Hypoplastic and/or cystic kidneys have been found in both LDL receptor-related protein 6 (Lrp6)-and b-catenin-mutant mouse embryos, and these proteins are key molecules for Wnt signaling. However, the underlying mechanisms of Lrp6/b-catenin signaling in renal development and cystic formation remain poorly understood. In this study, we found evidence that diminished cell proliferation and increased apoptosis occur before cystic dysplasia in the renal primordia of Lrp6-deficient mouse embryos. The expression of Ret protooncogene (Ret), a critical receptor for thegrowth factor glial cell line-derivedneurotrophic factor(GDNF),which is required for early nephrogenesis, was dramatically diminished in themutant renal primordia. The activities of other representative nephrogenic genes, incluDing Lim1, Pax2, Pax8, GDNF, and Wnt11, were subsequently diminished in the mutant renal primordia. Molecular biology experiments demonstrated that Ret is a novel transcriptional target ofWnt/b-catenin signaling. Wnt agonist lithium promoted Ret expression in vitro and in vivo. Furthermore, Lrp6-knockdown or lithium treatment in vitro led to downregulation or upregulation, respectively, of the phosphorylated mitogen-activated protein kinases 1 and 3, which act downstream of GDNF/Ret signaling. Mice with single and double mutations of Lrp6 and Ret were perinatal lethal and demonstrated gene dosage-dependent effects on the severity of renal hypoplasia during embryogenesis. Taken together, these results suggest that Lrp6-mediated Wnt/b-catenin signaling modulates or interacts with a signaling network consisting of Ret cascades and related nephrogenic factors for renal development, and the disruption of these genes or signaling activities may cause a spectrum of hypoplastic and cystic kidney disorders.
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