(Section A: Molecular, Structural, and Cellular Biology of Drug Transporters) Mammalian Nucleoside Transporters

Kong, Wei; Engel, Karen; Wang, Joanne

doi:10.2174/1389200043489162

Cited by 218 publications

(61 citation statements)

References 135 publications

(224 reference statements)

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“…Cloning of PMAT-We initially identified PMAT from the draft human genome data base because of its sequence homology to ENTs (18,19). Using 5Ј-and 3Ј-rapid amplification of cDNA ends and RT-PCR, we isolated a 2,112 base pair complementary DNA from human kidney poly(A) ϩ RNA.…”

Section: Resultsmentioning

confidence: 99%

Identification and Characterization of a Novel Monoamine Transporter in the Human Brain

Engel

Zhou

Wang

2004

Journal of Biological Chemistry

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247

362

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Precise control of monoamine neurotransmitter levels in the extracellular fluids of the brain is critical in maintaining efficient and robust neurotransmission. High affinity transporters in the solute carrier SLC6A family function in removing monoamines from the neurosynaptic cleft. Emerging evidence suggests that these transporters are only one part of a system of transporters that work in concert to maintain brain homeostasis of monoamines. Here we report the cloning and characterization of a new human plasma membrane monoamine transporter, PMAT. The PMAT cDNA encodes a protein of 530 amino acid residues with 10 -12 transmembrane segments. PMAT is not homologous to known neurotransmitter transporters but exhibits low homology to members of the equilibrative nucleoside transporter family. When expressed in Madin-Darby canine kidney cells and Xenopus laevis oocytes, PMAT efficiently transports serotonin (K m ‫؍‬ 114 M), dopamine (K m ‫؍‬ 329 M), and the neurotoxin 1-methyl-4-phenylpyridinium (K m ‫؍‬ 33 M). In contrast, there is no significant interaction of PMAT with nucleosides or nucleobases. PMAT-mediated monoamine transport does not require Na ؉ or Cl ؊ but appears to be sensitive to changes in membrane potential. Northern blot analysis showed that PMAT is predominantly expressed in the human brain and widely distributed in the central nervous system. These studies demonstrate that PMAT may be a novel low affinity transporter for biogenic amines, which, under certain conditions, might supplement the role of the high affinity transporters in the brain.In the central nervous system, monoamine neurotransmitters, including dopamine, serotonin, and norepinephrine, control a variety of physiological, behavioral, and endocrine functions (1, 2). Monoamine-mediated neurotransmission is also critically involved in a number of brain pathological processes such as Parkinson's disease, depression, schizophrenia, and drug addiction (1, 2). A key step that determines the intensity and duration of monoamine signaling is the transport of released monoamines into brain cells. This process is carried out by cell surface transporters that transport monoamines into presynaptic nerve terminals or neighboring glial cells where they can be recycled through repackaging into secretory vesicles or degraded by intracellular enzymes (3-5).Uptake of released monoamines into presynaptic neurons is mainly carried out by a family of Na ϩ -and Cl Ϫ -dependent high affinity plasma membrane transporters, which includes the dopamine transporter (DAT), 1 the serotonin transporter (SERT), and the norepinephrine transporter (NET) (3-5). These transporters, which share high sequence similarity and belong to the solute carrier 6A (SLC6A) family, are the known targets for many psychostimulants, antidepressants, and neurotoxins (3-5).Several lines of evidence suggest that in addition to the SLC6A high affinity transporters (i.e. DAT, SERT, and NET), the brain expresses other transporters to regulate extracellular monoamine levels. First, a number of...

