Quantitation of an alpha subunit splicing intermediate: evidence for transcriptional activation in the control of acetylcholine receptor expression in denervated chick skeletal muscle.

We have investigated the mechanisms responsible for the increase in acetylcholine receptor subunit mRNAs during the induction of denervation supersensitivity in skeletal muscle. Using a cRNA probe specific for exon 7 (224 nucleotides; with flanking intron sequences of 105 nucleotides on the 3' end, and of 70 nucleotides on the 5' end) of the alpha subunit of the chicken muscle acetylcholine receptor gene, we were able to quantitate the concentration of mature alpha subunit mRNA and its precursor. In 3-wk-old chicks, the concentration of alpha subunit message in leg muscle was found to be 4.0 attomoles per microgram total RNA, and to increase 40-fold within 1 wk after section of the sciatic nerve. The molar ratio of precursor/mature mRNA, which was approximately 0.023 in innervated as well as denervated muscle, transiently rose to 0.047 at the beginning of the second postoperative day when mature message content increased 20-fold; the rise in precursor level preceded the increase in mature message content. These findings suggest that an accelerated rate of transcription of the message coding for the alpha subunit causes increased message content and the stimulation of receptor synthesis characteristic of denervated muscle.

Mapping the main immunogenic region and toxin-binding site of the nicotinic acetylcholine receptor.

The alpha-chain of the nicotinic acetylcholine receptor carries the binding sites both for cholinergic ligands and for most experimentally induced or naturally occurring antibodies to the native receptor. By means of expression cloning in Escherichia coli, fusion proteins were derived from specific fragments of a complementary DNA encoding the mouse alpha-chain, allowing the mapping of the toxin-binding site to residues 160-216 and the main immunogenic region to residues 6-85. This approach permits the independent study of different functional domains of a complex receptor molecule and should be generally applicable to other proteins for which complementary DNA clones are available.

Functional expression of a new pharmacological subtype of brain nicotinic acetylcholine receptor.

A new type of agonist-binding subunit of rat neuronal nicotinic acetylcholine receptors (nAChRs) was identified. Rat genomic DNA and complementary DNA encoding this subunit (alpha 2) were cloned and analyzed. Complementary DNA expression studies in Xenopus oocytes revealed that the injection of messenger RNAs (mRNAs) for alpha 2 and beta 2 (a neuronal nAChR subunit) led to the generation of a functional nAChR. In contrast to the other known neuronal nAChRs, the receptor produced by the injection of alpha 2 and beta 2 mRNAs was resistant to the alpha-neurotoxin Bgt3.1. In situ hybridization histochemistry showed that alpha 2 mRNA was expressed in a small number of regions, in contrast to the wide distribution of the other known agonist-binding subunits (alpha 3 and alpha 4) mRNAs. These results demonstrate that the alpha 2 subunit differs from other known agonist-binding alpha-subunits of nAChRs in its distribution in the brain and in its pharmacology.

Expression and regulation of neuronal acetylcholine receptor mRNA in chick ciliary ganglia.

A chicken genomic clone encoding a portion of the neuronal acetylcholine receptor (AChR) alpha 3 subunit was used to identify homologous mRNA in embryonic chick ciliary ganglia. In situ hybridization indicated that the mRNA was neuronal. Northern blot analysis revealed a major hybridizing species of 3.5 kb. Protection experiments confirmed that ganglionic RNA contained material indistinguishable by RNAase digestion from the 300 nucleotide probe used. No transcripts were detected by in situ hybridization or Northern blot analysis for chick neuronal AChR alpha 2 or alpha 4 genes. alpha 3 transcripts were present at all times examined (E6 to 1 year posthatch). Both postganglionic axotomy and preganglionic denervation of ciliary ganglia in newly hatched chicks produced declines in alpha 3 mRNA levels, implying regulation of neuronal AChR mRNA by cell-cell interactions.

A cell type-specific enhancer drives expression of the chick muscle acetylcholine receptor alpha-subunit gene.

