The incorporation of solubilized choline-transport activity into liposomes

Choline on the Move: Perspectives on the Molecular Physiology and Pharmacology of the Presynaptic Choline Transporter

Genetic, biochemical, physiological, and pharmacological approaches have advanced our understanding of cholinergic biology for over 100 years. High-affinity choline uptake (HACU) was one of the last features of cholinergic signaling to be defined at a molecular level, achieved through the cloning of the choline transporter (CHT, SLC5A7). In retrospect, the molecular era of CHT studies initiated with the identification of hemicholinium-3 (HC-3), a potent, competitive CHT antagonist, though it would take another 30 years before HC-3, in radiolabeled form, was used by Joseph Coyle's laboratory to identify and monitor the dynamics of CHT proteins. Though HC-3 studies provided important insights into CHT distribution and regulation, another 15 years would pass before the structure of CHT genes and proteins were identified, a full decade after the cloning of most other neurotransmitter-associated transporters. The availability of CHT gene and protein probes propelled the development of cell and animal models as well as efforts to gain insights into how human CHT gene variation affects the risk for brain and neuromuscular disorders. Most recently, our group has pursued a broadening of CHT pharmacology, elucidating novel chemical structures that may serve to advance cholinergic diagnostics and medication development. Here we provide a short review of the transformation that has occurred in HACU research and how such advances may promote the development of novel therapeutics.

Molecular properties of the high-affinity choline transporter CHT1

This article summarizes molecular properties of the high-affinity choline transporter (CHT1) with reference to the historical background focusing studies performed in laboratories of the author. CHT1 is present on the presynaptic terminal of cholinergic neurons, and takes up choline which is the precursor of acetylcholine. The Na(+)-dependent uptake of choline by CHT1 is the rate-limiting step for synthesis of acetylcholine. CHT1 is the integral membrane protein with 13 transmembrane segments, belongs to the Na(+)/glucose co-transporter family (SLC5), and has 20-25% homology with members of this family. A single nucleotide polymorphism (SNP) for human CHT1 has been identified, which has a replacement from isoleucine to valine in the third transmembrane segment and shows the choline uptake activity of 50-60% as much as that of wild-type CHT1. The proportion of this SNP is high among Asians. Possible importance of choline diet for those with this SNP was discussed.

Choline transport in Pseudomonas aeruginosa

Choline used as the sole carbon or carbon and nitrogen source induces in Pseudomonas aeruginosa an active transport system. The induction of the choline uptake is repressed by succinate independently of the presence of ammonium ion in the culture medium. The repression mediated by succinate was insensitive to cyclic AMP. Substitution for dibutyryl-cyclic AMP was without effect. Choline metabolites that also support the growth of Pseudomonas aeruginosa were poor inducer agents of the choline transport. Kinetic evidence and the employment of choline metabolites as effectors indicated that the choline uptake system of this bacterium is formed by at least two components: one of high affinity (Km = 3 microM) and another of low affinity (Km = 400 microM). Contrary to what occurs in the synaptosome system, the high affinity form for the choline uptake was not dependent on Na+ ions and is not inhibited by hemicholinium-3. Since Pseudomonas aeruginosa can utilize choline as the sole carbon and nitrogen source, the induction of the choline transport with two components in this bacterium may be related to its own strategy to survive and grow in an adverse environment.

