Myasthenic serum selectively blocks acetylcholine receptors with long channel open times at developing rat endplates

Autoimmunity and neurological disease: antibody modulation of synaptic transmission

Over the past three decades, compelling evidence has emerged that the immune system can attack the nervous system with devastating consequences for human health. Either cell-mediated or humoral (antibody-mediated) autoimmune mechanisms may predominate in effecting a given disease, and either glia or neurons may fall under immune attack. A subset of these diseases has been particularly useful for understanding fundamental neuroscience as well as mechanisms of human disease. This subset involves humoral autoimmune attack on cell surface molecules subserving transmembrane signaling of excitable cells; special emphasis is placed here on proteins involved in synaptic transmission. We begin by reviewing the prototypic humoral autoimmune disease of synaptic transmission, myasthenia gravis. This provides a context for insights obtained from the study of diseases targeting molecules that regulate synaptic transmission at the neuromuscular junction and in the central nervous system. We also explore a disease where autoimmunity produces agonist antibodies acting at two distinct G-protein-coupled receptors. We conclude with an exploration of the vital issue of access of antibodies to targets within the central nervous system and the implications that such access may have in the pathogenesis of poorly understood idiopathic central nervous system diseases.

Detection of antibodies directed against the cytoplasmic region of the human acetylcholine receptor in sera from myasthenia gravis patients

The nicotinic acetylcholine receptor (AChR) is the autoantigen in the human autoimmune disease myasthenia gravis (MG). Anti-AChR antibodies in MG sera bind mainly to conformational epitopes, therefore the determination of their specificities requires the use of native AChR. Antibody competition studies suggest that most MG antibodies are directed against the extracellular part of the molecule, whereas antibodies directed against the cytoplasmic region of the AChR have not been detected. To determine whether even small quantities of such antibodies exist in MG sera, we performed competition experiments based on the inhibition by MG sera of the binding of MoAbs to the human AChR, rather than inhibition by MoAbs of the binding of MG sera performed earlier. When MoAbs directed against cytoplasmic epitopes on the alpha or beta subunits (alpha 373-380 and beta 354-360) were used as test MoAbs, 17% or 9% of MG sera inhibited the binding of the anti-alpha or anti-beta subunit MoAbs, respectively, by > or = 50%. Non-specific inhibition was excluded. These results suggest the presence, in several MG sera, of antibodies directed against cytoplasmic regions of the AChR; yet these antibodies seemed to represent a relatively small proportion of the total anti-AChR antibodies. The corresponding epitopes may be involved in the inducing mechanisms in certain MG cases, and knowledge of the presence of such antibodies may be useful in understanding the autoimmune mechanism involved in 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.

Pre- and postsynaptic blockade of neuromuscular transmission by Miller-Fisher syndrome IgG at mouse motor nerve terminals

Miller-Fisher syndrome, a variant of an acute inflammatory neuropathy is often associated with serum antibodies to the ganglioside GQ1b, but the pathogenic role of these antibodies and other serum factors is unclear. We here investigated the effect of highly purified immunoglobulin G (IgG) from patients with typical Miller-Fisher syndrome, recording quantal endplate currents by means of a perfused macro-patch-clamp electrode on hemidiaphragms of adult mice. The GQ1b-positive and the GQ1b-negative Miller-Fisher IgG as well as its monovalent Fab-fragments depressed evoked quantal release in a fast and fully reversible, concentration and voltage dependent manner. The time-course of quantal release was changed with the late releases becoming more frequent. The extent of depression of release followed a Michaelis-Menten kinetic and depended on the extracellular calcium concentration. In addition the amplitude of quanta was reduced postsynaptically. IgG and sera from healthy subjects had no effect. Our results indicate that in Miller-Fisher syndrome, IgG antibodies to an undetermined antigen depress the release process, most likely by interfering with the presynaptic Ca2+ inflow or by interacting with proteins of the exocytotic apparatus, and prevent the activation of postsynaptic channels. Antibodies thus seem to be one pathogenic factor for muscle weakness in Miller-Fisher syndrome and our findings may explain why muscle strength recovers rapidly after therapeutical plasmapheresis.

