NPHS2, encoding the glomerular protein podocin, is mutated in autosomal recessive steroid-resistant nephrotic syndrome

Flotillins and the PHB Domain Protein Family: Rafts, Worms and Anaesthetics

While our understanding of lipid microdomains has advanced in recent years, many aspects of their formation and dynamics are still unclear. In particular, the molecular determinants that facilitate the partitioning of integral membrane proteins into lipid raft domains are yet to be clarified. This review focuses on a family of raft-associated integral membrane proteins, termed flotillins, which belongs to a larger class of integral membrane proteins that carry an evolutionarily conserved domain called the prohibitin homology (PHB) domain. A number of studies now suggest that eucaryotic proteins carrying this domain have affinity for lipid raft domains. The PHB domain is carried by a diverse array of proteins including stomatin, podocin, the archetypal PHB protein, prohibitin, lower eucaryotic proteins such as the Dictyostelium discoideum proteins vacuolin A and vacuolin B and the Caenorhabditis elegans proteins unc-1, unc-24 and mec-2. The presence of this domain in some procaryotic proteins suggests that the PHB domain may constitute a primordial lipid recognition motif. Recent work has provided new insights into the trafficking and targeting of flotillin and other PHB domain proteins. While the function of this large family of proteins remains unclear, studies of the C. elegans PHB proteins suggest possible links to a class of volatile anaesthetics raising the possibility that these lipophilic agents could influence lipid raft domains. This review will discuss recent insights into the cell biology of flotillins and the large family of evolutionarily conserved PHB domain proteins.

Cell junction-associated proteins IQGAP1, MAGI-2, CASK, spectrins, and alpha-actinin are components of the nephrin multiprotein complex

Nephrin is a cell surface receptor of the Ig superfamily that localizes to slit diaphragms, the specialized junctions between the interdigitating foot processes of the glomerular epithelium (podocytes) in the kidney. Mutations in the NPHS1 gene encoding nephrin lead to proteinuria and congenital nephrotic syndrome, indicating that nephrin is essential for normal glomerular development and function. To identify nephrin-binding proteins, we performed mass spectrometry on proteins obtained from pull-down assays with GST-nephrin cytoplasmic domain. Nephrin specifically pulled down six proteins from glomerular lysates, MAGI-2/S-SCAM (membrane-associated guanylate kinase inverted 2/synaptic scaffolding molecule), IQGAP1 (IQ motif-containingGTPase-activatingprotein1),CASK(calcium/calmodulin-dependent serine protein kinase), alpha-actinin, alphaII spectrin, and betaII spectrin. All of these scaffolding proteins are often associated with cell junctions. By immunofluorescence these proteins are expressed in glomerular epithelial cells, where they colocalize with nephrin in the foot processes. During glomerular development, IQGAP1 is expressed in the junctional complexes between the earliest identifiable podocytes, MAGI-2/S-SCAM is first detected in junctional complexes in podocytes after their migration to the base of the cells. Thus, the nephrin-slit diaphragm protein complex contains a group of scaffolding proteins that function to connect junctional membrane proteins to the actin cytoskeleton and signaling cascades. Despite their special morphology and function, there is considerable compositional similarity between the podocyte slit diaphragm and typical junctional complexes of other epithelial cells.

Functional consequences of integrin-linked kinase activation in podocyte damage

BACKGROUND

The delicate foot process architecture of glomerular podocytes critically depends on integrin mediated cell-glomerular basement membrane (GBM) interaction. Integrin signaling via the integrin-linked kinase (ILK) is activated in podocyte damage and associated with considerable podocyte phenotype alterations. ILK has been shown to regulate cell fate via nuclear interaction of beta-catenin with lymphoid enhancer factor (LEF-1) transcription factors. The aim of this study was to elucidate the molecular mechanisms of ILK dependant phenotype regulation in podocytes.

METHODS

ILK function was evaluated in conditionally immortalized murine glomerular epithelial cells using overexpression of ILK and a small molecule ILK inhibitor in puromycin/adriamycin-induced podocyte damage in vitro and in vivo.

RESULTS

Kinase active, but not mutant ILK induced translocation of beta-catenin to the cell nucleus, de novo expression of LEF-1, and nuclear colocalization of beta-catenin and LEF-1. The role of ILK signaling in podocyte damage was evaluated using puromycin, an agent known to cause selective proteinuria and to increase ILK activity. The small molecular ILK inhibitor MC-5 blocked puromycin-induced nuclear translocation of beta-catenin, podocyte detachment, cell proliferation, and repression of the slit membrane molecules P-cadherin and CD2ap. In vivo activation of the beta-catenin pathway could be shown by nuclear colocalization of beta-catenin with WT-1 in adriamycin nephropathy.

CONCLUSION

ILK regulates podocyte cell matrix interaction, proliferation, and slit membrane gene expression in podocyte damage. As this pathway is amendable to pharmacologic intervention, further detailed studies of in vivo ILK function in glomerular disease appear justified.

New insights into the pathogenesis and the therapy of recurrent focal glomerulosclerosis

Recurrent focal glomerulosclerosis (FSGS) in renal allografts has remained a frustrating and enigmatic disease. Recent studies on gene mutations encoding podocin and other components of the slit-diaphragm in patients with native kidney nephrotic syndrome have underscored the heterogenecity of the idiopathic form of FSGS. While familial FSGS rarely recurs following transplantation, the sporadic variety of FSGS is associated with a 30% recurrence rate. The patients with the sporadic variety of FSGS who have homozygous or complex heterozygous podocin mutations have a low recurrence rate. In the other patients with sporadic FSGS, a more complex and likely multifactorial etiology accounts for the recurrence of FSGS. The role of CD80 expression on podocytes is intriguing but requires confirmation in kidney biopsies of patients with recurrent FSGS. Recent findings on podocin genomics, the permeability factor and CD80 expression may ultimately lead to a better understanding of recurrent FSGS as well as a more effective approach to its prevention and treatment.

TRPC6 is a glomerular slit diaphragm-associated channel required for normal renal function

Progressive kidney failure is a genetically and clinically heterogeneous group of disorders. Podocyte foot processes and the interposed glomerular slit diaphragm are essential components of the permeability barrier in the kidney. Mutations in genes encoding structural proteins of the podocyte lead to the development of proteinuria, resulting in progressive kidney failure and focal segmental glomerulosclerosis. Here, we show that the canonical transient receptor potential 6 (TRPC6) ion channel is expressed in podocytes and is a component of the glomerular slit diaphragm. We identified five families with autosomal dominant focal segmental glomerulosclerosis in which disease segregated with mutations in the gene TRPC6 on chromosome 11q. Two of the TRPC6 mutants had increased current amplitudes. These data show that TRPC6 channel activity at the slit diaphragm is essential for proper regulation of podocyte structure and function.

A mutation in the TRPC6 cation channel causes familial focal segmental glomerulosclerosis

Focal and segmental glomerulosclerosis (FSGS) is a kidney disorder of unknown etiology, and up to 20% of patients on dialysis have been diagnosed with it. Here we show that a large family with hereditary FSGS carries a missense mutation in the TRPC6 gene on chromosome 11q, encoding the ion-channel protein transient receptor potential cation channel 6 (TRPC6). The proline-to-glutamine substitution at position 112, which occurs in a highly conserved region of the protein, enhances TRPC6-mediated calcium signals in response to agonists such as angiotensin II and appears to alter the intracellular distribution of TRPC6 protein. Previous work has emphasized the importance of cytoskeletal and structural proteins in proteinuric kidney diseases. Our findings suggest an alternative mechanism for the pathogenesis of glomerular disease.

