Glucose-6-Phosphate Dehydrogenase in the Pentose Phosphate Pathway Is Localized in Vanadocytes of the Vanadium-Rich Ascidian, Ascidia sydneiensis samea

Ascidians are sessile marine animals known to accumulate high levels of vanadium selectively in vanadium-containing blood cells (vanadocytes). Almost all the vanadium accumulated in the vacuoles of vanadocytes is reduced to the +3 oxidation state via the +4 oxidation state, although vanadium is dissolved in the +5 oxidation state in sea water. Some of the reducing agents that participate in the reduction have been proposed. By chemical study, vanadium in the +5 oxidation state was reported to be reduced to the +4 oxidation state in the presence of NADPH. The present study revealed the existence of glucose-6-phosphodehydrogenase (G6PDH), the first enzyme to produce NADPH in the pentose phosphate pathway, in vanadocytes of a vanadium-rich ascidian. The results suggested that G6PDH conjugates the reduction of vanadium from the +5 through to the +4 oxidation state in vanadocytes of ascidians.

Identification of differentially expressed genes in psoriasis using expression profiling approaches

To identify differentially expressed genes which play causal roles in pathogenesis and maintenance for psoriasis, we used BodyMapping and introduced amplified fragment length polymorphism approaches. From the BodyMap database, we selected 2007 genes which specifically expressed in epithelial tissues. Among 2007 genes, we surveyed genes which differentially expressed in involved or uninvolved psoriatic lesional skin samples compared with atopic dermatitis, mycosis fungoides, and normal skin samples. As a result of surveying 2007 genes, 241 genes were differentially expressed only in involved psoriatic skin but not in the other samples. Hierarchical cluster analysis of gene expression profiles showed that 13 independent psoriatic-involved skin samples clustered tightly together, reflecting highly similar expression profiles. Using the same 2007 gene set, we examined gene expression levels in five serial lesions from distal uninvolved psoriatic skin to involved psoriatic plaque. We identified seven genes such as alpha-1-microglobulin/bikunin precursor, calnexin, claudin 1, leucine zipper down-regulated in cancer 1, tyrosinase-related protein 1, Yes-associated protein 1, and unc-13-like protein (Coleonyx elegans) which show high-expression levels only in uninvolved psoriatic lesions. These seven genes, which were reported to be related to apoptosis or antiproliferation, might have causal roles in pathophysiology in psoriasis.

Subunit C of the vacuolar-type ATPase from the vanadium-rich ascidian Ascidia sydneiensis samea rescued the pH sensitivity of yeast vma5 mutants

A vanadium-accumulating ascidian, Ascidia sydneiensis samea, expresses vacuolar-type H(+)-ATPases (V-ATPases) on the vacuole membrane of the vanadium-containing blood cells known as vanadocytes. Previously, we showed that the contents of their vacuoles are extremely acidic and that a V-ATPase-specific inhibitor, bafilomycin A(1), neutralized the contents of the vacuoles. To understand the function of V-ATPase in vanadocytes, we isolated complementary DNA encoding subunit C of V-ATPase from vanadocytes because this subunit has been known to be responsible for the assembly of V-ATPases and to regulate the ATPase activity of V-ATPases. The cloned cDNA was 1443 nucleotides in length, and encoded a putative 384 amino acid protein. By expressing the ascidian cDNA for subunit C under the control of a galactose-inducible promoter, the pH-sensitive phenotype of the corresponding vma5 mutant of a budding yeast was rescued. This result showed that the ascidian cDNA for subunit C functioned in yeast cells.

Human Ca2+/calmodulin-dependent phosphodiesterase PDE1A: novel splice variants, their specific expression, genomic organization, and chromosomal localization

