Proteinuria and perinatal lethality in mice lacking NEPH1, a novel protein with homology to NEPHRIN

Preferential adhesion mediated by Hibris and Roughest regulates morphogenesis and patterning in the Drosophila eye

Cell adhesion is essential for morphogenesis; however, the mechanisms by which cell adhesion coordinates precisely regulated morphogenesis are poorly understood. Here we analyze the morphogenetic processes that organize the interommatidial precursor cells (IPCs) of the Drosophila pupal eye. We demonstrate that the Drosophila immunoglobulin superfamily members Hibris and Roughest are essential for IPC morphogenesis in the eye. The two loci are expressed in complementary cell types, and Hibris and Roughest proteins bind directly in vivo. Primary pigment cells employ Hibris to function as organizers in this process; IPCs minimize contacts with neighboring IPCs and utilize Roughest to maximize contacts with primaries. In addition, we provide evidence that interactions between Hibris and Roughest promote junction formation and that levels of Roughest in individual cells determine their capacity for competition. Our results demonstrate that preferential adhesion mediated by heterophilic interacting cell-adhesion molecules can create a precise pattern by minimizing surface free energy.

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.

Statins prevent oxidized LDL-induced injury of glomerular podocytes by activating the phosphatidylinositol 3-kinase/AKT-signaling pathway

The injury of podocytes is associated with alterations of the glomerular size-selective barrier to proteins. In this study, oxidized LDL (oxLDL) but not native LDL induced apoptosis in human cultured podocytes and reduced Akt activity and P-Akt/Akt ratio. Moreover, oxLDL-induced redistribution and loss of nephrin, an adhesion molecule specific for the glomerular slit diaphragm. Nephrin reduction was preceded by inhibition of nephrin tyrosine phosphorylation and of its association with p85 phosphatidylinositol 3-kinase (PI3K). Moreover, three different statins, mevastatin, pravastatin, and simvastatin, inhibited in a dose-dependent manner apoptosis and loss of nephrin induced by oxLDL by stimulating Akt activity. In addition, simvastatin significantly increased the expression of nephrin protein and mRNA by podocytes. The protective effects of statins were blocked by treatment of podocytes with two unrelated pharmacologic inhibitors of PI3K, LY294002 and wortmannin, suggesting a role for PI3K, and by mevalonate, indicating dependency on HMG-CoA reductase activity. Statins directly stimulated Akt phosphorylation ad activity. Finally, oxLDL induced a retraction of cultured podocytes and an increase in the albumin diffusion across their monolayer that was inhibited by treatment with statins. In conclusion, statins reduced the oxLDL-induced apoptosis and loss of nephrin in glomerular podocytes. The statin-induced Akt activation may protect from the loss of nephrin by an inhibition of its redistribution and shedding and by a stimulation of its synthesis. These data provide a rationale for the anti-proteinuric effect of statins.

NEPH2 Is Located at the Glomerular Slit Diaphragm, Interacts with Nephrin and Is Cleaved from Podocytes by Metalloproteinases

The NEPH family comprises three transmembrane proteins of the Ig superfamily interacting with the glomerular slit diaphragm proteins podocin and ZO-1. NEPH1 binds to nephrin, another component of the slit diaphragm, and loss of either partner causes heavy proteinuria. NEPH2, which is strongly conserved among a large number of species, is also expressed in the kidney; however, its function is unknown. The authors raised NEPH2 antisera to demonstrate NEPH2 expression in a variety of mouse tissues, including the kidney and a podocyte cell line. The authors localized the expression of NEPH2 to the glomerular slit diaphragm by electron microscopy and show NEPH2 homodimerization and specific interactions with the extracellular domain of nephrin in vitro and in vivo. NEPH1, however, failed to interact with NEPH2. The authors detected immunoreactive NEPH2 in urine of healthy subjects, suggesting that the extracellular domain is cleaved under physiologic conditions. These findings were confirmed in vitro in podocyte cell culture. Shedding is increased by tyrosine phosphatase inhibitors and diminished by GM6001, an inhibitor of metalloproteinases. Overexpression experiments indicate an involvement of the MT1-matrix metalloproteinase. The results suggest a role for NEPH2 in the organization and/or maintenance of the glomerular slit diaphragm that may differ from the functions of NEPH1 and nephrin.

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.

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.

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.

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.

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.

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.

Unraveling the genetics of chronic kidney disease using animal models

Identifying genes underlying common forms of kidney disease in humans has proven difficult, expensive, and time consuming. Quantitative trait loci (QTL) for several complex traits are concordant among mice, rats, and humans, suggesting that genetic findings from these animal models are relevant to human disease. Therefore, we reviewed the literature on genetic studies of kidney disease in rat and mouse and examined the concordance between kidney disease QTL across species. Fifteen genomic regions contribute to kidney disease in the rat, with 12 replicated either in a separate rat cross or in another species. Five loci found in humans were concordant to QTL found in the rat. Two of these were found by homology to a previously identified rat QTL on chromosome 1, demonstrating that kidney disease loci in animal models can predict the location of kidney disease loci in humans. In contrast to the rat, the mouse has been underutilized in the genetic analysis of polygenic kidney disease, although mutagenesis and QTL analysis in the mouse are likely to contribute new findings in the near future. Knowledge of kidney disease loci conserved between the mouse and rat will identify prime candidate loci to test for association with chronic kidney disease in humans.

