|M.Sc Student||Shifman Anastasia|
|Subject||FoxD1 Protein Regulates Neural Cell Fate Specification|
during Early Xenopus Laevis Development
|Department||Department of Medicine||Supervisor||Professor Dale Frank|
|Full Thesis text|
Transcription factors of the Fox (forkhead box) family have been identified in all metazoans. In Xenopus, all the known Fox proteins are involved in cell specification and differentiation. Xenopus FoxD1, also known as XBF-2 (Xenopus brain factor - 2) is a winged-helix transcription factor expressed in the early mesoderm and nervous system. In other vertebrate systems, the role of FoxD1 protein was studied in later embryonic development. Little is known about the function of FoxD1 in early embryonic development.
Previous studies showed that FoxD1 is expressed in the anterior end of the Xenopus embryo nervous system, in the forebrain. Ectopic FoxD1 expression in animal cap explants induces anterior neural marker gene expression, but surprisingly it also induces expression of general neural and posterior neural markers. In this study, we have shown that canonical Wnt activity can regulate the eventual activity outcome of FoxD1 protein in regulating neural A-P patterning. In the presence of Wnt activity, FoxD1 protein is potent posteriorizer. In contrast, when FoxD1 is expressed in regions depleted of Wnt activity, FoxD1 protein is a robust anteriorizer. We also show that in conditions when the neuralizing activity of FoxD1 is inhibited by BMP4, FoxD1 protein not only induces posterior neural gene marker expression, but this expression is enhanced in the presence of BMP activity.
When we knocked down FoxD1 protein expression in Xenopus embryos, morphant embryos had a short anterior as well short posterior axis, since the head and trunk were reduced. At the gene expression level these results were verified. Surprisingly, the expression of the most anterior XANF gene marker in the forebrain was up-regulated, suggesting that FoxD1 regulates pattern in the most anterior forebrain regions. In addition, there was down-regulation of dorsal mesoderm structures, like notochord and dorso-lateral mesoderm, such as muscle. These posterior neural and mesodermal disruptions led us to investigate the role of FoxD1 at early gastrula stages. At gastrula stages, the knock down of FoxD1 protein causes a strong reduction of dorsal and dorso-lateral mesoderm gene expression, with a concomitant expansion of ventral mesoderm, suggesting that FoxD1 protein regulates mesoderm patterning. We also found that FoxD1 is necessary for Wnt3a and Wnt8 gene expression in lateral mesoderm. These two ligands are critical for regulating mesodermal and subsequent posterior neural patterning. These results suggest, that FoxD1 controls lateral mesoderm formation and further neural patterning via induction of Wnt3 and Wnt8 gene expression.