Adaptive regulation of taurine transport in two continuous renal epithelial cell lines

Biomarker Profiling with Targeted Metabolomic Analysis of Plasma and Urine Samples in Patients with Type 2 Diabetes Mellitus and Early Diabetic Kidney Disease

Background: Over the years, it was noticed that patients with diabetes have reached an alarming number worldwide. Diabetes presents many complications, including diabetic kidney disease (DKD), which can be considered the leading cause of end-stage renal disease. Current biomarkers such as serum creatinine and albuminuria have limitations for early detection of DKD. Methods: In our study, we used UHPLC-QTOF-ESI+-MS techniques to quantify previously analyzed metabolites. Based on one-way ANOVA and Fisher's LSD, untargeted analysis allowed the discrimination of six metabolites between subgroups P1 versus P2 and P3: tryptophan, kynurenic acid, taurine, l-acetylcarnitine, glycine, and tiglylglycine. Results: Our results showed several metabolites that exhibited significant differences among the patient groups and can be considered putative biomarkers in early DKD, including glycine and kynurenic acid in serum (p < 0.001) and tryptophan and tiglylglycine (p < 0.001) in urine. Conclusions: Although we identified metabolites as potential biomarkers in the present study, additional studies are needed to validate these results.

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 and the renal system

Taurine participates in a number of different physiologic and biologic processes in the kidney, often reflected by urinary excretion patterns. The kidney is key to aspects of taurine body pool size and homeostasis. This review will examine the renal-taurine interactions relative to ion reabsorption; renal blood flow and renal vascular endothelial function; antioxidant properties, especially in the glomerulus; and the role of taurine in ischemia and reperfusion injury. In addition, taurine plays a role in the renal cell cycle and apoptosis, and functions as an osmolyte during the stress response. The role of the kidney in adaptation to variations in dietary taurine intake and the regulation of taurine body pool size are described. Finally, the protective function of taurine against several kidney diseases is reviewed.

SNAT2 amino acid transporter is regulated by amino acids of the SLC6 gamma-aminobutyric acid transporter subfamily in neocortical neurons and may play no role in delivering glutamine for glutamatergic transmission

System A transporters SNAT1 and SNAT2 mediate uptake of neutral alpha-amino acids (e.g. glutamine, alanine, and proline) and are expressed in central neurons. We tested the hypothesis that SNAT2 is required to support neurotransmitter glutamate synthesis by examining spontaneous excitatory activity after inducing or repressing SNAT2 expression for prolonged periods. We stimulated de novo synthesis of SNAT2 mRNA and increased SNAT2 mRNA stability and total SNAT2 protein and functional activity, whereas SNAT1 expression was unaffected. Increased endogenous SNAT2 expression did not affect spontaneous excitatory action-potential frequency over control. Long term glutamine exposure strongly repressed SNAT2 expression but increased excitatory action-potential frequency. Quantal size was not altered following SNAT2 induction or repression. These results suggest that spontaneous glutamatergic transmission in pyramidal neurons does not rely on SNAT2. To our surprise, repression of SNAT2 activity was not limited to System A substrates. Taurine, gamma-aminobutyric acid, and beta-alanine (substrates of the SLC6 gamma-aminobutyric acid transporter family) repressed SNAT2 expression more potently (10x) than did System A substrates; however, the responses to System A substrates were more rapid. Since ATF4 (activating transcription factor 4) and CCAAT/enhancer-binding protein are known to bind to an amino acid response element within the SNAT2 promoter and mediate induction of SNAT2 in peripheral cell lines, we tested whether either factor was similarly induced by amino acid deprivation in neurons. We found that glutamine and taurine repressed the induction of both transcription factors. Our data revealed that SNAT2 expression is constitutively low in neurons under physiological conditions but potently induced, together with the taurine transporter TauT, in response to depletion of neutral amino acids.

Seven days of oral taurine supplementation does not increase muscle taurine content or alter substrate metabolism during prolonged exercise in humans

This study examined 1) the plasma taurine response to acute oral taurine supplementation (T), and 2) the effects of 7 days of T on muscle amino acid content and substrate metabolism during 2 h of cycling at approximately 60% peak oxygen consumption (VO2peak). In the first part of the study, after an overnight fast, 7 volunteers (28+/-3 yr, 184+/-2 cm, 88.0+/-6.6 kg) ingested 1.66 g oral taurine doses with breakfast (8 AM) and lunch (12 noon), and blood samples were taken throughout the day. In the second part of the study, eight men (22+/-1 yr, 181+/-1 cm, 80.9+/-3.8 kg, 4.21+/-0.16 l/min VO2peak) cycled for 2 h after 7 days of placebo (P) ingestion (6 g glucose/day) and again following 7 days of T (5 g/day). In the first part of the study, plasma taurine was 64+/-4 microM before T and rose rapidly to 778+/-139 microM by 10 AM and remained elevated at noon (359+/-56 microM). Plasma taurine reached 973+/-181 microM at 1 PM and was 161+/-31 microM at 4 PM. In the second part of the study, seven days of T had no effect on muscle taurine content (mmol/kg dry muscle) at rest (P, 44+/-15 vs. T, 42+/-15) or after exercise (P, 43+/-12 vs. T, 43+/-11). There was no difference in muscle glycogen or other muscle metabolites between conditions, but there were notable interaction effects for muscle valine, isoleucine, leucine, cystine, glutamate, alanine, and arginine amino acid content following exercise after T. These data indicate that 1) acute T produces a 13-fold increase in plasma taurine concentration; 2) despite the ability to significantly elevate plasma taurine for extended periods throughout the day, 7 days of T does not alter skeletal muscle taurine content or carbohydrate and fat oxidation during exercise; and 3) T appears to have some impact on muscle amino acid response to exercise.

Accelerated NaCl-induced hypertension in taurine-deficient rat: Role of renal function

Taurine modulates blood pressure and renal function. As the kidney plays a pivotal role in long-term control of arterial pressure, we tested the hypothesis that taurine-deficient rats display maladaptive renal and blood pressure responses to uninephrectomy. Control and taurine-deficient (i.e., beta-alanine-treated) rats with either one or two remaining kidneys were fed diets containing basal or high (8%) NaCl diet. Urine osmolality was greater in the taurine-deficient than controls fed a normal NaCl diet; proteinuria and blood pressure were unaffected by uninephrectomy. Following 6 weeks on an 8% NaCl diet, the uninephrectomized (UNX) animals developed significant hypertension, which was more severe in the taurine-deficient group; baroreflex function was unaffected. However, the UNX taurine-deficient rats displayed impaired ability to dispose of an acute isotonic saline volume load before a switchover to a high NaCl diet. Nonetheless, a more protracted exposure (i.e., 14 weeks) to dietary NaCl excess eliminated the blood pressure differential between the two groups; at this stage, renal excretory responses to an acute saline volume load or to atrial natriuretic peptide were similar in the two groups. Nonetheless, hypertensive taurine-deficient rats displayed greater proteinuria, although both groups excreted proteins of similar molecular weights ( approximately 15-66 kDa). Further, taurine-deficient kidney specimens displayed periarterial mononuclear cell infiltrates with strong immunoreactivity to the histiocyte marker CD68, suggestive of increased phagocytic activity. In conclusion, taurine deficiency modulates renal adaptation to combined uninephrectomy and dietary NaCl excess, resulting in an accelerated development of hypertension.

