Identification of three prolactin-related hormones as markers of invasive trophoblasts in the rat

An expressed-sequence tag database search has identified three rat cDNA clo nes in the prolactin/growth hormone family, including a homologue of mouse proliferin-related protein (PRP). The encoded proteins of the two novel clo nes, designated prolactin-like proteins L (PLP-L) and M (PLP-M), are predic ted to be synthesized as precursors of 229 and 227 amino acids, modified by N-Iinked glycosylation, and secreted as mature glycoproteins of 199 and 20 0 residues, respectively. Murine homologues to PLP-L and PLP-M were also id entified. The open reading frame of rat PRP encodes a precursor protein of 245 amino acids and predicts a secreted 215-amino acid glycoprotein with 81 % identity to mouse PRP. All three rat mRNAs are expressed in the placenta , and expression is not detected in other tissues. PLP-L mRNA expression is observed from Days 11-20, with highest levels at Day 13; highest levels of PLP-M are observed from Day 11 until parturition, with peak revels also on Day 13; and highest levels of PRP are also observed from Day 11 until term , with maximal expression on Day 17. All three genes are most highly expres sed in invasive trophoblast cells lining the central placental vessel. The identification of molecular markers for endovascular trophoblasts serves to highlight the invasive nature of rodent placentation and may prove useful for future studies of placental function.