Ligand induced changes in stability and distribution of acetylcholine receptors on surface membranes of muscle cells

Extracellular synaptic factors induce clustering of acetylcholine receptors stably expressed in fibroblasts

The clustering of nicotinic acetylcholine receptors (AChRs) is one of the first events observed during formation of the neuromuscular junction. To determine the mechanism involved in AChR clustering, we established a nonmuscle cell line (mouse fibroblast L cells) that stably expresses just one muscle-specific gene product, the AChR. We have shown that when Torpedo californica AChRs are expressed in fibroblasts, their immunological, biochemical, and electrophysiological properties all indicate that fully functional cell surface AChRs are produced. In the present study, the cell surface distribution and stability of Torpedo AChRs expressed in fibroblasts (AChR-fibroblasts) were analyzed and shown to be similar to nonclustered AChRs expressed in muscle cells. AChR-fibroblasts incubated with antibodies directed against the AChR induced the formation of small AChR microclusters (less than 0.5 micron 2) and caused an increase in the internalization rate and degradation of surface AChRs (antigenic modulation) in a manner similar to that observed in muscle cells. Two disparate sources of AChR clustering factors, extracellular matrix isolated from Torpedo electric organ and conditioned media from a rodent neuroblastoma-glioma hybrid cell line, each induced large (1-3 microns 2), stable AChR clusters with no change in the level of surface AChR expression. By exploiting the temperature-sensitive nature of Torpedo AChR assembly, we were able to demonstrate that factor-induced clusters were produced by mobilization of preexisting surface AChRs, not by directed insertion of newly synthesized AChRs. AChR clusters were never observed in the absence of extracellular synaptic factors. Our results suggest that these factors can interact directly with the AChR.

Binding characteristics of 3H-dihydroalprenolol to beta-adrenoceptors of rat heart treated with neuraminidase

Binding characteristics of the beta-blockers and beta-agonists with the beta-adrenoceptors were investigated in 3H-dihydroalprenolol (3H-DHA) binding to rat heart membranes treated with neuraminidase. When 60% of the total sialic acid content in the membranes was removed, reproducibility of the binding assay became much better than was attainable without neuraminidase treatment, and the maximum density of beta-adrenoceptors was increased. These data suggest that the binding of the test compounds with the beta-adrenergic receptors in cardiac muscle was under the influence of the sialic acid of the glycocalyx of the membrane. The 3H-DHA binding sites in membranes treated with neuraminidase showed a strict stereo-specificity when tested with propranolol. The ranking order of inhibition of beta-antagonists or agonists is: dl-propranolol greater than oxprenolol greater than alprenolol greater than pindolol greater than YM-09538 greater than labetalol greater than acebutolol greater than atenolol greater than metoprolol greater than sotalol greater than butoxamine greater than practolol as antagonists or l-isoproterenol greater than l-epinephrine greater than l-norepinephrine as agonists. A good correlation (r = 0.91, P less than 0.001) was observed between the Ki values observed by the present binding assay and the pA2 observed in the guinea-pig atria relative to the positive inotropic effect by Bieth et al. (Br. J. Pharmacol. 68, 563-569, 1980), indicating that the present method will be useful for screening new beta-adrenergic receptor antagonists or agonists.

Interaction of monoclonal antibodies to Torpedo acetylcholine receptor with the receptor of skeletal muscle

Several monoclonal antibodies (mcAbs) elicited against the nicotinic acetylcholine receptor (AChR) from Torpedo react also with skeletal muscle AChR. Such mcAbs were used to define antigenic determinants on muscle AChR and to elucidate their effect on muscle AChR functions. Primary chick muscle cultures were used as a model for skeletal muscle. Of the four mcAbs studies only mcAb 5.5, which is directed against the cholinergic site in Torpedo AChR, blocks the binding of alpha-bungarotoxin (alpha-Bgt) to AChR in chick muscle cultures and inhibits carbamylcholine-induced sodium transport in these cells. The interaction of mcAb 5.5 with the cholinergic site on muscle AChR demonstrates the conservation of this site. Two mcAbs, 5.5 and 5.34, each of a different antigenic specificity but both directed against conformation-dependent antigenic determinants, accelerate the degradation of AChR in muscle cultures. From the reactivity of the various mcAbs with Triton-solubilized and membranous AChR it appears that there are some antigenic differences between the detergent solubilized and membranous forms of the receptor.

