Role of the Xlim-1 and Xbra genes in anteroposterior patterning of neural tissue by the head and trunk organizer

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Abstract
Anteroposterior patterning of neural tissue is thought to be directed by the axial mesoderm which is functionally divided into head and trunk organizer. The LIM class homeobox gene Xlim-1 is expressed in the entire axial mesoderm, whereas the distinct transcription factor Xbra is expressed in the notochord but not in the prechordal mesoderm. mRNA injection experiments showed that Xenopus animal explants (caps) expressing an activated form of Xlim-1 (a LIM domain mutant named 3m) induce anterior neural markers whereas caps coexpressing Xlim-1/3m and Xbra induce posterior neural markers. These data indicate that, in terms of neural inducing ability, Xlim-1/3m-expressing caps correspond to the head organizer and Xlim-1/3m plus Xbra-coexpressing caps to the trunk organizer. Thus the expression domains of Xlim-1 and Xbra correlate with, and possibly define, the functional domains of the organizer. In animal caps Xlim-1/3m initiates expression of a neuralizing factor, chordin, whereas Xbra activates embryonic fibroblast growth factor (eFGF) expression, as reported previously; these factors could mediate the neural inducing and patterning effects that were observed. A dominant-negative FGF receptor (XFD) inhibits posteriorization by Xbra in a dose-dependent manner, supporting the suggestion that eFGF or a related factor has posteriorizing influence.