Model of the extracellular domain of the human alpha7 nAChR based on the crystal structure of the mouse alpha1 nAChR extracellular domain

Neuronal nicotinic acetylcholine receptors (nAChRs) are important therapeutic targets for various diseases, including Alzheimer's disease, Parkinson's disease, and schizophrenia, as well as for cessation of smoking. Based on the recently determined crystal structure of the extracellular domain (ECD) of the mouse nAChR alpha1 subunit complexed with alpha-bungarotoxin at 1.94A resolution, we have constructed three-dimensional models of the ECD of the monomer, homodimer, and homopentamer of the human alpha7 nAChR and investigated in detail the interface between the two alpha7 subunits. The docking of the agonist in the ligand-binding pocket of the human alpha7 dimer was also performed and found consistent with results from labeling and mutagenesis experiments. Since the nAChR ligand-binding site is a useful target for mutagenesis studies and the rational design of drugs against diseases, these models provide useful information for future work.

Circular dichroism studies of extracellular domains of human nicotinic acetylcholine receptors provide an insight into their structure

The extracellular domains (ECDs) of human nicotinic acetylcholine receptors (nAChRs) are of major pharmacological interest as drug targets in the autoimmune disease myasthenia gravis and in various neurological disorders. We have previously expressed and purified the human muscle alpha1-, beta1-, gamma- and epsilon-nAChR-ECDs, as well as the wild type and a mutant of neuronal alpha7-ECD, in yeast Pichia pastoris. The far-UV circular dichroism (CD) studies of these ECDs, presented here, revealed a major prevalence of beta-sheet ( approximately 40%) and a small proportion of alpha-helical ( approximately 5%) structure for all ECDs, in good agreement with the secondary structure composition of the Torpedo muscle-type nAChR-ECDs and in less, but considerable, agreement with that of the homologous invertebrate acetylcholine-binding proteins (AChBPs). The near-UV CD studies of these nAChR-ECDs indicated well-defined tertiary structures, as was previously suggested by biochemical and immunochemical studies. Furthermore, the binding of cholinergic ligands to the mutant of alpha7-ECD resulted in no changes in its secondary structure, but revealed significant local conformational changes. Our present studies probe the structure of human nAChR-ECDs for the first time and further suggest that our expressed proteins fold to a near-native conformation, thus being suitable for further structural studies.

Extracellular domains of the β, γ and ε subunits of the human acetylcholine receptor as immunoadsorbents for myasthenic autoantibodies: A combination of immunoadsorbents results in increased efficiency

Myasthenia gravis (MG) is usually caused by autoantibodies against the human muscle acetylcholine receptor (AChR). Plasmapheresis offers a therapeutic option, but, as well as removing the pathogenic anti-AChR autoantibodies, it non-specifically removes indispensable immunoglobulins. An attractive alternative to plasmapheresis would be the extracorporeal specific removal of the autoantibodies using AChR-based immunoadsorbents. Previously, we used the N-terminal extracellular domain (ECD) of the AChR alpha subunit to immunoadsorb anti-alpha subunit autoantibodies from MG sera. In this study, we immobilised the beta -, gamma- and epsilon-AChR ECDs on Sepharose and tested them as immunoadsorbents on 50 MG sera. A given ECD removed a different percentage of autoantibodies from different sera and different ECDs removed different percentages from the same serum; on average, the beta-, gamma- and epsilon-ECDs removed 22%, 20% and 15.5% of the autoantibodies, respectively. Immunoadsorption was completed in 3 min, 1 mug of ECD removed approximately 2 pmol of autoantibodies, and the immunoadsorbent could be recycled approximately 4 times. The combined use of two (alpha+gamma) or four (alpha+beta+gamma+epsilon) ECDs in a single immunoadsorbent resulted in much higher (often additive) immunoadsorption. These results show that MG sera have autoantibodies against several AChR subunits, and suggest that the combined use of all AChR ECDs could provide the basis for a novel, antigen-specific therapy for MG.

