An immunologically induced defect of neuromuscular transmission in rats and rabbits

Suppression of the CD28/B7 pathway reduces the occurrence and development of myasthenia gravis and cytokine levels

BACKGROUND

Myasthenia gravis (MG) is an antibody-mediated, autoimmune neuromuscular disease. Reports have indicated that the CD28/B7 ligand interactions play a crucial role during primary immune responses. Hence, the aim of the present study was to investigate the possible effects of the CD28/B7 pathway on the occurrence and development of MG and its associated cytokine factors.

METHODS

An experimental autoimmune myasthenia gravis (EAMG) was initially established by immunization of Lewis rats with acetylcholine receptor (AChR) α97-116 peptide. Then the rats were treated with dexamethasone and CTLA4-Ig (used for inhibiting the CD28/B7 pathway). Serum levels of AChR IgG and AChR IgG2b were then detected using ELISA. The clinical features, muscle contraction function, AChR content, expression of CD28, CTLA4, B7.1 and B7.2 in mononuclear cells of peripheral blood and the secretion of cytokines (INF-γ, IL-2, IL-10 and IL-12) in serum of rats were measured. Finally, lymphocyte proliferation upon CTLA4 IgG treatment was examined in vitro.

RESULTS

Inhibition of the CD28/B7 pathway and dexamethasone were found to significantly improve clinical symptoms of EAMG rats, reduce serum levels of AChR IgG, AChR IgG2b, INF-γ, IL-2, IL-10 and IL-12, the expression of CD28, CTLA4, B7.1 and B7.2 in mononuclear cells of peripheral blood, and enhance muscle contraction function and AChR content in the muscle in vivo. Meanwhile, CTLA4 IgG could abolish the increased lymphocyte proliferation following AChR stimulation in vitro.

CONCLUSION

Overall, the suppression of the CD28/B7 pathway by CTLA4-Ig can have the potential to retard the occurrence and development of MG.

Ocular myasthenia: a protean disorder

Ocular myasthenia is a localized form of myasthenia clinically involving only the extraocular, levator palpebrae superioris, and/or orbicularis oculi muscles. Ocular manifestations can masquerade as a variety of ocular motility disorders, including cranial nerve and gaze palsies. A history of variable and fatiguable muscle weakness suggests this diagnosis, which may be confirmed by the edrophonium (Tensilon) test and acetylcholine receptor antibody titer. Anticholinesterases, corticosteroids and other immunosuppressive agents, and other therapeutic modalities, including thymectomy and plasmapheresis, are used in treatment. As the pathophysiology of myasthenia has been elucidated in recent years, newer treatment strategies have evolved, resulting in a much more favorable prognosis than several decades ago. This review provides historical background, pathophysiology, immuno-genetics, diagnostic testing, and treatment options for ocular myasthenia, as well as a discussion of drug-induced myasthenic syndromes.

Immunisation with Torpedo acetylcholine receptor

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

Neuromuscular blocking properties of suxamethonium and decamethonium in normal and myasthenic rat muscle

Patients with myasthenia gravis (MG) have increased tolerance to the neuromuscular blocking properties of suxamethonium (SCh) and decamethonium (C10) and exhibit a reversal of the C10-induced block by neostigmine. The effects of these drugs were compared in forelimb flexor digitorum longus muscle from normal rats and from rats with experimental autoimmune myasthenia gravis (EAMG) to investigate the similarity of EAMG to MG. The depolarization induced by 1, 5, 10 and 25 microM SCh or C10 at the motor end-plates was significantly higher in normal than in EAMG muscle. However, both normal and EAMG end-plates responded in a similar qualitative manner to each drug. The depolarization produced by SCh was typically maintained until the drug was washed from the bath. The depolarization produced by C10 tended to decrease after reaching its peak despite continued application of the drug. With both drugs, miniature end-plate potential (MEPP) amplitude reduction is maintained until a saline wash. Neostigmine interaction with SCh and C10 in normal and EAMG muscle was compared by measuring isometric twitch tension in vitro. Neostigmine potentiated the neuromuscular block produced by either SCh or C10 in both normal and EAMG muscle. Thus muscle from rats with EAMG shares with MG an increased tolerance to SCh and C10 when compared to normal muscle but does not exhibit the qualitatively different interaction of C10 affected muscle with neostigmine that is found in MG patients. This and other studies comparing EAMG and MG indicate that EAMG is an appropriate model of MG but differences such as we have noted should be considered when extrapolating data from EAMG to the human disease.

