The Wnt signaling cascade plays an important role in many developmental processes by regulating the expression of specific Wnt-responsive target genes. Furthermore, inappropriate activation of the pathway is found in many types of cancer. The interaction of Wnt with its receptor Frizzled (Fz) leads to the inhibition of a complex consisting of the tumor suppressor protein APC, the scaffolding protein Axin and the kinase GSK3beta. The effector beta-catenin, which is normally targeted for degradation by this complex, translocates into the nucleus where it interacts with Tcf transcription factors to activate Wnt target gene expression. In C. elegans , a canonical Wnt pathway consisting of EGL-20/Wnt, LIN-17/Fz, BAR-1/beta-catenin and POP-1/Tcf activates the expression of target genes like the homeobox gene
mab-5 in the Q neuroblast lineage (1) .
mab-5 specifies the direction of the migration of the Q daughter cells. Thus,
mab-5 is expressed in the Q cell lineage on the left side of the animal (QL) and induces posterior migration of these cells.
mab-5 is however not expressed in the right Q cell lineage (QR), which results in migration in the opposite, anterior direction. This asymmetric
mab-5 expression is the result of a difference in sensitivity to the EGL-20/Wnt signal between the QL and QR cells (2). We are interested in the mechanism and regulation of the Wnt pathway. In a yeast-two-hybrid screen we have isolated a protein, C37A5.9, which specifically interacts with BAR-1. C37A5.9(RNAi) results in the posterior migration of both the QL and QR daughters, a phenotype that is also seen in
mab-5 gain-of-function mutants and when a constitutively active form of BAR-1 is overexpressed. In addition, this phenotype is similar to the phenotype of the known negative regulator
pry-1 (3). We have shown that C37A5.9 is mutated in the
pry-1 allele
mu38 (see abstract Rik Korswagen). C37A5.9 contains a DIX and RGS domain, and is similar to fly and vertebrate Axin. We propose that C37A5.9 is a functional Axin homologue and that a conserved APC/Axin/GSK3beta-like complex may regulate BAR-1 stability in C.elegans . To isolate additional regulatory components of the Wnt pathway, we screened for mutations that show a phenotype which is similar to that of C37A5.9/
pry-1 . We used the final positions of the Q daughter cells as a readout. To rapidly assess the positions of the Q cells, we used an integrated
mec-7::gfp reporter transgene which is specifically expressed in the touch receptor neurons (including the Q daughter cells AVM and PVM). We isolated 10 independent mutations that show posterior migration of both the QL and QR daughter cells in an
egl-20 /Wnt mutant background. Complementation tests and preliminary mapping results suggest that we have hit at least 5 genes, which we are currently mapping. This approach should identify new repressors of the Wnt pathway and provide us with further insight in the mechanisms of Wnt target gene regulation. Korswagen et al. (2000) Distinct ß-catenins mediate adhesion and signalling functions in C. elegans . Nature 406:527-532 Whangbo et al. (1999) A Wnt signaling system that specifies two patterns of cell migration in C. elegans . Mol Cell 4(5):851-8 Maloof et al. (1999) A Wnt signalling pathway controls Hox gene expression and neuroblast migration in C. elegans . Development 126:37-49.