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.

The taurine transporter: mechanisms of regulation

Taurine transport undergoes an adaptive response to changes in taurine availability. Unlike most amino acids, taurine is not metabolized or incorporated into protein but remains free in the intracellular water. Most amino acids are reabsorbed at rates of 98-99%, but reabsorption of taurine may range from 40% to 99.5%. Factors that influence taurine accumulation include ionic environment, electrochemical charge, and post-translational and transcriptional factors. Among these are protein kinase C (PKC) activation and transactivation or repression by proto-oncogenes such as WT1, c-Jun, c-Myb and p53. Renal adaptive regulation of the taurine transporter (TauT) was studied in vivo and in vitro. Site-directed mutagenesis and the oocyte expression system were used to study post-translational regulation of the TauT by PKC. Reporter genes and Northern and Western blots were used to study transcriptional regulation of the taurine transporter gene (TauT). We demonstrated that (i) the body pool of taurine is controlled through renal adaptive regulation of TauT in response to taurine availability; (ii) ionic environment, electrochemical charge, pH, and developmental ontogeny influence renal taurine accumulation; (iii) the fourth segment of TauT is involved in the gating of taurine across the cell membrane, which is controlled by PKC phosphorylation of serine 322 at the post-translational level; (iv) expression of TauT is repressed by the p53 tumour suppressor gene and is transactivated by proto-oncogenes such as WT1, c-Jun, and c-Myb; and (v) over-expression of TauT protects renal cells from cisplatin-induced nephrotoxicity.

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.

Effect of taurine deficiency on adenosine receptor-mediated relaxation of the rat aorta

We recently demonstrated that chronic taurine supplementation or deficiency causes alterations in reactivity of the rat aorta to several vasoactive agents. In the present investigation, we examined the effects beta-alanine-induced endogenous taurine deficiency on the mechanical responsiveness of the isolated rat aorta to adenosine receptor stimulation with 2-chloroadenosine (CAD), 5'-N-ethylcarboxyamidoadenosine (NECA), and N(6)-cyclopentyladenosine (CPA). The adenosine analogs produced concentration-dependent (1 x 10(-9)-3 x 10(-3) M) relaxations of aortas from both control and beta-alanine-treated rats with the rank order of potencies NECA>CAD>CPA, which was consistent with A(2) receptor identification. CAD and NECA induced both endothelium-dependent and -independent relaxations of the aortas. The endothelium-dependent responses to both agents and the independent responses to CAD were significantly attenuated by beta-alanine treatment. The relaxation responses of the aortas from control and taurine-deficient rats to CAD and NECA were markedly antagonized by ZM241385 (10(-5) M), suggesting the involvement of A(2A) adenosine receptors. Further, N-nitro-L-arginine methyl ester (L-NAME; 10(-5) M) significantly attenuated the endothelium-mediated relaxation produced by CAD and NECA in both groups. However, the inhibitory effect of L-NAME was less on the beta-alanine-treated tissues, providing evidence that the effect of taurine deficiency was linked to a reduction in nitric oxide generation. As in the aorta, CAD produced both endothelium-dependent and -independent relaxation responses in the rat superior mesenteric artery, and both responses were inhibited by chronic beta-alanine treatment, suggesting that not only similar responses can be generated by a given adenosine agonist in different vascular beds, but also beta-alanine treatment modulates these responses. On the other hand, while CPA elicited only endothelium-independent aortic relaxation, this response was not altered by taurine deficiency. The results indicate that endogenous taurine deficiency causes differential inhibitory effects on adenosine receptor-mediated vasorelaxation, depending upon the agonists used. Given the recognized role of adenosine in the vasculature, these alterations suggest taurine-mediated modulation of blood flow regulation.

Taurine depletion alters vascular reactivity in rats

We recently showed that chronic taurine supplementation is associated with attenuation of contractile responses of rat aorta to norepinephrine and potassium chloride. However, the potential involvement of endogenous taurine in modulation of vascular reactivity is not known. Therefore, we examined the effect of β-alanine-induced taurine depletion on the in vitro reactivity of rat aorta to selected vasoactive agents. The data indicate that both norepinephrine- and potassium-chloride-induced maximum contractile responses of endothelium-denuded aortae were enhanced in taurine-depleted rats compared with control animals. However, taurine depletion did not affect tissue sensitivity to either norepinephrine or potassium chloride. By contrast, sensitivity of the endothelium-denuded aortae to sodium nitroprusside was attenuated by taurine depletion. Similarly, taurine deficiency reduced the relaxant responses of endothelium-intact aortic rings elicited by submaximal concentrations of acetylcholine, and this effect was associated with decreased nitric oxide production. Taken together, the data suggest that taurine depletion augments contractility but attenuates relaxation of vascular smooth muscle in a nonspecific manner. Impairment of endothelium-dependent responses, which is at least in part associated with reduced nitric oxide generation, may contribute to the attenuation of the vasorelaxant responses. These vascular alterations could be of potential consequence in pathological conditions associated with taurine deficiency.Key words: rat aorta, β-alanine, taurine depletion, vascular reactivity.

Chronic taurine treatment ameliorates reduction in saline-induced diuresis and natriuresis

BACKGROUND

Taurine is an osmolyte found in high concentration in the kidney. Both the modulation of tissue taurine stores and the exogenous administration of taurine are known to affect renal function. Therefore, it is likely that taurine therapy could benefit the dysfunctional kidney.

METHODS

To test this idea, the present study examined the effect of chronic taurine administration on the excretory responses to acute saline volume loading by the unilaterally nephrectomized (UNX) control and the UNX glucose-intolerant rat (ages 3 to 9 months).

RESULTS

Sham-operated animals excreted similar amounts of the administered fluid and sodium loads with age. However, unilateral nephrectomy was associated with a significant reduction in the response to saline volume loading. This defect was prominent at a younger age (that is, 3 months) in the UNX glucose intolerant than the UNX control (6 months old) rat. Chronic taurine treatment ameliorated the reduction in saline-induced diuresis and natriuresis by both the UNX control and the UNX glucose intolerant rat. Both an increase in glomerular filtration and a reduction in tubular reabsorption of fluid and sodium caused this taurine-mediated improvement in renal excretory function.

CONCLUSION

Taurine treatment protects the kidney of the UNX rat against an age-dependent decline in excretory efficiency.