Morphological changes observed in rats immunized with the Torpedo acetylcholine receptor alpha-chain

Lewis rats were immunized with nicotinic acetylcholine receptor (nAChR) polypeptides purified from the electric tissue of Torpedo marmorata. Animals immunized with the acetylcholine binding, 41.500 daltons, polypeptide showed comparable fragmentation of the postsynaptic membrane and indications of rearrangement of the adjacent muscle fiber as earlier found in rabbits and rats immunized with the nAChR complex. Thus, these results demonstrate that the alpha-polypeptide chain carries all determinants necessary for onset of Experimental Autoimmune Myasthenia Gravis (EAMG).

On-line analysis of neuromuscular bioelectric potentials

A computer system is presented which provides for on-line data capture and analysis of evoked end-plate potentials and action potentials, and on-line data capture with off-line analysis of spontaneously occurring miniature end-plate potentials at the end-plate region of the neuromuscular junction. Sampling of evoked waveforms begins after an adjustable delay following the stimulus. Spontaneously occurring waveforms are captured by 'freezing' the contents of a circular buffer. The software provides MENU selectable support functions including storage and retrieval of data and calculated parameters, analog and digital display of waveforms, data calibration and gain modification, data editing, file management, and hardcopy output. Calculated parameters of the waveform are optionally placed in a data base file by the analysis programs. The data base may be used for editing, arithmetic operations, and subsetting of variables as well as statistical analysis and plotting of any selected variables.

Recent advances in the molecular mechanisms of human and animal models of myasthenia gravis

The receptor-channel molecule is a dynamic system which exists in multiple conformations and that is the way we should think of it when we study antibody interaction with the molecule. The results presented here suggest that some antibodies may affect receptor function by occupying sites other than the receptor site. Some of these sites may by exposed only in certain conformations, and occupation of some site by antibodies may effect conformational changes. These small but perhaps important differences in cholinergic channel properties of the myasthenic muscle from the normal one are revealed by studying the effect of myasthenic sera on drug interactions with the channel sites. The sera of myasthenics are able to react with certain channel conformations and are able to affect the interaction of channel antagonists such as H12HTX and QNB. The sera appear to act preferentially with the open conformation of the channel. As a consequence of such an effect, important conformational changes of the channel may fail to occur upon activation.

Properties of end-plate channels in rats immunized against acetylcholine receptors

