Phenoloxidases in ascidian hemocytes: characterization of the pro-phenoloxidase activating system

The phenoloxidase (PO) activity of the hemocytes lysate supernatant from three ascidians species, assayed by means of 3-methyl-2-benzothiazolinone hydrazone hydrochloride, have been compared. PO-containing hemocytes were identified by a cytochemical reaction and the enzymatic activity measured by a spectrophotometric assay of lysate supernatant from hemocyte populations separated on a discontinuous Percoll density gradient. In Styela plicata, the enzyme appeared to be contained in morula cells only. In Ciona intestinalis, PO activity was shown in univacuolar refractile granulocyte and granular hemocyte. In Phallusia mammillata both compartment cell and granular hemocytes were positive. Enzymatic assay following electrophoretic analysis on polyacrylamide gel electrophoresis (PAGE) or SDS-PAGE indicated that hemocyte lysate presented orthodiphenoloxidase (catecholase) activity. The enzymes from the three species differed in molecular size, activating substances and trypsin sensitivity.

Ascidians as a vertebrate-like model organism for physiological studies of Rho GTPase signaling

GTPases of the Rho family are evolutionarily conserved proteins that control cell shape dynamics during physiological processes as diverse as cell migration and polarity, axon outgrowth and guidance, apoptosis and phagocytosis. In mammals, 18 Rho proteins are distributed in 7 subfamilies. Rho, Rac and Cdc42 are the best-characterized ones, benefiting from the use of worm and drosophila, which only express these 3 subfamilies. An additional model would therefore help understand the physiological role of other mammalian subfamilies. We identified in genome databases the complete Rho family of two ascidians, Ciona intestinalis and Ciona savignyi, and showed that these families contain single ancestors of most mammalian Rho subfamilies. In Ciona intestinalis, all Rho genes are expressed and display specific developmental variations of mRNA expression during tadpole formation. Although C. intestinalis expresses five additional Rac compared to the closely related Ciona savignyi, only two appeared fully active in functional assays. Last, we identified in Ciona intestinalis database more than 50 Rho regulators (RhoGEFs and RhoGAPs) and 20 effector targets, whose analysis further supports the notion that Rho signaling components are of comparable complexity in mammals and ascidians. Since the tadpole of ascidians combines vertebrate-like developmental features with reduced cell number, particularly adapted to evolutionary and developmental biology studies, our data advocate this model for physiological studies of Rho signaling pathways.

Spatio-temporal pattern of MAP kinase activation in embryos of the ascidian Halocynthia roretzi

To understand developmental mechanisms, it is important to know when and where signaling pathways are activated. The spatio-temporal pattern of activation of mitogen-activated protein kinase (MAPK/ERK) was investigated during embryogenesis of the ascidian Halocynthia roretzi, using an antibody specific to the activated form of MAPK. During cleavage stages, activated MAPK was transiently observed in nuclei of the precursor blastomeres of endoderm, notochord, mesenchyme, brain, secondary muscle, trunk lateral cells and trunk ventral cells. These sites of MAPK activation are consistent with results of previous studies that have analyzed the embryonic induction of various tissues, and with results of inhibition of MAPK kinase (MEK) in ascidians. Activation of MAPK in notochord and mesenchyme blastomeres was observed in a short period in a single cell cycle. In contrast, in brain and secondary muscle lineages, MAPK activation spanned two or three cell cycles, and upon each cleavage, MAPK was asymmetrically activated in only one of the two daughter cells that remained brain or secondary muscle lineages. During later stages, MAPK activation was predominantly observed in the central nervous system. A conspicuous feature at this stage was that activation appeared to alternate between positive and negative along the anterior-posterior axis of the neural tube. During the tail elongation stage, MAPK was quiescent.

