Taurine deficiency damages retinal neurones: cone photoreceptors and retinal ganglion cells

The potential impact of a vegetarian diet on glaucoma

Treatment of primary open-angle glaucoma has centered on the lowering of intraocular pressure that damages the optic nerve; however, this strategy is not uniformly successful, especially in normal tension glaucoma, and there is interest in antioxidant, anti-inflammatory, and other neuroprotective strategies. Vegetarian diets are known to be rich in antioxidant and anti-inflammatory components and have a number of established health benefits. Thus, it would be reasonable to assume that vegetarian diets would be beneficial in glaucoma, but this approach has not been well studied. We examine the possible role of vegetarian diets and their components in the incidence and progression of glaucoma.

Taurine: a promising nutraceutic in the prevention of retinal degeneration

Taurine is considered a non-essential amino acid because it is synthesized by most mammals. However, dietary intake of taurine may be necessary to achieve the physiological levels required for the development, maintenance, and function of certain tissues. Taurine may be especially important for the retina. The concentration of taurine in the retina is higher than that in any other tissue in the body and taurine deficiency causes retinal oxidative stress, apoptosis, and degeneration of photoreceptors and retinal ganglion cells. Low plasma taurine levels may also underlie retinal degeneration in humans and therefore, taurine administration could exert retinal neuroprotective effects. Taurine has antioxidant, anti-apoptotic, immunomodulatory, and calcium homeostasis-regulatory properties. This review summarizes the role of taurine in retinal health and disease, where it appears that taurine may be a promising nutraceutical.

Plasma Metabolites Associated with OCT Features of Age-Related Macular Degeneration

Purpose

The most widely used classifications of age-related macular degeneration (AMD) and its severity stages still rely on color fundus photographs (CFPs). However, AMD has a wide phenotypic variability that remains poorly understood and is better characterized by OCT. We and others have shown that patients with AMD have a distinct plasma metabolomic profile compared with controls. However, all studies to date have been performed solely based on CFP classifications. This study aimed to assess if plasma metabolomic profiles are associated with OCT features commonly seen in AMD.

Design

Prospectively designed, cross-sectional study.

Participants

Subjects with a diagnosis of AMD and a control group (> 50 years old) from Boston, United States, and Coimbra, Portugal.

Methods

All participants were imaged with CFP, used for AMD staging (Age-Related Eye Disease Study 2 classification scheme), and with spectral domain OCT (Spectralis, Heidelberg). OCT images were graded by 2 independent graders for the presence of characteristic AMD features, according to a predefined protocol. Fasting blood samples were collected for metabolomic profiling (using nontargeted high-resolution mass spectrometry by Metabolon Inc). Analyses were conducted using logistic regression models including the worst eye of each patient (AREDS2 classification) and adjusting for confounding factors. Each cohort (United States and Portugal) was analyzed separately and then results were combined by meta-analyses. False discovery rate (FDR) was used to account for multiple comparisons.

Main Outcome Measures

Plasma metabolite levels associated with OCT features.

Results

We included data on 468 patients, 374 with AMD and 94 controls, and on 725 named endogenous metabolites. Meta-analysis identified significant associations (FDR < 0.05) between plasma metabolites and 3 OCT features: hyperreflective foci (6), atrophy (6), and ellipsoid zone disruption (3). Most associations were seen with amino acids, and all but 1 metabolite presented specific associations with the OCT features assessed.

Conclusions

To our knowledge, we show for the first time that plasma metabolites have associations with specific OCT features seen in AMD. Our results support that the wide spectrum of presentations of AMD likely include different pathophysiologic mechanisms by identifying specific pathways associated with each OCT feature.

Financial Disclosures

Proprietary or commercial disclosure may be found after the references.

Insights into the liver-eyes connections, from epidemiological, mechanical studies to clinical translation

Maintenance of internal homeostasis is a sophisticated process, during which almost all organs get involved. Liver plays a central role in metabolism and involves in endocrine, immunity, detoxification and storage, and therefore it communicates with distant organs through such mechanisms to regulate pathophysiological processes. Dysfunctional liver is often accompanied by pathological phenotypes of distant organs, including the eyes. Many reviews have focused on crosstalk between the liver and gut, the liver and brain, the liver and heart, the liver and kidney, but with no attention paid to the liver and eyes. In this review, we summarized intimate connections between the liver and the eyes from three aspects. Epidemiologically, we suggest liver-related, potential, protective and risk factors for typical eye disease as well as eye indicators connected with liver status. For molecular mechanism aspect, we elaborate their inter-organ crosstalk from metabolism (glucose, lipid, proteins, vitamin, and mineral), detoxification (ammonia and bilirubin), and immunity (complement and inflammation regulation) aspect. In clinical application part, we emphasize the latest advances in utilizing the liver-eye axis in disease diagnosis and therapy, involving artificial intelligence-deep learning-based novel diagnostic tools for detecting liver disease and adeno-associated viral vector-based gene therapy method for curing blinding eye disease. We aim to focus on and provide novel insights into liver and eyes communications and help resolve existed clinically significant issues.

Taurine: A Source and Application for the Relief of Visual Fatigue

According to reports, supplementation with appropriate doses of taurine may help to reduce visual fatigue. Presently, some progress has been made in research related to taurine in eye health, but the lack of systematic summaries has led to the neglect of its application in the relief of visual fatigue. This paper, therefore, provides a systematic review of the sources of taurine, including the endogenous metabolic and exogenous dietary pathways, as well as a detailed review of the distribution and production of exogenous taurine. The physiological mechanisms underlying the production of visual fatigue are summarized and the research progress of taurine in relieving visual fatigue is reviewed, including the safety of consumption and the mechanism of action in relieving visual fatigue, in order to provide some reference basis and inspiration for the development and application of taurine in functional foods for relieving visual fatigue.

Systemic taurine treatment affords functional and morphological neuroprotection of photoreceptors and restores retinal pigment epithelium function in RCS rats

The aim of our work was to study whether taurine administration has neuroprotective effects in dystrophic Royal College of Surgeons (RCS) rats, suffering retinal degeneration secondary to impaired retinal pigment epithelium phagocytosis caused by a MERTK mutation. Dystrophic RCS-p + female rats (n = 36) were divided into a non-treated group (n = 16) and a treated group (n = 20) that received taurine (0.2 M) in drinking water from postnatal day (P)21 to P45, when they were processed. Retinal function was assessed with electroretinogram. Retinal morphology was assessed in cross-sections using immunohistochemical techniques to label photoreceptors, retinal microglial and macroglial cells, active zones of conventional and ribbon synaptic connections, and oxidative stress. Retinal pigment epithelium function was examined using intraocular fluorogold injections. Our results document that taurine treatment increases taurine plasma levels and photoreceptor survival in dystrophic rats. The number of photoreceptor nuclei rows at P45 was 3-5 and 6-11 in untreated and treated animals, respectively. Electroretinograms showed increases of 70% in the rod response, 400% in the a-wave amplitude, 30% in the b-wave amplitude and 75% in the photopic b-wave response in treated animals. Treated animals also showed decreased numbers of microglial cells in the outer retinal layers, decreased glial fibrillary acidic protein (GFAP) expression in Müller cells, decreased oxidative stress in the outer and inner nuclear layers and improved maintenance of synaptic connections. Treated animals showed increased FG phagocytosis in the retinal pigment epithelium cells. In conclusion, systemic taurine treatment decreases photoreceptor degeneration and increases electroretinographic responses in dystrophic RCS rats and these effects may be mediated through various neuroprotective mechanisms.

