Sox9 Activation Highlights a Cellular Pathway of Renal Repair in the Acutely Injured Mammalian Kidney

After acute kidney injury (AKI), surviving cells within the nephron proliferate and repair. We identify Sox9 as an acute epithelial stress response in renal regeneration. Translational profiling after AKI revealed a rapid upregulation of Sox9 within proximal tubule (PT) cells, the nephron cell type most vulnerable to AKI. Descendants of Sox9+ cells generate the bulk of the nephron during development and regenerate functional PT epithelium after AKI-induced reactivation of Sox9 after renal injury. After restoration of renal function post-AKI, persistent Sox9 expression highlights regions of unresolved damage within injured nephrons. Inactivation of Sox9 in PT cells pre-injury indicates that Sox9 is required for the normal course of post-AKI recovery. These findings link Sox9 to cell intrinsic mechanisms regulating development and repair of the mammalian nephron

Yap- and Cdc42-dependent nephrogenesis and morphogenesis during mouse kidney development.

Yap is a transcriptional co-activator that regulates cell proliferation and apoptosis downstream of the Hippo kinase pathway. We investigated Yap function during mouse kidney development using a conditional knockout strategy that specifically inactivated Yap within the nephrogenic lineage. We found that Yap is essential for nephron induction and morphogenesis, surprisingly, in a manner independent of regulation of cell proliferation and apoptosis. We used microarray analysis to identify a suite of novel Yap-dependent genes that function during nephron formation and have been implicated in morphogenesis. Previous in vitro studies have indicated that Yap can respond to mechanical stresses in cultured cells downstream of the small GTPases RhoA. We find that tissue-specific inactivation of the Rho GTPase Cdc42 causes a severe defect in nephrogenesis that strikingly phenocopies loss of Yap. Ablation of Cdc42 decreases nuclear localization of Yap, leading to a reduction of Yap-dependent gene expression. We propose that Yap responds to Cdc42-dependent signals in nephron progenitor cells to activate a genetic program required to shape the functioning nephron

Sox11 gene disruption causes congenital anomalies of the kidney and urinary tract (CAKUT)

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<i>Yap</i> is required for kidney development.

<p>(A) Stages of nephrogenesis and their relationship to the UB (black) tips. Signals released from UB tips induce mesenchyme cells to condense around UB tips forming the CM (blue). Some of these CM cells aggregate forming the PA that converts into epithelial RV. The late RV fuses with UB tips and develops into comma (CSB) and S-shaped (SSB) body. (A′) Schematic diagram of the nephron components. (B) Confocal images for Yap, E-cadherin and DAPI staining in late RV at E14.5. Nuclear Yap is observed in the proximal segment of the RV (arrowheads), while Yap expression disappears in Six2:Cre expressing cells (D - arrows point to CM cells, arrowhead points to an early nephron). (C) Confocal images of p-Yap/E-cadherin/DAPI staining shows ubiquitous p-Yap expression. Individual channels images are in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1003380#pgen.1003380.s002" target="_blank">Figure S2</a>. (E) Immunohistochemistry using Yap/Taz antibody in RV and SSB shows a similar expression pattern observed with Yap antibody in previous panels (arrowheads). (F–F″) Confocal images for Yap/E-cadherin/DAPI staining in SSB at E14.5. Nuclear Yap is observed in proximal and distal segments of the SSB (arrowheads). (G,H) Macroscopic view of the urogenital system from wild-type and <i>Yap</i> mutant kidneys at P0. Note bilateral reduction in kidney size of mutant compared to control and empty bladder in mutant animals. (I,J) PAS staining of P0 kidneys from wild-type and <i>Yap^CM−/−</i> animals. Arrows point to the papilla. (K,L) Closer view of the cortical zone shows limited nephrogenesis in <i>Yap^CM−/−</i>. (M,N) Higher magnification shows abnormal glomeruli structure and tubules with barely discernable lumens (asterisk) in <i>Yap^CM−/−</i>. k: kidney; b: bladder; cd: collectiong duct; csb: comma-shaped body; d: distal; g: glomeruli; ic: inner cortex; ma: medulla; m: medial; nz: nephrogenic zone; p: proximal; pt: proximal tubule; ssb: S-shaped body. Scale bars represent 25 µm (B–F″; M–N), 1 mm (G–J), 200 µm (K,L).</p

Loss of CM-derived epithelial structures and abnormal morphogenesis in <i>Yap</i> mutants.

