Myasthenogenic significance of synthetic alpha-subunit peptide 183-200 of Torpedo californica and human acetylcholine receptor

Myasthenia Gravis: From the Viewpoint of Pathogenicity Focusing on Acetylcholine Receptor Clustering, Trans-Synaptic Homeostasis and Synaptic Stability

Myasthenia gravis (MG) is a disease of the postsynaptic neuromuscular junction (NMJ) where nicotinic acetylcholine (ACh) receptors (AChRs) are targeted by autoantibodies. Search for other pathogenic antigens has detected the antibodies against muscle-specific tyrosine kinase (MuSK) and low-density lipoprotein-related protein 4 (Lrp4), both causing pre- and post-synaptic impairments. Agrin is also suspected as a fourth pathogen. In a complex NMJ organization centering on MuSK: (1) the Wnt non-canonical pathway through the Wnt-Lrp4-MuSK cysteine-rich domain (CRD)-Dishevelled (Dvl, scaffold protein) signaling acts to form AChR prepatterning with axonal guidance; (2) the neural agrin-Lrp4-MuSK (Ig1/2 domains) signaling acts to form rapsyn-anchored AChR clusters at the innervated stage of muscle; (3) adaptor protein Dok-7 acts on MuSK activation for AChR clustering from “inside” and also on cytoskeleton to stabilize AChR clusters by the downstream effector Sorbs1/2; (4) the trans-synaptic retrograde signaling contributes to the presynaptic organization via: (i) Wnt-MuSK CRD-Dvl-β catenin-Slit 2 pathway; (ii) Lrp4; and (iii) laminins. The presynaptic Ca²⁺ homeostasis conditioning ACh release is modified by autoreceptors such as M1-type muscarinic AChR and A2A adenosine receptors. The post-synaptic structure is stabilized by: (i) laminin-network including the muscle-derived agrin; (ii) the extracellular matrix proteins (including collagen Q/perlecan and biglycan which link to MuSK Ig1 domain and CRD); and (iii) the dystrophin-associated glycoprotein complex. The study on MuSK ectodomains (Ig1/2 domains and CRD) recognized by antibodies suggested that the MuSK antibodies were pathologically heterogeneous due to their binding to multiple functional domains. Focussing one of the matrix proteins, biglycan which functions in the manner similar to collagen Q, our antibody assay showed the negative result in MG patients. However, the synaptic stability may be impaired by antibodies against MuSK ectodomains because of the linkage of biglycan with MuSK Ig1 domain and CRD. The pathogenic diversity of MG is discussed based on NMJ signaling molecules.

Effects of simvastatin on nuclear receptors, drug metabolizing enzymes and transporters expression in Human Umbilical Vein Endothelial Cells

BACKGROUND

Vascular endothelial cells (EC) are constantly exposed to endo- and exogenous compounds, which may disturb EC function. One of the protecting mechanisms against chemicals consists of drug metabolizing enzymes and transporter proteins regulated by nuclear receptors and transcription factors. Therefore, the aim of the current study was to assess the regulation of nuclear receptors and their coordinated genes in Human Umbilical Vein Endothelial Cells (HUVEC).

METHODS

HUVEC were exposed to TCDD (10 nM), oltipraz (100 μM) and simvastatin (1 μM) for 24 h. Gene expressions were evaluated using quantitative real-time PCR. The protein expression levels were determined by Western blotting. Enzymatic activity of CYP1A1/CYP1B1 was assessed by luciferin-labelled CYPs substrate.

RESULTS

Our study confirmed that nuclear receptor AhR and nuclear factor Nrf2 are highly expressed in HUVECs. Treatment of HUVECs with TCDD (AhR inducer) resulted in a significant induction of AHR target genes CYP1A1, CYP1B1 and NQO1. Oltipraz (Nrf2 inducer) also markedly increased expression of NQO1 but did not affect Nrf2 mRNA nor protein levels. Under simvastatin stimulation PXR and NRF2 target transcripts were not altered, however AHR-regulated genes: CYP1A1, CYP1B1 and MDR1 were significantly induced. Western blot analysis confirmed CYP1B1 induction in TCDD-treated HUVECs, but not in the simvastatin group. Moreover, HUVEC exposure to TCDD resulted in induction of CYP1A1/CYP1B1 enzymatic activity.