Taurine in submandibular gland of the rat: effect of muscarinic drugs

Taurine exerts a number of actions in mammalian cells, including regulation of ion transport and osmoregulation. The production and secretion of saliva involve transepithelial ion transport, thereby making the plasma-like primary saliva hypotonic before secretion. Therefore, it is plausible to suggest modulation of salivary taurine by muscarinic agents that affect salivary gland function. One of the objectives of this study was to determine tissue content and localization of taurine in the submandibular gland of the rat. Further, we determined whether treatment with muscarinic drugs that either increase (e.g., pilocarpine) or decrease (e.g., propantheline) saliva secretion affects the submandibular gland taurine content. The results indicate that the submandibular gland contains an appreciable amount of taurine (8.9 +/- 0.3 micromoles/g wet wt). Further, acute treatment of the rats with either of the muscarinic drugs did not significantly affect tissue taurine content compared to the control group. By contrast, chronic treatment with propantheline, but not pilocarpine, reduced the tissue content of taurine compared to the control rats (p<0.05). Utilizing light microscopic immunohistochemical techniques, intense immunoreactivity was found primarily in the striated ducts of the submandibular gland. Neither pilocarpine nor propantheline treatment led to differential distribution of immunoreactivity in this tissue. In conclusion, the submandibular gland contains an appreciable amount of taurine, primarily in the striated ducts, that can be decreased by chronic muscarinic receptor blockade.

Regulation of high affinity taurine transport in goldfish and rat retinal cells

Adaptive regulation and modulation by phosphorylation are mechanisms by which some cells control taurine transport. Goldfish and rat retinal cells were incubated with the activator of protein kinase C, phorbol 12,13-dibutyrate (PDBu), or the inhibitor of protein phosphatases, okadaic acid (OKA). OKA, 1 nM, inhibited the uptake of taurine at short period of incubation in goldfish retinal cells, and at low concentrations in rat retinal cells incubated with the inhibitor for 1 h. PDBu treatment did not produce significant effects. Isolated Müller cells from the goldfish retina presented a clear adaptive regulation and a decrease of taurine uptake by increasing phosphorylation either by the stimulation of PKC with PDBu or the inhibition of phosphatases with OKA.

Dietary taurine manipulations in aged male Fischer 344 rat tissue: taurine concentration, taurine biosynthesis, and oxidative markers11Abbreviations: CA, cysteic acid; CSA, cysteine sulfinic acid; CSD, cysteine sulfinic acid decarboxylase; CDO, cysteine dioxygenase; DNPH, 2,4-dinitrophenylhydrazin; DPPH, α,α-diphenyl-β-picrylhydrazyl; F344, Fischer 344; HPLC-ECD, high performance liquid chromatography with electrochemical detection; MDCK, Madin Darby canine kidney; PCA, perchloric acid; TAU, taurine; TBARS, thiobarbituric acid; and TCA, trichloroacetic acid

Taurine (TAU) is a ubiquitous sulfur-containing amino acid that has been proposed to be an antioxidant. The concentration of TAU decreases during aging, which may increase susceptibility to oxidative stress. Our study attempted to elucidate the mechanism for the age-dependent decrease in TAU content by examining TAU biosynthesis in aged rats. We also examined the effects of dietary TAU manipulations on TAU content and oxidative markers in aged male Fischer 344 (F344) rats. Adult (9 months) and aged (26 months) rats fed control diets, aged rats fed control diet and TAU-supplemented (1.5%) water, and aged rats fed a TAU-deficient diet were used. We observed a significant age-related decrease in TAU content in liver, kidney, and cerebellum. Dietary TAU supplementation increased tissue TAU content, whereas dietary TAU restriction had no effect. Enzyme-dependent TAU synthesis showed an age-dependent reduction in liver that was decreased further by TAU supplementation. Protein carbonyl content was elevated in the cerebral cortex and kidney of aged rats and was attenuated by TAU supplementation. A trend for a decrease in protein and acid-soluble thiol contents in hepatic tissue of aged rats was observed, and this was attenuated with dietary TAU supplementation. Our data show that a decrease in hepatic TAU biosynthesis may cause, in part, the observed decline in tissue TAU content in aged F344 rats, and TAU supplementation can restore TAU levels. Our study indicates that a decline in TAU content may exacerbate oxidative stress in aged rats, which can be reversed by dietary TAU supplementation.

Taurine modulates arginine vasopressin-mediated regulation of renal function

Taurine has been implicated in the regulation of arginine vasopressin (AVP) secretion, and we have previously shown altered renal excretory function in the taurine-depleted rat. To further elucidate the role of taurine in AVP-mediated renal responses, the effects of an antagonist for renal AVP receptors were examined in four groups of conscious rats: control, taurine-supplemented, taurine-depleted, and taurine-repleted. Control and taurine-supplemented rats displayed similar and significant AVP receptor antagonist-induced elevations in fluid excretion, sodium excretion, and free water clearance but a marked reduction in urine osmolality. These effects are consistent with inhibition of endogenous AVP activity. By contrast, in the taurine-depleted rats, the magnitude and the time course of drug-induced renal excretory responses lagged behind those of the control and taurine-supplemented groups. Further, baseline urine osmolality was significantly higher in the taurine-depleted compared with the control or taurine-supplemented groups. However, after administration of the antagonist, taurine-depleted rats manifested a delayed but more marked reduction in urine osmolality, thereby eliminating the baseline differential that existed between the taurine-depleted rats and control or taurine-supplemented groups. Consistent with these observations, plasma AVP was significantly increased in the taurine-depleted compared with the control rats. Interestingly, taurine repletion shifted all responses closer to the control group. Analysis of the data suggests that the effect of the antagonist on renal excretory function is related primarily to altered tubular reabsorption activity. These observations suggest that taurine modulates renal function, and, thereby, body fluid homeostasis, through an AVP-dependent mechanism.

Taurine down-regulates basal and osmolarity-induced gene expression of its transporter, but not the gene expression of its biosynthetic enzymes, in astrocyte primary cultures

Taurine content of astrocytes is primarily regulated by transport from the extracellular medium and endogenous biosynthesis from cysteine. We have investigated the gene expression of the taurine transporter (TauT) and the taurine biosynthetic enzymes, cysteine dioxygenase (CDO) and cysteine sulfinate decarboxylase (CSD), in astrocyte primary cultures in relationship to cell taurine content. TauT, CDO, and CSD mRNA levels were determined through quantitative RT-PCR. Cell taurine content was depleted by adapting the cells to a taurine-free chemically defined medium and increased by incubating the cells in the same medium containing exogenous taurine. With increased cell taurine content the level of TauT mRNA decreased, whereas the levels of CDO and CSD mRNA remained unchanged. In astrocytes exposed to a hyperosmotic medium the TauT mRNA level increased, whereas the CDO and CSD mRNA levels were not significantly altered. The osmolarity-induced up-regulation of TauT mRNA expression was fully prevented by increasing cell taurine content. Thus, the gene expression of the taurine transporter, but not that of the taurine biosynthetic enzymes, appears to be under the control of two antagonistic regulations, namely, a taurine-induced down-regulation and an osmolarity-induced up-regulation.

