Endplate Membrane Research Articles

Sorting nexin 17 increases low-density lipoprotein receptor-related protein 4 membrane expression: A novel mechanism of acetylcholine receptor aggregation in myasthenia gravis.

Myasthenia gravis (MG) is characterized by autoimmune damage to the postsynaptic membrane of the neuromuscular junction (NMJ) with impaired postsynaptic acetylcholine receptor (AChR) aggregation. Low-density lipoprotein receptor-related protein 4 (LRP4) plays an important role in AChR aggregation at endplate membranes via the Agrin–LRP4–muscle-specific receptor tyrosine kinase (MuSK) cascade. Sorting nexin 17 (SNX17) regulates the degradation and recycling of various internalized membrane proteins. However, whether SNX17 regulates LRP4 remains unclear. Therefore, we examined the regulatory effects of SNX17 on LRP4 and its influence on AChR aggregation in MG. We selected C2C12 myotubes and induced LRP4 internalization via stimulation with anti-LRP4 antibody and confirmed intracellular interaction between SNX17 and LRP4. SNX17 knockdown and overexpression confirmed that SNX17 promoted MuSK phosphorylation and AChR aggregation by increasing cell surface LRP4 expression. By establishing experimental autoimmune MG (EAMG) mouse models, we identified that SNX17 upregulation improved fragmentation of the AChR structure at the NMJ and alleviated leg weakness in EAMG mice. Thus, these results reveal that SNX17 may be a novel target for future MG therapy.

A new tool to sense pH changes at the neuromuscular junction synaptic cleft

Synaptic transmission triggers transient acidification of the synaptic cleft. Recently, it has been shown that pH affects the opening of postsynaptic channels and therefore the production of tools that allow to study these behaviors should result of paramount value. We fused α-bungarotoxin, a neurotoxin derived from the snake Bungarus multicinctus that binds irreversibly to the acetylcholine receptor extracellular domain, to the pH sensitive GFP Super Ecliptic pHluorin, and efficiently expressed it in Pichia pastoris. This sensor allows synaptic changes in pH to be measured without the need of incorporating transgenes into animal cells. Here, we show that incubation of the mouse levator auris muscle with a solution containing this recombinant protein is enough to fluorescently label the endplate post synaptic membrane. Furthermore, we could physiologically alter and measure post synaptic pH by evaluating changes in the fluorescent signal of pHluorin molecules bound to acetylcholine receptors. In fact, using this tool we were able to detect a drop in 0.01 to 0.05 pH units in the vicinity of the acetylcholine receptors following vesicle exocytosis triggered by nerve electrical stimulation. Further experiments will allow to learn the precise changes in pH during and after synaptic activation.

Chemical and Pharmacological Screening of Rhinella icterica (Spix 1824) Toad Parotoid Secretion in Avian Preparations.

The biological activity of Rhinella icterica parotoid secretion (RIPS) and some of its chromatographic fractions (RI18, RI19, RI23, and RI24) was evaluated in the current study. Mass spectrometry of these fractions indicated the presence of sarmentogenin, argentinogenin, (5β,12β)-12,14-dihydroxy-11-oxobufa-3,20,22-trienolide, marinobufagin, bufogenin B, 11α,19-dihydroxy-telocinobufagin, bufotalin, monohydroxylbufotalin, 19-oxo-cinobufagin, 3α,12β,25,26-tetrahydroxy-7-oxo-5β-cholestane-26-O-sulfate, and cinobufagin-3-hemisuberate that were identified as alkaloid and steroid compounds, in addition to marinoic acid and N-methyl-5-hydroxy-tryptamine. In chick brain slices, all fractions caused a slight decrease in cell viability, as also seen with the highest concentration of RIPS tested. In chick biventer cervicis neuromuscular preparations, RIPS and all four fractions significantly inhibited junctional acetylcholinesterase (AChE) activity. In this preparation, only fraction RI23 completely mimicked the pharmacological profile of RIPS, which included a transient facilitation in the amplitude of muscle twitches followed by progressive and complete neuromuscular blockade. Mass spectrometric analysis showed that RI23 consisted predominantly of bufogenins, a class of steroidal compounds known for their cardiotonic activity mediated by a digoxin- or ouabain-like action and the blockade of voltage-dependent L-type calcium channels. These findings indicate that the pharmacological activities of RI23 (and RIPS) are probably mediated by: (1) inhibition of AChE activity that increases the junctional content of Ach; (2) inhibition of neuronal Na+/K+-ATPase, leading to facilitation followed by neuromuscular blockade; and (3) blockade of voltage-dependent Ca2+ channels, leading to stabilization of the motor endplate membrane.

