Antigen-specific therapy of experimental myasthenia gravis with acetylcholine receptor-gelonin conjugates in vivo

Rats suffering from experimental autoimmune myasthenia gravis (EAMG) induced by previous immunization with foreign acetylcholine receptor (AChR) were treated with AChR-toxin conjugates using the plant toxin gelonin. This led to a marked improvement of clinical symptoms as well as a significant increase in functional AChR compared to untreated rats with EAMG as determined 6 to 10 weeks later. No therapeutic effect was observed after treatment with gelonin or AChR alone. The immune response to irrelevant control antigens was not altered by this treatment.

Hormonal regulation of calcium current in freshly isolated airway smooth muscle cells

Single freshly isolated smooth muscle cells of adult bovine trachea were voltage clamped, and the calcium inward current was separated from K+ currents by blocking the large outward currents with intra- and extracellular Cs+ and extracellular tetraethylammonium chloride. Isoproterenol stimulated peak calcium current (ICa) in a dose-dependent manner through the beta-adrenergic receptor. The isoproterenol effect was not mediated or caused by the stimulation of a K+ or Na+ current, a decrease in the intracellular concentrations of Ca2+ or H+, the stimulation of the Na(+)-H+ or the Na(+)-Ca2+ exchanger. Neither basal nor isoproterenol-stimulated ICa was affected by internal dialysis of the cell with adenosine 3',5'-cyclic monophosphate (cAMP), cAMP analogues, or the catalytic subunit of cAMP-kinase. Internal dialysis of the cells with guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) blocked the stimulation of isoproterenol whereas dialysis with guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) induced an isoproterenol-like maximal increase of ICa. These results show that the beta-adrenergic receptor stimulates the L-type calcium current of isolated tracheal smooth muscle cells independent of cAMP and cAMP-kinase through a GTP/GDP regulated protein.

Beta-adrenergic receptor stimulates L-type calcium current in adult smooth muscle cells

The hormonal regulation of L-type calcium current was investigated in freshly isolated tracheal smooth muscle cells using the whole-cell configuration of the patch-clamp technique. Isoproterenol stimulated the L-type calcium current 2.6-fold through beta-adrenoceptors. Dialysis of these cells with cyclic AMP, cyclic AMP analogues or the catalytic subunit of cyclic AMP kinase had no effect on basal or isoproterenol-stimulated calcium current. The calcium current was stimulated and inhibited by dialysis of the cells with GTP gamma S and GDP beta S, respectively. Evidently, in some smooth muscle cells the beta-adrenoceptor couples directly to L-type calcium channels via a G protein.

Antigen-gelonin conjugates. Preparation and application in experimental myasthenia gravis

The antigen-specific immune suppression by gelonin-antigen conjugates was tested in two different systems: (i) the horseradish-peroxidase-stimulated T-cell proliferation in vitro and (ii) in vivo with experimental autoimmune myasthenia gravis (EAMG) in the rat. For this, the phytotoxin gelonin, a glycoprotein from Gelonium multiflorum, was purified and linked to the respective antigens. For the in-vitro assay a lymph node cell suspension from rats immunized with horseradish peroxidase was cultured in the presence of this protein and proliferation was measured by [3H]thymidine uptake. In-vitro proliferation was significantly inhibited by adding gelonin-horseradish peroxidase conjugates. The therapeutic effects of antigen-gelonin conjugates were tested in the rat model EAMG. For these experiments rats were immunized with purified nicotinic acetylcholine receptor from electric fish in order to develop EAMG. The success of the immunization was monitored by the change in physical performance tests, the change in anti-acetylcholine receptor antibody titer, and by the change in the number of ionic endplate channels using a novel electrophysiological method. The latter method permits a very accurate assay of functional damage of acetylcholine receptor at the endplate and correlates well with the clinical severity of the disease. Rats were conventionally immunized with acetylcholine receptor from electric fish. After the onset of EAMG as measured by physical performance tests and rise in antibody titer a group of the animals was injected with an acetylcholine receptor-gelonin conjugate and this treatment was repeated seven days later. The loss in functional acetylcholine receptor was significantly smaller in the therapy group than in the untreated EAMG group.(ABSTRACT TRUNCATED AT 250 WORDS)

