Taurine: retinal function

Exogenous glutamate and taurine exert differential actions on light-induced release of two endogenous amino acids in isolated rat retina

A dark-adapted isolated rat retina, preloaded with [(14)C]glutamate ([(14)C]Glu) and [(3)H]taurine ([(3)H]Tau), was superfused with artificial cerebrospinal fluid (ACSF) in the absence and presence of Glu (1 mM) or Tau (1 mM), as well as the Glu uptake inhibitors dihydrokainic acid (DHK, 0.04 mM) and trans-L-pyrrolidine-2,4-dicarboxylate (t-PDC, 0.004 mM). After 10 min of light stimulation, the extracellular level of [(14)C]Glu and [(3)H]Tau was reduced to 82 +/- 2% and 65 +/- 4% of the control, respectively. Basal release was enhanced when Tau and t-PDC were applied together, although none of the compounds had any effect when applied individually. Glu and DHK had no effect. The decrease of [(14)C]Glu efflux evoked by light stimuli was enhanced by t-PDC and Tau, either added separately or together, whereas Glu and DHK were without effect. In contrast, [(3)H]Tau efflux variations induced by light stimuli were reduced markedly by both Tau and Glu. These findings suggest distinctly different roles of Tau and Glu in light-induced responses in mammalian retina, including a possible role for Tau in light adaptation processes.

Effects of taurine deficiency and chronic methanol administration on rat retina, optic nerve and brain amino acids and monoamines

A chronic methanol (MeOH) intoxication scheme (2 g/kg/day ip for 2 weeks) was carried out in Sprague-Dawley rats, previously depleted of folates with methotrexate (MTX). beta-Alanine (beta-Ala), 5%, was also administered to some animals in the drinking water. Amino acids were determined in plasma, retina, optic nerve, hippocampus and posterior cortex by HPLC with fluorescence detection and monoamines in retina, hippocampus and posterior cortex by electrochemical detection. Beta-Ala administration reduced taurine (Tau) levels in plasma, hippocampus and posterior cortex, but not in retina and optic nerve. Aspartate (Asp) concentration in the optic nerve was increased in MTX-MeOH treated animals, and the administration of beta-Ala did not modify this elevation. The association of beta-Ala with MTX-MeOH produced an increase of threonine, and a decrease of 5-hydroxytryptamine (5-HT) in the retina without modifying 5-hydroxyindoleacetic acid, whereas in the hippocampus an elevation of asparagine was observed. We conclude that, in the retina, beta-Ala in combination with MTX-MeOH increased serotonin and decreased dopamine (DA) turnover rate, and resulted in changes in the amino acid balance, that could affect glycinergic activity. On the other hand, in the hippocampus, Asp metabolism could be affected by Tau depletion with beta-Ala.

Plasma sulfur amino acids in relation to cardiovascular disease, nutritional status, and diabetes mellitus in patients with chronic renal failure at start of dialysis therapy

Sulfur amino acids (sAAs) are potential candidates as risk factors for cardiovascular disease (CVD). However, we recently reported that chronic hemodialysis patients with CVD had a greater prevalence of malnutrition, hypoalbuminemia, and lower plasma total homocysteine (tHcy) levels than those without CVD. In this cross-sectional study, we examined the relationship of plasma sAAs to CVD and nutritional status in 151 patients with chronic renal failure (CRF) close to the start of regular dialysis treatment (33 +/- 7 days before the first dialysis treatment). Clinical signs of CVD were present in 32% of patients with CRF, 41% had malnutrition assessed by subjective global nutritional assessment (SGNA) score, and 26% had diabetes mellitus (DM). Plasma tHcy levels were high in 91% of patients, as were plasma total cysteine (tCys) levels, whereas plasma methionine (Met) and taurine (Tau) levels were normal. Patients with CRF who had CVD were older, more often malnourished, and had lower tHcy and serum albumin (s-albumin) levels and a greater frequency of DM than those without CVD. Plasma tCys, Met, and Tau levels did not differ between patients with CRF with and without CVD. The tCys-tHcy ratio was higher in patients with CVD and related to SGNA score and DM. Moreover, this ratio, but not tHcy or tCys level, correlated with age and triglyceride, total cholesterol, and apolipoprotein B levels. Malnutrition and hypoalbuminemia were associated with low plasma sAA levels (tHcy, Met, and Tau); tCys was related to s-albumin level, but not SGNA score. Among patients with diabetes, sAA levels did not differ between patients with and without CVD or between malnourished and well-nourished patients. In conclusion, patients with CRF at the start of dialysis treatment with CVD were more often diabetic, malnourished, and had lower s-albumin and tHcy levels and a higher tCys-tHcy ratio than patients with no CVD. tCys-tHcy ratio, but not tHcy or tCys levels per se, was related to cardiovascular risk factors, suggesting that cysteine may have a role in the development of CVD. Malnutrition, hypoalbuminemia, and DM in patients with CRF influence sAA levels, mainly plasma tHcy, which should be considered when evaluating hyperhomocysteinemia as a cardiovascular risk factor.