Structure-activity studies on inhibition of choline uptake by a mouse brain synaptosomal preparation: basic data

Over 80 substances were studied for their inhibition of high-affinity uptake of [3H]choline into a mouse brain synaptosomal fraction. Kinetic experiments tested a number of them for competitive behavior. A minimal provisional model for the choline uptake process is envisioned that is consistent with current data and with relevant observations in the literature. There are two hydrophilic anionic sites on the choline transporter that are separated from each other by a cationic hydrophobic domain. Association of choline in a Na+-dependent manner with one or both of the sites is necessary for the transport of choline to take place. The choline binding anionic sites are sufficiently large and/or flexible to allow attachment of cationic moieties larger than choline. The cationic hydrophobic domain of the transporter is flexible, probably tending to planarity. The length of the hydrophobic region between the anionic sites is approximately that of 10 extended methylene groups, and the minimal width is approximated by the distance across the condensed ring system of chlorpromazine. The probability of attachment of the highly hydrophilic choline to its binding sites is increased both by hydrogen-bonding to a proton-acceptor within the anionic sites and by repulsion from the cationic hydrophobic region. A number of substances that potently and competitively inhibit high affinity choline uptake possess quaternary ammonium groups and neutral or negatively charged lipophilic groups. In general, substances in which two quaternary ammonium groups are separated by an appropriately configured hydrophobic group and which can combine with both anionic sites and the hydrophobic region between them are more potent inhibitors than monoquaternary substance with the same or similar groups. However, substances with a single high-affinity quaternary group and an appropriately structured hydrophobic group, e.g. the trimethoxy-3-butynyl quaternary ammonium compounds, possess inhibitory efficacies similar to those shown by the most potent bisquaternaries. The above suggests that further delineation of the characteristics of the structures of the above sites of the transporter could lead to devisal of more potent reversible inhibitors of choline uptake than now are available.

Is the acetylcholine releasing protein mediatophore present in rat brain?

Mediatophore is a protein purified from the nerve terminal membranes of Torpedo electric organ. It confers to artificial membranes a calcium-dependent mechanism that translocates acetylcholine. When similar reconstitution experiments are applied to rat brain synaptosomal membranes they reveal the presence of mediatophore activity with properties close to those described for the Torpedo protein (extractability, sensitivity to calcium, and effect of the drug cetiedil). The activity was more abundant in synaptosomal membranes than in mitochondrial or myelinic membranes and in cholinergic areas as compared to cerebellum.

Solubilization and characterization of a [3H]hemicholinium-3 binding site in rat brain

A sodium-dependent high-affinity [3H]-hemicholinium-3 ([3H]HCh-3) binding site was solubilized from rat striatal synaptic plasma membranes by 0.2% deoxycholate. Deoxycholate solubilization of the [3H]HCh-3 binding site was dependent upon both detergent concentration and ionic strength of the solubilization medium. Specific [3H]HCh-3 binding to the solubilized preparation was both sodium- and chloride-dependent and saturable, exhibiting an affinity of 14.2 nM and a capacity (Bmax) of 695 fmol/mg protein. Choline and other analogs inhibited specific [3H]HCh-3 binding to the solubilized preparation in a concentration-dependent manner with the similar rank order of potency observed in crude synaptic membranes. Treatments known to disrupt both protein and lipid moieties resulted in diminished specific [3H]HCh-3 binding. These results suggest that the characteristics of the solubilized [3H]HCh-3 binding site are similar to those of the membrane-bound site.

Calcium-induced desensitization of acetylcholine release from synaptosomes or proteoliposomes equipped with mediatophore, a presynaptic membrane protein

A "fatigue" of acetylcholine (ACh) release is described in cholinergic synaptosomes stimulated with the calcium ionophore A23187 or gramicidin. A small conditioning calcium entry, which did not trigger a large ACh release, led to a decrease of transmitter release elicited by a second large calcium influx. This fatigue was half-maximal at approximately 30 microM external calcium and developed in a few minutes. In contrast, activation of release by calcium was very rapid and was half-maximal at approximately 0.5 mM external calcium. Activation and desensitization of release could be attributed to the recently identified presynaptic membrane protein, the "mediatophore." Proteoliposomes equipped with purified mediatophore showed a calcium-dependent activation and "fatigue" of ACh release similar to that of synaptosomes. It was found that the ionophore A23187 rapidly equilibrated internal and external calcium concentrations in proteoliposomes. Thus, the external calcium concentration gave the internal concentration required for activation or desensitization of proteoliposomal ACh release. The mediatophore showed remarkable calcium binding properties (20 sites/molecule) with a KD of 25 microM. The physiological implications of desensitization on the organization of release sites are discussed.

