Antigenic role of single residues within the main immunogenic region of the nicotinic acetylcholine receptor

Structural insights into the molecular mechanisms of myasthenia gravis and their therapeutic implications

The nicotinic acetylcholine receptor (nAChR) is a major target of autoantibodies in myasthenia gravis (MG), an autoimmune disease that causes neuromuscular transmission dysfunction. Despite decades of research, the molecular mechanisms underlying MG have not been fully elucidated. Here, we present the crystal structure of the nAChR α1 subunit bound by the Fab fragment of mAb35, a reference monoclonal antibody that causes experimental MG and competes with ~65% of antibodies from MG patients. Our structures reveal for the first time the detailed molecular interactions between MG antibodies and a core region on nAChR α1. These structures suggest a major nAChR-binding mechanism shared by a large number of MG antibodies and the possibility to treat MG by blocking this binding mechanism. Structure-based modeling also provides insights into antibody-mediated nAChR cross-linking known to cause receptor degradation. Our studies establish a structural basis for further mechanistic studies and therapeutic development of MG.

DOI:http://dx.doi.org/10.7554/eLife.23043.001

Myasthenia gravis is a disease that causes chronic weakness in muscles. It affects more than 20 in every 100,000 people and diagnosis is becoming more common due to increased awareness of the disease. However, most current treatments only temporarily relieve symptoms so there is a need to develop more effective therapies.

The disease occurs when the immune system produces molecules called antibodies that bind to and destroy a receptor protein called nAChR. This receptor is normally found at the junctions between nerve cells and muscle cells, and its destruction disrupts communication between the nervous system and the muscle. However, it is not known exactly how these antibodies bind to nAChR, partly due to the lack of a detailed three-dimensional structure of the antibodies and nAChR together.

The human nAChR protein is made up of several subunits, including one called alpha1 that is the primary target of Myasthenia gravis antibodies. Noridomi et al. used a technique known as X-ray crystallography to generate a highly detailed three-dimensional model of the structure of the alpha1 subunit with an antibody from rats that acts as in a similar way to human Myasthenia gravis antibodies. The structure reveals the points of contact between the antibodies and a core region of the nAChR alpha1 subunit and suggests that many different Myasthenia gravis antibodies may bind to nAChR in the same way.

These findings may aid the development of drugs that bind to and disable Myasthenia gravis antibodies to relieve the symptoms of the disease.

DOI:http://dx.doi.org/10.7554/eLife.23043.002

Mapping autoantigen epitopes: molecular insights into autoantibody-associated disorders of the nervous system

Background

Our knowledge of autoantibody-associated diseases of the central (CNS) and peripheral (PNS) nervous systems has expanded greatly over the recent years. A number of extracellular and intracellular autoantigens have been identified, and there is no doubt that this field will continue to expand as more autoantigens are discovered as a result of improved clinical awareness and methodological practice. In recent years, interest has shifted to uncover the target epitopes of these autoantibodies.

Main body

The purpose of this review is to discuss the mapping of the epitope targets of autoantibodies in CNS and PNS antibody-mediated disorders, such as N-methyl-D-aspartate receptor (NMDAR), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), leucine-rich glioma-inactivated protein 1 (Lgi1), contactin-associated protein-like 2 (Caspr2), myelin oligodendrocyte glycoprotein (MOG), aquaporin-4 (AQP4), 65 kDa glutamic acid decarboxylase (GAD65), acetylcholine receptor (AChR), muscle-specific kinase (MuSK), voltage-gated calcium channel (VGCC), neurofascin (NF), and contactin. We also address the methods used to analyze these epitopes, the relevance of their determination, and how this knowledge can inform studies on autoantibody pathogenicity. Furthermore, we discuss triggers of autoimmunity, such as molecular mimicry, ectopic antigen expression, epitope spreading, and potential mechanisms for the rising number of double autoantibody-positive patients.

Conclusions

Molecular insights into specificity and role of autoantibodies will likely improve diagnosis and treatment of CNS and PNS neuroimmune diseases.

