Cell-free synthesis of acetylcholine receptor polypeptides

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.

Acetylcholine receptor antibody: clinical and experimental aspects

Anti-acetylcholine (ACh) receptor antibody is the specific antibody in myasthenia gravis (MG). Groups of patients distinguished by thymic pathology and age of onset have shown differences in sex and HLA antigen incidence and in anti-ACh receptor antibody levels. Group differences in the characteristics of this antibody, including the percentage of kappa and gamma light chains, IgG subclass and reactivity with other ACh receptor preparations, were detected only in patients with ocular MG. This group alone showed a possible association with Gm allotype in Caucasians; the anti-ACh receptor antibody had a greater proportion of kappa light chain and better reactivity with human ocular ACh receptor than did generalized MG. The results indicate heterogeneity of this disease. Thymic cells from myasthenic patients with thymic hyperplasia spontaneously synthesize anti-ACh receptor antibody in culture and, after irradiation to abrogate antibody production and any suppressor effects, can selectively enhance the synthesis of anti-ACh receptor antibody by autologous blood lymphocytes in co-culture. The cell types that underlie these responses have been investigated by deleting cell subsets by complement-mediated lysis using monoclonal antibodies. Neither cortical (NA1/34+) thymocytes nor mature T cells (MBG6+) are essential for antibody production in vitro by thymic cells. The enhancement of antibody production by irradiated thymic cells may depend on antigen-presenting cells not expressing the HLA-DR surface marker, or possibly antigen-specific helper T cells, or both.

Repression of nicotinic acetylcholine receptor expression by antisense RNAs and an oligonucleotide

Four antisense RNAs, synthesized from cDNA clones coding for the four subunits of the acetylcholine receptor of Torpedo electroplaques, were used to study their effect on the expression of functional Torpedo acetylcholine receptors in Xenopus oocytes. All antisense RNAs inhibited the appearance of functional receptors in the oocyte's surface membrane for at least 1 week. This inhibition was specific because the antisense RNAs did not block the expression of the Cl- channels, also encoded by Torpedo electroplaque mRNA. Experiments with incomplete antisense RNAs and a synthetic oligonucleotide indicate that covering the ribosome binding site or the initiation codon in the mRNA is not a necessary requirement for efficient blocking. Thus, the use of antisense RNAs combined with the Xenopus oocyte system provides a novel approach to screen cDNA libraries for the genes coding for multisubunit neurotransmitter receptors.

Creatine kinase protein sequence encoded by a cDNA made from Torpedo californica electric organ mRNA

Creatine kinase (ATP creatine N-phosphotransferase, EC 2.7.3.2) is important in the maintenance of ATP levels in high energy-requiring tissues such as muscle and brain. A complete understanding of its function requires knowledge of its amino acid sequence. To obtain cDNA clones encoding creatine kinase sequences, a cDNA bank was constructed using mRNA from the electric organ of Torpedo californica and was screened by comparing differential colony hybridization of electric organ and liver-derived 32P-labeled cDNAs. Cloned DNAs have been isolated that can arrest the abundant synthesis of Mr 40,000-43,000 material seen after in vitro translation of electric organ mRNA. One of the clones, CK52g8, was sequenced by the dideoxy M13 method and was found to encode a Mr 42,941 protein, which is 68% homologous to a known partial sequence of rabbit muscle creatine kinase and which has a composition similar to creatine kinases from chicken and rabbit tissues. By contrast, no significant homology was found with the known sequences of kinases that use other substrates. RNA blot hybridization analysis indicated that CK52g8 is complementary to a 1600-base-pair mRNA. Primer extension analysis indicated that CK52g8 is only 5 nucleotides short of a full-length cDNA, implying that it encodes a complete protein sequence. The availability of this complete sequence should be useful in further studies of creatine kinase structure and function using techniques such as site-specific mutagenesis.

Acetylcholine receptor: an allosteric protein

The nicotine receptor for the neurotransmitter acetylcholine is an allosteric protein composed of four different subunits assembled in a transmembrane pentamer alpha 2 beta gamma delta. The protein carries two acetylcholine sites at the level of the alpha subunits and contains the ion channel. The complete sequence of the four subunits is known. The membrane-bound protein undergoes conformational transitions that regulate the opening of the ion channel and are affected by various categories of pharmacologically active ligands.

Cell surface expression of murine, rat, and human Fc receptors by Xenopus oocytes

We report that Xenopus laevis oocytes can efficiently translate and insert heterologous membrane receptors into the oocyte plasma membrane, where they can be detected by the binding of either monoclonal antibodies or ligands. Thus, oocytes injected with mRNA from the mouse J774 macrophage-like cell line, the rat RBL-1 basophilic leukemia, and the U937 promonocyte cell line, bound 2.4G2 Fab, rat IgE, and mouse IgG2a, respectively. The increase in the high avidity Fc gamma R observed after gamma-interferon induction of U937 cells was also observed after injection of mRNA from gamma-interferon-induced U937 cells into oocytes. This suggests either much greater message stability or a greater rate of transcription of Fc gamma Rhi mRNA in the gamma-interferon-induced cells. The assay affords a sensitive method for the detection of rare mRNA species that code for plasma membrane proteins.

