Properties of monoclonal antibodies to nicotinic acetylcholine receptor from chick muscle

Myasthenia gravis: an autoimmune response against the acetylcholine receptor

Myasthenia gravis (MG) is an organ-specific autoimmune disease caused by an antibody-mediated assault on the muscle nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Binding of antibodies to the AChR leads to loss of functional AChRs and impairs the neuromuscular signal transmission, resulting in muscular weakness. Although a great deal of information on the immunopathological mechanisms involved in AChR destruction exists due to well-characterized animal models, it is not known which etiological factors determine the susceptibility for the disease. This review gives an overview of the literature on the AChR, MG and experimental models for this autoimmune disease.

Effects of electrical stimulation and tetrodotoxin paralysis on antigenic properties of acetylcholine receptors in rat skeletal muscle

To examine the role of muscle activity in the expression of fetal- and adult-type acetylcholine receptors (AChRs), we studied the effects of muscle stimulation in cell culture and of tetrodotoxin (TTX)-induced paralysis and denervation in adult rat muscles. The AChR content of these muscles was determined using [125I]alpha-bungarotoxin and the proportion of fetal-type receptors was estimated using a radioimmunoprecipitation assay with a myasthenic serum that was highly specific for fetal-type receptors. We found that both stimulated, aneural muscle cells in vitro and inactive muscles in vivo produced predominantly fetal-type AChRs. However the TTX-paralysed muscles had a lower proportion of fetal-type receptors than the denervated muscles. We conclude that neither muscle activity nor innervation alone, but a combination of both, is required for full regulation of AChR antigenicity.

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.

Loss of antigenic properties of acetylcholine receptors in rat skeletal muscle after birth

Antibodies in a myasthenic serum were used to follow the changes in antigenic properties of acetylcholine receptors in rat skeletal muscle during development. In binding assays at saturating concentrations of antigen or antibody, the antibodies reacted with extrajunctional receptors of fetal and denervated adult muscle but showed little binding to junctional receptors of adult rat muscle. They did, however, bind to junctional receptors of adult chicken muscle which, unlike rat receptors, do not appear to undergo a change in their channel properties during development. Binding studies with acetylcholine receptors of developing rat muscle carried out at saturating concentrations of antibody showed that the loss of antigenic determinant(s) begins at 1-2 days after birth.

Action of antibodies directed against the acetylcholine receptor on channel function at mouse and rat motor end-plates

1. The acute effects of antibodies (both polyclonal and monoclonal) raised against the acetylcholine receptor were studied at mouse and rat end-plates. Isolated muscles were incubated in solutions containing antibody for 2 1/4 to 3 1/2 h. Intracellular microelectrode techniques were then used to record miniature end-plate potentials (MEPPs) and voltage noise. 2. Most antibody preparations investigated did not reduce MEPP amplitudes as compared with controls. One monoclonal (C7) and one polyclonal (J) preparation irreversibly reduced MEPP amplitudes. Both preparations caused reductions in acetylcholine-induced depolarization and associated channel opening frequency (from voltage noise analysis). Single-channel depolarization was not altered by these antibodies. 3. On the basis of these and previous results, four antibody binding regions on the receptor surface were distinguished according to whether channel function and/or alpha-bungarotoxin binding were affected. Although most antibody preparations did not affect channel function, monoclonal antibody C7 appeared to alter function by acting on the channel itself so as to prevent channel opening.