Cysteine sulfinate decarboxylase and cysteine dioxygenase activities do not correlate with strain-specific changes in hepatic and cerebellar taurine content in aged rats

Taurine is a free sulfur-containing amino acid that is found in abundance in mammalian tissues and fluids. Many biological roles have been proposed for this amino acid, including reducing oxidative stress and cytotoxicity. Taurine has previously been reported to decline in tissues during aging which could exacerbate an age-related increase in oxidative stress. The aim of the present study was to elucidate the mechanism responsible for the observed decline in tissue taurine content. We measured the activity of the major taurine biosynthetic enzymes, cysteine sulfinate decarboxylase and cysteine dioxygenase, in liver and cerebellar tissues of rats. Tissues from male adult and aged Fischer 344 (F344; 10 and 28 months), Sprague-Dawley (SD; 5, 20 and 25 months), and F344/Brown-Norway hybrid (FBNF1; 14 and 33.5 months) rats were used. We observed a significant decline in hepatic taurine content of the F344 animals but the decline in the liver of SD and FBNF1 animals was non-significant. Hepatic cysteine sulfinate decarboxylase and cysteine dioxygenase activities were significantly lower in aged F344 rats but not in the other strains. Cerebellar taurine content was significantly lower in aged F344 and SD rats without a concomitant decline in cysteine sulfinate decarboxylase activity. These results suggest that a decline in hepatic de novo taurine biosynthesis might be partially responsible for a reduction in tissue taurine content in F344 rats.

Renal excretory responses of taurine-depleted rats to hypotonic and hypertonic saline infusion

Male Wistar-Kyoto rats were given either tap water (control) or 3% beta-alanine (taurine-depleted) for three weeks. To prepare for the kidney function studies, the animals were then implanted with femoral vessels and bladder catheters. Two days after surgery, each rat was given an intravenous infusion of saline at the rate of 50 microliter/min and urine samples were collected at specific time intervals. An isotonic saline solution (0.9% NaCl) was infused for determination of baseline parameters and was followed by the infusion of a hypotonic saline solution (0.45% NaCl). Two days later, the infusion protocol was repeated in the same animals; however, a hypertonic saline solution (1.8% NaCl) was substituted for the hypotonic saline solution. Renal excretion of fluid and sodium increased in the control, but not taurine-depleted, rats during the hypotonic saline infusion. Interestingly, diuretic and natriuretic responses were similar between the groups during hypertonic saline infusion. The results suggest that taurine-depletion in rats affects renal excretory responses to a hypotonic, but not a hypertonic, saline solution.

The role of taurine in infant nutrition

The importance of taurine in the diet of pre-term and term infants has not always been clearly understood and is a topic of interest to students of infant nutrition. Recent evidence indicates that it should be considered one of the "conditionally essential" amino acids in infant nutrition. Plasma values for taurine will fall if infants are fed a taurine-free formula or do not have taurine provided in the TPN solution. Urine taurine values also fall, which is indicative of an attempt by the kidney to conserve taurine. The very-low-birth-weight infant, for a variety of reasons involving the maturation of tubular transport function, cannot maximally conserve taurine by enhancing renal reabsorption and, hence, is potentially at greater risk for taurine depletion than larger pre-term or term infants, and certainly more than older children who have taurine in their diet. Taurine has an important role in fat absorption in pre-term and possibly term infants and in children with cystic fibrosis. Because taurine-conjugated bile acids are better emulsifiers of fat than glycine-conjugated bile acids, the dietary (or TPN) intake has a direct influence on absorption of lipids. Taurine supplementation of formulas or TPN solutions could potentially serve to minimize the brain phospholipid fatty acid composition differences between formula-fed and human milk-fed infants. Taurine appears to have a role in infants, children, and even adults receiving most (> 75%) of their calories from TPN solutions in the prevention of granulation of the retina and electroencephalographic changes. Taurine has also been reported to improve maturation of auditory-evoked responses in pre-term infants, although this point is not fully established. Clearly, taurine is an important osmolyte in the brain and the renal medulla. At these locations, it is a primary factor in the cell volume regulatory process, in which brain or renal cells swell or shrink in response to osmolar changes, but return to their previous volume according to the uptake or release of taurine. While there is a dearth of clinical studies in man concerning this volume regulatory response, studies in cats, rats, and dog kidney cells indicate the protective role of taurine in hyperosmolar stress. The infant depleted of taurine may not be able to respond to hyper- or hyponatremic stress without massive changes in neuronal volume, which has obvious clinical significance. The fact that the brain content of taurine is very high at birth and falls with maturation may be a protective feature, or compensation for renal immaturity Defining an amino acid as "conditionally essential" requires that deficiency result in a clinical consequence or consequences which can be reversed by supplementation. In pre-term and term infants, taurine insufficiency results in impaired fat absorption, bile acid secretion, retinal function, and hepatic function, all of which can be reversed by taurine supplementation. Therefore, this small beta-amino acid, taurine, is indeed conditionally essential.

Renal excretory responses to saline load in the taurine-depleted and the taurine-supplemented rat

Taurine is found in high concentrations in mammalian cells. Despite recognition of its role as an organic osmolyte in the kidney, information regarding its effects on renal fluid and electrolyte excretion is sparse. Therefore, the objective of the first series of experiments was to determine the effects of taurine depletion on renal excretory responses to a saline load. To induce taurine depletion, male Wistar-Kyoto (WKY) rats were treated with tap water containing 3% beta-alanine for 3 weeks. Taurine depletion reduced the initial rates of fluid and sodium excretion after an intravenous saline load. This effect was attributed to taurine depletion since maintenance of the taurine-depleted rats on tap water for 2 days to remove the effects of beta-alanine yielded the same pattern as the taurine-depleted rats exposed to beta-alanine at the time of the experiment. Nonetheless, rats exposed to short-term beta-alanine treatment, which has no influence on kidney taurine content, demonstrated a larger (approximately 25%) natriuretic but not diuretic response to the isotonic saline load than either the control or taurine-depleted rats. These data suggest that beta-alanine-induced inhibition of tubular reabsorption of taurine may result in subsequent excretion of taurine with attendant natriuresis early in the course of beta-alanine treatment. We also tested the hypothesis that taurine potentiates the renal excretory responses to an isotonic saline load in WKY rats. Inclusion of taurine in the infusate significantly increased natriuresis and diuresis after a saline load. This effect was greater in animals fed a basal than a high NaCl diet. Our data support a role for taurine as a natriuretic and diuretic agent.