Antiproteinuric effect of RAS blockade: New mechanisms

Experimental and clinical studies have shown that blockade of the renin-angiotensin system (RAS) is effective in reducing proteinuria in conditions such as diabetes by reducing systemic and intraglomerular hydrostatic pressure. However, increasing evidence suggests that nonhemodynamic effects, such as preservation of the podocyte slit diaphragm structure and function, may also mediate the antiproteinuric effects of RAS blockade. In this review, we analyze in detail the evidence for known and novel mechanisms considered to play important roles in mediating the antiproteinuric effect of RAS blockers, with a particular focus on diabetic nephropathy.

Induction of B7-1 in podocytes is associated with nephrotic syndrome

Kidney podocytes and their slit diaphragms form the final barrier to urinary protein loss. This explains why podocyte injury is typically associated with nephrotic syndrome. The present study uncovered an unanticipated novel role for costimulatory molecule B7-1 in podocytes as an inducible modifier of glomerular permselectivity. B7-1 in podocytes was found in genetic, drug-induced, immune-mediated, and bacterial toxin-induced experimental kidney diseases with nephrotic syndrome. The clinical significance of our results is underscored by the observation that podocyte expression of B7-1 correlated with the severity of human lupus nephritis. In vivo, exposure to low-dose LPS rapidly upregulates B7-1 in podocytes of WT and SCID mice, leading to nephrotic-range proteinuria. Mice lacking B7-1 are protected from LPS-induced nephrotic syndrome, suggesting a link between podocyte B7-1 expression and proteinuria. LPS signaling through toll-like receptor-4 reorganized the podocyte actin cytoskeleton in vitro, and activation of B7-1 in cultured podocytes led to reorganization of vital slit diaphragm proteins. In summary, upregulation of B7-1 in podocytes may contribute to the pathogenesis of proteinuria by disrupting the glomerular filter and provides a novel molecular target to tackle proteinuric kidney diseases. Our findings suggest a novel function for B7-1 in danger signaling by nonimmune cells.

Alpha-actinin-4-mediated FSGS: an inherited kidney disease caused by an aggregated and rapidly degraded cytoskeletal protein

Focal segmental glomerulosclerosis (FSGS) is a common pattern of renal injury, seen as both a primary disorder and as a consequence of underlying insults such as diabetes, HIV infection, and hypertension. Point mutations in theα-actinin-4 gene ACTN4 cause an autosomal dominant form of human FSGS. We characterized the biological effect of these mutations by biochemical assays, cell-based studies, and the development of a new mouse model. We found that a fraction of the mutant protein forms large aggregates with a high sedimentation coefficient. Localization of mutant α-actinin-4 in transfected and injected cells, as well as in situ glomeruli, showed aggregates of the mutant protein. Video microscopy showed the mutant α-actinin-4 to be markedly less dynamic than the wild-type protein. We developed a “knockin” mouse model by replacing Actn4 with a copy of the gene bearing an FSGS-associated point mutation. We used cells from these mice to show increased degradation of mutant α-actinin-4, mediated, at least in part, by the ubiquitin–proteasome pathway. We correlate these findings with studies of α-actinin-4 expression in human samples. “Knockin” mice with a disease-associated Actn4 mutation develop a phenotype similar to that observed in humans. Comparison of the phenotype in wild-type, heterozygous, and homozygous Actn4 “knockin” and “knockout” mice, together with our in vitro data, suggests that the phenotypes in mice and humans involve both gain-of-function and loss-of-function mechanisms.

Transgenic experiments in mice suggest that the human kidney disorder, focal segmental glomerulosclerosis (FSGS), is a result of both gain- and loss-of-function mechanisms

Podocyte migration during nephrotic syndrome requires a coordinated interplay between cathepsin L and alpha3 integrin

Podocyte foot process effacement and disruption of the slit diaphragm are typically associated with glomerular proteinuria and can be induced in rats by the injection of puromycin aminonucleoside. Here, we show that the induction of puromycin aminonucleoside nephrosis involves podocyte migration conducted by a coordinated interplay between the cysteine protease cathepsin L and alpha(3) integrin. Puromycin aminonucleoside treatment up-regulates cathepsin L expression in podocytes in vivo as well as expression and enzymatic activity of cathepsin L in podocytes in vitro. Isolated podocytes from mice lacking cathepsin L are protected from cell puromycin aminonucleoside-induced cell detachment. The functional significance of cathepsin L expression was underscored by the observation that puromycin aminonucleoside-induced cell migration was slowed down in cathepsin L-deficient podocytes and by the preservation of cell-cell contacts and expression of vital slit diaphragm protein CD2AP. Cathepsin L expression and activity were induced in podocytes lacking alpha(3) integrin. Similarly, acute functional inhibition of alpha(3) integrin in wild type podocytes with a blocking antibody increased the expression of cathepsin L activity. Down-regulation of alpha(3) integrin protected against puromycin aminonucleoside-induced podocyte detachment. In summary, these data establish that podocyte foot process effacement is a migratory event involving a novel interplay between cathepsin L and alpha(3) integrin.

Synaptic adhesion molecules

Formation, differentiation and plasticity of synapses, the specialized cell-cell contacts through which neurons communicate, all require interactions between pre- and post-synaptic partners. Several synaptically localized adhesion molecules potentially capable of mediating these interactions have been identified recently. Functional studies suggest roles for some of them in target recognition (e.g. SYG-1 and sidekicks), formation and alignment of synaptic specializations (e.g. SynCAM, neuroligin and neurexin), and regulation of synaptic structure and function (e.g. cadherins and syndecan).

