Sodium-dependent nucleoside transport in choroid plexus from rabbit. Evidence for a single transporter for purine and pyrimidine nucleosides

Modulation of neuroimmunity by adenosine and its receptors: metabolism to mental illness

Adenosine is a pleiotropic bioactive with potent neuromodulatory properties. Due to its ability to easily cross the blood-brain barrier, it can act as a signaling molecule between the periphery and the brain. It functions through four (A1, A2A, A2B, and A3) cell surface G protein-coupled adenosine receptors (ARs) that are expressed in some combination on nearly all cells types within the CNS. By regulating the activity of adenylyl cyclase and changing the intracellular concentration of cAMP, adenosine can alter neuronal function and neurotransmission. A variety of illnesses related to metabolic dysregulation, such as type 1 diabetes and Alzheimer's disease, are associated with an elevated serum concentration of adenosine and a pathogenesis rooted in inflammation. This review describes the accepted physiologic function of adenosine in neurological disease and explores its new potential as a peripheral to central danger signal that can activate the neuroimmune system and contribute to symptoms of sickness and psychopathologies.

Functional impairment of rat taurine transporter by activation of nitrogen oxide through superoxide

The objective of this study was to identify the nitrogen oxide form(s) involved in the functional impairment of the rat taurine transport system. Taurine uptake activity in the rat renal brush border membrane vesicle (RBBMV) preparation or Xenopus laevis oocytes that express the rat taurine transporter was compared after the pretreatment with nitrogen oxide donors from which nitric oxide (NO) is released at different rates. The functional impairment was associated with a reduced Vmax, but did not involve an alteration in the Km, of taurine uptake in the RBBMV preparation that had been pretreated with sodium nitroprusside, a slow release nitric oxide (NO) donor. When the preparation was pretreated with S-nitroso-N-acetyl penicillamine, a rapid release NO donor, the activity of taurine uptake was unaffected. The activity was not statistically different from the control after the pretreatment with sodium nitroprusside and superoxide dismutase. Consistent with the study with RBBMV, a similar alteration in the activity of taurine uptake by NO donors was observed in oocytes expressing the transporter. Considering the fact that peroxynitrite, a highly reactive nitrogen oxide form, is formed by the reaction between NO and superoxide, the taurine transporter, and probably other transport systems as well, may be functionally impaired by peroxynitrite.

Cation coupling properties of human concentrative nucleoside transporters hCNT1, hCNT2 and hCNT3

The SLC28 family of concentrative nucleoside transporter (CNT) proteins in mammalian cells contains members of two distinct phylogenic subfamilies. In humans, hCNT1 and hCNT2 belong to one subfamily, and hCNT3 to the other. All three CNTs mediate inwardly-directed Na(+)/nucleoside cotransport, and are either pyrimidine nucleoside-selective (hCNT1), purine nucleoside-selective (hCNT2), or broadly selective for both pyrimidine and purine nucleosides (hCNT3). While previous studies have characterized cation interactions with both hCNT1 and hCNT3, little is known about the corresponding properties of hCNT2. In the present study, heterologous expression in Xenopus oocytes in combination with radioisotope flux and electrophysiological techniques has allowed us to undertake a side-by-side comparison of hCNT2 with other hCNT family members. Apparent K (50) values for Na(+) activation were voltage-dependent, and similar in magnitude for all three transporters. Only hCNT3 was also able to couple transport of uridine to uptake of H(+). The Na(+)/nucleoside stoichiometry of hCNT2, as determined from both Hill coefficients and direct charge/flux measurements, was 1:1. This result was the same as for hCNT1, but different from that of hCNT3 (2:1). The charge-to-(22)Na(+) uptake stoichiometry was 1:1 for all three hCNTs. In parallel with their division into two separate CNT subfamilies, hCNT2 shares common cation specificity and coupling characteristics with hCNT1, which differ markedly from those of hCNT3.

Synapse formation is enhanced by oral administration of uridine and DHA, the circulating precursors of brain phosphatides

OBJECTIVE

The loss of cortical and hippocampal synapses is a universal hallmark of Alzheimer's disease, and probably underlies its effects on cognition. Synapses are formed from the interaction of neurites projecting from "presynaptic" neurons with dendritic spines projecting from "postsynaptic" neurons. Both of these structures are vulnerable to the toxic effects of nearby amyloid plaques, and their loss contributes to the decreased number of synapses that characterize the disease. A treatment that increased the formation of neurites and dendritic spines might reverse this loss, thereby increasing the number of synapses and slowing the decline in cognition.

DESIGN SETTING, PARTICIPANTS, INTERVENTION, MEASUREMENTS AND RESULTS

We observe that giving normal rodents uridine and the omega-3 fatty acid docosahexaenoic acid (DHA) orally can enhance dendritic spine levels (3), and cognitive functions (32). Moreover this treatment also increases levels of biochemical markers for neurites (i.e., neurofilament-M and neurofilament-70) (2) in vivo, and uridine alone increases both these markers and the outgrowth of visible neurites by cultured PC-12 cells (9). A phase 2 clinical trial, performed in Europe, is described briefly.

DISCUSSION AND CONCLUSION

Uridine and DHA are circulating precursors for the phosphatides in synaptic membranes, and act in part by increasing the substrate-saturation of enzymes that synthesize phosphatidylcholine from CTP (formed from the uridine, via UTP) and from diacylglycerol species that contain DHA: the enzymes have poor affinities for these substrates, and thus are unsaturated with them, and only partially active, under basal conditions. The enhancement by uridine of neurite outgrowth is also mediated in part by UTP serving as a ligand for neuronal P2Y receptors. Moreover administration of uridine with DHA activates many brain genes, among them the gene for the m-1 metabotropic glutamate receptor [Cansev, et al, submitted]. This activation, in turn, increases brain levels of that gene's protein product and of such other synaptic proteins as PSD-95, synapsin-1, syntaxin-3 and F-actin, but not levels of non-synaptic brain proteins like beta-tubulin. Hence it is possible that giving uridine plus DHA triggers a neuronal program that, by accelerating phosphatide and synaptic protein synthesis, controls synaptogenesis. If administering this mix of phosphatide precursors also increases synaptic elements in brains of patients with Alzheimer 's disease, as it does in normal rodents, then this treatment may ameliorate some of the manifestations of the disease.

Oral administration of circulating precursors for membrane phosphatides can promote the synthesis of new brain synapses

Although cognitive performance in humans and experimental animals can be improved by administering omega-3 fatty acid docosahexaenoic acid (DHA), the neurochemical mechanisms underlying this effect remain uncertain. In general, nutrients or drugs that modify brain function or behavior do so by affecting synaptic transmission, usually by changing the quantities of particular neurotransmitters present within synaptic clefts or by acting directly on neurotransmitter receptors or signal-transduction molecules. We find that DHA also affects synaptic transmission in mammalian brain. Brain cells of gerbils or rats receiving this fatty acid manifest increased levels of phosphatides and of specific presynaptic or postsynaptic proteins. They also exhibit increased numbers of dendritic spines on postsynaptic neurons. These actions are markedly enhanced in animals that have also received the other two circulating precursors for phosphatidylcholine, uridine (which gives rise to brain uridine diphosphate and cytidine triphosphate) and choline (which gives rise to phosphocholine). The actions of DHA aere reproduced by eicosapentaenoic acid, another omega-3 compound, but not by omega-6 fatty acid arachidonic acid. Administration of circulating phosphatide precursors can also increase neurotransmitter release (acetylcholine, dopamine) and affect animal behavior. Conceivably, this treatment might have use in patients with the synaptic loss that characterizes Alzheimer's disease or other neurodegenerative diseases or occurs after stroke or brain injury.

Uridine and cytidine in the brain: their transport and utilization

The pyrimidines cytidine (as CTP) and uridine (which is converted to UTP and then CTP) contribute to brain phosphatidylcholine and phosphatidylethanolamine synthesis via the Kennedy pathway. Their uptake into brain from the circulation is initiated by nucleoside transporters located at the blood-brain barrier (BBB), and the rate at which uptake occurs is a major factor determining phosphatide synthesis. Two such transporters have been described: a low-affinity equilibrative system and a high-affinity concentrative system. It is unlikely that the low-affinity transporter contributes to brain uridine or cytidine uptake except when plasma concentrations of these compounds are increased several-fold experimentally. CNT2 proteins, the high-affinity transporters for purines like adenosine as well as for uridine, have been found in cells comprising the BBB of rats. However, to date, no comparable high-affinity carrier protein for cytidine, such as CNT1, has been detected at this location. Thus, uridine may be more available to brain than cytidine and may be the major precursor in brain for both the salvage pathway of pyrimidine nucleotides and the Kennedy pathway of phosphatide synthesis. This recognition may bear on the effects of cytidine or uridine sources in neurodegenerative diseases.

