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

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

Ubiquilin-1 regulates nicotine-induced up-regulation of neuronal nicotinic acetylcholine receptors

Chronic exposure to nicotine, as in tobacco smoking, up-regulates nicotinic acetylcholine receptor surface expression in neurons. This up-regulation has been proposed to play a role in nicotine addiction and withdrawal. The regulatory mechanisms behind nicotine-induced up-regulation of surface nicotinic acetylcholine receptors remain to be determined. It has recently been suggested that nicotine stimulation acts through increased assembly and maturation of receptor subunits into functional pentameric receptors. Studies of muscle nicotinic acetylcholine receptors suggest that the availability of unassembled subunits in the endoplasmic reticulum can be regulated by the ubiquitin-proteosome pathway, resulting in altered surface expression. Here, we describe a role for ubiquilin-1, a ubiquitin-like protein with the capacity to interact with both the proteosome and ubiquitin ligases, in regulating nicotine-induced up-regulation of neuronal nicotinic acetylcholine receptors. Ubiquilin-1 interacts with unassembled alpha3 and alpha4 subunits when coexpressed in heterologous cells and interacts with endogenous nicotinic acetylcholine receptors in neurons. Coexpression of ubiquilin-1 and neuronal nicotinic acetylcholine receptors in heterologous cells dramatically reduces the expression of the receptors on the cell surface. In cultured superior cervical ganglion neurons, expression of ubiquilin-1 abolishes nicotine-induced up-regulation of nicotinic acetylcholine receptors but has no effect on the basal level of surface receptors. Coimmunostaining shows that the interaction of ubiquilin-1 with the alpha3 subunit draws the receptor subunit and proteosome into a complex. These data suggest that ubiquilin-1 limits the availability of unassembled nicotinic acetylcholine receptor subunits in neurons by drawing them to the proteosome, thus regulating nicotine-induced up-regulation.     

Inflammatory cytokines decrease the expression of nicotinic acetylcholine receptor during the cell maturation

It is known that the nervous system significantly attenuates systemic inflammatory responses through the parasympathetic nervous system. Furthermore, it has been reported that the alpha7 subunit of a nicotinic acetylcholine receptor is required for a cholinergic inhibition against cytokine synthesis in a macrophage. As antigen-presenting cells (APCs) play a central role in the generation of primary T cell responses and the maintenance of immunity, in this study, we investigated the expression level of nicotinic receptors of a p53-deficient APC cell line (JawsII) derived from a mouse bone marrow. We showed that stimulation of the JawsII cells with lipopolysaccharide (LPS) and tumor necrosis factor alpha (TNF-α) led increase of CD80 and CD86 expression while diminishment of the surface nicotinic receptor. On the other hand, stimulation of nicotinic receptor had no effect on these phenomena. Furthermore, we examined the ability of the cells to release cytokine when stimulated with both nicotine and LPS and showed that the stimulation with LPS augmented the secretion of IL-1a, IL-1b, IL-6, and TNF-α. These results suggested that nicotinic stimulation had no effect on the diminishment of alpha7 nicotinic acetylcholine receptor on JawsII cells by LPS stimulation.     

Dual role of the RIC-3 protein in trafficking of serotonin and nicotinic acetylcholine receptors

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

Assembly of nicotinic alpha7 subunits in Xenopus oocytes is partially blocked at the tetramer level

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

Expression of muscarinic and nicotinic acetylcholine receptors in the mouse urothelium

Acetylcholine (ACh) and its receptors play a crucial role in bladder physiology. Here, we investigated the presence of muscarinic receptor subtypes (MR) and nicotinic acetylcholine receptor (nAChR) alpha-subunits in the mouse urothelium by RT-PCR and immunohistochemistry. With RT-PCR, we detected mRNAs coding for all of the five different MR subtypes and for the nicotinic receptor subunits alpha2, alpha4, alpha5, alpha6, alpha7, alpha9 and alpha10, whereas the alpha3-subunit was not expressed. Using immunohistochemistry, we localised a panel of acetylcholine receptors in the different layers of the murine bladder urothelium, with predominant appearance in the basal plasma membrane of the basal cell layer and in the apical membrane of the umbrella cells. M2R and subunit alpha9 were observed exclusively in the umbrella cells, whereas the MR subtypes 3-5 and the nAChR subunits alpha4, alpha7 and alpha10 were also detected in the intermediate and basal cell layers. The subunit alpha5 was localised only in the basal cell layer. In conclusion, the murine urothelium expresses multiple cholinergic receptors, including several subtypes of both MR and nAChR, which are differentially distributed among the urothelial cell types. Since these receptors have different electrophysiological and pharmacological properties, and therefore are considered to be responsible for different cellular responses to ACh, this differential distribution is expected to confer cell type-specificity of cholinergic regulation in the bladder urothelium.     

