The Precision Toxicology Initiative

The goal of PrecisionTox is to overcome conceptual barriers to replacing traditional mammalian chemical safety testing by accelerating the discovery of evolutionarily conserved toxicity pathways that are shared by descent among humans and more distantly related animals. An international consortium is systematically testing the toxicological effects of a diverse set of chemicals on a suite of five model species comprising fruit flies, nematodes, water fleas, and embryos of clawed frogs and zebrafish along with human cell lines. Multiple forms of omics and comparative toxicology data are integrated to map the evolutionary origins of biomolecular interactions, which are predictive of adverse health effects, to major branches of the animal phylogeny. These conserved elements of adverse outcome pathways (AOPs) and their biomarkers are expect to provide mechanistic insight useful for regulating groups of chemicals based on their shared modes of action. PrecisionTox also aims to quantify risk variation within populations by recognizing susceptibility as a heritable trait that varies with genetic diversity. This initiative incorporates legal experts and collaborates with risk managers to address specific needs within European chemicals legislation, including the uptake of new approach methodologies (NAMs) for setting precise regulatory limits on toxic chemicals.

Incorporation of acetylcholine receptors into liposomes. Vesicle structure and acetylcholine receptor function

Functionally intact acetylcholine receptors can be solubilized from electric organ membranes of Torpedo californica and incorporated into liposomes by the cholate dialysis technique. Freezing and thawing of the reconstituted preparation appears to seal a population of initially leaky vesicles and leads to vesicle fusion. Inclusion of supplementary cholesterol at an optimal concentration of 20% (w/w) greatly enhances vesicle fusion during the freeze-thaw cycle. Size analysis by electron microscopy of negatively stained preparations indicates that fusion is accompanied by shifts in size and volume distributions of the vesicle population. Liposomes formed in the absence of acetylcholine receptors are distributed over a substantially smaller size range than liposomes containing receptors. Acetylcholine receptors appear in those liposomes as dimers of 80 A doughnut-shaped particles. Freeze-fracture replicas of reconstituted preparations reveal the presence of large vesicles containing particles which correspond in size to acetylcholine receptors and smaller liposomes devoid of particles. The distribution of particles in the reconstituted membranes is sparse compared to their dense packing in native electric organ membranes. The activation and desensitization of reconstituted acetylcholine receptors mediated by acetylcholine or carbamylcholine is dose dependent. The reconstituted receptors distinguish between these agonists in terms of binding affinity in a way similar to receptors in the native membrane. Correlation of the fractional occupancy of ligand binding sites by cobratoxin with inhibition of receptor function is used to demonstrate that in the reconstituted system the doubly liganded acetylcholine receptor prevails in controlling channel gating. The potential experimental advantages as well as limitations of this reconstituted system are discussed.

Stabilization of acetylcholine receptor channels by lipids in cholate solution and during reconstitution in vesicles

Acetylcholine receptors were solubilized from electric organ membranes of Torpedo californica in mixed micelles of sodium cholate and soybean lipids. Sodium cholate, when supplemented with relatively low amounts of soybean lipids (cholate:lipid, 20:1, molar ratio), was effective in solubilizing receptors without denaturing their agonist-regulated cation channels. Another dialyzable detergent, octylglucoside, denatured the ion channel even in the presence of excess lipids. Reassembly of receptors and lipids into vesicles was achieved by cholate dialysis. About 70% of the receptors were oriented with their toxin binding sites on the external surface of the vesicles. Evidence suggests that all of the receptors in a single vesicle were oriented either right side out or inside out. During the reassembly process about 10-fold greater lipid concentrations were required for the preservation of channel function. At lipid/protein ratios greater than 16:1 (w/w), receptors reassembled into vesicles at a constant protein/lipid ratio. These vesicles contained approximately 7% receptors by weight, 5-fold less than the native membrane. The remainder of the lipid assembled into small vesicles which did not contain receptors. At lipid/protein ratios less than 16:1 (w/w), receptors reassociated with lipids in higher weight ratios. Irreversible inactivation of a fraction of the acetylcholine receptor channels occurred in proportion to the greater packing density. This channel denaturation was accompanied by a lowered susceptibility of the disulfide bond between the delta subunits of the acetylcholine receptor dimer to reducing agents. Toxin binding and the orientation of the receptors in the reconstituted vesicles was not affected by reduced lipid/protein ratios. The unexpected constant acetylcholine receptor/lipid ratio in the reconstituted vesicles and the unexpected uniform orientation of the acetylcholine receptors within a vesicle are discussed in terms of the interactions occurring during the initial nucleation events of the reassembly process.

