The 7 Nicotinic Acetylcholine Receptor Subunit Exists in Two Isoforms that Contribute to Functional Ligand-Gated Ion Channels

University of South Florida, Tampa, Florida, United States
Molecular Pharmacology (Impact Factor: 4.13). 10/2004; 66(3):420-9. DOI: 10.1124/mol.104.000059
Source: PubMed


Fast synaptic transmission in mammalian autonomic ganglia is mediated primarily by nicotinic receptors, and one of the most abundant nicotinic acetylcholine receptor subtypes in these neurons contains the alpha7 subunit (alpha7-nAChRs). Unlike alpha7-nAChRs expressed in other cells, the predominant alpha7-nAChR subtype found in rat intracardiac and superior cervical ganglion neurons exhibits a slow rate of desensitization and is reversibly blocked by alpha-bungarotoxin (alphaBgt). We report here the identification of an alpha7 subunit sequence variant in rat autonomic neurons that incorporates a novel 87-base pair cassette exon in the N terminus of the receptor and preserves the reading frame of the transcript. This alpha7 isoform was detected using reverse transcriptase-polymerase chain reaction techniques in neonatal rat brain and intracardiac and superior cervical ganglion neurons. Immunoblot experiments using a polyclonal antibody directed against the deduced amino acid sequence of the alpha7-2 insert showed a pattern of expression consistent with alpha7-2 subunit mRNA distribution. Moreover, the alpha7-2 subunit could be immunodepleted from protein extracts by solid-phase immunoprecipitation techniques using the anti-alpha7 monoclonal antibody 319. The alpha7-2 subunit was shown to form functional homomeric ion channels that were activated by acetylcholine and blocked by alpha-bungarotoxin when expressed in Xenopus laevis oocytes. This alpha7 isoform exhibited a slow rate of desensitization, and inhibition of these channels by alphaBgt reversed rapidly after washout. Taken together, these data indicate that the alpha7-2 subunit is capable of forming functional alphaBgt-sensitive acetylcholine receptors that resemble the alpha7-nAChRs previously identified in rat autonomic neurons. Furthermore, the distribution of the alpha7-2 isoform is not limited to peripheral neurons.

