Connecting Wnt to the Cytoskeleton

Abstract

The planar cell polarity pathway is important for the proper orientation of cells in various tissues. Wnt is an extracellular signal that triggers this process by stimulating rearrangement of the actin cytoskeleton through the actions of the guanine triphosphatases (GTPases) Rho and Rac. However, the guanine nucleotide exchange factors (GEFs) that connect Wnt signaling to these GTPases are unknown. While screening Xenopus embryos for genes expressed in the notochord--a tissue that undergoes convergent extension, which is a process that mediates the anterior-posterior extension of the body axis during development--Tanegashima et al . identified a gene homologous to human WGEF (encoding weak-similarity GEF). XWGEF was expressed at the early gastula stage, became restricted to the notochord at the end of gastrulation, and then in the tail-bud stages expression was detected in the notochord and head region. When a tagged version of XWGEF was overexpressed, the protein was found colocalized with actin and appeared to be associated with the cell membrane. When expressed in human 293 cells, XWGEF coimmunoprecipitated and increased the abundance of GTP-bound RhoA but did not interact with and had no effect on Rac or Cdc42. When mRNA for hWGEF or XWGEF was injected into Xenopus embryos, RhoA activation was observed and the embryos exhibited phenotypes similar to those observed in which a constitutively active RhoA mRNA was injected. Injection of mRNA encoding a dominant-negative RhoA or a dominant-negative Rok (a downstream effector of RhoA), but not mRNA encoding a dominant-negative Rac1, rescued the body axis phenotype produced by overexpression of XWGEF. Using morpholino oligonucleotides to reduce XWGEF, the authors found that the embryos exhibited neural tube closure defects and a shortened axis, consistent with a defect in convergent extension. Epistasis experiments placed XWGEF downstream of Wnt and upstream of Daam-1 (a formin-homology protein). Tagged proteins expressed in mammalian cultured cells showed that WGEF interacted with both Dishevelled (Dsh, a component downstream of Wnt) and Daam-1. The authors propose a model whereby activation of the Frizzled receptor by Wnt leads to the formation of a complex of Dsh, Daam-1, and WGEF at the membrane, which allows WGEF to stimulate RhoA and regulate the actin cytoskeleton. K. Tanegashima, H. Zhao, I. B. Dawid, WGEF activates Rho in the Wnt-PCP pathway and controls convergent extension in Xenopus gastrulation. EMBO J. 27 , 606-617 (2008). [PubMed]