Nonoisotopic assay for the presynaptic choline transporter reveals capacity for allosteric modulation of choline uptake

Structural and functional annotation of solute carrier transporters: implication for drug discovery

INTRODUCTION

Solute carriers (SLCs) represent the largest group of membrane transporters in the human genome. They play a central role in controlling the compartmentalization of metabolism and most of this superfamily is linked to human disease. Despite being in general considered druggable and attractive therapeutic targets, many SLCs remain poorly annotated, both functionally and structurally.

AREAS COVERED

The aim of this review is to provide an overview of functional and structural parameters of SLCs that play important roles in their druggability. To do this, the authors provide an overview of experimentally solved structures of human SLCs, with emphasis on structures solved in complex with chemical modulators. From the functional annotations, the authors focus on SLC localization and SLC substrate annotations.

EXPERT OPINION

Recent progress in the structural and functional annotations allows to refine the SLC druggability index. Particularly the increasing number of experimentally solved structures of SLCs provides insights into mode-of-action of a significant number of chemical modulators of SLCs.

Drug-nutrient interactions: discovering prescription drug inhibitors of the thiamine transporter ThTR-2 (SLC19A3)

BACKGROUND

Transporter-mediated drug-nutrient interactions have the potential to cause serious adverse events. However, unlike drug-drug interactions, these drug-nutrient interactions receive little attention during drug development. The clinical importance of drug-nutrient interactions was highlighted when a phase III clinical trial was terminated due to severe adverse events resulting from potent inhibition of thiamine transporter 2 (ThTR-2; SLC19A3).

OBJECTIVE

In this study, we tested the hypothesis that therapeutic drugs inhibit the intestinal thiamine transporter ThTR-2, which may lead to thiamine deficiency.

METHODS

For this exploration, we took a multifaceted approach, starting with a high-throughput in vitro primary screen to identify inhibitors, building in silico models to characterize inhibitors, and leveraging real-world data from electronic health records to begin to understand the clinical relevance of these inhibitors.

RESULTS

Our high-throughput screen of 1360 compounds, including many clinically used drugs, identified 146 potential inhibitors at 200 μM. Inhibition kinetics were determined for 28 drugs with half-maximal inhibitory concentration (IC50) values ranging from 1.03 μM to >1 mM. Several oral drugs, including metformin, were predicted to have intestinal concentrations that may result in ThTR-2-mediated drug-nutrient interactions. Complementary analysis using electronic health records suggested that thiamine laboratory values are reduced in individuals receiving prescription drugs found to significantly inhibit ThTR-2, particularly in vulnerable populations (e.g., individuals with alcoholism).

CONCLUSIONS

Our comprehensive analysis of prescription drugs suggests that several marketed drugs inhibit ThTR-2, which may contribute to thiamine deficiency, especially in at-risk populations.

The Druggability of Solute Carriers

The transport of materials across membranes is a vital process for all aspects of cellular function, including growth, metabolism, and communication. Protein transporters are the molecular gates that control this movement and serve as key points of regulation for these processes, thus representing an attractive class of therapeutic targets. With more than 400 members, the solute carrier (SLC) membrane transport proteins are the largest family of transporters, yet, they are pharmacologically underexploited relative to other protein families and many of the available chemical tools possess suboptimal selectivity and efficacy. Fortuitously, there is increased interest in elucidating the physiological roles of SLCs as well as growing recognition of their therapeutic potential. This Perspective provides an overview of the SLC superfamily, including their biochemical and functional features, as well as their roles in various human diseases. In particular, we explore efforts and associated challenges toward drugging SLCs, as well as highlight opportunities for future drug discovery.

