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Development 129, 5227-5239 (2002)
Copyright © 2002 The Company of Biologists Limited

The Xenopus receptor tyrosine kinase Xror2 modulates morphogenetic movements of the axial mesoderm and neuroectoderm via Wnt signaling

Hiroki Hikasa*, Mikihito Shibata, Ichiro Hiratani and Masanori Taira{dagger}

Department of Biological Sciences, Graduate School of Science, University of Tokyo, and Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Corporation, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan
* Present address: Department of Microbiology and Molecular Genetics, Harvard Medical School, and Molecular Medicine Unit, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, Boston, MA 02215, USA

{dagger} Author for correspondence (e-mail: m_taira{at}biol.s.u-tokyo.ac.jp)

Accepted 14 July 2002

The Spemann organizer plays a central role in neural induction, patterning of the neuroectoderm and mesoderm, and morphogenetic movements during early embryogenesis. By seeking genes whose expression is activated by the organizer-specific LIM homeobox gene Xlim-1 in Xenopus animal caps, we isolated the receptor tyrosine kinase Xror2. Xror2 is expressed initially in the dorsal marginal zone, then in the notochord and the neuroectoderm posterior to the midbrain-hindbrain boundary. mRNA injection experiments revealed that overexpression of Xror2 inhibits convergent extension of the dorsal mesoderm and neuroectoderm in whole embryos, as well as the elongation of animal caps treated with activin, whereas it does not appear to affect cell differentiation of neural tissue and notochord. Interestingly, mutant constructs in which the kinase domain was point-mutated or deleted (named Xror2-TM) also inhibited convergent extension, and did not counteract the wild-type, suggesting that the ectodomain of Xror2 per se has activities that may be modulated by the intracellular domain. In relation to Wnt signaling for planar cell polarity, we observed: (1) the Frizzled-like domain in the ectodomain is required for the activity of wild-type Xror2 and Xror2-TM; (2) co-expression of Xror2 with Xwnt11, Xfz7, or both, synergistically inhibits convergent extension in embryos; (3) inhibition of elongation by Xror2 in activin-treated animal caps is reversed by co-expression of a dominant negative form of Cdc42 that has been suggested to mediate the planar cell polarity pathway of Wnt; and (4) the ectodomain of Xror2 interacts with Xwnts in co-immunoprecipitation experiments. These results suggest that Xror2 cooperates with Wnts to regulate convergent extension of the axial mesoderm and neuroectoderm by modulating the planar cell polarity pathway of Wnt.

Key words: Xenopus laevis, Spemann organizer, Convergent extension, Neural plate closure, Planar cell polarity, Xlim-1, Receptor tyrosine kinase, Xror2, Xwnt11, Xfz7, Cdc42






© The Company of Biologists Ltd 2002