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Molecular Mechanisms of Forebrain Development in Vertebrates
Summary: Andrey Zaraisky investigates the genetic cascade connected with the functioning of the homeobox gene Xanf-1, a key regulator of anterior neurectoderm patterning
My research focuses primarily on defining the genetic network connected with the functioning of a key brain regulator, the homeobox gene Anf/Rpx/Hesx1, and its role in early development of the most anterior brain unit, the rostral forebrain. The rostral forebrain, which includes human cerebral hemispheres, is an important evolutionary innovation of vertebrates and is responsible for their higher cognitive functions. Therefore, understanding the mechanisms that underlie development of the rostral forebrain in vertebrates is a challenging goal of modern developmental biology. At the same time, revealing the principles of rostral forebrain development is important for understanding many human hereditable and developmental diseases. Despite impressive progress in functional genomics, the genetic mechanisms underlying early steps of rostral forebrain development, which include establishment of the anterior neural plate, its regionalization, and morphogenesis, remain insufficiently understood.
The homeobox gene Anf is one of few known regulators that is activated in the early rudiment of the rostral forebrain; the gene's product controls both neural specification and patterning in that region. The importance of Anf for forebrain development is underscored by the fact that mutations in this gene in mice and humans result in the severe forebrain disorder septo-optic dysplasia. Furthermore, we have demonstrated that overexpression of Xanf-1, the Xenopus laevis Anf, elicited enlargement of the rostral forebrain rudiment.
We have begun a systematic investigation of the Anf genetic network in the Xenopus experimental model. Our approach is based on a combination of four technologies: (1) bicolor fluorescent reporters in living transgenic Xenopus embryos to precisely reveal gene cis-regulatory elements; (2) yeast one-hybrid screening to identify transcription factors that bind to such cis-elements; (3) subtractive hybridization of cDNA from tissue explants to identify genes differentially expressed in embryonic areas with different developmental fates; and (4) yeast two-hybrid screening for protein partners of these transcription factors.
With this approach, we have been able to define the network of transcription factors responsible for restricting expression of Xanf-1 within the rostral forebrain rudiment. In addition, we have revealed several previously unknown genes regulated by the Xanf-1 homeodomain protein and its protein partners. This broad screen for genes involved in the Xanf-1 regulatory cascade yielded several very interesting candidates, further investigation of which promises to elucidate important new aspects of early forebrain development. Specifically, we are currently addressing the following issues.
First, we are investigating roles of two novel homologues of noggin, the well-known antagonist of the bone morphogenetic protein, in the regulation of anterior neural plate development. Investigation of these secreted factors, noggin2 and noggin4, whose genes are regulated by Xanf-1, looks very promising, because discovering novel regulators of the BMP cascade is generally recognized to be of great significance for better understanding embryonic development and for use in biomedical applications. Second, we are studying the small GTPase Ras-dva, which is negatively regulated by Xanf-1 in the rostral forebrain rudiment—an important investigation, given that Ras-dva, which represents a novel sub-family of small GTPases, may be involved in a signaling pathway that regulates early brain development. Third, we are investigating a possible novel mechanism coupling cell-morphogenetic movements with gene expression and based on interaction of the transcription factor Xanf-1 with the cytoskeleton protein zyxin. The importance of this study is underscored by the fact that the mechanisms coordinating morphogenesis of the anterior neurectoderm with gene expression are still poorly understood. Finally, we are studying the role of Xanf-1, in cells of the rostral forebrain rudiment, in inhibiting the expression of some genes encoding caudal forebrain regulators. We hypothesize that this inhibition helps to remove from the prospective territory of the rostral forebrain regulators that specify forebrain fate; this inhibition is thus a critical event of rostral forebrain development in all vertebrates.
Last updated September 2008
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