WT1-interacting protein and ZO-1 translocate into podocyte nuclei after puromycin aminonucleoside treatment

Gene regulatory networks in disease and ageing

The precise control of gene expression is required for the maintenance of cellular homeostasis and proper cellular function, and the declining control of gene expression with age is considered a major contributor to age-associated changes in cellular physiology and disease. The coordination of gene expression can be represented through models of the molecular interactions that govern gene expression levels, so-called gene regulatory networks. Gene regulatory networks can represent interactions that occur through signal transduction, those that involve regulatory transcription factors, or statistical models of gene-gene relationships based on the premise that certain sets of genes tend to be coexpressed across a range of conditions and cell types. Advances in experimental and computational technologies have enabled the inference of these networks on an unprecedented scale and at unprecedented precision. Here, we delineate different types of gene regulatory networks and their cell-biological interpretation. We describe methods for inferring such networks from large-scale, multi-omics datasets and present applications that have aided our understanding of cellular ageing and disease mechanisms.

Essential role of Wtip in mouse development and maintenance of the glomerular filtration barrier

Wilms' tumor interacting protein (Wtip) has been implicated in cell junction assembly and cell differentiation and interacts with proteins in the podocyte slit diaphragm, where it regulates podocyte phenotype. To define Wtip expression and function in the kidney, we created a Wtip-deleted mouse model using β-galactosidase-neomycin (β-geo) gene trap technology. Wtip gene trap mice were embryonic lethal, suggesting additional developmental roles outside kidney function. Using β-geo heterozygous and normal mice, Wtip expression was identified in the developing kidneys, heart, and eyes. In the kidney, expression was restricted to podocytes, which appeared initially at the capillary loop stage coinciding with terminal podocyte differentiation. Heterozygous mice had an expected lifespan and showed no evidence of proteinuria or glomerular pathology. However, heterozygous mice were more susceptible to glomerular injury than wild-type littermates and developed more significant and prolonged proteinuria in response to lipopolysaccharide or adriamycin. In normal human kidneys, WTIP expression patterns were consistent with observations in mice and were lost in glomeruli concurrent with loss of synaptopodin expression in disease. Mechanistically, we identified the Rho guanine nucleotide exchange factor 12 (ARHGEF12) as a binding partner for WTIP. ARHGEF12 was expressed in human podocytes and formed high-affinity interactions through their LIM- and PDZ-binding domains. Our findings suggest that Wtip is essential for early murine embryonic development and maintaining normal glomerular filtration barrier function, potentially regulating slit diaphragm and foot process function through Rho effector proteins.NEW & NOTEWORTHY This study characterized dynamic expression patterns of Wilms' tumor interacting protein (Wtip) and demonstrates the novel role of Wtip in murine development and maintenance of the glomerular filtration barrier.

Circulating extracellular vesicles of patients with steroid-sensitive nephrotic syndrome have higher RAC1 and induce recapitulation of nephrotic syndrome phenotype in podocytes

Since previous research suggests a role of a circulating factor in the pathogenesis of steroid-sensitive nephrotic syndrome (NS), we speculated that circulating plasma extracellular vesicles (EVs) are a candidate source of such a soluble mediator. Here, we aimed to characterize and try to delineate the effects of these EVs in vitro. Plasma EVs from 20 children with steroid-sensitive NS in relapse and remission, 10 healthy controls, and 6 disease controls were obtained by serial ultracentrifugation. Characterization of these EVs was performed by electron microscopy, flow cytometry, and Western blot analysis. Major proteins from plasma EVs were identified via mass spectrometry. Gene Ontology classification analysis and Ingenuity Pathway Analysis were performed on selectively expressed EV proteins during relapse. Immortalized human podocyte culture was used to detect the effects of EVs on podocytes. The protein content and particle number of plasma EVs were significantly increased during NS relapse. Relapse NS EVs selectively expressed proteins that involved actin cytoskeleton rearrangement. Among these, the level of RAC-GTP was significantly increased in relapse EVs compared with remission and disease control EVs. Relapse EVs were efficiently internalized by podocytes and induced significantly enhanced motility and albumin permeability. Moreover, relapse EVs induced significantly higher levels of RAC-GTP and phospho-p38 and decreased the levels of synaptopodin in podocytes. Circulating relapse EVs are biologically active molecules that carry active RAC1 as cargo and induce recapitulation of the NS phenotype in podocytes in vitro.NEW & NOTEWORTHY Up to now, the role of extracellular vesicles (EVs) in the pathogenesis of steroid-sensitive nephrotic syndrome (NS) has not been studied. Here, we found that relapse NS EVs contain significantly increased active RAC1, induce enhanced podocyte motility, and increase expression of RAC-GTP and phospho-p38 expression in vitro. These results suggest that plasma EVs are biologically active molecules in the pathogenesis of NS.

EP300/CBP is crucial for cAMP-PKA pathway to alleviate podocyte dedifferentiation via targeting Notch3 signaling

Podocyte injury is the hallmark of proteinuric glomerular diseases. Notch3 is neo-activated simultaneously in damaged podocytes and podocyte's progenitor cells of FSGS, indicating a unique role of Notch3. We previously showed that activation of cAMP-PKA pathway alleviated podocyte injury possibly via inhibiting Notch3 expression. However, the mechanisms are unknown. In the present study, Notch3 signaling was significantly activated in ADR-induced podocytes in vitro and in PAN nephrosis rats and patients with idiopathic FSGS in vivo, concomitantly with podocyte dedifferentiation. In cultured podocytes, pCPT-cAMP, a selective cAMP-PKA activator, dramatically blocked ADR-induced activation of Notch3 signaling as well as inhibition of cAMP-PKA pathway, thus alleviating the decreased cell viability and podocyte dedifferentiation. Bioinformatics analysis revealed EP300/CBP, a transcriptional co-activator, as a central hub for the crosstalk between these two signaling pathways. Additionally, CREB/KLF15 in cAMP-PKA pathway competed with RBP-J the major transcriptional factor of Notch3 signaling for binding to EP300/CBP. EP300/CBP siRNA significantly inhibited these two signaling transduction pathways and disrupted the interactions between the above major transcriptional factors. These data indicate a crucial role of EP300/CBP in regulating the crosstalk between cAMP-PKA pathway and Notch3 signaling and modulating the phenotypic change of podocytes, and enrich the reno-protective mechanisms of cAMP-PKA pathway.

WT1-interacting protein inhibits cell proliferation and tumorigenicity in non-small-cell lung cancer via the AKT/FOXO1 axis

Lung cancer is the most common cancer and the leading cause of cancer‐related death worldwide; hence, it is imperative that the mechanisms underlying the malignant properties of lung cancer be uncovered in order to efficiently treat this disease. Increasing evidence has shown that WT1‐interacting protein (WTIP) plays important roles both physiologically and pathologically in humans; however, the role of WTIP in cancer is unknown. Here, we investigated the role and mechanism of WTIP in cell proliferation and tumorigenesis of non‐small‐cell lung cancer (NSCLC). We report that WTIP is a tumor suppressor in human NSCLC. We found that WTIP expression was significantly reduced in both NSCLC cell lines and clinical specimens compared to that in normal controls; this reduction was largely attributed to promoter hypermethylation. Downregulation of WTIP significantly correlates with poor prognosis and predicts a shorter overall survival and progression‐free survival among NSCLC patients. Moreover, ectopic overexpression of WTIP dramatically inhibits cell proliferation and tumorigenesis in vitro and in vivo; conversely, depletion of WTIP expression shows the opposite effects. Mechanistically, WTIP impairs AKT phosphorylation and activation, leading to enhanced expression and transcriptional activity of FOXO1, which further increases p21Cip1 and p27Kip1, and decreases cyclin D1, which consequently results in cell cycle arrest. Collectively, the results of the current study indicate that WTIP is an important proliferation‐related gene and that WTIP expression may represent a novel prognostic biomarker for NSCLC.