Design, synthesis, and characterization of a 39 amino acid peptide mimic of the main immunogenic region of the Torpedo acetylcholine receptor

We have designed a 39 amino acid peptide mimic of the conformation-dependent main immunogenic region (MIR) of the Torpedo acetylcholine receptor (TAChR) that joins three discontinuous segments of the Torpedo α-subunit, α(1-12), α(65-79), and α(110 - 115) with two GS linkers: This 39MIR-mimic was expressed in E. coli as a fusion protein with an intein-chitin-binding domain (IChBD) to permit affinity collection on chitin beads. Six MIR-directed monoclonal antibodies (mAbs) bind to this complex and five agonist/antagonist site directed mAbs do not. The complex of MIR-directed mAb-132A with 39MIR has a Kd of (2.11±0.11)×10(-10)M, which is smaller than (7.13±1.20)×10(-10)M for the complex of mAb-132A with α(1-161) and about the same as 3.4×10(-10)M for that of mAb-132A with TAChR. Additionally, the 39MIR-IChBD adsorbs all MIR-directed antibodies (Abs) from an experimental autoimmune myasthenia gravis (EAMG) rat serum. Hence, the 39MIR-mimic has the potential to inactivate or remove pathogenic Torpedo MIR-directed Abs from EAMG sera and to direct a magic bullet to the memory B-cells that produce those pathogenic Abs. The hope is to use this as a guide to produce a mimic of the human MIR on the way to an antigen specific therapeutic agent to treat MG.

Structural characterization of the main immunogenic region of the Torpedo acetylcholine receptor

To develop antigen-specific immunotherapies for autoimmune diseases, knowledge of the molecular structure of targeted immunological hotspots will guide the production of reagents to inhibit and halt production of antigen specific attack agents. To this end we have identified three noncontiguous segments of the Torpedo nicotinic acetylcholine receptor (AChR) α-subunit that contribute to the conformationally sensitive immunological hotspot on the AChR termed the main immunogenic region (MIR): α(1-12), α(65-79), and α(110-115). This region is the target of greater than 50% of the anti-AChR Abs in serum from patients with myasthenia gravis (MG) and animals with experimental autoimmune myasthenia gravis (EAMG). Many monoclonal antibodies (mAbs) raised in one species against an electric organ AChR cross react with the neuromuscular AChR MIR in several species. Probing the Torpedo AChR α-subunit with mAb 132A, a disease inducing anti-MIR mAb raised against the Torpedo AChR, we have determined that two of the three MIR segments, α(1-12) and α(65-79), form a complex providing the signature components recognized by mAb 132A. These two segments straddle a third, α(110-115), that seems not to contribute specific side chains for 132A recognition, but is necessary for optimum antibody binding. This third segment appears to form a foundation upon which the three-dimensional 132A epitope is anchored.

Molecular modeling of the complex between Torpedo acetylcholine receptor and anti-MIR Fab198

Myasthenia gravis is a neuromuscular disorder caused by an antibody-mediated autoimmune response to the muscle-type nicotinic acetylcholine receptor (AChR). The majority of monoclonal antibodies (mAbs) produced in rats immunized with intact AChR compete with each other for binding to an area of the alpha-subunit called the main immunogenic region (MIR). The availability of a complex between the AChR and Fab198 (Fab fragment of the anti-MIR mAb198) would help understand how the antigen and antibody interact and in designing improved antibody fragments that protect against the destructive activity of myasthenic antibodies. In the present study, we modeled the Torpedo AChR/Fab198 complex, based primarily on the recent 4A resolution structure of the Torpedo AChR. In order to computationally dock the two structures, we used the ZDOCK software. The total accessible surface area change of the complex compared to those of experimentally determined antigen-antibody complexes indicates an intermediate size contact surface. CDRs H3 and L3 seem to contribute most to the binding, while L2 seems to contribute least. These data suggest mutagenesis experiments aimed at validating the model and improving the binding affinity of Fab198 for the AChR.