Oxidative stress induces cytotoxicity during rejection reaction in the compound ascidian Botryllus schlosseri

When genetically incompatible colonies of the compound ascidian Botryllus schlosseri contact each other, a rejection reaction occurs, characterised by the appearance of cytotoxic foci along the touching borders. In the course of this reaction, morula cells, a common haemocyte-type in ascidians, release their vacuolar content, mainly phenoloxidase and its polyphenol substrata, upon the recognition of soluble factors diffusing from the alien colony through the partially fused tunic. In a previous paper, we demonstrated the relationship between phenoloxidase and cytotoxicity. Here, we investigated the effects of superoxide dismutase, catalase and sorbitol (scavengers of superoxide anions, peroxides and hydroxyl radicals, respectively) on the cytotoxicity observed in haemocyte cultures incubated with heterologous blood plasma. Although the above compounds have no effects on morula cell degranulation and phenoloxidase activity, they suppress cell death, suggesting that oxidative stress plays a key role in in vitro cytotoxicity. In addition, sorbitol reduces the extent of the cytotoxicity occurring in the rejection reaction between incompatible colonies, which stresses the important role of hydroxyl radicals in this process. The observation of a decrease in total and non-protein thiols in haemocytes previously incubated with heterologous blood plasma fits the hypothesis of oxidative stress as the main cause of phenoloxidase-related cytotoxicity.

Sperm-egg binding mediated by sperm alpha-L-fucosidase in the ascidian, Halocynthia roretzi

Spermatozoa bind to the vitelline coat in the ascidians and many other animals. The binding of sperm in Halocynthia roretzi is mediated by a sperm alpha-L-fucosidase and complementary-L-fucosyl residues of glycoproteins in the vitelline coat. cDNA clones for alpha-L-fucosidase were isolated from growing testis mRNA. It contained a 1398 bp full-length cDNA insert (HrFuc'ase) that encoded the 466 amino acid residues of H. roretzi sperm alpha-L-fucosidase. A putative signal peptide of 21 amino acid residues proceeded the sequence for the mature protein (M.W. 52.4 kDa). The coding sequence for HrFuc'ase showed 47.7% sequence identity to the human liver fucosidase sequence. The polyclonal antibody was prepared against a lacZ-HrFuc'ase fusion protein expressed in E. coli. The antibody crossed to a 54 kDa protein in sperm on western blotting and inhibited fertilization in a dose dependent manner. These data suggest that sperm-egg binding is mediated by the sperm alpha-L-fucosidase, HrFuc'ase in the ascidian, H. roretzi.

Effects of tributyltin and other metals on the phenoloxidase activating system of the tunicate, Styela plicata

Toxic metals, such as tributyltin (TBT), contribute substantially to anthropogenic pollution in many estuarine environments. Animals that live in those environments, particularly invertebrate filter feeders like tunicates, are likely to be exposed to substantial metal contamination. This study investigates the effects of TBT and other metals on the phenoloxidase activity of the estuarine tunicate, Styela plicata, in an effort to identify a biochemical marker of metal pollution. Hemocytes harvested from S. plicata that were exposed to tributyltin or copper in aquaria had significantly enhanced phenoloxidase activities relative to non-exposed controls. This enhanced phenoloxidase activity could be explained by an increased frequency of morula cells, which contain high levels of phenoloxidase's proenzyme, prophenoloxidase. Unlike those from tunicates exposed to metals in aquaria, the phenoloxidase activities of hemocytes incubated with tributyltin in vitro were significantly reduced when compared with hemocytes cultured without tributyltin. The ability of tributyltin to decrease phenoloxidase activity in tissue culture may reflect its known inhibitory effects on calcium-dependent signaling systems such as those involved in the exocytosis of prophenoloxidase from morula cells.

Characterization of cytochrome b(5) in the ascidian Polyandrocarpa misakiensis and budding-specific expression

A cDNA for cytochrome b(5) was cloned from a cDNA library of buds of the ascidian, Polyandrocarpa misakiensis, by a hybridization method involving a digoxigenin-labeled cDNA probe of human soluble cytochrome b(5). The nucleotide sequence of the cDNA for the ascidian cytochrome b(5) (Pmb5) consisted of about 1,800 base pairs including 5'- and 3'-noncoding regions, and a coding sequence of 405 base pairs. The amino acid sequence of 135 residues deduced from the coding nucleotide sequence exhibited 54% identity and 76% similarity to chicken cytochrome b(5). A highly conserved amino acid sequence was observed in the amino-terminal domain of 96 residues containing two heme-binding histidine residues. The putative soluble form of the recombinant Pmb5 expressed in Escherichia coli was purified to homogeneity by column chromatographies on an anion-exchanger and gel filtration. The purified Pmb5 showed the typical absorption spectrum of cytochrome b(5) with an asymmetric peak at 556 nm and a shoulder at 560 nm upon reduction with NADH and NADH-cytochrome b(5) reductase. The low temperature spectrum of the dithionite-reduced form of the protein contained the split peaks at 551 and 555 nm, this spectrum being very similar to that of mammalian liver cytochrome b(5). Expression of Pmb5 in the ascidian was examined immunohistochemically with a monoclonal antibody against the Pmb5. Apparently high level expression of Pmb5 was found in the developing buds, but the levels of cytochrome b(5) in the parents and juvenile adults were very low. This is the first report on the characterization of Pmb5, and the increased expression of Pmb5 in the ascidian.