Distribution of alpha7 and alpha4 nicotinic acetylcholine receptor subunits in several tissues of Triturus carnifex (Amphibia, Urodela)

The distribution of neuronal and non-neuronal mRNAs for alpha7 and alpha4 nicotinic acetylcholine receptor subunits was investigated in Triturus carnifex tissues using the in situ hybridization approach. The findings reveal a composite pattern of expression only partially overlapping for the two subunits; subunit alpha7 seems to be expressed widely throughout nervous, gastrointestinal and skin tissues, while alpha4 is present in a restricted number of cells of nervous and gastrointestinal tissue. We also found a specific pattern for each subunit; alpha7 and alpha4 associated exclusively to the epidermal glands and hypophysis, respectively; this is probably due to alternative roles that nicotinic acetylcholine receptors play in regulating physiological functions of non-neuronal amphibian tissues, rather than as mere neurotransmitters in the nervous system.     

Administration of keratinocyte growth factor (KGF) modulates the pulmonary expression of nicotinic acetylcholine receptor subunits alpha7, alpha9 and alpha10

Administration of recombinant human keratinocyte growth factor (rHuKGF, Delta23N-KGF, palifermin) protects the lung against a variety of injurious stimuli. The exact mechanisms leading to lung protection are unknown. Alterations in the non-neuronal cholinergic system of the lung might be involved, as vital pulmonary functions are regulated by acetylcholine. Here, we investigated the effect of KGF on the expression of nicotinic acetylcholine receptor subunits alpha7, alpha9 and alpha10 in rat lungs. Adult rats were treated via intratracheal instillation with rHuKGF or with an equivalent volume of PBS. The expression of nicotinic acetylcholine receptor subunits was analyzed by real-time RT-PCR, immunoblotting and immunohistochemistry. Treatment with rHuKGF led to a decreased expression of nicotinic receptor subunit alpha7 in the total lung. In contrast, the expression of the receptor subunits alpha9 and alpha10 was up-regulated. In conclusion, nicotinic acetylcholine receptors are differentially regulated by KGF treatment in vivo, which might result in changes in the biological effects of acetylcholine.     

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

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

Purification and characterization of a nicotinic acetylcholine receptor from chick brain

Immunohistochemical studies have previously shown that both the chick brain and chick ciliary ganglion neurons contain a component which shares antigenic determinants with the main immunogenic region of the nicotinic acetylcholine receptor from electric organ and skeletal muscle. Here we describe the purification and initial characterization of this putative neuronal acetylcholine receptor. The component was purified by monoclonal antibody affinity chromatography. The solubilized component sediments on sucrose gradients as a species slightly larger than Torpedo acetylcholine receptor monomers. It was affinity labeled with bromo[3H]acetylcholine. Labeling was prevented by carbachol, but not by alpha-bungarotoxin. Two subunits could be detected in the affinity-purified component, apparent molecular weights 48 000 and 59 000. The 48 000 molecular weight subunit was bound both by a monoclonal antibody directed against the main immunogenic region of electric organ and skeletal muscle acetylcholine receptor and by antisera raised against the alpha subunit of Torpedo receptor. Evidence suggests that there are two alpha subunits in the brain component. Antisera from rats immunized with the purified brain component exhibited little or no cross-reactivity with Torpedo electric organ or chick muscle acetylcholine receptor. One antiserum did, however, specifically bind to all four subunits of Torpedo receptor. Experiments to be described elsewhere (J. Stollberg et al., unpublished results) show that antisera to the purified brain component specifically inhibit the electrophysiological function of acetylcholine receptors in chick ciliary ganglion neurons without inhibiting the function of acetylcholine receptors in chick muscle cells. All of these properties suggest that this component is a neuronal nicotinic acetylcholine receptor with limited structural homology to muscle nicotinic acetylcholine receptor.     

