Trans-interaction of nephrin and Neph1/Neph3 induces cell adhesion that associates with decreased tyrosine phosphorylation of nephrin

Phosphorylation of PACSIN2 at S313 Regulates Podocyte Architecture in Coordination with N-WASP

Changes in the dynamic architecture of podocytes, the glomerular epithelial cells, lead to kidney dysfunction. Previous studies on protein kinase C and casein kinase 2 substrates in neurons 2 (PACSIN2), a known regulator of endocytosis and cytoskeletal organization, reveal a connection between PACSIN2 and kidney pathogenesis. Here, we show that the phosphorylation of PACSIN2 at serine 313 (S313) is increased in the glomeruli of rats with diabetic kidney disease. We found that phosphorylation at S313 is associated with kidney dysfunction and increased free fatty acids rather than with high glucose and diabetes alone. Phosphorylation of PACSIN2 emerged as a dynamic process that fine-tunes cell morphology and cytoskeletal arrangement, in cooperation with the regulator of the actin cytoskeleton, Neural Wiskott-Aldrich syndrome protein (N-WASP). PACSIN2 phosphorylation decreased N-WASP degradation while N-WASP inhibition triggered PACSIN2 phosphorylation at S313. Functionally, pS313-PACSIN2 regulated actin cytoskeleton rearrangement depending on the type of cell injury and the signaling pathways involved. Collectively, this study indicates that N-WASP induces phosphorylation of PACSIN2 at S313, which serves as a mechanism whereby cells regulate active actin-related processes. The dynamic phosphorylation of S313 is needed to regulate cytoskeletal reorganization.

A critical role of the podocyte cytoskeleton in the pathogenesis of glomerular proteinuria and autoimmune podocytopathies

Selective glomerular filtration relies on the membrane separating the glomerular arterioles from the Bowman space. As a major component of the glomerular filtration barrier, podocytes form foot processes by the actin cytoskeleton, which dynamically adjusts in response to environmental changes to maintain filtration barrier integrity. The slit diaphragms bridge the filtration slits between neighboring foot processes and act as signaling hubs interacting with the actin cytoskeleton. Focal adhesions relay signals to regulate actin dynamics while allowing podocyte adherence to the basement membrane. Mutations in actin regulatory and signaling proteins may disrupt the actin cytoskeleton, resulting in foot process retraction, effacement, and proteinuria. Large-scale gene expression profiling platforms, transgenic animal models, and other in vivo gene delivery methods now enhance our understanding of the interactions among podocyte focal adhesions, slit diaphragms, and actin dynamics. In addition, our team found that at least 66% of idiopathic nephrotic syndrome (INS) children have podocyte autoantibodies, which was defined as a new disease subgroup-, autoimmune podocytopathies. This review outlines the pathophysiological mechanisms of podocyte cytoskeleton protein interactions in proteinuria and glomerular podocytopathy.

Cell adhesion molecule KIRREL1 is a feedback regulator of Hippo signaling recruiting SAV1 to cell-cell contact sites

The Hippo/YAP pathway controls cell proliferation through sensing physical and spatial organization of cells. How cell-cell contact is sensed by Hippo signaling is poorly understood. Here, we identified the cell adhesion molecule KIRREL1 as an upstream positive regulator of the mammalian Hippo pathway. KIRREL1 physically interacts with SAV1 and recruits SAV1 to cell-cell contact sites. Consistent with the hypothesis that KIRREL1-mediated cell adhesion suppresses YAP activity, knockout of KIRREL1 increases YAP activity in neighboring cells. Analyzing pan-cancer CRISPR proliferation screen data reveals KIRREL1 as the top plasma membrane protein showing strong correlation with known Hippo regulators, highlighting a critical role of KIRREL1 in regulating Hippo signaling and cell proliferation. During liver regeneration in mice, KIRREL1 is upregulated, and its genetic ablation enhances hepatic YAP activity, hepatocyte reprogramming and biliary epithelial cell proliferation. Our data suggest that KIRREL1 functions as a feedback regulator of the mammalian Hippo pathway through sensing cell-cell interaction and recruiting SAV1 to cell-cell contact sites.

