Received: 6 March, 2007. Accepted: 25 April, 2007. Special Feature Functional Development and Embryology ©2007 Global Science Books The Roles of Wnt Signaling in Early Mouse Development and Embryonic Stem Cells Caroline R. Kemp • Marijke Hendrickx • Erik Willems • Danuta Wawrzak • Mourad Métioui • Luc Leyns * Lab for Cell Genetics, Vrije Universiteit Brussel (VUB), 2 Pleinlaan, 1050 Brussels, Belgium Corresponding author: * lleyns@vub.ac.be ABSTRACT The Wnt family of secreted signaling molecules is conserved throughout the animal kingdom. Wnt signaling plays critical roles during embryonic development and mutations leading to the overactivation of the Wnt pathway have been linked to cancer. Wnt signals are transduced intracellularly by the Frizzled family of receptors. Moreover, proteoglycans and the co-receptors LRP5 and -6 participate in the transmission of Wnt signals, whereas a series of secreted antagonists can block Wnt signaling directly (i.e. Frzb and Sfrps) or indi- rectly (i.e. Dkks). Some of the biochemical interactions of the Wnts with their receptors and antagonists have recently been characterized, permitting further elucidation of how these proteins function in vivo. Expression pattern analyses in mouse embryos have shown that Wnt genes are active during most, if not all, developmental processes and gene inactivation has uncovered some of their key roles throughout mouse embryogenesis. Importantly, knock-out and overexpression studies have proven the importance of Wnt signaling during mesoderm, neurectoderm and body axis formation. With their ability to differentiate into all adult cell types in vitro, mouse embryonic stem (ES) cells have been used to mimic the developing embryo. In this ES cell system, it has recently been shown that Wnt signals contribute to meso- derm induction and neural inhibition. Here we will provide an overview of the Wnt signaling pathway and its roles during mouse embryo- nic development, focusing on gastrulation. Functional studies in the mouse, including gene ablation and overexpression experiments, will be reviewed. Finally, we will discuss the latest reports on the application of ES cells to study the Wnt pathway during development. _____________________________________________________________________________________________________________ Keywords: Dkk, Frizzled, LRP, mesoderm, neural, pluripotency, Sfrp Abbreviations: APC, adenomatous polyposis coli; aVE, anterior visceral endoderm; CRD, cysteine-rich domain; Dkk, Dickkopf; dpc, days post coitum; dVE, distal visceral endoderm; EGO, early gastrula organizer; ES, embryonic stem; Fzd, Frizzled; GSK3β, glycogen synthase kinase 3β; LIF, leukemia inhibitory factor; LRP, low-density lipoprotein-related receptor protein; PCP, planar cell polarity; Sfrp, secreted frizzled related protein; Tcf/Lef, T cell factor/lymphoid enhancing factor CONTENTS INTRODUCTION.......................................................................................................................................................................................... 1 THE WNT SIGNALING PATHWAY ............................................................................................................................................................ 2 The Wnt family.......................................................................................................................................................................................... 2 Canonical and noncanonical Wnt signaling ............................................................................................................................................... 2 Wnt receptors and co-receptors ................................................................................................................................................................. 2 Receptor-ligand interactions ...................................................................................................................................................................... 3 Wnt activity modulators ............................................................................................................................................................................ 4 Binding properties of Wnt modulators ...................................................................................................................................................... 4 Alternative Wnt pathway activators........................................................................................................................................................... 5 ROLES OF WNTS DURING DEVELOPMENT .......................................................................................................................................... 5 Overview of early mouse embryonic development ................................................................................................................................... 5 Wnt signaling in embryonic axis formation............................................................................................................................................... 6 Other Wnt functions in the embryo ........................................................................................................................................................... 8 WNTS AND ES CELL DIFFERENTIATION ............................................................................................................................................... 8 Overview of embryonic stem cells ............................................................................................................................................................ 8 Wnts maintain pluripotency....................................................................................................................................................................... 9 Wnt signals promote mesodermal differentiation ...................................................................................................................................... 9 Wnts inhibit neural differentiation ............................................................................................................................................................. 9 WNTS AND DISEASES ............................................................................................................................................................................. 10 CONCLUDING REMARKS ....................................................................................................................................................................... 10 ACKNOWLEDGEMENTS ......................................................................................................................................................................... 10 REFERENCES............................................................................................................................................................................................. 10 _____________________________________________________________________________________________________________ INTRODUCTION More than 20 years ago, int-1 was identified by its onco- genic properties when ectopically activated by the mouse mammary tumor virus (Nusse and Varmus 1982; Nusse and Varmus 1992). Later renamed Wnt1, it was one of the first oncogenes ‘discovered’ that was activated in tumors by a provirus. Since then, Wnt homologs have been shown to be conserved throughout the animal kingdom, from hydra, worms and sea urchin to humans, with 19 members identi-