Identification and Characterization of Inhibitors of a Neutral Amino Acid Transporter, SLC6A19, Using Two Functional Cell-Based Assays

SLC6A19 (B⁰AT1) is a neutral amino acid transporter, the loss of function of which results in Hartnup disease. SLC6A19 is also believed to have an important role in amino acid homeostasis, diabetes, and weight control. A small-molecule inhibitor of human SLC6A19 (hSLC6A19) was identified using two functional cell-based assays: a fluorescence imaging plate reader (FLIPR) membrane potential (FMP) assay and a stable isotope-labeled neutral amino acid uptake assay. A diverse collection of 3440 pharmacologically active compounds from the Microsource Spectrum and Tocriscreen collections were tested at 10 µM in both assays using MDCK cells stably expressing hSLC6A19 and its obligatory subunit, TMEM27. Compounds that inhibited SLC6A19 activity in both assays were further confirmed for activity and selectivity and characterized for potency in functional assays against hSLC6A19 and related transporters. A single compound, cinromide, was found to robustly, selectively, and reproducibly inhibit SLC6A19 in all functional assays. Structurally related analogs of cinromide were tested to demonstrate structure-activity relationship (SAR). The assays described here are suitable for carrying out high-throughput screening campaigns to identify modulators of SLC6A19.

Truncating SLC5A7 mutations underlie a spectrum of dominant hereditary motor neuropathies

Objective

To identify the genetic cause of disease in 2 previously unreported families with forms of distal hereditary motor neuropathies (dHMNs).

Methods

The first family comprises individuals affected by dHMN type V, which lacks the cardinal clinical feature of vocal cord paralysis characteristic of dHMN-VII observed in the second family. Next-generation sequencing was performed on the proband of each family. Variants were annotated and filtered, initially focusing on genes associated with neuropathy. Candidate variants were further investigated and confirmed by dideoxy sequence analysis and cosegregation studies. Thorough patient phenotyping was completed, comprising clinical history, examination, and neurologic investigation.

Results

dHMNs are a heterogeneous group of peripheral motor neuron disorders characterized by length-dependent neuropathy and progressive distal limb muscle weakness and wasting. We previously reported a dominant-negative frameshift mutation located in the concluding exon of the SLC5A7 gene encoding the choline transporter (CHT), leading to protein truncation, as the likely cause of dominantly-inherited dHMN-VII in an extended UK family. In this study, our genetic studies identified distinct heterozygous frameshift mutations located in the last coding exon of SLC5A7, predicted to result in the truncation of the CHT C-terminus, as the likely cause of the condition in each family.

Conclusions

This study corroborates C-terminal CHT truncation as a cause of autosomal dominant dHMN, confirming upper limb predominating over lower limb involvement, and broadening the clinical spectrum arising from CHT malfunction.

Discovery of Compounds that Positively Modulate the High Affinity Choline Transporter

Cholinergic hypofunction is associated with decreased attention and cognitive deficits in the central nervous system in addition to compromised motor function. Consequently, stimulation of cholinergic neurotransmission is a rational therapeutic approach for the potential treatment of a variety of neurological conditions. High affinity choline uptake (HACU) into acetylcholine (ACh)-synthesizing neurons is critically mediated by the sodium- and pH-dependent high-affinity choline transporter (CHT, encoded by the SLC5A7 gene). This transporter is comparatively well-characterized but otherwise unexplored as a potential drug target. We therefore sought to identify small molecules that would enable testing of the hypothesis that positive modulation of CHT mediated transport would enhance activity-dependent cholinergic signaling. We utilized existing and novel screening techniques for their ability to reveal both positive and negative modulation of CHT using literature tools. A screening campaign was initiated with a bespoke compound library comprising both the Pfizer Chemogenomic Library (CGL) of 2,753 molecules designed specifically to help enable the elucidation of new mechanisms in phenotypic screens and 887 compounds from a virtual screening campaign to select molecules with field-based similarities to reported negative and positive allosteric modulators. We identified a number of previously unknown active and structurally distinct molecules that could be used as tools to further explore CHT biology or as a starting point for further medicinal chemistry.

