Disruption of synaptosomal calcium channel function by Lambert-Eaton myasthenic immunoglobulin is serum-dependent

Sandra J. Hewett, William D. Atchison

Research output: Contribution to journalArticlepeer-review

7 Scopus citations

Abstract

An autoantibody to nerve terminal Ca2+ channels has been suggested to mediate the pathogenesis of the neuromuscular disorder Lambert-Eaton Myasthenic Syndrome (LEMS). We demonstrated previously that in the presence of control human serum, immunoglobulins isolated from a patient with LEMS reduced flux of Ca2+ into isolated nerve terminals during depolarization. The objective of the present study was to determine the role of serum in reducing uptake of 45Ca2+ into rat brain synaptosomes by LEMS IgG. Depolarization-dependent uptake of 45Ca2+ through voltage-gated Ca2+ channels was determined using synaptosomes incubated with control (disease-free) and LEMS IgG with or without control human serum. In the absence of human serum, LEMS IgG did not reduce uptake of 45Ca2+ into synaptosomes. However, in the presence of control human serum (10% of total incubation volume), 45Ca2+ uptake was reduced significantly by LEMS IgG (2 and 4 mg/ml), but not by IgG from disease-free patients or by 10% (v/v) control human serum alone. This concentration of serum was found to be optimal; higher concentrations produced significant reductions in Ca2+ uptake, whereas at lower concentrations the serum/IgG combination was ineffective. The depressant effect of high concentrations of serum alone on 45Ca2+ uptake was mimicked by equal concentrations of bovine serum albumin suggesting that deficits in 45Ca2+ uptake produced by high concentrations of serum were due to increased protein binding of the radiolabel. Heat-inactivating the serum abolished its ability to interact with the LEMS immunoglobulins to depress 45Ca2+ uptake. This suggested a role for complement in this effect. To test whether the membrane attack complex (MAC) or the alternative pathway of complement (APC) contributed to the alteration in Ca2+ channel function, 45Ca2+ uptake into synaptosomes was measured following incubation with LEMS IgG and C5- or Factor B-deficient serum, respectively. LEMS IgG reduced K+-stimulated uptake in both cases. Thus, neither the MAC nor the APC contributed to disruption of Ca2+ channel function by LEMS IgG in synaptosomes. In contrast, incubation of synaptosomes with LEMS IgG and C3-deficient serum prevented the reduction in 45Ca2+ uptake suggesting that the C3 component of the complement pathway may participate in Ca2+ channel dysfunction in synaptosomes following incubation with serum and LEMS IgG.

Original languageEnglish (US)
Pages (from-to)317-323
Number of pages7
JournalBrain Research
Volume599
Issue number2
DOIs
StatePublished - Dec 25 1992
Externally publishedYes

Keywords

  • Calcium channel
  • Complement
  • Lambert-Eaton Myasthenic Syndrome
  • Neuromuscular disorder
  • Neurotransmitter release
  • Synaptosome

ASJC Scopus subject areas

  • Neuroscience(all)
  • Molecular Biology
  • Clinical Neurology
  • Developmental Biology

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