Agonist and blocking effects of choline at the neuromuscular junction

The effects of choline chloride were studied at the voltage-clamped frog neuromuscular junction by measuring miniature endplate currents and equilibrium dose response curves for acetylcholine applied by microionophoresis. Choline reduced the amplitude and shortened the time constant of miniature endplate currents in a dose dependent manner. Dose response curves carried out in the presence of low doses of choline (200 microM) were shifted to the right and the apparent dissociation constant for ACh was increased without affecting the Hill coefficient or the maximum conductance at the endplate. Higher doses of choline shifted the curve even further to the right but reduced the Hill coefficient and maximum conductance. Choline ionophoretic dose response curves were carried out but the conductance response was only about 1% of the response to comparable concentrations of ACh. In the presence of ethanol, which reduces the agonist dissociation constant, choline responses were increased and the dose response curve analysis revealed that the efficacy of choline was about 17% in comparison to ACh. Similar effects were measured at rat endplates. Rat nerve-muscle preparations were used to investigate the effects of choline upon neuromuscular transmission.

Agonist and inhibitory effects of pyridostigmine at the neuromuscular junction

When all of the AChE at the endplate is irreversibly inhibited by phospholine iodide the ionophoretically induced ACh endplate currents are increased more than 10-fold in amplitude. The reversible AChE inhibitor pyridostigmine only increases the current to about half this value because its effects are obscured by receptor blocking. It was found that pyridostigmine can activate the receptor ion channels when released by ionophoresis at the endplate, thus suggesting that agonist-like desensitization could contribute to the blocking effects.

Effector mechanisms in myasthenia gravis: end-plate function after passive transfer of IgG, Fab, and F(ab')2 hybrid molecules

Using the quantitative ionophoresis technique and the mouse passive transfer model of myasthenia gravis, end-plate function was measured in mice transferred with myasthenic IgG, Fab, or F(ab')2 hybrid molecules prepared by recombination of one acetylcholine receptor (AChR) specific Fab and one Fab directed against irrelevant antigen. The Hill coefficient (a measure for the cooperativity between AChR subunits) and the apparent dissociation constant K for the ACh-AChR interaction were essentially unaltered after passive transfer of IgG or IgG fragments. In contrast, myasthenic IgG and Fab, but not control IgG or Fab, markedly reduced the mean number of end-plate channels. A similar effect was observed after passive transfer of F(ab')2 hybrid molecules. These results show that passive transfer of myasthenic IgG or IgG fragments leads to a quantitative reduction of ACh-controlled end-plate channels, whereas the function of the remaining receptors/channels remains unchanged. The results suggest further that cross-linking of AChR by divalent antibody may not be an absolute requirement for the reduction of AChR at the functional neuromuscular synapse.

Experimental autoimmune myasthenia gravis: can pretreatment with 125I-labeled receptor prevent functional damage at the neuromuscular junction?

Rats were immunized with purified receptor from electric fish to induce experimental autoimmune myasthenia gravis (EAMG). It is implied by the clonal selection theory that antigens react only with receptors on specific immunocompetent cell subpopulations. In an attempt to damage these specific cells with the aid of highly radioactive antigen, one group of rats was pretreated with an additional injection of radiolabeled receptor of high specific activity 3 days before the basic immunization. The success of the immunization was monitored by measuring changes in the following three parameters: antibody titers against nicotinic acetylcholine receptor; number of alpha-bungarotoxin-binding sites at endplates; and number of acetylcholine-operated ionic endplate channels, using quantitative electrophysiologic methods. Conventionally immunized animals showed the classical signs of EAMG: elevated antibody titers against nicotinic acetylcholine receptor and a reduction of the number of alpha-bungarotoxin-binding sites, as well as reduction of the number of acetylcholine-operated ionic channels. The same symptoms were found in animals pretreated with unlabeled receptor and in animals pretreated with radioactive albumin. Animals pretreated with radioactively labeled receptor showed far less reduction of functional nicotinic acetylcholine receptor and only slightly raised antibody titers. This study suggests that preimmunization with radioactive antigen selectively eliminates immunocompetent cells, thus precluding the production of antibodies by a subsequent immunization procedure. The same protective effect cannot be obtained by either preimmunization with unlabeled antigen or by radioactively labeled unspecific antigen.