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    • "The alternative splicing has been also reported for various mammalian nAChR genes, and can be associated with human diseases. For example, alternative splicing in the neuronal α4 and α7 nAChR subunits is associated with Tourette syndrome [32] and schizophrenia [33], respectively, and that of the α1 and ε subunits of the muscle-type nAChR—with congenital myasthenic syndromes [34], [35], [36]. Indeed, the presence of non-functional α1 nAChR resulting from RNA splicing was found in muscle cells [37]. "
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    ABSTRACT: Isolation of polyadenilated mRNA from human immortalized bronchial epithelial cell line BEP2D revealed the presence of multiple isoforms of RNA coded by the CHRNA9 gene for α9 nicotinic acetylcholine receptor (nAChR). BEP2D cells were homozygous for the rs10009228 polymorphism encoding for N442S amino acid substitution, and also contained mRNA coding for several truncated isoforms of α9 protein. To elucidate the biologic significance of the naturally occurring variants of α9 nAChR, we compared the biologic effects of overexpression of full-length α9 N442 and S442 proteins, and the truncated α9 variant occurring due to a loss of the exon 4 sequence that causes frame shift and early termination of the translation. These as well as control vector were overexpressed in the BEP2D cells that were used in the assays of proliferation rate, spontaneous vs. tobacco nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced cellular transformation, and tumorigenicity in cell culture and mice. Overexpression of the S442 variant significantly increased cellular proliferation, and spontaneous and NNK-induced transformation. The N442 variant significantly decreased cellular transformation, without affecting proliferation rate. Overexpression of the truncated α9 significantly decreased proliferation and suppressed cellular transformation. These results suggested that α9 nAChR plays important roles in regulation of bronchial cell growth by endogenous acetylcholine and exogenous nicotine, and susceptibility to NNK-induced carcinogenic transformation. The biologic activities of α9 nAChR may be regulated at the splicing level, and genetic polymorphisms in CHRNA9 affecting protein levels, amino acid sequence and RNA splicing may influence the risk for lung cancer.
    PLoS ONE 11/2011; 6(11):e27978. DOI:10.1371/journal.pone.0027978 · 3.23 Impact Factor
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    • "These mutations exist within different regions of the receptor, such as within the second transmembrane domain lining the pore of the channel (Revah et al. 1991), as well as in the D-loop near the ligand-binding domain (Gay et al. 2008). Other studies have shown that α7 nAChR desensitization kinetics can be affected by co-assembly with non-α7 subunits such as β2 (Khiroug et al. 2002), or by alternative splicing of the α7 receptor (Severance et al. 2004). In this study, we have identified a critical proline residue near the middle of the β9 strand which, when mutated to specific amino acids, confers markedly slower desensitization to the rat α7 receptor. "
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    ABSTRACT: The rat α7 nicotinic acetylcholine receptor (nAChR) has a proline residue near the middle of the β9 strand. The replacement of this proline residue at position 180 (P180) by either threonine (α7-P180T) or serine (α7-P180S) slowed the onset of desensitization dramatically, with half-times of ~930 and 700 ms, respectively, compared to 90 ms for the wild-type receptor. To investigate the importance of the hydroxyl group on the position 180 side-chains, the mutant receptors α7-P180Y and α7-P180F were studied and showed half-times of desensitization of 650 and 160 ms, respectively. While a position 180 side-chain OH group may contribute to the slow desensitization rates, α7-P180S and α7-P180V resulted in receptors with similar desensitization rates, suggesting that increased backbone to backbone H bonding expected in the absence of proline at position 180 would likely exert a great effect on desensitization. Single channel recordings indicated that for the α7-P180T receptor there was a significantly reduced closed time without any change in single channel conductance (as compared to wild-type). Kinetic simulations indicated that all changes observed for the mutant channel behaviour were reproduced by decreasing the rate of desensitization, and increasing the microscopic affinity to resting receptors. Molecular dynamics (MD) simulations on a homology model were used to provide insight into likely H bond interactions within the outer β-sheet that occur when the P180 residue is mutated. All mutations analysed increased about twofold the predicted number of H bonds between the residue at position 180 and the backbone of the β10 strand. Moreover, the α7-P180T and α7-P180S mutations also formed some intrastrand H bonds along the β9 strand, although H bonding of the OH groups of the threonine or serine side-chains was predicted to be infrequent. Our results indicate that rapid desensitization of the wild-type rat α7 nAChR is facilitated by the presence of the proline residue within the β9 strand.
    The Journal of Physiology 11/2010; 588(Pt 22):4415-29. DOI:10.1113/jphysiol.2010.195495 · 5.04 Impact Factor
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    • "With the tip of the superfusion placed 120 μm above our cells we reach 75 % of the final concentration of solutions within 35 msec. This is considerably slower than the rapid application system used by others (full concentration reached within 5 msec) to record the currents carried by the fast desensitizing splice variant α7-1 of the α7 nAChR (Zhang et al., 1994; Severance et al., 2004). By comparing their rapid application with a conventional but slower puffer pipette, these authors concluded that the high speed of the superfusion is a prerequisite for the detection of currents in response to α7-1 activation in chick ciliary ganglion neurons (Zhang et al., 1994). "
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    ABSTRACT: Nicotinic acetylcholine receptors (nAChRs) mediate fast synaptic transmission in ganglia of the autonomic nervous system. Here, we determined the subunit composition of hetero-pentameric nAChRs in the mouse superior cervical ganglion (SCG), the function of distinct receptors (obtained by deletions of nAChR subunit genes) and mechanisms at the level of nAChRs that might compensate for the loss of subunits. As shown by immunoprecipitation and Western blots, wild-type (WT) mice expressed: alpha 3 beta 4 (55%), alpha 3 beta 4 alpha 5 (24%) and alpha 3 beta 4 beta 2 (21%) nAChRs. nAChRs in beta 4 knockout (KO) mice were reduced to < 15% of controls and no longer contained the alpha 5 subunit. Compound action potentials, recorded from the postganglionic (internal carotid) nerve and induced by preganglionic nerve stimulation, did not differ between alpha 5 beta 4 KO and WT mice, suggesting that the reduced number of receptors in the KO mice did not impair transganglionic transmission. Deletions of alpha 5 or beta2 did not affect the overall number of receptors and we found no evidence that the two subunits substitute for each other. In addition, dual KOs allowed us to study the functional properties of distinct alpha 3 beta4 and alpha 3 beta 2 receptors that have previously only been investigated in heterologous expression systems. The two receptors strikingly differed in the decay of macroscopic currents, the efficacy of cytisine, and their responses to the alpha-conotoxins AuIB and MII. Our data, based on biochemical and functional experiments and several mouse KO models, clarify and significantly extend previous observations on the function of nAChRs in heterologous systems and the SCG.
    European Journal of Neuroscience 03/2010; 31(6):978-93. DOI:10.1111/j.1460-9568.2010.07133.x · 3.18 Impact Factor
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