Podocalyxin governs the barrier function of human endometrial epithelium and critically regulates receptivity for embryo implantation

Embryo implantation is a key step in establishing pregnancy and a major limiting factor in IVF. Implantation requires the endometrium, the inner lining of the uterus, to transform from a non-receptive to a receptive state, to allow embryos to attach to the surface and enter into the tissue. However, the fundamental mechanisms governing receptivity are not well understood. Here we show that transmembrane protein podocalyxin is a major and clinically significant factor regulating human endometrial receptivity. Podocalyxin is expressed in all endometrial epithelial cells in the non-receptive state but is selectively down-regulated in the luminal epithelium at receptivity. We present evidence that podocalyxin critically governs the barrier function of the endometrial epithelium, likely as an intrinsic protective mechanism, rendering it non-receptive to an embryo. In addition, podocalyxin suppresses genes promoting receptivity (eg LIF, CSF1) but stimulates those inhibiting implantation (eg WNT7A, LEFTY2). Down-regulation of podocalyxin in the luminal epithelium, likely mediated by progesterone, selectively converts the endometrial surface to a more adhesive state that facilitates embryo attachment and penetration. Furthermore, inadequate down-regulation of podocalyxin in the endometrial luminal epithelium is associated with poorer implantation rates in IVF. We thus propose that podocalyxin promotes the barrier function of human endometrial epithelial cells and critically regulates receptivity for embryo implantation. Overall design: RNA-seq (control vs PCX-overexpression) 4 replicates per group