46 EXPRESSION OF TRANSCRIPTION FACTORS SPECIFIC TO THE TROPHOBLAST LINEAGE IN MOUSE SOMATIC NUCLEAR TRANSFER EMBRYOS

Somatic cell nuclear transfer (SCNT) embryos can develop at relatively high rates during the preimplantation period; however, most of these fail after implantation. Development of extraembryonic tissue is indispensable for normal embryonic development. Hence, an abnormality of trophoblast development might be a significant factor in post-implantation lethality of SCNT embryos. A transcription factor, caudal-related homeobox 2 (Cdx2), appears to be involved in the segregation of ICM and trophectoderm (TE) in preimplantation embryos (Niwa et al. 2005 Cell 123, 917–929). Both Cdx2 and Oct3/4 are expressed in all cells at the morula stage, and then Cdx2 expression becomes restricted to the TE and Oct3/4 to the ICM as the blastocyst develops. Mouse embryos deficient in Cdx2 are able to develop to normal blastocysts but die soon after implantation, probably because of defects in the TE lineage. Moreover, dysplasia of the spongiotrophoblast layer might attribute to an abnormality of Tpbpa expression in mouse SCNT embryos (Wakisaka-Saito et al. 2006 Biochem. Biophys. Res. Commun. 349, 106–114). In this study, we examined the expression profiles of transcription factors implicated in trophoblast development in mouse SCNT embryos and intracytoplasmic sperm injection (ICSI) embryos by immunohistochemistry and real-time PCR analysis. SCNT embryos were produced according to the method reported previously (Wakayama et al. 1998 Nature 394, 369–374). In brief, B6D2F1 and B6C3F1 female mice were used for the collection of recipient oocytes and donor cells, respectively. After nuclear transfer, the oocytes were activated and cultured in KSOM to the morula and blastocyst stages. Immunohistochemical analysis demonstrated that in ICSI embryos Cdx2 was only partially expressed at the 8-cell stage but completely in early morulae. In contrast, in SCNT embryos, it was absent at the 8-cell stage and appeared partially at the early morula stage. Thereafter, Cdx2 expression became restricted to the TE cells in both the ICSI and the SCNT blastocysts. However, ectopic expression of Oct3/4 was observed in the TE cells of SCNT, but not in ICSI blastocysts. Real-time PCR analysis showed that at the 8-cell stage, Cdx2 was expressed in ICSI but not in SCNT embryos. In addition, the expression level of Cdx2 in SCNT embryos at the blastocyst stage was only half that in ICSI embryos (P < 0.05). However, there was no significant difference in expression level of Oct3/4 between ICSI and SCNT embryos. Eomesodermin (Eomes) is also implicated in trophoblast development and its expression depends on Cdx2, BMP4, and FGF4. In SCNT embryos, the expression level of Eomes was also only half that in ICSI embryos. These results indicate that the delayed expression of Cdx2 in SCNT embryos may lead to the ectopic expression of Oct3/4 in blastocysts and, along with the limited expression of Cdx2 and Eomes, may contribute to disorders in the function of the trophoblast lineage for normal placental development. This work was supported by a Grant-in-Aid for the 21st Century Center of Excellence Program of the MEXT, Japan, and by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science.