Metformin Protects against Podocyte Injury in Diabetic Kidney Disease

Metformin is the most commonly prescribed drug for treating type 2 diabetes mellitus (T2D). Its mechanisms of action have been under extensive investigation, revealing that it has multiple cellular targets, either direct or indirect ones, via which it regulates numerous cellular pathways. Diabetic kidney disease (DKD), the serious complication of T2D, develops in up to 50% of the individuals with T2D. Various mechanisms contribute to the development of DKD, including hyperglycaemia, dyslipidemia, oxidative stress, chronic low-grade inflammation, altered autophagic activity and insulin resistance, among others. Metformin has been shown to affect these pathways, and thus, it could slow down or prevent the progression of DKD. Despite several animal studies demonstrating the renoprotective effects of metformin, there is no concrete evidence in clinical settings. This review summarizes the renoprotective effects of metformin in experimental settings. Special emphasis is on the effects of metformin on podocytes, the glomerular epithelial cells that are central in maintaining the glomerular ultrafiltration function.

Metformin increases glucose uptake and acts renoprotectively by reducing SHIP2 activity

Metformin, the first-line drug to treat type 2 diabetes (T2D), inhibits mitochondrial glycerolphosphate dehydrogenase in the liver to suppress gluconeogenesis. However, the direct target and the underlying mechanisms by which metformin increases glucose uptake in peripheral tissues remain uncharacterized. Lipid phosphatase Src homology 2 domain-containing inositol-5-phosphatase 2 (SHIP2) is upregulated in diabetic rodent models and suppresses insulin signaling by reducing Akt activation, leading to insulin resistance and diminished glucose uptake. Here, we demonstrate that metformin directly binds to and reduces the catalytic activity of the recombinant SHIP2 phosphatase domain in vitro. Metformin inhibits SHIP2 in cultured cells and in skeletal muscle and kidney of db/db mice. In SHIP2-overexpressing myotubes, metformin ameliorates reduced glucose uptake by slowing down glucose transporter 4 endocytosis. SHIP2 overexpression reduces Akt activity and enhances podocyte apoptosis, and both are restored to normal levels by metformin. SHIP2 activity is elevated in glomeruli of patients with T2D receiving nonmetformin medication, but not in patients receiving metformin, compared with people without diabetes. Furthermore, podocyte loss in kidneys of metformin-treated T2D patients is reduced compared with patients receiving nonmetformin medication. Our data unravel a novel molecular mechanism by which metformin enhances glucose uptake and acts renoprotectively by reducing SHIP2 activity.-Polianskyte-Prause, Z., Tolvanen, T. A., Lindfors, S., Dumont, V., Van, M., Wang, H., Dash, S. N., Berg, M., Naams, J.-B., Hautala, L. C., Nisen, H., Mirtti, T., Groop, P.-H., Wähälä, K., Tienari, J., Lehtonen, S. Metformin increases glucose uptake and acts renoprotectively by reducing SHIP2 activity.

PACSIN2 accelerates nephrin trafficking and is up-regulated in diabetic kidney disease

Nephrin is a core component of podocyte (glomerular epithelial cell) slit diaphragm and is required for kidney ultrafiltration. Down-regulation or mislocalization of nephrin has been observed in diabetic kidney disease (DKD), characterized by albuminuria. Here, we investigate the role of protein kinase C and casein kinase 2 substrate in neurons 2 (PACSIN2), a regulator of endocytosis and recycling, in the trafficking of nephrin and development of DKD. We observe that PACSIN2 is up-regulated and nephrin mislocalized in podocytes of obese Zucker diabetic fatty (ZDF) rats that have altered renal function. In cultured podocytes, PACSIN2 and nephrin colocalize and interact. We show that nephrin is endocytosed in PACSIN2-positive membrane regions and that PACSIN2 overexpression increases both nephrin endocytosis and recycling. We identify rabenosyn-5, which is involved in early endosome maturation and endosomal sorting, as a novel interaction partner of PACSIN2. Interestingly, rabenosyn-5 expression is increased in podocytes in obese ZDF rats, and, in vitro, its overexpression enhances the association of PACSIN2 and nephrin. We also show that palmitate, which is elevated in diabetes, enhances this association. Collectively, PACSIN2 is up-regulated and nephrin is abnormally localized in podocytes of diabetic ZDF rats. In vitro, PACSIN2 enhances nephrin turnover apparently via a mechanism involving rabenosyn-5. The data suggest that elevated PACSIN2 expression accelerates nephrin trafficking and associates with albuminuria.—Dumont, V., Tolvanen, T. A., Kuusela, S., Wang, H., Nyman, T. A., Lindfors, S., Tienari, J., Nisen, H., Suetsugu, S., Plomann, M., Kawachi, H., Lehtonen, S. PACSIN2 accelerates nephrin trafficking and is up-regulated in diabetic kidney disease.

