Positioning of the midbrain-hindbrain boundary organizer through global posteriorization of the neuroectoderm mediated by Wnt8 signaling.

The organizing center located at the midbrain-hindbrain boundary (MHB) patterns the midbrain and hindbrain primordia of the neural plate. Studies in several vertebrates showed that the interface between cells expressing Otx and Gbx transcription factors marks the location in the neural plate where the organizer forms, but it is unclear how this location is set up. Using mutant analyses and shield ablation experiments in zebrafish, we find that axial mesendoderm, as a candidate tissue, has only a minor role in positioning the MHB. Instead, the blastoderm margin of the gastrula embryo acts as a source of signal(s) involved in this process. We demonstrate that positioning of the MHB organizer is tightly linked to overall neuroectodermal posteriorization, and specifically depends on Wnt8 signaling emanating from lateral mesendodermal precursors. Wnt8 is required for the initial subdivision of the neuroectoderm, including onset of posterior gbx1 expression and establishment of the posterior border of otx2 expression. Cell transplantation experiments further show that Wnt8 signaling acts directly and non-cell-autonomously. Consistent with these findings, a GFP-Wnt8 fusion protein travels from donor cells through early neural plate tissue. Our findings argue that graded Wnt8 activity mediates overall neuroectodermal posteriorization and thus determines the location of the MHB organizer.

Isolation and expression of the homeobox gene Gbx1 during mouse development.

In zebrafish, gbx1 and otx2 are among the earliest genes expressed in the neuroectoderm, dividing it into an anterior and a posterior domain with a common border that marks the midbrain-hindbrain boundary (MHB) primordium. Here, we describe the sequence and expression pattern of Gbx1 in mouse. The first transcripts are found at embryonic day 7.75 in the hindbrain. Later on, expression of Gbx1 is detectable in the hindbrain (rhombomeres 2 to 7), spinal cord, optic vesicles, and in the ventral telencephalon. In mouse, Gbx1 expression is not observed at the MHB as is the case during early zebrafish development. We suggest that an evolutionary switch occurred: in mouse Gbx2 is involved in the early specification of the MHB primordium, whereas in zebrafish, gbx1 is required instead of gbx2.