GCM1標的基因high temperature requirement A4 (HtrA4)調控胎盤細胞融合及入侵的機制

Abstract

細胞融合和細胞入侵在胎盤發育的過程扮演重要且不可或缺的角色。絨毛膜絨毛(chorionic villi)上的滋養葉胞層細胞 (cytotrophoblast)可透過細胞融合分化成多核的滋養葉融合層細胞 (syncytiotrophoblast)，進而幫助養份和氣體的交換,另一方面，滋養葉胞層細胞透過分化形成外絨毛滋養細胞 (extravillous trophoblast)，促使滋養細胞入侵到子宮組織，幫助母體和胎兒血液循環系統的交通。在我們過去的研究中發現，人類胎盤特殊轉錄因子GCM1 (Glial cells missing homolog 1)可透過控制融合蛋白syncytin-1和syncytin-2的基因表現來調控胎盤滋養葉細胞的細胞融合。值得一題的是，GCM1和syncytin-1蛋白都會在外絨毛滋養細胞中表現，然而，我們卻不知道它們在生理上的功能。為了探討此一問題，我們首先在胎盤細胞中證實GCM1具有促進胎盤細胞入侵的功能。我們進一步利用染色質免疫沉澱技術結合晶片分析(ChIP-chip analysis)，鑑定發現HtrA4 (high temperature requirement A4)是GCM1獨特的標的基因，HtrA4蛋白具有絲氨酸蛋白酶的活性，透過切割fibronectin可幫助胎盤細胞的入侵功能。重要的是，透過免疫組織染色方法偵測到HtrA4蛋白位於胎盤和子宮的接觸面，並且發現HtrA4蛋白在缺氧環境的細胞和子癇先兆症病患的胎盤表現量明顯減少，顯示缺氧環境下HtrA4蛋白的減少可能造成外絨毛滋養細胞淺層入侵子宮組織，進而導致子癇先兆症。我們更進一步證實，HtrA4蛋白可直接和syncytin-1蛋白的SU區域結合,達到抑制細胞融合的功能。因此，HtrA4蛋白可能透過抑制細胞融合和促進細胞入侵的雙重角色，完成調控外絨毛滋養細胞分化的功能。在我們的研究中揭露了GCM1和HtrA4蛋白的新功能，具有調控外絨毛滋養細胞入侵的能力，並且證實異常HtrA4蛋白的表現可能導致子癇先兆症的形成。

Cell-cell fusion and cell invasion are essential for placental development. Human cytotrophoblasts in the chorionic villi may undergo cell-cell fusion to form syncytiotrophoblast to facilitate nutrient-gas exchange or differentiate into extravillous trophoblasts (EVTs) to facilitate maternal-fetal circulation. In our previous studies, the placental transcription factor, glial cell missing 1 (GCM1), regulates syncytin-1 and -2 expression to mediate trophoblast fusion. Interestingly, GCM1 and syncytin-1 are also expressed in EVTs with unknown physiological functions. We firstly demonstrated that GCM1 promote placental cells invasion. We also performed ChIP-chip analysis and identify high temperature requirement A4 (HtrA4) as a GCM1 novel target gene, which encodes a serine protease facilitating cleavage of fibronectin and invasion of placental cells. Importantly, HtrA4 is immunolocalized in EVTs at the maternal-fetal interface and its expression is decreased by hypoxia and in preeclampsia, a pregnancy complication associated with placental hypoxia and shallow trophoblast invasion. We further demonstrate that HtrA4 interacts with SU domain of syncytin-1 and suppresses cell-cell fusion. Therefore, HtrA4 may be crucial for EVT differentiation by playing a dual role in prevention of cell-cell fusion of EVTs and promotion of their invasion into the uterus. Our study reveals a novel function of GCM1 and HtrA4 in regulation of trophoblast invasion and that abnormal HrtA4 expression may contribute to shallow trophoblast invasion in preeclampsia.