Abstract
Transient maintenance of a pluripotent embryonic cell population followed by the onset of multi-lineage commitment is a fundamental aspect of development. However, molecular regulation of this transition is not well characterized in vivo. Here, we demonstrate that the nuclear protein Geminin is required to restrain commitment and spatially restrict mesoderm, endoderm and non-neural ectoderm to their proper locations in the Xenopus embryo. We used microarray analyses to demonstrate that Geminin overexpression represses many genes associated with cell commitment and differentiation, while elevating expression levels of genes that maintain pluripotent early and immature neurectodermal cell states. We characterized the relationship of Geminin to cell signaling and found that Geminin broadly represses Activin-, FGF- and BMP-mediated cell commitment. Conversely, Geminin knockdown enhances commitment responses to growth factor signaling and causes ectopic mesodermal, endodermal and epidermal fate commitment in the embryo. We also characterized the functional relationship of Geminin with transcription factors that had similar activities and found that Geminin represses commitment independent of Oct 4 ortholog (Oct25/60) activities, but depends upon intact Polycomb repressor function. Consistent with this, chromatin immunoprecipitation assays directed at mesodermal genes demonstrate that Geminin promotes Polycomb binding and Polycomb-mediated repressive histone modifications, while inhibiting modifications associated with gene activation. This work defines Geminin as an essential regulator of the embryonic transition from pluripotency through early multi-lineage commitment, and demonstrates that functional cooperativity between Geminin and Polycomb contributes to this process.