Developmental regulation of integrin expression at the time of implantation in the mouse embryo

The trophectoderm layer of the mouse blastocyst differentiates at the late blastocyst stage to form the invasive trophoblast that mediates implantation of the embryo into the uterine wall. The first sign that trophoblast cells have developed an invasion-specific cell behavior appears about 10-15 hours after the embryo hatches from the zona pellucida, when the quiescent, non-adherent trophectoderm cells initiate protrusive activity and become adhesive to extracellular matrix. Our previous findings that trophoblast outgrowth on extracellular-matrix-coated substrata involves the integrin family of adhesion receptors (Sutherland, A. E., Calarco, P. G. and Damsky, C. H., 1988, J. Cell Biol. 106, 1331-1348), suggested that the onset of trophoblast adhesive and migratory behavior at the time of implantation may be due to changes in expression or distribution of integrin receptors. We have thus examined the mRNA and protein expression of individual integrin subunits during pre- and periimplantation development (E0-E7.5). A basic repertoire of integrins, including receptors for fibronectin (alpha 5 beta 1), laminin (alpha 6B beta 1) and vitronectin (alpha v beta 3), was expressed continuously throughout this period, whereas the expression of five other integrin subunits was developmentally regulated. The mRNA for three of these (alpha 2, alpha 6A and alpha 7) was first detected in the late blastocyst, coincident with endoderm differentiation and development of attachment competence. The mRNA for another (alpha 1) was not detected until after trophoblast outgrowth had begun, suggesting that its expression may be induced by contact with matrix. At E7.5, three of the temporally regulated integrins (alpha 1, apha 6A, alpha 7), all of which can form receptors for laminin, were detected only in the ectoplacental cone (differentiating trophoblast), and may thus play specific roles in trophoblast adhesion and/or differentiation. Because laminin expression is upregulated in decidualized uterine stroma in response to the implanting embryo, we examined trophoblast-laminin interactions, using laminin fragments and integrin antibodies to determine which integrin receptors were involved. Trophoblast cells attached and spread on both the E8 and P1' fragments of laminin; however, the P1' binding site was cryptic in intact laminin. Interaction with P1' was RGD- and alpha v beta 3-dependent, whereas outgrowth on E8 was RGD-independent and not inhibited by antibodies to the laminin receptor alpha 6 beta 1, suggesting that alpha 7 beta 1 is the major trophoblast integrin E8 receptor.(ABSTRACT TRUNCATED AT 400 WORDS)