Pigment cell-specific expression of the tyrosinase gene in ascidians has a different regulatory mechanism from vertebrates

Tyrosinase is the key enzyme required for the synthesis of melanin pigments. Sequence comparison and functional analysis of the 5' upstream regions of vertebrate tyrosinase genes have revealed the importance of conserved E-box motifs in regulating their specific expression in pigment cells, optic cup-derived retinal pigment epithelium (RPE) and neural crest-derived melanocytes. In ascidians (more basal protochordates), two pigment cells that resemble vertebrate RPE cells are formed and specifically express the orthologous tyrosinase gene (HrTyr) in the cerebral vesicle located at the anterior end of the neural tube. To define regulatory sequences required for pigment cell-lineage-specific expression of HrTyr during embryogenesis, a series of mutations of the 5' upstream region of HrTyr were fused to the lacZ reporter gene and were microinjected into fertilized eggs. We found that the -152bp upstream of the translational start site is essential for expression in pigment cell precursors of tailbud-stage embryos. Further, additional positive and unique restriction elements were identified in the region up to -1.8kb. Surprisingly, in the -152bp minimal promoter or in other regions with regulatory activities, there are no E-box motifs or sequences correlating with other conserved elements regulating vertebrate tyrosinase promoters. The possibility that Pax proteins regulate HrTyr expression is also discussed.

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.

Overlapping expression of amphioxus homologs of the thyroid transcription factor-1 gene and thyroid peroxidase gene in the endostyle: insight into evolution of the thyroid gland

The endostyle is a pharyngeal organ of uro- chordates, cephalochordates, and primitive vertebrates. This organ has iodine-concentrating and iodine-metabolism activities, and therefore the endostyle is considered to be homologous to the follicle of the thyroid gland. In higher vertebrates the genes for both thyroid transcription factor-1 (TTF-1) and thyroid peroxidase (TPO) are expressed in the thyroid gland follicle. TTF-1 regulates the expression of TPO, which encodes an iodinating enzyme associated with thyroid hormone synthesis. A recent study showed that the ascidian TTF-1 and TPO genes are specifically expressed in the endostyle, but that the expression domains of these genes are not overlapping, suggesting that ascidian TPO is not regulated by TTF-1. To investigate the molecular mechanisms involved in the formation and function of the endostyle, with special reference to the evolution of the follicle of the thyroid gland, I isolated and characterized cDNA clones for the amphioxus homologs of the TTF-1 gene (BbTTF-1) and TPO gene (BbTPO) from Branchiostoma belcheri. Reverse transcriptase-polymerase chain reaction/Southern blotting revealed that both amphioxus TTF-1 and TPO genes are expressed mainly in the adult endostyle. Spatial and temporal expression patterns assessed by in situ hybridization revealed that BbTTF-1 is expressed in the endodermal cells during early embryogenesis and is maintained in all zones of the adult endostyle. On the other hand, expression of BbTPO is chiefly in zones 5 and 6 of the adult endostyle where it overlaps with that of BbTTF-1, and to a lesser extent in zones 1 and 3. This restriction of the expression of BbTTF-1 and BbTPO to the endostyle strongly suggests that the endostyle is homologous to the follicle of the thyroid gland. Moreover, the spatial and temporal expression patterns of these genes suggest that TTF-1 regulates TPO expression. The coexpression of these genes in amphioxus suggests that regulation of TPO by TTF-1 was present in the common ancestor of cephalochordates (acraniates) and craniates.

