GENE ACTIVATION DURING EARLY STAGES OF LENS INDUCTION IN XENOPUS

Several stages in the lens determination process have been defined, th ough it is not known which gene products control these events. At mid- gastrula stages in Xenopus, ectoderm is transiently competent to respo nd to lens-inducing signals. Between late gastrula and neural tube sta ges, the presumptive lens ectoderm acquires a lens-forming bias, becom es specified to form lens and begins differentiation. Several genes ha ve been identified, either by expression pattern, mutant phenotype or involvement in crystallin gene regulation, that may play a role in len s bias and specification, and we focus on these roles here. Fate mappi ng shows that the transcriptional regulators Otx-2, Pax-6 and Sox-3 ar e expressed in the presumptive lens ectoderm prior to lens differentia tion. Otx-2 appears first, followed by Pax-6, during the stages of len s bias (late neural plate stages); expression of Sox-3 follows neural tube closure and lens specification. We also demonstrate the expressio n of these genes in competent ectoderm transplanted to the lens-formin g region, Expression of these genes is maintained or activated prefere ntially in ectoderm in response to the anterior head environment. Fina lly, we examined activation of these genes in response to early and la te lens-inducing signals. Activation of Otx-2, Pax-6 and Sox-3 in comp etent ectoderm occurs in response to the early inducing tissue, the an terior neural plate. Since Sox-3 is activated following neural tube cl osure, we tested its dependence on the later inducing tissue, the opti c vesicle, which contacts lens ectoderm at this stage. Sox-3 is not ex pressed in lens ectoderm, nor does a lens form, when the optic vesicle anlage is removed at late neural plate stages. Expression of these ge nes demarcates patterning events preceding differentiation and is tigh tly coupled to particular phases of lens induction.