We report here the identification of novel human PDE1A splice variants, their tissue distribution patterns, genomic structure, and chromosomal localization of the gene. We identified one N-terminus (N3) and one C-terminus (C3) by cDNA library screening and dbEST database search. These N- and C-termini, including the reported N-termini (N1 and N2) and C-termini (C1 and C2), combined to generate nine different PDE1A cDNAs. N1 and N2 are similar to the 5' ends of the bovine PDE1A proteins of 61 kDa and 59 kDa, respectively, and C1 and C2 are the 3' ends of the reported human PDE1A variants. The results of PCR and Southern blot analysis show that nine PDE1A splice variants exhibit distinctive tissue distribution patterns by the difference of the N-terminus. PDE1As with N2 were widely expressed in various tissues, mainly in the kidney, liver, and pancreas. On the other hand, PDE1As with N1 and N3 were particularly expressed at a high level in the brain and testis, respectively. These findings suggest that the distinct expression patterns among PDE1A variants depend on the several promoters situated upstream of exons encoding 5' ends of the variants. The PDE1A gene spans over 120 kb of genomic DNA, and consists of at least 17 exons and 16 introns. The PDE1A gene was located on human chromosome 2q32 by fluorescent in situ hybridization analysis.

Exclusive expression of transketolase in the vanadocytes of the vanadium-rich ascidian, Ascidia sydneiensis samea

Ascidians, especially those belonging to the Ascidiidae, are known to accumulate extremely high levels of vanadium in vanadocytes, one type of blood (coelomic) cell. Vanadium, which exists in the +5 oxidation state in seawater, is accumulated in the vanadocytes and reduced to the +3 oxidation state. We have been trying to characterize all of the polypeptides specific to vanadocytes and to specify the proteins that participate in the accumulation and reduction of vanadium. To date, we have localized three enzymes in vanadocytes: 6-phosphogluconate dehydrogenase (6-PGDH: EC 1.1.1.44), glucose-6-phosphate dehydrogenase (G6PDH: EC 1.1.1.49), and glycogen phosphorylase (GP: EC 2.4.1.1), all of which are involved in the pentose phosphate pathway. In the current study, we cloned a cDNA for transketolase, an essential and rate-limiting enzyme in the non-oxidative part of the pentose phosphate pathway, from vanadocytes. The cDNA encoded a protein of 624 amino acids, which showed 61.8% identity to the human adult-type transketolase gene product. By immunocytochemistry and immunoblot analyses, the transketolase was revealed to be a protein that was expressed only in vanadocytes and not in any of the more than ten other types of blood cell. This finding, taken together with the localized expression of the other three enzymes, strongly supports the hypothesis that the pentose phosphate pathway functions exclusively in vanadocytes.

Characterization and effects of methyl-2- (4-aminophenyl)-1, 2-dihydro-1-oxo-7- (2-pyridinylmethoxy)-4-(3,4, 5-trimethoxyphenyl)-3-isoquinoline carboxylate sulfate (T-1032), a novel potent inhibitor of cGMP-binding cGMP-specific phosphodiesterase (PDE5)

An isoquinolone derivative, methyl-2-(4-aminophenyl)-1, 2-dihydro-1-oxo-7-(2-pyridinylmethoxy)-4-(3,4, 5-trimethoxyphenyl)-3-isoquinoline carboxylate sulfate (T-1032), was found to be a novel potent inhibitor of cyclic GMP (cGMP)-binding cGMP-specific phosphodiesterase (PDE5). We investigated the inhibitory effects of T-1032 on six PDE isozymes isolated from canine tissues. T-1032 specifically inhibited the hydrolysis of cGMP by PDE5 partially purified from canine lung, at a low concentration (IC(50) = 1.0 nM, K(i) = 1.2 nM), in a competitive manner. In contrast, the IC(50) values of T-1032 for PDE1, PDE2, PDE3, and PDE4 were more than 1 microM. T-1032 also inhibited PDE6 from canine retina with an IC(50) of 28 nM, which is of the same order of magnitude as the IC(50) of sildenafil. cGMP hydrolytic activities of two alternative splice variants of canine PDE5 expressed in COS-7 cells were inhibited by this compound to a similar extent. T-1032 increased the intracellular concentration of cGMP in cultured rat vascular smooth muscle cells in the presence and absence of C-type natriuretic peptide, an activator of membrane-bound guanylate cyclase, whereas the compound did not change cyclic AMP levels. These data indicated that T-1032, which belongs to a new structural class of PDE5 inhibitors, is a potent and selective PDE5 inhibitor. This compound may be useful in pharmacological studies to examine the role of a cGMP/PDE5 pathway in tissues.

