Prolactin receptor expression and its role in trophoblast outgrowth in human embryos

Research question: What is the gene expression pattern of prolactin receptor (PRLR) in human pre-implantation embryos and what are its functions during the embryonic development and adhesion process?

Design: A total of 405 discarded human vitrified oocytes and embryos donated for research by consenting couples were used in this study. The oocytes and embryos were used to analyse PRLR expression and to evaluate the influence of prolactin (PRL) supplementation in the embryo culture medium on embryo developmental competence and viability. The rates of blastocyst development and adhesion, outgrowth area, cytoskeletal reorganization and nascent adhesion formation were compared between groups.

Results: PRLR expression increased significantly after embryo compaction (P < 0.0001) and blastulation (P < 0.0001). Supplementation of the embryo culture medium with PRL did not improve the developmental rate and morphological grade. In contrast, blastocyst outgrowth was significantly increased in embryos cultured with PRL (P = 0.0004). Phosphorylation of JAK2, downstream of the prolactin receptor family, was markedly higher in the PRL-treated embryos than in embryos cultured without PRL. Furthermore, the expression of mRNAs encoding ezrin-radixin-moesin proteins and epithelial-mesenchymal transition-related genes was stimulated by the activation of PRL-JAK2 signalling. The PRL-treated embryos had higher mRNA expression of integrins than non-treated embryos, and transcriptional repression of cadherin 1 was observed after PRL treatment. More nascent adherent cells expressed focal adhesion kinase and paxillin in PRL-treated embryos than in non-treated embryos.

Conclusions: Human embryos express PRLR at the morula and blastocyst stages, and PRLR signalling stimulates blastocyst adhesion by promoting integrin-based focal adhesions and cytoskeletal organization during trophoblast outgrowth.