1. Rats injected with purified acetylcholine receptors (AChR) extracted from electric organs of Torpedo marmorata showed clinical symptoms consistent with the development of experimental myasthenia gravis.2. Sera of rats with this disease contain high levels of anti-AChR antibodies. However, no simple correlation was found between antibody titre and miniature end-plate current (m.e.p.c.) amplitude.3. M.e.p.c.s. at the end-plates of rats injected with AChR (Anti-R), emulsified in complete Freund Adjuvant (CFA), were reduced to about one third the size of controls taken from rats injected only with CFA (Anti-CFA). Mean m.e.p.c. (Anti-R) = 0.73 +/- 0.06 nA; mean m.e.p.c. (Anti-CFA) = 2.43 +/- 0.12 nA (V(m) = -80 mV, T = 20 degrees C).4. The m.e.p.c. decay time constant, tau(m.e.p.c.), is similar at immunized and control rat end-plates. tau(m.e.p.c.) (Anti-R) = 1.32 +/- 0.06 msec; tau(m.e.p.c.) (Anti-CFA) = 1.31 +/- 0.06 msec (V(m) = -80 mV, T = 20 degrees C).5. The end-plate current decay time constant, tau(e.p.c.), is similar at immunized and control end-plates and in both cases depends exponentially on membrane potential. The change in membrane potential required to produce an e-fold change in tau(e.p.c.) is 102.0 +/- 5.72 mV at immunized (Anti-R) end-plates and 92.3 +/- 6.14 mV at control (Anti-CFA) end-plates at T = 10 degrees C.6. Acetylcholine noise was examined at immunized and control rat end-plates at 10 degrees C. Analysis of noise indicates that the single channel conductance, gamma, and mean channel life-time, tau(noise), are essentially unchanged by immunization against AChR. gamma (Anti-R) = 13.15 +/- 0.53 pS; gamma (Anti-CFA) = 12.50 +/- 0.50 pS; tau(noise) (Anti-R) = 2.9 +/- 0.18 msec; tau(noise) (Anti-CFA) = 2.68 +/- 0.14 msec (V(m) = -80 mV, T = 10 degrees C).7. Mean quantal content and Ca(2+) dependence of the end-plate potential are unchanged at immunized end-plates.8. It is concluded that at immunized end-plates the number of activated receptor-channel complexes is reduced without modification of single channel properties. In this respect the immunized rat end-plate is a good model for myasthenia gravis affected human end-plates.

Facilitatory effects of 4-aminopyridine on neuromuscular transmission in disease states

The in vitro effects of 4-aminopyridine (4-AP) on neuromuscular transmission were determined by microelectrode techniques in intercostal muscles from patients with myasthenia gravis (MG) and the Eaton-Lambert syndrome (ELS), and in forelimb muscles from rats with experimental autoimmune myasthenia gravis (EAMG). In MG and EAMG, the amplitudes of miniature endplate potentials (MEPPs) and endplate potentials (EPPs) were reduced, and there was increased sensitivity to the blocking action of d-tubocurarine (dTc). In ELS, MEPP amplitude was normal but the average number of acetylcholine quanta released by nerve impulses was reduced, causing subthreshold EPPs. In EAMG muscle, 4-AP produced dose-dependent increases in EPP amplitude and in the duration of indirectly elicited muscle action potentials but no changes in MEPP amplitude and resting membrane potential. 4-AP completely reversed the postsynaptic blockade produced by dTc and EAMG. 4-AP appears to facilitate neuromuscular transmission in EAMG, MG, and ELS by increasing the neurally evoked transmitter release, thus overcoming either the pre- or the postsynaptic neuromuscular blockade.

Inhibition of alpha-bungarotoxin binding to acetylcholine receptors by antisera from animals with experimental autoimmune myasthenia gravis

Conditions are described for an assay that allows the percent inhibition of alpha-bungarotoxin binding to acetylcholine receptors by antisera and monovalent antigen-binding fragments of antibody molecules (Fab) to be determined. Anti-Torpedo californica acetylcholine-receptor antisera, prepared in New Zealand White rabbits and Lewis rats, were tested for the ability to inhibit [125I]-alpha-bungarotoxin binding to membrane-associated and detergent-solubilized T californica acetylcholine receptors. Similar inhibition studies were performed using rabbit antisera and antigen-binding fragments prepared against each of the four acetylcholine receptor subunits. Antisera and antigen-binding fragments prepared against intact receptor could inhibit a maximum of 50% of the alpha-bungarotoxin binding to solubilized receptor. The results using monovalent antigen-binding fragments indicated that the inhibition was not due to antibody-mediated aggregation of receptor molecules. Rabbits and rats immunized with receptor denatured by sodium dodecyl sulfate all produced antisera that could bind to nondenatured receptor, but none of these animals developed experimental autoimmune myasthenia gravis. These results suggest that the antigenic determinants present on acetylcholine receptors responsible for induction of experimental autoimmune myasthenia gravis are lost with sodium dodecyl sulfate denaturation. A strong correlation was also observed between the presence of experimental autoimmune myasthenia gravis in rats and rabbits and the ability of the antisera from these animals to inhibit 50% of alpha-bungarotoxin binding to solubilized acetylcholine receptors.

