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The novel homeobox-containing gene Vax1, a member of the Emx/Not gene family, is specifically expressed in the developing basal forebrain and optic nerve. Here, we show that Vax1 is essential for normal development of these structures. Mice carrying a targeted mutation of Vax1 show dysgenesis of the optic nerve, coloboma, defects in the basal telencephalon, and lobar holoprosencephaly. With the help of molecular markers we determined that in the developing visual system, the absence of Vax1 results in a proximal expansion of the activity of Pax6 and Rx. This observation suggests that Vax1 may interfere negatively with the expression of Pax6 and Rx. In reciprocal gain-of-function experiments, injection of Xvax1 mRNA or Shh into Xenopus embryos primarily affects the brain at the level of the eyeprimordium. Consistent with the loss-of-function results, the injection of Xvax1 results in a down-regulation of Rx. Similarly, Shh injection expands the Vax1 and Pax2 territory at the expense of the Pax6 and Rx region. On the basis of these results, we propose a model for a molecular cascade involved in the establishment of structures of the visual system.
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10601036
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Effect of ectopic Xvax1 overexpression on the transcription of Xrx. Normal expression pattern ofXrx at stages 17 (a) and stage 34 (g). A horizontal vibratome section of a stage 17 embryo is shown ind (anterior is up). Embryos were injected with 250 pg (b,e,h) or 500 pg [c,d(is),f,i] of Xvax1encoding mRNA into both blastomeres at the two-cell stage. The resulting effects were dose dependent. At stage 17, repression ofXrx is either moderate (b,e) or strong [c,d(is),f], resulting in a mild (h) or strong phenotype (i) at stage 34. (nis) Noninjected side; (is) injected side.
Xvax1 expression is positively regulated byX-bhh. Xenopus embryos were injected into one cell (c–g) or into both cells (h) at the two-cell stage with synthetic mRNA encoding X-bhh. (a) Control embryos (stage 34) express Xvax1 in the optic stalk and disc, as well as in the ventralforebrain. (b) Transverse section from a stage 34 control embryo at the level of the eye define differentially regulated Xvax1 (purple) andXpax6 (red) expression domains. Xvax1 is expressed in the ventralhypothalamus and in the eye disc, whereas Xpax6 is strongly expressed in the neural retina and in the lens, as well as in the dorsal midbrain. (c–h) Injection ofX-bhh leads to a strong induction of Xvax1 (purple) expression (d,e,g,h) and to a severe inhibition of Xpax6 (red) expression (g,h) in the optic primordium. (c) Uninjected side of the embryo shown in d. (e) Transverse section of the embryo in c and d.Xvax1 (purple) is ectopically expressed in the whole remaining eye vesicle. (f) The uninjected side of the same embryo ing reveals almost normal Xpax6 expression (red) in the eye. (h) Transverse section of an embryo injected into both cells at the two-cell stage. Xvax1 is strongly expressed in both eye vesicles. (le) Lens; (od) optic disc; (os) optic stalk; (ov) optic vesicle; (vh) ventralhypothalamus.
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