<p>(A–J) Sections of P0 kidneys stained using late nephron markers confirm abnormal nephron formation in <i>Yap^CM−/−</i> kidneys. Glomeruli (Podocin, A,B; Podocin-WT1-Tomato lectin, C–D′). Proximal tubules (LTL, E,F). Henle's loop (<i>Slc12a1</i>, G,H). Distal tubules (<i>Slc12a3</i>, I, J). (K,L) Overview of an E14.5 nephrogenic zone reveals the presence of CM cells (arrows) in both genotypes, but CM-derived epithelial structures (arrowheads) are greatly reduced in mutant when compared to control littermates. (M–N′) Higher magnification shows histological morphology defects of mutant SSB compared to wild-type controls at E13.5. Scale bars represent 500 µm (A,B), 50 µm (C–D′), 200 µm (E–J), 100 µm (K–L).</p

<i>Cdc42</i> is necessary for Yap to be normally localized and active.

<p>(A–B′″) Staining for Six2 and Yap shows reduce nuclear Yap staining in most of the Six2 positives cells (arrows) of <i>Cdc42^CM−/−</i> compared to wild-type at E12.5. Control (C–C′″) and Cre infected (D–D′″) <i>Cdc42^flox/flox</i> mouse embryonic fibroblasts (MEFs) stained with Yap antibody and doubly counterstained with phalloidin and Hoechst 33258. (E) Quantification from panels A–B′″ of Yap nuclear staining in CM and UB cells from controls (black columns) and <i>Cdc42^CM−/−</i> (white columns) kidneys at E12.5. Data represent mean fluorescence intensity per nucleus area (100 nuclei for each genotype - ***p<0.0001). (F–M) Expression of Cited1 (F), <i>Capn6</i> (H), <i>Traf1</i> (J), <i>Meox2</i> (L) in control E14.5 kidneys, demonstrating expression in nephron progenitor cells. <i>Cdc42</i> deletion results in loss of expression of these genes in CM cells (G, I, K, M), similar to what is seen in <i>Yap^CM−/−</i> mutant. (N,O) <i>IS</i>H reveals increase in levels of <i>Fgf10</i> expression specifically in CM cells of mutant kidneys compared to wild-type controls. Scale bars represent 25 µm (A–B′″), 10 µm (C–D′″), 100 µm (F,G), 200 µm (H–O).</p

<i>Yap</i> deletion impairs nephron induction, without affecting self-renewal of the CM population.

<p>(A,B) Immunostaining analysis for Six2 (E14.5) shows no change in expression pattern in both genotypes (arrows). E-cadherin was used to visualize the UB compartment. (C,D) Dramatic reduction in nephrogenesis visualized by loss of NCAM-expressing structures (arrowheads) in the nephrogenic zone of <i>Yap</i> mutant compared to wild-type (E16.5). Note the reduced NCAM expression in CM cells. Calbindin highlights the UB and CD. (E) Quantification of early nephron structures in E15.5 controls (black columns) and <i>Yap</i> mutants (white columns) based on NCAM staining. Total***: p<0.0001; PA***: p<0.0001; RV*: p = 0.0209; CSB*: p = 0.0018; SSB***: p<0.0001. (F,G) ISH analysis shows maintained <i>Gdnf</i> expression in CM of control and <i>Yap</i> mutants (E15.5). (H,I) WT1 staining (E18.5) reveals staining in CM cells (arrows) for both genotypes, and dramatic reduction in number of renal MET-derived structures in mutants compared to wild-type. (J,K) Immunostaining analysis for the CM marker Sall1 (E14.5) shows no change in expression pattern in both genotypes. E-cadherin was used to visualize the UB compartment. Scale bars represent 100 µm.</p

Characterization of segmentation in <i>Yap</i> mutant nephrons.

<p>(A–B′) Double staining for E-cadherin and Calbindin in RV and SSB. Co-staining for Hnf1ß/WT1 (C–D′) and Sox9/WT1 (E–F′) reveals normal segmentation of the RV with both proximal and distal segments. Similarly, SSB show normal segmentation. Note the reduced size of the proximal domain in <i>Yap</i>-null SSB (compare WT1 positive segment in <i>Yap</i> mutants (D′, F′) to controls (C′, E′). This is also apparent in B′ and J′. (G–H′) Immunofluorescence for E-cadherin and Jag1 reveals no change in specification of the distal RV and the medial segment of the SSB in both genotypes. Note the aberrant morphology (asterisk) of the site where the connection occurred between the SSB and the UE (B′,D′,F′,H′ and J′). (I–J′) Immunofluorescence using antibodies to Cytokeratin (UE) and Laminin (BM) shows that fusion occurred before the comma-shaped stages. All staining performed at E15.5. CSB: comma-shaped body; RV: renal vesicle; SSB: S-shaped body. Scale bars represent 25 µm. DAPI was used to counterstain nuclei.</p