Nucleoside transport in primary cultured rabbit tracheal epithelial cells

The present study aimed at elucidating the mechanisms of nucleoside transport in primary cultured rabbit tracheal epithelial cells (RTEC) grown on a permeable filter support. Uptake of (3)H-uridine, the model nucleoside substrate, from the apical fluid of primary cultured RTEC was examined with respect to its dependence on Na(+), substrate concentration, temperature and its sensitivity to inhibitors, other nucleosides and antiviral nucleoside analogs. Apical (3)H-uridine uptake in primary cultured RTEC was strongly dependent on an inward Na(+) gradient and temperature. Ten micromolar nitro-benzyl-mercapto-purine-ribose (NBMPR) (an inhibitor of es-type nucleoside transport in the nanomolar range) did not further inhibit this process. (3)H-uridine uptake from apical fluid was inhibited by basolateral ouabain (10 microM) and apical phloridzin (100 microM), indicating that uptake may involve a secondary active transport process. Uridine uptake was saturable with a K(m) of 3.4 +/- 1.8 microM and the V(max) of 24.3 +/- 5.2 pmoles/mg protein/30 s. Inhibition studies indicated that nucleoside analogs that have a substitution on the nucleobase competed with uridine uptake from apical fluid, but those with modifications on the ribose sugar including acyclic analogs were ineffective. The pattern of inhibition of apical (3)H-uridine, (3)H-inosine and (3)H-thymidine uptake into RTEC cells by physiological nucleosides was consistent with multiple systems: A pyrimidine-selective transport system (CNT1); a broad nucleoside substrate transport system that excludes inosine (CNT4) and an equilibrative NBMPR-insensitive nucleoside transport system (ei type). These results indicate that the presence of apically located nucleoside transporters in the epithelial cells lining the upper respiratory tract can lead to a high accumulation of nucleosides in the trachea. At least one Na(+)-dependent, secondary, active transport process may mediate the apical absorption of nucleosides or analogous molecules.

Polarized distribution of nucleoside transporters in rat brain endothelial and choroid plexus epithelial cells

This study investigated mRNA expression and protein localization of equilibrative and concentrative nucleoside transporters (ENTs, CNTs) in primary cultures of rat brain endothelial cells (RBEC) and rat choroid plexus epithelial cells (RCPEC). Reverse transcriptase PCR analysis revealed that RBEC and RCPEC contained mRNA for rENT1, rENT2 and rCNT2 and for rENT1, rENT2, rCNT2 and rCNT3, respectively. Immunoblotting of membrane fractions of RBEC, fresh RCPEC and primary cultures of RCPEC revealed the presence of rENT1, rENT2 and rCNT2 proteins in all samples. Measurement of [14C]adenosine uptake into cells grown as monolayers on permeable plastic supports revealed a polarized distribution of Na+-dependent adenosine uptake in that CNT activity was associated exclusively in membranes of RBEC facing the lower chamber (which corresponds to the surface facing the interstitial fluid in situ) and in membranes of RCPEC facing the upper chamber (which corresponds to the surface facing the cerebrospinal fluid in situ). In both RBEC and RCPEC, adenosine uptake from the opposite chambers was Na+-independent and partially inhibited by nitrobenzylthioinosine, indicating the presence of the equilibrative sensitive transporter rENT1.

Oral uridine-5'-monophosphate (UMP) increases brain CDP-choline levels in gerbils

We examined the biochemical pathways whereby oral uridine-5'-monophosphate (UMP) increases membrane phosphatide synthesis in brains of gerbils. We previously showed that supplementing PC12 cells with uridine caused concentration-related increases in CDP-choline levels, and that this effect was mediated by elevations in intracellular uridine triphosphate (UTP) and cytidine triphosphate (CTP). In the present study, adult gerbils received UMP (1 mmol/kg), a constituent of human breast milk and infant formulas, by gavage, and plasma samples and brains were collected for assay between 5 min and 8 h thereafter. Thirty minutes after gavage, plasma uridine levels were increased from 6.6 +/- 0.58 to 32.7 +/- 1.85 microM (P < 0.001), and brain uridine from 22.6 +/- 2.9 to 89.1 +/- 8.82 pmol/mg tissue (P < 0.001). UMP also significantly increased plasma and brain cytidine levels; however, both basally and following UMP, these levels were much lower than those of uridine. Brain UTP, CTP, and CDP-choline were all elevated 15 min after UMP (from 254 +/- 31.9 to 417 +/- 50.2, [P < 0.05]; 56.8 +/- 1.8 to 71.7 +/- 1.8, [P < 0.001]; and 11.3 +/- 0.5 to 16.4 +/- 1, [P < 0.001] pmol/mg tissue, respectively), returning to basal levels after 20 and 30 min. The smallest UMP dose that significantly increased brain CDP-choline was 0.05 mmol/kg. These results show that oral UMP, a uridine source, enhances the synthesis of CDP-choline, the immediate precursor of PC, in gerbil brain.

The Broadly Selective Human Na+/Nucleoside Cotransporter(hCNT3) Exhibits Novel Cation-coupled Nucleoside TransportCharacteristics

The concentrative nucleoside transporter (CNT) protein family in humans is represented by three members, hCNT1, hCNT2, and hCNT3. hCNT3, a Na+/nucleoside symporter, transports a broad range of physiological purine and pyrimidine nucleosides as well as anticancer and antiviral nucleoside drugs, and belongs to a different CNT subfamily than hCNT1/2. H+-dependent Escherichia coli NupC and Candida albicans CaCNT are also CNT family members. The present study utilized heterologous expression in Xenopus oocytes to investigate the specificity, mechanism, energetics, and structural basis of hCNT3 cation coupling. hCNT3 exhibited uniquely broad cation interactions with Na+, H+, and Li+ not shared by Na+-coupled hCNT1/2 or H+-coupled NupC/CaCNT. Na+ and H+ activated hCNT3 through mechanisms to increase nucleoside apparent binding affinity. Direct and indirect methods demonstrated cation/nucleoside coupling stoichiometries of 2:1 in the presence of Na+ and both Na+ plus H+, but only 1:1 in the presence of H+ alone, suggesting that hCNT3 possesses two Na+-binding sites, only one of which is shared by H+. The H+-coupled hCNT3 did not transport guanosine or 3'-azido-3'-deoxythymidine and 2',3'-dideoxycytidine, demonstrating that Na+- and H+-bound versions of hCNT3 have significantly different conformations of the nucleoside binding pocket and/or translocation channel. Chimeric studies between hCNT1 and hCNT3 located hCNT3-specific cation interactions to the C-terminal half of hCNT3, setting the stage for site-directed mutagenesis experiments to identify the residues involved.

The Choroid Plexus‐Cerebrospinal Fluid System: From Development to Aging

The function of the cerebrospinal fluid (CSF) and the tissue that secretes it, the choroid plexus (CP), has traditionally been thought of as both providing physical protection to the brain through buoyancy and facilitating the removal of brain metabolites through the bulk drainage of CSF. More recent studies suggest, however, that the CP-CSF system plays a much more active role in the development, homeostasis, and repair of the central nervous system (CNS). The highly specialized choroidal tissue synthesizes trophic and angiogenic factors, chemorepellents, and carrier proteins, and is strategically positioned within the ventricular cavities to supply the CNS with these biologically active substances. Through polarized transport systems and receptor-mediated transcytosis across the choroidal epithelium, the CP, a part of the blood-CSF barrier (BCSFB), controls the entry of nutrients, such as amino acids and nucleosides, and peptide hormones, such as leptin and prolactin, from the periphery into the brain. The CP also plays an important role in the clearance of toxins and drugs. During CNS development, CP-derived growth factors, such as members of the transforming growth factor-beta superfamily and retinoic acid, play an important role in controlling the patterning of neuronal differentiation in various brain regions. In the adult CNS, the CP appears to be critically involved in neuronal repair processes and the restoration of the brain microenvironment after traumatic and ischemic brain injury. Furthermore, recent studies suggest that the CP acts as a nursery for neuronal and astrocytic progenitor cells. The advancement of our knowledge of the neuroprotective capabilities of the CP may therefore facilitate the development of novel therapies for ischemic stroke and traumatic brain injury. In the later stages of life, the CP-CSF axis shows a decline in all aspects of its function, including CSF secretion and protein synthesis, which may in themselves increase the risk for development of late-life diseases, such as normal pressure hydrocephalus and Alzheimer's disease. The understanding of the mechanisms that underlie the dysfunction of the CP-CSF system in the elderly may help discover the treatments needed to reverse the negative effects of aging that lead to global CNS failure.