A podocentric view of nephrology

PURPOSE OF REVIEW

The glomerular visceral epithelial cell plays a central role in ultrafiltration of the blood and in a wide variety of inherited and acquired diseases of the kidney. The discovery of nephrin and other slit diaphragm proteins has led to an explosion of knowledge in the biology of this cell type. The most significant recent discoveries are reviewed in this paper.

RECENT FINDINGS

Together with the glomerular endothelial cells and intervening glomerular basement membrane, the podocyte constitutes a major portion of the glomerular filtration barrier that separates blood from the urinary space. A number of proteins have been identified that are localized to the slit diaphragms that separate podocyte foot processes. Although it has been suggested that the slit diaphragm represents the ultimate filtration barrier, additional roles for this structure as a signaling centre and in endocytosis have been identified. Mutations in genes that reside in the slit pore or interact with the actin cytoskeleton have been linked to a variety of inherited diseases of the podocyte. Additional mutations in these genes have been linked to sporadic forms of nephrotic syndrome and proposed as modifiers of renal risk. The generation of podocyte-specific transgenic models and genomic tools for the murine podocyte provide important resources for the glomerular biologist.

SUMMARY

Over the past year, studies using human genetics, conditional gene targeting and cell biological approaches have led to a rapid increase in our understanding of podocyte and glomerular biology, which should lead to the development of novel therapies for individuals with glomerular disease.

Induction of B7-1 in podocytes is associated with nephrotic syndrome

Kidney podocytes and their slit diaphragms form the final barrier to urinary protein loss. This explains why podocyte injury is typically associated with nephrotic syndrome. The present study uncovered an unanticipated novel role for costimulatory molecule B7-1 in podocytes as an inducible modifier of glomerular permselectivity. B7-1 in podocytes was found in genetic, drug-induced, immune-mediated, and bacterial toxin-induced experimental kidney diseases with nephrotic syndrome. The clinical significance of our results is underscored by the observation that podocyte expression of B7-1 correlated with the severity of human lupus nephritis. In vivo, exposure to low-dose LPS rapidly upregulates B7-1 in podocytes of WT and SCID mice, leading to nephrotic-range proteinuria. Mice lacking B7-1 are protected from LPS-induced nephrotic syndrome, suggesting a link between podocyte B7-1 expression and proteinuria. LPS signaling through toll-like receptor-4 reorganized the podocyte actin cytoskeleton in vitro, and activation of B7-1 in cultured podocytes led to reorganization of vital slit diaphragm proteins. In summary, upregulation of B7-1 in podocytes may contribute to the pathogenesis of proteinuria by disrupting the glomerular filter and provides a novel molecular target to tackle proteinuric kidney diseases. Our findings suggest a novel function for B7-1 in danger signaling by nonimmune cells.

Synaptic specificity is generated by the synaptic guidepost protein SYG-2 and its receptor, SYG-1

Synaptic connections in the nervous system are directed onto specific cellular and subcellular targets. Synaptic guidepost cells in the C. elegans vulval epithelium drive synapses from the HSNL motor neuron onto adjacent target neurons and muscles. Here, we show that the transmembrane immunoglobulin superfamily protein SYG-2 is a central component of the synaptic guidepost signal. SYG-2 is expressed transiently by primary vulval epithelial cells during synapse formation. SYG-2 binds SYG-1, the receptor on HSNL, and directs SYG-1 accumulation and synapse formation to adjacent regions of HSNL. syg-1 and syg-2 mutants have defects in synaptic specificity; the HSNL neuron forms fewer synapses onto its normal targets and forms ectopic synapses onto inappropriate targets. Misexpression of SYG-2 in secondary epithelial cells causes aberrant accumulation of SYG-1 and synaptic markers in HSNL adjacent to the SYG-2-expressing cells. Our results indicate that local interactions between immunoglobulin superfamily proteins can determine specificity during synapse formation.

Early glomerular filtration defect and severe renal disease in podocin-deficient mice

Podocytes are specialized epithelial cells covering the basement membrane of the glomerulus in the kidney. The molecular mechanisms underlying the role of podocytes in glomerular filtration are still largely unknown. We generated podocin-deficient (Nphs2-/-) mice to investigate the function of podocin, a protein expressed at the insertion of the slit diaphragm in podocytes and defective in a subset of patients with steroid-resistant nephrotic syndrome and focal and segmental glomerulosclerosis. Nphs2-/- mice developed proteinuria during the antenatal period and died a few days after birth from renal failure caused by massive mesangial sclerosis. Electron microscopy revealed the extensive fusion of podocyte foot processes and the lack of a slit diaphragm in the remaining foot process junctions. Using real-time PCR and immunolabeling, we showed that the expression of other slit diaphragm components was modified in Nphs2-/- kidneys: the expression of the nephrin gene was downregulated, whereas that of the ZO1 and CD2AP genes appeared to be upregulated. Interestingly, the progression of the renal disease, as well as the presence or absence of renal vascular lesions, depends on the genetic background. Our data demonstrate the crucial role of podocin in the establishment of the glomerular filtration barrier and provide a suitable model for mapping and identifying modifier genes involved in glomerular diseases caused by podocyte injuries.

