Muscle endplate cholinoreceptors

Mapping the Nicotinic Acetylcholine Receptor Nanocluster Topography at the Cell Membrane with STED and STORM Nanoscopies

The cell-surface topography and density of nicotinic acetylcholine receptors (nAChRs) play a key functional role in the synapse. Here we employ in parallel two labeling and two super-resolution microscopy strategies to characterize the distribution of this receptor at the plasma membrane of the mammalian clonal cell line CHO-K1/A5. Cells were interrogated with two targeted techniques (confocal microscopy and stimulated emission depletion (STED) nanoscopy) and single-molecule nanoscopy (stochastic optical reconstruction microscopy, STORM) using the same fluorophore, Alexa Fluor 647, tagged onto either α-bungarotoxin (BTX) or the monoclonal antibody mAb35. Analysis of the topography of nanometer-sized aggregates (“nanoclusters”) was carried out using STORMGraph, a quantitative clustering analysis for single-molecule localization microscopy based on graph theory and community detection, and ASTRICS, an inter-cluster similarity algorithm based on computational geometry. Antibody-induced crosslinking of receptors resulted in nanoclusters with a larger number of receptor molecules and higher densities than those observed in BTX-labeled samples. STORM and STED provided complementary information, STED rendering a direct map of the mesoscale nAChR distribution at distances ~10-times larger than the nanocluster centroid distances measured in STORM samples. By applying photon threshold filtering analysis, we show that it is also possible to detect the mesoscale organization in STORM images.

Autoimmune Attack of the Neuromuscular Junction in Myasthenia Gravis: Nicotinic Acetylcholine Receptors and Other Targets

The nicotinic acetylcholine receptor (nAChR) family, the archetype member of the pentameric ligand-gated ion channels, is ubiquitously distributed in the central and peripheral nervous systems, and its members are the targets for both genetic and acquired forms of neurological disorders. In the central nervous system, nAChRs contribute to the pathological mechanisms of neurodegenerative disorders, such as Alzheimer and Parkinson diseases. In the peripheral nerve-muscle synapse, the vertebrate neuromuscular junction, "classical" myasthenia gravis (MG) and other forms of neuromuscular transmission disorders are antibody-mediated autoimmune diseases. In MG, antibodies to the nAChR bind to the postsynaptic receptors and activate the classical complement pathway culminating in the formation of the membrane attack complex, with the subsequent destruction of the postsynaptic apparatus. Divalent nAChR-antibodies also cause internalization and loss of the nAChRs. Loss of receptors by either mechanism results in the muscle weakness and fatigability that typify the clinical manifestations of the disease. Other targets for antibodies, in a minority of patients, include muscle specific kinase (MuSK) and low-density lipoprotein related protein 4 (LRP4). This brief Review analyzes the current status of muscle-type nAChR in relation to the pathogenesis of autoimmune diseases affecting the peripheral cholinergic synapse.

Localization of histidine residues relevant for the binding of alpha-bungarotoxin to the acetylcholine receptor alpha-subunit in V8-proteolytic fragments

Histidine residues have been shown to be critical for alpha-BgTx binding to the acetylcholine receptor (Lacorazza et al., 1992; Bouzat et al., 1993; Lacorazza et al., 1995). Receptor subunits from Discopyge tschudii were modified with diethylpyrocarbonate (DEP). DEP treatment produces a concentration-dependent decrease of [125I] alpha-BgTx binding to the alpha-subunit. The neurotoxin binding capacity was fully restored by adding the nucleophile hydroxylamine. By proteolytic mapping of the alpha-subunit with V8-protease, we determined that the binding capacity to the fragment alpha V8-19 decreased 80% by DEP treatment. In addition, the [125I] alpha-BgTx binding to the same fragment decreased by 70% when the subunits were reduced and affinity-alkylated. We report the N-terminal sequence of both subunits and V8-fragments (alpha V8-10, alpha V8-13, and alpha V8-18), which constitute a first contribution to the knowledge of the primary structure of the Discopyge tschudii receptor. We propose that the fragment alpha V8-19 contains one or more of the histidine residues involved in the alpha-BgTx binding and probably includes the Cys alpha 192-193 disulfide bond. Only two histidine residues are present in the extracellular sequence of Torpedo californica for such fragments: His alpha 186 and alpha 204.