Isoform-specific Na,K-ATPase and membrane cholesterol remodeling in motor endplates in distinct mouse models of myodystrophy.

Na,K-ATPase is a membrane transporter that is critically important for skeletal muscle function. Mdx and Bla/J mice are the experimental models of Duchenne muscular dystrophy and dysferlinopathy that are known to differ in the molecular mechanism of the pathology. This study examines the function of α1- and α2-Na,K-ATPase isozymes in respiratory diaphragm and postural soleus muscles from mdx and Bla/J mice compared with control С57Bl/6 mice. In diaphragm muscles, the motor endplate structure was severely disturbed (manifested by defragmentation) in mdx mice only. The endplate membrane of both Bla/J and mdx mice was depolarized due to specific loss of the α2-Na,K-ATPase electrogenic activity and its decreased membrane abundance. Total FXYD1 subunit (modulates Na,K-ATPase activity) abundance was decreased in both mouse models. However, the α2-Na,K-ATPase protein content as well as mRNA expression were specifically and significantly reduced only in mdx mice. The endplate membrane cholesterol redistribution was most pronounced in mdx mice. Soleus muscles from Bla/J and mdx mice demonstrated reduction of the α2-Na,K-ATPase membrane abundance and mRNA expression similar to the diaphragm muscles. In contrast to diaphragm, the α2-Na,K-ATPase protein content was altered in both Bla/J and mdx mice; membrane cholesterol re-distribution was not observed. Thus, the α2-Na,K-ATPase is altered in both Bla/J and mdx mouse models of chronic muscle pathology. However, despite some similarities, the α2-Na,K-ATPase and cholesterol abnormalities are more pronounced in mdx mice.

Myasthenia Gravis: Analytical Methods for the Determination in Biological Fluids of First-Line Therapeutic Agents Used in the Management of the Disease

Myasthenia Gravis (MG) an autoimmune disease is the prototype of neuromuscular junction disease caused by auto antibodies against the nicotinic Acetylcholine Receptor (AChR) located in the postsynaptic muscle endplate membrane [1,2]. The autoimmune disorder is characterized by weakness and fatigability of the voluntary muscle arising from the distortion and simplification of the postynaptic muscle membrane and consequent attachment of antibodies and complement to the membrane.

Methanolic extract of Rhinella schneideri (cururú toad) poison crude cause ultrastructural changes in nerve terminals of mouse phrenic nerve-diaphragm preparations

Rhinella schneideri (or Bufo paracnemis), popularly known in Brazil as cururu toad, is also found in other countries in South America. The cardiovascular effects of this poison are largely known and recently was shown that it is capable to affect the neuromuscular junction on avian and mice isolated preparation. In this work, we used transmission electron microscopy to investigate the ultrastructure of the motor nerve terminal and postsynaptic junctional folds of phrenic nerve-hemidiaphragm preparations incubated for either 5 or 60 min with the methanolic extract of R. schneideri (50 µg/mL). In addition, the status of the acetylcholine receptors (AChR) was examined by TRITC-α-bungarotoxin immunofluorescence location at the endplate membrane. The results show that 5 min of incubation with the gland secretion extract significantly decreased (32 %) the number of synaptic vesicles into the motor nerve terminal, but did not decrease the electron density on the top of the junctional folds where nicotinic receptors are concentrated; however, 60 min of incubation led to significant nerve terminal reloading in synaptic vesicles whereas the AChR immunoreactivity was not as marked as in control and after 5 min incubation. Muscle fibers were well-preserved but intramuscular motor axons were not. The findings corroborated pharmacological data since the decrease in the number of synaptic vesicles (5 min) followed by recovery (60 min) is in accordance with the transient increase of MEPPs frequency meaning increased neurotransmitter release. These data support the predominant presynaptic mode of action of the R. schneideri, but do not exclude the possibility of a secondary postsynaptic action depending on the time the preparation is exposed to poison.