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.

Disruption of the taurine transporter gene (taut) leads to retinal degeneration in mice

Taurine is involved in cell volume homeostasis, antioxidant defense, protein stabilization, and stress responses. High levels of intracellular taurine are maintained by a Na+-dependent taurine transporter (TAUT) in the plasma membrane. In view of the immunomodulatory and cytoprotective effects of taurine, a mouse model with a disrupted gene coding for the taurine transporter (taut-/- mice) was generated. These mice show markedly decreased taurine levels in a variety of tissues, a reduced fertility, and loss of vision due to severe retinal degeneration. In particular, the retinal involvement identifies the taurine transporter as an important factor for the development and maintenance of normal retinal functions and morphology.

Taurine blocks spontaneous cone contraction but not horizontal cell dark suppression in isolated goldfish retina

The objective of this study was to investigate the effects of taurine on cone retinomotor movements and the responses of cone-driven horizontal cells in dark-adapted teleost retina. In isolated goldfish retina preparations maintained in the dark, cones spontaneously contracted, and the responses of horizontal cells were suppressed. Addition of 5 mM taurine to the physiological solution blocked the spontaneous contraction of cones in the dark but did not block the dark-suppression of horizontal cell responses. These results indicate that the mechanism that leads to horizontal cell dark suppression is not sensitive to taurine. Although both cone retinomotor position and horizontal cell responsiveness are known to be modulated by dopamine, the present results do not support the hypothesis that taurine inhibits dopamine release in the dark because only spontaneous cone contraction was affected by taurine. These results also indicate that spontaneous cone contraction in the dark is not the cause of horizontal cell dark suppression because, in the presence of taurine, cones were elongated yet horizontal cell responses were still suppressed. Consequently, these results make it clear that horizontal cell dark suppression is not an artifact produced by incubating isolated teleost retina preparations in taurine-free physiological solution.

Immunocytochemical localization of taurine in the developing retina of the lefteye flounder Paralichthys olivaceus

Light microscopic immunolocalization of taurine, a sulfur-containing free amino acid, was investigated in the developing retina of a lefteye flounder, Paralichthys olivaceus, which exhibits metamorphic changes with rod cell addition for 3-5 weeks after hatching. This immunocytochemical study of the developing retina revealed: 1) From 3 to 13 days after hatching, intense immunostaining was shifted from the surroundings of neural cells to the neural somata and processes in the inner retina. 2) Intense immunoreactivity appeared also in the outer and inner segments and basal processes (pedicles) of cone cells within 6 days or 13 days after hatching. 3) Lack of immunoreactivity was found in the outer segment of rod cells from their appearance during metamorphosis. These findings are discussed with the possible functional roles of taurine in the fish retina: 1) involvement in cell differentiation and/or development; 2) protection of the outer segments against light stimuli; and 3) regulation of neural transmission.

Taurine and its trophic effects in the retina

The sulphur amino acid taurine possesses variable functions during development and regeneration of the central nervous system. The retina synthesize and uptake taurine, which is the amino acid present in the highest concentration in this tissue. Deficiency of taurine alters the structure and the function of the cerebral and cerebellar cortex, as well as the retina. Taurine increases outgrowth of postcrush goldfish retina in culture, partially by elevating calcium influx, and also by the modulation of protein phosphorylation. Its concentration increases in the retina after the lesion of the optic nerve, and the intraocular injection of it, between the crush and the explantation, stimulates the outgrowth of neurites. Taken together, although there are a great number of unresolved questions on the mechanisms of action of this amino acid as a trophic substance, the results support the role of taurine during regeneration of the optic nerve.