T helper cell recognition of muscle acetylcholine receptor in myasthenia gravis. Epitopes on the gamma and delta subunits

We tested the response of CD4+ cells and/or total lymphocytes from the blood of 22 myasthenic patients and 10 healthy controls to overlapping synthetic peptides, 20 residues long, to screen the sequence of the gamma and delta subunits of human muscle acetylcholine receptor (AChR). The gamma subunit is part of the AChR expressed in embryonic muscle and is substituted in the AChRs of most adult muscles by an epsilon subunit. The delta subunit is present in both embryonic and adult AChRs. Adult extrinsic ocular muscles, which are preferentially and sometimes uniquely affected by myasthenic symptoms, and thymus, which has a still obscure but important role in the pathogenesis of myasthenia gravis, express the embryonic gamma subunit. Anti-AChR CD4+ responses were more easily detected after CD8+ depletion. All responders recognized epitopes on both the gamma and delta subunits and had severe symptoms. In four patients the CD4+ cell response was tested twice, when the symptoms were severe and during a period of remission. Consistently, the response was only detectable, or larger, when the patients were severely affected.

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.

Myasthenia gravis. CD4+ T epitopes on the embryonic gamma subunit of human muscle acetylcholine receptor

In myasthenia gravis (MG) an autoimmune response against muscle acetylcholine receptor (AChR) occurs. Embryonic muscle AChR contains a gamma subunit, substituted in adult muscle by a homologous epsilon subunit. Antibodies and CD4+ cells specific for embryonic AChR have been demonstrated in MG patients. We identified sequence segments of the human gamma subunit forming epitopes recognized by four embryonic AChR-specific CD4+ T cell lines, propagated from MG patients' blood by stimulation with synthetic peptides corresponding to the human gamma subunit sequence. Each line had an individual epitope repertoire, but two 20-residue sequence regions were recognized by three lines of different HLA haplotype. Most T epitope sequences were highly diverged between the gamma and the other AChR subunits, confirming the specificity of the T cells for embryonic AChR. These T cells may have been sensitized against AChR expressed by a tissue other than innervated skeletal muscle, possibly the thymus, which expresses an embryonic muscle AChR-like protein, containing a gamma subunit. Several sequence segments forming T epitopes are similar to regions of microbial and/or mammalian proteins unrelated to the AChR. These findings are consistent with the possibility that T cell cross-reactivity between unrelated proteins ("molecular mimicry"), proposed as a cause of autoimmune responses, is not a rare event.

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.

A myasthenia gravis plasma immunoglobulin reduces miniature endplate potentials at human endplates in vitro

A particular myasthenia gravis (MG) plasma Ig has previously been shown to block a single alpha-bungarotoxin (alpha-BuTx) binding site on embryonic rat muscle acetylcholine receptor (AChR). We have investigated its effect on embryonic/denervated and adult human AChR both in extracts and in situ. Plasma Ig blocked 125I-alpha-BuTx binding by greater than 85% to the AChR extracted from denervated muscle, but only by 55% to AChR extracted from normal human muscle. Incubation of intact human muscle fibers with the plasma Ig reduced 125I-alpha-BuTx binding to the endplate AChRs by 63%, and substantially decreased the amplitude of miniature endplate potentials. We conclude that anti-alpha-BuTx site antibodies, when present, can be important in the pathophysiology of the disease.