Role of cholesterol in the maintenance of endplate electrogenesis in rat diaphragm.

Methyl-β-cyclodextrin (0.1 mM) reduced resting potential of muscle fibers and abolished local endplate membrane hyperpolarization in rat diaphragm. This effect was associated with selective reduction of electrogenic activity of α2-isoform of Na,K-ATPase without changes in the level of intracellular acetylcholine. Experiments with cholesterol marker filipin showed that methyl-β-cyclodextrin in this dose induced cholesterol translocation from lipid rafts to liquid phase of the membrane without its release into extracellular space. This modification of lipid rafts by methyl-β-cyclodextrin presumably impaired the mechanism maintaining electrogenesis in endplates mediated by modulation of Na,K-ATPase by non-quantum acetylcholine. Cholesterol can serve as a molecular component of this mechanism.

Structural and Pathogenic Mechanisms Centered on Muscle-Specific Tyrosine Kinase in Autoimmune Neuromuscular Junction Disorders

Myasthenia gravis (MG), an autoimmune neuromuscular junction disorder characterized by fatigable weakness of voluntary muscles including ocular, facial, oropharyngeal, limb and respiratory muscles with a prevalence of about 150-200 per million, is a disease of the postsynaptic neuromuscular junction where acetylcholine receptors (AChRs) are targeted by autoantibodies in 80-85% of MG patients. We face unresolved issues in the efforts to clarify the pathogenesis of remaining MG patients (seronegative MG). In this regard, we have paied attention to the findings that part of seronegative MG patients have antibodies against other postsynaptic proteins: the muscle-specific tyrosine kinase (MuSK) which is activated by Dok-7 (muscle adaptor protein) and plays a key role in AChR clustering at the end-plate membrane, and the low-density lipoprotein receptor-related protein 4 (Lrp4) which is the agrin receptor and mediates the neural signal to MuSK. To search a clue to further understanding of the immunopathological mechanisms underlying postsynaptic junction disorders, we need to understand a complex architecture formed by diversity of elements through signals orchestrated by sophisticated interactions in the synapse. The present review focuses on the signaling downstream from MuSK which is originated from the activation by agrin/Lrp4 signaling (via MuSK immunoglobulin-like domains 1 and 2) and by Wnt signaling (via MuSK cysteine-rich domain (CRD)), and also focuses on the synaptic stability by extracellular matrix proteins (such as collagen Q, perlecan and biglycan) and their receptors (via dystroglycans). Also, the review may shed light on possible mechanisms (including Wnt/MuSK CRD, Lrp4 and laminin-network) to modify the presynaptic function in autoimmune postsynaptic disorders. Keywords: Acetylcholine receptor, Agrin, laminin-network, Lrp4, MuSK, myasthenia gravis, neuromuscular junction, Wnts.

Safety Factor for Neuromuscular Transmission Is Reduced in Mild and Moderate Myasthenia Gravis Due to Loss of Both Sodium Channels and Acetylcholine Receptors (S35.006)

