|Ph.D Student||Fonar Yuri|
|Subject||Focal Adhesion Kinase (FAK) Protein Regulates Wnt Signaling|
During Xenopus Laevis Development
|Department||Department of Medicine||Supervisor||Professor Dale Frank|
|Full Thesis text|
Focal adhesion kinase (FAK) is a cytoplasmatic thyrosine kinase protein, recruited to the membrane at regions of cell adhesions in the extracellular matrix, called focal adhesions. Besides its kinase function, FAK mediates protein-protein interactions, acting as scaffold to transfer adhesion dependent and growth factor signals to the cell. FAK's role in locomotion, migration and metastasis has been extensively studied. However, little is known about FAK's function in early embryonic development and morphogenesis. Knockout of FAK in mouse has an early lethal phenotype, making analysis of its role difficult.
Using antisense morpholino oligonucleotides, we have knocked down FAK protein expression in the frog Xenopus laevis, a model organism of vertebrate development. In Xenopus, FAK is expressed in the developing neural plate during the neurulation process. FAK protein knockdown had striking features, suggesting an anteriorized embryo phenotype. FAK morphant embryos had a short anterior-posterior axis, widening of the neural plate with postponed folding, short or absent posterior structures and extended head and anterior tissues. At the gene expression level, there was diminished expression of posterior neural marker genes with upregulation of anterior neural marker genes. These features were very similar to another known phenotype, inhibition of the canonical Wnt signaling pathway. Canonical Wnt signaling acts as a potent posteriorizer of the nervous system during vertebrate embryonic development. Like FAK, Wnt3a is also expressed in the neural plate. The knockdown of FAK protein leads to a strong reduction of Wnt3a gene expression in the neural plate, suggesting that the Wnt pathway is targeted for inhibition in FAK morphant embryos. Experiments also suggest the possibility that FAK knockdown also inhibits the canonical Wnt signaling cascade. Overexpression of exogenous Wnt3 protein in FAK morphant embryos rescues the anteriorized phenotype. This result strongly suggests that FAK protein lies upstream to the Canonical Wnt signaling pathway by regulating expression of the Wnt3a gene in the neural plate.
In this study, we show that FAK protein functions during early stages of development to regulate canonical Wnt signaling. FAK is essential for Wnt3a expression in the neural plate and perhaps FAK modulates the canonical Wnt signaling cascade at some undetermined point. This is a first description of connection between the FAK and Wnt proteins. There are a number of disease and pathological states independently associated with FAK or Wnt overactivation. The confirmation that FAK dependent regulation of Wnt signaling or Wnt3a gene expression is a general phenomenon could have wide potential biological and therapeutic implications.