Towards an understanding of kidney diseases associated with WT1 mutations

Abstract

Mutations in Wilms' tumor 1 (WT1) cause a wide spectrum of renal manifestations, eventually leading to end-stage kidney failure. Insufficient understanding of WT1's molecular functions in kidney development has hampered efficient therapeutic applications for WT1-associated diseases. Recently, the generation and characterization of mouse models and application of multiple state-of-the-art approaches have significantly expanded our understanding of the molecular mechanisms of how WT1 mutations lead to kidney failure. Here, we discuss the WT1 binding consensus and illustrate the major roles of WT1 in different cell populations in kidney biology. WT1 controls metanephric mesenchyme (MM) self-renewal and proliferation mainly by regulating FGF and BMP-pSMAD signaling pathways as well as Sall1 and Pax2, encoding key transcription factors; WT1 drives MM differentiation and mesenchyme-epithelial transition by targeting Fgf8 and Wnt4; WT1 defines podocyte identity by activation of other podocyte-specific transcription factors, including Mafb, Lmx1b, FoxC2, and Tcf21. These factors potentially cooperate with WT1 regulating the expression of components and regulators of the cytoskeleton for establishing podocyte polarity, slit diaphragm structure, and focal adhesion to the glomerular basement membrane. Understanding of WT1's function in kidney biology including WT1-regulated pathways will give insights that will eventually help therapeutic applications.

Figures

Figure 1

Wilms' tumor 1 (WT1) protein structure and Wt1 expression pattern in developing murine kidney. (a) Structure of WT1 protein showing alternative splice regions consisting of 17 amino acids encoded by exon 5 and amino acids lysine, threonine, and serine (KTS) encoded by the 3′-end of exon 9 and the different functional domains. (b) The expression of Wt1 in developing kidney (E15.5) detected by in-situ hybridization (picture provided by Dagmar Kruspe). Wt1 is expressed in cap mesenchyme (CM) surrounding the ureteric bud (UB) and at very low level in stromal mesenchyme (SM). Wt1 expression is increased in pre-tubular aggregates (PTAs) that begin to undergo mesenchymal to epithelial transition to form the renal vesicle (RV). Wt1 is expressed in the proximal part of renal vesicles and the S-shaped bodies (S), which give rise to podocyte precursors in the capillary loop (C). The highest expression of Wt1 is found in podocytes within glomeruli (G). Scale bar=20 μm.

Figure 2

Wilms' tumor 1 (WT1) contributes to metanephric mesenchyme self-renewal and its differentiation. WT1, Sall1, Pax2, Six2, and Hoxd11 are individually required for survival and self-renewal of metanephric mesenchyme (MM). Among them, WT1 is upstream of Sall1 and Pax2 transcription factors. BMP4 induces the apoptosis of MM; however, BMP7 promotes the MM differentiation. In balance of the BMP-pSMAD signaling in MM, WT1 directly regulates Bmper, a BMP-pSMAD inhibitor, and activates the FGF signaling pathway via regulating Fgf20/Fgf16 and Gas1 expression to antagonize BMP-pSMAD signaling for the survival and self-renewal of MM. High expression level of Wnt9b beneath the ureteric tip increases the Wnt singling activity nearing the MM; there, the β-catenin-LEF/TCF complex works together with Six2 and WT1 activating the critical differentiation factors, Fgf8 and Wnt4, to form pre-tubular aggregate (PTA). Signals produced by PTA promote the mesenchyme–epithelial transition (MET) to form renal vesicles. The proximal part of the renal vesicle expressing Wt1 will develop into the epithelial cells of the glomerulus and the distal part of the renal vesicle expressing Lhx1 will develop into the epithelial cells of the nephron tubule.

Figure 3

WT1 defines the podocyte-specific morphology. (a) Ultrastructural kidney analysis of Wt1 mutant and control adult mice treated with doxycycline for 6 days. Podocytes (arrow) and endothelial cells (arrowheads) were abnormally distant from the GBM (G), which was itself irregularly shaped in the mutant tissue. Black dots indicate negative charges by polyethylenimine staining. No obvious charge differences between the GBMs of mutant and control glomeruli are detectable. Scale bar=0.5 μm. This figure is adapted with permission from Dong et al. (b) Schematic WT1-controlled networks for establishing podocyte-specific morphology. WT1 regulates the slit diaphragm complex components, including Nephrin (Nphs1), Podocin (Nphs2), Membrane-associated guanylate kinase, WW and PDZ domain–containing protein 2 (Magi2), CD2-associated protein (CD2AP), NCK adaptor protein 2 (NCK2), Kin of IRRE-like protein 1/2/3 (Kirrel) and phospholipase C, and epsilon 1 (Plce1). WT1 controls focal adhesion by regulating Integrin α3 (Itga3), Integrin β1 (Itgb1), Laminin α5 (Lama5), and Laminin β2 (Lamb2). WT1 also controls the cytoskeleton components and its regulators to establish the podocyte polarity—namely, Synaptopodin (Synpo), α-actinin-4, myosin, heavy polypeptide 9, non-muscle (Myh9), Rho GTPase-activating protein 24 (Arhgap24), and Atypical protein kinase C (aPKC). (c) Multiple Wt1 binding sites to cis-regulatory regions of Synpo and Magi2 as viewed in the UCSC browser within the indicated genomic intervals. Conservation denotes placental mammal basewise conservation by phastCons score. The peaks indicate the overlapping WT1 binding sites generated from Kreidberg and colleagues' and our ChIP-seq data.,