Objective: To evaluate the contributions of loss of Na + channels and acteylcholine receptors (AChR) to reduction in safety factor for neuromuscular transmission (SF) in patients with MGFA class IIa and IIIa myasthenia gravis (MG). I hypothesized that loss of endplate Na+ channels would contribute to the reduction of SF in patients with MG. Background Endplate Na + channels improve neuromuscular transmission by reducing the threshold depolarization needed to trigger an action potential (E AP ). In MG, AChRs and Na + channels are lost from the endplate. Na+ channels as well as AChRs were lost from the endplate membrane. Design/Methods: I used standard electrophysiological techniques to compare membrane currents and potentials in intercostal nerve-muscle preparations from 5 MGFA IIIa, 2 MGFA IIa and 7 control adult male subjects. Na + currents recorded using loose patch voltage clamp. Action potential (AP) characteristics measured with a 2 electrode clamp. Endplate potentials (EPPs) studied using μ-conotoxin GIIIb to block muscle Na + channels. The motor nerve was stimulated using a suction electrode. I report the findings for recordings from fast-glycolytic muscle fibers. Results: In MG, EPP was reduced from 40.2 ±1.3 to 30.1±1.3 mV (IIa) and 23.5 ±1.7 mV (IIIa) (p + channel loss increased E AP from −71.9 ±2.2 mV to -66.1 ±2.7 mV (IIa; p AP was the same for MG fibers and controls. The SF for neuromuscular transmission was reduced from 2.98 to 1.58 (IIa) and 1.09 (IIIa). Loss of Na + channels accounted for 53% (IIa) or 41% of the SF reduction. Reduction of EPP produced 47% (IIa) or 59% (IIIa) of the SF decrease. Conclusions: Loss of endplate Na + channels contributes to the reduced SF in MG to a similar extent as does the loss of endplate AChRs. Supported by: Department Veterans Affairs. Disclosure: Dr. Ruff has nothing to disclose.

One-Step Vital Staining of Presynaptic Terminals and Post-Synaptic Receptors at Neuromuscular Junctions in Mouse Skeletal Muscle.

This protocol describes vital staining of neuromuscular junctions in the mouse triangularis sterni muscle in one incubation step, combining presynaptic, motor nerve terminal staining with the styryl dye FM1-43, which labels recycling synaptic vesicles, and TRITC-α-bungarotoxin, which labels acetylcholine receptors in the motor endplate membrane. Curr. Protoc. Mouse Biol. 1:489-496 © 2011 by John Wiley & Sons, Inc.

Endplate Na + Channels Loss Impairs Neuromuscular Transmission in Myasthenia Gravis

Myasthenia gravis (MG) is caused by acetylcholine receptor (AChR) antibodies inducing complement-medicated destruction of endplate membrane. Endplate Na+ channels enhance the safety factor for neuromuscular transmission by reducing the threshold depolarization needed to trigger an action potential (EAP). I previously demonstrated that MG antibodies do not directly alter the function of Na+ channels and that Na+ channels are lost by complement-mediated loss of endplate membrane. I examined the impact of postsynaptic voltage-gated Na+ channel loss on the SF. Comparing intercostal nerve-muscle preparations from controls and MG patients with moderate (M) or severe (S) MG, I found that endplate AChR loss decreased the endplate potential (EPP) from 40.2 ±1.3 to 30.1±1.3 mV (M) and 23.5 ±1.7 (S) (p < 0.001 for both). Endplate Na+ channel loss increased EAP from −71.9 ±2.2 to −66.1 ±2.7 (M; p < 0.01) and 62.3 ±2.7 mV (S; p < 0.001). In contrast, on extrajunctional membrane (> 0.1 cm from the endplate), EAP was the same for MG fibers and controls. The SF for neuromuscular transmission was reduced from 2.98 to 1.58 (M) and 1.09 (S). If the EPP was the same as control, the SF for MG fibers would have been 2.12 (M) or 1.86 (S). If EAP for MG fibers were the same as control fibers, the SF for MG fibers would have been 2.23 (M) or 1.74(S). In moderate MG, reduced EPP accounted for 47% of the SF reduction and increased EAP 53% of the SF reduction. In severe MG, reduced EPP accounted for 59% of the SF reduction and increased EAP 40% of the SF reduction. Conclusion: loss of endplate Na+ channels in MG contributes to the reduction in SF to a similar extent as does the loss of endplate AChRs.