Isolation and characterization of two novel phosphodiesterase PDE11A variants showing unique structure and tissue-specific expression

cDNAs encoding a novel phosphodiesterase, phosphodiesterase 11A (PDE11A), were isolated by a combination of reverse transcriptase-polymerase chain reaction using degenerate oligonucleotide primers and rapid amplification of cDNA ends. Their catalytic domain was identical to that of PDE11A1 (490 amino acids) reported during the course of this study. However, the cDNAs we isolated had N termini distinct from PDE11A1, indicating two novel N-terminal variants of PDE11A. PDE11A3 cDNA encoded a 684-amino acid protein including one complete and one incomplete GAF domain in the N-terminal region. PDE11A4 was composed of 934 amino acids including two complete GAF domains and shared 630 C-terminal amino acids with PDE11A3 but had a distinct N terminus containing the putative phosphorylation sites for cAMP- and cGMP-dependent protein kinases. PDE11A3 transcripts were specifically expressed in testis, whereas PDE11A4 transcripts were particularly abundant in prostate. Recombinant PDE11A4 expressed in COS-7 cells hydrolyzed cAMP and cGMP with K(m) values of 3.0 and 1.4 microm, respectively, and the V(max) value with cAMP was almost twice that with cGMP. Although PDE11A3 showed the same K(m) values as PDE11A4, the relative V(max) values of PDE11A3 were approximately one-sixth of those of PDE11A4. PDE11A4, but not PDE11A3, was phosphorylated by both cAMP- and cGMP-dependent protein kinases in vitro. Thus, the PDE11A gene undergoes tissue-specific alternative splicing that generates structurally and functionally distinct gene products.

Identification of a conserved residue responsible for the autoinhibition of cGMP-dependent protein kinase Ialpha and beta

We isolated a constitutively active form of cGMP-dependent protein kinase Ialpha (cGK Ialpha) by PCR-driven random mutagenesis. The replacement of Ile-63 by Thr in the autoinhibitory domain results in the enhancement of autophosphorylation and the basal kinase activity in the absence of cGMP. The hydrophobicity at position 63 is essential for the inactive state of cGK Ialpha, and Ile-78 of cGK Ibeta is also required for the autoinhibitory property. Furthermore, cGK Ialpha (Ile-63-Thr) is constitutively active in vivo. These findings suggest that a conserved residue in the autoinhibitory domain was involved in the autoinhibition of both cGK Is.

A novel interaction of cGMP-dependent protein kinase I with troponin T

cGMP-dependent protein kinase (cGK) is a major intracellular receptor of cGMP and is implicated in several signal transduction pathways. To identify proteins that participate in the cGMP/cGK signaling pathway, we employed the yeast two-hybrid system with cGK Ialpha as bait. cDNAs encoding slow skeletal troponin T (skTnT) were isolated from both mouse embryo and human skeletal muscle cDNA libraries. The skTnT protein interacted with cGK Ibeta but not with cGK II nor cAMP-dependent protein kinase. The yeast two-hybrid and in vitro binding assays revealed that the N-terminal region of cGK Ialpha, containing the leucine zipper motif, is sufficient for the association with skTnT. In vivo analysis, mutations in cGK Ialpha, which disrupted the leucine zipper motif, were shown to completely abolish the binding to skTnT. Furthermore, cGK I also interacted with cardiac TnT (cTnT) but not with cardiac troponin I (cTnI). Together with the observations that cTnI is a good substrate for cGK I and is effectively phosphorylated in the presence of cTnT in vitro, these findings suggest that TnT functions as an anchoring protein for cGK I and that cGK I may participate in the regulation of muscle contraction through phosphorylation of TnI.

Cloning and characterization of a novel human phosphodiesterase that hydrolyzes both cAMP and cGMP (PDE10A)

