Trophic signals acting via phosphatidylinositol-3 kinase are required for normal pre-implantation mouse embryo development

Abstract

The growth and survival of the preimplantation mammalian embryo may be regulated by several autocrine trophic factors that have redundant or overlapping actions. One of the earliest trophic factors to be produced is embryo-derived platelet-activating factor (1-O-alky-2-acetyl-sn-glyceryl-3-phosphocholine). The addition of platelet-activating factor to embryo culture media exerted a trophic effect, but structurally related lipids (3-O-alky-2-acetyl-sn-glyceryl-1-phosphocholine, 1-O-alky-sn-glyceryl-3-phosphocholine, octadecyl-phosphocholine) had no effect. Platelet-activating factor induced a pertussis toxin-sensitive [Ca(2+)](i) transient in two-cell embryos that did not occur in platelet-activating factor-receptor null (Pafr-/-) genotype embryos. Fewer Pafr-/- mouse zygotes developed to the blastocyst stage in vitro compared with Pafr+/+ zygotes (P<0.02), those that developed to blastocysts had fewer cells (P<0.001) and more cells with fragmented nuclei (P<0.001). The inhibition of 1-O-phosphatidylinositol 3-kinase (LY294002 (3 microM and 15 microM) and wortmannin (10 nM and 50 nM)) caused a dose-dependent inhibition of platelet-activating factor-induced [Ca(2+)](i) transients (P<0.001). The two-cell embryo expressed 1-O-phosphatidylinositol 3-kinase catalytic subunits p110 alpha, beta, gamma and delta, and regulatory subunits p85 alpha and beta. LY294002 and wortmannin each caused a significant reduction in the proportion of embryos developing to the morula and blastocyst stages in vitro, reduced the number of cells within each blastocyst, and significantly increased the proportion of cells in blastocysts with fragmented nuclei. The results indicate that embryo-derived platelet-activating factor (and other embryotrophic factors) act through its membrane receptor to enhance embryo survival through a 1-O-phosphatidylinositol 3-kinase-dependent survival pathway.