Effect of functional unloading and the deficit of dystrophin on the local hyperpolarization of the postsynaptic membrane of a skeletal muscle fiber

The data obtained confirm the unity of the electrogenic mechanism of hyperpolarization by nanomolar concentrations of cholinergic ligands in the extrasynaptic region and by endogenous nonquantal acetylcholine in the synaptic region of a skeletal muscle fiber. In both cases, this mechanism involves the α2 isoform of Na,K-ATPase and is operative in the absence of Na+ entry through membrane channels. At the same time, there are peculiar properties manifested under functional disturbances. Thus, the efficiency of this mechanism in the synaptic region selectively increases in rats under hindlimb unloading, and decreases in the case of dystrophin deficit in mdx mice. The latter suggests that dystrophin is a molecular component essential for local hyperpolarization of the end-plate membrane.

How Myasthenia Gravis Alters the Safety Factor for Neuromuscular Transmission

Myasthenia gravis (MG), the best understood autoimmune disease, is characterized by antibodies directed against the skeletal muscle acetylcholine receptors (AChRs). An elevated concentration of Na+ channels at the endplate ensures the efficiency of neuromuscular transmission by reducing the threshold depolarization needed to trigger an action potential (EAP). Postsynaptic AChRs and voltage-gated Na+ channels are lost from the neuromuscular junction in MG. We studied the impact of postsynaptic voltage-gated Na+ channel loss on the safety factor for neuromuscular transmission (SF). Comparing intercostal nerve-muscle preparations from controls and MG patients, endplate AChR loss decreased the size of the endplate potential (EPP) from 40.2 ±1.3 mV to 23.5 ±1.7 mV (p < 0.001). Endplate Na+ channel loss in patients with MG increased EAP from −71.9 ±2.2 mV to 62.3 ±2.7 mV (p < 0.001). The SF was reduced from 2.98 to 1.09. If the EPP was the same as control, the SF for MG fibers would have been 1.86 rather than the observed value of 1.09. If EAP for MG fibers were the same as control fibers, the SF for MG fibers would have been 1.74. The reduction in EPP accounted for 59% of the reduction in SF and the increase in EAP produced 40% of the SF reduction. To evaluate whether AChR-specific antibody impaired the function of Na+ channels, we tested omohyoid nerve-muscle preparations from rats injected with monoclonal myasthenogenic IgG (passive transfer model of MG [PTMG]). AChR antibody that produced PTMG did not alter the function of Na+ channels. We conclude that loss of endplate Na+ channels in MG is due to complement-mediated loss of endplate membrane rather than a direct effect of myasthenogenic antibodies on Na+ channels. Loss of endplate Na+ channels caused 40% of the SF reduction in MG.

How myasthenia gravis alters the safety factor for neuromuscular transmission

Myasthenia gravis (MG), the most common of autoimmune myasthenic syndromes, is characterized by antibodies directed against the skeletal muscle acetylcholine receptors (AChRs). Endplate Na + channels ensure the efficiency of neuromuscular transmission by reducing the threshold depolarization needed to trigger an action potential. Postsynaptic AChRs and voltage-gated Na + channels are both lost from the neuromuscular junction in MG. This study examined the impact of postsynaptic voltage-gated Na + channel loss on the safety factor for neuromuscular transmission. In intercostal nerve–muscle preparations from MG patients, we found that endplate AChR loss decreases the size of the endplate potential, and endplate Na + channel loss increases the threshold depolarization needed to produce a muscle action potential. To evaluate whether AChR-specific antibody impairs the function of Na + channels, we tested omohyoid nerve–muscle preparations from rats injected with monoclonal myasthenogenic IgG (passive transfer model of MG [PTMG]). The AChR antibody that produces PTMG did not alter the function of Na + channels. We conclude that loss of endplate Na + channels in MG is due to complement-mediated loss of endplate membrane rather than a direct effect of myasthenogenic antibodies on endplate Na + channels.

