The requirement of Smad4 in mouse early embryonic development

This dissertation is divided into two Chapters: Chapter I focuses on the requirement of maternal Smad4 in mouse preimplantation development; Chapter II focuses on the function of zygotic Smad4 in trophoblast lineage development.;Chapter I: Maternally contributed transcripts and proteins are crucial for shaping the earliest developmental programs. Smad4 is a central mediator of the Transforming Growth Factor beta (TGF-ss) superfamily, molecules of which are important regulators of development and tissue homeostasis. We found that Smad4 gene products are abundant in unfertilized eggs as well as cleaving blastomeres of mouse preimplantation embryos. To investigate the role of maternal Smad4 during mouse preimplantation development, we have conditionally inactivated Smad4 in mouse female germline. Here we report that Smad4 is a new member of mouse maternal effect genes that lacking of maternal Smad4 results in peri-implantation lethality severe preimplantation development defects in mitosis, polarization, and first lineage segregation between trophectoderm (TE) and inner cell mass (ICM). Not only have our studies uncovered novel functions of Smad4-dependent signaling in preimplantation development, but also have provided important insights into how maternal effect genes influence the earliest development programs in mammals.;Chapter II: Trophoblast lineage gives rise to embryonic portion of the placenta, the function of which is essential for the establishment and maintenance of pregnancy. Mouse trophoblast stem (TS) cells established from the outgrowth of the polar trophectoderm of blastocysts or the extraembryonic ectoderm of post-implantation embryos are the precursors of trophoblasts and can contribute to all trophoblast lineage derivatives in vivo, providing a powerful in vitro system for the study of trophoblast stem cell self-renewal and differentiation. Although it is known that TGF-ss/Nodal-related signaling together with FGF4 signaling is critical for TS self-renewal, the function of Smad4, the central mediator of the TGF-ss related signaling pathways, in TS cell has not been fully investigated, partially due to the early lethality of Smad4 null embryos around peri-gastrulation stage. By studying Smad4-deficient TS cells, we have uncovered a critical requirement for Smad4 mediated TGF-ss related signaling in trophoblast lineage development. We show that Smad4 deficiency alters the differentiation kinetics and trophoblast lineage distribution of the mutant TS cells. Moreover, Smad4 null TS cells display enhanced mobility and invasiveness and undergo epithelial-mesenchymal transition (EMT). Consistent with the changes in cellular properties, transcriptome profiling reveals misregulation of genes associated with EMT, tumorigenesis and cancer progression in Smad4 null TS cells. Our data provides evidence linking the differentiation defects and EMT properties seen in the Smad4 null TS cells to alterations in several signaling pathways, including the WNT pathway, whose role in TS cell biology has not been well appreciated previously. Taken together, our studies revealed a novel requirement for Smad4 in maintaining the differentiation potential, epithelial integrity and homeostasis of TS cells through Smad4-depedent TGF-ss related signaling and its crosstalks with other developmentally important signaling pathways. Our analysis will provide insights into the molecular mechanisms that regulate trophoblast lineage development in vivo.