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

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.

Monoclonal antibodies against the high affinity choline transport system

Monoclonal antibodies have been isolated that specifically block the high affinity, Na(+)-dependent transport of choline in insect synaptosomes and synaptosomal ghosts. Antibodies were derived after immunization of mice with synaptosomal membranes from locust. Antibody VIB6F5, an IgG isotype, significantly inhibited the high affinity translocation of choline, the effect exhibited saturation at increased antibody concentrations. Antibodies recognized a 80 kDa antigen identified by Western blot analysis of synaptosomal membranes. In immunocytochemical approaches VIB6F5 specifically stained distinct areas in the neuropil of head and thoracic ganglia.

Transport of choline and its relationship to the expression of the organic cation transporters in a rat brain microvessel endothelial cell line (RBE4)

The present study was undertaken to elucidate the functional characteristics of choline uptake and deduce the relationship between choline uptake and the expression of organic cation transporters in the rat brain microvessel endothelial cell line RBE4. Confluent RBE4 cells were found to express a high affinity choline uptake system. The system is Na(+)-independent and shows a Michaelis-Menten constant of approx. 20 microM for choline. The choline analogue hemicholinium-3 inhibits choline uptake in these cells with an inhibition constant of approx. 50 microM. The uptake system is also susceptible for inhibition by various organic cations, including 1-methyl-4-phenylpyridinium, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, clonidine, procainamide, and tetramethylammonium. The prototypical organic cation tetraethylammonium shows very little affinity for the choline uptake system in these cells. The inhibition of choline uptake by hemicholinium-3 is competitive. Northern analysis and RT-PCR show that these cells do not express the organic cation transporters OCT2 and OCT3. These cells do express, however, low levels of OCT1, but the functional characteristics of choline uptake in these cells are very different from the known properties of choline uptake via OCT1. The Na(+)-coupled high affinity choline transporter CHT1 is not expressed in these cells as evidenced by RT-PCR. This corroborates the Na(+)-independent nature of choline uptake in these cells. It is concluded that RBE4 cells express an organic cation transporter that is responsible for choline uptake in these cells and that this transporter is not identical to any of the organic cation transporters thus far identified at the molecular level in mammalian cells.

Functional characterization of the human high-affinity choline transporter

Na(+)-dependent, high-affinity choline uptake in cholinergic neurons is the rate-limiting step in acetylcholine synthesis. Here we report the molecular cloning and functional characterization of the human high-affinity choline transporter (hCHT1). The hCHT1 exhibits significant homology with known members of the Na(+)-dependent glucose transporter family, but not with members of the neurotransmitter transporter family. The human CHT1 gene is 25 kb in length with 9 exons and was assigned to chromosome II at position IIq11-12. Northern blot analysis showed that a 5.4 kb hCHT1 transcript was expressed exclusively in tissues containing cholinergic neurons. When expressed in Xenopus oocytes, the human clone induced Na(+)- and Cl(-)-dependent, high-affinity choline uptake, which was sensitive to the specific inhibitor hemicholinium-3, with a K(i) of 1.3 nM. The hCHT1-mediated choline uptake increased with increasing concentrations of choline, Na(+) and Cl(-), with EC(50) values of 2.0 microM, 76 mM, and 48 mM, and with apparent Hill coefficients of 1, 2.5 and 2.3, respectively.

Conductive choline transport by alveolar epithelial plasma membrane vesicles

Choline is an important substrate in alveolar epithelia for both surfactant production and cellular maintenance. The underlying mechanisms of uptake and sites of membrane transport remain uncertain. To test the hypothesis that choline transport occurs at the basolateral side of alveolar epithelia by both Na+-independent and -dependent mechanisms, plasma membrane vesicles were prepared from the apical and basolateral membranes of mature porcine type II pneumocytes. Choline+ transport was assayed by uptake of [3H]choline+ by enriched apical or basolateral vesicles. In the presence of imposed, inside-negative charge gradients, basolateral vesicles exhibited early overshoot of [3H]choline+ uptake unaffected by the presence or absence of external Na+ (541 +/- 53 vs 564 +/- 79 pmol/mg protein (NS)). High sensitivity to hemicholinium-3 was observed in the presence or absence of Na+. In the absence of inside-negative charge gradients, uptake was reduced 12-fold in the presence or absence of Na+, and external choline+ induced internal alkalization of acidified basolateral vesicles. Accumulative [3H]choline+ uptakes by apical vesicles in the presence or absence of inside-negative charge gradients and Na+ were insignificant. We conclude that predominant choline+ uptake by type II pneumocytes occurs at the basolateral membrane by Na+-independent, electrogenic choline+ conductance. The presence of electroneutral choline+/H+ exchange is suggested.

