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 its nearly restricted expression in the anterior ectodermal placode, presumptive 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 putative signals from the anterior neural plate that transform lens-competent ectoderm to lens-biased ectoderm; (2) further steps in the process of lens-forming bias restrict Xlens1 expression to the presumptive lens ectoderm (PLE) during later neural plate stages; (3) interactions with the optic vesicle 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 unlike pax6, Xlens1 cannot induce the expression of the lens differentiation marker beta-crystallin. In the whole embryo, overexpression of Xlens1 in the lens ectoderm causes it to thicken and maintain gene expression characteristics of the PLE. Also, this overexpression suppresses differentiation in the lens ectoderm, suggesting that Xlens1 functions to maintain specified lens ectoderm in an undifferentiated state. Misexpression of Xlens1 in other regions causes hypertrophy of restricted tissues but only occasionally leads ectopic sites of gamma-crystallin protein expression in select anterior head regions. These results indicate that Xlens1 expression alone does not specify lens ectoderm. Lens specification and differentiation likely depends on a combination of other gene products and an appropriate level of Xlens1 activity.
Kenyon
K L
KL
Neuroscience Program, The George Washington University Medical Center, Institute for Biomedical Sciences, NW, Washington DC 20037, USA.
Moody
S A
SA
Jamrich
M
M
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002454:Cell Differentiation; D004268:DNA-Binding Proteins / Q000502:physiology; D004475:Ectoderm / Q000502:physiology; D005136:Eye Proteins; D051858:Forkhead Transcription Factors; D018507:Gene Expression Regulation, Developmental; D018398:Homeodomain Proteins; D007908:Lens, Crystalline / Q000166:cytology / Q000196:embryology*; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000235:genetics* / Q000502:physiology; D000071841:PAX6 Transcription Factor; D051761:Paired Box Transcription Factors; D012097:Repressor Proteins; D017386:Sequence Homology, Amino Acid; D014157:Transcription Factors / Q000235:genetics* / Q000502:physiology; D014981:Xenopus; D029867:Xenopus Proteins
D004268:DNA-Binding Proteins; D005136:Eye Proteins; C497673:FOXE3 protein, Xenopus; D051858:Forkhead Transcription Factors; D018398:Homeodomain Proteins; D009687:Nuclear Proteins; D000071841:PAX6 Transcription Factor; D051761:Paired Box Transcription Factors; D012097:Repressor Proteins; D014157:Transcription Factors; D029867:Xenopus Proteins