Molecular forms and hydrodynamic properties of acetylcholine receptor from electric tissue

Untangling Direct and Domain-Mediated Interactions Between Nicotinic Acetylcholine Receptors in DHA-Rich Membranes

At the neuromuscular junction (NMJ), the nicotinic acetylcholine receptor (nAChR) self-associates to give rise to rapid muscle movement. While lipid domains have maintained nAChR aggregates in vitro, their specific roles in nAChR clustering are currently unknown. In the present study, we carried out coarse-grained molecular dynamics simulations (CG-MD) of 1-4 nAChR molecules in two membrane environments: one mixture containing domain-forming, homoacidic lipids, and a second mixture consisting of heteroacidic lipids. Spontaneous dimerization of nAChRs was up to ten times more likely in domain-forming membranes; however, the effect was not significant in four-protein systems, suggesting that lipid domains are less critical to nAChR oligomerization when protein concentration is higher. With regard to lipid preferences, nAChRs consistently partitioned into liquid-disordered domains occupied by the omega-3 ([Formula: see text]-3) fatty acid, docosahexaenoic acid (DHA); enrichment of DHA boundary lipids increased with protein concentration, particularly in homoacidic membranes. This result suggests dimer formation blocks access of saturated chains and cholesterol, but not polyunsaturated chains, to boundary lipid sites.

The initiation of the muscle action potential

The specific functional properties of the nicotinic acetylcholine receptors (AChR) and the particular oligomeric membrane organization of AChR are suggested to be the basis for the steep electrical depolarisation, required for the initiation of the postsynaptic action potentials causing muscle contraction and discharge of electric organs. The dimer (M(r) approximately 580,000) and the monomer (M(r) approximately 290,000) of the detergent-solubilized, affinity-purified AChR of Torpedo californica electrocytes exhibit different channel conductances and larger oligochannels. Patch clamp data of the dimer, reconstituted in large lipid vesicles, show that the dimer is a double-channel protein causing single-channel events of conductance G(D) = 84 +/- 6 pS at 0.11 M K+ and 0.1 mM Ca2+ at 293 K (20 degrees C). At the same ionic conditions the vesicle-reconstituted monomer, if prevented from aggregation, exhibits a channel conductance, G(M) = 42 +/- 3 pS, which is only half of that of the dimer. The dimer conductivity event thus reflects the synchronous switching of its two constituent monomeric parts. The K(+)-conductance of the reconstituted Torpedo dimer is the same, and shows the same inhibitory Ca(2+)-dependence, as that of the Torpedo AChR expressed in Xenopus laevis oocytes (Imoto et al., Nature, 324, 670-674, 1986). In terms of Ca(2+)-binding, reducing K(+)-transport, the equilibrium constant is KCa = 0.48 mM at 0.11 m K+, 20 degrees C; G0([Ca]-->0) = 98 +/- 6 pS and G infinity ([Ca]-->infinity) = 27 +/- 6 pS. The ratio G0/G infinity and an estimate of the lateral surface area of the channel vestibule yields about 16 negatively charged groups in an average distance of 1.8 nm. These negative charges cause an accumulation of K+ ions in the channel vestibule by a factor of about 4. Our results and the comparison with the oocyte data reveal that it is also the dimer which is the physiological opening-closing unit of the AChR in the oocyte membrane. The larger macrochannel events are multiples of the dimer or of the monomer conductances. The occurrence of such oligochannels from AChR protein oligomers could guarantee the steep electrical depolarisation necessary to generate the action potential by the Na(+)-channel system.

