Global, Survival, and Apoptotic Transcriptome during Mouse and Human Early Embryonic Development.

Survival and cell death signals are crucial for mammalian embryo preimplantation development. However, the knowledge on the molecular mechanisms underlying their regulation is still limited. Mouse studies are widely used to understand preimplantation embryo development, but extrapolation of these results to humans is questionable. Therefore, we wanted to analyse the global expression profiles during early mouse and human development with a special focus on genes involved in the regulation of the apoptotic and survival pathways. We used DNA microarray technology to analyse the global gene expression profiles of preimplantation human and mouse embryos (metaphase II oocytes, embryos at the embryonic genome activation stage, and blastocysts). Components of the major apoptotic and survival signalling pathways were expressed during early human and mouse embryonic development; however, most expression profiles were species-specific. Particularly, the expression of genes encoding components and regulators of the apoptotic machinery were extremely stable in mouse embryos at all analysed stages, while it was more stage-specific in human embryos. CASP3, CASP9, and AIF were the only apoptosis-related genes expressed in both species and at all studied stages. Moreover, numerous transcripts related to the apoptotic and survival pathway were reported for the first time such as CASP6 and IL1RAPL1 that were specific to MII oocytes; CASP2, ENDOG, and GFER to blastocysts in human. These findings open new perspectives for the characterization and understanding of the survival and apoptotic signalling pathways that control early human and mouse embryonic development.

Developmental regulated expression of anti- and pro-apoptotic BCL-2 family genes during human early embryonic development.

Apoptotic cell death has been reported in human oocytes and preimplantation embryos under in vivo and in vitro conditions. BCL-2 family proteins comprise both anti- and pro-apoptotic members, which are likely to play a key role in controlling oocyte and early embryo survival. However, very limited data are available on their expression kinetics during human early embryonic development. Using our DNA microarray data, we analyzed the expression pattern of 21 BCL-2 family genes in human mature MII oocytes, day 3 embryos and day 5/6 blastocysts from patients who underwent in vitro fertilization (IVF). Selected genes were further validated by qRT-PCR and their subcellular localization analyzed by immunofluorescence confocal microscopy. Our results suggest a switch from oocyte-inherited BCL-2 family transcripts, such as BCL2L10, to embryo-produced transcripts after embryonic genome activation, including BIK, BCL2L11 and NOXA. Moreover, the pro-apoptotic gene BCL2L13 was constitutively expressed throughout human early embryonic development. Remarkably, day 3 embryos expressed more BCL-2 pro-apoptotic genes than mature MII oocytes and day 5/6 blastocysts, suggesting that embryos at this stage are more prone to apoptosis. This is further supported by an absence of cleaved Caspase-3 in the oocyte and its presence in the embryo. Using a drug that induces apoptosis (gambogic acid), we were able to show activated Caspase-3 in the oocyte in addition to an alteration of BCL2L13 protein localization. Similarly BCL2L13 localization was altered in degenerated oocytes. This study opens new perspectives for understanding the molecular regulation of human oocyte and pre-implantation embryo survival and death.

[The Bcl-2 family pathway in gametes and preimplantation embryos]. / Les régulateurs d'apoptose de la famille Bcl-2 dans les gamètes et lors du développement embryonnaire précoce.

Apoptosis, a form of cell death by self-destruction, has been reported in gametes and preimplantation embryos both in vitro and in vivo. Recent evidence suggests that cell death processes, whose control deserves to be elucidated, can impact embryo developmental competence. Moreover, quality of the gametes (particularly of the oocytes) is relevant not only for their survival rates but exert an influence during the early stages of embryo development. Thus, the investigation of apoptosis-related genes and mechanisms in early embryos is crucial. BCL-2 family proteins, through balanced interactions between pro- and anti-death members, play a pivotal role in controlling cell life and death. In this article, we review the literature concerning the expression of Bcl-2 family members in gametes and early embryos. Research results indicate that the various Bcl-2 subfamilies (pro- and anti-apoptotic "multidomain" family members and "BH3-only" death factors) exhibit a dynamic expression pattern during male and female gamete differentiation and early embryo development. While pro-apoptotic Bax protein plays a critical role in germ cell and early embryo degeneration, the relative importance of the prosurvival (Bcl-2, Bcl-xL, Bcl-w, Mcl-1) and "BH3-only" (Bim, Bad, Bik) members is not clear. Although information on expression patterns of Bcl-2 family transcripts and proteins is necessary, other elements such as transcriptional control (by environmental stimuli), subcellular localization and post-translational modifications should also be taken into account. Aside from basic research, a better understanding of apoptosis-related proteins and mechanisms involved in gamete and embryo viability at the molecular level may provide new guides for diagnosis and therapeutic strategies.