Steroids induce acetylcholine receptors on cultured human muscle: implications for myasthenia gravis

Effect of protracted dexamethasone exposure and its withdrawal on rocuronium-induced neuromuscular blockade and sugammadex reversal: an ex vivo rat study

Studies have reported that protracted dexamethasone treatment induces resistance to nondepolarizing neuromuscular blocking agents (NMBAs) and the association with nicotinic acetylcholine receptors in the diaphragm of rats. Here, we investigated the effect of protracted dexamethasone administration on the sensitivity to rocuronium and the recovery profile when reversed by sugammadex; additionally, we observed the recovery period of pharmacodynamic change after withdrawal. Sprague-Dawley rats received daily intraperitoneal injections of dexamethasone or saline for 14 days. On days 1, 3, and 7 after the last dexamethasone treatment (Dexa1, Dexa3, and Dexa7, respectively) or 1 day after saline (control group), the phrenic nerve-hemidiaphragm preparation was dissected for assay. The dose-response curve of rocuronium in Dexa1 was shifted to the right compared to controls, but curves in Dexa3 and Dexa7 were not significantly different. Groups were not significantly different in attaining the train-of-four ratio ≥ 0.9, but the recovery index in Dexa7 was shorter than that in control and Dexa1. Recovery profiles (period of sugammadex reversal) were not correlated with resistance properties but rather with total administered drugs (binding capacity of NMBAs and sugammadex). Protracted dexamethasone exposure induced resistance to rocuronium but seemed to have no effect on sugammadex reversal in the rat diaphragm.

Myasthenia Gravis and Associated Diseases

BACKGROUND:

Myasthenia gravis (MG) is an autoimmune disease caused by the action of specific antibodies to the postsynaptic membrane of the neuromuscular junction, leading to impaired neuromuscular transmission. Patients with MG have an increased incidence of other autoimmune diseases.

AIM:

to determine the presence of other associated diseases in patients with MG.

METHOD:

A group of 127 patients with MG followed in 10 years period, in which the presence of other associated diseases has been analysed.

RESULTS:

The sex ratio is in favour of the female sex, the average age of the initial manifestation of the disease is less than 50 years, 65.4% of the patients with MG have another disease. 15.0% patients have associated another autoimmune disease. Thyroid disease is the most common associated with MG, rarely rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and other autoimmune diseases. Other diseases include hypertension, heart disease, diabetes, respiratory diseases, dyslipidemia. 10.2% of the patients are diagnosed with extrathymic tumours of various origins.

CONCLUSION:

Associated diseases are common in patients with MG, drawing attention to the possible common basis for their coexistence, as well as their impact on the intensity and treatment of the disease.

Complete resistance after maximal dose of rocuronium

Rocuronium is a non-depolarizing neuromuscular blocking agent (NDNMBA), employed in the clinic as an adjunct to general anesthesia to facilitate tracheal intubation rapid sequence, and to provide skeletal muscle relaxation during surgery. Many cases of resistance to neuromuscular blocking agents (NMBAs) have been anecdotally reported. There are specific pathologic states, such as upper motor neuron lesions, severe thermal injuries, liver disease, renal failure, disuse atrophy, all of which show an increased resistance to the effects of nondepolarizing muscle relaxants. Also concurrent drug therapy can alter the efficacy of NMBAs such as some classes of antibiotics, furosemide, β receptor agonists, phosphodiesterase inhibitors, calcium antagonists, respiratory stimulants but also ketamine, propofol and barbiturates at high concentrations. In this scenario we describe an unusual case of 20-years-old man who showed a complete resistance to rocuronium maybe due to a glucocorticoids concomitant therapy.

Glucocorticoids in myasthenia gravis - if, when, how, and how much?

Glucocorticoids (GC) are the most commonly used immune-directed therapy in myasthenia gravis (MG). However, to date, GC have not proven their effectiveness in the setting of a randomized clinical trial that complies with currently accepted standards. The rationale for the use of GC in MG is the autoimmune nature of the disease, which is supported by consistent positive results from retrospective studies. Well-defined recommendations for treatment of MG with GC are lacking and further hampered by inter- and intra-individual differences in the disease course and responses to GC treatment. Uncertainties concerning GC treatment in MG encompass the indication for treatment initiation, exact dosage, dose adjustment in specific conditions (e.g., pregnancy, thymectomy), mode of tapering, and surveillance of adverse events (AE). This review illustrates the mode of action of GC in the treatment for MG, presents the currently available data on GC treatment in MG, and attempts to translate the currently available information into clinical recommendations.

