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), thatregulates 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 nullembryos develop to full term, embryonic stem cells (ESCs) derived from Kit nullblastocysts 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 ofESC derivation from inner cell mass (ICM). We found that transgenic mice carrying Kitdifferent promoter regions fused to EGFP (Kit-EGFP) initiate EGFP expression at morulastage. EGFP expression is then maintained in the blastocyst, within the ICM, and its levelsincrease in the presence of MAPK and GSK3? inhibitors (2i) and LIF compared to the LIFonly condition. Kit-EGFP ESCs but not primordial germ cells (PGCs), showed nonhomogeneousEGFP expression pattern when cultured in LIF condition, and they upregulatedEGFP 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 cellsinduced a decrease and an increase of Kit levels, respectively. Finally, Kit constitutiveactivation induced by the D814Y mutation increased ESC proliferation in vitro and interatoma assays in vivo. Our results show that Kit regulatory regions respond to groundstate culture conditions and suggest a role of Kit in the regulation of ESC proliferation.