Experimental autoimmune myasthenia gravis: can pretreatment with 125I-labeled receptor prevent functional damage at the neuromuscular junction?

Rats were immunized with purified receptor from electric fish to induce experimental autoimmune myasthenia gravis (EAMG). It is implied by the clonal selection theory that antigens react only with receptors on specific immunocompetent cell subpopulations. In an attempt to damage these specific cells with the aid of highly radioactive antigen, one group of rats was pretreated with an additional injection of radiolabeled receptor of high specific activity 3 days before the basic immunization. The success of the immunization was monitored by measuring changes in the following three parameters: antibody titers against nicotinic acetylcholine receptor; number of alpha-bungarotoxin-binding sites at endplates; and number of acetylcholine-operated ionic endplate channels, using quantitative electrophysiologic methods. Conventionally immunized animals showed the classical signs of EAMG: elevated antibody titers against nicotinic acetylcholine receptor and a reduction of the number of alpha-bungarotoxin-binding sites, as well as reduction of the number of acetylcholine-operated ionic channels. The same symptoms were found in animals pretreated with unlabeled receptor and in animals pretreated with radioactive albumin. Animals pretreated with radioactively labeled receptor showed far less reduction of functional nicotinic acetylcholine receptor and only slightly raised antibody titers. This study suggests that preimmunization with radioactive antigen selectively eliminates immunocompetent cells, thus precluding the production of antibodies by a subsequent immunization procedure. The same protective effect cannot be obtained by either preimmunization with unlabeled antigen or by radioactively labeled unspecific antigen.

The effects of alcohols and diols at the nicotinic acetylcholine receptor of the neuromuscular junction

A series of straight chain aliphatic alcohols from ethanol to octanol were tested at voltage-clamped frog endplates. In the presence of high concentrations of ethanol (greater than 1 M) the individual current responses to ionophoretic pulses of ethanol were reduced in amplitude and the dose-response curve for acetylcholine was shifted to the right. All the alcohols tested had this effect and their potency increased with the length of the carbon chain. The results were interpreted to indicate that as the molecular weight of the alcohol increased, its potency as a channel blocker also increased. The diol derivative of ethanol, which is ethylene glycol (ethanediol), was totally inactive up to 400 mM. However, 1,3-propanediol was a more potent blocker than propanol. After dose-response curves were carried out in high doses of ethanol and propanediol, the number of receptors was found to be permanently reduced. This effect could be due to irreversible denaturation of the receptor and therefore reversible denaturation could account for some of the reversible blocking effects caused by such drugs. An additional effect on the receptor was observed in that low concentrations of ethanol and propanol reduced the apparent dissociation constant for acetylcholine, thus increasing the amplitude of individual responses and shifting the dose-response curve to the left.

Postjunctional characteristics of the endplates in mammalian fast and slow muscles

