PKC signalling regulates tight junction membrane assembly in the pre-implantation mouse embryo

Epithelial differentiation including tight junction (TJ) formation occurs exclusively within the trophectoderm (TE) lineage of the mouse blastocyst. Here we examine mechanisms by which TJ protein membrane assembly might be regulated by protein kinase C (PKC) in the embryo. To overcome the inherent staging asynchrony of individual blastomeres within intact embryos, we have used isolated inner cell masses (ICMs) from early blastocysts to induce epithelial differentiation in their outer cells responding to their new cell contact pattern. Two TJ proteins examined retain their order of membrane assembly in isolated ICMs in culture as during normal development (early-assembling ZO-2 and late-assembling ZO-1alpha(+)), but this process is highly accelerated. Using six chemical modulators of PKC activity, we show here that PKC signalling is involved in the regulation of TJ membrane assembly. While indolactam-mediated PKC activation stimulates membrane assembly of both TJ proteins, TPA-mediated PKC activation stimulates only that of ZO-1alpha(+). The PKC inhibitors Ro-31-8220, Ro-31-8425 and Go 6983 suppress the stimulatory effect of both PKC activators on membrane assembly to varying extents according to inhibitor and TJ protein examined. Go 6983 similarly inhibits ZO-2 and ZO-1alpha(+) membrane assembly. PKC inhibition by G6 6976 appeared to stimulate TJ membrane assembly. Despite the broad PKC isotype specificity of the inhibitors used, these data suggest that the two TJ proteins are differently regulated by PKC isotypes or subfamilies. As Go 6983 uniquely affects aPKC (particularly PKCzeta) and we find that both PKCdelta and relocate upon activator treatment to co-localise partially with the TJ proteins in isolated ICMs, we suggest that at least PKCdelta and zeta may play a central role in regulating TJ membrane assembly.