Distribution of cathepsin E in the larval and adult organs of the bullfrog with special reference to the mature form in the larval fore-gut

Expression of CCAAT/enhancer binding protein delta is closely associated with degeneration of surface mucous cells of larval stomach during the metamorphosis of Xenopus laevis

CCAAT/enhancer binding protein delta (C/EBP delta) is one of the transcription factors that have a basic-leucine zipper domain. In mammals, it has been suggested that this transcription factor plays a role in differentiation of adipocytes or in apoptosis of mammary gland epithelial cells. The factor also plays a role in acute-phase response in injury, infection and inflammation. We cloned Xenopus homologues of the C/EBP delta gene from metamorphosing stomach by subtractive hybridization and analyzed spatio-temporal expression pattern of the homologues. Two isoforms of C/EBP delta were isolated and named C/EBP delta-1 and -2. Their deduced amino acid sequences were highly similar to each other (identity, 91.2%). Expression of the C/EBP delta mRNAs in the stomach transiently increased during its metamorphosis-associated remodeling, and the transient up-regulation was also found in thyroid hormone-induced metamorphosis. The C/EBP delta mRNAs were exclusively localized in degenerating larval surface mucous cells, not in newly proliferating and differentiating adult-type epithelial cells. The result suggests a possibility that Xenopus C/EBP delta plays a role in apoptotic cell death of larval-type epithelium during the stomach remodeling.

Stomach remodeling-associated changes of H+/K+-ATPase beta subunit expression in Xenopus laevis and H+/K+-ATPase-dependent acid secretion in tadpole stomach

Through subtractive hybridization, H+/K+-ATPase beta subunit mRNA, highly expressed in the larval stomach of Xenopus laevis, was isolated. In situ hybridization demonstrated that the H+/K+-ATPase beta subunit mRNA was exclusively expressed in manicotto gland cells of the larval stomach, not in any other cell. Northern blot analysis showed that metamorphosis-associated changes of the H+/K+-ATPase beta subunit mRNA expression in the stomach were characterized by high expression in tadpoles, a considerably lower expression in metamorphosing tadpoles, and a re-increase of expression in froglets. Further in situ hybridization showed that the decrease of expression correlated with the degeneration of larval type epithelium in the manicotto gland, while the re-increase correlated with the differentiation of oxynticopeptic cells of the adult type stomach. Moreover, the H+/K+-ATPase beta subunit mRNA was expressed in adult epithelial primordia. Such changes were found in thyroid hormone-induced precocious metamorphosis. Based on studies using this ATPase as well as xP1 and PgC (pepsinogen C) as molecular markers, this study discusses a probable cell lineage involved in metamorphosis-associated stomach remodeling. The pH of luminal contents of the larval stomach was found to be lower than 2. In addition, the pH of an isolated stomach changed from 7.2 to lower than 4 after incubation in Ringer's solution, suggesting acid production from the larval stomach. This is the first demonstration of the H+/K+-ATPase-mediated acid production and secretion in the larval stomach of Xenopus laevis.

Molecular cloning of preprocathepsin E cDNA from the stomach of bullfrog Rana catesbeiana

A cDNA library was constructed from a poly(A)(+) RNA fraction of the gastric mucosa of bullfrog Rana catesbeiana. We cloned a cDNA encoding preprocathepsin E (Pre-Pro-CE) from the library. The present study is the first demonstration of the Pre-Pro-CE cDNA of lower vertebrate such as amphibian. Amino acid sequence deduced from the cDNA was compared with partial amino acid sequence determined by Edman degradation, suggesting that the cDNA comprises an open reading frame encoding a signal peptide (16 amino acids), a pro-sequence (33 amino acids) and a mature protein region (348 amino acids). Two consensus tri-peptide sequences (FDT and VDT) as active site and positions of seven cysteine residues were conserved in this amphibian CE. Although the bullfrog CE was deduced to contain one potential N-linked glycosylation site, its position (Asn(139)-Leu(140)-Thr(141)) was different from that of mammalian CEs. Molecular phylogenetic analysis showed that the bullfrog Pro-CE belongs to the typical Pro-CE group among various aspartic proteinases.