The transcription factor Lmx1b maintains Wnt1 expression within the isthmic organizer

Cells in the caudal mesencephalon and rostral metencephalon become organize d by signals emanating from the isthmus organizer (IsO). The IsO is associa ted with the isthmus, a morphological constriction of the neural tube which eventually defines the mesencephalic/ metencephalic boundary (MMB), Here w e report that the transcription factor Lmx1b is expressed and functions in a distinct region of the IsO, Lmx1b expression is maintained by the glycopr otein Fgf8, a signal capable of mediating IsO signaling. Lmx1b, in turn, ma intains the expression of the secreted factor Wnt1, Our conclusions are sub stantiated by the following: (i) Lmx1b mRNA becomes localized to the isthmu s immediately after Fgf8 initiation, (ii) Wnt1 expression is localized to t he Lmx1b expression domain, but with slightly later kinetics, (iii) Fgf8-so aked beads generate similar domains of expression for Lmx1b and Wnt1 and (i v) retroviral-mediated expression of Lmx1b (Lmx1b/RCAS) maintains Wnt1 expr ession in the mesencephalon, Ectopic Lmx1b is insufficient to alter the exp ression of a number of other genes expressed at the IsO, suggesting that it does not generate a new signaling center. Instead, if we allow Lmx1b/RCAS- infected brains to develop longer, we detect changes in mesencephalic morph ology, Since both ectopic and endogenous Lmx1b expression occurs in regions of the isthmus undergoing morphological changes, it could normally play a role in this process. Furthermore, a similar phenotype is not observed in W nt1/RCAS-infected brains, demonstrating that ectopic Wnt1 is insufficient t o mediate the effect of ectopic Lmx1b in our assay. Since Wnt1 function has been linked to the proper segregation of mesencephalic and metencephalic c ells, we suggest that Lmx1b and Wnt1 normally function in concert to affect IsO morphogenesis.