Specific antiserum and monoclonal antibodies against the taurine biosynthesis enzyme cysteine sulfinate decarboxylase: identity of brain and liver enzyme

Identification and expression of cysteine sulfinate decarboxylase, possible regulation of taurine biosynthesis in Crassostrea gigas in response to low salinity

Taurine has been reported high amounts in marine animals to maintain osmotic balance between osmoformers and sea water. Approximately 80% of the total amino-acid content is taurine in Pacific oyster Crassostrea gigas, an intertidal and euryhaline species. In this study, we cloned the two copies of cysteine sulfinate decarboxylase (CSAD), the key enzyme in taurine biosynthesis pathway, screened in oyster genome data. Sequentially, we compared the expression patterns of CgCSAD1 and CgCSAD2 under low salinity treatment (8‰ and 15‰) using different families from two populations. There was no correlation between the expression of CSAD and the different population. Notably, CgCSAD1 increased significantly in treated groups for 24 h, but CgCSAD2 had no significant differentiation. Moreover, the results of CgCSAD1 interference provided the evidence of the positive correlation between CgCSAD1 expressions and taurine contents. The zinc finger domain showed in multi-alignment results may be the important character of CgCSAD1 as the key enzyme in taurine biosynthesis to regulate taurine pool in response to low salinity. This study provides a new evidence for the important role of taurine in adaptation to low salinity in oyster. In addition, it is a good model to discuss the function and evolution of the duplication in mollusks.

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.

Sulfur amino acid metabolism: pathways for production and removal of homocysteine and cysteine

Tissue concentrations of both homocysteine (Hcy) and cysteine (Cys) are maintained at low levels by regulated production and efficient removal of these thiols. The regulation of the metabolism of methionine and Cys is discussed from the standpoint of maintaining low levels of Hcy and Cys while, at the same time, ensuring an adequate supply of these thiols for their essential functions. S-Adenosylmethionine coordinately regulates the flux through remethylation and transsulfuration, and glycine N-methyltransferase regulates flux through transmethylation and hence the S-adenosylmethionine/S-adenosylhomocysteine ratio. Cystathionine beta-synthase activity is also regulated in response to the redox environment, and transcription of the gene is hormonally regulated in response to fuel supply (insulin, glucagon, and glucocorticoids). The H2S-producing capacity of cystathionine gamma-lyase may be regulated in response to nitric oxide. Cys is substrate for a variety of anabolic and catabolic enzymes. Its concentration is regulated primarily by hepatic Cys dioxygenase; the level of Cys dioxygenase is upregulated in a Cys-responsive manner via a decrease in the rate of polyubiquitination and, hence, degradation by the 26S proteasome.

Taurine biosynthetic enzymes and taurine transporter: molecular identification and regulations

Many biological effects of taurine rely upon its cellular concentration, which is primarily controlled by taurine biosynthetic enzymes cysteine dioxygenase (CDO) and cysteine sulfinate decarboxylase (CSD) and taurine transporter (TauT). The cloning of CDO, CSD and TauT in various species provided first-hand information on these proteins, as well as molecular tools to investigate their regulations. CDO upregulation in hepatocytes in response to high sulfur amino acids appears clearly as the most spectacular among the regulations of the biosynthetic enzymes. Downregulation of TauT activity by activation of PKC appears particularly well documented. A unique serine residue could be identified as a phosphorylation site that leads to an inactive form of TauT. The previously revealed downregulation of TauT expression by taurine and hypertonicity-induced upregulation of TauT expression were shown to result from a modified transcription rate of TauT gene, but the precise molecular mechanisms are not yet formally established. Other regulations of taurine transporter expression were more recently reported, which involve glucose, tumor suppressor protein p53, tumor necrosis factor-alpha, and nitric oxide. This review reports the experimental models and data that support these various regulations but also points out the aspects that remain poorly understood or unknown concerning their molecular basis and physiological significance.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.

