Differential effect of taurine and serotonin on the outgrowth from explants or isolated cells of the retina

Taurine-Responsive Genes Related to Signal Transduction as Identified by cDNA Microarray Analyses of HepG2 Cells

Taurine-induced changes in the expression profiles of HepG2 cells were assessed using a cDNA microarray technology, and confirmed by real-time reverse transcription-polymerase chain reaction (RT-PCR) analyses. Of 8,298 human genes on the microarray, 128 genes (87 known genes) were up-regulated, and 349 (206 known genes) were down-regulated more than 2.0-fold by taurine. Among the 293 known genes regulated by taurine, a total of 44 genes were involved in signal transduction; 16 genes were up-regulated greater than 2.0-fold, and 28 genes were down-regulated more than 2.0-fold by taurine. The results of RT-PCR analyses for the five genes selected were consistent with our microarray data, although the fold changes in the expression level differed somewhat between the two analytical methods. Among signal transduction-related genes affected by taurine, four genes--mitogen-activated protein kinase (MAPK) kinase kinase 7, p21-activated kinase 4, sprouty homolog 2, and MAPK kinase 1--are implicated in the MAPK signaling pathway. Taurine also regulated the expression of signal transducer and activator of transcription (STAT) 3 gene involved in the Janus kinase-STAT pathway, and diacylglycerol kinase, zeta 104 kDa, the downstream mediator of the protein kinase C transmembrane signaling pathway. In conclusion, gene expression profiling of HepG2 cells treated with taurine provided us with new insights into the novel aspects of taurine as a possible regulator of MAPK signaling cascades and protein kinase C signaling pathways involved in cellular processes such as cell growth, differentiation, and apoptosis.

Taurine effect on neuritic outgrowth from goldfish retinal explants in the absence and presence of fetal calf serum

Post-crush goldfish retinal explants were cultured in the absence of fetal calf serum and in the presence of calcium, albumin, glucose or taurine. There was an increase in length and density of the neurites in a dose-dependent manner when fetal calf serum was added to the medium. The addition of calcium to the basic medium, Leibovitz, produced a small increase in neuritic outgrowth at low concentrations, but inhibited outgrowth at higher levels. Albumin did not significantly modify neuritic outgrowth. Glucose, at variable concentrations, produced a bell-shaped increase in length and density of the neurites. Taurine, in the absence of fetal calf serum, produced a small increase in neuritic outgrowth at 10 mM. In the presence of glucose, taurine stimulated outgrowth was less efficient than in the presence of fetal calf serum. In order to investigate the mechanisms of action of taurine as a trophic agent, the culture medium in the absence of fetal calf serum will be supplemented with glucose.

Medium requirements for neuritic outgrowth from goldfish retinal explants and the trophic effect of taurine

The use of culture media of known composition are necessary for studying the role of trophic molecules. Since most of the in vitro research on regeneration of the optic nerve has been performed in the presence of fetal calf serum, the aim of this study was to obtain a medium in which the neuritic outgrowth from post-crush goldfish retinal explants could take place without adding fetal calf serum. After the lesion of the optic nerve (10 days), the retina of goldfish was dissected and explants were cultured for 5 and 10 days in the absence or in the presence of fetal calf serum, at which time the neuritic outgrowth was determined. Various concentrations and combinations of glucose, albumin, calcium, HEPES and taurine were used. The highest neuritic outgrowth was observed in the presence of fetal calf serum, in which condition the amino acid taurine increased length and density of neurites. Media supplemented with albumin, calcium or HEPES did not modify the outgrowth of neurites from the explants. However, glucose favored the neuritic outgrowth in a bell-shaped manner, although fibers were thinner than those observed in the presence of fetal calf serum. Taurine did not stimulate outgrowth of neurites from explants growing in a medium with optimal concentrations of glucose, indicating that elements of the fetal calf serum are determinant for the trophic effect of taurine. The present results contribute to further studies, such as those related to the effect of taurine and of trophic factors derived from the optic tectum, which would be performed in the presence of a medium free of fetal calf serum.

Taurine as a micronutrient in development and regeneration of the central nervous system

Taurine is an amino acid known to possess trophic properties in the central nervous system. The relevance of its presence in maternal milk is related to its role as an essential nutrient. Taurine deficiency around birth produces anatomical and functional modifications in the brain and in the retina. In addition, taurine favors neuron proliferation and survival, as well as neurite extension. The mechanisms by which taurine exerts its trophic role in the regenerating retina are related to increases in calcium fluxes, to modifications of protein phosphorylation, and to influence of the target organ. Moreover, taurine-zinc interaction might be crucial in the development of structures such as the hippocampal formation. Thus, taurine can be considered as one of the determinant nutritional molecules during development and regeneration of the central nervous system.

Regulation of high affinity taurine transport in goldfish and rat retinal cells

Adaptive regulation and modulation by phosphorylation are mechanisms by which some cells control taurine transport. Goldfish and rat retinal cells were incubated with the activator of protein kinase C, phorbol 12,13-dibutyrate (PDBu), or the inhibitor of protein phosphatases, okadaic acid (OKA). OKA, 1 nM, inhibited the uptake of taurine at short period of incubation in goldfish retinal cells, and at low concentrations in rat retinal cells incubated with the inhibitor for 1 h. PDBu treatment did not produce significant effects. Isolated Müller cells from the goldfish retina presented a clear adaptive regulation and a decrease of taurine uptake by increasing phosphorylation either by the stimulation of PKC with PDBu or the inhibition of phosphatases with OKA.