Identification of novel inhibitors of the amino acid transporter B0 AT1 (SLC6A19), a potential target to induce protein restriction and to treat type 2 diabetes

BACKGROUND AND PURPOSE

The neutral amino acid transporter B⁰ AT1 (SLC6A19) has recently been identified as a possible target to treat type 2 diabetes and related disorders. B⁰ AT1 mediates the Na⁺ -dependent uptake of all neutral amino acids. For surface expression and catalytic activity, B⁰ AT1 requires coexpression of collectrin (TMEM27). In this study, we established tools to identify and evaluate novel inhibitors of B⁰ AT1.

EXPERIMENTAL APPROACH

A CHO-based cell line was generated, stably expressing collectrin and B⁰ AT1. Using this cell line, a high-throughput screening assay was developed, which uses a fluorescent dye to detect depolarisation of the cell membrane during amino acid uptake via B⁰ AT1. In parallel to these functional assays, we ran a computational compound screen using AutoDock4 and a homology model of B⁰ AT1 based on the high-resolution structure of the highly homologous Drosophila dopamine transporter.

KEY RESULTS

We characterized a series of novel inhibitors of the B⁰ AT1 transporter. Benztropine was identified as a competitive inhibitor of the transporter showing an IC₅₀ of 44 ± 9 μM. The compound was selective with regard to related transporters and blocked neutral amino acid uptake in inverted sections of mouse intestine.

CONCLUSION AND IMPLICATIONS

The tools established in this study can be widely used to identify new transport inhibitors. Using these tools, we were able to identify compounds that can be used to study epithelial transport, to induce protein restriction, or be developed further through medicinal chemistry.

Optimization of the choline transporter (CHT) inhibitor ML352: Development of VU6001221, an improved in vivo tool compound

This Letter describes the further lead optimization of the CHT inhibitor probe, ML352 (VU0476201), and the development of VU6001221, an improved in vivo tool. A multi-dimensional optimization effort encountered steep SAR, and ultimately, subtle tuning of the electronics of the central phenyl core provided VU6001221, a CHT inhibitor with comparable potency for choline uptake inhibition as ML352, yet improved PK and CNS penetration. Moreover, VU6001221 enabled evaluation, for the first time, of a CHT inhibitor in a standard preclinical rodent cognition model, novel object recognition (NOR). We observed VU6001221 to elicit a dose-responsive increase in NOR, raising the possibility of agonism of synaptic α7 nicotinic ACh receptors by elevated extracellular choline, that if confirmed would represent a novel molecular strategy to enhance cognition.

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.

Differential impact of genetically modulated choline transporter expression on the release of endogenous versus newly synthesized acetylcholine

The efficient import of choline into cholinergic nerve terminals by the presynaptic, high-affinity choline transporter (CHT, SLC5A7) dictates the capacity for acetylcholine (ACh) synthesis and release. Tissue levels of ACh are significantly reduced in mice heterozygous for a loss of function mutation in Slc5a7 (HET, CHT(+/-)), but significantly elevated in overexpressing, Slc5a7 BAC-transgenic mice (BAC). Since the readily-releasable pool of ACh is thought to constitute a small fraction of the total ACh pool, these genotype-dependent changes raised the question as to whether CHT expression or activity might preferentially influence the size of reserve pool ACh vesicles. In the current study, we approached this question by evaluating CHT genotype effects on the release of ACh from suprafused mouse forebrain slices. We treated slices from HET, BAC or wildtype (WT) controls with elevated K(+) and monitored release of both newly synthesized and storage pools of ACh. Newly synthesized ACh produced following uptake of [(3)H]choline was quantified by scintillation spectrometry whereas release of endogenous ACh storage pools was quantified by an HPLC-MS approach, from the same samples. Whereas endogenous ACh release scaled with CHT gene dosage, preloaded [(3)H]ACh release displayed no significant genotype dependence. Our findings suggest that CHT protein levels preferentially impact the capacity for ACh release afforded by mobilization of reserve pool vesicles.