Effects of a monoclonal anti-acetylcholine receptor antibody on the avian end-plate

1. The effects of anti-acetylcholine receptor (AChR) monoclonal antibodies (mAbs) 370 and 132A on miniature end-plate potentials (MEPPs) and end-plate currents (EPCs) in the posterior latissimus dorsi muscle of adult chickens were investigated. 2. After incubation of the electrophysiological preparation with mAb 370 (5-50 micrograms/ml), which blocks both agonist (carbamylcholine) and alpha-bungarotoxin (alpha-BTX) binding and induces a hyperacute form of experimental autoimmune myasthenia gravis (EAMG), MEPP and EPC amplitudes were irreversibly reduced. 3. This effect was not associated with any significant change in the time constant describing EPC decay (tau EPC), current reversal potential, or the voltage dependence of tau EPC. The tau EPC at -80 mV was 5.9 +/- 0.6 ms before incubation with mAb 370 (50 micrograms/ml) and 6.0 +/- 0.9 ms afterwards. Current reversal potential was -3.9 +/- 0.4 mV before mAb incubation and -4.8 +/- 1.5 mV afterwards. The change in membrane potential required to produce an e-fold change in tau EPC was 128 +/- 2.3 mV before antibody incubation compared to 125 +/- 6.6 mV after incubation. 4. A second anti-AChR mAb, 132A (50 micrograms/ml), which is capable of inducing the classically described form of EAMG without blocking agonist or alpha-BTX binding, or inducing hyperacute EAMG, produced no significant change in MEPP amplitude, EPC amplitude, tau EPC or EPC reversal potentials. 5. The mAb 370 (50 micrograms/ml) induced a partially reversible decrease of the quantal content of the neurally evoked end-plate potential (EPP). This effect was not observed with mAb 132A, (+)tubocurarine (10(-7)-10(-5) g/ml) or an irrelevant anti-oestrogen receptor mAb. 6. These data suggest that the rapid onset of weakness observed in chicken hatchlings after the injection of mAb 370 (Gomez & Richman, 1983) can be attributed to a combined effect of a block of acetylcholine (ACh)-induced ion channel activity in the postsynaptic membrane and a reduction of the neurally evoked release of acetylcholine from the nerve terminal.

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

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

Epitopes on human acetylcholine receptor defined by monoclonal antibodies and myasthenia gravis sera

The mouse monoclonal antibody (m.ab) binding sites on human acetylcholine receptor have been mapped by inhibition by F(ab')2 m.ab fragments, by competition with a rat m.ab against the main immunogenic region (anti-m.i.r), and by their ability to protect the a-Bungarotoxin (a-BuTx) binding sites from inhibition by a myasthenia gravis (MG) plasma. Two m.abs (C3 and D6) that bind to two distinct but overlapping regions on the AChR, were inhibited by the anti-m.i.r. m.ab M35. M.abs binding to three other regions protected the a-BuTx sites by up to 50%. In further inhibition assays these m.abs were used to define the binding sites for MG anti-AChR antibodies. There was considerable heterogeneity in the antigenic specificity of the MG sera. The inhibition of MG anti-AChR by anti-m.i.r. M35 correlated highly with inhibition by mouse m.ab D6, but not with inhibition by m.ab C3. There was a correlation between inhibition by m.abs F8 and B3, although these m.abs bind to two non-overlapping regions. Some MG anti-AChR antibodies bound to epitopes that only partially overlapped those defined by m.abs. Inhibition of human antibody binding by m.abs raised against specific antigens is a useful approach that should help define the epitopes to which autoantibodies bind.

