EVIDENCE THAT FGF8 SIGNALING FROM THE MIDBRAIN-HINDBRAIN JUNCTION REGULATES GROWTH AND POLARITY IN THE DEVELOPING MIDBRAIN

The developing vertebrate mesencephalon shows a rostrocaudal gradient in the expression of a number of molecular markers and in the cytoarch itectonic differentiation of the tectum, where cells cease proliferati ng and differentiate in a rostral to caudal progression. Tissue grafti ng experiments have implicated cell signalling by the mesencephalic-me tencephalic (mid-hindbrain) junction (or isthmus) in orchestrating the se events. We have explored the role of Wnt-1 and FGF8 signalling in t he regulation of mesencephalic polarity. Wnt-1 is expressed in the cau dal mesencephalon and Fgf8 in the most rostral metencephalon. Wnt-1 re gulates Fgf8 expression in the adjacent metencephalon, most likely via a secondary mesencephalic signal. Ectopic expression of Fgf8 in the m esencephalon is sufficient to activate expression of Engrailed-2 (En-2 ) and ELF-I, two genes normally expressed in a decreasing caudal to ro stral gradient in the posterior mesencephalon. Ectopic expression of E ngrailed-1 (En-1), a functionally equivalent homologue of En-2 is suff icient to activate ELF-I expression by itself. These results indicate the existence of a molecular hierarchy in which FGF8 signalling establ ishes the graded expression of En-2 within the tectum. This in turn ma y act to specify other aspects of A-P polarity such as graded ELF-I ex pression. Our studies also reveal that FGF8 is a potent mitogen within the mesencephalon: when ectopically expressed, neural precursors cont inue to proliferate and neurogenesis is prevented. Taken together our results suggest that FGF8 signalling from the isthmus has a keg role i n coordinately regulating growth and polarity in the developing mesenc ephalon.