Heparan sulfate of perlecan is involved in glomerular filtration

Perlecan is a heparan sulfate proteoglycan and a major component of the glomerular basement membrane. To understand the role of heparan sulfate chains of perlecan in glomerular filtration, detailed analyses were performed of the kidneys of Hspg2(Delta)(3/)(Delta)(3) mice, whose perlecan lacks heparan sulfate attachment sites in N-terminal domain I. Macroscopic, histologic, and electron microscopic observations, as well as immunohistochemical and immunoelectron microscopic analyses using specific antibodies against perlecan and agrin core proteins, revealed no significant abnormalities in these mice under physiologic conditions. Polyethyleneimine staining demonstrated no significant changes in charge density in the glomerular basement membrane. Transcripts of other heparan sulfate proteoglycans, agrin, and collagen type XVIII, as well as perlecan, were expressed at similar levels to those in the wild-type littermates. Approximately 40% of the perlecan synthesized by Hspg2(Delta)(3/)(Delta)(3) fibroblasts was substituted with heparin sulfate and 60% was substituted with chondroitin sulfate. All of the perlecan synthesized by wild-type fibroblasts contained heparin sulfate, indicating an altered substitution of glycosaminoglycans on Hspg2(Delta)(3/)(Delta)(3) perlecan. Immunostaining indicated that the level of chondroitin sulfate was actually increased in the Hspg2(Delta)(3/)(Delta)(3) glomerular basement membrane. When administered intraperitoneally with BSA, Hspg2(Delta)(3/)(Delta)(3) mice exhibited remarkable proteinuria. These findings suggest that heparan sulfate chains of perlecan play an important role in glomerular filtration, especially of a large amount of protein.

NPHS2 (Podocin) mutations in nephrotic syndrome. Clinical spectrum and fine mechanisms

Nephrotic syndrome (NS) is the most frequent cause of proteinuria in children and is emerging as a leading cause of uremia. Molecular studies in families with recessive NS have led to the discovery of specialized molecules endowed in podocytes that play a role in proteinuria. This review focalizes the key position of podocin (NPHS2 gene) in this rapidly evolving field and furnishes a compendium to those involved in clinics and genetics of NS. Screening for NPHS2 mutations have been done in sporadic NS and familial cases with recessive inheritance, documenting a mutation detection rate of 45-55% in families and 8-20% in sporadic NS according to the different groups and considering all the clinical phenotypes. Almost 50 NPHS2 mutations have been reported and variants and/or non silent polymorphisms potentially involved in proteinuria were recognized. Personalized data on clinical aspects related to responsiveness to drugs, evolution to end stage renal failure and post-transplant outcome are reported. Functional studies and cell sorting experiments demonstrated retention in the endoplasmic reticulum of most mutants involving the stomatin domain. Pull-down experiments with the common R229Q polymorphism demonstrated an altered interaction with nephrin that affects the stability of the functional unit. Overall, data are here presented that underscore a major role of inherited defects of NPHS2 in NS in children (including a relevant impact in sporadic cases) and give the functional rationale for the association. A practical compendium is also given to clinicians involved in the management of NS that should modify the classic therapeutic approach.

Insulin-like growth factors inhibit podocyte apoptosis through the PI3 kinase pathway

BACKGROUND

Abnormal podocyte development and progressive podocyte injury have been implicated in a number of human kidney diseases. Factors necessary for regulating development and maintenance of this cell type are only beginning to emerge.

METHODS

To study the role of the insulin-like growth factor (IGF) system in regulating podocyte survival, we induced human fetal podocytes to undergo apoptosis. We demonstrated a significant increase in apoptosis when these cells were incubated in the presence of etoposide, as measured by DNA fragmentation and nuclear membrane condensation and blebbing.

RESULTS

Podocyte apoptosis was reduced to control levels when the cells were coincubated in the presence of IGF-1. We showed that the protective effect of IGFs in this cell type was mediated through the activation of the phosphatidylinositol 3'-kinase (PI3K) pathway. IGF-1 stimulation resulted in the formation of the insulin receptor substrate (IRS)-1-p85 complex, an increase in PI3 kinase activity, and activation of protein kinase B (AKT/PKB) and the bcl-2 family member bad. Incubation of the podocytes with inhibitors of the PI3 kinase pathway resulted in a loss of this IGF-1 protective effect.

CONCLUSION

These data demonstrate an important role for the IGF system in fetal podocyte survival in vitro, and suggest potential mediators to slow or alleviate the loss or damage of the podocyte in progressive renal disease.

Respiratory chain deficiency presenting as congenital nephrotic syndrome

Nephrotic syndrome (NS) in infancy includes NS of Finnish type (mutation of the nephrin gene), diffuse mesangial sclerosis (idiopathic or linked to WT1 mutation), idiopathic NS, most often steroid resistant, and NS related to infections during pregnancy (virus, syphilis, toxoplasmosis). Later in life, NS has a large variety of etiologies. It has been described in association with neuromuscular symptoms, deafness, and diabetes in a few children and adults with respiratory chain (RC) disorders. To date, however, NS has never been observed in neonates with RC disorders. Here, we report RC deficiency in one infant with certain congenital NS and two siblings with acute neonatal cardiac and renal disease with probable NS. Although clinical and histopathological presentations were initially close to congenital NS of Finnish type, clinical outcome was atypical and nephrin mutation was excluded. Mitochondrial RC complex II+V deficiency was identified in the three patients. Based on these observations, we suggest that RC disorders should be considered in patients with congenital NS.

How Does the Kidney Filter Plasma?

The kidneys filter the plasma in special filtration units—glomeruli—and thereby excrete low-molecular-weight waste products into the urine. The mechanisms of glomerular filtration have been a matter of controversy for several decades, but recent data have revealed new details about the molecular nature of the filter and have demonstrated a central role for the podocyte slit diaphragm in the filtration process.

Mutations in NPHS2 in sporadic steroid-resistant nephrotic syndrome in Chinese children

BACKGROUND

Since the identification of the NPHS2 gene, various investigators have demonstrated that an NPHS2 mutation is a frequent cause of sporadic steroid-resistant nephrotic syndrome (SRNS), and occurs in 10.5-28% of children with the syndrome. Idiopathic nephrotic syndrome (INS) is also the most frequent glomerular disease in Chinese children, of which approximately 20% of cases show steroid resistance. To our knowledge, however, whether or not NPHS2 is the causative gene in Chinese sporadic SRNS has not been established. This study aims to examine mutations in NPHS2 in Chinese children with sporadic SRNS.

METHODS

We examined 23 Chinese children with sporadic SRNS for mutations in NPHS2. The mutational analysis of NPHS2 was performed by polymerase chain reaction, denaturing high-performance liquid chromatography and DNA sequencing.

RESULTS

A heterozygous missense mutation of L361P in exon 8 of NPHS2 was detected in one of 23 children with sporadic SRNS, whereas it was not found in 53 controls. We also identified seven NPHS2 polymorphisms, -51G>T, 288C>T, IVS3-46C>T, IVS3-21C>T, IVS7-74G>C, 954T>C and 1038A>G, in some patients and controls. There was no significant difference in the genotypic and allelic frequencies of these polymorphisms between the patients and controls.

CONCLUSION

The results demonstrate that NPHS2 mutations are also present in Chinese sporadic SRNS. Our investigation supports the necessity of searching for mutations in NPHS2 in Chinese children with sporadic SRNS.

Genetic engineering of glomerular sclerosis in the mouse via control of onset and severity of podocyte-specific injury

This study aimed to generate a mouse model of acquired glomerular sclerosis. A model system that allows induction of podocyte injury in a manner in which onset and severity can be controlled was designed. A transgenic mouse strain (NEP25) that expresses human CD25 selectively in podocytes was first generated. Injection of anti-Tac (Fv)-PE38 (LMB2), an immunotoxin with specific binding to human CD25, induced progressive nonselective proteinuria, ascites, and edema in NEP25 mice. Podocytes showed foot process effacement, vacuolar degeneration, detachment and downregulation of synaptopodin, WT-1, nephrin, and podocalyxin. Mesangial cells showed matrix expansion, increased collagen, mesangiolysis, and, later, sclerosis. Parietal epithelial cells showed vacuolar degeneration and proliferation, whereas endothelial cells were swollen. The severity of the glomerular injury was LMB2 dose dependent. With 1.25 ng/g body wt or more, NEP25 mice developed progressive glomerular damage and died within 2 wk. With 0.625 ng/g body wt of LMB2, NEP25 mice survived >4 wk and developed focal segmental glomerular sclerosis. Thus, the study has established a mouse model of acquired progressive glomerular sclerosis in which onset and severity can be preprogrammed by experimental maneuvers.

