The putative window of embryo implantation in the human opens between days 19--24 of the menstrual cycle. During this period, the endometrium undergoes distinctive structural and functional changes orchestrated by steroid hormones, growth factors, and cytokines to attain a receptive phase in which it acquires the ability to implant the developing embryo. A major challenge in the study of human reproduction is to identify the molecular signals that participate in the establishment of this critical receptive phase in the context of the natural cycle. Toward this goal, we analyzed human endometrial biopsies at various days of the menstrual cycle by employing messenger RNA (mRNA) differential display technique. We isolated several complementary DNAs representing genes that are either up- or down-regulated within the putative window of implantation. We identified one of these genes as that encoding interferon (IFN)-inducible guanylate-binding protein 1 (or GBP1), which possesses GTPase activity. Analysis of endometrial biopsies by Northern blotting and RT-PCR demonstrated that GBP1 mRNA is specifically induced at the midsecretory phase of the menstrual cycle. In situ hybridization analysis revealed that GBP1 mRNA expression is localized in the glandular epithelial cells as well as in the stroma in the immediate vicinity of the glands. We observed that treatment of human endometrial adenocarcinoma cell, Ishikawa, with IFN-gamma or IFN-alpha markedly induced the expression of GBP1 mRNA. IFN-gamma was, however, a more potent inducer of GBP1 than IFN-alpha. Consistent with this finding, the temporal profile of GBP1 expression during the menstrual cycle resembled that of IFN-gamma mRNA more closely than that of IFN-alpha, predicting a regulatory role of IFN-gamma in GBP1 expression in midsecretory human endometrium. Although the precise function of GBP1 in the receptive human uterus remains unclear, its unique expression overlapping the putative window of implantation suggests that it might serve as a useful marker of uterine receptivity in the human.

The human endometrium acquires the ability to implant the developing embryo within a specific time window that is thought to open between days 19-24 of the secretory phase of the menstrual cycle. During this period the endometrium undergoes pronounced structural and functional changes induced by the ovarian steroids, estrogen and progesterone, that prepare it to be receptive to invasion by the embryo. The identification of reliable biochemical markers to assess this critical receptive phase in the context of the natural cycle remains one of the major challenges in the study of human reproduction. Our previous studies in a rat model system demonstrated that the expression of calcitonin, a peptide hormone involved in calcium homeostasis, is transiently induced by progesterone in the glandular epithelium at the onset of implantation. Attenuation of calcitonin synthesis in the uterus during the preimplantation phase by administration of calcitonin antisense oligodeoxynucleotides severely impairs implantation of rat embryos, suggesting that this peptide hormone plays a critical role in uterine receptivity. To investigate whether calcitonin is also expressed in the human endometrium during implantation, we monitored the spatio-temporal expression of calcitonin on various days of the menstrual cycle. Our studies employing RT-PCR showed that calcitonin messenger ribonucleic acid is expressed in human endometrium during the postovulatory midsecretory phase (days 17-25) of the menstrual cycle, with maximal expression occurring between days 19-21. Very little calcitonin expression was detected in the endometrium in either the preovulatory proliferative (days 5-14) or the late secretory (days 26-28) phase. In situ hybridization and immunocytochemical analyses localized the calcitonin expression predominantly in the glandular epithelial cells of the endometrium. Our studies further showed that calcitonin expression in the human endometrium is under progesterone regulation. Treatment of women with an antiprogestin, mifepristone (RU-486), drastically reduced calcitonin expression in the endometrium. Collectively, these findings reveal that progesterone-induced expression of calcitonin in the secretory endometrium temporally coincides with the putative window of implantation in the human.