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Ira O. Daar, Ph.D.
Developmental Signal Transduction
Our current research interests are aimed toward examining the mechanism by which Eph receptor tyrosine kinases and their ephrin ligands signal events affecting cell-cell adhesion and morphogenetic movements. From the elucidation of these signal transduction pathways we may improve our understanding of oncogenesis. The cell-cell adhesion system plays a major role in normal development and morphogenesis. Inactivation of this adhesion system is thought to play a critical role in cancer invasion and metastasis. The Xenopus embryo is well suited for investigations of these processes because the frog has a well characterized and invariant cell fate map and cell lineage can be easily traced during experiments. Mutant receptors, ligands, and other proteins can be ectopically expressed in embryos. Thus, their effects on signal transduction, motility, and differentiation can be assessed morphologically and histologically as well as biochemically in a developing vertebrate.
Our laboratory is currently investigating the role of the Xenopus Eph receptor tyrosine kinases and ephrinB transmembrane ligands in cell signaling and function using the Xenopus oocyte and embryo systems. At present, our emphasis is placed upon the mechanism by which these Eph family members send signals affecting morphogenetic movements and exhibit crosstalk with the fibroblast growth factor (FGF) signaling pathway.
Members of the Eph family have been implicated in regulating numerous developmental processes and have been found to be de-regulated in metastatic cancers. Recent studies from our laboratory have identified a member of another tyrosine kinase family-the fibroblast growth factor (FGF) receptor-that can interact with and regulate the ephrin signaling molecule. FGF, like the ephrins, has been implicated in axon guidance, neural cell migration, and angiogenesis. Our research demonstrates that crosstalk between the FGF receptor and an Eph ligand can exist in both neural and nonneural tissues, and that when tyrosine phosphorylated in response to FGF receptor activation, Eph ligand signals that reduce adhesion can be inhibited. This study is the first evidence for direct interaction between an Eph transmembrane ligand and members of the FGF receptor tyrosine kinase family.
In collaboration with Sally Moody's laboratory, we demonstrated that both the FGF and ephrin pathways impact cell movement. Activating the FGF pathway before gastrulation represses cellular movements in the presumptive anterior neural plate and prevents retinal stem cells from expressing a retinal fate, independent of mesoderm induction or anterior-posterior patterning. Inhibiting the FGF pathway promotes cell dispersal and significantly increases eye field contribution. EphrinB1 reverse signaling is required to promote cellular movements, and can rescue the FGF receptor-induced repression of retinal fate. These results indicate that FGF modulation of ephrin signaling can regulate cell movement and positioning, and thus cell fate.
Our laboratory has continued these studies examining proximal and distal signaling from ephrinB1 that controls cell adhesion and cell movement. We recently found evidence that ephrinB1 signals via its intracellular domain to control retinal progenitor movement into the eye field by interacting with Dishevelled (dsh), and co-opting the planar cell polarity (PCP) pathway. Using biochemical analysis and gain or loss of function experiments, our data suggest that dsh associates with ephrinB1 and mediates ephrinB1 signaling via downstream members of the PCP pathway during eye field formation.
A body of evidence is emerging that shows a requirement for ephrin ligands in the proper formation of cell and tissue boundaries. These processes are dependent upon the cell-cell adhesion system that plays a critical role in normal morphogenetic processes during development, as well as in invasion and metastasis. Although ephrinB ligands are bi-directional signaling molecules, the precise mechanism by which ephrinB1 signals through its intracellular domain to regulate cell-cell adhesion in epithelial cells has remained elusive. We have found that ephrinB1 associates with the Par polarity complex protein, Par-6, a scaffold protein required for establishing tight junctions, and can compete with the small GTPase Cdc42 for an association with Par-6. This competition results in inactivation of the Par complex, resulting in the loss of tight junctions. Moreover, the interaction between ephrinB1 and Par-6 is disrupted upon tyrosine phosphorylation of the intracellular domain of ephrinB1. Thus, we have identified a mechanism by which ephrinB1 signaling regulates cell-cell junctions in epithelial cells, and this may impact how we devise therapeutic interventions regarding these molecules in metastatic disease.
This page was last updated on 3/27/2013.