A functional variant in NEPH3 gene confers high risk of renal failure in primary hematuric glomerulopathies. Evidence for predisposition to microalbuminuria in the general population

Background

Recent data emphasize that thin basement membrane nephropathy (TBMN) should not be viewed as a form of benign familial hematuria since chronic renal failure (CRF) and even end-stage renal disease (ESRD), is a possible development for a subset of patients on long-term follow-up, through the onset of focal and segmental glomerulosclerosis (FSGS). We hypothesize that genetic modifiers may explain this variability of symptoms.

Methods

We looked in silico for potentially deleterious functional SNPs, using very strict criteria, in all the genes significantly expressed in the slit diaphragm (SD). Two variants were genotyped in a cohort of well-studied adult TBMN patients from 19 Greek-Cypriot families, with a homogeneous genetic background. Patients were categorized as “Severe” or “Mild”, based on the presence or not of proteinuria, CRF and ESRD. A larger pooled cohort (HEMATURIA) of 524 patients, including IgA nephropathy patients, was used for verification. Additionally, three large general population cohorts [Framingham Heart Study (FHS), KORAF4 and SAPHIR] were used to investigate if the NEPH3-V353M variant has any renal effect in the general population.

Results and conclusions

Genotyping for two high-scored variants in 103 TBMN adult patients with founder mutations who were classified as mildly or severely affected, pointed to an association with variant NEPH3-V353M (filtrin). This promising result prompted testing in the larger pooled cohort (HEMATURIA), indicating an association of the 353M variant with disease severity under the dominant model (p = 3.0x10^-3, OR = 6.64 adjusting for gender/age; allelic association: p = 4.2x10^-3 adjusting for patients’ kinships). Subsequently, genotyping 6,531 subjects of the Framingham Heart Study (FHS) revealed an association of the homozygous 353M/M genotype with microalbuminuria (p = 1.0x10^-3). Two further general population cohorts, KORAF4 and SAPHIR confirmed the association, and a meta-analysis of all three cohorts (11,258 individuals) was highly significant (p = 1.3x10^-5, OR = 7.46). Functional studies showed that Neph3 homodimerization and Neph3-Nephrin heterodimerization are disturbed by variant 353M. Additionally, 353M was associated with differential activation of the unfolded protein response pathway, when overexpressed in stressed cultured undifferentiated podocyte cells, thus attesting to its functional significance. Genetics and functional studies support a “rare variant-strong effect” role for NEPH3-V353M, by exerting a negative modifier effect on primary glomerular hematuria. Additionally, genetics studies provide evidence for a role in predisposing homozygous subjects of the general population to micro-albuminuria.

Septin 7 reduces nonmuscle myosin IIA activity in the SNAP23 complex and hinders GLUT4 storage vesicle docking and fusion

Glomerular epithelial cells, podocytes, are insulin responsive and can develop insulin resistance. Here, we demonstrate that the small GTPase septin 7 forms a complex with nonmuscle myosin heavy chain IIA (NMHC-IIA; encoded by MYH9), a component of the nonmuscle myosin IIA (NM-IIA) hexameric complex. We observed that knockdown of NMHC-IIA decreases insulin-stimulated glucose uptake into podocytes. Both septin 7 and NM-IIA associate with SNAP23, a SNARE protein involved in GLUT4 storage vesicle (GSV) docking and fusion with the plasma membrane. We observed that insulin decreases the level of septin 7 and increases the activity of NM-IIA in the SNAP23 complex, as visualized by increased phosphorylation of myosin regulatory light chain. Also knockdown of septin 7 increases the activity of NM-IIA in the complex. The activity of NM-IIA is increased in diabetic rat glomeruli and cultured human podocytes exposed to macroalbuminuric sera from patients with type 1 diabetes. Collectively, the data suggest that the activity of NM-IIA in the SNAP23 complex plays a key role in insulin-stimulated glucose uptake into podocytes. Furthermore, we observed that septin 7 reduces the activity of NM-IIA in the SNAP23 complex and thereby hinders GSV docking and fusion with the plasma membrane.

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Septin 7, nonmuscle myosin heavy chain IIA (NMHC-IIA) and SNAP23 form a complex.