Control of type II transforming growth factor-beta receptor expression by integrin ligation

Ectopic expression of the alpha5 integrin subunit in cancer cells with little or no endogenous expression of this integrin often results in reduced proliferation as well as reduced malignancy. We now show that inhibition resulting from ectopic expression of alpha5 integrin is due to induction of autocrine negative transforming growth factor-beta (TGF-beta) activity. MCF-7 breast cancer cells do not express either alpha5 integrin or type II TGF-beta receptor and hence are unable to generate TGF-beta signal transduction. Ectopic expression of alpha5integrin expression enhanced cell adhesion to fibronectin, reduced proliferation, and increased the expression of type II TGF-beta receptor mRNA and cell surface protein. Receptor expression was increased to a higher level in alpha5 transfectants by growth on fibronectin-coated plates. Induction of type II TGF-beta receptor expression also resulted in the generation of autocrine negative TGF-beta activity because colony formation was increased after TGF-beta neutralizing antibody treatment. Transient transfection with a TGF-beta promoter response element in tandem with a luciferase cDNA into cells stably transfected with alpha5 integrin resulted in basal promoter activities 5-10-fold higher than those of control cells. Moreover, when alpha5 transfectants were treated with a neutralizing antibody to either TGF-beta or integrin alpha5, this increased basal promoter activity was blocked. Autocrine TGF-beta activity also induced 3-fold higher endogenous fibronectin expression in alpha5 transfectants relative to that of control cells. Re-expression of type II receptor by alpha5 transfection also restored the ability of the cells to respond to exogenous TGF-beta and led to reduced tumor growth in athymic nude mice. Taken together, these results show for the first time that TGF-beta type II receptor expression can be controlled by alpha5beta1 ligation and integrin signal transduction. Moreover, TGF-beta and integrin signal transduction appear to cooperate in their tumor-suppressive functions.     

Primary structure and expression of beta 2: a novel subunit of neuronal nicotinic acetylcholine receptors

A new subunit, beta 2, of the neuronal nicotinic receptor family has been identified. This subunit has the structural features of a non-agonist-binding subunit. We provide evidence that beta 2 can substitute for the muscle beta 1 subunit to form a functional nicotinic receptor in Xenopus oocytes. Expression studies performed in oocytes have demonstrated that three different neuronal nicotinic acetylcholine receptors can be formed by the pairwise injection of beta 2 mRNA and each of the neuronal alpha subunit mRNAs. The beta 2 gene is expressed in PC12 cells and in areas of the central nervous system where the alpha 2, alpha 3, and alpha 4 genes are expressed. These results lead us to propose that the nervous system expresses diverse forms of neuronal nicotinic acetylcholine receptors by combining beta 2 subunits with different agonist-binding alpha subunits.     

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

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

Functional nicotinic acetylcholine receptor expression in stem and progenitor cells of the early embryonic mouse cerebral cortex.

The adult cerebral cortex contains nicotinic acetylcholine (ACh) receptors vital to cortical function. However, little is known about the assembly of embryonic nicotinic receptor subunits into functional receptors or whether they play an active role in cortical development. We now report evidence of functional nicotinic acetylcholine receptor channels in fetal mouse cerebral cortex as early as embryonic day 10 (E10), when the cortex consists of dividing stem and progenitor cells. Patch-clamp electrophysiological measurements indicate that nicotine and ACh evoke sizable inward currents characteristic of nicotinic receptors, that are strongly rectifying with a reversal potential near 0 mV. Three different nicotinic agonists, ACh, nicotine, and dimethylphenylpiperazinium, evoked cytosolic Ca(2+) signals. Agonist-evoked Ca(2+) signals and electrophysiological responses were found in greater than 70% of all E10-E11 cells tested and were blocked by nicotinic receptor antagonists. The Ca(2+) response to nicotinic agonists was markedly prolonged in cells from early embryonic stages relative to later stages of development. alpha3, alpha4, and alpha7 receptor subunit proteins were detected immunocytochemically in cortical cells from E10 to birth. The incidence of each subunit declined with embryonic age, suggesting a role in early development. We discuss the possible function of nicotinic receptors in early cortical development and their role as a target for nicotine in the developmental pathologies associated with the fetal tobacco syndrome.     

An extracellular RRR motif flanking the M1 transmembrane domain governs the biogenesis of homomeric neuronal nicotinic acetylcholine receptors

We have previously demonstrated that the highly conserved R209, that flanks the M1 transmembrane segment of nicotinic acetylcholine (ACh) receptors, is required for the transport of assembled homomeric neuronal α7 nicotinic ACh receptors to the cell surface. In the present paper we show that basic residues at positions 208 and 210 are necessary for the assembly of α7 receptors. On the contrary, a basic residue at position 210 of α3 subunit decreases the assembly of heteromeric neuronal α3β4 nicotinic ACh receptors. A basic residue at position 210 of the β4 subunit slightly decreases α3β4 receptor expression. We conclude that a pre-M1 RRR motif is necessary for the biogenesis of homomeric α-bungarotoxin-sensitive neuronal α7 nicotinic ACh receptors.