Antiviral and antitumor compounds from tunicates

Tunicates provide a rich source of biologically active compounds with potentially useful medicinal properties. The most interesting compounds identified thus far are the didemnins, depsipeptides from a Caribbean Trididemnum species, which are potent inhibitors of L1210 leukemia cells in vitro and are also active in vivo against P388 leukemia and B16 melanoma. In addition the didemnins inhibit growth of a variety of RNA and DNA viruses in vitro and protect mice infected intravaginally with herpes simplex virus type 2. Didemnin B, a derivative of didemnin A, is far more active than A, which argues for the likelihood of further useful chemical modifications in the series.     

Galactan sulfate from the test of tunicates

Methyl and benzyl 3-O-β-d-xylopyranosyl-α-d-mannopyranoside were prepared by way of d-xylosylation (Koenigs-Knorr) of methyl and benzyl 4,6-O-benzylidene-α-d-mannopyranoside (1 and 17). Analogous 2-O-β-d-xylopyranosyl-α-d-mannopyranosides could not be prepared efficiently by this procedure. However, methyl and benzyl 3-O-acetyl-4,6-O-benzylidene-α-d-mannopyranoside, prepared by limited acetylation of 1 and 17, respectively, could be d-xylosylated by the same method, and afforded, after removal of protective groups, methyl and benzyl 2-O-β-d-xylopyranosyl-α-d-mannopyranoside. Hydrogenolysis of benzyl 2-O- and 3-O-β-d-xylopyranosyl-α-d-mannopyranoside yielded the corresponding, reducing disaccharides. In addition to these disaccharides, disaccharides containing an α-d-xylopyranosyl group, and trisaccharides having d-xylopyranosyl groups at both O-2 and O-3 were obtained as minor products.     

Unconventional signalling in tunicates

The zooids in colonial tunicates do not appear to be directly interconnected by nerves, but this has not prevented the evolution of coordinated behaviour in several groups. In Botryllus and other colonial styelid asci‐dians the endothelium lining the blood vessels is excitable and transmits action potentials from cell to cell via gap junctions. These signals mediate protective contractions of the zooids and synchronize contractions of the vascular ampullae. In didemnid ascidians such as Diplosoma a network of myocytes in the tunic serves to transmit excitation and to cause contractions of the cloacal apertures. Individual zooids of Pyrosoma protect themselves by closing their siphons and arresting their branchial cilia when stimulated. At the same time a flash of light is emitted. Neighbouring zooids sense the flash with their photoreceptors and respond in turn with protective responses and light emission. Protective responses thus spread by photic signalling and propagate from zooid to zooid through the colony in a saltatory manner. In chains of Salpafusifortnis, changes in the direction and/or speed of swimming are transmitted from zooid to zooid via adhesion plaques. When a zooid is stimulated, its body‐wall epithelium conducts action potentials to the plaque connecting it to the next zooid, exciting receptor neurons in that zooid. These receptors have sensory processes that bridge the gap between the two zooids. The sensory neurons so excited in the second zooid conduct impulses to the brain where they alter the motor output pattern, and at the same time generate epithelial action potentials that travel to the next zooid in line, where the same thing happens.

It is not clear why these unconventional signalling methods have evolved but the tunic may be an inhospitable environment for nerves, making conventional nervous links impossible.     

Body muscle-cell differentiation from coelomic stem cells in colonial tunicates

Body muscle-cell differentiation was ultrastructurally examined in palleal buds of the colonial tunicate Symplegma reptans. Undifferentiated coelomic cells accumulate near the primordial oral siphon and associate with the basal lamina beneath the epidermis. They initially display the characteristics of hemoblast cells that have a large nucleus with a prominent nucleolus and narrow cytoplasm filled with polysomes. However, they soon become unique due to the development of an indented contour of the nucleus. When the basal lamina of the epidermis develops into the fibrous extracellular matrix (ECM), the muscle precursor cell has the deeply-notched nucleus, and thick and thin filaments in the cytoplasm facing the ECM. Collagen fibril-like structures appear in the ECM. Myofilaments are arranged with the ratio of thick to thin filaments being 1:2.5. Dense bodies and plaques become evident before the oral siphon is perforated. These results show that in S. reptans, the sphincter muscle cells arise from undifferentiated hemoblasts, and that their differentiation begins with a morphological change in their nuclei. Epidermal cells and/or the ECM may have an inductive effect on muscle cell differentiation.     

