Regulation of the wnt signaling pathway in Xenopus laevis

Wnt signaling has been implicated in both axis determination and patterning of mesodermal tissue in Xenopus as well as the onset of cancer. A combination of genetic and biochemical approaches in multiple organisms has yielded the intracellular mediators of the pathway. They include frizzled, dishevelled, glycogen synthase kinase-3 beta (GSK-3β), β-catenin, APC, and Tcf-LEF family members. GSK-3β is a serine-threonine kinase and a negative regulator of the wnt signaling pathway. Our work has focused on the regulation of GSK-3β activity by lithium and a newly identified gene, axin. Lithium perturbs the development of a number of organisms including Dictyostelium, sea urchin, and Xenopus and it has been shown that lithium directly inhibits GSK-3β, providing an potential explanation for its developmental actions. Here, we extend this finding to show that lithium can potently inhibit GSK-3β activity in vivo resulting in an accumulation of β-catenin protein and activation of the wnt signaling pathway. To prove that GSK-3β is the only target of lithium important in mediating its developmental effects, we have designed a screen in Saccharomyces cerevisiae to identify mutants of MDS-1, a GSK-3β homologue, which are resistant to lithium treatment. In addition, in a yeast two hybrid screen using GSK-3β as bait, we have identified Xenopus axin. Axin is a maternal gene which is also expressed strongly in the anterior midbrain. In vivo, axin binds GSK-3β and mutants of axin lacking key regulatory domains act as dominant negatives by binding GSK-3β and inhibiting its activity. These mutant forms of axin provide alternative, in vivo GSK-3βB inhibitors which mimic the effect of lithium. We also provide evidence that an axin-GSK-3β complex is important in the regulation of GSK-3β activity. Our data suggests that axin may mediate inhibition as well as activation of wnt signaling through its interactions with GSK-3β