Specificity of Lambert-Eaton Myasthenic Syndrome immunoglobulin for nerve terminal calcium channels

Abstract Lambert-Eaton Myasthenic Syndrome (LEMS) is a presynaptic, neuromuscular disorder characterized by impaired nerve-evoked release of ACh. Repetitive nerve stimulation, which increases the probability of quantal release, improves the transmission defect. An autoantibody to Ca 2+ channels of presynaptic motor nerve terminals is thought to mediate the pathogenesis of this disease. The goal of the present study was to examine the specificity of LEMS autoantibodies for nerve terminal Ca 2+ channels as compared to other voltage-sensitive ion channels in nerve terminals, and to determine if non-specific membrane damage contributed to the pathogenesis of LEMS. The ion channel specificity of LEMS autoantibody was assessed by comparing the ability of acute application of IgG isolated from the plasma of a patient with LEMS to reduce depolarization-dependent uptake of 45 Ca 2+ and 22 Na + into or efflux of 86 Rb + from rat forebrain synaptosomes. The clinical diagnosis of LEMS was confirmed electrophysiologically by treatment of mice for 30 days with plasma (1.5 ml/day) taken from this patient. Characteristic reduction of quantal content elicited at 1 Hz and facilitation at 20 Hz was observed in mice treated wtih LEMS plasma compared to those treated with control plasma. One s, K + -stimulated 45 Ca 2+ uptake was inhibited36.5 ± 14.5%and44.5 ± 9.8% by acute application of 2 and 4 mg/ml LEMS IgG, respectively; IgG from patients with small cell carcinoma of the lung (SCC) had no effect on 45 Ca 2+ entry. The same concentrations of LEMS IgG affected neither voltage-dependent uptake of 22 Na + into veratridine-depolarized synaptosomes nor 86 R + efflux from K + -depolarized synaptosomes. Lactate dehydrogenase release from synaptosomes incubated with varying concentrations of LEMS IgG was not increased over control suggesting that the reduction in 45 Ca 2+ uptake was not a result of disruption of synaptosomal membrane integrity. Likewise, normal activation of Ca 2+ channels was not prevented by IgG-induced alterations in synaptosomal membrane potentials as the fluorescence of a membrane potential-sensitive carbocyanine dye (diS-C 2 (5)) was unaltered following acute application of 2 mg/ml LEMS or control IgG. These results indicate that immunoglobulins isolated from a patient with LEMS alter presynaptic nerve terminals Ca 2+ channel activity. These deficits are not due to decreases in synaptosomal integrity nor to IgG-induced alterations in the synaptosomal membrane potential but appear to result from direct action on voltage-dependent Ca 2+ channels. The antibody from this patient appears to be specific for nerve terminal Ca 2+ channels as it did not affect ion flux through voltage-dependent Na + or K + channels.