Expression of the Four Subunits of the Torpedo Californica Nicotinic Acetylcholine Receptor in Saccharomyces cerevisiae

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.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

Functional expression in frog oocytes of human rho 1 receptors produced in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae was engineered to express the rho 1 subunit of the human gamma-aminobutyric acid rho 1 (GABA rho 1) receptor. RNA that was isolated from several transformed yeast strains produced fully functional GABA receptors in Xenopus oocytes. The GABA currents elicited in the oocytes were fast, nondesensitizing chloride currents; and the order of agonist potency was GABA > beta-alanine > glycine. Moreover, the receptors were resistant to bicuculline, strongly antagonized by (1,2,5,6 tetrahydropyridine-4-yl)methylphosphinic acid, and modulated by zinc and lanthanum. Thus, the GABA receptors expressed by the yeast mRNA retained all of the principal characteristics of receptors expressed by cRNA or native retina mRNAs. Western blot assays showed immunoreactivity in yeast plasma membrane preparations, and a rho 1-GFP fusion gene showed mostly intracellular distribution with a faint fluorescence toward the plasma membrane. In situ immunodetection of rho 1 in yeast demonstrated that some receptors reach the plasma membrane. Furthermore, microtransplantation of yeast plasma membranes to frog oocytes resulted in the incorporation of a small number of functional yeast rho 1 receptors into the oocyte plasma membrane. These results show that yeast may be useful to produce complete functional ionotropic receptors suitable for structural analysis.

Future Therapeutic Strategies in Autoimmune Myasthenia Gravis

Antibodies against muscle acetylcholine receptor (AChR) undoubtedly play a critical role in the pathology of most myasthenia gravis (MG) cases. Selective elimination of the majority of these antibodies should result in a considerable improvement of the MG symptoms. Such a specific elimination could be achieved by AChR-based immunoadsorbents. However, sufficient quantities of native human AChR are not available while bacterially expressed recombinant domains of the AChR are unable to bind satisfactorily MG antibodies. We have undertaken the production of the extracellular domains of human AChR subunits in eukaryotic systems, in native-like conformation, for their use as potent immunoadsorbents. The N-terminal extracellular domain (amino acids 1-210; alpha(1-210)) of the alpha(1) subunit of the human muscle AChR was expressed in the yeast Pichia pastoris. The polypeptide was water-soluble, glycosylated, and in monomer form. The alpha(1-210) bound 125I-alpha-bungarotoxin (125I-alpha-BTX) with a high affinity (Kd = 5.1 +/- 2.4 nM), and this binding was blocked by unlabeled d-tubocurarine and gallamine. Several conformation-dependent anti-AChR antibodies were able to bind alpha(1-210) as did antibodies from a large proportion of MG patients. The purified protein was subsequently immobilized on Sepharose-CNBr and was used to immunoadsorb anti-AChR antibodies from 64 MG sera. It eliminated more than 50% (50-94%) of the anti-AChR antibodies in 20% of the sera, whereas from another 30% of the sera it eliminated 20-60% of their anti-AChR antibodies. Work is in progress for the expression of the extracellular domain of all other muscle AChR subunits. It is expected that their combined use may eliminate the great majority of the anti-AChR antibodies from most MG patients.

Yeast expression and NMR analysis of the extracellular domain of muscle nicotinic acetylcholine receptor alpha subunit

