Identification and characterization of a glomerular-specific promoter from the human nephrin gene

Adeno-associated virus gene therapy prevents progression of kidney disease in genetic models of nephrotic syndrome

Gene therapy for kidney diseases has proven challenging. Adeno-associated virus (AAV) is used as a vector for gene therapy targeting other organs, with particular success demonstrated in monogenic diseases. We aimed to establish gene therapy for the kidney by targeting a monogenic disease of the kidney podocyte. The most common cause of childhood genetic nephrotic syndrome is mutations in the podocyte gene NPHS2, encoding podocin. We used AAV-based gene therapy to rescue this genetic defect in human and mouse models of disease. In vitro transduction studies identified the AAV-LK03 serotype as a highly efficient transducer of human podocytes. AAV-LK03-mediated transduction of podocin in mutant human podocytes resulted in functional rescue in vitro, and AAV 2/9-mediated gene transfer in both the inducible podocin knockout and knock-in mouse models resulted in successful amelioration of kidney disease. A prophylactic approach of AAV 2/9 gene transfer before induction of disease in conditional knockout mice demonstrated improvements in albuminuria, plasma creatinine, plasma urea, plasma cholesterol, histological changes, and long-term survival. A therapeutic approach of AAV 2/9 gene transfer 2 weeks after disease induction in proteinuric conditional knock-in mice demonstrated improvement in urinary albuminuria at days 42 and 56 after disease induction, with corresponding improvements in plasma albumin. Therefore, we have demonstrated successful AAV-mediated gene rescue in a monogenic renal disease and established the podocyte as a tractable target for gene therapy approaches.

The ShGlomAssay Combines High-Throughput Drug Screening With Downstream Analyses and Reveals the Protective Role of Vitamin D3 and Calcipotriol on Podocytes

Chronic kidney disease (CKD) is a major public health burden affecting more than 500 million people worldwide. Podocytopathies are the main cause for the majority of CKD cases due to pathogenic morphological as well as molecular biological alterations of postmitotic podocytes. Podocyte de-differentiation is associated with foot process effacement subsequently leading to proteinuria. Since currently no curative drugs are available, high throughput screening methods using a small number of animals are a promising and essential tool to identify potential drugs against CKD in the near future. Our study presents the implementation of the already established mouse GlomAssay as a semi-automated high-throughput screening method-shGlomAssay-allowing the analysis of several hundreds of FDA-verified compounds in combination with downstream pathway analysis like transcriptomic and proteomic analyses from the same samples, using a small number of animals. In an initial prescreening we have identified vitamin D3 and its analog calcipotriol to be protective on podocytes. Furthermore, by using RT-qPCR, Western blot, and RNA sequencing, we found that mRNA and protein expression of nephrin, the vitamin D receptor and specific podocyte markers were significantly up-regulated due to vitamin D3- and calcipotriol-treatment. In contrast, kidney injury markers were significantly down-regulated. Additionally, we found that vitamin D3 and calcipotriol have had neither influence on the expression of the miR-21 and miR-30a nor on miR-125a/b, a miRNA described to regulate the vitamin D receptor. In summary, we advanced the established mouse GlomAssay to a semi-automated high-throughput assay and combined it with downstream analysis techniques by using only a minimum number of animals. Hereby, we identified the vitamin D signaling pathway as podocyte protective and to be counteracting their de-differentiation.

Thrombospondin Type 1 Domain-Containing 7A Localizes to the Slit Diaphragm and Stabilizes Membrane Dynamics of Fully Differentiated Podocytes

BACKGROUND

About 3%-5% of adults with membranous nephropathy have autoantibodies directed against thrombospondin type 1 domain-containing 7A (THSD7A), a podocyte-expressed transmembrane protein. However, the temporal and spatial expression of THSD7A and its biologic function for podocytes are unknown, information that is needed to understand the effects of THSD7A autoantibodies in this disease.

METHODS

Using a variety of microscopic techniques, we analyzed THSD7A localization in postnatal, adult, and autoantibody-injected mice as well as in human podocytes. We also analyzed THSD7A function in human podocytes using confocal microscopy; Western blotting; and adhesion and migration assays.

RESULTS

We found that THSD7A expression begins on glomerular vascularization with slit diaphragm formation in development. THSD7A localizes to the basal aspect of foot processes, closely following the meanders of the slit diaphragm in human and mice. Autoantibodies binding to THSD7A localize to the slit diaphragm. In human podocytes, THSD7A expression is accentuated at filopodia and thin arborized protrusions, an expression pattern associated with decreased membrane activity of cytoskeletal regulators. We also found that, phenotypically, THSD7A expression in human podocytes is associated not only with increases in cell size, enhanced adhesion, and reduced detachment from collagen type IV-coated plates but also, with decreased ability to migrate.

CONCLUSIONS

Our findings suggest that THSD7A functions as a foot process protein involved in the stabilization of the slit diaphragm of mature podocytes and that autoantibodies to THSD7A, on the basis of their localization, might structurally and functionally alter the slit diaphragm's permeability to protein.

Podocyte-specific expression of Cre recombinase promotes glomerular basement membrane thickening

Conditional gene targeting using Cre recombinase has offered a powerful tool to modify gene function precisely in defined cells/tissues and at specific times. However, in mammalian cells, Cre recombinase can be genotoxic. The importance of including Cre-expressing control mice to avoid misinterpretation and to maximize the validity of the experimental results has been increasingly recognized. While studying the role of podocytes in the pathogenesis of glomerular basement membrane (GBM) thickening, we used Cre recombinase driven by the podocyte-specific podocin promoter (NPHS2-Cre) to generate a conditional knockout. By conventional structural and functional measures (histology by periodic acid-Schiff staining, albuminuria, and plasma creatinine), we did not detect significant differences between NPHS2-Cre transgenic and wild-type control mice. However, surprisingly, the group that expressed Cre transgene alone developed signs of podocyte toxicity, including marked GBM thickening, loss of normal foot process morphology, and reduced Wilms tumor 1 expression. GBM thickening was characterized by altered expression of core structural protein laminin isoform α₅β₂γ₁. RNA sequencing analysis of extracted glomeruli identified 230 genes that were significant and differentially expressed (applying a q < 0.05-fold change ≥ ±2 cutoff) in NPHS2-Cre mice compared with wild-type control mice. Many biological processes were reflected in the RNA sequencing data, including regulation of the extracellular matrix and pathways related to apoptosis and cell death. This study highlights the importance of including the appropriate controls for potential Cre-mediated toxicity in conditional gene-targeting experiments. Indeed, omitting the Cre transgene control can result in critical errors during interpretation of experimental data.

Nephron, Wilms' tumor-1 (WT1), and synaptopodin expression in developing podocytes of mice

Newborn mouse glomeruli are still immature with a morphological feature of an early capillary loop stage, but infant mice do not manifest proteinuria. Little is known about the molecular mechanism whereby infant mice are resistant to proteinuria. Nephrin and synaptopodin are crucial for slit diaphragm and foot process (FP) formation for avoiding proteinuria. Nephrin tyrosine phosphorylation means a transient biological signaling required for FP repair or extension during nephrotic disease. Using an immunohistochemical technique, we examined the natural course of nephrin, Wilms’ tumor-1 (WT1) and synaptopodin at 16.5 days of embryonic age (E16.5d) and E19.5d, 7 days of post-neonatal age (P7d) and P42d during renal development of mice. As a result, nephrin and synaptopodin were detected at E19.5d in S-shaped bodies. WT1, a transcriptional factor for nephrin, was detected in nucleus in podocyte-like cells in all stages. Nephrin tyrosine phosphorylation was evident in glomeruli at P7d, and this was associated with an early-stage of FP extension. Inversely, nephrin phosphorylation became faint at P42d, along with maturated FP. Based on the present results, we suggest the sequential molecular mechanism to protect growing mice from proteinuria: (i) WT1-induced nephrin production by podocytes in S-shaped bodies at E19.5d; (ii) Synchronized induction of synaptopodin at the same period; and (iii) FP extension is initiated at a milk-suckling stage under a nephrin tyrosine-phosphorylated condition, while it is arrested at an adult stage, associated with a loss of nephrin-based signaling.

