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Mech Dev
2004 Dec 01;12112:1425-41. doi: 10.1016/j.mod.2004.08.002.
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The homeodomain-containing transcription factor X-nkx-5.1 inhibits expression of the homeobox gene Xanf-1 during the Xenopus laevis forebrain development.
Bayramov AV
,
Martynova NY
,
Eroshkin FM
,
Ermakova GV
,
Zaraisky AG
.
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Expression of the homeobox gene Xanf-1 starts within the presumptive forebrainprimordium of the Xenopus embryo at the midgastrula stage and is inhibited by the late neurula. Such stage-specific inhibition is essential for the normal development as the experimental prolongation of the Xanf-1 expression elicits severe brain abnormalities. To identify transcriptional regulators that are responsible for the Xanf-1 inhibition, we have used the yeast one-hybrid system and identified a novel Xenopus homeobox gene X-nkx-5.1 that belongs to a family of Nkx-5.1 transcription factors. In terms of gene expression, X-nkx-5.1 shares many common features with its orthologs in other species, including expression in the embryonic brain and in the ciliated cells of the otic and lateral line placodes. However, we have also observed several features specific for X-nkx-5.1, such as expression in precursors of the epidermal ciliated cells that may indicate a possible common evolutionary origin of all ciliated cells derived from the embryonic ectoderm. Another specific feature is that the X-nkx-5.1 expression in the anterior neural plate starts early, within the area overlapping the Xanf-1 expression territory at the midneurula stage, and it correlates with the beginning of the Xanf-1 inhibition. Using various loss and gain-of-function techniques, including microinjections of antisense morpholino oligonucleotides and mRNA encoding for the X-nkx-5.1 and its dominant repressor and activator versions, we have shown that X-nkx-5.1 can indeed play a role of stage-specific inhibitor of Xanf-1 in the anterior neural plate during the Xenopus development.
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15511636
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Fig. 2. The analysis of temporal and spatial expression patterns of X-nkx-5.1. (A) RT-PCR assay of X-nkx-5.1 expression at the stages indicated. (B–L, Q–W) In situ hybridization analysis of spatio-temporal expression of X-nkx-5.1. (B) At stage 10 1/2 the expression is detected in scattered cells of the animal hemisphere. (C) Scattered epidermal cells at the ventral side of the embryo at stage 12 1/2 continue to express X-nkx-5.1 even stronger than at stage 10 1/2. (D) At stage 12 1/2 the X-nkx-5.1 expression is absent at the dorsal side of embryo. (E,F-close-up.) Expression of X-nkx-5.1 in the scattered epidermal cells continues during neurulation. (E) The whole embryo at the midneurula stage (anterior to the left). (F) The fragment of the epidermis of the embryo at the midneurula stage. (G) Epidermal cells expressing X-nkx-5.1 (arrows) are localized exclusively in the internal layer (the horizontal cut of embryo at stage15). (H) The saggital section of embryo at the end of neurulation. The X-nkx-5.1 expression in the anterior neural fold is marked by the white arrow. (I–L) Anterior views of the embryos (dorsal up) at successive stages between the end of gastrulation and the end of neurulation show gradual increase of the X-nkx-5.1 expression in the anteriorneurectoderm and headepidermis. Note that no expression could yet be seen in the anteriorneurectoderm at the end of gastrulation (I). The dotted lines on I and J indicate the approximate position of the anterior margin of the neural plate). (M–P) Anterior views of the embryos hybridized at the same stages as shown on I-L with the Xanf-1 mRNA probe. This row demonstrates a negative correlation between the time courses of Xanf-1 and X-nkx-5.1 expression: in contrast to X-nkx-5.1, Xanf-1 is intensively expressed in the presumptive anteriorneurectoderm at the end of gastrulation (M), but its expression is gradually declined thereafter (N–P). The dotted lines on M and N indicate approximate position of the anterior margin of the neural plate. The Xanf-1 expression in the primordium of the anteriorpituitary is shown by the arrows. (Q) After the neural fold closing X-nkx-5.1 expression is detected in the anterior side of neural tube and spreads along the dorsal side. (R) The expression of X-nkx-5.1 within the area of the headepidermis at stage 22. The arrow shows the expression in the otic ganglion and the arrowhead-the expression in the cement gland. (S) At stage 23 the longitudinal row of scattered cells expressing X-nkx-5.1 is revealed on both side of neural cord (arrow). (T,U) At the tailbud stage, the X-nkx-5.1 expression increases in two domains of the neural tube: one in the prospective forebrain (see the arrowhead) and the other in the ventralmesencephalon (the black arrow). It also sharply increases in the anterior part of otic vesicle (the green arrows), in otic ganglion (the yellow arrows) and in the areas, corresponding to the middle (the red arrows) and anterioventral (the red arrowhead) lateral line placodes. (V,W) At the tadpole stage, the X-nkx-5.1 expression is localized in the ventralforebrain in two domains corresponding to the lamina terminalis (the black arrowheads) and ventralthalamus (the red arrowheads), respectively. The comparatively weak expression is seen in the ventral part of the rostral hindbrain (the red arrows) and in scattered cells within the dorsal and lateral sides of posteriorhindbrain and spinal cord (the black arrows).