Syndromes néphrotiques congénitaux et infantiles

Congenital nephrotic syndrome is present at birth or appears during the first three months of life and infantile nephrotic syndrome during the first year. Finnish type congenital nephrotic syndrome is an autosomal recessive disease. Nephrotic syndrome is present at birth, severe and does not respond to therapy. Infectious and nutritional complications are frequent. Renal function deteriorates necessitating a dialysis-transplantation program. Between age five and eight. The disease does not recur after transplantation. Diffuse mesangial sclerosis is the second cause of congenital and infantile nephrotic syndrome. It may be isolated or part of a Denys-Drash syndrome (association of the nephropathy with male pseudohermaphroditism and Wilm's tumor). Nephrotic syndrome is resistant to therapy. Renal failure develops in early childhood. Therapy is aimed to prevent oedema, denutrition, infections and thrombosis. Proteinuria does not recur after renal transplantation. Other causes are less frequent.

Rapid development of glomerular injury and renal failure in mice lacking p53R2

The Rrm2b gene encodes p53R2, a catalytic subunit of ribonucleotide reductase that is required for DNA repair. Embryonic stem (ES) cells containing a retroviral insertion in the Rrm2b locus were used to generate mutant mice. Analysis of kidney RNA from Rrm2b (-/-) mice showed that the retroviral insertion disrupted expression of Rrm2b transcripts. Rrm2b (-/-) pups were represented at the expected Mendelian ratios at 10-12 days of age and grew normally past weaning. Mice failed to thrive after 6 weeks of age and began to die by 8 weeks of age. Phenotyping revealed that Rrm2b (-/-) mice died from a severe glomerular lesion that led to nephrotic syndrome and chronic renal failure. In kidneys of Rrm2b (-/-) mice, podocytes were enlarged and there was evidence of foot process effacement by 6 weeks of age. By 8 weeks of age, progressive podocyte hypertrophy and loss of foot processes was accompanied by hypertrophy of glomerular capillary endothelial cells that was extensive enough to restrict capillary blood flow. Collapsing glomerulopathy with avascular glomeruli was widespread in mice surviving beyond 9 weeks of age. Additional abnormalities in other organ systems were minor or consistent with secondary effects of renal failure. These findings suggest that lack of p53R2, the protein encoded by Rrm2b, has early and relatively selective detrimental effects on the kidney glomerulus that lead to rapid death from progressive renal failure.

Congenital nephrosis, mesangial sclerosis, and distinct eye abnormalities with microcoria: an autosomal recessive syndrome

We observed the occurrence of congenital nephrotic syndrome (CNS) and distinct ocular anomalies in two unrelated families. Eleven children from both families presented with a similar course of renal disease starting with nephrotic syndrome and renal failure prenatally or immediately after birth that resulted in death before the age of 2 months. Kidney histopathology showed diffuse mesangial sclerosis (DMS). Clinically obvious eye abnormalities were recognized in six of the eight patients in whom sufficient clinical data were available. Ocular anomalies included enlarged or large appearing corneae in some cases suggesting buphthalmos, and extremely narrow, nonreactive pupils (microcoria). Pathological examination of the eyes of two aborted fetuses revealed a more complex ocular maldevelopment including posterior lenticonus as well as anomalies of cornea and retina. On the basis of these observations and other cases in the literature, we delineate a previously unrecognized distinct entity characterized by congenital nephrotic syndrome, DMS, and eye abnormalities with microcoria as the leading clinical feature. Pedigrees of affected families with parental consanguinity support autosomal recessive inheritance. We propose that this syndrome should be designated microcoria-congenital nephrosis syndrome or Pierson syndrome. Possible overlap with Galloway-Mowat syndrome and relations to other oculo-renal syndromes are discussed.

Nephrin and podocin expression around the onset of puromycin aminonucleoside nephrosis

Decreased expression levels of the glomerular slit membrane proteins, nephrin and podocin, have been reported after the onset of puromycin aminonucleoside (PA) nephrosis. We examined nephrin and podocin expressions prior to the onset of proteinuria of PA nephrosis to elucidate the proteinuria induction mechanism of PA. PA nephrosis was induced by a subcutaneous single injection of 120 mg kg(-1) PA. The mRNA levels of nephrin and podocin in whole kidney total RNA were quantified by the TaqMan real time PCR quantification system. The localization and levels of nephrin and podocin molecules were analyzed by immunofluorescence and Western blotting, respectively. Albuminuria and proteinuria were significant on days 3 and 4 in PA nephrosis rats. The protein levels of nephrin and podocin decreased significantly at day 3. The protein localization of nephrin and podocin changed at day 2 and day 1, respectively. The mRNA level of nephrin increased at day 2 and subsequently decreased at day 4. The podocin mRNA level did not change significantly. In conclusions, the protein level of nephrin and podocin decreased at the onset of albuminuria in the PA nephrosis. However, the first change induced by PA was the change of podocin localization from a linear pattern to a dot-like one prior to the onset of albuminuria.

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.

Nephrotic plasma alters slit diaphragm-dependent signaling and translocates nephrin, Podocin, and CD2 associated protein in cultured human podocytes

Podocytes are critical in maintaining the filtration barrier of the glomerulus and are dependent on the slit diaphragm (SD) proteins nephrin, podocin, and CD2-associated protein (CD2AP) to function optimally. The effects of normal human plasma and nephrotic plasma on podocytes were tested, focusing particularly on the SD complex. With the use of a conditionally immortalized human podocyte cell line, it first was shown that exposure to normal and non-nephrotic human plasma leads to a concentration of nephrin, podocin, CD2AP, and actin at the cell surface. Next, the effects of plasma from patients with nephrotic conditions to non-nephrotic conditions were compared. When exposed to all nephrotic plasma samples (and a non-human serum control), nephrin podocin and CD2AP assumed a cytoplasmic distribution; nephrin and synaptopodin were selectively downregulated, and the relocation of nephrin induced by nephrotic plasma could be rescued back to the plasma membrane by co-incubation with non-nephrotic plasma. Furthermore, intracellular calcium signaling was altered by nephrotic plasma, which was mediated by tyrosine kinase phosphorylation. With the use of nephrin mutant human cell lines, it was shown that this signaling and translocation response to normal plasma is nephrin dependent. This work demonstrates that nephrotic plasma seems to be deficient in factors that act via the podocyte SD complex, which are essential in maintaining its physiologic function.

Genetic analysis of 22 candidate genes for hypertension in the Japanese population

OBJECTIVE

We performed association studies between 118 single-nucleotide polymorphisms (SNPs) of 22 candidate genes (or gene family) and hypertension in a Japanese population.

DESIGN AND PARTICIPANTS

The study population consisted of 1880 subjects representing the general population in Japan, recruited from the Suita study. The candidate genes were selected based on their functions, including insulin resistance (APM1, CD36, HSD11B1), oxidative stress (CYBA, GPX1, GSTMs), steroid hormone (ESR1, ESR2, HSD11B2), renal functions (PTGS2, KLK1, NPHS1, NPHS2, SGK, SLC12A1, PTGES), and others related to cardiovascular physiology (GJA4, NOS1, NTRK3, P2RX4, SPP1, ALDH2).

RESULTS

Multiple logistic analyses, with age and body mass index as covariates, indicated that 13 SNPs (eight genes), six SNPs (four genes) and 11 SNPs (four genes) were associated with hypertension (P < 0.05) in the total, male, and female populations, respectively. PTGS2 seems to be a promising candidate gene for hypertension in men. GSTM3 and SLC12A1 seem to be promising candidate genes for hypertension in women. Especially, a polymorphism in SLC12A1 was significantly associated with hypertension in women even after correction by the Bonferroni method (corrected P = 0.0236). Multiple logistic analyses, with age and body mass index as covariates, indicated that the prevalence of hypertension in females was significantly higher in subjects with the CC genotype than in those with the TT + TC genotypes (P < 0.0001, odds ratio = 1.967, 95% confidence interval = 1.430-2.712).