Phosphoinositides in the kidney

Phosphoinositides (PIs) play pivotal roles in the regulation of many biological processes. The quality and quantity of PIs is regulated in time and space by the activity of PI kinases and PI phosphatases. The number of PI-metabolizing enzymes exceeds the number of PIs with, in many cases, more than one enzyme controlling the same biochemical step. This would suggest that the PI system has an intrinsic ability to buffer and compensate for the absence of a specific enzymatic activity. However, there are several examples of severe inherited human diseases caused by mutations in one of the PI enzymes, although other enzymes with the same activity are fully functional. The kidney depends strictly on PIs for physiological processes, such as cell polarization, filtration, solute reabsorption, and signal transduction. Indeed, alteration of the PI system in the kidney very often results in pathological conditions, both inherited and acquired. Most of the knowledge of the roles that PIs play in the kidney comes from the study of KO animal models for genes encoding PI enzymes and from the study of human genetic diseases, such as Lowe syndrome/Dent disease 2 and Joubert syndrome, caused by mutations in the genes encoding the PI phosphatases, OCRL and INPP5E, respectively.

RBBP7 is a prognostic biomarker in patients with esophageal squamous cell carcinoma

Retinoblastoma-binding protein 7 (RBBP7) is an important component of several complexes that regulate chromatin metabolism. It is overexpressed in certain cancer types and serves conflicting roles in tumor progression. In the present study, the expression and roles of RBBP7 were explored in esophageal squamous cell carcinoma (ESCC). Immunohistochemical staining was used to detect RBBP7 expression in ESCC tissues. The mRNA sequencing profiles from the Cancer Genome Atlas and Genotype-Tissue Expression databases were mined to analyze the mRNA expression of RBBP7 in tissues. Proliferation, clone formation, apoptosis and Transwell invasion/migration assays were performed to explore the roles of RBBP7 in ESCC. RBBP7 was highly expressed in ESCC tissues. The protein and mRNA expression levels of RBBP7 were significantly elevated in tumor tissues compared with paired adjacent normal tissues. RBBP7 overexpression was associated with a poor overall survival in patients with ESCC. Furthermore, higher RBBP7 expression was significantly correlated with poor tumor differentiation, advanced regional lymph node involvement, and pathological TNM staging. Knockdown of RBBP7 in ESCC cells did not affect tumor apoptosis or tumor growth. However, the overexpression of RBBP7 significantly enhanced the invasion and migration of ESCC cells, whereas the knockdown of RBBP7 resulted in significantly decreased invasion and migration. The present study indicated that RBBP7 is a novel biomarker and prognosticator for patients with ESCC. Furthermore, RBBP7 serves crucial roles in promoting ESCC invasion and migration.

Increased autophagy is cytoprotective against podocyte injury induced by antibody and interferon-α in lupus nephritis

OBJECTIVE

More recent studies suggested that defects in autophagy contribute to the pathogenesis of SLE, especially in adaptive immunity. Occurrence and progression of lupus nephritis (LN) is the end result of complex interactions between regulation of immune responses and pathological process by renal resident cells, but there is still a lot of missing information for an establishment on the role of autophagy in pathogenesis of LN and as a therapy target.

METHODS

Systemic and organ-specific aetiologies of autophagy were first evaluated by autophagy protein quantification in tissue homogenates in MRL ^lpr/lpr lupus prone and female C57BL mice. Analysis of gene expression was also adopted in human blood and urine sediments. Then, some key mediators of the disease, including complement inactivated serum, IgG from patients with LN (IgG-LN) and interferon (IFN)-α were chosen to induce podocyte autophagy. Podocyte injuries including apoptosis, podocin derangement, albumin filtration and wound healing were monitored simultaneously with autophagy steady-state and flux.

RESULTS

Elevated LC3B in kidney homogenates and increased autophagosomes in podocyte from MRL ^lpr/lpr were observed. In humans, mRNA levels of some key autophagy genes were increased in blood and urinary sediments, and podocyte autophagosomes were observed in renal biopsies from patients with LN. Complement inactivated serum, IgG-LN and IFN-α could induce podocyte autophagy in a time-dependent and dosage-dependent manner, and by reactive oxygen species production and mTORC1 inhibition, respectively. Autophagy inhibition aggravated podocyte damage whereas its inducer relieved the injury.

CONCLUSION

Podocyte autophagy is activated in lupus-prone mice and patients with lupus nephritis. Increased autophagy is cytoprotective against antibody and interferon-α induced podocyte injury.

Fyn-binding protein ADAP supports actin organization in podocytes

The renal podocyte is central to the filtration function of the kidney that is dependent on maintaining both highly organized, branched cell structures forming foot processes, and a unique cell-cell junction, the slit diaphragm. Our recent studies investigating the developmental formation of the slit diaphragm identified a novel claudin family tetraspannin, TM4SF10, which is a binding partner for ADAP (also known as Fyn binding protein Fyb). To investigate the role of ADAP in podocyte function in relation to Fyn and TM4SF10, we examined ADAP knockout (KO) mice and podocytes. ADAP KO mice developed glomerular pathology that began as hyalinosis and progressed to glomerulosclerosis, with aged male animals developing low levels of albuminuria. Podocyte cell lines established from the KO mice had slower attachment kinetics compared to wild-type cells, although this did not affect the total number of attached cells nor the ability to form focal contacts. After attachment, the ADAP KO cells did not attain typical podocyte morphology, lacking the elaborate cell protrusions typical of wild-type podocytes, with the actin cytoskeleton forming circumferential stress fibers. The absence of ADAP did not alter Fyn levels nor were there differences between KO and wild-type podocytes in the reduction of Fyn activating phosphorylation events with puromycin aminonucleoside treatment. In the setting of endogenous TM4SF10 overexpression, the absence of ADAP altered the formation of cell-cell contacts containing TM4SF10. These studies suggest ADAP does not alter Fyn activity in podocytes, but appears to mediate downstream effects of Fyn controlled by TM4SF10 involving actin cytoskeleton organization.

Adhesion GPCRs in Kidney Development and Disease

Chronic kidney disease (CKD) represents the fastest growing pathology worldwide with a prevalence of >10% in many countries. In addition, kidney cancer represents 5% of all new diagnosed cancers. As currently no effective therapies exist to restore kidney function after CKD- as well as cancer-induced renal damage, it is important to elucidate new regulators of kidney development and disease as new therapeutic targets. G protein-coupled receptors (GPCRs) represent the most successful class of pharmaceutical targets. In recent years adhesion GPCRs (aGPCRs), the second largest GPCR family, gained significant attention as they are present on almost all mammalian cells, are associated to a plethora of diseases and regulate important cellular processes. aGPCRs regulate for example cell polarity, mitotic spindle orientation, cell migration, and cell aggregation; all processes that play important roles in kidney development and/or disease. Moreover, polycystin-1, a major regulator of kidney development and disease, contains a GAIN domain, which is otherwise only found in aGPCRs. In this review, we assess the potential of aGPCRs as therapeutic targets for kidney disease. For this purpose we have summarized the available literature and analyzed data from the databases The Human Protein Atlas, EURExpress, Nephroseq, FireBrowse, cBioPortal for Cancer Genomics and the National Cancer Institute Genomic Data Commons data portal (NCIGDC). Our data indicate that most aGPCRs are expressed in different spatio-temporal patterns during kidney development and that altered aGPCR expression is associated with a variety of kidney diseases including CKD, diabetic nephropathy, lupus nephritis as well as renal cell carcinoma. We conclude that aGPCRs present a promising new class of therapeutic targets and/or might be useful as diagnostic markers in kidney disease.

Exosomes from high glucose-treated glomerular endothelial cells trigger the epithelial-mesenchymal transition and dysfunction of podocytes

New data indicate that abnormal glomerular endothelial cell (GEC)-podocyte crosstalk plays a critical role in diabetic nephropathy (DN). The aim of our study is to investigate the role of exosomes from high glucose (HG)-treated GECs in the epithelial-mesenchymal transition (EMT) and dysfunction of podocytes. In this study, exosomes were extracted from GEC culture supernatants and podocytes were incubated with the GEC-derived exosomes. Here, we demonstrate that HG induces the endothelial-mesenchymal transition (EndoMT) of GECs and HG-treated cells undergoing the EndoMT secrete more exosomes than normal glucose (NG)-treated GECs. We show that GEC-derived exosomes can be internalized by podocytes and exosomes from HG-treated cells undergoing an EndoMT-like process can trigger the podocyte EMT and barrier dysfunction. Our study reveals that TGF-β1 mRNA is enriched in exosomes from HG-treated GECs and probably mediates the EMT and dysfunction of podocytes. In addition, our experimental results illustrate that canonical Wnt/β-catenin signaling is involved in the exosome-induced podocyte EMT. Our findings suggest the importance of paracrine communication via exosomes between cells undergoing the EndoMT and podocytes for renal fibrosis in DN. Thus, protecting GECs from the EndoMT and inhibiting TGF-β1-containing exosomes release from GECs is necessary to manage renal fibrosis in DN.

