Molecular Analysis and Structure-Activity Relationship Modeling of the Substrate/Inhibitor Interaction Site of Plasma Membrane Monoamine Transporter

Abstract

Plasma membrane monoamine transporter (PMAT) is a new polyspecific transporter that interacts with a wide range of structurally diverse organic cations. To map the physicochemical descriptors of cationic compounds that allow interaction with PMAT, we systematically analyzed the interactions between PMAT and three series of structural analogs of known organic cation substrates including phenylalkylamines, n-tetraalkylammonium (n-TAA) compounds, and ␤-carbolines. Our results showed that phenylalkylamines with a distance between the aromatic ring and the positively charged amine nitrogen atom of ϳ6.4 Å confer optimal interactions with PMAT, whereas studies with n-TAA compounds revealed an excellent correlation between IC 50 values and hydrophobicity. The five ␤-carbolines that we tested, which possess a pyridinium-like structure and are structurally related to the neurotoxin 1-methyl-4-phenylpyridinium, inhibited PMAT with high affinity (IC 50 values of 39.1-65.5 M). Cytotoxicity analysis further showed that cells expressing PMAT are 14-to 15-fold more sensitive to harmalan and norharmanium, suggesting that these two ␤-carbolines are also transportable substrates of PMAT. We then used computer-aided modeling to generate qualitative and quantitative three-dimensional pharmacophore models on the basis of 23 previously reported and currently identified PMAT inhibitors and noninhibitors. These models are characterized by a hydrogen bond donor and two to three hydrophobic features with distances between the hydrogen bond donor and hydrophobic features ranging between 5.20 and 7.02 Å. The consistency between the mapping results and observed PMAT affinity of a set of test compounds indicates that the models performed well in inhibitor prediction and could be useful for future virtual screening of new PMAT inhibitors.

Impaired Monoamine and Organic Cation Uptake in Choroid Plexus in Mice with Targeted Disruption of the Plasma Membrane Monoamine Transporter (Slc29a4) Gene

“…By removing its substrates from the CSF, PMAT could play an important role in protecting the brain from cationic neurotoxins and other potentially toxic organic cations. In this context, it is noteworthy to point out that we recently reported that several ␤-carbolines (e.g., harmalan and norharmanium) are transportable substrates of PMAT (44). ␤-Carbolines are naturally found analogs of MPP ϩ , a synthetic neurotoxin that produces Parkinson's syndrome in humans and animal models.…”

Electrophysiological Characterization of the Polyspecific Organic Cation Transporter Plasma Membrane Monoamine Transporter

“…By gene ontology, PMAT (SLC29A4) belongs to the equilibrative nucleoside transporter (ENT/ SLC29) family and is alternatively named ENT4 (Kong et al, 2004). However, detailed functional characterization work demonstrated that PMAT functions as a polyspecific organic cation transporter and shares large overlapping substrate and inhibitor specificity with the OCTs Engel and Wang, 2005;Zhou et al, 2007b;Ho et al, 2011). For example, many well established PMAT substrates, such as MPP ϩ , monoamine neurotransmitters, and metformin, are also classic substrates for the OCTs.…”

“…To eliminate hydrophilic OCs from the body, mammalian cells have evolved complex OC transport systems including the classic organic cation transporters 1 to 3 (OCT1-3) from the solute carrier 22 (SLC22) family and the multidrug and toxin extrusion proteins 1 and 2 from the solute carrier 47 (SLC47) family (Koepsell and Endou, 2004;Wright and Dantzler, 2004;Otsuka et al, 2005;Fujita et al, 2006). Our laboratory recently cloned and characterized a novel polyspecific organic cation transporter, the plasma membrane monoamine transporter (PMAT) Engel and Wang, 2005;Ho et al, 2011). By gene ontology, PMAT (SLC29A4) belongs to the equilibrative nucleoside transporter (ENT/ SLC29) family and is alternatively named ENT4 (Kong et al, 2004).…”

Computer-Aided Drug Design and Discovery, Screening and Synthesis of Small Molecule Inhibitors of Nucleoside Transporters

“…69 A hydrophobic aryl group spaced 0.5 to 7.7 Ǻ appeared to be key, with IC 50 s decreasing with increasing hydrophobicity. 69 It is important to note that the IC 50 range of the substrates and inhibitors tested ranged from roughly 9 M to upwards of 1000 M or more. In contrast, Wang et al, which looked at hENT4 activity under the acidic conditions one might find in ischemic conditions identified potent nanomolar dipyridamole derivatives.…”

“…67,68 Ho et al did a study on hENT4 in which they looked at substrate and inhibitor structure-activity relationships at physiological pH. 69 A hydrophobic aryl group spaced 0.5 to 7.7 Ǻ appeared to be key, with IC 50 s decreasing with increasing hydrophobicity. 69 It is important to note that the IC 50 range of the substrates and inhibitors tested ranged from roughly 9 M to upwards of 1000 M or more.…”