Treatment of myasthenia gravis

Myasthenia gravis is a prototypic antibody-mediated neurological autoimmune disorder. Its pathogenesis is much better understood than that of multiple sclerosis or immune neuropathies. Currently two targets in the endplate membrane are considered as autoantigens, the acetylcholine receptor (in up to 90%) and the muscle-specific kinase (MuSK, in about 5%). Fortunately, substantial therapeutic progress has been made even before the era of molecular and translational medicine. In this review we characterise modern treatment algorithms that are adapted to disease severity and introduce the principle of escalating treatment strategies for myasthenia gravis. In mild cases and ocular forms of myasthenia gravis treatment with acetylcholine esterase inhibitors may be sufficient, at least temporarily. In generalised myasthenia gravis a wide array of immunosuppressive treatments have been established but most have never been tested in a full-size prospective randomised trial. Up to 10% of patients with myasthenia gravis are associated with a thymoma, i.e. of paraneoplastic origin, and this has to be looked for by CAT scan or MRI. In non-thymoma patients younger than about 50 years of age and with generalised weakness a complete early (but not urgent) thymectomy is considered as state of the art based on circumstantial evidence and expert opinion; the best type of procedure is still under debate. Usually, pretreatment with immunosuppressive medication or plasma-pheresis is recommended. Myasthenic crisis is best treated by plasma-pheresis, mostly combined with immunoadsorption techniques. Intravenous immunoglobulins are a reasonable therapeutic alternative, but a shortage in supply and high prices limit its use. With regard to immunosuppression azathioprine is still the standard base-line treatment, often combined with initial corticosteroids. In rare patients with inborn hepatic enzyme deficiency of thiomethylation azathioprine it is not well tolerated and may be substituted by mycophenolate mofetil. Severe cases may profit from combined immunosuppression with corticosteroids, cyclosporine A and even moderate doses of methotrexate or cyclophosphamide. Tacrolimus is under investigation. All such combination therapies need to be supervised by an experienced academic neuroimmunological centre. Serial measurements of anti-acetylcholine receptor antibodies, once these are elevated, or MuSK antibodies are a useful adjunct for monitoring treatment success. In escalating therapy for very severe cases one may employ monoclonal anti-CD 20 antibodies (rituximab). In highly refractory cases also immunoablation via high-dose cyclophosphamide, followed by trophic factors such as G-CSF has been suggested.

How acetylcholine gives rise to current at the motor end‐plate

How acetylcholine gives rise to current at the motor end‐plate

Effect of N‐acetylaspartylglutamate (NAAG) on non‐quantal and spontaneous quantal release of acetylcholine at the neuromuscular synapse of rat

N-Acetylaspartylglutamate (NAAG), known to be present in rat motor neurons, may participate in neuronal modulation of non-quantal secretion of acetylcholine (ACh) from motor nerve terminals. Non-quantal release of ACh was estimated by the amplitude of the endplate membrane hyperpolarization (H-effect) caused by inhibition of nicotinic receptors by (+)-tubocurarine and acetylcholinesterase by armin (diethoxy-p-nitrophenyl phosphate). Application of exogenous NAAG decreased the H-effect in a dose-dependent manner. The reduction of the H-effect by NAAG was completely removed when N-acetyl-beta-aspartylglutamate (betaNAAG) or 2-(phosphonomethyl)-pentanedioic acid (2-PMPA) was used to inhibit glutamate carboxypeptidase II (GCP II), a presynaptic Schwann cell membrane-associated ectoenzyme that hydrolyzes NAAG to glutamate and N-acetylaspartate. Bath application of glutamate decreased the H-effect similarly to the action of NAAG but N-acetylaspartate was without effect. Inhibition of NMDA receptors by dl-2-amino-5-phosphopentanoic acid, (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine (MK801), and 7-chlorokynurenic acid or inhibition of muscle nitric oxide synthase (NO synthase) by N(G)-nitro-l-arginine methyl ester and 3-bromo-7-nitroindazole completely prevented the decrease of the H-effect by NAAG. These results suggest that glutamate, produced by enzymatic hydrolysis of bath-applied NAAG, can modulate non-quantal secretion of ACh from the presynaptic terminal of the neuromuscular synapse via activation of postsynaptic NMDA receptors and synthesis of nitric oxide (NO) in muscle fibers. NAAG also increased the frequency of miniature endplate potentials (mEPPs) generated by spontaneous quantal secretion of ACh, whereas the mean amplitude and time constants for rise time and for decay of mEPPs did not change.

