Gastrokine-2 is transiently expressed in the endodermal and endothelial cells of the maturing mouse yolk sac.

The yolk sac (YS) is a thin, bi-layered membrane encompassing the developing mammalian embryo in utero. The outer layer of the YS is composed of visceral endodermal cells derived from the primitive endoderm. The inner mesenchymal layer is highly vascularised and the first source of haematopoietic cells. YS haematopoiesis takes place from shortly after gastrulation (E7.5) until after midgestation (~E12.5) when the placenta and sites within the embryo proper predominate as sites of blood cell production. Here, we have assessed gene expression in the developing YS to determine what factors are associated with the end of haematopoietic cell production in this extra-embryonic tissue. We have observed that transcripts encoding gastrokine-2 (Gkn2) are up-regulated in the YS at E11.5, peak at E12.5 and are then reduced. Low levels of Gkn2 transcript were detected in purified YS endothelial cells. Gastrokine-2 protein was detected in the EpCAM-expressing visceral endodermal layer. Gastrokine-2 protein predominantly clustered at the basal side of the visceral endodermal cells facing the endothelial cell layer though did not appear to be strongly associated with secretory lysosomes. Gastrokine-1 protein binds F-actin. In contrast, gastrokine-2 in the YS is predominantly expressed at the basal side of the visceral endodermal cells, whereas F-actin in the YS is mostly found in the inner leaf of the luminal side of visceral endoderm. We have therefore identified expression of Gkn2 transcript and protein in the visceral endodermal layer and at lower levels in YS endothelial cells. This previously unreported expression of a "stomach-specific" secreted protein may be linked to the changes in the YS niche coinciding with the shift of haematopoiesis away from the YS into the embryo.