Mechanisms of Wnt signaling in development

Wnt genes encode a large family of secreted, cysteine-rich proteins that pl ay key roles as intercellular signaling molecules in development. Genetic s tudies in Drosophila and Caenorhabditis elegans, ectopic gene expression in Xenopus, and gene knockouts in the mouse have demonstrated the involvement of Wnts in processes as diverse as segmentation, CNS patterning, and contr ol of asymmetric cell divisions. The transduction of Wnt signals between ce lls proceeds in a complex series of events including post-translational mod ification and secretion of Wnts, binding to transmembrane receptors, activa tion of cytoplasmic effecters, and, finally, transcriptional regulation of target genes. Over the past two years our understanding of Wnt signaling ha s been substantially improved by the identification of Frizzled proteins as cell surface receptors for Wnts and by the finding that beta-catenin, a co mponent downstream of the receptor, can translocate to the nucleus and func tion as a transcriptional activator. Here we review recent data that have s tarted to unravel the mechanisms of Wnt signaling.