Wnt8 is required in lateral mesendodermal precursors for neural posteriorization in vivo

The dorsal ectoderm of the vertebrate gastrula was proposed by Nieuwkoop to be specified towards an anterior neural fate by an activation signal, with its subsequent regionalization along the anteroposterior (AP) axis regulat ed by a graded transforming activity, leading to a properly patterned foreb rain, midbrain, hindbrain and spinal cord. The activation phase involves in hibition of BMP signals by dorsal antagonists, but the later caudalization process is much more poorly characterized. Explant and overexpression studi es in chick, Xenopus, mouse and zebrafish implicate lateral/paraxial mesode rm in supplying the transforming influence, which is largely speculated to be a Wnt family member. We have analyzed the requirement for the specific ventrolaterally expressed Wnt8 ligand in the posteriorization of neural tissue in zebrafish wild-typ e and Nodal-deficient embryos (Antivin overexpressing or cyclops;squint dou ble mutants), which show extensive AP brain patterning in the absence of do rsal mesoderm. In different genetic situations that vary the extent of meso dermal precursor formation, the presence of lateral wnt8-expressing cells c orrelates with the establishment of AP brain pattern. Cell tracing experime nts show that the neuroectoderm of Nodal-deficient embryos undergoes a rapi d anterior-to-posterior transformation in vivo during a short period at the end of the gastrula stage. Moreover, in both wild-type and Nodal-deficient embryos, inactivation of Wnt8 function by morpholino (MOwnt8) translationa l interference dose-dependently abrogates formation of spinal cord and post erior brain fates, without blocking ventrolateral mesoderm formation. MOwnt 8 also suppresses the forebrain deficiency in bozozok mutants, in which ina ctivation of a homeobox gene causes ectopic wnt8 expression. In addition, t he bozozok forebrain reduction is suppressed in bozozok;squint;cyclops trip le mutants, and is associated with reduced wnt8 expression, as seen in cycl ops;squint mutants. Hence, whereas boz and Nodal signaling largely cooperat e in gastrula organizer formation, they have opposing roles in regulating w nt8 expression and forebrain specification. Our findings provide strong sup port for a model of neural transformation in which a planar gastrula-stake Wnt8 signal, promoted by Nodal signaling and dorsally limited by Bozozok, a cts on anterior neuroectoderm from the lateral mesoderm to produce the AP r egional patterning of the CNS.