The HMG-box transcription factor XTcf-4 demarcates the forebrain-midbrain boundary.

A small subfamily of HMG-box transcription factors, the LEF/TCF group, serves as nuclear transducer of the Wnt-1/Wg signaling cascade. Upon Wnt-1/Wg signaling their members interact with beta-catenin and regulate the expression of Xenopus target genes siamois, twin, nodal related-3 or fibronectin. We have isolated a new HMG-box transcription factor in Xenopus that will be addressed XTcf-4 based on its homology to human and murine Tcf-4. Unlike XTcf-3, which is a maternal gene, and XLef-1 that is expressed after mid blastula transition (Molenaar et al., 1998. Mech. Dev. 75, 151-154), XTcf-4 expression starts at late neurula stage and is restricted to the anterior most midbrain demarcating the forebrain-midbrain boundary. The expression partially overlaps with a broad set of Xenopus Wnt family members in distinct patterns. XTcf-4 transcripts were also found partially co-localized with those of Xaxin, an intracellular antagonist of Wnt-1/Wg signaling.