| Pax6 is a transcription factor required for vertebrate and invertebrate eye development. In Drosophila, three downstream targets of the Pax6 homologue eyeless ( ey) have been identified: sine oculis ( so), eyes absent (eya) and dachshund (dac). Their evolutionary conservation has lead to the proposal that vertebrate eye development is controlled by a genetic network analogous to that in Drosophila.; To test this hypothesis, I studied the interdependencies of protein expression of mouse homologs of Drosophila eyeless, sine oculis, dachshund and eyes absent—Pax6, Six3, Dach1 and Eya1, respectively—in mouse eye and nasal development. I also analyzed molecular interactions between these proteins using an in vitro system where nuclear translocation of Eya1 can be induced by co-transfection with an interacting protein,; Pax6 is present in the developing lens and optic vesicle. Six3 and Dach1 expression in the lens ectoderm is Pax6-dependent, whereas that in the optic vesicle is not. Moreover, Six3 shows a cytoplasmic distribution in Pax6 mutant ectoderm at E9.5, instead of its normal nuclear localization. Therefore different regulatory mechanisms operate in these two ocular tissues. In accord with the Drosophila model, the expression of Six3, Dach1 and Eya1 in the nasal placode is Pax6-dependent. In contrast to the Drosophila model, however, the initial expression of these proteins is Pax6-independent.; In Dach1 knockouts, Pax6 and Six3 ocular expression is unaffected. In Eya1/2 double knockouts, Pax6, Six3 and Dach1 ocular and nasal expression are unaffected. In contrast to the Drosophila model in which dac and so are downstream of eya, these results indicate that in mammalian development, Pax6, Six3 and Dach1 expression does not depend on the function of either Dach1 or on that of Eya1 and Eya2.; In vitro, Eya1 can be nuclear translocated by all Six family members tested, and the Eya domain, is sufficient for this translocation. An interaction between these two proteins was confirmed by co-immunoprecipitations.; These experiments establish the molecular interdependencies of Pax6, Six3, Eya1and Dach1 gene expression in the early developing eye and nose and support a model in which Eya1 requires Six to act in the nucleus as a transcriptional activator. |
