Tauret. In: Huxtable RJ, Azuma J, Kuriyama K, Nakagawa M, Baba A, editors. Taurine 2. Boston: Springer US; 1996. pp. 333-42. [DOI: 10.1007/978-1-4899-0182-8_35]

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.

Vitamin A aldehyde-taurine adduct and the visual cycle

Taurine is a sulfur-containing amino acid that is not incorporated into protein but is abundant in retina. Schiff base adducts that form nonenzymatically and reversibly from reactions between taurine and vitamin A aldehyde (A1T) are increased under conditions in which the visual chromophore 11-cis-retinal is more abundant. These settings include black versus albino mice, dark-adapted versus light-adapted mice, and mice expressing the Rpe65-Leu450 versus Rpe65-Met450 variant. Conversely, A1T is less abundant in mouse models deficient in 11-cis-retinal. As an amphiphile, protonated A1T may serve to facilitate retinoid trafficking and could constitute a small-molecule reserve of mobilizable 11-cis-retinal in photoreceptor cells.

Visual pigment consists of opsin covalently linked to the vitamin A-derived chromophore, 11-cis-retinaldehyde. Photon absorption causes the chromophore to isomerize from the 11-cis- to all-trans-retinal configuration. Continued light sensitivity necessitates the regeneration of 11-cis-retinal via a series of enzyme-catalyzed steps within the visual cycle. During this process, vitamin A aldehyde is shepherded within photoreceptors and retinal pigment epithelial cells to facilitate retinoid trafficking, to prevent nonspecific reactivity, and to conserve the 11-cis configuration. Here we show that redundancy in this system is provided by a protonated Schiff base adduct of retinaldehyde and taurine (A1-taurine, A1T) that forms reversibly by nonenzymatic reaction. A1T was present as 9-cis, 11-cis, 13-cis, and all-trans isomers, and the total levels were higher in neural retina than in retinal pigment epithelium (RPE). A1T was also more abundant under conditions in which 11-cis-retinaldehyde was higher; this included black versus albino mice, dark-adapted versus light-adapted mice, and mice carrying the Rpe65-Leu450 versus Rpe65-450Met variant. Taurine levels paralleled these differences in A1T. Moreover, A1T was substantially reduced in mice deficient in the Rpe65 isomerase and in mice deficient in cellular retinaldehyde-binding protein; in these models the production of 11-cis-retinal is compromised. A1T is an amphiphilic small molecule that may represent a mechanism for escorting retinaldehyde. The transient Schiff base conjugate that the primary amine of taurine forms with retinaldehyde would readily hydrolyze to release the retinoid and thus may embody a pool of 11-cis-retinal that can be marshalled in photoreceptor cells.

Taurine deficiency and apoptosis: findings from the taurine transporter knockout mouse

Apoptosis is characterized by cell shrinkage, nuclear condensation, DNA-fragmentation and apoptotic body formation. Compatible organic osmolytes, e.g. taurine, modulate the cellular response to anisotonicity and may protect from apoptosis. Taurine transporter knockout mice (taut-/- mice) show strongly decreased taurine levels in a variety of tissues. They develop clinically important age-dependent diseases and some of them are characterized by apoptosis. Increased photoreceptor apoptosis leads to blindness of taut-/- mice at an early age. The taurine transporter may not be essential for the differentiation of photoreceptor cells, but many mature cells do not survive without an intact taurine transporter. The olfactory epithelium of taut-/- mice also exhibits structural and functional abnormalities. When compared with wild-types, taut-/- mice have a significantly higher proliferative activity of immature olfactory receptor neurons and an increased number of apoptotic cells. This is accompanied by electrophysiological findings indicating a reduced olfactory sensitivity. Furthermore, taut-/- and taut+/- mice develop moderate unspecific hepatitis and liver fibrosis beyond 1 year of age where hepatocyte apoptosis and activation of the CD95 system are pronounced.

