Transgenic Wnt/TCF pathway reporters: all you need is Lef?

The Wnt signaling pathway controls a large and diverse set of cell fate decisions in embryonic development, adult organ maintenance and disease. At the transcriptional level, Wnt/beta-catenin signaling is primarily mediated by the T-cell factor (TCF)/Lef-1 family of transcription factors, referred to here as TCFs. In order to track Wnt pathway activity during animal development, several laboratories have built transgenic reporter constructs containing multimerized TCF binding sites. Most of these reporters are active at multiple known sites of Wnt signaling, and several act as faithful reporters of pathway activity in specific contexts. However, multimerized TCF reporters should not be assumed to give a complete or definitive readout of Wnt signaling in vivo. Direct comparisons reveal discrepancies among reporters; in addition, there is good reason to expect that some important types of pathway activity, including target gene de-repression and TCF-independent Wnt or beta-catenin signaling, will not be accurately reported by such constructs. This review will discuss various transgenic Wnt/beta-catenin/TCF reporters, address the fidelity and completeness of their Wnt responsiveness, and contrast their in vivo transcriptional responses with those of natural Wnt target genes. Finally, three caveats to the interpretation of multimerized TCF reporter expression patterns will be proposed.