Glycogen synthase kinase‑3β is required for epithelial‑mesenchymal transition and barrier dysfunction in mouse podocytes under high glucose conditions

Epithelial-mesenchymal transition (EMT) is important for diabetic nephropathy (DN). Podocytes are specialized epithelial cells, which form a major component of the glomerular filtration barrier. Podocyte damage has been suggested to be the primary mechanism behind the albuminuria associated with DN. The present study aimed to determine the function of glycogen synthase kinase (GSK)-3β in EMT and barrier dysfunction of mouse podocytes exposed to high glucose (HG) conditions. Matured and differentiated podocytes were treated with normal glucose (NG), HG or NG + mannitol. Podocytes were also transfected with a small interfering RNA (siRNA) against GSK-3β or a scrambled siRNA, or were treated with lithium chloride (LiCl), a GSK-3β inhibitor, under NG or HG conditions. The expression levels of the epithelial cell markers, nephrin and podocin, and the myofibroblast cell markers, α-smooth muscle actin (SMA) and fibronectin, in podocytes by western blot analysis and immunofluorescence staining, respectively. The monolayer barrier function was assessed by albumin inflow. The phosphorylation and activity levels of GSK-3β were also quantified. It was observed that HG promotes EMT in podocytes, due to the increased levels of podocin and nephrin expression and the reduced α-SMA and fibronectin expression levels. HG also induced barrier dysfunction and increased the expression level of total GSK-3β, Try216-phosphorylated-GSK-3β and the GSK-3β activity in podocytes. Transfection of GSK-3β siRNA or treatment with LiCl reversed the HG-induced EMT and barrier dysfunction in podocytes. In conclusion, the present study determined that GSK-3β is required for EMT and barrier dysfunction in podocytes under HG conditions; therefore, GSK-3β may be a novel target for the treatment of DN.

Serum response factor induces endothelial-mesenchymal transition in glomerular endothelial cells to aggravate proteinuria in diabetic nephropathy

We investigated the expression and function of serum response factor (SRF) in endothelial-mesenchymal transition (EndMT) in glomerular endothelial cells (GEnCs) of diabetic nephropathy (DN). The expression of SRF, endothelial markers (VE-cadherin, CD31), and mesenchymal markers (α-SMA, FSP-1, fibronectin) was examined in GEnCs following high glucose or in renal cortex tissues of DN rats. SRF was upregulated by SRF plasmids and downregulated by CCG-1423 (a small molecule inhibitor of SRF) to investigate how SRF influenced EndMT in GEnCs of DN. Streptozocin (STZ) was used to generate diabetes mellitus DM in rats. In GEnCs after high glucose treatment and in renal cortex tissues of diabetic rats, SRF, α-SMA, FSP-1, and fibronectin increased, while VE-cadherin and CD31 declined. SRF overexpression in GEnCs induced expression of Snail, an important transcription factor mediating EndMT. Blockade of SRF reduced Snail induction, protected GEnCs from EndMT, and ameliorated proteinuria. Together, increased SRF activity provokes EndMT and barrier dysfunction of GEnCs in DN. Targeting SRF by small molecule inhibitor may be an attractive therapeutic strategy for DN.

WTIP interacts with BRCA2 and is essential for BRCA2 centrosome localization in cervical cancer cell

AIM

Breast cancer 2, early onset (BRCA2) has been reported to be associated with familial breast and ovarian cancer. Several proteins interact with conserved regions of BRCA2, which play significant roles in DNA damage repair and centrosomal localization. This study was aimed to identify a novel protein, Wilms tumor 1 interacting protein (WTIP), which might interact with the conserved regions of BRCA2, as well as the functional role of silencing of WTIP in response to centrosomal localization.

MATERIALS AND METHODS

Hela S3 cells were used in our study. A yeast two-hybrid screening was used to identify a novel BRCA2-interacting protein. Coimmunoprecipitation and glutathione S-transferase (GST) pull-down assays were performed to detect protein-protein interaction between BRCA2 and hemaglutinin (HA)-WTIP. The expression of WTIP was silenced by short hairpin RNA (shRNA) and the levels of WTIP were confirmed by Western blot. Immunofluorescence microscopy was performed to study the centrosome localization. The functional role of knocking down WTIP expression in response to centrosomal localization was then investigated.

RESULTS

The results showed that there was an interaction between WTIP and BRCA2 (amino acids 2750-2864) in Hela S3 cells. We found that WTIP interacted with BRCA2 in both exogenous and endogenous level. The expression levels of WTIP were significantly decreased by siRNA compared to the control group. Downregulation of WTIP abolished BRCA2 centrosome localization and abnormal cell division.

CONCLUSION

This study indicates that WTIP interacts with BRCA2 and might be responsible for BRCA2 centrosome localization in cervical cancer cell.

Activation of the IL-2 Receptor in Podocytes: A Potential Mechanism for Podocyte Injury in Idiopathic Nephrotic Syndrome?

The renal podocyte plays an important role in maintaining the structural integrity of the glomerular basement membrane. We have previously reported that patients with idiopathic nephrotic syndrome (INS) have increased IL-2 production. We hypothesized that podocytes express an IL-2 receptor (IL-2R) and signaling through this receptor can result in podocyte injury. To confirm the presence of the IL-2R, we tested a conditionally immortalized murine podocyte cell line by flow cytometry, qPCR, and Western blot. To test for the presence of the IL-2R in vivo, immunohistochemical staining was performed on human renal biopsies in children with FSGS and control. Podocytes were stimulated with IL-2 in vitro, to study signaling events via the JAK/STAT pathway. The results showed that stimulation with IL-2 resulted in increased mRNA and protein expression of STAT 5a, phosphorylated STAT 5, JAK 3, and phosphorylated JAK 3. We then investigated for signs of cellular injury and the data showed that pro-apoptotic markers Bax and cFLIP were significantly increased following IL-2 exposure, whereas LC3 II was decreased. Furthermore, mitochondrial depolarization and apoptosis were both significantly increased following activation of the IL-2R. We used a paracellular permeability assay to monitor the structural integrity of a podocyte monolayer following IL-2 exposure. The results showed that podocytes exposed to IL-2 have increased albumin leakage across the monolayer. We conclude that murine podocytes express the IL-2R, and that activation through the IL-2R results in podocyte injury.

Melanocortin 1 receptor agonist protects podocytes through catalase and RhoA activation

Drugs containing adrenocorticotropic hormone have been used as therapy for patients with nephrotic syndrome. We have previously shown that adrenocorticotropic hormone and a selective agonist for the melanocortin 1 receptor (MC1R) exert beneficial actions in experimental membranous nephropathy with reduced proteinuria, reduced oxidative stress, and improved glomerular morphology and function. Our hypothesis is that MC1R activation in podocytes elicits beneficial effects by promoting stress fibers and maintaining podocyte viability. To test the hypothesis, we cultured podocytes and used highly specific agonists for MC1R. Podocytes were subjected to the nephrotic-inducing agent puromycin aminonucleoside, and downstream effects of MC1R activation on podocyte survival, antioxidant defense, and cytoskeleton dynamics were studied. To increase the response and enhance intracellular signals, podocytes were transduced to overexpress MC1R. We showed that puromycin promotes MC1R expression in podocytes and that activation of MC1R promotes an increase of catalase activity and reduces oxidative stress, which results in the dephosphorylation of p190RhoGAP and formation of stress fibers through RhoA. In addition, MC1R agonists protect against apoptosis. Together, these mechanisms protect the podocyte against puromycin. Our findings strongly support the hypothesis that selective MC1R-activating agonists protect podocytes and may therefore be useful to treat patients with nephrotic syndromes commonly considered as podocytopathies.

Critical role of serum response factor in podocyte epithelial-mesenchymal transition of diabetic nephropathy

PURPOSE

To investigate the expression and function of serum response factor in podocyte epithelial-mesenchymal transition of diabetic nephropathy.

METHODS

The expression of serum response factor, epithelial markers and mesenchymal markers was examined in podocytes or renal cortex tissues following high glucose. Serum response factor was upregulated by its plasmids and downregulated by CCG-1423 to investigate how it influenced podocyte epithelial-mesenchymal transition in diabetic nephropathy. Streptozotocin was used to generate diabetes mellitus in rats.

