Takaharu Numakunai

Evidence for participation of sperm proteinases in fertilization of the solitary ascidian, Halocynthia roretzi: effects of protease inhibitors.

Abstract Effects of 15 proteinase inhibitors and an inhibitor against aminopeptidases on fertilization of the solitary ascidian, Halocynthia roretzi were studied in search of lysins. Fertilization of intact eggs was blocked by three trypsin inhibitors, leupeptin, antipain, and soybean trypsin inhibitor, and by two chymotrypsin inhibitors, chymostatin and potato proteinase inhibitor I. On the other hand, the fertilization of naked eggs was not blocked at all by leupeptin and was only partially blocked by chymostatin at the concentrations sufficient for blocking that of intact eggs. This indicates that spermatozoa utilize trypsin-like and chymotrypsin-like proteinases probably as lysins for penetrating through the chorion. The chymotrypsin-like activity appears to be also required for some step besides sperm penetration through the egg investments.

Ascidian tyrosinase gene: its unique structure and expression in the developing brain.

Tadpole larvae of ascidians have two sensory pigment cells in the brain. One is the otolith cell that functions as a gravity receptor, the other pigment cell is part of a primitive photosensory structure termed the ocellus. These sensory cells, like vertebrate pigment cells, contain membrane‐bounded melanin granules and are considered to reflect a crucial position in the evolutionary process of this cell type. To investigate the molecular changes accompanying the evolution of pigment cells, we have isolated from Halocynthia roretzi a gene encoding tyrosinase, a key enzyme in melanin biosynthesis. The cDNA has an open reading frame (ORF) of 596 amino acids, which is 36–39% identical in amino acid sequence to vertebrate tyrosinases. In addition, the sequence analysis of both cDNA and genomic clones reveals an unusual organization of the tyrosinase gene, an extraordinary 3′ untranslated region of the transcripts with significant homology to the coding sequence, and a single short intron in the sequence encoding a cytoplasmic domain. Expression of the gene is detected first in two pigment precursor cells positioned in the neural plate of early neurulae, and later in two melanin‐containing pigment cells within the brain of late tailbud embryos. Its expression pattern correlates well with the appearance of tyrosinase enzyme activity in the developing brain. These results provide the first description of pigment cell differentiation at the molecular level in the ascidian embryo, and also will contribute to a better understanding of the evolution of chordate pigment cells. Dev. Dyn. 208:363–374, 1997.

Galactose-specific lectin in the hemolymph of solitary ascidian, Halocynthia roretzi: Isolation and characterization

Abstract The lectin from the hemolymph of solitary ascidian, Halocynthia roretzi, has been isolated in an electrophoretically homogeneous form by affinity chromatography on a column of acid-treated Sepharose. It is a large protein with s20,w=24 S, composed of subunits with a molecular weight of 41,000. D-Galactose and various disaccharides containing D-galactose inhibit the hemagglutinating activity of the lectin. Thus, H. roretzi lectin is a D-galactose-specific lectin.

Expression of the Otx gene in the ciliary bands during sea cucumber embryogenesis

Summary: The Otx gene encodes a homeodomain transcription factor that has a highly conserved role in brain formation of both flies and vertebrates. To deduce evolutionary relationship of the chordate central nervous system to the larval or adult nervous system of nonchordate deuterostomes, we characterized the expression of the Otx gene (Sj‐Otx) throughout the entire embryonic and larval development of the sea cucumber Stichopus japonicus. Sj‐Otx transcripts were detected in fertilized eggs and in the posterior part of the archenteron of gastrulae. However, the expression was downregulated as embryos developed into auricularia larvae. Sj‐Otx was expressed again in the ciliary bands of late auricularia larvae, just before metamorphosis to doliolaria larvae. The expression domain corresponded to the domains moving to the mouth during metamorphosis and sinking into the buccal cavity, but not to the five transverse ciliary bands of the doliolaria. The expression gradually disappeared during further development and was not detected in juveniles. These results indicate that the gene responsible for chordate brain formation is expressed in the ciliary bands of auricularia larvae. genesis 27:58–63, 2000.