Juvenile myasthenia: diagnosis and treatment

Myasthenia in children can be juvenile (autoimmune) or congenital. Juvenile myasthenia (JM) is an autoimmune disorder characterised by fluctuating weakness and fatigue in the ocular, facial, bulbar or limb muscles. Diagnosis is confirmed by electromyography (EMG), single fibre EMG and the patient's clinical response to anticholinesterase medication. Serology is less helpful in children because acetylcholine receptor antibodies, usually positive in adults, are frequently absent in patients with prepubertal onset of the disease. Treatment methods in JM include anticholinesterase drugs, thymectomy and immunomodulatory agents. Plasmapheresis and intravenous immunoglobulin are used in myasthenic crisis. The prognosis of patients with JM is usually good, clinical remission being achieved in the majority of patients with the current treatment methods.

Altered gene expression in steroid-treated denervated muscle

In rats treated with high-dose corticosteroids, skeletal muscle that is denervated in vivo (steroid-denervated) develops electrical inexcitability similar to that seen in patients with acute quadriplegic myopathy. To determine whether changes in muscle gene transcription might underlie inexcitability of steroid-denervated muscle we performed RNase protection assays to quantitate adult (SkM1) and embryonic (SkM2) sodium channel isoforms and chloride channel (CLC-1) mRNA levels in control, denervated, steroid-innervated, and steroid-denervated skeletal muscle. While SkM1 mRNA levels were relatively unaffected by denervation or steroid treatment, SkM2 mRNA levels were increased by both. These effects were synergistic and high levels of SkM2 mRNA were expressed in denervated muscle exposed to corticosteroids. Skeletal muscle CLC-1 mRNA levels were decreased by denervation. To better understand the marked upregulation of SkM2 in steroid-denervated muscle we examined changes in myogenin and glucocorticoid receptor mRNA levels. However, changes in these mRNA levels cannot account for the upregulation of SkM2 in steroid-denervated muscle.

Neuroactive neurosteroids as endogenous effectors for the sigma1 (sigma1) receptor: pharmacological evidence and therapeutic opportunities

Neuroactive neurosteroids, including progesterone, allopregnanolone, pregnenolone and dehydroepiandrosterone, represent steroid hormones synthesized de novo in the brain and acting locally on nervous cells. Neurosteroids modulate several neurotransmitter systems such as gamma-aminobutyric acid type A (GABA(A)), N-methyl-D-aspartate (NMDA) and acetylcholine receptors. As physiologic consequences, they are involved in neuronal plasticity, learning and memory processes, aggression and epilepsy, and they modulate the responses to stress, anxiety and depression. The sigma1-receptor protein was recently purified and its cDNA was cloned in several species. The amino-acid sequences are structurally unrelated to known mammalian proteins, but shared homology with a fungal sterol C8-C7 isomerase. The sigma1-receptor ligands exert a potent neuromodulation on excitatory neurotransmitter systems, including the glutamate and cholinergic systems. Consequently, selective sigma1 agonists show neuroprotective properties and beneficial effects in memory processes, stress and depression. The evidence of a direct interaction between neurosteroids and sigma1 receptors was first suggested by the ability of several steroids to inhibit the binding of sigma1-receptor radioligands in vitro and in vivo. A crossed pharmacology between neurosteroids and sigma1-receptor ligands was described in several physiological tests and behavioral responses. This review will detail the recent evidence for a common mechanism of action between neurosteroids and sigma1-receptor ligands and focus on the potential therapeutic interests of such interaction in the physiopathology of learning and memory impairments, stress, depression and neuroprotection.

