β-catenin overexpression within the metanephric mesenchyme causes renal dysplasia via upregulation of the Gdnf signalling axis

Abstract

<p>Renal dysplasia, a developmental disorder characterized by defective nephrogenesis and branching morphogenesis, ranks as one of the major causes of renal failure among the pediatric population. The molecular mechanisms underlying the pathogenesis of renal dysplasia are not well understood; however, changes in gene expression are a major contributing factor. In this study, we demonstrate that the levels of activated β-catenin, a transcriptional co-regulator, are elevated in the nuclei of ureteric, stromal, and mesenchymal cells within dysplastic human kidney tissue. To determine the mechanisms by which mesenchymal β-catenin over-expression leads to renal dysplasia, we generated a conditional mouse model in which β-catenin was stabilized exclusively in the metanephric mesenchyme. Kidneys from these mutant mice are remarkably similar to dysplastic human kidneys. In addition, these mutant mice also demonstrate the formation of 4 to 6 ectopic kidneys. While nephrogenesis appeared normal, investigation of ureteric branch pattern revealed ectopic ureteric budding off the Wolffian duct, ectopic branching off the initial ureteric bud stalk and a disorganization of branch patterning. In-situ hybridization of mutant kidneys revealed increased expression of Gdnf, Cret, and Wnt11, key factors that regulate ureteric branch patterning. We further demonstrate that β-catenin directly binds to TCF consensus binding sites within the Gdnf promoter region located 4.9kb, 2.25kb and 2.1kb upstream of the Gdnf transcriptional start site. Molecular cloning of the 4.9kb fragment upstream of a luciferase gene revealed that ß-catenin regulates gene transcription from the 4.9kb consensus site. Consistent with these findings, genetic deletion of β-catenin from the metanephric mesenchyme cell lineage lead to decreased Gdnf expression and a reduction in ureteric branching morphogenesis resulting in renal hypoplasia. Taken together, our findings establish that β-catenin is an essential regulator of Gdnf expression within the metanephric mesenchyme. Furthermore, we have identified a novel disrupted signalling pathway that contributes to the pathogenesis of renal dysplasia. In this pathway, an over-expression of β-catenin directly leads to an over-expression of Gdnf, causing ectopic and disorganized branching morphogenesis and, consequently, renal dysplasia.</p>