Neurophysiological and immunohistochemical studies of IgG anti-GM1 monoclonal antibody on neuromuscular transmission: effects in rat neuromuscular junctions.

Guillain-Barré syndrome, which is a variant of acute inflammatory neuropathy, is associated with anti-GM1 antibodies and causes ataxia. We investigated the effects of IgG anti-GM1 monoclonal antibody (IgG anti-GM1 mAb) on spontaneous muscle action potentials in a rat spinal cord-muscle co-culture system and the localization of IgG anti-GM1 mAb binding in the rat hemi-diaphragm. The frequency of spontaneous muscle action potentials in innervated muscle cells was acutely inhibited by IgG anti-GM1 mAb. When cultures were pretreated with GM2 synthase antisense oligodeoxynucleotide, IgG anti-GM1 mAb failed to inhibit spontaneous muscle action potentials, demonstrating the importance of the GM1 epitope in the action of IgG anti-GM1 mAb. Immunohistochemistry of rat hemi-diaphragm showed that IgG anti-GM1 mAb binding overlapped with neurofilament 200 (NF200) antibodies staining, but not α-bungarotoxin (α-BuTx) staining, demonstrating that IgG anti-GM1 mAb was localized at the presynaptic nerve terminal. IgG anti-GM1 mAb binding overlapped with syntaxin antibody and S-100 antibody in the nerve terminal. After collagenase treatment, IgG anti-GM1 mAb and NF200 antibodies did not show staining, but α-BuTx selectively stained the hemi-diaphragm. IgG anti-GM1 mAb binds to the presynaptic nerve terminal of neuromuscular junctions. Therefore, we suggest that the inhibitory effect of IgG anti-GM1 mAb on spontaneous muscle action potentials is related to the GM1 epitope in presynaptic motor nerve terminals at the NMJs.

Effect of sera from seronegative myasthenia gravis patients on neuromuscular junctions.

About 85 % of patients with generalized myasthenia gravis (MG) have anti-nicotinic acetylcholine receptor (nAChR) antibodies in their sera (seropositive MG; SPMG). The other 15 % (seronegative MG; SNMG) are also considered to have antibody-mediated disease, but the nature of the antibodies in SNMG is not fully understood. We investigated the effect of sera from patients with MG on spontaneous muscle action potentials and acetylcholine (ACh)-induced potentials, and we examined the localization of epitopes recognized by SPMG sera or SNMG sera. SPMG sera and SNMG sera inhibited spontaneous muscle action potentials and ACh-induced potentials in the spinal-muscle co-culture system. However, spontaneous muscle action potentials and ACh-induced potentials in the neuromuscular junctions were strongly blocked by SPMG serum, whereas they were weakly blocked by SNMG serum. Both types of sera reacted strongly with the neuromuscular junctions in normal rat muscles, as shown by double immunostaining with serum and α-bungarotoxin. The SPMG epitope remained in the neuromuscular junctions, whereas that of SNMG disappeared after denervation of the sciatic nerve. Therefore, we suggest that the skeletal muscle weakness in SNMG may be due to an interaction with presynaptic neuromuscular transmission and nAChR.