A novel assay to assess the effect of pharmaceutical compounds on the differentiation of podocytes

BACKGROUND AND PURPOSE

Therapeutic options for treating glomerulopathies, the main cause of chronic kidney disease, are limited. Podocyte dedifferentiation is a major event in the pathogenesis of glomerulopathies. The goal of the present study was, therefore, to develop an assay to monitor podocyte differentiation suitable for compound screening.

EXPERIMENTAL APPROACH

We isolated and cultured glomeruli from transgenic mice, expressing cyan fluorescent protein (CFP) under the control of the promoter of nephrin, a marker of podocyte differentiation. Mean CFP fluorescence intensity per glomerulus (MFG) was determined by summation of all glomerular voxels from confocal z-stacks in the absence and presence of pharmaceutical compounds.

KEY RESULTS

In untreated cultured glomeruli, MFG remained fairly stable during the first 5 days, when foot processes were already effaced, and the level of many podocyte-specific proteins was only mildly affected, as revealed by proteomics. Between day 6 and 9, MFG decreased to almost zero. The decrease in MFG was paralleled by a decrease in CFP and nephrin expression, as determined by RT-PCR, western blots and proteomics. Puromycin aminonucleoside (PAN), which damages podocytes, concentration-dependently induced a complete loss of MFG. Dexamethasone (25 μM) and pioglitazone (10 μM) markedly attenuated the effect of 0.6 μg·mL^-1 PAN on MFG.

CONCLUSION AND IMPLICATIONS

In summary, we established a novel assay to assess the effect of pharmaceutical compounds on the differentiation of podocytes in situ. Our assay is suitable for compound screening to identify drugs for the treatment of glomerulopathies.

Overexpression of Mafb in podocytes protects against diabetic nephropathy

We previously showed that the transcription factor Mafb is essential for podocyte differentiation and foot process formation. Podocytes are susceptible to injury in diabetes, and this injury leads to progression of diabetic nephropathy. In this study, we generated transgenic mice that overexpress Mafb in podocytes using the nephrin promoter/enhancer. To examine a potential pathogenetic role for Mafb in diabetic nephropathy, Mafb transgenic mice were treated with either streptozotocin or saline solution. Diabetic nephropathy was assessed by renal histology and biochemical analyses of urine and serum. Podocyte-specific overexpression of Mafb had no effect on body weight or blood glucose levels in either diabetic or control mice. Notably, albuminuria and changes in BUN levels and renal histology observed in diabetic wild-type animals were ameliorated in diabetic Mafb transgenic mice. Moreover, hyperglycemia-induced downregulation of Nephrin was mitigated in diabetic Mafb transgenic mice, and reporter assay results suggested that Mafb regulates Nephrin directly. Mafb transgenic glomeruli also overexpressed glutathione peroxidase, an antioxidative stress enzyme, and levels of the oxidative stress marker 8-hydroxydeoxyguanosine decreased in the urine of diabetic Mafb transgenic mice. Finally, Notch2 expression increased in diabetic glomeruli, and this effect was enhanced in diabetic Mafb transgenic glomeruli. These data indicate Mafb has a protective role in diabetic nephropathy through regulation of slit diaphragm proteins, antioxidative enzymes, and Notch pathways in podocytes and suggest that Mafb could be a therapeutic target.

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.

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.

Mouse models to study kidney development, function and disease

PURPOSE OF REVIEW

The mouse is the most widely used model organism to study gene function in the kidney in vivo. Here we review recent advances in technologies to manipulate the mouse genome and gene function to study renal biology. We discuss strengths and weaknesses of the approaches and provide examples in which they have been used to address renal questions. In addition, we provide a summary of the available resources of mouse tools and gene-targeting consortia.

RECENT FINDINGS

Although conventional gene-targeting and spontaneous genetic mutations in mice have provided great insights into kidney function over several decades, the addition of powerful renal-specific gene-targeting tools and the advent of RNA technologies to manipulate gene function in vivo allow investigators to address research questions more precisely in the laboratory. Together with the establishment of multiple international consortia to target all the genes in the mouse genome and the development of gene trap and N-ethyl-N-nitrosourea resources, genetic manipulation in mice has become more efficient.

SUMMARY

The availability of newer technologies and tremendous resources for mouse strains and reagents ensures that the mouse will remain a key model organism to study renal function.

[Podocyte research in rheumatic diseases]

Podocytes are glomerular visceral epithelial cells, which function as molecular sieve with foot process (FT) and slit diaphragm (SD) spanning FT, not to allow high molecular weight protein to be filtrated through glomerular capillary loop. Pathological proteinuria is caused by discoordinated tertiary podocyte structure such as disappearance of FT and/or SD, and irreversible glomeular sclerosis is caused by podocyte loss due to cell death and/or detachment from capillary wall. With recent advance of nephrological research technology such as podocyte cell culture system, genetically engineered transgenic mice with podocyte-specific regulation of gene expression, podocyte-associated biomarkers, the new isolation method of glomeruli, laser capture microdissection, multiphoton imaging and extracellular flux analyzer, new findings of pathogenesis of glomerular lesions will be expected, not only in primary glomerulonephritis, but also in secondary glomerulonephritis or glomerulopathy due to rheumatic diseases.

Podocyte-specific VEGF down-regulation and pathophysiological development

It is well-known that vascular endothelial growth factor (VEGF) plays a key role in development and pathology, but its function in normal adult tissues is rarely understood. Increased use of anti-angiogenic therapies targeting VEGF in human pathologies have shown more and more adverse effects. In this report, a conditional expression model (Tet-On system) was used to down-regulate podocyte VEGF in adult mice, which resulted in many kidney problems, characterized by glomerular morphological changes, proteinuria, reduced water consumption and urination, increased urine electro-conductivity, as well as high susceptibility to BSA stress. Our findings indicated that podocyte-specific VEGF down-regulation resulted in poor kidney performance and led mice to be more susceptible to further kidney damages.