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Knockdown of septin 7 increases NM-IIA activity in the SNAP23 complex.

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Insulin decreases septin 7 level and increases NM-IIA activity in the SNAP23 complex.

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Septin 7 hinders GSV docking/fusion by reducing NM-IIA activity in the SNAP23 complex.

Integrin Ligation Results in Nephrin Tyrosine Phosphorylation In Vitro

Nephrin is expressed at the basolateral aspect of podocytes and is an important signaling protein at the glomerular slit diaphragm. In vitro studies have demonstrated that Nephrin phosphorylation-dependent signaling is able to assemble a protein complex that is able to polymerize actin. However, proximal signaling events that result in nephrin tyrosine phosphorylation are not well understood. Nephrin deletion in mice and human nephrin mutations result in developmental failure of the podocyte intercellular junction resutling in proteinuria. This has been presumed to be due to a failure to respond to an external polarized cue in the absence of nephrin or a failure to transduce an outside-in signal in patients with nephrin mutations. The nephrin extracellular domain binds to itself or neph1 across the foot process intercellular junction. Nephrin is tyrosine phosphorylation-silent in healthy glomeruli when presumably the nephrin extracellular domain is in an engaged state. These observations raise the possibility of an alternate proximal signaling mechanism that might be responsible for nephrin tyrosine phosphorylation. Here we present data showing that integrin engagement at the basal aspect of cultured podocytes results in nephrin tyrosine phosphorylation. This is abrogated by incubating podocytes with an antibody that prevents integrin β1 ligation and activation in response to binding to extracellular matrix. Furthermore, nephrin tyrosine phosphorylation was observed in podocytes expressing a membrane-targeted nephrin construct that lacks the extracellular domain. We propose, integrin-activation based signaling might be responsible for nephrin phosphorylation rather than engagment of the nephrin extracellular domain by a ligand.

Nephrin Contributes to Insulin Secretion and Affects Mammalian Target of Rapamycin Signaling Independently of Insulin Receptor

Nephrin belongs to a family of highly conserved proteins with a well characterized function as modulators of cell adhesion and guidance, and nephrin may have a role in metabolic pathways linked to podocyte and pancreatic β-cell survival. However, this role is incompletely characterized. In this study, we developed floxed nephrin mice for pancreatic β-cell-specific deletion of nephrin, which had no effect on islet size and glycemia. Nephrin deficiency, however, resulted in glucose intolerance in vivo and impaired glucose-stimulated insulin release ex vivo Glucose intolerance was also observed in eight patients with nephrin mutations compared with three patients with other genetic forms of nephrotic syndrome or nine healthy controls.In vitro experiments were conducted to investigate if nephrin affects autocrine signaling through insulin receptor A (IRA) and B (IRB), which are both expressed in human podocytes and pancreatic islets. Coimmunoprecipitation of nephrin and IRB but not IRA was observed and required IR phosphorylation. Nephrin per se was sufficient to induce phosphorylation of p70S6K in an phosphatidylinositol 3-kinase-dependent but IR/Src-independent manner, which was not augmented by exogenous insulin. These results suggest a role for nephrin as an independent modulator of podocyte and pancreatic β-cell nutrient sensing in the fasting state and the potential of nephrin as a drug target in diabetes.

Kin of IRRE-like Protein 2 Is a Phosphorylated Glycoprotein That Regulates Basal Insulin Secretion

Direct interactions among pancreatic β-cells via cell surface proteins inhibit basal and enhance stimulated insulin secretion. Here, we functionally and biochemically characterized Kirrel2, an immunoglobulin superfamily protein with β-cell-specific expression in the pancreas. Our results show that Kirrel2 is a phosphorylated glycoprotein that co-localizes and interacts with the adherens junction proteins E-cadherin and β-catenin in MIN6 cells. We further demonstrate that the phosphosites Tyr(595-596) are functionally relevant for the regulation of Kirrel2 stability and localization. Analysis of the extracellular and intracellular domains of Kirrel2 revealed that it is cleaved and shed from MIN6 cells and that the remaining membrane spanning cytoplasmic domain is processed by γ-secretase complex. Kirrel2 knockdown with RNA interference in MIN6 cells and ablation of Kirrel2 from mice with genetic deletion resulted in increased basal insulin secretion from β-cells, with no immediate influence on stimulated insulin secretion, total insulin content, or whole body glucose metabolism. Our results show that in pancreatic β-cells Kirrel2 localizes to adherens junctions, is regulated by multiple post-translational events, including glycosylation, extracellular cleavage, and phosphorylation, and engages in the regulation of basal insulin secretion.