Functional reconstitution of the nicotinic acetylcholine receptor by CHAPS dialysis depends on the concentrations of salt, lipid, and protein

The detergent CHAPS was found to be the preferable surfactant for the efficient purification and reconstitution of the Torpedo californica nicotinic acetylcholine receptor (AChR). The main result is that the incorporation of the AChR proteins into lipid vesicles by CHAPS dialysis was strongly dependent on the salt and protein concentrations. As monitored by sucrose gradients, by electron microscopy, and by agonist-induced lithium ion flux, the best reconstitution yields were obtained in 0.5 M NaCl at a protein concentration of 0.5 g/L and in 0.84 M NaCl at 0.15 g/L protein. Electron micrographs of receptor molecules, which were incorporated into vesicles, showed single, nonaggregated dimer (M(r) = 580,000) and monomer (M(r) = 290,000) species. CHAPS dialysis at NaCl concentrations less than 0.5 M largely reduced the receptor incorporation concomitant with protein aggregation. Electron micrographs of these preparations revealed large protein sheets or ribbons not incorporated into vesicles. The analysis of static and dynamic light scattering demonstrated that the detergent-solubilized AChR molecules aggregate at low lipid contents (less than or equal to 500 phospholipids/AChR dimer), independent of the salt concentration. AChR proteins eluted from an affinity column with a solution containing 8 mM CHAPS (but no added lipid) still contained 130 +/- 34 tightly bound phospholipids per dimer. The aggregates (about 10 dimers on the average) could be dissociated by readdition of lipid and, interestingly, also by increasing the CHAPS concentration up to 15 mM. This value is much higher than the CMC of CHAPS = 4.0 +/- 0.4 mM, which was determined by surface tension measurements. The data clearly suggest protein-micelle interactions in addition to the association of monomeric detergents with proteins. Furthermore, the concentration of the (free) monomeric CHAPS at the vesicle-micelle transformation in 0.5 M NaCl ([Dw]c = 3.65 mM) was higher than in 50 mM NaCl ([Dw]c = 2.8 mM). However, it is suggested that the main effect of high salt concentrations during the reconstitution process is an increase of the fusion (rate) of the ternary protein/lipid/CHAPS complexes with mixed micelles or with vesicular structures, similar to the salt-dependent fusion of vesicles.

T cells from normal and myasthenic individuals recognize the human acetylcholine receptor: heterogeneity of antigenic sites on the alpha-subunit

The alpha-subunit of the nicotinic acetylcholine receptor is the major target of the autoimmune response in myasthenia gravis. We investigated the proliferative response of T cells from patients with myasthenia gravis and healthy volunteers to recombinant polypeptides of the human acetylcholine receptor including the full-length alpha-subunit (alpha 1-437). T cells from 20 (71%) of 28 patients and 7 (37%) of 19 healthy volunteers responded in primary cultures. Subsequently, specific T-cell lines were established: CD4+, CD8-, UCHL1+, and major histocompatibility complex (MHC) class II-restricted. Using a set of fragments of the alpha-subunit, major antigenic sites could be localized on the extracellular, N-terminal part of the molecule as well as close to the C-terminus. The T-cell response was heterogeneous, both among different individuals and among T-cell lines from a single donor. These T cells did not cross-react with Torpedo acetylcholine receptor, which was previously used as a substitute for human muscle acetylcholine receptor, suggesting that the T cells had a bias for unique human sequences. A single antigenic fragment could be presented in the context of different MHC class II molecules, and different fragments could be presented in the context of the same MHC molecule. This supports earlier observations of considerable heterogeneity in dealing with acetylcholine receptor as an autoantigen on the level of both T cells and antigen-presenting cells. The data also demonstrate that acetylcholine receptor-specific T cells are present in the normal immune repertoire, and emphasize the role of immune regulation for maintaining a state of tolerance.

Acetylcholine receptor-specific T-lymphocyte clones in the normal human immune repertoire: target epitopes, HLA restriction, and membrane phenotypes

Potentially autoimmune T-lymphocyte lines specific for the nicotinic acetylcholine receptor of the neuromuscular junction have been isolated previously from patients with myasthenia gravis. We report on the isolation and expansion of T cells specific for the acetylcholine receptor of Torpedo californica or for a recombinant mammalian acetylcholine receptor alpha chain peptide (X4), from the peripheral blood of 11 healthy donors. Two major T-cell epitopes, located between amino acid positions 44-104 and 141-172, were identified using a panel of overlapping mammalian alpha chain fusion proteins. Most T lines recognized the acetylcholine receptor epitopes in the molecular context of HLA-DR molecules. Unexpectedly, all the T. californica acetylcholine receptor-specific T lines obtained from one DR4 (DRw53), DQw3 donor and two DR4, w8 (DRw53), DQw3 donors were restricted by DRw53 product(s). Using DR gene-transfected L cells as antigen presenters, in 4 lines, a close relationship between the recognized epitope and the restricting DR element was revealed. The membrane phenotype of the T. californica acetylcholine receptor-and X4-specific T lines was predominantly CD4+CD8-, with some CD4+CD8+ components. It did not significantly differ from that of control, tuberculin purified protein derivate-specific T lines raised from the same donors. These findings are in harmony with previous ones demonstrating the presence of potentially autoimmune T-lymphocyte clones within normal immune repertoires.

