Adrenomedullin: A Novel Peptide Hormone for Uterine Receptivity and Conceptus Elongation in Pigs?

Sudikshya Paudel, Bangmin Liu, Magdalina J Cummings, Xiaoqiu Wang

Adrenomedullin (ADM) is an evolutionarily conserved multi-functional peptide hormone in mammalian species. The implantation sites of conceptuses (embryo/fetus and its extra-embryonic membranes) in pregnant Adm+/− female mice are abnormally spaced leading to fetal crowding, intrauterine growth restriction (IUGR) and increases in pregnancy loss. As litter-bearing species, pigs exhibit a high incidence of early embryonic death (30-40%) and naturally occurring IUGR (15-25%), and significant numbers of stillborn piglets (3-9%). Therefore, it is imperative that the roles of ADM in regulation of uterine receptivity, as well as growth and development of conceptus in pigs be established. This study determined abundances of ADM in uterine luminal fluid, and the patterns of expression of ADM and its receptor components in uteri from cyclic and pregnant gilts, as well as conceptuses during the peri-implantation period of pregnancy. ADM receptor components include: calcitonin receptor-like receptor (CALCRL; G protein-coupled receptor bound by ADM), receptor activity modifying protein (RAMP2; dimerized with CALCRL to form ADM1 receptor) and RAMP3 (dimerized with CALCRL to form ADM2 receptor with lower specificity to ADM) and atypical chemokine receptor 3 (ACKR3; a decoy receptor that serves as a cell-autonomous molecular rheostat to dampen ADM signaling). Total recoverable ADM was greater in the uterine fluid of pregnant compared with cyclic gilts between days 10 and 16 post-estrus and was from uterine luminal epithelial (LE) and conceptus trophectoderm (Tr) cells. Uterine expression of CALCRL, RAMP2, and ACKR3 were affected by day (P < 0.05), pregnant status (P < 0.01) and/or day x status (P < 0.05). Within porcine conceptuses, expression of CALCRL, RAMP2 and ACKR3 increased between days 10 and 16 of pregnancy. Using an established porcine trophectoderm (pTr1) cell line isolated from elongating porcine conceptuses recovered on day 2 post-breeding, it was determined that 10-7 M ADM stimulated proliferation of pTr1 cells (P < 0.05) at 48 h, and increased phosphorylated mechanistic target of rapamycin (p-MTOR) and 4E binding protein (p-4EBP1) by 6.1- and 4.9-fold (P < 0.0001), respectively. These novel results indicate a significant role for ADM in uterine receptivity for implantation and conceptus growth and development in pigs. They also provide a framework for future studies of ADM signaling to affect proliferation and migration of Tr cells, spacing of blastocysts, implantation and placentation in pigs.