Vitamin A aldehyde-taurine adducts function in photoreceptor cells

To facilitate the movement of retinoids through the visual cycle and to limit nonspecific chemical reaction, multiple mechanisms are utilized to handle these molecules when not contained within the binding pocket of opsin. Vitamin A aldehyde is sequestered by reversible Schiff base formation with phosphatidylethanolamine (PE) and subsequently undergoes NADPH-dependent reduction. Otherwise inefficient handling of retinaldehyde can lead to the formation of fluorescent di-retinal compounds within the outer segments of photoreceptor cells. These bisretinoid fluorophores initiate photooxidative processes having adverse consequences for retina. Various carrier proteins confer water solubility and maintain the 11-cis-retinoid configuration. Mechanisms for sequestration of retinoid include the formation of a reversible Schiff base between retinaldehyde and taurine (A1-taurine, A1T), the most abundant amino acid in photoreceptor cells. Here we have undertaken to examine the effects of taurine depletion using the transport inhibitors guanidinoethyl sulfonate (GES) and β-alanine. Oral treatment of BALB/cJ mice with β-alanine reduced ocular A1T and the mice exhibited significantly lower scotopic and photopic a-wave amplitudes. As a secondary effect of retinal degeneration, A1T was not detected and taurine was significantly reduced in mice carrying a P23H opsin mutation. The thinning of ONL that is indicative of reduced photoreceptor cell viability in albino Abca4^-/- mice was more pronounced in β-alanine treated mice. Treatment of agouti and albino Abca4^-/- mice with β-alanine and GES was associated with reduced bisretinoid measured chromatographically. Consistent with a reduction in carbonyl scavenging activity by taurine, methylglyoxal-adducts were also increased in the presence of β-alanine. Taken together these findings support the postulate that A1T serves as a reservoir of vitamin A aldehyde, with diminished A1T explaining reduced photoreceptor light-sensitivity, accentuated ONL thinning in Abca4^-/- mice and attenuated bisretinoid formation.

Therapeutic effect of the mesenchymal stem cells on vigabatrin-induced retinopathy in adult male albino rat

Vigabatrin (VGB) is an effective antiepileptic drug used mainly to treat infantile spasms and refractory complex partial seizures. However, using VGB was restricted as it was known to cause retinal toxicity that appears as a severe peripheral visual field defect. Accordingly, this study was conducted to examine the histopathological and biochemical effects of VGB on the retina in adult male albino rats and assess the possible therapeutic role of mesenchymal stem cells (MSCs) against this potential toxicity. The rats were divided into three groups (control group, VGB group, and VGB/MSCs group), one week after 65 days of VGB treatment ±MSCs. The right eyeballs were prepared for histological and immunohistochemical examination, whereas the left eyeballs were prepared for real-time polymerase chain reaction analysis. Our results demonstrated that MSCs ameliorated retinal pathological changes and downregulated the expression of glial fibrillary acidic protein, vascular endothelial growth factor, and synaptophysin after VGB administration suggesting MSCs function and vascular modulating effect. Moreover, MSCs regulate retinal tissue gene expression of BAX, Bcl-2, BDNF, NGF, synapsin, interleukin (IL)-6, IL-1β, and occludin suggesting MSCs antiapoptotic and immunomodulating effect. In conclusion, MSCs administration could be a suitable therapeutic line to ameliorate VGB-induced retinopathy.

Glial Cell Activation and Oxidative Stress in Retinal Degeneration Induced by β-Alanine Caused Taurine Depletion and Light Exposure

We investigate glial cell activation and oxidative stress induced by taurine deficiency secondary to β-alanine administration and light exposure. Two months old Sprague-Dawley rats were divided into a control group and three experimental groups that were treated with 3% β-alanine in drinking water (taurine depleted) for two months, light exposed or both. Retinal and external thickness were measured in vivo at baseline and pre-processing with Spectral-Domain Optical Coherence Tomography (SD-OCT). Retinal cryostat cross sections were immunodetected with antibodies against various antigens to investigate microglial and macroglial cell reaction, photoreceptor outer segments, synaptic connections and oxidative stress. Taurine depletion caused a decrease in retinal thickness, shortening of photoreceptor outer segments, microglial cell activation, oxidative stress in the outer and inner nuclear layers and the ganglion cell layer and synaptic loss. These events were also observed in light exposed animals, which in addition showed photoreceptor death and macroglial cell reactivity. Light exposure under taurine depletion further increased glial cell reaction and oxidative stress. Finally, the retinal pigment epithelial cells were Fluorogold labeled and whole mounted, and we document that taurine depletion impairs their phagocytic capacity. We conclude that taurine depletion causes cell damage to various retinal layers including retinal pigment epithelial cells, photoreceptors and retinal ganglion cells, and increases the susceptibility of the photoreceptor outer segments to light damage. Thus, beta-alanine supplements should be used with caution.

Retinoic acid and taurine enhance differentiation of the human bone marrow stem cells into cone photoreceptor cells and retinal ganglion cells

Degeneration and apoptotic death of the photoreceptor cell-layer of retina are a major cause of irreversible blindness in the development era. The stem cell replacement therapy is one of the strategies for the retinal repairing. In addition, exogenous signals critically contribute to the direction of lineage decisions that causes the fate-restricted photoreceptor progenitors from stem cell progeny in culture. It has been found that epidermal growth factor (EGF), taurine, and retinoic acid (RA) initially act in the instructive as well as lineage-restricted way in the progenitor lineage for producing neuroretinal cells or photoreceptor like cells from stem cell. The study aims to investigate the effect of RA and taurine in differentiation of the human bone marrow stem cell into cone photoreceptors cells and retinal ganglion cells. Mesenchymal stem cell was derived from human bone marrow of the term delivery. Therefore, the cultured cells have been treated with Dulbecco's modified Eagle's medium (DMEM)/high glucose (H⁺ ). After the four-cell passage, basal medium was replaced with DMEM/F12 complemented with 50 μmol/L taurine, RA (1 µM) and EGF (1 µg/ml). Subsequently cellular change morphology was detected following 7 and 14 days. Then, gene expression of neuroretinal and photoreceptor cell biomarkers (CRX, OTX2, PKC-α, recoverin, and Rho) were examined by quantitative polymerase chain reaction (Q-PCR). Also, cells were cultured, fixed, and then immunocytochemical analyzed. Primary antibodies included CRX and Rho. Cellular morphology demonstrated spindle elongated morphology. Taurine alone and combination of RA upregulate neuroretinal and photoreceptor cell biomarkers in messenger RNA and protein levels but along with EGF have not significant effect. Our data showed that taurine combination with RA can differentiate bone marrow mesenchymal stem cells into neuroretinal or photoreceptor like cells in vitro that can offer an attractive treatment ground for transplantation in the cell-replacement therapy for some forms of the retinal degeneration.