Dynamic (re)organization of the podocyte actin cytoskeleton in the nephrotic syndrome

The visceral glomerular epithelial cell, also known as the podocyte, plays an important role in the maintenance of renal glomerular function. This cell type is highly specialized and its foot processes together with the interposed slit diaphragm (SD) form the final barrier to urinary protein loss. Effacement of foot processes is associated with the development of proteinuria and-if not reversed in a certain time-with permanent deterioration of the glomerular filter. To maintain an intact glomerular filter barrier, podocyte-podocyte interactions and podocyte interactions with the glomerular basement membrane (GBM) are essential. Recent years have highlighted podocyte functions by unraveling the molecular composition of the SD, but have also clarified the important role of the podocyte actin cytoskeleton, and the podocyte-GBM interaction in the development of foot process (FP) effacement. This review provides an update of podocyte functions with respect to novel podocyte-specific proteins and also focuses on the dynamic interaction between the actin cytoskeleton of podocytes, their cell surface receptors and the GBM.

Nephrin promotes cell-cell adhesion through homophilic interactions

Nephrin is a type-1 transmembrane protein and a key component of the podocyte slit diaphragm, the ultimate glomerular plasma filter. Genetic and acquired diseases affecting expression or function of nephrin lead to severe proteinuria and distortion or absence of the slit diaphragm. Here, we showed by using a surface plasmon resonance biosensor that soluble recombinant variants of nephrin, containing the extracellular part of the protein, interact with each other in a specific and concentration-dependent manner. This molecular interaction was increased by twofold in the presence of physiological Ca(2+)concentration, indicating that the binding is not dependent on, but rather promoted by Ca(2+). Furthermore, transfected HEK293 cells and an immortalized mouse podocyte cell line overexpressing full-length human nephrin formed cellular aggregates, with cell-cell contacts staining strongly for nephrin. The distance between plasma membranes at the nephrin-containing contact sites was shown by electron microscopy to be 40 to 50 nm, similar to the width of glomerular slit diaphragm. The cell contacts could be dissociated with antibodies reacting with the first two extracellular Ig-like domains of nephrin. Wild-type HEK293 cells were shown to express slit diaphragm components CD2AP, P-cadherin, FAT, and NEPH1. The results show that nephrin molecules exhibit homophilic interactions that could promote cellular contacts through direct nephrin-nephrin interactions, and that the other slit diaphragm components expressed could contribute to that interaction.

Complement mediates nephrin redistribution and actin dissociation in experimental membranous nephropathy11See Editorial by Quigg, p. 2318

BACKGROUND

The onset of proteinuria in passive Heymann nephritis, (PHN), a rat model of human membranous nephropathy (MN), is complement-dependent and is associated with altered podocyte slit diaphragm integrity and dissociation of nephrin from the actin cytoskeleton. These studies examined if complement is responsible for these podocyte changes.

METHODS

PHN was induced with sheep anti-Fx1A. Controls were injected with normal sheep globulin. A third group was injected with anti-Fx1A and depleted of complement with cobra venom factor. Four days later, proteinuria was measured, slit diaphragm integrity was examined by electron microscopy, nephrin distribution was studied by immunofluorescence, and the glomerular content of nephrin and its association with actin were assessed by sequential extraction of isolated glomeruli and Western blotting.

RESULTS

Four days after immunization, seven out of eight PHN rats were proteinuric, whereas none of the complement depleted group had proteinuria despite similar levels of antibody deposition. Complement depletion preserved slit diaphragm morphology. Immunofluorescence microscopy with an antibody to the extracellular domain of nephrin showed a normal staining pattern in the rats depleted of complement and a shift to a more dispersed and clustered pattern in the PHN group. Western blot analysis of the glomerular extracts showed a significant reduction in the total amount of nephrin and in the fraction of actin-associated nephrin in the PHN group, whereas the amounts in the complement-depleted rats were similar to normal controls.

CONCLUSION

The onset of proteinuria in the PHN model of MN is coincident with complement-dependent alterations in the association of nephrin with the actin cytoskeleton and loss of podocyte slit diaphragm integrity.

CD2-associated protein and glomerular disease

CONTEXT

Proteinuria is a major cause of progression in renal disease. The glomerular ultrafiltration barrier, containing highly differentiated podocytes, normally restricts protein access to the urine. Patients with urinary protein in the nephrotic range (>3.5 g daily) often have effaced podocyte foot-processes. Slit diaphragms span the gaps between foot processes as a barrier to macromolecules. Nephrin and podocin are slit-diaphragm proteins identified in families with congenital nephrotic syndromes. CD2-associated protein (CD2AP) is an adapter protein originally identified as a novel ligand interacting with the T-cell-adhesion protein CD2. CD2AP knockout (-/-) mice develop a congenital nephrotic syndrome with podocyte foot-process effacements and die at 6 weeks of age from renal failure. CD2AP localises to the slit diaphragm and links nephrin and podocin to phosphoinositide 3-OH kinase; this complex has cell-signalling properties.

