XENOPUS BRN-3.0, A POU-DOMAIN GENE EXPRESSED IN THE DEVELOPING RETINAAND TECTUM - NOT REGULATED BY INNERVATION

Purpose. To study the effect of denervation on the expression of the P OU-domain gene Brn-3.0 in the Xenopus visual system. Methods. An oligo nucleotide probe was used to identify homologs of the murine gene Brn- 3.0 in the retina. In situ hybridization was used to deter-mine the sp atial distribution of the mRNA within the developing embryo. To study the effects of denervation on Brn-3.0 expression and cell fate, embryo nic eyes were transplanted to an ectopic location on the animal (the f lank) before the onset of retinal ganglion cell (RGC) axonogenesis. Ge ne expression in ectopic eyes and denervated tecta was analyzed over t ime using in situ hybridization. Results. The deduced, partial amino a cid sequence for Xenopus Brn-3.0 shows 100% identity with the mouse Br n-3.0 and the human Brn-3a gene products. It is expressed during early embryonic development in distinct populations of the neural crest and later in specific cranial ganglia. It also is expressed in RGCs and i n the optic tectum, beginning before the first RGC axons have reached the tectum and continuing without interruption throughout the period w hen retino-tectal connections are established and refined. If the reti no-tectal projection is kept from forming by transplanting one eye to an ectopic location, Brn-3.0 expression is unaffected in both the ecto pic eye and the denervated side of the tectum. Conclusions. Coordinate d expression of Brn-3.0 in afferent and efferent pathways suggests mut ual regulation. However, the authors' evidence shows that expression i n the retina is not regulated by target-derived factors nor is express ion in the tectum regulated by retinal innervation.