Isolation and functional characterization of anti-acetylcholine receptor subunit-specific autoantibodies from myasthenic patients: Receptor loss in cell culture

The muscle nicotinic acetylcholine receptor (nAChR) is the major autoantigen in the autoimmune disease myasthenia gravis (MG), in which autoantibodies bind to, and cause loss of, nAChRs. Antibody-mediated nAChR loss is caused by the action of complement and by the acceleration of nAChR internalization caused by antibody-induced cross-linking of nAChR molecules (antigenic modulation). To obtain an insight into the role of the various anti-nAChR antibody specificities in MG, we have studied nAChR antigenic modulation caused by isolated anti-subunit autoantibodies. Autoantibodies against the nAChR alpha or beta subunits were isolated from four MG sera by affinity chromatography on columns carrying immobilized recombinant extracellular domains of human nAChR expressed in the yeast Pichia pastoris. The isolated anti-alpha and anti-beta autoantibodies, as well as untreated MG sera, induced nAChR antigenic modulation in TE671 cells. Partially antibody-depleted sera exhibited reduced modulating activity, whereas a serum completely depleted of anti-nAChR antibodies exhibited no nAChR modulation. Interestingly, the anti-alpha autoantibodies were, on average, approximately 4.3 times more effective than the anti-beta autoantibodies. The present work supports the notion that anti-nAChR autoantibodies may be the sole nAChR-reducing factor in anti-nAChR antibody-seropositive MG, and that anti-alpha-subunit autoantibodies are the dominant pathogenic autoantibody specificity.

Muscle and neuronal nicotinic acetylcholine receptors

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

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.

Recent advances in the understanding and therapy of myasthenia gravis

Myasthenia gravis (MG) is a T-cell dependent autoimmune disease mediated by autoantibodies, which mainly target muscle nicotinic acetylcholine receptors (AChR) and cause loss of functional AChRs in the neuromuscular junction. Both MG and its major autoantigen are studied extensively, yet the etiology of the disease remains unclear, although it is known to be associated with the thymus. A genetic predisposition, combined with several unidentified environmental stimuli, likely creates a favorable milieu in which the disease can appear. Current research focusses on elucidating the cellular and molecular pathways of immune dysregulation, which underly MG outburst and progression. Considerable progress has been made concerning the involvement of the thymus, the identification of impaired mechanisms of immune control and the B–T-cell interaction in MG pathogenesis, while the role of chemokines arises as an intriguing new puzzle. Recent findings fueled the development of novel therapeutic approaches with some encouraging, although preliminary, results. This review summarizes recent achievements in the fields of both basic research and therapeutics.

Expression and characterization of soluble forms of the extracellular domains of the beta, gamma and epsilon subunits of the human muscle acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) is a ligand-gated ion channel found in muscles and neurons. Muscle AChR, formed by five homologous subunits (alpha2 beta gamma delta or alpha2 beta gamma epsilon), is the major antigen in the autoimmune disease, myasthenia gravis (MG), in which pathogenic autoantibodies bind to, and inactivate, the AChR. The extracellular domain (ECD) of the human muscle alpha subunit has been heterologously expressed and extensively studied. Our aim was to obtain satisfactory amounts of the ECDs of the non-alpha subunits of human muscle AChR for use as starting material for the determination of the 3D structure of the receptor ECDs and for the characterization of the specificities of antibodies in sera from patients with MG. We expressed the N-terminal ECDs of the beta (amino acids 1-221; beta1-221), gamma (amino acids 1-218; gamma1-218), and epsilon (amino acids 1-219; epsilon1-219) subunits of human muscle AChR in the yeast, Pichia pastoris. beta1-221 was expressed at approximately 2 mg.L(-1) culture, whereas gamma1-218 and epsilon1-219 were expressed at 0.3-0.8 mg.L(-1) culture. All three recombinant polypeptides were glycosylated and soluble; beta1-221 was mainly in an apparently dimeric form, whereas gamma1-218 and epsilon1-219 formed soluble oligomers. CD studies of beta1-221 suggested that it has considerable beta-sheet secondary structure with a proportion of alpha-helix. Conformation-dependent mAbs against the ECDs of the beta or gamma subunits specifically recognized beta1-221 or gamma1-218, respectively, and polyclonal rabbit antiserum raised against purified beta1-221 bound to (125)I-labeled alpha-bungarotoxin-labeled human AChR. Moreover, immobilization of each ECD on Sepharose beads and incubation of the ECD-Sepharose matrices with MG sera caused a significant reduction in the concentrations of autoantibodies in the sera, showing specific binding to the recombinant ECDs. These results suggest that the expressed proteins present some near-native conformational features and are thus suitable for our purposes.

