Essential and opposing roles of zebrafish β-catenins in the formation of dorsal axial structures and neurectoderm

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Abstract
In Xenopus, Wnt signals and their transcriptional effectorβ -catenin are required for the development of dorsal axial structures. In zebrafish, previous loss-of-function studies have not identified an essential role for β-catenin in dorsal axis formation, but the maternal-effect mutation ichabod disrupts β-catenin accumulation in dorsal nuclei and leads to a reduction of dorsoanterior derivatives. We have identified and characterized a second zebrafish β-catenin gene,β -catenin-2, located on a different linkage group from the previously studied β-catenin-1, but situated close to the ichabod mutation on LG19. Although the ichabod mutation does not functionally alter the β-catenin-2 reading frame, the level of maternal β-catenin-2, but not β-catenin-1, transcript is substantially lower in ichabod, compared with wild-type, embryos. Reduction of β-catenin-2 function in wild-type embryos by injection of morpholino antisense oligonucleotides (MOs) specific for this gene (MO2) results in the same ventralized phenotypes as seen in ichabod embryos, and administration of MO2 to ichabod embryos increases the extent of ventralization. MOs directed against β-catenin-1 (MO1), by contrast, had no ventralizing effect on wild-type embryos. β-catenin-2 is thus specifically required for organizer formation and this function is apparently required maternally, because the ichabod mutation causes a reduction in maternal transcription of the gene and a reduced level of β-catenin-2 protein in the early embryo. A redundant role of β-catenins in suppressing formation of neurectoderm is revealed when both β-catenin genes are inhibited. Using a combination of MO1 and MO2 in wild-type embryos, or by injecting solely MO1 in ichabod embryos, we obtain expression of a wide spectrum of neural markers in apparently appropriate anteroposterior pattern. We propose that the early, dorsal-promoting function ofβ -catenin-2 is essential to counteract a later, dorsal- and neurectoderm-repressing function that is shared by both β-catenin genes.