Biology of the Placental Proteins in Domestic Ruminants: Expression, Proposed Roles and Practical Applications

Abstract

The placenta, a lodgment at the feto-maternal interface, is a temporal organ that plays crucial roles in maintaining gestation and fetal development. To achieve these purposes, the placenta produces an array of proteins with distinct spatio-temporal characteristics. In this review, the authors focus on the paralog families of prolactin (PRL) and aspartic proteinase, exclusively produced by the placenta, and discuss their biological roles and practical significance in animal husbandry. The bovine placental PRL family consists of one classical member and at least ten non-classical members. Bovine placental lactogen (PL) is a unique classical member due to its lactogenic activity and potentially involved in partitioning nutrients to maintain fetal development. In contrast, the biological roles of non-classical members in bovine placental PRL family proteins remain unclear. Recent papers have reported that a non-classical member protein named prolactin-related protein -I (PRP-I) exhibited angiogenic activity following C-terminal cleavage by proteolytic enzymes. These results suggest that non-classical members of the placental PRL family exert their biological activities via specific mechanisms other than the PRL receptor-signaling pathway. The bovine genome contains a hundred or more aspartic proteinase-like genes and at least 22 mRNAs with close structural relationships are transcribed in the placenta. These molecules correspond to pregnancy-specific protein (PSP)-B, PSP-60 and pregnancy-associated glycoprotein (PAG) as reported previously. The true character of PSPs and PAG is thought to be a mixture of placental aspartic proteinase-like proteins. Immunological detection of PAG protein in maternal serum is the basis for early pregnancy diagnosis in ruminants. Currently, early pregnancy diagnosis by PAG assay is available for cows, buffaloes and deer.