Isolation and characterization of human anti-acetylcholine receptor monoclonal antibodies from transgenic mice expressing human immunoglobulin loci

The isolation of human antibodies against muscle acetylcholine receptor (AChR), the autoantigen involved in myasthenia gravis (MG), is important for the development of therapeutically useful reagents. Monovalent antibody fragments from monoclonal antibodies against the main immunogenic region (MIR) of AChR protect the receptor from the destructive activity of MG autoantibodies. Human anti-AChR alpha-subunit antibody fragments with therapeutic potential have been isolated using phage display antibody libraries. An alternative approach for obtaining human mAb has been provided by the development of humanized mice. In this report, we show that immunization of transgenic mouse strains with the extracellular domain of the human AChR alpha-subunit results in antibody responses and isolation of hybridomas producing human mAb. Four specific IgM mAb were isolated and analyzed. mAb170 recognized the native receptor the best and was capable of inducing AChR antigenic modulation, suggesting its specificity for a pathogenic epitope. Moreover, the recombinant antigen-binding (Fab) fragment of this mAb competed with an anti-MIR mAb, revealing that its antigenic determinant lies in or near the MIR. Finally, Fab170 was able to compete with MG autoantibodies and protect the AChR against antigenic modulation induced by MG sera. This approach will be useful for isolating additional mAb with therapeutic potential against the other AChR subunits.

Isolation of potent human Fab fragments against a novel highly immunogenic region on human muscle acetylcholine receptor which protect the receptor from myasthenic autoantibodies

In the autoimmune disease myasthenia gravis (MG), antibodies against the muscle nicotinic acetylcholine receptor (AChR) cause loss of functional AChR in the neuromuscular junction. To isolate AChR-specific human antibody fragments (Fab), a phage-display library was constructed from an MG patient's thymic B lymphocytes. The first Fab isolated had a low affinity for human AChR, but two sequential antibody chain shufflings using the MG donor heavy and light chain gene repertoires resulted in isolating two new Fab with an approximately 30-fold higher binding ability. The selected Fab contained extensively mutated heavy and light chains and probably represent intraclonal variants of a common progenitor having diverged in vivo by somatic hypermutation. Interestingly, the isolated Fab bound to an extracellular highly immunogenic region located either on an alpha-subunit site affected by the gamma/epsilon-subunits or on the interface between alpha- and gamma/epsilon-subunits. This region is not the previously described "main immunogenic region" (MIR), although it seems to be close to it, as one improved Fab and an anti-MIR mAb competed for AChR binding with distinctly different subpopulations of MG sera. Furthermore, this Fab protected surface AChR in cell cultures against MG autoantibody-induced antigenic modulation, suggesting a potential therapeutic use in MG, especially in combination with a human anti-MIR Fab.

Specific immunoadsorption of the autoantibodies from myasthenic patients using the extracellular domain of the human muscle acetylcholine receptor α-subunit. Development of an antigen-specific therapeutic strategy

Antibodies against the acetylcholine receptor (AChR) are the main pathogenic factor in myasthenia gravis (MG). Clinical improvement correlates well with a reduction in levels of circulating anti-AChR antibodies, and plasmapheresis is an efficient short-term MG treatment. The Sepharose-immobilized N-terminal extracellular domain of human muscle AChR alpha-subunit was used to immunoadsorb anti-AChR autoantibodies from 50 MG patients sera. The immunoadsorbents removed 60-94% of the anti-AChR antibodies in 10 sera and a mean of 35% from all samples combined. Immunoadsorption was fast, efficient, and the columns could be used repeatedly without any release or proteolysis of the polypeptide, suggesting the feasibility of antigen-specific MG immunoadsorption therapy.