Taurine treatment of retinal degeneration and cardiomyopathy in a consanguineous family with SLC6A6 taurine transporter deficiency

In a consanguineous Pakistani family with two affected individuals, a homozygous variant Gly399Val in the eighth transmembrane domain of the taurine transporter SLC6A6 was identified resulting in a hypomorph transporting capacity of ~15% compared with normal. Three-dimensional modeling of this variant has indicated that it likely causes displacement of the Tyr138 (TM3) side chain, important for transport of taurine. The affected individuals presented with rapidly progressive childhood retinal degeneration, cardiomyopathy and almost undetectable plasma taurine levels. Oral taurine supplementation of 100 mg/kg/day resulted in maintenance of normal blood taurine levels. Following approval by the ethics committee, a long-term supplementation treatment was introduced. Remarkably, after 24-months, the cardiomyopathy was corrected in both affected siblings, and in the 6-years-old, the retinal degeneration was arrested, and the vision was clinically improved. Similar therapeutic approaches could be employed in Mendelian phenotypes caused by the dysfunction of the hundreds of other molecular transporters.

Taurine Attenuates the Hypotaurine-Induced Progression of CRC via ERK/RSK Signaling

Colorectal cancer (CRC) is one of the most common malignant tumors, and previous metabolomics work has demonstrated great promise in identifying specific small molecules of tumor phenotype. In the present study, we analyzed the metabolites of resected tissues through gas chromatography-mass spectrometry (GC-MS), and found that the concentration of taurine in CRC tissues diminished whereas the concentration of hypotaurine increased. The results in vitro demonstrated that taurine significantly suppressed cellular proliferation, metastasis, and colony formation whereas it induced apoptosis in CRC cells. Furthermore, taurine regulated the expression levels of epithelial mesenchymal transition (EMT)-associated genes in a dose-dependent manner. Taurine also alleviated hypotaurine-induced CRC progression, which was linked to the inhibition of the ERK/RSK-signaling pathway and diminution in intracellular hypotaurine. Taurine additionally attenuated hypotaurine-induced tumor growth and metastasis in vivo. Patients with CRC exhibited lower levels of serum taurine, suggesting that taurine might be a promising biomarker reflecting a poor prognosis in CRC. Collectively, our results demonstrated that taurine-attenuated, hypotaurine-induced CRC progression provides a potential target for CRC therapy.

Taurine and oxidative stress in retinal health and disease

Retinal disorders are leading causes of blindness and are due to an imbalance between reactive oxygen species and antioxidant scavenger (in favor of pro‐oxidant species) or a disruption of redox signaling and control. Indeed, it is well known that oxidative stress is one of the leading causes of retinal degenerative diseases. Different approaches using nutraceuticals resulted in protective effects in these disorders. This review will discuss the impact of oxidative stress in retinal neurodegenerative diseases and the potential strategies for avoiding or counteracting oxidative damage in retinal tissues, with a specific focus on taurine. Increasing data indicate that taurine may be effective in slowing down the progression of degenerative retinal diseases, thus suggesting that taurine can be a promising candidate for the prevention or as adjuvant treatment of these diseases. The mechanism by which taurine supplementation acts is mainly related to the reduction of oxidative stress. In particular, it has been demonstrated to improve retinal reduced glutathione, malondialdehyde, superoxide dismutase, and catalase activities. Antiapoptotic effects are also involved; however, the protective mechanisms exerted by taurine against retinal damage remain to be further investigated.

Growth Factors as Tools in Photoreceptor Cell Regeneration and Vision Recovery

Photoreceptor loss is a major cause of blindness around the world. Stem cell therapy offers a new strategy in retina degenerative disease. Retinal progenitors can be derived from embryonic stem cells (ESC) in vitro, but cannot be processed to a mature state. In addition, the adult recipient retina presents a very different environment than the photoreceptor precursor donor. It seems that modulation of the recipient environment by ectopic development regulated growth factors for transplanted cells could generate efficient putative photoreceptors. The purpose of this review article was to investigate the signaling pathway of growth factors including: insulin-like growth factors (IGFs), fibroblast growth factors (FGF), Nerve growth factor (NGF), Brain-derived neurotrophic factor (BDNF), Taurin and Retinoic acid (RA) involved in the differentiation of neuroretina cell, like; photoreceptor and retinal progenitor cells. Given the results available in the related literature, the differentiation efficacy of ESCs toward the photoreceptor and retinal neurons and the important role of growth factors in activating signaling pathways such as Akt, Ras/Raf1/ and ERKs also inhibit the ASK1/JNK apoptosis pathway. Manipulating differentiated culture, growth factors can influence photoreceptor transplantation efficiency in retinal degenerative disease.

New solutions for old challenges in glaucoma treatment: is taurine an option to consider?

Glaucoma is a range of progressive optic neuropathies characterized by progressive retinal ganglion cell loss and visual field defects. It is recognized as a leading cause of irreversible blindness affecting more than 70 million people worldwide. Currently, reduction of intraocular pressure, a widely recognized risk factor for glaucoma development, is the only pharmacological strategy for slowing down retinal ganglion cell loss and disease progression. However, retinal ganglion cell death and visual field loss have been observed in normotensive glaucoma, suggesting that the disease process is partially independent of intraocular pressure. Taurine is one of the agents that have attracted attention of researchers recently. Taurine has been shown to be involved in multiple cellular functions, including a central role as a neurotransmitter, as a trophic factor in the central nervous system development, as an osmolyte, as a neuromodulator, and as a neuroprotectant. It also plays a role in the maintenance of the structural integrity of the membranes and in the regulation of calcium transport and homeostasis. Taurine is known to prevent N-methyl-D-aspartic acid-induced excitotoxic injury to retinal ganglion cells. A recently published study clearly demonstrated that taurine prevents retinal neuronal apoptosis both in vivo and in vitro. Protective effect of taurine may be attributed to direct inhibition of apoptosis, an activation of brain derived neurotrophic factor-related neuroprotective mechanisms and reduction of retinal oxidative and nitrosative stresses. Further studies are needed to fully explore the potential of taurine as a neuroprotective agent, so that it can be applied in clinical practice, particularly for the treatment of glaucoma. The objective of current review was to summarize recent evidence on neuroprotective properties of taurine in glaucoma.

