|Ph.D Student||Elkouby Yaniv|
|Subject||The Embryonic Posterior Nervous System is Induced by|
Paraxial Mesoderm via Wnt3a and Meis3
|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, antagonism of BMP4 signaling in the ectoderm provides the first activation step, which induces anterior neural tissue. During the second transformation step, this anterior neural tissue is patterned to more posterior neural fates. Three signaling molecules, Wnt, FGF and RA are well established as "posteriorizers" that transform anterior neural tissue (forebrain) to more posterior neural fates (midbrain, hindbrain and spinal cord). Primary neurons and neural crest cells also differentiate in the posterior nervous system and require these same posteriorizers for their induction. Meis3, a member of the Meis family of proteins acts as a posteriorizer during nervous system development and its activity is required for the induction of posterior neural tissues, such as hindbrain, primary neurons and neural crest.
Meis3 and canonical Wnt signaling are both necessary and sufficient for the proper inductions of posterior neural cell fates. Their loss- and gain-of-function phenotypes are strikingly similar. Importantly, critical molecular targets of the canonical Wnt signaling have not been defined. Altogether, these suggest that the caudalizing activity of the canonical Wnt signaling might be mediated by the Meis3 transcription factor in the specification of posterior neural cell fates.
In this study, we identify Wnt3a as the crucial Wnt ligand for neural caudalizition. We show that Wnt3a mediated canonical Wnt signaling is necessary and sufficient for the activation of Meis3 expression. Biochemical and reporter analyses determine that Meis3 is a direct target gene of this pathway. Epistasis experiments show that Meis3 acts functionally downstream of canonical Wnt3a signaling to mediate its caudalizing activity. Furthermore, while Wnt-signaling in the neural plate is sufficient for posterior identity, the source of its caudalizing activity remains uncertain. Ablation experiments show that a special mesodermal domain, the paraxial mesoderm is crucial for neural caudalization; explants experiments show that the source of the initial Wnt3a caudalizing signal is localized to this tissue. This suggests a new model for neural A-P patterning, in which Wnt3a from the paraxial mesoderm induces posterior cell fates via direct activation of Meis3, in the overlying neural plate. In addition, we show that within the neural plate during the later neurula stages Wnt3a and Meis3 act in a feedback loop, then auto-regulated by Meis3.