Soluble, oligomeric, and ligand-binding extracellular domain of the human alpha7 acetylcholine receptor expressed in yeast: replacement of the hydrophobic cysteine loop by the hydrophilic loop of the ACh-binding protein enhances protein solubility

The N-terminal extracellular domain (ECD; amino acids 1-208) of the neuronal nicotinic acetylcholine receptor (AChR) alpha7 subunit, the only human AChR subunit known to assemble as a homopentamer, was expressed as a glycosylated form in the yeast Pichia pastoris in order to obtain a native-like model of the extracellular part of an intact pentameric nicotinic AChR. This molecule, alpha7-ECD, although able to bind the specific ligand alpha-bungarotoxin, existed mainly in the form of microaggregates. Substitution of Cys-116 in the alpha7-ECD with serine led to a decrease in microaggregate size. A second mutant form, alpha7-ECD(C116S,Cys-loop), was generated in which, in addition to the C116S mutation, the hydrophobic Cys-loop (Cys(128)-Cys(142)) was replaced by the corresponding hydrophilic Cys-loop from the snail glial cell acetylcholine-binding protein. This second mutant protein was water-soluble, expressed at a moderate level (0.5 +/- 0.1 mg/liter), and had a size corresponding approximately to a pentamer as judged by gel filtration and electron microscopy studies. It also bound (125)I-alpha-bungarotoxin with relatively high affinity (K(d) = 57 nm), the binding being inhibited by unlabeled alpha-bungarotoxin, d-tubocurarine, or nicotine (K(i) = 0.8 x 10(-7) m, K(i) = 1 x 10(-5) m, and K(i) = 0.9 x 10(-2) m, respectively). All three constructs were expressed as glycosylated forms, but in vitro deglycosylation reduced the heterogeneity without affecting their ligand binding properties. These results show that alpha7-ECD(C116S,Cys-loop) was expressed in P. pastoris as an oligomer (probably a pentamer) with a near native conformation and that its deglycosylated form seems to be suitable starting material for structural studies on the ligand-binding domain of a neurotransmitter receptor.

Future Therapeutic Strategies in Autoimmune Myasthenia Gravis

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

Solution conformation of the antibody-bound tyrosine phosphorylation site of the nicotinic acetylcholine receptor beta-subunit in its phosphorylated and nonphosphorylated states

Phosphorylation of the acetylcholine receptor (AChR) seems to be responsible for triggering several effects including its desensitization and aggregation at the postsynaptic membrane and probably initiates a signal transduction pathway at the postsynaptic membrane. To study the structural and functional role of the tyrosine phosphorylation site of the AChR beta-subunit and contribute to the in-depth understanding of the structural basis of the ion channel function, we synthesized four peptides containing the phosphorylated and nonphosphorylated sequences (380-391) of the human and Torpedo AChR beta-subunits and studied their interaction with a monoclonal antibody (mAb 148) that is known to bind to this region and that is capable of blocking ion channel function. All four peptides were efficient inhibitors of mAb 148 binding to AChR, although the nonphosphorylated human peptide was considerably less effective than the three others. We then investigated the conformation acquired by all four peptides in their antibody-bound state, which possibly illustrates the local conformation of the corresponding sites on the intact AChR molecule. The phosphorylated human and Torpedo peptides adopted a distorted 3(10) helix conformation. The nonphosphorylated Torpedo peptide, which is also an efficient inhibitor, was also folded. In contrast, the nonphosphorylated human peptide (a less efficient inhibitor) presented an extended structure. It is concluded that the phosphorylation of the AChR at its beta-subunit Tyr site leads to a significant change in its conformation, which may affect several functions of the AChR.

Identification of regions involved in the binding of alpha-bungarotoxin to the human alpha7 neuronal nicotinic acetylcholine receptor using synthetic peptides

The neuronal alpha7 nicotinic acetylcholine receptor (AChR) binds the neurotoxin alpha-bungarotoxin (alpha-Bgt). Fine mapping of the alpha-Bgt-binding site on the human alpha7 AChR was performed using synthetic peptides covering the entire extracellular domain of the human alpha7 subunit (residues 1-206). Screening of these peptides for (125)I-alpha-Bgt binding resulted in the identification of at least two toxin-binding sites, one at residues 186-197, which exhibited the best (125)I-alpha-Bgt binding, and one at residues 159-165, with weak toxin-binding capacity; these correspond, respectively, to loops C and IV of the agonist-binding site. Toxin binding to the alpha7(186-197) peptide was almost completely inhibited by unlabelled alpha-Bgt or d -tubocurarine. Alanine substitutions within the sequence 186-198 revealed a predominant contribution of aromatic and negatively charged residues to the binding site. This sequence is homologous to the alpha-Bgt binding site of the alpha1 subunit (residues 188-200 in Torpedo AChR). In competition experiments, the soluble peptides alpha7(186-197) and Torpedo alpha1(184-200) inhibited the binding of (125)I-alpha-Bgt to the immobilized alpha7(186-197) peptide, to native Torpedo AChR, and to the extracellular domain of the human alpha1 subunit. These results suggest that the toxin-binding sites of the neuronal alpha7 and muscle-type AChRs bind to identical or overlapping sites on the alpha-Bgt molecule. In support of this, when synthetic alpha-Bgt peptides were tested for binding to the recombinant extracellular domains of the human alpha7 and alpha1 subunits, and to native Torpedo and alpha7 AChR, the results indicated that alpha-Bgt interacts with both neuronal and muscle-type AChRs through its central loop II and C-terminal tail.

