Characterization of the rat Na+/nucleoside cotransporter 2 and transport of nucleoside-derived drugs using electrophysiological methods

Who Is Who in Adenosine Transport

Extracellular adenosine concentrations are regulated by a panel of membrane transporters which, in most cases, mediate its uptake into cells. Adenosine transporters belong to two gene families encoding Equilibrative and Concentrative Nucleoside Transporter proteins (ENTs and CNTs, respectively). The lack of appropriate pharmacological tools targeting every transporter subtype has introduced some bias on the current knowledge of the role of these transporters in modulating adenosine levels. In this regard, ENT1, for which pharmacology is relatively well-developed, has often been identified as a major player in purinergic signaling. Nevertheless, other transporters such as CNT2 and CNT3 can also contribute to purinergic modulation based on their high affinity for adenosine and concentrative capacity. Moreover, both transporter proteins have also been shown to be under purinergic regulation via P1 receptors in different cell types, which further supports its relevance in purinergic signaling. Thus, several transporter proteins regulate extracellular adenosine levels. Moreover, CNT and ENT proteins are differentially expressed in tissues but also in particular cell types. Accordingly, transporter-mediated fine tuning of adenosine levels is cell and tissue specific. Future developments focusing on CNT pharmacology are needed to unveil transporter subtype-specific events.

Molecular determinants of acidic pH-dependent transport of human equilibrative nucleoside transporter 3

Equilibrative nucleoside transporters (ENTs) translocate hydrophilic nucleosides across cellular membranes and are essential for salvage nucleotide synthesis and purinergic signaling. Unlike the prototypic human ENT members hENT1 and hENT2, which mediate plasma membrane nucleoside transport at pH 7.4, hENT3 is an acidic pH-activated lysosomal transporter partially localized to mitochondria. Recent studies demonstrate that hENT3 is indispensable for lysosomal homeostasis, and that mutations in hENT3 can result in a spectrum of lysosomal storage-like disorders. However, despite hENT3's prominent role in lysosome pathophysiology, the molecular basis of hENT3-mediated transport is unknown. Therefore, we sought to examine the mechanistic basis of acidic pH-driven hENT3 nucleoside transport with site-directed mutagenesis, homology modeling, and [³H]adenosine flux measurements in mutant RNA-injected Xenopus oocytes. Scanning mutagenesis of putative residues responsible for pH-dependent transport via hENT3 revealed that the ionization states of Asp-219 and Glu-447, and not His, strongly determined the pH-dependent transport permissible-impermissible states of the transporter. Except for substitution with certain isosteric and polar residues, substitution of either Asp-219 or Glu-447 with any other residues resulted in robust activity that was pH-independent. Dual substitution of Asp-219 and Glu-447 to Ala sustained pH-independent activity over a broad range of physiological pH (pH 5.5-7.4), which also maintained stringent substrate selectivity toward endogenous nucleosides and clinically used nucleoside drugs. Our results suggest a putative pH-sensing role for Asp-219 and Glu-447 in hENT3 and that the size, ionization state, or electronegative polarity at these positions is crucial for obligate acidic pH-dependent activity.

Cause-effect relationships in nutritional intervention studies for health claims substantiation: guidance for trial design

The growing worldwide interest on functional food research has been accompanied by increasing regulatory guidelines in this area, with the aim of ensuring that any claimed effect in foods, beyond their nutritional role, is based on scientific unequivocal evidence. In order to assess the cause-effect relationship between the regular consumption of a food or a food component and the beneficial outcome for the consumer, an appropriate study design is required. Previous knowledge and research on the specific claimed food or product may be an adequate basis for defining a hypothesis and accurate objectives. Other key factors to take into account are based on the outcomes studied, the length of the trial, sample size and type, as well as the transparency on reporting the results obtained. Based on the Consolidated Standards on Reporting Trials statement (CONSORT), together with the specific guidelines published by the European Food Safety Authority (EFSA) Panel on Dietetic Products, Nutrition and Allergies, the present article aims at summarizing key questions conducting to the most appropriate study design for solid health claim substantiation.

Further characterization of the electrogenicity and pH sensitivity of the human organic anion-transporting polypeptides OATP1B1 and OATP1B3

Organic anion-transporting polypeptides (OATPs) are involved in the liver uptake of many endogenous and xenobiotic compounds, such as bile acids and drugs, respectively. Using Xenopus laevis oocytes and Chinese hamster ovary (CHO) cells expressing rat Oatp1a1, human OATP1B1, or OATP1B3, the sensitivity of these transporters to extracellular/intracellular pH (pHo/pHi) and changes in plasma membrane potential (ΔΨ) was investigated. In X. laevis oocytes, nonspecific plasma membrane permeability increased only at pHo below 4.5. Above this value, both using oocytes and CHO cells, extracellular acidification affected differently the specific transport of taurocholic acid (TCA) and estradiol 17β-d-glucuronide (E(2)17βG) by Oatp1a1 (stimulation), OATP1B1 (inhibition), and OATP1B3 (stimulation). Changes in substrate uptake in the presence of valinomycin (K(+)-ionophore), carbonyl cyanide 3-chlorophenylhydrazone and nigericin (protonophores), and amiloride (Na(+)/H(+)-inhibitor) and cation replacement in the medium were studied with fluorescent probes for measuring substrate uptake (cholylglycyl amidofluorescein) and changes in pHi (SNARF-4F) and ΔΨ [DilC(1)(5)]. The results suggest that activity of these three carriers is sodium/potassium-independent and affected differently by changes in pHo and ΔΨ: Oatp1a1 was confirmed to be an electroneutral anion exchanger, whereas the function of both OATP1B1 and OATP1B3 was markedly affected by the magnitude of ΔΨ. Moreover, electrophysiological measurements revealed the existence of a net anion influx associated to OATP1B1/OATP1B3-mediated transport of TCA, E(2)17βG, and estrone-3-sulfate. Furthermore, a leakage of Na(+) through OATP1B1 and OATP1B3, which is not coupled to substrate transport, was found. In conclusion, these results suggest that OATP1B1 and OATP1B3 are electrogenic transporters whose activity may be strongly affected under circumstances of displacement of local pH.

