Effects of taurine on the phosphorylation of specific proteins in subcellular fractions of the rat retina

Taurine depletion increases phosphorylation of a specific protein in the rat retina

Partial depletion of the taurine content in the rat retina was accomplished for up to 22 weeks by introduction of 1.5% guanidinoethanesulfonate (GES) in the drinking water. Taurine levels decreased by 50% after 1 week of GES treatment and by 80% at 16 weeks. Replacement of GES by taurine to the GES-treated rats from week 16 to 22 returned their taurine content to the control value. Whereas addition of taurine (1.5%) to the drinking water of control rats from week 16 to 22 elevated the retinal taurine content to 118% of the control value, the administration of untreated water to GES-treated animals for the 16 to 22 week time period increased the retinal taurine content to only 76% of the control value.The amplitude of the electroretinogram (ERG) b-wave was decreased by 60% after GES-treatment for 16 weeks and maintained this reduced level for up to 22 weeks. Administration of taurine in the drinking water from week 16 to 22 returned the b-wave amplitude to a range not statistically different from the control values whereas the administration of untreated water produced less improvement.After 6 weeks of GES treatment when the retinal taurine content was reduced by 70% and the amplitude of the b-wave was reduced by 50% (extrapolated from Figure 1), phosphorylation of a specific protein with an approximate molecular weight of 20K was increased by 94%. The increased phosphorylation of the ~20K protein observed after GES treatment was reversed when the animals were treated with taurine (1 1/2%) in the drinking water for an additional 6 weeks. There was no change in the phosphorylation of the ~20K protein when animals were treated with taurine for 6 weeks. The data obtained support the theory that taurine may have a regulatory effect on retinal protein phosphorylation.

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.

Mode of action of taurine as a neuroprotector

Previously, it has been shown that taurine exerts its protective function against glutamate-induced neuronal excitotoxicity through its action in reducing glutamate-induced elevation of intracellular free calcium, [Ca2+]i. Here, we report the mechanism underlying the effect of taurine in reducing [Ca2+]i. We found that taurine inhibited glutamate-induced calcium influx through L-, P/Q-, N-type voltage-gated calcium channels (VGCCs) and NMDA receptor calcium channel. Surprisingly, taurine had no effect on calcium influx through NMDA receptor calcium channel when cultured neurons were treated with NMDA in Mg2+-free medium. Since taurine was found to prevent glutamate-induced membrane depolarization, we propose that taurine protects neurons against glutamate excitotoxicity by preventing glutamate-induced membrane depolarization, probably through its effect in opening of chloride channels and, therefore, preventing the glutamate-induced increase in calcium influx and other downstream events.

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.

Ethanol and amino acids in the central nervous system: assessment of the pharmacological actions of acamprosate

Ethanol induces alterations in the central nervous system by differentially interfering with a number of neurotransmitter systems, although the mechanisms by which such effects are executed are not well understood. The present review therefore, is designed to ascertain the effect of ethanol on both excitatory and inhibitory amino acid neurotransmitters, as well as the sulphonated amino acid taurine, assayed by the microdialysis technique within specific brain regions of rat during different types of alcohol intoxication, acute and chronic, as well as during the withdrawal period. Such an understanding of these pharmacological actions of ethanol on neurotransmitters is essential in order to provide the impetus for the development of appropriate therapeutic intervention to ameliorate the multitude of neurochemical disorders induced by ethanol. In addition the possible mode of action of a new therapeutic drug for the treatment of alcoholism, acamprosate will be discussed. The first part of this review will be limited to studies of the effect of ethanol on both amino acid neurotransmitters and the sulphonated amino acid taurine, a possible neuromodulator. While, the second part will seek to establish the possible mechanism of action of a new therapeutic drug, acamprosate, which is used to combat the effects of ethanol, particularly during the craving period, as well as maintaining abstinence in weaned alcoholics.