Preferential accumulation of the active S-(+) isomer in murine retina highlights novel mechanisms of vigabatrin-associated retinal toxicity

((S)-(+)/(R)-(-)) vigabatrin (Sabril^R; γ-vinyl GABA), an antiepileptic irreversibly inactivating GABA-transaminase, was administered to male C57Bl6 J mice via continuous infusion (0, 40, 80 mg/kg/d) for 12 days. Our study design pooled retina, eye (minus retina), whole brain and plasma from n = 24 animals for each dose to provide n = 8 triplicates per treatment group. Hypothesizing that (S)-(+) VGB (active isomer) would preferentially accumulate in retina, we determined VGB isomers, comprehensive amino acids, and pharmacokinetic parameters. In brain, eye and plasma, the ((S)-(+)/(R)-(-)) ratio varied from 0.73 to 1.29 and 13.3 in retina, accompanied by a partition coefficient (tissue/plasma, ((S)-(+);(R)-(-))) of 5.8;0.34, 0.63;0.49, and 0.51;0.34 in retina, eye and brain, respectively. Racemic VGB (nmol/g; plasma, nmol/mL, range of means for dose) content was: retina, 25-36; eye (minus retina), 4.8-8.0; brain, 3.1-6.8 and plasma, 8.7-14.9. GABA tissue content (nmol/g) was 1246-3335, 18-64 and 2615-3200 as a function of VGB dose for retina, eye (minus retina) and brain, respectively. The retinal glial cell toxin 2-aminoadipic acid also increased with VGB dose (76-96 nmol/g). Partitioning of active (S)-(+) VGB to retina suggests the involvement of a stereospecific transporter, the identification of which could reveal new therapeutic paradigms that might mitigate VGB's well-known retinal toxicity and expand its clinical utility.

Vigabatrin-Induced Retinal Functional Alterations and Second-Order Neuron Plasticity in C57BL/6J Mice

Purpose

Vigabatrin (VGB) is an effective antiepileptic that increases concentrations of inhibitory γ-aminobutyric acid (GABA) by inhibiting GABA transaminase. Reports of VGB-associated visual field loss limit its clinical usefulness, and retinal toxicity studies in laboratory animals have yielded conflicting results.

Methods

We examined the functional and morphologic effects of VGB in C57BL/6J mice that received either VGB or saline IP from 10 to 18 weeks of age. Retinal structure and function were assessed in vivo by optical coherence tomography (OCT), ERG, and optomotor response. After euthanasia, retinas were processed for immunohistochemistry, and retinal GABA, and VGB quantified by mass spectrometry.

Results

No significant differences in visual acuity or total retinal thickness were identified between groups by optomotor response or optical coherence tomography, respectively. After 4 weeks of VGB treatment, ERG b-wave amplitude was enhanced, and amplitudes of oscillatory potentials were reduced. Dramatic rod and cone bipolar and horizontal cell remodeling, with extension of dendrites into the outer nuclear layer, was observed in retinas of VGB-treated mice. VGB treatment resulted in a mean 3.3-fold increase in retinal GABA concentration relative to controls and retinal VGB concentrations that were 20-fold greater than brain.

Conclusions

No evidence of significant retinal thinning or ERG a- or b-wave deficits were apparent, although we describe significant alterations in ERG b-wave and oscillatory potentials and in retinal cell morphology in VGB-treated C57BL/6J mice. The dramatic concentration of VGB in retina relative to the target tissue (brain), with a corresponding increase in retinal GABA, offers insight into the pathophysiology of VGB-associated visual field loss.

Taurine Protects Retinal Cells and Improves Synaptic Connections in Early Diabetic Rats

Purpose: Taurine has long been thought to be involved in retinal protection from retinal degenerative diseases, but the underlying molecular mechanisms remain unclear. Retinal neurodegeneration is an early event in the pathogenesis of diabetic retinopathy (DR) that precedes and participates in the microcirculatory abnormalities that occur in DR. Our objective was to investigate the role and mechanisms of taurine in early diabetic retinas.Methods: Eight-week-old STZ-induced diabetic rats and control animals were randomly assigned to receive taurine or vehicle by intraperitoneal injection or by intragastric administration. The retinal function and retinal cell counts were evaluated using an electroretinography (ERG) and immunofluorescence microscopy. Plasma amino acids were measured by ion-exchange chromatography (IEC). The expression levels of retinal taurine transporter (Tau-T), mitochondria-dependent apoptosis-associated genes and reactive gliosis markers were studied by western blotting and immunofluorescence. Pre- and post-synaptic markers (PSD95 and mGluR6) in outer plexiform layer (OPL), and the bipolar cell marker protein kinase C alpha (PKCα) were localized by immunofluorescence. Levels of PSD95 and mGluR6 were determined by quantitative western blot.Results: Taurine significantly prevented the reduction of photopic b-wave amplitude and retinal cone cells and ganglion cells loss and maintained the Bcl-2/Bax ratio balance in diabetic rats. Taurine also prevented the upregulation of glial fibrillary acidic protein (GFAP) and reduced retinal reactive gliosis. Taurine reduced plasma glutamate and tyrosine levels, which were elevated in diabetic rats. Moreover, mGluR6 levels reduction detected by western blot and immunofluorescence in diabetic retinas was inhibited and the displacement of mGluR6 in OPL into the inner nuclear layer (INL) detected by immunofluorescence was reduced by Taurine treatment.Conclusion: Taurine may protect retinal cells from diabetic attacks by activating Tau-T, reducing retinal reactive gliosis, improving retinal synaptic connections and decreasing retinal cell apoptosis. Thus, taurine treatment may be a novel approach for early DR.

β-alanine supplementation induces taurine depletion and causes alterations of the retinal nerve fiber layer and axonal transport by retinal ganglion cells

To study the effect of taurine depletion induced by β-alanine supplementation in the retinal nerve fiber layer (RNFL), and retinal ganglion cell (RGC) survival and axonal transport. Albino Sprague-Dawley rats were divided into two groups: one group received β-alanine supplementation (3%) in the drinking water during 2 months to induce taurine depletion, and the other group received regular water. After one month, half of the rats from each group were exposed to light. Retinas were analyzed in-vivo using Spectral-Domain Optical Coherence Tomography (SD-OCT). Prior to processing, RGCs were retrogradely traced with fluorogold (FG) applied to both superior colliculi, to assess the state of their retrograde axonal transport. Retinas were dissected as wholemounts, surviving RGCs were immunoidentified with Brn3a, and the RNFL with phosphorylated high-molecular-weight subunit of the neurofilament triplet (pNFH) antibodies. β-alanine supplementation decreases significantly taurine plasma levels and causes a significant reduction of the RNFL thickness that is increased after light exposure. An abnormal pNFH immunoreactivity in some RGC bodies, their proximal dendrites and axons, and a further diminution of the mean number of FG-traced RGCs compared with Brn3a⁺RGCs, indicate that their retrograde axonal transport is affected. In conclusion, taurine depletion causes RGC loss and axonal transport impairment. Finally, our results suggest that care should be taken when ingesting β-alanine supplements due to the limited understanding of their potential adverse effects.