RESULTS

In podocytes after high glucose treatment, serum response factor and mesenchymal markers increased, while epithelial markers declined. Similar changes were observed in vivo. Serum response factor overexpression in podocytes induced expression of Snail, an important transcription factor mediating epithelial-mesenchymal transition. Blockade of serum response factor reduced Snail induction, protected podocytes from epithelial-mesenchymal transition and ameliorated proteinuria.

CONCLUSION

Together, increased serum response factor activity provokes podocytes' epithelial-mesenchymal transition and dysfunction in diabetic nephropathy. Targeting serum response factor by small-molecule inhibitor may be an attractive therapeutic strategy for diabetic nephropathy.

Role of Asxl1 in kidney podocyte development via its interaction with Wtip

Additional sex comb-like (ASXL) family proteins are chromatin factors that function in transcriptional activation and repression. However, the underlying mechanisms and biological implications have not been well established. Here, we identified a LIM domain-containing protein, Wilms tumor 1-interacting protein (WTIP), as an ASXL1-binding partner. Biochemical assays confirmed an interaction between the murine homologs Asxl1 and Wtip. The suppressive role of Wtip in WT1 function and the expression of Wtip in kidney podocytes prompted us to investigate the role of Asxl1 in the kidney using Asxl1-null mice. In homozygous Asxl1(-/-) embryos, defects in kidney size and glomerular podocyte morphology were observed. Furthermore, up-regulation of Wt1/Wtip target genes was observed in the kidneys of Asxl1-null embryos. Overall, these findings implicate Asxl1 in the maintenance of podocyte structure via its association with Wtip and in the regulation of WT1 signaling during early kidney development.

Interaction of thrombospondin1 and CD36 contributes to obesity-associated podocytopathy

Obesity is associated with podocyte injury and the development of proteinuria. Elevated plasma free fatty acid is one of the characteristics of obesity and has been linked to podocyte dysfunction. However, the mechanisms remain unclear. In the current study, we examined the effect of saturated free fatty acid (FFA) on human podocyte apoptosis and function in vitro. The mechanism and its in vivo relevance were also determined. We found that FFA treatment induced human podocyte apoptosis and dysfunction, which was associated with increased expression of a matricellular protein-thrombospondin1 (TSP1). FFA stimulated TSP1 expression in podocytes at the transcriptional levels through activation of MAPK pathway. Addition of purified TSP1 to cell culture media induced podocyte apoptosis and dysfunction. Tis effect is though a TGF-β independent mechanism. Moreover, peptide treatment to block TSP1 binding to its receptor-CD36 attenuated FFA induced podocyte apoptosis, suggesting that TSP1/CD36 interaction mediates FFA-induced podocyte apoptosis. Importantly, using a diet-induced obese mouse model, in vivo data demonstrated that obesity-associated podocyte apoptosis and dysfunction were attenuated in TSP1 deficient mice as well as in CD36 deficient mice. Taken together, these studies provide novel evidence that the interaction of TSP1 with its receptor CD36 contributes to obesity--associated podocytopathy.

Lipopolysaccharide induces inducible nitric oxide synthase-dependent podocyte dysfunction via a hypoxia-inducible factor 1α and cell division control protein 42 and Ras-related C3 botulinum toxin substrate 1 pathway

Urine protein loss in immune complex-mediated diseases such as lupus nephritis is associated with podocyte foot process effacement (podocytopathy) but is not always dependent on glomerular immune complex deposition. Several murine and human studies have associated lupus nephritis with inducible nitric oxide synthase (iNOS) expression in what appear to be podocytes. This study was conducted to determine mechanisms of immune-complex-independent and iNOS-dependent podocyte dysfunction. Conditionally immortalized podocytes were cultured with lipopolysaccharide (LPS) and nitric oxide (NO), superoxide (SO), or peroxynitrite donors in the presence or absence of inhibitors of iNOS, reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase or monocyte chemotactic protein 1 (MCP-1), or with sepiapterin to increase coupling of iNOS homodimers. Podocyte NO, SO, and MCP-1 production and nitrotyrosine modifications were determined. The podocytopathy phenotype was determined by measuring cell motility and membrane permeability to albumin. This study determined that NO produced by iNOS is sufficient and necessary to induce podocytopathy. NO probably induces this phenotype via hypoxia-inducible factor 1α and cell division control protein 42 and Ras-related C3 botulinum toxin substrate 1 pathways. With LPS stimulation, neither SO nor peroxynitrite produced by uncoupled iNOS or NADPH oxidase nor MCP-1 was sufficient to induce the full phenotype. This study supports the notion that iNOS may induce autocrine podocyte dysfunction. Thus, targeting iNOS or the pathways of its induction may have therapeutic benefit.

Apelin promotes diabetic nephropathy by inducing podocyte dysfunction via inhibiting proteasome activities

Podocyte injuries are associated with progression of diabetic nephropathy (DN). Apelin, an adipocyte-derived peptide, has been reported to be a promoting factor for DN. In this study, we aim to determine whether apelin promotes progression of DN by inducing podocyte dysfunction. kk-Ay mice were used as models for DN. Apelin and its antagonist, F13A were intraperitoneally administered for 4 weeks, respectively. Renal function and foot process proteins were analysed to evaluate the effects of apelin on kk-Ay mice and podocytes. Apelin increased albuminuria and decreased podocyte foot process proteins expression in kk-Ay mice, which is consistent with the results that apelin receptor (APLNR) levels increased in glomeruli of patients or mice with DN. In cultured podocytes, high glucose increased APLNR expression and apelin administration was associated with increased permeability and decreased foot process proteins levels. All these dysfunctions were associated with decreased 26S proteasome activities and increased polyubiquitinated proteins in both kk-Ay mice and cultured podocytes, as demonstrated by 26S proteasome activation with cyclic adenosine monophosphate (cAMP) or oleuropein. These effects seemed to be related to endoplasmic reticulum (ER) stress, as apelin increased C/EBP homologous protein (CHOP) and peiFα levels while cAMP or oleuropein reduced it in high glucose and apelin treated podocytes. These results suggest that apelin induces podocyte dysfunction in DN through ER stress which was induced by decreased proteasome activities in podocytes.

Podocyte endocytosis in the regulation of the glomerular filtration barrier

Severe defects in the glomerular filtration barrier result in nephrotic syndrome, which is characterized by massive proteinuria. The podocyte, a specialized epithelial cell with interdigitating foot processes separated by a slit diaphragm, plays a vital role in regulating the passage of proteins from the capillary lumen to Bowman's space. Recent findings suggest a critical role for endocytosis in podocyte biology as highlighted by genetic mouse models of disease and human genetic mutations that result in the loss of the integrity of the glomerular filtration barrier. In vitro podocyte studies have also unraveled a plethora of constituents that are differentially internalized to maintain homeostasis. These observations provide a framework and impetus for understanding the precise regulation of podocyte endocytic machinery in both health and disease.

Effect of benfotiamine in podocyte damage induced by peritoneal dialysis fluid

Background: In peritoneal dialysis (PD), residual renal function (RRF) fundamentally contributes to improved quality of life and patient survival. High glucose and advanced glycation end-products (AGE) contribute locally to peritoneal and systemically to renal damage. Integrity of podocyte structure and function is of special importance to preserve RRF. Benfotiamine could counteract the glucose and AGE-mediated toxicity by blocking hyperglycemia-associated podocyte damage via the pentose-phosphate pathway.

Methods: A human differentiated podocyte cell line was incubated with control solution (control), 2.5% glucose solution (glucose), and 2.5% peritoneal dialysis fluid (PDF) for 48 h either ±50 μM benfotiamine. Podocyte damage and potential benefit of benfotiamine were analyzed using immunofluorescence, western blot analysis, and a functional migration assay. For quantitation, a semiquantitative score was used.

Results: When incubating podocytes with benfotiamine, glucose, and PDF-mediated damage was reduced, resulting in lower expression of AGE and intact podocin and ZO-1 localization. The reorganization of the actin cytoskeleton was restored in the presence of benfotiamine as functional podocyte motility reached control level. Decreased level of inflammation could be shown as well as reduced podocyte apoptosis.

Conclusion: These data suggest that benfotiamine protects podocytes from glucose and PDF-mediated dysfunction and damage, in particular, with regard to cytoskeletal reorganization, motility, inflammation, and podocyte survival.