CONCLUSIONS

This study revealed functional expression of AhR and Nrf2 in HUVECs. Moreover, it was defined that simvastatin induced AhR and its related genes.

Extracorporeal Whole Blood Immunoadsorption of Autoimmune Myasthenia Gravis by Cellulose Tryptophan Adsorbent

Whole blood immunoadsorption (WBIA) system, using an adsorbent to remove pathogenic antibodies of myasthenia gravis (MG), was studied. Cellulose-tryptophan adsorbent was synthesized and its adsorption capacity of binding with acetylcholine receptor in the plasma of MG patient was evaluated. Experimental autoimmune myasthenia gravis (EAMG) rabbits were induced by Ta183-200 peptide. The rabbits underwent extracorporeal whole blood adsorption for 2 h. Results showed no significant damages on blood cells and no changes in the concentration of electrolytes. Total protein decreased by 12.0% (P < 0.05), and globulin protein decreased 23.9 +/- 5.6% (P < 0.05). The mean overall removal of antibodies against Ta183-200 was 41.12%. The percentage of decrement of compound muscle action potential in 3, 5, 10Hz of EAMG rabbits all dropped down after the treatment. In conclusion, the adsorbent is biocompatible, was safe for whole blood immunoadsorption, and can remove antibodies in an MG patient effectively. Whole blood immunoadsorption improved clinical manifestation and neuromuscular function of the EAMG rabbits.

Preparation of an immunoadsorbent coupled with a recombinant antigen to remove anti-acetylcholine receptor antibodies in abnormal serum

An immunoadsorbent that removes anti-acetylcholine receptor antibodies (AChRAb) in abnormal serum of myasthenia gravis (MG) patient was efficiently prepared by an expression product, the functional fragment of AChR(alpha205) fused with maltose binding protein (MBP). The ligand can then covalently bind to amylose resin through MBP fusion protein. It was shown from the result of this study with anti-AChR mice sera that the removal rate of AChRAb on this immunoadsorbent reached 87+/-10% (mean value of 10 mice) and the maximally binding capacity of AChRAb was approximately 260 microg/g immunoadsorbent (wet weight). Moreover, the immunoadsorption test of sera in two MG patients indicated that about 90% and 96% of abnormal AChRAb could be eliminated, while other serum components such as albumin, IgG, IgM and IgA only dropped 18%, 35%, 22%, 15% and 24%, 27%, 15%, 12%, respectively, for two MG patient sera. It is anticipated from this study that the immunoadsorbent reported here could, with further development, find its clinical application for removal of AChRAb from patient serum.

Removal of Pathogenic Autoantibodies by Immunoadsorption

Autoimmune diseases result from disrupted tolerance to self-antigens and subsequent damage to tissues and organs. In several diseases, specific autoantibodies have been either proved or suspected to play a role in this process. Consequently, several strategies have been devised in an attempt to discard the destructive immunoglobulins. Currently, both nonselective and epitope-specific methods are applied in several diseases. In this review, we provide a summary of the available data on elimination of pathogenic autoantibodies and discuss the advantages and pitfalls of the different approaches.