cDNA encoding a novel phosphodiesterase (PDE) was isolated from a human fetal lung cDNA library and designated PDE10A. The deduced amino acid sequence contains 779 amino acids, including a putative cGMP binding sequence in the amino-terminal portion of the molecule and a catalytic domain that is 16-47% identical in amino acid sequence to those of other PDE families. Recombinant PDE10A transfected and expressed in COS-7 cells hydrolyzed cAMP and cGMP with Km values of 0.26 and 7.2 microM, respectively, and Vmax with cGMP was almost twice that with cAMP. Of the PDE inhibitors tested, dipyridamole was most effective, with IC50 values of 1.2 and 0.45 microM for inhibition of cAMP and cGMP hydrolysis, respectively. cGMP inhibited hydrolysis of cAMP, and cAMP inhibited cGMP hydrolysis with IC50 values of 14 and 0.39 microM, respectively. Thus, PDE10A exhibited properties of a cAMP PDE and a cAMP-inhibited cGMP PDE. PDE10A transcripts were particularly abundant in the putamen and caudate nucleus regions of brain and in thyroid and testis, and in much lower amounts in other tissues. The PDE10A gene was located on chromosome 6q26 by fluorescent in situ hybridization analysis. PDE10A represents a new member of the PDE superfamily, exhibiting unique kinetic properties and inhibitor sensitivity.

Genomic origin and transcriptional regulation of two variants of cGMP-binding cGMP-specific phosphodiesterases

We have reported alternative splice variants of cGMP-binding cGMP-specific phosphodiesterases (PDE5A), i.e. rat PDE5A2, human PDE5A1, canine PDE5A1 and PDE5A2, which possess distinct N-terminal sequences. In this study, the DNA sequences corresponding to the unique N-terminal portions of PDE5A1 and PDE5A2 were shown to be tandemly located upstream of exons encoding the common region of PDE5A in both human and rat PDE5A genes. The presence of human PDE5A2 and rat PDE5A1 transcripts in lung was confirmed by reverse transcriptase-PCR. These results indicated that two variant forms of PDE5A exist in humans, canines and rats. We examined the tissue distribution of the two variants of human PDE5A in adult and fetal humans. The patterns of expression of the two alternatively spliced transcripts of human PDE5A in human tissues differed. Many putative regulatory elements including cAMP response elements were observed in the 5'-untranslated region and intron of the PDE5A gene. The levels of the PDE5A transcripts, especially the PDE5A2 transcripts, were increased by a cAMP analogue in cultured rat vascular smooth muscle cells, indicating that the PDE5A2 is an inducible variant of PDE5A in rats.

6-Phosphogluconate dehydrogenase is a 45-kDa antigen recognized by S4D5, a monoclonal antibody specific to vanadocytes in the vanadium-rich ascidian Ascidia sydneiensis samea

We previously prepared a monoclonal antibody, S4D5, specific to vanadocytes, vanadium-containing blood cells, in the vanadium-rich ascidian Ascidia sydneiensis samea. Here, we demonstrate that a 45-kDa antigen recognized by S4D5 is 6-phosphogluconate dehydrogenase (6-PGDH), an enzyme of the pentose phosphate pathway, based on cDNA isolation of RNA samples from blood cells of the ascidian. Western blot analysis confirmed an abundance of 6-PGDH protein in the vanadocytes and localization of 6-PGDH in the soluble extract of the blood cells. Soluble protein exhibited a correspondingly high level of 6-PGDH enzymatic activity. Ascidians are known to selectively accumulate high levels of vanadium in vanadocytes, and the highest recorded concentration of accumulated vanadium is 350 mM, which is 10(7) times the concentration in sea water. Almost all vanadium ions are reduced to the +3 oxidation state via the +4 oxidation state in vanadocytes, indicating that reducing agents must participate in the accumulation. On the other hand, vanadium ions in the +5 oxidation state are reduced to the +4 oxidation state by the presence of NADPH in vitro. Together, these observations suggest that NADPH produced in the pentose phosphate pathway may conjugate the reduction of vanadium from the +5 oxidation state through the +4 oxidation state in vanadocytes of ascidians.

