Passive transfer of Lambert-Eaton myasthenic syndrome with IgG from man to mouse depletes the presynaptic membrane active zones.

Abstract

In the Lambert-Eaton myasthenic syndrome (LEMS), there is a decreased release of acetylcholine quanta from the nerve terminal by nerve impulse. Recently, an autoimmune origin of LEMS was documented by passive transfer of its electrophysiologic features from man to mousewith IgG. Freeze-fracture electron microscopy of LEMS neuromuscular junctions has revealed a paucity of presynaptic membrane active zones. Thus, the active zones might be the targets of the pathogenic autoantibodies in LEMS. To test this assumption, freeze-fracture electron microscopic studies were done in mice injected with 10 mg of IgG daily from each of three LEMS patients and in control mice treated with normal human IgG or no IgG. IgG from patients 1 and 2 impaired neuromuscular transmission in mice, but IgG from patient 3 failed to do so. After 52-69 days of treatment, diaphragm or anterior tibial muscles were removed and coded. Paired muscles from control mice and mice receiving LEMS IgG were studied "blindly." Satisfactory freeze-fracture replicas of 185 presynaptic membrane P-faces were analyzed by stereometric methods. In mice treated with LEMS IgG that was pathogenic by electrophysiologic criteria, there was a selective depletion of active zones and activezone particles but not of other membrane particles and there was a concomitant increase of large membrane particles aggregated into clusters. These findings provide additional evidence that the active zones facilitate quantal transmitter release by nerve impulse, lendiurther support to the assumption that the active-zone particles are Ca2+ channels, and establish mediation of the membrane lesions in LEMS by IgG.

Heterogeneity in Synaptic Vesicle Release at Neuromuscular Synapses of Mice Expressing SynaptopHluorin

“…Although we also observed spots of greater intensity as compared with other areas of the terminal, it is unlikely that these represent individual active zones. Previous studies have estimated the density of active zones in mouse neuromuscular terminals to be ϳ2.5 per m 2 (Fukunaga et al, 1982(Fukunaga et al, , 1983. Based on our imaging conditions, our limit of resolution is on the order of 1 m. Therefore, each spH florescence "spot" likely encompasses several active zones.…”

Autoimmune disorders of the neuromuscular junction

“…[68] Mice injected with LEMS IgG showed similar changes to those described in the patients. [69] Moreover, IgG could be demonstrated on the motor nerve terminal, apparently at the position where VGCCs are thought to be concentrated. [70] Other evidence implicated divalent binding of antibodies leading to reduction in the VGCCs, and complement did not seem to be involved.…”

Lambert-Eaton sera reduce low-voltage and high-voltage activated Ca2+ currents in murine dorsal root ganglion neurons.

“…In freezefracture views of neuromuscular synapses, intramembrane particles, which are thought to include Ca2+ channels, are organized in linear arrays at active zones (5)(6)(7). The hypothesis that LES antibodies target the Ca2+ channels that control transmitter release arose from the observation that LES antibodies decrease the number of active zones and disrupt active zone organization in mice and humans (8)(9)(10). Further support for this hypothesis has been based on the demonstration that LES antibodies decrease Ca2+ influx in transformed cells such as small cell lung carcinoma (11,12), rat (13) and human (14) neuroblastoma, and rat thyroid cells (15), as well as in secretory cells such as adrenal chromaffin (16)(17)(18) and rat pituitary cells (19).…”