Mutational and pH analysis of ionic residues in transmembrane domains of vesicular acetylcholine transporter

Nemacol is a small molecule inhibitor of C. elegans vesicular acetylcholine transporter with anthelmintic potential

Nematode parasites of humans and livestock pose a significant burden to human health, economic development, and food security. Anthelmintic drug resistance is widespread among parasites of livestock and many nematode parasites of humans lack effective treatments. Here, we present a nitrophenyl-piperazine scaffold that induces motor defects rapidly in the model nematode Caenorhabditis elegans. We call this scaffold Nemacol and show that it inhibits the vesicular acetylcholine transporter (VAChT), a target recognized by commercial animal and crop health groups as a viable anthelmintic target. We demonstrate that it is possible to create Nemacol analogs that maintain potent in vivo activity whilst lowering their affinity to the mammalian VAChT 10-fold. We also show that Nemacol enhances the ability of the anthelmintic Ivermectin to paralyze C. elegans and the ruminant nematode parasite Haemonchus contortus. Hence, Nemacol represents a promising new anthelmintic scaffold that acts through a validated anthelmintic target.

Identification of a mammalian vesicular polyamine transporter

Spermine and spermidine act as neuromodulators upon binding to the extracellular site(s) of various ionotropic receptors, such as N-methyl-d-aspartate receptors. To gain access to the receptors, polyamines synthesized in neurons and astrocytes are stored in secretory vesicles and released upon depolarization. Although vesicular storage is mediated in an ATP-dependent, reserpine-sensitive fashion, the transporter responsible for this process remains unknown. SLC18B1 is the fourth member of the SLC18 transporter family, which includes vesicular monoamine transporters and vesicular acetylcholine transporter. Proteoliposomes containing purified human SLC18B1 protein actively transport spermine and spermidine by exchange of H(+). SLC18B1 protein is predominantly expressed in the hippocampus and is associated with vesicles in astrocytes. SLC18B1 gene knockdown decreased both SLC18B1 protein and spermine/spermidine contents in astrocytes. These results indicated that SLC18B1 encodes a vesicular polyamine transporter (VPAT).

Vesicular neurotransmitter transporters: mechanistic aspects

Secondary transporters driven by a V-type H⁺-ATPase accumulate nonpeptide neurotransmitters into synaptic vesicles. Distinct transporter families are involved depending on the neurotransmitter. Monoamines and acetylcholine on the one hand, and glutamate and ATP on the other hand, are accumulated by SLC18 and SLC17 transporters, respectively, which belong to the major facilitator superfamily (MFS). GABA and glycine accumulate through a common SLC32 transporter from the amino acid/polyamine/organocation (APC) superfamily. Although crystallographic structures are not yet available for any vesicular transporter, homology modeling studies of MFS-type vesicular transporters based on distantly related bacterial structures recently provided significant advances, such as the characterization of substrate-binding pockets or the identification of spatial clusters acting as hinge points during the alternating-access cycle. However, several basic issues, such as the ion stoichiometry of vesicular amino acid transporters, remain unsettled.

Regulation of cholinergic activity by the vesicular acetylcholine transporter

Acetylcholine, the first chemical to be identified as a neurotransmitter, is packed in synaptic vesicles by the activity of VAChT (vesicular acetylcholine transporter). A decrease in VAChT expression has been reported in a number of diseases, and this has consequences for the amount of acetylcholine loaded in synaptic vesicles as well as for neurotransmitter release. Several genetically modified mice targeting the VAChT gene have been generated, providing novel models to understand how changes in VAChT affect transmitter release. A surprising finding is that most cholinergic neurons in the brain also can express a second type of vesicular neurotransmitter transporter that allows these neurons to secrete two distinct neurotransmitters. Thus a given neuron can use two neurotransmitters to regulate different physiological functions. In addition, recent data indicate that non-neuronal cells can also express the machinery used to synthesize and release acetylcholine. Some of these cells rely on VAChT to secrete acetylcholine with potential physiological consequences in the periphery. Hence novel functions for the oldest neurotransmitter known are emerging with the potential to provide new targets for the treatment of several pathological conditions.

Spiroindolines identify the vesicular acetylcholine transporter as a novel target for insecticide action

The efficacy of all major insecticide classes continues to be eroded by the development of resistance mediated, in part, by selection of alleles encoding insecticide insensitive target proteins. The discovery of new insecticide classes acting at novel protein binding sites is therefore important for the continued protection of the food supply from insect predators, and of human and animal health from insect borne disease. Here we describe a novel class of insecticides (Spiroindolines) encompassing molecules that combine excellent activity against major agricultural pest species with low mammalian toxicity. We confidently assign the vesicular acetylcholine transporter as the molecular target of Spiroindolines through the combination of molecular genetics in model organisms with a pharmacological approach in insect tissues. The vesicular acetylcholine transporter can now be added to the list of validated insecticide targets in the acetylcholine signalling pathway and we anticipate that this will lead to the discovery of novel molecules useful in sustaining agriculture. In addition to their potential as insecticides and nematocides, Spiroindolines represent the only other class of chemical ligands for the vesicular acetylcholine transporter since those based on the discovery of vesamicol over 40 years ago, and as such, have potential to provide more selective tools for PET imaging in the diagnosis of neurodegenerative disease. They also provide novel biochemical tools for studies of the function of this protein family.