Taurine uptake activity in the rat retina: protein kinase C-independent inhibition by chelerythrine

Taurine, a regulatory amino acid of various biochemical processes in the retina, requires an efficient uptake system to maintain the high physiological concentration of taurine in the retina. Taurine uptake was characterized in both whole retinal preparations and in isolated rod outer segments (ROS) in terms of uptake kinetics and possible protein kinase C (PKC)-dependent regulation. Two uptake systems, a high- and a low-affinity system, were found in whole retinal preparations while only the high-affinity system was found in the isolated ROS. All the uptake systems characterized were inhibited by guanidinoethane sulfonate (GES), a well-known competitive inhibitor of taurine uptake. Stimulation and inhibition of PKC activity with phorbol myristate acetate and with staurosporine, respectively, produced no significant effect on taurine uptake. On the other hand, chelerythrine (CHT), a documented potent PKC inhibitor, was found to cause significant inhibition of the two taurine uptake systems, presumably through a PKC-independent mechanism. The data demonstrate that CHT may be a useful tool in studying taurine uptake in the retina and specifically in the ROS.

Pharmacological characterization of the effects of taurine on calcium uptake in the rat retina

Taurine is known to increase ATP-dependent calcium ion (Ca2+) uptake in retinal membrane preparations and in isolated rod outer segments (ROS) under low calcium conditions (10 microM) (Pasantes-Morales and Ordóñez, 1982; Lombardini, 1991). In this report, ATP-dependent Ca2+ uptake in retinal membrane preparations was found to be inhibited by 5 microM cadmium (Cd2+), suggesting the involvement of cation channel activation. The activation of cGMP-gated cation channels, which are found in the ROS, is a crucial step in the phototransduction process. An inhibitor of cGMP-gated channels, LY83583, was found to inhibit taurine-stimulated ATP-dependent Ca2+ uptake but had no effect on ATP-dependent Ca2+ uptake in the absence of taurine, indicating that taurine may be increasing ATP-dependent Ca2+ uptake through a mechanism of action involving the opening of cGMP-gated channels. The activation of cGMP-gated channels with dibutyryl-cGMP and with phosphodiesterase inhibition using zaprinast caused an increase in ATP-dependent Ca2+ uptake in isolated ROS, but not in taurine-stimulated ATP-dependent Ca2+ uptake. LY83583 had the same effects in isolated ROS as in retinal membrane preparations. Another inhibitor of cGMP-gated channels, Rp-8-Br-PET-cGMPS, produced the same pattern of inhibition in isolated ROS as LY83583. Thus, there appears to be a causal link between taurine and the activation of the cGMP-gated channels in the ROS under conditions of low calcium concentration, a connection that suggests an important role for taurine in the visual signalling function of the retina.

Effects of in vivo taurine depletion on induced-chemiluminescence production in macrophages isolated from rat lungs

Alveolar macrophages isolated by pulmonary lavage from partially taurine-depleted rats demonstrated increased (2-2.6 fold) chemiluminescence due to the extracellular reaction between exogenous zymosan and various reactive forms of oxygen compared to macrophages isolated from control animals. Partial taurine depletion was achieved by adding 3% beta-alanine to the drinking water of the rats for 5 weeks prior to harvesting the macrophages. Superoxide dismutase activity was not increased in the lung tissue of the taurine-depleted rats. These data suggest that taurine has antioxidant properties and that taurine depletion is potentially deleterious to alveolar macrophages and pulmonary tissue.

Immunohistochemical localization of taurine in the retina of developing and adult human and adult monkey

The localization of taurine in the retina of fetal (12-25 weeks of gestation), postnatal (five-month-old infant) and adult human (35- and 65-year-old) was examined by immunohistochemistry. Additionally, retinas of fresh adult monkey, which served as positive controls, were employed. No immunoreactivity was found in the fetal retinas from 12-15 weeks of gestation. At 1617 weeks of gestation, the ganglion cells and some of their axons were conspicuously labelled for taurine. At 18-19 weeks, Muller glial endfeet, the inner plexiform layer, some amacrine and putative horizontal cells and photoreceptors showed moderate immunoreactivity. With further development at 20-21 and 24-25 weeks of gestation, the immunoreactivity was prominent in Muller cell endfeet, some bipolar cells and in horizontal cells that were aligned in a row in the inner nuclear layer, close to the fovea. At both fetal stages, the photoreceptors and horizontal cells showed strong immunoreactivity. In the postnatal infant retina, taurine immunoreactivity was present in some amacrine cells and photoreceptor inner segments and nuclei, but not in ganglion and horizontal cells, which was also the pattern noted in the adult monkey and human retinas. With development, a shift in the intensity of taurine immunoreactivity was noted towards the outer retina. The expression of taurine immunoreactivity in most fetal retinal neurons implies a role for this amino acid in the normal development as well as maturation of human retina.