Interaction of nucleoside analogues with nucleoside transporters in rat brain endothelial cells

A number of nucleoside analogues, consisting of antiviral compounds and agents designed as adenosine A1 receptor agonists, were examined for nucleoside transporter affinity using an in vitro model of the blood-brain barrier (BBB), the rat brain endothelial cell line, RBE4. Structure-activity relationships (SAR) were also performed to identify the key structural requirements for transporter recognition and the suitability of these systems for carrier-mediated strategies to deliver therapeutics across the BBB. Adenosine receptor agonists did not show transport affinity for concentrative nucleoside carriers, but exhibited affinity for equilibrative systems (Ki=10.8-97.9 microM) within the range of Kms for natural substrates. However, none of the antiviral compounds tested in this study showed affinity for either class of nucleoside transporter. SAR studies suggest that the hydroxyl group located at the 3'-position of the ribose moiety is an essential requirement for transporter recognition. This may explain the inability of nucleoside derived anti-viral compounds to use these systems despite the significant structural homology with naturally occurring nucleosides. Sites have also been identified which accommodate structural additions with retention of carrier affinity, suggesting that compounds which fail to penetrate the BBB could be attached to these sites for carrier-mediated delivery using a prodrug strategy.

Nucleoside transporters in the disposition and targeting of nucleoside analogs in the kidney

Systemic disposition of nucleosides and nucleoside analogs is dependent on renal handling of these compounds. There are five known, functionally characterized nucleoside transporters with varying substrate specificities for nucleosides: concentrative nucleoside transporters (CNT1-CNT3; Solute Carrier (SLC) 28A1-28A3), which mediate the intracellular flux of nucleosides, and equilibrative nucleoside transporters (ENT1-ENT2; SLC29A1-SLC29A2), which mediate bi-directional facilitated diffusion of nucleosides. All five of these transporters are expressed in the kidney. Concentrative nucleoside transporters primarily localize to the apical membrane of renal epithelial cells while equilibrative nucleoside transporters primarily localize to the basolateral membrane. These transporters work in concert to mediate reabsorptive flux of naturally occurring nucleosides and nucleoside analogs. In addition, equilibrative transporters also participate in secretory flux of some nucleoside analogs. Nucleoside transporters also serve in the targeting of nucleoside analog therapies to renal tumors. This review examines the role that these transporters play in renal disposition of nucleosides and nucleoside analogs in both systemic and kidney-specific therapies.

Functional characterization of a H+/nucleoside co-transporter (CaCNT) from Candida albicans, a fungal member of the concentrative nucleoside transporter (CNT) family of membrane proteins

Human and other mammalian concentrative (Na(+)-linked) nucleoside transport proteins belong to a membrane protein family (CNT, TC 2.A.41) that also includes Escherichia coli H(+)-dependent nucleoside transport protein NupC. Here, we report the cDNA cloning and functional characterization of a CNT family member from the pathogenic yeast Candida albicans. This 608 amino acid residue H(+)/nucleoside symporter, designated CaCNT, contains 13 predicted transmembrane domains (TMs), but lacks the exofacial, glycosylated carboxyl-terminus of its mammalian counterparts. When produced in Xenopus oocytes, CaCNT exhibited transport activity for adenosine, uridine, inosine and guanosine but not cytidine, thymidine or the nucleobase hypoxanthine. Apparent K(m) values were in the range 16-64 micro M, with V(max) : K(m) ratios of 0.58-1.31. CaCNT also accepted purine and uridine analogue nucleoside drugs as permeants, including cordycepin (3'-deoxyadenosine), a nucleoside analogue with anti-fungal activity. Electrophysiological measurements under voltage clamp conditions gave a H(+) to [(14)C]uridine coupling ratio of 1 : 1. CaCNT, obtained from logarithmically growing cells, is the first described cation-coupled nucleoside transporter in yeast, and the first member of the CNT family of proteins to be characterized from a unicellular eukaryotic organism.

Coupling of CFTR-mediated anion secretion to nucleoside transporters and adenosine homeostasis in Calu-3 cells

The purpose of this study was to characterize the role of adenosine-dependent regulation of anion secretion in Calu-3 cells. RT-PCR studies showed that Calu-3 cells expressed mRNA for A2A and A2B but not A1 or A3 receptors, and for hENT1, hENT2 and hCNT3 but not hCNT1 or hCNT2 nucleoside transporters. Short-circuit current measurements indicated that A2B receptors were present in both apical and basolateral membranes, whereas A2A receptors were detected only in basolateral membranes. Uptake studies demonstrated that the majority of adenosine transport was mediated by hENT1, which was localized to both apical and basolateral membranes, with a smaller hENT2-mediated component in basolateral membranes. Whole-cell current measurements showed that application of extracellular nitrobenzylmercaptopurine ribonucleoside (NBMPR), a selective inhibitor of hENT1-mediated transport, had similar effects on whole-cell currents as the application of exogenous adenosine. Inhibitors of adenosine kinase and 5'-nucleotidase increased and decreased, respectively, whole-cell currents, whereas inhibition of adenosine deaminase had no effect. Single-channel studies showed that NBMPR and adenosine kinase inhibitors activated CFTR Cl- channels. These results suggested that the equilibrative nucleoside transporters (hENT1, hENT2) together with adenosine kinase and 5'-nucleotidase play a crucial role in the regulation of CFTR through an adenosine-dependent pathway in human airway epithelia.

Mechanism of the reduced elimination clearance of benzylpenicillin from cerebrospinal fluid in rats with intracisternal administration of lipopolysaccharide

The mechanism responsible for the reduced clearance of benzylpenicillin (BPC) from the cerebrospinal fluid (CSF) was investigated in rats that received an intracisternal administration of lipopolysaccharide (LPS). BPC was intraventricularly injected and its elimination from the CSF studied. During the inflammation created by the LPS administration to the cisterna magna, the clearance of BPC and taurine from the CSF was significantly reduced but reverted to the control level when N-nitro-L-arginine, a nitric oxide (NO) synthase inhibitor, was intracisternally administered. The in vitro uptake of BPC and taurine was significantly reduced in the choroid plexus (CP, the blood-CSF barrier) of rats with experimental inflammation and in control CP that had been pretreated with sodium nitroprusside (SNP, an NO donor). Interestingly, the clearance and CP uptake of formycin B, a substrate for a nucleoside transporter, were not affected by the experimental inflammation or by pretreatement with SNP. These observations suggest that the BPC transporter, and probably other transport systems as well, is functionally sensitive to NO in the blood-CSF barrier. Therefore, functional impairment of BPC transport in the CP by NO may be partly responsible for the increase in BPC concentration in the CSF during inflammation such as that caused by meningitis.

Transport of the organic cations gonyautoxin 2/3 epimers, a paralytic shellfish poison toxin, through the human and rat intestinal epitheliums

The aim of this work is to study the mechanisms involved in gonyautoxins (GTXs) intestinal absorption. For this purpose, we studied the transport of GTX 2/3 epimers by intestinal epithelial cell lines (IEC-6 and Caco-2) cultured on polycarbonate filters. Specific transport was calculated by subtracting from the flux of GTX 2/3 measured at 37 degrees C that occurring at 4 degrees C, this being an indication of transcellular transport. The transcellular apical-to-basolateral (A-B) flux in Caco-2 cell monolayers, was greater than that in the opposite direction, suggesting the involvement of an active transport system favoring the absorption of the toxin. However, in IEC-6 cells the transcellular basolateral-to-apical (B-A) specific transport of the toxin was greater than that in the opposite direction. The A-B and B-A fluxes were, respectively, 127 +/- 26 and 205 +/- 23 nmol/min, suggesting the presence of a prevalent secretive process of the toxin in IEC-6 cells. The A-B transport of GTX 2/3 epimers in Caco-2 cells, but not in IEC-6 cells, was partially Na(+)-dependent and significantly inhibited by adenosine. TEA and verapamil in both Caco-2 and IEC-6 cells failed to affect the A-B and B-A transport of GTX 2/3 epimers. Cyanine in IEC-6 cells, but not in Caco-2 cells, increased the A-B flux of the toxin, suggesting the involvement of the organic cation transporter in the absorption of GTX 2/3 epimers. The mitochondrial energetic uncoupler 2,4-dinitrophenol significantly inhibited the A-B and the B-A transport in both cell lines. In conclusion, IEC-6 cells secrete actively the toxins, whereas Caco-2 cells were found to absorb the toxins in a process that was inhibited in the presence of adenosine and the absorption was dependent of Na(+).