Effects of D-penicillamine on neuromuscular transmission in rats

Treatment of patients with D-penicillamine (D-P) has been associated with a syndrome similar to myasthenia gravis (MG). To explore this association, we examined the effects of D-P on neuromuscular transmission in rat muscle. In the first experiment, bath-applied D-P had no significant effect on either miniature endplate potential (MEPP) amplitude or action potential (AP) amplitude. Endplate potential (EPP) amplitude and spontaneous MEPP frequency decreased significantly at concentrations approximately 40 times the maximum human therapeutic level. In the second experiment, rats receiving D-P by daily injections for 33 to 37 days did not differ from controls in any of the measured electrophysiologic characteristics. Electron microscopy of muscle endplates from rats treated with D-P showed no evidence of degeneration or simplification. In all cases, thymus histology by light microscopy was normal, and no antireceptor antibodies were found. Thus, D-P has a mild direct presynaptic effect on neuromuscular transmission at high concentrations, but this effect is too small to account for the weakness seen in the myasthenia-like syndrome in humans.

Short-term effects of prednisolone on neuromuscular transmission in normal rats and those with experimental autoimmune myasthenia gravis

Electrophysiological investigations of the effects of bath-applied prednisolone at the neuromuscular junction were performed in muscles from normal rats and rats with experimental autoimmune myasthenia gravis (EAMG). In muscles from both groups, prednisolone reversible and significantly depressed the amplitudes of minature end-plate potentials (MEPPs), end-plate potentials (EPPs) and indirectly elicited action potentials (APs) without affecting resting membrane potentials. Prednisolone also caused a significant reduction in EPP rise time to peak and half-decay time while markedly increasing MEPP frequency and AP rise time to peak and duration. These effects were shown to be dose-dependent. The percentage decrease in amplitude after prednisolone perfusion was similar for EPPs and MEPPs, indicating that the depressive effect of prednisolone at the junction is postsynaptic. In all of the parameters studied, the percentage effect of prednisolone was the same in EAMG and normal preparations. No stimulus-linked repetitive EPPs or APs were observed after prednisolone. It is concluded that prednisolone has a depressive effect on neuromuscular transmission, but that this occurs only at high concentrations of the drug which are not achieved during the treatment of myasthenia gravis.

Myasthenia in frogs immunized against cholinergic-receptor protein

Frogs immunized with cholinergic-receptor protein developed myasthenia in 116--175 days. The muscular weakness was overcome by subcutaneous administration of 20 microgram of neostigmine. Electromyograms showed a decline in action potential amplitude during a 2-Hz train. Nerve stimulation evoked subthreshold end-plate potentials (EPPs) averaging 10.4 +/- 7.4 mV, but at many junctions no EPP was obtained. Miniature EPP amplitude had a modal value of 0.15 mV compared with 0.35 mV for the controls. The corresponding means were 0.24 +/- 0.23 mV and 0.48 +/- 0.23 mV. Microperfusion with edrophonium (5 mg/l) increased the amplitude of EPPs and miniature end-plate potentials (MEPPS). Postjunctional response tested with 20 muM carbamylcholine was 56% of control. Postjunctional response by carbamylcholine iontophoresis gave 19 +/- 22 mV/nC compared with 76 +/- 50 mV/nC for the controls. The data indicate that the neuromuscular transmission deficits in receptor-immunized frogs are mainly postsynaptic in origin, but there may be additional presynaptic contributions. This amphibian model of myasthenia gravis offers many opportunities and advantages in the study of receptor-immunized animals.

