Antigenic similarities between the subunits of acetylcholine receptor from Torpedo marmorata

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.

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.

Properties of monoclonal antibodies to nicotinic acetylcholine receptor from chick muscle

Four stable, hybrid-cell lines secreting monoclonal antibodies to distinct determinants on the nicotinic acetylcholine receptor from chick muscle have been established. These were characterised by the following criteria: immunoglobulin isotype, ability to produce experimental autoimmune myasthenia gravis in mice and reactivity towards homologous and heterologous acetylcholine receptor proteins. Two monoclonal antibodies were found to inhibit the reaction of alpha-bungarotoxin with homologous acetylcholine receptor; in addition one of these, on binding to receptor-toxin, induced a rapid dissociation of the complex (t1/2 = 0.5 h at 23 degrees C). Three of the antibody preparations recognised epitopes on this receptor from muscle of other species and two of these caused experimental autoimmune myasthenia gravis in BALB/c mice following passive transfer. The latter two recognised to significant extents the alpha-bungarotoxin binding component purified from chick optic lobe and brain cortex. Sedimentation analysis demonstrated that two of the monoclonal antibodies form a distinct size (s20, w = 12S) of complex with the receptor of chick muscle which most probably corresponds to a 1:1 attachment of antibody and receptor; this may involve cross-linking of two determinants within the same oligomer. A similar observation was made with the alpha-bungarotoxin binding component from optic lobe using one of the cross-reacting antibodies. Another monoclonal antibody was found to be capable of forming much heavier complexes with the receptor from chick muscle, these are thought to involve inter-molecular cross-linking of oligomers. The observed properties of these antibodies are discussed in relation to their myasthenogenicity and with reference to the extent of structural similarities between the peripheral nicotinic acetylcholine receptor and the alpha-bungarotoxin binding protein from brain.

Immunisation with Torpedo acetylcholine receptor

Acetylcholine mediates the transfer of information between neurons in the electric organ of, for example, Torpedo as well as in vertebrate skeletal muscle. The nicotinic acetylcholine receptor complex translates the binding of acetylcholine into ion permeability changes. This leads to an action potential in the muscle fibre. The nicotinic acetylcholine receptor protein has been purified from Torpedo by use of affinity chromatography. The receptor is an intrinsic membrane glycoprotein composed of five polypeptide chains. When various animals are immunised with the receptor they demonstrate clinical signs of severe muscle weakness coincident with high antibody titres in their sera. The symptoms resemble those found in the autoimmune neuromuscular disease myasthenia gravis in humans. This animal model has constituted a unique model for studying autoimmune diseases. This paper reviews some of the work using Torpedo acetylcholine receptor in order to increase the understanding of the motor nervous system function and myasthenia gravis. It is now known that the nicotinic acetylcholine receptor protein is the antigen involved in myasthenia gravis. The mechanism of immune damage involves a direct block of the receptor function. This depends on the presence of antibodies which crosslink the postsynaptic receptors leading to their degradation. The questions to be answered in the future are; (a) what initiates or triggers the autoimmune response, (b) how do the antibodies cause the symptoms--is there a steric hindrance of the interaction of acetylcholine and the receptor, (c) why is there not a strict relationship between antibody titre and severity of symptoms, and (d) why are some muscles affected and other spared? With help of the experimental model, answers to these questions may result in improved strategies for the treatment of the autoimmune disease myasthenia gravis.

Lack of cross-reactivity of a monoclonal antibody against the neurotransmitter site of Torpedo nicotinic acetylcholine receptor with muscle receptors from several sources

A monoclonal antibody raised against Torpedo nicotinic acetylcholine receptor has been used to study the neurotransmitter binding site of the acetylcholine receptor from several sources. When tested on Torpedo receptor, this monoclonal antibody inhibited binding of alpha-bungarotoxin to 50% of the available sites. The failure to completely inhibit binding of the toxin is attributed to the orientation of the determinant for the monoclonal antibody on the receptor molecule. This monoclonal antibody bound very poorly to the acetylcholine receptor from muscle tissues. This suggests either a modification or a reduced accessibility of the determinant to the monoclonal antibody.