The regulation of acetylcholine receptor alpha-subunit gene expression was analyzed by transient expression assays. Using rabbit beta-globin cDNA as a reporter gene, we have confirmed that the 5'-flanking sequence of the chicken acetylcholine receptor alpha-subunit gene directs specific expression in differentiated C2C12 cells, a mouse muscle cell line, but not in undifferentiated C2C12 cells and mouse 3T3 fibroblasts. Testing chimeric plasmids containing Bal31 deletion mutants of the alpha-subunit gene upstream sequence, we found the -116 to -81 region of the alpha-subunit to be responsible for tissue- and stage-specific expression. This 36 bp fragment stimulates the activity of both alpha-subunit and SV40 promoters in a distance- and orientation-independent manner, thus fulfilling the criteria of an enhancer.

Electrophysiology of a chick neuronal nicotinic acetylcholine receptor expressed in Xenopus oocytes after cDNA injection.

Brain nicotinic acetylcholine receptors (nAChRs) are made up of protein subunits that differ from those constituting muscle nAChRs. To characterize the physiological properties of one class of avian brain nicotinic receptor, we injected the nuclei of Xenopus oocytes with full-length cDNAs for the ligand binding (alpha 4) and structural (n alpha) subunits. Injected oocytes had large ACh-induced currents in the microampere range that were insensitive to alpha-bungarotoxin, as expected for neuronal nAChRs. We found that these brain nAChRs incorporate at least two alpha 4 subunits and that their functional properties differ from muscle nAChRs in at least two respects: the elementary conductance is considerably smaller (20 pS), and channels in outside out patches stop functioning within a few minutes.

A neuronal nicotinic acetylcholine receptor subunit (alpha 7) is developmentally regulated and forms a homo-oligomeric channel blocked by alpha-BTX.

cDNA and genomic clones encoding alpha 7, a novel neuronal nicotinic acetylcholine receptor (nAChR) alpha subunit, were isolated and sequenced. The mature alpha 7 protein (479 residues) has moderate homology with all other alpha and non-alpha nAChR subunits and probably assumes the same transmembrane topology. alpha 7 transcripts transiently accumulate in the developing optic tectum between E5 and E16. They are present in both the deep and the superficial layers of E12 tectum. In Xenopus oocytes, the alpha 7 protein assembles into a homo-oligomeric channel responding to acetylcholine and nicotine. The alpha 7 channel desensitizes very rapidly, rectifies strongly above -20 mV, and is blocked by alpha-bungarotoxin. A bacterial fusion protein encompassing residues 124-239 of alpha 7 binds labeled alpha-bungarotoxin. We conclude that alpha-bungarotoxin binding proteins in the vertebrate nervous system can function as nAChRs.

Autoimmune T lymphocytes in myasthenia gravis. Determination of target epitopes using T lines and recombinant products of the mouse nicotinic acetylcholine receptor gene.

Oligoclonal and cloned T lines from peripheral blood or thymuses of patients with myasthenia gravis (MG) were selected for reactivity against nicotinic acetylcholine receptors (AChR) from Torpedo california, or against a recombinant fusion peptide, X4, representing the extracellular portion of the mouse AChR alpha-chain. All cell lines expressed the CD4 membrane phenotype, and their antigen reactivity was blocked by antibodies against monomorphic HLA DR/DP determinants. Using a panel of fusion proteins of different, overlapping mouse AChR alpha-chain sequences, a major T cell epitope was localized between amino acid positions 85 and 142. This determinant was distinct from the humoral main immunogenic region, which has been identified on the sequence 61-76. The response pattern of uncloned T lines from three patients with different HLA haplotypes suggests, however, that in any one MG patient T lymphocytes may recognize more than one autoantigenic epitope on the AChR alpha-chain, and that the T lymphocyte response profiles vary among individual patients.

Nicotinic acetylcholine receptors in the nematode Caenorhabditis elegans.

