2003). cells displayed an epithelial-to-mesenchymal transition (EMT)-like process characterized by the loss of cell polarity, cell ingression, QX 314 chloride and the up-regulation of the EMT and the mesodermal marker genesEomes,Brachyury/T, andFGF8. These results suggest that the AVE acts as a morphogenetic boundary to prevent EMT and mesoderm induction in the anterior Mouse monoclonal to SNAI2 epiblast by maintaining the integrity of the BM. We propose that this novel function cooperates with the signaling activities of the AVE to restrict EMT and mesoderm induction to the posterior epiblast. Keywords:EMT, FLRT3, anterior visceral endoderm, basement membrane, epithelial-to-mesenchymal transition, morphogenesis Gastrulation results in the formation of the three primary germ layersectoderm, mesoderm, and endodermand in the establishment of the basic body plan of the mouse embryo (Beddington and Robertson 1999;Tam and Loebel 2007). Prior to gastrulation, at embryonic day 5.5 (E5.5), a group of visceral endoderm (VE) cells at the distal tip of the embryo differentiates into a morphologically distinct tissue termed distal VE (DVE) (Srinivas 2006). The DVE expresses characteristic molecular markers such as Hex, Lefty1, and Dkk1 and migrates from the distal tip of the embryo to a more proximal region to give rise to the anterior VE (AVE), which at E6.5 positions itself above the prospective anterior epiblast (Tam and Loebel 2007). Failure of AVE cells to migrate as in the case of embryos deficient in Lim1, Otx2, or Foxa2/Hnf3 results in loss of QX 314 chloride anterior neural induction due to lack of AP patterning in the epiblast (Tam and Loebel 2007). Recent work has shown that this AVE (and the chick comparative, the hypoblast) controls anteriorposterior (AP) patterning by distinct processes. For instance, the AVE restricts primitive streak (PS) formation and mesoderm induction to the posterior side of the epiblast by expressing antagonists (Cer1, Lefty1, and Dkk1) of the posteriorizing activities of Nodal and Wnts (Lu et al. 2001;Bertocchini and Stern 2002;Perea-Gomez et al. 2002;Rossant and Tam 2004). Also, the AVE has been proposed to direct epiblast movements through induction of the Wnt planar cell polarity pathway (Voiculescu et al. 2007). Finally, the AVE can also initiate the transient expression of neural markers (Albazerchi and Stern 2007). Epiblast cells undergo epithelial-to-mesenchymal transition (EMT) and ingress into the PS region to give rise to mesoderm and definitive endoderm (DE). While the mesoderm migrates over a long distance to eventually give rise to the mesodermal organs, the DE intercalates into the overlying VE and gradually displaces the VE into the extraembryonic region, where it forms the endodermal component of the yolk sac (YS). EMT is usually characterized by the loss of cell polarity and the initiation of cell migration (Thiery and Sleeman 2006). At the molecular level, EMT is usually associated with FGF-induced down-regulation of the cellcell adhesion protein E-cadherin, the breakdown of the basement membrane (BM), a thin sheet of extracellular matrix that underlie epithelia, and the up-regulation of EMT and mesendodermal genes such asBrachyury(T),Foxa2,Snail,Eomes, andFGF8(Ciruna and Rossant 2001; Tam and Loebel 2007;Arnold et al. 2008). The BM controls cell migration, differentiation, and cell fate during early embryogenesis. In the pregastrulation embryo, epiblast cells in contact with the BM produced by the VE polarize and differentiate into ectoderm epithelium, whereas epiblast cells that fail to contact the BM undergo apoptosis (Li et al. 2003). For EMT to occur in the PS, the formation and integrity of the BM require dynamic regulation. The BM has to break down locally to allow the separation of the newly formed mesoderm and endoderm from QX 314 chloride the remaining ectoderm (Fujiwara et al. 2007). The molecular cues that regulate BM dynamics during gastrulation are poorly characterized. Fibronectin leucine-rich transmembrane protein 3 (FLRT3) belongs to a small subfamily (FLRT1-3) of putative type I transmembrane proteins (Lacy et al. 1999). While the functions of FLRT1 and FLRT2 are essentially unknown,XenopusFLRT3 has been proposed to form a complex with fibroblast growth factor receptors (FGFRs) and to activate intracellular signals, such as the canonical QX 314 chloride MAPK pathway, which results in ectopic tail formation (Bottcher et al. 2004). FLRTs also promote homotypic cell sorting, impartial of FGFR signaling, in mammalian cells orXenopusembryos, possibly by acting as homophilic cell adhesion molecules.