Expression, structure and chromosomal localization of the human cGMP-binding cGMP-specific phosphodiesterase PDE5A gene

cGMP-binding, cGMP-specific phosphodiesterase which is encoded by the PDE5A gene plays important roles in cardiovascular system, and is a significant target molecule of therapeutic agents. However, little is known about molecular characteristics of the human PDE5A gene. The 4.4-kb cDNA encoding human PDE5A was isolated from lung and placenta cDNA libraries. The deduced amino acid sequence analysis demonstrated that N-terminal amino acid sequence is dissimilar to that of rat PDE5A [Kotera, J., Yanaka, N., Fujishige, K., Imai, Y., Akatsuka, H., Ishizuka, T., Kawashima, K. & Omori, K. (1997) Eur. J. Biochem. 249, 434-442]. Human PDE5A mRNA is produced in high amounts in various tissues such as pancreas, skeletal muscle, placenta, heart, thyroid, adrenal cortex, testis, small intestine and stomach. In addition, the megakaryocyte-like cell line Dami cells and two types of human vascular smooth muscle cells also produce the mRNA. Over 100-kb chromosomal DNA corresponding to the human PDE5A gene was isolated and analyzed. The human PDE5A gene was revealed to contain 21 exons. Comparison of genomic organization with the rod photoreceptor phosphodiesterase beta-subunit gene (PDE6B), which is another kind of cGMP-specific phosphodiesterase, has shown that the PDE5A and PDE6B genes are very similar in their relative exon intron organization. In particular, the evolutionary relatedness of these genes was suggested in the catalytic domain. Furthermore, chromosomal location of the PDE5A gene was defined as being chromosome 4q26 by fluorescent in situ hybridization analysis.

Autocrine/paracrine role of adrenomedullin in cultured endothelial and mesangial cells

Adrenomedullin (AM), a potent vasorelaxant and natriuretic peptide isolated from human pheochromocytoma, is present in the kidney and secreted from endothelial cells (EC) and vascular smooth muscle cells (VSMC), but the functional role of AM is still unclear. To clarify the significance of AM as a local regulator, we investigated its secretion and action in cultured cells, and examined the effects of neutralization using a specific monoclonal antibody against AM. The prepared antibody directed against the ring structure showed a high affinity for human and rat AM. Using radioimmunoassay with this antibody, we found significant secretion from cultured rat mesangial cells (MC) of a 6-kDa mature form of AM as seen from EC and VSMC. The addition of AM into cultured cells dose-dependently increased cAMP production and potently inhibited PDGF-stimulated thymidine incorporation. Pretreatment with the monoclonal antibody completely abolished cAMP increase induced by exogenous AM. Moreover, antibody neutralization of endogenously secreted AM in cultured EC, but not in MC or VSMC, markedly (by approximately 70%) reduced basal cAMP production and significantly (1.7-fold) enhanced DNA synthesis. These results indicate that AM, acting as an autocrine/paracrine regulator, exerts an antiproliferative action on EC and MC, and suggest its role as a local modulator of endothelial and mesangial function.

L-aspartate but not the D form is secreted through microvesicle-mediated exocytosis and is sequestered through Na+-dependent transporter in rat pinealocytes

Rat pinealocytes accumulate glutamate in microvesicles and secrete it through exocytosis so as to transmit signals intercellularly. Glutamate is involved in the negative regulation of norepinephrine-stimulated melatonin production. In this study, we found that aspartate is also released from cultured rat pinealocytes during the exocytosis of glutamate. The release of aspartate was triggered by addition of KCI or A23187 (a Ca2+ ionophore) in the presence of Ca2+ and was proportional to the amount of L-glutamate released. Furthermore, the release of aspartate was inhibited by both botulinum neurotoxin type E and L- or N-type voltage-gated Ca2+ channel blockers. Bay K 8644, an agonist for the L-type Ca2+ channel, stimulated the release of aspartate 2.1-fold. Immunohistochemical analyses with antibodies against aspartate and synaptophysin revealed that aspartate is colocalized with synaptophysin in a cultured pinealocyte. HPLC with fluorometric detection indicated that the released aspartate is of the L form, although pinealocytes also contain the D form in their cytoplasm, corresponding to approximately 30% of the total free aspartate. Radiolabeled L-aspartate was taken up by the microsomal fraction from bovine pineal glands in a Na+-dependent manner. The Na+-dependent uptake of L-aspartate was strongly inhibited by L-cysteine sulfinate, beta-hydroxyaspartate, and L-serine-O-sulfate, inhibitors for the Na+-dependent glutamate/aspartate transporter on the plasma membrane. Na+-dependent sequestration of L-aspartate was also observed in cultured rat pinealocytes, which was inhibited similarly by these transporter inhibitors. These results strongly suggest that L-aspartate is released through microvesicle-mediated exocytosis from pinealocytes and is taken up again through the Na+-dependent transporter at the plasma membrane. The possible role of L-aspartate as an intercellular chemical transmitter in the pineal gland is discussed.

