Six3functions in anterior neural plate specification by promoting cell proliferation and inhibitingBmp4expression

Abstract

Summary Although it is well established that Six3 is a crucial regulator of vertebrate eye and forebrain development, it is unknown whether this homeodomain protein has a role in the initial specification of the anterior neural plate. In this study, we show that exogenous Six3 can expand the anterior neural plate in both Xenopus and zebrafish, and that this occurs in part through Six3-dependent transcriptional regulation of the cell cycle regulators cyclinD1 and p27Xic1, as well as the anti-neurogenic genes Zic2 and Xhairy2. However, Six3 can still expand the neural plate in the presence of cell cycle inhibitors and we show that this is likely to be due to its ability to repress the expression of Bmp4 in ectoderm adjacent to the anterior neural plate. Furthermore, exogenous Six3 is able to restore the size of the anterior neural plate in chordino mutant zebrafish, indicating that it has the ability to promote anterior neural development by antagonising the activity of the BMP pathway. On its own, Six3 is unable to induce neural tissue in animal caps, but it can do so in combination with Otx2. These results suggest a very early role for Six3 in specification of the anterior neural plate, through the regulation of cell proliferation and the inhibition of BMP signalling.

SIX2 and BMP4 Mutations Associate With Anomalous Kidney Development

“…It remains unclear how six2.1 and bmp4 act in patterning the pronephros, but recent papers hint at links between six genes and bmp regulation: bmp4 expression is downregulated in otic vesicles in dog-eared (eya1) mutant zebrafish embryos, 40 and six3 has been shown to repress bmp4 transcription directly in Xenopus and zebrafish during anterior neural place specification. 41 It is worth noting that while bmp4 and six2.1 are expressed at the right place and time to affect pronephric development during somite stages, knockdown of bmp4 and six2.1 could also be affecting pronephric development by altering global mesoderm patterning in the early embryo leading to an expansion of the intermediate mesoderm region that will express wt1. However, the MO-injected embryos we analyzed did not show global mesoderm patterning defects nor alterations of pax2.1 in the intermediate mesoderm ( Figure 6; data not shown), suggesting the observed effects were due to later functions of bmp4 and six2.1 in pronephric development.…”

Emerging roles for zic genes in early development

“…The six1 gene is expressed in the pre-placodal ectoderm, rostral and lateral to the anterior neural plate, while the six3 gene is expressed in the anterior neurectoderm (Pandur and Moody, 2000;Zhou et al, 2000). In Xenopus, both genes have been shown to induce transcription of the zic2 gene, while repressing neural crest marker expression (Brugmann et al, 2004;Gestri et al, 2005). Ectopic Six1 acts as a transcriptional activator to induce expression of zic2, although reducing Six1 protein levels also expands zic2 expression in Xenopus embryos.…”

“…Six3 acts as a transcriptional repressor to induce expression of zic2, which indicates that it is not a direct regulator. In addition, Six3 represses BMP4 and wnt expression (Gestri et al, 2005). Interestingly, mutation of the human SIX3 gene causes holoprosencephaly, as do mutations in the ZIC2 gene (Dubourg et al, 2004).…”

Expression of Six3 Opposite Strand (Six3OS) during mouse embryonic development

“…In the mouse, functional inactivation of Six3 abolishes telencephalon formation, a phenotype most likely resulting from the abnormal anterior expansion of posteriorizing Wnt activity (Lagutin et al, 2003). In Xenopus, Six3 activity participates in the specification of the anterior neural plate by promoting cell proliferation and inhibiting BMP4 expression (Gestri et al, 2005). In medaka fish, Six3 acts as an antagonist of the replication-initiation inhibitor Geminin to control the balance between proliferation and differentiation during retina formation (Del Bene et al, 2004).…”