XMeis3 protein activity is required for proper hindbrain patterning in Xenopus laevis embryos

Meis-family homeobox proteins have been shown to regulate cell fate specifi cation in vertebrate and invertebrate embryos. Ectopic expression of RNA en coding the Xenopus Meis3 (XMeis3) protein caused anterior neural truncation s with a concomitant expansion of hindbrain and spinal cord markers in Xeno pus embryos. In naive animal cap explants, XMeis3 activated expression of p osterior neural markers in the absence of pan-neural markers. Supporting it s role as a neural caudalizer, XMeis3 is expressed in the hindbrain and spi nal cord. We show that XMeis3 acts like a transcriptional activator, and it s caudalizing effects can be mimicked by injecting RNA encoding a VP16-XMei s3 fusion protein. To address the role of endogenous XMeis3 protein in neur al patterning, XMeis3 activity was antagonized by injecting RNA encoding an Engrailed-XMeis3 antimorph fusion protein or XMeis3 antisense morpholino o ligonucleotides. In these embryos, anterior neural structures were expanded and posterior neural tissues from the midbrain-hindbrain junction through the hindbrain were perturbed. In neuralized animal cap explants, XMeis-anti morph protein modified caudalization by basic fibroblast growth factor and Wnt3a. XMeis3-antimorph protein did not inhibit caudalization per se, but r e-directed posterior neural marker expression to more anterior levels; it r educed expression of spinal cord and hindbrain markers, yet increased expre ssion of the more rostral En2 marker. These results provide evidence that X Meis3 protein in the hindbrain is required to modify anterior neural-induci ng activity, thus, enabling the transformation of these cells to posterior fates.