Development of an assay for modulating anti-acetylcholine receptor autoantibodies using human rhabdomyosarcoma cell line

Three types of autoantibodies against the acetylcholine receptors (AChR) of skeletal muscle are detectable in patients with myasthenia gravis including binding, blocking, and modulating anti-AChR antibodies. Modulating autoantibodies correlate best with the severity of the disease, but are also technically most difficult to measure because the assay generally requires fresh human muscle cells. We have developed an assay for the modulation of anti-AChR antibodies using a rhabdomyosarcoma (RD) cell line expressing AChR on the cell surface. By decreasing the FetalClone III serum from 10% to 0.5% in Eagles Minimal Essential Medium (EMEM) we were able to increase the number of AChR on RD cells to meet the need of sensitivity of the assay. The extent of modulation was determined as the percent of AChR internalized in the presence or absence of modulating autoantibodies. Less than 6% modulation was found with the normal serum (n = 42). The CVs of both the intra- and day-to-day precision were less than 20%. When clinical samples (n = 105) were assayed in our laboratory and also at Nichols Institute, a correlation coefficient of 0.816 was obtained. The selection of RD cell line, the success of increasing the expression of the AChR on RD cells and the use of 125I alpha-bungarotoxin of high specific activity allowed the establishment of an assay which can be used in routine clinical laboratory for the measurement of modulating anti-AChR autoantibodies for the management of patients with myasthenia gravis.

Calcium influx inhibition by steroids and analogs in C2C12 skeletal muscle cells

Glucocorticoids, namely alpha-methylprednisolone (PDN) and deflazacort, are the only drugs reported to have a beneficial effect on the degenerative course of Duchenne muscular dystrophy (DMD). Increased cytosolic calcium concentrations ([Ca2+]c) have been implicated as one of the pathological events responsible for the degeneration of dystrophic skeletal muscles. In previous studies, we have demonstrated that PDN treatment of both normal and dystrophic murine skeletal muscle cells was able to normalize elevated [Ca2+]c and improved myogenesis. Here we have investigated the mechanism underlying the effects of glucocorticoids on cellular Ca2+ influx into C2C12 skeletal muscle cells. Long-term incubation of C2C12 myocytes with PDN was necessary to observe a reduction of 45Ca2+ influx. PDN was most effective in inhibiting 45Ca2+ uptake when added for 4 days (at the time of fusion of myoblasts into myotubes) and to a lesser extent, when added after fusion. It was ineffective when added to C2C12 cells at the myoblast stage. Short PDN incubation times, at the time of fusion were insufficient to elicit a response. Several steroids were tested for their ability to inhibit 45Ca2+ influx in C2C12 myocytes. All four glucocorticoids examined were able to reduce Ca2+ influx, dexamethasone being the most potent (IC50 3.14+/-0.34 x 10(-8) M). Mineralocorticoids (aldosterone and 11-deoxycorticosterone) were also able to reduce Ca2+ influx. The vitamin E-derived lazaroid U-83836E and the glucocorticoid-derived lazaroid U-74389G also elicited a decrease in Ca2+ influx, but higher concentrations were necessary. Because both glucocorticoids and lazaroids display antioxidant properties, but U-83836E is devoid of glucocorticoid activity, the reduction in Ca2+ influx was suspected to be triggered via an antioxidant mechanism. To test this hypothesis, we assessed the action of several antioxidants, such as vitamin E, vitamin C, 2-tert.-butyl-4-methoxyphenol (BHA), 2,6-di-tert.-butyl-4-methyl-phenol (BHT) and nordihydroguaiaretic acid (NDGA), on 45Ca2+ influx. None of these agents had an effect on 45Ca2+ influx. In addition, several oxidants were tested (either acutely or chronically) for their ability to elicit 45Ca2+ influx in C2C12 myocytes and were found to be inactive. The involvement of the glucocorticoid receptor on the modulation of Ca2+ influx was investigated. The glucocorticoid receptor antagonist mifepristone (code name RU38486, 10(-6) M) caused a shift of two orders of magnitude of the PDN response. However, neither actinomycin D nor cycloheximide affected the response to PDN. Results with the phospholipase A2 inhibitor, manoalide, suggest that glucocorticoid-induced protein synthesis (e.g. enhanced stimulation of lipocortin) does not play a role in the reduction of calcium influx. Our results suggest that steroids elicit a decrease in calcium influx in C2C12 skeletal muscle cells. This decrease is not due to an antioxidant mechanism or to a mechanism which requires gene expression. Since mineralocorticoids and U-83836E also had similar effects, the mechanism could belong to the non-genomic effects of corticoids (e.g. membrane stabilization). The beneficial effect of glucocorticoids in DMD could be attributed to a reduction of the pathological increase in Ca2+ influx via an effect on the sarcolemma.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Glucocorticoid effects on the skeletal muscle differentiation program: analysis of clonal proliferation, morphological differentiation and the expression of muscle-specific and regulatory genes