Germ-cell warfare in ascidians: sperm from one species can interfere with the fertilization of a second species

Ascidians (invertebrate chordates) are very abundant in many marine subtidal areas. They often live in dense multispecies clumps; thus, interspecific competition for space may be intense. Although most noncolonial species are broadcast spawners, their eggs can be fertilized only by sperm of the same species (1). Multiple fertilization is lethal and all animals have evolved blocks to polyspermy. Ascidian eggs block polyspermy by enzymatic (2) and electrical mechanisms (3). Sperm bind to N-acetylglucosamine groups on the vitelline coat (4, 5, 6, 7). Follice cells surrounding the vitelline coat release N-acetylglucosaminidase during egg activation (8), preventing the binding of all sperm but a few (2). I show here that this interaction is not species-specific; sperm from one species can cause glycosidase release from follicle cells of a second species. Furthermore, once glycosidase release has been induced, the subsequent addition of sperm from the egg-producing species fails to fertilize a substantial proportion of these eggs. This leads to the hypothesis that sperm from one species of ascidian can interfere with fertilization of a second species. While intraspecific sperm competition has been well documented in several taxa (9, 10), this is the first record of sperm competition between species, or interspecific sperm competition.

A retinoic acid-inducible modular protease in budding ascidians

Retinoic acid-treated mesenchyme cells of the budding ascidian Polyandrocarpa misakiensis acquire an organizer activity to induce a secondary body axis when implanted into developing buds. We identified several different mRNAs that were upregulated in the mesenchyme cells after retinoic acid treatment. We isolated a cDNA clone corresponding to one of these mRNAs. The C-terminal region of the predicted protein product is homologous to the catalytic domain of serine proteases that belong to the trypsin family. The N-terminal region contains several types of protein-protein interaction domains. We therefore named this protein tunicate retinoic acid-inducible modular protease (TRAMP). Expression of the TRAMP mRNA in mesenchyme cells during budding and its upregulation by retinoic acid were demonstrated by reverse transcription-PCR and in situ hybridization. A glutathione S-transferase-TRAMP fusion protein showed a protease activity with trypsin-like substrate specificity and stimulated proliferation of the cell line established in this species.

Ascidian homologs of mammalian thyroid peroxidase genes are expressed in the thyroid-equivalent region of the endostyle

The endostyle is a pharyngeal organ for the internal filter feeding of urochordates, cephalochordates, and larval lamprey. This organ is also considered to be homologous to the follicular thyroid gland of higher vertebrates. Thyroglobulin (Tg) and thyroid peroxidase (TPO) are specifically expressed in the thyroid gland of higher vertebrates, and they play an important role in iodine metabolism for the synthesis of thyroid hormones. Previous histochemical observations showed that iodine-concentrating and peroxidase activities were detected in zones 7, 8, and 9 of the ascidian endostyle, suggesting that these zones contains cells that are equivalent to those in the vertebrate follicular thyroid. In order to investigate the molecular developmental mechanisms involved in the formation and function of the endostyle, with special reference to the evolution of the thyroid gland, in the present study, we isolated and characterized cDNA clones for TPO genes, CiTPO from Ciona intestinalis and HrTPO from Halocynthia roretzi. Northern blot and in situ hybridization analyses revealed that the expression of the ascidian TPO genes was restricted to zone 7, one of the elements equivalent to the thyroid. These results provide the first evidence at the gene expression level for shared function between a part of the ascidian endostyle and the vertebrate follicular thyroid gland. J. Exp. Zool. ( Mol. Dev. Evol. ) 285:158-169, 1999.

Structure and developmental expression of the ascidian TRP gene: insights into the evolution of pigment cell-specific gene expression

The tyrosinase family in vertebrates consists of three related melanogenic enzymes: tyrosinase, tyrosinase-related protein-1 (TRP-1), and TRP-2. These proteins control melanin production in pigment cells and play a crucial role in determining vertebrate coloration. We have isolated a gene from the ascidian Halocynthia roretzi which encodes a tyrosinase-related protein (HrTRP) with 45-49% identity with vertebrate TRP-1 and TRP-2. The expression of the HrTRP gene in pigment lineage a8.25 cells starts at the early-mid gastrula stage, which coincides with the stage when these cells are determined as pigment precursor cells; therefore, it provides the earliest pigment lineage-specific marker, which enables us to trace the complete cell lineage leading to two pigment cells in the larval brain. In addition, the expression pattern of the HrTRP gene appears to share similar characteristics with the mouse TRP-2 gene although structurally the HrTRP gene is more closely related to mammalian TRP-1 genes. Based on these observations and on results from molecular phylogenetic and hybridization analyses, we suggest that triplication of the tyrosinase family occurred during the early radiation of chordates. Initially, duplication of an ancestral tyrosinase gene produced a single TRP gene before the urochordate and cephalochordate-vertebrate divergence, and a subsequent duplication of the ancestral TRP gene in the vertebrate lineage gave rise to two TRP genes before the emergence of teleost fishes. Evolution of the melanin synthetic pathway and possible phylogenetic relationships among chordate pigment cells that accommodate the metabolic process are discussed. Dev Dyn 1999;215:225-237.