CONCLUSION

Although the present results should be replicated in other study populations for confirmation, the present results suggest that SLC12A1 may contribute to hypertension in Japanese women.

Stable expression of nephrin and localization to cell-cell contacts in novel murine podocyte cell lines

BACKGROUND

Cell culture of podocytes has become an indispensable tool in the study of podocyte biology. To date, however, podocyte cell lines with stable expression of the crucial slit diaphragm protein nephrin and localization of nephrin to cell-cell contacts are not available.

METHODS

Conditionally immortalized cells were grown from isolated glomeruli of mice, harboring the temperature-sensitive SV40 large T antigen. About 60 clonal cell lines were generated by limiting dilution.

RESULTS

Among 30 Wilm's tumor (WT)-1- and podocalyxin-positive cell clones, two cell clones stably expressed nephrin as assessed by reverse transcription-polymerase chain reaction (RT-PCR), Northern and Western blotting, and immunofluorescence. In addition, expression of the following podocyte proteins was demonstrated: NEHP1, FAT, P-cadherin, podocin, CD2AP, ZO-1 (alpha(-) isoform), Lmx1b, podoplanin, synaptopodin, cortactin, and vimentin. The nephrin-positive podocyte cell lines formed a monolayer with abundant cell-cell contacts. Transmission electron microscopy revealed formation of primitive foot process-like interdigitations and slit diaphragm-like junctions. Nephrin colocalized with F-actin at cell-cell contacts as demonstrated by immunofluorescence. Intriguingly, nephrin and actin-associated proteins (synaptopodin, CD2AP, and cortactin) were recruited to and accumulated at the entire cell margin only in confluent cells, but not in dispersed cells.

CONCLUSION

We present novel murine podocyte cell lines with stable expression of nephrin and abundant formation of cell-cell contacts, possessing several features of in situ podocyte cell-cell contacts. Furthermore, our data suggest that the accumulation of certain proteins in podocyte foot processes is linked to formation of cell-cell contacts.

Altered transcapillary escape of albumin and microalbuminuria reflects two different pathogenetic mechanisms

We studied the following in normo- and microalbuminuric hypertensive type 2 diabetic patients: 1) transcapillary escape rate of albumin (TERalb) and 2) expression of mRNA slit diaphragm and podocyte proteins in renal biopsies. Normoalbuminuric subjects had renal cancer, and kidney biopsy was performed during surgery. TERalb was evaluated by clearance of (125)I-albumin. Real-time PCR of mRNA slit diaphragm was measured in kidney specimens. Albumin excretion rate (AER) was by definition lower in normoalbuminuric subjects than in microalbuminuric subjects with typical diabetic glomerulopathy (group 1), in microalbuminuric subjects with normal or near-normal glomerular structure (group 2), and in microalbuminuric subjects with atypical diabetic nephropathy (group 3). This classification was based on light microscopy analysis of renal tissue. TERalb (%/h) was similar in normoalbuminuric and microalbuminuric group 1, 2, and 3 diabetic patients (medians: 14.1 vs. 14.4 vs. 15.7 vs. 14.9, respectively) (ANOVA, NS). mRNA expression of slit diaphragm proteins CD2AP, FAT, Actn 4, NPHS1, and NPHS2 was higher in normoalbuminuric patients than in microalbuminuric patients (groups 1, 2, and 3) (ANOVA, P < 0.001). All diabetic patients had greater carotid artery intimal thickness than normal control subjects using ultrasound technique (ANOVA, P < 0.01). In conclusion, the present study suggests that microalbuminuria identifies a subgroup of hypertensive type 2 diabetic patients who have altered mRNA expression of slit diaphragm and podocyte proteins, even before glomerular structure shows abnormalities using light microscopy analysis. On the contrary, altered TERalb and increased carotid artery intimal thickness are shown by all hypertensive type 2 diabetic patients, both with normal and altered patterns of AER.

Congenital nephrotic syndrome of Finnish type: detection of new nephrin mutations and prenatal diagnosis in an Italian family

OBJECTIVES

Congenital nephrotic syndrome of the Finnish type (CNF) is a rare autosomal recessive disorder, caused by mutations in the NPHS1 gene, coding for nephrin. The aim of this work was to investigate the disease mutations in a CNF Italian family and to perform genetic prenatal diagnosis in the second pregnancy.

METHODS

Polymerase chain reaction (PCR) and automatic sequence analysis were used to screen the CNF Italian family for NPHS1 mutations.

RESULTS

Two novel heterozygous mutations, including a single nucleotide insertion (c.248insA) and a missense mutation (p.S572N), were detected in the proband. Molecular prenatal diagnosis was performed on fetal DNA sample: the fetus resulted compound heterozygous for the same proband mutations.

CONCLUSION

To the best of our knowledge, this is the first report of a molecular prenatal diagnosis performed in an Italian family with congenital nephrotic syndrome of Finnish type (CNF). Our findings indicate that, even though CNF is not very common outside Finland, availability and reliability of DNA diagnostics are important issues to confirm the AFP results in prenatal diagnosis.

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.

Molecular analysis of NPHS2 and ACTN4 genes in a series of 33 Italian patients affected by adult-onset nonfamilial focal segmental glomerulosclerosis

BACKGROUND

Mutations in the NPHS2 gene, encoding podocin, and in the ACTN4 gene, encoding alpha-actinin-4, have been identified in familial childhood-onset forms of focal and segmental glomerulosclerosis (FSGS). NPHS2 may be also responsible for some sporadic cases. The role of NPHS2 and ACTN4 in the adult sporadic form of the disease is being clarifying.

METHODS

Thirty-three adult subjects affected by sporadic FSGS were studied at molecular level. At biopsy, 12 patients had nephrotic syndrome, 5 patients had isolated proteinuria and 16 patients showed proteinuria and hematuria. Glomerular filtration rate (GFR) was in the normal range in 19 subjects and 14 patients had a variable degree of renal failure. Multiplex families presenting with a clear familial inheritance for proteinuria or other congenital nephrotic syndrome were excluded. The whole coding region, all intron/exon boundaries and flanking intronic regions of NPHS2 gene and the exon 8, i.e. hot-spot mutations of the ACTN4 gene, were analyzed in all patients by denaturing high-performance liquid chromatography (DHPLC) to search disease-causing defects.

RESULTS

The analysis identified four already described and two new polymorphisms, IVS3-21C>T and IVS3-46C>T, on the NPHS2 gene. Moreover, the R229Q allele was identified in 3/33 patients and in 7/124 controls, accounting for an allelic frequency of 0.045 and 0.028, respectively. The new intronic polymorphism IVS7-54C>T was also found in the exon 8 of the ACTN4 gene.

CONCLUSIONS

In this study, we exhaustively analyzed the NPHS2 and the exon 8 of the ACTN4 genes in a series of sporadic 'adult-onset' FSGS patients. No causative mutations were found while the R229Q allele was identified in 3 patients confirming its possible role as a 'disease-associated NPHS2 allele' although its pathogenetic involvement needs to be further clarified. Moreover, the description of new intronic polymorphisms in both genes is reported.

Selective impairment of gene expression and assembly of nephrin in human diabetic nephropathy

BACKGROUND

Recent disclosure of podocyte proteins has unraveled previously rather mysterious mechanisms that govern glomerular perm-selectivity in health and disease. Here we addressed the role of nephrin, CD2-associated protein (CD2AP), and podocin together with the integrity of the slit diaphragm in the pathogenesis of proteinuria of patients with diabetes and nephropathy.