Characteristics of chloride-dependent incorporation of glutamate into brain membranes argue against a receptor binding site

Although membrane sites from brain, labelled with [3H]glutamate (Glu) under sodium-free conditions, are thought to represent excitatory receptors, certain anomalous characteristics of the kinetics of apparent binding raised the question of whether transport might contribute to this process, prompting a closer examination of it. Hyperosmolar media and low incubation temperatures (4 degrees C) both led to decreases in the apparent specific binding of [3H]glutamate to membranes from the brain of the rat in the presence of chloride. Furthermore, only 15% of the [3H]glutamate, bound at 37 degrees C, was dissociable when the membranes were then cooled to 4 degrees C. The binding of [3H]glutamate was increased in the presence of certain dipeptides such as L-phenylalanyl-L-glutamate (Phe-Glu); and the binding augmented by the presence of Phe-Glu, was also sensitive to temperature and osmolarity of the incubation buffer. Sonication of membranes in 5 mM glutamate increased the apparent binding of [3H]glutamate and abolished the stimulatory effect of Phe-Glu. These findings are consistent with the hypothesis that chloride-dependent association of [3H]glutamate with membranes from brain reflects, in part, a sequestration process, which may be driven by glutamate exchange.

Purification of a presynaptic membrane protein that mediates a calcium-dependent translocation of acetylcholine

A protein, which we call "mediatophore," that mediates calcium-dependent release of acetylcholine from proteoliposomes has been purified from the presynaptic plasma membrane. About 250 micrograms of this material was obtained from 500 g of Torpedo marmorata electric organ. Precipitation of the protein and subsequent removal of associated lipids inactivated the protein, which then became water soluble; this permitted evaluation of its Stokes radius (52 A) and its sedimentation coefficient (9.8 +/- 0.75 S) and, hence, an approximate molecular mass of 210 +/- 16 kDa could be determined. PAGE analysis showed that the protein is made of 17-kDa subunits, not linked by disulfide bonds. When this material was observed by electron microscopy after negative staining, the apparently pentameric structures had an average diameter of about 7 nm.

A cell-surface antigen of cholinergic nerve terminals recognized by antisera to choline acetyltransferase

An antiserum to choline acetyltransferase, partially purified from bovine brain (anti-ChAT 1), when incubated with synaptosomes prepared from rat cerebral cortex in the presence of complement caused release of 10% of lactate dehydrogenase and 27% of ChAT. Sodium-dependent uptake of choline was totally abolished. Similar results were obtained when an antiserum to ChAT highly purified from pig brain (anti-ChAT 2) was used under similar conditions. These treatments had no effect on noradrenaline uptake and did not release glutamate decarboxylase. The results suggest selective lysis of cholinergic synaptosomes had occurred. A similar selective lysis was also observed when synaptosomes prepared from guinea pig cerebral cortex were used. Anti-ChAT 2 also inhibited choline uptake in a complement-independent manner.

Rapid regulation of [3H]hemicholinium-3 binding sites in the rat brain

The effects of various drugs known to affect the sodium-dependent high-affinity choline-uptake system (SDHACU) in the brain were examined for their action upon the [3H]hemicholinium-3 [( 3H]HCh-3) binding site, which is associated with the choline carrier. The [3H]HCh-3 binding sites are affected in a similar way to the SDHACU system. Thus, alterations in the velocity of choline-uptake are mediated through changes in the apparent number of available transport sites at cholinergic terminals.