Diabetic foot ulcers: effects of hyperoxia and SDF-1α on endothelial progenitor cells

Diabetes mellitus is a common disease afflicting many people. In addition to coronary artery disease, diabetic retinopathy and renal failure, diabetic patients face abnormal wound healing and have increased lower extremity ulcers and amputations. In diabetes, wound healing is altered due to both macrovascular and microvascular processes. While the former can be addressed with surgical intervention, the latter is more difficult to correct. Neovascularization within the granulation tissue via angiogenesis and vasculogenesis is critical for wound healing. Endothelial progenitor cells (EPCs) have been implicated in vasculogenesis. Mobilization of EPCs from the bone marrow is impaired in diabetes and homing of EPCs to the wound is also abnormal. Recent studies show that hyperoxia and administration of exogenous stromal-derived factor-1α increases circulatory and wound levels of EPCs and improves wound healing in diabetic mice. These findings have great potential for translation into human counterparts as the treatment for this prevalent disease matures.

Use of phosphatide precursors to promote synaptogenesis

New brain synapses form when a postsynaptic structure, the dendritic spine, interacts with a presynaptic terminal. Brain synapses and dendritic spines, membrane-rich structures, are depleted in Alzheimer's disease, as are some circulating compounds needed for synthesizing phosphatides, the major constituents of synaptic membranes. Animals given three of these compounds, all nutrients-uridine, the omega-3 polyunsaturated fatty acid docosahexaenoic acid, and choline-develop increased levels of brain phosphatides and of proteins that are concentrated within synaptic membranes (e.g., PSD-95, synapsin-1), improved cognition, and enhanced neurotransmitter release. The nutrients work by increasing the substrate-saturation of low-affinity enzymes that synthesize the phosphatides. Moreover, uridine and its nucleotide metabolites activate brain P2Y receptors, which control neuronal differentiation and synaptic protein synthesis. A preparation containing these compounds is being tested for treating Alzheimer's disease.

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.

Adenosine and Catecholamine Agonists Speed the Flagellar Beat of Mammalian Sperm by a Non-Receptor-Mediated Mechanism1

Activation of rapid motility apparently is one of the first steps of sperm capacitation and can be studied in vitro. Previously we found that 2-chloro-2'-deoxyadenosine or the catecholamine isoproterenol activates mouse sperm motility in vitro via a pathway mediated by cAMP that requires extracellular Ca2+, the atypical sperm adenylyl cyclase, and sperm-specific protein kinase A. We now show that several other adenosine analogs and catecholamines accelerate the flagellar beat of mouse and human sperm. Unexpectedly, the potent adenosine receptor agonist CGS21680 does not accelerate the beat, and the adenosine receptor antagonist DPCPX does not diminish the accelerating action of 2-chloro-2'-deoxyadenosine. The pharmacological profile for activation by catecholamines is also unusual. Both agonists and antagonists of beta-adrenergic receptors elevate the beat frequency. Moreover, both l-(-) and d-+ isomers of epinephrine, norepinephrine, and isoproterenol produce similar acceleration of the beat. In contrast, inhibitors of equilibrative nucleoside transporters effectively slow the onset of the accelerating action of adenosine analogs. Replacement of external Na+ with Li+ also diminishes the accumulation of cAMP and slows the resultant accelerating action of 2-chloro-2'-deoxyadenosine, suggesting the involvement of a Na+-dependent concentrative nucleoside transporter. Our results show that adenosine and catecholamine agonists act in a novel signaling pathway that does not involve G protein-coupled cell-surface receptors that link to conventional adenylyl cyclases. Instead, adenosine and analogs may be transported into sperm via equilibrative and concentrative nucleoside transporters to act on unknown intracellular targets.

Exogenous nucleosides accelerate differentiation of rat intestinal epithelial cells

Several studies have reported differing data on the effect of exogenous nucleosides and nucleotides on the proliferation and differentiation in various intestinal cell lines and explants. To study whether exogenous nucleosides modulate intestinal cell differentiation, IEC-6 cells were differentiated in the presence or absence of a nucleoside mixture (cytidine, uridine, guanosine and inosine, 30 microM each), and the concentrations of nucleoside derivatives were determined by HPLC. Cell differentiation was assessed by electron microscopy, alkaline phosphatase activity and Rnd3 gene expression. The present results showed that uridine, guanosine and inosine were cleared from culture media (up to 32, 63 and 100 % in proliferating cells, and 31, 80 and 94 % in differentiated cells, respectively) whereas cytidine concentrations increased. Differentiation of IEC-6 cells was associated with a significant increase in intracellular nucleotide concentrations. Clearance of nucleosides correlated with a significant increase in the intracellular nucleotide pool in proliferating and differentiated IEC-6 cells. Intracellular guanosine nucleotides increased 2.5- and 5-fold in nucleoside-supplemented proliferating and differentiated cells, respectively. At 24 h, nucleoside-supplemented differentiated IEC-6 cells had significantly higher energy charge and GTP levels than non-supplemented ones. These modifications paralleled changes in cell differentiation as indicated by increased alkaline phosphatase activity, prolonged microvilli formation and accelerated down-regulation of Rnd3 gene expression. The present findings suggest that exogenous nucleosides were selectively taken up by IEC-6 cells, increased the intracellular nucleotide pool, GTP and energy charge, and favoured cell morphological and functional changes during differentiation.