Design, synthesis, and conformational study of biologically active photolabeled analogues of the main immunogenic region of the acetylcholine receptor

Photoaffinity labeling is a powerful tool for the characterization of the molecular basis of ligand binding to acceptor molecules, which provides important insights for mapping the bimolecular interfaces. The autoimmune disease myasthenia gravis is caused by autoantibodies against the acetylcholine receptor (AChR). The majority of the anti-AChR antibodies bind to the "main immunogenic region" (MIR) of the AChR. To identify the contact points between the complementarity determining regions of the anti-MIR antibodies that recognize the MIR contact sites of the AChR, we present here three photoreactive dodecapeptide MIR analogues containing the photolabel p-benzoyl-L-phenylalanine (Bpa) moiety, either in position 1 or 11. The structure of the produced 12-mers was analyzed using two-dimensional (1)H-NMR spectroscopy, whereas their binding to anti-MIR monoclonal antibodies (mAbs) was determined by immunochemical assays. In all cases the modifications resulted in conservation of the beta-turn conformation of the N-terminus, which has been proved essential for antibody recognition and increased anti-MIR binding relative to the MIR decapeptide.

The conformation of the main immunogenic region on the alpha-subunit of muscle acetylcholine receptor is affected by neighboring receptor subunits

Myasthenia gravis (MG) is caused by autoantibodies to the acetylcholine receptor (AChR). Experiments with fetal (alpha(2)betagammadelta) and adult (alpha(2)betaepsilondelta) AChR and with recombinant subunit dimers showed that some monoclonal antibodies (mAbs) against the main immunogenic region (MIR), located on the alpha-subunit of the AChR, bind better to fetal AChR and to alphagamma subunit dimer than to adult AChR and alphaepsilon dimer and equally to both alphabeta and alphadelta. However, other anti-MIR mAbs prefer adult AChR and alphaepsilon dimer, bind well to alphabeta but weakly to alphadelta. These results suggest that the MIR conformation is affected by the neighboring gamma/epsilon- and delta-subunits and may contribute to understanding the antibody specificities in MG.

Functional immobilisation of the nicotinic acetylcholine receptor in tethered lipid membranes

The nicotinic acetylcholine receptor from Torpedo was immobilised in tethered membranes. Surface plasmon resonance was used to quantify the binding of ligands and antibodies to the receptor. The orientation and structural integrity of the surface-reconstituted receptor was probed using monoclonal antibodies, demonstrating that approximately 65% of the receptors present their ligand-binding site towards the lumen of the flow cell and that at least 85% of these receptors are structurally intact. The conformation of the receptor in tethered membranes was investigated with Fourier transform infrared spectroscopy and found to be practically identical to that of receptors reconstituted in lipid vesicles. The affinity of small receptor ligands was determined in a competition assay against a monoclonal antibody directed against the ligand-binding site which yielded dissociation constants in agreement with radioligand binding assays. The presented method for the functional immobilisation of the nicotinic acetylcholine receptor in tethered membranes might be generally applicable to other membrane proteins.

Compared structures of the free nicotinic acetylcholine receptor main immunogenic region (MIR) decapeptide and the antibody-bound [A76]MIR analogue: a molecular dynamics simulation from two-dimensional NMR data

Monoclonal antibodies against the main immunogenic region (MIR) of the muscle acetylcholine receptor (AChR) are capable of inducing experimental myasthenia gravis (MG) in animals. The epitope of these antibodies has been localized between residues 67 and 76 of the AChR alpha-subunit. The conformation in solution of the Torpedo californica MIR peptide and of its [A76] MIR analogue have been analyzed using molecular modeling based on nmr interproton distances and J-derived phi dihedral angles. Molecular dynamics simulations including dimethyl-sulfoxide as explicit solvent have been carried out on the free MIR peptide. Calculation of the structure of the [A76]MIR analogue bound to an anti-MIR monoclonal antibody have been performed in the presence of water molecules. A tightly folded structure appears for both peptides with alpha beta-folded N-terminal N68-P-A-D71 sequence of type I in the free state and type III in the mAb6-bound state. The C-terminal sequence is folded in two different ways according to the result in the free and bound state of the peptides: two overlapping beta/beta or beta/alpha turns result in a short helical sequence in the free MIR peptide, whereas the bound analogue is folded by uncommon hydrogen bond closing an 11-membered cycle. This structural evolution is essentially the result of the reorientation of the hydrophobic side chains that are probably directly involved in peptide--antibody recognition.

