|Ph.D Student||Volovik Yael|
|Subject||Functional Interactions between Meis3 and Tsh1 Proteins|
Control Anterior-Posterior Pattern Formation
during Early Nervous System Development
in Vertebrate Embryos
|Department||Department of Medicine||Supervisor||Professor Dale Frank|
|Full Thesis text|
In the vertebrate, Xenopus laevis (frog), the embryonic nervous system is induced early in development by a two-step process. Initially, during gastrulation, BMP signaling antagonism in the ectoderm provides the first “activation” step, which induces anterior neural tissue. During the second “transformation” step, this anterior neural tissue is re-specified to more posterior neural fates. Three signaling molecules, Wnt, fibroblast growth factor (FGF) and retinoic acid (RA) act as posteriorizers to “transform” anterior neural tissue (forebrain) to more posterior neural fates (midbrain, hindbrain and spinal cord). Meis3, a member of the TALE-class homebox family of transcription factors acts downstream to canonical Wnt signaling as a neural posteriorizer, being necessary and sufficient for hindbrain induction. meis3 gene expression in the neural plate is directly activated by the mesodermal Wnt3a ligand. Later, Meis3 activates wnt3a expression in the neural plate and Wnt3a and Meis3 act in a positive feedback loop, enhancing each other’s gene expression. At high levels, Meis3 activates transcription of the Teashirt1 (Tsh1) co-repressor protein. Tsh1 is recruited to the meis3 promoter by Meis3 protein to repress meis3 transcription, and to terminate the hindbrain developmental program.
Tsh proteins are found in all the Bilateria. Tsh1 proteins interact with Wnt signaling in several developmental processes. In Drosophila, Tsh protein interacts with homeotic genes to control prothoracic segment specification. Planarian Tsh protein is an early target of Wnt signaling required for Wnt dependent tail versus head regeneration. In Xenopus, Tsh1 over-expression or knock-down caused a similar phenotype of hindbrain perturbation at neurula stages.
In this study, we examined the Meis3/Tsh1 interactions during early neural development. Over-expression and knock-down experiments showed that Tsh1 protein acts as a switch to regulate Meis3 hindbrain versus spinal cord inducing activity, with an important regulatory role in hindbrain-spinal cord junction determination. Furthermore, we show that active Tsh1 is crucial for proper hindbrain development and patterning, already at gastrula stages. In Tsh1 hypomorph embryos, we revealed that Tsh1 interacts with the Wnt and RA signaling pathways in the neural plate. We show that Tsh1 is required for Wnt mediated anterior field inhibition, and crucial for activating down-stream expression of specific homeobox genes that are RA targets. Finally, we show that zygotic tsh1 transcription begins earlier than was previously reported and is tightly auto-regulated by Tsh1 protein itself. These multifunctional activities place Tsh1 protein in the center of many regulatory circuits that control AP pattern formation in the developing CNS.