Podocyte injury and albuminuria in mice with podocyte-specific overexpression of the Ste20-like kinase, SLK

SLK expression and activity are increased during kidney development and recovery from renal ischemia-reperfusion injury. In cultured cells, SLK promotes F-actin destabilization as well as apoptosis, partially via the p38 kinase pathway. To better understand the effects of SLK in vivo, a transgenic mouse model was developed where SLK was expressed in a podocyte-specific manner using the mouse nephrin promoter. Offspring of four founder mice carried the SLK transgene. Among male transgenic mice, 66% developed albuminuria at approximately 3 months of age, and the albuminuric mice originated from three of four founders. Overall, the male transgenic mice demonstrated about fivefold greater urinary albumin/creatinine compared with male non-transgenic mice. Transgenic and non-transgenic female mice did not develop albuminuria, suggesting that females were less susceptible to glomerular filtration barrier damage than their male counterparts. In transgenic mice, electron microscopy revealed striking podocyte injury, including poorly formed or effaced foot processes, and edematous and vacuolated cell bodies. By immunoblotting, nephrin expression was decreased in glomeruli of the albuminuric transgenic mice. Activation-specific phosphorylation of p38 was increased in transgenic mice compared with non-transgenic animals. Glomeruli of SLK transgenic mice showed around 30% fewer podocytes, and a reduction in F-actin compared with control glomeruli. Thus, podocyte SLK overexpression in vivo results in injury and podocyte loss, consistent with the effects of SLK in cultured cells.

Kidney dendritic cell activation is required for progression of renal disease in a mouse model of glomerular injury

The progression of kidney disease to renal failure correlates with infiltration of mononuclear immune cells into the tubulointerstitium. These infiltrates contain macrophages, DCs, and T cells, but the role of each cell type in disease progression is unclear. To investigate the underlying immune mechanisms, we generated transgenic mice that selectively expressed the model antigens ovalbumin and hen egg lysozyme in glomerular podocytes (NOH mice). Coinjection of ovalbumin-specific transgenic CD8+ CTLs and CD4+ Th cells into NOH mice resulted in periglomerular mononuclear infiltrates and inflammation of parietal epithelial cells, similar to lesions frequently observed in human chronic glomerulonephritis. Repetitive T cell injections aggravated infiltration and caused progression to structural and functional kidney damage after 4 weeks. Mechanistic analysis revealed that DCs in renal lymph nodes constitutively cross-presented ovalbumin and activated CTLs. These CTLs released further ovalbumin for CTL activation in the lymph nodes and for simultaneous presentation to Th cells by distinct DC subsets residing in the kidney tubulointerstitium. Crosstalk between tubulointerstitial DCs and Th cells resulted in intrarenal cytokine and chemokine production and in recruitment of more CTLs, monocyte-derived DCs, and macrophages. The importance of DCs was established by the fact that DC depletion rapidly resolved established kidney immunopathology. These findings demonstrate that glomerular antigen-specific CTLs and Th cells can jointly induce renal immunopathology and identify kidney DCs as a mechanistic link between glomerular injury and the progression of kidney disease.

The alternatively spliced anti-angiogenic family of VEGF isoforms VEGFxxxb in human kidney development

BACKGROUND/AIM

Vascular endothelial growth factor (VEGF), required for renal development, is generated by alternative splicing of 8 exons to produce two families, pro-angiogenic VEGF(xxx), formed by proximal splicing in exon 8 (exon 8a), and anti-angiogenic VEGF(xxx)b, generated by distal splicing in exon 8 (exon 8b). VEGF(165)b, the first described exon 8b-containing isoform, antagonises VEGF(165) and is anti-angiogenic in vivo.

METHODS

Using VEGF(xxx)b-specific antibodies, we investigated its expression quantitatively and qualitatively in developing kidney, and measured the effect of VEGF(165)b on renal endothelial and epithelial cells.

RESULTS

VEGF(xxx)b formed 45% of total VEGF protein in adult renal cortex, and VEGF(165)b does not increase glomerular endothelial cell permeability, it inhibits migration, and is cytoprotective for podocytes. During renal development, VEGF(xxx)b was expressed in the condensed vesicles of the metanephros, epithelial cells of the comma-shaped bodies, invading endothelial cells and epithelial cells of the S-shaped body, and in the immature podocytes. Expression reduced as the glomerulus matured.

CONCLUSION

These results show that the anti-angiogenic VEGF(xxx)b isoforms are highly expressed in adult and developing renal cortex, and suggest that the VEGF(xxx)b family plays a role in glomerular maturation and podocyte protection by regulating the pro-angiogenic pro-permeability properties of VEGF(xxx) isoforms.

Molecular mechanisms of epithelial cell-specific expression and regulation of the human anion exchanger (pendrin) gene

Pendrin, a Cl(-)/anion exchanger encoded by the gene PDS, is highly expressed in the kidney, thyroid, and inner ear epithelia and is essential for bicarbonate secretion, iodide accumulation, and endolymph ion balance, respectively. This study aimed to define promoter regulatory elements essential for renal, thyroid, and inner ear epithelial cell-specific expression of human PDS (hPDS) and to explore the effect of ambient pH and aldosterone on hPDS promoter activity. Endogenous pendrin mRNA and protein were detected in renal HEK293, thyroid LA2, and inner ear VOT36 epithelial cell lines, but not in the fibroblast cell line, NIH3T3. A 4.2-kb hPDS 5'-flanking DNA sequence and consecutive 5'-deletion products were cloned into luciferase reporter vectors and transiently transfected into the above cell lines. Distinct differences in expression/activity of deduced positive/negative regulatory elements within the hPDS promoter between HEK293, LA2, and VOT36 cells were demonstrated, with only basal activity in NIH3T3 cells. Acidic pH (7.0-7.1) decreased and alkaline pH (7.6-7.7) increased hPDS promoter activity in transfected HEK293 and VOT36, but not in LA2 cells. Aldosterone (10(-8) M) reduced hPDS promoter activity in HEK293 but had no effect in LA2 and VOT36 cells. These pH and aldosterone-induced effects on the hPDS promoter occurred within 96-bp and 89-bp regions, respectively, which likely contain distinct response elements to these modulators. Acidic pH and aldosterone decreased, and alkaline pH increased, endogenous pendrin mRNA level in HEK293 cells. In conclusion, pendrin-mediated HCO3(-) secretion in the renal tubule and anion transport in the endolymph may be regulated transcriptionally by systemic pH and aldosterone.

Nephrin--a unique structural and signaling protein of the kidney filter

Since the discovery of nephrin, the first integral component of the slit diaphragm to be identified, the podocyte slit pore has become a major focus in research concerning the glomerular filtration barrier. Nephrin is a central component of the glomerular ultrafilter, with both structural and signaling functions. The extracellular domain of nephrin and other components of the slit diaphragm seem to form a porous molecular sieve. The intracellular domain of nephrin is associated with linker proteins, such as CD2-associated protein and Nck proteins that can connect nephrin to the actin cytoskeleton. Alterations in nephrin interactions with other proteins during development or injury can lead to complex signaling reactions aimed at establishing or restoring the filter function.

Deletion of protein kinase C-beta isoform in vivo reduces renal hypertrophy but not albuminuria in the streptozotocin-induced diabetic mouse model

The protein kinase C (PKC)-beta isoform has been implicated to play a pivotal role in the development of diabetic kidney disease. We tested this hypothesis by inducing diabetic nephropathy in PKC-beta-deficient (PKC-beta(-/-)) mice. We studied nondiabetic and streptozotocin-induced diabetic PKC-beta(-/-) mice compared with appropriate 129/SV wild-type mice. After 8 weeks of diabetes, the high-glucose-induced renal and glomerular hypertrophy, as well as the increased expression of extracellular matrix proteins such as collagen and fibronectin, was reduced in PKC-beta(-/-) mice. Furthermore, the high-glucose-induced expression of the profibrotic cytokine transforming growth factor (TGF)-beta1 and connective tissue growth factor were significantly diminished in the diabetic PKC-beta(-/-) mice compared with diabetic wild-type mice, suggesting a role of the PKC-beta isoform in the regulation of renal hypertrophy. Notably, increased urinary albumin-to-creatinine ratio persisted in the diabetic PKC-beta(-/-) mice. The loss of the basement membrane proteoglycan perlecan and the podocyte protein nephrin in the diabetic state was not prevented in the PKC-beta(-/-) mice as previously demonstrated in the nonalbuminuric diabetic PKC-alpha(-/-) mice. In summary, the differential effects of PKC-beta deficiency on diabetes-induced renal hypertrophy and albuminuria suggest that PKC-beta contributes to high-glucose-induced TGF-beta1 expression and renal fibrosis, whereas perlecan, as well as nephrin, expression and albuminuria is regulated by other signaling pathways.

