Taube nuss is a novel gene essential for the survival of pluripotent cellsof early mouse embryos

The cells of the inner cell mass constitute the pluripotent cell population of the early embryo. They have the potential to form all of the tissues of the embryo proper and some extra-embryonic tissues. They can be considered a transient stem cell population for the whole of the embryo, and stem cel ls maintaining the same capacity can be isolated from these cells. We have isolated, characterised and mutated a novel gene, taube nuss (Tbn), that is essential for the survival of this important cell population. The taube nu ss protein sequence (TBN) was highly conserved between human, mouse, Xenopu s laevis, Drosophila melanogaster, Caenorhabditis elegans and Arabidopsis t haliana, particularly in a domain that is not present in any published prot eins, showing that TEN is the founding member of a completely new class of proteins with an important function in development. The Tbn gene was expres sed ubiquitously as early as E2.5 and throughout embryonic development. It was also expressed in adult brain,vith slightly higher levels in the hippoc ampus. The Tbn mutant embryos developed normally to the blastocyst stage an d contained inner cell masses. They hatched from the zonae pellucidae, impl anted and induced decidual reactions, but failed to develop beyond E4.0, At this time the trophoblast cells were viable, but inner cell masses were no t detectable. At E3.75, massive TUNEL-positive DNA degradation and chromati n condensation were visible within the inner cell masses, whereas the cell membranes where intact. Caspase 3 was expressed in these cells. In vitro, t he inner cell mass of mutant embryos failed to proliferate and died after a short period in culture. These results indicate that the novel protein, ta ube nuss, is necessary for the survival of the inner cell mass cells and th at inner cell mass cells died of apoptosis in the absence of the taube nuss protein. As cell pruning by apoptosis is a recognised developmental proces s at this stage of development, the taube nuss protein may be one of the fa ctors regulating the extent of programmed cell death at this time point.