The development of polyploid cells has been most extensively studied in model invertebrates such as the fruit fly and nematode, where polyploidy is more common than it is in mammals. Trophoblast giant (TG) cells, which are required for embryonic implantation and give rise to the placenta, are one of only two mammalian cell types that undergo endoreduplication. Cultured murine trophoblast stem (TS) cells differentiate into TG cells when they are deprived of fibroblast growth factor 4 (FGF4). Ullah et al. report that inhibition of cyclin-dependent kinase 1 (CDK1) in TS cells with the CDK1 inhibitor RO3306 caused the cells to increase in size, undergo endoreduplication, and initiate TG cell-specific gene expression. RO3306 treatment was not sufficient to induce endoreduplication in embryonic stem cells; therefore, some developmental programming was required to make TS cells competent for endoreduplication. The switch to endocycles was not simply a result of mitotic arrest, because treating TS cells with nocodazole was not sufficient to induce endoreduplication. The activity of CDK1 immunoprecipitated from TS cell extracts decreased after FGF4 deprivation, as determined by a histone H1 phosphorylation assay. Western blotting indicated that the amount of the CDK1 inhibitors p21 (CIP1) and p57 (KIP2) increased during endoreduplication induced by FGF4 deprivation. Both p21-CDK and p57-CDK1 complexes were immunoprecipitated from TG, but not TS, cells, which suggests that one or both of these proteins could be responsible for inhibiting CDK1 activity to induce endocycling. p57 was required for endoreduplication in response to FGF4 deprivation but not for endoreduplication induced by RO3660. p21 was not required for FGF4 deprivation-induced endoreduplication but instead appeared to play a role in suppressing activation of the DNA damage response in endocycling cells. These data suggest a model in which endoreduplication in mammals is triggered by p57 inhibition of CDK1 and maintained by p21. Inhibition of CDK1 activity induces endoreplication in both flies and mammals, but the mechanisms by which it is inhibited differ.
Z. Ullah, M. J. Kohn, R. Yagi, L. T. Vassilev, M. L. DePamphilis, Differentiation of trophoblast stem cells into giant cells is triggered by p57/Kip2 inhibition of CDK1 activity. Genes Dev. 22, 3024-3036 (2008). [Abstract] [Full Text]