Insulin Regulates the Activity of the High-Affinity Choline Transporter CHT

Studies in humans and animal models show that neuronal insulin resistance increases the risk of developing Alzheimer’s Disease (AD), and that insulin treatment may promote memory function. Cholinergic neurons play a critical role in cognitive and attentional processing and their dysfunction early in AD pathology may promote the progression of AD pathology. Synthesis and release of the neurotransmitter acetylcholine (ACh) is closely linked to the activity of the high-affinity choline transporter protein (CHT), but the impact of insulin receptor signaling and neuronal insulin resistance on these aspects of cholinergic function are unknown. In this study, we used differentiated SH-SY5Y cells stably-expressing CHT proteins to study the effect of insulin signaling on CHT activity and function. We find that choline uptake activity measured after acute addition of 20 nM insulin is significantly lower in cells that were grown for 24 h in media containing insulin compared to cells grown in the absence of insulin. This coincides with loss of ability to increase phospho-Protein Kinase B (PKB)/Akt levels in response to acute insulin stimulation in the chronic insulin-treated cells. Inhibition of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3-kinase) in cells significantly lowers phospho-PKB/Akt levels and decreases choline uptake activity. We show total internal reflection microscopy (TIRF) imaging of the dynamic movement of CHT proteins in live cells in response to depolarization and drug treatments. These data show that acute exposure of depolarized cells to insulin is coupled to transiently increased levels of CHT proteins at the cell surface, and that this is attenuated by chronic insulin exposure. Moreover, prolonged inhibition of PI3-kinase results in enhanced levels of CHT proteins at the cell surface by decreasing their rate of internalization.

Identification and characterization of ML352: a novel, noncompetitive inhibitor of the presynaptic choline transporter

The high-affinity choline transporter (CHT) is the rate-limiting determinant of acetylcholine (ACh) synthesis, yet the transporter remains a largely undeveloped target for the detection and manipulation of synaptic cholinergic signaling. To expand CHT pharmacology, we pursued a high-throughput screen for novel CHT-targeted small molecules based on the electrogenic properties of transporter-mediated choline transport. In this effort, we identified five novel, structural classes of CHT-specific inhibitors. Chemical diversification and functional analysis of one of these classes identified ML352 as a high-affinity (K_i = 92 nM) and selective CHT inhibitor. At concentrations that fully antagonized CHT in transfected cells and nerve terminal preparations, ML352 exhibited no inhibition of acetylcholinesterase (AChE) or cholineacetyltransferase (ChAT) and also lacked activity at dopamine, serotonin, and norepinephrine transporters, as well as many receptors and ion channels. ML352 exhibited noncompetitive choline uptake inhibition in intact cells and synaptosomes and reduced the apparent density of hemicholinium-3 (HC-3) binding sites in membrane assays, suggesting allosteric transporter interactions. Pharmacokinetic studies revealed limited in vitro metabolism and significant CNS penetration, with features predicting rapid clearance. ML352 represents a novel, potent, and specific tool for the manipulation of CHT, providing a possible platform for the development of cholinergic imaging and therapeutic agents.

Synthesis and structure-activity relationships of a series of 4-methoxy-3-(piperidin-4-yl)oxy benzamides as novel inhibitors of the presynaptic choline transporter

The synthesis and SAR of 4-methoxy-3-(piperidin-4-yl) benzamides identified after a high-throughput screen of the MLPCN library is reported. SAR was explored around the 3-piperidine substituent as well as the amide functionality of the reported compounds. Starting from the initial lead compounds, 1-7, iterative medicinal chemistry efforts led to the identification of ML352 (10m). ML352 represents a potent and selective inhibitor of CHT based on a drug-like scaffold.

Importing, caring, breeding, genotyping, and phenotyping a genetic mouse in a Chinese university

The genetic manipulation of the laboratory mouse has been well developed and generated more and more mouse lines for biomedical research. To advance our science exploration, it is necessary to share genetically modified mouse lines with collaborators between institutions, even in different countries. The transfer process is complicated. Significant paperwork and coordination are required, concerning animal welfare, intellectual property rights, colony health status, and biohazard. Here, we provide a practical example of importing a transgenic mice line, Dynamin 1 knockout mice, from Yale University in the USA to Perking University in China for studying cell secretion. This example including the length of time that required for paper work, mice quarantine at the receiving institution, and expansion of the mouse line for experiments. The procedure described in this paper for delivery live transgenic mice from USA to China may serve a simple reference for transferring mouse lines between other countries too.