Phenotype of the Taurine Transporter Knockout Mouse

This chapter reports present knowledge on the properties of mice with disrupted gene coding for the taurine transporter (taut-/- mice). Study of those mice unraveled some of the roles of taurine and its membrane transport for the development and maintenance of normal organ functions and morphology. When compared with wild-type controls, taut-/- mice have decreased taurine levels in skeletal and heart muscle by about 98%, in brain, kidney, plasma, and retina by 80 to 90%, and in liver by about 70%. taut-/- mice exhibit a lower body mass as well as a strongly reduced exercise capacity compared with taut+/- and wild-type mice. Furthermore, taut-/- mice show a variety of pathological features, for example, subtle derangement of renal osmoregulation, changes in neuroreceptor expression, and loss of long-term potentiation in the striatum, and they develop clinically relevant age-dependent disorders, for example, visual, auditory, and olfactory dysfunctions, unspecific hepatitis, and liver fibrosis. Taurine-deficient animal models such as acutely dietary-manipulated foxes and cats, pharmacologically induced taurine-deficient rats, and taurine transporter knockout mouse are powerful tools allowing identification of the mechanisms and complexities of diseases mediated by impaired taurine transport and taurine depletion (Chapman et al., 1993; Heller-Stilb et al., 2002; Huxtable, 1992; Lake, 1993; Moise et al., 1991; Novotny et al., 1991; Pion et al., 1987; Timbrell et al., 1995; Warskulat et al., 2004, 2006b). Taurine, which is the most abundant amino acid in many tissues, is normally found in intracellular concentrations of 10 to 70 mmol/kg in mammalian heart, brain, skeletal muscle, liver, and retina (Chapman et al., 1993; Green et al., 1991; Huxable, 1992; Timbrell et al., 1995). These high taurine levels are maintained by an ubiquitous expression of Na(+)-dependent taurine transporter (TAUT) in the plasma membrane (Burg, 1995; Kwon and Handler, 1995; Lang et al., 1998; Liu et al., 1992; Ramamoorthy et al., 1994; Schloss et al., 1994; Smith et al., 1992; Uchida et al., 1992; Vinnakota et al., 1997; Yancey et al., 1975). Taurine is not incorporated into proteins. It is involved in cell volume regulation, neuromodulation, antioxidant defense, protein stabilization, stress responses, and via formation of taurine-chloramine in immunomodulation (Chapman et al., 1993; Green et al., 1991; Huxtable, 1992; Timbrell et al., 1995). On the basis of its functions, taurine may protect cells against various types of injury (Chapman et al., 1993; Green et al., 1991; Huxtable, 1992; Kurz et al., 1998; Park et al., 1995; Stapleton et al., 1998; Timbrell et al., 1995; Welch and Brown, 1996; Wettstein and Häussinger, 1997). In order to examine the multiple taurine functions, murine models have several intrinsic advantages for in vivo research compared to other animal models, including lower cost, maintenance, and rapid reproduction rate. Further, experimental reagents for cellular and molecular studies are widely available for the mouse. In particular, mice can be easily genetically manipulated by making transgene and knockout mice. This chapter focuses on the phenotype of the TAUT-deficient murine model (taut-/-; Heller-Stilb et al., 2002), which may help researchers elucidate the diverse roles of taurine in development and maintenance of normal organ functions and morphology.

Study of taurine and tauret content in the compound eye of locust with light and dark adaptation

Taurine as well as tauret (retinyliden taurine) levels were measured in locust Locusta migratoria compound eyes. HPLC measurements revealed relatively low taurine levels (1.9 +/- 0.16 mM) in dark-adapted eyes. Glutamate, aspartate and glycine levels were 2.0 +/- 0.2, 2.7 +/- 0.4 and 3.0 +/- 0.37 mM, respectively, while GABA was present only in trace amounts. After about 4 h of light adaptation at 1500-2000 lx, amino acid levels in the compound eye were as follows: taurine, 1.8 +/- 0.17 mM; glutamate, no change at 2.1 +/- 0.2 mM; aspartate sharply increased to 4.7 +/- 0.7 mM; glycine slightly decreased to 2.8 +/- 0.3 mM; and GABA trace levels. In the compound eye of locust Locusta migratoria, the existence of endogenous tauret in micro-molar range was established. In the dark, levels were several times higher compared with compound eye after light adaptation 1500 lx for 3 h, as estimated by TLC in combination with spectral measurements. Existence of tauret in compound eye is of special interest because in the compound eye, rhodopsin regeneration is based on photoregeneration.

New HPLC evidence on endogenous tauret in retina and pigment epithelium

This investigation was improve the separation for tauret (retinylidene taurine) and to compare its content in the retina under dark and light adaptation. To prevent tauret hydrolysis, retinal samples were quickly frozen and lyophilized. Methanol extracts of dried retina and pigment epithelium from both dark- or light-adapted frogs, Rana ridibunda, were injected onto HPLC. Synthetic standard tauret appeared at 4.7 min after the solvent front. At the same time, an endogenous substance was eluted from the mixed retinal and pigment epithelial samples. The UV spectra of this endogenous compound matched with the spectra of synthetic tauret obtained under identical conditions, with lambda(max) = 446 nm at peak. We conclude that the HPLC system used permitted full separation of tauret from the methanol extracts of the retina and pigment epithelium. TLC and further HPLC analysis have shown that tauret quantities were several times higher in the retina and pigment epithelium of the frogs adapted to dark compared with those light-adapted (about 4 h under 1000 1x illumination). Tauret based vitamin A transport is probably involved in other systems as well, where along with its other known beneficial effects taurine probably is necessary to facilitate vitamin A transport.

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.