Two cDNAs (Ce21 and Ce13) were isolated from a Caenorhabditis elegans library screened with a probe encoding conserved domains of the avian alpha5 neuronal nicotinic acetylcholine receptor (nAChR). Alignments to all nAChR subunits in the EMBL/Swissprot data base demonstrate that the Ce21 protein most resembles the vertebrate alpha7 subunit, whereas Ce13 is closest to the ARD subunit of Drosophila. The corresponding genes were isolated and hybridized to YAC grids: Ce21 maps on chromosome V near the his-23 gene, and Ce13 on chromosome I very near or at unc-29. The structure of the Ce21 gene was compared with that of other vertebrate and invertebrate nAChR genes and found to share by far the largest number of conserved splice sites with the vertebrate alpha7 gene. Upon expression in the Xenopus oocyte system, the Ce21 subunit assembled into a functional homomeric nAChR, whose properties were compared with those of the chicken alpha7 receptor. The anthelmintic nicotinic agonist levamisole is unable to activate the Ce21 and alpha7 receptors, but efficiently antagonises their responses to ACh. Both receptors desensitise quickly upon agonist application, are more sensitive to nicotine than to acetylcholine, and are efficiently blocked by dihydro-beta-erythroidine. Unlike the alpha7 receptor, however, the Ce21 receptor is relatively insensitive to methyllycaconitine and to alpha-bungarotoxin. The similarities in protein sequence, gene structure and physiological properties between alpha7 and Ce21 suggest a very ancient lineage for the alpha7 class of nAChR subunits.

Acetylcholine receptor synthesis rate and levels of receptor subunit messenger RNAs in chick muscle.

Levels of mRNAs specific for the alpha-, gamma- and delta-subunit of the nicotinic acetylcholine receptor were measured in chick skeletal muscle by solution hybridization, using a genomic DNA probe containing the intramembrane segments M2 and M3 of the alpha-subunit and probes comprising exons 2-6 and exons 4-8, respectively, of the gamma- and delta-subunit. In the innervated calf musculature of adult chickens, receptor-specific messages were detected in approx. 100-fold excess over the amount required to account for the observed synthesis rate. Within 1 week after section of the sciatic nerve, alpha-, gamma- and delta-subunit message levels rose 112-, 42- and 24-fold, respectively, while receptor expression rate increased about 150-fold. The rise in message levels preceded the denervation-induced increase in receptor concentration. In differentiating myogenic cells all three messages were found in excess over the amounts required for the observed rate of receptor synthesis. Treatment of differentiated myotubes with drugs that change receptor synthesis rate selectively affects alpha-subunit mRNA. In all situations in vitro and in vivo the alpha-subunit mRNA was found to reach final levels faster, and to be from 3 to over 30 times more abundant, than the other messages. These observations corroborate earlier evidence for a regulatory mechanism in which the supply of mRNA determines acetylcholine receptor synthesis rate. They also suggest that receptor expression is not simply proportional to acetylcholine receptor subunit mRNA concentrations, but rather is controlled, to a considerable extent, by the efficiency with which the receptor-specific mRNAs and/or the subunits they code for are subsequently utilized.

Steroids inhibit nicotinic acetylcholine receptors.

Application of progesterone to Xenopus oocytes expressing a cloned neuronal nicotinic acetylcholine (nAChR) revealed two effects. The first effect was a fully reversible reduction of the current induced by acetylcholine (ACh), its onset being nearly instantaneous. The second effect, which developed in a few hours, was an irreversible suppression of ACh-evoked currents. The transient inhibition had an apparent Ki of 7 microM when tested with 50 nM ACh, but the percentage of inhibition was positively correlated to the ACh concentration. A reduction of ACh-induced currents which appeared immediately upon progesterone application was also observed with muscle nAChR expressed in oocytes and with nAChR on membrane patches isolated from ciliary ganglion neurons. Thus nAChRs are modulated by progesterone and steroids may play an important role in nicotinic cholinoception.