An ancient prevertebrate Na+-nucleoside cotransporter (hfCNT) from the Pacific hagfish (Eptatretus stouti)

The human concentrative (Na+-linked) plasma membrane transport proteins hCNT1, hCNT2, and hCNT3 are pyrimidine nucleoside-selective (system cit), purine nucleoside-selective (system cif), or broadly selective for both pyrimidine and purine nucleosides (system cib), respectively. All have orthologs in other mammalian species and belong to a gene family (CNT) that has members in insects, nematodes, pathogenic yeast, and bacteria. Here, we report the cDNA cloning and functional characterization of a CNT family member from an ancient marine prevertebrate, the Pacific hagfish (Eptatretus stouti). This Na+-nucleoside symporter, designated hfCNT, is the first transport protein to be characterized in detail in hagfish and is a 683-amino acid residue protein with 13 predicted transmembrane helical segments (TMs). hfCNT was 52, 50, and 57% identical in sequence to hCNT1, hCNT2, and hCNT3, respectively. Similarity to hCNT3 was particularly marked in the TM 4-13 region. When produced in Xenopus oocytes, hfCNT exhibited the transport properties of system cib, with uridine, thymidine, and inosine apparent K(m) values of 10-45 microM. The antiviral nucleoside drugs 3'-azido-3'-deoxythymidine, 2',3'-dideoxycytidine, and 2',3'-dideoxyinosine were also transported. Simultaneous measurement of uridine-evoked currents and radiolabeled uridine uptake under voltage-clamp conditions gave a Na+-to-uridine coupling ratio of 2:1 (cf. 2:1 for hCNT3 and 1:1 for hCNT1/2). The apparent K50 value for Na+ activation was >100 mM. A 50:50 chimera between hfCNT and hCNT1 (TMs 7-13 of hfCNT replaced by those of hCNT1) exhibited hCNT1-like cation interactions, establishing that the structural determinants of cation stoichiometry and binding affinity were located within the carboxy-terminal half of the protein. The high degree of sequence similarity between hfCNT and hCNT3 may indicate functional constraints on the primary structure of the transporter and suggests that cib-type CNTs fulfill important physiological functions.

Regulation of K(+) current in human airway epithelial cells by exogenous and autocrine adenosine

The regulatory actions of adenosine on ion channel function are mediated by four distinct membrane receptors. The concentration of adenosine in the vicinity of these receptors is controlled, in part, by inwardly directed nucleoside transport. The purpose of this study was to characterize the effects of adenosine on ion channels in A549 cells and the role of nucleoside transporters in this regulation. Ion replacement and pharmacological studies showed that adenosine and an inhibitor of human equilibrative nucleoside transporter (hENT)-1, nitrobenzylthioinosine, activated K(+) channels, most likely Ca(2+)-dependent intermediate-conductance K(+) (I(K)) channels. A(1) but not A(2) receptor antagonists blocked the effects of adenosine. RT-PCR studies showed that A549 cells expressed mRNA for I(K)-1 channels as well as A(1), A(2A), and A(2B) but not A(3) receptors. Similarly, mRNA for equilibrative (hENT1 and hENT2) but not concentrative (hCNT1, hCNT2, and hCNT3) nucleoside transporters was detected, a result confirmed in functional uptake studies. These studies showed that adenosine controls the function of K(+) channels in A549 cells and that hENTs play a crucial role in this process.

Characterization of nucleoside transport systems in cultured rat epididymal epithelium

The nucleoside transport systems in cultured epididymal epithelium were characterized and found to be similar between the proximal (caput and corpus) and distal (cauda) regions of the epididymis. Functional studies revealed that 70% of the total nucleoside uptake was Na(+) dependent, while 30% was Na(+) independent. The Na(+)-independent nucleoside transport was mediated by both the equilibrative nitrobenzylthioinosine (NBMPR)-sensitive system (40%) and the NBMPR-insensitive system (60%), which was supported by a biphasic dose response to NBMPR inhibition. The Na(+)-dependent [(3)H]uridine uptake was selectively inhibited 80% by purine nucleosides, indicating that the purine nucleoside-selective N1 system is predominant. Since Na(+)-dependent [(3)H]guanosine uptake was inhibited by thymidine by 20% and Na(+)-dependent [(3)H]thymidine uptake was broadly inhibited by purine and pyrimidine nucleosides, this suggested the presence of the broadly selective N3 system accounting for 20% of Na(+)-dependent nucleoside uptake. Results of RT-PCR confirmed the presence of mRNA for equilibrative nucleoside transporter (ENT) 1, ENT2, and concentrative nucleoside transporter (CNT) 2 and the absence of CNT1. It is suggested that the nucleoside transporters in epididymis may be important for sperm maturation by regulating the extracellular concentration of adenosine in epididymal plasma.

A novel proton-dependent nucleoside transporter, CeCNT3, from Caenorhabditis elegans

In this study, we describe the cloning and characterization of a proton-dependent, broadly selective nucleoside transporter from Caenorhabditis elegans. Recently, we constructed a broadly selective nucleoside transporter which accepts both purine and pyrimidine nucleosides. Based on these studies, we hypothesized that CNTs with novel substrate selectivities exist in nature and that a CNT homolog in the C. elegans genomic database may function as a broadly selective nucleoside transporter. We cloned the cDNA for this transporter, termed CeCNT3 because of its broad selectivity, using polymerase chain reaction-based methods. CeCNT3 is predicted to have 575 amino acid residues (63.4 kDa) with 11 to 14 putative transmembrane domains and exhibits approximately 30% identity to members of the mammalian CNT family. This transporter exhibits a novel substrate selectivity, transporting a wide range of purine and pyrimidine nucleosides (inosine, guanosine, adenosine, uridine, and thymidine) but not cytidine. The apparent Km values for inosine and thymidine are 15.2 +/- 5.3 microM and 11.0 +/- 2.4 microM, respectively. Kinetic studies demonstrate that purine and pyrimidine nucleosides share a common recognition site in the transporter. In contrast to all known members of the mammalian CNT family, CeCNT3-mediated transport of nucleosides is proton-, but not sodium-, dependent. Mutation of tyrosine 332 in CeCNT3 decreased both the maximum uptake rate and apparent Km of thymidine, suggesting that this residue is in the domain of nucleoside recognition and translocation. The broad nucleoside specificity of CeCNT3 may be explained by this and other residues that restrict purine and pyrimidine nucleoside uptake and that discriminate among pyrimidine nucleosides.

Molecular identification and characterization of novel human and mouse concentrative Na+-nucleoside cotransporter proteins (hCNT3 and mCNT3) broadly selective for purine and pyrimidine nucleosides (system cib)

The human concentrative (Na(+)-linked) plasma membrane transport proteins hCNT1 and hCNT2 are selective for pyrimidine nucleosides (system cit) and purine nucleosides (system cif), respectively. Both have homologs in other mammalian species and belong to a gene family (CNT) that also includes hfCNT, a newly identified broad specificity pyrimidine and purine Na(+)-nucleoside symporter (system cib) from the ancient marine vertebrate, the Pacific hagfish (Eptatretus stouti). We now report the cDNA cloning and characterization of cib homologs of hfCNT from human mammary gland, differentiated human myeloid HL-60 cells, and mouse liver. The 691- and 703-residue human and mouse proteins, designated hCNT3 and mCNT3, respectively, were 79% identical in amino acid sequence and contained 13 putative transmembrane helices. hCNT3 was 48, 47, and 57% identical to hCNT1, hCNT2, and hfCNT, respectively. When produced in Xenopus oocytes, both proteins exhibited Na(+)-dependent cib-type functional activities. hCNT3 was electrogenic, and a sigmoidal dependence of uridine influx on Na(+) concentration indicated a Na(+):uridine coupling ratio of at least 2:1 for both hCNT3 and mCNT3 (cf 1:1 for hCNT1/2). Phorbol myristate acetate-induced differentiation of HL-60 cells led to the parallel appearance of cib-type activity and hCNT3 mRNA. Tissues containing hCNT3 transcripts included pancreas, bone marrow, trachea, mammary gland, liver, prostate, and regions of intestine, brain, and heart. The hCNT3 gene mapped to chromosome 9q22.2 and included an upstream phorbol myristate acetate response element.