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Section: Resultsmentioning

confidence: 99%

Identification and Characterization of a Novel Monoamine Transporter in the Human Brain

Engel

Zhou

Wang

2004

Journal of Biological Chemistry

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247

362

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“…The implications for this study are many. First, drugs that enhance adenosine signaling are of clinical interest in treatment of inflammation, the reduction of infarct size following myocardial or cerebral ischemia (20,32), and as anti-seizure therapy (33). Nucleoside transporter inhibitors are also intriguing as adjunct therapy in cancer patients to potentiate the effectiveness of antifolate drugs (34).…”

Section: Discussionmentioning

confidence: 99%

“…Because CBD inhibits both adenosine and thymidine uptake with similar efficacy, the drug could inhibit a common nucleoside transporter. There are two subtypes of nucleoside transporter: ENT, which are blocked by the drug S-(4-nitrobenzyl)-6-thioinosine (NBMPR), and concentrative transporters, which are dependent on the presence of extracellular sodium (20). In EOC-20 and RAW264.7 cells, the majority of adenosine and thymidine transport was NBMPR-sensitive and not affected by sodium removal, suggesting ENT are the primary transporters functioning in these cells ( Fig.…”

Section: Analysis Of Proliferationmentioning

confidence: 99%

Inhibition of an equilibrative nucleoside transporter by cannabidiol: A mechanism of cannabinoid immunosuppression

Carrier

Auchampach

Hillard

2006

Proc. Natl. Acad. Sci. U.S.A.

416

317

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The plant-derived cannabinoids ⌬ 9 -tetrahydrocannabinol (THC) and cannabidiol (CBD) both have immunosuppressive effects; although some effects of THC are mediated by the CB2 receptor, CB2 binds CBD weakly. In examining the effects of THC and CBD on microglial proliferation, we found that these compounds potently inhibit [ 3 H]thymidine incorporation into a murine microglial cell line with no effect on cell cycle. Because adenosine agonists have antiinflammatory effects, and because uptake of adenosine is a primary mechanism of terminating adenosine signaling, we tested the hypothesis that CBD is immunosuppressive because it enhances endogenous adenosine signaling. In vivo treatment with a low dose of CBD decreases TNF␣ production in lipopolysaccharide-treated mice; this effect is reversed with an A2A adenosine receptor antagonist and abolished in A2A receptor knockout mice. These studies demonstrate that CBD has the ability to enhance adenosine signaling through inhibition of uptake and provide a non-cannabinoid receptor mechanism by which CBD can decrease inflammation.adenosine ͉ lipopolysaccharide ͉ tetrahydrocannabinol ͉ thymidine ͉ tumor necrosis factor-␣ T he marijuana-derived cannabinoids ⌬ 9 -tetrahydrocannabinol (THC) and cannabidiol (CBD) each have immunosuppressive effects (1) and are currently in clinical trials for treatment of multiple sclerosis (2). Although the CB 2 cannabinoid receptor partially mediates the antiinflammatory effects of THC (3), CBD does not bind well to the known cannabinoid receptors (4); this low affinity results in the inability of CBD to produce the subjective ''high'' and cognitive effects that are characteristic of marijuana and THC (5, 6). To date, the mechanism by which CBD decreases inflammation is unknown, although micromolar concentrations of CBD have been shown to inhibit lipoxygenase activity (7).In addition to multiple sclerosis, CBD has shown promise in several rodent models of inflammation. Oral treatment with CBD decreases edema and hyperalgesia in a rat paw model of carrageenan-induced inflammation (8). A single dose of CBD also decreases serum TNF␣ production in lipopolysaccharide (LPS)-treated mice (9). CBD improves arthritis symptoms and joint pathology in murine collagen-induced arthritis while inhibiting IFN-␥ production and lymph node cell proliferation, as measured by [ 3 H]thymidine incorporation (9). Treatment with THC also decreases proliferation of lymph node cells as well as splenocytes (10).Given the potential of THC and CBD to decrease proliferation in immune cells, we initially examined the effects of several natural and synthetic cannabinoids on microglial cell proliferation by using [ 3 H]thymidine incorporation. Microglia are the resident immune cells of the brain, and their proliferation has been linked to a number of neurodegenerative diseases (11). However, CBD and THC decreased [ 3 H]thymidine incorporation into EOC-20 microglial cells with no effect on cell cycle. Further experiments demonstrate that this decrease in [ 3 H]thymidine incor...

