A novel fork head gene mediates early steps during Xenopus lens formation

Xlens1 is a novel Xenopus member of the fork head gene family, named for it s nearly restricted expression in the anterior ectodermal placode, presumpt ive lens ectoderm (PLE), and anterior epithelium of the differentiated lens . The temporal and spatial restriction of its expression suggests that: (1) Xlens1 is transcribed initially at neural plate stages in response to puta tive signals from the anterior neural plate that transform lens-competent e ctoderm to lens-biased ectoderm; (2) further steps in the process of lens-f orming bias restrict Xlens1 expression to the presumptive lens ectoderm (PL E) during later neural plate stages; (3) interactions with the optic vesicl e maintain Xlens1 expression in the lens placode; and (4) Xlens1 expression is downregulated as committed lens cells undergo terminal differentiation. Induction assays demonstrate that pax6 induces Xlens1 expression, but unli ke pax6, Xlens1 cannot induce the expression of the lens differentiation ma rker beta-crystallin. In the whole embryo, overexpression of Xlens1 in the lens ectoderm causes it to thicken and maintain gene expression characteris tics of the PLE, Also, this overexpression suppresses differentiation in th e lens ectoderm, suggesting that Xlens1 functions to maintain specified len s ectoderm in an undifferentiated state. Misexpression of Xlens1 in other r egions causes hypertrophy of restricted tissues but only occasionally leads ectopic sites of gamma-crystallin protein expression in select anterior he ad regions, These results indicate that Xlens1 expression alone does not sp ecify lens ectoderm, Lens specification and differentiation likely depends on a combination of other gene products and an appropriate level of Xlens1 activity.