Membrane transporters

Carrier-mediated drug transport is relatively unexplored in comparison with passive transcellular and paracellular drug transport. Yet, there is a host of transporter proteins that can be targeted for improving epithelial drug absorption. Generally, these are transport mechanisms for amino acids, dipeptides, monosaccharides, monocarboxylic acids, organic cations, phosphates, nucleosides, and water-soluble vitamins. Among them, the dipeptide transporter mechanism has received the most attention. Dipeptide transporters are H(+)-coupled, energy-dependent transporters that are known to play an essential role in the oral absorption of beta-lactam antibiotics, angiotensin-converting enzyme (ACE) inhibitors, renin inhibitors, and an anti-tumor drug, bestatin. Moreover, several investigators have demonstrated the utility of the dipeptide transporter as a platform for improving the oral bioavailability of drugs such as zidovudine and acyclovir through dipeptide prodrug derivatization. Thus far, at least four proton-coupled peptide transporters have been cloned. The first one cloned was PepT1 from the rabbit small intestine. The focus of this presentation will be structure-function, intracellular trafficking, and regulation of PepT1. Disease, dietary, and possible excipient influences on PepT1 function will also be discussed.

Involvement of multiple transporters in the oral absorption of nucleoside analogues

Many nucleoside analogues such as azt, ddI, ddC, d4T, 3TC, acv and vacv are currently being used in the treatment of patients infected with HIV, suffering from AIDS, or AIDS-related opportunistic infections. The transport of nucleoside analogues across the gastrointestinal tract is mediated by a number of transporters that fall into three broad categories, i.e., Na(+)-dependent concentrative transporters, Na(+)-independent equilibrative transporters and H(+)/peptide transporters. The first two transporter classes contain a large number of subtypes that are based on the substrate specificity. Recent studies have shown that most of the anti-HIV nucleoside analogues are transported by one or more of the nucleoside transporters. Furthermore, certain analogues, such as acv, appear to be absorbed by non-carrier-mediated diffusion, whereas vacv is apparently transported by non-nucleoside transporters (e.g., the oligopeptide transporter, PepT1 and possibly others). Thus, it is desirable to understand the precise nature of the absorption mechanism of these drugs to improve bioavailability and reduce the variability that is commonly observed in vivo in human patients. A complete understanding of the complex interactions of nucleoside analogues with the various transporters will help in designing better delivery systems and strategies to improve efficacy. In the current report, the mechanisms of nucleoside and nucleoside-analogue transport are reviewed. Also, methods of exploiting prodrugs to improve the bioavailability characteristics of drugs are highlighted.

Mechanisms of transport of nucleosides and nucleoside analogues in choroid plexus

Nucleoside and nucleobase analogues are being used to treat a number of viral infections of the central nervous system (CNS) including herpes-simplex encephalitis, cytomegalovirus retinitis, and AIDS-related dementia complex. Delivery of nucleoside analogues to the CNS is considered a key challenge in the treatment of these diseases. In this review, we focus on the mechanisms of transport of nucleosides and nucleoside analogues in the choroid plexus. First, the structure and function of the choroid plexus are reviewed. Then, we focus on the mechanisms of nucleoside transport in mammalian cells. Specific emphasis is placed on the molecular and functional characteristics of various nucleoside transporters. A discussion is then devoted to the mechanisms of nucleoside transport in choroid plexus. Current knowledge of nucleoside transport systems in choroid plexus in several animal species is summarized followed by a short section on the mechanisms of transport of nucleobases in the choroid plexus. Finally, a brief section on future directions is included.

Investigation of distribution, transport and uptake of anti-HIV drugs to the central nervous system

The distribution of currently available anti-HIV drugs into the CNS is reviewed with a focus on transport mechanisms. Among these drugs, nucleoside analogs are most well studied for their CNS distribution. The average reported values of the CSF/plasma steady-state concentration or corresponding AUC ratios are 0.23 (AZT), 0.06 (ddI), 0.04 (ddC), 0.49 (d4T), and 0.08 (3TC). Active efflux transport out of the CNS appears to be a predominant mechanism limiting nucleoside access to the CNS, although poor penetration may contribute to some extent for some polar nucleosides. The nature of the efflux pump for these drugs is speculated to be MRP-like transporter(s) in blood-brain and blood-CSF barriers. For non-nucleoside and protease inhibitors, much research remains to be done on the extent, time course, and mechanisms of their CNS distribution. The CNS penetration of some protease inhibitors is restricted by P-glycoprotein. A better understanding of transport mechanisms of anti-HIV drugs in the CNS is essential to develop approaches to enhance CNS delivery of available drugs and to identify new drugs less subject to active efflux transporter(s) in the CNS.

Identification of amino acid residues responsible for the pyrimidine and purine nucleoside specificities of human concentrative Na(+) nucleoside cotransporters hCNT1 and hCNT2

hCNT1 and hCNT2 mediate concentrative (Na(+)-linked) cellular uptake of nucleosides and nucleoside drugs by human cells and tissues. The two proteins (650 and 658 residues, 71 kDa) are 72% identical in sequence and contain 13 putative transmembrane helices (TMs). When produced in Xenopus oocytes, recombinant hCNT1 is selective for pyrimidine nucleosides (system cit), whereas hCNT2 is selective for purine nucleosides (system cif). Both transport uridine. We have used (i) chimeric constructs between hCNT1 and hCNT2, (ii) sequence comparisons with a newly identified broad specificity concentrative nucleoside transporter (system cib) from Eptatretus stouti, the Pacific hagfish (hfCNT), and (iii) site-directed mutagenesis of hCNT1 to identify two sets of adjacent residues in TMs 7 and 8 of hCNT1 (Ser(319)/Gln(320) and Ser(353)/Leu(354)) that, when converted to the corresponding residues in hCNT2 (Gly(313)/Met(314) and Thr(347)/Val(348)), changed the specificity of the transporter from cit to cif. Mutation of Ser(319) in TM 7 of hCNT1 to Gly enabled transport of purine nucleosides, whereas concurrent mutation of Gln(320) to Met (which had no effect on its own) augmented this transport. The additional mutation of Ser(353) to Thr in TM 8 converted hCNT1/S319G/Q320M, from cib to cif, but with relatively low adenosine transport activity. Additional mutation of Leu(354) to Val (which had no effect on its own) increased the adenosine transport capability of hCNT1/S319G/Q320M/S353T, producing a full cif-type transporter phenotype. On its own, the S353T mutation converted hCNT1 into a transporter with novel uridine-selective transport properties. Helix modeling of hCNT1 placed Ser(319) (TM 7) and Ser(353) (TM 8) within the putative substrate translocation channel, whereas Gln(320) (TM 7) and Leu(354) (TM 8) may exert their effects through altered helix packing.

Characterization of a bioengineered chimeric Na+-nucleoside transporter

Na+-dependent nucleoside transporters mediate the intracellular uptake of purine and pyrimidine nucleosides. The N1, N2, and N3 Na+-nucleoside transporters differ in substrate selectivity. N1 is purine-selective, N2 is pyrimidine-selective, and N3 is broadly selective. Recently, we created a chimeric transporter, T8, from the cloned rat N1 and N2 transporters. Whereas most chimeric proteins exhibit the characteristics of one of the two parent proteins, limited studies suggested that T8 possesses either a combined substrate selectivity of N1 and N2 or the selectivity of N3. The purpose of this study was to determine the substrate profile, transport mechanisms, and Na+-coupling stoichiometry of T8 and to compare these measurements with those of wild-type N1, N2, and N3. In Xenopus laevis oocytes expressing T8, Na+-dependent uptake of 3H-labeled purine (adenosine, inosine, and guanosine) and pyrimidine nucleosides (uridine, thymidine, and cytidine) was significantly enhanced (3.5-18.6-fold), which suggests that T8 accepts both purine and pyrimidine nucleosides as permeants. T8-mediated uptake of [3H]thymidine was competitively inhibited by inosine, and T8-mediated uptake of [3H]inosine was competitively inhibited by thymidine, which suggests that purine and pyrimidine nucleosides share a common binding site. Base-modified ribo- and 2'-deoxyribonucleosides were potent inhibitors of T8. In contrast, 2',3'-dideoxyinosine, 2',3'-dideoxycytidine, and 3'-azidothymidine, which are known inhibitors of N1 or N2, did not inhibit T8-mediated uptake. These data suggest that the substrate profile of T8 is not a combination of those of N1 and N2; rather, it is similar to that of N3. However, the Na+/nucleoside stoichiometric ratio of T8 was determined to be 1, consistent with both N1 and N2 but different from N3.