NFAT2 inhibitor ameliorates diabetic nephropathy and podocyte injury in db/db mice

BACKGROUND AND PURPOSE

Podocyte injury plays a key role in the development of diabetic nephropathy (DN). We have recently shown that 11R-VIVIT, an inhibitor of cell-permeable nuclear factor of activated T-cells (NFAT), attenuates podocyte apoptosis induced by high glucose in vitro. However, it is not known whether 11R-VIVIT has a protective effect on DN, especially podocyte injury, under in vivo diabetic conditions. Hence, we examined the renoprotective effects of 11R-VIVIT in diabetic db/db mice and the possible mechanisms underlying its protective effects on podocyte injury in vivo and in vitro.

EXPERIMENTAL APPROACH

Type 2 diabetic db/db mice received i.p. injections of 11R-VIVIT (1 mg·kg(-1)) three times a week and were killed after 8 weeks. Immortalized mouse podocytes were cultured under different experimental conditions.

KEY RESULTS

11R-VIVIT treatment markedly attenuated the albuminuria in diabetic db/db mice and also alleviated mesangial matrix expansion and podocyte injury. However, body weight, food and water intake, and glucose levels were unaffected. It also attenuated the increased NFAT2 activation and enhanced urokinase-type plasminogen activator receptor (uPA receptor) expression in glomerulor podocytes. In cultured podocytes, the increased nuclear accumulation of NFAT2 and uPA receptor expression induced by high glucose treatment was prevented by 11R-VIVIT or NFAT2-knockdown; this was accompanied by improvements in the filtration barrier function of the podocyte monolayer.

CONCLUSIONS AND IMPLICATIONS

The NFAT inhibitor 11R-VIVIT might be a useful therapeutic strategy for protecting podocytes and treating DN. The calcinerin/NFAT2/uPA receptor signalling pathway should be exploited as a therapeutic target for protecting podocytes from injury in DN.

Mechanisms of transcriptional regulation by WT1 (Wilms' tumour 1)

The WT1 (Wilms' tumour 1) gene encodes a zinc finger transcription factor and RNA-binding protein that direct the development of several organs and tissues. WT1 manifests both tumour suppressor and oncogenic activities, but the reasons behind these opposing functions are still not clear. As a transcriptional regulator, WT1 can either activate or repress numerous target genes resulting in disparate biological effects such as growth, differentiation and apoptosis. The complex nature of WT1 is exemplified by a plethora of isoforms, post-translational modifications and multiple binding partners. How WT1 achieves specificity to regulate a large number of target genes involved in diverse physiological processes is the focus of the present review. We discuss the wealth of the growing molecular information that defines our current understanding of the versatility and utility of WT1 as a master regulator of organ development, a tumour suppressor and an oncogene.

ZO-1 and ZO-2 are required for extra-embryonic endoderm integrity, primitive ectoderm survival and normal cavitation in embryoid bodies derived from mouse embryonic stem cells

The Zonula Occludens proteins ZO-1 and ZO-2 are cell-cell junction-associated adaptor proteins that are essential for the structural and regulatory functions of tight junctions in epithelial cells and their absence leads to early embryonic lethality in mouse models. Here, we use the embryoid body, an in vitro peri-implantation mouse embryogenesis model, to elucidate and dissect the roles ZO-1 and ZO-2 play in epithelial morphogenesis and de novo tight junction assembly. Through the generation of individual or combined ZO-1 and ZO-2 null embryoid bodies, we show that their dual deletion prevents tight junction formation, resulting in the disorganization and compromised barrier function of embryoid body epithelial layers. The disorganization is associated with poor microvilli development, fragmented basement membrane deposition and impaired cavity formation, all of which are key epithelial tissue morphogenetic processes. Expression of Podocalyxin, which positively regulates the formation of microvilli and the apical membrane, is repressed in embryoid bodies lacking both ZO-1 and ZO-2 and this correlates with an aberrant submembranous localization of Ezrin. The null embryoid bodies thus give an insight into how the two ZO proteins influence early mouse embryogenesis and possible mechanisms underlying the embryonic lethal phenotype.

KLF4-dependent epigenetic remodeling modulates podocyte phenotypes and attenuates proteinuria

The transcription factor Kruppel-like factor 4 (KLF4) has the ability, along with other factors, to reprogram somatic cells into induced pluripotent stem (iPS) cells. Here, we determined that KLF4 is expressed in kidney glomerular podocytes and is decreased in both animal models and humans exhibiting a proteinuric. Transient restoration of KLF4 expression in podocytes of diseased glomeruli in vivo, either by gene transfer or transgenic expression, resulted in a sustained increase in nephrin expression and a decrease in albuminuria. In mice harboring podocyte-specific deletion of Klf4, adriamycin-induced proteinuria was substantially exacerbated, although these animals displayed minimal phenotypical changes prior to adriamycin administration. KLF4 overexpression in cultured human podocytes increased expression of nephrin and other epithelial markers and reduced mesenchymal gene expression. DNA methylation profiling and bisulfite genomic sequencing revealed that KLF4 expression reduced methylation at the nephrin promoter and the promoters of other epithelial markers; however, methylation was increased at the promoters of genes encoding mesenchymal markers, suggesting selective epigenetic regulation of podocyte gene expression. Together, these results suggest that KLF4 epigenetically modulates podocyte phenotype and function and that the podocyte epigenome can be targeted for direct intervention and reduction of proteinuria.

Slit diaphragm protein Neph1 and its signaling: a novel therapeutic target for protection of podocytes against glomerular injury

Podocytes are specialized epithelial cells that are critical components of the glomerular filtration barrier, and their dysfunction leads to proteinuria and renal failure. Therefore, preserving podocyte function is therapeutically significant. In this study, we identified Neph1 signaling as a therapeutic target that upon inhibition prevented podocyte damage from a glomerular injury-inducing agent puromycin aminonucleoside (PAN). To specifically inhibit Neph1 signaling, we used a protein transduction approach, where the cytoplasmic domain of Neph1 (Neph1CD) tagged with a protein transduction domain trans-activator of transcription was transduced in cultured podocytes prior to treatment with PAN. The PAN-induced Neph1 phosphorylation was significantly reduced in Neph1CD-transduced cells; in addition, these cells were resistant to PAN-induced cytoskeletal damage. The biochemical analysis using subfractionation studies showed that unlike control cells Neph1 was retained in the lipid raft fractions in the transduced cells following treatment with PAN, indicating that transduction of Neph1CD in podocytes prevented PAN-induced mislocalization of Neph1. In accordance, the immunofluorescence analysis further suggested that Neph1CD-transduced cells had increased ability to retain endogenous Neph1 at the membrane in response to PAN-induced injury. Similar results were obtained when angiotensin was used as an injury-inducing agent. Consistent with these observations, maintaining high levels of Neph1 at the membrane using a podocyte cell line overexpressing chimeric Neph1 increased the ability of podocytes to resist PAN-induced injury and PAN-induced albumin leakage. Using a zebrafish in vivo PAN and adriamycin injury models, we further demonstrated the ability of transduced Neph1CD to preserve glomerular function. Collectively, these results support the conclusion that inhibiting Neph1 signaling is therapeutically significant in preventing podocyte damage from glomerular injury.

Cellular effects of everolimus and sirolimus on podocytes

Everolimus (EVL) and Sirolimus (SRL) are potent immunosuppressant agents belonging to the group of mammalian target of rapamycin (mTOR) inhibitors used to prevent transplant rejection. However, some patients develop proteinuria following a switch from a calcineurin inhibitor regimen to mTOR inhibitors. Whether different mTOR inhibitors show similar effects on podocytes is still unknown. To analyze this, human podocytes were incubated with different doses of EVL and SRL. After incubation with EVL or SRL, podocytes revealed a reduced expression of total mTOR. Phosphorylation of p70S6K and Akt was diminished, whereas pAkt expression was more reduced in the SRL group. In both groups actin cytoskeletal reorganization was increased. Synaptopodin and podocin expression was reduced as well as nephrin protein, particularly in the SRL group. NFκB activation and IL-6 levels were lower in EVL and SRL, and even lower in SRL. Apoptosis was more increased in SRL than in the EVL group. Our data suggests that mTOR inhibitors affect podocyte integrity with respect to podocyte proteins, cytoskeleton, inflammation, and apoptosis. Our study is the first to analyze both mTOR inhibitors, EVL and SRL, in parallel in podocytes. Partially, the impact of EVL and SRL on podocytes differs. Nevertheless, it still remains unclear whether these differences are of relevance regarding to proteinuria in transplant patients.

