhabditis elegans as the protein encoded by the gene resistance to inhibitors of cholinesterase (ric-3), can enhance functional expression of alpha7 nicotinic acetylcholine receptors (nAChRs). In the present study, the influence of C. elegans and human RIC-3 upon multiple homomeric (alpha7, alpha8, and alpha9) and heteromeric (alpha3beta2, alpha3beta4, alpha4beta2, alpha4beta4, and alpha9alpha10) nAChR subtypes has been examined in transfected mammalian cells by radioligand binding and functional characterization. Coexpression of RIC-3 facilitates a dramatic enhancement of the ability of alpha7 (and the closely related alpha8 subunit) to generate functional nAChRs in otherwise nonpermissive mammalian cells. In contrast, coexpression of RIC-3 did not facilitate functional expression of either homomeric alpha9 or heteromeric alpha9alpha10 nAChRs in mammalian cell lines. It is noteworthy that whereas RIC-3 has been reported to cause a marked functional inhibition of heteromeric nAChRs such as alpha3beta4 and alpha4beta2 expressed in Xenopus laevis oocytes, RIC-3 significantly enhances levels of functional expression of these and other (alpha3beta2 and alpha4beta4) heteromeric nAChRs when expressed in mammalian cell lines. In addition, the interaction of multiple nAChR subunits (alpha3, alpha4, alpha7, beta2, and beta4) with RIC-3 has been demonstrated by coimmunoprecipitation from metabolically labeled transfected cells. It is significant that coimmunoprecipitation experiments have provided evidence that RIC-3 associates with unassembled nAChR subunits, a finding that is consistent with previous suggestions that RIC-3 may act by enhancing the maturation (subunit folding and assembly) of nAChRs. We conclude that RIC-3 is an nAChR-associated protein that can enhance functional expression of multiple nAChR subtypes in transfected mammalian cells.