The endostyle is a pharyngeal organ of Urochordata, Cephalochordata and larval Cyclostomata. This organ secretes mucus-proteins for internal filter feeding, a feeding system that must have developed in the common ancestor of these subphyla. Therefore, the endostyle is a key structure to understanding the origin and evolution of chordates. A previous study of the overall gene expression in Ciona intestinalis young adults yielded several candidates for ascidian endostyle-specific genes. In the present study, we determined in detail the expression profiles of six novel endostyle-specific genes. Ci-VWFL1 and CiVWFL2 encode related proteins similar to vertebrate von Willebrand factor, and were continuously expressed in zones 4 and 2 of the developing endostyle, respectively. The expression of Ci-Ends8 was observed in the entire region of zone 6 in young adults; however, the expression of this gene was restricted to the dorsal- and ventral-regions of zone 6 in the adult endostyle. The expression of Ci-Ends9 and Ci-Ends10 was observed in zones 6 and 4 in young adults, respectively, and was downregulated in the adult endostyle. Ci-Ends11 showed an expression pattern similar to that of Ciona TTF-1, which encodes a thyroid-related transcription factor. The predicted amino acid sequence of Ci-Ends10 showed similarity to Trip230, and that of Ci-Ends11 resembled Ptp4E. These molecules might be useful for further analysis of the development, function and evolution of the endostyle.
Urochordates (ascidians) represent one of the basal chordates in addition to cephalochordates (amphioxus) and cyclostomates (lampreys), and have some primitive features of chordates. One of these features, the endostyle, which is located in the ventral midline of the adult pharynx (Fig. 1A, B), is a characteristic organ for mucoprotein secretion. This organ is generally thought to have arisen in the common ancestor of chordates during the shift to internal feeding. Furthermore, the endostyle has functions that parallel those of the vertebrate follicular thyroid, such as iodine uptake and thyroid peroxidase activity (reviewed by Eales, 1997). Therefore, the endostyle is a key structure for understanding the origin and evolution of chordates. Cells of the ascidian endostyle are differentiated into nine zones that run parallel to one another in longitudinal orientation (e.g., Dunn, 1974). The cells of zones 1, 3 and 5 are considered to be supporting elements involved in catching and transporting food. Cells of zones 2, 4 and 6 are protein-secreting glandular cells used for filter feeding. Cells of zones 7, 8 and 9 have iodine-binding and peroxidase activities, like the follicular thyroid cells of vertebrates (Fig. 1C).
Several cDNA clones for ascidian endostyle-specific genes (HrEnds1 and HrEnds2) have been isolated from Halocynthia roretzi (Ogasawara et al., 1996) and homologous cDNA clones have been isolated from Ciona intestinalis (Ogasawara and Satoh, 1998). The proteins deduced for these genes have no similarity to reported proteins, and therefore no insights into their function or phylogenetic relationships have been obtained by comparison to known proteins. Ciona homologs of thyroid-related transcription factor TTF-1 and differentiation marker TPO have been isolated (Ogasawara et al., 1999a, 1999b; Ristoratore et al., 1999). Exclusive expression of these genes in the endostyle strongly suggests that this organ is homologous to the follicular thyroid. However, the molecular mechanisms of the development, function and evolution of the endostyle are still unclear.
Ciona intestinalis has a small genome of about 1.6×108 bp/haploid genome containing approximately 15,800 genes (Simmen et al., 1998; Dehal et al., 2002). Comprehensive analyses of expressed sequence tags (ESTs) and gene expression profiles have been performed for several developmental stages of this animal (Satou et al., 2002). We have determined the expression profiles of 976 non-redundant genes in young adults 2-3 weeks after metamorphosis (Ogasawara et al., 2002), and this analysis identified several candidates for endostyle-specific genes. In the present study, we characterized cDNAs and the expression profiles of the novel endostyle-specific genes.
MATERIALS AND METHODS
Characterization of endostyle-specific cDNA clones
Candidates for endostyle-specific cDNA clones of Ciona intestinalis were identified by previous global analyses of gene expression profiles in Ciona young adults (Ogasawara et al., 2002). Reconfirmation of candidates for endostyle-specific gene was performed by in situ hybridization in young adults and adult endostyles, and by dot blotting in adults, leading to the identification of six candidate clones. Nucleotide sequences were determined for both strand by standard procedures using BigDye terminators and an ABI PRISM 377 sequencer (Applied Biosystems). Public protein databases were searched with each sequence using the BLASTX algorithm at the NCBI server.