METHODS

Nephrin mRNA and protein expression were evaluated in parallel in adult diabetic patients by in situ hybridization and immunohistochemistry. For comparison, nondiabetic patients with minimal change nephrosis and normal control patients were evaluated. CD2AP and podocin expression by immunohistochemistry was also assessed. The filtration slit was analyzed by morphometry and transmission electron microscopy.

RESULTS

Extracellular nephrin mRNA and protein were markedly reduced in diabetic patients. No changes were found in patients with minimal change versus controls. CD2AP and podocin were comparable in all subjects. Ultrastructural analysis showed in diabetic patients a remarkable reduction in the percentage of electron dense slit diaphragms, despite a frequency of the filtration slits comparable to control patients.

CONCLUSION

Down-regulation of nephrin and loss of the electron dense structure of slit diaphragm indicate a novel mechanism accounting for proteinuria in diabetic nephropathy. To the extent that glomerular protein trafficking contributes to renal disease progression, our findings may have clinical relevance. Reduction of nephrin in the context of normal expression of CD2AP and podocin can be taken reasonably as a specific marker of renal disease in diabetes. Therapies targeted at correcting podocyte nephrin might be of value for diabetic medicine.

Nephrin strands contribute to a porous slit diaphragm scaffold as revealed by electron tomography

Nephrin is a key functional component of the slit diaphragm, the structurally unresolved molecular filter in renal glomerular capillaries. Abnormal nephrin or its absence results in severe proteinuria and loss of the slit diaphragm. The diaphragm is a thin extracellular membrane spanning the approximately 40-nm-wide filtration slit between podocyte foot processes covering the capillary surface. Using electron tomography, we show that the slit diaphragm comprises a network of winding molecular strands with pores the same size as or smaller than albumin molecules, as demonstrated in humans, rats, and mice. In the network, which is occasionally stratified, immunogold-nephrin antibodies labeled individually detectable globular cross strands, about 35 nm in length, lining the lateral elongated pores. The cross strands, emanating from both sides of the slit, contacted at the slit center but had free distal endings. Shorter strands associated with the cross strands were observed at their base. Immunolabeling of recombinant nephrin molecules on transfected cells and in vitrified solution corroborated the findings in kidney. Nephrin-deficient proteinuric patients with Finnish-type congenital nephrosis and nephrin-knockout mice had only narrow filtration slits that lacked the slit diaphragm network and the 35-nm-long strands but contained shorter molecular structures. The results suggest the direct involvement of nephrin molecules in constituting the macromolecule-retaining slit diaphragm and its pores.

Nephrin gene (NPHS1) in patients with minimal change nephrotic syndrome (MCNS)

BACKGROUND

Minimal change nephrotic syndrome (MCNS) is a major problem in pediatric nephrology. While the pathogenesis of MCNS is not known, the latest discoveries in the genetic diseases indicate that glomerular epithelial cells (podocytes) and the slit diaphragm play a primary role in development of proteinuria. Because nephrin is known to be a major component of the slit diaphragm, we analyzed the structure of nephrin gene (NPHS1) in patients with MCNS of different severity.

METHODS

Clinical data and DNA samples were collected from 25 adults who had biopsy-proven MCNS in childhood. A direct sequencing was performed to all 29 exons of the NPHS1 gene. The significance of the findings was evaluated by similar analysis of DNA samples from 25 healthy control patients.

RESULTS

The analysis of NPHS1 revealed no specific MCNS-associated mutation. However, 5 of the 25 MCNS patients had heterozygous allelic variants leading to nonconservative amino acid substitutions not previously reported (G879R; R800C; T294I; A916S). One of the five patients also had the Fin-major mutation, and two had new, conservative amino acid substitutions (S786N; A342G). Three of the five patients were classified as steroid sensitive, one was an early nonresponder, and one patient showed clear resistance to steroid treatment. Six known polymorphic changes in NPHS1 were also found, three of them leading to amino acid changes. The number of allelic variants was high both in MCNS patients and control patients (mean 3.0 and 2.6).

CONCLUSION

The results suggest that genetic changes in nephrin may have a pathogenetic role in some patients with MCNS.

Changes in glomerular perm-selectivity induced by angiotensin II imply podocyte dysfunction and slit diaphragm protein rearrangement

Molecular mechanisms governing the loss of glomerular membrane perm selectivity during progression of proteinuric kidney diseases are so far poorly defined. Discovery of the proteins of the podocyte slit diaphragm, including the nephrin-CD2AP-podocin complex, has represented a major breakthrough in understanding the crucial role of the glomerular epithelial layer in the pathogenesis of proteinuria in human congenital disorders. A number of studies have tried to address the role of nephrin in acquired proteinuric disorders with conflicting results. In human diabetic nephropathy a defect of nephrin gene and protein expression has been consistently reported, which translates in profound changes of filtration slit ultrastructural architecture. The exclusive effect of angiotensin II inhibitors of restoring deficient nephrin expression in proteinuric diseases underlines a close interaction between angiotensin II and podocyte proteins and indicates a fresh way to look at the renoprotective properties of these molecules.

The Wt1+/R394W mouse displays glomerulosclerosis and early-onset renal failure characteristic of human Denys-Drash syndrome

Renal failure is a frequent and costly complication of many chronic diseases, including diabetes and hypertension. One common feature of renal failure is glomerulosclerosis, the pathobiology of which is unclear. To help elucidate this, we generated a mouse strain carrying the missense mutation Wt1 R394W, which predisposes humans to glomerulosclerosis and early-onset renal failure (Denys-Drash syndrome [DDS]). Kidney development was normal in Wt1(+/R394W) heterozygotes. However, by 4 months of age 100% of male heterozygotes displayed proteinuria and glomerulosclerosis characteristic of DDS patients. This phenotype was observed in an MF1 background but not in a mixed B6/129 background, suggestive of the action of a strain-specific modifying gene(s). WT1 encodes a nuclear transcription factor, and the R394W mutation is known to impair this function. Therefore, to investigate the mechanism of Wt1 R394W-induced renal failure, the expression of genes whose deletion leads to glomerulosclerosis (NPHS1, NPHS2, and CD2AP) was quantitated. In mutant kidneys, NPHS1 and NPHS2 were only moderately downregulated (25 to 30%) at birth but not at 2 or 4 months. Expression of CD2AP was not changed at birth but was significantly upregulated at 2 and 4 months. Podocalyxin was downregulated by 20% in newborn kidneys but not in kidneys at later ages. Two other genes implicated in glomerulosclerosis, TGFB1 and IGF1, were upregulated at 2 months and at 2 and 4 months, respectively. It is not clear whether the significant alterations in gene expression are a cause or a consequence of the disease process. However, the data do suggest that Wt1 R394W-induced glomerulosclerosis may be independent of downregulation of the genes for NPHS1, NPHS2, CD2AP, and podocalyxin and may involve other genes yet to be implicated in renal failure. The Wt1(R394W) mouse recapitulates the pathology and disease progression observed in patients carrying the same mutation, and the mutation is completely penetrant in male animals. Thus, it will be a powerful and biologically relevant model for investigating the pathobiology of the earliest events in glomerulosclerosis.

Characterization of the interactions of the nephrin intracellular domain

Nephrin is a signalling cell-cell adhesion protein of the Ig superfamily and the first identified component of the slit diaphragm that forms the critical and ultimate part of the glomerular ultrafiltration barrier. The extracellular domains of the nephrin molecules form a network of homophilic and heterophilic interactions building the structural scaffold of the slit diaphragm between the podocyte foot processes. The intracellular domain of nephrin is connected indirectly to the actin cytoskeleton, is tyrosine phosphorylated, and mediates signalling from the slit diaphragm into the podocytes. CD2AP, podocin, Fyn kinase, and phosphoinositide 3-kinase are reported intracellular interacting partners of nephrin, although the biological roles of these interactions are unclarified. To characterize the structural properties and protein-protein interactions of the nephrin intracellular domain, we produced a series of recombinant nephrin proteins. These were able to bind all previously identified ligands, although the interaction with CD2AP appeared to be of extremely low stoichiometry. Fyn phosphorylated nephrin proteins efficiently in vitro. This phosphorylation was required for the binding of phosphoinositide 3-kinase, and significantly enhanced binding of Fyn itself. A protein of 190 kDa was found to associate with the immobilized glutathione S-transferase-nephrin. Peptide mass fingerprinting and amino acid sequencing identified this protein as IQGAP1, an effector protein of small GTPases Rac1 and Cdc42 and a putative regulator of cell-cell adherens junctions. IQGAP1 is expressed in podocytes at significant levels, and could be found at the immediate vicinity of the slit diaphragm. However, further studies are needed to confirm the biological significance of this interaction and its occurrence in vivo.

