MECHANISM OF TRANSDIFFERENTIATION OF MULTIPOTENT EPITHELIUM OF ASCIDIANS.

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During bud development of the ascidian Polyandrocarpa misakiensis, most tissues are formed from the differentiated multipotent atrial epithelium via a process called transdifferentiation. Recently a cell line was established from the atrial epithelium of this species. Using this cell line as an assay system, an aminopeptidase was purified. This enzyme induced proliferation and differentiation of the cell line. A cDNA clone encoding the enzyme was isolated. The amino acid sequence deduced from the cDNA contained the consensus sequence for active site of thiol proteases and five repeats of trefoil growth factor motifs. Since aminopeptidase activity increased during early stages of bud development, this growth factor is suggested to play an important role in bud development.The aminopeptidase activity can be induced by retinoic acid. Several lines of evidence suggests that retinoic acid is actually synthesized in the developing bud. Complementary DNA clones encoding several types of aldehyde dehydrogenases, which are candidates for retinoic acid synthase, were isolated and characterized. The amounts of mRNAs for two of the clones increased during bud development. Complementary DNAs encoding a homologue of retinoic acid receptors and retinoidX receptors were also cloned. The ascidian RAR was expressed in the mesenchymal cells of developing buds, and the expression was induced by retinoic acid.Differential display was performed to screen cDNAs of which expression were induced or enhanced by retinoic acid. One of the isolated clones encoded a homologue of trypsin-like serine proteases. Since trypsin can induce dedifferentiation of the atrial epithelium, the polypeptide encoded by this cDNA may be involved in bud development.