Molecular characterization of taurine transport in bovine aortic endothelial cells

GABAA receptors as plausible molecular targets and mediators for taurine and homotaurine actions

Dementia and autoimmune diseases are prevalent conditions with limited treatment options. Taurine and homotaurine (HT) are naturally occurring sulfonate amino acids, with taurine being highly abundant in animal tissues, but declining with age in the blood. HT is a blood-brain barrier permeable drug under investigation for Alzheimer's disease. HT also has beneficial effects in a mouse model of multiple sclerosis likely through an anti-inflammatory mechanism mediated by GABAA receptor (GABAAR) agonism in immune cells. While both taurine and HT are structural GABA analogs and thought to be GABA mimetics at GABAARs, there is uncertainty concerning their potency as GABA mimetics on native GABAARs. We show that HT is a very potent GABA mimetic, as it evokes GABAAR-mediated currents with an EC50 of 0.4 μM (vs. 3.7 μM for GABA and 116 µM for taurine) in murine cerebellar granule cells in brain slices, with both taurine and HT having similar efficacy in activating native GABAARs. Furthermore, HT displaces the high affinity GABAAR ligand [3H]muscimol at similarly low concentrations (HT IC50 of 0.16 μM vs. 125 μM for taurine) in mouse brain homogenates. The potency of taurine and HT as GABAAR agonists aligns with endogenous concentrations of taurine in the blood and with HT concentrations achieved in the brain following oral administration of HT or the HT pro-drug ALZ-801. Consequently, we discuss that GABAARs subtypes, similar to the ones we studied here in neurons, are plausible targets for mediating the potential beneficial effects of taurine in health and life-span extension and the beneficial HT effects in dementia and autoimmune conditions.

Significance of taurine transporter (TauT) in homeostasis and its layers of regulation (Review)

Taurine (2‑aminoethanesulfonic acid) contributes to homeostasis, mainly through its antioxidant and osmoregulatory properties. Taurine's influx and efflux are mainly mediated through the ubiquitous expression of the sodium/chloride‑dependent taurine transporter, located on the plasma membrane. The significance of the taurine transporter has been shown in various organ malfunctions in taurine‑transporter‑null mice. The taurine transporter differentially responds to various cellular stimuli including ionic environment, electrochemical charge, and pH changes. The renal system has been used as a model to evaluate the factors that significantly determine the regulation of taurine transporter regulation.

Taurine Attenuates Streptococcus uberis-Induced Bovine Mammary Epithelial Cells Inflammation via Phosphoinositides/Ca2+ Signaling

Taurine may alleviate the inflammatory injury induced by Streptococcus uberis (S. uberis) infection by regulating intracellular Ca²⁺ levels. However, the underlying mechanisms remain unclear. Infection leads to subversion of phosphoinositides (PIs) which are closely related to Ca²⁺ signaling. In order to investigate whether taurine regulates inflammation by means of PIs/ Ca²⁺ systems, competitive inhibitors of taurine (β-alanine) siTauT, siPAT1, siPLC, siCaN, siPKC, and inhibitors of PLC (U73122), PKC (RO31-8220), and CaN (FK 506) were used. The results indicate that taurine transfers the extracellular nutrient signal for intercellular innate immunity to phosphoinositides without a need to enter the cytoplasm while regulating intracellular Ca²⁺ levels during inflammation. Both the Ca²⁺-PKCα-NF-κB, and Ca²⁺-CaM-CaN-NFAT signaling pathways of S. uberis infection and the regulatory roles of taurine follow activation of PIs/Ca²⁺ systems. These data increase our understanding on the mechanisms of multifunctional nutrient, taurine attenuated inflammatory responses caused by S. uberis infection, and provide theoretical support for the prevention of this disease.

Biomarkers of oxidative stress, inflammation, and vascular dysfunction in inherited cystathionine β-synthase deficient homocystinuria and the impact of taurine treatment in a phase 1/2 human clinical trial

STUDY OBJECTIVE

A phase 1/2 clinical trial was performed in individuals with cystathionine β synthase (CBS) deficient homocystinuria with aims to: (a) assess pharmacokinetics and safety of taurine therapy, (b) evaluate oxidative stress, inflammation, and vascular function in CBS deficiency, and (c) evaluate the impact of short-term taurine treatment.