Investigation on the mechanism of exacerbation of myasthenia gravis by aminoglycoside antibiotics in mouse model

To investigate the underlying mechanism of the exacerbation of myasthenia gravis by aminoglycoside antibiotics. C57/BL6 mice were immunized with acetylcholine receptor (AChR), extracted from electric organ of Narcine timilei according to Xu Haopeng's methods, in complete Fruends adjuvant (CFA) to establish experimental autoimmune myasthenia gravis (EAMG). EAMG mice were divided randomly into 5 groups: MG group, NS group and three antibiotics groups. The clinical symptom scores of mice were evaluated on d7 after the last immunization and d14 of antibiotics treatment. Repetitive nerve stimulation (RNS) was performed and the levels of anti-AChR antibody (AChR-Ab) were tested at the same time. The mean clinical symptom grades of gentamycin group (1.312, 2.067), amikacin group (1.111, 1.889) and etimicin group (1.263, 1.632) were significantly higher than those of MG group (1.000, 1.200) (P<0.05). The positive rates of RNS of three antibiotics groups were 69.23%, 58.82% and 63.16% respectively, which were significantly higher than those of MG group and NS group (40.00%, 40.00%, P<0.05). The AChR-Ab level in serum and the expression of AChR on neuromuscular junction (NMJ) of mice in three antibiotics groups were also higher than those of MG group. Our results indicated that aminoglycoside antibiotics could aggravate the symptom of myasthenia gravis. The exacerbation of myasthenia gravis by these antibiotics probably involves competitively restraining the release of acetylcholine from presynaptic membrane, impairing the depolarization of postsynaptic membrane, depressing the irritability of myocyte membrane around the end-plate membrane and consequently leading to the blockade of neuromuscular junction.

Treatment Principles in the Management of Autoimmune Myasthenia Gravis

The pathogenesis of myasthenia gravis (MG) involves a T cell-directed antibody-mediated autoimmune attack on the nicotinic acetylcholine receptor (AChR) or, occasionally, on other postsynaptic antigens. The antibodies induce their effects through complement-mediated destruction of the postsynaptic endplate membrane with resultant reduction in endplate AChR, and to a lesser degree by increased turnover of endplate AChR or blockade of AChR function. Considerable progress in the treatment of MG has accrued from so-called symptomatic treatments, including improved critical care of seriously ill patients and medications (e.g., cholinesterase inhibitors) increasing the concentration of acetylcholine at the remaining endplate AChRs. Information from other autoimmune diseases and from the response of the normal immune system to invading pathogens supports the view that the course of MG is characterized by exacerbations and remissions. Therefore, the goal in MG treatment is to induce and maintain a remission. This usually involves combinations of short-term and long-term immunosuppressive agents. Selection of the particular combinations of agents in a given patient is guided by the goal of minimizing the cost/benefit ratio of the regimen in an individual patient. In general, the plan involves an initial forceful attack followed by a slow and measured withdrawal.

Effect of tetraalkylammonium derivative of 6-methyluracil on amplitude and temporal parameters of miniature endplate potentials in frog neuromuscular junction.

The effect of C-547, a tetraalkylammonium derivative of 6-methyluracil, a novel highly selective acetylcholinesterase inhibitor, on frog neuromuscular junction was studied. In concentrations 10(-9)-10(-7)M the preparation increased the amplitude and temporal parameters of miniature endplate potentials. In contrast to the effect of C-574 on purified acetylcholinesterase from mammals, the effect of this agent on frog neuromuscular junction was reversible. In a concentration of 10(-6)M the preparation apart from anticholinesterase activity produced a parasympatholytic effect manifested in a decrease in amplitude and decay time constant of miniature endplate potentials accompanied by a decrease in spontaneous transmitter secretion. After washout, the parasympatholytic effect recovered more slowly, but disappeared more rapidly compared to anticholinesterase activity. These findings suggest that parasympatholytic effect of C-547 results from direct action on receptor-channel complexes in the endplate membrane.