Natural Peptide antibiotics from tunicates: structures, functions and potential uses

Because tunicates rely on innate immunity, their hemocytes areimportant contributors to host defense. Styela clava, a solitaryascidian, have eight hemocyte subtypes. Extracts of their totalhemocyte population contained multiple small (2–4 kDa)antimicrobial peptides. When purified, these fell into two distinctfamilies that were named styelins and clavanins. Styelins A-E are phenylalanine-rich, 32 residue peptides withactivity against marine bacteria and human pathogens. They showconsiderable sequence homology to pleurocidins, antimicrobialpeptides of the flounder, Pseudopleuronectes americanus. StyelinD, one of the five styelins identified by peptide isolationand cDNA cloning, was remarkable in containing 12 post-translationallymodified residues, including a 6-bromotryptophan, two monohydroxylysines,four 3,4-dihydroxyphenylalanines (DOPA), four dihydroxylysinesand one dihydroxyarginine. These modifications enhanced StyelinD's bactericidal ability at acidic pH and high salinity. A novelhistochemical stain for DOPA suggested that Styelin D was restrictedto granulocytes. Clavanins A-E are histidine-rich, 23 residue peptides that areC-terminally amidated and most effective at acidic pH. Clavaspirinis a newly described family member that also has potent cytotoxicproperties. By immunocytochemistry, clavanins were identifiedin the granules of five eosinophilic granulocyte subtypes andin macrophage cytoplasm. Transmission and scanning electron micrographs of methicillin-resistantStaphylococcus aureus (MRSA) and E. coli that had been treatedwith Styelin D and clavaspirin suggested that both peptidesinduced osmotic disregulation. Treated bacteria manifested cytoplasmicswelling and extrusion of cytoplasmic contents through theirpeptidoglycan cell wall. The diverse array of antimicrobialpeptides in S. clava hemocytes constitutes an effective hostdefense mechanism.     

High-resolution vertical profiles of pelagic tunicates

The objectives of our research were to obtain high-resolutionvertical profiles of the abundance of pelagic tunicates andto relate their distribution to physical and biological variables.The abundance of doholids (Tunicata: Thaliacea) from near thesurface to near the seafloor was determined by obtaining continuousvideotape recording from a descending submersible. These verticalprofiles, accompanied by temperature and fluorescence measurements,revealed (i) pronounced changes in dololiolid concentrationwithin 2 m depth intervals, and (ii) two general modes of verticaldistribution, one characterized by a peak in abundance in thethennocline and the second by a more or less even distributionthroughout much of the water column. This in situ approach canprovide estimates of zooplankton abundance in near real-time     

Amphioxus and tunicates as evolutionary model systems

One important question in evolutionary biology concerns the origin of vertebrates from invertebrates. The current consensus is that the proximate ancestor of vertebrates was an invertebrate chordate. Today, the invertebrate chordates comprise cephalochordates (amphioxus) and tunicates (each a subphylum in the phylum Chordata, which also includes the vertebrate subphylum). It was widely accepted that, within the chordates, tunicates represent the sister group of a clade of cephalochordates plus vertebrates. However, recent studies suggest that the evolutionary positions of tunicates and cephalochordates should be reversed, the implications of which are considered here. We also review the two major groups of invertebrate chordates and compare relative advantages (and disadvantages) of each as model systems for elucidating the origin of the vertebrates.     