Expression of cysteine sulfinate decarboxylase mRNA in rat mammary gland

Our previous report demonstrated that high concentration of taurine is present in rat milk for the first few days of lactation and plays an important role in the body growth of rat pups. In the present study, gene expression of rate-limiting enzyme for taurine biosynthesis, cysteine sulfinate decarboxylase (CSD) were examined in rat mammary gland. By RT-PCR, CSD mRNA was found to be expressed in rat mammary gland like that in the liver. The expression level of CSD mRNA in the mammary gland was higher in the earlier lactational stage (days 1 and 6 of lactation) than that in the later lactational stage (day 14). CSD mRNA expression in the mammary gland of non-pregnant rats was only a trace level. By in situ hybridization analysis, CSD mRNA was demonstrated in the epithelial cells of the mammary gland. These results suggest that high concentrations of taurine in the milk are at least partially resulted from de novo synthesis of taurine in mammary gland epithelial cells and that the expression pattern of CSD mRNA may be responsible for the changes in taurine levels in the milk during a lactational period.

Characterization of the cDNA coding for rat brain cysteine sulfinate decarboxylase: brain and liver enzymes are identical proteins encoded by two distinct mRNAs

Cysteine sulfinate decarboxylase (CSD) is considered as the rate-limiting enzyme in the biosynthesis of taurine, a possible osmoregulator in brain. Through cloning and sequencing of RT-PCR and RACE-PCR products of rat brain mRNAs, a 2,396-bp cDNA sequence was obtained encoding a protein of 493 amino acids (calculated molecular mass, 55.2 kDa). The corresponding fusion protein showed a substrate specificity similar to that of the endogenous enzyme. The sequence of the encoded protein is identical to that encoded by liver CSD cDNA. Among other characterized amino acid decarboxylases, CSD shows the highest homology (54%) with either isoform of glutamic acid decarboxylase (GAD65 and GAD67). A single mRNA band, approximately 2.5 kb, was detected by northern blot in RNA extracts of brain, liver, and kidney. However, brain and liver CSD cDNA sequences differed in the 5' untranslated region. This indicates two forms of CSD mRNA. Analysis of PCR-amplified products of genomic DNA suggests that the brain form results from the use of a 3' alternative internal splicing site within an exon specifically found in liver CSD mRNA. Through selective RT-PCR the brain form was detected in brain only, whereas the liver form was found in liver and kidney. These results indicate a tissue-specific regulation of CSD genomic expression.

Taurine synthesis and cysteine metabolism in cultured rat astrocytes: effects of hyperosmotic exposure

We investigated mechanisms controlling taurine synthesis in cultured rat cerebral astrocytes. The mean +/- SE rate of taurine synthesis from extracellular cysteine was 21.2 +/- 2.0 pmol.mg protein-1.min-1, whereas taurine degradation was < 1.3% of this rate. Eliminating cellular glutathione and inhibiting glutathione biosynthesis increased taurine synthesis from extracellular cysteine by 39%. In cell homogenates, cysteine dioxygenase (CDO) and cysteine-sulfinate decarboxylase activities were 2.4 +/- 0.2 and 8.3 +/- 2.8 nmol.mg protein-1.min-1, respectively. CDO activity was strongly dependent on cysteine concentration over physiological and pathophysiological ranges of intracellular cysteine concentration. Growth in hyperosmotic medium caused a greater increase in culture medium taurine content than that measured from cells in isosmotic growth medium. Hyperosmotic treatment transiently increased the rate of cysteine accumulation and cellular cysteine and glutathione contents but had no effect on the synthesis rate of taurine from extracellular cysteine. Thus cysteine is accumulated and then metabolized to taurine through CDO, whose activity depends on the intracellular cysteine concentration and appears to be rate limiting for taurine synthesis. Hyperosmotic exposure increases net taurine production yet has no effect on taurine synthesis from exogenously applied cysteine. Availability of substrate from intracellular pools must contribute to maintenance of high intracellular taurine during hyperosmotic exposure.