Unique pharmacological interactions of taurine and chelerythrine in the retina

The effects of taurine and chelerythrine (CHT) on ATP-dependent calcium uptake and the phosphorylation of the approximately 20 kDa phosphoprotein in the retina were compared. In the absence of the CHT, taurine stimulated ATP-dependent calcium uptake and attenuated the phosphorylation of the approximately 20 kDa phosphoprotein. On the other hand, CHT produced the opposite results in the absence of taurine. When the two agents were used simultaneously, it was found that CHT non-competitively inhibited the action of taurine to stimulate calcium uptake, while taurine non-competitively inhibited the action of CHT to stimulate the phosphorylation of the approximately 20 kDa phosphoprotein. The data present an unusual pharmacological mechanism for controlling the signal transduction pathway involving the two distinct cellular processes being studied. Given the unique data, a control system is proposed in which the function of the approximately 20 kDa phosphoprotein is linked to the stimulation of ATP-dependent calcium uptake.

Effect of taurine on chelerythrine inhibition of calcium uptake and ATPase activity in the rat retina

Taurine potentiates calcium uptake in whole retinal homogenates as well as in rod outer segments and mitochondrial fractions. The aim of this study was to correlate taurine potentiation of calcium uptake with its effects on other cellular processes through the use of chelerythrine (CHT), a modulator of protein kinase C (PKC), ATPase activity, and, as recently shown, of retinal protein phosphorylation. CHT inhibited calcium uptake only when ATP was present, and inhibition increased significantly in conditions of ATP excess. Taurine potentiated ATP-dependent calcium uptake but decreased the potency of ATP to induce uptake activity. CHT inhibition of calcium uptake exhibited similar potencies in the presence and absence of taurine, and this inhibition seemed to be independent of PKC inhibition. Because of the ATP-dependence of the observed effect, total ATPase activity was studied using similar treatments. In the absence of taurine, CHT inhibited ATPase activity with the same potency (IC50 approximately 59.3 microM) as with calcium uptake inhibition (IC50 approximately 87.9 microM), presenting a possible mechanism of action of CHT. In the presence of taurine, no such correlation was observed, suggesting an ATPase-independent mechanism of action. In fact, taurine did not potentiate ATPase activity, but rather it decreased the potency of CHT inhibition of ATPase, effects incongruent with the effects of taurine on calcium uptake and on CHT inhibition of calcium uptake. Enzyme kinetic experiments provided more supporting data. Taurine was found to cause an increase in the affinity of the ATP substrate for the ATPase enzyme, contradicting the aforementioned effect of taurine to decrease the potency of ATP to induce calcium uptake. Thus, taurine seems to increase calcium uptake through a hitherto unreported mechanism distinct from its modulation of ATPase activity.

The differential osmoregulation and localization of taurine transporter mRNA and Na+/myo-inositol cotransporter mRNA in rat eyes

We studied the cellular localization and osmotic regulation of taurine transporter (TauT) mRNA in the rat eyes using in situ hybridization. TauT mRNA signals were expressed in the ciliary body, and the outer part of the inner nuclear layer (INL), the outer nuclear layer (ONL) and the inner segment (IS) of the adult rat retina. Chronic hypernatrema, induced by gavaging with 1 ml/100 g body weight of 5% NaCl every other day for 7 days, markedly increased in TauT mRNA in the retina compared with control rats. However, there was little change in TauT mRNA in the eyes in acute hypernatremic state that is induced by single injection of high concentration of NaCl. On the contrary, acute hypernatremic rats displayed markedly elevated Na+/myo-inositol cotransporter (SMIT) mRNA in the retina and the iris-ciliary body and the lens epithelium. Under chronic hypernatremic conditions, there was no significant increase in SMIT mRNA in rat eyes. These findings suggest that TauT mRNA is osmotically regulated in vivo to protect retinal neuronal function, especially against chronic hypernatremic conditions, in contrast to rapid up-regulation of SMIT mRNA in acute hypernatremic rats.