Synthesis and binding affinities of 5-(3-pyridinyl)- and 5-(3-quinolinyl)-4-azahomoadamantanes to alpha7 nicotinic acetylcholine receptors

A general synthetic route that can lead to nicotinic ligands bearing a variety of bulky aza-ring systems was developed. This methodology was applied to obtain 5-(3-pyridinyl)- and 5-(3-quinolinyl)-4-azahomoadamantanes 2a, 3a and 2b, 3b. The parent 5-(3-pyridinyl)-4-azahomoadamantane 2a (Ki = 5.0 microM) binds with about 100 times lower affinity than (+)-epibatidine 1 (Ki = 0.045 microM) to alpha7 nicotinic acetylcholine receptors (nAChRs). N-methyl substitution of 2a gives compound 3a which has about nine times lower binding affinity. The replacement of pyridinyl with a quinolinyl ring (compounds 2b, 3b) results in a dramatic reduction in potency (Ki > 1000 microM).

Expression of soluble ligand- and antibody-binding extracellular domain of human muscle acetylcholine receptor alpha subunit in yeast Pichia pastoris. Role of glycosylation in alpha-bungarotoxin binding

The N-terminal extracellular domain (amino acids 1-210; halpha-(1-210)) of the alpha subunit of the human muscle nicotinic acetylcholine receptor (AChR), bearing the binding sites for cholinergic ligands and the main immunogenic region, the major target for anti-AChR antibodies in patients with myasthenia gravis, was expressed in the yeast, Pichia pastoris. The recombinant protein was water-soluble and glycosylated, and fast protein liquid chromatography analysis showed it to be a monomer. halpha-(1-210) bound (125)I-alpha-bungarotoxin with a high affinity (K(d) = 5.1 +/- 2.4 nm), and this binding was blocked by unlabeled d-tubocurarine and gallamine (K(i) approximately 7.5 mm). Interestingly, (125)I-alpha-bungarotoxin binding was markedly impaired by in vitro deglycosylation of halpha-(1-210). Several monoclonal antibodies that show partial or strict conformation-dependent binding to the AChR were able to bind to halpha-(1-210), as did antibodies from a large proportion of myasthenic patients. These results suggest that the extracellular domain of the human AChR alpha subunit expressed in P. pastoris has an apparently near native conformation. The correct folding of the recombinant protein, together with its relatively high expression yield, makes it suitable for structural studies on the nicotinic acetylcholine receptor and for use as an autoantigen in myasthenia gravis studies.

Production of antibodies to alpha(181-192) peptides of neuronal nicotinic acetylcholine receptor coupled to protein carriers in different orientations

The antibodies to nicotinic acetylcholine receptor alpha(181-192) synthetic peptides were elicited in rabbits and mice using the peptides conjugated to protein carriers in different orientations, either through C-terminal Cys (S-conjugates), or through amino groups (N-conjugates). S-conjugated peptides were less potent in eliciting peptide-specific antibodies compared to N-conjugates and this type of conjugation resulted in antibodies to the coupling reagent. However, the epitopes present in either S- or N-conjugated peptides appeared to be similar, indicating that amino acid residues, which form the epitope, were located in the middle part of the peptide and did not include both N- and C-terminal residues. Peptide conjugation to a protein carrier did not play a role in stabilizing the peptide conformation, but was necessary to concentrate the peptide epitopes on the carrier surface enabling bivalent antibody binding.

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.

Selective lesions of rabbit extraocular muscles injected with the anti-AChR immunotoxin saporin-mAb 73

PURPOSE

To evaluate the effects on extraocular muscles of a skeletal muscle-specific immunotoxin, saporin-mAb 73, as an alternative to botulinum toxin to induce a permanent correction of oculo-facial dystonias or some forms of ocular motility disorders.

