How to visualize Wnt signaling in animals
There are various "reporter transgenes" that respond to Wnt signals in intact animals and therefore, as far as one can know, reflect endogenous Wnt signaling (reviewed in Barolo, 2006). These reporters are based on a multimerized TCF binding site, driving expression of LacZ or GFP. Several transgenic mouse lines have been described, one called TOP-GAL by Gupta and Fuchs (1999) and one called BAT-GAL by Maretto et al (2003), while Moriyama (2007) describe a GFP-based reporter mouse strain. A LEF-EGFP WNT Reporter to detect Wnt signaling is described by Currier et al, (2010).
Because the expression of Axin2 is under the control of Wnt signaling and Axin2 may even be a universal Wnt target, the transgenic line made by Jho et al, (2002), based on the Axin2 promoter and GFP, also acts as a useful Wnt reporter. Similarly, Lustig et al, (2002) inserted LacZ into the endogenous Axin2/Conductin gene to visualize expression of this Wnt target in animals.
The gene LGR5 (sometimes called GPR49) is another target gene of the Wnt pathway, in particular in stem cells of various tissues. This gene has been targeted by LacZ by Barker et al (2007) and provides an in vivo read-out of the Wnt pathway.
In response to the often-asked question "what is the best Wnt reporter", there is no simple answer. In our experience, Axin2/Conductin is widely expressed in areas where one would expect it, and might be an optimal reporter for many tissues. However, there are cells (in the lung for example) that are positive for the TOP-GAL reporter and negative for Axin2-based reporters (unpublished observations). The problem of reliably detecting Wnt signaling in vivo is compounded by difficulties in assessing Wnt gene expression itself (which is best done by in situ hybridization as antibodies are rarely good enough), or in finding other independent hallmarks of Wnt signaling (see Barolo, 2006 for a discussion).
A TOPdGFP Zebrafish line was generated by Dorsky and Moon (2002) and also described by Hurlstone et al (2003)
In Drosophila, Chang et al (2008) have used fragments of the naked cuticle gene to generate Wnt/Wingless reporter lines. Just like Axin2 in the mouse, naked cuticle is a negative feedbakc regulator and the expression of this gene follows Wingless signaling widely.
TOPGAL mice (from Elaine Fuchs) can be obtained from Jackson labs.
Lgr5tm1(cre/ERT2) mice (from Hans Clevers) can be obtained from Jackson labs.Axin2-LacZ (Conductin-LacZ) mice (from Walter Birchmeier) are provided by Jackson Labs and by EMMA.
This figure, provided by Cati Logan, shows TOP-GAL expression in a mouse embryo