Structural Analogues Of Taurine Research Articles

The taurine transporter substrate guanidinoethyl sulfonate mimics the action of taurine on long-term synaptic potentiation.

Taurine is especially abundant in rodent brain where it appears to be involved in osmoregulation and synaptic plasticity mechanisms. The demonstration of a physiological role for taurine has been hampered by the difficulty in modifying taurine levels in most tissues, including the brain. We used an experimental strategy to reduce taurine levels, involving treatment with guanidinoethyl sulfonate (GES), a structural analogue of taurine that, among other properties, acts as a competitive inhibitor of taurine transport. GES delivered in the drinking water of rats for 1month effectively reduced taurine levels in brain structures (hippocampus, cerebellum and cortex) and outside the brain (heart, muscle, kidney, liver and plasma) by between 50 and 80%, depending on the tissue. This partial taurine depletion did not affect either basal synaptic transmission or the late phase of long-term potentiation (late-LTP) in hippocampal slices. In vivo microdialysis studies in the hippocampus revealed that GES treatment reduced extracellular taurine levels and the magnitude of taurine released in response to the application of either N-methyl-D-aspartate (NMDA) or a hypoosmotic solution, without affecting release mechanisms. Finally, we demonstrated in hippocampal slices that a brief GES application can mimic taurine action on the conversion of a decremental LTP into a perdurable late-LTP, concluding that GES might replace taurine function in some mechanisms such as those implicated in synaptic plasticity.

Modulation of GABA-A receptors of astrocytes and STC-1 cells by taurine structural analogs.

Taurine activates and modulates GABA receptors in vivo as well as those expressed in heterologous systems. This study aimed to determine whether the structural analogs of taurine: homotaurine and hypotaurine, have the ability to activate GABA-A receptors that include GABAρ subunits. The expression of GABA-A receptors containing GABAρ has been reported in the STC-1 cells and astrocytes. In both cell types, taurine, homo-, and hypotaurine gated with low efficiency a picrotoxin-sensitive GABA-A receptor. The known bimodal modulatory effect of taurine on GABAρ receptors was not observed; however, differences between the activation and deactivation rates were detected when they were perfused together with GABA. In silico docking simulations suggested that taurine, hypo-, and homotaurine do not form a cation-π interaction such as that generated by GABA in the agonist-binding site of GABAρ. This observation complements the electrophysiological data suggesting that taurine and its analogs act as partial agonists of GABA-A receptors. All the observations above suggest that the structural analogs of taurine are partial agonists of GABA-A receptors that occupy the agonist-binding site, but their structures do not allow the proper interaction with the receptor to fully gate its Cl(-) channel.

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.

Enhancing effect of taurine on CYP7A1 mRNA expression in Hep G2 cells

Taurine has been reported to enhance cholesterol 7alpha-hydroxylase (CYP7A1) mRNA expression in animal models. However, no in vitro studies of this effect have been reported. The Hep G2 human hepatoma cell line has been recognized as a good model for studying the regulation of human CYP7A1. This work characterizes the effects of taurine on CYP7A1 mRNA levels of Hep G2 cells in a dose- and time-dependent manner. In the dose-dependent experiment, Hep G2 cells were treated with 0, 2, 10 or 20 mM taurine in the presence or absence of cholesterol 0.2 mM for 48 h. In the time-dependent experiment, Hep G2 cells were treated with 0 or 20 mM taurine for 4, 24 and 48 h with and without cholesterol 0.2 mM. Our data revealed that taurine showed time- and dose-response effects on CYP7A1 mRNA levels in Hep G2 cells. However, glycine - a structural analogue of taurine - did not have an effect on CYP7A1 gene expression. These results show that, in agreement to previous studies on animal models, taurine induces the mRNA levels of CYP7A1 in Hep G2 cells, which could enhance cholesterol conversion into bile acids. Also, Hep G2 cell line may be an appropriate model to study the effects of taurine on human cholesterol metabolism.

A specific taurine recognition site in the rabbit brain is responsible for taurine effects on thermoregulation.

