Apoptosis in mouse amniotic epithelium is induced by activated macrophages through the TNF receptor type 1/TNF pathway

The amniotic epithelium is in direct contact with the amniotic fluid and has tight junctions. The amniotic tight junctions function as a barrier to restrict fluid flux via the amniotic membrane during midpregnancy in the mouse. However, during late pregnancy, amniotic fluid volume significantly decreases in association with the disruption of amniotic tight junctions. The disruption of amniotic tight junctions is caused by apoptosis in the amniotic epithelium on Embryonic Day 17 (E17). In this study, we examine the molecular mechanisms underlying apoptosis of the amniotic epithelium of the mouse. We found that from E16, the number of activated macrophages that express high levels of NOS2 and tumor necrosis factor (TNF) increase in amniotic fluid. TNF receptor type 1 (TNFR1) was detectable from E16 onward. On E17, amniotic epithelial cells expressing TNFR1 became TUNEL positive, suggesting that TNF/TNFR1 signaling may initiate apoptosis. To further confirm the role of TNF/TNFR1 signaling, WP9QY, a TNFR1 antagonist, was injected into the amniotic cavity and was found to significantly reduce the numbers of apoptotic cells in the E17 amniotic epithelium. Furthermore, dehydroxymethylepoxyquinomicin, a specific nuclear factor-kappa B inhibitor, was found to inhibit TNF production in macrophages and amniotic apoptosis in vivo. Finally, we showed that injection of TNF into the amniotic cavity induces early onset of apoptosis. These results indicate that amniotic apoptosis is induced by the TNF pathway via TNFR1 expressed in the amniotic epithelial cells and that activation of macrophages may trigger amniotic apoptosis.