Opsonic complement system of the solitary ascidian, Halocynthia roretzi

To elucidate the molecular architecture and function of the possibly primitive complement system of the solitary ascidian. Halochynthia roretzi, cDNA clones for the third component (C3) and mannose-binding lectin (MBL)-associated serine protease (MASP) were isolated from the hepatopancreas cDNA library. The deduced primary structure of ascidian C3 (AsC3) shows overall similarity to mammalian C3 including a typical thioester site. Two distinct ascidian MASPs, termed AsMASPa and AsMASPb, have the same domain structure as mammalian Clr/ Cls/MASP-1/MASP-2. Both of them show a closer similarity to mammalian MASP-1 than to mammalian Clr/Cls/ MASP-2. Ascidian body fluid contains an opsonic activity which enhances phagocytosis of yeast by ascidian blood cells, and an antibody against AsC3 inhibits this opsonic activity. These results indicate that the lectin-dependent, opsonic complement system was present prior to the emergence of the vertebrates and well ahead of the establishment of adaptive immunity.

Primary structure and function of superoxide dismutase from the ascidian Halocynthia roretzi

A protein with a molecular weight of 17K, immunoreactive with the S-1B2 antibody, has been isolated from hemocytes of Halocynthia roretzi. Its amino acid sequence has been determined by sequential Edman degradation analysis of peptide fragments derived from proteolytic fragmentation. The 17K protein is a single chain protein consisting of 151 amino acids with an acylated N-terminal serine. A comparison of the amino acid sequence of H. roretzi 17K protein with those of other proteins reveals that the 17K protein is Cu,Zn-SOD. The protein was found to have a KCN-inhibited SOD activity. Cu,Zn-SOD has been purified from H. roretzi plasma. The molecular weight is 17K and the activity is inhibited with KCN and diethyldithiocarbamate. It has been demonstrated that it can enhance phagocytosis by H. roretzi hemocytes. Thus, plasma Cu,Zn-SOD plays a role in H. roretzi as a defense molecule.

Phenoloxidase and cytotoxicity in the compound ascidian Botryllus schlosseri

The vacuoles of morula cells (MC) of the colonial ascidian Botryllus schlosseri contain phenoloxidase (PO). As the release of their vacuolar content at the border of incompatible contacting colonies is associated with the formation of necrotic masses which characterize the rejection reaction, the role of PO in Botryllus cytotoxicity was investigated. When hemocytes are incubated with blood plasma from incompatible (heterologous) colonies, MC degranulate and, after 60 min, the cytotoxicity index becomes significantly greater than that observed in controls incubated with autologous plasma. The rise in cell mortality is completely inhibited by the addition of PO inhibitors sodium benzoate, tropolone and phenylthiourea, and serine protease inhibitors phenylmethylsulfonyl fluoride, benzamidine, N-tosyl-L-phenylalanine chloromethyl ketone and N-tosyl-L-lysine chloromethyl ketone. The addition of either reducing agents L-cysteine and ascorbic acid or reactive oxygen species scavenger enzymes superoxide dismutase and catalase has a similar effect. Significant inhibition of cytotoxicity is also observed with the quinone scavenger, 3-methyl-2-benzothiazolinone hydrazone. In the presence of sodium benzoate and phenylthiourea, there is a significant reduction in the number, size and color intensity of necrotic masses along the contact border of incompatible colonies. A significant increase in superoxide anion production, completely inhibited by sodium benzoate, is observed when hemocytes are incubated with heterologous blood plasma. These results indicate that: (i) PO is the enzyme responsible for the cytotoxicity observed in both hemocyte cultures and rejection reactions; (ii) PO is present inside MC vacuoles as a proenzyme which is activated, upon release, by humoral proteases; (iii) cytotoxicity appears to be mainly due to oxidative stress generated by PO during oxidation of polyphenols to quinones without the involvement of other oxidases such as NADPH oxidase and peroxidase.

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.

