THE OPTX2 HOMEOBOX GENE IS EXPRESSED IN EARLY PRECURSORS OF THE EYE AND ACTIVATES RETINA-SPECIFIC GENES

Vertebrate eye development begins at the gastrula stage, when a region known as the eye field acquires the capacity to generate retina and l ens. Optx2, a homeobox gene of the sine oculis-Six family, is selectiv ely expressed in this early eye field and later in the lens placode an d optic vesicle. The distal and ventral portion of the optic vesicle a re fated to become the retina and optic nerve, whereas the dorsal port ion eventually loses its neural characteristics and activates the synt hesis of melanin, forming the retinal pigment epithelium. Optx2 expres sion is turned off in the future pigment epithelium but remains expres sed in the proliferating neuroblasts and differentiating cells of the neural retina. When an Optx2-expressing plasmid is transfected into em bryonic or mature chicken pigment epithelial cells, these cells adopt a neuronal morphology and express markers characteristic of developing neural retina and photoreceptors. One explanation of these results is that Optx2 functions as a determinant of retinal precursors and that it has induced the transdifferentiation of pigment epithelium into ret inal neurons and photoreceptors. We also have isolated optix, a Drosop hila gene that is the closest insect homologue of Optx2 and Six3. Opti x is expressed during early development of the fly head and eye primor dia.