The Eph family of receptor tyrosine kinases and their membrane-bound ligands, the ephrins, have been implicated in regulating cell adhesion and migration during development by mediating cell-to-cell signaling events. We have shown that ectopic expression of X-ephrinB1, a Xenopus homologue of the murine ephrin-B1 transmembrane ligand, causes dissociation of Xenopus embryonic blastomeres by the mid-blastula transition. Moreover, a mutant which lacks the extracellular receptor binding domain can induce this phenotype. Basic fibroblast growth factor (bFGF), but not activin, can rescue both the loss of cell adhesion and mesoderm induction in ectodermal explants expressing XLerk. Genetic evidence from other laboratories suggests that ephrins may transduce signals, and become tyrosine phosphorylated during embryogenesis. However, the induction and functional significance of ephrin phosphorylation is not yet clear. Our studies reveal that an activated fibroblast growth factor (FGF) receptor associates with and phosphorylates ephrinB1 on tyrosine. Moreover, this phosphorylation is a regulatory event that alters ephrinB1 function in cell adhesion. In addition, we identify a region in the cytoplasmic tail of ephrinB1 that is critical for direct interaction with the FGF receptor. We also find that FGF treatment of neural tissue expressing ephrinB1 reduces cell binding to a cognate Eph receptor substrate. This is the first demonstration of direct communication between the FGF receptor family and the Eph ligand family and implicates crosstalk between these two cell surface molecules in regulating cell adhesion. My laboratory is also currently studying a novel homeobox protein, Meis 1, that has some homology to the human oncogene product, pbx-1, discovered in human pre-B cell leukemias. Meis1 (Myeloid Ecotropic viral Integration Site 1) is a homeobox gene that was originally isolated as a common site of viral integration in myeloid tumors of the BXH-2 recombinant inbred mice strain. We previously isolated a Xenopus homolog of Meis1 (Xmeis1) and have shown that Xmeis1 plays a critical role in neural crest development. In animal cap explants, overexpression of Xmeis1b, an alternatively spliced form of Xmeis1, induces expression of neural crest marker genes in the absence of mesoderm. Moreover, Xmeis1b induces XGli-3 and XZic3, pre-pattern genes involved at the earliest stages of neural crest development. Misexpression of Xmeis1b also induces ectopic expression of neural crest markers along the antero-posterior axis of the neural tube in developing Xenopus embryos. In contrast, Xmeis1a, another splice variant, is much less effective at inducing these effects. These data suggest that Xmeis1b is involved in neural crest cell fate specification during embryogenesis, and can functionally intersect with the Gli/Zic signal transduction pathway. Pbx proteins bind DNA cooperatively as heterodimers with Meis family members and also with Hox proteins, and are believed to specify cell identity during development. This year, we present evidence that Pbx1, in partnership with Meis1b, can regulate neural crest marker genes during Xenopus development. A Xenopus homolog of the homeodomain protein Pbx1b, was isolated and shown to be expressed throughout embryogenesis, and overlaps with Xmeis1 in several areas including the lateral neural plate, caudal branchial arch, hindbrain, and optic cup. When ectopically expressed, Xpbx1b can synergize with Xmeis1b to promote posterior neural and neural crest cell fates in embryonic tissue. Further, a physical interaction between these two homeodomain proteins is necessary for neural crest induction in embryos. Finally, expression of mutant constructs in which Xpbx1b and Xmeis1b proteins were fused to the repressor domain from Drosophila Engrailed was found to inhibit neural crest gene expression. These data indicate that Xpbx1b and its partner, Xmeis1b, function in a transcriptional activation complex during neural crest formation.
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