This thesis investigates mechanisms of Dickkopf-1 (Dkk1) action in heart development. Dkk1 is notable for being able to initiate heart development in normally non-cardiogenic mesoderm of Xenopus laevis and recapitulate normal program up to and including formation of a beating heart tube. Although Dkk1 is a secreted antagonist of canonical Wnt signaling pathway, its heart-forming ability may require additional signaling. Since Wnt-antagonism of Dkk1 is mediated by the C-terminal cysteine-rich domain of Dkk1 (C1), we focused in particular on Dkk1's other cysteine-rich domain, the N-terminal domain (N1). In a quantitative assay measuring Wnt activity, N1 interfered with signaling of some canonical Wnts, such as Wnt8, but much less efficiently than C1. N1's ability to interfere with Wnt8 signaling, however, did not extend to all canonical Wnts: Wnt3a was not antagonized by N1. In contrast to intact Dkk1, C1 domain alone also differentially antagonized Wnt8 and Wnt3a. Dkk1 was shown to activate cJun terminal kinase (JNK), both in Xenopus embryos and in tumor cell lines. Our studies indicate that both domains of Dkk1 can induce JNK activation in Xenopus .

Previous studies have shown that injection of N1 domain did not alter gross morphology of the embryo, suggesting that N1 may be inert. In Chapter 3, I show that N1 possesses biological activity in the embryo. First, N1 cooperated with truncated bone morphogenetic protein receptor (tBR) to maintain markers of axial and prechordal mesoderm. N1 also cooperated with several Wnt antagonists in the context of BMP inhibition to expand the expression domain of markers of axial and prechordal mesoderm, as well as to increase the frequency of heart marker expression. N1 also cooperated with Wnt antagonists Crescent and C1 to increase the extent of heart marker induction in ventral marginal zone (VMZ) explants. During early stages of gastrulation, N1 altered some markers of the dorsal-ventral patterning, either alone or in combination with BMP inhibition. These data suggest that N1 possesses novel activity and that Dkk1 has to be considered not a simple Wnt antagonist but a dual-function protein.