The alpha subunit of the nicotinic acetylcholine receptor (AChR) from Torpedo electric organ and mammalian muscle contains high affinity binding sites for alpha-bungarotoxin and for autoimmune antibodies in sera of patients with myasthenia gravis. To obtain sufficient materials for structural studies of the receptor-ligand complexes, we have expressed part of the mouse muscle alpha subunit as a soluble, secretory protein using the yeast Pichia pastoris. By testing a series of truncated fragments of the receptor protein, we show that alpha211, the entire amino-terminal extracellular domain of AChR alpha subunit (amino acids 1-211), is the minimal segment that could fold properly in yeast. The alpha211 protein was secreted into the culture medium at a concentration of >3 mg/liter. It migrated as a 31-kDa polypeptide with N-linked glycosylation on SDS-polyacrylamide gel. The protein was purified to homogeneity by isoelectric focusing electrophoresis (pI 5.8), and it appeared as a 4.5 S monomer on sucrose gradient at concentrations up to 1 mm ( approximately 30 mg/ml). The receptor domain bound monoclonal antibody mAb35, a conformation-specific antibody against the main immunogenic region of the AChR. In addition, it formed a high affinity complex with alpha-bungarotoxin (k(D) 0.2 nm) but showed relatively low affinity to the small cholinergic ligand acetylcholine. Circular dichroism spectroscopy of alpha211 revealed a composition of secondary structure corresponding to a folded protein. Furthermore, the receptor fragment was efficiently (15)N-labeled in P. pastoris, and proton cross-peaks were well dispersed in nuclear Overhauser effect and heteronuclear single quantum coherence spectra as measured by NMR spectroscopy. We conclude that the soluble AChR protein is useful for high resolution structural studies.

Identification of PK-A phosphorylation sites in the carboxyl terminus of L-type calcium channel alpha 1 subunits

Full length L-type calcium channel alpha 1 subunits are rapidly phosphorylated by protein kinase A (PK-A) in vitro and in vivo at sites located in their long carboxyl terminal tails. In skeletal muscle, heart, and brain the majority of biochemically isolated alpha 1 subunits lacks these phosphorylation sites due to posttranslational proteolytic processing. Truncation may therefore modify the regulation of channel activity by PK-A. We combined site-directed mutagenesis and heterologous expression to investigate the extent to which putative cAMP-dependent phosphorylation sites in the C-terminus of alpha 1 subunits from skeletal muscle, heart, and brain are phosphorylated in vitro. The full length size form of wild-type and mutant calcium channel alpha 1 subunits was obtained at high yield after heterologous expression in Saccharomyces cerevisiae. Like in fetal rabbit myotubes [Rotman, E.I., et al. (1995) J. Biol. Chem. 270, 16371-16377], the rabbit skeletal muscle alpha 1 C-terminus was phosphorylated at serine residues 1757 and 1854. In the carboxyl terminus of alpha 1S from carp skeletal muscle and alpha 1C from rabbit heart a single serine residue was phosphorylated by PK-A in vitro. The C-terminus of alpha 1D was phosphorylated at more than one site. Employing deletion mutants, most of the phosphorylation ( > 70%) was found to occur between amino acid residues 1805 and 2072. Serine 1743 was identified as additional phosphorylation site in alpha 1D. We conclude that in class S and C calcium channels the most C-terminal phosphorylation sites are substrate for PK-A in vitro, whereas in class D calcium channels phosphorylation also occurs at a site which is likely to be retained even after posttranslational truncation.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

Heterologous expression of the membrane proteins that control cellular excitability

Versatile and potent expression systems are needed to decipher the structure and functions of the many excitability proteins that have been identified through molecular cloning. This article reviews the use of recombinant vaccinia viruses (VV), which have been recently explored for the heterologous expression of eukaryotic proteins. Vaccinia viruses feature a series of favourable properties, most of all a broad host range and high efficiency of infection, that make them uniquely suited as flexible expression vectors. In one type of experiment, the recombinant virus simply harbors the cDNA for the foreign protein; in a second type the virus harbors the cDNA for the specific and efficient RNA polymerase of bacteriophage T7, which in turn generates RNA from a separate introduced plasmid or virus. Both variations have been successfully applied to the expression and analysis of voltage-dependent ion channels, neurotransmitter receptors and other excitability proteins in many cell lines and postmitotic cells in culture. VV vectors promise to be particularly useful to study membrane proteins that require posttranslational processing, association with cell-specific subunits or coupling to endogenous second messengers pathways.