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.

The role of taurine in osmotic, mechanical, and chemical protection of the retinal rod outer segments

The ability of the photoreceptor cell to resist osmotic stress was examined by incubating isolated frog retina in medium of varying osmolality. An electron microscopic analysis of the rod outer segment following a severe hypoosmotic insult revealed connections between adjacent disks and between disk rims and the plasma membrane, which presumably provide mechanical stability to the rod outer segment. One surprising result was the extent of the damage incurred by the electrical signaling pathway of the photoreceptor cells subjected to a 50 mOsm insult; only the distal P111 component of the ERG remained unaffected. Thus, the rod outer segment is particularly resistant to osmotic-induced injury, presumably because of the effective osmoregulatory actions of taurine. Incubation of retina with tauret, retinylidentaurine, uncovered rose-like hexagonal structures on the surface of the rod outer segment. These structures purportedly consist of connections between disk rims and the plasma membrane of the rod outer segments. Based on the influence of tauret, it is likely that the calcium dependence of these channels is selective for retinoids. These data are discussed relative to taurine's role in the process of rhodopsin regeneration and in the protection of the rod outer segments against osmotic, mechanical and light induced damage.

Review of some actions of taurine on ion channels of cardiac muscle cells and others

1. Taurine has recently been known to protect against ischemia and heart failure. Taurine possesses plenty of actions on the ion channels and transports, but is very non-specific. 2. Taurine may directly and indirectly help to regulate the [Ca]i level by modulating the activity of the voltage-dependent Ca2+ channels (also dependent on [Ca]i/[Ca]o), by regulation of Na+ channels, and secondly via Na-Ca exchange and Na(+)-taurine cotransport. 3. Taurine can prevent the Ca2+ ([Ca]o or [Ca]i)-induced cardiac functions. 4. Therefore, it seems possible that taurine could exert the potent cardioprotective actions even under the condition of low [Ca]i levels as well as under the Ca2+ overload condition. 5. The electrophysiological actions of taurine on cardiomyocytes, smooth muscle cells, and neurons from recent studies are summarized.

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.

Immunoreactivity for taurine in the cochlea: its abundance in supporting cells

Taurine is the second most abundant free amino acid in the brain where its osmoregulatory function is well established. Taurine-deprived kittens show retinal pathology leading to blindness. In the inner ear, taurine has been reported to be the most abundant free amino acid although its role in inner ear function is not known. Immunohistochemistry was employed here to investigate the localisation of taurine in normal cochleae of the guinea pig compared with two different conditions: experimentally induced endolymphatic hydrops and after oral administration of glycerol. In normal cochleae, by light microscopy, taurine-like immunoreaction was never observed in the sensory outer hair cells and appeared absent from the inner hair cells. In contrast taurine-like immunolabeling was found to be present in all supporting tissue with the striking exception of the tectorial membrane and the outer pillar cell which had no or little taurine immunoreactivity respectively. In early experimental endolymphatic hydrops, the distribution of taurine-like immunoreactivity appeared similar to that observed for normal cochleae. In long-term hydrops, degenerated outer hair cells were replaced by the swelling of the phalangeal process of the Deiters' cells which became highly immunoreactive to taurine. After glycerol administration, the tectorial membrane became more tightly bound to the apical surface of the sensory hair cells and distinctly immunoreactive to taurine. The localisation of taurine in the organ of Corti shown here is consistent with taurine being involved in the maintenance of osmotic equilibrium in the normal and perhaps also in the restructuration of the pathological organ of Corti.

Localisation of amino acid neurotransmitters during postnatal development of the rat retina