STARTING POINT

The CD2AP +/- heterozygous mice developed by Jeong Kim and colleagues (Science 2003; 300: 1298-300) are haploinsufficient and develop glomerular changes at 9 months of age with a histological pattern similar to that in human focal segmental glomerulosclerosis. These researchers found that 2 of 30 African-American patients with idiopathic focal segmental glomerulosclerosis had a CD2AP mutation that ablated expression of one allele. WHERE NEXT? Further studies should address the normal distribution of the CD2AP heterozygous mutation in different ethnic populations, because the association with human idiopathic focal segmental glomerulosclerosis could be accidental. Decreased expression of CD2AP in podocytes of individuals with the CD2AP heterozygous mutation would help to understand how the haploinsufficiency translates into increased susceptibility to renal disease. Transfection of podocytes with mutated CD2AP or study of cultured podocytes from CD2AP +/- mice would provide further insight into whether the nephrin-podocin-CD2AP signal-transduction pathway is altered and leads to increased apoptosis of podocytes. Assuming that a decrease in CD2AP attenuates clearance of glomerular immune complexes, patients with other types of idiopathic glomerulonephritis should also have a CD2AP mutation. However, first studies looking at the most common form of glomerulonephritis, IgA nephropathy, have failed to show decreased renal CD2AP expression.

Plasma Membrane Targeting of Podocin Through the Classical Exocytic Pathway: Effect of NPHS2 Mutations

Podocytes are specialized epithelial cells of the glomerulus in the kidney, which interconnect at the top of the glomerular basement membrane through the slit diaphragm, an adherens-like junction that plays a crucial role in the glomerular filtration process. Podocin, a plasma membrane anchored stomatin-like protein, is expressed in lipid rafts at the insertion of the slit diaphragm in podocytes. Mutations in NPHS2, the gene encoding podocin, are associated with inherited and sporadic cases of steroid-resistant nephrotic syndrome. Here, we show that brefeldin A induces accumulation of newly synthesized podocin in the endoplasmic reticulum, suggesting that podocin biosynthesis follows the classical secretory pathway, and we study the effect of 12 NPHS2 mutations associated with steroid-resistant nephrotic syndrome on the trafficking of the protein. We found that 9 podocin mutants were not targeted to the plasma membrane, 8 being retained in the endoplasmic reticulum and one being localized in late endosomes. Furthermore, by screening our database of patients with NPHS2 mutations, we found that podocin mutants retained in the endoplasmic reticulum are associated with earlier onset of the disease than those correctly targeted to the cell membrane. Our data suggest that most of NPHS2 mutations lead to retention of podocin in the endoplasmic reticulum and therefore provide a rationale for devising therapeutic approaches aimed at correcting the protein processing defect.

Molecular structure-function relationship in the slit diaphragm

The past 5 years have witnessed an exponential increase in our understanding of the structure and function of the glomerular slit diaphragm. The identification of nephrin as the first transmembrane slit diaphragm protein was a watershed event in slit diaphragm biology. This article correlates some of the observations of the prenephrin era with more recent studies, and elaborates on the individual characteristics of each slit diaphragm protein. Recent studies on protein-protein interactions related to slit diaphragm permeability and cell signaling are elaborated, along with observations on their expression in human disease and experimental models of proteinuria. Developmental expression of components of the slit diaphragm in normal and knockout mice also is discussed. Finally, some areas of future investigation are proposed.

Intron disruption of the annexin IV gene reveals novel transcripts

Annexin IV (AIV), a Ca2+-dependent membrane-binding protein, is expressed in many epithelia. Annexin IV modifies membrane bilayers by increasing rigidity, reducing water and H+ permeability, promoting vesicle aggregation, and regulating ion conductances, all in a Ca2+-dependent manner. We have characterized a mouse in which a gene trap has been inserted into the first intron of annexin IV. Processing of the primary transcript is disrupted. Northern blot and immunoblot data indicated that annexin IV expression was eliminated in many but not all tissues. Immunohistochemical analysis, however, demonstrated that annexin IV expression was eliminated in some cell types, but was unaltered in others. 5'-Rapid amplification of cDNA ends analysis of intestinal and kidney RNA revealed three transcripts, AIVa, AIVb, and AIVc. AIVa is widely distributed. AIVb is expressed only in the digestive tract. AIVc expression is very restricted. A selected number of epithelial cells of unique morphology demonstrate high concentrations. All three transcripts produce an identical annexin IV protein. The different tissue and cell-specific expression profiles of the three transcripts suggest that regulation of both the annexin IV gene expression and the cellular role of the protein are complex. The AIVa-/- mouse may become a valuable model to further study transcription and the physiological role of annexin IV.

[Pathogenetic aspectics of nephrotic syndrome]

Nephrotic syndrome is characterized by protein loss in the urine, hypoalbuminemia, hyperlipidemia and edema. Several diseases cause a nephrotic syndrome, as they damage the glomerular podocytes. These specialized epithelial cells, together with endothelial cells of the glomerular capillaries and the basal membrane, form a filter that retains plasma proteins in the circulation. A disturbance of this filter causes proteinuria. The three most common primary glomerular diseases are minimal change, membranous glomerulonephritis, and the primary focal segmental glomerulosclerosis. The familiar forms are rare; however, the identification of the relevant gene defects has greatly advanced our understanding of podocyte function as well as the pathogenesis of nephrotic syndrome.

Causes and consequences of proteinuria: the kidney filtration barrier and progressive renal failure

The past few years have witnessed a major breakthrough in the understanding of the molecular mechanisms and ultrastructural changes behind the development of proteinuria. The discovery of several proteins in the glomerular podocyte and slit diaphragm, where mutations lead to disease, has revealed the importance of this cell with its diaphragm as the major filtration barrier as opposed to the glomerular basement membrane (GBM) previously ascribed this function. Furthermore, accumulating clinical as well as experimental evidence points to the harmful effects of proteinuria, irrespective of the original damage. The purpose of this review is to shed light on what we know today about the two sides of this 'coin', the causes and the consequences of proteinuria.