A factor from neurons induces partial immobilization of nonclustered acetylcholine receptors on cultured muscle cells

A factor or factors released by cultured NG108-15 neuroblastoma X glioma hybrid cells and added to the medium of rat myotube primary cultures was found to immobilize some of the previously mobile acetylcholine receptors in the myotube membrane. Partial receptor immobilization occurred within 3 h after the beginning of treatment with the NG108-15-conditioned medium factor and persisted for at least 24 h of continuous treatment. A similarly derived conditioned medium concentrate from the non-neuronal parent glioma cell line did not immobilize receptors, relative to untreated controls. Acetylcholine receptors were visualized by fluorescent alpha-bungarotoxin and their lateral motion was observed by the technique of fluorescence photobleaching recovery.

Carbohydrate requirement for expression and stability of acetylcholine receptor on the surface of embryonic muscle cells in culture

We have investigated the significance of protein glycosylation for metabolism of acetylcholine receptors (AcChoR) in primary cultures of embryonic chicken muscle cells. Tunicamycin, a specific inhibitor of the glycosylation of asparagine residues on glycoproteins, decreased AcChoR accumulation and accelerated its degradation. In contrast, there was no evidence that tunicamycin treatment affected AcChoR biosynthesis, intracellular transport, or incorporation into surface membranes. Leupeptin, an inhibitor of intracellular proteases, markedly increased accumulation of AcChoR on the external surface of muscle cells treated with tunicamycin. Our findings indicate that impairment of protein glycosylation prevents accumulation of AcChoR by increasing its susceptibility to degradation by cellular proteases.

Crosslinkage and visualization of acetylcholine receptors on myotubes with biotinylated alpha-bungarotoxin and fluorescent avidin

A biotinylated derivative of alpha-bungarotoxin and tetramethylrhodamine-labeled avidin were employed to fluorescence label the acetylcholine receptors (AcChoR) on the surface of rat myotubes in primary culture. Because of the multivalency of both the biotinylated bungarotoxin and the avidin, this treatment extensivey crosslinks the AcChoR. AcChoR crosslinking immobilizes more than 90% of the normally laterally mobile AcChoR as verified by the fluorescence photobleaching recovery technique; it also redistributes the AcChoR into visible micropatches. Biotinylated alpha-bungarotoxin/avidin-induced AcChoR crosslinking greatly accelerates the rate of internalization of surface AcChoR; this rapid internalization affects both the normally immobile AcChoR in areas of diffuse distribution and the normally immobile AcChoR in preexisting patches. The peculiar pattern of fluorescent avidin binding to AcChoR patches previously bound with biotinylated bungarotoxin suggests that almost all AcChoR patches are in very close contact (< 70 A) with the glass substrate. AcChoR immobilization leads to a partial immobilization of concanavalin A receptors in the myotube membrane.

Interrelationship of concanavalin-A-binding and antigenic sites on the acetylcholine receptor from Torpedo marmorata

A preparation of purified 125I-labelled acetylcholine receptor was shown to bind to concanavalin A and to be totally bound by rabbit antiserum to Torpedo acetylcholine receptor. Pre-incubation of the receptor with F(ab')2 and Fab fragments from antibodies against Torpedo acetylcholine receptor, or with corresponding fragments from control immunoglobulin G showed that subsequent binding of the receptor to concanavalin A was specifically inhibited to a maximum of approximately 25% by the immune fragments. Treatment of acetylcholine receptor with periodate or with glycosidases apparently destroyed or removed carbohydrate residues without affecting the antigenicity of the receptor as assessed by radioimmunoassay. These results suggest that although there is a steric interrelatonship between the antigenic and concanavalin-A-binding sites of the receptor the latter sites do not contain its major antigenic determinants.

Experimental autoimmune myasthenia gravis

Injection of animals with purified acetylcholine receptor in complete Freund's adjuvant causes development of antibodies which crossreact with receptors in muscle. The crossreacting antibodies impair neuromuscular transmission. Animals with experimental autoimmune myasthenia gravis (EAMG) are excellent models for studying the complex mechanisms by which the autoimmune response to receptor in myasthenia gravis causes muscle weakness. This review first briefly describes the discovery of EAMG. Then, to provide the necessary perspective, receptor structure and function and properties of anti-receptor antibodies are discussed, followed by a brief review of the pathological mechanisms in EAMG.