Nephrin loss in experimental diabetic nephropathy is prevented by deletion of protein kinase C alpha signaling in-vivo

Albuminuria in diabetic nephropathy is due to endothelial dysfunction, a loss of negative charges in the basement membrane, and changes a of the slit-membrane diaphragm composition. We have recently shown that protein kinase C alpha (PKCalpha)-deficient mice are protected against the development of albuminuria under diabetic conditions. We here tested the hypothesis that PKCalpha mediates the hyperglycemia-induced downregulation of the slit-diaphragm protein nephrin. After 8 weeks of streptozotocin (STZ)-induced hyperglycemia the expression of glomerular nephrin was significantly reduced. In contrast, other slit-diaphragm proteins such as podocin and CD2AP were unaltered in diabetic state. In PKCalpha-/- mice, hyperglycemia-induced downregulation of nephrin was prevented. Podocin and CD2AP remained unchanged. In addition, the nephrin messenger RNA expression was also reduced in hyperglycemic wild-type mice but remained unaltered in PKCalpha-/- mice. We postulate that the underlying mechanism of the hyperglycemia-induced regulation of various proteins of the glomerular filtration barrier is a PKCalpha-dependent regulation of the Wilms' Tumor Suppressor (WT1) which previously has been shown to act as a direct transcription factor on the nephrin promoter. Our data suggest that PKCalpha activation may be an important intracellular signaling pathway in the regulation of nephrin expression and glomerular albumin permeability in the diabetic state.

Pierson syndrome: a novel cause of congenital nephrotic syndrome

In this report, we describe a newborn infant who presented with congenital nephrotic syndrome and renal insufficiency, as well as bilateral microcoria. This constellation of findings is a hallmark of Pierson syndrome, a newly recognized genetic disorder that is caused by a deficiency of beta2 laminin in the basement membrane. Our patient demonstrated classic histopathologic findings of Pierson syndrome on renal biopsy, including absence of beta2 laminin on immunofluorescent staining, and genetic testing confirmed the diagnosis. We conclude that Pierson syndrome should be included in the differential diagnosis for congenital nephrotic syndrome, especially in patients with ocular abnormalities.

Direct effects of dexamethasone on human podocytes

Glucocorticoids are widely used in the treatment of human glomerular diseases, but their mode of action is poorly understood particularly in steroid-sensitive nephrotic syndrome, which is most common in childhood and is characterized by a lack of inflammation in the kidney. The podocyte is a key cell in the glomerulus in health and disease: until recently, human podocytes have been difficult to study in vitro. We have developed a conditionally immortalized human podocyte cell line transfected with a temperature-sensitive simian virus 40 transgene: when the transgene is inactivated in vitro, these cells adopt the phenotype of differentiated podocytes. We have used these cells to evaluate, using immunocytochemistry, reverse transcriptase-polymerase chain reaction, and Western blotting, direct effects of the glucocorticoid dexamethasone at concentrations designed to mimic in vivo therapeutic corticosteroid levels. Dexamethasone upregulated expression of nephrin and tubulin-alpha, and downregulated vascular endothelial growth factor. Effects on cell cycle were complex with downregulation of cyclin kinase inhibitor p21 and augmentation of podocyte survival, without any effect on apoptosis. We report cytokine production by human podocytes, especially interleukin (IL)-6 and -8; IL-6 expression was suppressed by dexamethasone. These potent direct effects on podocytes illustrate a novel mode of action of glucocorticoids and suggest potential new therapeutic strategies for glomerular disease.

Transcriptional Regulation of Nephrin Gene by Peroxisome Proliferator–Activated Receptor-γ Agonist: Molecular Mechanism of the Antiproteinuric Effect of Pioglitazone

The renoprotective potential of the peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonist pioglitazone was explored in an immune model of progressive nephropathy, passive Heymann nephritis (PHN), compared with that of an angiotensin II receptor antagonist, taken as standard therapy for renoprotection. PHN rats received orally vehicle, pioglitazone (10 mg/kg twice daily), or candesartan (1 mg/kg twice daily) from months 2 to 8. Pioglitazone reduced proteinuria as effectively as candesartan and limited renal functional and structural changes. Kidneys from untreated PHN rats showed lower nephrin mRNA and protein than controls, both restored by pioglitazone. The effect was seen both early and late during the course of the disease. Whether the antiproteinuric effect of pioglitazone could be due to its effect on nephrin gene transcription also was investigated. HK-2 cells were transfected with plasmids that harbor the luciferase gene under portions (2-kb or 325-bp) of human nephrin gene promoter that contain putative peroxisome proliferator-responsive elements (PPRE) and incubated with pioglitazone (10 muM). Transcriptional activity of luciferase gene was highly increased by pioglitazone, with the strongest expression achieved with the 325-bp fragment. Increase in luciferase activity was prevented by bisphenol A diglycidyl ether, a PPAR-gamma synthetic antagonist. Electrophoretic mobility shift assay experiments showed a direct interaction of PPAR/retinoid X receptor heterodimers to PPRE present in the enhancer region of the nephrin promoter. In conclusion, pioglitazone exerts an antiproteinuric effect in immune-mediated glomerulonephritis as angiotensin II receptor antagonist does. Enhancement of nephrin gene transcription through specific PPRE in its promoter discloses a novel mechanism of renoprotection for PPAR-gamma agonists.

Comparative promoter analysis allows de novo identification of specialized cell junction-associated proteins

Shared transcription factor binding sites that are conserved in distance and orientation help control the expression of gene products that act together in the same biological context. New bioinformatics approaches allow the rapid characterization of shared promoter structures and can be used to find novel interacting molecules. Here, these principles are demonstrated by using molecules linked to the unique functional unit of the glomerular slit diaphragm. An evolutionarily conserved promoter model was generated by comparative genomics in the proximal promoter regions of the slit diaphragm-associated molecule nephrin. Phylogenetic promoter fingerprints of known elements of the slit diaphragm complex identified the nephrin model in the promoter region of zonula occludens-1 (ZO-1). Genome-wide scans using this promoter model effectively predicted a previously unrecognized slit diaphragm molecule, cadherin-5. Nephrin, ZO-1, and cadherin-5 mRNA showed stringent coexpression across a diverse set of human glomerular diseases. Comparative promoter analysis can identify regulatory pathways at work in tissue homeostasis and disease processes.

