Cathepsin B, Cathepsin L, and Cystatin C in the Porcine Uterus and Placenta: Potential Roles in Endometrial/Placental Remodeling and in Fluid-Phase Transport of Proteins Secreted by Uterine Epithelia Across Placental Areolae

Cathepsins (CTSB and CTSL1) and their inhibitor, cystatin C (CST3), remodel uterine endometrium and placenta for transport of gases, micronutrients, and macromolecules essential for development and growth of the conceptus (embryo/fetus and placental membranes). We examined the temporal/spatial control of expression for CTSB, CTSL1, and CST3 mRNAs in endometria and placentae of pigs using three developmental models: 1) pigs were hysterectomized during the estrous cycle or pregnancy; 2) cyclic pigs were injected with estrogen to induce pseudopregnancy and were hysterectomized; and 3) pigs were ovariectomized, injected with progesterone, and hysterectomized. The abundance of CTSB, CTSL1, and CST3 mRNAs increased in endometrial epithelia during pregnancy and in response to exogenous progesterone but not estrogen. CST3 was also expressed in cells scattered within the stratum compactum stroma. Progesterone decreased epithelial but increased stromal compartment expression of CST3. CTSB increased in all chorionic epithelia, but CTSL1 was limited to chorionic epithelia that form areolae to absorb secretions from uterine glands. Based on the placental and endometrial distribution of CTSL1, we examined expression in the neonatal enterocytes known to transport immunoglobulins from colostrum. CTSL1 was also expressed in enterocytes of intestine from neonatal piglets. Therefore, CTSL1 is expressed by endometrial epithelia, placental areolae, and neonatal intestine, and it may function in the transport of macromolecules across these epithelia. Our results support the idea that reciprocal interactions between CSTL1, CTSB, and CST3 may be required to remodel endometrial and placental tissues for close apposition between maternal and fetal vasculatures and to facilitate transplacental transport of gases, micronutrients (amino acids, glucose), and macromolecules (proteins). Cysteine proteases and their inhibitors may also specifically modify proteins for successful utilization and fluid-phase transport across uterine, placental, and neonatal gut epithelia.