Systemic taurine treatment provides neuroprotection against retinal photoreceptor degeneration and visual function impairments

Objective

Retinitis pigmentosa causes progressive photoreceptor degeneration in the subjects while no clinical therapy exists. The present study sought to evaluate the potential protective effects of taurine on a pharmacologically induced RP animal model.

Methods

Photoreceptor degeneration in mice was induced by an intraperitoneal injection of N-methyl-N-nitrosourea (MNU). The MNU-administrated mouse received taurine treatment and then they were examined by electroretinography, spectral-domain optical coherence tomography, optokinetic test, and histological and immunohistochemistry assay.

Results

Prominent taurine deficiency was found in the retinas of MNU-administered mice. Intravenous taurine treatment increased significantly the retinal taurine level. Morphological studies showed that taurine could alleviate the retinal disorganizations in the MNU-induced mice. Taurine also ameliorated the visual impairments in the MNU-induced mice as evidenced by functional examinations. Immunostaining experiments demonstrated that both the M-cone and S-cone populations in the degenerative retinas are rescued by taurine. In particular, the M-cone photoreceptors in superior-temporal quadrant and the S-cone photoreceptors in inferior-nasal quadrant were preferentially rescued. Mechanism study showed that the photoreceptor apoptosis and oxidative stress in the degenerative retina were effectively alleviated by taurine treatment.

Conclusion

Taurine is protective against the MNU-induced photoreceptor degeneration. Systemic taurine administration may act as a promising therapeutic potion for retinopathies with chronic cycle.

Human Plasma Metabolomics in Age-Related Macular Degeneration: Meta-Analysis of Two Cohorts

The pathogenesis of age-related macular degeneration (AMD), a leading cause of blindness worldwide, remains only partially understood. This has led to the current lack of accessible and reliable biofluid biomarkers for diagnosis and prognosis, and absence of treatments for dry AMD. This study aimed to assess the plasma metabolomic profiles of AMD and its severity stages with the ultimate goal of contributing to addressing these needs. We recruited two cohorts: Boston, United States (n = 196) and Coimbra, Portugal (n = 295). Fasting blood samples were analyzed using ultra-high performance liquid chromatography mass spectrometry. For each cohort, we compared plasma metabolites of AMD patients versus controls (logistic regression), and across disease stages (permutation-based cumulative logistic regression considering both eyes). Meta-analyses were then used to combine results from the two cohorts. Our results revealed that 28 metabolites differed significantly between AMD patients versus controls (false discovery rate (FDR) q-value: 4.1 × 10-2-1.8 × 10-5), and 67 across disease stages (FDR q-value: 4.5 × 10-2-1.7 × 10-4). Pathway analysis showed significant enrichment of glycerophospholipid, purine, taurine and hypotaurine, and nitrogen metabolism (p-value ≤ 0.04). In conclusion, our findings support that AMD patients present distinct plasma metabolomic profiles, which vary with disease severity. This work contributes to the understanding of AMD pathophysiology, and can be the basis of future biomarkers and precision medicine for this blinding condition.

Taurine and its analogs in neurological disorders: Focus on therapeutic potential and molecular mechanisms

Taurine is a sulfur-containing amino acid and known as semi-essential in mammals and is produced chiefly by the liver and kidney. It presents in different organs, including retina, brain, heart and placenta and demonstrates extensive physiological activities within the body. In the several disease models, it attenuates inflammation- and oxidative stress-mediated injuries. Taurine also modulates ER stress, Ca2+ homeostasis and neuronal activity at the molecular level as part of its broader roles. Different cellular processes such as energy metabolism, gene expression, osmosis and quality control of protein are regulated by taurine. In addition, taurine displays potential ameliorating effects against different neurological disorders such as neurodegenerative diseases, stroke, epilepsy and diabetic neuropathy and protects against injuries and toxicities of the nervous system. Several findings demonstrate its therapeutic role against neurodevelopmental disorders, including Angelman syndrome, Fragile X syndrome, sleep-wake disorders, neural tube defects and attention-deficit hyperactivity disorder. Considering current biopharmaceutical limitations, developing novel delivery approaches and new derivatives and precursors of taurine may be an attractive option for treating neurological disorders. Herein, we present an overview on the therapeutic potential of taurine against neurological disorders and highlight clinical studies and its molecular mechanistic roles. This article also addresses the neuropharmacological potential of taurine analogs.

Metabolomic analyses of vigabatrin (VGB)-treated mice: GABA-transaminase inhibition significantly alters amino acid profiles in murine neural and non-neural tissues

The anticonvulsant vigabatrin (VGB; Sabril^R) irreversibly inhibits GABA transaminase to increase neural GABA, yet its mechanism of retinal toxicity remains unclear. VGB is suggested to alter several amino acids, including homocarnosine, β-alanine, ornithine, glycine, taurine, and 2-aminoadipic acid (AADA), the latter a homologue of glutamic acid. Here, we evaluate the effect of VGB on amino acid concentrations in mice, employing a continuous VGB infusion (subcutaneously implanted osmotic minipumps), dose-escalation paradigm (35-140 mg/kg/d, 12 days), and amino acid quantitation in eye, visual and prefrontal cortex, total brain, liver and plasma. We hypothesized that continuous VGB dosing would reveal numerous hitherto undescribed amino acid disturbances. Consistent amino acid elevations across tissues included GABA, β-alanine, carnosine, ornithine and AADA, as well as neuroactive aspartic and glutamic acids, serine and glycine. Maximal increase of AADA in eye occurred at 35 mg/kg/d (41 ± 2 nmol/g (n = 21, vehicle) to 60 ± 8.5 (n = 8)), and at 70 mg/kg/d for brain (97 ± 6 (n = 21) to 145 ± 6 (n = 6)), visual cortex (128 ± 6 to 215 ± 19) and prefrontal cortex (124 ± 11 to 200 ± 13; mean ± SEM; p < 0.05), the first demonstration of tissue AADA accumulation with VGB in mammal. VGB effects on basic amino acids, including guanidino-species, suggested the capacity of VGB to alter urea cycle function and nitrogen disposal. The known toxicity of AADA in retinal glial cells highlights new avenues for assessing VGB retinal toxicity and other off-target effects.