METHODS

An immunotoxin was prepared with a monoclonal antibody (mAb 73) against acetylcholine receptors of skeletal muscle, linked to saporin, a type 1 ribosome-inactivating protein (RIP) from Saponaria officinalis. Sixteen New Zealand white rabbits were treated with a single injection of immunotoxin directly into the medial rectus muscle of one eye. Four different dosages of 2, 5, 20, or 50 ng saporin-mAb 73 were used. The rabbits were sacrificed at two, 7 and 14 days post-injection. The medial rectus muscle and the retractor bulbi muscle of both the injected and the fellow eyes were taken and serial sections were examined by light microscopy in a blinded manner.

RESULTS

Saporin-mAb 73, even at the dosage of 2 ng, brought about focal damage in the extraocular muscles of rabbits without histological changes in adjacent muscles. The histological examination revealed necrotic/apoptotic lesions restricted to the sites of inoculation and largely infiltrated by macrophages. No evident inflammatory reaction was detected at any time and neutrophils were substantially absent. At 14 days after injection, necrosis/apoptosis was still evident and the sclerotic reaction was minimal.

CONCLUSIONS

The immunotoxin saporin-mAb 73 injections into the extraocular muscles of rabbits caused focal damage to the muscles. There was no significant inflammatory reaction and muscle fiber loss was present even at the lower doses. Although the lesions were followed for only 14 days, our results suggest that saporin-mAb 73 has potential to cause safe focal muscle damage but longer-term follow-up are needed to investigate the persistence of muscle weakness.

Conjugation of acetylcholine receptor-protecting Fab fragments with polyethylene glycol results in a prolonged half-life in the circulation and reduced immunogenicity

Antibodies to the acetylcholine receptor (AChR) cause AChR loss, resulting in the disease, myasthenia gravis (MG). The majority of the pathogenic antibodies seem to be directed against the main immunogenic region (MIR) of the AChR. In contrast to the intact antibodies, Fab fragments of anti-AChR antibodies are not themselves pathogenic and such fragments of anti-MIR monoclonal antibodies (mAbs) protect the AChR in vitro and in vivo against the pathogenic antibodies. However, Fab fragments have a very short in vivo half-life and are immunogenic, obstacles which must be overcome before their clinical use can be envisaged. We investigated the effect of conjugating Fab fragments to polyethylene glycol (PEG), a method known to increase the in vivo half-life and reduce the immunogenicity of proteins. When the Fab' fragments of two rat anti-MIR mAbs (nos. 35 and 195) were conjugated to methoxy-PEG-maleimide, the conjugates retained about 10% of their AChR binding activity and efficiently protected the AChR against the binding and modulating activity of myasthenic antibodies. Their in vivo half-life in rats was approximately 15 times longer than that of the unconjugated Fab' fragment and they were much less immunogenic in mice. This work represents an important step towards the clinical use of AChR-protective anti-MIR Fabs, but further improvements are needed before their clinical use is attempted.

Epidemiology of seropositive myasthenia gravis in Greece

OBJECTIVES

To study the epidemiological characteristics of myasthenia gravis in Greece.

METHODS

A population based study was carried out of seropositive myasthenia gravis in Greece for the period from 1 January 1983 to 30 June 1997; 843 patients were studied.

RESULTS

The average annual incidence for the period 1992-7, for which the database is complete, was 7.40/million population/year (women 7.14; men 7.66). On 1 July 1997, there were 740 prevalent cases. The point prevalence rate was 70.63/million (women 81.58; men 59.39). The average overall annual mortality rate in the patients was 0.67/million population (women 0.53; men 0.82), and the mortality rate attributed to myasthenia gravis was 0.43/million population (women 0.41; men 0.45). The average age at onset was 46.50 years (women 40.16; men 54.46), and the mean age of the prevalent patients was 52.58 (women 47.65; men 59.48). The women:men incidence ratio was 1:1.04, and the prevalence ratio was 1.41:1. It is predicted that the prevalence and women: men prevalence ratio would increase if the patient list included all patients with a date of onset before 1983.

CONCLUSIONS

The largest epidemiological study ever performed on myasthenia gravis is presented. The most important epidemiological indexes are provided.