Reversal of snake neurotoxin binding to mammalian acetylcholine receptor by specific antiserum

Snake curaremimetic toxins are known to bind to the nicotinic acetylcholine receptor (AcChoR) [Changeux et al. (1970) Proc. Natl Acad. Sci. USA, 67, 1241-1247], thus blocking neuromuscular transmission, and producing respiratory failure in mammals. In the present paper we show that the toxic effects of Naja nigricollis toxin alpha to mammals can be efficiently reversed by toxin-alpha-specific antibodies. In vivo we observed that return to normal breathing in toxin-alpha-intoxicated and ventilated rats was 12 times faster after injection of specific antiserum or monoclonal antibody (M-alpha 1) as compared with control animals. Ex vivo we observed that return to normal contraction of a toxin-alpha-blocked phrenic nerve-hemidiaphragm preparation was 14 times more rapid after treatment with specific antiserum than after washings. In vitro we observed that antibodies accelerated the reversal of binding of [3H]toxin alpha to AcChoR prepared from rat diaphragm. The observation made in vitro furthermore indicates that antibodies are capable of destabilizing the [3H]toxin-AcChoR complex. A similar destabilization phenomenon occurs also in vivo, as inferred from measurements of receptor occupancy by [3H]toxin alpha in diaphragm of anaesthetized rats in the presence or absence of antibodies. The property of antibodies to reverse neurotoxin binding to AcChoR may be considered as a critical test for evaluation of the quality of a neurotoxin-specific antisera.

Molecular studies of the neuronal nicotinic acetylcholine receptor family

Nicotinic acetylcholine receptors on neurons are part of a gene family that includes nicotinic acetylcholine receptors on skeletal muscles and neuronal alpha bungarotoxin-binding proteins that in many species, unlike receptors, do not have an acetylcholine-regulated cation channel. This gene superfamily of ligand-gated receptors also includes receptors for glycine and gamma-aminobutyric acid. Rapid progress on neuronal nicotinic receptors has recently been possible using monoclonal antibodies as probes for receptor proteins and cDNAs as probes for receptor genes. These studies are the primary focus of this review, although other aspects of these receptors are also considered. In birds and mammals, there are subtypes of neuronal nicotinic receptors. All of these receptors differ from nicotinic receptors of muscle pharmacologically (none bind alpha bungarotoxin, and some have very high affinity for nicotine), structurally (having only two types of subunits rather than four), and, in some cases, in functional role (some are located presynaptically). However, there are amino acid sequence homologies between the subunits of these receptors that suggest the location of important functional domains. Sequence homologies also suggest that the subunits of the proteins of this family all evolved from a common ancestral protein subunit. The ligand-gated ion channel characteristic of this superfamily is formed from multiple copies of homologous subunits. Conserved domains responsible for strong stereospecific association of the subunits are probably a fundamental organizing principle of the superfamily. Whereas the structure of muscle-type nicotinic receptors appears to have been established by the time of elasmobranchs and has evolved quite conservatively since then, the evolution of neuronal-type nicotinic receptors appears to be in more rapid flux. Certainly, the studies of these receptors are in rapid flux, with the availability of monoclonal antibody probes for localizing, purifying, and characterizing the proteins, and cDNA probes for determining sequences, localizing mRNAs, expressing functional receptors, and studying genetic regulation. The role of nicotinic receptors in neuromuscular transmission is well understood, but the role of nicotinic receptors in brain function is not. The current deluge of data using antibodies and cDNAs is beginning to come together nicely to describe the structure of these receptors. Soon, these techniques may combine with others to better reveal the functional roles of neuronal nicotinic receptors.

Interactions of anti-nicotinic acetylcholine receptor antibodies at alpha-bungarotoxin binding sites across species and tissues

Two antisera prepared against the nicotinic acetylcholine receptor (nAcChoR) from Electrophorus exhibit comparable ability to inhibit high-affinity alpha-bungarotoxin binding to membrane fractions from rat brain or muscle, PC12 or TE671 cells, or Torpedo electric tissue. Only one of several monoclonal antibodies raised against nAcChoR from Torpedo inhibits toxin binding to membranes from rat brain or muscle or TE671 cells, but is considerably more potent as an inhibitor of toxin binding to Torpedo nAcChoR. These results indicate that some antibodies prepared against nAcChoR from electric tissue recognize epitopes near the high-affinity toxin binding sites. Some of these toxin binding site epitopes are preserved across species and tissues. The positive outcome of this study supports the continued use of toxin as a probe for at least a subset of mammalian neuronal nAcChoR.