METHODS

Individuals with pyridoxine-nonresponsive CBS deficiency with homocysteine >50 μM, without inflammatory disorder or on antioxidant therapy were enrolled. Biomarkers of oxidative stress and inflammation, endothelial function (brachial artery flow-mediated dilation [FMD]), and disease-related metabolites obtained at baseline were compared to normal values. While maintaining current treatment, patients were treated with 75 mg/kg taurine twice daily, and treatment response assessed after 4 hours and 4 days.

RESULTS

Fourteen patients (8-35 years; 8 males, 6 females) were enrolled with baseline homocysteine levels 161 ± 67 μM. The study found high-dose taurine to be safe when excluding preexisting hypertriglyceridemia. Taurine pharmacokinetics showed a rapid peak level returning to near normal levels at 12 hours, but had slow accumulation and elevated predosing levels after 4 days of treatment. Only a single parameter of oxidative stress, 2,3-dinor-8-isoprostaglandin-F2α, was elevated at baseline, with no elevated inflammatory parameters, and no change in FMD values overall. Taurine had no effect on any of these parameters. However, the effect of taurine was strongly related to pretreatment FMD values; and taurine significantly improved FMD in the subset of individuals with pretreatment FMD values <10% and in individuals with homocysteine levels >125 μM, pertinent to endothelial function.

CONCLUSION

Taurine improves endothelial function in CBS-deficient homocystinuria in patients with preexisting reduced function.

Physiological role of taurine--from organism to organelle

Taurine is often referred to as a semi-essential amino acid as newborn mammals have a limited ability to synthesize taurine and have to rely on dietary supply. Taurine is not thought to be incorporated into proteins as no aminoacyl tRNA synthetase has yet been identified and is not oxidized in mammalian cells. However, taurine contributes significantly to the cellular pool of organic osmolytes and has accordingly been acknowledged for its role in cell volume restoration following osmotic perturbation. This review describes taurine homeostasis in cells and organelles with emphasis on taurine biophysics/membrane dynamics, regulation of transport proteins involved in active taurine uptake and passive taurine release as well as physiological processes, for example, development, lung function, mitochondrial function, antioxidative defence and apoptosis which seem to be affected by a shift in the expression of the taurine transporters and/or the cellular taurine content.

Taurine and atherosclerosis

Taurine is abundantly present in most mammalian tissues and plays a role in many important physiological functions. Atherosclerosis is the underlying mechanism of cardiovascular disease including myocardial infarctions, strokes and peripheral artery disease and remains a major cause of morbidity and mortality worldwide. Studies conducted in laboratory animal models using both genetic and dietary models of hyperlipidemia have demonstrated that taurine supplementation retards the initiation and progression of atherosclerosis. Epidemiological studies have also suggested that taurine exerts preventive effects on cardiovascular diseases. The present review focuses on the effects of taurine on the pathogenesis of atherosclerosis. In addition, the potential mechanisms by which taurine suppress the development of atherosclerosis will be discussed.

Hypertonicity enhances GABA uptake by cultured rat retinal capillary endothelial cells

We have reported previously that taurine transporter (TauT) mediates γ-aminobutyric acid (GABA) as a substrate in a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2 cells). This study investigates how TauT-mediated GABA transport is regulated in TR-iBRB2 cells under hypertonic conditions. [³H]GABA uptake by TR-iBRB2 cells exposed to 12 h- to 24 h-hypertonic culture medium was significantly greater than that of isotonic culture medium. [³H]GABA uptake by TR-iBRB2 cells was Na(+)-, Cl(-)-, and concentration-dependent with a Michaelis-Menten (K(m)) constant of 3.5 mM under isotonic conditions and K(m) of 0.324 and 5.48 mM under hypertonic conditions. Under hypertonic conditions, [³H]GABA uptake by TR-iBRB2 cells was more potently inhibited by substrates of TauT, such as taurine and β-alanine, than those of GABA transporters such as GABA, nipecotic acid, and betaine. These results suggest that an unknown high-affinity GABA transport process and TauT-mediated GABA transport are enhanced under hypertonic conditions. In conclusion, hypertonicity enhances GABA uptake by cultured rat retinal capillary endothelial cells.

