胎盤融合性蛋白syncytin 1和syncytin 2 的功能分析

Abstract

中文摘要 在胎盤發育過程中，細胞滋養層細胞會進行細胞與細胞間融合而形成多核的融合滋養層細胞，融合滋養層細胞是母體與胎兒物質交換的主要媒介，因此細胞滋養層細胞間的融合對於胎盤發育是必須的過程。在先前的研究中已知人類內生性反轉錄病毒HERV-W的外鞘蛋白syncytin 1可能參與及調控細胞滋養層細胞的融合，而近幾年所發現另外的人類內生性反轉錄病毒HERV-FRD的外鞘蛋白syncytin 2也被發現大量表現於胎盤，暗示著syncytin 2調控細胞滋養層細胞融合的可能。 在過去本實驗室對於syncytin 1研究中發現syncytin 1 C端胺基酸序列會抑制syncytin 1的融合功能，為了解syncytin 2在功能與結構上和syncytin 1的異同，本研究首先探究syncytin 2的C端序列是否也抑制syncytin 2 的融合功能，經融合檢驗發現syncytin 2的C端胺基酸序列並不會抑制其融合功能，反而對於syncytin 2調控細胞間融合是必須的。 在syncytin 1與syncytin 2的蛋白生合成過程中，本研究發現syncytin蛋白皆會被內生性蛋白酶Furin所剪裁修飾，而形成surface（SU）及transmembrane（TM）兩次單元，並且兩次單元藉由形成雙硫鍵連接，藉由突變SU次單元上CXXC的結構發現突變蛋白會喪失調控細胞與細胞間融合的功能，並且都具有顯性抑制效應，代表著syncytin 1與syncytin 2在和受器作用與調控細胞融合上有著相似的機制。 綜合以上結果可以了解syncytin 2與syncytin 1在蛋白生合成及融合機制上有許多相似，進一步的暗示syncytin 2與syncytin 1在胎盤發育上扮演相似角色的可能性，並有助於未來繼續研究和釐清syncytin蛋白在胎盤發育中所扮演的角色。

Abstract Fusion of cytotrophoblast into the multinucleated syncytiotrophoblast layer is essential for the development of a functional placenta. Previous studies of placental development have shown that the envelope protein of a human endogenous retrovirus W (HERV-W), syncytin 1, may mediate placental cell fusion. Recently, the envelope protein of HERV-FRD, syncytin 2, has been identified and shown to be highly expressed in placenta. This implies that syncytin 2 may be involved in placental cell fusion. Our previous studies have demonstrated that the cytoplasmic domain (CTM) of syncytin 1 inhibits its fusogenic activity. In this study, we first tested whether the CTM of syncytin 2 regulates its fusogenic activity. We found that the CTM of syncytin 2 is required for its fusogenic activity. We further studied the biosynthesis of syncytins and demonstrated that Furin is involved in the proteolytic cleavage of syncytin 1 and syncytin 2 by using furin inhibitor and in the Furin-defective human colon carcinoma LoVo cell line. Finally, we studied the role of the disulfide-forming CXXC and CX6CC motifs in syncytins and found that the disulfide linkage between SU and TM subunits is essential for the fusogenic activity of syncytins because mutation in the CXXC motif abolishes the fusogenic activity of syncytins and causes a dominant-negative effect. Overall, this study provides a better understanding of the biology of syncytin 1 and syncytin 2.