UT-A urea transporter promoter, UT-Aalpha, targets principal cells of the renal inner medullary collecting duct

The urea transporters, UT-A1 and UT-A3, two members of the UT-A gene family, are localized to the terminal portion of the inner medullary collecting duct (IMCD). In this manuscript, we demonstrate that 4.2 kb of the 5'-flanking region of the UT-A gene (UT-Aalpha promoter) is sufficient to drive the IMCD-specific expression of a heterologous reporter gene, beta-galactosidase (beta-Gal), in transgenic mice. RT-PCR, immunoblotting, and immunohistochemistry demonstrate that within the kidney, transgene expression is confined to the terminal portion of the IMCD. Colocalization studies with aquaporin-2 show that expression is localized to the principal cells of the IMCD2 and IMCD3 regions. Utilizing beta-Gal activity assays, we further show that within the kidney, the beta-Gal transgene can be regulated by both water restriction and glucocorticoids, similar to the regulation of the endogenous UT-A gene. These results demonstrate that 4.2 kb of the UT-Aalpha promoter is sufficient to drive expression of a heterologous reporter gene in a tissue-specific and cell-specific fashion in transgenic mice.

PPARalpha activation upregulates nephrin expression in human embryonic kidney epithelial cells and podocytes by a dual mechanism

Nephrin is an important member of the glomerular ultrafiltration complex and changes in its expression are associated with severe proteinuria. In this study, we show that synthetic PPARalpha agonists, but not PPARgamma agonists, stimulate an increased nephrin mRNA and protein expression in cultures of human podocytes and A293 human embryonic kidney epithelial cells which are blocked by the PPARalpha antagonist Ru486. Furthermore, the PPARalpha agonists have an additive effect on the interleukin-1beta (IL-1beta)-induced nephrin upregulation. Luciferase-reporter assays reveal that human nephrin promoter activity is stimulated by the PPARalpha agonists. Neither IL-1beta nor TNFalpha alone has an effect on nephrin promoter activity suggesting that additional posttranscriptional regulatory events might be operative. The role of nephrin mRNA stability regulation was evaluated in cells treated with actinomycin D to stop further RNA transcription. In the presence of PPARalpha agonists, IL-1beta or TNFalpha, the decay of nephrin mRNA was drastically reduced thus arguing for an additional posttranscriptional mode of action. In summary, these data show that PPARalpha activation causes an increased nephrin expression by a dual action, on the one hand by stimulating nephrin promoter activity and on the other hand by reducing nephrin mRNA degradation. These findings may have importance for treatment strategies of renal diseases affecting the expression of nephrin and subsequently the proper action of the glomerular filtration apparatus.

Rapid isolation of glomeruli coupled with gene expression profiling identifies downstream targets in Pod1 knockout mice

Mouse mutations have provided tremendous insights into the molecular basis of renal and glomerular development. However, genes often play important roles during multiple stages of nephrogenesis, making it difficult to determine the role of a gene in a specific cell lineage such as the podocyte. Conditional gene targeting and chimeric analysis are two possible approaches to dissect the function of genes in specific cell populations. However, these are labor-intensive and costly and require the generation, validation, and analysis of additional transgenic lines. For overcoming these shortcomings and, specifically, for studying the role of gene function in developing glomeruli, a technique to isolate and purify glomeruli from murine embryos was developed. Combined with gene expression profiling, this method was used to identify differentially expressed genes in glomeruli from Pod1 knockout (KO) mice that die in the perinatal period with multiple renal defects. Glomeruli from early developing stages (late S-shape/early capillary loop) onward can be isolated successfully from wild-type and KO kidneys at 18.5 d postcoitus, and RNA can readily be obtained and used for genome-wide microarray analysis. With this approach, 3986 genes that are differently expressed between glomeruli from Pod1 KO and wild-type mice were identified, including a four-fold reduction of alpha 8 integrin mRNA in glomeruli from Pod1 KO mice that was confirmed by immunostaining. This procedure may be adapted to any transgenic strain, providing a rapid and efficient method to dissect the function of specific genes in glomerular development.

Molecular cloning and characterization of an endogenous antisense transcript of Nphs1

Mutations of NPHS1, the gene encoding the kidney glomerular filtration barrier protein nephrin, cause congenital nephrotic syndrome of the Finnish type. Nephrin is a component of the interpodocyte-spanning slit diaphragm: it mediates outside-in signaling and forms a nexus for homo- and heterotypic molecular interactions. When studying the nephrin-deficient mouse line generated by random insertional mutagenesis we unexpectedly discovered an endogenous antisense transcript originating from the nephrin-encoding locus. Further evidence of the antisense transcript (Nphs1as) was obtained by searching for Nphs1-like expressed sequence tags. Surprisingly, one clone showed exact complementarity in the antisense orientation. Nphs1as is expressed in the brain, thymus, and peripheral lymph nodes as well as in the embryonic stem cells. However, the mesenteric lymph nodes and the main sites of nephrin expression, the kidney and pancreas, were negative. Nphs1as is a continuous, polyadenylated mRNA that spans Nphs1 exons from 7 to 12 in the reverse orientation. The relative amounts of sense and antisense mRNAs as well as nephrin protein were determined by semiquantitative RT-PCR and immunoblotting, respectively, in various mouse tissues. These results suggest that Nphs1as may be important for the regulation of the appropriate tissue- and cell-type-specific expression of nephrin.

Nephrin expression is increased in anti-Thy1.1-induced glomerulonephritis in rats

Nephrin is an important constituent of the glomerular filtration barrier and alteration of its expression is associated with severe proteinuria. In this study we show that injection of an anti-Thy1.1 antibody in rats not only induces a mesangioproliferative glomerulonephritis associated with increased proteinuria, but also leads to a sustained increase of nephrin mRNA and protein expression in renal glomeruli over a time period of 29 days. In contrast, podocin and CD2AP, two proteins shown to interact with nephrin in the slit diaphragm, are acutely downregulated at days 3-7 and, thereafter, recovered again to normal levels after 29 days. Interestingly, immunofluorescence staining of kidney sections at day 10 of the disease shows a highly heterogeneous pattern, in that some podocytes show complete absence of nephrin, whereas others show highly accumulated staining for nephrin compared to control sections, which in total results in an increased level of nephrin per glomerulus. In summary, our data show that in the course of mesangioproliferative glomerulonephritis in rats, an upregulation of nephrin expression occurs with a concomitant transient downregulation of podocin and CD2AP which may account for a highly dysregulated filtration barrier and increased proteinuria.

Deciphering the Renal Code: Advances in Conditional Gene Targeting

Several powerful new techniques can examine gene function in mammals. Recombinase systems and kidney-specific promoters enable gene knockout and overexpression. Genetic systems induced on administration or removal of antibiotics or hormones permit control of gene expression. Gene silencing using short interfering RNA expression systems should accelerate loss-of-function studies. Thorough characterization of animals that have undergone conditional gene targeting has already provided insights into renal development and diseases. Here we discuss the advantages and pitfalls of currently available gene-targeting systems.