The third-dimensional structure of the complex between an Fv antibody fragment and an analogue of the main immunogenic region of the acetylcholine receptor: a combined two-dimensional NMR, homology, and molecular modeling approach

Binding of autoantibodies to the acetylcholine receptor (AChR) plays a major role in the autoimmune disease Myasthenia gravis (MG). In this paper, we propose a structure model of a putative immunocomplex that gives rise to the reduction of functional AChR molecules during the course of MG. The model complex consists of the [G(70), Nle(76)] decapeptide analogue of the main immunogenic region (MIR), representing the major antigenic epitope of AChR, and the single chain Fv fragment of monoclonal antibody 198, a potent MG autoantibody. The structure of the complexed decapeptide antigen [G(70), Nle(76)]MIR was determined using two-dimensional nmr, whereas the antibody structure was derived by means of homology modeling. The final complex was constructed using calculational docking and molecular dynamics. We termed this approach "directed modeling," since the known peptide structure directs the prestructured antibody binding site to its final conformation. The independently derived structures of the peptide antigen and antibody binding site already showed a high degree of surface complementarity after the initial docking calculation, during which the peptide was conformationally restrained. The docking routine was a soft algorithm, applying a combination of Monte Carlo simulation and energy minimization. The observed shape complementarity in the docking process suggested that the structure assessments already led to anti-idiotypic conformations of peptide antigen and antibody fragment. Refinement of the complex by dynamic simulation yielded improved surface adaptation by small rearrangements within antibody and antigen. The complex presented herein was analyzed in terms of antibody-antigen interactions, properties of contacting surfaces, and segmental mobility. The structural requirements for AChR complexation by autoantibodies were explored and compared with experimental data from alanine scans of the MIR peptides. The analysis revealed that the N-terminal loop of the peptide structure, which is indispensable for antibody recognition, aligns three hydrophobic groups in a favorable arrangement leading to the burial of 40% of the peptide surface in the binding cleft upon complexation. These data should be valuable in the rational design of an Fv mutant with much improved affinity for the MIR and AChR to be used in therapeutic approaches in MG.

Prevention of passively transferred experimental autoimmune myasthenia gravis by a phage library-derived cyclic peptide

Many pathogenic antibodies in myasthenia gravis (MG) and its animal model, experimental autoimmune MG (EAMG), are directed against the main immunogenic region (MIR) of the acetylcholine receptor (AcChoR). These antibodies are highly conformation dependent; hence, linear peptides derived from native receptor sequences are poor candidates for their immunoneutralization. We employed a phage-epitope library to identify peptide-mimotopes capable of preventing the pathogenicity of the anti-MIR mAb 198. We identified a 15-mer peptide (PMTLPENYFSERPYH) that binds specifically to mAb 198 and inhibits its binding to AcChoR. A 10-fold increase in the affinity of this peptide was achieved by incorporating flanking amino acid residues from the coat protein as present in the original phage library. This extended peptide (AEPMTLPENYFSERPYHPPPP) was constrained by the addition of cysteine residues on both ends of the peptide, thus generating a cyclic peptide that inhibited the binding of mAb 198 to AcChoR with a potency that is three orders of magnitude higher when compared with the parent library peptide. This cyclic peptide inhibited the in vitro binding of mAb 198 to AcChoR and prevented the antigenic modulation of AcChoR caused by mAb 198 in human muscle cell cultures. The cyclic peptide also reacted with several other anti-MIR mAbs and the sera of EAMG rats. In addition, this peptide blocked the ability of mAb 198 to passively transfer EAMG in rats. Further derivatization of the cyclic peptide may aid in the design of suitable synthetic mimotopes for modulation of MG.