Na(+)-dependent and Na(+)-independent systems of choline transport by plasma membrane vesicles of A549 cell line

Membrane vesicles of A549 lung cells accumulate choline by two pathways: the Na(+)-independent uphill uptake of choline [Michaelis-Menten constant (Km) approximately 44 microM; steady-state gradient approximately 45 at 5 microM external choline] is dependent on a transmembrane H+ gradient, is relatively insensitive to hemicholinium-3, is amiloride sensitive, and is abolished by valinomycin plus carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). The Na(+)-dependent active choline uptake (Km approximately 4 microM, inhibitor constant for hemicholinium-3 approximately 0.1 microM), is specific for Na+, is amiloride and FCCP sensitive, and is electrogenic: the overshoot using K(+)-loaded vesicles and NaCl gradient was increased by valinomycin. The time of the overshoot peak, T was approximately 90 s in a NaSCN medium (or in presence of other lipid-soluble anions), a value close to that for alpha-aminoisobutyrate as substrate (T = approximately 1.5 min). T was lengthened in NaCl medium to approximately 10 min, and the overshoot was abolished by impermeant anions. External Cl- is not required for the choline uptake: valinomycin produced an overshoot in the presence of only impermeant anions, with T approximately 90 s. Most of the above properties are shared by the high-affinity Na(+)-dependent choline transport in synaptosomes. The characteristics of the Na(+)-dependent choline uptake by membrane vesicles of A549 cells are consistent with an electrogenic choline(+)-Na+ cotransport, with the rate-limiting anion (e.g., Cl-) influx balancing the positive charges transferred into the vesicles. The data are also consistent with an involvement of an amiloride-sensitive choline+/H+ antiport (or choline(+)-OH- symport) in the low- and high-affinity choline uptake pathways.

High-affinity choline transport sites: use of [3H]hemicholinium-3 as a quantitative marker

High-affinity choline transport (HAChT), the rate-limiting and regulatory step in acetylcholine (ACh) synthesis, is selectively localized to cholinergic neurons. Hemicholinium-3 (HC3), a potent and selective inhibitor of HAChT, has been used as a specific radioligand to quantify HAChT sites in membrane binding and autoradiographic studies. Because both HAChT velocity and [3H]HC3 binding change as in vivo activity of cholinergic neurons is altered, these markers are also useful measures of cholinergic neuronal activity. Evidence that [3H]HC3 is a specific ligand for HAChT sites on cholinergic terminals is reviewed. The ion requirements of HAChT and [3H]HC3 binding indicate that sodium and chloride are required for recognition of both choline and [3H]HC3. A common recognition site is also indicated by the close correspondence of the potency of HC3 and choline analogues for inhibiting both HAChT and [3H]HC3 binding. The parallel regional distributions of both markers in adult brain, during development and after specific lesions, all indicate specific cholinergic localization. The close association of HAChT and [3H]HC3 binding sites is also supported by parallel regulatory changes occurring after in vivo drug treatments and in vitro depolarization. Overall, the data indicate a close association between HAChT and [3H]HC3 binding and are consistent with the sites being identical. Methodologic considerations in using [3H]HC3 as a ligand and considerations in interpretation of results are also discussed.

The ligand binding site of the synaptosomal choline transporter: a provisional model based on inhibition studies

A topographic model of the ligand binding site of the choline transporter was deduced from inhibition studies with the help of CPK molecular models. It is posited that there are two identical or closely similar hydrophilic anionic sites separated from each other by an hinged, essentially planar but conformationally flexible cationic hydrophobic domain. Subsequently to attachment of external choline to either one of the anionic sites, both sites cooperate in enveloping the ligand by a Venus fly-trap mechanism. This leads to rapid configurational changes by which the closed-liganded form of the transporter opens up to the interior to release the bound choline. Intracellular K+, a ligand for the choline-binding site, is proposed to be counter-transported by a reversal of the above mechanism.

Isolation and reconstitution of the high-affinity choline carrier

Monoclonal antibodies, which block the high-affinity uptake of choline in synaptosomal ghosts, have been used to purify a membrane polypeptide (80 kDa) from insect synaptosomal membranes. This isolated protein was found to catalyse the sodium-dependent, hemicholinium-sensitive accumulation of choline after reconstitution into liposomes, thus, apparently represents the high-affinity choline transporter.

Binding of [3H]hemicholinium-3 to the high-affinity choline transporter in electric organ synaptosomal membranes

Sodium-dependent binding of [3H]hemicholinium-3 was observed to be 10-fold higher with presynaptic membranes from the electric organ than with electroplaque membranes and this binding site copurified with synaptosomal membranes. The KD for specific [3H]hemicholinium-3 binding was found to be 31 +/- 4 nM and the Bmax, 5.0 +/- 0.2 pmol/mg protein; a Ki of 16 nM was estimated for hemicholinium-3 as a competitive inhibitor of high-affinity choline transport in electric organ synaptosomes. Choline and choline analogues were equally potent as inhibitors of [3H]choline uptake and [3H]hemicholinium-3 binding. Tubocurarine and oxotremorine also inhibited uptake and binding, but carbachol was without effect in both tests. These findings suggest that [3H]hemicholinium binds to the high-affinity choline transporter present at the cholinergic nerve terminal membrane. A comparison of maximal velocities for choline transport and the maximal number of hemicholinium-3 binding sites indicated that the high-affinity choline transporter has an apparent turnover number of about 3s-1 at 20 degrees C under resting conditions. The high transport rates observed in electric organ synaptosomes are likely due to the high density of high-affinity choline transporters in this tissue, estimated on the basis of [3H]hemicholinium-3 binding to be of the order of 100/micron2 of synaptosomal membrane.

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.

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.

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.