Breakdown of tolerance to a self-peptide of acetylcholine receptor alpha-subunit induces experimental myasthenia gravis in rats

Experimental autoimmune myasthenia gravis (EAMG), a model for human myasthenia (MG), is routinely induced in susceptible rat strains by a single immunization with Torpedo acetylcholine receptor (TAChR). TAChR immunization induces anti-AChR Abs that cross-react with self AChR, activate the complement cascade, and promote degradation of the postsynaptic membrane of the neuromuscular junction. In parallel, TAChR-specific T cells are induced, and their specific immunodominant epitope has been mapped to the sequence 97-116 of the AChR alpha subunit. A proliferative T cell response against the corresponding rat sequence (R97-116) was also found in TAChR-immunized rats. To test whether the rat (self) sequence can be pathogenic, we immunized Lewis rats with R97-116 or T97-116 peptides and evaluated clinical, neurophysiological, and immunological parameters. Clinical signs of the disease were noted only in R97-116-immunized animals and were confirmed by electrophysiological signs of impaired neuromuscular transmission. All animals produced Abs against the immunizing peptide, but anti-rat AChR Abs were observed only in animals immunized with the rat peptide. These findings suggested that EAMG in rats can be induced by a single peptide of the self AChR, that this sequence is recognized by T cells and Abs, and that breakdown of tolerance to a self epitope might be an initiating event in the pathogenesis of rat EAMG and MG.

Adsorption column for myasthenia gravis treatment: Medisorba MG-50

Adsorption column Medisorba MG-50 (Kuraray Medical Inc.) for the treatment of myasthenia gravis (MG) is introduced. The adsorbent in this column is composed of cellulose beads as carrier material and covalent-bound synthetic peptide as a ligand that has a specific affinity to the pathogenic anti-acetylcholine receptor antibody of MG. The amino acid sequence of the peptide is modified from the segment of alpha 183-200 of the torpedo acetylcholine receptor (AChR) protein, and the segment is the acetylcholine binding site on AChR and the target site of anti-AChR antibody. The adsorbent showed specific adsorption characteristics to the anti-ACHR antibody (blocking antibody) in vitro. Clinically, MG-50 is used in plasma-perfusion therapy, and it is recognized that MG-50 specifically reduces blocking antibody titer and improves MG symptoms. MG-50 is approved in Japan.

Immunoadsorption in myasthenia gravis based on specific ligands mimicking the immunogenic sites of the acetylcholine receptor

A specific system for antibody removal from blood circulation in myasthenia gravis (MG) patients was devised by use of the immunoadsorbent bound to an acetylcholine receptor (AChR) peptide that was synthesized corresponding to the sequence of residues 183-200 of the AChR alpha-subunit (alpha 183-200), antibodies which prevent the binding of ACh to AChR. The alpha 183-200 peptide was confirmed to be immunogenic for induction of an animal model of the disease and for reactivity with MG autoantibodies. We then made use of these results for immunoadsorption therapy through the antigen-antibody reaction on the molecular level, having given patients relief from myasthenic weakness. The greatest care was taken for the selection of an antigenic region in the molecular structure among various myasthenogenic domains of AChR and for the antigenic conformation of synthetic peptide as the adsorbent to react with antibodies raised against the native protein.

Current topics on immunoadsorption therapy

Immunoadsorption therapy has become popular for the treatment of various autoimmune diseases as the number of clinically available immunoadsorption columns increases. Immunoadsorption columns for plasma perfusion or direct hemoperfusion available for clinical use such as the Immusorba TR and PH, Selesorb, Prosorba, Immunosorba, Ig-Therasorb, and Medisorba MG are reviewed in the first part of this article. The peptide columns which immobilize peptides as the ligands are described in the second half. The peptide column has the potential to deliver specific adsorbents if a peptide is appropriately designed to remove the pathogenic substances specifically responsible for the given disease.