Anatomy of the antigenic structure of a large membrane autoantigen, the muscle-type nicotinic acetylcholine receptor

The neuromuscular junction nicotinic acetylcholine receptor (AChR), a pentameric membrane glycoprotein, is the autoantigen involved in the autoimmune disease myasthenia gravis (MG). In animals immunized with intact AChR and in human MG, the anti-AChR antibody response is polyclonal. However, a small extracellular region of the AChR alpha-subunit, the main immunogenic region (MIR), seems to be a major target for anti-AChR antibodies. A major loop containing overlapping epitopes for several anti-MIR monoclonal antibodies (mAbs) lies within residues alpha 67-76 at the extreme synaptic end of each alpha-subunit: however, anti-MIR mAbs are functionally and structurally quite heterogeneous. Anti-MIR mAbs do not affect channel gating, but are very effective in the passive transfer of MG to animals; in contrast, their Fab or Fv fragments protect the AChR from the pathogenic effects of the intact antibodies. Antibodies against the cytoplasmic region of the AChR can be elicited by immunization with denatured AChR and the precise epitopes of many such mAbs have been identified; however, it is unlikely that such antibodies are present in significant amounts in human MG. Antibodies to other extracellular epitopes on all AChR subunits are present in both experimental and human MG; these include antibodies to the acetylcholine-binding site which affect AChR function in various ways and also induce acute experimental MG. Finally, anti-AChR antibodies cross-reactive with non-AChR antigens exist, suggesting that MG may result from molecular mimicry. Despite extensive studies, many gaps remain in our understanding of the antigenic structure of the AChR; especially in relation to human MG. A thorough understanding of the antigenic structure of the AChR is required for an in-depth understanding, and for possible specific immunotherapy, of MG.

Triphosgene: an efficient carbonylating agent for liquid and solid-phase aza-peptide synthesis. Application to the synthesis of two aza-analogues of the AChR MIR decapeptide

The N alpha/C alphaH exchange in aza-peptides has the advantage of preserving the side chain. Bis(trichloromethyl)carbonate or triphosgene is a solid, stable phosgene substitute which retains its high reactivity. Temperature and coupling times are greatly reduced with reference to other usually recommended carbonylating agents, while purity and yield are increased. It has been used, in both liquid- and solid-phase procedures, for the synthesis of various aza-analogues of dipeptides, tripeptides and decapeptides containing the alanine, aspartic acid and asparagine aza-residue.

Construction and application of a new class of sequential oligopeptide carriers (SOCn) for multiple anchoring of antigenic peptides--application to the acetylcholine receptor (AChR) main immunogenic region

A new class of sequential oligopeptide carriers (SOCn), namely (Lys-Aib-Gly)n (n = 2-7), for anchoring antigenic peptides, is presented. These SOCn have been designed in order to assume a determined structural motif, exhibiting defined spatial orientations of the Lys-N epsilon H2 anchoring groups. The NMR study showed that SOCn adopt a rigid conformation with some regularity, initiated from the C-terminus of the carrier, while molecular dynamics simulation confirmed the occurrence of a distorted 3(10)-helix. It was also demonstrated, by 1HNMR, that all the antigenic peptides bound to the SOCn retain their original, folded active, structure and that probably they do not interact to each other. It is concluded that the beneficial structural elements of the SOCn impose a favorable disposition of the anchored peptides so that potent antigens with maximum molecular recognition are generated.

Antigenic mimicry of natural L-peptides with retro-inverso-peptidomimetics

Three analogues of the model peptide of sequence IRGERA corresponding to the COOH-terminal residues 130-135 of histone H3 were synthesized, and their antigenicity, immunogenicity, and resistance to trypsin were compared to those of the natural L-peptide. The three analogues correspond to the D-enantiomer, containing only D-residues, and two retro-peptides containing NH-CO bonds instead of natural peptide bonds. The chirality of each residue was maintained in the retro-peptide and inverted in the retro-inverso-peptide. Antibodies to the four peptide analogues were produced by injecting BALB/c mice with peptides covalently coupled to small unilamellar liposomes containing monophosphoryl lipid A. Each of the four peptide analogues induced IgG antibodies of various subclasses. The IgG3 antibodies reacted similarly with the four analogues, whereas antibodies of the IgG1, IgG2a, and IgG2b isotypes showed strong conformational preferences for certain peptides. The retro-inverso-peptide IRGERA mimicked the structure and antigenic activity of the natural L-peptide but not of the D- and retro-peptides, whereas the retro-peptide IRGERA mimicked the D-peptide but not the L- and retro-inverso-peptides. The equilibrium affinity constants (Ka) of three monoclonal antibodies generated against the L- and D-peptides with respect to the four peptide analogues were measured in a biosensor system. Large differences in Ka values were observed when each monoclonal antibody was tested with respect to the four peptides. The use of retro-inverso-peptides to replace natural L-peptides is likely to find many applications in immunodiagnosis and as potential synthetic vaccines.