Regulation of the taurine transporter gene in the S3 segment of the proximal tubule

Traditionally, bulk amino acid reabsorption in the kidney has been thought to be localized to the early portions of the proximal nephron. Adult Sprague-Dawley rats were fed diets with low, normal, and high taurine content for two weeks. Kidneys were hybridized with an 35S-radiolabeled complementary RNA probe to the rB16a subclone encoding the extracellular and transmembrane domains of the rat brain taurine transporter. Identical fragments were generated by RT-PCR from rat brain and kidneys as confirmed by DNA sequencing. Hybridization was localized to the outer zone of the medulla of all the kidneys. In the normal diet animals, taurine transporter mRNA was localized to the S3 segment of the proximal tubule, to the loop of Henle in the medulla, and to the glomerular epithelial cell layer. With taurine restriction, taurine transporter mRNA expression was up-regulated predominantly in the S3 segment and was virtually absent in this segment in animals supplemented with taurine. These experiments have precisely localized the rat kidney taurine transporter gene, demonstrating regulation that is limited to the S3 segment of the proximal tubule.

Functional expression of rat renal cortex taurine transporter in Xenopus laevis oocytes: adaptive regulation by dietary manipulation

Renal brush border taurine transport adapts to changes in the dietary intake of sulfur amino acids with increased rates after dietary restriction and reduced transport after dietary surplus. The Xenopus laevis oocyte expression system was used to define the renal adaptive response to dietary manipulation. Injection of poly(A)+ RNA isolated from rat kidney cortex resulted in a time- and dose-dependent increase in NaCl-taurine cotransport in oocytes. The Km of the expressed taurine transporter was 22.5 microM. In oocytes, injection of 40 ng of poly(A)+ RNA from kidneys of low taurine diet (LTD)-fed rats elicited 2-fold the taurine uptake of normal taurine diet (NTD)-fed rats and >3-fold the uptake of high taurine diet (HTD)-fed rats. Northern blots of rat kidneys using a riboprobe derived from an rB16a (rat brain taurine transporter) subclone revealed 6.2- and 2.4-kb transcripts, the abundance of which were increased or decreased in LTD- or HTD-fed rats, respectively, as compared with NTD-fed rats. A approximately 70-kD protein was detected by Western blot using an antibody derived from a synthetic peptide corresponding to a conserved intracellular segment of rB16a. The abundance of the approximately 70-kD protein was increased or decreased in LTD- or HTD-fed rats, respectively, as compared with NTD-fed rats. In conclusion, expression of the rat renal taurine transporter is regulated by dietary taurine at the level of mRNA accumulation and protein synthesis.

Adaptive regulation of taurine and beta-alanine uptake in a human kidney cell line from the proximal tubule

1. The underlying mechanisms involved in the adaptive regulation of beta-amino acid uptake in the human proximal tubule were examined by use of an immortalized human embryonic kidney epithelial cell line (IHKE). 2. The results indicated that the adaptive response to maintain whole-body taurine homeostasis occurs predominantly via changes in the activity of the high-affinity taurine transport system by alterations in the uptake capacity and with an unaffected half-saturation constant. An adaptive response was not observed for the structurally related beta-alanine. 3. Only colchicine, which interferes with microtubule organization, was capable of blocking the response to alterations of taurine in cell medium, whereas inhibition of protein and nucleic acid synthesis by cycloheximide and actinomycin D, respectively, did not change the adaptive pattern. 4. Phorbol 12-myristate 13-acetate (PMA), mimicking the effects of diacylglycerol, induced inhibition of both beta-alanine and taurine uptake. By contrast, the Ca2(+)-ionophore A23187, mimicking the effects of IP3, only stimulated the uptake of taurine but not the influx of beta-alanine. However, the effect of PMA down-regulation and A23187 up-regulation was rapid and short-lived in contrast to the adaptive response, suggesting that the inositol phospholipid pathway involving diacetylglycerol and IP3 is less likely to be linked directly to the adaptive regulation, but rather plays a role in short-term regulation.

Adaptive regulation of MDCK cell taurine transporter (pNCT) mRNA: transcription of pNCT gene is regulated by external taurine concentration

NaCl-dependent taurine transporter (pNCT) activity of MDCK cells (Madin-Darby canine kidney) is up- or down-regulated by medium taurine manipulation. In this study we found that the abundance of pNCT mRNA was up- or down-regulated after cells were incubated in media containing 0 microM taurine or 500 microM taurine for 24 h. Down-regulation was observed after 12 h exposure to high taurine (500 microM) and mRNA abundance was appreciably reduced after 72 h exposure. Nuclear run-off assays show that the gene for pNCT is induced at the transcriptional level by taurine. Addition of cycloheximide blocked the adaptive response and reduced transcription of pNCT mRNA in MDCK cells. Cycloheximide had virtually no effect on pNCT mRNA stability, suggesting that ongoing protein synthesis is required for adaptive regulation of pNCT gene transcription.

Taurine transport in cultured choroid plexus

PURPOSE

Taurine, a beta-amino acid, is a neuromodulator which interacts functionally with the glycinergic, GABAergic, cholinergic and adrenergic systems. Although a continuous cell culture model is not available for the choroid plexus epithelia, we recently described a primary cell culture of rabbit choroid plexus epithelia. The goal of the current study was to determine the suitability of this primary cell culture for the study of the Na(+)-taurine transporter in the rabbit choroid plexus.

METHODS

A primary cell culture of rabbit choroid plexus epithelial cells was grown on semi-permeable filters and kinetics of 3H-taurine uptake were ascertained.

RESULTS

Taurine transport in the cultured choroid plexus cell was Na(+)-dependent and saturable (Km = 156 microM). The beta-amino acids, beta-alanine and taurine, significantly inhibited Na(+)-driven taurine transport whereas L-alanine partially inhibited taurine transport in the cultured cells. In addition, we observed that the activity of the Na(+)-taurine transporter is affected by exposure to taurine in the media.

CONCLUSIONS

These results-demonstrate that a Na(+)-taurine transporter with characteristics similar to those in the intact tissue is expressed in cultured choroid plexus epithelial cells. The transporter may undergo adaptive regulation and play a role in taurine homeostasis in the central nervous system.