Review series: The cell biology of renal filtration

The function of the kidney, filtering blood and concentrating metabolic waste into urine, takes place in an intricate and functionally elegant structure called the renal glomerulus. Normal glomerular function retains circulating cells and valuable macromolecular components of plasma in blood, resulting in urine with just trace amounts of proteins. Endothelial cells of glomerular capillaries, the podocytes wrapped around them, and the fused extracellular matrix these cells form altogether comprise the glomerular filtration barrier, a dynamic and highly selective filter that sieves on the basis of molecular size and electrical charge. Current understanding of the structural organization and the cellular and molecular basis of renal filtration draws from studies of human glomerular diseases and animal models of glomerular dysfunction.

Nephrin, a transmembrane protein, is involved in pancreatic beta-cell survival signaling

Nephrin, a cell surface signaling receptor, regulates podocyte function in health and disease. We study the role of nephrin in β-cell survival signaling. We report that in mouse islet β-cells and the mouse pancreatic beta-cell line (βTC-6 cells) nephrin is associated and partly co-localized with PI3-kinase. Incubation of cells with functional anti-nephrin antibodies induced nephrin clustering at the plasma membrane, nephrin phosphorylation and recruitment of PI3-kinase to nephrin thus resulting in increased PI3K-dependent Akt phosphorylation and augmented phosphorylation/inhibition of pro-apoptotic Bad and FoxO. Nephrin silencing abolished Akt activation and increased susceptibility of cells to apoptosis. High glucose impaired nephrin signaling, increased nephrin internalization and up-regulated PKCα expression. Interestingly, a marked decrease in nephrin expression and phosphorylated Akt was observed in pancreatic islets of db/db lepr-/- diabetic mice. Our findings revealed that nephrin is involved in β-cell survival and suggest that glucose-induced changes in nephrin signaling may contribute to gradual pancreatic β-cell loss in type 2 diabetes.

Podocyte proteins in congenital and minimal change nephrotic syndrome

BACKGROUND

Podocyte foot process effacement is a uniform finding in kidneys with heavy proteinuria. Its molecular mechanisms, however, are unsolved. We analyzed the expression of podocyte proteins in two kidney disorders: Congenital nephrotic syndrome of the Finnish type (CNF) and minimal change nephrotic syndrome (MCNS).

METHODS

Immunoperoxidase and immunofluorescence stainings were used to semiquantitatively analyze the expression of 13 and 4 podocyte proteins from different cellular compartments in CNF and MCNS, respectively.

RESULTS

The expression of a major slit diaphragm (SD) protein, Neph 1, showed a 46-fold decrease (p < 0.0001) in CNF kidneys as compared to controls. The three cytosolic adaptor proteins, podocin, NCK1/2, CD2AP, connecting SD proteins to the actin cytoskeleton were slightly upregulated (1.1-fold, 1.4-fold, and 3.3-fold, respectively). Also, the staining of the two actin-regulator proteins, ACTN4 and INF2, was modestly increased (2.2-fold and 1.7-fold, respectively, p < 0.0001). Staining for α3-integrin showed 1.9-fold increase (p < 0.0001) indicating that the major podocyte anchoring complex, α3β1, was well preserved in CNF glomeruli. In contrast to CNF kidneys, Neph1 FAT1, ACTN4, and CD2AP were quite normally expressed in proteinuric and non-proteinuric MCNS kidneys.

CONCLUSION

CNF kidneys lacking nephrin show decreased expression of other SD proteins but not cytosolic podocyte proteins involved in the foot process architecture or function. In MCNS kidneys, these changes in expression were not observed.

Functions of the podocyte proteins nephrin and Neph3 and the transcriptional regulation of their genes

Nephrin and Neph-family proteins [Neph1–3 (nephrin-like 1–3)] belong to the immunoglobulin superfamily of cell-adhesion receptors and are expressed in the glomerular podocytes. Both nephrin and Neph-family members function in cell adhesion and signalling, and thus regulate the structure and function of podocytes and maintain normal glomerular ultrafiltration. The expression of nephrin and Neph3 is altered in human proteinuric diseases emphasizing the importance of studying the transcriptional regulation of the nephrin and Neph3 genes NPHS1 (nephrosis 1, congenital, Finnish type) and KIRREL2 (kin of IRRE-like 2) respectively. The nephrin and Neph3 genes form a bidirectional gene pair, and they share transcriptional regulatory mechanisms. In the present review, we summarize the current knowledge of the functions of nephrin and Neph-family proteins and transcription factors and agents that control nephrin and Neph3 gene expression.