Transgenic overexpression of the presynaptic choline transporter elevates acetylcholine levels and augments motor endurance

The hemicholinium-3 (HC-3) sensitive, high-affinity choline transporter (CHT) sustains cholinergic signaling via the presynaptic uptake of choline derived from dietary sources or from acetylcholinesterase (AChE)-mediated hydrolysis of acetylcholine (ACh). Loss of cholinergic signaling capacity is associated with cognitive and motor deficits in humans and in animal models. Whereas genetic elimination of CHT has revealed the critical nature of CHT in maintaining ACh stores and sustaining cholinergic signaling, the consequences of elevating CHT expression have yet to be studied. Using bacterial artificial chromosome (BAC)-mediated transgenic methods, we generated mice with integrated additional copies of the mouse Slc5a7 gene. BAC-CHT mice are viable, appear to develop normally, and breed at wild-type (WT) rates. Biochemical studies revealed a 2 to 3-fold elevation in CHT protein levels in the CNS and periphery, paralleled by significant increases in [(3)H]HC-3 binding and synaptosomal choline transport activity. Elevations of ACh in the BAC-CHT mice occurred without compensatory changes in the activity of either choline acetyltransferase (ChAT) or AChE. Immunohistochemistry for CHT in BAC-CHT brain sections revealed markedly elevated CHT expression in the cell bodies of cholinergic neurons and in axons projecting to regions known to receive cholinergic innervation. Behaviorally, BAC-CHT mice exhibited diminished fatigue and increased speeds on the treadmill test without evidence of increased strength. Finally, BAC-CHT mice displayed elevated horizontal activity in the open field test, diminished spontaneous alteration in the Y-maze, and reduced time in the open arms of the elevated plus maze. Together, these studies provide biochemical, pharmacological and behavioral evidence that CHT protein expression and activity can be elevated beyond that seen in wild-type animals. BAC-CHT mice thus represent a novel tool to examine both the positive and negative impact of constitutively elevated cholinergic signaling capacity.

The presynaptic choline transporter imposes limits on sustained cortical acetylcholine release and attention

Functional variation in the gene encoding the presynaptic choline transporter (CHT) has been linked to attention-deficit/hyperactivity disorder. Here, we report that a heterozygous deletion in the CHT gene in mice (CHT(+/-)) limits the capacity of cholinergic neurons to sustain acetylcholine (ACh) release and attentional performance. Cortical microdialysis and amperometric methods revealed that, whereas wild-type and CHT(+/-) animals support equivalent basal ACh release and choline clearance, CHT(+/-) animals exhibit a significant inability to elevate extracellular ACh following basal forebrain stimulation, in parallel with a diminished choline clearance capacity following cessation of stimulation. Consistent with these findings, the density of CHTs in cortical synaptosomal plasma membrane-enriched fractions from unstimulated CHT(+/-) animals matched those observed in wild-type animals despite reductions in CHT levels in total extracts, achieved via a redistribution of CHT from vesicle pools. As a consequence, in CHT(+/-) animals, basal forebrain stimulation was unable to mobilize wild-type quantities of CHT to the plasma membrane. In behavioral studies, CHT(+/-) mice were impaired in performing a sustained attention task known to depend on cortical cholinergic activity. In wild-type mice, but not CHT(+/-) mice, attentional performance increased the density of CHTs in the synaptosomal membrane in the right frontal cortex. Basal CHT levels in vesicle-enriched membranes predicted the degree of CHT mobilization as well as individual variations in performance on the sustained attention task. Our findings demonstrate biochemical and physiological alterations that underlie cognitive impairments associated with genetically imposed reductions in choline uptake capacity.