(1) Taurine and GABA are recognized as endogenous cryogens. In a previous study, some structural analogues of taurine, namely 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine 1,1-dioxide (TAG), 2-aminoethylarsonic (AEA), 2-hydroxyethanesulfonic (ISE) and (+/-)cis-2-aminocyclohexane sulfonic acids (CAHS) have been shown to displace [(3)H]taurine binding from rabbit brain synaptic membrane preparations, without interacting either with GABA-ergic systems, nor with taurine uptake mechanism, thus behaving like direct taurinergic agents. (2) To answer the question whether the role of taurine as an endogenous cryogen depends on the activation of GABA receptors or that of specific taurine receptor(s), taurine or the above structural analogues were injected intracerebroventricularly in conscious, restrained rabbits singularly or in combination and their effects on rectal (RT)- and ear-skin temperature and gross motor behavior (GMB) were monitored. (3) Taurine (1.2 x 10(-6)-4.8 x 10(-5) mol) induced a dose-related hypothermia, vasodilation at ear vascular bed and inhibition of GMB. CAHS, at the highest dose tested (4.8 x 10(-5) mol) induced a taurine-like effect either on RT or GMB. On the contrary ISE, injected at the same doses of taurine, induced a dose-related hyperthermia, vasoconstriction and excitation of GMB. AEA and TAG caused a dose-related hyperthermia, but at doses higher than 1.2 x 10(-7) mol caused death within 24 h after treatment. (4) CAHS (4.8 x 10(-5) mol) antagonized the hyperthermic effect induced by TAG (1.2 x 10(-6) mol), AEA (1.2 x 10(-8) mol) or ISE (4.8 x 10(-5) mol). (5) In conclusion, these findings may indicate the existence of a recognition site specific for taurine, responsible for its effects on thermoregulation.

Suppressive effect of taurine on platelet-derived growth factor (PDGF) BB-induced c-fos and c-jun mRNA expressions through extracellular signal-regulated kinase (ERK) in mesenchymal cell lines.

Platelet-derived growth factor (PDGF) plays an important role in the pathogenic course of atherosclerosis, pulmonary fibrosis, and glomerulonephritis, and increased activity of the PDGF signaling pathway has been implicated as a contributing factor in the progression of the diseases. Taurine may be a prophylactic amino acid for atherosclerosis not only by decreasing plasma cholesterol level, but also by inhibiting the cell proliferation-signaling pathway. To elucidate how taurine affects the signaling pathway, we investigated the effect of taurine on the expression of immediate-early genes and activation of mitogen-activated protein kinases (MAPKs) in NIH/3T3 cells as standard mesenchymal cells. Taurine inhibited PDGF-BB-induced c-fos and c-jun mRNA expressions dose-dependently, although structural analogues of taurine did not. Taurine decreased the PDGF-induced p44/p42 ERK (extracellular signal-regulated kinase) phosphorylation state dose-dependently, although no phosphorylation was observed on JNK/SAPK (c-Jun N-terminal kinase/stress-activated protein kinase) and p38 MAPK. Further, PDGF-BB-induced tyrosine phosphorylation of the PDGF-beta receptor was not influenced by treatment with taurine, indicating that taurine never affects ligand-receptor interaction, and may act downstream of the PDGF receptor. Thus, the inhibitory mechanism of taurine on PDGF-induced c-fos and c-jun mRNA expressions may depend on the p44/p42 ERK pathway, but not on PDGF-beta receptor tyrosine phosphorylation, JNK/SAPK or p38 MAPK pathway. These results suggest that taurine may suppress the cell proliferation-signaling pathway through the inhibition of ERK activity and immediate-early gene expression.

Taurine inhibition of metal-stimulated catecholamine oxidation.

