| Axin is a key negative regulator of the canonical Wnt signaling pathway that functions by promoting the degradation of beta-catenin, the major effector in the signaling cascade. The related protein Axin2 is thought to function in a similar way to Axin and appears to have overlapping functions. Loss of Axin results in early embryonic lethality, while Axin2 null mice are viable but display craniofacial defects. Mutations in either gene cause cancer in humans. The lack of redundancy between Axin and Axin2 could be due to their different modes of expression: while Axin is expressed ubiquitously, Axin2 is expressed in tissue- and developmental stage-specific pattern, and its transcription is induced by canonical Wnt signaling. Alternatively, the two proteins might have partially different functions, a hypothesis supported by the observation that they differ in their subcellular localization in colon epithelial cells. To test the functional equivalence of Axin and Axin2 in vivo, I generated knockin mice in which the Axin gene was replaced with an Axin2 cDNA. The mutant mice that express no endogenous Axin, but produce Axin2 under control of the Axin locus, were viable, fertile and apparently normal, demonstrating that the Axin and Axin2 proteins are functionally equivalent.; The functional replacement of Axin by Axin2 suggested that only regions that are conserved between the two genes are important for the function of Axin/Axin2. In order to study the importance of different domains for the function of Axin, I next attempted to generate Axin knockin mutant mice that contain a deletion of one of the following regions: (1) the RGS domain (2) the DIX domain (3) the KVEKVD C-terminal motif. The RGS domain of Axin confers binding to APC, which is thought to be required for its ability to negatively regulate the levels of beta-catenin. The DIX domain is a region of homology with the protein disheveled (Dv1), which allows Axin to interact with LRP and Dv1. The KVEKVD C-terminal motif of Axin was reported to be important for the ability of Axin to activate the JNK signaling pathway, and also to be a potential site for the SUMOylation of Axin. (Abstract shortened by UMI.)... |
