Estrogen-Dependent Uterine Secretion of Osteopontin Activates Blastocyst Adhesion Competence

Abstract

Embryo implantation is a highly orchestrated process that involves blastocyst-uterine interactions. This process is confined to a defined interval during gestation referred to as the “window of embryo implantation receptivity”. In mice this receptive period is controlled by ovarian estrogen and involves a coordination of blastocyst adhesion competence and uterine receptivity. Mechanisms coordinating the acquisition of blastocyst adhesion competence and uterine receptivity are largely unknown. Here, we show that ovarian estrogen indirectly regulates blastocyst adhesion competence. Acquisition of blastocyst adhesion competence was attributed to integrin activation (e.g. formation of adhesion complexes) rather than de novo integrin synthesis. Osteopontin (OPN) was identified as an estrogen-dependent uterine endometrial gland secretory factor responsible for activating blastocyst adhesion competence. Increased adhesion complex assembly in OPN-treated blastocysts was mediated through focal adhesion kinase (FAK)- and phosphatidylinositol 3-kinase (PI3K)-dependent signaling pathways. These findings define for the first time specific regulatory components of an estrogen-dependent pathway coordinating blastocyst adhesion competence and uterine receptivity.

Embryo–epithelium interactions during implantation at a glance

“…Integrins αvβ3, α5β1 and α6Bβ1 are present in the early and late mouse blastocyst, whereas the α2, α6A and α7 subunits appear in the late blastocyst, and α1 appears only in outgrowths (Sutherland et al, 1988(Sutherland et al, , 1993; see also Box 2). In mice, binding of fibronectin, vitronectin or OPN to the blastocyst stimulates the trafficking of integrins to the mouse trophectodermal surface and the assembly of focal contact-like structures, as indicated by the presence of the focal adhesion markers vinculin and talin (Chaen et al, 2012;Schultz and Armant, 1995;Schultz et al, 1997;Yelian et al, 1995). Integrin signalling modulates trophoblast adhesion through the activation of phospholipase Cγ to initiate phosphoinositide signalling and intracellular Ca 2+ mobilisation Wang et al, 2007).…”

Uterine receptivity and embryo–uterine interactions in embryo implantation: lessons from mice

“…This estrogen spike leads to stromal edema and luminal closure placing the blastocyst in close apposition with the luminal epithelium, and makes Fig. 2 The hormonal status in early pregnancy the uterus produce some blastocyst activators like osteopontin (OPN) [6]. It is followed by an intimate adherence of the blastocyst trophectoderm to the luminal epithelium, marking the first discernible sign of implantation on the night of day 4 (2200-2400 h).…”

“…Among the estrogen-responsive genes, it is known that LIF and OPN are functionally important secreted proteins during implantation [4,6]. Estrogen injection up-regulates both genes in the glandular epithelium in the delayed implantation mouse model [6,9].…”

“…In mice, OPN is expressed at uterine glands in an estrogen-dependent manner immediately before implantation, stimulates integrins on the cell surface of trophoblast to activate an intracellular PI3K/AKT signaling pathway (Fig. 3) [6]. Since an activated blastocyst has more active integrins on the surface of trophectoderm than a dormant one [6], it is speculated that a major role of OPN in integrin is to activate a blastocyst.…”

“…3) [6]. Since an activated blastocyst has more active integrins on the surface of trophectoderm than a dormant one [6], it is speculated that a major role of OPN in integrin is to activate a blastocyst. However, OPN null females do not show any reproductive defects and, therefore, other integrin activators must induce blastocyst activation corporately with OPN.…”