Identification and characterization of novel Kirrel isoform during myogenesis

Somatic cell fusion is an essential component of skeletal muscle development and growth and repair from injury. Additional cell types such as trophoblasts and osteoclasts also require somatic cell fusion events to perform their physiological functions. Currently we have rudimentary knowledge on molecular mechanisms regulating somatic cell fusion events in mammals. We therefore investigated during in vitro murine myogenesis a mammalian homolog, Kirrel, of the Drosophila Melanogaster genes Roughest (Rst) and Kin of Irre (Kirre) which regulate somatic muscle cell fusion during embryonic development. Our results demonstrate the presence of a novel murine Kirrel isoform containing a truncated cytoplasmic domain which we term Kirrel B. Protein expression levels of Kirrel B are inverse to the occurrence of cell fusion events during in vitro myogenesis which is in stark contrast to the expression profile of Rst and Kirre during myogenesis in Drosophila. Furthermore, chemical inhibition of cell fusion confirmed the inverse expression pattern of Kirrel B protein levels in relation to cell fusion events. The discovery of a novel Kirrel B protein isoform during myogenesis highlights the need for more thorough investigation of the similarities and potential differences between fly and mammals with regards to the muscle cell fusion process.

Glomerulopathy and mutations in NPHS1 and KIRREL2 in soft-coated Wheaten Terrier dogs

Dogs of the soft-coated wheaten terrier breed (SCWT) are predisposed to adult-onset, genetically complex, protein-losing nephropathy (average onset age = 6.3 ± 2.0 years). A genome-wide association study using 62 dogs revealed a chromosomal region containing three statistically significant SNPs (p(raw) ≤ 4.13 × 10(-8); p(genome) ≤ 0.005) when comparing DNA samples from affected and geriatric (≥14 years) unaffected SCWTs. Sequencing of candidate genes in the region revealed single nucleotide changes in each of two closely linked genes, NPHS1 and KIRREL2, which encode the slit diaphragm proteins nephrin and Neph3/filtrin, respectively. In humans, mutations in nephrin and decreased expression of Neph3 are associated with podocytopathy and protein-losing nephropathy. The base substitutions change a glycine to arginine in the fibronectin type 3 domain of nephrin and a proline to arginine in a conserved proline-rich region in Neph3. These novel mutations are not described in other species, nor were they found in 550 dogs of 105 other breeds, except in 3 dogs, including an affected Airedale terrier, homozygous for both substitutions. Risk for nephropathy is highest in dogs homozygous for the mutations (OR = 9.06; 95 % CI = 4.24-19.35). This is the first molecular characterization of an inherited podocytopathy in dogs and may serve as a model for continued studies of complex genetic and environmental interactions in glomerular disease.

Regulation of nephrin gene by the Ets transcription factor, GA-binding protein

BACKGROUND

Transcription factor GA-binding protein (GABP) is suggested to be involved in the formation of the neuromuscular junctions by regulating the transcription of synapse genes. Interestingly, neurons and podocytes share molecular and functional similarities that led us to investigate the expression and function of GABP in podocytes and its role in transcriptional regulation of nephrin, the key molecule of the podocyte slit diaphragm that is essential for normal glomerular ultrafiltration.

METHODS

The expression and localization of GABP in the rat and human kidney as well as in human embryonic kidney A293 cells and undifferentiated and differentiated human podocytes were analysed by immunoblotting and immunostaining. The role of GABP in activating the nephrin promoter was investigated by reporter gene assay and site-directed mutagenesis of the GABP-binding elements, and the interaction of GABP with the nephrin promoter was analysed by chromatin immunoprecipitation. The function of GABP in podocytes was studied by knocking down GABPα in differentiated human podocytes using lentiviral shRNA targeting GABPα.

RESULTS

GABP is expressed in the nuclei in rat and human glomeruli. In addition, in A293 cells and undifferentiated and differentiated human podocytes, GABP highly enriches in the nucleus. GABP activates and binds nephrin proximal promoter and Ets sites are essential for this activity. Knock-down of GABPα stimulates apoptosis in cultured podocytes.