WT1 promotes cell proliferation in non-small cell lung cancer cell lines through up-regulating cyclin D1 and p-pRb in vitro and in vivo

The Wilms’ tumor suppressor gene (WT1) has been identified as an oncogene in many malignant diseases such as leukaemia, breast cancer, mesothelioma and lung cancer. However, the role of WT1 in non-small-cell lung cancer (NSCLC) carcinogenesis remains unclear. In this study, we compared WT1 mRNA levels in NSCLC tissues with paired corresponding adjacent tissues and identified significantly higher expression in NSCLC specimens. Cell proliferation of three NSCLC cell lines positively correlated with WT1 expression; moreover, these associations were identified in both cell lines and a xenograft mouse model. Furthermore, we demonstrated that up-regulation of Cyclin D1 and the phosphorylated retinoblastoma protein (p-pRb) was mechanistically related to WT1 accelerating cells to S-phase. In conclusion, our findings demonstrated that WT1 is an oncogene and promotes NSCLC cell proliferation by up-regulating Cyclin D1 and p-pRb expression.

Signaling from the podocyte intercellular junction to the actin cytoskeleton

Observations of hereditary glomerular disease support the contention that podocyte intercellular junction proteins are essential for junction formation and maintenance. Genetic deletion of most of these podocyte intercellular junction proteins results in foot process effacement and proteinuria. This review focuses on the current understanding of molecular mechanisms by which podocyte intercellular junction proteins such as the nephrin-neph1-podocin-receptor complex coordinate cytoskeletal dynamics and thus intercellular junction formation, maintenance, and injury-dependent remodeling.

Mild electrical stimulation and heat shock ameliorates progressive proteinuria and renal inflammation in mouse model of Alport syndrome

Alport syndrome is a hereditary glomerulopathy with proteinuria and nephritis caused by defects in genes encoding type IV collagen in the glomerular basement membrane. All male and most female patients develop end-stage renal disease. Effective treatment to stop or decelerate the progression of proteinuria and nephritis is still under investigation. Here we showed that combination treatment of mild electrical stress (MES) and heat stress (HS) ameliorated progressive proteinuria and renal injury in mouse model of Alport syndrome. The expressions of kidney injury marker neutrophil gelatinase-associated lipocalin and pro-inflammatory cytokines interleukin-6, tumor necrosis factor-α and interleukin-1β were suppressed by MES+HS treatment. The anti-proteinuric effect of MES+HS treatment is mediated by podocytic activation of phosphatidylinositol 3-OH kinase (PI3K)-Akt and heat shock protein 72 (Hsp72)-dependent pathways in vitro and in vivo. The anti-inflammatory effect of MES+HS was mediated by glomerular activation of c-jun NH₂-terminal kinase 1/2 (JNK1/2) and p38-dependent pathways ex vivo. Collectively, our studies show that combination treatment of MES and HS confers anti-proteinuric and anti-inflammatory effects on Alport mice likely through the activation of multiple signaling pathways including PI3K-Akt, Hsp72, JNK1/2, and p38 pathways, providing a novel candidate therapeutic strategy to decelerate the progression of patho-phenotypes in Alport syndrome.

Genipin inhibits mitochondrial uncoupling protein 2 expression and ameliorates podocyte injury in diabetic mice

Diabetic nephropathy (DN) is one of the most common causes of end stage renal disease (ESRD) in China, which requires renal replacement therapy. Recent investigations have suggested an essential role of podocyte injury in the initial stage of DN. This study investigated the potential therapeutic role of genipin, an active extract from a traditional Chinese medicine, on progression of DN in diabetic mice induced by intraperitoneally injection of streptozocin (STZ). In diabetic mice, orally administration of genipin postponed the progression of DN, as demonstrated by ameliorating body weight loss and urine albumin leakage, attenuating glomerular basement membrane thickness, restoring the podocyte expression of podocin and WT1 in diabetic mice. The protective role of genipin on DN is probably through suppressing the up-regulation of mitochondrial uncoupling protein 2 (UCP2) in diabetic kidneys. Meanwhile, through inhibiting the up-regulation of UCP2, genipin restores podocin and WT1 expression in cultured podocytes and attenuates glucose-induced albumin leakage through podocytes monolayer. Therefore, these results revealed that genipin inhibited UCP2 expression and ameliorated podocyte injury in DN mice.

Calcineurin regulation of cytoskeleton organization: a new paradigm to analyse the effects of calcineurin inhibitors on the kidney

Calcineurin is a serine/threonine phosphatase originally involved in the immune response but is also known for its role as a central mediator in various non-immunological intracellular signals. The nuclear factor of activated T cell (NFAT) proteins are the most widely described substrates of calcineurin, but ongoing work has uncovered other substrates among which are the cytoskeleton organizing proteins (i.e. cofilin, synaptopodin, WAVE-1). Control over cytoskeletal proteins is of outmost interest because the phenotypic properties of cells are dependent on cytoskeleton architecture integrity, while rearrangements of the cytoskeleton are implicated in both physiological and pathological processes. Previous works investigating the role of calcineurin on the cytoskeleton have focused on neurite elongation, myocyte hypertrophic response and recently in kidney cells structure. Nuclear factor of activated T cell activation is expectedly identified in the signalling pathways for calcineurin-induced cytoskeleton organization, however new NFAT-independent pathways have also been uncovered. The aim of this review is to summarize the current knowledge on the effects of calcineurin on cytoskeletal proteins and related intracellular pathways. These newly described properties of calcineurin on cytoskeletal proteins may explain some of the beneficial or deleterious effects observed in kidney cells associated with the use of the calcineurin inhibitors, cyclosporine and tacrolimus.

Hydrogen peroxide induces dimerization of protein kinase G type Iα subunits and increases albumin permeability in cultured rat podocytes

Podocytes help regulate filtration barrier permeability in the kidneys. They express contractile proteins that are characteristic of smooth muscle cells as well as receptors for vasoactive factors such as angiotensin II and atrial natriuretic peptide (ANP). The later one generates intracellular cGMP, with subsequent activation of cGMP-dependent protein kinase; PKG (isoform PKGIα and PKGIβ). In this study, we asked whether hydrogen peroxide (H(2)O(2)), a physiological vasorelaxing factor, affected podocyte permeability and the podoctye PKGIα signaling pathway. Expression of PKGIα was confirmed in cultured rat podocytes using RT-PCR, immunofluorescence, and Western blotting. Exposure of podocytes to exogenous H(2)O(2) (100 µM) in non-reducing conditions increased the formation of PKGIα interprotein disulfide bonds, affected the phosphorylation of PKG target proteins, namely MYPT1 (maximal increase of about 57% at 30 min) and MLC (maximal decrease of about 62% at 10 min). Furthermore, H(2)O(2) increased the permeability of a layer of podocytes to albumin: Transmembrane flux for albumin increased five-fold (106.6 ± 5.2 µg/ml vs. 20.2 ± 2.5 µg/ml, P < 0.05, n = 5), and the PKG inhibitor Rp-8-Br-cGMPS (100 µM) prevented the flux increase. These data suggest that oxidative modulation of PKGIα in podocytes plays an important

Defining the role of TRIP6 in cell physiology and cancer

Integrating signals from the ECM (extracellular matrix) via the cell surface into the nucleus is an essential feature of multicellular life and often malfunctions in cancer. To date many signal transducers known as shuttle proteins have been identified that act as both: a cytoskeletal and a signalling protein. Here, we highlight the interesting member of the Zyxin family TRIP6 [thyroid receptor interactor protein 6; also designated ZRP-1 (zyxin-related protein 1)] and review current literature to define its role in cell physiology and cancer. TRIP6 is a versatile scaffolding protein at FAs (focal adhesions) involved in cytoskeletal organization, coordinated cell migration and tissue invasion. Via its LIM and TDC domains TRIP6 interacts with different components of the LPA (lysophosphatidic acid), NF-κB (nuclear factor κB), glucocorticoid and AMPK (AMP-activated protein kinase) signalling pathway and thereby modulates their activity. Within the nucleus TRIP6 acts as a transcriptional cofactor regulating the transcriptional responses of these pathways. Moreover, intranuclear TRIP6 associates with proteins ensuring telomere protection and hence may contribute to genome stability. Accordingly, TRIP6 is engaged in key cellular processes such as cell proliferation, differentiation and survival. These diverse functions of TRIP6 are found to be dysregulated in various cancers and may have pleiotropic roles in tumour initiation, tumour growth and metastasis, which turn TRIP6 into an attractive candidate for cancer diagnosis and targeted therapy.