We have studied the postjunctional characteristics of motor endplates in the extensor digitorum longus (EDL) and soleus muscles of the rat. At voltage clamped endplates, equilibrium interactions between acetylcholine (ACh) and the ACh receptor were determined from the dose-response curves obtained by quantitative ionophoresis of ACh. These results showed that the maximum ACh induced conductance change per unit endplate surface, gmax, was 21.8 +/- 0.9 nS/microns2 in EDL and 8.2 +/- 0.9 nS/microns2 in soleus, the apparent dissociation constant, K, was 65.9 +/- 4.3 microM in EDL and 43.5 +/- 3.3 microM in soleus, and the Hill-coefficient, nH, was 2.3 +/- 0.1 in EDL and 2.2 +/- 0.1 in soleus. Single channel characteristics were derived from analysis of the ACh-induced endplate current noise. The results showed that at room temperature the mean conductance of the single channel, gamma, was 24.6 +/- 1.2 pS in EDL and 23.9 +/- 1.2 pS in soleus, and the mean life time of the channel, tau, was 0.80 +/- 0.05 ms in EDL and 0.71 +/- 0.03 ms in soleus. Of all the properties studied, the maximum conductance per unit endplate surface, gmax, was significantly smaller at the soleus endplate than at the EDL endplate. The calculated density of functional ACh receptors was 62% less, and the total number of the functional ACh receptors was 60% less at the soleus endplates than at the EDL endplates. These results suggest that the soleus has a lower margin of safety for neuromuscular transmission than the EDL.

Effects of ethidium bromide on the nicotinic acetylcholine receptor

Ethidium bromide was tested for electrophysiological effects at the postsynaptic membrane of the frog muscle endplate. At low concentrations ethidium bromide blocked the open ion channel of the nicotinic acetylcholine receptor and reduced its open time. The rates for channel blocking and unblocking were calculated giving a dissociation constant of 139 nM at -80 mV membrane potential.

Neuromuscular Transmission in experimental autoimmune myasthenia gravis (EAMG). Quantitative ionophoresis and current fluctuation analysis at normal and myasthenic rat end-plates

Chronic experimental autoimmune myasthenia gravis (EAMG) was induced in rats by immunization with acetylcholine receptor (AChR) purified from the electroplax of Torpedo californica. 35--40 days after immunization, serum anti-AChR antibody titers were about 40 nM. At this stage, electrophysiology was performed on isolated M. omohyoideus muscle-preparations from myasthenic and from normal (control) rats. For the study of the equilibrium interaction between acetylcholine (ACh) and AChR, dose-response curves were obtained by quantitative ionophoretic application of ACh to voltage-clamped end-plates. Analysis of dose-response curves yielded the following parameters: maximum end-plate conductance per unit surface gmax (EAMG) = 10.3 +/- 1.1 nS/micrometer 2, gmax (normal) = 20.2 +/- 1.8 nS/micrometer 2; apparent dissociation constant K (EAMG) = 96 +/- 5 microM, K (normal) = 58 +/- 6 microM; Hill-coefficient nH (EAMG) = 2.3 +/- 0.1, nH (normal) = 2.3 +/- 0.1. Single channel properties were derived from an analysis of ACh-induced end-plate current noise: the mean single channel conductance was gamma (EAMG) = 20.1 +/- 2.2 pS, gamma (normal) = 27.6 +/- 1.8 pS and the mean channel life-time tau (EAMG) = 1.39 +/- 0.09 ms, tau (normal) = 1.32 +/- 0.08 ms (T = 22 degrees C). The electrophysiological data are interpreted as follows: (1) At myasthenic end-plates there is a 50--60% reduction of functioning AChR (decrease of gmax). A total number of about 2 x 10(6) (1 x 10(6)) channels per end-plate was calculated for control (myasthenic) rats. (2) The affinity of AChR for ACh is reduced and/or there is an impediment of the conformational change from the closed- to the open-channel configuration (increase of K). (3) Single channel properties are essentially unaffected.

Postsynaptic effects of ethanol at the frog neuromuscular junction

The molecular mode of action of alcohol in the central nervous system (CN) is unclear. The effects of ethanol on axonal action potentials can only be measured at concentrations which are very much higher than those required to produce central effects. At the frog neuromuscular junction similar concentrations increase the open time (tau) of the ion channel associated with the nicotinic acetylcholine (ACh) receptor. We have now investigated the effect of ethanol on the postsynaptic membrane of the frog neuromuscular junction by measuring equilibrium dose-response curves for the interaction between the neuro-transmitter (ACh) and the ACh receptors. Using this system, we found that ethanol produces significant changes in receptor function. Moreover, we found that an ethanol concentration which can be physiologically tolerated by man (0.2%) the dose-response curve is measurably affected.

