|Ph.D Student||Gutkovich Yoni Evgeni|
|Subject||Transcription Factor Network and Signaling Pathway|
Interactions Specify Posterior Neural Cell Fates
in the Xenopus Embryo
|Department||Department of Medicine||Supervisor||Professor Dale Frank|
|Full Thesis text|
The Xenopus laevis nervous system is induced during early embryonic development in a two-step process. During the first step, called "activation", BMP antagonists secreted by the Spemann organizer induce anteriorly fated neural cells in the overlying ectoderm. The second step, called "transformation", involves RA, FGF and Wnt signaling. These signaling molecules act to “transform” (caudalize) the anterior forebrain fated cells to more posterior cell fates, like hindbrain and spinal cord. Present paradigms suggest that these “transformers” caudalize different parts of the posterior nervous system through a concentration gradient combined with a constant level of BMP antagonism. The caudalizers would be at their highest concentration levels in the most posterior end of the embryo to induce spinal cord; intermediate levels would induce hindbrain/midbrain structures and their absence/low concentration allows the differentiation of the forebrain. Formation of other sub-groups of neural cells, such as primary neuron and neural crest cells, occurs in parallel to the differentiation of hindbrain and spinal cord. Both neural crest and primary neurons, along with the hindbrain fated cells, are induced posterior to the midbrain-hindbrain junction by the same caudalizing signaling pathways. Hence, it is reasonable to speculate that these three distinct cell types are specified via a common regulatory pathway. The Meis3 transcription factor was shown to be essential for hindbrain differentiation, whereas Zic and Pax3 transcription factors were shown to be necessary for neural crest development. In this study we show that Pax3, Zic family proteins and Meis3 are all individually required for the induction of neural crest, primary neuron and hindbrain cell fates. We show that Meis3 is situated downstream to Pax3 and Zic proteins in the pathway hierarchy, and that Meis3 rescues the loss of posterior neural cell fate phenotype in Zic protein deficient embryos. FGF3 and FGF8 ligands, whose expression is induced by Meis3, were shown to be Meis3 direct target genes. We show that these FGFs rescue the loss of posterior neural cell fate phenotype in Meis3 morphant embryos. Surprisingly, Zic deficient embryos displayed elevated spinal cord marker expression, and explant experiments showed that spinal cord marker expression, unlike that of hindbrain, was optimally induced by BMP signaling and not BMP antagonism. We propose that neural crest, primary neuron and hindbrain cell fates are specified via a common regulatory pathway, while spinal cord induction does not follow the simple two-step model for neural induction, forcing us to refine the accepted paradigm.