We examined the effect of glucocorticoids on the proliferation and differentiation of skeletal muscle cells using the C2C12 cell line. We found that treatment with glucocorticoids enhanced muscle cell differentiation but had only minor effects on the clonal growth rate of C2C12 cells. The stimulatory effect of glucocorticoids on myogenic differentiation was reflected in the increased expression of muscle-specific genes, creatine kinase (CK) and acetylcholine receptor gamma subunit (AChR). Dexamethasone had no effect on CK and AChR mRNA stability and enhanced transcription from a CAT reporter genes containing the 3.3kb 5' flanking region of the murine CK gene (-3300MCK-CAT). Since dexamethasone did not affect the expression levels of the myogenic regulatory genes such as myoD and myogenin, the enhancement of muscle-specific transcription might reflect an increase in the functional activity of the regulatory proteins. Other possible mechanisms involved in the differentiation-enhancing effect of glucocorticoids are discussed.

Myasthenia gravis as a prototype autoimmune receptor disease

Myasthenia gravis (MG) is an organ-specific autoimmune disease in which autoantibodies against nicotinic acetylcholine receptors (AChR) at the postsynaptic membrane cause loss of functional AChR and disturbed neuromuscular transmission. The immunopathogenic mechanisms responsible for loss of functional AChR include antigenic modulation by anti-AChR antibodies, complement-mediated focal lysis of the postsynaptic membrane, and direct interference with binding of acetylcholine to the AChR or with ion channel function. The loss of AChR and subsequent defective neuromuscular transmission is accompanied by increased expression of the different AChR subunit genes, suggesting a role for the target organ itself in determining susceptibility and severity of disease. Experimental autoimmune myasthenia gravis (EAMG) is an animal model for the disease MG, and is very suitable to study the immunopathogenic mechanisms leading to AChR loss and the response of the AChR to this attack. In this article the current concepts of the structure and function of the AChR and the immunopathological mechanisms in MG and EAMG are reviewed.

Modulation by prednisolone of calcium handling in skeletal muscle cells

1. Increased calcium (Ca2+) influx has been incriminated as a potential pathological mechanism in the chronic skeletal muscle degeneration exhibited by Duchenne muscular dystrophy (DMD) patients. We have studied the influence of the glucocorticoid alpha-methylprednisolone (PDN), the only drug known to have a beneficial effect on the degenerative course of DMD, on Ca2+ handling in the C2 skeletal muscle cell line. 2. PDN, when added 3 days (when myoblasts start to fuse into myotubes) after cell seeding, led to a 2 to 4 fold decrease in cellular Ca2+ uptake. This decrease was independent of the extracellular Ca2+ concentration applied to cells. The effect took at least 24 h in order to become established (PDN of 10(-5) M) and took longer for lower PDN concentrations (EC50 of ca. 10(-6) M at day 5, 10(-6.5) M at day 7 and 10(-7.5) M at day 9 in culture). 3. Cellular calcium accumulation was also decreased in PDN-treated myotubes exposed to 45Ca(2+)-containing medium for 1 to 6 days. 4. No effect of PDN was seen on 45Ca2+ efflux; a decrease in the amount of 45Ca2+ released was observed due to the reduction of cellular 45Ca2+ loading. 5. PDN treatment led to an approximately 2 fold decrease in basal cytosolic Ca2+ concentration. 6. Three antioxidant drugs (lazaroids), previously shown to enhance in vitro skeletal muscle cell differentiation to the same extent as PDN, induced a similar decrease in Ca2+ influx. 7. Our results suggest that long-term incubation of C2 cells with PDN leads to a decrease of the size of the cellular Ca2+ pools and to reduced resting cytosolic Ca2+ levels. Part of the beneficial effect of PDN in DMD patients could be attributed to a reduction of Ca2+ influx and of the size of Ca2+ pools in dystrophic muscle fibres.