Gene expression analysis of human renal biopsies: recent developments towards molecular diagnosis of kidney disease

PURPOSE OF REVIEW

The analysis of renal tissue from kidney biopsies by histology, electron microscopy and immunohistology represents the current standards used to establish a specific diagnosis in nephrology. Recent progress in gene expression-based tissue analysis may provide fundamentally novel information in renal biopsy interpretation. In this review, progress towards the routine application of this approach is summarized.

RECENT FINDINGS

Renal disease is characterized by closely interrelated mechanisms of inflammation, repair, scarring and atrophy affecting over 20 different intrinsic renal cell types. The renal biopsy sample represents a 'snapshot' of these dynamic processes. A central question for molecular diagnosis is whether specific gene expression patterns can adequately define segments of these disease processes. Can molecular markers be extracted as effectively as has been shown in oncology? Several studies have been able to correlate renal gene expression patterns with clinical parameters, renal histological findings and patient follow-up data. In small populations, molecular markers have been able to provide novel diagnostic, prognostic and differential therapeutic information beyond conventional histology.

SUMMARY

A growing number of renal gene expression projects are generating targets for the integration of molecular approaches into kidney biopsy evaluation. If these molecular makers can pass rigorous testing for their diagnostic value, they should become an indispensable part of the management of the renal patient.

Genetic forms of nephrotic syndrome

Mutations of NPHS1, NPHS2, or WT1 may be responsible for severe forms of nephrotic syndrome in children, progressing to end-stage renal failure. Recent studies have shown that congenital nephrotic syndrome may be secondary to mutations of one of these three genes and that some patients have a digenic inheritance of NPHS1 and NPHS2 mutations. The clinical spectrum of NPHS2 mutations has broadened, with the demonstration that mutations in the respective gene podocin may be responsible for nephrotic syndrome occurring at birth, in childhood, or in adulthood. It is now well recognized that podocin mutations are found in 10%-30% of sporadic cases of steroid-resistant nephrotic syndrome with focal segmental glomerulosclerosis. Data from large cohorts indicate that the risk of recurrence of nephrotic syndrome after renal transplantation in patients with podocin mutations is very low.

No evidence for genotype/phenotype correlation in NPHS1 and NPHS2 mutations

Primary steroid-resistant nephrotic syndrome (SRNS) is characterized by childhood onset of proteinuria and progression to end-stage renal disease. In 26% of cases it is caused by recessive mutations in NPHS2 (podocin). Congenital nephrotic syndrome (CNS) is caused by mutations in NPHS1 (nephrin) or NPHS2. In three families mutations in NPHS1 and NPHS2 had been reported to occur together, and these tri-allelic mutations were implicated in genotype/phenotype correlations. To further test the hypothesis of tri-allelism, we examined a group of 62 unrelated patients for NPHS1 mutations, who were previously shown to have NPHS2 mutations; 15 of 62 patients had CNS. In addition, 12 CNS patients without NPHS2 mutation were examined for NPHS1 mutations. Mutational analysis yielded three different groups. (1) In 48 patients with two recessive NPHS2 mutations (11 with CNS), no NPHS1 mutation was detected, except for 1 patient, who had one NPHS1 mutation only. This patient was indistinguishable clinically and did not have CNS. (2) In 14 patients with one NPHS2 mutation only (4 with CNS), we detected two additional recessive NPHS1 mutations in the 4 patients with CNS. They all carried the R229Q variant of NPHS2. The CNS phenotype may be sufficiently explained by the presence of two NPHS1 mutations. (3) In 12 patients without NPHS2 mutation (all with CNS), we detected two recessive NPHS1 mutations in 11 patients, explaining their CNS phenotype. We report ten novel mutations in the nephrin gene. Our data do not suggest any genotype/phenotype correlation in the 5 patients with mutations in both the NPHS1 and the NPHS2 genes.

The cellular basis of albuminuria

The appearance of albumin in the urine has long been recognized as a cardinal feature of kidney disease and more recently has been shown to also be an independent cardiovascular risk factor associated with insulin resistance. Recent studies on rare human genetic variants, targeted gene disruption in mouse models and cultured glomerular cells in vitro have dramatically improved our understanding of the cellular and molecular basis of albuminuria. This review aims to summarize the current state of knowledge, to illustrate known mechanisms of proteinuria in disease states and to suggest a possible explanation for the link between albuminuria and insulin resistance.

Effect of the knockdown of podocin mRNA on nephrin and alpha-actinin in mouse podocyte

Recently, the novel podocyte proteins podocin, nephrin, and alpha-actinin-4 have been identified in three congenital/family nephrotic syndromes, respectively. Further studies showed that these podocyte proteins were involved in some acquired nephrotic syndromes and various experimental models of proteinuria. However, the molecular interactions among these podocyte proteins remain unclear. In this study, to investigate the molecular interactions among podocin, nephrin, and alpha-actinin-4, we reconstructed the RNA interference (RNAi) expression vector, pSilencer 2.1-U6, specifically targeting podocin mRNA, and it was transfected into the mouse podocyte clone (MPC5). Immunofluorescence staining, double-immunolabeling, confocal microscopy, semiquantitative reverse transcription polymerase chain reaction (RT-PCR), and Western blotting were used to detect the distribution and expression of podocin, nephrin, alpha-actinin-4, and glyseraldehyde-3-phosphate dehydrogenase (GAPDH)/beta-actin. The fluorescence intensity of podocin and nephrin decreased obviously, along with the evident distribution change from the cell membrane surface to the nucleus circumference in podocyte. In relation to GAPDH, the mRNA reductions of podocin and nephrin were observed by about 65% and 70%, respectively. The expression of podocin protein was too low to be detected in the interference group. In relation to beta-actin, the protein level of nephrin decreased by about 78%. The distribution and the mRNA and protein level of alpha-actinin showed no appreciable change. Alpha-actinin localized mainly in the cytoplasm and also extended to the processes. Thus, the significant decreased expression of nephrin along with the redistribution were detected with the knockdown of podocin mRNA, whereas the expression and distribution of alpha-actinin-4 showed no change. These results suggest that podocin may interact directly with nephrin, but not with alpha-actinin.

Molecular basis of proteinuria

The glomerular filtration barrier is composed of endothelial cells, basement membrane, and podocytes. In recent years, remarkable progress has been made in our understanding of the molecular structure of the filtration barrier and its relation to the effectiveness of the barrier function. The glomerular basement membrane is composed of a multitude of proteins, including collagen IV, heparan sulfate proteoglycans, and laminin, among others. The slit diaphragm, which is seen as a membrane covering the space between adjacent foot processes close to the basement membrane, is an extremely important structure with a crucial role in permselectivity of the filtration barrier. Its composition is now understood to consist primarily of a unique protein called nephrin. Mutations in the gene-encoding nephrin are known to result in the Finnish type of nephrotic syndrome. The exact mechanism by which nephrin controls permselectivity is not yet clear, but it is known to interact with several podocyte proteins including CD2AP, podocin, and alpha-actinin-4. Abnormalities of any of these proteins may result in proteinuria. The role of nephrin and its associated proteins in the pathogenesis of common acquired glomerulopathies in humans is still under investigation. Normal function of podocyte also depends upon maintaining a fully mature and terminally differentiated phenotype. A host of transcription factors, especially WT1 and PAX2, play a significant role in modulating podocyte function.