Effect of beta-alanine treatment on mitochondrial taurine level and 5-taurinomethyluridine content

BACKGROUND

The beta-amino acid, taurine, is a nutritional requirement in some species. In these species, the depletion of intracellular stores of taurine leads to the development of severe organ dysfunction. The basis underlying these defects is poorly understood, although there is some suggestion that oxidative stress may contribute to the abnormalities. Recent studies indicate that taurine is required for normal mitochondrial protein synthesis and normal electron transport chain activity; it is known that defects in these events can lead to severe mitochondrial oxidative stress. The present study examines the effect of taurine deficiency on the first step of mitochondrial protein synthesis regulation by taurine, namely, the formation of taurinomethyluridine containing tRNA.

METHODS

Isolated rat cardiomyocytes were rendered taurine deficient by incubation with medium containing the taurine transport inhibitor, beta-alanine. The time course of cellular and mitochondrial taurine depletion was measured. The primer extension method was employed to evaluate the effect of beta-alanine treatment on taurinomethyluridine content of tRNALeu. The protein levels of ND6 were also determined by Western blot analysis.

RESULTS

beta-alanine caused a time-dependent decrease in cellular taurine content, which were reduced in half after 48 hrs of incubation. The amount of taurine in the mitochondria was considerably less than that in the cytosol and was unaffected by beta-alanine treatment. Approximately 70% of the tRNALeu in the untreated cell lacked taurinomethyluridine and these levels were unchanged following beta-alanine treatment. Protein content of ND6, however, was significantly reduced after 48 hours incubation with beta-alanine.

CONCLUSIONS

The taurine levels of the cytosol and the mitochondria are not directly coupled. The beta-alanine-mediated reduction in taurine levels is too small to affect taurinomethyluridine levels. Nonetheless, it interferes with mitochondrial protein synthesis, as exemplified by a decrease in ND6 protein content. Thus, beta-alanine does not cause alterations in mitochondrial protein synthesis through the lowering of taurine levels.

Inhibition of taurine transport by cyclosporin A is due to altered surface abundance of the taurine transporter and is reversible

We have investigated the underlying mechanism of the CsA-induced inhibition of taurine transport using a cell line permanently expressing the mouse taurine transporter (mTauT) tagged with the green-fluorescence protein (GFP). CsA inhibited the uptake activity of the expressed mTauT.GFP fusion protein in both dose and time dependent manner. Surface biotinylation assay revealed that the CsA-treatment reduced the relative surface abundance of the taurine transporter without affecting its total expression level. CsA treatment reduced both the taurine uptake and the relative surface abundance of the transporter by similar magnitudes. Conversely, when the CsA was washed off, both the uptake and the relative surface abundance of the transporter recovered fully to the control level. Remarkably, the recovery process was insensitive to the protein synthesis inhibitor cycloheximide. These results suggested that the CsA inhibited taurine transport by altering the surface abundance, possibly by internalization of the expressed taurine transporters.

Function and regulation of taurine transport at the inner blood-retinal barrier

In the retina, taurine exerts a number of neuroprotective functions as an osmolyte and antioxidant. The purpose of the present study was to elucidate the taurine transport system(s) at the inner blood-retinal barrier (BRB). [(3)H]Taurine transport at the inner BRB was characterized using in vivo integration plot analysis and a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2 cells). The expression of the taurine transporter (TauT) was demonstrated by RT-PCR and immunoblot analyses. The apparent influx permeability clearance of [(3)H]taurine in the rat retina was found to be 259 muL/(ming retina), supporting carrier-mediated influx transport of taurine at the BRB. [(3)H]Taurine uptake by TR-iBRB2 cells was Na(+)-, Cl(-)- and concentration-dependent with a K(m) of 22.2 muM and inhibited by TauT inhibitors, such as beta-alanine and hypotaurine. RT-PCR and immunoblot analyses demonstrated that TauT is expressed in TR-iBRB2 and primary cultured human retinal endothelial cells. The uptake of [(3)H]taurine and the expression of TauT mRNA in TR-iBRB2 cells increased under hypertonic conditions but decreased following pretreatment with excess taurine. In conclusion, TauT most likely mediates taurine transport and regulate taurine transport at the inner BRB.