WT1-interacting protein (Wtip) regulates podocyte phenotype by cell-cell and cell-matrix contact reorganization

Podocytes respond to environmental cues by remodeling their slit diaphragms and cell-matrix adhesive junctions. Wt1-interacting protein (Wtip), an Ajuba family LIM domain scaffold protein expressed in the podocyte, coordinates cell adhesion changes and transcriptional responses to regulate podocyte phenotypic plasticity. We evaluated effects of Wtip on podocyte cell-cell and cell-matrix contact organization using gain-of- and loss-of-function methods. Endogenous Wtip targeted to focal adhesions in adherent but isolated podocytes and then shifted to adherens junctions after cells made stable, homotypic contacts. Podocytes with Wtip knockdown (shWtip) adhered but failed to spread normally. Noncontacted shWtip podocytes did not assemble actin stress fibers, and their focal adhesions failed to mature. As shWtip podocytes established cell-cell contacts, stable adherens junctions failed to form and F-actin structures were disordered. In shWtip cells, cadherin and β-catenin clustered in irregularly distributed spots that failed to laterally expand. Cell surface biotinylation showed diminished plasma membrane cadherin, β-catenin, and α-catenin in shWtip podocytes, although protein expression was similar in shWtip and control cells. Since normal actin dynamics are required for organization of adherens junctions and focal adhesions, we determined whether Wtip regulates F-actin assembly. Undifferentiated podocytes did not elaborate F-actin stress fibers, but when induced to overexpress WTIP, formed abundant stress fibers, a process blocked by the RhoA inhibitor C3 toxin and a RhoA kinase inhibitor. WTIP directly interacted with Rho guanine nucleotide exchange factor (GEF) 12 (Arhgef12), a RhoA-specific GEF enriched in the glomerulus. In conclusion, stable assembly of podocyte adherens junctions and cell-matrix contacts requires Wtip, a process that may be mediated by spatiotemporal regulation of RhoA activity through appropriate targeting of Arhgef12.

TM4SF10 and ADAP interaction in podocytes: role in Fyn activity and nephrin phosphorylation

TM4SF10 [transmembrane tetra(4)-span family 10] is a claudin-like cell junction protein that is transiently expressed during podocyte development where its expression is downregulated in differentiating podocytes coincident with the appearance of nephrin at the slit diaphragm. In a yeast two-hybrid screen, we identified adhesion and degranulation-promoting adaptor protein (ADAP), a well-known Fyn substrate and Fyn binding partner, as a TM4SF10 interacting protein in mouse kidney. Using coimmunoprecipitation and immunohistochemistry experiments in cultured human podocytes, we show that TM4SF10 colocalizes with Fyn and ADAP but does not form a stable complex with Fyn. Cytoskeletal changes and phosphorylation events mediated by Fyn activity were reversed by TM4SF10 overexpression, including a decrease in the activating tyrosine phosphorylation of Fyn (Y(421)), suggesting TM4SF10 may have a regulatory role in suppressing Fyn activity. In addition, TM4SF10 was reexpressed following podocyte injury by puromycin aminonucleoside treatment, and its expression enhanced the abundance of high-molecular-weight forms of nephrin indicating it may participate in a mechanism controlling nephrin's appearance at the plasma membrane. Therefore, these studies have identified ADAP as another Fyn adapter protein expressed in podocytes, and that TM4SF10, possibly through ADAP, may regulate Fyn activity. Since TM4SF10 expression is temporally regulated during kidney development, these studies may help define a mechanism by which the slit diaphragm matures as a highly specialized cell junction during podocyte differentiation.

Common variation in GPC5 is associated with acquired nephrotic syndrome

Severe proteinuria is a defining factor of nephrotic syndrome irrespective of the etiology. Investigation of congenital nephrotic syndrome has shown that dysfunction of glomerular epithelial cells (podocytes) plays a crucial role in this disease. Acquired nephrotic syndrome is also assumed to be associated with podocyte injury. Here we identify an association between variants in GPC5, encoding glypican-5, and acquired nephrotic syndrome through a genome-wide association study and replication analysis (P value under a recessive model (P(rec)) = 6.0 × 10(-11), odds ratio = 2.54). We show that GPC5 is expressed in podocytes and that the risk genotype is associated with higher expression. We further show that podocyte-specific knockdown and systemic short interfering RNA injection confers resistance to podocyte injury in mouse models of nephrosis. This study identifies GPC5 as a new susceptibility gene for nephrotic syndrome and implicates GPC5 as a promising therapeutic target for reducing podocyte vulnerability in glomerular disease.

PINCH1 is transcriptional regulator in podocytes that interacts with WT1 and represses podocalyxin expression

Background

PINCH1, an adaptor protein containing five LIM domains, plays an important role in regulating the integrin-mediated cell adhesion, migration and epithelial-mesenchymal transition. PINCH1 is induced in the fibrotic kidney after injury, and it primarily localizes at the sites of focal adhesion. Whether it can translocate to the nucleus and directly participate in gene regulation is completely unknown.

Methodology/Principal Findings

Using cultured glomerular podocytes as a model system, we show that PINCH1 expression was induced by TGF-β1, a fibrogenic cytokine that promotes podocyte dysfunction. Interestingly, increased PINCH1 not only localized at the sites of focal adhesions, but also underwent nuclear translocation after TGF-β1 stimulation. This nuclear translocation of PINCH1 was apparently dependent on the putative nuclear export/localization signals (NES/NLS) at its C-terminus, as deletion or site-directed mutations abolished its nuclear shuttling. Co-immunoprecipitation and pull-down experiments revealed that PINCH1 interacted with Wilms tumor 1 protein (WT1), a nuclear transcription factor that is essential for regulating podocyte-specific gene expression in adult kidney. Interaction of PINCH1 and WT1 was mediated by the LIM1 domain of PINCH1 and C-terminal zinc-finger domain of WT1, which led to the suppression of the WT1-mediated podocalyxin expression in podocytes. PINCH1 also repressed podocalyxin gene transcription in a promoter-luciferase reporter assay.

Conclusion/Significance

These results indicate that PINCH1 can shuttle into the nucleus from cytoplasm in podocytes, wherein it interacts with WT1 and suppresses podocyte-specific gene expression. Our studies reveal a previously unrecognized, novel function of PINCH1, in which it acts as a transcriptional regulator through controlling specific gene expression.

Subcellular and regional location of "brain" proteins BASP1 and MARCKS in kidney and testis

Proteins BASP1 and MARCKS are abundant in axonal endings of neurons. Similarly to brain-specific protein GAP-43, BASP1 and MARCKS are reversibly bound to the plasma membrane. These proteins control both actin polymerization and actin cytoskeleton binding to the membrane. Performing these functions, BASP1 and MARCKS take part in growth cone guidance during development and in neurotransmitter secretion in adults. These activities predetermine the pivotal role of BASP1 and MARCKS in learning and memory. BASP1 and MARCKS were also found in non-nerve tissues, in particular, in the kidney and testis. Evidently, the physiological roles of these proteins differ in different tissues. Correspondingly, their intracellular location and activities may not be similar to those in neurons. In this paper, we analyze subcellular fractions (cytoplasm and nuclei) of rat kidney and testis with the purpose of determining the intracellular location of BASP1 and MARCKS. Western blots demonstrated that in these tissues, as in the brain, both proteins are present in the cytoplasm of the cell. According to our immunohistochemical study, BASP1 and MARCKS are specifically distributed in the tissues studied. In kidney, both proteins are present in cells located in glomeruli. In the testicular tubules, BASP1 is mainly expressed at the late stage of spermatogenesis (in spermatids) and is preserved in mature spermatozoa, while MARCKS appears equally during all stages of spermatogenesis. MARCKS is not found in mature spermatozoa. The results indicate that study of functions of BASP1 and MARCKS in the kidney and in the reproduction system holds much promise.