Taurine Promotes Retinal Ganglion Cell Survival Through GABAB Receptor Activation

Retinal ganglion cell (RGC) degeneration occurs in numerous retinal diseases, either as a primary process like in glaucoma, or secondary to photoreceptor loss and no efficient compound targeting directly RGC neuroprotection is yet available. We previously described that taurine exerts a direct protective effect on RGCs cultured under serum-deprived conditions. Because taurine was known to have an agonist-like activity for GABA/glycine receptors, we investigated here if the taurine-elicited neuroprotective effect may be mediated through the activation of these receptors using selective antagonist ligands. RGCs were purified, seeded in 96-well plate and maintained in culture during 6 days in vitro. Viable cells were labelled with calcein and densities in full-well area were then automatically counted. Here we show that the protective effect of taurine against RGC loss observed under serum deprivation can be mediated through the GABAB receptor stimulation. Hence, two selective agonists, including baclofen, at this metabotropic GABAB receptor were found to reproduce taurine action by enhancing RGC survival in culture. This study suggests that GABAB receptor stimulation provides direct neuroprotection for RGCs. Accordingly, drugs targeting GABAB receptor may represent a new way for the prevention of RGC degeneration.

Taurine exhibits an apoptosis-inducing effect on human nasopharyngeal carcinoma cells through PTEN/Akt pathways in vitro

Nasopharyngeal carcinoma (NPC) is a distinctive type of head and neck malignancy with a high incidence in southern China. Previous studies have confirmed that taurine shows an anti-cancer effect on a variety of human tumors by inhibiting cell proliferation and inducing apoptosis. However, the underlying molecular mechanism of its anti-cancer effect on NPC is not well understood. To clarify these anti-cancer mechanisms, we performed cell viability and colony formation assays. Apoptotic cells were quantified by flow cytometry. The expression levels of apoptosis-related proteins were evaluated by Western blot. The results showed that taurine markedly inhibited cell proliferation in NPC cells, but only slightly in an immortalized normal nasopharyngeal cell line. Taurine suppressed colony formation and induced apoptosis of NPC cell lines in a dose-dependent manner. Furthermore, taurine increased the active form of caspase-9/3 in a dose-dependent manner. Taurine down-regulated the anti-apoptotic protein Bcl-xL and up-regulated the pro-apoptotic protein Bax and GRP78, a major endoplasmic reticulum (ER) chaperone. These results suggest the involvement of mitochondrial and ER stress signaling in apoptosis. In addition, taurine increased the levels of PTEN (phosphatase and tensin homolog deleted on chromosome 10) and p53, and reduced phosphorylated Akt (protein kinase B). In conclusion, taurine may inhibit cell proliferation and induce apoptosis in NPC through PTEN activation with concomitant Akt inactivation.

Protective effect of magnesium acetyltaurate and taurine against NMDA-induced retinal damage involves reduced nitrosative stress

PURPOSE

Retinal nitrosative stress associated with altered expression of nitric oxide synthases (NOS) plays an important role in excitotoxic retinal ganglion cell loss in glaucoma. The present study evaluated the effects of magnesium acetyltaurate (MgAT) on changes induced by N-methyl-D-aspartate (NMDA) in the retinal expression of three NOS isoforms, retinal 3-nitrotyrosine (3-NT) levels, and the extent of retinal cell apoptosis in rats. Effects of MgAT with taurine (TAU) alone were compared to understand the benefits of a combined salt of Mg and TAU.

METHODS

Excitotoxic retinal injury was induced with intravitreal injection of NMDA in Sprague-Dawley rats. All treatments were given as pre-, co-, and post-treatment with NMDA. Seven days post-injection, the retinas were processed for measurement of the expression of NOS isoforms using immunostaining and enzyme-linked immunosorbent assay (ELISA), retinal 3-NT content using ELISA, retinal histopathological changes using hematoxylin and eosin (H&E) staining, and retinal cell apoptosis using terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) staining.

RESULTS

As observed on immunohistochemistry, the treatment with NMDA caused a 4.53-fold increase in retinal nNOS expression compared to the PBS-treated rats (p<0.001). Among the MgAT-treated groups, only the pretreatment group showed significantly lower nNOS expression than the NMDA-treated group with a 2.00-fold reduction (p<0.001). Among the TAU-treated groups, the pre- and cotreatment groups showed 1.84- and 1.71-fold reduction in nNOS expression compared to the NMDA-treated group (p<0.001), respectively, but remained higher compared to the PBS-treated group (p<0.01). Similarly, iNOS expression in the NMDA-treated group was significantly greater than that for the PBS-treated group (2.68-fold; p<0.001). All MgAT treatment groups showed significantly lower iNOS expression than the NMDA-treated groups (3.58-, 1.51-, and 1.65-folds, respectively). However, in the MgAT co- and post-treatment groups, iNOS expression was significantly greater than in the PBS-treated group (1.77- and 1.62-folds, respectively). Pretreatment with MgAT caused 1.77-fold lower iNOS expression compared to pretreatment with TAU (p<0.05). In contrast, eNOS expression was 1.63-fold higher in the PBS-treated group than in the NMDA-treated group (p<0.001). Among all treatment groups, only pretreatment with MgAT caused restoration of retinal eNOS expression with a 1.39-fold difference from the NMDA-treated group (p<0.05). eNOS expression in the MgAT pretreatment group was also 1.34-fold higher than in the TAU pretreatment group (p<0.05). The retinal NOS expression as measured with ELISA was in accordance with that estimated with immunohistochemistry. Accordingly, among the MgAT treatment groups, only the pretreated group showed 1.47-fold lower retinal 3-NT than the NMDA-treated group, and the difference was significant (p<0.001). The H&E-stained retinal sections in all treatment groups showed statistically significantly greater numbers of retinal cell nuclei than the NMDA-treated group in the inner retina. However, the ganglion cell layer thickness in the TAU pretreatment group remained 1.23-fold lower than that in the MgAT pretreatment group (p<0.05). In line with this observation, the number of apoptotic cells as observed after TUNEL staining was 1.69-fold higher after pretreatment with TAU compared to pretreatment with MgAT (p<0.01).

CONCLUSIONS

MgAT and TAU, particularly with pretreatment, reduce retinal cell apoptosis by reducing retinal nitrosative stress. Pretreatment with MgAT caused greater improvement in NMDA-induced changes in iNOS and eNOS expression and retinal 3-NT levels than pretreatment with TAU. The greater reduction in retinal nitrosative stress after pretreatment with MgAT was associated with lower retinal cell apoptosis and greater preservation of the ganglion cell layer thickness compared to pretreatment with TAU.

Cone degeneration is triggered by the absence of USH1 proteins but prevented by antioxidant treatments

Usher syndrome type 1 (USH1) is a major cause of inherited deafness and blindness in humans. The eye disorder is often referred to as retinitis pigmentosa, which is characterized by a secondary cone degeneration following the rod loss. The development of treatments to prevent retinal degeneration has been hampered by the lack of clear evidence for retinal degeneration in mutant mice deficient for the Ush1 genes, which instead faithfully mimic the hearing deficit. We show that, under normal housing conditions, Ush1g^−/− and Ush1c^−/− albino mice have dysfunctional cone photoreceptors whereas pigmented knockout animals have normal photoreceptors. The key involvement of oxidative stress in photoreceptor apoptosis and the ensued retinal gliosis were further confirmed by their prevention when the mutant mice are reared under darkness and/or supplemented with antioxidants. The primary degeneration of cone photoreceptors contrasts with the typical forms of retinitis pigmentosa. Altogether, we propose that oxidative stress probably accounts for the high clinical heterogeneity among USH1 siblings, which also unveils potential targets for blindness prevention.