Origin and evolution of the vertebrate leukocyte receptors: the lesson from tunicates

Two selected receptor genes of the immunoglobulin superfamily (IgSF), one CTX/JAM family member, and one poliovirus receptor-like nectin that have features of adhesion molecules can be expressed by Ciona hemocytes, the effectors of immunity. They can also be expressed in the nervous system (CTX/JAM) and in the ovary (nectin). The genes encoding these receptors are located among one set of genes, spread over Ciona chromosomes 4 and 10, and containing other IgSF members homologous to those encoded by genes present in a tetrad of human (1, 3 + X, 11, 21 + 19q) or bird chromosomes (1, 4, 24, 31) that include the leukocyte receptor complex. It is proposed that this tetrad is due to the two rounds of duplication that affected a single prevertebrate ancestral region containing a primordial leukocyte receptor complex involved in immunity and other developmental regulatory functions. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

Involvement of vasa homolog in germline recruitment from coelomic stem cells in budding tunicates

We investigated the mechanism by which germline cells are recruited in every asexual reproductive cycle of the budding tunicate Polyandrocarpa misakiensis using a vasa homolog (PmVas) as the germline-specific probe. A presumptive gonad of Polyandrocarpa arose as a loose cell aggregate in the ventral hemocoel of a 1-week-old developing zooid. It developed into a compact clump of cells and then separated into two lobes, each differentiating into the ovary and the testis. The ovarian tube that was formed at the bottom of the ovary embedded the oogonia and juvenile oocytes, forming the germinal epithelium. PmVas was expressed strongly by loose cell aggregates, compact clumps, and peripheral germ cells in the testis and germinal epithelium. No signals were detected in growing buds and less than 1-week-old zooids, indicating that germ cells arise de novo in developing zooids of P. misakiensis. Cells of the loose cell aggregates were 5–6 μm in diameter. They looked like undifferentiated hemoblasts in the hemocoel. To examine the involvement of PmVas in the germline recruitment at postembryonic stages, both growing buds and 1-week-old developing zooids were soaked with double-stranded PmVas RNA. The growing buds developed into fertile zooids expressing PmVas, whereas the 1-week-old zooids developed into sterile zooids that did not express PmVas. In controls (1-week-old zooids) soaked with double-stranded lacZ RNA, the gonad developed normally. These results strongly suggest that in P. misakiensis, PmVas plays a decisive role in switching from coelomic stem cells to germ cells.     

Characterization of interleukin-1 activity in tunicates

1. Eight North American species of tunicates were examined for the presence of interleukin-1 (IL-1) like activity. 2. The tunicates studied produce molecules with readily detectable lymphocyte activation factor (LAF) activity. 3. G50 column chromatography separated molecular species that were directly mitogenic for thymocytes (mol. wt greater than 50,000) from those that were comitogenic in an IL-1 assay (mol. wt 20,000). 4. Tunicate fractions with LAF activity induced increased vascular permeability in rabbit skin. 5. Tunicate LAF activity was neutralized by polyclonal anti-human IL-1 antisera. 6. These data further support the conclusion that IL-1 is an ancient and functionally conserved molecule.     

Evolutionary crossroads in developmental biology: the tunicates

The tunicates, or urochordates, constitute a large group of marine animals whose recent common ancestry with vertebrates is reflected in the tadpole-like larvae of most tunicates. Their diversity and key phylogenetic position are enhanced, from a research viewpoint, by anatomically simple and transparent embryos, compact rapidly evolving genomes, and the availability of powerful experimental and computational tools with which to study these organisms. Tunicates are thus a powerful system for exploring chordate evolution and how extreme variation in genome sequence and gene regulatory network architecture is compatible with the preservation of an ancestral chordate body plan.     

