Expression of Cysteine Proteases in Extraembryonic Tissues during Mouse Embryogenesis

The expression of cathepsin B- and L-specific mRNAs as well as active forms of the enzymes was determined in mouse placenta and visceral yolk sac from 7.5 through 17.5 days postconception, a period marked by major anatomic transitions in the mouse conceptus. The level of specific mRNA was determined relative to the 28S ribosomal RNA in a series of multiprobe ribonuclease protection assays using high-specific-activity antisense cathepsin B and L riboprobes. The molecular forms of active cysteine proteases present in the tissues at the time of extraction were detected using a membrane-permeant radiolabeled active site-specific inhibitor, Fmoc-[125I2]Tyr–Ala-CHN2. The results of this study show that the expression of active cathepsin L relative to active cathepsin B is significantly higher in visceral yolk sac than in placenta, consistent with a higher proteolytic requirement for the former tissue. Active cathepsin L was highest at Day 9.5 in visceral yolk sac, a stage at which it has been shown that proteolysis in this organ is required for production of amino acids for embryonic protein synthesis. Cathepsin L mRNA was also elevated in the Day 9.5 placenta, but paradoxically this did not result in an increase in cellular active enzyme. An unknown protein, termed p14, highly expressed in placenta, also reacted with the inhibitor. Expression of this protein was highest early during gestation in the ectoplacental cone, suggesting that p14 may be important in the implantation process.