Podocyte biology and the emerging understanding of podocyte diseases

The understanding of the unique molecular apparatus of the podocyte has increased dramatically in recent years. This new knowledge has improved the diagnosis and classification of the diseases that have been termed podocytopathies. Podocyte injury frequently leads to reorganization of the slit diaphragm and reorganization of the foot process structure. Four major causes of foot process effacement can be identified, with some due to genetic mutations and others due to acquired conditions: (1) impaired formation of the slit diaphragm complex; (2) abnormalities of the glomerular basement membrane or the adhesion of podocytes to the glomerular basement membrane; (3) abnormalities of the actin cytoskeleton and associated proteins, and (4) alterations in the apical membrane domain of the podocyte. The major podocytopathies can also be organized into four categories, including those with a normal glomerular histology, diffuse mesangial sclerosis, focal segmental glomerulosclerosis, and collapsing glomerulopathy.

Kirrel2, a novel immunoglobulin superfamily gene expressed primarily in beta cells of the pancreatic islets

A novel immunoglobulin superfamily (Igsf) protein gene was discovered by computational analysis of human draft genomic DNA, and multiple cDNA clones were obtained. The protein encoded by this gene contains five Ig domains, one transmembrane domain, and an intracellular domain. It has significant similarity with several known Igsf proteins, including Drosophila RST (irregular chiasm C-roughest) protein and mammalian KIRREL (kin of irregular chiasm C-roughest), NEPH1, and NPHS1 (nephrin) proteins. All these proteins have multiple Ig domains, possess properties of cell adhesion molecules, and play important roles in organ development. RT-PCR and Northern blot results indicate this gene is predominantly expressed in pancreas, and public sequence databases indicate there is also expression in the nervous system. We have named this gene Kirrel2 (kin of irregular chiasm-like 2), to reflect its similarity to irregular chiasm C-roughest and Kirrel. Four splice forms of Kirrel2 were observed, including two that we cloned from pancreas mRNA as well as two GenBank entries, one from the brain and one from a retinoblastoma cell line. A partial cDNA clone of the mouse orthologue was obtained by RT-PCR from mouse brain, and the inferred protein sequence has 85% sequence identity to the human protein. Immunohistochemical staining results indicate that the KIRREL2 protein is conserved from rodents to primates, and it is highly expressed in pancreatic islets. RT-PCR results on mouse pancreatic cell lines indicate that expression in the pancreas is restricted to beta cells. Thus, KIRREL2 protein is a beta-cell-expressed Ig domain protein and may be involved in pancreas development or beta cell function.

Neph1 and nephrin interaction in the slit diaphragm is an important determinant of glomerular permeability

Neph1-deficient mice develop nephrotic syndrome at birth, indicating the importance of this protein in the development of a normal glomerular filtration barrier. While the precise subcellular localization of Neph1 remains unknown, its relationship with other components of the glomerular filtration barrier is of great interest in this field. In this paper, we localize the expression of Neph1 to the glomerular slit diaphragm by immunogold electron microscopy in rodents and describe its direct interaction with two other components of the slit diaphragm, nephrin and ZO-1. Both native and recombinant Neph1 associate with each other as dimers and multimers and interact with nephrin via their extracellular segments. Disruption of the Neph1-nephrin interaction in vivo by injecting combinations of individual subnephritogenic doses of anti-Neph1 and anti-nephrin results in complement- and leukocyte-independent proteinuria with preserved foot processes. This disruption modestly reduces Neph1 and nephrin protein expression in podocytes and dramatically reduces ZO-1 protein expression via the interaction of ZO-1 PDZ domains with the cytoplasmic tail of Neph1, independent of changes in mRNA expression of all three genes. The interaction between nephrin and Neph1 is specific and not shared by either protein with P-cadherin, another integral slit diaphragm protein. The interaction between nephrin and Neph1 therefore appears to be an important determinant of glomerular permeability.

A novel protein, densin, expressed by glomerular podocytes

With the recent molecular findings, the podocyte is emerging as a key cell type involved in glomerular damage, but protein complexes involved remain poorly understood. To systematically search for additional podocyte molecules interacting with nephrin, a key structural molecule of the interpodocyte filtration slit, precipitation of glomerular lysates was set out with anti-nephrin antibodies to identify members of the nephrin-associated protein complex. Proteins of the precipitate were subsequently identified with MALDI-TOF mass analysis. One of the proteins thus obtained showed identity with densin, a protein originally purified from rat forebrain postsynaptic density fraction and so far shown to be highly brain-specific. The expression of densin appeared distinctly in the glomerulus and cultured podocytes by RT-PCR. Immunoblotting studies revealed a specific band of 185 kD in brain and cultured podocytes; in human glomerulus, densin appeared as a 210-kD band. By immunocytochemistry, densin localizes in glomeruli in a podocyte-like pattern. Electron microscopic studies revealed densin localization in the slit diaphragm area. Due to its known involvement in the synaptic organization, maintenance of cell shape and polarity in nerve cells, together with its demonstrated interactions with alpha-actinin-4, densin may share the same functions in podocytes by associating with the nephrin interacting protein complex at the slit diaphragm.

