Abstract
ZO-1, ZO-2 and ZO-3 are closely related scaffolding proteins that link tight junction (TJ) transmembrane proteins such as occludin, claudins and junctional adhesion molecules to the actin cytoskeleton. Despite being among the first TJ proteins to have been identified and having undergone extensive biochemical analysis, little is know about the physiological roles of individual ZO proteins in different tissues or during vertebrate development. Here, we show that ZO-3-/- mice lack an obvious phenotype. In contrast, embryos deficient for ZO-2 die shortly after implantation due to an arrest in early gastrulation. ZO-2-/- embryos show decreased proliferation at E6.5, increased apoptosis at E7.5 and altered architecture of the apical junctional complex as compared to wild-type. ZO-1-/- mice are currently unavailable, while chimeric mice derived from ZO-1-/- embryonic stem (ES) cells are embryonic lethal. Because of the embryonic lethality of the ZO-1-/- and ZO-2-/- mice, we also generated knockout ES cell lines. We have obtained ZO-1-/-, ZO-2-/-, ZO-3-/-, ZO-1-/-ZO-2-/-, and ZO-2-/-ZO-3-/- ES cells. These cell lines have shown various defects in their ability to differentiate into epithelial cells, cardiomyocytes or skeletal muscle cells.

Figure Legend: Compromised blood-testis barrier function in testis from ZO-2 chimera. (A-D) (Electron micrographs of lanthanum tracer studies. At a few sites in the WT seminiferous tubule, the lanthanum tracer (black) permeates across the incomplete system of TJs of the myoid cell layer and into the intercellular space between Sertoli cells and spermatogonia (A and B). Diffusion is restricted at the levels of the BTB (B, arrow) and the tracer was never observed in the adluminal compartment. In contrast, in testis from ZO-2 chimera, an increased leakage of lanthanum tracer into the adluminal compartment and between Sertoli cells and multiple spermatocytes is observed (C and D).

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