Carrier-mediated choline uptake by Krebs II ascites cells

Krebs II ascites cells have a low affinity uptake system for choline (Km = 36 microM, Vm = 76 nmol/min per 2 X 10(8). Choline entered the cells and was rapidly phosphorylated (95% of total intracellular soluble label). Trans acceleration of labeled choline from cells preloaded with radiolabeled choline and postincubated in the presence of unlabeled choline indicates that choline transport in Krebs II ascites cell is carrier mediated. Ethanolamine competed for the choline carrier. The uptake was reduced by hemicholinium-3, iodoacetamide and ouabain. The mechanism of choline transport in Krebs II ascites cells is in agreement with a linear transport model.

Reconstitution of carrier-mediated choline transport in proteoliposomes prepared from presynaptic membranes of Torpedo electric organ, and its internal and external ionic requirements

Proteoliposomes made by a butanol-sonication technique from electric organ presynaptic membranes showed choline transport activity. In contrast to intact nerve terminals, the uptake of choline was dissociated from its conversion to acetylcholine in this preparation. The kinetics of choline uptake by proteoliposomes was best described by two Michaelis-Menten components. At a low concentration of choline, uptake was inhibited by hemicholinium-3 and required external Na+ and, thus, closely resembled high-affinity choline uptake by intact cholinergic nerve terminals. Choline transport could be driven by the Na+ gradient and by the transmembrane potential (inside negative) but did not directly require ATP. External Cl-, but not a Cl- gradient, was needed for choline transport activity. It is suggested that internal K+ plays a role in the retention of choline inside the proteoliposome. Proteoliposomes should prove a useful tool for both biochemical and functional studies of the high-affinity choline carrier.

Choline transport across a carbon tetrachloride phase containing a chloroform-methanol extract of brain

The presence of a chloroform-methanol extract of cat brain in a carbon tetrachloride phase separating two aqueous phases resulted in an increased passage of [3H]choline across the organic phase which was inhibited by the choline transport inhibitor hemicholinium-3 and by high concentrations of non-radioactive choline. In the absence of cat brain extract, [3H]choline passage across carbon tetrachloride was neither inhibited by hemicholinium-3, nor by non-radioactive choline.

Kinetics of irreversible inhibition of choline transport in synaptosomes by ethylcholine mustard aziridinium

Ethylcholine mustard aziridinium (ECMA) inhibits choline transport in synaptosomes at a half-maximal concentration of about 20 microM. The rate of inhibition falls off rapidly after 10 min and the concentration dependency reaches a plateau at about 100 microM. The inhibition is not removed by washing the synaptosomes, and choline and hemicholinium-3 protect the carrier against attack by the mustard. Choline efflux, particularly that stimulated by choline in the medium (transactivation) is also inhibited by the aziridinium compound. Similarly choline influx activated by preloaded internal choline is inhibited by ECMA. The mustard can enter the synaptosomes in an active form but most of the carrier is alkylated when facing the outside. Prior depolarization of the synaptosomes causes an increase in the rate of inhibition by ECMA which is proportionally about the same as the increase in choline influx also caused by depolarization. At low ECMA concentrations the rate of inhibition is that of a first-order reaction with the carrier but at high ECMA concentrations the translocation of the carrier to the outward-facing conformation controls the rate of inhibition. Using a model of choline transport with some simplifying assumptions it is possible to estimate the amount of carrier; cholinergic synaptosomes carry about six times the concentration of carrier found in noncholinergic ones. In noncholinergic synaptosomes the carrier faces predominately out, the reverse in cholinergic ones. The rate constant of carrier translocation is increased by combination with choline some six- to sevenfold to about 3.5 min-1. The rate constant of ECMA attack on the carrier is about 440 M-1 sec-1.

