Zic3 is involved in the left-right specification of the Xenopus embryo

Establishment of left-right (L-R) asymmetry is fundamental to vertebrate de velopment. Several genes involved in L-R asymmetry have been described. In the Xenopus embryo, Vg1/activin signals are implicated upstream of asymmetr ic nodal related 1 (Xnr1) and Pitx2 expression in L-R patterning, We report here that Zic3 carries the left-sided signal from the initial activin-like signal to determinative factors such as Pitx2. Overexpression of Zic3 on t he right side of the embryo altered the orientation of heart and gut loopin g, concomitant with disturbed laterality of expression of Xnr1 and Pitx2, b oth of which are normally expressed in the left lateral plate mesoderm, The results indicate that Zic3 participates in the left-sided signaling upstre am of Xnr1 and Pitx2, At early gastrula, Zic3 was expressed not only in pre sumptive neuroectoderm but also in mesoderm. Correspondingly, overexpressio n of Zic3 was effective in the L-R specification at the early gastrula stag e, as revealed by a hormone-inducible Zic3 construct. The Zic3 expression i n the mesoderm is induced by activin beta or Vg1, which are also involved i n the left-sided signal in L-R specification. These findings suggest that a n activin-like signal is a potent upstream activator of Zic3 that establish es the L-R axis, Furthermore, overexpression of the zinc-finger domain of Z ic3 on the right side is sufficient to disturb the L-R axis, while overexpr ession of the N-terminal domain on the left side affects the laterality. Th ese results suggest that Zic3 has at least two functionally important domai ns that play different roles and provide a molecular basis for human hetero taxy, which is an L-R pattern anomaly caused by a mutation in human ZIC3.