Serine 318 is essential for the pyrimidine selectivity of the N2 Na+-nucleoside transporter

Molecular cloning has isolated two subtypes of Na+-nucleoside transporters; one is pyrimidine-selective (N2), and the other is purine-selective (N1). Using chimeric rat N2/N1 transporters, we previously demonstrated that transmembrane domains (TM) 8 and 9 are the major sites for substrate binding and discrimination. Interestingly, when TM8 of N2 was replaced by that of N1, the resulting chimera, T8, lost the pyrimidine selectivity of N2 and accepted both purine and pyrimidine nucleosides. Five residues differ between rat N2 and N1 in TM8. To identify the critical residues responsible for transport selectivity, the five residues in N2 were systematically changed to their equivalents in N1. Replacing the serine residue at position 318 to its equivalent N1 residue, glycine, caused N2 to lose its selectivity for pyrimidine nucleosides and accept purine nucleosides as substrates. In contrast, replacing the other four residues did not change the pyrimidine selectivity of N2. Furthermore, when glycine 318 in chimera T8 was changed back to serine, the chimeric transporter regained pyrimidine selectivity. These observations suggest that serine 318 is located in the nucleoside permeation pathway and is responsible for the substrate selectivity of N2. An adjacent residue, glutamine 319, was found to be important in modulating the apparent affinity for nucleosides.

Nucleoside transporters of mammalian cells

In this review, we have summarized recent advances in our understanding of the biology of nucleoside transport arising from new insights provided by the isolation and functional expression of cDNAs encoding the major nucleoside transporters of mammalian cells. Nucleoside transporters are required for permeation of nucleosides across biological membranes and are present in the plasma membranes of most cell types. There is growing evidence that functional nucleoside transporters are required for translocation of nucleosides between intracellular compartments and thus are also present in organellar membranes. Functional studies during the 1980s established that nucleoside transport in mammalian cells occurs by two mechanistically distinct processes, facilitated diffusion and Na(+)-nucleoside cotransport. The determination of the primary amino acid sequences of the equilibrative and concentrative transporters of human and rat cells has provided a structural basis for the functional differences among the different transporter subtypes. Although nucleoside transporter proteins were first purified from human erythrocytes a decade ago, the low abundance of nucleoside transporter proteins in membranes of mammalian cells has hindered analysis of relationships between transporter structure and function. The molecular cloning of cDNAs encoding nucleoside transporters and the development of heterologous expression systems for production of recombinant nucleoside transporters, when combined with recombinant DNA technologies, provide powerful tools for characterization of functional domains within transporter proteins that are involved in nucleoside recognition and translocation. As relationships between molecular structure and function are determined, it should be possible to develop new approaches for optimizing the transportability of nucleoside drugs into diseased tissues, for development of new transport inhibitors, including reagents that are targeted to the concentrative transporters, and, eventually, for manipulation of transporter function through an understanding of the regulation of transport activity.

The transport of the anti-HIV drug, 2',3'-didehydro-3'-deoxythymidine (D4T), across the blood-brain and blood-cerebrospinal fluid barriers

1. The brain is a site of infection, viral replication and sanctuary for HIV-1. The treatment of HIV-1 infection therefore requires that an effective agent be delivered to the brain. 2',3'-Didehydro-3'-deoxythymidine (D4T) is a nucleoside analogue which has been shown to have beneficial clinical effects in the treatment of HIV infection. However, although D4T has been detected in human CSF, the ability of this drug to cross both the blood-brain and blood-cerebrospinal fluid (CSF) barriers and gain entrance into the brain tissue is not known. 2. This study examined the CNS entry of D4T by means of the bilateral vascular brain perfusion technique in the anaesthetized guinea-pig. 3. The results indicated that [3H]-D4T had a limited ability to cross the blood-brain barrier (BBB), which was not significantly greater than D-[14C]-mannitol (a slowly penetrating marker molecule). Although D4T was found to cross the blood-CSF barrier, the presence of D4T in the CSF did not reflect levels of the drug in the brain tissue. 4. These results can be related to the measured low lipophilicity of D4T, the higher paracellular permeability characteristics of the choroid plexus (blood-CSF barrier) compared to the BBB, and the sink action nature of the CSF to the brain tissue. 5. In conclusion, these animal studies suggest that D4T may only penetrate the brain tissue to a limited extent and consideration should be given to these findings in the clinical situation.

Endogenous nucleosides in the guinea-pig eye: analysis of transport and metabolites

This study investigates the transport of endogenous nucleosides and deoxynucleosides from the capillaries of the eye into the aqueous humour and the lens using the in situ vascular eye perfusion technique in the guinea-pig. The transport of [3H] adenosine and [3H] thymidine across the blood-aqueous barrier proved to be very rapid with a volume of distribution after 4 minutes perfusion reaching 11.9+/-3.0% and 9.93+/-1.1%, respectively. However, the transport of [3H] guanosine and [3H] cytidine was slower, with volumes of distribution reaching only 3.38+/-0.58% and 4.8+/-1.41%. The values for the entry of deoxyadenosine and deoxyguanosine were not significantly different from the values obtained for corresponding ribonucleosides (adenosine and guanosine) so that a change in the pentose sugar does not change the affinity of the nucleoside for the transport protein. Perfusion with a low sodium medium inhibited the transport of [3H] adenosine and [3H] thymidine into the aqueous humour. The presence of 800 nM NBTI also caused a decrease in adenosine transport into the aqueous humour, so that the volume of distribution after 2 minutes reached only 3.78+/-1.87%. These findings suggest that the transfer of adenosine across the blood-aqueous barrier has both concentrative and equilibrative components. The presence of 0.1 mM thymidine had no effect on the [3H] adenosine transport, whereas 0.1 mM of adenosine resulted in a marked decrease on the [3H] thymidine transport which suggests that the concentrative nucleotide transport is probably mediated by both cif and cit transport systems. The cellular uptake of nucleosides into the lens was very rapid and the volume of distribution of purine nucleosides was within the range of 30-50% whereas that for thymidine uptake was somewhat lower, reaching 20-30%. HPLC analysis of the eye structures in the guinea-pig showed that lens, vitreous body and the rest of the eye do not contain either free nucleosides or purine bases in detectable quantities, except for xanthine which was detected in aqueous humour at a concentration of 2.51+/-0.51 mM. However, serum of the anaesthetised guinea-pig did not contain xanthine in detectable amount so it seems that the metabolic degradation of the nucleosides in the guinea-pig eye progresses as far as xanthine, which is then accumulated in the aqueous humour.

Na(+)-dependent purine nucleoside transporter from human kidney: cloning and functional characterization

Many purine nucleosides and their analogs are actively transported in the kidney. Using homology cloning strategies and reverse transcriptase-polymerase chain reactions, we isolated a cDNA encoding a Na(+)-dependent nucleoside transporter, hSPNT1, from human kidney. Functional expression in Xenopus laevis oocytes identified hSPNT1 as a Na(+)-dependent nucleoside transporter that selectively transports purine nucleosides but also transports uridine. The Michaelis constant (K(m)) of uridine (80 microM) in interacting with hSPNT1 was substantially higher than that of inosine (4.5 microM). hSPNT1 (658 amino acids) is 81% identical to the previously cloned rat Na(+)-nucleoside transporter, SPNT, but differs markedly from SPNT in terms of its primary structure in the NH2 terminus. In addition, an Alu repetitive element (approximately 282 bp) is present in the 3'-untranslated region of the hSPNT1 cDNA. Northern analysis revealed that multiple transcripts of hSPNT1 are widely distributed in human tissues including human kidney. In contrast, rat SPNT transcripts are absent in kidney and highly localized to liver and intestine. The hSPNT1 gene was localized to chromosome 15. This is the first demonstration of a purine nucleoside transporter in human kidney.