Choroid plexus taurine transport

The putative osmoregulatory agent, taurine, is lost from the brain during hypo-osmotic stress or ischemia, but the regulatory mechanisms involved in this loss have not been fully elucidated. In this study, we have examined taurine transport by the isolated rat choroid plexus, one element of the brain-blood interface, and examined how it may be regulated as part of brain volume regulation. Choroid plexus taurine uptake was Na- and Cl-dependent with a Vmax and Km of 6.5 +/- 0.3 pmol/mg/min and 232 +/- 33 microM. The latter is substantially greater than the normal CSF taurine concentration and this may be important in removing taurine released into the CSF during parenchymal cell swelling. Taurine uptake also appears calmodulin dependent as it was reduced by 84 and 91% in the presence of 25 microM trifluoperazine and 100 microM W-7, two calmodulin inhibitors. Taurine efflux from choroid plexus was stimulated by trifluoperazine, taurine, and hypo-osmotic stress. The latter two effects were reduced by niflumic acid, suggesting that taurine and hypo-osmotic stress act on the same pathway. The stimulation of efflux by hypo-osmotic stress decreased with time, whereas the effect of external taurine was sustained. If this efflux pathway is involved in the movement of taurine from choroid plexus to blood, these results suggest that changes in extracellular taurine may be more important than the direct effect of hypo-osmolality in the long-term loss of taurine from the brain.

Regulation of expression of taurine transport in two continuous renal epithelial cell lines and inhibition of taurine transporter by a site-directed antibody

The renal tubular epithelium adapts to changes in the sulfur amino acid composition of the diet, particularly in terms of reabsorption of taurine. The adaptive response is expressed by enhanced or decreased NaCl-dependent taurine transport by rat renal brush border membrane vesicles (BBMV). Taurine transport activity in two cultured renal epithelial cell lines (MDCK and LLC-PK1) is up- or down-regulated by extracellular taurine concentration as the result of reciprocal changes in the Vmax of the transporter. In MDCK cells, abundance of taurine transporter mRNA (pNCT mRNA) was up- or down-regulated after incubation in media containing 0, 50, or 500 microM taurine. Decreased mRNA was observed in both cell lines after 12 h, and it was appreciably reduced after 72 h exposure to 500 microM taurine. Northern blot analysis of mRNA from LLC-PK1 cells using pNCT cDNA as a riboprobe showed that two transcripts, 9.6 kb and 7.2 kb, were expressed; the abundance of mRNA was increased or decreased after incubation in taurine-free or high taurine medium, respectively. Down-regulation was observed primarily in the 7.2 kb transcript after 24 h incubation. Rapid up-regulation occurred in the 9.6 kb transcript within 12 h of transfer from high to low taurine. Nuclear run-off assays showed that the gene for pNCT is induced at the transcriptional level by taurine. Regulation of expression of the taurine transporter was also studied by injection of pNCT cRNA into Xenopus laevis oocytes. Expression of transport activity was significantly reduced (64%) when oocytes were incubated in 50 microM taurine as compared to 0 microM taurine. Transport activity was totally blocked when pNCT cRNA-injected oocytes were exposed to an active phorbol ester, PMA (10(-6) M). Inhibition of uptake was reversed by staurosporine, an inhibitor of protein kinase C activity. An inactive phorbol ester, 4 alpha-phorbol, had no effect on taurine transport. A polyclonal antibody directed a highly conserved intracellular segment between homologous transmembrane domains VI and VII inhibited taurine transport activity in both pNCT cRNA-injected oocytes and BBMV. Incubation of oocytes with 10 micrograms/ml antibody (Ab) reduced taurine uptake to 46% of control, and 20-80 micrograms/ml Ab reduced uptake to 20% of control. In BBMV, active taurine uptake (10 microM) was inhibited approximately 30% by 10 pg Ab/mg protein, whereas none specific IgG had no significant effect. Proline uptake (20 microM) by BBMV was not inhibited by the Ab, nor was GABA uptake (50 microM). Two pNCT proteins, approximately 70 kD and approximately 30 kD, were detected by Western blot, and the abundance of both was regulated by medium taurine.

IN CONCLUSION

(i) regulation of taurine transport activity in LLC-PK1 cells by medium taurine occurs at a level of mRNA transcription; (ii) regulation of pNCT occurs at both transcriptional and translational levels; (iii) pNCT expression is regulated by protein kinase C-dependent phosphorylation; and (iv) the intracellular segment between domains VI and VII may be required for activation of the taurine transporter; this segment may function as a gate in taurine transport.

Cyclic AMP-dependent up-regulation of the taurine transporter in a human retinal pigment epithelial cell line

This investigation was undertaken to study the role of cAMP in the regulation of the taurine transporter expressed in a human retinal pigment epithelial (HRPE) cell line. Treatment of the HRPE cells with cholera toxin for 24 h was found to stimulate the taurine transporter activity, as measured by taurine transport into the cells in the presence of NaCl, to a significant extent. The stimulation was 50-60% at 100 ng/ml cholera toxin. This stimulation was specific to the taurine transporter since the transport of two other amino acids (leucine and alanine), which are not substrates for the taurine transporter, was not affected by cholera toxin under similar conditions. Exposure of the cells to cholera toxin for a time period > 4 h was needed to elicit the stimulatory effect. The cholera toxin-induced stimulation of the taurine transporter activity was associated with an increase in the maximal velocity of the transport system. The affinity of the transporter for taurine was not altered by the treatment. The stimulatory effect was markedly blunted when the treatment of the cells with cholera toxin was done in the presence of actinomycin D, an inhibitor of transcription, or cycloheximide, an inhibitor of translation. The increase in the taurine transporter activity induced by cholera toxin was associated with a 2.6-fold increase in the steady state levels of the transporter mRNA. Measurement of cyclic nucleotides in control and cholera toxin-treated cells revealed that the toxin caused a 20-fold increase in the cellular levels of cAMP, the levels of cGMP remaining unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

Taurine in pediatric nutrition

The past 20 years have seen the status of taurine change from an end product of methionine and cysteine metabolism and substance conjugated to bile acids to that of an important, and sometimes essential, nutrient. It is now added to most synthetic human infant formulas and pediatric parenteral solutions throughout the world. This article describes the research that led to this end.

Substrate-specific regulation of the taurine transporter in human placental choriocarcinoma cells (JAR)

Exposure of the JAR human placental choriocarcinoma cells to taurine leads to a marked decrease in the activity of the taurine transporter in these cells. The ability to induce this adaptive response is not unique to taurine but is shared by other substrates of the transporter as well. Compounds such as betaine and alpha-aminoisobutyric acid which are not substrates for the transporter do not produce this effect. The change in the taurine transporter activity induced by taurine exposure is however unique to the taurine transporter because the activities of many other transport systems remain unaffected under these conditions. The adaptive regulation is not associated with any change in the dependence of the transporter activity on Na+ and Cl-, in the Na+/Cl-/taurine stoichiometry and in the affinities of the transporter for Na+ and Cl-. The decrease in the transporter activity caused by taurine exposure is due to a decrease in the maximal velocity of the transporter, and to a lesser extent, in the substrate affinity of the transporter. The decrease in the transporter activity observed in intact cells is demonstrable in plasma membrane vesicles after isolation from control and taurine-exposed cells. Cycloheximide and actinomycin D block the adaptive response in intact cells to a significant extent, but not completely. Northern blot analysis of mRNA from control and taurine-exposed cells shows that taurine exposure causes a significant decrease in the steady state levels of the taurine transporter mRNA. It is concluded that the activity of the taurine transporter in JAR cells is subject to substrate-specific adaptive regulation and that transcriptional as well as posttranscriptional events are involved in this regulatory process.

