Myasthenic patients' IgG causes redistribution of acetylcholine receptors: freeze-fracture studies

Abstract

Immunoglobulin from patients with myasthenia gravis (MG) accelerates the degradation of acetylcholine receptors (AChRs) of skeletal muscle by a mechanism dependent on receptor cross-linking. However, the intermediate steps between cross-linking and endocytosis and degradation are not known. We have used freeze-fracture electron microscopy to study the effects of myasthenic immunoglobulin on the distribution of AChRs in the surface membranes of cultured rat skeletal muscle. Large, angular intramembrane particles were identified as AChRs. These particles were present in three configurations: distributed individually; in "hot spots"; and in clusters of two to 60 particles. Large depressions (greater than 100 nm in diameter) were thought to be endocytotic pits; they occurred preferentially in hot spots, and approximately half of the depressions were closely associated with clusters of AChRs. In control myotubes, hot spots contained a high density of AChR particles that were evenly, rather than randomly, distributed. Exposure to myasthenic patients' IgG induced clustering of the particles within such hot spots. In non-hot spot regions, control myotubes had mainly individually distributed AChRs, while clusters were seen only rarely. The addition of myasthenic IgG caused a rapid increase, followed by a gradual decrease, in the incidence of individual clusters as well as an increase in cluster size. This redistribution of particles was dependent on cross-linking by the myasthenic anti-AChR antibodies. Monovalent Fab fragments of myasthenic IgG had no effect on the distribution of AChR particles. However, addition of a second "piggyback" antibody to cross-link Fab-AChR complexes caused a redistribution of AChR particles like that due to the original myasthenic IgG. These findings indicate that cross-linking of AChRs by divalent antibody causes clustering of AChR particles, which may play an important role in their accelerated endocytosis and degradation.