Structural and functional crosstalk between acetylcholine receptor and its membrane environment

Nicotinic acetylcholine receptor (AChR) is a transmembrane protein belonging to the superfamily of rapid, ligand-operated channels. Theoretical models based on thermodynamic criteria assign portions of the polypeptide chains to the lipid bilayer region. From an experimental point of view, however, the relationship between the two moieties remains largely unexplored. Current studies from our laboratory are aimed at defining the structural, dynamic, and functional relationship between membrane lipids and AChR. We are particularly interested in establishing the characteristics of and differences between the lipids in each leaflet of the bilayer and the belt or "annular" lipids immediately surrounding AChR and the bulk bilayer lipids. We are also interested in determining the possible implications of lipid modifications on AChR channel properties. Toward these ends, fluorescence and other spectroscopic techniques, together with biochemical analyses and patch-clamp studies, are currently being undertaken. Correlations can be established between structural aspects of phospholipid packing in the immediate perimeter of AChR and other properties of these annular lipids revealed by dynamic spectroscopic and molecular modeling techniques. Lipid compositional analyses of the clonal muscle cell line BC3H-1 and chemical modification studies have been carried out by incubation of intact cells in culture and of membrane patches excised therefrom with liposomes of different lipid composition. These studies have been combined with electrophysiological measurements using the patch-clamp technique, with the aim of determining the possible effects of lipids on the channel properties of muscle-type AChR. A variety of experimental conditions, involving polar head and fatty acyl chain substitution of phospholipids and cholesterol incorporation, are being assayed in the BC3H-1 cells.

Effects of organophosphorus anticholinesterases on nicotinic receptor ion channels at adult mouse muscle endplates

1. The effects of a range of organophosphorus anticholinesterases on the nicotinic acetylcholine receptor ion channel at the adult mouse muscle endplate were investigated by use of single-channel recording techniques. Diisopropylfluorophosphate (DFP), sarin and soman had no effect on open times at concentrations of up to 100 microM, but ecothiopate (Eco) and O-ethyl S-[2-(diisopropylamino)ethyl]methyl phosphonothiolate (VX) were found to have voltage- and concentration-dependent open channel-blocking actions at concentrations of 1-50 microM. In addition to its channel-blocking action, Eco (50 microM) had a weak agonist effect: it is suggested that this may be attributable to thiocholine produced by hydrolysis of Eco. 2. Rate constants for blockade by Eco and VX were determined according to a sequential model. The greater voltage-dependence of the block by Eco was due to a greater voltage sensitivity of the blocking rate constant compared to VX: the voltage-dependence of the unblocking rate constant was similar for both compounds. 3. In control recordings, the frequency of channel opening declined exponentially with time after formation of the gigaseal. Sarin and soman both increased the rate of this decline, indicating that they accelerated the rate of desensitization of the receptors. Eco and VX reduced the initial frequency of opening, which may have been due to enhancement of a fast phase of desensitization during gigaseal formation, or to blockade of closed channels. 4. It is concluded that the direct actions of organophosphates on nicotinic receptor ion channels are of little importance for their toxicity under normal conditions, since they occur only at much higher concentrations than those which cause inhibition of acetylcholinesterase. Such actions may become apparent, however, when therapies against the anticholinesterase effects of organophosphates increase their lethal dose sufficiently. These direct actions should also be taken into account when the effects of organophosphates on cholinergic transmission are studied.

Asymmetric distribution of phospholipids in acetylcholine receptor-rich membranes from T. marmorata electric organ

1. The distribution of phospholipids between the two leaflets of the lipid bilayer in acetylcholine receptor (AChR)-rich membranes from T. marmorata has been examined with two complementary techniques: chemical derivatization with the membrane-impermeable reagent trinitrobenzenesulphonate (TNBS) and B.cereus phospholipase C hydrolysis. 2. AChR-membranes were reacted with TNBS at 0-4 and 37 degrees C and the accessibility of their aminophospholipids was compared to that of rod outer segment and erythrocyte membranes. The results indicate that more of the total ethanolamine glycerophospholipid (EGP) than of the total phosphatidylserine (PS) is located in the outer monolayer. 3. Nearly half the phospholipid content of AChR membranes is hydrolyzed by phospholipase C with a half-time of ca. 1.6 min at 25 degrees C. Consistent with the TNBS results, more of the total EGP than of the total PS is degraded. Beyond 3 min the reaction slows down, relatively smaller additional amounts of lipids are hydrolyzed, and all phospholipid classes are attacked to a similar extent, indicating that after half the lipid is removed all phospholipids become accessible to the enzyme. 4. The results indicate that the outer leaflet of the bilayer is richer in ethanolamine and choline glycerophospholipids, whereas phosphatidylinositol, most of the sphingomyelin, and ca 65% of the PS are located on the inner leaflet.