[Application of the technics of molecular genetics in neurochemistry]

Research in molecular neurobiology has recently entered a new phase of rapid development as a result of the application of the techniques of molecular genetics. This is illustrated by recent work on the electric ray (Torpedo marmorata and T. californica), whose electric organ is a rich source of cholinergic synapses. Other examples from recent literature of the application of the recombinant DNA technique to the mammalian central nervous system are given and possible future developments are discussed.

The electric organ of Discopyge tschudii: its innervated face and the biology of acetylcholinesterase

An ultrastructural, histochemical, and biochemical study of the electric organ of the South American Torpedinid ray, Discopyge tschudii, was carried out. Fine structural cytochemical localization of acetylcholinesterase (AChE) indicated that most of the esterase was associated with the basal lamina. Electron microscopy indicated no marked differences in the electrocyte ultrastructure between Discopyge and Torpedo californica. Discopyge electric organ possessed three molecular forms, two asymmetric forms (16 S and 13 S) and one globular hydrophobic form (6.5 S). The asymmetric 16 S AChE form was solubilized by heparin, a sulfated glycosaminoglycan, suggesting that heparin-like macromolecules are involved in the binding of the enzyme to the basal lamina. Our results show that cell-free translated AChE peptides, synthesized using Discopyge electric organ poly(A+) RNA, correspond to a main band of 62,000 daltons which probably represents the catalytic subunit of the asymmetric AChE.

A system for the translation of receptor messenger-RNA and the study of the assembly of functional receptors

The Xenopus oocyte translation system has been developed for the recognition of receptor messenger-RNA molecules. Observations with the nicotinic acetylcholine, GABA, glycine, glutamate and serotonin receptors show that the mRNAs coding for their subunits can be faithfully translated in the oocyte and the products processed, glycosylated, assembled to a receptor structure, inserted correctly in the cell membrane and organised to form the functional ion channel. The specific regulatory interactions between different sites on one receptor, as exemplified in the GABAA receptor, are established in this process. This system is of value for the detection of mRNAs for receptor gene cloning and for the study of receptor assembly and of the expression of multiple receptor genes. The receptor ion channels formed can also be studied in their own right in a highly accessible and controlled situation.

A method for isolation of intact, translationally active ribonucleic acid

A method for isolation of large, translationally active RNA species is presented. The procedure involves homogenization of cells or tissues in 5 M guanidine monothiocyanate followed by direct precipitation of RNA from the guanidinium by 4 M LiCl. Modifications are described for use with tissue culture cells, yeast, tissues, or isolated nuclei. The advantages of the procedure include speed, simplicity, avoidance of an ultracentrifugation, and its applicability to large numbers of small samples. The procedure yields large mRNA precursors up to 10 kb and mRNA species which translate very well. However, small (less than 300 nucleotides) RNA species are recovered with a poor yield.

The A12 acetylcholinesterase and polypeptide composition of electric organ basal lamina of Electrophorus and some Torpedinae fishes

Basal lamina (BL) of Torpedo, Discopyge and Electrophorus electric organs was purified in order to establish polypeptide composition and association with acetylcholinesterase (AChE). Results indicate that BL presents a distinct peptide pattern and that the A12 form of AChE is directly attached to it. Comparison of the species studied demonstrated similarities both in polypeptide composition and AChE content of the purified BL. Extractions of BL with solutions of high ionic strength, guanidine-HCl and acetic acid indicated the differential solubilization of various domains of BL polypeptides.

Differentiation-dependent changes of nicotinic synapse-associated proteins

Developmentally regulated changes were followed by analyzing the protein composition in vivo of the electric organ of Torpedo marmorata. A 45 000-Mr component, most likely a form of actin, is found to decrease during synaptogenesis, whereas a 43 000-Mr component increases significantly at later embryonic stages, to become the most abundant protein of electric organ. The 43 000-Mr polypeptides are heterogeneous in their solubilization properties and isoelectric points. Translation in vitro of mRNA isolated from embryonic electric organ shows that the appearance of these proteins during development is regulated by the amount of translatable mRNA available. The close correlation between the translatable amounts of mRNA in vitro and the protein synthesis observed in vivo during synaptogenesis suggests that the functional maturation of the electric organ is linked to the appearance of 43 000-Mr polypeptides.

Analysis of brain and pituitary RNA metabolism: a review of recent methodologies

Recent characterization of brain and pituitary RNA metabolism is reviewed. Relative to other tissues, the brain transcribes more of the unique, single-copy DNA. This transcriptional diversity reflects the inherent heterogeneity in organization and development of the brain. The end product of transcriptional regulation in the brain is a population of functional cytoplasmic mRNAs with multiple components, differing in complexity and abundance. Analysis of nuclear and cytoplasmic RNA provides evidence that both brain-specific synthesis and processing may determine the final mRNA population. Both polyadenylated and non-polyadenylated RNA classes contribute significantly to the total brain polysomal mRNA fraction. Characterizations of individual species of mRNA from both brain and pituitary are described. One possible transcriptional modulator in both the pituitary and brain is the presence of steroid hormone at responsive sites. Functional consequences of steroid accumulation within the brain may be (1) interactions with neurotransmitter, especially catecholamine, metabolism and function, (2) developmental interactions with neuronal systems, and (3) differentiation of glial cell function. The pleiotropic nature of steroid hormone effects (both transcriptional and non-transcriptional) within one brain region is considered by examining the biochemical effects of glucocorticoids in the hippocampus.

