Taurine and nifedipine protect retinal rod outer segment structure altered by removal of divalent cations

Light deprivation slows but does not prevent the loss of photoreceptors in taurine transporter knockout mice

Taurine transporter knockout mice show severe retinal degeneration at an early age. The study was designed to determine whether degeneration also takes place in the absence of light. Mice were maintained up to 6 weeks of age in cyclic lighting or in total darkness. Degeneration took place in both groups, but was more rapid in animals exposed to standard cyclic illumination. At the ultrastructural level the retinas showed features characteristic of apoptosis but not of necrosis.

CONCLUSIONS

Cell differentiation is not seriously affected by the lack of a functional taurine transporter but mature photoreceptor cells do not survive without an intact transporter, even in the dark.

Myo-inositol restores the inflammation-induced down-regulation of taurine transport by the murine macrophage cell line, RAW 264.7

The role of myo-inositol in the regulation of taurine transport in activated murine macrophage cell line, RAW 264.7, was studied. Challenge of RAW 264.7 murine macrophages for 24 hr with phorbol ester 12-myristate 13-acetate (PMA) (10 ng/ml), a PKC activator, resulted in a 62% decrease in taurine transport activity. Among the various monosaccharides (1 mM) tested in the presence of PMA, myo-inositol was most effective in restoring the PMA-induced down-regulation of taurine transport in murine macrophages (82% increase compared to the value for cells treated with PMA Alone, p < 0.01). The protective role of myo-inositol against stress-induced down-regulation of taurine transport by macrophages was further investigated in conditions mimicking bacterial infection, inflammation, and immune-suppressed circumstances. A challenge of murine macrophages with lipopolysaccharide (LPS) (0.1 and 10 microg/ml) resulted in a 60% decrease in taurine transport activity compared to the value for untreated control cells (p < 0.01). When cells were co-treated with myo-inositol (100 nM approximately 10 mM) in the presence of LPS for 24 hrs, taurine transport activity increased in a dose-dependent manner compared to the value for cells treated with LPS only. Taurine transport activity in cells treated with LPS (10 microg/ml) plus interferon-gamma (IFN-gamma) (150 unit/ml) for 24 hrs was 13% of the value for untreated control cells (p < 0.01). Again, this inflammation-induced down-regulation of taurine transport activity was completely antagonized with co-administration of 100 nM or higher levels of myo-inositol in the culture medium. Similarly, myo-inositol effectively restored the taurine transport activity suppressed by cyclosporin A (0.5 and 50 nM) in murine macrophages (p < 0.01). From these results, myo-inositol appears to be a common accelerator of taurine transport by murine macrophages in diverse conditions of down-regulated taurine transport.

Beneficial effect of intravenous taurine infusion on electroretinographic disorder in taurine deficient rats

We investigated the effect of intravenous taurine infusion on the electroretinogram (ERG) of taurine-deficient rats produced by treatment with guanidinoethyl sulfonate (GES), a taurine transport inhibitor. Mother rats were fed a taurine-free diet and given drinking water containing 1% GES from 2 weeks of gestation to weaning. The same feeding conditions were applied to male offspring after weaning. Both ERG measurement and continuous infusion of taurine at a dose of 10, 30 or 100 mg/animal/day were performed for 3 weeks from 7 to 10 weeks of age. GES-treatment reduced a- and b-wave amplitudes to 50% of the control levels and also increased b-wave latencies. Intravenous infusion of taurine improved these ERG abnormalities in a dose-dependent manner. Taurine concentrations in plasma, eyes and brain were also decreased by treatment with GES, and dose-dependent recovery was observed after infusion with taurine, although the concentrations of other amino acids were not affected by GES-treatment and infusion of taurine. Observations of morphological changes revealed that the retinal damage in GES-treated animals was decreased by taurine infusion. These results indicate that the changes in ERG and retinal structure observed in taurine deficiency are improved by intravenous infusion of taurine.

Taurine: retinal function

The status and potential functions of taurine in the retina have been reviewed. Taurine is present in high concentrations in the retina of all species tested, while the retinal concentrations of the enzymes necessary to synthesize taurine are presumed to vary among those species. The documented low activity of cysteinesulfinic acid decarboxylase, a key enzyme in taurine biosynthesis, in the livers of the cat, monkey and human possibly reflect low activity in their retinas, indicating reliance on the diet as an important source of taurine. Both high- and low-affinity binding proteins and uptake systems have been described for taurine in retinal tissue. Evoked release of taurine by light and other depolarizing stimuli have been well documented. Retinal pathologies including diminished ERGs and morphologic changes have been reported for animals and man deficient in taurine. Possible functions for taurine in the retina include: (1) protection of the photoreceptor - based on the shielding effects of taurine on rod outer segments exposed to light and chemicals; (2), regulation of Ca2+ transport - based on the modulatory effects of taurine on Ca2+ fluxes in the presence and absence of ATP; and (3) regulation of signal transduction - based on the inhibitory effects of taurine on protein phosphorylation.