The development of a multicellular organism from a single-celled zygote depends upon morphogenetic movements that generate the body plan and the elaboration of cell fate decisions. These events are regulated by highly conserved intercellular signals. One such family of secreted signaling molecules, encoded by Wnt genes, plays pivotal roles during embryogenesis to regulate the proliferation, differentiation and migration of embryonic cells. Interestingly, aberrant signaling by the Wnt signal transduction pathway leads to tumor formation, demonstrating that embryogenesis and carcinogenesis rely on cell communication via identical signaling pathways. Recent studies indicate that the Wnt signaling pathway may also regulate morphogenesis by orienting the mitotic spindle and planar cell polarity. The primary focus of our research is to understand the molecular mechanisms by which Wnts regulate cell proliferation, fate determination and cell polarity during normal mouse development. The mammalian embryo grows very rapidly during early development. During gastrulation (the process that generates the three germ layers that ultimately give rise to the entire organism), cells take only 6 hours, on average, to complete a cell cycle. Several Wnt genes are co-expressed during gastrulation and, as such, are good candidates for growth regulators. Targeted mutations generated in several different Wnt genes revealed that while Wnt8 is not essential, Wnt5a is required for the proper outgrowth of the embryonic trunk and tail. Furthermore, Wnt5a mutants display defects in the outgrowth of the face, tongue, limbs, external ear, external genitalia, the skeleton and the gastrointestinal tract. The abnormal morphologies observed in these disparate structures bear a striking resemblance to each other, suggesting that Wnt5a plays a fundamental role in regulating growth and morphogenesis in numerous tissues. Our current studies are directed toward understanding how Wnt5a may regulate stem/progenitor cell proliferation and differentiation during gastrulation and gastrointestinal tract development. Furthermore, we intend to explore putative roles for Wnt5a in the orientation of the mitotic spindle and in planar cell polarity during vertebrate gastrulation. In contrast to the Wnt5a mutants, embryos lacking Wnt3a completely lack trunk and tail somites, forming ectopic neural structures at the expense of mesoderm. T is a classic mouse mutation which, when homozygous, generates a paraxial mesoderm phenotype similar to the Wnt3a mutant phenotype. T disrupts the Brachyury gene, which encodes a highly conserved transcription factor expressed during gastrulation. We have shown that Brachyury is a direct transcriptional target of the Wnt3a, but not the Wnt5a, signaling pathway during the specification of mesoderm fates. Although several Wnts are coexpressed during gastrulation, it is clear that at least two Wnts have unique functions during gastrulation. This begs the question of how Wnt signaling specificity is achieved. Our future studies will address this problem. We are currently developing novel methodology that will allow for gain-of-function and complementation approaches in the mouse in vivo by combining electroporation-based gene transfer methods with whole embryo in vitro cultures. Finally, microarray hybridization screens and expression cloning approaches will be employed to identify new downstream targets of the Wnt3a and Wnt5a pathways.
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