In vivo expression of podocyte slit diaphragm-associated proteins in nephrotic patients with NPHS2 mutation

BACKGROUND

Mutations in NPHS2, encoding podocin, are a prevalent cause of autosomal-recessive steroid-resistant nephrotic syndrome (SRNS). Podocin is a protein associated with the slit diaphragm that interacts with nephrin and CD2-associated protein (CD2AP) within lipid rafts.

METHODS

Using renal biopsies of six patients, we analyzed the in vivo consequences of different types of NPHS2 mutations on (1) the podocyte expression and distribution of podocin using in situ hybridization and immunohistology and (2) the distribution of related podocyte proteins and glomerular extracellular matrix components.

RESULTS

In two patients with homozygous 855_856delAA or 419delG mutation, absence of podocyte labeling with the antibodies against the C-terminal domain contrasted with the normal expression of the N-terminal domain of the protein along the glomerular basement membrane (GBM). In patients carrying compound heterozygous mutations or variants (R168S/467_468insT, R138Q/V180M, and R291W/R229Q), or single heterozygous 976_977insA, podocin transcription appeared unchanged but the distribution of the protein was modified. Podocin was restricted to the podocyte body in the patient carrying the R168S/467_468insT mutation whereas strong immunolabeling of the podocyte body was associated with discrete labeling along the GBM in the three others. In all cases, podocin defect was associated with changes in the distribution of nephrin, CD2AP, and alpha-actinin: the proteins were mainly detected in the podocyte body, with mild expression along the GBM. There were no detectable changes in the distribution of other podocyte proteins or glomerular extracellular matrix components.

CONCLUSION

NPHS2 mutations result in profound alteration of podocin expression and/or distribution. Secondary changes in the distribution of nephrin, CD2AP, and alpha-actinin are additional evidences for the scaffolding role of podocin in the organization of the slit diaphragm.

Disease-causing missense mutations in NPHS2 gene alter normal nephrin trafficking to the plasma membrane

BACKGROUND

Podocin is a membrane-integrated protein that is located at the glomerular slit diaphragm and directly interacts with nephrin. The gene encoding podocin, NPHS2, is mutated in patients with autosomal-recessive steroid-resistant nephrotic syndrome (SRN). In order to study a potential pathomechanism of massive proteinuria in patients with SRN, we have investigated the trafficking and subcellular localization of five common disease-causing missense mutants of human podocin.

METHODS

Site-directed mutagenesis was applied to generate cDNA constructs encoding five different missense mutations of human podocin (P20L, G92C, R138Q, V180M, and R291W). To identify the subcellular localization of each mutant in transfected human embryonic kidney (HEK)293 cells, we have generated and characterized a rabbit polyclonal antibody against the human podocin. Specificity of the antibody was determined by light and immunoelectron microscopy, as well as immunoblot analysis using human glomeruli. Confocal microscopy was applied to determine subcellular localization of the wild-type and the mutated podocin molecules, as well as wild-type nephrin in transfected cells. Immunoprecipitation and pull-down studies were carried out to investigate the molecular interaction of podocin mutants and wild-type nephrin.

RESULTS

Immunofluorescence and confocal microscopy showed that wild-type podocin located to the plasma membrane when expressed in HEK293 cells. Two missense mutations, P20L and G92C, located at the N-terminus part of the molecule, were also present at the plasma membrane, indicating that these mutations did not affect the subcellular localization of the mutated podocin molecules. In contrast, subcellular localization of three other missense mutants located in the proximal C-terminus part of the protein was drastically altered, in which R138Q was retained in the endoplasmic reticulum (ER), V180M formed inclusion bodies in the cytoplasm, and the R291W mutant was trapped both in the ER and in small intracellular vesicles. Interestingly, this abnormal subcellular localization of podocin missense mutants also resulted in alteration in protein trafficking of wild-type nephrin in cotransfected cells through the strong protein binding between both molecules.

CONCLUSION

In patients with SRN, some missense mutations in the NPHS2 gene not only lead to misfolding and mislocalization of the mutated podocin, but they can also interfere with slit diaphragm structure and function by altering the proper trafficking of nephrin to the plasma membrane.

Podocyte foot process effacement is not correlated with the level of proteinuria in human glomerulopathies

BACKGROUND

Nephrotic syndromes result from increased glomerular permeability to proteins and are structurally believed to be associated with podocyte foot process effacement. Despite increasing knowledge of the molecular composition of the glomerular filtration barrier, the relationship between proteinuria and foot process effacement is unclear.

METHODS

We conducted a morphologic study on the relationship between podocyte foot process effacement and proteinuria. Electron microscope pictures of glomerular capillaries were randomly taken from 27 cases in various stages of minimal change nephrotic syndrome (MCNS), from six cases of IgA nephropathy (IgAN) with high proteinuria and from seven control kidneys. From each picture, the mean width of the foot processes (FPW) was quantitated.

RESULTS

In normal kidney the mean FPW was 580 +/- 40 nm. In biopsies from patients with MCNS without treatment, foot processes were diffusely effaced, reflected by a FPW of 1600 +/- 440 nm. In biopsies from patients with MCNS relapsing under prednisolone treatment, foot processes were significantly less effaced than in untreated MCNS (FPW 920 +/- 200 nm). In biopsies displaying IgAN, effacement was significantly more segmental than in untreated MCNS (FPW 800 +/- 170 nm). Proteinuria did not differ significantly among the groups. Neither in MCNS nor in IgAN was the extent of foot process effacement correlated with the level of proteinuria.

CONCLUSION

Podocyte foot process effacement is not correlated with proteinuria. The differences in podocyte effacement between MCNS, MCNS relapsing under prednisolone treatment, and IgAN may point to different mechanisms of podocyte injury in these diseases.

A novel mutation of NPHS2 identified in a Chinese family

Since the identification of the NPHS2 gene,which encodes podocin, several groups from European, Middle Eastern, and North American countries have reported NPHS2 mutations in families with steroid-resistant nephrotic syndrome (SRNS) or focal segmental glomerulo sclerosis (FSGS). Families with SRNS have also been reported in China with a population of more than1.3 billion. However, to our knowledge, there is no mutational analysis of the NPHS2 gene in familial SRNS orFSGS in China. We identified a novel mutation of NPHS2(467_468insT and 503G>A) in a Chinese family with autosomal recessive SRNS using polymerase chain re-action, denaturing high-performance liquid chromatography, and DNA sequencing techniques. The results demonstrate that there is also NPHS2 mutation in Chinese familial SRNS. Therefore, Chinese SRNS patients with a familial history of NS should also be screened for possible mutations of NPHS2. We also detected clearly decreased staining with a specific podocin C-terminal antibody(P35) and negative staining with a specific podocin N-terminal antibody (P21). These results were contrary to those predicted from the mutated sites. Further studies are needed to explore the mechanism and impact of the mutant gene on the expression and localization of the relevant protein.

Prevalence of WT1 mutations in a large cohort of patients with steroid-resistant and steroid-sensitive nephrotic syndrome

BACKGROUND

Nephrotic syndrome (NS) represents the association of proteinuria, hypoalbuminemia, edema, and hyperlipidemia. Steroid-resistant nephrotic syndrome (SRNS) is defined by primary resistance to standard steroid therapy. It remains one of the most intractable causes for end-stage renal disease (ESRD) in the first two decades of life. Sporadic mutations in the Wilms' tumor suppressor gene WT1 have been found to be present in patients with SRNS in association with Wilms' tumor (WT) and urinary or genital malformations, as well as in patients with isolated SRNS.

METHODS

To further evaluate the incidence of WT1 mutations in patients with NS we performed mutational analysis in 115 sporadic cases of SRNS and in 110 sporadic cases of steroid-sensitive nephrotic syndrome (SSNS) as a control group. Sixty out of 115 (52%) patients with sporadic SRNS were male, 55/115 (48%) were female. Sex genotype was verified by haplotype analysis. Mutational analysis was performed by direct sequencing and by denaturing high-performance liquid chromatography (DHPLC).