Retinoid X receptor-mediated transdifferentiation cascade in budding tunicates

In the budding tunicate, Polyandrocarpa misakiensis, retinoic acid (RA) applied to buds promotes transdifferentiation of somatic cells to form the secondary body axis. This study investigated the gene cascade regulating such RA-triggered transdifferentiation in tunicates. Genes encoding retinoic acid receptor (RAR) and retinoid X receptor (RXR) were induced during transdifferentiation, and they responded to all-trans RA or 13-cis RA in vivo, whereas 9-cis RA had the least effects, demonstrating differences in the ligand preference between budding tunicates and vertebrates. In contrast to RAR mRNA, RXR mRNA could induce transdifferentiation-related genes such as RXR itself, ERK, and MYC in an RA-dependent manner and also induced β-catenin (β-CTN) RA-independently when it was introduced in vitro into tunicate cell lines that do not express endogenous RAR or RXR. Small interfering RNA (siRNA) of RXR dramatically attenuated not only RXR but also ERK and β-CTN gene activities. An ERK inhibitor severely blocked wound healing and dedifferentiation. β-CTN siRNA suppressed morphogenesis and redifferentiation, similar to RXR siRNA. These results indicate that in P. misakiensis, the main function of RA is to trigger positive feedback regulation of RXR rather than to activate RAR for unlocking downstream pathways for transdifferentiation. Our results may reflect an ancient mode of RA signaling in chordates.     

浮游被囊动物的分类及其生态学研究进展

被囊动物(Tunicata)是一类低等脊索动物,包括3个纲:有尾纲、海樽纲和海鞘纲;全部生活在海洋里,其中有尾纲和海樽纲营浮游生活。综述了国内外浮游被囊动物分类和生态研究的现状和进展,综述介绍了有尾纲和海樽纲的分类依据、研究现状、趋势和在海洋生态系统中的作用。浮游被囊动物是热带和亚热带海域重要的浮游动物类群,种类和数量的分布变化受物理和生物环境因素的影响;它一方面大量摄食浮游细菌和微小浮游植物,另一方面被一些经济动物摄食,因此在海洋食物链的传递和生态系统的物质循环中占有重要位置。     

Structural heterogeneity among unique sulfated L-galactans from different species of ascidians (tunicates)

The sulfated polysaccharides that occur in the tunic of ascidians differ markedly in molecular weight and chemical composition. A high molecular weight fraction (F-1), which has a high galactose content and a strong negative optical rotation, is present in all species. Several structural differences were observed among the F-1 fractions obtained from three species of ascidians that were studied in detail. Large numbers of alpha-L-galactopyranose residues sulfated at position 3 and linked glycosidically through position 1----4 are present in F-1 from all three ascidians. However, alpha-L-galactopyranose units, 1----3-linked and partially sulfated at position 4, comprise about half of the sugar units in the central core of F-1 from Ascidian nigra. In addition, L-galactopyranose nonreducing end units occur in F-1 from Styela plicata and A. nigra, but comprise only a minor fraction of F-1 from Clavelina sp. The combination of these various component units gives a complex structure for F-1 from S. plicata and A. nigra, whereas F-1 from Clavelina sp. possesses a simpler structure. The structures of these ascidian glycans are unique among all previously described sulfated polysaccharides, since they are highly branched (except that from Clavelina sp), sulfated at position 3, and contain large amounts of L-galactose without its D-enantiomorph. These data show unusual examples of polyanionic glycans with structural function in animal tissues.     

Chemical defenses of tunicates of the genus Aplidium from the Weddell Sea (Antarctica)

Predation and competition are important factors structuring Antarctic benthic communities and are expected to promote the production of chemical defenses. Tunicates are subject to little predation, and this is often attributed to chemical compounds, although their defensive activity has been poorly demonstrated against sympatric predators. In fact, these animals, particularly the genus Aplidium, are rich sources of bioactive metabolites. In this study, we report the natural products, distribution and ecological activity of two Aplidium ascidian species from the Weddell Sea (Antarctica). In our investigation, organic extracts obtained from external and internal tissues of specimens of A. falklandicum demonstrated to contain deterrent agents that caused repellency against the Antarctic omnivorous predator, the sea star Odontaster validus. Chemical analysis performed with Antarctic colonial ascidians Aplidium meridianum and Aplidium falklandicum allowed the purification of a group of known bioactive indole alkaloids, meridianins A-G. These isolated compounds proved to be responsible for the deterrent activity.