Immunolocation of phenoloxidase in vacuoles of the compound ascidianBotryllus schlosserimorula cells

Functional amyloidogenesis in immunocytes from the colonial ascidian Botryllus schlosseri: Evolutionary perspective

Cytotoxic morula cells (MCs) and phagocytes are the circulating immunocytes of the colonial ascidian Botryllus schlosseri: Both these cells can synthesise amyloid fibrils, supporting the idea that physiological amyloidogenesis is involved in inflammation and modulation of immune responses. Intriguingly, amyloid of B. schlosseri immunocytes is made of two different proteins. MCs, the first cells to sense non-self and involved in the allorejection reaction between contacting genetically incompatible colonies, use melanin encapsulation as the principal method to fight non-self. They release amyloid fibrils formed by p102 protein that allow the packaging and deposit of melanin and other toxic molecules nearby the invader or in the contact region of incompatible colonies. Phagocytes release amyloid-based extracellular traps when challenged with microbes: their amyloid fibrils harbour BsAPP, an orthologue of the vertebrate amyloidogeneic protein APP. This strategy of immune response, present also in human neutrophils, allows phagocytes to block and engulf bacteria and fungi.

Looking for putative phenoloxidases of compound ascidians: haemocyanin-like proteins in Polyandrocarpa misakiensis and Botryllus schlosseri

Phenoloxidases (POs) and haemocyanins constitute a family of copper-containing proteins widely distributed among invertebrates. Both of them are able, under appropriate conditions, to convert polyphenols to quinones and induce cytotoxicity through the production of reactive oxygen species, a fundamental event in many immune responses. In ascidians, PO activity has been described and studied in both solitary and colonial species and the enzyme is involved in inflammatory and cytotoxic reactions against foreign cells or molecules, and in the formation of the cytotoxic foci which characterise the nonfusion reaction of botryllids. Expressed genes for two putative POs (CiPO1 and CiPO2) have been recently identified in C. intestinalis. In the present study, we determined the cDNA sequences of two haemocyanin-like proteins from two colonial ascidians: Botryllus schlosseri from the Mediterranean Sea and Polyandrocarpa misakiensis from Japan. Multiple sequence alignments evidenced the similarity between the above sequences and crustacean proPOs whereas the analysis of the three-dimensional structure reveals high similarity with arthropod haemocyanins which share common precursors with arthropod proPOs. Botryllus HLP grouped in the same cluster with Ciona POs, whereas Polyandrocarpa HLP clustered with arthropod haemocyanins; all of them share the full conservation of the six histidines at the two copper-binding sites as well as of other motifs, also found in arthropod haemocyanin subunits, involved in the regulation of enzyme activity. In situ hybridisation indicated that the genes are transcribed inside morula cells, a characteristic haemocyte type in ascidians where PO activity is located, at the beginning of their differentiation. These results represent a first attempt to identify candidate molecules responsible of the PO activity in compound ascidians.

Structure, biosynthesis and possible function of tunichromes and related compounds

Several species of ascidians (phylum Chordata, subphylum Urochordata) contain a group of oligopeptides called "tunichromes" in their blood cells. These peptides have been implicated in (a) metal chelation and accumulation/sequestration of vanadium or iron; (b) crosslinking of structural fibers in tunic formation, (c) wound healing and (d) defense reactions. However, their biosynthesis, metabolism, and biological function remain largely un-elucidated due to their extreme instability and high reactivity. Tunichromes and related compounds uniquely possess dehydrodopamine moieties, all originating from post-translational modification of peptidyl tyrosine. It is conceivable that the presence of such novel post-translationally modified groups provide attributes that are crucial for their biological roles. Therefore, we examined the chemistry and reactivity of tunichromes in light of the available knowledge of the biochemistry of simple monomeric dehydro-N-acyldopamine units. Based on the reactivity of such simple compounds, the potential biological activities of tunichromes are predicted. Their possible biosynthetic route from peptidyl tyrosine is critically evaluated to provide a better basis for unraveling their biological functions. Prevalence of dehydro-N-acyldopamine units in different tunichromes, some marine antibiotic compounds, insect cuticular sclerotizing precursors and some bioadhesive marine proteins may aid in the de novo design of unique biomaterials with potential antibiotic/adhesive properties.

Rejection patterns in botryllid ascidian immunity: the first tier of allorecognition

Botryllid ascidians, a small but geographically widely distributed group of compound tunicates, are being used as a model system for the study of allorecognition. Botryllid ascidians possess a unique type of immunity. Pairs of colonies that meet through their extending ampullae either fuse to form a chimera or develop cytotoxic lesions at contact zones (rejection). This first tier of allorecognition is succeeded (in cases of fusion) by two additional tiers, not reviewed here (the colony resorption phenomenon and the phenomenon of somatic and germ cell parasitism). Fusion and rejection are controlled by a single highly polymorphic gene locus termed the fusibility/histocompatibility (Fu/HC) locus. One shared allele on the Fu/HC locus is enough for fusion. Rejecting colonies do not share any Fu/HC alleles. To date, 14 botryllid ascidians have been studied for their fusibility patterns; of these, the cosmopolitan species Botryllus schlosseri (Pallas, 1766) has emerged as the most studied taxon. This review summarizes studies revealing the various types of noncompatible responses that are expressed following the application of the "colony allorecognition assay" and the "cut surface assay". These include divergent alloresponses related to different populations of the same botryllid species, distinctive allorecognition sites, polymorphism and a repertoire of Fu/HC alleles, a state of low responsiveness as opposed to the expected immunological memory, the retreat growth phenomenon, and the irreversible nature of the rejection process. A detailed description of the accumulated knowledge on the effector cells (morula cells and macrophages), the humoral and cellular molecules (at the biochemical and molecular levels), and the prophenoloxidase system is given. Links between allogeneic responses and the evolutionary ecology of botryllid ascidians are revealed. Since tunicates occupy a key phylogenetic position in the origin of the vertebrates, the study of colony allorecognition in this group may shed light on self/nonself recognition elements in other multicellular organisms, including vertebrates.

Colonial allorecognition, hemolytic rejection, and viviparity in botryllid ascidians

Allorecognition is a fundamental system that animals use to maintain individuality. Although embryos are usually semiallogeneic with their mother, viviparous animals are required to allow these embryos to develop inside the maternal body, but must also eliminate an "invasion" by nonself. In colonial ascidians of the family Botryllidae, when two colonies are brought into contact at their growing edges, a hemolytic rejection reaction occurs between allogeneic colonies. Morula cells, a type of hemocyte, are the major effector cells in the hemolytic rejection. Morula cells infiltrate and aggregate where the two colonies make contact, and then discharge their vacuolar contents, which contain phenoloxidase and quinones. In viviparous botryllids, colonial contact at artificially cut surfaces always results in colonial fusion and establishment of a common vascular network even between allogeneic colonies in which the growing-edge contact results in rejection. This colonial fusion between incompatible colonies (surgical fusion) suggests that the allorecognition sites are not distributed in the vascular system in which the embryos are brooded. It is supposed that a common ancestor of the viviparous species lost the capacity for allorecognition in their vascular system to protect its embryos from alloreactivity, when it changed from ovoviviparous to viviparous in the course of evolution. The limited distribution of allorecognition sites would be a solution to the embryo-parent histoincompatibility in viviparity.