Stimulation by chelerythrine of the phosphorylation of the amino acid serine in an approximately 20 kDa protein present in the mitochondrial fraction of the rat retina

It has been reported that chelerythrine chloride, a benzophenanthridine alkaloid, with a wide variety of biologic effects stimulates the phosphorylation of an approximately 20 kDa protein present in the mitochondrial fraction of the rat retina. It has also been shown previously that both the serine and threonine resides in this phosphoprotein are phosphorylated when the retinal preparation is incubated in the presence of [gamma-32P]ATP. Phosphorylation of the serine residue(s) was determined to predominate over phosphorylation of the threonine residue(s). In the present investigation, it was demonstrated that chelerythrine stimulates the incorporation of radioactive phosphate into the serine residue(s), increasing the radioactivity in the phosphoserine/ phosphothreonine ratio by 80%. This observation represents a novel and apparently contradictory effect for chelerythrine, which is used normally as a selective protein kinase C inhibitor. In addition to testing chelerythrine for its effects on the phosphorylation of the approximately 20 kDa protein, a number of other protein kinase inhibitors and activators were investigated. The results suggest that the enzyme responsible for the phosphorylation of the approximately 20 kDa protein is not a well-characterized or documented kinase.

Quantitative analysis of the combination dose-effects of taurine and taurine analogues on the phosphorylation of an approximately 44-Kd protein present in a mitochondrial subfraction of rat heart

Combinations of taurine and analogues of taurine that partially contain the N-C-C-S moiety within a semirigid saturated ring structure were tested for their effects on the phosphorylation of an approximately 44-Kd protein present in the mitochondrial fraction of rat heart. (+/-)Piperidine-3-sulfonic acid (PiP), an inhibitor of the phosphorylation of the approximately 44-Kd protein with activity approximately similar to that of taurine, was observed to be mutually exclusive with taurine, i.e., to have a similar mode of action. The combination of taurine plus PiP in a fixed ratio mixture of 1:1 was slightly antagonistic at all concentrations. (+/-)Aminotetrahydrothiopyran-1,1-dioxide (APS), a sulfone derivative of taurine with a net positive charge, also has approximately the same inhibitory activity as taurine. However, APS was mutually nonexclusive with taurine when tested in combination and thus appears to act independently of taurine. Taurine plus APS in a fixed ratio mixture of 3:1 was highly antagonistic at low concentrations of the mixture, approached an additive relation at 50% saturation, and became synergistic at high concentrations of the mixture. Three analogues of taurine, pyridine-3-sulfonic acid (PyS), quinoline-8-sulfonic acid (QS), and 2-aminobenzenesulfonic acid (ABS), that have the basic taurine structure (N-C-C-S) partially in a semirigid unsaturated ring structure stimulate the phosphorylation of the approximately 44-Kd protein. Due to the unsaturated ring structure, these analogues of taurine have a net negative charge at physiologic pH and are not zwitterions. When PyS, QS, or ABS was titrated in the presence of a fixed concentration of taurine (10 mM), there was a competitive relation even though their electronic nature is quite different than that of taurine. The combination of QS plus PyS (1:5) appears to progress through a transition from being synergistic at low concentrations of the fixed ratio mixture, additive at 50% saturation, and finally antagonistic at high concentration of the fixed ratio mixture.

Paradoxical stimulatory effect of the kinase inhibitor chelerythrine on the phosphorylation of a approximately 20 K M(r) protein present in the mitochondrial fraction of rat retina