Molecular determinants of substrate selectivity in Na+-dependent nucleoside transporters

In mammalian cells, the salvage of purine and pyrimidine nucleosides is mediated by both facilitated and Na+-dependent nucleoside transporters. These transporters also play important roles in the transmembrane flux of therapeutic nucleoside analogs, which are widely used in the treatment of cancer and viral infections. The N1, N2, and N3 Na+-dependent nucleoside transporters differ in terms of their transport selectivity for purine and pyrimidine nucleosides. N1 is purine-selective, N2 is pyrimidine-selective, and N3 is broadly selective. To identify structural domains involved in substrate binding and molecular determinants responsible for distinct transport selectivity, chimeric transporters were made from the cloned rat N1 and N2 transporters. Of the 14 transmembrane domains (TM) of N1 and N2, transplanting TM8-9 of N1 into N2 converted N2 from a pyrimidine- to a purine-selective transporter. Transplanting only TM8 generated a chimera with characteristics similar to the N3 transporter that has yet to be cloned. These data suggest that TM8-9 confer substrate selectivity and may form at least part of a substrate-binding site in Na+-dependent nucleoside transporters.

Na+-dependent gamma-aminobutyric acid (GABA) transport in the choroid plexus of rabbit

The goal of this study was to examine the mechanisms of transport of gamma-aminobutyric acid (GABA) in the choroid plexus. Choroid plexus slices from the rabbit were depleted of ATP with 2,4-dinitrophenol. GABA accumulated in the choroid plexus slices in a concentrative manner in the presence of an inwardly-directed Na+ gradient. Uptake occurred in the presence of Cl-; replacement of Cl- with gluconate abolished uptake. SCN-, NO3- or Br- were able to support uptake in the absence of Cl- to a significant extent (80, 68 and 61% of control, respectively). GABA uptake was saturable (Km of 37 +/- 8.5 microM, Vmax of 409 +/- 43 nmol/g/min). Na+-driven GABA uptake was inhibited by beta-alanine (IC50 = 22.9 microM) and hypotaurine (IC50 = 21.9 microM) but less potently by nipecotic acid (IC50 = 244 microM) and hydroxy-nipecotic acid (IC50 = 284 microM). Betaine, L-(2,4)-diaminobutyric acid, guvacine and 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol were weak inhibitors (IC50 > 500 microM). GABA inhibited Na+-driven uptake of taurine (IC50 = 230 microM); taurine, however, did not inhibit GABA uptake (IC50 > 1 mM). RT-PCR, using degenerate primers for cloned GABA transporters, did not result in the amplification of a band from rat choroid plexus RNA. The location of the choroid plexus in the ventricles of the brain, and its role in the secretion of the cerebrospinal fluid, suggest a role for the choroid plexus Na+-GABA transporter in the disposition of GABA in the brain.

The characteristics of basolateral nucleoside transport in the perfused sheep choroid plexus and the effect of nitric oxide inhibition on these processes

The single pass paired dilution technique was used to measure the uptake of nucleosides across the basolateral face of the isolated in situ perfused sheep choroid plexus (CP). The uptake of labelled adenosine and guanosine into the CP was large (approximately 35%) whereas that of thymidine was less (approximately 15%). The addition of 0.5 mM unlabelled adenosine to the perfusate inhibited the uptake of labelled adenosine by 66%, guanosine by 100% and that of thymidine by 50%, whereas the addition of 0.5 mM unlabelled thymidine caused complete self-inhibition. The backflux of adenosine was very small which may indicate a high rate of cellular metabolism or a flux into cerebrospinal fluid (CSF). The addition of 0.5 mM unlabelled adenosine did not alter the backflux of adenosine, but increased that of guanosine and thymidine. The entry of radioactivity derived from adenosine across the apical side of the CP cells into the newly formed CSF was determined as a 'CSF uptake index' relative to [14C]butanol and found to be about 25%; however, HPLC analysis revealed that the majority of this activity was hypoxanthine, and not adenosine. The complete inhibition of nitric oxide synthase caused a significant reduction in adenosine uptake into the CP and an increase in backflux for this molecule. It would appear that the uptake for adenosine by the CP is governed by the rate of cellular metabolism and not by the rate of transport into the cells of the choroid plexus whereas for guanosine and thymidine, transport is of greater importance.

Characterization of a novel Na+-dependent, guanosine-specific, nitrobenzylthioinosine-sensitive transporter in acute promyelocytic leukemia cells

NB4 cells are the only bona fide in vitro model of human acute promyelocytic leukemia. We have examined cytidine and guanosine transport in this cell line and characterized a novel guanosine-specific transporter. Cytidine transport occurred predominately by equilibrative nitrobenzylthioinosine (NBMPR)-sensitive (es) transport. In the presence of Na+, guanosine at various concentrations accumulated at least 6-fold above equilibrium. The initial rate of guanosine transport in Na+ buffer decreased by 75% with the addition of 1 microM NBMPR and the IC50 for NBMPR inhibition was 0.7 +/- 0.1 nM. Replacement of Na+ with choline also resulted in a 75% decrease in total guanosine transport. The potent inhibition of guanosine transport by NBMPR and the loss of transport in choline suggested that a Na+-dependent NBMPR-sensitive transporter was responsible for the majority of guanosine uptake. This concentrative, sensitive transporter is Na+ dependent with a stoichiometric coupling ratio of 1:1. This novel transporter, referred to as csg, is guanosine-specific with total guanosine transport inhibited by only 50% in the presence of 1 mM competing nucleosides. HL-60, acute myelocytic leukemia cells, do not exhibit csg activity while L1210, murine acute lymphocytic leukemia cells, exhibit csg transport. The presence of the csg transporter suggests an important role for guanosine in particular forms of leukemia and may provide a new target for cytotoxic therapy.

Saturation kinetics, specificity and NBMPR sensitivity of thymidine entry into the central nervous system

It was not until the development of a technique that could measure the brain uptake of slowly moving substrates, that the saturable transport system at the blood-brain barrier (BBB) for the pyrimidine deoxyribonucleoside, thymidine, was demonstrated. The aim of this present study was to further characterize this saturable uptake system at the blood-brain and blood-CSF barriers in terms of specificity, 6-(4-nitrobenzyl)thio-9-beta-D-ribofuranosylpurine (NBMPR) sensitivity and saturation kinetics by means of the in situ brain perfusion technique in anaesthetized guinea pigs. The results indicated that the transport system identified for [3H]thymidine can also transport other pyrimidine deoxyribonucleosides (deoxycytidine) and pyrimidine ribonucleosides (uridine) and is partially NBMPR-sensitive. In addition, guanosine, monocarboxylic acids, hexoses or amino acids were not substrates for the transport system. Further studies revealed that the transport system for [3H]thymidine at the BBB has a low affinity (Km 0.20 +/- 0.06 mM), but a relatively high capacity (Vmax 1.06 +/- 0.08 nmol min(-1) g(-1)). Overall, this study is indicative of a NBMPR-sensitive (es) facilitative transport system for [3H]thymidine and the likely presence of a NBMPR-insensitive and/or sodium-dependent transport system of the N2 (cit) type at the blood-brain and blood-CSF barriers of the guinea pig.

Functional expression of human intestinal Na+-dependent and Na+-independent nucleoside transporters in Xenopus laevis oocytes

We have shown previously that the human jejunal brush border membrane expresses both the N1 (cif) and the N2 (cit) Na+-dependent (concentrative) nucleoside transporters but not the Na+-independent (facilitative) nitrobenzylmercaptopurineriboside (NBMPR)-sensitive (es) transporter (Patil SD and Unadkat JD, Am J Physiol, 272: 1314-1320, 1997). In the present study, we have demonstrated that when Xenopus laevis oocytes are microinjected with human jejunal mRNA, four nucleoside transporters are expressed simultaneously, namely the N1 and N2 Na+-dependent nucleoside transporters and the es and the NBMPR-insensitive (ei) Na+-independent transporters. The expressed Na+-dependent nucleoside transporters showed substrate specificity identical to that previously described by us using jejunal brush border membrane vesicles (Patil SD and Unadkat JD, Am J Physiol, 272: 1314-1320, 1997). The expressed es and ei Na+-independent transporters demonstrated broad substrate selectivity with both purines and pyrimidines capable of inhibiting the uptake of guanosine and thymidine mediated by this transporter. The expressed Na+-dependent nucleoside transporters mediated the transport of their respective nucleoside substrates with a high affinity and a low capacity, whereas the es and the ei transporters mediated the transport of nucleosides with a low affinity and a high capacity. Collectively, these observations suggest that the Na+-independent nucleoside transporters are expressed in the basolateral membrane of the human jejunal epithelium. Based on these data, we hypothesize that the concentrative transporters in the brush border membrane and equilibrative transporters in the basolateral membrane are arranged in series in the human jejunal epithelium to allow efficient vectorial transport of nucleosides from the lumen to the blood. The simultaneous expression of four nucleoside transporters in X. laevis oocytes establishes a basis for molecular cloning of these four human nucleoside transporters.

