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

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.