Analytical and semi-preparative separation of diastereomeric lipid amino acid conjugates

A series of biologically active peptides and their conjugates with lipidic amino acids were investigated by systematic change of the mobile phase composition using traditional octadecylsilica stationary phase and the newly developed Supelcosil LC-ABZ column. The mobile phases contained various concentrations of methanol and acetonitrile combined with 0.1% trifluoroacetic acid (TFA). Better peak shapes and higher resolution of the isomers could be observed when the mobile phase contained 0.1% TFA. More symmetrical peaks and much higher S values (slope of the log k' vs organic phase concentration plots) were obtained on the special reversed-phase column developed for anionic, basic or zwitterionic compounds. The optimum separation conditions were scaled up to a semi-preparative reversed-phase column (15 mm i.d.) to collect mg quantities of isomers for further studies.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Bacterial expression of a single-chain Fv fragment which efficiently protects the acetylcholine receptor against antigenic modulation caused by myasthenic antibodies

Monoclonal antibodies (mAb) against the main immunogenic region (MIR) of the acetylcholine receptor (AChR) are very potent in inducing antigenic modulation of the AChR in animals and in muscle cell cultures. A recombinant antibody fragment of the rat anti-MIR mAb198 was cloned by polymerase chain reaction and expressed as soluble single-chain Fv fragment (scFv198) in E. coli and affinity purified. DNA sequencing was used to define the VH (IB) and VL (K2) chain gene usage. scFv198 was found immunologically and biologically active. Its binding affinity for the Torpedo AChR (KD = 2 +/- 0.6 nM) was very similar with that of the intact mAb198 (KD = 1.8 +/- 0.6 nM) while for the human AChR (KD = 80.7 +/- 16.6 nM) it was about four times lower than that of the intact mAb198 (KD = 21.6 +/- 6.6 nM). This fragment was capable of efficiently protecting the AChR in human cell cultures, against antigenic modulation caused by the intact mAb198 or by the antibodies from a myasthenic patient. The produced scFv198 fragment is, therefore, potentially useful in therapeutic applications for myasthenia gravis after appropriate genetic manipulations.

Conformational requirements for molecular recognition of acetylcholine receptor main immunogenic region (MIR) analogues by monoclonal anti-MIR antibody: a two-dimensional nuclear magnetic resonance and molecular dynamics approach

The conformational properties of two [D-A70, A76] and [Aib70, A76] analogues of the alpha 67-76 Torpedo acetylcholine receptor fragment, with low binding capacity for the anti main immunogenic region (MIR) antibodies, were studied in DMSO by two-dimensional nmr techniques and molecular dynamics simulations. The results were compared to the free and bound conformations of the [A76] analogue, which has twice more affinity for the anti-MIR monoclonal antibody 6 (mAb6), than the natural Torpedo sequence. It appeared that a single substitution of the A70, at a crucial position, by the D-A70 or Aib70, could modify completely the conformational behavior of the peptide and reduced its recognition by the anti-MIR antibody. The WNPADY rigid structure at the N-terminal part was essential for antibody recognition. The adjacent more flexible C-terminal sequence (GGIK) gives additional stability to the monoclonal antibody-peptide complex probably due to an adequate orientation of the peptide side chains in the complex, by setting them in close contact with the antibody.

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.

