Retroviral Misexpression ofengrailedGenes in the Chick Optic Tectum Perturbs the Topographic Targeting of Retinal Axons

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Abstract
We have investigated the role of the homeodomain transcription factor genesEn-1andEn-2, homologs of theDrosophilasegment polarity geneengrailed, in regulating the development of the retinotopic map in the chick optic tectum. The En proteins are distributed in a gradient along the rostral–caudal axis of the developing tectum, with highest amounts found caudally. Previous evidence suggests that En-1 and En-2 may regulate the polarity of the rostral–caudal axis of the tectum and the subsequent topographic mapping of retinal axons. We have tested this hypothesis by using a recombinant replication-competent retrovirus to overexpress theEn-1orEn-2genes in the developing tectum. Anterograde labeling with the axon tracer DiI was used to analyze the topographic mapping of retinal axons after the time that the retinotectal projection is normally topographically organized. Overexpression of eitherEn-1orEn-2perturbed the topographic targeting of retinal axons. InEn-infected tecta, nasal retinal axons form an abnormally diffuse projection with numerous aberrant axons, branches, and arbors found at topographically incorrect locations, colocalized with domains of viral infection. In contrast, temporal axons did not form a diffuse projection or discrete aberrant arbors; however, many temporal axons were stunted and ended aberrantly rostral to their appropriate TZ, or in other cases either did not enter the tectum or formed a dense termination at its extreme rostral edge. These findings indicate that En-1 and En-2 are involved in regulating the development of the retinotopic map in the tectum. Furthermore, they support the hypothesis thatEngenes regulate the polarity of the rostral–caudal axis of the tectum, most likely by controlling the expression of retinal axon guidance molecules.