Cell polarity signaling in the regulation of syncytiotrophoblast homeostasis and inflammatory response.

Maintenance of cell polarity and the structure of the apical surface of epithelial cells is a tightly regulated process necessary for tissue homeostasis. The syncytiotrophoblast of the human placenta is an entirely unique epithelial layer. It is a single giant multinucleate syncytial layer that comprises the maternal-facing surface of the human placenta. Like other epithelia, the syncytiotrophoblast is highly polarized with the apical surface dominated by microvillar membrane protrusions. Syncytiotrophoblast dysfunction is a key feature of pregnancy complications like preeclampsia. Preeclampsia is commonly associated with a heightened maternal immune response and pro-inflammatory environment. Importantly, reports have observed disruption of syncytiotrophoblast apical microvilli in placentas from preeclamptic pregnancies, indicating a loss of apical polarity, but little is known about how the syncytiotrophoblast regulates polarity. Here, we review the evolutionarily conserved mechanisms that regulate apical-basal polarization in epithelial cells, and the emerging evidence that PAR polarity complex components are critical regulators of syncytiotrophoblast homeostasis and apical membrane structure. Pro-inflammatory cytokines have been shown to disrupt the expression of polarity regulating proteins. We also discuss initial data showing that syncytiotrophoblast apical polarity can be disrupted by the addition of the pro-inflammatory cytokine tumor necrosis factor-α, revealing that physiologically relevant signals can modulate syncytiotrophoblast polarization. Since disrupted polarity is a feature of preeclampsia, further elucidation of the syncytiotrophoblast-specific polarity signaling network and testing whether the disruption of polarity-factor signaling networks may contribute to the development of preeclampsia is warranted.