Recognition of inter-transmembrane regions of acetylcholine receptor alpha subunit by antibodies, T cells and neurotoxins. Implications for membrane-subunit organization

Analysis of exposed regions on the main extracellular domain of mouse acetylcholine receptor alpha subunit in live muscle cells by binding profiles of antipeptide antibodies

To study the structural organization of the main extracellular domain of the nicotinic acetylcholine receptor (AChR) alpha subunit in live muscle cells, we examined the native membrane-bound receptors in cultured mouse skeletal muscle cells for their ability to bind a panel of antibodies against uniform-sized overlapping synthetic peptides which collectively represent this entire domain. The binding profile indicated that the regions alpha 23-49, alpha 78-126, alpha 146-174, and alpha 182-210 are accessible to binding with antibody. Residues alpha 23-49, alpha 78-126, and alpha 194-210 contain binding regions for alpha-neurotoxin and some myasthenia gravis autoantibodies. A comparison of this binding profile with the profile obtained for membrane-bound Torpedo californica AChR in isolated membrane fractions showed some similarities as well as significant differences between the subunit organization in the isolated membrane fraction and that in the membrane of live muscle cells. Regions alpha 89-104 and alpha 158-174, which are exposed in the isolated membrane fraction, are also exposed in the live cell. On the other hand, regions alpha 23-49, and alpha 182-210, which are exposed in the live cell, are not accessible in the isolated membrane and, furthermore, the region alpha 1-16, which has marginal accessibility in the cell, becomes highly accessible in the membrane isolates. The exposed regions defined by this study may be the primary targets for the initial autoimmune attack on the receptors in experimental autoimmune myasthenia gravis.

Profile of the regions of acetylcholine receptor alpha chain recognized by T-lymphocytes and by antibodies in EAMG-susceptible and non-susceptible mouse strains after different periods of immunization with the receptor

C57BL/6 (B6) mice develop a neuromuscular disease, experimental autoimmune myasthenia gravis (EAMG), after two or more immunizations with Torpedo californica acetylcholine receptor (AChR). To determine whether EAMG is related to recognition of particular region(s) on the main extracellular domain of the alpha chain (residues alpha 1-210) in prolonged immunization, we have examined the differences in the antibody and T cell recognition profiles of B6 and SJL (a strain that does not develop EAMG) mice after different periods and a number of immunizations with Torpedo AChR. In a given strain, antibodies and T cells recognized immunodominant regions, which may coincide or may be uniquely B cell or T cell determinants. Both B6 and SJL exhibited similar antibody recognition profiles after the second and through the fourth immunizations with AChR. Major differences between the two strains were found in their T cell recognition of regions in the second part (residues 100-210) of the main extracellular domain of the alpha chain. T cells of SJL recognized consistently only one region (111-126) within this part of the alpha chain, whereas in B6, T cell recognition of three peptides (111-126, 146-162 and 182-198) and next neighbor regions to them persisted throughout the period. Of these three peptides, 146-162 was an immunodominant peptide unique to B6, as the other two peptides (111-126 and 182-198) were also recognized by either T cells or antibodies in SJL. To study the role of the T cells recognizing region 146-162 in EAMG, a T cell line was generated against this region and the cells transferred into B6 mice followed by one Torpedo AChR injection. Enhancement of antibody production toward alpha chain peptides was observed as an influence of T cell transfer compared to profiles at 1 week. In addition, one out of three mice examined showed signs of EAMG. These results suggest the importance of T cells recognizing residues 146-162 in EAMG. It is concluded that the presence of persistent T cell responses to the second half (residues (100-210) of the main extracellular domain of the alpha chain is associated with the development of EAMG in B6 mice, while absence of these responses in SJL mice may enable them to escape the disease. The preservation of the immunodominance of peptide 146-162 in the T cell recognition of B6 is probably most important for the pathogenesis of EAMG in this strain.

Cell-lytic and antibacterial peptides that act by perturbing the barrier function of membranes: facets of their conformational features, structure-function correlations and membrane-perturbing abilities

Almost all hemolytic and antimicrobial peptides form part of the defense mechanism of species widely distributed across the evolutionary scale. Although these peptides are of varying lengths and composition, they form amphiphilic structures in a hydrophobic environment. They also have the ability to form channels in natural and model membranes. Hemolytic peptides have proven to be very useful in studying the mechanism of hemolysis and the permeability properties of red blood cells. Preliminary investigations indicate that these peptides may also be useful in the investigation of complex cellular phenomena like exocytosis and neurotransmission. Although molecules like vancomycin, bacitracin and penicillins have been extensively used as antibiotics for therapeutic purposes, most species throughout the evolutionary scale use peptides as antimicrobial agents. These peptides exert their activity by altering the permeability properties of the bacterial plasma membrane and do not interfere with macro molecular synthesis like the other antibiotics that are presently used in therapies. Hence it is likely that resistance to peptide antibacterial agents may not develop easily. Since the problem of antibiotic resistance is presently a particularly severe one, peptide antibiotics may be the drugs of choice in the future.