Functional expression of rat M5 muscarinic acetylcholine receptor in yeast

We have produced the rat M5 muscarinic acetylcholine receptor, an integral membrane protein, in the yeast Saccharomyces cerevisiae. This was achieved by placing an M5 gene in the yeast vector under the control of the yeast alpha-factor promoter and leader sequence. Northern blotting revealed the presence of M5 transcripts in yeast transformed with the M5 plasmid constructs. Crude extract prepared from the transformant yeasts showed saturable binding of the muscarinic antagonist [3H]-N-methyl scopolamine ([3H]NMS) with a kd of 22.77 nM and Bmax of 134.76 fmole per mg protein. Results deduced from saturation binding assay of intact cell demonstrated clearly that the M5 receptor was translocated across the membrane of the endoplasmic reticulum using the secretion signal on alpha-leader sequence and its binding site was still functional. Yeast expressing M5 receptor did not exhibit cell-cycle arrest in the presence of carbachol, a acetylcholine agonist, indicating that the recombinant M5 receptor could not couple directly to the endogenous yeast pheromone signaling G-protein.

Receptors for neurotransmitters and related substances

Advances in techniques for cloning neurotransmitter receptors have revealed new targets for selective drug design. Cell systems for more efficient expression of cloned receptor genes have also been developed. Knowledge of the nature of ligand-binding sites is now becoming available and this should aid in the design of better drugs with fewer side effects.

Expression and characterization of the chick nicotinic acetylcholine receptor alpha-subunit in insect cells using a baculovirus vector

A baculovirus transfer vector was constructed containing an entire cDNA copy of the chick nicotinic acetylcholine receptor (nAChR) alpha-subunit under control of the Autographa californica nuclear polyhedrosis virus (AcNPV) polyhedrin gene promoter. Recombinant baculovirus was obtained by co-transfection of Spodoptera frugiperda cells with infectious, wild-type AcNPV DNA and the transfer vector. Polyhedrin-negative, recombinant viruses were identified which expressed the nAChR alpha-subunit. The insect cell-expressed alpha-subunit protein had a molecular mass of 42 kDa and was shown to be targeted to the plasma membrane by fluorescence microscopy and toxin-binding assays. The levels of expression were low, approximately 1-2% of cell proteins, when compared with the levels of natural polyhedrin protein. The expressed receptor alpha-subunit was recognised by polyclonal antisera raised against purified Torpedo nAChR alpha-subunit and carried the binding site for the snake venom toxin, alpha-bungarotoxin. Bound alpha-bungarotoxin was displaced in competition binding assays by alpha-cobra toxin, carbamylcholine and d-tubocurarine, and thus had a similar pharmacological profile to that obtained with authentic receptors in muscle cells and receptors expressed in other systems i.e. Xenopus oocytes and mammalian cells. We have also shown that when the chick nAChR alpha-subunit is expressed in the absence of other receptor subunits, unexpectedly high concentrations of nicotine (10 mM) were required to displace bound alpha-bungarotoxin.

Expression and pharmacological characterization of the human M1 muscarinic receptor in Saccharomyces cerevisiae

The yeast S. cerevisiae has been examined as a heterologous host for the expression of mammalian neurotransmitter receptors which couple to guanine nucleotide regulatory (G) proteins. A cloned gene encoding the M1 subtype of human muscarinic receptor (HM1) was transformed into S. cerevisiae on a high copy plasmid under the control of the promoter for the yeast alcohol dehydrogenase (ADH) gene. Northern blotting demonstrated the presence of HM1 transcripts in transformants, and crude membranes prepared from these cells showed saturable binding of the muscarinic antagonist [3H]N-methyl scopolamine with a Kd of 179 pM and Bmax of 20 fmol/mg protein. Competition binding studies revealed pharmacological properties for these sites which were comparable to those reported for the M1 site in mammalian tissues. Yeast expressing HM1 did not exhibit high affinity agonist binding or cell-cycle arrest in the presence of muscarinic agonists, indicating that the mammalian receptor did not couple to the endogenous yeast G protein.