Autoimmune T lymphocytes in myasthenia gravis. Determination of target epitopes using T lines and recombinant products of the mouse nicotinic acetylcholine receptor gene

Oligoclonal and cloned T lines from peripheral blood or thymuses of patients with myasthenia gravis (MG) were selected for reactivity against nicotinic acetylcholine receptors (AChR) from Torpedo california, or against a recombinant fusion peptide, X4, representing the extracellular portion of the mouse AChR alpha-chain. All cell lines expressed the CD4 membrane phenotype, and their antigen reactivity was blocked by antibodies against monomorphic HLA DR/DP determinants. Using a panel of fusion proteins of different, overlapping mouse AChR alpha-chain sequences, a major T cell epitope was localized between amino acid positions 85 and 142. This determinant was distinct from the humoral main immunogenic region, which has been identified on the sequence 61-76. The response pattern of uncloned T lines from three patients with different HLA haplotypes suggests, however, that in any one MG patient T lymphocytes may recognize more than one autoantigenic epitope on the AChR alpha-chain, and that the T lymphocyte response profiles vary among individual patients.

T cell epitopes in experimental autoimmune myasthenia gravis of the rat: strain-specific epitopes and cross-reaction between two distinct segments of the alpha chain of the nicotinic acetylcholine receptor (Torpedo californica)

T cell epitopes on the nicotinic acetylcholine receptor (A ChR) of Torpedo californica were analyzed using T cell lines isolated from Lewis, BN, and (Lewis X BN)F1 rats. All lines selected for reactivity against either native or denatured AChR or for 6 selected synthetic peptides of the AChR alpha chain expressed the CD4 membrane phenotype and recognized their antigen in the context of major histocompatibility complex class II determinants. They were tested in vitro for reactivity with each of these antigens. The results indicate that parental Lewis and BN rat T lymphocytes recognize distinct molecular epitopes on the AChR protein, whereas (Lewis X BN)F1 hybrids respond against both sets of epitopes. Two peptides (P10 and P11) which represent distinct amino acid sequences on the putatively extracellular part of the AChR alpha chain, and which share only 4 common amino acids, two of them contiguous, showed an unexpected cross-reactivity in the Lewis rat. T cells selected for either peptide co-recognize the other peptide in vitro. In addition, these cells are responsive against full length AChR. P11, in particular, appears to be a major epitope for Lewis rats immunized with AChR.

Thymus in myasthenia gravis. Isolation of T-lymphocyte lines specific for the nicotinic acetylcholine receptor from thymuses of myasthenic patients

The thymus is believed to play a central role in the pathogenesis of Myasthenia gravis (MG). According to a previous hypothesis, MG is initiated within the thymus by immunogenic presentation of locally produced nicotinic acetylcholine receptor (AChR) to potentially autoimmune T cells. Data of 10 consecutive MG patients demonstrate two critical features of MG thymuses that support the concept of intrathymic activation of autoreactive, AChR-specific lymphocytes. Morphologically, the thymuses showed lympho-follicular hyperplasia in nine cases and benign thymoma in one case. The paramount feature revealed by immunohistological double marker analyses was the intimate association of myoid cells (antigen producing) with interdigitating reticulum cells (potentially antigen presenting cells), both of which were surrounded by T3+ lymphocytes in thymus medulla. All 10 thymuses contained T lymphocytes reactive with AChR. This was in contrast to the peripheral immune compartment (blood) where in only 3 of 10 patients, significant T cell responses to AChR were observed. AChR-specific T cell lines could be established from 8 of 10 thymuses, all members of the helper/inducer subset as indicated by the expression of markers T3 and T4.