Alpha 5 subunit forms functional alpha 3 beta 4 alpha 5 nAChRs in transfected human cells.

nAChRs heterologously expressed in human cells after transient transfection with alpha 3 beta 4 alpha 5 or alpha 3 beta 4 subunit cDNAs exhibited similar sensitivities to antagonists and comparable functional channel profiles. However, the sum of two Hill equations was required for best fitting the ACh dose-current response curves after co-expression of alpha 5, alpha 3 and beta 4 subunits. One component was comparable to that obtained in alpha 3 beta 4-transfected cells, while the additional component, putatively attributed to an alpha 3 beta 4 alpha 5 nAChR population, showed a Hill coefficient > 2 and a nine-fold greater half-maximal ACh concentration (EC50). These results suggest that the alpha 5 subunit participates in the assembly of alpha 3 beta 4 alpha 5 nAChRs complexes in human cells, adding a new member to the family of neuronal nicotinic receptors.

On the transcriptional regulation of neuronal nAChR genes.

The promoters driving transcription of the neuronal nicotinic genes alpha 7 and beta 3 have been characterized in the chicken. Although their regulatory modalities are thoroughly different, they nevertheless lead to co-expression in the same neurons.

Nicotinic acetylcholine receptor gene expression in developing chick autonomic ganglia.

The developmental expression patterns of ten genes encoding nicotinic acetylcholine receptor subunits were analyzed using Northern blots and in situ hybridization in chick peripheral ganglia of neural crest, placodal and dual embryonic origin. The superior cervical and ciliary ganglia were investigated in detail because they accumulated relatively abundant transcripts of the alpha3, beta4, alpha5 and alpha7 genes. In the superior cervical ganglion, these four mRNA species had similar developmental time-courses. They appeared at embryonic day 8 (E8), increased steadily until E16 and maintained a rather high plateau level until E18. In the ciliary ganglion, alpha7 transcripts were already abundant at E6, increased until E10, and considerably decreased thereafter. High-resolution in situ hybridization showed that alpha7 transcripts were present in all cell types of the E6 ciliary ganglion, whereas they were restricted to large neuronal somas at E16. Transfections with a reporter gene under the control of the alpha7 promoter demonstrated that a sharp developmental divide occurred at E11-12, after which stage the promoter was activatable in neurons exclusively.

Pharmacological properties of the homomeric alpha 7 receptor.

The pharmacological properties of the alpha-bungarotoxin sensitive alpha 7 neuronal nicotinic acetylcholine receptor (nAChR) were studied upon reconstitution in Xenopus oocytes. Channels formed by alpha 7 are about 10-fold more sensitive to nicotine and cytisine than to ACh but are little, if at all, activated by the ganglionic agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP). Tubocurarine (TC) was found to act as a non-competitive inhibitor, whereas dihydro-beta-erythroidine (DH beta E) behaves as a pure competitive inhibitor whose blockade is fast and fully reversible. In addition, the alpha 7 receptor displays a poor sensitivity to methonium salts. The pharmacological properties of the alpha 7 channels are readily distinguishable from those of other identified neuronal nicotinic receptors.

Purification and properties of phage P22 c2 repressor.

The c2 repressor of phage P22 has been purified to homogeneity. It specifically binds to lambdaimm21 and P22 DNA. Its affinity for the presumed operator mutant P22 virB is reduced. The initial dissociation rates of the complex between c2 repressor and lambdaimm21 DNA are 0.02 min-1 at 0 degrees C, 0.08 min-1 at 20 degrees C and 0.17 min-1 at 32 degrees C. The dissociation rates of complexes formed between the c2 repressor and the lambdaimm21 operators OR, OL and OR vira were measured and compared to the corresponding rates obtained with 21 cI repressor.

Agonist and antagonist effects of nicotine on chick neuronal nicotinic receptors are defined by alpha and beta subunits.