The human paracellin-1 gene (hPCLN-1): renal epithelial cell-specific expression and regulation

Tubular reabsorption of Mg2+ is mediated by the tight junction protein paracellin-1, which is encoded by the gene PCLN-1 (CLDN16) and exclusively expressed in the kidney. Tubular Mg2+ reclamation is modulated by many hormones and factors. The aim of this study was to define regulatory elements essential for renal tubular cell-specific expression of human PCLN-1 (hPCLN-1) and to explore the effect of Mg2+ transport modulators on the paracellin-1 gene promoter. Endogenous paracellin-1 mRNA and protein were detected in renal cell lines opossom kidney (OK), HEK293, and MDCT, but not in the fibroblast cell line NIH3T3. A 7.5-kb hPCLN-1 5'-flanking DNA sequence along with seven 5'-deletion products were cloned into luciferase reporter vectors and transiently transfected into the renal and nonrenal cells. The highest levels of luciferase activity resulted from transfection of a 5'-flanking 2.5-kb fragment (pJ2M). This activity was maximal in OK cells, was orientation dependent, and was absent in NIH3T3 cells. Mg2+ deprivation significantly increased pJ2M-driven activity in transfected OK cells, whereas Mg2+ load decreased it compared with conditions of normal Mg2+. Deletion analysis along with electrophoretic mobility-shift assay demonstrated that OK cells contain nuclear proteins, which bind a 70-bp region between -1633 and -1703 of major functional significance. Deleting this 70-bp segment, which contains a single peroxisome proliferator-response element (PPRE), or mutating the PPRE, caused a 60% reduction in luciferase activity. Stimulating the 70-bp sequence with 1,25(OH)2 vitamin D decreased luciferase activity by 52%. This effect of 1,25(OH)2 vitamin D was abolished in the absence of PPRE or in the presence of mutated PPRE. We conclude that the PPRE within this 70-bp DNA region may play a key role in the cell-specific and regulatory activity of the hPCLN-1 promoter. Ambient Mg2+ concentration and 1,25(OH)2 vitamin D may modulate paracellular, paracellin-1-mediated, Mg2+ transport at the transcriptional level. 1,25(OH)2 vitamin D exerts its activity on the hPCLN-1 promoter likely via the PPRE site.

Insights into the physiological role of WT1 from studies of genetically modified mice

Discenza, Maria Teresa, and Jerry Pelletier. Insights into the physiological role of WT1 from studies of genetically modified mice. Physiol Genomics 16: 287-300, 2004; 10.1152/physiolgenomics.00164.2003.—The identification of WT1 gene mutations in children with WAGR and Denys-Drash syndromes pointed toward a role for WT1 in genitourinary system development. Biochemical analysis of the different WT1 protein isoforms showed that WT1 is a transcription factor and also has the ability to bind RNA. Analysis of WT1 complexes identified several target genes and protein partners capable of interacting with WT1. Some of these studies placed WT1, its downstream targets, and protein partners in a transcriptional regulatory network that controls urogenital system development. We review herein studies on WT1 knockout and transgenic models that have been instrumental in defining a physiological role for WT1 in normal and abnormal urogenital development.

Inflammatory cytokines upregulate nephrin expression in human embryonic kidney epithelial cells and podocytes

Nephrin is an important regulator of the glomerular filtration barrier and its malfunction is associated with severe proteinuria. In this study we show that exposure of human embryonic kidney epithelial A293 cells to the proinflammatory cytokine interleukin-1beta (IL-1beta) causes a dose-dependent upregulation of nephrin mRNA level. Time-course analyses reveal first significant increases in nephrin mRNA levels after 4h of stimulation. Furthermore, nephrin protein is also elevated by IL-1beta treatment. Tumor necrosis factor-alpha (TNFalpha) exerted a comparable effect on nephrin mRNA and protein expression. The IL-1beta-induced upregulation of nephrin expression occurs independently of nitric oxide (NO) generation, since the NO-synthase inhibitor N(G)-monomethyl-L-arginine does not block the IL-1beta effect. Mechanistically, we found that the IL-1beta-induced response does not involve protein kinase C, protein kinase A, the classical mitogen-activated protein kinase (MAPK), the stress-activated p38-MAPK, or the NF-kappaB cascade, since selective inhibitors of these pathways were unable to alter the IL-1beta response. Moreover, neither unselective cyclooxygenase (COX) inhibitors, like indomethacin, nor COX-2-selective inhibitors, like flosulide and NS 398, nor the anti-inflammatory glucocorticoid dexamethasone were able to alter IL-1beta-induced nephrin expression. The only inhibitor that was able to block IL-1beta- and TNFalpha-induced nephrin upregulation was rottlerin, which has been suggested to act as a selective PKCdelta inhibitor. However, concerning cytokine-triggered nephrin expression, rottlerin action involved inhibition of another still to be identified protein kinase. Importantly, cytokine-induced upregulation of nephrin expression was also confirmed in primary human podocytes. In summary, these data show for the first time that inflammatory cytokines like IL-1beta or TNFalpha can upregulate nephrin expression and this mechanistically involves a rottlerin-sensitive protein kinase.

Focal and segmental glomerulosclerosis in mice with podocyte-specific expression of mutant alpha-actinin-4

Mutations in the gene encoding alpha-actinin-4 (ACTN4), an actin crosslinking protein, are associated with a form of autosomal dominant focal segmental glomerulosclerosis (FSGS). To better study its progression, a transgenic mouse model was developed by expressing murine alpha-actinin-4 containing a mutation analogous to that affecting a human FSGS family in a podocyte-specific manner using the murine nephrin promoter. Consistent with human ACTN4-associated FSGS, which shows incomplete penetrance, a proportion of the transgenic mice exhibited significant albuminuria (8 of 18), while the overall average systolic BP was elevated in both proteinuric and non-proteinuric ACTN4-mutant mice. Immunofluorescence confirmed podocyte-specific expression of mutant alpha-actinin-4, and real-time RT-PCR revealed that HA-ACTN4 mRNA levels were higher in proteinuric versus non-proteinuric ACTN4-mutant mice. Only proteinuric mice exhibited histologic features consistent with human ACTN4-associated FSGS, including segmental sclerosis and tuft adhesion of some glomeruli, tubular dilatation, mesangial matrix expansion, as well as regions of podocyte vacuolization and foot process fusion. Consistent with such podocyte damage, proteinuric ACTN4-mutant kidneys exhibited significantly reduced mRNA and protein levels of the slit diaphragm component, nephrin. This newly developed mouse model of human ACTN4-associated FSGS suggests a cause-and-effect relationship between actin cytoskeleton dysregulation by mutant alpha-actinin-4 and the deterioration of the nephrin-supported slit diaphragm complex.

Glomerular-specific alterations of VEGF-A expression lead to distinct congenital and acquired renal diseases

Kidney disease affects over 20 million people in the United States alone. Although the causes of renal failure are diverse, the glomerular filtration barrier is often the target of injury. Dysregulation of VEGF expression within the glomerulus has been demonstrated in a wide range of primary and acquired renal diseases, although the significance of these changes is unknown. In the glomerulus, VEGF-A is highly expressed in podocytes that make up a major portion of the barrier between the blood and urinary spaces. In this paper, we show that glomerular-selective deletion or overexpression of VEGF-A leads to glomerular disease in mice. Podocyte-specific heterozygosity for VEGF-A resulted in renal disease by 2.5 weeks of age, characterized by proteinuria and endotheliosis, the renal lesion seen in preeclampsia. Homozygous deletion of VEGF-A in glomeruli resulted in perinatal lethality. Mutant kidneys failed to develop a filtration barrier due to defects in endothelial cell migration, differentiation, and survival. In contrast, podocyte-specific overexpression of the VEGF-164 isoform led to a striking collapsing glomerulopathy, the lesion seen in HIV-associated nephropathy. Our data demonstrate that tight regulation of VEGF-A signaling is critical for establishment and maintenance of the glomerular filtration barrier and strongly supports a pivotal role for VEGF-A in renal disease.

