|M.Sc Student||Gefen Aharon|
|Subject||XMeis3 Protein Activates a Hindbrain Inducing Center|
Formation by Turning on FGF3 Expression during
|Department||Department of Medicine||Supervisor||Professor Dale Frank|
|Full Thesis text|
In the vertebrate Xenopus laevis (frog), the embryonic central nervous system (CNS) is induced by a two-step process. Initially, antagonism of BMP4 signaling in the ectoderm provides activates an anterior neural tissue. During the second transformation step, forebrain tissue is patterned to midbrain, hindbrain and spinal cord. Three signaling molecules: Wnt, fibroblast growth factors (FGF) and retinoic acid are well established as "posteriorizers".
The XMeis3 protein, a Tale-class homeobox transcription factor, is required for proper antero-posterior pattern formation of the CNS by forming a hindbrain-inducing center. XMeis3 acts as an activator of posterior neural marker expression and reduces anterior neural marker expression in both embryos and animal cap explants neuralized by BMP antagonists. The posteriorizing activity of XMeis3 is FGF/MAP kinase signaling dependant. In Xenopus, FGF3 expression overlaps with XMeis3 in the early hindbrain. FGF8 is an additional FGF-member that is also expressed in the proper temporal and regional manner. Yet, little is known about these FGF molecules’ role in Xenopus hindbrain formation.
The goal of this project was to reveal FGF family proteins that participate in this process, and to determine the hierarchy between XMeis3 and these FGF molecules.
We demonstrate that FGF3 and / or FGF8 protein activities are necessary for proper posteriorizing of the CNS. The loss-of-function phenotypes of XMeis3 and FGF (by using antisense Morpholino Oligonucleotides (MO) - specific inhibitors to FGF3 and/ or FGF8 translation) are strikingly similar. The XMeis3 posteriorizing activity is FGF3 and / or FGF8 dependent, both in whole embryos and in animal cap explants. FGF3 expression is XMeis3 activity dependent. Overexpression of FGF3 mRNA partially rescues posterior cell fates in XMeis3 morphant embryos. Inhibition of FGF3 and / or FGF8 overcomes the caudalizing activity of XMeis3 mRNA overexpression in animal cap explants. Thus, FGF3 and /or FGF8 act downstream of XMeis3 protein to induce posterior cell fates. We also show the mutual relationships between HoxD1, an XMeis3 direct target gene and FGF3.
We also show the involvement of FGF signaling in two other processes in the nervous system. The loss-of-function of the combination of FGF3 and FGF8, but neither alone, has a role in neural crest development. FGF3 activity is also necessary for proper primary neurogenesis.
In conclusion, we show that XMeis3 protein regulates antero-posterior patterning in the nervous system by forming a hindbrain-inducing center. By activating FGF3 and/or FGF8 expression, hindbrain, primary neuron and neural crest cell-fates are specified from more anteriorly fated tissues.