In order to characterize the phosphorylation of a approximately 20k M(r) protein present in the mitochondrial fraction of the rat retina, chelerythrine chloride, a well-known protein kinase C inhibitor, was tested for activity. Instead of the expected inhibition of the kinase reaction by chelerythrine the phosphorylation of the approximately 20k M(r) protein was stimulated by a factor of 3 at 150 microM. This unique stimulatory action of chelerythrine could be eliminated by the addition of 10 mM dithiothreitol. A suggested mechanism of action for dithiothreitol in the elimination of the increased phosphorylation of the approximately 20k M(r) protein by chelerythrine is the addition of the thiol group of dithiothreitol to the iminium bond of chelerythrine. Taurine, a known inhibitor of the phosphorylation of retinal proteins was also tested in combination with chelerythrine for its effects on the phosphorylation of the approximately 20k M(r) protein. A non-competitive relationship was observed when chelerytrhine was used as the variable activator and taurine as the fixed inhibitor (30 mM). The stimulatory effect of chelerythrine on the phosphorylation of proteins was not limited to retinal tissue but was also observed in the P2 fraction of brain cortex. Chelerythrine demonstrated only inhibitory effects on the phosphorylation of proteins in a heart mitochondrial fraction.

Partial characterization of an approximately 20 K M(r) retinal protein whose phosphorylation is inhibited by taurine

It has been demonstrated previously that taurine (2-aminoethanesulfonic acid) is an inhibitor of protein phosphorylation in a mitochondrial fraction of the rat retina. It appears that taurine is most effective in inhibiting the phosphorylation of an approximately 20 K apparent molecular weight (M(r)) protein found in the retinal tissue. This study further characterizes the location of the approximately 20 K phosphoprotein by phase separation using Triton X-114 and also characterizes the nature of the phosphate bond by various solvent extractions and by exposure to acid and base conditions. Triton X-114 experiments indicated that the approximately 20K phosphoprotein is located in the aqueous phase and, consequently, is probably not an integral protein of the mitochondrial membranes. Treatment of the phosphoprotein with solvents, acid, and/or base determined the phosphate linkage to be through a phosphoester bond rather than an acylphosphate bond. The approximately 20 K M(r) phosphoprotein was also isolated from one-dimensional polyacrylamide gels and subsequently digested with trypsin and hydrolyzed with 1 M HCl to break all peptide bonds. Analysis of the phosphoamino acids by two-dimensional high voltage electrophoresis on cellulose plates revealed that it is both the serine and threonine residues that are phosphorylated. However, phosphorylation of the serine residue(s) is predominant.

Spontaneous and evoked release of [3H]taurine from a P2 subcellular fraction of the rat retina

The effects of spontaneous and evoked [3H]taurine release from a P2 fraction prepared from rat retinas were studied. The P2 fraction was preloaded with [3H]taurine under conditions of high-affinity uptake and then examined for [3H]taurine efflux utilizing superfusion techniques. Exposure of the P2 fraction to high K+ (56 mM) evoked a Ca(2+)-independent release of [3H]taurine. Li+ (56 mM) and veratridine (100 microM) had significantly less effect (8-15% and 15-30%, respectively) on releasing [3H]taurine compared to the K(+)-evoked release. 4-Aminopyridine (1 mM) had no effect on the release of [3H]taurine. The spontaneous release of [3H]taurine was also Ca(2+)-independent. When Na+ was omitted from the incubation medium K(+)-evoked [3H]taurine release was inhibited by approximately 40% at the first 5 minute depolarization period but was not affected at a second subsequent 5 minute depolarization period. The spontaneous release of [3H]taurine was inhibited by 60% in the absence of Na+. Substitution of Br- for Cl- had no effect on the release of either spontaneous or K(+)-evoked [3H]taurine release. However, substitution of the Cl- with acetate, isethionate, or gluconate decreased K(+)-evoked [3H]taurine release. Addition of taurine to the superfusion medium (homoexchange) resulted in no significant increase in [3H]taurine efflux. The taurine-transport inhibitor guanidinoethanesulfonic acid increased the spontaneous release of [3H]taurine by approximately 40%. These results suggest that the taurine release of [3H]taurine is not simply a reversal of the carrier-mediated uptake system. It also appears that taurine is not released from vesicles within the synaptosomes but does not rule out the possibility that taurine is a neurotransmitter. The data involving chloride substitution with permeant and impermeant anions support the concept that the major portion of [3H]taurine release is due to an osmoregulatory action of taurine while depolarization accounts for only a small portion of [3H]taurine release.