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“…uridine, adenosine, etc.) and their structural analogs (4,5). ENT1 and ENT2 play important roles in nucleoside salvage pathways, regulation of adenosine signaling, and cellular disposition of anticancer and antiviral nucleoside analogs (4,5).…”

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confidence: 99%

“…and their structural analogs (4,5). ENT1 and ENT2 play important roles in nucleoside salvage pathways, regulation of adenosine signaling, and cellular disposition of anticancer and antiviral nucleoside analogs (4,5). ENT3 is an intracellular nucleoside transporter, which may play a role in lysosomal transport of nucleosides (6).…”

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confidence: 99%

Molecular Determinants of Substrate Selectivity of a Novel Organic Cation Transporter (PMAT) in the SLC29 Family

Zhou¹,

Engel³

et al. 2007

Journal of Biological Chemistry

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Plasma membrane monoamine transporter (PMAT or ENT4) is a newly cloned transporter assigned to the equilibrative nucleoside transporter (ENT) family (SLC29). Unlike ENT1-3, PMAT mainly functions as a polyspecific organic cation transporter. In this study, we investigated the molecular mechanisms underlying the unique substrate selectivity of PMAT. By constructing chimeras between human PMAT and ENT1, we showed that a chimera consisting of transmembrane domains (TM) 1-6 of PMAT and TM7-11 of hENT1 behaved like PMAT, transporting 1-methyl-4-phenylpyridinium (MPP ؉ , an organic cation) but not uridine (a nucleoside), suggesting that TM1-6 contains critical domains responsible for substrate recognition. To identify residues important for the cation selectivity of PMAT, 10 negatively charged residues were chosen and substituted with alanine. Five of the alanine mutants retained PMAT activity, and four were non-functional due to impaired targeting to the plasma membrane. However, alanine substitution at Glu 206 in TM5 abolished PMAT activity without affecting cell surface expression. Eliminating the charge at Glu 206 (E206Q) resulted in loss of organic cation transport activity, whereas conserving the negative charge (E206D) restored transporter function. Interestingly, mutant E206Q, which possesses the equivalent residue in ENT1, gained uridine transport activity. Thr 220 , another residue in TM5, also showed an effect on PMAT activity. Helical wheel analysis of TM5 revealed a distinct amphipathic pattern with Glu 206 and Thr 220 clustered in the center of the hydrophilic face. In summary, our results suggest that Glu 206 functions as a critical charge sensor for cationic substrates and TM5 forms part of the substrate permeation pathway in PMAT.Membrane transporters play pivotal roles in sustaining the normal life of cells (1, 2). The solute carrier (SLC) 4 proteins constitute a large series of membrane transporters currently consisting of 360 members in 46 gene families in humans (1). Transporters from the same SLC gene family share at least 20 -25% amino acid sequence identity and are thought to be evolved from a common ancestor (1, 2). Whereas it is generally anticipated that transporters from the same gene family possess similar functional properties, several SLC families appear to consist of members with great functional diversity (2, 3). For example, the Na ϩ -glucose transporter family, SLC5, comprises not only Na ϩ -coupled glucose transporters, but also transporters for iodide, choline, vitamins, and short-chain fatty acids (3).We recently discovered an interesting functional divergence in the equilibrative nucleoside transporter (ENT) family, SLC29. The human and rodent genomes encode four SLC29 isoforms, SLC29A1-4. SLC29A1-3, known as ENT1-3, are nucleoside transporters that specifically transport nucleosides (e.g. uridine, adenosine, etc.) and their structural analogs (4, 5). ENT1 and ENT2 play important roles in nucleoside salvage pathways, regulation of adenosine signaling, and cellular disposition of a...

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(Section A: Molecular, Structural, and Cellular Biology of Drug Transporters) Mammalian Nucleoside Transporters

Cited by 218 publications

References 135 publications

Identification and Characterization of a Novel Monoamine Transporter in the Human Brain

Identification and Characterization of a Novel Monoamine Transporter in the Human Brain

Inhibition of an equilibrative nucleoside transporter by cannabidiol: A mechanism of cannabinoid immunosuppression

Molecular Determinants of Substrate Selectivity of a Novel Organic Cation Transporter (PMAT) in the SLC29 Family

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