Determination of dose-response curves by quantitative ionophoresis at the frog neuromuscular junction

1. Quantitative ionophoresis at the neuromuscular junction is possible when (a) the drug is released from appropriate distances (15--20 micrometer for most drugs), (b) the topology of receptors is known and (c) high resistance drug pipettes (100--200 M omega) are sued. 2. With this method, drug concentration-endplate conductance relations were determined in voltage-clamped end-plates of the frog for the agonists ACh, carbamylcholine (CCh) and suberyldicholine (SubCh). 3. Based on the co-operative and independent model, theoretical dose-response curves were computed using as parameters the Hill coefficient nH, maximum conductance gmax., and apparent dissociation constant K. It was found that the co-operative model fitted the data much better than the independent model. 4. Based on the co-operative model, the mean maximum conductance for ACh was gmax. = 169 nS/micrometer, equivalent to 9000 ionic channels/micrometer length of a nerve terminal which can be opened at high drug concentrations. 5. The maximum conductance for CCh at--80 mV membrane potential was, on the average, 78% of that for ACh measured at the same end-plates. This value is termed the relative efficacy of CCh. 6. The mean values for the apparent dissociation constant K were 27.8 micrometer for ACh, 336 micrometer for CCh and 18 micrometer for SubCh. 7. The inhibition of the acetylcholinesterase activity by edrophonium (3--10 micrometer) affected only the local ACh concentration at the receptor sites, but not gmax. and nH. 8. Dose-response curves measured before and after removal of single nerve terminals in collagenase-treated muscle fibres showed no change in the nH, gmax. and K. A slight increase in gmax. to a value of 218 nS/micrometer observed comparing collagenase-treated and untreated end-plate. 9. Desensitization of receptors may occur in the range of several tens of milli-seconds.

The M. omohyoideus of the mouse as a convenient mammalian muscle preparation. A study of junctional and extrajunctional acetylcholine receptors by noise analysis and cooperativity

Muscles from cats, rats, guinea pigs and mice have been investigated as preparations for visualizing mammalian neuromuscular junctions with the aid of Nomarski interference optics. The M. omohyoideus of the mouse was found to be most convenient. Electrophysiological investigations showed that an endplate is normally surrounded by a population of perijunctional receptors. For junctional receptors in the endplate, a Hill coefficient of nH = 2.6 for acetylcholine was determined at 38 degrees C, decreasing to a value of 2.3 at room temperature. For both perijunctional and extrajunctional receptors (the latter occurring after denervation), the coefficient nH was 1.9. Noise analysis revealed a channel conductance gamma which changed abruptly from 22.4 +/- 1.0 pS (10--23 degrees C) to 45.6 +/- 3.9 pS (34--39 degrees C) in a very small temperature range around 25.5 degrees C. The mean channel lifetime tau was 0.3 ms at 39 degrees C and 1.0 ms at 23 degrees C.

Junctional and extrajunctional acetylcholine receptors in normal and denervated frog muscle fibres. Noise analysis experiments with different agonists

Ionic channel properties of acetylcholine receptors located in, in the vicinity of, or far away from a frog neuromuscular junction were investigated by noise analysis of drug induced current fluctuations. For drugs applied to the junction, in certain cases two Lorentzian curves were necessary to describe the data. It is postulated that the reason for this observation is that a contribution from perijunctional receptors was being observed. The conductance of a single channel in the junction was independent of the nature of the agonist and had an average value of 17.9 pS (temperature range 8-25 degrees C, solution buffered with Tris). After denervation for 21 days the conductance gamma was 7.5 pS at extrajunctional locations. In the close neighbourhood of the junction (peri-junctional receptors) values were found between 4 and 19 pS. The mean value of the open channel life-time tau in the endplate exposed to acetylcholine was 2.4 ms at 8-11 degrees C. This value was 0.90 ms with carbachol, 0.50 ms with succinylcholine, 0.28 ms with decamethonium and 0.45 ms with nicotine. The receptors outside the endplate exhibited tau-values which at a given temperature were 2-3 times larger than those at the endplate. Raising the temperature to 23 degrees C reduced all tau-values by factors of 2-3. It is concluded that at least two types of ACh-receptors with different properties exist in the muscle membrane, possibly produced by ACh-receptive units in different states of aggregation.

