Functional analysis of transforming growth factor-beta related molecules during early mouse development

Embryogenesis requires cell-cell communication in which individual cells send and receive signals that determine their final fate. The transforming growth factor-β (TGF-β) superfamily of molecules encodes secreted factors which are capable of regulating cellular growth and differentiation. Recently, members of the TGF-β superfamily have emerged as candidates for many postulated embryonic inductive interactions. As a step towards understanding such interactions, this thesis investigates the hypothesis that TGF-β superfamily molecules may act as embryonic inducing factors in the mouse. The embryonic expression of a number of TGF-β superfamily members and a putative inhibitor of one of these, follistatin, were investigated. While activin (a mesoderm inducing factor in Xenopus animal caps) was not expressed in the mouse embryo, follistatin, BMP-7, BMP-2 and Vgr-2 were expressed during gastrulation and early organogenesis. Strategies were designed to ectopically express two of these molecules, BMP-7 and Vgr-2 in embryogenesis. Firstly, embryonic stem (ES) cells were used as a vehicle to secrete Vgr-2 throughout the embryo. No effect on embryonic morphology was detected, although conditioned medium from the ES cells is capable of inducing mesoderm in Xenopus. Secondly, COS cells were used to deliver a localised source of BMP-7 to the cranial neural plate in cultured embryos. Such ectopic expression resulted in the induction of ectopic dorsal neural marker expression (Msx1 and AP-2), the diminished expression of a ventral marker (Shh), and a localised expansion of neurectodermal tissue. The localisation of endogenous BMP-7 mRNA during formation and patterning of the neural axis, together with the activities shown in this assay, suggest that BMP-7 may direct one or more aspects of early central nervous system formation in the mouse.