Abstract
The signal transduction adapter protein Disabled-2 (Dab2) is one of the two mammalian orthologs of the
Drosophila Disabled. The brain-specific Disabled-1 (Dab1) functions in positional organization of brain cells during development. Dab2 is widely distributed and is highly expressed in many epithelial cell types. The
dab2 gene was interrupted by in-frame insertion of β-galactosidase (LacZ) in embryonic stem cells and transgenic mice were produced. Dab2 expression was first observed in the primitive endoderm at E4.5, immediately following implantation. The homozygous Dab2-deficient mutant is embryonic lethal (earlier than E6.5) due to defective cell positioning and structure formation of the visceral endoderm. In E5.5
dab2 (−/−) conceptus, visceral endoderm-like cells are present in the deformed primitive egg cylinder; however, the visceral endoderm cells are not organized, the cells of the epiblast have not expanded, and the proamniotic cavity fails to form. Disorganization of the visceral endodermal layer is evident, as cells with positive visceral endoderm markers are scattered throughout the
dab2 (−/−) conceptus. Only degenerated remains were observed at E6.5 for
dab2 (−/−) embryos, and by E7.5, the defective embryos were completely reabsorbed. In blastocyst
in vitro culture, initially cells with characteristics of endoderm, trophectoderm, and inner cell mass were observed in the outgrowth of the hatched
dab2 (−/−) blastocysts. However, the
dab2 (−/−) endodermal cells are much more dispersed and disorganized than those from wild-type blastocysts, the inner cell mass fails to expand, and the outgrowth degenerates by day 7. Thus, Dab2 is required for visceral endodermal cell organization during early mouse development. The absence of an organized visceral endoderm in Dab2-deficient conceptus leads to the growth failure of the inner cell mass. We suggest that Dab2 functions in a signal pathway to regulate endodermal cell organization using endocytosis of ligands from the blastocoel cavity as a positioning cue.