Mapping the extracellular topography of the alpha-chain in free and in membrane-bound acetylcholine receptor by antibodies against overlapping peptides spanning the entire extracellular parts of the chain

The extracellular surface of the alpha-chain of Torpedo california acetylcholine receptor (AChR) was mapped for regions that are accessible to binding with antibodies against a panel of synthetic overlapping peptides which encompassed the entire extracellular parts of the chain. The binding of the antipeptide antibodies to membrane-bound AChR (mbAChR) and to isolated, soluble AChR was determined. The specificity of each antiserum was narrowed down by determining the extent of its cross-reaction with the two adjacent peptides that overlap the immunizing peptide. With mbAChR, high antibody reactivity was obtained with antisera against peptides alpha 1-16, alpha 89-104, alpha 158-174, alpha 262-276, and alpha 388-408. Lower, but significant, levels of reactivity were obtained with antibodies against peptides alpha 67-82, alpha 78-93, alpha 100-115, and alpha 111-126. On the other hand, free AChR bound high levels of antibodies against peptides alpha 34-49, alpha 78-93, alpha 134-150, alpha 170-186, and alpha 194-210. It also bound moderate levels of antibodies against peptides alpha 262-276 and alpha 388-408. Low, yet significant, levels of binding were exhibited by antibodies against peptides alpha 45-60, alpha 111-126, and alpha 122-138. These binding studies, which enabled a comparison of the accessible regions in mbAChR and free AChR, revealed that the receptor undergoes considerable changes in conformation upon removal from the cell membrane. The exposed regions found here are discussed in relation to the functional sites of AChR (i.e., the acetylcholine binding site, the regions that are recognized by anti-AChR antibodies, T-cells and autoimmune responses and the regions that bind short and long neurotoxins).

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Profile of the regions on the alpha-chain of human acetylcholine receptor recognized by autoantibodies in myasthenia gravis

Eighteen synthetic overlapping peptides encompassing the entire extracellular part (residues alpha 1-210) of the alpha-chain of human acetylcholine receptor (AChR) and a 19th peptide (residues alpha 262-276) corresponding to an extracellular connection between two transmembrane regions were prepared and used for the measurement, by solid-phase radioimmunoassay, of the binding of autoantibodies in plasma from myasthenia gravis (MG) patients. Autoantibodies were found to recognize only a limited number of the synthetic peptides. The regions recognized resided predominantly within the areas alpha 10-30, alpha 111-145 and alpha 175-198 and, less frequently, region alpha 45-77. Differences in the recognition profile of the peptides from patient to patient indicated that the autoantibody responses were under genetic control. However, by using a mixture of the appropriate peptides, it was possible to determine autoantibodies in all 15 myasthenia sera and to distinguish between these, normal human sera and other neurological or autoimmune diseases. The mapping of the continuous antigenic regions recognized by autoantibodies on the alpha-chain of human AChR has permitted a comparison of the regions recognized by autoantibodies and autoimmune T-cells from the same donor. It also provided a peptide-based direct antibody binding method for diagnosis of MG.

Alpha-bungarotoxin binds to human acetylcholine receptor alpha-subunit peptide 185-199 in solution and solid phase but not to peptides 125-147 and 389-409

The nicotinic acetylcholine receptor (AChR) of human skeletal muscle has a reducible disulfide bond near the neurotransmitter binding site in each of its alpha-subunits. By testing a panel of overlapping synthetic peptides encompassing the alpha-subunit segment 177-208 (containing cysteines 192 and 193) we found that specific binding of 125I-labelled alpha-bungarotoxin (alpha-BTx) was maximal in the region 185-199. Binding was inhibited by unlabelled alpha-BTx greater than d-tubocurarine greater than atropine greater than carbamylcholine. Peptide 193-208 did not bind alpha-BTx, whereas 177-192 retained 40% binding activity. Peptides corresponding to regions 125-147 (containing cysteines 128 and 142) and 389-409, or peptides unrelated to sequences of the AChR failed to bind alpha-BTx. No peptide bound 125I-alpha-labelled parathyroid hormone. The apparent affinity (KD) of alpha-BTx binding to immobilized peptides 181-199 and 185-199 was approximately 25 microM and 80 microM, respectively, in comparison with alpha-BTx binding to native Torpedo ACh receptor (apparent KD approximately 0.5 nM). In solution phase, both peptides effectively competed with solubilized native human AChR for binding of alpha-BTx, and peptide 185-199 showed little evidence of dissociation after 24 h. Peptides that bound alpha-BTx did so when sulfhydryls were reduced. Cysteine modification, by N-ethylmaleimide or acetamidomethylation, abolished alpha-BTx-binding activity. The data implicate the region of cysteines 192 and 193 in the binding of neurotransmitter to the human receptor.