Prednisolone enhances myogenesis and dystrophin-related protein in skeletal muscle cell cultures from mdx mouse

The differentiation of skeletal muscle cells from mdx mice which lack dystrophin expression was examined after glucocorticoid treatment, namely alpha-methylprednisolone (PDN). Primary skeletal muscle cell cultures were established from newborn mdx, congenic C57BL/10, and allogenic BALB/C mice. We show that PDN promotes the myogenesis of both mdx- and control mice-derived cultures as determined by 1) the number of myotubes, 2) acetylcholine receptors, and 3) dystrophin and dystrophin-related protein levels. These results support the hypothesis that PDN could enhance the myogenesis of satellite cells and increase dystrophin-related protein expression in DMD treated patients.

Long-term treatment with glucocorticoids increases synthesis and stability of junctional acetylcholine receptors on innervated cultured human muscle

We have studied the effect of long-term treatment with hydrocortisone on the expression of acetylcholine receptors (AChRs) at the neuromuscular junctions of human muscle cultured in monolayer and innervated de novo by fetal rat spinal cord motoneurons. Hydrocortisone increased accumulation of junctional AChRs in a dose- and time-dependent fashion. This increase was due to both decreased degradation and increased synthesis of AChRs. Other glucocorticoids, dexamethasone and prednisolone, exerted similar effects. Our study demonstrates a novel action of glucocorticoids on human junctional AChRs.

Uncoupling of growth inhibition and differentiation in dexamethasone-treated human rhabdomyosarcoma cells

The effects of dexamethasone, a synthetic glucocorticoid, and of N,N-dimethylformamide on in vitro growth and differentiation and on proto-oncogene expression of human rhabdomyosarcoma cells were studied. RD/18 clone cells (derived from the embryonal rhabdomyosarcoma cell line RD) treated with 100 nM dexamethasone showed an almost complete block of differentiation: about 5% myosin-positive cells were observed after 2 weeks of culture in dexamethasone-supplemented differentiation medium, compared to 20% of untreated cultures. Dexamethasone also induced a 20-30% growth inhibition and a more flattened morphology. The treatment with N,N-dimethylformamide induced a significantly increased proportion of myosin-positive cells (reaching about 30%) and a 40% growth inhibition. Induction of differentiation inversely correlated with the levels of c-myc proto-oncogene expression: after a 2 week culture dexamethasone-treated cells showed the highest c-myc expression and N,N-dimethylformamide-treated cells the lowest. Culture conditions per se down-modulated c-erbB1 and up-regulated c-jun expression, with no relationship to the differentiation pattern. Other proto-oncogenes were not expressed (c-sis, N-myc, c-mos, c-myb) or were not modulated (c-fos, c-raf). Therefore dexamethasone and N,N-dimethylformamide, both causing a decreased growth rate, showed opposing actions on myogenic differentiation and on c-myc proto-oncogene expression of human rhabdomyosarcoma cells.

Specific effect of corticoids on acetylcholine receptor expression in rat skeletal muscle cell cultures

The potential effect of different classes of steroids on the expression of acetylcholine receptors (AChR) was studied in different primary cultures of newborn-rat skeletal muscle cells. Comparison among three techniques for preparing newborn skeletal muscle cells showed that these systems were equivalent to study AChR expression. Only corticoids stimulated myogenesis as a twofold increase in AChR expression indicated. Among the corticoids, the glucocorticoids were the more potent, whereas the mineralocorticoid aldosterone had less marked effect. The sex hormones progesterone and testosterone partially blocked these effects, without inducing any significant effect when given alone. The steroids tested differed in efficacy in correlation with their different chemical structures. Among the glucocorticoids a clear structure-activity relationship could be established. These results emphasize the specificity of corticoid action on muscle cells and suggest an explanation for the effects induced by glucocorticoids used in treating human muscular or neuromuscular diseases.