Multiplicity of Brain Cysteine Sulfinic Acid Decarboxylase - Purification, Characterization and Subunit Structure

Cysteine sulfinic acid decarboxylase (CSAD), the rate-limiting enzyme in taurine biosynthesis, appears to be present in the brain in multiple isoforms. Two distinct forms of CSAD, referred to as CSAD I and CSAD II, were obtained on Sephadex G-100 column. CSAD I and CSAD II differ in: (1) the elution profile on Sephadex G-100 column; (2) the sensitivity towards Mn(2+), methione, and other sulfur-containing amino acids, and (3) their immunologic properties. CSAD II has been purified to about 2,500-fold by a combination of column chromatographies and polyacrylamide gel electrophoresis (PAGE). The purity of the enzyme preparation was established as judged from the following observations: (1) a single protein band was observed under various electrophoretic conditions, e.g., 5-20% nondenaturing PAGE, 7% nondenaturing PAGE and 10% SDS-PAGE, and (2) in nondenaturing PAGE, the protein band comigrated with CSAD activity. CSAD II has a molecular weight of 90 kDa and is a homodimer consisting of two 43 +/- 2 kDa subunits. CSAD appears to require Mn(2+) for its maximum activity. Other divalent cations fail to replace Mn(2+) in activation of CSAD activity. However, the precise role of Mn(2+) in the action of CSAD remains to be determined. Copyright 1996 S. Karger AG, Basel

Immunocytochemical localization of cysteine sulfinate decarboxylase in astrocytes in the cerebellum and hippocampus: a quantitative double immunofluorescence study with glial fibrillary acidic protein and S-100 protein

Immunocytochemistry of cysteine sulfinate decarboxylase was performed with a new rabbit antiserum that we have recently produced and characterized using as antigen an 11,000-fold purified fraction isolated from rat liver. This antiserum precipitated cysteine sulfinate decarboxylase enzymatic activity, labeled one band (mol. wt 51,000) on immunoblots of crude tissue extracts and did not stain any cells in peripheral tissues devoid of cysteine sulfinate decarboxylase. According to these criteria, this antiserum appeared to be specific for cysteine sulfinate decarboxylase. Numerous cells were immunolabeled in the cerebellum and the hippocampus. Most notable was the labeling of the small cells surrounding the Purkinje cells and sending radial fibers up to the pial surface of the cerebellar cortex or the staining of small star-shaped cells with thin immunolabeled processes abutting on blood vessels. Identified nerve cells such as the Purkinje cells and granule cells in the cerebellum or the pyramidal and granule cells in the hippocampus were devoid of any immunoreactivity. simultaneous double immunofluorescence was carried out using anti-glial fibrillary acidic protein or anti-S-100 monoclonal antibodies. Cysteine sulfinate decarboxylase as well as glial fibrillary acidic protein- or S-100-immunopositive cells were plotted independently for the same section. Quantitative analysis of the maps indicated that the overwhelming majority of cysteine sulfinate decarboxylase-immunolabeled cells were positive for the established astrocytes markers, glial fibrillary acidic protein or S-100. Between 82 and 98% of cysteine sulfinate decarboxylase-immunolabeled cells were also glial fibrillary acidic protein-positive, depending upon the layer. Cysteine sulfinate decarboxylase immunostaining was localized within the cytoplasm, while that of glial fibrillary acidic protein was linked to the cytoskeleton. Since both labels could not be fully superposed, some double immunolabeled cells may have escaped our analysis. More than 94% up to 99% of cysteine sulfinate decarboxylase-immunolabeled cells were simultaneously S-100-immunopositive. Our quantitative data establish that cysteine sulfinate decarboxylase is strictly localized in astrocytes in the cerebellum and in the hippocampus. This finding suggests that taurine is synthesized by astrocytes in the brain and accordingly may play a role in relation to glial function, possibly within the framework of glial-neuronal interactions.