Podocyte differentiation and hereditary proteinuria/nephrotic syndromes

The study of familial nephrotic syndromes (NS) and the analysis of murine models of glomerular diseases resulted in major progresses in the knowledge of podocyte physiology and pathology. Numerous proteins participating in the composition of the slit diaphragm region have been identified. The importance of several of them (nephrin, podocin, CD2AP, and Neph1) in the maintenance of the glomerular filtration barrier has been demonstrated by the occurrence of massive proteinuria when they are defective. The role of the cytoskeleton has been revealed by the development of proteinuria/NS in patients with ACTN4 mutation and the occurrence of early and severe NS in alpha-actinin-4-deficient mice. Given the genetic heterogeneity of familial NS and the many other genes to be identified, further insights in the molecular basis of the role of the podocyte in the maintenance of the glomerular filtration barrier may be expected in the near future.

Clinical, histopathologic, and genetic studies in nine families with focal segmental glomerulosclerosis

BACKGROUND

Familial forms of focal segmental glomerulosclerosis (FSGS) are caused by mutations in genes at 1q25-31 (gene for steroid-resistant nephrotic syndrome 2 [NPHS2]), 11q21-22, 19q13 (gene for alpha-actinin 4 and NPHS1), and at additional unidentified chromosomal loci.

METHODS

We describe clinical and histopathologic features and results of linkage analysis in nine consecutive index cases with familial FSGS who, together with their families, were referred for genetic studies.

RESULTS

Two of the index cases presented in childhood (22%) and seven cases presented in adolescence or adulthood (78%). Six of their families (67%), including the two cases with childhood-onset disease, showed probable autosomal recessive inheritance. FSGS segregated at the 1q25-31 locus in two of these families and at the 11q21-22 locus in four families. None had disease caused by mutations in genes at the 19q13 locus, and no locus was identified in the three remaining families. Clinical features of proteinuria, minimal hematuria, hypertension, preeclampsia, and progressive renal impairment were usually present with autosomal recessive or dominant inheritance and with disease that segregated at the different loci. Eighteen renal biopsies from affected members of eight families showed a strong correlation between tubulointerstitial damage and percentage of obsolescent glomeruli (rho = +0.76; P < 0.01). None of the 13 patients from eight families who underwent transplantation developed recurrent FSGS in their grafts. In general, carriers of autosomal recessive disease had no distinctive clinical features apart from the development of preeclampsia in successive pregnancies.

CONCLUSION

Familial forms of FSGS are not uncommon, and presentation frequently is in adolescence or adulthood, even when inheritance is autosomal recessive. Furthermore, carriers of autosomal recessive FSGS often have no distinctive phenotype.

Nephrin and Neph1 co-localize at the podocyte foot process intercellular junction and form cis hetero-oligomers

Glomerular visceral epithelial cells (podocytes) appear to play a central role in maintaining the selective filtration barrier of the renal glomerulus. While the immunoglobulin superfamily member Nephrin was proposed to act as a cell adhesion molecule at the podocyte intercellular junction necessary for maintaining glomerular perm selectivity, the Nephrin ligand has not been identified. The existence of a new subfamily of Nephrin-like molecules including Neph1 was recently described. Genetic deletion of Nephrin or Neph1 resulted in similar phenotypes of podocyte foot process effacement and proteinuria. The subcellular localization of Neph1 and the possibility that Nephrin and Neph1 interact was investigated. Polyclonal antiserum for Neph1 was raised and characterized. Neph1 migrated as a 90-kDa protein on SDS-PAGE under reducing conditions. Neph1 was identified in a glomerular and podocyte-specific distribution in adult rat kidney. Like Nephrin and Podocin, Neph1 was enriched in Triton X-100 detergent-resistant membrane fractions. Consistent with this observation, immunogold electron microscopy demonstrated that Neph1 localized exclusively to lateral margins of podocyte foot processes at the insertion of the slit diaphragm. Neph1 and Nephrin participate in a direct cis-interaction involving their cytoplasmic domains. In addition, interactions between the extracellular domain of Nephrin and itself and between the extracellular domain of Nephrin and that of Neph1 were detected. Neph1 did not interact via a homophilic interaction. These observations suggest that Nephrin and Neph1 form a hetero-oligomeric receptor complex in the plane of the membrane that might interact across the foot process intercellular junction through interactions between Nephrin with itself and Neph1.

Update in podocyte biology: putting one's best foot forward

PURPOSE OF REVIEW

The rapidly developing field of podocyte cell biology is reviewed, focusing on papers published in the last 12 months.

RECENT FINDINGS

Four areas of particular progress can be discerned. First, podocytes proliferate during early metanephric development, are quiescent after the capillary loop stage, and re-enter the cell cycle only in the disease group termed collapsing glomerulopathy. We have learned that control of the podocyte cell cycle involves both expression of cell-cycle regulating proteins and the process of cytokinesis. Second, the podocyte slit diaphragm is the final component of the filtration barrier. The structure and maintenance of the slit diaphragm has been a major focus of research activity, and a multiplicity of relevant molecular interactions have been defined. Significant advances have been made in understanding the complex and interacting role of nephrin and podocin mutations in the genesis of clinical glomerular disease. Third, several proteins essential to controlling discrete podocyte transcriptional programs have been defined. Finally, conditionally-immortalized podocyte cell lines, derived from mouse and human tissue, have proven their worth as models to advance investigations of podocyte biology.

SUMMARY

Podocyte injury occurs as a consequence of genetic mutation, immunological injury, viral infection, or abnormal hemodynamic forces within the glomerulus. As we understand more about the podocyte proteome and cell biology, we gain an increasingly detailed molecular understanding of podocyte structure and function. In this drama we have many molecular players and increasing stretches of molecular dialogue, but the script remains largely to be deciphered. Nevertheless, we do understand the consequences that arise when the podocyte cannot put its best foot (processes) forward.