We used postembedding immunocytochemistry to determine the localisation of the amino acid neurotransmitters glutamate, gamma-aminobutyrate (GABA), and glycine, potential neurotransmitter precursors (aspartate and glutamine), and taurine in the rat retina during postnatal development. All amino acids investigated were present at birth; however, only the inhibitory neurotransmitters GABA and glycine displayed neuronal localisation. GABA was localised in a sparse population of amacrine cells, and glycine immunoreactivity was found in cells within the ventricular zone that appeared to migrate through the neuroblastic layer. Glutamate labelling was diffuse across the retina until postnatal day (PND) 8. Localisation of glutamine was evident within Müller's cells by PND 6, in agreement with the known age of onset of glutamine synthetase activity. Based on the findings of uptake of radiolabelled glutamate and GABA by PND 8 and changes in immunoreactivity, we propose that Müller's cells evolve at PND 6-8 from their precursor cells, the radial glial cells. Evidence for differences in glutamate turnover in the infant retina was seen on examination of aspartate and glutamine immunoreactivity. Aspartate labelling was weak until PND 11, when ganglion cells and some amacrine cells were labelled. Unlike the mature retina, a large number of amacrine cells were glutamine immunoreactive in the PND 6 retina. One reason for the observed differences in precursor pooling may be a lack of neuronal neurotransmitter release and overall low metabolic activity. We also investigated the response of the developing retina to ischaemic insult to test the physiological hypoxia model of vascular development. Our findings are consistent with the hypothesis that the developing retina has increased tolerance to ischaemic insult. Our findings suggest that, although the retina is morphologically adult like by PND 8, there are differences in neurotransmitter turnover in the immature rat retina.

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.

Assessment of neurotoxicity and "neuroprotection"

Coronal brain slices allow the study of neurotoxicity and "neuroprotection" under conditions where the differentiation-state and interrelationships of the neurones and glial cells are closer to those occurring in the intact tissue than is the case for co-cultured cell systems. The involvement of glial cells in the excitotoxicity of kainate and the potentiation of this toxicity by inhibition of glutamine synthase can be demonstrated. Longer-term toxicity of kainate may also be compounded by depletion of glutathione levels resulting from inhibition of gamma-glutamylcysteine synthase. The involvement of nitric oxide formation in the toxicity of N-methyl-D-aspartate can also be shown. The neurotoxicity of 1-methyl-4-phenylpyridinium can be readily demonstrated in coronal slice preparations. Taurine affords protection against this neurotoxicity. The possible mechanisms of these effects are considered in terms of the cyclic interrelationships between the different events which can lead to cell death.

Neurochemical architecture of the normal and degenerating rat retina

We used post-embedding immunocytochemistry to determine the cellular localization of glutamate, gamma-amino butyric acid (GABA), glycine, aspartate, glutamine, arginine, and taurine in the normal and degenerating rat retina. Müller's cell function was also evaluated by determining the uptake and degradation characteristics for glutamate. Immunocytochemical localization of amino acids in adult Royal College of Surgeons (RCS) and control rat retinas were similar with respect to cell classes. Differences in the intensity of labelling for glutamate, aspartate, glutamine, and glycine were observed in several classes of neurons, but the most prominent differences were shown by bipolar cells of the adult RCS rat retina. In addition, glutamine labelling within Müller's cells was higher in the RCS rat than the control. These changes may have occurred because of alterations in the glutamate production or degradation pathways. We tested this hypothesis by determining Müller's cells glutamate uptake and degradation characteristics in adult and postnatal day 16 RCS retinas. High affinity uptake of 3[H]-glutamate revealed an accumulation of grains over Müller's cell bodies in the adult RCS retina implying glutamate degradation anomalies. We confirmed anomalies in glutamate metabolism in RCS Müller's cells by showing that exogenously applied glutamate was degraded over a longer time course in postnatal day 16 RCS retinas, compared to control retinas. Differences in arginine immunoreactivity in adult and immature RCS retinas conform to the presumed dysfunction of Müller's cells in these degenerating retinas. The anomalies of amino acid localization, uptake and degradation lead us to conclude that Müller's cells in the RCS retina show abnormal function by postnatal day 16; an earlier time to previously reported anatomical and functional changes in this animal model of retinal degeneration.

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.

Isolation of a cDNA encoding a taurine transporter in the human retinal pigment epithelium