Ascidian phenoloxidase: its release from hemocytes, isolation, characterization and physiological roles

Hemocytes of the solitary ascidian Halocynthia roretzi released phenoloxidase in response to sheep red blood cells and yeast cells but not to latex beads. Phenoloxidase was also released from the hemocytes by treatments with zymosan and lipopolysaccharides but not with beta 1-3 glucan. EDTA scarcely inhibited the activity of phenoloxidase but inhibited the release of the enzyme. Phenoloxidase was purified from H. roretzi hemocytes by SP-Sephadex chromatography and Sephadex G-100 gel filtration. The molecular weight of the purified enzyme was estimated to be 62,000. Phenoloxidase activity was strongly inhibited by diethyldithiocarbamate, phenylthiourea and reducing agents. H. roretzi phenoloxidase was characterized as a metalloenzyme that required copper ions for the expression of full activity. The phenoloxidase showed antibacterial activity in the presence of L-(3,4-dihydroxy)-phenylalanine and H. roretzi plasma. Thus, it can be concluded that phenoloxidase released from H. roretzi hemocytes functions as a humoral factor in the defense system of H. roretzi.

Lactate dehydrogenase (LDH) gene duplication during chordate evolution: the cDNA sequence of the LDH of the tunicate Styela plicata

L-Lactate dehydrogenase (L-LDH, E.C. 1.1.1.27) is encoded by two or three loci in all vertebrates examined, with the exception of lampreys, which have a single LDH locus. Biochemical characterizations of LDH proteins have suggested that a gene duplication early in vertebrate evolution gave rise to Ldh-A and Ldh-B and that an additional locus, Ldh-C arose in a number of lineages more recently. Although some phylogenetic studies of LDH protein sequences have supported this pattern of gene duplication, others have contradicted it. In particular, a number of studies have suggested that Ldh-C represents the earliest divergence among vertebrate LDHs and that it may have diverged from the other loci well before the origin of vertebrates. Such hypotheses make explicit statements about the relationship of vertebrate and invertebrate LDHs, but to date, no closely related invertebrate LDH sequences have been available for comparison. We have attempted to provide further data on the timing of gene duplications leading to multiple vertebrate LDHs by determining the cDNA sequence of the LDH of the tunicate Styela plicata. Phylogenetic analyses of this and other LDH sequences provide strong support for the duplications giving rise to multiple vertebrate LDHs having occurred after vertebrates diverged from tunicates. The timing of these LDH duplications is consistent with data from a number of other gene families suggesting widespread gene duplication near the origin of vertebrates. With respect to the relationships among vertebrate LDHs, our data are not consistent with previous claims that Ldh-C represented the earliest divergence. However, the precise relationships among some of the main lineages of vertebrate LDHs were not resolved in our analyses.

Phenoloxidase-dependent cytotoxic mechanism in ascidian (Styela plicata) hemocytes active against erythrocytes and K562 tumor cells

The cytotoxic activity against rabbit erythrocytes (RE) and human K562 tumor cells by Styela plicata hemocytes was significantly related to the phenoloxidase (PO) which converts phenols to quinone and initiates the melanogenic pathway. The effector hemocyte population, separated in a Percoll density gradient band, enriched in a granulocyte type named "morula cells", was examined with RE in a hemocyte cytotoxic assay and plaque forming cell assay. Inhibition experiments with the copper chelating agents 1-phenyl-2-thiourea and tropolone, the substrate analogue sodium benzoate and sodium ascorbate support the notion that hemocyte cytotoxic activity is a PO-dependent mechanism. Treatments of hemocytes with the antioxidant enzymes, superoxide dismutase and catalase rule out oxy radicals produced by the melanogenic process as responsible of erythrolysis. Such a result suggests that quinone compounds derived from the melanogenic pathway might be the cytotoxic molecules. The PO-dependent anti-RE activity was also shown in a plaque forming assay in which "morula cells", containing polyphenols and PO, were identified as cytotoxic.