B-T lymphocyte interactions in experimental autoimmune myasthenia gravis: antigen presentation by rat/mouse hybridoma lines secreting monoclonal antibodies against the nicotinic acetylcholine receptor

Many, but not all rat/mouse B hybridoma cells, producing monoclonal antibodies against determinants on the nicotinic acetylcholine receptor (AChR) of the electric organ of Torpedo californica, were able to immunologically present antigen to AChR-specific, Ia-compatible CD4+ T lymphocyte lines. Most of the hybridomas presented AChR in a privileged manner, i.e. they present AChR even more effectively than macrophages or dendritic cells. However, in the presentation of antigens other than AChR, they were inferior to macrophages. Moreover, some hybridomas were able to present AChR not only in soluble state, but also in membrane vesicles. Privileged presentation of AChR by hybridomas depended on the reactivity of the secreted immunoglobulins with epitopes of the AChR alpha chain, and on the expression of major histocompatibility complex class II antigens on the hybridoma cell surface. There was, however, no quantitative correlation between the actual AChR presentation capacity of one clone and the density of its surface Ia. Neither fine specificity nor isotype of hybridoma immunoglobulin are critical in determining privileged AChR presentation. We postulate that different hybridomas vary in their ability to take up soluble and particulate antigen, to process and to re-express them on the cellular membrane. This capacity may determine their efficiency to present antigen to T cells.

Reconstitution of membrane receptor systems

This review makes an attempt to summarize the present status of the field of receptor reconstitution. First a general discussion on the problem of receptor to effector coupling is discussed with an emphasis on the approaches used to solubilize, purify and reconstitute receptors with their respective biochemical effectors. Two categories of receptors have thus far been studied in great detail: (1) receptors linked to ion channels best represented by the nicotinic acetylcholine receptor and (2) receptors linked to adenylate cyclase. Through a detailed discussion of these two receptor systems the reader should get an idea of where the field of receptor reconstitution is headed. Only in the beta-adrenergic-receptor-dependent adenylate cyclase have the receptor and the effector systems been completely separated, purified and reconstituted. Therefore, a detailed discussion on that system occupies a very significant portion of this article. A summary of the state-of-the-art on a number of other receptor systems is also given in the last part of the review.

Antibodies against preselected peptides to map functional sites on the acetylcholine receptor

Rabbit immune sera and mouse monoclonal antibodies were raised against the synthetic peptide Tyr-Cys-Glu-Ile-Ile-Val matching in sequence residues 127-132 of the alpha-subunit of all nicotinic acetylcholine receptors sequenced so far. Representative cholinergic ligands did not interfere with the binding of these antibodies to the receptor from Torpedo marmorata, indicating that this sequence is not part of the binding sites for cholinergic ligands. The applicability of antigenic sites analysis to the mapping of functional sites on receptor proteins is discussed.

Reconstitution of pure acetylcholine receptor in phospholipid vesicles and comparison with receptor-rich membranes by the use of a potentiometric dye

Acetylcholine receptor, isolated in Triton X-100 on a cobra alpha-neurotoxin affinity column was incorporated into unilamellar phospholipid vesicles by a detergent depletion method using Amberlite XAD-2. Vesicles of an average diameter of 25 nm were formed, as verified by freeze-fracture electron microscopy and gel filtration. 85 to 95% of the alpha-bungarotoxin binding sites of the reconstituted acetylcholine receptor were oriented towards the outside of the vesicles. In the reconstituted receptor one molecule of residual Triton X-100 per 2.5 alpha-bungarotoxin binding sites on the receptor molecule could be assessed. The reconstituted protein was not accessible to papain digestion, whereas the pure acetylcholine receptor, solubilized by Triton X-100 was split into smaller polypeptides under the same condition. Reconstituted acetylcholine receptor and receptor-rich membranes did not exhibit the same behavior as measured by use of a potentiometric dye. This is interpreted as an irreversible alteration of at least 95% of the receptors purified in the presence of Triton X-100. Furthermore, it could be shown that the fluorescence intensity changes induced by carbamylcholine in receptor-rich membranes did not reflect ion fluxes, but conformational changes of the protein or a displacement of the dye from the protein.