In situ hybridization
Young adult specimens 2–3 weeks after metamorphosis and adult specimens of 2–3 months after metamorphosis were prepared as described in Ogasawara et al. (2002). DIG-labeled RNA probes were synthesized from PCR-amplified templates which contained a T7 RNA polymerase promoter, and purified using a centrifugal ultra-filter as described in Ogasawara et al. (2001). Whole-mount in situ hybridization of young adult specimens was performed by the method described by Ogasawara et al. (2002). In situ hybridization of adult endostyle specimens and sections was carried out essentially as described in Ogasawara and Satoh (1998).
mRNA dot blotting and Northern blotting
Poly(A)+ RNA of adult organs (endostyle, pharyngeal-gill, body-wall muscle, intestine, and gonad) was isolated using Oligotex dT30 beads (Roche). For dot blotting, 100 ng of mRNA of each organ was blotted on a Hybond-N+ nylon membrane (Amersham Biosciences). Northern blotting using 10 μg of mRNA of each organ was carried out using standard procedures (Sambrook et al., 1989). Hybridization was carried out using a DIG-labeled RNA probe, followed by washing of the filter under high-stringency conditions (hybridization: 6x SSPE, 0.1% SDS, 1× Denhardt's solution, 50% formamide at 68°C for 16 hr; washing: 2× SSC, 0.1% SDS at 68°C for 30 min, 1× SSC, 0.1% SDS at 68°C for 30 min, 0.5× SSC, 0.1% SDS at 68°C for 30 min).
RESULTS AND DISCUSSION
Twenty-one cDNA candidates for endostyle-specific genes were identified in a previous study of gene expression profiles in Ciona young adults (Ogasawara et al., 2002). The expression of these genes was prominent in the endostyle; however, several of these genes were also expressed in other organs of young adults (data not shown). Reconfirmation of the expression pattern by means of in situ hybridization, mRNA dot blotting analyses, and sequencing of cDNA clones revealed that six of these genes were novel endo-style-specific genes.
von Willebrand factor-like genes were expressed exclusively in the glandular cells of the ascidian endostyle
Whole-mount in situ hybridization using Ciona young adults showed that genes designated ID03415 and ID05762 were expressed exclusively in the endostyle (Fig. 2A, F). Although Northern blotting using mRNAs of adult organs showed several transcripts of these genes, the expression of these genes was detected only in the endostyle, and this was confirmed by dot blotting (Fig. 2D, E, I, J). The predicted amino acid sequences of these genes indicated that both genes encoded sequences similar to vertebrate von Willebrand factor (VWF) which is expressed in the vascular endothelial cells. Therefore, genes ID03415 and ID05762 were named Ci-VWFL1 (Ciona intestinalis von Willebrand Factor Like gene 1) and Ci-VWFL2 (Ciona intestinalis von Willebrand Factor Like gene 2), respectively. Ci-VWFL1 was expressed in the medial region of the young adult endostyle (Fig. 2A, B). In adults, expression of Ci-VWFL1 was restricted to the entire region of zone 4, which is a medial-glandular zone of the endostyle (Fig. 2C). Ci-VWFL2 was expressed in the ventral region of the young adult endostyle (Fig. 2F, G). In adults, Ci-VWFL2 was expressed in the entire region of zone 2, which is the ventral-glandular zone of the endostyle (Fig. 2H). Therefore, Ci-VWFL1 and CiVWFL2 are markers specific for medial- and ventral-glandular zones of the endostyle, respectively.
Comparison between Ci-VWFL1 and Ci-VWFL2 and reported proteins in the public protein database revealed that both Ci-VWFL1 and Ci-VWFL2 have similar amino acid sequences to the C-terminal half of vertebrate VWF (Fig. 2K), which is a glycoprotein involved in the blood coagulation system (reviewed in Denis, 2002). Vertebrate VWF has several A domains, C domains and D domains, and also has a cystine-knot domain in the C-terminus. Both Ci-VWFL1 and Ci-VWFL2 have several C domains, an A domain and a cystine-knot domain with similar compositions to the corresponding vertebrate domains. Therefore, these genes encode proteins related to each other. This is a first instance of glandular-zone-specific genes with similarity to other reported proteins, and might facilitate the functional and molecular phylogenetic analysis of the endostyle. Aros and Virágh (1969) pointed out that zones 2 and 4 have identical cytoplasmic structures and secretion granules. The existence of transcripts that probably produce similar proteins in zones 2 and 4 supports their suggestion. Although the domain compositions of Ciona VWF-like proteins and vertebrate VWF were basically conserved, Ciona VWF-like proteins have no A domain repeat or D domain repeat, but have an additional C domain in their N-termini. We could not find any other VWF-like genes or VWF orthologs (data not shown) in the Ciona genome (Dehal et al., 2002). In reported vertebrate genomes, we found VWF orthologs in human, mouse, rat and fugu. However, we could find no vertebrate VWF-like gene which had the same domain structure as Ci-VWFL1 and Ci-VWFL2. Although it is still unclear whether vertebrate VWF and Ciona VWF-like genes are homologous or not, further analyses in other basal chordates might provide insights into the evolution of VWF-like proteins and the endostyle. The ascidian genome contains a basic set of genes with less redundancy than the vertebrate genome, but some Ciona genes have been duplicated or further multiplied in the ascidian lineage (Dehal et al., 2002). Analyses of these related genes, including Ci-VWFL1 and Ci-VWFL2, in other ascidians closely related to Ciona might help us to understand the evolution of the ascidian genome.