Monoclonal antibodies that distinguish between normal and denervated human acetylcholine receptor

Ten monoclonal anti-human acetylcholine receptor (AChR) monoclonal antibodies (m.abs) all exhibited high avidity binding to the human AChR. None was able to inhibit alpha-bungarotoxin (alpha-Butx) binding to the receptor. Five distinct but partially overlapping antibody-binding regions were defined by competition experiments. Four antibodies, which competed with each other for one region on denervated human AChR and also bound to human fetal AChR, failed to bind appreciably to normal human AChR in solution, to normal AChR solubilized from 6 other species, or to human endplates in frozen sections.

High- and low-affinity binding of [3H]acetylcholine at nicotinic cholinergic receptors in rat brain

There is both high-affinity and low-affinity nicotinic cholinergic binding of [3H]acetylcholine [( 3H]ACh) in rat brain membrane preparations. As determined by a filtration binding assay, [3H]ACh bound with Kd = 36.0 +/- 8.4 nM and Bmax = 19.4 +/- 4.5 fmol/mg protein or 3.3 +/- 0.7 fmol/mg tissue for high-affinity binding and Kd about 10(-7) to 10(-6) M and Bmax about 6-10 fmol/mg tissue or 40-60 fmol/mg protein for low-affinity binding. d-Tubocurarine (1 mM) inhibits high- as well as low-affinity binding, whereas 10 microM alpha-bungarotoxin does not compete at both binding sites. Substance P had no effect on the binding parameters of high-affinity nicotinic cholinergic binding.

Monoclonal antibodies to Torpedo acetylcholine receptor. Characterisation of antigenic determinants within the cholinergic binding site

Thirteen monoclonal antibodies (mAb) to the acetylcholine receptor (AChR) from Torpedo marmorata showed high avidity for the receptor but none exhibited binding to muscle AChR solubilised from seven other animal species. Five mAb and Fab monomer fragments prepared from two of them, inhibited alpha-bungarotoxin (alpha BuTx) binding to receptor by a maximum of 50%. In the presence of excess mAb the 125I-alpha BuTx bound could be precipitated by anti-IgG indicating that the mAb bound to only one of the two alpha BuTx binding sites on each AChR monomer. This site appeared to have a lower affinity for d-tubocurarine and decamethonium than the non-mAb site. Binding of five anti-site mAb was mutually competitive and four of them (AS2-AS5) were inhibited by other cholinergic ligands and influenced by four non-toxin binding site antibodies. One (AS1) bound within the toxin binding site yet outside the main neurotransmitter binding region. It is concluded that these five mAb distinguish between the two alpha BuTx binding sites on the Torpedo AChR, and bind only to the site which displays lower affinity for d-tubocurarine and other competitive ligands.

Brain and muscle nicotinic acetylcholine receptors are different but homologous proteins

An alpha-bungarotoxin-binding protein was purified from chick optic lobe and brain by an improved method. Previous and present observations justify its designation as a brain nicotinic acetylcholine receptor (AcChoR). It contains subunits whose apparent molecular weights are somewhat larger than those of subunits of peripheral AcChoRs. The size of the optic lobe AcChoR complex is greater than that of the peripheral receptor when estimated from its sedimentation behavior. Brain AcChoR subunits can be specifically precipitated by a monoclonal antibody directed against chick muscle AcChoR. Amino-terminal amino acid sequence analysis was performed on AcChoR preparations and isolated subunits from the optic lobe and from the rest of the chick brain. The sequences obtained demonstrate that, at least for the lowest molecular weight component, the AcChoRs from different brain areas are identical and they are highly homologous to muscle AcChoR. It is concluded that the brain alpha-bungarotoxin-binding protein is indeed a nicotinic AcChoR and is encoded by a set of genes that is different from, but strongly related to, that for the muscle AcChoR.