A Role for (alpha)v(beta)3 Integrin During Implantation in the Rabbit Model

The study of implantation has been facilitated by the identification of specific biomarkers that are associated with uterine receptivity. The (alpha)v(beta)3 integrin is a cell surface adhesion receptor, whose expression has been shown to be elevated in the endometrium at the time of implantation in both humans and other mammalian species; however, the distribution of (alpha)v(beta)3 in the rabbit model is unknown. The rabbit has been shown to be an excellent model for the study of implantation. As an obligate ovulator, the timing of pregnancy can be precisely established, and embryonic attachment occurs through specialized trophoblast-endometrial structures known as trophoblastic knobs. In the present study, the expression of (alpha)v(beta)3 integrin subunit in the rabbit uterus was examined by reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry, and in situ hybridization. Expression of the (alpha)v(beta)3 integrin was examined in Day 6.5 embryos, flushed from pregnant does. Immunofluorescence demonstrated strong immunostaining on the rabbit blastocyst (Day 6.5). RTPCR analyses showed higher levels of mRNA for (beta)3 subunit at the implantation site, with reduced expression in nonimplantation sites and in nonpregnant adult and immature endometrium. Immunohistochemistry demonstrated little, if any, (beta)3 immunoreactivity on the endometrial epithelium. In contrast, in situ hybridization showed expression of the (beta)3 integrin subunit mRNA in the uterine myometrium and on the trophoblast. To further determine the functional significance of (alpha)v(beta)3 integrin expression during implantation, pregnant female rabbits that underwent ventral laparotomy on the morning of Day 6 received intrauterine injection of the following into the right uterine horn: 1) the monoclonal (alpha)v(beta)3 neutralizing antibody (LM609), 2) arg-gly-asp (RGD) hexapeptides (GRGDSP), 3) non-RGD hexapeptides (GRGESP), and 4) IgG isotype matched control antibody. The left horn served as a control and received only saline injections. A significant reduction in the number of implantation sites was observed in the horns receiving anti-(alpha)v(beta)3 antibody (P < 0.001) and the RGD peptides (P = 0.03). In the rabbit, the (alpha)v(beta)3 integrin is present on the embryo and trophoblast and appears to be involved in early embryo-maternal interaction.