Taurine is an abundant amino acid found in mammalian tissues and it has been suggested to have cytoprotective functions. The aim of the present study was to determine if taurine had the potential to reduce oxidative stress associated with metal-stimulated catecholamine oxidation. Taurine and structural analogs of taurine were tested for their ability to inhibit metal-stimulated quinone formation from dopamine or L-dopa. Oxidative damage to proteins and lipids were also assessed in vitro and the effects of taurine were determined. Taurine (20 mM) was found to decrease significantly ferric iron (50-500 microM)- and manganese (10 microM)-stimulated L-dopa or dopamine oxidation. Taurine had no effect on zinc-induced dopamine oxidation and slightly potentiated copper- and NaIO(4)-stimulated quinone formation. Ferric iron-stimulated lipid peroxidation was not affected by taurine (1-20 mM). Protein carbonyl formation induced by ferric iron (500 microM) and L-dopa (500 microM) was significantly reduced by 10 mM taurine. The cytotoxicity of L-dopa (250 microM) and ferric chloride (75 microM) to LLC-PK(1) cells was attenuated by 10 mM taurine or hypotaurine. Homotaurine alone stimulated L-dopa oxidation and potentiated the cytotoxic effects of ferric iron. Homotaurine was found to be cytotoxic when combined with L-dopa or L-dopa/iron. In contrast, hypotaurine inhibited quinone formation and protected LLC-PK(1) cells. These studies suggest that taurine may exhibit cytoprotective effects against the oxidation products of catecholamines by acting as a scavenger for free radicals and cytotoxic quinones.

Depression of the electroretinogram in rats deficient in zinc and taurine during prenatal and postnatal life

The objective of this study was to investigate whether zinc interacts with taurine to influence the development of the electroretinogram. Virgin female Sprague-Dawley rats were bred overnight and assigned to 1 of 4 treatments in a 2 × 2 factorial design with two levels of zinc (50 μg/g through gestation and 50 μg/g after parturition; 15 μg/g through gestation and 7.5 μg/g after parturition) and two levels of taurine (2 or 0 μmol/g). Guanidinoethyl sulfonate (10 g/L), a structural analogue of taurine, was added to the drinking water of the animals receiving 0 μmol/g taurine. At postnatal day 23, male pups ( n = 10) were weaned onto their respective diets. Dark-adapted electroretinograms were recorded as a function of stimulus intensity on 7 1/2–8 1/2-week-old anesthetized pups. Two-factor analysis of variance demonstrated no interaction between zinc and taurine for a- or b-wave amplitudes or latencies ( P < 0.05). Zinc and taurine deficiencies each independently depressed electroretinogram a-wave and b-wave amplitudes but not latencies. The amplitude of the b-wave was plotted as a function of log stimulus intensity, and an iterative curve-fitting procedure was used to determine the maximum response, slope, and half-saturation constant. No interaction was noted. A significant treatment effect on maximum response was demonstrated for zinc ( P = 0.0498) and taurine ( P = 0.0014). No treatment effects were evident for the half-saturation constant or slope. These findings indicate that zinc and taurine deficiencies are not synergistic in their depressing effects on the electroretinogram in this model.

Taurine transport at the blood-brain barrier: an in vivo brain perfusion study

Taurine transport into six brain regions of equithesin-anesthetized rats was studied by the in situ brain perfusion technique. This technique gives both accurate measurements of cerebrovascular amino acid transport and allows complete control of the perfusate amino acid composition. Final wash procedure showed that taurine efflux occurred rapidly from endothelial cells. The taurine influx into endothelial cells was sodium and chloride dependent suggesting that the sodium and chloride gradients are the principal source of energy for taurine transport into endothelial cells. Taurine transport could be fitted by a model with saturable components. The kinetic constants in the parietal cortex were 1.4 × 10 −4 μmol/s/g for the apparent V max and 0.078 mM for the apparent K m . Competition experiments in the presence of sodium ions showed that [ 14C]taurine uptake was strongly inhibited by the structural analogs of taurine, hypotaurine and β-alanine.

Effect of taurine deficiency on tissue taurine concentrations and pregnancy outcome in the rat.

Taurine status and pregnancy outcome were assessed in rats fed low dietary taurine and varying doses of guanidinoethyl sulfonate (GES), a structural analogue of taurine. Female Sprague-Dawley rats (225-270 g) were mated overnight and assigned to one of four groups from day 0 to 20 of gestation. Taurine-deficient animals were fed a basal diet containing < 0.001 mumol taurine/g and 0.5 (n = 7), 1.0 (n = 8), or 2.0% (n = 7) GES in their drinking water, ad libitum. Control animals (n = 8) received similar treatment, with 2 mumol taurine/g added to the diet and no GES in their water. Taurine was analyzed by reverse-phase HPLC, using electrochemical detection after precolumn derivatization with ortho-phthalaldehyde. Treatment of rats with varying doses of GES produced a sharp decline in maternal liver and brain taurine to 15 and 55% of that of control levels, and in fetal liver and brain taurine to 75 and 50% of that of control levels, respectively (p = 0.0001; one-way ANOVA). The 2% group had a smaller mean (+/- SEM) litter weight than the control group (35.8 +/- 6.1 vs. 51.9 +/- 2.8 g; p = 0.042) as a result of a smaller litter size. The decrease in litter size was associated with confinement of implantation sites to either the left or right uterine horn in four of seven dams. Taurine deficiency did not result in intrauterine growth retardation or significant external, visceral, or skeletal malformations. Developmental defects were not found in any of the taurine-deficient groups, but reproductive abnormalities were present at the highest dose of the analogue.