A structural and dynamic model for the nicotinic acetylcholine receptor

Folding of the five polypeptide subunits (alpha 2 beta gamma delta) of the nicotinic acetylcholine receptor (AChR) into a functional structural model is described. The principles used to arrange the sequences into a structure include: (1) hydrophobicity----membrane-crossing segments; (2) amphipathic character----ion-carrying segments (ion channel with single group rotations); (3) molecular shape (elongated, pentagonal cylinder)----folding dimensions of exobilayer portion; (4) choice of acetylcholine binding sites----specific folding of exobilayer segments; (5) location of reducible disulfides (near agonist binding site)----additional specification of exobilayer arrangement; (6) genetic homology----consistency of functional group choices; (7) noncompetitive antagonist labeling----arrangement of bilayer helices. The AChR model is divided into three parts: (a) exobilayer consisting of 11 antiparallel beta-strands from each subunit; (b) bilayer consisting of four hydrophobic and one amphiphilic alpha-helix from each subunit; (c) cytoplasmic consisting of one (folded) loop from each subunit. The exobilayer strands can form a closed 'flower' (the 'resting state') which is opened ('activated') by agonists bound perpendicular to the strands. Rearrangement of the agonists to a strand-parallel position and partial closing of the 'flower' leads to a desensitized receptor. The actions of acetylcholine and succinoyl and suberoyl bis-cholines are clarified by the model. The opening and closing of the exobilayer 'flower' controls access to the ion channel which is composed of the five amphiphilic bilayer helices. A molecular mechanism for ion flow in the channel is given. Openings interrupted by short duration closings (50 microseconds) depend upon channel group motions. The unusual photolabeling of intrabilayer serines in alpha, beta and delta subunits but not in gamma subunits near the binding site for non-competitive antagonists is explained along with a mechanism for the action of these antagonists such as phencyclidine. The unusual alpha 192Cys-193Cys disulfide may have a special peptide arrangement, such as a cis-peptide bond to a following proline (G.A. Petsko and E.M. Kosower, unpublished results). The position of phosphorylatable sites and proline-rich segments are noted for the cytoplasmic loops. The dynamic behavior of the AChR channel and many different experimental results can be interpreted in terms of the model. An example is the lowering of ionic conductivity on substitution of bovine for Torpedo delta M2 segment. The model represents a useful construct for the design of experiments on AChR.

Heterogeneous kinetic properties of acetylcholine receptor channels in Xenopus myocytes

We have used the cell-attached patch-clamp technique to examine the kinetic and conductance properties of acetylcholine receptor channel currents in cultured Xenopus myocytes. At high agonist concentrations (5-100 microM) the currents occurred in bursts of openings. The probability that a channel existed in an ion-conducting conformation during a burst (Po) was adopted as an empirical measure of channel kinetic behaviour. All openings within a given burst were to the same mean current amplitude. However, different bursts could have openings with a mean conductance of either 46 pS (gamma 40) or 64 pS (gamma 60). For gamma 40 bursts there were three predominant populations which could be distinguished by their mean Po values (approximately 0.9, approximately 0.3, and less than 0.01 at 20 microM-acetylcholine). Po values increased as the acetylcholine concentration within the micropipette was increased. About 80% of bursts were from the highest Po population. gamma 60 bursts also occurred in three predominant modes. The highest Po population accounted for greater than 80% of all bursts and had a mean Po of approximately 0.6 at 20 microM-acetylcholine. For both gamma 40 and gamma 60 channels, bursts from the highest Po population had open-interval durations which were approximately 4 times longer than those from bursts from the medium Po population. Closed intervals from gamma 40, high Po bursts were approximately 4 times shorter than those from medium Po bursts. Occasional examples of switching between different kinetic modes were observed, suggesting that the Po populations may represent different activity patterns of a homogeneous channel population.

Apparent acetylcholine receptor channel conversion at individual rat soleus end-plates in vitro

Miniature end-plate currents (m.e.p.c.s.) were recorded extracellularly from individual fibres in neonatal rat soleus muscles for 2-24 h. In agreement with previous studies, the decay phases of m.e.p.c.s at many end-plates were doubly exponential with time constants of approximately 6 ms and approximately 1.5 ms at 21 degrees C. Earlier studies have shown that doubly exponential decays are due to the combined action of embryonic-type acetylcholine (ACh) receptors (AChRs) with long channel open times and adult-type AChRs with brief open times. When individual end-plates with doubly exponential m.e.p.c.s were studied for several hours or more, the relative size of the slow decay component frequently decreased with time. There was no evidence for a corresponding decrease in total m.e.p.c. amplitude. The time constants of the fast and slow components did not change. M.e.p.c. decays were stable at end-plates that were either very mature (small slow decay component) or very immature (small fast decay component). In these cases, the decay phases were virtually singly exponential and the time constant did not change. These data indicate that at end-plates with a mixture of adult-type and embryonic-type channels, the fraction of adult-type AChRs increases with time. This is similar to what occurs at end-plates developing in vivo. The results of ACh noise analysis experiments support this interpretation.