Taurine transporter is expressed in osteoblasts

Taurine influences bone metabolism and is taken up by cells via a specific transport system, the taurine transporter (TAUT). We report a link between taurine and bone homeostasis by demonstrating transcription and translation of TAUT in bone-forming cells. TAUT was expressed in human primary osteoblasts, the human osteosarcoma osteoblast-like cell line MG63, and the mouse osteoblastic cell line MC3T3-E1. Immunostaining with polyclonal antibodies also demonstrated the presence of TAUT in both human and murine osteoblasts. TAUT mRNA expression and [(3)H]taurine uptake increased during differentiation of MG63 cells in culture. Supplementation of culture medium with taurine enhanced alkaline phosphatase activity and osteocalcin secretion. The regulation and detailed function of taurine and TAUT in bone remain unclear, but our findings suggest a functional role for them in bone homeostasis.

Regulation of taurine transport in rat hippocampal neurons by hypo-osmotic swelling

Taurine, an important mediator of cellular volume regulation in the central nervous system, is accumulated into neurons and glia by means of a highly specific sodium-dependent membrane transporter. During hyperosmotic cell shrinkage, net cellular taurine content increases as taurine transporter activity is enhanced via elevated gene expression of the transporter protein. In hypo-osmotic conditions, taurine is rapidly lost from cells by means of taurine-conducting membrane channels. We reasoned that changes in taurine transporter activity also might accompany cell swelling to minimize re-accumulation of taurine from the extracellular space. Thus, we determined the kinetic and pharmacological characteristics of neuronal taurine transport and the response to osmotic swelling. Accumulation of radioactive taurine is strongly temperature dependent and occurs via saturable and non-saturable pathways. At concentrations of taurine expected in extracellular fluid in vivo, 98% of taurine accumulation would occur via the saturable pathway. This pathway obeys Michaelis-Menten kinetics with a Km of 30.0 +/- 8.8 microm (mean +/- SE) and Jmax of 2.1 +/- 0.2 nmol/mg protein min. The saturable pathway is dependent on extracellular sodium with an effective binding constant of 80.0 +/- 3.1 mm and a Hill coefficient of 2.1 +/- 0.1. This pathway is inhibited by structural analogues of taurine and by the anion channel inhibitors, 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS) and 5-nitro-2-(3 phenylpropylamino) benzoic acid (NPPB). NPPB, but not DIDS, also reduces the ATP content of the cell cultures. Osmotic swelling at constant extracellular sodium concentration reduces the Jmax of the saturable transport pathway by approximately 48%, increases Kdiff for the non-saturable pathway by 77%, but has no effect on cellular ATP content. These changes in taurine transport occurring in swollen neurons in vivo would contribute to net reduction of taurine content and resulting volume regulation.

Taurine biosynthetic enzymes and taurine transporter: molecular identification and regulations

Many biological effects of taurine rely upon its cellular concentration, which is primarily controlled by taurine biosynthetic enzymes cysteine dioxygenase (CDO) and cysteine sulfinate decarboxylase (CSD) and taurine transporter (TauT). The cloning of CDO, CSD and TauT in various species provided first-hand information on these proteins, as well as molecular tools to investigate their regulations. CDO upregulation in hepatocytes in response to high sulfur amino acids appears clearly as the most spectacular among the regulations of the biosynthetic enzymes. Downregulation of TauT activity by activation of PKC appears particularly well documented. A unique serine residue could be identified as a phosphorylation site that leads to an inactive form of TauT. The previously revealed downregulation of TauT expression by taurine and hypertonicity-induced upregulation of TauT expression were shown to result from a modified transcription rate of TauT gene, but the precise molecular mechanisms are not yet formally established. Other regulations of taurine transporter expression were more recently reported, which involve glucose, tumor suppressor protein p53, tumor necrosis factor-alpha, and nitric oxide. This review reports the experimental models and data that support these various regulations but also points out the aspects that remain poorly understood or unknown concerning their molecular basis and physiological significance.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Expression and location of taurine transporters and channels in the epididymis of infertile c-ros receptor tyrosine kinase-deficient and fertile heterozygous mice