Reverse transcription-polymerase chain reaction (RT-PCR) was performed to amplify a cDNA encoding a taurine transporter in the human retinal pigment epithelium (HRPE). The coding region of a PCR product was found to be 1863 bp long, predicting a 620-amino acid protein (69,826 Da). This cDNA sequence is almost identical to those taurine transporters recently determined in the human thyroid and placenta: 12 and 1 base pair(s) different from the reported thyroid and placenta transporter clones, respectively. The injection of mRNA in vitro transcribed from the PCR product markedly increased taurine uptake in Xenopus laevis oocytes. Taurine uptake is Na+ and Cl- dependent. Unlabeled taurine, beta-alanine and gamma-amino-n-butyric acid at 100 microM inhibited the uptake of radiolabeled taurine whereas 100 microM alpha-alanine and alpha-aminoisobutyric acid did not. A kinetic study showed that taurine uptake is mediated by a single carrier system with the apparent Michaelis-Menten constant of approximately 2 microM. These results suggest that the PCR product encodes a functional taurine transporter whose characteristics are similar to those of taurine uptake observed in the original HRPE cells. A DNA encoding the reported placental transporter was made from the PCR product by site-directed mutagenesis but it was not functional in the oocyte expression. A similar RT-PCR was performed with poly (A)+ mRNA isolated from JAR human placenta choriocarcinoma cells. This PCR product was identical to that from the HRPE. In addition, the clone of the human thyroid transporter was obtained and re-sequenced. Its translation coding region was also identical to that of the PCR product from the HRPE, showing that taurine transporters are identical in the human RPE, thyroid and placenta.

Taurine in gerbil retina: changes during ischemia reperfusion/insult (I.R.I.) and aging

The retina contains a high amount of taurine which suggests a role in retinal function. The mongolian gerbil is well studied in stroke because of its incomplete circle of Willis. Two groups of gerbils were used: one served as control and the other was subjected to unilateral left carotid occlusion during 30 minutes, followed by 60 minutes of reperfusion. Gerbils were selected by ocular fundus and only sensitive gerbils were retained for the experimentation. We studied the level of taurine in gerbil retina of different ages: 3, 9, 15 and 24 months old (sham operated and ischemic groups). Level of taurine was determined by HPLC/electrochemical method. Compared to sham operated groups, level of taurine was significantly increased in ischemic groups for all ages studied. In the sham operated groups, level of taurine was low at birth, reached a plateau, and then decreased with aging. In the ischemic groups, level of taurine regularly increased from 3 to 24 months of age. With comparison evaluated for each age (modification ischemic versus sham operated groups expressed in percentage), level of taurine was quite equal at 3 and 9 months of age, but increased in 15 and 24 months old gerbils.

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.

Early appearance and transient expression of putative amino acid neurotransmitters and related molecules in the developing rabbit retina: an immunocytochemical study

We have studied, by immunocytochemistry, the ontogeny of GABA, glycine, glutamate, glutamine, and taurine-containing cells in the rabbit retina. Amacrine cells show GABA immunoreactivity by embryonic day 25 (E25) and throughout postnatal life. By contrast, ganglion cells and horizontal cells are only transiently GABA-immunoreactive (-IR); few appear GABA-IR by the third postnatal week. At maturity, glycine is present in amacrine cells and in some bipolar cells. During development, putative ganglion cells transiently contained glycine between E25 and postnatal day 3 (P3), whereas immunolabelling in presumed amacrine cells and bipolar cells persists after birth. Ganglion cells, bipolar cells, photoreceptors, and some amacrine cells are glutamate-IR in the adult retina. Glutamate immunoreactivity first appears in the somata and processes of cytoblastic cells by E20 and is prominent by E25. Surprisingly, ganglion cells are not strongly glutamate-IR until just before eye-opening, at postnatal day 10 (P10), coincident with the appearance of glutamine in their somata and in Müller glial cells. Bipolar cells are glutamate-IR before they or Müller cells contain high levels of glutamine (at P10). Glutamate immunoreactivity in photoreceptors is progressively restricted to the inner segments by eye-opening. At no stage are presumed horizontal cells glutamate-IR or glutamine-IR, but some amacrine cells show glutamate- and glutamine-IR by P10. Taurine is localized to photoreceptors and Müller glial in the adult retina. Some cytoblasts are taurine-IR at E20; with ensuing development, taurine labelling becomes restricted primarily to Müller cells and photoreceptors; some putative bipolar cells may also be labelled. However, for a few days around birth, cells resembling horizontal cells, also show taurine immunoreactivity. The early appearance and often transient expression of these amino acids in retinal cells suggests that these neuroactive molecules may be involved in the structural and functional development of the retina.