Uptake and incorporation of choline by Schistosoma mansoni adults

Choline uptake and incorporation into Schistosoma mansoni is used as a model for investigating transport across and formation of a double bilayer surface of a syncytial transporting epithelium. Choline uptake reached a maximal rate during the first 2 min (Vmax = 0.27 mumol mg-1 protein min-1; Km = 36 microM). Choline uptake during a 30 min incubation was similar to that of single bilayer transport systems described in the literature. Choline incorporation into phosphatidylcholine was saturated above 40 microM external choline concentration (Vmax = 3.7 pmol mg-1 protein min-1; Km = 7 microM). The low rate of choline efflux and the half life of the tissue choline pool (T 1/2 = 3 h), suggests that free choline pools available for efflux in S. mansoni are small. This model allows the determination of whether a proposed effector of membrane phosphatidylcholine synthesis and turnover alters surface bilayer formation through changes in transport of the precursor across the apical surface.

A fluorescence polarization study of calcium and phase behaviour in synaptosomal lipids

Steady-state fluorescence polarization of the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene reported temperature-dependent lipid order in L-alpha-dimyristoylphosphatidylcholine, egg phosphatidylcholine and synaptosomal membranes. No change in lipid order was detected after depolarization of synaptosomes by veratridine (150 microM) even in the presence of 2 mM CaCl2. However, Ca2+ reduced the mobility of a second probe, dansylated dipalmitoylphosphatidylethanolamine, in dispersions of synaptosomal lipids. This effect, which was seen at a Ca2+/total phospholipid ratio as low as 0.1, may represent an interaction between the cation and negatively-charged phospholipids. It is suggested that Ca2+ promotes a phase separation in synaptosomal lipids which may be relevant to the process of neurotransmitter release.

Choline uptake by lipid liposomes and its diffusion across chloroform are saturable and inhibited by hemicholinium-3

Uptake of choline by lipid liposomes formed by cholate dialysis was saturable, inhibited by hemicholinium-3 and increased by preloading with unlabelled choline. Diffusion of choline between two aqueous phases separated by chloroform was also saturable, inhibited by high concentrations of hemicholinium-3, and increased by choline on the opposite side of the chloroform. These results indicate that choline diffusion may sometimes demonstrate properties qualitatively similar to those of low affinity carrier mediated transport systems.

Reconstitution of a functional synaptosomal membrane possessing the protein constituents involved in acetylcholine translocation

Reconstitution of a functional presynaptic membrane possessing calcium-dependent acetylcholine release properties has been achieved. The proteoliposomal membrane obtained gains its acetylcholine-releasing capabilities from presynaptic membrane proteins. At the peak of acetylcholine release, intramembrane particles became more numerous in one of the proteoliposomal membrane faces. This phenomenon resembles the intramembrane particle rearrangements found in stimulated synaptosomes. No visible structures capable of releasing acetylcholine as a result of the calcium influx were found inside the proteoliposomes. This supports the view that the release of free cytosolic acetylcholine from stimulated nerve terminals can be directly attributed to presynaptic membrane proteins. These proteins were extracted in a functional form from the synaptosomal membrane.

Choline transport by synaptosomal membrane vesicles isolated from insect nervous tissue

Membrane vesicles derived from insect nervous tissue are capable of accumulating choline via a high affinity, carrier-mediated process with ion gradients as the sole driving force. The transport is strictly dependent on the presence of Na+ and Cl- in the medium and is stimulated by a membrane potential.

Acetylcholine release from proteoliposomes equipped with synaptosomal membrane constituents

A lyophilized presynaptic membrane powder prepared from Torpedo electric organ synaptosomes was incorporated into liposomes. These proteoliposomes had a large internal volume. The P and E faces of their membrane showed particles which were comparable to the presynaptic membrane ones. The synaptosomal ecto-esterase activity was also incorporated. A large amount of acetylcholine could be entrapped in the proteoliposome which became permeable to acetylcholine in the presence of calcium. Acetylcholine was released in preference to choline. The calcium-induced acetylcholine release depended on the incorporation of a presynaptic membrane constituent. Proteoliposomes prepared from postsynaptic membrane powders gave a much slower acetylcholine efflux. The protein pattern of presynaptic and postsynaptic membrane proteoliposomes were compared.