Biosynthesis of acetylcholine receptor in vitro

Torpedo acetylcholine receptor is accessible to analysis by cell-free protein synthesis, like other membrane proteins studied previously. The factors deserving particular attention in this system are the small amounts of messenger RNA contained in electroplax tissue, the inherent complexity of an oligomeric membrane protein (necessitating the use of subunit-specific antibodies), the anomalous electrophoretic behavior of the primary translation products (attributable in part to the high--SH content of sAChRs), and the multiple glycosylated forms that appear upon processing by microsomal membranes. Although not all these complexities can be satisfactorily explained, they have been empirically characterized and can be well controlled if the appropriate procedures are followed.

The molecular cloning and characterisation of cDNA coding for the alpha subunit of the acetylcholine receptor

A rare cDNA coding for most of the alpha subunit of the Torpedo nicotinic acetylcholine receptor has been cloned into bacteria. The use of a mismatched oligonucleotide primer of reverse transcriptase facilitated the design of an efficient, specific probe for recombinant bacteria. DNA sequence analysis has enabled the elucidation of a large part of the polypeptide primary sequence which is discussed in relation to its acetylcholine binding activity and the location of receptor within the plasma membrane. When used as a radioactive probe, the cloned cDNA binds specifically to a single Torpedo mRNA species of about 2350 nucleotides in length but fails to show significant cross-hybridisation with alpha subunit mRNA extracted from cat muscle.

Molecular cloning of cDNA coding for the gamma subunit of Torpedo acetylcholine receptor

From the electric organ of Torpedo californica, we purified mRNA that, when translated in vitro, produces polypeptides immunoprecipitable by antibodies against purified acetylcholine receptor. A novel cloning system [Okayama, H. & Berg, P. (1982) Mol. Cell. Biol. 2, 161-170] was used to produce a cDNA library from this mRNA. This library contained clones with receptor sequences identified by differential hybridization and hybridization-selection. We describe a clone of 2,030 base pairs with sequences appropriate for the amino-terminal amino acids of the gamma subunit of acetylcholine receptor. This clone contains 82 bases 5' of the codon for the amino-terminal amino acid of the mature protein. A portion of this sequence codes for a methionine followed by a 16-amino acid polypeptide that is contiguous to the amino-terminal amino acid of the mature protein and that has the characteristics of a leader peptide. The cDNA insert hybridizes to a 2,100-base RNA present in electric organ but not in the brain of T. californica.

Translation of exogenous messenger RNA coding for nicotinic acetylcholine receptors produces functional receptors in Xenopus oocytes

Messenger RNA extracted from the electric organ of Torpedo was injected into Xenopus oocytes. This led to the synthesis and incorporation of functional acetylcholine receptors into the membrane of the oocyte. When activated by acetylcholine these Torpedo acetylcholine receptors in the oocyte membrane opened channels whose ionic permeability resembled that of nicotinic receptors in other cells.

In vitro synthesis, glycosylation, and membrane insertion of the four subunits of Torpedo acetylcholine receptor

We have characterized the early biosynthetic forms of the Torpedo electroplax acetylcholine receptor by using a cell-free protein synthesizing system. We obtained primary translation products of approximately 38, 50, 49, and 60 kilodaltons for the alpha, beta, gamma, and delta polypeptides, respectively, by using immunoprecipitation with subunit-specific antisera. These chains could each be labeled by the formylated initiator [35S]Met-tRNA. On cotranslational incubation with pancreatic rough microsomes, glycosylated forms of each subunit were obtained that had molecular weights close to those of their mature authentic counterparts. Extensive trypsinization reduced the glycosylated forms of the receptor subunits to glycosylated membrane-protected fragments of approximately 35 (alpha), 37 (beta), 45 (gamma), and 44 (delta) kilodaltons. In this system, then, each receptor chain spans the membrane at least once. This in vitro-synthesized material apparently exhibited neither oligomeric assembly nor alpha-bungarotoxin binding.

Structural models of the nicotinic acetylcholine receptor and its toxin-binding sites

Models of the protein structure of agonist-, competitive antagonist-, and snake neurotoxin-binding sites were designed using the sequence of the first 54 residues of the acetylcholine receptor (AChR) alpha subunit from Torpedo californica. These models are based on the premise that the N-terminal portions of the subunits form the outermost extracellular surface of the AChR and that agonists bind to this portion. The models were developed by predicting the secondary structure of the alpha-subunit N-terminal segment from its sequence, then using these predictions to fold the segment into tertiary structures that should bind snake neurotoxins, agonists, and antagonists. Possible gating mechanisms and quaternary structures are suggested by the proposed tertiary structures of the subunits. Experiments are suggested to test aspects of the models.