| Summary: | <p>Abstract</p> <p>Background</p> <p>The <it>pygopus </it>gene of <it>Drosophila </it>encodes an essential component of the Armadillo (β-catenin) transcription factor complex of canonical Wnt signaling. To better understand the functions of <it>Pygopus</it>-mediated canonical Wnt signaling in kidney development, targeted mutations were made in the two mammalian orthologs, <it>Pygo1 </it>and <it>Pygo2</it>.</p> <p>Results</p> <p>Each mutation deleted >80% of the coding sequence, including the critical PHD domain, and almost certainly resulted in null function. <it>Pygo2 </it>homozygous mutants, with rare exception, died shortly after birth, with a phenotype including lens agenesis, growth retardation, altered kidney development, and in some cases exencephaly and cleft palate. <it>Pygo1 </it>homozygous mutants, however, were viable and fertile, with no detectable developmental defects. Double <it>Pygo1</it>/<it>Pygo2 </it>homozygous mutants showed no apparent synergy in phenotype severity. The BAT-gal transgene reporter of canonical Wnt signaling showed reduced levels of expression in <it>Pygo1</it><sup>-/-</sup>/<it>Pygo2</it><sup>-/- </sup>mutants, with tissue-specific variation in degree of diminution. The <it>Pygo1 </it>and <it>Pygo2 </it>genes both showed widespread expression in the developing kidney, with raised levels in the stromal cell compartment. Confocal analysis of the double mutant kidneys showed disturbance of both the ureteric bud and metanephric mesenchyme-derived compartments. Branching morphogenesis of the ureteric bud was altered, with expanded tips and reduced tip density, probably contributing to the smaller size of the mutant kidney. In addition, there was an expansion of the zone of condensed mesenchyme capping the ureteric bud. Nephron formation, however, proceeded normally. Microarray analysis showed changed expression of several genes, including <it>Cxcl13</it>, <it>Slc5a2</it>, <it>Klk5</it>, <it>Ren2 </it>and <it>Timeless</it>, which represent candidate Wnt targets in kidney development.</p> <p>Conclusion</p> <p>The mammalian <it>Pygopus </it>genes are required for normal branching morphogenesis of the ureteric bud during kidney development. Nevertheless, the relatively mild phenotype observed in the kidney, as well as other organ systems, indicates a striking evolutionary divergence of <it>Pygopus </it>function between mammals and <it>Drosophila</it>. In mammals, the <it>Pygo1</it>/<it>Pygo2 </it>genes are not absolutely required for canonical Wnt signaling in most developing systems, but rather function as quantitative transducers, or modulators, of Wnt signal intensity.</p>
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