Search for the acetylcholine and vesamicol binding sites in vesicular acetylcholine transporter: the region around the lumenal end of the transport channel

Vesicular acetylcholine transporter (VAChT; TC 2.A.1.2.13) mediates storage of acetylcholine (ACh) by synaptic vesicles. A three-dimensional homology model of VAChT is available, but the binding sites for ACh and the allosteric inhibitor (-)-trans-2-(4-phenylpiperidino)cyclohexanol (vesamicol) are unknown. In previous work, mutations of invariant W331 in the lumenal beginning of transmembrane helix VIII (TM VIII) of rat VAChT led to as much as ninefold loss in equilibrium affinity for ACh and no loss in affinity for vesamicol. The current work investigates the effects of additional mutations in and around W331 and the nearby lumenal end of the substrate transport channel. Mutants of human VAChT were expressed in the PC12(A123.7) cell line and characterized using radiolabeled ligands and filtration assays for binding and transport. Properties of a new and a repeat mutation in W331 are consistent with the original observations. Of 16 additional mutations in 13 other residues (Y60 in the beginning of lumenal Loop I/II, F231 in the lumenal end of TM V, W315, M316, K317, in the lumenal end of TM VII, M320, A321, W325, A330 in lumenal Loop VII/VIII, A334 in the lumenal beginning of TM VIII, and C388, C391, F392 in the lumenal beginning of TM X), only A334F impairs binding. This mutation decreases ACh and vesamicol equilibrium binding affinities by 14- and 4-fold, respectively. The current results, combined with previous results, demonstrate existence of a spatial cluster of residues close to vesicular lumen that decreases affinity for ACh and/or vesamicol when the cluster is mutated. The cluster is composed of invariant W331, highly conserved A334, and invariant F335 in TM VIII and invariant C391 in TM X. Different models for the locations of the ACh and vesamicol binding sites relative to this cluster are discussed.

Possible important pair of acidic residues in vesicular acetylcholine transporter

Invariant E309 is in contact with critical and invariant D398 in a three-dimensional homology model of vesicular acetylcholine transporter (VAChT, TC 2.A.1.2.13) [Vardy, E., et al. (2004) Protein Sci. 13, 1832-1840]. In the work reported here, E309 and D398 in human VAChT were mutated singly and together to test their functions, assign pK values to them, and determine whether the residues are close to each other in three-dimensional space. Mutants were stably expressed in the PC12(A123.7) cell line, and transport and binding properties were characterized at different pH values using radiolabeled ligands and filtration assays. Contrary to a prior conclusion, the results demonstrate that most D398 mutants do not bind the allosteric inhibitor vesamicol even weakly. Earlier work showed that most D398 mutants do not transport ACh. D398 therefore probably is the residue that must deprotonate with a pK of 6.5 for binding of vesamicol and with a pK of approximately 5.9 for transport of ACh. Because E309Q has no effect on VAChT functions at physiological pH, E309 has no apparent critical role. However, radical mutations in E309 cause decreases in ACh and vesamicol affinities and total loss of ACh transport. Unlike wild-type VAChT, which exhibits a peak of [(3)H]vesamicol binding centered at pH 7.4, mutants E309Q, E309D, E309A, and E309K all exhibit peaks of binding centered at pH >or=9. The combination of high pH and mutated E309 apparently produces a relaxed (in contrast to tense) conformation of VAChT that binds vesamicol exceptionally tightly. No compensatory interactions between E309 and D398 in double mutants were discovered. Proof of a close spatial relationship between E309 and D398 was not found. Nevertheless, the data are more consistent with the homology model than an alternative hydropathy model of VAChT that likely locates E309 far from D398 and the ACh binding site in three-dimensional space. Also, a probable network of interactions involving E309 and an unknown residue having a pK of 10 has been revealed.

Equilibrium binding and transport by vesicular acetylcholine transporter

The method describes production and the selection of neurosecretory PC12A123.7 cells stably transfected with human vesicular acetylcholine transporter (hVAChT). Transfected cells provide postnuclear supernatant used to characterize equilibrium binding of the neurotransmitter acetylcholine (ACh), the pH dependence for transport of ACh, and the rate behavior for dissociation of the allosteric, high-affinity inhibitor vesamicol. Retention of radiolabeled ACh or vesamicol, mediated by hVAChT in synaptic-like microvesicles of postnuclear supernatant, is measured using filter assays. The procedure for regression analysis of data also is described.