Factors controlling axonal and dendritic arbors

The sculpting and maintenance of axonal and dendritic arbors is largely under the control of molecules external to the cell. These factors include both substratum-associated and soluble factors that can enhance or inhibit the outgrowth of axons and dendrites. A large number of factors that modulate axonal outgrowth have been identified, and the first stages of the intracellular signaling pathways by which they modify process outgrowth have been characterized. Relatively fewer factors and pathways that affect dendritic outgrowth have been described. The factors that affect axonal arbors form an incompletely overlapping set with those that affect dendritic arbors, allowing selective control of the development and maintenance of these critical aspects of neuronal morphology.

Differential taurine effect on outgrowth from goldfish retinal ganglion cells after optic crush or axotomy. Influence of the optic tectum

The interaction between innervated tissues, targets and nerves is crucial in the maintenance of physiological conditions, and the disturbance of this harmony causes the production of morphological and biochemical changes. After lesion of the optic nerve, several modifications take place in the retina, the optic tectum and the optic nerve. The influence of the tectum on the outgrowth from the goldfish retina and the possible role of taurine was studied. Ganglion retinal cells were identified by retrolabeling with Dil. Crushing the optic nerve 10 days prior to plating retinal cells, as compared with optic axotomy, did not affect the survival of cultured retinal cells, as well as the length of the neurites. However, the number of neurites per cell and the branching of the longest fiber were higher after axotomy than after crushing. The addition of taurine to the medium did not modify this response at 5 days in culture. At early periods in culture, the stimulatory effect on isolated ganglion cell outgrowth produced by taurine was enhanced after axotomy respecting crushing of the optic nerve, but was not affected in retinal explants. The addition of medium from cultured optic tectum several days post-crush of the optic nerve to retinal explants from intact retinas or coming from post-crush retina modified the outgrowth, being inhibitory or stimulatory in a time-dependent manner. The co-culture of optic tectum and retina also affected the outgrowth from the retina with a byphasic shape. The results support the differential response of the retina facing partial or complete interruption with the target and limit the effect of taurine to early periods in culture. In addition, the production of inhibitory factors from the tectum, plus the stimulatory ones, are strongly supported by this work.

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, glutamate and GABA modulate the outgrowth from goldfish retinal explants and its concentrations are affected by the crush of the optic nerve

The amino acid taurine plays an important trophic role during development and regeneration of the central nervous system. Other amino acid systems, such as those for glutamate and gamma-aminobutyric acid (GABA), are modified during the same physiological and pathological processes. After crushing the optic nerve, goldfish retinal explants were plated in the absence and in the presence of different amino acids and amino acid receptor agonists. The length and the density of the neurites were measured at 5 days in culture. Taurine increased the length and the density of neurites. Glutamate and glycine increased them at low concentration, but were inhibitors at higher concentration. The combination of N-methyl-D-aspartate (NMDA) and glycine produced a greater inhibitory effect than NMDA alone. NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) added simultaneously with taurine impaired the stimulatory effect of the latter. GABA stimulated the emission of neurites in a concentration dependent manner. Hypotaurine also elevated the length of neurites, but cysteinsesulfinic acid did not produce a significant effect. The concentrations of taurine, glutamate and GABA were determined by HPLC with fluorescent detection in the retina of goldfish at various days post-crushing the optic nerve. The levels of taurine were significantly increased at 48h after the crush, and were elevated up to 20 days. Glutamate level decreased after the lesion of the optic nerve and was still low at 20 days. GABA concentration was not significantly different from the control. The interaction of these amino acids during the regenerative period, especially the balance between taurine and glutamate, may be a determinant in restoring vision after the crush.

Taurine-stimulated outgrowth from the retina is impaired by protein kinase C activators and phosphatase inhibitors

Taurine increases neurite elongation of post-crush goldfish retinal explants, as well as the number of outgrowing isolated cells from goldfish and rat retina in culture. The trophic effect of taurine is related to an elevation in calcium flux rather than increased cell proliferation. Since taurine regulates phosphorylation in rat retina, we investigated if this process could be involved in the mechanism of taurine action on outgrowth. Control and taurine-supplemented post-crush goldfish retinal explants were cultured in the presence of protein kinase C activators or phosphatase inhibitors, and the length of neurites was measured after five days in culture. In some cases, there was an inhibition of the stimulatory effect of taurine without a modification in basal outgrowth. In others, outgrowth of control explants was also reduced. A certain level of protein phosphorylation seems to be critical for the trophic effect of taurine in the retina.

Plasma concentration of taurine is higher in malnourished than control children: differences between kwashiorkor and marasmus

Plasma free amino acids were determined in the plasma of severely malnourished children under two years of age. A total of thirty-one patients and eleven controls were evaluated: seventeen cases of kwashiorkor, eight cases of marasmus, and six cases of marasmic-kwashiorkor. Fasting plasma samples were taken in the morning on the day of admission. Fasting plasma samples were also taken from nine patients at discharge after two months in the hospital where they received a balanced diet as treatment. A partial reversal of the signs of malnutrition was observed at discharge. In the whole group of patients ad admission, lower concentrations of tyrosine, methionine, tryptophan, and leucine and higher concentrations of aspartate, glutamate, and taurine were observed compared to controls. Taurine continued to be elevated in the malnourished group at the time of discharge. Marasmic children, as compared to controls, had high aspartate and low tryptophan levels, but taurine levels were not significantly different from controls. Kwashiorkor patients had low tyrosine, methionine, tryptophan, and lysine, and significantly higher taurine plasma levels. The elevated concentration of taurine might be the result of a redistribution of this amino acid to provide specific tissues with the required amount for development.