Purification of acetylcholine receptors, reconstitution into lipid vesicles, and study of agonist-induced cation channel regulation

We report the purification of acetylcholine receptors with active agonist-regulated cation channels from Torpedo californica electric organ tissue by five methods. In one method, previously used by others, contaminating proteins were removed from partially purified membranes by alkaline extraction, preserving membrane integrity throughout the procedure. In the other four methods, acetylcholine receptors were purified after solubalization with sodium cholate. The continual presence of soybean lipid in mixed micelles with cholate was required to prevent irreversible inactivation of the cation channel. Solubilized receptors were purified by affinity chromatography using either Naga naja siamensis toxin III or concanavalin A coupled to agarose. Sucrose gradient centrifugation was also used to purify solubilized receptors. The best method combined affinity chromatography on toxin-agarose and concanavalin A agarose. Receptors purified by all five methods were incorporated into soybean lipid vesicles by the cholate dialysis technique. The agonist-regulated cation channels of the receptors were equally active after reconstitution, independent of the method used for purification. All reconstituted vesicle preparations were similar in preferential orientation of acetylcholine receptor toward the external surface, dose-response to carbamylcholine, carbamylcholine-induced desensitization, and carbamycholine-induced influx of 22Na+ per mol of receptor. Carbamylcholine-induced 22Na+ influx/receptor was greater after reconstitution than in native vesicles. This was because, in native vesicles, carbamylcholine-induced 22Na+ influx was limited by equilibration of the internal volume of the vesicles with the external 22Na+ concentration, whereas in reconstituted vesicles 22Na+ influx was limited by desensitization of the receptor molecule. We demonstrate that only one of the two toxin binding sites on the receptor monomer, the one which can be affinity alkylated with 4-(N-maleimido)benzyltrimethylammonium, controls the carbamylcholine-induced opening of the cation channel.

Functional equivalence of monomeric and dimeric forms of purified acetylcholine receptors from Torpedo californica in reconstituted lipid vesicles

Acetylcholine receptors from Torpedo californica electric organ were solubilized and purified under conditions which prevent inactivation of the agonist-regulated cation channels. The dimer form of the receptors was preserved during purification. Treatment with reducing agents converted dimers into monomers. Receptor monomers and dimers were separately reconstituted into soybean lipid vesicles by the cholate dialysis technique. Reconstituted monomers and dimers were functionally equivalent with respect to their carbamylcholine-induced dose-dependent uptake of 22Na+, the total flux of 22Na+ per receptor during the permeability response, and the occurrence of desensitization. Evidence against non-covalent association of monomers to produce dimeric functional units was obtained using glutaraldehyde as a crosslinking agent. These results show that both the acetylcholine-binding sites and the agonist-regulated cation-specific channel are contained within the alpha 2 beta gamma delta subunit structure of the acetylcholine receptor monomer.

Reconstitution of purified acetylcholine receptors with functional ion channels in planar lipid bilayers

Acetylcholine receptor, solubilized and purified from Torpedo californica electric organ under conditions that preserve the activity of its ion channel, was reconstituted into vesicles of soybean lipid by the cholate-dialysis technique. The reconstituted vesicles were then spread into monolayers at an air-water interface and planar bilayers were subsequently formed by apposition of two monolayers. Addition of carbamoylcholine caused an increase in membrane conductance that was transient and relaxed spontaneously to the base level (i.e., became desensitized). The response to carbamoylcholine was dose dependent and competitively inhibited by curare. Fluctuations of membrane conductance corresponding to the opening and closing of receptor channels were observed. Fluctuation analysis indicated a single-channel conductance of 16 +/- 3 pS (in 0.1 M NaCl) with a mean channel open time estimated to be 35 +/- 5 ms. Thus, purified acetylcholine receptor reconstituted into lipid bilayers exhibited the pharmacological specificity, activation, and desensitization properties expected of this receptor in native membranes.