Finding of the same antigens in the polychaete, Pseudopotamilla occelata, as those in the vanadium-rich ascidian, Ascidia sydneiensis samea

The polychaete Pseudopotamilla occelata is the first animal revealed to contain high levels of vanadium besides ascidians. The present experiment disclosed that P. occelata has the same antigens with those in the ascidian Ascidia syndneiensis samea, which were recognized by two types of antibodies, a polyclonal antibody against vanadium-associated proteins extracted from blood cells and a monoclonal antibody against vanadocytes in the vanadium-rich ascidian A. sydneiensis samea. There is, therefore, a possibility that similar mechanism works on the accumulation of vanadium between the Polychaeta and the Ascididae.

Identification of a vanadium-associated protein from the vanadium-rich ascidian, Ascidia sydneiensis samea

Ascidians are known to accumulate vanadium in their blood cells (vanadocytes) at extremely high levels which correspond to about 10(6) to 10(7) times the levels of vanadium ions in seawater. The route for the accumulation of vanadium ions from the outside environment into the blood system in ascidians has not yet been discovered. In the present experiments, using a combined technique of anion exchange column and atomic absorption spectrometry, we first extracted a vanadium-associated protein (VAP) from the blood cells of the ascidian Ascidia sydneiensis samea. VAP was estimated to associate with vanadium at an approximate ratio of 1 mol:16 mole. SDS-PAGE and a polyclonal antibody against VAP (anti-VAP) revealed that VAP is composed of at least two types of peptides estimated to be 12.5 kDa and 15 kDa with a minor peptide of 18 kDa and that VAP is localized in the cytoplasm of the vanadocytes.

Microvesicle-mediated exocytosis of glutamate is a novel paracrine-like chemical transduction mechanism and inhibits melatonin secretion in rat pinealocytes

Mammalian pinealocytes are neuroendocrine cells that synthesize and secrete melatonin, these processes being positively controlled by norepinephrine derived from innervating sympathetic neurons. Previously, we showed that pinealocytes contain a large number of microvesicles (MVs) that specifically accumulate L-glutamate through a vesicular glutamate transporter and contain proteins for exocytosis such as synaptobrevin 2 (VAMP2). These findings suggested that the MVs are counterparts of synaptic vesicles and are involved in paracrine-like chemical transduction in the pineal gland. Here, we show that pinealocytes actually secrete glutamate upon stimulation by KCl in the presence of Ca2+ at 37 degrees C. The ability of glutamate secretion disappeared when the cells were incubated at below 20 degrees C. Loss of the activity was also observed on successive stimulation, but it was recovered after 12 hr incubation. A low concentration of cadmium chloride or omega-conotoxin GVIA inhibited the secretion. Botulinum neurotoxin E cleaved synaptic vesicle-associated protein 25 (SNAP-25) and thus inhibited the secretion. The released L-glutamate stimulated pinealocytes themselves via glutamate receptor(s) and inhibited norepinephrine-stimulated melatonin secretion. These results strongly suggest that pinealocytes are glutaminergic paraneurons, and that the glutaminergic system regulates negatively the synthesis and secretion of melatonin. The MV-mediated paracrine-like chemical transduction seems to be a novel mechanism that regulates hormonal secretion by neuroendocrine cells.

The L-type Ca2+ channel is involved in microvesicle-mediated glutamate exocytosis from rat pinealocytes