Molecular cloning of the canine nicotinic acetylcholine receptor alpha-subunit gene and development of the ELISA method to diagnose myasthenia gravis

We investigated the molecular structure of canine nicotinic acetylcholine receptor (AChR) alpha-subunit gene and developed an enzyme-linked immunosorbent assay (ELISA) as an immunological method to diagnose myasthenia gravis (MG). Canine AChR alpha-subunit cDNA was constructed from mRNA isolated from skeletal muscle of five dogs using the reverse transcriptase-polymerase chain reaction and its molecular structure was determined. The canine AChR alpha-subunit gene had 1371 base pairs encoding 457 amino acids and had a 96.1% homology to the human AChR alpha-subunit gene at the amino acid level. From the results of sequencing the DNA, specific antibodies to the acetylcholine binding domain of the canine AChR alpha-subunit were produced by immunizing rabbits with synthetic oligopeptides (alpha-subunit 183-200 amino acids). The specificity of the rabbit anti-oligopeptide serum was examined by Western blot analysis using an E. coli-expressed AChR alpha-subunit protein and an AChR alpha-subunit protein fraction prepared from canine skeletal muscle as an antigen. An ELISA assay was developed using oligopeptides corresponding to the binding domain to diagnose canine MG; specific antibodies were detected from two dogs with MG, one diabetic dog and two healthy dogs among 25 dogs examined. Further examinations of the ELISA using a large number of samples of clinically MG-positive and MG-negative dogs are needed to establish its usefulness in MG diagnosis.

Specific immunoadsorbent for myasthenia gravis treatment: development of synthetic peptide designed to remove antiacetylcholine receptor antibody

We have developed Medisorba MG, a new immunoadsorbent column for myasthenia gravis (MG). The alpha 183-200 segment of the Torpedo Californica acetylcholine receptor (AChR) is recognized as the acetylcholine binding site by the blocking antibody, which is one of the anti-AChR antibodies involved in the pathogenesis of MG. As a specific affinity ligand to remove the blocking antibody, Torpedo alpha 183-200 was synthesized and immobilized covalently to porous cellulose beads. This immunoadsorbent showed specific removal of the blocking antibody without reducing IgG and albumin levels significantly in clinical evaluation and in vitro study. Clinical improvement was found in 78% of the cases, and no adverse effects were observed in any case. The Medisorba MG column has been confirmed as a useful device for the treatment of MG.

Synthetic peptides fail to induce nasal tolerance to experimental autoimmune myasthenia gravis

Nasal administration of Torpedo acetylcholine receptor (AChR) to Lewis rats prior to induction of experimental autoimmune myasthenia gravis (EAMG) is highly efficient in prevention of clinical weakness, and suppression of AChR-specific T and B cell responses. To identify possible antigenic determinants within the receptor which can modulate EAMG and anti-AChR response, we evaluated the effects of nasal administration of alpha 61-76, alpha 100-116, alpha 146-162, delta 354-367, and alpha 261-277 of Torpedo AChR at different doses on the tolerance induction against EAMG irrespective if given at lower, the same or higher doses than whole Torpedo AChR protein, that was confirmed to be highly efficient as tolerogen to EAMG. None of these peptides, neither administrated alone nor in combination, induced tolerance to EAMG. Peptide administration did not affect the levels or affinities of anti-AChR antibodies when compared with non-tolerized control EAMG rats, while administration of whole AChR protein affected both variables. The results may indicate that the T and B cell heterogeneity of AChR epitopes makes it difficult to induce tolerance using synthetic peptide.

Comparative study of clinical effects between plasma adsorption and double filtration plasmapheresis in patients with myasthenia gravis

Plasma exchange (PE) has been one of the most powerful treatments for patients with myasthenia gravis (MG) since Pinching et al. reported its clinical usefulness in 1976, despite the need for supplemental human plasma. However, new apheresis techniques, e.g., plasma adsorption (PA) and double filtration plasmapheresis (DFPP), which do not need human plasma, were developed and have been introduced for clinical use in MG. We compared the effects of these plasma purification therapies in patients with MG and found that DFPP improved such subjective symptoms as chest compression and general fatigue better than PA while both of them could decrease the serum level of acetylcholine receptor (AChR) antibodies and relieve objective muscle weakness to a similar degree. It may be that DFPP can remove some circulating pathogenic factors other than AChR antibodies more efficiently than PA.