Defective sperm volume regulation causes infertility in c-ros knockout (KO) mice lacking the initial segment of the epididymis (IS). As taurine is in high concentration in the male tract and acts as an organic osmolyte in somatic cells, the taurine transporter (TauT), taurine content and taurine channel phospholemman (PLM) were examined in the epididymides of these and fertile heterozygous (+/-) mice. TauT and PLM genes were demonstrated by RT-PCR in the caput of +/- and c-ros knockout males. Northern blots revealed one transcript of TauT (8 kb) in all regions with different expression between segments (cauda > corpus > IS > caput) and no differences in expression between genotypes. Western blotting revealed three translation products of TauT in all epididymal regions (107, 69, 49 kDa) with higher expression of the 69 kDa and 49 isoforms in the -/- than +/- caput. Immunohistochemical staining revealed staining of principal cells and stronger staining of apical and clear cells in all epididymal regions. The expression of the PLM transcript (0.75 kb: cauda = corpus > caput > IS) was upregulated in the proximal caput and cauda of KO mice. Tissue taurine was higher in the cauda >corpus>IS congruent with caput in fertile males and significantly higher in the proximal caput of the KO male. By contrast, taurine concentrations in cauda epididymidal fluid and content per 10(6) sperm did not differ between genotypes. TauT and PLM may be involved in taurine regulation in the normal epididymis and the proximal accumulation of taurine in the infertile males.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Taurine as a constituent of mitochondrial tRNAs: new insights into the functions of taurine and human mitochondrial diseases

Taurine (2-aminoethanesulphonic acid), a naturally occurring, sulfur-containing amino acid, is found at high concentrations in mammalian plasma and tissues. Although taurine is involved in a variety of processes in humans, it has never been found as a component of a protein or a nucleic acid, and its precise biochemical functions are not fully understood. Here, we report the identification of two novel taurine-containing modified uridines (5-taurinomethyluridine and 5-taurinomethyl-2-thiouridine) in human and bovine mitochondrial tRNAs. Our work further revealed that these nucleosides are synthesized by the direct incorporation of taurine supplied to the medium. This is the first reported evidence that taurine is a constituent of biological macromolecules, unveiling the prospect of obtaining new insights into the functions and subcellular localization of this abundant amino acid. Since modification of these taurine-containing uridines has been found to be lacking in mutant mitochondrial tRNAs for Leu(UUR) and Lys from pathogenic cells of the mitochondrial encephalomyopathies MELAS and MERRF, respectively, our findings will considerably deepen our understanding of the molecular pathogenesis of mitochondrial encephalomyopathic diseases.

Taurine: evidence of physiological function in the retina

Taurine is a free amino acid found in high millimolar concentrations in mammalian tissue and is particularly abundant in the retina. Mammals synthesize taurine endogenously with varying abilities, with some species more dependent on dietary sources of taurine than others. Human children appear to be more dependent on dietary taurine than adults. Specifically, it has been established that visual dysfunction in both human and animal subjects results from taurine deficiency. Moreover, the deficiency is reversed with simple nutritional supplementation with taurine. The data suggest that taurine is an important neurochemical factor in the visual system. However, the exact function or functions of taurine in the retina are still unresolved despite continuing scientific study. Nevertheless, the importance of taurine in the retina is implied in the following experimental findings: (1) Taurine exhibits significant effects on biochemical systems in vitro. (2) The distribution of taurine is tightly regulated in the different retinal cell types through the development of the retina. (3) Taurine depletion results in significant retinal lesions. (4) Taurine release and uptake has been found to employ distinct regulatory mechanisms in the retina.