CONCLUSIONS

The results show that GABP is expressed in podocytes and is involved in the regulation of nephrin gene expression. Furthermore, GABP may be important in the maintenance of podocyte function by regulating apoptosis.

Dynamin-mediated Nephrin phosphorylation regulates glucose-stimulated insulin release in pancreatic beta cells

We have previously demonstrated a role for Nephrin in glucose stimulated insulin release (GSIR). We now hypothesize that Nephrin phosphorylation is required for GSIR and that Dynamin influences Nephrin phosphorylation and function. MIN6-C3 Nephrin-deficient pancreatic beta cells and human islets were transfected with WT-Nephrin or with a mutant Nephrin in which the tyrosine residues responsible for SH2 domain binding were substituted with phenylalanine (3YF-Nephrin). GSIR and live images of Nephrin and vesicle trafficking were studied. Immunoprecipitation experiments and overexpression of WT-Dynamin or dominant negative Dynamin mutant (K44A-Dynamin) in WT-Nephrin, 3YF-Nephrin, or Nephrin siRNA-transfected cells were utilized to study Nephrin-Dynamin interaction. In contrast to WT-Nephrin or to single tyrosine mutants, 3YF-Nephrin did not positively affect GSIR and led to impaired cell-cell contacts and vesicle trafficking. K44A-Dynamin prevented the effect of Nephrin on GSIR in the absence of protein-protein interaction between Nephrin and Dynamin. Nephrin gene silencing abolished the positive effects of WT-Dynamin on GSIR. The effects of protamine sulfate and vanadate on Nephrin phosphorylation and GSIR were studied in MIN6 cells and human islets. WT-Nephrin phosphorylation after glucose occurred at Tyr-1176/1193 and resulted in improved GSIR. On the contrary, protamine sulfate-induced phosphorylation at Tyr-1176/1193/1217 was associated with Nephrin degradation and impaired GSIR. Vanadate, which prevented Nephrin dephosphorylation after glucose stimulation, improved GSIR in human islets and MIN6 cells. In conclusion, Dynamin-dependent Nephrin phosphorylation occurs in response to glucose and is necessary for Nephrin-mediated augmentation of GSIR. Pharmacological modulation of Nephrin phosphorylation may thus facilitate pancreatic beta cell function.

Transcription of nephrin-Neph3 gene pair is synergistically activated by WT1 and NF-κB and silenced by DNA methylation

BACKGROUND

Nephrin and Neph3 are homologous molecules expressed in the podocyte slit diaphragms that are essential for normal glomerular ultrafiltration. Nephrin and Neph3 genes form a bidirectional gene pair suggesting that they may share key features in their regulation. We investigated if nephrin and Neph3 genes have similar mechanisms in their transcriptional regulation focussing on transcription factor Wilms' tumour 1 (WT1) and nuclear factor-κB (NF-κB) and DNA methylation.

METHODS

Transcriptional regulation of nephrin and Neph3 by WT1 and NF-κB was analysed by overexpression studies, reporter gene assay and chromatin immunoprecipitation using A293 cells and cultured podocytes. The interaction between WT1 and NF-κB was studied by co-immunoprecipitation. The effect of NF-κB activator tumour necrosis factor-α (TNF-α) with or without NF-κB pathway inhibitor (BAY 11-7082) on nephrin and Neph3 messenger RNA (mRNA) expression and on cellular distribution of NF-κB was determined by quantitative polymerase chain reaction (PCR) and immunostaining, respectively. The role of DNA methylation in regulating nephrin and Neph3 genes was studied by demethylating agent (5-aza-2'-deoxycytidine) treatment and quantitative PCR.

RESULTS

WT1 and NF-κB interact with nephrin and Neph3 promoter and cooperatively regulate nephrin and Neph3. The cooperation was further supported by the physical interaction between WT1 and NF-κB. TNF-α increased nephrin and Neph3 mRNA expression and this effect was mediated by NF-κB. Furthermore, DNA methylation played a role in silencing nephrin and Neph3 expression in a cell-type and differentiation stage-dependent manner.

CONCLUSION

These results provide novel insights into the transcriptional regulation of nephrin and Neph3 genes and indicate that nephrin and Neph3 share the same mechanisms in their regulation.