Myasthenia gravis as a prototype autoimmune receptor disease

Myasthenia gravis (MG) is an organ-specific autoimmune disease in which autoantibodies against nicotinic acetylcholine receptors (AChR) at the postsynaptic membrane cause loss of functional AChR and disturbed neuromuscular transmission. The immunopathogenic mechanisms responsible for loss of functional AChR include antigenic modulation by anti-AChR antibodies, complement-mediated focal lysis of the postsynaptic membrane, and direct interference with binding of acetylcholine to the AChR or with ion channel function. The loss of AChR and subsequent defective neuromuscular transmission is accompanied by increased expression of the different AChR subunit genes, suggesting a role for the target organ itself in determining susceptibility and severity of disease. Experimental autoimmune myasthenia gravis (EAMG) is an animal model for the disease MG, and is very suitable to study the immunopathogenic mechanisms leading to AChR loss and the response of the AChR to this attack. In this article the current concepts of the structure and function of the AChR and the immunopathological mechanisms in MG and EAMG are reviewed.

Specific removal of anti-acetylcholine receptor antibodies in patients with myasthenia gravis

In short-term therapy for myasthenia gravis caused by an antibody-mediated attack on the acetylcholine receptor (AChR) in skeletal muscle, a specific system for antibody removal, the use of tryptophan-bound immunoadsorbent and synthetic AChR peptide-bound immunoadsorbent, offers advantages over plasma exchange. These two types of immunoadsorption provided selective or semi-selective removal of pathogenic substances from the circulation without the use of plasma products, and minimized side-effects. A difficulty is that the former removed about 65% of the total IgG, and the latter removed only a fraction of the pathogenic antibodies. In neither case can a radical method of treatment for myasthenia gravis be expected. Hopefully, an adsorbent which has a well-balanced bioimmunological specific binding reaction and physicochemical adsorptive affinity will be developed in the future.

Clustering of B and T epitopes within short sequence regions of the nicotinic acetylcholine receptor

The epitope repertoire of B cells, due to their selective ability to process their specific antigen and the potential bias imposed on the resulting peptides by the surface immunoglobulins bound to the antigen, may influence the T-helper repertoire. Immunization of C57B1/6 mice with Torpedo acetylcholine receptor (TAChR) causes experimental autoimmune myasthenia gravis (EAMG). Anti-TAChR CD4+ cells recognize epitopes within three sequence regions of the TAChR alpha subunit ('dominant epitopes'). Immunization of mice with denatured or synthetic TAChR antigens sensitizes CD4+ cells to other TAChR sequence regions ('cryptic epitopes'). We investigated here whether clustering of B and T epitopes within the same short sequence segments occurs during the anti-TAChR response, as previously described for the response to hexogenous antigens unrelated to homologous self proteins. Twelve 19-20 residue synthetic sequences of the TAChR alpha, gamma and delta subunits, containing dominant or cryptic CD4+ epitopes for C57B1/6 mice, were tested for ability to induce anti-peptide antibody production. C57B1/6 mice were immunized with the individual peptides. Ten peptides stimulated antibody production. Therefore > 80% of these short TAChR sequences also contain B epitopes. Therefore also in the anti-TAChR response leading to EAMG T and B cell epitopes frequently reside within the same short sequence segment.

Response to human acetylcholine receptor alpha 138-199: determinant spreading initiates autoimmunity to self-antigen in rabbits

NZW rabbits immunised with a mixture of synthetic peptides representing alpha 138-199 of the human acetylcholine receptor (AChR) alpha-subunit exhibited clinical, biochemical and electrophysiological signs of experimental autoimmune myasthenia gravis (EAMG), with raised levels of anti-rabbit AChR antibodies. Surprisingly, these were partly directed at the main immunogenic region (MIR, thought to be alpha 67-76) and alpha-Bungarotoxin binding sites on rabbit AChR, and reacted less well with human AChR. Moreover, they could be separated from the anti-peptide antibodies by fractionation on immobilised peptide. We conclude that immunisation with these peptides led, by 'determinant spreading', to a response directed at self-AChR. Similar phenomena may have been overlooked in previous studies of responses to synthetic or recombinant AChR sequences. These findings suggest that autoimmunity could be induced by low-affinity, cross-reacting epitopes even when the observed serum response is highly specific for the autoantigen.