Polymorphisms in the nephrin gene and diabetic nephropathy in type 1 diabetic patients

BACKGROUND

Several mutations in the nephrin gene are responsible for the lack of slit membrane of the glomeruli leading to massive proteinuria present already in utero. Variations in the nephrin gene may also affect the degree of proteinuria in acquired kidney diseases. We tested the hypothesis of whether any of the polymorphisms identified in the coding region of the nephrin gene were associated with diabetic nephropathy.

METHODS

In a case-control, cross-sectional study, 996 Finnish type 1 diabetic patients from the FinnDiane Study were genotyped by standard polymerase chain reaction protocol.

RESULTS

The frequencies of the rare alleles in the E117K, R408Q, and N1077S polymorphisms in the entire cohort were 34%, 8%, and 12%, respectively. When comparing patients with a mutant allele with the wild genotype there was no difference between the patients with end-stage renal disease, proteinuria, microalbuminuria, and those with a normal albumin excretion rate (df =3, chi2 =1.62, 1.31 and 0.77). Neither were the polymorphisms associated with the progression of kidney disease, nor with creatinine clearance and albumin excretion rate.

CONCLUSION

This study does not support an involvement of the coding region of the nephrin gene in the pathogenesis of diabetic nephropathy in type 1 diabetic patients.

Retinoids regulate the repairing process of the podocytes in puromycin aminonucleoside-induced nephrotic rats

The foot processes forming the slit diaphragm are disrupted in diseases associated with proteinuria. Although they are often repairable, regulators for the repairing process remain unknown. By extrapolating from the fact that vitamin A is essential for the nephrogenesis, this study examined whether or not injured podocytes in the middle of the repairing process require retinaldehyde dehydrogenase type 2 (RALDH2), one of the key enzymes to produce all-trans-retinoic acid (ATRA). RALDH2 was dramatically upregulated in podocytes of puromycin aminonucleoside-induced nephrosis (PAN nephrosis) rats. On day 5 of PAN nephrosis, RALDH2 showed the remarkable induction, whereas glomerular expression levels of nephrin and midkine, one of the ATRA target genes, were downregulated. Daily administration of ATRA ameliorated proteinuria, which was accompanied by the improvement in the effacement of the foot processes and by the induction of nephrin and midkine. In contrast, recovery from PAN nephrosis was delayed in rats fed with a vitamin A-deficient diet. Consistently, the promoter region of human nephrin gene (NPHS1) contained three putative retinoic acid response elements (RARE) and showed the enhancer activity in response to ATRA in a dose-dependent manner. This transcriptional activation was regulated through the receptors for retinoids because BMS-189453, an antagonist to the retinoid receptors, counteracted it in a dose-dependent manner. In conclusion, active metabolites of vitamin A, especially ATRA produced by RALDH2 play relevant roles during the repairing process of injured podocytes. The results obtained from PAN nephrosis rats might be applicable to human renal diseases.

Glomerular-specific alterations of VEGF-A expression lead to distinct congenital and acquired renal diseases

Kidney disease affects over 20 million people in the United States alone. Although the causes of renal failure are diverse, the glomerular filtration barrier is often the target of injury. Dysregulation of VEGF expression within the glomerulus has been demonstrated in a wide range of primary and acquired renal diseases, although the significance of these changes is unknown. In the glomerulus, VEGF-A is highly expressed in podocytes that make up a major portion of the barrier between the blood and urinary spaces. In this paper, we show that glomerular-selective deletion or overexpression of VEGF-A leads to glomerular disease in mice. Podocyte-specific heterozygosity for VEGF-A resulted in renal disease by 2.5 weeks of age, characterized by proteinuria and endotheliosis, the renal lesion seen in preeclampsia. Homozygous deletion of VEGF-A in glomeruli resulted in perinatal lethality. Mutant kidneys failed to develop a filtration barrier due to defects in endothelial cell migration, differentiation, and survival. In contrast, podocyte-specific overexpression of the VEGF-164 isoform led to a striking collapsing glomerulopathy, the lesion seen in HIV-associated nephropathy. Our data demonstrate that tight regulation of VEGF-A signaling is critical for establishment and maintenance of the glomerular filtration barrier and strongly supports a pivotal role for VEGF-A in renal disease.

Alternatively used promoters and distinct elements direct tissue-specific expression of nephrin

Nephrin, an essential component of the glomerular ultrafilter, the slit diaphragm, has also been found to be expressed in the central nervous system and pancreas. This study examined the regulation of the nephrin gene by analyzing the expression of different length nephrin promoter-lacZ reporter constructs in transgenic mice. An upstream segment between -4 kb and -4 bp was shown to be sufficient for driving expression in all three tissues. Surprisingly, a 5.7-kb construct lacking the transcription initiation site and the immediate upstream region of the gene could drive expression in the central nervous system. This led to the identification of a novel, alternatively used exon 1B located 1871 bp upstream of the ATG codon of the previously known first exon, now termed exon 1A. The existence and functionality of exon 1B was verified in nephrin knockout mice in which exon 1A is deleted. Deletion of exon 1B and its immediate surrounding sequence, introduced in the 4-kb promoter-lacZ reporter construct, abolished the expression of the transgene in pancreas and spinal cord but not in kidney and brain in transgenic mice. Analysis of five promoter-reporter gene constructs showed that regulatory elements driving expression encoded by exon 1A in kidney and brain are localized in the region between -4 kb and 2.1 kb.

Filtrin is a novel member of nephrin-like proteins

NPHS1 encodes nephrin, the core protein of the interpodocyte slit diaphragm of the kidney glomerulus. NPHS1 is the causative gene for congenital nephrotic syndrome of the Finnish type (CNF) with massive, treatment resistant proteinuria. We report here the establishment of a novel nephrin-like gene, NLG1 encoding filtrin, a protein with substantial homology to human nephrin. Filtrin is a type I transmembrane protein consisting of 708 amino acids. Together with the recently cloned NEPH1, NLG1 establishes a new nephrin-like subgroup of genes belonging to the immunoglobulin superfamily of cell adhesion molecules. The RNA dot blot experiment revealed that the NLG1 mRNA expression is widely distributed but most prominently observed in the pancreas and lymph nodes. The expression of NLG1 mRNA in kidney glomeruli was verified with RT-PCR. Further immunoblotting studies with antifiltrin antibody showed a specific band at 107kDa in the human and rat glomeruli. In immunofluorescence microscopy specific staining of glomeruli but also proximal and distal parts of the nephron was seen in human kidney cortex. Due to its structural similarity and sequence homology as well as partially consistent expression pattern with nephrin we propose that filtrin belongs to a functionally important complex of proteins of the glomerular filtration barrier.

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

The mouse Kreisler (Krml1/MafB) segmentation gene is required for differentiation of glomerular visceral epithelial cells

Molecular components of the glomerular filtration mechanism play critical roles in renal diseases. Many of these components are produced during the final stages of differentiation of glomerular visceral epithelial cells, also known as podocytes. While basic domain leucine zipper (bZip) transcription factors of the Maf subfamily have been implicated in cellular differentiation processes, Kreisler (Krml1/MafB), the gene affected in the mouse kreisler (kr) mutation, is known for its role in hindbrain patterning. Here we show that mice homozygous for the kr(enu) mutation develop renal disease and that Kreisler is essential for cellular differentiation of podocytes. Consistent with abnormal podocyte differentiation, kr(enu) homozygotes show proteinuria, and fusion and effacement of podocyte foot processes, which are also observed in the nephrotic syndrome. Kreisler acts during the final stages of glomerular development-the transition between the capillary loop and mature stages-and downstream of the Pod1 basic domain helix-loop-helix transcription factor. The levels of Podocin, the gene mutated in autosomal recessive steroid-resistant nephrotic syndrome (NPHS2), and Nephrin, the gene mutated in congenital nephrotic syndrome of the Finnish type (NPHS1), are slightly reduced in kr(enu)/kr(enu) podocytes. However, these observations alone are unlikely to account for the aberrant podocyte foot process formation. Thus, Kreisler must regulate other unknown genes required for podocyte function and with possible roles in kidney disease.

