Characterization of gene of anuran cathepsin D as a metamorphosis-associated enzyme

Cathepsin D is a member of the lysosomal aspartic proteinases and has been claimed to play a crucial role in the breakdown of larval tissues during anuran metamorphosis. The present study aimed to characterize the gene of anuran cathepsin D as an approach for elucidating the molecular mechanism of involution of larval tissues. Three overlapping cDNA of Xenopus cathepsin D were amplified by the polymerase chain reaction and were cloned, which covered 590 bp, 355 bp and 790 bp sequences of the region between two enzyme active sites, a 5'-end and 3'-end regions of the gene, respectively. Altogether they revealed 1.6 kbp encoding 422 amino acids of the enzyme. Xenopus cathepsin D showed about 60% homology to mammalian enzymes at the amino acid sequence level. Northern blot analyses revealed the gradual increase of cathepsin D transcripts in the tail during prometamorphosis, reaching a peak at stage 61 and 62. Thyroid hormone notably enhanced the signals of northern blotting of the tail. The in situ hybridization histochemistry showed that both epidermal cells in the basal layer and subepidermal mesenchymal cells embedded in the thick layer of collagen fibers produce high levels of mRNA of cathepsin D in response to thyroid hormone, suggesting active contribution of the cells closely located at the basement membrane to the breakdown of tail tissues.