In vivo functional analysis of ezrin during mouse blastocyst formation

During mouse blastocyst formation, a layer of outer cells differentiates in less than 48 h into a functional epithelium (the trophectoderm). Ezrin, an actin-binding structural component of microvilli in epithelial cells, is a lso involved in signal transduction and ionic pump control. In the mouse em bryo, ezrin becomes restricted to the apical cortex of all blastomeres at c ompaction and of outer cells at later stages. Here we investigated the func tion of ezrin in living embryos during epithelial differentiation using mut ant forms of ezrin tagged with green fluorescent protein (GFP). GFP-tagged wild-type ezrin (Ez/GFPc) behaved like endogenous ezrin and did not interfe re with development. Deletion of the last 53 amino acids (Delta 53/GFP) cha nged the localization of ezrin: after compaction, Delta 53/GFP remained ass ociated with the apical and basolateral cortex in all blastomeres, and its expression slightly disturbed the cavitation process. Finally, full-length ezrin with GFP inserted at position 234 (Ez/GFPi) was localized all around the cortex throughout development, although it was concentrated at tile api cal pole after compaction. In embryos expressing Ez/GFPi, the duration of t he 16-cell stage was reduced, while the onset of cavitation was delayed. Mo reover, cavitation was abnormal, and the blastocoele was small and retracte d almost completely several times as if there were major leakages of blasto coelic fluid. Our results suggest that, in addition to its role in microvil li organization, ezrin is involved in the formation of a functional epithel ium through a still unknown mechanism. (C) 2001 Academic Press.