The choroid plexus epithelium is the site of the organic anion transport protein in the brain

The mRNA for organic anion transport protein (oatp) was previously shown to be present in abundance in liver and kidney, and in small amounts in brain. Data obtained from experiments with reverse transcriptase-PCR techniques and in situ hybridization analysis showed that the oatp mRNA is present within the brain, localized to the choroid plexus. A sequence-specific antibody to the oatp polypeptide demonstrated the presence of the expected polypeptide with a molecular weight of 80,000 plus an immunoreactive species with a higher molecular weight in preparations of choroid plexus membranes. Examination of the choroid plexus by fluorescence confocal microscopy revealed that immunoreactive oatp polypeptide is localized to the apical surface of the choroid plexus epithelial cells, which contacts the cerebrospinal fluid. This localization of oatp is consistent with previous experiments showing vectorial transport of organic anions between the choroid plexus and the cerebrospinal fluid.

Demonstration of the existence of mRNAs encoding N1/cif and N2/cit sodium/nucleoside cotransporters in rat brain

Nucleoside transport may be involved in the regulation of extracellular levels of adenosine, an inhibitory neuromodulator in the central nervous system. Previous reports have provided functional evidence for Na+-dependent nucleoside transport in rat brain. We isolated total RNA from various regions of rat brain and tested for the presence of mRNA for two recently cloned Na+/nucleoside cotransporters using reverse transcriptase PCR (RT-PCR). Messenger RNA for a pyrimidine-selective Na+/nucleoside cotransporter mRNA (rCNT1) was detected in samples from each brain region tested by RT-PCR amplification of a 309-bp DNA product. Southern blot and sequence analysis confirmed that this product was derived from rCNT1 mRNA. A purine-selective Na+/nucleoside cotransporter mRNA (rCNT2, also termed SPNT) was detected throughout brain by amplifying a 235-bp DNA product, the sequence of which was identical to that published. These experiments demonstrate the presence of both rCNT1 and rCNT2 mRNA in rat brain.

Identification of a saturable uptake system for deoxyribonucleosides at the blood-brain and blood-cerebrospinal fluid barriers

Substances can enter the brain either directly across the blood-brain barrier or indirectly across the choroid plexuses and arachnoid membrane (blood-CSF barrier) into the CSF and then by diffusion into the brain. Earlier studies have demonstrated a saturable thymidine uptake across the blood-CSF barrier, but not across the blood-brain barrier. In this study transport of [3H]thymidine across both barriers was measured in vivo by means of a bilateral vascular brain perfusion technique in the anaesthetised guinea-pig. This method allows simultaneous and quantitative measurement of slowly penetrating solutes into both brain and CSF, under controlled conditions of arterial inflow. The results of the present study carried out over perfusion periods of up to 30 min indicated a progressive uptake of [3H]thymidine into brain and CSF, which was found to be significantly greater than the transport of D-[14C]mannitol (a plasma space marker). Furthermore, the addition of 1 mM unlabelled thymidine in the perfusate caused saturation of [3H]thymidine uptake into both brain and CSF. In conclusion, these findings suggest that thymidine can cross both the blood-brain and blood-CSF barriers in the guinea-pig by carrier-mediated transport systems.

Adenosine transporters

1. In mammals, nucleoside transport is an important determinant of the pharmacokinetics, plasma and tissue concentration, disposition and in vivo biological activity of adenosine as well as nucleoside analogues used in antiviral and anticancer therapies. 2. Two broad types of adenosine transporter exist, facilitated-diffusion carriers and active processes driven by the transmembrane sodium gradient. 3. Facilitated-diffusion adenosine carriers may be sensitive (es) or insensitive (ei) to nanomolar concentrations of the transport inhibitor nitrobenzylthioinosine (NBMPR). Dipyridamole, dilazep and lidoflazine analogues are also more potent inhibitors of the es carrier than the ei transporter in cells other than those derived from rat tissues. 4. The es transporter has a broad substrate specificity (apparent Km for adenosine approximately 25 microM in many cells at 25 degrees C), is a glycoprotein with an average apparent Mr of 57,000 in human erythrocytes that has been purified to near homogeneity and may exist in situ as a dimer. However, there is increasing evidence to suggest the presence of isoforms of the es transporter in different cells and species, based on kinetic and molecular properties. 5. The ei transporter also has a broad substrate specificity with a lower affinity for some nucleoside permeants than the es carrier, is genetically distinct from es but little information exists as to the molecular properties of the protein. 6. Sodium-dependent adenosine transport is present in many cell types and catalysed by four distinct systems, N1-N4, distinguished by substrate specificity, sodium coupling and tissue distribution. 7. Two genes have been identified which encode sodium-dependent adenosine transport proteins, SNST1 from the sodium/glucose cotransporter (SGLT1) gene family and the rat intestinal N2 transporter (cNT1) from a novel gene family including a bacterial nucleoside carrier (NupC). Transcripts of cNT1, which encodes a 648-residue protein, are found in intestine and kidney only. 8. Success in cloning the remaining adenosine transporter genes will improve our understanding of the diversity of nucleoside transport processes, with a view to better targeting of therapeutic nucleoside analogues and protective use of transport inhibitors.

Identification of a novel membrane transporter associated with intracellular membranes by phenotypic complementation in the yeast Saccharomyces cerevisiae

A partial mouse cDNA was isolated by its ability to functionally complement a thymidine transport deficiency in plasma membranes of the yeast, Saccharomyces cerevisiae. The full-length cDNA encoded a previously unidentified 27-kDa protein (mouse transporter protein (MTP)) with four predicted transmembrane-spanning domains. MTP mRNA was detected in cells of several mammalian species, and its predicted protein sequence exhibited near identity (98%) with that of a human cDNA (HUMORF13). MTP and its homologs evidently reside in an intracellular membrane compartment because a protein (about 24 kDa) that was recognized by MTP-specific antibodies was observed in a subcellular fraction of rat hepatocytes enriched for Golgi membranes. Deletion of the hydrophilic C terminus of MTP, which encompassed two putative signal motifs for intracellular localization (Tyr-X-X-hydrophobic amino acid), allowed expression of recombinant protein (MTP deltaC) in plasma membranes of Xenopus laevis oocytes. MTP deltaC-expressing oocytes exhibited greater fragility than nonexpressing oocytes, and those that survived the experimental manipulations were capable of mediated uptake of thymidine, uridine, and adenosine. Thymidine uptake by MTP deltaC-expressing oocytes was inhibited by thymine and dTMP. MTP may function in the transport of nucleosides and/or nucleoside derivatives between the cytosol and the lumen of an intracellular membrane-bound compartment.

Characterization of equilibrative and concentrative Na+-dependent (cif) nucleoside transport in acute promyelocytic leukemia NB4 cells

Nucleoside transport processes can be classified by the transport mechanism, e=equilibrative and c=concentrative, by the sensitivity to inhibition by nitrobenzylthioinosine (NBMPR), s=sensitive and i=insensitive, and also by permeant selectivity. To characterize nucleoside transport in acute promyelocytic NB4 cells, nucleoside transport was resolved into different components by selective elimination of transport processes with NBMPR and with Na+-deficient media. Initial transport rates were estimated from time course experiments. For adenosine, uridine, and formycin B, equilibrative transport accounted for approximately 60% of their uptake, with ei and es transport contributing almost equally, and Na+-dependent transport accounting for the remaining 40% of the total uptake. Thymidine uptake was mediated exclusively by equilibrative systems with ei and es systems each contributing 50% to total uptake. Adenosine accumulated above equilibrative concentrations, suggesting that a concentrative transport process was active and/or that metabolism led to adenosine's accumulation. Formycin B, a nonmetabolizable analog, also accumulated in the cells, supporting the concentrative potential of the Na+-dependent transporter. Kinetic analyses also provided evidence for three distinct high affinity transport mechanisms. NBMPR binding assays indicated the presence of two high affinity (Km 0.10 and 0.35 nM) binding sites. In conclusion, NB4 cells express ei and es transport, as well as a large ci transport component, which appears to correspond to cif (f=formycin B or purine selective) nucleoside transport, not previously described in human cells.