Taurine analog modulation of taurine uptake by two different mechanisms in cultured glial cells

Previous data have shown that HEPES, a taurine structural analog, inhibits the uptake of taurine by cultured cells differently, depending on its addition either to the culture medium or to the Krebs-Ringer buffer used for cell incubation during taurine uptake measurements (Lleu and Rebel, J Neurosci Res 23: 78-86, 1989). An extensive study of the effect of numerous other taurine structural analogs on taurine uptake by cultured glial cells was carried out. Our results show that taurine uptake modulation by structural analogs follows two different mechanisms. For the first mechanism, observable after the simultaneous presence of taurine and of its analog during the incubation time of the uptake experiment (10 min), the amine function on the molecule is essential. The sulfonate group could be replaced either by a sulfinic group or by a carboxylic group. beta-Alanine, hypotaurine, acetyltaurine, guanidinoethanesulfonate and guanidinopropionate are the most potent inhibitors in this first mechanism. For the second mechanism, which requires the presence of the analog in the culture medium during the 48 hr preceding the taurine uptake measurement, the simultaneous presence of an amine and of a sulfonate group or of an amine and a sulfinate group is required. Carboxylates are ineffective in modulating taurine uptake in this mechanism. The sulfonate buffers synthesized by Good et al. (Biochemistry 5: 467-477, 1966) also affect taurine uptake in both mechanisms.

Taurine and beta-alanine transport in an established human kidney cell line derived from the proximal tubule

The transport mechanisms of taurine and beta-alanine by an immortalized human embryonic kidney epithelial cell line (IHKE) were examined. The uptake of these beta-amino acids was characterized by two Na(+)-dependent transport components, whereas an inwardly directed H(+)-gradient only stimulated amino acid influx to a small extent and in the absence of sodium. Competition experiments revealed that taurine and beta-alanine drastically reduced the uptake of one another by the high-affinity Na(+)-dependent transport system. However, some alpha-amino acids could also compete with the beta-amino acids, but with a low affinity. Examinations of the effect of different anions on the Na(+)-dependent uptake of taurine at a low amino acid concentration (240 nM) revealed a specific requirement for Cl-, whereas Cl- had no measurable effect at a higher concentration (1.0 mM) of taurine. In addition, activation of taurine transport as a function of Na+ and Cl- concentration indicated a probable coupling ratio of 3 Na+/1 Cl-/1 taurine for the high-affinity carrier. Finally, cellular regulation of taurine transport was indicated by the finding that pretreatment with taurine containing media decreased the activity of the taurine transporter(s).

A comparative study on the uptake of alpha-aminoisobutyric acid by normal and immortalized human embryonic kidney cells from proximal tubule

We investigated whether an immortalized human kidney epithelial cell line (IHKE), compared with normal embryonic cells (NHKE), can be used as a representative system with which to characterize the transport of neutral amino acids in the proximal tubule of the human kidney. The IHKE cell line, immortalized by treatment with NiSO4, exhibited microvilli and enzyme markers specific for highly specialized tubule cells. The Na(+)-dependent uptake of alpha-aminoisobutyric acid (AIB) by IHKE and NHKE cells occurred by means of a single transport system with identical half-saturation constants, but the capacity for uptake was higher in the immortalized cells. Proton-dependent influx of AIB was also mediated by a single transport component with similar uptake characteristics in both types of cells. Imposition of an H(+)-gradient to a Na(+)-gradient reduced the sodium dependent uptake of AIB with the exception of short incubation time (1 min), where addition of a proton gradient produced a marked increase in the Na(+)-dependent influx of AIB in NHKE but not in IHKE cells. Competition experiments revealed that the Na(+)-dependent uptake at 50 microM AIB was reduced by neutral alpha-amino acids in the two cell lines. L-Glutamate, L-aspartate, L-arginine and the beta-amino acid taurine had no effect. Only in the IHKE cell line did addition of 5 mM L-lysine produce a slight inhibition. Except for L-proline all of the neutral and acidic amino acids tested reduced the H(+)-dependent uptake of AIB in the IHKE cell line. By contrast, addition of L-aspartate did not influence the transport of AIB in NHKE cells. L-Arginine, but not L-lysine decreased the influx in both cell lines. We conclude that the IHKE cell line has retained the capability to accumulate AIB by transport protein(s) similar to those present for neutral alpha-amino acids in NHKE cells.

Renal amino acid transport: cellular and molecular events from clearance studies to frog eggs

This article reviews recent advances in the mechanisms of renal amino acid transport. Renal amino acid transport is necessary to efficiently reclaim approximately 450 mmol amino acids from the glomerular ultrafiltrate each day in man. In general, individual amino acids are transported across the epithelial membrane of the proximal tubule by a sodium (Na+) dependent mechanism. This cotransport process utilizes the energy of the Na+ gradient to enter the cell. The amino acid then exits the basolateral surface and Na+ is pumped out by the Na(+)-K(+)-ATPase located in the basolateral membrane. In addition to the cellular accumulation of amino acids across the luminal membrane, these compounds may be taken up by the cell from the basolateral surface. Most amino acids are transported both individually and in a series of seven group specific processes. Human disorders of amino acid transport have been described for six of the seven transport systems. The process of ontogeny of amino acid accumulation by the proximal tubule is a complex one and will be further discussed in this review. A number of factors including pH, ion dependency, electrogenicity of transport process, as well as a variety of hormonal factors, may contribute to the regulation of amino acid transport. Gene expression of several amino acid transporters has been successfully performed using the oocyte of the frog Xenopus laevis. Using this system, a number of transporters have been cloned. Such a strategy will permit the cloning of virtually all transporter molecules, and thus we can anticipate the elucidation of the structure of the transporters. However, for a comprehensive understanding of cytoskeletal interactions protein phosphorylation and phospholipid domains and their linkage to the primary structure of the transporter need to be studied. The future for research in this area is indeed a bright one.

Activation of amino acid diffusion by a volume increase in cultured kidney (MDCK) cells

When MDCK cells are cultured in MEM, they maintain a high concentration of three amino acids: glutamate (25 mM), taurine (19 mM) and glycine (9 mM). With incubation of the cells in hypotonic media, the contents of these amino acids measured by HPLC are reduced in different time courses: taurine decreases most rapidly, followed by glutamate and glycine. All these losses are Na+ independent. To determine the transport mechanism activated by the hypotonic media, increasing external concentrations reaching 60 mM for nine different amino acids in Na(+)-free media were tested separately. For the five neutral (zwitterionic) amino acids, taurine, glycine, alanine, phenylalanine and tryptophan, cell contents increased linearly with external concentrations in hypotonic media, whereas in isotonic media only a slight rise was observed. The two anionic amino acids, glutamate and aspartate, were also increased linearly with their external concentrations in hypotonic media, but the changes were lower than those found for neutral amino acids. The presence of a negative membrane potential was responsible for this behavior since, using a K+ hypotonic medium which clamps the potential to zero, the glutamate content was found to increase linearly with an amplitude similar to the one observed for neutral amino acid. When external concentrations of two cationic amino acids, arginine and lysine, were increased in hypotonic media, only a small change, similar to that in isotonic media, was observed. These results indicate that a diffusion process for neutral and anionic amino acids is activated by a volume increase and it is suggested that an anion channel is involved.