Acetylcholine receptor and ionic channel of Torpedo electroplax: binding of perhydrohistrionicotoxin to membrane and solubilized preparations

The electric organ of the ray, Torpedo ocellata, can serve as a source for both the acetylcholine (ACh) receptor and its ionic channel. The two entities were identified by their specific binding of [3H]ACh and [3H]perhydrohistrionicotoxin ([3H]H12-HTX), respectively. Binding of [3H]H12-HTX was inhibited by certain drugs and toxins, e.g., histrionicotoxin (HTX), amantadine, and tetraethylammonium (TEA) ions at concentrations that did not inhibit [3H]ACh binding. However, the specific carbamoylcholine-induced 22Na efflux from microsacs from the electric organ membranes was blocked by inhibitors of either the receptor or its ionic channel. The ionic channel had the properties of a protein as judged by heat sensitivity and the inhibition of [3H]H12-HTX binding, after incubation of the electric organ membranes with protein reagents such as p-chloromercuribenzenesulfonic acid (PCMBS) or N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). The "binding" of [3H]H12-HTX at 4 X 10(-8) M to lipids in the microsacs was 12% of the total binding to intact microsacs and was nonsaturable and insensitive to heat or specific drugs. After solubilization with cholate, the [3H]H12-HTX binding subunits retained the same affinities for toxins and drugs. The Kd for [3H]H12-HTX was 3 X 10(-7) M. The majority of the ionic channel could be separated from the ACh receptors in the cholate extract by incubation with ACh-receptor affinity gel and ACh-receptor antibodies. The ACh receptor purified by this affinity gel contained only a few active ionic channel units as judged by low levels of high affinity binding of [3H]H12-HTX. On the other hand, after solubilization with Triton X-100, all the ionic channel molecules were either separated or denatured so that the purified ACh receptor did not exhibit high affinity binding for [3H]H12-HTX.

Myasthenia gravis: further electrophysiological and ultrastructural analysis of transmission failure in the mouse passive transfer model

Using the mouse passive transfer model the mean amplitude of miniature endplate potentials and endplate potentials of mice treated with myasthenic immunoglobulins was markedly decreased. Miniature endplate potential frequency and quantum content of endplate potentials were normal, arguing against a major presynaptic disarrangement. Under electron-microscopy no gross structural alterations of endplates were demonstrated. It is concluded that the mouse passive transfer model closely resembles human myasthenia gravis of recent onset.

Neonatal experimental autoimmune myasthenia gravis

Neonatal rats born of and nursed by mothers immunized with Torpedo acetylcholine receptor protein developed a defect of neuromuscular transmission as indicated by reduced miniature endplate potential amplitudes. It is likely that antibodies to the Torpedo receptor protein were passively transferred to the neonates in the milk. With the exception of the route of transfer, this neonatal form of experimental autoimmune myasthenia gravis appears to be similar to its human counterpart, and thus can serve as an experimental model.

Study of two different subpopulations of anti-acetylcholine receptor antibodies in a rabbit with experimental auto-immune myasthenia gravis

Two different subpopulations of anti-acetylcholine receptor antibodies were studied during the evolution of experimental autoimmune myasthenia in one rabbit immunized with Torpedo acetylcholine receptor. The results show that the subpopulation of antibodies directed against the toxin-binding site of the receptor might play a role in the appearance of the paralysis observed in this particular case.

Myasthenic immunoglobulin accelerates acetylcholine receptor degradation

Degradation of acetylcholine receptors by cultured rat skeletal muscle cells was determined from the release of 125I from bound 125I-labeled alpha-bungarotoxin. Addition of immunoglobulin from patients with myasthenia gravis to the culture medium accelerated the degradation rate to a mean of 8.51 +/- 0.44 percent per hour, compared with the mean control rate of 3.97 +/- 0.14 percent per hour (P less than .001). A similar mechanism may possibly be involved in the autoimmune pathogenesis of myasthenia gravis in man.

