[Molecular immunology of voltage-gated calcium channel and Lambert-Eaton myasthenic syndrome]

Abstract

Lambert-Eaton myasthenic syndrome(LEMS), an autoimmune disease that is often associated with small cell lung carcinoma(SCLC), impairs the quantal release of acetylcholine by directing antibodies against voltage-gated calcium channels (VGCC) in the motor nerve terminal. We focused attention on the P/Q type VGCC, to which there are antibodies in LEMS patients in higher frequency than antibodies to other types of VGCC. To search for antigenic sites in the molecular structure of alpha 1A subuuit of P/Q type VGCC in LEMS, we synthesized 4 peptides corresponding to the extracellular region (S5-S6 linker) of each of 4 domains that form alpha 1A subunit of VGCC. Also, LEMS patients' sera were studied by immunoprecipitation assay using these antigens. Peptides corresponding to the extracellular region (S5-S6 linker) of domains II and IV were specifically reactive with LEMS antibodies; their titiers respectively correlated with those of anti-P/Q type calcium channel (omega-conotoxin MVIIC-sensitive human cerebellum extret). Lewis rats were immunized with the domain II S5-S6 linker peptides conjugated with KLH. The immunized rats showed LEMS features characterized by reduced acetylcholine quantum content of endplate potentials and antibodies reactive with P/Q type VGCC. Our observations suggest 2 potential epitopes of LEMS antibodies. Synaptotagmin is a Ca2+ and phospholipid binding protein integrated in synaptic vesicle membranes. It plays a crucial role in neurotransmitter release, probably as a Ca2+ sensor for exocytosis of synaptic vesicles. The extracellular region of synaptotagmin was found antigenic for the induction of a rat model of LEMS. A proportion of human LEMS antibodies reacted with the recombinant synaptotagmin in immunoblot.