RESULTS

Mutations in WT1 were found in 3/60 (5%) male (sex genotype) cases and 5/55 (9%) female (sex genotype) cases of sporadic SRNS, and 0/110 (0%) sporadic cases of SSNS. One out of five female patients with mutations in WT1 developed a WT, 2/3 male patients presented with the association of urinary and genital malformations, 1/3 male patients presented with sexual reversal (female phenotype) and bilateral gonadoblastoma, and 4/5 female patients presented with isolated SRNS.

CONCLUSION

According to the data acquired in this study, patients presenting with a female phenotype and SRNS and male patients presenting with genital abnormalities should especially be screened to take advantage of the important genetic information on potential Wilms' tumor risk and differential therapy. This will also help to provide more data on the phenotype/genotype correlation in this patient population.

Role of truncating mutations in MME gene in fetomaternal alloimmunisation and antenatal glomerulopathies

BACKGROUND

Membranous glomerulonephritis is an immune-mediated disease. In a recent case of antenatal membranous glomerulonephritis, we identified neutral endopeptidase (NEP) as the podocyte target antigen of circulating antibodies produced by the mother who failed to express NEP on granulocytes. We aimed to investigate whether the disease could affect other families, to search for mutations in the metallomembrane endopeptidase (MME) gene for NEP, and to analyse the outcome of the antenatal renal insult.

METHODS

From three families with a case of neonatal membranous glomerulopathy, we detected mutations by direct sequencing of genomic PCR products. Single nucleotide polymorphism (SNP) analysis was undertaken with five SNPs located in the MME gene. IgG subclasses with anti-NEP activity were determined by western blotting.

FINDINGS

In five mothers, we identified two compound heterozygous or homozygous mutations in the MME gene. The first, a 1342C-->T nonsense mutation, was detected in one family. The second, 446delC, was detected in all three families; all chromosomes bearing this mutation had the same alleles for the five SNPs. Severity of neonatal renal disease was determined by the mothers' IgG response to fetal NEP antigens expressed on glomerular podocytes. The oldest affected individual, now aged 20 years, has developed severe chronic renal failure.

INTERPRETATION

Truncating mutations in the MME gene are the cause of alloimmunisation during pregnancy. Idiopathic renal failure in early adulthood might be caused by immune-mediated fetal nephron loss. We show that disease caused by fetomaternal alloimmunisation secondary to a genetic defect is not restricted to blood cells.

RELEVANCE TO CLINICAL PRACTICE

During pregnancy, the absence of the NEP protein induces an alloimmunisation process against NEP presented by fetal cells, including syncytiotrophoblasts. The fetal podocyte insult and ensuing nephron loss could lead to chronic renal failure in early adulthood. Alloimmunisation against NEP should be considered as a leading cause of membranous glomerulopathy early in life. Concentrations of circulating anti-NEP antibodies should be carefully monitored during subsequent pregnancies, and specific therapeutic approaches developed. This new disease might also account for idiopathic chronic renal failure detected during adolescence, in individuals who can be identified by searching for anti-NEP antibodies in their mother and by MME gene mutation analysis. NEP deficiency should also be considered in patients developing de-novo membranous glomerulopathy after renal transplantation.

Case Report. A novel NPHS2 gene mutation in Turkish children with familial steroid-resistant nephrotic syndrome

We report in this paper two siblings aged 8 and 17 months who were clinically diagnosed with familial steroid-resistant nephrotic syndrome (SRNS). By mutation screening of the NPHS2 gene, a homozygous missense mutation, P118L, was detected in both children. This study is the first systematic investigation of NPHS2 gene mutations in Turkish children with familial SRNS. If this mutation is a hot spot of mutation in the Turkish population, screening this novel mutation in Turkish children with SRNS may be of great clinical use to prevent unnecessary treatment modalities, provide accurate genetic counselling and predict the prognosis of the disease.

Ultrastructural study on nephrin expression in experimental puromycin aminonucleoside nephrosis

BACKGROUND

Nephrin is a recently identified protein that is a key component of the slit diaphragm. This protein may play a crucial role in maintaining the glomerular filtration barrier, and mutations in the gene for nephrin reportedly lead to congenital nephrosis. However, the expression of nephrin in acquired glomerular disease has not yet been fully clarified. To address this issue, we analysed the expression and localization of nephrin by morphological analysis based on immunoelectron microscopy in normal glomeruli and in glomeruli from proteinuric experimental models.

METHODS

Twenty rats were divided into three experimental groups (n = 16 total) and a control group (n = 4). Rats in the experimental groups received a single intravenous injection of puromycin aminonucleoside (PAN), and were sacrificed at 1 (n = 4), 2 (n = 6) and 3 weeks (n = 6) post-injection. Nephrin expression was assessed by immunoelectron microscopy using a polyclonal antibody against nephrin and gold particles. It was quantified by counting the gold particles and the slit diaphragms and by measuring the average foot process width in microphotographs.

RESULTS

The average foot process width in the 1 week group (5924.5 +/- 1523.9 nm) was far greater than that of controls (1112.9 +/- 79.8 nm), but decreased thereafter. The average number of total gold particles per unit length (10 000 nm) of the glomerular basement membrane (GBM) underlying the foot processes was reduced at 1 week (26.0 +/- 9.5), compared with controls (335.3 +/- 125.5), but increased thereafter. Also, the average number of junctional gold particles per unit length of the GBM was lower than controls (208.4 +/- 1.7) at 1 week (10.1 +/- 3.5), but increased thereafter. There were no significant differences between the numbers of junctional gold particles per slit diaphragm among the groups, but significant differences were observed in the distributions of gold particles among the groups. Gold particles were more frequently seen in cytoplasm at 1 week.

CONCLUSIONS

The present ultrastructural studies showed that nephrin expression and its distribution were altered in PAN-treated rats, and this occurred in parallel with foot process effacement. Nephrin expression returned to normal with improved resolution of the effacement. Nephrin expression was found to be rather preserved in areas without foot process effacement, even in PAN-treated rats. The significance of the above findings in terms of proteinuria and foot process effacement needs further clarification.

Transcriptional control of kidney development

In recent years, gene inactivation in the mouse and other model systems has shed new light on the processes of inductive tissue interactions and morphogenesis. These studies have been especially fruitful for understanding the kidney, as this organ has been a classical model of organogenesis for more than 50 years and is thus well characterized in terms of morphology and inductive properties. One outcome of these molecular genetic experiments is that the coordination of kidney development appears to be in good part performed at the transcriptional level. Many of the gene mutations associated with kidney malformations and disease are indeed transcription factors regulating key tissue interaction events. This review primarily addresses the role of the most significant transcription factors in mouse nephrogenesis.

Key molecular events in puromycin aminonucleoside nephrosis rats

Nephrin, podocin and alpha-actinin are all involved in proteinuria, but it is unclear which molecular event plays a crucial role during the development of proteinuria. Immunofluorescence staining and real-time quantitative polymerase chain reaction were used to study the glomerular expression of these molecules in puromycin aminonucleoside (PAN) nephrosis. Morphometric methods were applied to evaluate the podocyte foot process (FP) morphology. Two days after PAN injection, nephrin and podocin staining became discontinuous, podocin intensity decreased and FP swelled. Nephrin protein and mRNA decreased at day 5. Both podocin and nephrin intensity decreased dramatically when heavy proteinuria occurred, but nephrin mRNA was regained. When proteinuria disappeared, podocin recovered whereas nephrin did not (P = 0.02); alpha-actinin intensity increased (P = 0.009) and the distribution changed. The podocyte FP volume density correlated negatively with nephrin (r = -0.78, P = 0.0001) and podocin immunofluorescence intensity (r = -0.76, P = 0.0001). We conclude that, before the onset of proteinuria, the first response was the nephrin and podocin distribution change, podocin protein decrease and swollen FP; the podocin quantitative change was earlier than nephrin. Podocin and nephrin distribution and the protein level was associated with proteinuria more closely than their mRNA level. The delayed alpha-actinin induction might be a reparative response.