Angiopoietin-like protein 3 regulates the motility and permeability of podocytes by altering nephrin expression in vitro

It is well known that podocyte injury plays a vital role in massive proteinuria. The increase of podocyte motility results in podocyte foot process (FP) effacement, a typical form of podocyte injury. Our previous studies demonstrated that glomerular podocytes can express angiopoietin-like protein 3 (ANGPTL3) and that the increase of ANGPTL3 in dysfunctional glomerulus is correlated with podocyte FP effacement. Little is known, however, about the role of ANGPTL3 in podocyte injury. In this study, we investigated ANGPTL3's effect on the motility and permeability of podocytes and on the expression of nephrin, a key molecule in podocytes. By scrape-wound and transwell migration assay, we found that ANGPTL3 over-expression significantly increased podocyte motility, whereas after ANGPTL3 knockdown by RNA interference, motility remained the same as that of the control group. Adriamycin (ADR) treatment significantly promoted podocyte motility. However, the same dose of ADR treatment could not promote motility after the knockdown of ANGPTL3. In addition, we assayed the diffusion of FITC-BSA across the podocytes' monolayer to investigate whether ANGPTL3 could promote protein loss by means of an increase in podocyte motility. The results showed that the changes in the FITC-BSA permeability of the podocytes corresponded to changes in motility. Furthermore, we found that ANGPTL3 over-expression dramatically increased the expression of nephrin but that the up-regulation of nephrin induced by ADR was significantly inhibited when ANGPTL3 was diminished by RNAi. In conclusion, we found ANGPTL3 to be capable of regulating the motility and permeability of podocytes and that the mechanism of ANGPTL3's regulation could be associated with the altered expression of nephrin.

Inhibition of integrin-linked kinase blocks podocyte epithelial-mesenchymal transition and ameliorates proteinuria

Proteinuria is a primary clinical symptom of a large number of glomerular diseases that progress to end-stage renal failure. Podocyte dysfunctions play a fundamental role in defective glomerular filtration in many common forms of proteinuric kidney disorders. Since binding of these cells to the basement membrane is mediated by integrins, we determined the role of integrin-linked kinase (ILK) in podocyte dysfunction and proteinuria. ILK expression was induced in mouse podocytes by various injurious stimuli known to cause proteinuria including TGF-beta1, adriamycin, puromycin, and high ambient glucose. Podocyte ILK was also found to be upregulated in human proteinuric glomerular diseases. Ectopic expression of ILK in podocytes decreased levels of the epithelial markers nephrin and ZO-1, induced mesenchymal markers such as desmin, fibronectin, matrix metalloproteinase-9 (MMP-9), and alpha-smooth muscle actin (alpha-SMA), promoted cell migration, and increased the paracellular albumin flux across podocyte monolayers. ILK also induced Snail, a key transcription factor mediating epithelial-mesenchymal transition (EMT). Blockade of ILK activity with a highly selective small molecule inhibitor reduced Snail induction and preserved podocyte phenotypes following TGF-beta1 or adriamycin stimulation. In vivo, this ILK inhibitor ameliorated albuminuria, repressed glomerular induction of MMP-9 and alpha-SMA, and preserved nephrin expression in murine adriamycin nephropathy. Our results show that upregulation of ILK is a convergent pathway leading to podocyte EMT, migration, and dysfunction. ILK may be an attractive target for therapeutic intervention of proteinuric kidney diseases.

Podocyte injury induces nuclear translocation of WTIP via microtubule-dependent transport

Podocyte structural and transcriptional phenotype plasticity characterizes glomerular injury. Transcriptional activity of WT1 (Wilm's tumor 1) is required for normal podocyte structure and is repressed by the podocyte adherens junction protein, WTIP (WT1 interacting protein). Here we show that WTIP translocated into podocyte nuclei in lipopolysaccharide (LPS)-treated mice, a model of transient nephrotic syndrome. Cultured podocytes, which stably expressed an epitope-tagged WTIP, were treated with LPS. Imaging and cellular fractionation studies demonstrated that WTIP translocated from podocyte cell contacts into nuclei within 6 h and relocalized to cell contacts within 24 h after LPS treatment. LPS-stimulated WTIP nuclear translocation required JNK activity, which assembled a multiprotein complex of the scaffolding protein JNK-interacting protein 3 and the molecular motor dynein. Intact microtubule networks and dynein activity were necessary for LPS-stimulated WTIP translocation. Podocytes expressing sh-Wtip change morphology and demonstrate altered actin assembly in cell spreading assays. Stress signaling pathways initiate WTIP nuclear translocation, and the concomitant loss of WTIP from cell contacts changes podocyte morphology and dynamic actin assembly, suggesting a mechanism that transmits changes in podocyte morphology to the nucleus.

Impaired transcription factor interplay in addition to advanced glycation end products suppress podocalyxin expression in high glucose-treated human podocytes

Podocalyxin represents a Wilms’ tumor suppressor protein (WT1)-regulated differentiation marker for glomerular epithelium. We provide evidence concerning mechanisms involved in the regulation of podocalyxin expression following long-term exposure to increased (25 mM) glucose levels. Prolonged culture of conditionally immortalized human podocytes in 25 mM glucose induced suppression of podocalyxin expression both at the protein and mRNA levels, whereas WT1 protein levels remained unaltered. WT1 interacted with another transcription factor, CRE-binding protein (CBP). This association was decreased by 40% in the presence of 25 mM glucose. Chromatin immunoprecipitation assays on chromatin from podocytes cultured in 25 mM glucose revealed reduced WT1 binding to podocalyxin promoter sequences, probably resulting from impaired WT1-CBP interactions. We explored the possible role of glucose-induced adducts (advanced glycation end products; AGEs) in impairing interactions between WT1 and CBP, with the use of aminoguanindine, an inhibitor of AGE formation. Podocytes were cultured in the simultaneous presence of 20 mM aminoguanidine and 25 mM glucose, and podocalyxin protein levels were examined. Aminoguanidine effectively prevented downregulation of podocalyxin protein levels but could not restore podocalyxin levels once expression was suppressed. Thus increased glucose apparently impaired the ability of WT1 to initiate transcription in part by decreased association of WT1 with CBP. Administration of aminoguanidine concomitant with increasing glucose levels in our in vitro model system protected from glucose-induced “silencing” of the podocalyxin gene, suggesting that AGEs play an important role in suppressing its expression in diabetic conditions.

Ischemic injury to kidney induces glomerular podocyte effacement and dissociation of slit diaphragm proteins Neph1 and ZO-1

Glomerular injury is often characterized by the effacement of podocytes, loss of slit diaphragms, and proteinuria. Renal ischemia or the loss of blood flow to the kidneys has been widely associated with tubular and endothelial injury but rarely has been shown to induce podocyte damage and disruption of the slit diaphragm. In this study, we have used an in vivo rat ischemic model to demonstrate that renal ischemia induces podocyte effacement with loss of slit diaphragm and proteinuria. Biochemical analysis of the ischemic glomerulus shows that ischemia induces rapid loss of interaction between slit diaphragm junctional proteins Neph1 and ZO-1. To further understand the effect of ischemia on molecular interactions between slit diaphragm proteins, a cell culture model was employed to study the binding between Neph1 and ZO-1. Under physiologic conditions, Neph1 co-localized with ZO-1 at cell-cell contacts in cultured human podocytes. Induction of injury by ATP depletion resulted in rapid loss of Neph1 and ZO-1 binding and redistribution of Neph1 and ZO-1 proteins from cell membrane to the cytoplasm. Recovery resulted in increased Neph1 tyrosine phosphorylation, restoring Neph1 and ZO-1 binding and their localization at the cell membrane. We further demonstrate that tyrosine phosphorylation of Neph1 mediated by Fyn results in significantly increased Neph1 and ZO-1 binding, suggesting a critical role for Neph1 tyrosine phosphorylation in reorganizing the Neph1-ZO-1 complex. This study documents that renal ischemia induces dynamic changes in the molecular interactions between slit diaphragm proteins, leading to podocyte damage and proteinuria.

The role of the Wilms' tumour-suppressor protein WT1 in apoptosis

The Wilms' tumour-suppressor gene (WT1), encodes a zinc-finger transcription factor that is critical for the development of several organs, including the kidneys, gonads and spleen. Despite its identification as a tumour suppressor that plays a crucial role in the formation of a paediatric malignancy of the kidneys (Wilms' tumour), it has also emerged as an oncogenic factor influencing proliferation and apoptosis in a large variety of adult cancers. This review focuses on new insights into WT1's role in early development and its potential oncogenic role in adult cancer.