Pinealocytes, parenchymal cells of the pineal gland, secrete glutamate through microvesicle-mediated exocytosis upon depolarization by KCl in the presence of Ca2+, which is involved in a novel paracrine-like intercellular signal transduction mechanism in neuroendocrine organs. In the present study, we investigated whether or not the L-type Ca2+ channel is involved in the microvesicle-mediated glutamate secretion from cultured rat pinealocytes. Nifedipine, a specific antagonist of the L-type Ca2+ channel, inhibited the Ca(2+)-dependent glutamate exocytosis by 48% at 20 microM. Other L-type Ca2+ channel antagonists, such as nitrendipine, showed similar effects. 1,4-Dihydro-2,6-dimethyl-5-nitro-4 [2-(trifluoromethyl)-phenyl]-3-pyridinecarboxylic acid methyl ester (BAY K8644), an agonist of the L-type Ca2+ channel, at 1 microM, on the other hand, stimulated the glutamate exocytosis about 1.6-fold. Consistently, these Ca2+ channel antagonists inhibited about 50% of the Ca2+ uptake, whereas BAY K8644 increased the uptake 5.3-fold. An antibody against the carboxyl-terminal region of the rabbit L-type Ca2+ channel recognized polypeptides of pinealocytes with apparent molecular masses of 250 and 270 kDa, respectively, and immunostained the plasma membrane region of the pinealocytes. These results strongly suggested that the entry of Ca2+ through L-type Ca2+ channel(s), at least in part, triggers microvesicle-mediated glutamate exocytosis in pinealocytes.

Localization of N-ethylmaleimide-sensitive fusion protein in pinealocytes

N-ethylmaleimide-sensitive fusion protein (NSF), a protein necessary for vesicular docking and/or fusion, was detected immunohistochemically in pinealocytes. NSF was distributed similarly to synaptophysin and vacuolar-type H(+)-ATPase (V-ATPase), marker proteins for synaptic-like microvesicles (MVs) abundantly present in pinealocytes. A subcellular fractionation study indicated that .> 95% of NSF was present as a membrane-bound form and that some NSF was associated with MVs. Like neuronal NSF, the protein was not solubilized from membranes with either 2 mM Mg-ATP or 2% sodium carbonate, suggesting that NSF was tightly bound to the membranes. NSF was also detected in purified MVs from bovine posterior pituitaries. Since MVs are the organelles in which transmitters are stored, these results suggest that NSF is involved in the MV-mediated exocytosis of transmitters from endocrine cells.

Immunological detection of a vacuolar-type H(+)-ATPase in vanadocytes of the ascidian Ascidia sydneiensis samea

Ascidians belonging to the family Ascidiidae are known to accumulate vanadium from seawater in their blood cells, concentrating vanadium by a factor of 10(7). Among several different types of blood cell, the signet ring cells have both high levels of vanadium and a low pH. These observations suggest the possibility that proton ions concentrated by a H(+)-ATPase are energetically linked to the accumulation of vanadium. In the present experiments, therefore, we made an immunological search for a H(+)-ATPase in the vacuolar membranes of the signet ring cells, as a first step in our attempts to clarify the energetics of the accumulation of vanadium by these cells. Antibodies raised against the 72-kDa and 57-kDa subunits of a vacuolar-type H(+)-ATPase from bovine chromaffin granules reacted with the vacuolar membranes of signet ring cells. Immunoblotting analysis confirmed that specific antigens in ascidian blood cells actually reacted with the antibodies. Furthermore, addition of bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase, inhibited the uptake of protons by the vacuoles of signet ring cells. Thus, the addition of bafilomycin A1 inhibited the pumping function of the vacuoles of signet ring cells, with resultant neutralization of the contents of the vacuoles.

Accumulation of vanadium during embryogenesis in the vanadium-rich ascidian, Ascidia gemmata

It is a remarkable and previously unrecognized fact that ascidians, which are known to contain high levels of vanadium in their blood cells, begin to accumulate vanadium during embryogenesis. This study revealed that the accumulation starts quite dramatically 2 wk after fertilization, and 2 mo later, the amount of vanadium in larvae is 600,000 times higher than that in the unfertilized egg. These results were obtained by neutron activation analysis, a highly sensitive method for determining levels of vanadium, in the Ascidia gemmata, the ascidian that contains the highest known levels of vanadium and accumulates vanadium at 150 mM in its blood cells, a concentration that corresponds to 4,000,000 times the concentration in seawater.