Mapping of the polypeptide chain organization of the main extracellular domain of the alpha-subunit in membrane-bound acetylcholine receptor by antipeptide antibodies spanning the entire domain

To study the organization of the polypeptide chain of the main extracellular domain of the nicotinic acetylcholine receptor (AChR) alpha-subunit, we examined the ability of the native membrane-bound AChR of Torpedo californica (T-AChR) to bind a panel of antibodies against overlapping synthetic peptides which collectively encompassed this entire domain. Antibodies against the alpha-chain peptides alpha 1-16, alpha 89-104 and alpha 158-174 were able to bind to membrane-bound T-AChR. Other anti-peptide antibodies showed little or no binding to T-AChR in the membrane. It is concluded that regions alpha 1-16, alpha 89-104 and alpha 158-174 are highly exposed on the surface of the alpha subunit of membrane-bound AChR.

Effect of myasthenic IgG on degradation of junctional acetylcholine receptor

We investigated the effect of the IgG from patients with myasthenia gravis (MG) on the degradation of normal rat junctional acetylcholine receptor (AChR) labeled with 125I-alpha-bungarotoxin (BuTx) and calculated the degradation rate (DR). The DR for the IgG from these patients was significantly higher than that from healthy volunteers and patients with other autoimmune diseases. For MG, DR was significantly correlated with the severity of the disease but not with anti-AChR antibody titer. DR was accelerated by IgG from patients with generalized MG whose antibody titers were in the normal range and by IgG from patients with ocular MG. These results indicate that measurement of the DR of junctional AChR in normal rats is more closely correlated with the severity of the disease than is measurement of anti-AChR antibody and that the former is a sensitive and confirmatory method for evaluating MG.

Sodium dodecyl sulfate- and carbamylcholine-induced changes in circular dichroism spectra of acetylcholine receptor synthetic peptides

The effect of sodium dodecyl sulfate (SDS) on the conformation of acetylcholine receptor alpha-subunit synthetic peptides was investigated by circular dichroism. In the presence of SDS (0.01-0.02%), the affinity of a 173-204 32 residue peptide and a 172-227 56 residue peptide for the competitive antagonist alpha-bungarotoxin increases about 10-fold to the nanomolar range. Circular dichroism spectroscopy of these peptides revealed significant changes in the secondary structure of the peptides in the presence of SDS at concentrations below the critical micelle concentration. It is concluded that SDS induces a conformation of the peptides that is conductive to high affinity binding. Carbamylcholine, an acetylcholine analog, produced small but significant changes in the spectrum of the 173-204 peptide. This change could be the result of agonist-induced conformational changes in this region of the acetylcholine receptor alpha-subunit or to changes in the asymmetric environments of aromatic chromophores in the binding site. These studies demonstrate that synthetic peptides alone are capable of retaining significant functional activity and contain significant secondary structure.

T-cell immunity to acetylcholine receptor and its subunits in Lewis rats over the course of experimental autoimmune myasthenia gravis

Lymph nodes, spleen and thymus obtained from Lewis rats were examined over the course of experimental autoimmune myasthenia gravis (EAMG) for the distribution and the number of antigen-reactive CD4+ T helper cells which, upon recognition of Torpedo acetylcholine receptor (AChR) or the alpha, beta, gamma or delta subunits of Torpedo AChR, responded by secretion of interferon-gamma (IFN-gamma). T cells with these specificities were detected in these three immune organs. Numbers were highest in lymph nodes. In spleen and thymus, numbers of antigen-reactive T cells did not differ. T cells reacting against the intact AChR were more frequent than T cells recognizing any of the subunits. The immunogenicity between the four subunits did not differ, with the exception that the alpha subunit induced a slightly higher T-cell response. No restriction of the T-cell repertoire to the four subunits was detected during early compared to late phases of EAMG. The AChR and subunit-reactive T cells could--via secretion of effector molecules including IFN-gamma--play an important role in the initiation and perpetuation of EAMG, and consequently also of human myasthenia gravis. T cells with the same specificities were also detected in control animals injected with adjuvant only, but at much lower numbers which were within the range of T cells recognizing the control antigen myelin basic protein. They could represent naturally occurring autoimmune T cells.