Studies of human myasthenia gravis: electrophysiological and ultrastructural evidence compatible with antibody attachment to acetylcholine receptor complex

Neuromuscular junctions from patients with early onset and chronic myasthenia gravis were examined by electrophysiological and ultrastructural techniques. Acetylcholine (AcCh) sensitivities were reduced by 34-63% in early onset myasthenia and 60-80% in chronic myasthenia. Ultrastructural analysis revealed that virtually all junctional folds of the early onset patients were intact but that the AcCh-receptor-rich crests of these folds were uniformly covered by an attached layer of 30 X 70 A particles arranged in small tufts or rosettes. In chronic myasthenic endplates, however, junctional fold crests were destroyed, apparently being replaced by vesicular membrane debris similarly labeled by tufts of 30 X 70 A particles. Thus, the initial reduction in junctional AcCh sensitivity observed in early onset myasthenia gravis may be attributed at least in part to in situ masking or inactivation of AcCh receptors, whereas the marked decrease in AcCh sensitivity observed in the chronic myasthenic patient may represent a combination of two factors: (a) in situ masking of AcCh receptors and (b) destruction of the receptor-containing crests of the junctional folds. These observations are compatible with an autoimmune etiology of myasthenia gravis initially involving an apparent antibody attachment to one or more components of the functional AcCh receptor complex, followed by systematic destruction and removal of junctional folds by both humoral and cell-mediated autoimmune responses.

Antibodies to acetylcholine receptors in rabbits: immunochemical and electrophysiological studies

Acetylcholine receptors (AChR), purified from electric eel and Torpedo by affinity chromatography using a synthetic quaternary ammonium ligand, bound 10 nmole alpha-bungarotoxin (alpha-BuTx) per mg of protein and demonstrated a common subunit. Rabbits, immunized with either eel or Torpedo/AChR, developed flaccid paralysis barely altered by anticholinesterases and died 48 hours after the first sign of paralysis. In paralyzed animals, repetitive stimulation at low rates induced a 50% to 90% decrement of evoked potentials, temporarily reversed by edrophonium. Extracellular and introcellular MEPP amplitudes were 40% to 50% of controls. Serum and isolated IgG formed single immunoprecipitin lines against the antigens. AChR-antibody complexes did not bind alpha-BuTx, whereas alpha-BuTx-AChR complexes bound antibody if the antibody was specific for that AChR. Torpedo and eel AChR showed partial idenity with both antisera. Rat diaphragms and eel electroplax incubated with antisera to Torpedo and eel AChR showed a 50%-60% reduction in carbamylcholine depolarization. These studies demonstrate differences between eel and Torpedo AChR induce a block in neuromuscular transmission in rabbits with features of myasthenia gravis.

Carbamylcholine and acetylcholine-sensitive, cation-selective ionophore as part of the purified acetylcholine receptor

Black lipid membranes were formed with oxidized cholesterol in the presence of either the acetylcholine receptor, purified from the electric organ of the electric ray Torpedo californica or its tryptic digest. In both cases, conductance of cations increased and was dependent on the concentration of the receptor protein. Conductance of Ca++ was dependent on the concentration, but addition of carbamylcholine gave no reproducible of consistent effects. Only in the case of the tryptic digest of the acetylcholine receptor did carbamylcholine and acetylcholine consistently induce monovalent cation selective conductance (PNa,K: PCl=4.4). The induced monovalent cationic conductance due to carbamylcholine (10 muM) varied from 10- to over 100-fold. Curare (10muM) prevented the action of carbamylcholine. Na-dodecyl sulfate gel electrophoresis of the acetylcholine receptor, before and after tryptic digestion, indicated that this mild enzyme treatment hydrolyzed the receptor molecule subunits. Nevertheless, the receptor molecule retained its full binding of [acetyl(-3)H]acetylcholine; and analytical gel electrophoresis indicated that it remained intact possibly through hydrogen, hydrophobic and disulfice bonding.