Cloning and functional analysis of ascidian Mitf in vivo: insights into the origin of vertebrate pigment cells

The microphthalmia-associated transcription factor (Mitf) is a basic-helix-loop-helix-leucine zipper (bHLH-ZIP) transcription factor essential for the development and function of all melanin-producing pigment cells in vertebrates. To elucidate the evolutionary history of Mitf and the antiquity of its association with pigment cells, we have isolated and characterized HrMitf, a sole member of the Mitf-TFE bHLH-ZIP subfamily in the ascidian Halocynthia roretzi. Maternal HrMitf mRNA is detected in the fertilized egg and in the animal hemisphere from 4-cell stage through the gastrula stage. From the neurula through the early tailbud stage, HrMitf is preferentially expressed in the pigment-lineage cells that express the lineage-specific melanogenesis genes tyrosinase (HrTyr) and Tyrp. Overexpression of HrMitf induced ectopic expression of HrTyr enzyme activity in mesenchymal cells where the same enzyme activity was induced by overexpression of HrPax3/7, suggesting that a part(s) of the Pax3-Mitf-tyrosinase gene regulatory cascade seen in vertebrate melanocytes is operative during ascidian embryogenesis. We also show HrMitf and mouse Mitf-A, a Mitf isoform abundantly expressed in pigmented epithelial cells, share similar functional characteristics. These results suggest antiquity of the association of the Mitf-TFE subfamily with pigment cells and may support the idea that acquisition of multiple promoters (isoforms) by an ancestral Mitf gene has allowed the evolution of multiple pigment cell types.

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.

Behavior and cellular morphology of the test cells during embryogenesis of the ascidian Halocynthia roretzi

During the early stages of embryogenesis of the ascidian Halocynthia roretzi the test cells creep exclusively on the inner surface of the chorion. Concomitant with elongation of the embryonic tail, however, the test cells begin to gather around the embryo and finally cover the whole embryo. The time at which the test cells surround the embryo almost coincides with that of initiation of larval tunic formation. Scanning electron microscope observations revealed that the test cells extend numerous cytoplasmic processes or pseudopodia. During larval tunic formation, the test cells compose a net by intertwining their filopodia, and the cell net covers the whole embryo.

Allogeneic cellular reactions between intra-specific types of a solitary ascidian, halocynthia roretzi

Coelomic cells from a solitary ascidian, Halocynthia roretzi, exibit a nonphagocytic cellular reaction against coelomocytes from different species and another individuals of the same species. The reaction was denoted contact reaction. While xenogeneic contact reactions were always observed, allogeneic reactions were observed in most but not all combinations of individuals. Three variant types of H. roretzi (Type A, B, C) inhabit the coast of northern Japan. They are reproductively isolated under natural conditions. Non-reactive combinations exist between the different types, in all possible combinations. The paterns and frequencies of reactivities between different variants are almost the same to those observed within a single type. The results of the alloreactivity suggest that the three types of H. roretzi became separate from each other very recently and still remain intra-specific variants.

Sperm chymotrypsin-like enzymes of different inhibitor-susceptibility as lysins in ascidians

Inhibitory effects of three peptidyl phenylalaninals on fertilization and on chymotrypsin-like enzyme activity of sperm in three species of ascidians were examined. The results suggest that a sperm chymotrypsin-like enzyme is indispensable for the fertilization in each of the ascidians, and that these enzymes have different susceptibilities to inhibitors.

Self-sterility of eggs induced by exogenous and endogenous protease in the solitary ascidian, Halocynthia roretzi

The unfertilized eggs (UFE) of the solitary ascidian, Halocynthia roretzi, which are released naturally, are strictly self‐sterile. However, ovarian eggs isolated after spawning, which are expected to develop into UFE on the following day, are self‐fertile. Some exogenous proteases‐trypsin, chymotrypsin, papain and elastase‐induced self‐sterility in the self‐fertile ovarian eggs within an hour in vitro. The establishment of self‐sterility by the exogenous protease did not require the synthesis of new protein, or the participation of follicle cells. Some of the ovarian eggs were able to differentiate into self‐sterile eggs spontaneously in vitro. The protein synthesis inhibitors puromycin and cycloheximide had no effect on the spontaneous establishment of self‐sterility. However, several protease inhibitors such as leupeptin, soybean trypsin inhibitor (SBTI) and antipain, did inhibit the spontaneous establishment of self‐sterility. The possible participation of trypsin‐like protease in the establishment of self‐sterility in the ovary is discussed. Mol. Reprod. Dev. 52:99–106, 1999.