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.

Myasthenogenicity in the main immunogenic region of acetylcholine receptor as modified by conformational design: an approach to antigenic synthetic peptides

Myasthenogenic regions in the acetylcholine receptor (AChR) alpha-subunit were studied in view of the conformation-dependent B-cell epitope expected at beta-turn and the MHC class II-restricted T-cell epitope expected at alpha-helix. Torpedo AChR alpha 67-76 and alpha 107-116 were synthesized as the main immunogenic region and the site specific for T-cell epitope in Lewis rat, respectively. Model peptides, synthesized by combining these natural sequence segments or by intervening the segment aligned as Asn-Pro-Gly-Gly (NPGG) in natural sequence segments, were tested in terms of antigenic conformation. The model peptide, alpha 107-116.alpha 67-76.alpha 107-116, was immunogenic in the induction of the animal model of myasthenia, accompanied by the anti-peptide antibody cross-reactive with the native AChR. High antigenicity in antibody assays for various peptide- and native AChR-immunized rats was found when the model peptides, alpha 107-116.alpha 67-76 and/or alpha 107-116.NPGG.alpha 67-76 were used for measurement as antigens. Antigenic conformation for the induction of the disease may thus be different from that for the reactivity to antibody.

Myasthenogenicity of a human acetylcholine receptor ?-subunit peptide: Morphology and immunology

Each of 10 rats inoculated with a synthetic peptide comprising residues 125-147 (without a disulfide bond) of human acetylcholine receptor (AChR) alpha-subunit (H alpha) had deposits of IgG and C3 (immune complexes) and showed morphological changes in the fine structure at the motor end-plates 5 weeks after a single immunization. Antibody to the H alpha peptides was elevated 1 week after immunization, but, antibody levels to solubilized human or rat AChR were very low in 8 of the 10 rats. These results suggest that the immune response to peptide H alpha is the myasthenogenic site, which induces morphological change at the end-plates.

Substitution of Torpedo acetylcholine receptor alpha 1-subunit residues with snake alpha 1- and rat nerve alpha 3-subunit residues in recombinant fusion proteins: effect on alpha-bungarotoxin binding

A fusion protein consisting of the TrpE protein and residues 166-211 of the Torpedo acetylcholine receptor alpha 1 subunit was produced in Escherichia coli using a pATH10 expression vector. Residues in the Torpedo sequence were changed by means of oligonucleotide-directed mutagenesis to residues present in snake alpha 1 subunit and rat nerve alpha 3 subunit which do not bind alpha-bungarotoxin. The fusion protein of the Torpedo sequence bound 125I-alpha-bungarotoxin with high affinity (IC50 = 2.5 x 10(-8) M from competition with unlabeled toxin, KD = 2.3 x 10(-8) M from equilibrium saturation binding data). Mutation of three Torpedo residues to snake residues, W184F, K185W, and W187S, had no effect on binding. Conversion of two additional Torpedo residues to snake, T191S and P194L, reduced alpha-bungarotoxin binding to undetectable levels. The P194L mutation alone abolished toxin binding. Mutation of three Torpedo alpha 1 residues to neuronal alpha 3-subunit residues, W187E, Y189K, and T191N, also abolished detectable alpha-bungarotoxin binding. Conversion of Try-189 to Asn which is present in the snake sequence (Y189N) abolished toxin binding. It is concluded that in the sequence of the alpha subunit of Torpedo encompassing Cys-192 and Cys-193, Try-189 and Pro-194 are important determinants of alpha-bungarotoxin binding. Tyr-189 may interact directly with cationic groups or participate in aromatic-aromatic interactions while Pro-194 may be necessary to maintain a conformation conductive to neurotoxin binding.