High-resolution epitope mapping and fine antigenic characterization of the main immunogenic region of the acetylcholine receptor. Improving the binding activity of synthetic analogues of the region

The main immunogenic region (MIR) of the nicotinic acetylcholine receptor (AChR) is an immunodominant area of the molecule, both in human and in experimental autoimmune myasthenia gravis. Anti-MIR monoclonal antibody (mAb) binding has been earlier localized between amino acid residues 67-76 of the AChR alpha-subunit. A thorough study of the epitope(s) for anti-MIR mAbs, by the use of a large panel of overlapping synthetic peptides and multiple peptide analogues, is now presented and offers clues for potential therapeutic applications of the obtained data. Use of all possible overlapping hexapeptides within Torpedo and human alpha 40-91 AChR and of selected peptides of various sizes, showed that the shortest peptide capable of significant antibody binding is the pentapeptide alpha 67-71. Systematic screening of peptide analogues, where each amino acid residue within alpha 67-76 and alpha 67-74 of both Torpedo and human AChRs was substituted by various amino acids, was performed. Asn68 and Asp71 were found to be indispensable for anti-MIR mAb binding, whereas Pro69 and Ala/Asp 70 were less but still significantly important. mAb binding to alpha 67-76 from various AChR species further supported the significance of these results. An additional series of selected peptide analogues was then constructed, aiming at the identification of analogues with high antigenic activity. Many analogues with either single substitutions of alpha 76 or combinations of two substitutions at alpha 73 and alpha 76 were tested. Several of these analogues (mainly His76, Arg76, Val73Ala76, His73Ala76, Val73Arg76) exhibited dramatic mAb binding enhancement. Some anti-MIR mAbs that do not bind to alpha 67-76 bound significantly to certain analogues. Such analogues could find applications in studies of therapeutic models of myasthenia gravis.

Myasthenia gravis: effect on antibody binding of conservative substitutions of amino acid residues forming the main immunogenic region of the nicotinic acetylcholine receptor

In Myasthenia Gravis most anti-acetylcholine receptor (AChR) antibodies are against a highly conserved area of the AChR alpha-subunit called the Main Immunogenic Region (MIR). Amino acid residues critical for MIR formation have been located within the sequence alpha 67-76. In the present study, binding of anti-AChR monoclonal antibodies (mAbs) to synthetic peptide analogues of the sequence alpha 67-76 of human and Torpedo AChRs containing conservative single-residue substitutions identified the amino acid residues most important to the antigenicity of the MIR sequence, and offered clues to its tridimensional structure. Conservative substitutions of residues Asn68 and Asp71 greatly diminished mAb binding, identifying them as critical contact residues for anti-MIR mAbs. Substitutions at Asp70 and Tyr72 moderately affected binding. Cross-reactive mAbs originally raised against Electrophorus AChR bound single residue-substituted synthetic peptides in a manner consistent with the possibility that Electrophorus AChR may have a glutamic acid residue at position alpha 70 or alpha 71. Substitutions at residues Asp/Ala70 and Val/Ile70 between human and Torpedo alpha-subunits may be size-compensating, suggesting these amino acids in the native AChR may be in closer proximity than proposed in previous models of the MIR.

Amino acid residues within the sequence region alpha 55-74 of Torpedo nicotinic acetylcholine receptor interacting with antibodies to the main immunogenic region and with snake alpha-neurotoxins

The sequence region 55-74 of the alpha-subunit of the acetylcholine receptor (AChR) from Torpedo californica electroplax comprises the amino-terminal end of a sequence segment--residues alpha 67-76--forming the main immunogenic region (MIR), which is most frequently recognized by anti-AChR autoantibodies in myasthenia gravis. The synthetic sequence alpha 55-74 of Torpedo AChR binds alpha-bungarotoxin (alpha BTX), suggesting that amino acid residues within this sequence region may contribute to formation of an alpha BTX binding site. Using single-residue substituted synthetic analogues of the sequence alpha 55-74 of Torpedo AChR, in which each residue was sequentially substituted by either glycine or alanine, we sought identification of the amino acids involved in interaction with alpha-neurotoxins and with three different anti-MIR monoclonal antibodies (mAbs 6, 22, and 198). Substitution of Arg55, Arg57, Trp60, Arg64, Leu65, Arg66, Trp67, or Asn68 strongly inhibited alpha-toxin binding, whereas substitutions of Ile61, Val63, Pro69, Ala70, Asp71, or Tyr72 had marginal effects. Substitutions within the region alpha 68-72 significantly diminished binding of anti-MIR mAbs, although residue preferences differed among mAbs. Further, substituting Trp60 substantially reduced binding of mAb 198, and moderately affected binding of mAb 6, and substitution of Asp62 slightly but consistently affected binding of mAbs 6 and 22.

