Loss of polarity regulators initiates gasdermin-E-mediated pyroptosis in syncytiotrophoblasts.

The syncytiotrophoblast is a human epithelial cell that is bathed in maternal blood on the maternal-facing surface of the human placenta. It therefore acts as a barrier and exchange interface between the mother and fetus. Syncytiotrophoblast dysfunction is a feature of pregnancy pathologies, like preeclampsia. Dysfunctional syncytiotrophoblasts display a loss of microvilli, which is a marker of aberrant apical-basal polarization, but little data exist about the regulation of syncytiotrophoblast polarity. Atypical PKC isoforms are conserved polarity regulators. Thus, we hypothesized that aPKC isoforms regulate syncytiotrophoblast polarity. Using human placental explant culture and primary trophoblasts, we found that loss of aPKC activity or expression induces syncytiotrophoblast gasdermin-E-dependent pyroptosis, a form of programmed necrosis. We also establish that TNF-α induces an isoform-specific decrease in aPKC expression and gasdermin-E-dependent pyroptosis. Therefore, aPKCs are homeostatic regulators of the syncytiotrophoblast function and a pathogenically relevant pro-inflammatory cytokine leads to the induction of programmed necrosis at the maternal-fetal interface. Hence, our results have important implications for the pathobiology of placental disorders like preeclampsia.

Genome-Wide Analysis of Hypoxia-Inducible Factor Binding Reveals Targets Implicated in Impaired Human Placental Syncytiotrophoblast Formation under Low Oxygen.

Preeclampsia (PE) is a common and serious complication of pregnancy with no cure except premature delivery. The root cause of PE is improper development of the placenta-the temporary organ supporting fetal growth and development. Continuous formation of the multinucleated syncytiotrophoblast (STB) layer via differentiation and fusion of cytotrophoblasts (CTBs) is vital for healthy placentation and is impaired in preeclamptic pregnancies. In PE, there is reduced/intermittent placental perfusion, likely resulting in a persistently low O2 environment. Low O2 inhibits differentiation and fusion of CTBs into STB and may thus contribute to PE pathogenesis; however, the underlying mechanisms are unknown. Because low O2 activates a transcription factor complex in cells known as the hypoxia-inducible factor (HIF), the objective of this study was to investigate whether HIF signaling inhibits STB formation by regulating genes required for this process. Culture of primary CTBs, the CTB-like cell line BeWo, and human trophoblast stem cells under low O2 reduced cell fusion and differentiation into STB. Knockdown of aryl hydrocarbon receptor nuclear translocator (a key component of the HIF complex) in BeWo cells restored syncytialization and expression of STB-associated genes under different O2 levels. Chromatin immunoprecipitation sequencing facilitated the identification of global aryl hydrocarbon receptor nuclear translocator/HIF binding sites, including several near genes implicated in STB development, such as ERVH48-1 and BHLHE40, providing new insights into mechanisms underlying pregnancy diseases linked to poor placental O2 supply.

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.

Human placenta and trophoblasts simultaneously express three isoforms of atypical protein kinase-c.

Atypical protein kinase-c (aPKC) isoforms are important regulators of polarity and stem cell differentiation. There are three isoforms of aPKC: aPKC-ι, aPKC-ζ, and PKM-ζ. Recently, aPKC-ι was shown to regulate human trophoblast stem cell (TSC) differentiation. Compensation by remaining isoforms when a single aPKC isoform is lost can occur, but the expression pattern of aPKC-ζ in placenta is unknown. Here we show that aPKC-ι, aPKC-ζ and a new isoform, aPKC-ζ III, are expressed in trophoblasts. Therefore, studies examining the role of aPKC isoforms that control for potential compensation between aPKC isoforms are necessary to understand aPKC-mediated regulation of TSC differentiation.