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Oliver: Forebrain and Visual System Development

The forebrain is the most anterior portion of the central nervous system and gives rises to the telencephalon, diencephalon, and the eyes during development. All derivatives of the forebrain originate from the anterior neural plate. Therefore, mutations in genes that disrupt forebrain development provide a powerful tool for dissecting the mechanisms that regionalize the neural plate, establish fate restrictions, and determine the identities of all its main derivatives including the eyes.

Figure 1:Six3 expression in the anterior neural plate and developing eyes of mouse embryos.

In humans, mutations in the SIX3 gene encoding a homeodomain transcription factor have been associated with holoprosencephaly, the most common embryologic malformation of the forebrain in humans. The functional inactivation of Six3 in mice allowed us to determine that Six3 is a crucial regulator of vertebrate head development; its activity is essential to regulate key molecular events required for the regional patterning of the anterior neural plate.

The generated Six3 mutant mice lack the rostral forebrain and we determined that Six3 repression of Wnt signaling in the anterior neuroectoderm is essential for vertebrate forebrain development (Lagutin et al., 2003).

Following the specification of the anterior neural plate, the optic vesicles evaginate from the ventral forebrain to produce the lens and retina of the vertebrate eye.

Figure 2.Targeted inactivation of Six3 results in the absence of the eyes and nose and leads to postnatal lethality. Wild-type (a,c), Six3+/- (e), and Six3-/- (b,d,f) embryos were used.

Six3 expression in the developing eye suggested that most likely it is also an important regulator of the early stages of visual system development in vertebrates. This proposal was corroborated by the conditional deletion of mouse Six3 in the eye field. Using this approach we showed that neuroretina specification in mouse embryos requires Six3-mediated suppression of Wnt8b in the anterior neural plate. Using chromatin immunoprecipitation assays (ChIP), we identified Six3-responsive elements in the Wnt8b locus and demonstrated that Six3 directly represses Wnt8b expression in vivo.


Figure 3: Six3 activity is crucial for the development of the mammalian neural retina. (A) At E11.5, a normal-looking pigmented eye is seen in control embryos. In Six3 conditional–mutant littermates, the eye appears defective and composed of only the RPE (B, C). At E10.5, NR specification has already taken place in control embryos, as indicated by the expression of the NR markers Six3 (D), Rax (F), Sox2 (H), and Vsx2 (J). In the conditional-mutant littermates, this process is defective, as revealed by the lack of expression of any of these NR markers (arrows in E, G, I, K). At this stage, the RPE is also normally detected, as indicated by the expression of Mitf (L) and Pax6 (N). In Six3 conditional–mutant embryos, the expression of these two transcription factors highlights the fact that the mutant optic vesicle is exclusively composed of the RPE (arrows in M, O).