Taurine, amino acid transmitters, and related molecules in the retina of the Australian lungfish Neoceratodus forsteri: a light-microscopic immunocytochemical and electron-microscopic study

The morphology of the retina of the Australian lungfish Neoceratodus forsteri was investigated by means of light- and electron microscopy, whilst immunocytochemical studies were performed to determine the cellular distributions of the major amino acid neurotransmitters and other amino acids. The distributions of glycine and GABA were similar to those previously described for teleost, amphibian and mammalian retinae. Labelling was abundant in amacrine cells, whilst GABA was also present in one layer of horizontal cells and some bipolar cells. Taurine was present in both rods and cones, but, unlike the mammalian or avian retina, was absent from other cellular structures, including glial elements. Unexpectedly, the photoreceptor terminals lacked an apparent content of the excitatory amino acid transmitter glutamate. The glutamate that was present in the rods and cones occupied a crescentic arc corresponding to the location of glycogen-rich paraboloids. Asparagine was also present in rods, albeit in the modified mitochondria that formed the elipsoids of the rod inner segments. Arginine, the precursor for formation of nitric oxide, was present in glial cells, and in the paraboloids of both rods and cones.

Rapid postmortem changes in the cellular localisation of amino acid transmitters in the retina as assessed by immunocytochemistry

We have assessed by means of immunocytochemistry, the cellular distributions of the amino acid transmitters GABA, glycine and glutamate, and the free-radical scavenger taurine, in the retinae of adult rabbits at various times after death. Within 10 min of death, horizontal cells began to display immunoreactivity for GABA, whilst displaced amacrine cells began to display immunoreactivity for glycine. By 40 min postmortem, GABA was present in glial cells. Glutamate, which is not normally detectable in retinal glia, was detected in such glia by 20 min postmortem. By contrast immunocytochemically detectable glycine did not accumulate in glia. There was a gradual diminution of immunoreactivity for taurine in glial cells and photoreceptors. By 2 h postmortem, most immunoreactivity had disappeared from the retina. We conclude that amino acid transmitters show rapid changes in their distributions immediately after death, which may be related to changes in the patterns of transmitter release and uptake, and changes in degradation mechanisms. The rapid changes in cellular localisation of amino acid immunoreactivity illustrated in this study, indicate that the fixation of nervous tissues must be performed rapidly. Moreover, the massive loss of immunoreactivity by 2 h postmortem suggests that any assays for content of these transmitters at this, and subsequent time-points, will bear little resemblance to the values obtained at the time of death.

Retinoic acid promotes differentiation of photoreceptors in vitro

The results of several recent studies have demonstrated that cell commitment and differentiation in the developing vertebrate retina are influenced by cell-cell interactions within the microenvironment. Retinoic acid has been shown to influence cell fates during development of the nervous system, and retinoic acid has been detected in the embryonic retina. To determine whether retinoic acid mediates the differentiation of specific neuronal phenotypes during retinal histogenesis, we treated dissociated cell cultures of embryonic and neonatal rat retina with varying concentrations of all-trans or 9-cis retinoic acid and analyzed the effects on cell fate using neuron and photoreceptor-specific antibodies. Addition of exogenous retinoic acid caused a dose-dependent, specific increase in the number of cells that developed as photoreceptors in culture throughout the period of retinal neurogenesis. In the same cultures, retinoic acid also caused a dose-dependent decrease in the number of cells that developed as amacrine cells. Also, results of double-labeled immunohistochemical studies using bromodeoxyuridine demonstrated that the primary effect of retinoic acid was to influence progenitor cells to develop as newly generated rod photoreceptors. Since retinoic acid and at least one of the retinoic acid receptors (RAR alpha) have been localized to the developing neural retina, these results suggest that retinoic acid may play a role in the normal development of photoreceptor cells in vivo.

Taurine promotes the differentiation of a vertebrate retinal cell type in vitro

The retina offers a model system for investigating the mechanisms that control cell type determination and differentiation in the vertebrate central nervous system. Previously, rod photoreceptor development in vitro was found to require a diffusible activity released by retinal cells (D. Altshuler and C. Cepko, Development 114, 947–957, 1992). In this report, we show that retinal-cell-conditioned medium and extracts contain two separable activities that influence rod development: a > 10 kDa inhibitory activity, and a stimulatory activity that is < 1 kDa and heat stable. Taurine was found to be a component of the < 1 kDa fraction and to stimulate rod development when added to retinal cultures. Taurine was not the only rod-promoting factor in these retinal preparations, however, as conditioned medium and extracts stimulated a higher level of rod development than did taurine alone. Taurine uptake into cells could be blocked without inhibiting taurine's ability to stimulate rod development, arguing against an osmoregulatory or nutritive mechanism of action. Finally, a competitive antagonist of taurine's bioactivity was identified and shown partially to inhibit rod development in retinal explants, suggesting that taurine may normally act to stimulate rod development in the retina. These results provide evidence for three activities, one of which is taurine, that are candidate regulators of rod photoreceptor development in vivo.