Rational Basis for Nutraceuticals in the Treatment of Glaucoma

BACKGROUND

Glaucoma, the second leading cause of blindness worldwide, is a chronic optic neuropathy characterized by progressive retinal ganglion cell (RGC) axons degeneration and death. Primary open-angle glaucoma (OAG), the most common type, is often associated with increased intraocular pressure (IOP), however other factors have been recognized to partecipate to the patogenesis of the optic neuropathy. IOP-independent mechanisms that contribute to the glaucoma-related neurodegeneration include oxidative stress, excitotoxicity, neuroinflammation, and impaired ocular blood flow. The involvement of several and diverse factors is one of the reasons for the progression of glaucoma observed even under efficient IOP control with the currently available drugs.

METHODS

Current research and online content related to the potential of nutritional supplements for limiting retinal damage and improving RGC survival is reviewed.

RESULTS

Recent studies have suggested a link between dietary factors and glaucoma risk. Particularly, some nutrients have proven capable of lowering IOP, increase circulation to the optic nerve, modulate excitotoxicity and promote RGC survival. However, the lack of clinical trials limit their current therapeutic use. The appropriate use of nutraceuticals that may be able to modify the risk of glaucoma may provide insight into glaucoma pathogenesis and decrease the need for, and therefore the side effects from, conventional therapies.

CONCLUSION

The effects of nutrients with anti-oxidant and neuroprotective properties are of great interest and nutraceuticals may offer some therapeutic potential although a further rigorous evaluation of nutraceuticals in the treatment of glaucoma is needed to determine their safety and efficacy.

Taurine mitigates nitrite-induced methemoglobin formation and oxidative damage in human erythrocytes

Nitrite is present as a noxious contaminant in drinking water and causes oxidative damage in various tissues of humans and animals. It is a well-known methemoglobin-forming agent that has been shown to damage blood cells. The protective effect of taurine, a semi-essential sulfur-containing amino acid, was studied on sodium nitrite (NaNO₂)-induced oxidative damage in human erythrocytes. Erythrocytes were incubated with NaNO₂, in the presence and absence of taurine, and changes in oxidative stress parameters determined. Pretreatment with taurine significantly ameliorated NaNO₂-induced oxidative damage to lipids, proteins, and plasma membrane. It also reduced the NaNO₂-induced increase in methemoglobin levels and ROS production. Taurine improved the antioxidant capacity of cells, restored the alterations in the activities of various metabolic enzymes, and prevented morphological changes in erythrocytes. Thus, taurine can be potentially used as a protective agent against the damaging effects of nitrite.

Effects of taurine and housing density on renal function in laying hens

This study investigated the putative protective effects of supplemental 2-aminoethane sulfonic acid (taurine) and reduced housing density on renal function in laying hens. We randomly assigned fifteen thousand green-shell laying hens into three groups: a free range group, a low-density caged group, and a high-density caged group. Each group was further divided equally into a control group (C) and a taurine treatment group (T). After 15 d, we analyzed histological changes in kidney cells, inflammatory mediator levels, oxidation and anti-oxidation levels. Experimental data revealed taurine supplementation, and rearing free range or in low-density housing can lessen morphological renal damage, inflammatory mediator levels, and oxidation levels and increase anti-oxidation levels. Our data demonstrate that taurine supplementation and a reduction in housing density can ameliorate renal impairment, increase productivity, enhance health, and promote welfare in laying hens.

Methods for culturing retinal pigment epithelial cells: a review of current protocols and future recommendations

The retinal pigment epithelium is an important part of the vertebrate eye, particularly in studying the causes and possible treatment of age-related macular degeneration. The retinal pigment epithelium is difficult to access in vivo due to its location at the back of the eye, making experimentation with age-related macular degeneration treatments problematic. An alternative to in vivo experimentation is cultivating the retinal pigment epithelium in vitro, a practice that has been going on since the 1970s, providing a wide range of retinal pigment epithelial culture protocols, each producing cells and tissue of varying degrees of similarity to natural retinal pigment epithelium. The purpose of this review is to provide researchers with a ready list of retinal pigment epithelial protocols, their effects on cultured tissue, and their specific possible applications. Protocols using human and animal retinal pigment epithelium cells, derived from tissue or cell lines, are discussed, and recommendations for future researchers included.

Natural compounds and retinal ganglion cell neuroprotection

Glaucoma, the second leading cause of blindness in the world, is a chronic optic neuropathy often associated with increased intraocular pressure and characterized by progressive retinal ganglion cell (RGC) axons degeneration and death leading to typical optic nerve head damage and distinctive visual field defects. Although the pathogenesis of glaucoma is still largely unknown, it is hypothesized that RCGs become damaged through various insults/mechanisms, including ischemia, oxidative stress, excitotoxicity, defective axonal transport, trophic factor withdrawal, and neuroinflammation. In this review, we summarize the potential benefits of several natural compounds for RGCs neuroprotection.

Vigabatrin can enhance electroretinographic responses in pigmented and albino rats

PURPOSE

To evaluate the effects of the antiepileptic medication vigabatrin (VGB) on the retina of pigmented rats.

METHODS

Scotopic and photopic electroretinograms were recorded from dark- and light-adapted Long-Evans (pigmented) and Sprague Dawley (albino) rats administered, daily, 52-55 injections of 250 mg·kg(-1)·day(-1) VGB or 25-26 injections of 500 mg·kg(-1)·day(-1) VGB, or a corresponding number of sham injections. Sensitivity and saturated amplitude of the rod photoresponse (S, Rm(P3)) and postreceptor response (1/σ, Vm) were derived, as were sensitivity and amplitude of the cone-mediated postreceptor response (1/σ(cone), Vm(cone)). The oscillatory potentials and responses to a series of flickering lights (6.25, 12.5, 25 and 50 Hz) were studied in the time and frequency domains. A subset of rats' eyes was harvested for Western blotting or histology.

RESULTS

Of the parameters derived from dark-adapted ERG responses, in both pigmented and albino rats, VGB repeatedly and reliably enhanced electroretinographic parameters; no significant ERG deficits were noted. No significant alterations were observed in ER/oxidative stress or in the Akt cell death/survival pathway. There were migrations of photoreceptor nuclei toward the RPE and outgrowths of bipolar cell dendrites into the outer nuclear layer in VGB-treated rats; these were never observed in sham-treated animals.

CONCLUSIONS

Although VGB is associated with retinal dysfunction in patients and VGB toxicity has been demonstrated by other laboratories in the albino rat, in our pigmented and albino rats, VGB did not induce deficits in, but rather enhanced, retinal function. Nonetheless, retinal neuronal dysplasia was observed.

