Molecular Patterning of the Oikoplastic Epithelium of the Larvacean Tunicate Oikopleura dioica

Abstract

Appendicularia are protochordates that rely on a complex mucous secretion, the house, to filter food particles from seawater. A monolayer of cells covering the trunk of the animal, the oikoplastic epithelium, secretes the house. This epithelium contains a fixed number of cells arranged in characteristic patterns with distinct sizes and nuclear morphologies. Certain house structures appear to be spatially related to defined, underlying groups of cells in the epithelium. We show that the house is composed of at least 20 polypeptides, a number of which are highly glycosylated, with glycosidase treatments resulting in molecular mass shifts exceeding 100 kDa. Nanoelectrospray tandem mass spectrometric microsequencing of house polypeptides was used to design oligonucleotides to screen an adult Oikopleura dioica cDNA library. This resulted in the isolation of cDNAs coding for three different proteins, oikosin 1, oikosin 2, and oikosin 3. The latter two are novel proteins unrelated to any known data base entries. Oikosin 1 has 13 repeats of a Cys domain, previously identified as a subunit of repeating sequences in some vertebrate mucins. We also find one repeat of this Cys domain in human cartilage intermediate layer protein but find no evidence of this domain in any invertebrate species, including those for which entire genomes have been sequenced. The three oikosins show distinct and complementary expression patterns restricted to the oikoplastic epithelium. This easily accessible epithelium, with differential gene expression patterns in readily identifiable groups of cells with distinctive nuclear morphologies, is a highly attractive model system for molecular studies of pattern formation.Appendicularia, or larvaceans, are pelagic tunicates (Urochordata) that feed on dissolved organic carbon and microorganisms by filtering seawater through a transparent mucous structure called the house. The name "larvaceans" derives from the fact that the adult animal resembles the larval form more closely than in the other two classes of Urochordata. In the latter two classes, metamorphosis drastically redesigns the body plan, whereas in appendicularia, it consists of a simple switch in tail position (tailshift) from a straight posteriorly directed orientation to a definitive arrangement, where the tail is orthogonal to the trunk and retains the notochord as its axial structure. For this reason, appendicularia are thought to have diverged earlier from the chordate ancestor than their sister classes, and recent phylogenetic analyses of 28 and 18 S rRNA genes confirm this view (1, 2). Appendicularia have three features common to all chordates at some stage in the life cycle: gill slits, a tubular nerve chord, and a rod-shaped notochord.

Food limitation and growth in temperate epipelagic appendicularians (Tunicata)

“…However, house production can represent a high percentage of the biomass (Sato et al 2001) and should therefore be included in growth rate estimates. While house production (discarding an old house and expanding a new one) is an episodic event, house secretion (secretion of the mucous sleeve that covers the oikoblastic epithelium) is a continuous process (Fenaux 1985, Flood & Deibel 1998, Spada et al 2001. Thus, house secretion represents an energy compartment into which a proportion of the total growth is continuously devoted.…”

Histone mRNAs Do Not Accumulate during S Phase of either Mitotic or Endoreduplicative Cycles in the ChordateOikopleura dioica

“…Furthermore, higher-molecular-weight proteins, such as the ␤-actin (50 kDa) internal control, were revealed on immunoblots at all stages. Likely candidates for the problem with specific detection of low-molecular-weight, basic histones on Western blots of postmetamorphic stages were the acidic mucopolysaccharides (24) that are massively secreted from the oikoplastic epithelial cells for repetitive building of the house structure from days 1 to 5. Different attempts at partial purification to relieve blocking of histone detection while still maintaining quantitative recovery of the internal ␤-actin control were unsuccessful.…”

Genome Reference and Sequence Variation in the Large Repetitive Central Exon of Human MUC5AC

“…The CysD domains, which were shown by this work to vary in number between individuals, are highly conserved across many levels of eukaryotic taxonomies (42,44). Although the 10 cysteines within these domains are thought to be involved in intramolecular disulfide bonds (45), the specific function of these domains has not been established.…”