ROSTRAL OPTIC TECTUM ACQUIRES CAUDAL CHARACTERISTICS FOLLOWING ECTOPIC ENGRAILED EXPRESSION

Background: Expression of the homeobox-containing gene Engrailed (En) in an increasing rostral-to-caudal gradient in the dorsal mesencephalo n is the earliest known marker for polarity of the chick optic tectum. In heterotopic transplantation experiments, En protein expression cor relates well with the subsequent gradient of cytoarchitecture as well as the pattern of retinotectal projections. The En gradient also corre lates with the expression of two putative retinal axon-guidance molecu les, RAGS and ELF-1, which are Eph-like receptor tyrosine kinase ligan ds that may function in the establishment of retinotopic projections b y excluding temporal axons from the caudal tectum. Results: To examine the function of En in determining tectal polarity, we used the replic ation-competent retroviral vector RCAS to misexpress mouse En-1 throug hout the chick tectal primordium. Our results show that the rostral po rtion of the tectum adopts a caudal phenotype: the gradient of cytoarc hitectonic differentiation is abolished, and the molecular markers RAG S and ELF-1 are strongly expressed rostrally. In addition, cell membra nes from rostral tectum of RCAS En-1-infected embryos preferentially r epel temporal axons in in vitro membrane stripe assays. Conclusions: T hese results are consistent with a role for En in determining rostroca udal polarity of the developing tectum. The demonstration that both RA GS and ELF-1 are upregulated following En misexpression provides a mol ecular basis for understanding the previous observation, also based on retrovirus-mediated En misexpression, that nasal axons form ectopic c onnections in rostral tectum, from which temporal axons are excluded.