Endostyle-specific genes which have different expression patterns between young adults and adults
In situ hybridization of the Ciona endostyle revealed that genes ID06909, ID02629 and ID02772 have different expression patterns between young adults (2–3 weeks after metamorphosis) and adults (2–3 months after metamorphosis). A gene ID06909, renamed Ci-Ends8, was expressed in the entire region of zone 6, which is a dorsal protein-secreting element in the young adult endostyle (Fig. 3A, B). In adults, Ci-Ends8 was also expressed only in the endostyle (Fig. 3D, E), but its expression domain was changed and restricted to the dorsal and ventral parts of zone 6 (Fig. 3C). This observation suggests that the cells in zone 6 of the adult endostyle are not uniform. The predicted amino acid sequence of Ci-Ends8 has no similarity to other reported proteins.
Genes ID02629 and ID02772, which were renamed Ci-Ends9 and Ci-Ends10, were expressed in zones 6 and 4 in young adults, respectively (Fig. 3F, G, J, K). However, expression of these genes was not detected in adults under the conditions of our dot blotting and in situ hybridization (Fig. 3H, I, L, M). The transient expression pattern of these genes suggests that the nature of the endostyle differs between young adults and adults. The predicted amino acid sequence of Ci-Ends9 has no similarity to other reported proteins. On the other hand, Ci-Ends10 encodes a protein with sequence similar to the binding site for thyroid hormone receptor located in the Trip230 (data not shown). The relationship between these transient expression patterns and the molecular natures of these genes are not yet clear.
Ci-Ends11 has multiple expression domains which resemble those of CiTTF-1 expression
Gene ID06825, named Ci-Ends11, showed endostyle-specific expression (Fig. 4A, E) and had multiple expression domains in the endostyle (Fig. 4B, C). Ci-Ends11 was expressed in the entire region of zone 3, which is generally thought to be a supporting element, the dorsal region of zone 2, the ventral region of zone 4, and the ventral region of zone 5 (Fig. 4D). The predicted amino acid sequence of Ci-Ends11 shows similarity to the catalytic domain of Ptp4E, a receptor-linked protein-tyrosine phosphatase (data not shown). The expression pattern of Ci-Ends11 was similar to that of TTF-1, which encodes a thyroid-related transcription factor, except in the dorsal region of zone 5 (Fig. 4F-I).
Isolation of organ-specific genes based on cDNA project
Recent advances in characterizing the overall expression profiles and ESTs of Ciona genes (e.g., Satou et al., 2002) provide us opportunities for isolating tissue- or organ-specific genes in a comprehensive manner. In a previous study, only three Ciona endostyle-specific genes could be isolated by means of differential screening (Ogasawara et al., 2002). In the present study, we have characterized six novel cDNA clones for Ciona endostyle-specific genes based on the expression profiles of about 1,000 genes. Therefore, this approach might be more effective for isolating organ-specific genes than differential screening. Analysis of the rest of the 4,000 genes expressed in young adults should yield more endostyle-specific genes, which may be used to understand the evolution and development of the endostyle in chordates. The finding of genes which are expressed in different endostyle regions between young adults and adults suggests that a set of genes change their expression profiles during a development period from young adults to adults. Therefore, the screening of cDNAs not only from young adults but also from adults will enable us to find more endostyle-related genes, which facilitates studies of evolution and development of the endostyle globally.
We thank Kazuko Hirayama and all members in the Maizuru Fisheries Research Station, Kyoto University, Field Science Education and Research Center for their cooperation. This work was supported by a Grant-in-Aid from MEXT Japan (Grants 14034210) to M.O. We thank anonymous referees for their suggestions.