Current-voltage relation and reversal potential at junctional and extrajunctional ACh-receptors of the frog neuromuscular junction

The relationship between synaptic current and membrane potential has been examined at junctional and extrajunctional end-plate receptors of the frog. At junctional receptors, the synaptic current-membrane potential relationship is linear for close range iontophoretic ACh application and non-linear when it is delivered from some distance. At extra-junctional receptors the current-voltage relationship is always non-linear. These non-linearities can be related to the fact that in both cases [ACh] on membrane outlasts the mean life-time of the synaptic channels. When their mean life-time is varied, the current-voltage relationship obtained at junctional receptors is no longer linear and the peak synaptic conductance increases or decreases with hyperpolarization as the channel life time is shortened or lengthened, respectively.

The fine structure of the perineural endothelium

Fine strands of motor nerves were examined with the electron microscope using thin section as well as freeze-etching techniques. The specimens were taken from frog cutaneous pectoris nerve, rat sciatic nerve, mouse and shrew phrenic nerves and from human skin nerves. The perineural sheath (Henle, Ranvier, Key and Retzius) consists of one to several concentric laminae of endothelial cells; it encases nerve fascicles and eventually individual nerve fibers and terminals. The endothelial cells are extremely thin and fitted togeether smoothly by overlap and dove-tailing of their border zones. The cell contacts are formed by continuous zonulae occludentes, often reinforced by maculae adhaerentes, and in depth they comprise 3-15 strands with an average of 5-6 strands per junction. The membranes of endothelial cells are studded with attachment sites and stomata of plasmalemmal vesicles suggesting a high level of pinocytotic activity. This phenomenon is by no means restricted to the external laminae of the endothelial sheath. Each endothelial lamina is vested with basement membranes on both (epineural and endoneural) sides, and the spaces between laminae contain a few collagen fibers and fibroblasts. Occasionally, punctate tight junctions are seen between laminae. Cytological evidence supports the hypothesis that the perineural endothelium provides a relatively tight and highly selective barrier separating the peripheral nerves from surrounding tissue and its extracellular fluid spaces. This effect is achieved on the one hand by the sealing of pericellular spaces and on the other hand by a membrane controlled transcellular transport mechanism (pinocytosis), both of which are enhanced by their serial arrangement.

Acetylcholine receptor: modification of synaptic gating mechanism after treatment with a disulfide bond reducing agent

Reduction of a 'reactive' disulfide bond in the postsyraptic membrane of the frog neuromuscular junction by dithiothreitol (DTT) decreases both the sensitivity of the membrane to applied acetylcholine (ACh) and the amplitude of the single 'shot effect'. Analysis of ACh induced conductance fluctuations in voltage clamped frog endplates indicates that DTT reduces both the amplitude gamma and duration tau of the elementary conductance events. The mean control value of gamma was 18.5-10(-12) omega-1 with no significant dependence on temperature. The mean control values pi were 2.3 msec at 7-9 degrees C and 0.94 msec at 20-22 degrees C. At 7-9 degrees C 1m7 DTT (20-50 min after application) reduced gamma to 61% of the control value and at 20-22 degrees C to 39%, while tau was reduced to 70% at both temperature ranges. The dose-response curve for iontophoretically applied ACh indicates that neither the total number of ionic channels nor the cooperativity within the receptors are changed. However, the affinity of ACh for the receptor sites was reduced. All effects of DTT were fully reversed by the oxidizing agent 5,5'-dithie-bis-(2-nitro-benzoic acid) (DTNB).