MECHANISM OF TRANSDIFFERENTIATION OF MULTIPOTENT EPITHELIUM OF ASCIDIANS.

在海鞘的芽发育过程中，大多数组织是由多能心房上皮分化形成的，这一过程被称为转分化。最近从该物种的心房上皮细胞中建立了细胞系。利用该细胞系作为检测系统，纯化了一种氨基肽酶。这种酶诱导细胞系增殖和分化。分离到了一个编码该酶的cDNA克隆。由cDNA推导出的氨基酸序列包含巯基蛋白酶活性位点的一致序列和三叶生长因子基序的5个重复序列。由于氨肽酶活性在芽发育早期增加，因此该生长因子在芽发育中起重要作用。维甲酸可诱导氨基肽酶活性。一些证据表明维甲酸实际上是在发育中的芽中合成的。分离并鉴定了几种类型的醛脱氢酶的互补DNA克隆，这些酶是维甲酸合成酶的候选基因。其中两个克隆的mrna数量在芽发育过程中增加。还克隆了编码维甲酸受体和维甲酸x受体同源物的互补dna。RAR在海鞘芽发育的间充质细胞中表达，并通过维甲酸诱导表达。采用差异展示的方法筛选维甲酸诱导或增强表达的cdna。其中一个分离的克隆编码了胰蛋白酶样丝氨酸蛋白酶的同源物。由于胰蛋白酶可以诱导心房上皮的去分化，因此该cDNA编码的多肽可能参与了心房上皮的发育。

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.