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Background  Studies in mouse,  Xenopus  and chicken have shown that  Otx2  and  Gbx2  expression domains are fundamental for positioning the midbrain-hindbrain boundary (MHB) organizer. Of the two zebrafish  gbx  genes,  gbx1  is a likely candidate to participate in this event because its early expression is similar to that reported for  Gbx2  in other species. Zebrafish  gbx2 , on the other hand, acts relatively late at the MHB. To investigate the function of zebrafish  gbx1  within the early neural plate, we used a combination of gain- and loss-of-function experiments.    Results  We found that ectopic  gbx1  expression in the anterior neural plate reduces forebrain and midbrain, represses  otx2  expression and repositions the MHB to a more anterior position at the new  gbx1 / otx2  border. In the case of  gbx1  loss-of-function, the initially robust  otx2  domain shifts slightly posterior at a given stage (70% epiboly), as does MHB marker expression. We further found that ectopic juxtaposition of  otx2  and  gbx1  leads to ectopic activation of MHB markers  fgf8 ,  pax2.1  and  eng2 . This indicates that, in zebrafish, an interaction between  otx2  and  gbx1  determines the site of MHB development. Our work also highlights a novel requirement for  gbx1  in hindbrain development. Using cell-tracing experiments,  gbx1  was found to cell-autonomously transform anterior neural tissue into posterior. Previous studies have shown that  gbx1  is a target of Wnt8 graded activity in the early neural plate. Consistent with this, we show that  gbx1  can partially restore hindbrain patterning in cases of Wnt8 loss-of-function. We propose that in addition to its role at the MHB,  gbx1  acts at the transcriptional level to mediate Wnt8 posteriorizing signals that pattern the developing hindbrain.    Conclusion  Our results provide evidence that zebrafish  gbx1  is involved in positioning the MHB in the early neural plate by refining the  otx2  expression domain. In addition to its role in MHB formation, we have shown that  gbx1  is a novel mediator of Wnt8 signaling during hindbrain patterning.

Zebrafish gbx1 refines the Midbrain-Hindbrain Boundary border and mediates the Wnt8 posteriorization signal