Spontaneous muscle action potentials are blocked by N-type and P/Q-calcium channels blockers in the rat spinal cord–muscle co-culture system

Effects of IgG anti-GM1 monoclonal antibodies on neuromuscular transmission and calcium channel binding in rat neuromuscular junctions

Guillain-Barré syndrome is a type of acute inflammatory neuropathy that causes ataxia and is associated with the IgG anti-GM1 antibody. However, the pathogenic role of the IgG anti-GM1 antibody and calcium channels in neuromuscular junctions (NMJs) remains unclear. Thus, the aim of the present study was to investigate the effects of the IgG anti-GM1 monoclonal antibody (mAb) on spontaneous muscle action potentials (SMAPs), and the effects of calcium channel blockers, in a rat spinal cord-muscle co-culture system. In addition, the binding of IgG anti-GM1 mAb to calcium channels was investigated in the rat hemidiaphragm. The frequency of SMAPs in the innervated muscle cells was acutely inhibited by the IgG anti-GM1 mAb; however, this effect was blocked by the N-type calcium channel blocker, ω-conotoxin GVIA (30 nM). Furthermore, the P/Q-type calcium channel blocker, ω-agatoxin IVA (10 nM), was found to partially block the IgG anti-GM1 mAb-induced inhibitory effect in the spinal cord-muscle co-culture system. Immunohistochemical analysis of the rat hemidiaphragm indicated that IgG anti-GM1 mAb binding overlapped with anti-Ca_v2.2 (α_1B) antibody binding in the nerve terminal. In addition, IgG anti-GM1 mAb binding partially overlapped with anti-Ca_v2.1 (α_1A) antibody binding. Thus, the results demonstrated that the IgG anti-GM1 mAb binds to calcium channels in the nerve terminals of NMJs. Therefore, the inhibitory effect of IgG anti-GM1 mAb on SMAPs may involve N-type and P/Q-type calcium channels in motor nerve terminals at the NMJ.

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.

IgG anti-GalNAc-GD1a antibody inhibits the voltage-dependent calcium channel currents in PC12 pheochromocytoma cells

We investigated the effects of IgG anti-GalNAc-GD1a antibodies, produced by immunizing rabbits with GalNAc-GD1a, on the voltage-dependent calcium channel (VDCCs) currents in nerve growth factor (NGF)-differentiated PC12 pheochromocytoma cells. VDCCs currents in NGF-differentiated PC12 cells were recorded using the whole-cell patch-clamp technique. Immunized rabbit serum that had a high titer of anti-GalNAc-GD1a antibodies inhibited the VDCCs currents in the NGF-differentiated PC12 cells (36.0+/-9.6% reduction). The inhibitory effect of this serum was reversed to some degree within 3-4 min by washing with bath solution. Similarly, application of purified IgG from rabbit serum immunized with GalNAc-GD1a significantly inhibited the VDCCs currents in PC12 cells (30.6+/-2.5% reduction), and this inhibition was recovered by washing with bath solution. Furthermore, the inhibitory effect was also observed in the GalNAc-GD1a affinity column binding fraction (reduction of 31.1+/-9.85%), while the GalNAc-GD1a affinity column pass-through fraction attenuated the inhibitory effect on VDCCs currents. Normal rabbit serum and normal rabbit IgG did not affect the VDCCs currents in the PC12 cells. In an immunocytochemical study using fluorescence staining, the PC12 cells were stained using GalNAc-GD1a binding fraction. These results indicate that anti-GalNAc-GD1a antibodies inhibit the VDCCs currents in NGF-differentiated PC12 cells.