Organization of ligand binding sites at the acetylcholine receptor: a study with monoclonal antibodies

We have studied 20 monoclonal antibodies directed against both the solubilized and the membrane-bound receptor from Torpedo marmorata. We find the following: (i) Six of the antibodies compete with cholinergic ligands for receptor binding and, hence, are directed against the ligand binding regions. (ii) Of these six antibodies, two cross-react with receptor from Electrophorus electricus, rat myotubes, and chicken sympathetic ganglia. These two antibodies therefore define a preserved structure within the ligand binding regions. The other four antibodies bind to structures not common between the receptor preparations tested. (iii) From competition binding studies using internally 3H-labeled antibodies, nine nonoverlapping antigenic regions were defined at the surface of the receptor. Three of these regions overlap with the ligand binding regions. Since two of these three regions do not overlap with each other, two structurally distinct ligand binding regions must exist at the receptor. (iv) From competition binding studies with representative cholinergic ligands, the antibodies directed against the ligand binding regions can be subdivided into three groups: one group competes with all ligands tested; the second group competes with all ligands except the bismethonium compounds; the third group competes with all ligands except the bismethonium compounds and tubocurarine. The results are summarized in a model of the organization of ligand binding sites at the receptor: There are two ligand binding regions differing in their antigenic properties. Furthermore, either there exists separate sites for distinct groups of ligands within each of these binding regions or some ligands produce conformational changes of the receptor that reversibly abolish some antigenic sites. In any case, the cholinergic ligands must interact with the receptor by more and/or other structural determinants than are provided by the structure of acetylcholine.

On the interpretation of equilibrium binding studies

Equilibrium binding curves are discussed on the basis of their fundamental equations. The shape of these curves is not influenced by the number of conformational states of the receptor-ligand complexes. In contrast, the number of binding sites determines the shape of the equilibrium binding curve. For example, a curved Scatchard plot establishes the existence of more than one binding site at the receptor but cannot yield information on the number of conformational states. If there are discrepancies between equilibrium binding and equilibrium dose-response curves they indicate that the response is not initiated by the overall binding process. They may be explained assuming that only one particular receptor-ligand complex (e.g. the fully saturated one) will initiate the physiological response.

Nylon tube linked acetylcholine receptor: a tool for affinity isolations and immunosorption

We have covalently coupled acetylcholine receptor and other proteins to the inner surface of nylon tubes and employ these affinity tubes in binding assays and for chromatographic purposes. Here we describe two applications: (i) The concentration determination of toxins and antibodies directed against the acetylcholine receptor, and (ii) the isolation and chromatography of specific immunoglobulins.

Equilibrium binding of acetylcholine to the membrane-bound acetylcholine receptor

We have studied the binding of acetylcholine to membrane-bound acetylcholine receptor from Torpedo marmorata employing a highly accurate airfuge assay procedure. At equilibrium the receptor displays two classes of acetylcholine binding sites; these interact with only weak positive cooperativity. As a further difference to binding data deduced from electrophysiological dose/response curves, the equilibrium constants for the two classes of sites (Kd1 = 25 nM, Kd2 = 8 nM) are orders of magnitude lower than the concentration required for half-maximal response. Both the weaker-than-expected cooperativity of sites and the high binding affinities are likely to be due to desensitisation of the receptor during the period of incubation. The positively cooperative interaction of acetylcholine binding sites is only observed with membrane preparations obtained in the presence of appropriate chelating, sulfhydryl-blocking and active-serine-blocking agents. Aged membrane preparations loose the ability of site interactions while only small changes in the total number of binding sites are observed. In the absence of divalent ions, the affinity of binding of acetylcholine to the receptor is reduced. To assess the significance of the binding data obtained, several alternative reaction schemes for non-random binding to two sites at the receptor are considered. In addition, the effects of possible sources of experimental error on the shape of Scatchard plots are analysed.