Two-dimensional 1H-NMR study of antigen-antibody interactions: binding of synthetic decapeptides to an anti-acetylcholine receptor monoclonal antibody

Two-dimensional NMR experiments [correlated spectroscopy (COSY) and two-dimensional transferred nuclear Overhauser enhancement spectroscopy (TR-NOESY)] have been applied to study the interactions of a monoclonal antibody (mAb) directed to the main immunogenic region (MIR) of the acetylcholine receptor (AChR), and four synthetic decapeptides from the MIR. The decapeptides were the Torpedo AChR alpha 67-76 fragment (W67-N68-P69-A70-D71-Y72-G73-+ ++G74-I75-K76) and its three [A69], [A73], and [A76] analogues. The results led to the following conclusions: (1) the magnitude of the TR-NOE cross peaks does not depend only on the structuration of the peptide in the bound state, but also on restrictions of the mobility, i.e., on the correlation time tau c, which can be different for every residue; (2) the binding capacity of the synthetic peptides to mAbs measured by radioimmunoassay is directly correlated to the NOE magnitude; and (3) the combined interpretation of the COSY and TR-NOESY experiments gives a qualitative information about the nature and the overall conformation of the sequence which is in contact with the mAb binding site.

The main immunogenic region (MIR) of the nicotinic acetylcholine receptor and the anti-MIR antibodies

Myasthenia gravis (MG) is caused by autoantibodies against the nicotinic acetylcholine receptor (AChR) of the neuromuscular junction. The anti-AChR antibodies are heterogeneous. However, a small region on the extracellular part of the AChR alpha subunit, called the main immunogenic region (MIR), seems to be the major target of the anti-AChR antibodies, but not of the specific T-cells, in experimental animals and possibly in MG patients. The major loop of the overlapping epitopes for all testable anti-MIR monoclonal antibodies (MAbs) was localized within residues 67-76 (WNPADYGGIK for Torpedo and WNPDDYGGVK for human AChR) of the alpha subunit. The N-terminal half of alpha 67-76 is the most critical, Asn68 and Asp71 being indispensable for binding. Yet anti-MIR antibodies are functionally and structurally quite heterogeneous. Anti-MIR MAbs do not affect channel gating, but they are very potent in mediating acceleration of AChR degradation (antigenic modulation) in cell cultures and in transferring experimental MG in animals. Fab fragments of anti-MIR MAbs bound to the AChR prevent the majority of the MG patients' antibodies from binding to and causing loss of the AChR. Whether this inhibition means that most MG antibodies bind on the same small region or is a result of broad steric/allosteric effects is under current investigation.

The main immunogenic region of the acetylcholine receptor. Structure and role in myasthenia gravis

Auto-antibodies to the nicotine acetylcholine receptor (AChR) cause the disease myasthenia gravis (MG). Animals immunized with AChR or receiving anti-AChR antibodies acquire MG symptoms. The majority of the monoclonal antibodies (mAbs) raised in rats against intact AChR bind to a region on the extracellular side of the AChR's alpha-subunit, the main immunogenic region (MIR). The major loop of the overlapping epitopes for several anti-MIR mAbs has been localised between residues 67-76 of the alpha-subunit. Anti-MIR mAbs are very potent in accelerating AChR degradation (antigenic modulation) in muscle cell cultures and transferring experimental MG in animals. Fab fragments of single anti-MIR mAbs when bound to the AChR inhibit two-thirds of the MG patients' antibodies from binding and from inducing antigenic modulation of the AChR. This suggest that the majority of the human MG antibodies are also directed against the MIR. It has however to be verified by direct experiments.