Regulation of Kit expression in early mouse embryos and ES cells

Kit is an essential growth factor receptor, encoded at the White spotting locus (W), that regulates proliferation and/or survival of many embryonic and postnatal stem cell types. When mutated, it can induce malignant transformation of the host cells. Although Kit null embryos develop to full term, embryonic stem cells (ESCs) derived from Kit null blastocysts fail to survive without LIF. To understand the role of Kit in ESC pluripotency, we studied its expression during early mouse embryogenesis and during the process of ESC derivation from inner cell mass (ICM). We found that transgenic mice carrying Kit different promoter regions fused to EGFP (Kit-EGFP) initiate EGFP expression at morula stage. EGFP expression is then maintained in the blastocyst, within the ICM, and its levels increase in the presence of MAPK and GSK3? inhibitors (2i) and LIF compared to the LIF only condition. Kit-EGFP ESCs but not primordial germ cells (PGCs), showed nonhomogeneous EGFP expression pattern when cultured in LIF condition, and they upregulated EGFP expression, as well as that of Sox2, Nanog, Prdm14 but not Oct4, following 2i-LIF culture. Sox2 deletion or overexpression in ESCs or postnatal germ cells induced a decrease and an increase of Kit levels, respectively. Finally, Kit constitutive activation induced by the D814Y mutation increased ESC proliferation in vitro and in teratoma assays in vivo. Our results show that Kit regulatory regions respond to ground state culture conditions and suggest a role of Kit in the regulation of ESC proliferation.