Taurine: the comeback of a neutraceutical in the prevention of retinal degenerations

Taurine is the most abundant amino acid in the retina. In the 1970s, it was thought to be involved in retinal diseases with photoreceptor degeneration, because cats on a taurine-free diet presented photoreceptor loss. However, with the exception of its introduction into baby milk and parenteral nutrition, taurine has not yet been incorporated into any commercial treatment with the aim of slowing photoreceptor degeneration. Our recent discovery that taurine depletion is involved in the retinal toxicity of the antiepileptic drug vigabatrin has returned taurine to the limelight in the field of neuroprotection. However, although the retinal toxicity of vigabatrin principally involves a deleterious effect on photoreceptors, retinal ganglion cells (RGCs) are also affected. These findings led us to investigate the possible role of taurine depletion in retinal diseases with RGC degeneration, such as glaucoma and diabetic retinopathy. The major antioxidant properties of taurine may influence disease processes. In addition, the efficacy of taurine is dependent on its uptake into retinal cells, microvascular endothelial cells and the retinal pigment epithelium. Disturbances of retinal vascular perfusion in these retinal diseases may therefore affect the retinal uptake of taurine, resulting in local depletion. The low plasma taurine concentrations observed in diabetic patients may further enhance such local decreases in taurine concentration. We here review the evidence for a role of taurine in retinal ganglion cell survival and studies suggesting that this compound may be involved in the pathophysiology of glaucoma or diabetic retinopathy. Along with other antioxidant molecules, taurine should therefore be seriously reconsidered as a potential treatment for such retinal diseases.

Evaluation of the taurine concentrations in dog plasma and aqueous humour: a pilot study

In the 70s, the amino acid taurine was found essential for photoreceptor survival. Recently, we found that taurine depletion can also trigger retinal ganglion cell degeneration both in vitro and in vivo. Therefore, evaluation of taurine levels could be a crucial biomarker for different pathologies of retinal ganglion cells such as glaucoma. Because different breeds of dog can develop glaucoma, we performed taurine measurements on plasma and aqueous humour samples from pet dogs. Here, we exposed results from a pilot study on normal selected breed of pet dogs, without any ocular pathology. Samples were collected by veterinarians who belong to the Réseau Européen d'Ophtalmologie Vétérinaire et de Vision Animale. Following measurements by high-performance liquid chromatography (HPLC), the averaged taurine concentration was 162.3 μM in the plasma and 51.8 μM in the aqueous humour. No correlation was observed between these two taurine concentrations, which exhibited a ratio close to 3. Further studies will determine if these taurine concentrations are changed in glaucomatous dogs.

Review: taurine: a "very essential" amino acid

Taurine is an organic osmolyte involved in cell volume regulation, and provides a substrate for the formation of bile salts. It plays a role in the modulation of intracellular free calcium concentration, and although it is one of the few amino acids not incorporated into proteins, taurine is one of the most abundant amino acids in the brain, retina, muscle tissue, and organs throughout the body. Taurine serves a wide variety of functions in the central nervous system, from development to cytoprotection, and taurine deficiency is associated with cardiomyopathy, renal dysfunction, developmental abnormalities, and severe damage to retinal neurons. All ocular tissues contain taurine, and quantitative analysis of ocular tissue extracts of the rat eye revealed that taurine was the most abundant amino acid in the retina, vitreous, lens, cornea, iris, and ciliary body. In the retina, taurine is critical for photoreceptor development and acts as a cytoprotectant against stress-related neuronal damage and other pathological conditions. Despite its many functional properties, however, the cellular and biochemical mechanisms mediating the actions of taurine are not fully known. Nevertheless, considering its broad distribution, its many cytoprotective attributes, and its functional significance in cell development, nutrition, and survival, taurine is undoubtedly one of the most essential substances in the body. Interestingly, taurine satisfies many of the criteria considered essential for inclusion in the inventory of neurotransmitters, but evidence of a taurine-specific receptor has yet to be identified in the vertebrate nervous system. In this report, we present a broad overview of the functional properties of taurine, some of the consequences of taurine deficiency, and the results of studies in animal models suggesting that taurine may play a therapeutic role in the management of epilepsy and diabetes.

Taurine provides neuroprotection against retinal ganglion cell degeneration

Retinal ganglion cell (RGC) degeneration occurs in numerous retinal diseases leading to blindness, either as a primary process like in glaucoma, or secondary to photoreceptor loss. However, no commercial drug is yet directly targeting RGCs for their neuroprotection. In the 70s, taurine, a small sulfonic acid provided by nutrition, was found to be essential for the survival of photoreceptors, but this dependence was not related to any retinal disease. More recently, taurine deprivation was incriminated in the retinal toxicity of an antiepileptic drug. We demonstrate here that taurine can improve RGC survival in culture or in different animal models of RGC degeneration. Taurine effect on RGC survival was assessed in vitro on primary pure RCG cultures under serum-deprivation conditions, and on NMDA-treated retinal explants from adult rats. In vivo, taurine was administered through the drinking water in two glaucomatous animal models (DBA/2J mice and rats with vein occlusion) and in a model of Retinitis pigmentosa with secondary RGC degeneration (P23H rats). After a 6-day incubation, 1 mM taurine significantly enhanced RGCs survival (+68%), whereas control RGCs were cultured in a taurine-free medium, containing all natural amino-acids. This effect was found to rely on taurine-uptake by RGCs. Furthermore taurine (1 mM) partly prevented NMDA-induced RGC excitotoxicity. Finally, taurine supplementation increased RGC densities both in DBA/2J mice, in rats with vein occlusion and in P23H rats by contrast to controls drinking taurine-free water. This study indicates that enriched taurine nutrition can directly promote RGC survival through RGC intracellular pathways. It provides evidence that taurine can positively interfere with retinal degenerative diseases.

Advances in drug design based on the amino Acid approach: taurine analogues for the treatment of CNS diseases

Amino acids are well known to be an important class of compounds for the maintenance of body homeostasis and their deficit, even for the polar neuroactive aminoacids, can be controlled by supplementation. However, for the amino acid taurine (2-aminoethanesulfonic acid) this is not true. Due its special physicochemical properties, taurine is unable to cross the blood-brain barrier. In addition of injured taurine transport systems under pathological conditions, CNS supplementation of taurine is almost null. Taurine is a potent antioxidant and anti-inflammatory semi-essential amino acid extensively involved in neurological activities, acting as neurotrophic factor, binding to GABA A/glycine receptors and blocking the excitotoxicity glutamate-induced pathway leading to be a neuroprotective effect and neuromodulation. Taurine deficits have been implicated in several CNS diseases, such as Alzheimer’s, Parkinson’s, epilepsy and in the damage of retinal neurons. This review describes the CNS physiological functions of taurine and the development of new derivatives based on its structure useful in CNS disease treatment.