Multiple protonation states of vesicular acetylcholine transporter detected by binding of [3H]vesamicol

Vesicular acetylcholine transporter (VAChT) is inhibited by (-)-vesamicol [(-)-trans-2-(4-phenylpiperidino)cyclohexanol], which binds tightly to an allosteric site. The tertiary alkylamine center in (-)-vesamicol is protonated and positively charged at acidic and neutral pH and unprotonated and uncharged at alkaline pH. Deprotonation of the amine has been taken to explain loss of (-)-vesamicol binding at alkaline pH. However, binding data deviate from a stereotypical bell shape, and more binding occurs than expected at alkaline pH. The current study characterizes the binding of (-)-vesamicol from pH 5 to pH 10 using filter assays, (-)-[3H]vesamicol (hereafter called [3H]vesamicol), and human VAChT expressed in PC12(A123.7) cells. At acidic pH, protons and [3H]vesamicol compete for binding to VAChT. Preexposure or long-term exposure of VAChT to high pH does not affect binding, thus eliminating potential denaturation of VAChT and failure of the filter assay. The dissociation constant for the complex between protonated [3H]vesamicol and VAChT decreases from 12 nM at neutral pH to 2.1 nM at pH 10. The simplest model of VAChT that explains the behavior requires a proton at site 1 to dissociate with pK1 = 6.5 +/- 0.1, a proton at site A to dissociate with pKA = 7.6 +/- 0.2, and a proton at site B to dissociate with pKB = 10.0 +/- 0.1. Deprotonation of the site 1 proton is obligatory for [3H]vesamicol binding. Deprotonation of site A decreases affinity (2.2 +/- 0.5)-fold, and deprotonation of site B increases affinity (18 +/- 4)-fold. Time-dependent dissociation of bound [3H]vesamicol is biphasic, but equilibrium saturation curves are not. The contrasting phasicity suggests that the pathway to and from the [3H]vesamicol binding site exists in open and at least partially closed states. The potential significance of the findings to development of PET and SPECT ligands based on (-)-vesamicol for human diagnostics also is discussed.

Substrate specificity and mechanism of the intestinal clonidine uptake by Caco-2 cells

PURPOSE

This study was performed to characterize the substrate specificity and mechanism of the intestinal clonidine transport.

METHODS

Uptake of [3H]clonidine into Caco-2 cells was investigated. Interaction with drugs was studied in competition assays.

RESULTS

Uptake of [3H]clonidine was linear for up to 2 min, Na+-independent, and insensitive to changes in membrane potential, but strongly H+-dependent. The uptake rate of clonidine was saturable with kinetic parameters of 0.5+/-0.1 mM (Kt) and 16.6+/-1.8 nmol/2 min per mg of protein (Vmax) at an outside pH of 7.5. Many drugs such as clonidine, guanabenz, methamphetamine, imipramine, clomipramine, nortriptyline, quinine, xylazine, ephedrine, and diphenhydramine strongly inhibited the [3H]clonidine uptake with Ki values between 0.15 and 1 mM.

CONCLUSIONS

Clonidine is transported by a carrier-mediated process. Substrate specificity and mechanism are very similar to the transport described in blood-brain barrier endothelial cells. The transport characteristics do not correspond to carriers for organic cations of the SLC22 family or the choline transporters CHT1 and CLT1. The system might be identical to the H+/tertiary amine antiporter. It interacts with a large number of both hydrophilic and lipophilic cationic drugs, and also, interestingly, with opiates.

Mutational and bioinformatics analysis of proline- and glycine-rich motifs in vesicular acetylcholine transporter

The vesicular acetylcholine transporter (VAChT) contains six conserved sequence motifs that are rich in proline and glycine. Because these residues can have special roles in the conformation of polypeptide backbone, the motifs might have special roles in conformational changes during transport. Using published bioinformatics insights, the amino acid sequences of the 12 putative, helical, transmembrane segments of wild-type and mutant VAChTs were analyzed for propensity to form non-alpha-helical conformations and molecular notches. Many instances were found. In particular, high propensity for kinks and notches are robustly predicted for motifs D2, C and C'. Mutations in these motifs either increase or decrease Vmax for transport, but they rarely affect the equilibrium dissociation constants for ACh and the allosteric inhibitor, vesamicol. The near absence of equilibrium effects implies that the mutations do not alter the backbone conformation. In contrast, the Vmax effects demonstrate that the mutations alter the difficulty of a major conformational change in transport. Interestingly, mutation of an alanine to a glycine residue in motif C significantly increases the rates for reorientation across the membrane. These latter rates are deduced from the kinetics model of the transport cycle. This mutation is also predicted to produce a more flexible kink and tighter tandem notches than are present in wild-type. For the full set of mutations, faster reorientation rates correlate with greater predicted propensity for kinks and notches. The results of the study argue that conserved motifs mediate conformational changes in the VAChT backbone during transport.