Ascidian actin genes: developmental regulation of gene expression and molecular evolution

Actin is a ubiquitous protein in eukaryotic cells and plays an important role in cell structure, cell motility, and the generation of contractile force in both muscle and nonmuscle cells. Multiple genes encoding muscle or nonmuscle actins have been isolated from several species of ascidians and their expression patterns have been investigated. Sequence and expression analyses of muscle actin genes have shown that ascidians have at least two distinct isoforms of muscle actin, the larval muscle and body-wall isoforms. In the ascidian Halocynthia roretzi, two clusters of actin genes are expressed in the larval muscle cells. The HrMA2/4 cluster contains at least five actin genes and the HrMA1 cluster contains a pair of actin genes whose expression is regulated by a single bidirectional promoter. cis-Regulatory elements essential for muscle-specific expression of a larval muscle actin gene HrMA4a have been identified. The adult body-wall muscle actin is clearly distinguished from the larval muscle actin by diagnostic amino acids. The adult muscle actin genes may be useful tools to investigate the mechanisms of muscle development in ascidian adults. The evolution of chordate actin genes has been inferred by comparing the organization and sequences of actin genes and performing molecular phylogenetic analysis. The results suggest a close relationship between ascidian and vertebrate actins. The chordate ancestor seems to have evolved the "chordate-type" cytoplasmic and muscle actins before its divergence into vertebrates and urochordates. The phylogenetic analysis also suggests that the vertebrate muscle actin isoforms evolved after the separation of the vertebrates and urochordates. Muscle actin genes have been used to investigate the mechanism of muscle cell regression during the evolution of anural development. The results suggest that the regression of muscle cell differentiation is mediated by changes in the structure of muscle actin genes rather than in the trans-acting regulatory factors required for their expression. Actin genes have provided a unique system to study developmental and evolutionary mechanisms in chordates.

Evidence for cell surface and internal phospholipase activity in ascidian eggs

Upon fertilization, ascidian eggs release a cell surface glycosidase used in the block to polyspermy and undergo cortical contractions resulting from increased intracellular calcium levels. The glycosidase is released by fertilization, calcium ionophores or added phospholipase C (PLC) activity. The PLC inhibitor D609 blocks glycosidase release. Intact Ascidia ceratodes eggs cleave 4-methylumbelliferyl-phospho-choline when it is added to seawater. This yields highly fluorescent 4-methylumbelliferone. Authentic phospholipase C but not phospholipase D can cleave this substrate. Thus, the authors believe that cleavage of the substrate is specific for PLC activity. Eggs incubated in the fluorogenic substrate after having been washed and detergent extracted were not fluorescent. Therefore the substrate failed to enter intact cells. Glycosidase release and PLC activity were stimulated by ionomycin. Octylglucoside or Triton X-100 extracts of ascidian eggs had two forms of phospholipase activity as shown by ion affinity chromatography: PL1 eluting at 0.25 mol/L NaCl and PL2 eluting at 0.6 mol/L NaCl. The PL1 appeared to be isolated as a single protein. When surface proteins were labeled with non-penetrating biotin and were subsequently reacted with streptavidin, half of the PLC activity bound. This demonstrates that half the ascidian egg PLC activity is located on the surface of either the egg or follicle cell, and half is located within the egg.

Purification and characterization of a 39,000-Da serine proteinase from the hemolymph of a solitary ascidian, Halocynthia roretzi

A new endogenous serine proteinase from the cell-free hemolymph of a solitary ascidian, Halocythia roretzi, was purified by a combination of ammonium sulfate fractionation, hydrophobic interaction chromatography on TSKgel Toyopearl HW 65 F, ion exchange chromatography on TSKgel DEAE-Toyopearl 650 M, affinity chromatography on Arginine-Sepharose 4B, gel filtration on TSKgel Toyopearl HW 65F and hydroxyapatite chromatography on Bio-Gel HT. The serine proteinase is a single polypeptide chain whose molecular weight and isoelectric point are 39 kDa and about 7.6 pI, respectively. The most susceptible substrate was Boc-Leu-Gly-Arg-4-methyl-coumaryl-7-amide (MCA), and activity was optimal at pH 8. The enzyme was relatively stable at high temperatures; about 50% activity was retained even at 60 degrees C for 30 min in 50 mM Tris-HCl, pH 8.0, containing 0.5 M NaCl, and 0.05% Brij-35. The enzyme was characterized by the inhibitory effects of synthetic or natural inhibitors, substrate specificity toward 26 peptidyl-MCAs, proteinase activity toward natural proteins and complex formation with a serine proteinase inhibitor (58 kDa) previously found in H. roretzi hemolymph, indicating that the enzyme was a member of serine proteinases and strongly inhibited by the 58 kDa serine proteinase inhibitor as well as human antithrombin III. We also demonstrated the clotting enzyme activity of the purified serine proteinase toward bovine fibrinogen and Limulus coagulogen, a fibrinogen-like clottable protein of horseshoe crabs.