1. Functional neuronal nicotinic receptors were reconstituted in Xenopus oocytes by the nuclear injection of different combinations of chick and rat cDNAs encoding alpha and beta subunits. The pharmacology of these nicotinic receptors was investigated using two-electrode voltage clamp. 2. The sensitivity of the chick alpha 3/beta 2, alpha 3/beta 4, and alpha 4/beta 2 receptors to acetylcholine (ACh) and neuronal bungarotoxin differed markedly, indicating that both subunits contribute to the pharmacological properties of the receptors. 3. Nicotine acted as an agonist on the chick alpha 3/beta 4 and alpha 4/beta 2 receptors and rat alpha 3/beta 2 receptor. In contrast, nicotine (at concentrations > 3 microM) was only a weak partial agonist of the chick alpha 3/beta 2 receptor. Moreover, nicotine coapplied with 3 microM ACh on the chick alpha 3/beta 2 receptor acted as a potent competitive antagonist, with an IC50 of 0.43 microM. No antagonist effect of nicotine could be revealed on the other nicotinic receptors. 4. The effect of nicotine was tested on hybrid receptors obtained by coinjection of chick and rat cDNAs encoding the alpha 3 and beta 2 subunits (yielding the rat alpha 3/chick beta 2 and chick alpha 3/rat beta 2 receptors). Nicotine (10 microM) strongly inhibited both hybrid receptors. 5. Chimeric subunits were constructed by exchanging a segment located in the extracellular N-termini of chick alpha 3 and alpha 4 subunits and chick alpha 3 and rat alpha 3 subunits. These subunits were coexpressed in oocytes with chick or rat beta 2 subunits. The effect of nicotine on these receptors pointed to the importance of a 15 amino acid stretch located 3' of the first transmembrane segment in the determination of the agonist and antagonist action of nicotine. 6. Within this 15 amino acid segment, a single residue differs in chick and rat alpha 3 subunits, at position 198, within the ligand binding site of alpha subunits. Gln198 of the rat alpha 3 subunit was replaced by Thr as found in the chick alpha 3 subunit, using site-directed mutagenesis. The mutant subunit was coexpressed with the rat beta 2 subunit, yielding receptors which were inhibited by nicotine. 7. It is concluded that both alpha and beta subunits not only alter considerably the sensitivity of the nicotinic receptor to agonists and antagonists, but also can turn an agonist into a potent antagonist of one receptor subtype.(ABSTRACT TRUNCATED AT 400 WORDS)

Pentameric structure and subunit stoichiometry of a neuronal nicotinic acetylcholine receptor.

Neuronal nicotinic acetylcholine receptors are members of a gene family of ligand-gated transmitter receptors that includes muscle nicotinic receptors, GABAA receptors and glycine receptors. Several lines of evidence indicate that neuronal nicotinic receptors can be made up of only two subunits, an alpha (alpha) subunit which binds ligand, and a non-alpha (n alpha) or beta (beta) subunit. The stoichiometry of each subunit in the functional receptor has been difficult to assess, however. Estimates of the molecular weight of neuronal nicotonic receptor macromolecules suggest that these receptors contain at least four subunits but probably not more than five. We have examined the subunit stoichiometry of the chick neuronal alpha 4/n alpha 1 receptor by first using site-directed mutagenesis to create subunits that confer different single channel properties on the receptor. Co-injection with wild-type and mutant subunits led to the appearance of receptors with wild-type, mutant and hybrid conductances. From the number of hybrid conductances, we could deduce the number of each subunit in the functional receptor.

Conservation of neural nicotinic acetylcholine receptors from Drosophila to vertebrate central nervous systems.

Nicotinic acetylcholine receptors (nAChR) are found both in vertebrate and insect central nervous systems. We have isolated a Drosophila gene by crosshybridization with a vertebrate probe. Structural conservation of domains of the deduced protein and of intron/exon boundaries indicate that the Drosophila gene encodes an nAChR alpha-like subunit (ALS). That the Drosophila gene product most resembles the neuronal set of vertebrate nAChRs alpha-subunits is also indicated by the failure of an ALS-beta-galactosidase fusion protein to bind alpha-bungarotoxin on blots in contrast to vertebrate endplate alpha-subunit constructions. The ALS encoding gene exceeds 54 kb in length and the transcript has a very long and unusual 5' leader. As we found previously for a gene whose product is also involved in cholinergic synapses, acetylcholinesterase, the leader encodes short open reading frames, which might be involved in translation control. We also note the presence of opa repeats in the gene, as has been found for various Drosophila genes expressed in the nervous system.