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.

The carboxyl terminus of Neph family members binds to the PDZ domain protein zonula occludens-1

The PSD95/Dlg/ZO-1 (PDZ) domain-containing protein zonula occludens-1 (ZO-1) selectively localizes to the cytoplasmic basis of the slit diaphragm, a specialized cell-cell contact in between glomerular podocytes necessary to prevent the loss of protein in the urine. However, the function of ZO-1 at the slit diaphragm has remained elusive. Deletion of Neph1, a slit diaphragm protein of the immunoglobulin superfamily with a cytoplasmic PDZ binding site, causes proteinuria in mice. We demonstrate now that Neph1 binds ZO-1. This interaction was mediated by the first PDZ domain of ZO-1 and involved the conserved PDZ domain binding motif present in the carboxyl terminus of the three known Neph family members. Furthermore, Neph1 co-immunoprecipitates with ZO-1 from lysates of mouse kidneys, demonstrating that this interaction occurs in vivo. Both deletion of the PDZ binding motif of Neph1 as well as threonine-to-glutamate mutation of the threonine within the binding motif abrogated binding of ZO-1, suggesting that phosphorylation may regulate this interaction. ZO-1 binding was associated with a strong increase in tyrosine phosphorylation of the cytoplasmic tail of Neph1 and dramatically accelerated the ability of Neph1 to induce signal transduction. Thus, our data suggest that ZO-1 may organize Neph proteins and recruit signal transduction components to the slit diaphragm of podocytes.

Homodimerization and heterodimerization of the glomerular podocyte proteins nephrin and NEPH1

Nephrin and NEPH1, the gene products of NPHS1 and NEPH1, are podocyte membrane proteins of the Ig superfamily. Similar to the nephrin knockout, mice lacking NEPH1 show severe proteinuria leading to perinatal death. To identify the ligand of NEPH1, the extracellular domain of NEPH1 was fused to human IgG. This NEPH1-Ig fusion protein labeled the glomerular capillary wall of mouse kidneys in a staining pattern identical to NEPH1 and nephrin, prompting speculation that that NEPH1 might form homodimers and/or heterodimers with nephrin. In coimmunoprecipitation and pull-down assays, the NEPH1-Ig fusion protein precipitated wild-type NEPH1 from overexpressing HEK 293T cells. Truncational analysis revealed that the adhesive properties were not confined to a single Ig domain of NEPH1. Fusion proteins containing two Ig domains of NEPH1 were sufficient to immobilize NEPH1, but they failed to interact with control protein containing the phylogenetically related PKD repeats of polycystin-1. NEPH1 also precipitated nephrin, a protein with eight Ig domains and a fibronectin-like domain. Truncational analysis of nephrin revealed a very similar mode of interaction, i.e., two nephrin Ig domains fused to human IgG precipitated either nephrin or NEPH1, but not the control protein. Both NEPH1 and nephrin interactions were strictly dependent upon posttranslational glycosylation, and bacterially expressed protein failed to bind NEPH1. These findings demonstrate that the Ig domains of NEPH1 and nephrin form promiscuous homodimeric and heterodimeric interactions that may facilitate cis- and trans- homodimerizations and heterodimerizations of these molecules at the glomerular slit diaphragm.

The immunoglobulin superfamily protein SYG-1 determines the location of specific synapses in C. elegans

During nervous system development, neurons form reproducible synapses onto specific targets. Here, we analyze the development of stereotyped synapses of the C. elegans HSNL neuron in vivo. Postsynaptic neurons and muscles were not required for accurate synaptic vesicle clustering in HSNL. Instead, vulval epithelial cells that contact HSNL act as synaptic guidepost cells that direct HSNL presynaptic vesicles to adjacent regions. The mutant syg-1(ky652) has defects in synapse formation that resemble those in animals that lack vulval epithelial cells: HSNL synaptic vesicles fail to accumulate at normal synaptic locations and form ectopic anterior clusters. syg-1 encodes an immunoglobulin superfamily protein that acts in the presynaptic HSNL axon. SYG-1 protein is localized to the site of future synapses, where it initiates synapse formation and localizes synaptic connections in response to the epithelial signal. SYG-1 is related to Drosophila IrreC and vertebrate NEPH1 proteins, which mediate cell-cell recognition in diverse developmental contexts.

Inducible gene silencing in podocytes: a new tool for studying glomerular function

Glomerular filtration is one of the primary functions of the kidney. Podocytes, a highly specialized cell type found in glomeruli, are believed to play a critical role in that function. Null mutations of genes expressed in podocytes like WT1, nephrin, and NEPH1 result in an embryo and perinatal lethal phenotype and therefore do not allow the functional analysis of these genes in the adult kidney. Here is describes the generation of a model that will allow such studies. We have engineered transgenic mice in which the disruption of targeted genes can be induced in a temporally controlled fashion in podocytes. For this, a transgene encoding the mutated estrogen receptor-Cre recombinase fusion protein was introduced into the mouse genome. Animals were crossed with Z/AP reporter mice to test for efficient and inducible recombination. We found that, after injection of inducer drug tamoxifen, Cre fusion protein translocates to the nuclei of podocytes, where it becomes active and mediates recombination of DNA carrying loxP target sequences. These animals provide for the first time a tool for silencing genes selectively in podocytes of adult animals.

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.