Mutual Antagonism of Wilms’ Tumor 1 and β-Catenin Dictates Podocyte Health and Disease

Abstract

Activation of b-catenin, the intracellular mediator of canonical Wnt signaling, has a critical role in mediating podocyte injury and proteinuria. However, the underlying mechanisms remain poorly understood. Here, we show that b-catenin triggers ubiquitin-mediated protein degradation of Wilms' tumor 1 (WT1) and functionally antagonizes its action. In mice injected with adriamycin, WT1 protein was progressively lost in glomerular podocytes at 1, 3, and 5 weeks after injection. Notably, loss of WT1 apparently did not result from podocyte depletion but was closely associated with upregulation of b-catenin. This change in WT1/ b-catenin ratio was accompanied by loss of podocyte-specific nephrin, podocalyxin, and synaptopodin and acquisition of mesenchymal markers Snail1, a-smooth muscle actin, and fibroblast-specific protein 1. In vitro, overexpression of b-catenin induced WT1 protein degradation through the ubiquitin proteasomal pathway, which was blocked by MG-132. WT1 and b-catenin also competed for binding to common transcriptional coactivator CREB-binding protein and mutually repressed the expression of their respective target genes. In glomerular miniorgan culture, activation of b-catenin by Wnt3a repressed WT1 and its target gene expression. In vivo, blockade of Wnt/b-catenin signaling by endogenous antagonist Klotho induced WT1 and restored podocyte integrity in adriamycin nephropathy. These results show that b-catenin specifically targets WT1 for ubiquitin-mediated degradation, leading to podocyte dedifferentiation and mesenchymal transition. Our data also suggest that WT1 and b-catenin have opposing roles in podocyte biology, and that the ratio of their expression levels dictates the state of podocyte health and disease in vivo.

Wnt signaling orients the proximal-distal axis of kidney nephrons

“…Several published studies have reported an antagonistic relationship in mature podocytes between Wnt signaling and the transcription factor WT1, which is essential for podocyte function. β-catenin is activated during podocyte injury (Dai et al, 2009;Li and Siragy, 2014;Wang et al, 2011) and leads to ubiquitin-mediated degradation of Wt1 (Zhou et al, 2015), and Wnt inhibition restores podocyte integrity in an induced nephropathy model (He et al, 2011). Thus, inhibition of Wt1 by Wnt signaling, which was observed in the current study in the nephrogenic mesenchyme as well as in the coelomic epithelium within hours of Wnt delivery (Fig.…”

Extracellular Superoxide Dismutase Protects against Proteinuric Kidney Disease

“…Furthermore, ADR led to a depletion of WT1, a podocyte protein necessary for normal functioning and maintenance of podocyte differentiation. 20 This decrease was more accentuated in the KO compared with the WT mice ( Figure 4J). This indicates substantial podocyte injury after ADR injury that is accentuated in the KO mice.…”

“…After fixation with paraformaldehyde and blocking, sections were stained with antibody against WT1 (sc-192; Santa Cruz Biotechnology) and nephrin (2OR-NP002; Fitzgerald Industries International, Acton, MA) as previously described 20 or to desmin (D1033; Sigma-Aldrich) and followed by appropriate Cy2-and Cy3-labeled secondary antibodies (Jackson ImmunoResearch Laboratories, West Grove, PA), respectively. The number of WT1-positive cells was performed on 20 randomly selected glomeruli per animal and presented as average count per glomeruli.…”

Live or Let Die: Is There any Cell Death in Podocytes?

“…24,[126][127][128][129] WT1 expression is reduced in glomerular diseases characterized by nephrotic range proteinuria. Here, miRNA-193a as well as Wnt/β-catenin signaling might play an important role in regulating WT1 expression.…”

“…There is no doubt that podocytes are lost during glomerular disease, but the methodology to assess podocyte number already is discussed controversially and there has been no gold standard established. 24 Widely used markers such as WT1 (Wilms' Tumor protein) or TLE4 (TLE4: transducin-like enhancer of split 4) might detect only mature and differentiated podocytes, and therefore might not be reliable markers in states of disease because they potentially could underestimate podocyte numbers, as with loss of these marker upon dedifferentiation.…”