Molecular cloning of the cDNA for an MDCK cell Na(+)- and Cl(-)-dependent taurine transporter that is regulated by hypertonicity

Cells in the hypertonic renal medulla maintain their intracellular ion concentration at isotonic levels, despite much higher concentrations of extracellular electrolytes, by accumulating high concentrations of nonperturbing small organic solutes termed osmolytes. Taurine has been identified as a nonperturbing osmolyte in the renal medulla and Madin-Darby canine kidney (MDCK) cells. In hypertonic medium, the increased accumulation of taurine in MDCK cells is the result of increased activity of a Na(+)- and Cl(-)-dependent taurine transporter. We have isolated a cDNA encoding a Na(+)- and Cl(-)-dependent taurine transporter, whose sequence corresponds to a protein of 655 amino acids with significant amino acid sequence similarity to previously cloned Na(+)- and Cl(-)-dependent transporters, including the MDCK cell betaine/gamma-aminobutyric acid transporter and several brain neurotransmitter transporters. Northern hybridization indicates that mRNA for the taurine transporter is present in renal cortex and medulla, ileal mucosa, brain, liver, and heart. The abundance of mRNA for the taurine transporter is increased in MDCK cells cultured in hypertonic medium, suggesting that regulation of transport activity by medium hypertonicity occurs at the level of mRNA accumulation.

Characteristics of taurine transport in cultured renal epithelial cell lines: asymmetric polarity of proximal and distal cell lines

Taurine transport was determined in two continuous, renal epithelial cell lines: LLC-PK1 derived from the proximal tubule of the pig, and the Madin-Darby canine kidney cell (MDCK) from the distal tubule of the dog. In LLC-PK1, taurine transport is maximal at the apical surface, whereas in MDCK cells, transport is greatest at the basolateral surface. Transport is highly dependent on both sodium and chloride in the external medium, and is specific for beta-amino acids. The apical and basolateral surfaces of both cell lines show an adaptive response to extracellular taurine concentration, but only the basolateral surface of the MDCK cell responds to hyperosomolality by increased taurine accumulation. Thus, differential control of the beta-amino acid transport system by substrate and external tonicity exists. The role of the beta-amino acid transport system may differ according to the origin of the cell: in the proximal renal tubular cell, net transepithelial reabsorption of filtered taurine increases the body pool. By contrast, taurine accumulation by distal tubular cells may form a mechanism of cell volume regulation in response to osmotic stress.

Taurine behaves as an osmolyte in Madin-Darby canine kidney cells. Protection by polarized, regulated transport of taurine

Using a clonal growth assay, we demonstrated that taurine, a nonperturbing osmolyte accumulated in kidney medulla, brain, and some other tissues of hypertonic experimental animals can function as a nonperturbing osmolyte in Madin-Darby canine kidney (MDCK) cells. The taurine content of hypertonic MDCK cells is twice that of isotonic MDCK cells (isotonic 160 nmol/mg protein; hypertonic 320 nmol/mg protein). Therefore we studied taurine transport in MDCK cells grown on porous supports and then studied the effect of hypertonicity which is known to elicit increased uptake of some other nonperturbing osmolytes by MDCK cells. Basal uptake exceeded apical uptake, with Km and Vmax of 56 microM and 933 pmol/min.mg protein on the basal surface and 10 microM and 50 pmol/min.mg protein on the apical surface. On both surfaces, virtually all taurine uptake was Na+ and Cl- dependent. 24 h after cells were shifted to hypertonic medium (500 mosmol/kg), taurine uptake doubled on the basolateral surface without change on the apical surface. The response to hypertonicity was the result of an increase in Vmax without change in Km. There was no change in taurine efflux when cells were shifted from isotonic to hypertonic medium. When cells adapted to hypertonic medium were shifted to isotonic medium, a large transient basolateral efflux of taurine occurred within 10 min. We conclude that taurine can function as a nonperturbing osmolyte in MDCK cells and that tonicity-regulated taurine transport is a basolateral function in MDCK cells.

Characteristics of taurine transport in rat liver lysosomes

Taurine (2-aminoethanesulfonic acid) is a unique sulfur amino acid derivative that has putative nutritional, osmoregulatory, and neuroregulatory roles and is highly concentrated within a variety of cells. The permeability of Percoll density gradient purified rat liver lysosomes to taurine was examined. Intralysosomal amino acid analysis showed trace levels of taurine compared to most other amino acids. Taurine uptake was Na(+)-independent, with an overshoot between 5-10 minutes. Trichloroacetic acid extraction studies and detergent lysis confirmed that free taurine accumulated in the lysosomal space. Kinetic studies revealed heterogeneous uptake with values for Km1 = 31 +/- 1.82 and Km2 greater than 198 +/- 10.2 mM. The uptake had a pH optimal of 6.5 and was stimulated by the potassium specific ionophore valinomycin. The exodus rate was fairly rapid, with a t1/2 of 5 minutes at 37 degrees C. Analog inhibition studies indicated substrate specificity similar to the plasma membrane beta-alanine carrier system, with inhibition by beta-alanine, hypotaurine, and taurine. alpha-Alanine, 2-methylaminoisobutyric acid (MeAIB), and threonine were poor inhibitors. No effects were observed with sucrose and the photoaffinity derivative of taurine NAP-taurine [N-(4-azido-2-nitrophenyl)-2-aminoethanesulfonate]. In summary, rat liver lysosomes possess a high Km system for taurine transport that is sensitive to changes in K+ gradient and perhaps valinomycin induced diffusional membrane potential. These features may enable lysosomes to adapt to changing intracellular concentrations of this osmotic regulatory substance.

Amino acids as volume-regulatory osmolytes in mammalian cells

1. This review summarizes current knowledge relating to the volume-regulatory and osmoprotective functions of amino acids in mammalian cells exposed to anisosmotic fluids. 2. Experiments in vivo and in vitro have established that they play a significant role in regulating brain cell volume under these conditions, and that taurine may be of particular importance in this respect. 3. Their possible role in renal medulla is discussed, and it is suggested that they may protect cells against acute (but not long-term) osmotic variation. 4. Evidence is briefly presented regarding adaptive changes in amino acid content of other cell types.