Cell adhesion and signalling at implantation

Implantation failure is one of the major causes of infertility and contributes a major barrier to assisted reproduction success. Establishing a detailed understanding of embryo implantation is critical to our basic knowledge of human reproduction. The initial embryoendometrial interaction appears to be crucial in triggering the cascade of physiological and molecular events required to establish a stable maternal-conceptus relationship. I hypothesised that integrin αvβ3 and its ligand OPN mediate the initial interaction during implantation, and that this is modulated by growth factors secreted locally from the embryo. In order to overcome earlier difficulties in the investigation of implantation in vivo, in vitro models were established by transferring either mouse or human embryos onto confluent human uterine epithelial (Ishikawa) cells. Following 24 h of co-culture, embryos were loosely attached onto cells, with more stable attachment after 48 h. Embryo attachment stimulated local up-regulation of epithelial integrin αvβ3 and OPN as monitored by immunofluorescence. Furthermore, OPN was cleaved to produce an adhesion-active 45kDa fragment which translocated to the apical cell surface in response to IGF-I stimulation, but not to HB-EGF. In order to dissect associated mechanisms down to a single molecular interaction, ligand (IGF-I, HB-EGF, or OPN)-conjugated, embryo-sized microbeads were used in attachment assays. IGF-I/HB-EGF-coated beads enhanced levels of epithelial integrin αvβ3. Knock-down of αvβ3 reduced the rate of attachment of both embryos and beads carrying IGF-I, HB-EGF or OPN. Knock-down of OPN reduced the stability of embryo attachment and beads bearing IGF-I or OPN, but interestingly did not affect the rate of attachment of beads bearing HB-EGF. Additionally, knock-down of IGFIR or SHP-2 reduced attachment rate of both embryo and protein-coated beads. I have thus demonstrated a role for αvβ3-OPN interaction in embryo attachment as an adhesion receptor and a mediator of intracellular signalling via two independent pathways. Two embryo-derived growth factors, IGF-I and HB-EGF, can mediate initial attachment to the epithelium. Experimental data suggest therapeutic applications to improve female infertility including the development of novel diagnostic markers for evaluating uterine receptivity.