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.

Immunocytochemistry of amino-acids in the rodent pituitary using extremely specific, very high titre antisera

The compartmental interrelationships of the metabolically related amino-acids glutamate, GABA and glutamine and the metabolically unrelated amino-acids taurine and glycine in the rodent pituitary, were investigated by light microscopic immunocytochemistry using highly specific antisera. Glutamate-like immunoreactivity was abundant in astrocytes in the posterior pituitary. Glutamine immunoreactivity was present only at low levels in the posterior pituitary, but was abundant in astrocytes within the intermediate lobe. Other glia-like cells in the anterior pituitary were also glutamine-immunoreactive. GABA immunoreactivity was abundant in the intermediate lobe but absent from anterior and posterior lobes. The GABA immunoreactivity mainly took the form of small punctata, the majority of which were in intimate apposition to the glutamine-immunoreactive glia. Strong taurine immunoreactivity was present in astrocytes in the posterior pituitary but only weak labelling was present in intermediate and anterior lobes of the pituitary. Specific glycine immunoreactivity was not detected in the pituitary. These results suggest that glutamate-immunoreactive astrocytes in the posterior pituitary, unlike glia in loci such as the retina, do not convert much, if any, of their glutamate content into glutamine (or if they do, it is rapidly further metabolized to another compound), whereas those astrocytes in the intermediate lobe do contain glutamine. The spatial association of GABAergic fibres with glutamine-positive astrocytes raises the possibility that astrocytes in the intermediate lobe receive a GABAergic innervation. Glutamate, glutamine and taurine (or their metabolites) may have roles as neuroactive substances regulating pituitary secretion.

Extremely high titre polyclonal antisera against small neurotransmitter molecules: rapid production, characterisation and use in light- and electron-microscopic immunocytochemistry

We have produced polyclonal antibodies against the small amino acid neurotransmitters, GABA, glutamate, glycine and taurine, with a simple new technique using antigens co-adsorbed with an adjuvant peptide to gold particles, which causes rapid and massive immune responses in all animals that we have studied. These antibodies are all of extremely high titre; they are typically used in immunocytochemistry at dilutions from 1 in 250,000 to 1 in 1,000,000 which represents an increase in titre of at least two orders of magnitude compared to standard antibody production techniques. Such very high dilutions result in minimal background labeling and a high signal-to-noise ratio when applied to sections of aldehyde-fixed, epoxy resin-embedded tissues at both light- and electron-microscopic levels. Each antibody displays minimal cross-reactivity with other neurotransmitter molecules. We suggest that our technique may be broadly applicable for raising antibodies against a wide variety of antigens of interest to neuroscientists, particularly those that normally elicit weak immune responses. The technique may also assist in clonal expansion prior to generation of monoclonal antibodies and may be viable, with modifications, for use in human immunisations.

Cloning of the human taurine transporter and characterization of taurine uptake in thyroid cells

A cDNA clone encoding a taurine transporter, designated HTAU, has been isolated from human thyroid. It contains an open reading frame encoding a protein of 619 amino acids with a calculated molecular weight of 69,675 Da. The predicted amino acid sequence of HTAU is highly homologous to those of dog kidney and rat brain. The HTAU mRNA was detectable in various human tissues examined. Transient expression of HTAU in COS-7 cells conferred a higher taurine uptake. The taurine uptake in FRTL-5 cells appears to be regulated by thyrotropin through cAMP. Finally, a higher taurine uptake may be associated with a higher proliferation rate in some cultured cell lines.

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.

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

The effects of 20 mM taurine on the phosphorylation of specific proteins in mitochondrial and rod outer segment subcellular fractions of the rat retina were measured. A band of protein with an apparent molecular weight of approximately 20K was consistently inhibited by taurine. Densitometry measurements performed on gel electrophoresis autoradiograms from the mitochondrial fraction demonstrated a 42.7 +/- 8.3% decrease due to taurine (20 mM) in the area corresponding to radioactivity from the approximately 20K phosphoprotein. However, only a 21.2 +/- 9.0% decrease was observed due to taurine in the rod outer segment preparation. These data suggest that taurine is exerting its primary effect on the phosphorylation of the approximately 20K molecular weight protein in the mitochondria of the retina. In addition, calmodulin and phorbol ester had no effect on the phosphorylation of the approximately 20K molecular weight protein.