Uptake of [ 14C]taurine into tissues of schistocerca gregaria in vivo

Locusts ( Schistocerca americana gregaria) were injected with a tracer dose of 0.25 μCi of [ 14C]taurine into the haemolymph and its clearance from haemolymph and uptake into flight muscle, thoracic ganglia and eye were studied at times thereafter. Clearance of labelled taurine from haemolymph suggested that the amino acid was rapidly taken up by some tissues and more slowly by others. Thus uptake of [ 14C]taurine was rapid into nervous tissues and eye, but was much more gradual into flight muscle. Uptake was inhibited by cold and the structural analogues of taurine, ß-alanine, γ-aminobutyric acid and hypotaurine. Taurine was retained in all tissues over a long period, indicating a very slow rate of catabolism for this amino acid in the locust.

Na+-dependent transport of taurine by membrane vesicles of neuroblastoma x glioma hybrid cells.

The transport of taurine into membrane vesicles prepared from neuroblastoma x glioma hybrid cells 108CC5 was studied. A great part of the taurine uptake by the membrane preparation is due to the transport into an osmotically sensitive space of membrane vesicles. Taurine uptake by membrane vesicles is an active transport driven by the concentration gradient of Na+ across the membrane (outside concentration greater than inside). The Km value of 36 microM for Na+-dependent taurine uptake indicates a high-affinity transport system. The rate of taurine transport by the membrane vesicles is enhanced by the K+ gradient (inside concentration greater than outside) and the K+ ionophore valinomycin. Taurine transport is inhibited by several structural analogs of taurine: hypotaurine, beta-alanine, and taurocyamine. All these results indicate that the taurine transport system of the membrane vesicles displays properties almost identical to those of intact neuroblastoma X glioma hybrid cells.

Inhibition of the synthesis of taurocholic acid by structural analogues of taurine

Structural analogues of taurine that are inhibitors of the rat liver microsomal synthesis of taurocholic acid are reported. Isethionic acid, the hydroxy analogue of taurine, is the most potent inhibitor ( I 50 = 110 μM). Aminomethanesulfonic acid, a taurine analogue which has a single carbon bridge between the amino and sulfonic acid moieties, is also a very potent compound ( I 50 = 688 μM). Hydroxypropanesulfonic acid ( I 50 = 3125 μM), the higher homologue of isethionic acid, and aminoethane-sulfuric acid ( I 50 = 3750 μM) are considerably less potent inhibitors. Hypotaurine I 50 = 7500 μM) and glycine ( I 50 = 8800 μM), both substrates for the enzyme system, decrease taurocholate formation but only when used at extremely high concentrations.

Suppression of conditioned drinking by taurine and related compounds

Mice were conditioned to respond for water reinforcements on a FR-5 schedule. Taurine, injected intraperitoneally at doses of 9.0, 13.8, and 21.3 mmole/kg, 30 min prior to the experimental session, produced a dose-related decrease in both the initial response rate and total number of reinforcements received by mice deprived of water for 24 hr. The structural analogues of taurine (aminomethanesulfonic acid, 3-aminopropanesulfonic acid, β-alanine, cysteamine, and glycine) also produced a hypodipsia. Doses of taurine which produced depression of responding for water reinforcements were used which produced no suppression of spontaneous motor activity, rotarod performance, Sidman avoidance, or shuttle-box avoidance. After intraperitoneal injection, the concentration of taurine increased in the hypothalamus and medulla, but not in other brain areas. We suggest that taurine might be acting by specifically depressing areas of the hypothalamus which stimulate drinking.