Molecular cloning and sequence analysis of the cDNA encoding rat liver cysteine sulfinate decarboxylase (CSD)

The taurine biosynthesis enzyme, cysteine sulfinate decarboxylase (CSD), was purified to homogeneity from rat liver. Three CSD peptides generated by tryptic cleavage were isolated and partially sequenced. Two of them showed a marked homology with glutamate decarboxylase and their respective position on the CSD amino acid sequence was postulated accordingly. Using appropriate degenerated primers derived from these two peptides, a PCR amplified DNA fragment was generated from liver poly(A)+ mRNA, cloned and used as a probe to screen a rat liver cDNA library. Three cDNAs, length around 1800 bp, were isolated which all contained an open reading frame (ORF) encoding a 493 amino acid protein with a calculated molecular mass of 55.2 kDa close to the experimental values for CSD. The encoded protein contained the sequence of the three peptides isolated from homogenous liver CSD. Our data confirm and significantly extend those recently published (Kaisaki et al. (1995) Biochim. Biophys. Acta 1262, 79-82). Indeed, an additional base pair found 1371 bp downstream from the initiation codon led to a shift in the open reading frame which extended the carboxy-terminal end by 15 amino acid residues and altogether modified 36 amino acids. The validity of this correction is supported by the finding that the corrected reading frame encoded a peptide issued from CSD tryptic cleavage that was not encoded anywhere in the CSD sequence previously reported.

Specificity of cysteine sulfinate decarboxylase (CSD) for sulfur-containing amino-acids

Cysteine sulfinate decarboxylase (CSD) which decarboxylates cysteine sulfinic acid (CSA) to form hypotaurine is thought to be involved in the biosynthesis of taurine. It was recently localized in astrocytes in the cerebellum and hippocampus by immunocytochemistry. Another sulfur-containing amino-acid (SCAA), homocysteic acid (HCA), was also found in astrocytes in these regions. We therefore investigated the specificity of CSD vs CSA and HCA as well as the related analogs homocysteine sulfinic acid (HCSA) and cysteic acid (CA). CSD was immunotrapped from brain and liver tissue supernatant using a specific CSD antiserum and Protein-A Sepharose. It was then incubated with the L-form of the various SCAA. Reaction products were identified and quantified by pre-column o-phthalaldehyde derivatization HPLC. CA and HCA from 2.5 to 25 mM inhibited the formation of hypotaurine from CSA (0.25 mM). Moreover, the inhibition curves were parallel for liver and brain CSD. CA or HCA (25 mM) elicited a near-total inhibition. HCSA did not produce a significant inhibition up to 25 mM. Incubation with 25 mM CSA or CA led to the formation of hypotaurine and taurine, respectively. The ratio of formation of taurine to that of hypotaurine was similar for CSD from liver and brain. In contrast no homotaurine, the decarboxylated reaction product of HCA, could be detected following incubation with 25 mM HCA. According to the sensitivity of the HPLC analysis this indicates that the decarboxylation of HCA, if any, was 130-fold and 50-fold less than that of CSA by CSD from liver and brain, respectively, in our experimental conditions. Similarly, following incubation with HCSA, no new peak appeared on the chromatogram when compared to a blank sample. These results show that CSD from either brain or liver has a high specificity for CSA and CA, which are the SCAA involved in the biosynthesis of taurine. HCA is an inhibitor of CSD but does not appear to be a substrate for CSD in vitro. HCSA is neither a substrate nor an inhibitor of CSD in vitro. Accordingly, CSD is unlikely to play a role in the metabolism of HCA or HCSA in vivo.

Taurine immunoreactivity in the rat supraoptic nucleus: prominent localization in glial cells

