Molecular mechanism of placental Development: The role of TMED2, a vesicular transport protein during labyrinth layer formation

胎盘是哺乳类动物最早形成的器官，对胚胎的发育至关重要。小鼠胎盘，我们研究的模型系统，在组织学上分为三个不同的层：蜕层、海绵滋养层和迷路层。迷宫层是母婴环境中营养物质和气体交换的场所。它由靠近母体鼻窦的胚胎血管组成。在小鼠中，迷宫层的形成依赖于绒毛膜和尿囊这两个胚胎组织之间的附着和融合。我们发现，作为货物受体emp24结构域家族的一员，TMED2在绒毛膜和尿囊中表达，是迷宫层形成所必需的。因此，由于胎盘衰竭，Tmed2纯合突变胚胎在妊娠中期停滞。为了揭示TMED2在迷路层发育中的作用，我们建立了一个绒毛膜尿囊外植体模型，并表明它再现了与迷路层形成相关的早期分子和形态学变化。通过这种外植体模型，我们发现TMED2在绒毛膜中是绒毛膜尿囊附着所必需的，在两种组织中都是融合和迷路层形成所必需的。重要的是，我们发现尿囊中需要TMED2来分泌纤维连接蛋白（一种细胞外蛋白和潜在的货物）。

The placenta is the first organ formed in eutherian mammals and is essential for survival of the developing embryo. The mouse placenta, the model system that we study, is divided into three histologically distinct layers: decidual, spongiotrophoblast, and labyrinth. The labyrinth layer is the site of nutrient and gas exchange between the maternal and foetal environment. It consists of embryonic blood vessels that are in close proximity to maternal sinuses. In the mouse, labyrinth layer formation depends on attachment and fusion between two embryonic tissues: the chorion and the allantois. We showed that TMED2, a member of the emp24 domain family of cargo receptors, was expressed in the chorion and the allantois, and required for labyrinth layer formation. Thus, Tmed2 homozygous mutant embryos arrest at mid-gestation due to placental failure. To uncover the role of TMED2 in labyrinth layer development, we generated a chorioallantoic explant model and showed that it recapitulated the early molecular and morphological changes associated with labyrinth layer formation. Using this explant model, we found that TMED2 was required in the chorion for chorioallantoic attachment, and in both tissues for fusion and labyrinth layer formation. Importantly, we uncovered that TMED2 was required in the allantois for secretion of Fibronectin, an extracellular protein and potential cargo. Little is known about the function of TMED proteins during development and disease, although this family is conserved in all eukaryotes and essential for normal development. However, our work in the context of the placenta and the liver, combined with the work of others in multiple species and cell lines, indicates that TMED proteins are cargo receptors. Therefore, we will test two hypotheses: (1) TMED2-mediated transport of fibronectin is required in the allantois for chorioallantoic fusion; and (2) distinct TMED2-regulated pathways in the chorion and in the allantois are essential for labyrinth layer formation. The aims of this proposal are: (1) To determine if fibronectin is a TMED2 cargo protein and to uncover its role in labyrinth layer development; (2) To determine if TMED2 is required in both chorionic mesothelium and ectoderm for chorioallantoic attachment and fusion; and (3) To identify the TMED2-regulated pathways in the chorion and the allantois that are required for labyrinth layer formation. Our studies reveal a crucial role for TMED2 in placental development and, more specifically, in mediating communication between the chorion and allantois. Similar types of communication are essential for organogenesis. For example, during development of the lung where ventral foregut endodermal epithelium interacts with mesoderm to form the lungs. Since TMED2 and other members of this family are broadly expressed, we expect that our work will shed significant insight into the role of this protein and its cargoes in organogenesis.