Immunochemical properties of junctional and extrajunctional acetylcholine receptor

Association of neonatal myasthenia gravis with antibodies against the fetal acetylcholine receptor

The specificities of autoantibodies directed against the acetylcholine receptor (AChR) for embryonic and adult muscle AChR were studied in 22 mothers with myasthenia gravis (MG) and in their newborns using human fetus and normal adult muscle AChR preparations. 12 mothers had transmitted MG to their neonates with, in three cases, antenatal injury. A clear correlation was found between occurrence of neonatal MG (NMG) and the high overall level of anti-AChR antibodies (embryonic or adult muscle AChR). However, a strong correlation was also found between occurrence of NMG and the ratio of anti-embryonic AChR to anti-adult muscle (Te/Ta) AChR antibodies (P < 0.0002). Taken together, these data suggest that autoantibodies directed against the embryonic form of the AChR could play a predominant role in the pathogenesis of NMG. Paradoxically, the three cases with antenatal injury presumably the most severe form of NMG, were not associated with high Te/Ta. At the clinical level, these observations could prove helpful in the prediction of transmission of NMG.

A myasthenia gravis plasma immunoglobulin reduces miniature endplate potentials at human endplates in vitro

A particular myasthenia gravis (MG) plasma Ig has previously been shown to block a single alpha-bungarotoxin (alpha-BuTx) binding site on embryonic rat muscle acetylcholine receptor (AChR). We have investigated its effect on embryonic/denervated and adult human AChR both in extracts and in situ. Plasma Ig blocked 125I-alpha-BuTx binding by greater than 85% to the AChR extracted from denervated muscle, but only by 55% to AChR extracted from normal human muscle. Incubation of intact human muscle fibers with the plasma Ig reduced 125I-alpha-BuTx binding to the endplate AChRs by 63%, and substantially decreased the amplitude of miniature endplate potentials. We conclude that anti-alpha-BuTx site antibodies, when present, can be important in the pathophysiology of the disease.

Isolation and characterization of nicotinic acetylcholine receptor-like protein from fetal calf thymus

A nicotinic acetylcholine receptor-like protein (AChR-LP) was isolated from fetal calf thymus by affinity chromatography using cobrotoxin-Sepharose after alkaline extraction and solubilization with Triton X-100. The AChR-LP had a specificity of 1.61 +/- 1.12 nmol of alpha-bungarotoxin binding sites per mg of protein. The isoelectric point, sedimentation coefficient and amino acid composition of the purified AChR-LP were very similar to those of muscle and electric organ AChRs. Upon SDS-polyacrylamide gel electrophoresis purified thymus AChR-LP preparations contained up to 6 polypeptide bands of molecular weights of 40,000, 43,000, 51,000, 56,000, 58,000, and 66,000, respectively. The peptides of 40,000, 51,000, 56,000, and 66,000 dalton cross-reacted with the four subunits of Torpedo californica and fetal calf muscle AChR.

Regulation of acetylcholine receptor channel function during development of skeletal muscle

The nicotinic acetylcholine (ACh) receptor channel mediates synaptic transmission at the neuromuscular junction. During the development of skeletal muscle, ACh receptors undergo changes in distribution, antigenic determinants, degradation rate, and function. Now that these developmental hallmarks have been identified, attention has turned toward understanding both the structural bases for such changes and the role of nerve in triggering these changes. Recently, a much clearer understanding of one of these developmental processes, namely, the alterations in channel function, has emerged through both sensitive patch-clamp measurements and the application of recombinant DNA technology. In light of these new advances, we now reevaluate the processes governing the developmental changes in the functional properties of the ACh receptor.

Loss of antigenic properties of acetylcholine receptors in rat skeletal muscle after birth

Antibodies in a myasthenic serum were used to follow the changes in antigenic properties of acetylcholine receptors in rat skeletal muscle during development. In binding assays at saturating concentrations of antigen or antibody, the antibodies reacted with extrajunctional receptors of fetal and denervated adult muscle but showed little binding to junctional receptors of adult rat muscle. They did, however, bind to junctional receptors of adult chicken muscle which, unlike rat receptors, do not appear to undergo a change in their channel properties during development. Binding studies with acetylcholine receptors of developing rat muscle carried out at saturating concentrations of antibody showed that the loss of antigenic determinant(s) begins at 1-2 days after birth.

Immunological evidence for a change in subunits of the acetylcholine receptor in developing and denervated rat muscle

We used specific antibodies to gamma, delta, and epsilon subunits to characterize acetylcholine receptor (AChR) in extracts and at endplates of developing, adult, and denervated rat muscle. The AChRs in normal adult muscle were immunoprecipitated by anti-epsilon and anti-delta, but not by anti-gamma antibodies, whereas AChRs in denervated and embryonic muscles were precipitated by anti-gamma and anti-delta, but showed little or no reactivity to anti-epsilon antibodies. In immunofluorescence experiments, AChRs at neonatal endplates bound antibodies to gamma or delta, but not epsilon, subunit, whereas those in adult muscles bound antibodies to epsilon or delta, but not gamma, subunit. AChRs at denervated endplates and at developing endplates between postnatal days 9 and 16 bound all three antibodies. We conclude that the distribution of gamma and epsilon subunits of the AChR parallels the distribution of AChRs with embryonic and adult channel properties, respectively.

Myasthenic serum selectively blocks acetylcholine receptors with long channel open times at developing rat endplates

We have examined the physiological effects of antibodies from a highly specific myasthenic serum on acetylcholine receptors at developing rat endplates. The antibodies reduced the amplitude of miniature endplate potentials by 60% in 3- to 6-day-old animals but had no effect after day 14. Between days 7 and 12 the antibodies had an intermediate effect. This is the same period during which acetylcholine receptors with long channel open times (slow channels) disappear and receptors with short open times (fast channels) increase in number. Therefore, we examined the effect of the antibodies at endplates with a mixture of channel types more carefully. At all times tested, both noise analysis and analysis of miniature endplate currents showed that the antibodies reduced slow channel activity selectively. Single-channel recordings indicated that acetylcholine receptors that remained active after antibody treatment had normal gating properties. Thus, the antibodies appeared to silence slow channels selectively.