Conformational modification enhances myasthenogenicity in synthetic peptide of acetylcholine receptor alpha-subunit

The induction of myasthenia gravis depends on linked recognition of antigenic sites of acetylcholine receptor (AChR) by B-cells and T-cells. The former is conformationally restrained, and the latter is under the MHC class II restriction. We synthesized an artificially formed peptide (model peptide) by coupling the alpha 190-195 selected as B-cell site and cholinergic binding site and the alpha-107-116 selected as T-cell site and agretope with the intervening chain segment aligned as Asn-Pro-Gly-Gly (NPGG) to adopt beta-turn conformation. This model peptide, alpha 107-116-NPGG-alpha 190-195, was potently immunogenic in Lewis rats to provoke anti-peptide antibody reactive with native AChR and to induce the animal model of immunopharmacologic blockade of acetylcholine (ACh)-binding site. Low immunogenicity compared with this was found when using natural peptides predicted as sequences of B-cell site or T-cell site and the peptide synthesized by linking both without intervention of NPGG. The alpha 190-195 had no function of cholinergic binding either as a single segment or as part of the conformation-modified peptides; results suggest that the conformation modified for high immunogenicity does not assume the bioactive conformation for ACh-binding.

Alpha-bungarotoxin binds to human acetylcholine receptor alpha-subunit peptide 185-199 in solution and solid phase but not to peptides 125-147 and 389-409

The nicotinic acetylcholine receptor (AChR) of human skeletal muscle has a reducible disulfide bond near the neurotransmitter binding site in each of its alpha-subunits. By testing a panel of overlapping synthetic peptides encompassing the alpha-subunit segment 177-208 (containing cysteines 192 and 193) we found that specific binding of 125I-labelled alpha-bungarotoxin (alpha-BTx) was maximal in the region 185-199. Binding was inhibited by unlabelled alpha-BTx greater than d-tubocurarine greater than atropine greater than carbamylcholine. Peptide 193-208 did not bind alpha-BTx, whereas 177-192 retained 40% binding activity. Peptides corresponding to regions 125-147 (containing cysteines 128 and 142) and 389-409, or peptides unrelated to sequences of the AChR failed to bind alpha-BTx. No peptide bound 125I-alpha-labelled parathyroid hormone. The apparent affinity (KD) of alpha-BTx binding to immobilized peptides 181-199 and 185-199 was approximately 25 microM and 80 microM, respectively, in comparison with alpha-BTx binding to native Torpedo ACh receptor (apparent KD approximately 0.5 nM). In solution phase, both peptides effectively competed with solubilized native human AChR for binding of alpha-BTx, and peptide 185-199 showed little evidence of dissociation after 24 h. Peptides that bound alpha-BTx did so when sulfhydryls were reduced. Cysteine modification, by N-ethylmaleimide or acetamidomethylation, abolished alpha-BTx-binding activity. The data implicate the region of cysteines 192 and 193 in the binding of neurotransmitter to the human receptor.

Monoclonal antibodies directed against a synthetic peptide corresponding to the alpha-bungarotoxin binding region of the acetylcholine receptor

Murine monoclonal antibodies have been produced against a 32 amino acid synthetic peptide corresponding to residues 173-204 on the alpha-subunit of the nicotinic acetylcholine receptor from Torpedo californica. All of the monoclonal antibodies were of the IgM subtype and most cross-reacted with the purified native receptor. None of the antibodies were effective in blocking alpha-bungarotoxin binding to the receptor nor, conversely, did alpha-bungarotoxin interfere with antibody binding. However, two monoclonal antibodies, previously shown to bind near the ligand binding site on the native receptor, did compete partially (50%) with the binding of one of the IgM monoclonal antibodies.