Taurine is an inhibitory amino acid that hyperpolarizes magnocellular neurosecretory neurons. To determine which cell types in the rat supraoptic nucleus contain taurine, we used a monoclonal antibody raised against a taurine conjugate. Preembedding immunocytochemistry was carried out at the light and electron microscopic levels using diaminobenzidine and gold-substituted silver-intensified peroxidase as markers. We report the presence of taurine in all cellular compartments of the supraoptic nucleus, except axons, with variable labeling intensities among the different compartments. Few cell bodies of magnocellular neurons were immunoreactive, but many distal dendrites and some proximal ones showed weak-to-moderate levels of immunoreactivity. Strong immunoreactivity was found over glial cell bodies and their processes, in particular in the ventral glial lamina of the supraoptic nucleus. Large astrocytic processes labeled with the taurine antibody included the endfeet participating in the glial limitans around capillaries and at the ventral surface of the hypothalamus. Other types of immunoreactive astrocytic profiles were found scattered within the neuropil where these processes participated in different interactions with the neuronal elements of the supraoptic nucleus. Immunoreactive glial expansions, sometimes even the main process of the glial cell, engulfed axonal boutons. Other labeled glial processes were found between two magnocellular perikarya or closely apposed to the membrane of axonal boutons contacting the neuronal cell bodies. The frequent finding of closely apposed glial and dendritic elements bearing different levels of taurine-like immunoreactivity suggests that exchange of taurine between those two compartments may occur. We propose that taurine could be released from supraoptic glia by a small decrease in osmolarity or by receptor-mediated mechanisms during conditions of low hormonal (vasopressin and/or oxytocin) needs. Such released taurine could then act on presynaptic or postsynaptic sites, or both, to exert its neuromodulatory actions.

Production and characterization of a new specific antiserum against the taurine putative biosynthetic enzyme cysteine sulfinate decarboxylase

We have shown previously that cysteine sulfinate decarboxylase (CSD), the putative biosynthetic enzyme of taurine in the brain, is identical to the liver enzyme according to biochemical, kinetic, and immunochemical criteria. In the present work, CSD was purified in its native form from rat liver. The purification was performed in eight steps, which included conventional chromatography (diethylaminoethyl cellulose, hydroxylapatite), followed by HPLC (hydrophobic, adsorption, and ion-exchange HPLC). The purification factor was 11,000, and the final yield was around 2%. The procedure led to the enrichment of a protein, the molecular mass of which was 51,000 daltons as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The final fraction was more than 90% homogeneous. By using this fraction as the antigen, an antiserum was raised in rabbit that (a) quantitatively immunoprecipitated CSD activity from liver and brain extract, and (b) immunolabeled one band (51,000 daltons) on immunoblots of partially purified fractions from liver. Enrichment of CSD specific activity and that of the protein immunolabeled by the antiserum for a given step, e.g., hydrophobic HPLC, were consistently parallel. The antiserum was used to carry out CSD immunocytochemistry in cerebellum. Numerous small cells were labeled in the Purkinje cell layer, the granular layer, and the white matter. In the molecular layer, Bergmann radial fibers were immunostained. The Purkinje and stellate cells were devoid of any labeling at the cell body and terminal levels. The antiserum appears to be specific for CSD and suitable for immunocytochemical visualization of CSD in the brain.

Immunocytochemistry of the taurine biosynthesis enzyme, cysteine sulfinate decarboxylase, in the cerebellum: evidence for a glial localization

Immunocytochemistry of cysteine sulfinate decarboxylase was developed in the cerebellum with an antiserum that we have recently produced using as immunogen a homogeneous fraction purified about 2000-fold from liver. In the rat, this antiserum immunoprecipitated the enzymatic activity from brain, labeled one band (molecular weight = 51,000) on immunoblots of an enriched fraction from brain but none with a brain crude extract and thus appeared to be specific. In the cerebellum numerous immunolabeled cells were found in the white matter that were typically arranged in rows like oligodendrocytes. A few immunolabeled cells were scattered in the granular layer. Around the Purkinje cells numerous small satellite cells were immunostained that sent faintly labeled radial fibers through the molecular layer. These cells were thus identified as Golgi epithelial cells with their Bergmann fibers. Purkinje cells were not labeled either at the cell body or at the nerve ending levels. In the molecular layer no cells were found consistently and convincingly immunolabeled that could correspond to the stellate cells. No immunopositive punctae typical of the interneuron nerve endings were ever observed. A significant cysteine sulfinate decarboxylase activity was also measured in glial cell fractions enriched in oligodendrocytes and astrocytes respectively. The glial localization of taurine biosynthesis enzyme in the cerebellum does not support the hypothesis that it could be involved in neurotransmission, but rather suggests that taurine is manufactured by glial cells and accordingly may play a physiological role in relation to glial function.