Vanadobin, a Vanadium-Binding Substance, Extracted from the Blood Cells of an Ascidian, Can Reduce Vanadate(V) to Vanadyl(IV)

Ascidians specifically accumulate high levels of vanadium from seawater in their blood cells. Almost all of the vanadium is present in a reduced form in the blood cells, although the metal exists in a +5 oxidation state in seawater. It has, therefore, been assumed that agents that cause the reduction of vanadate(V) to vanadyl(IV) must be present within ascidian blood cells. In this regard, we have extracted a vanadium-binding substance, which we have called vanadobin, from the vanadocytes of ascidians. We examined whether vanadobin is involved in the reduction of vanadate(V) accumulated from seawater. Data obtained by spectrophotometry and ESR spectrometry revealed that not only a crude homogenate of vanadium-rich blood cells but also a purer form of vanadobin eluted from a column of Sephadex G-15 could reduce vanadate(V). Our experiments demonstrate that vanadobin, a vanadium-binding substance extracted from ascidian blood cells, can reduce vanadate(V) to vanadyl(IV) and maintain it in the reduced form.

Extraction of a Vanadium-Binding Substance (Vanadobin) from the Blood Cells of Several Ascidian Species

A combination of techniques, including chromatography on Sephadex G-15 and SE-cellulose columns and neutron activation analysis for vanadium determination, was used to extract (at low pH) a vanadium-binding substance (vanadobin) from the blood cells of the ascidian species: Ascidia ahodori OKA, A. gemmata SLUITER, A. zara OKA, Corella japonica HERDMAN, and Ciona intestinalis (LINNE). In general, ascidians can be classified into two different categories based on vanadium content: species of the family Ascidiidae contain high levels of vanadium, whereas those in the Cionidae and Corellidae do not always have such high amounts. Because Ciona intestinalis and Corella japonica do have vanadobin in their blood cells, vanadobin may well be a universal complex in ascidians, having the role of accumulating vanadium in blood cells and maintaining its concentration. The blood cells of A. gemmata contained the highest amount of vanadium. Vanadobin extracted from these cells exhibits absorption spectra, not only in the ultraviolet region, but also in the visible region; such spectra correspond to those observed in vanadium complexes in oxidation states of +3 and +4.

EPR characterization of vanadyl (IV) species in blood cells of ascidians

EPR spectra due to vanadyl species of blood cells of Ascidia ahodori collected from different places in Japan (Ushimado and Asamushi) were measured. The EPR spectral pattern as well as their EPR parameters for these two species were found to be different from each other. EPR analysis correlating g parallel and A parallel indicated that vanadyl ion in the animals from Ushimado was in a much stronger ligand field than those from Asamushi. Vanadyl species in the Ushimado animals was comparable to the stable complexes such as the vanadyl-ATP complex prepared at pH 12.

THE ACCUMULATION AND DISTRIBUTION OF VANADIUM, IRON, AND MANGANESE IN SOME SOLITARY ASCIDIANS

The vanadium, iron, and manganese contents of 15 species of solitary ascidians belonging to the suborders Phlebobranchia and Stolidobranchia were determined by thermal neutron activation analysis. Vanadium was detectable in all species examined. In general, the vanadium content in various tissues of the Phlebobranchia was considerably higher than the iron and manganese contents. The blood cells especially contained a large amount of vanadium. The highest value (21 µg vanadium/mg dry weight) was obtained from blood corpuscles of Ascidia ahodori. Species in the suborder Stolidobranchia, on the other hand, had smaller quantities of vanadium in comparison with those in the suborder Phlebobranchia. The iron and manganese contents did not differ greatly between the two suborders. The data are considered in the light of physiological roles of these transition metals in ascidians.

Purification of vanadium binding substance from the blood cells of the tunicate, Ascidia sydneiensis samea

Vanadium binding substance has been partially purified through chromatographies on Sephadex G-25 and SE-Cellulose at pH 2.3. The binding substance was colorless, relatively stable and maintained vanadium ion. The vanadium ion in the substance existed in vanadyl form (VO(IV)). Furthermore, the substance had an apparent affinity for exogenous vanadium ion(V) and contained a reducing sugar.

The accumulation of manganese from the environmental medium by the egg of Oryzias latipes

The manganese content of the egg and embryo of the Medaka, Oryzias latipes was determined by activation analysis. A remarkable increase in the amount of manganese in the egg was observed within one hour after fertilization. The rate of increase was reduced by the gastrula stage and the concentration of manganese remained unchanged at a later stage. The accumulation of manganese by the Oryzias egg was discussed in relation to the effect of manganese on respiratory enzyme systems.