Two gene fragments that direct podocyte-specific expression in transgenic mice

Transgenic manipulation of the glomerular visceral epithelial cell offers a powerful approach for studying the biology of this morphologically complex cell type. It has been previously demonstrated that an 8.3-kb and a 5.4-kb fragment of the murine Nphs1 (nephrin) promoter-enhancer drives lacZ expression in podocytes, brain, and pancreas of transgenic mice, recapitulating the expression pattern of the endogenous nephrin gene. In this present study, two truly podocyte-specific promoters were identified that drive transgene expression in podocytes without expression in extrarenal tissues in adult or embryonic mice. A 1.25-kb fragment driving a lacZ reporter gene (p1.25N-nlacF) was derived from murine Nphs1 promoter similar to a human NPHS1 promoter fragment previously reported. Transgenic mice were generated and beta-galactosidase (beta-gal) expression was analyzed. Four of twelve founder mice were found to express beta-gal in podocytes (33% penetrance). Expression in brain and pancreas was absent in all animals, suggesting that nephrin expression in these organs might be driven by distinct cis-regulatory elements that can be removed to obtain podocyte-specific expression. A 2.5-kb fragment derived from the human NPHS2 (podocin) gene was designed in a similar fashion to drive lacZ expression in transgenic mice (p2.5P-nlacF). Twelve of twlve NPHS2 mouse founder lines expressed beta-gal exclusively in podocytes (100% penetrance). Beta-gal activity was not observed extrinsic to the kidney in p1.25N-nlacF or p2.5P-nlacF mouse embryos at gestational time points between 8.5 d post coitus and birth. In conclusion, the 2.5-kb NPHS2 promoter fragment may be useful for podocyte-specific transgenic expression when extrarenal expression of a transgene is problematic.

Regulation of adhesive interaction between podocytes and glomerular basement membrane

Glomerular filtration depends on well-orchestrated cell-cell and cell-matrix contacts of glomerular podocytes. Over the last years critical constituents of these contacts have been identified via molecular approaches. Podocyte cell-matrix interactions have been shown to be mediated in part by alpha(3)beta(1)-integrin heterodimers. Disturbances of integrin matrix interaction lead to detachment of podocytes in vitro, corresponding to the critical event of foot process retraction and glomerular basement membrane (GBM) denudation in vivo. Further, dystroglycan-mediated matrix attachment appears to play a critical role for podocyte foot process architecture. Downstream signaling events are currently elucidated concentrating mainly on integrin-dependent cascades and their consequences for podocyte adhesion and proliferation. An activation of the integrin-linked kinase in podocyte damage in vivo and in vitro makes this molecule a particularly interesting candidate for integrin-mediated inside-out and outside-in signaling in podocytes. Podocyte cell-cell interaction has been characterized in a few studies in vitro, indicating the slit diaphragm to be a modified adherens junction. The structural link between the cell-matrix and cell-cell contacts is maintained by the actin cytoskeleton, which may also enable cross-talk between these two cell contact sites. Examining podocyte function in tissue culture, animal models and human expression studies should allow further detailed dissection of the molecular pathways responsible for maintenance and failure of the glomerular filtration barrier.

Transcriptional regulation of podocyte specification and differentiation

Glomerular visceral epithelial cells (podocytes) are highly specialized cells found in the vertebrate and invertebrate kidney and make up a major portion of the filtration barrier between blood and urinary spaces. During development, specification and differentiation of the podocyte lineage must be tightly orchestrated to produce highly specialized characteristics such as foot processes and slit diaphragms. Furthermore, podocytes are poised to direct incoming endothelial and mesangial cells during glomerular development. They express a number of growth factors that likely play a major role in these processes. Recent findings from transgenic and knockout mouse models and the identification of genes responsible for human podocyte disease have provided insight into transcriptional regulation of some of these processes. These transcription factors include Pax2, WT1 (the Wilms tumor suppressor gene), Pod1 (capsulin, epicardin), Kreisler (maf-1), lmx1b, and mf2. Furthermore, regulatory regions from a podocyte-restricted gene, NPHS1 (nephrin) that are required to direct podocyte-specific expression have been identified from both human and murine genes and provide a tool to further dissect the transcriptional regulation of podocyte-specific gene expression. This article reviews the present state of knowledge regarding transcriptional regulation of podocyte specification and differentiation.

Transcriptional induction of slit diaphragm genes by Lmx1b is required in podocyte differentiation

LMX1B encodes a LIM-homeodomain transcription factor. Mutations in LMX1B cause nail-patella syndrome (NPS), an autosomal dominant disease with skeletal abnormalities, nail hypoplasia, and nephropathy. Expression of glomerular basement membrane (GBM) collagens is reduced in Lmx1b(-/-) mice, suggesting one basis for NPS nephropathy. Here, we show that Lmx1b(-/-) podocytes have reduced numbers of foot processes, are dysplastic, and lack typical slit diaphragms, indicating an arrest in development. Using antibodies to podocyte proteins important for podocyte function, we found that Lmx1b(-/-) podocytes express near-normal levels of nephrin, synaptopodin, ZO-1, alpha3 integrin, and GBM laminins. However, mRNA and protein levels for CD2AP and podocin were greatly reduced, suggesting a cooperative role for these molecules in foot process and slit diaphragm formation. We identified several LMX1B binding sites in the putative regulatory regions of both CD2AP and NPHS2 (podocin) and demonstrated that LMX1B binds to these sequences in vitro and can activate transcription through them in cotransfection assays. Thus, LMX1B regulates the expression of multiple podocyte genes critical for podocyte differentiation and function. Our results indicate that reduced levels of proteins associated with foot processes and the glomerular slit diaphragm likely contribute, along with reduced levels of GBM collagens, to the nephropathy associated with NPS.

Glomerular-specific gene excision in vivo

Podocytes (glomerular visceral epithelial cells) are highly specialized cells that are found in the renal glomerulus and make up a major portion of the filtration barrier between the blood and urinary spaces. Recently, the identification of a number of genes responsible for both autosomal dominant and recessive forms of human nephrotic syndrome has provided insight into a number of molecules responsible for unique features of the podocyte such as the slit diaphragms. Despite these major advances in our understanding of podocyte biology, the function of many genes expressed in the podocyte remains unknown. Targeted gene disruption using homologous recombination in murine embryonic stem cells (ES cells) is a powerful tool to determine the biologic function of genes in vivo. However, resulting embryonic lethal or pleiotropic phenotypes often preclude the analysis of genes in specific renal cell types. To overcome this problem, a glomerular-specific Cre-recombinase transgenic murine line under the control of the Nphs1 (nephrin) promoter (Neph-Cre) was generated. This article reports successful Cre-mediated excision of a 'floxed' transgene specifically in podocytes in vivo. This murine founder line represents a powerful new tool for the manipulation of the expression of genes in podocytes and will provide valuable insight into podocyte biology in the whole animal.