Functional inhibition of acetylcholine receptors by antibodies in myasthenic sera

Autoimmunity and neurological disease: antibody modulation of synaptic transmission

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

Association of arthrogryposis multiplex congenita with maternal antibodies inhibiting fetal acetylcholine receptor function

Arthrogryposis multiplex congenita (AMC), characterized by multiple joint contractures developing in utero, results from lack of fetal movement. Some cases are genetically determined, but AMC occasionally complicates pregnancy in patients with myasthenia gravis (MG) suggesting involvement of circulating maternal antibodies. We previously demonstrated antibodies that inhibited the function of fetal acetylcholine receptor (AChR) in one healthy woman with an obstetric history of recurrent AMC. Here we study sera from this woman, from one other with a similar history, and from three (one asymptomatic) whose babies had neonatal MG and AMC. All five maternal sera had high titers of antibodies that inhibited alpha-Bungarotoxin (alpha-BuTx) binding to fetal AChR, and their sera markedly inhibited fetal AChR function with little effect on adult AChR function. Moreover, in a further survey, 3 of 20 sera from anti-AChR negative AMC mothers inhibited fetal AChR function significantly at 1:100 dilution. These results demonstrate the role of antibodies to fetal AChR and perhaps other muscle antigens in some cases of AMC. More generally, they suggest that placental transfer of antibodies directed at fetal antigens should be considered as a cause of other recurrent fetal or perinatal disorders.

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

Antibodies to the acetylcholine receptor (AChR), which are diagnostic of the human autoimmune disease myasthenia gravis, block AChR function and increase the rate of AChR degradation leading to impaired neuromuscular transmission. Steroids are frequently used to alleviate symptoms of muscle fatigue and weakness in patients with myasthenia gravis because of their well-documented immunosuppressive effects. We show here that the steroid dexamethasone significantly increases total surface AChRs on cultured human muscle exposed to myasthenia gravis sera. Our results suggest that the clinical improvement observed in myasthenic patients treated with steroids is due not only to an effect on the immune system but also to a direct effect on muscle. We propose that the identification and development of pharmacologic agents that augment receptors and other proteins that are reduced by human genetic or autoimmune disease will have broad therapeutic applications.

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

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

Plasma from myasthenia gravis patients reduces acetylcholine receptor agonist-induced Na+ flux into TE671 cell line

Plasma from myasthenia gravis patients was tested for its ability to inhibit agonist-induced 22Na+ influx into the TE671 cell line that expresses human acetylcholine receptors. Reduced 22Na+ influx correlated weakly with the total anti-acetylcholine receptor antibody level in the plasma, and was also related to the presence of antibody directed against the agonist binding site, as detected by inhibition of 125I-alpha-bungarotoxin binding. However, in some cases there was inhibition of 22Na+ flux without evident anti-alpha-bungarotoxin binding site antibody. We conclude that in most patients antibodies that interfere with 22Na+ influx do so by blocking the agonist binding site. However, in some cases antibodies may be directed at the Na+ ion channel or some important functional determinant.