Rescue of a Wnt mutation by an activated form of LEF-1: Regulation of maintenance but not initiation of Brachyury expression

Members of the LEF-1/TCF family of transcription factors have been implicat ed in mediating a nuclear response to Wnt signals by association with beta -catenin, consistent with this view, mice carrying mutations in either the Wnt3a gene or in both transcription factor genes Lef1 and Tcf1 were previou sly found to show a similar defect in the formation of paraxial mesoderm in the gastrulating mouse embryo. In addition, mutations in the Brachyury gen e, a direct transcriptional target of LEF-1, were shown to result in mesode rmal defects. However, direct evidence for the role of LEF-1 and Brachyury in Wnt3a signaling has been limiting. In this study, we genetically examine the function of LEF-1 in the regulation of Brachyury expression and in sig naling by Wnt3a. Analysis of the expression of Brachyury in Lef1(-/-)Tcf1(- /-) mice and studies of Brachyury:lacZ transgenes containing wild type or m utated LEF-1 binding sites indicate that Lef1 is dispensable for the initia tion, but is required for the maintenance of Brachyury expression. We also show that the expression of an activated form of LEF-1, containing the beta -catenin activation domain fused to the amino terminus of LEF-1, can rescu e a Wnt3a mutation. Together, these data provide genetic evidence that Lef1 mediates the Wnt3a signal and regulates the stable maintenance of Brachyur y expression during gastrulation.