Abstract
The functional role of activins in mesoderm induction has been shown in early amphibian embryos; however, its role in early mammalian embryogenesis is still unknown. The protein and mRNA for activin subunits are present in mouse preimplantation embryos. In this study we report on the expression of activin receptor type II (ActR-II) gene in pre- and post-implantation mouse embryos and the regulation of ActR-II mRNA in F9 embryonal carcinoma cells that served as a model for murine embryonic development. The results show that ActR-II gene is expressed in ovulated mature oocytes, becomes decreased at 2-celll stage, and is substantially activated in day-6 embryo at the stage when ectoderm and endoderm are formed. When F9 cells are treated with retinoic acid (RA) and undergo endodermal differentiation, ActR-II mRNA is induced. The 6.0 kb transcript is enhanced more than the 3.0 kb transcript. suggesting that these two mRNA species are controlled by different promotors. Addition of cyclic AMP analogue, which further induces differentiation of F9 cells into parietal endoderm, does not further enhance ActR-II gene expression. The effect of RA on ActR-II mRNA levels is direct and does not require ongoing protein synthesis. Furthermore, all-trans-RA is more effective than 9-cis-RA in the induction of ActR-II gene expression. The results of this study demonstrate that the ActR-II gene is activated during endodermal differentiation and suggest that activin action in mammalian embryonic development is mediated through increased activin receptor gene expression.
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