Monoclonal antibodies against the acetylcholine receptor gamma-subunit as site specific probes for receptor tyrosine phosphorylation

Solution conformation of the antibody-bound tyrosine phosphorylation site of the nicotinic acetylcholine receptor beta-subunit in its phosphorylated and nonphosphorylated states

Phosphorylation of the acetylcholine receptor (AChR) seems to be responsible for triggering several effects including its desensitization and aggregation at the postsynaptic membrane and probably initiates a signal transduction pathway at the postsynaptic membrane. To study the structural and functional role of the tyrosine phosphorylation site of the AChR beta-subunit and contribute to the in-depth understanding of the structural basis of the ion channel function, we synthesized four peptides containing the phosphorylated and nonphosphorylated sequences (380-391) of the human and Torpedo AChR beta-subunits and studied their interaction with a monoclonal antibody (mAb 148) that is known to bind to this region and that is capable of blocking ion channel function. All four peptides were efficient inhibitors of mAb 148 binding to AChR, although the nonphosphorylated human peptide was considerably less effective than the three others. We then investigated the conformation acquired by all four peptides in their antibody-bound state, which possibly illustrates the local conformation of the corresponding sites on the intact AChR molecule. The phosphorylated human and Torpedo peptides adopted a distorted 3(10) helix conformation. The nonphosphorylated Torpedo peptide, which is also an efficient inhibitor, was also folded. In contrast, the nonphosphorylated human peptide (a less efficient inhibitor) presented an extended structure. It is concluded that the phosphorylation of the AChR at its beta-subunit Tyr site leads to a significant change in its conformation, which may affect several functions of the AChR.

Anatomy of the antigenic structure of a large membrane autoantigen, the muscle-type nicotinic acetylcholine receptor

The neuromuscular junction nicotinic acetylcholine receptor (AChR), a pentameric membrane glycoprotein, is the autoantigen involved in the autoimmune disease myasthenia gravis (MG). In animals immunized with intact AChR and in human MG, the anti-AChR antibody response is polyclonal. However, a small extracellular region of the AChR alpha-subunit, the main immunogenic region (MIR), seems to be a major target for anti-AChR antibodies. A major loop containing overlapping epitopes for several anti-MIR monoclonal antibodies (mAbs) lies within residues alpha 67-76 at the extreme synaptic end of each alpha-subunit: however, anti-MIR mAbs are functionally and structurally quite heterogeneous. Anti-MIR mAbs do not affect channel gating, but are very effective in the passive transfer of MG to animals; in contrast, their Fab or Fv fragments protect the AChR from the pathogenic effects of the intact antibodies. Antibodies against the cytoplasmic region of the AChR can be elicited by immunization with denatured AChR and the precise epitopes of many such mAbs have been identified; however, it is unlikely that such antibodies are present in significant amounts in human MG. Antibodies to other extracellular epitopes on all AChR subunits are present in both experimental and human MG; these include antibodies to the acetylcholine-binding site which affect AChR function in various ways and also induce acute experimental MG. Finally, anti-AChR antibodies cross-reactive with non-AChR antigens exist, suggesting that MG may result from molecular mimicry. Despite extensive studies, many gaps remain in our understanding of the antigenic structure of the AChR; especially in relation to human MG. A thorough understanding of the antigenic structure of the AChR is required for an in-depth understanding, and for possible specific immunotherapy, of MG.

Binding of immunoglobulin G from patients with autoimmune thyroid disease to rat sodium-iodide symporter peptides: evidence for the iodide transporter as an autoantigen

The recent cloning of the rat sodium-iodide symporter (rNIS) from FRTL-5 cells makes possible studies of the role of this thyroid-specific protein as an antigen in autoimmune diseases of the thyroid (AITD). We generated 21 synthetic peptides replicating the entire sequence of the extramembranous domains (ExMD) of rNIS. Each was synthesized by automated chemistry, purified by high-pressure liquid chromatography (HPLC), and characterized by mass spectroscopy. Immunoglobulins were purified using protein A from serum of 27 patients with Graves' disease (GD), 27 patients with autoimmune hypothyroidism (HT), and 20 normal controls. Binding of IgG from patients and controls to each of the rNIS peptides was measured by enzyme-linked immunosorbent assay (ELISA). Binding of patient IgG significantly greater than control was observed with six peptides: peptide 262-280 (representing ExMD 8 between transmembrane [TM] domains VII and VIII), peptide 437-444 (ExMD 11), peptides 468-487, 483-602, and 498-517 from ExMD 12, and peptides 560-579 from the proximal portion of the carboxyl terminus (ExMD 13). 63% of GD patients and 26% of HT patients immunoglobulin G (IgG) bound peptide 498-517 compared to zero controls. Similarly, 59% of GD were positive against peptide 468-487 versus zero controls. Peptide 262-280 bound IgG from 44% of GD patients, 15% of HT patients, and none of the controls. The remaining peptides showed little or no binding of patient IgG. These data indicate that patients with GD and HT possess antibodies that recognize rNIS significantly greater than do normal individuals, suggesting that the iodide transporter represents an important autoantigen in AITD. They further suggest that the incidence of the antibodies is higher in GD than HT, and that the antigenic epitopes involve ExMD 8, 11, 12, and 13.