Search

IgA nephropathy (IgAN) is the most common primary glomerulonephritis worldwide and defined by the presence of IgA-containing immune complexes in the mesangium that induce an inflammation leading to glomerulonephritis. Since we poorly understand early mechanisms of glomerular injury in IgAN we performed single-cell RNA sequencing (scRNA-seq) analysis of glomerulus-associated cells using SMARTseq2-technology at the early stage of IgAN in grouped ddY-mice. Cell-specific molecular signatures unraveled a key role of endothelial cells in the early pathogenesis of IgAN, especially in the recruitment and infiltration of immune cells. Mesangial and podocyte cells demonstrated less molecular changes. Several intra-glomerular paracrine pathways were detected, such as mesangial cell-derived Slit3 potentially activating Robo-receptors in podocyte/endothelial cells. Surprisingly, proximal tubular cells were strongly affected at the early stage and potential glomerulo-tubular cell-cell crosstalk pathways were identified. Importantly, many of the cellular transcriptomic signatures identified in this well-established mouse model were also detected in published bulk transcriptomic data in human IgAN. Moreover, we validated the functionality of key cell-cell crosstalk pathways using cell culture models, such as the effect of the Slit-Robo signalling axis. Thus, our study provides important novel molecular insights into the pathogenesis of early IgAN-associated glomerulopathy.

Background Immunoglobulin A nephropathy (IgAN) and its systemic variant IgA vasculitis (IgAV) damage the glomeruli, resulting in proteinuria, hematuria and kidney impairment. Dendrin is a podocyte-specific protein suggested to be involved in the pathogenesis of IgAN. Upon cell injury, dendrin translocates from the slit diaphragm to the nucleus, where it is suggested to induce apoptosis and cytoskeletal changes, resulting in proteinuria and accelerated disease progression in mice. Here we investigated gene and protein expression of dendrin in relation to clinical and histopathological findings to further elucidate its role in IgAN/IgAV. Methods Glomerular gene expression was measured using microarray on 30 IgAN/IgAV patients, 5 patients with membranous nephropathy (MN) and 20 deceased kidney donors. Dendrin was spatially evaluated on kidney tissue sections by immunofluorescence (IF) staining (IgAN patients, n = 4; nephrectomized kidneys, n = 3) and semi-quantified by immunogold electron microscopy (IgAN/IgAV patients, n = 21; MN, n = 5; living kidney donors, n = 6). Histopathological grading was performed according to the Oxford and Banff classifications. Clinical data were collected at the time of biopsy and follow-up. Results Dendrin mRNA levels were higher (P = .01) in IgAN patients compared with MN patients and controls and most prominently in patients with preserved kidney function and fewer chronic histopathological changes. Whereas IF staining did not differ between groups, immunoelectron microscopy revealed that a higher relative nuclear dendrin concentration in IgAN patients was associated with a slower annual progression rate and milder histopathological changes. Conclusion Dendrin messenger RNA levels and relative nuclear protein concentrations are increased and associated with a more benign phenotype and progression in IgAN/IgAV patients.

Inflammatory pathways are activated in most glomerular diseases but molecular mechanisms driving them in kidney tissue are poorly known. We identified retinoic acid receptor responder 1 (Rarres1) as a highly podocyte-enriched protein in healthy kidneys. Studies in podocyte-specific knockout animals indicated that Rarres1 was not needed for the normal development or maintenance of the glomerulus filtration barrier and did not modulate the outcome of kidney disease in a model of glomerulonephritis. Interestingly, we detected an induction of Rarres1 expression in glomerular and peritubular capillary endothelial cells in IgA and diabetic kidney disease, as well as in ANCA-associated vasculitis. Analysis of publicly available RNA data sets showed that the induction of Rarres1 expression was a common molecular mechanism in chronic kidney diseases. A conditional knock-in mouse line, overexpressing Rarres1 specifically in endothelial cells, did not show any obvious kidney phenotype. However, the overexpression promoted the progression of kidney damage in a model of glomerulonephritis. In line with this, conditional knock-out mice, lacking Rarres1 in endothelial cells, were partially protected in the disease model. Mechanistically, Rarres1 promoted inflammation and fibrosis via transcription factor Nuclear Factor-κB signaling pathway by activating receptor tyrosine kinase Axl. Thus, induction of Rarres1 expression in endothelial cells is a prevalent molecular mechanism in human glomerulopathies and this seems to have a pathogenic role in driving inflammation and fibrosis via the Nuclear Factor-κB signaling pathway.

The cause of podocyte injury in idiopathic nephrotic syndrome (INS) remains unknown. Although recent evidence points to the role of B cells and autoimmunity, the lack of animal models mediated by autoimmunity limits further research. We aimed to establish a mouse model mimicking human INS by immunizing mice with Crb2, a transmembrane protein expressed at the podocyte foot process.C3H/HeN mice were immunized with the recombinant extracellular domain of mouse Crb2. Serum anti-Crb2 antibody, urine protein-to-creatinine ratio, and kidney histology were studied. For signaling studies, a Crb2-expressing mouse podocyte line was incubated with anti-Crb2 antibody.Serum anti-Crb2 autoantibodies and significant proteinuria were detected 4 weeks after the first immunization. The proteinuria reached nephrotic range at 9-13 weeks and persisted up to 29 weeks. Initial kidney histology resembled minimal change disease in humans, and immunofluorescence staining showed delicate punctate IgG staining in the glomerulus, which colocalized with Crb2 at the podocyte foot process. A subset of mice developed features resembling FSGS after 18 weeks. In glomeruli of immunized mice and in Crb2-expressing podocytes incubated with anti-Crb2 antibody, phosphorylation of ezrin, which connects Crb2 to the cytoskeleton, increased, accompanied by altered Crb2 localization and actin distribution.The results highlight the causative role of anti-Crb2 autoantibody in podocyte injury in mice. Crb2 immunization could be a useful model to study the immunologic pathogenesis of human INS, and may support the role of autoimmunity against podocyte proteins in INS.

Background Inflammatory processes play an important role in the pathogenesis of glomerulopathies. Finding novel ways to suppress glomerular inflammation may offer a new way to stop disease progression. However, the molecular mechanisms that initiate and drive inflammation in the glomerulus are still poorly understood. Methods We performed large-scale gene expression profiling of glomerulus-associated G protein-coupled receptors (GPCRs) to identify new potential therapeutic targets for glomerulopathies. The expression of Gprc5b in disease was analyzed using quantitative PCR and immunofluorescence, and by analyzing published microarray data sets. In vivo studies were carried out in a podocyte-specific Gprc5b knockout mouse line. Mechanistic studies were performed in cultured human podocytes. Results We identified an orphan GPCR, Gprc5b, as a novel gene highly enriched in podocytes that was significantly upregulated in common human glomerulopathies, including diabetic nephropathy, IgA nephropathy, and lupus nephritis. Similar upregulation of Gprc5b was detected in LPS-induced nephropathy in mice. Studies in podocyte-specific Gprc5b knockout mice showed that Gprc5b was not essential for normal development of the glomerular filtration barrier. However, knockout mice were partially protected from LPS-induced proteinuria and recruitment of inflammatory cells. Mechanistically, RNA sequencing in Gprc5b knockouts mice and experiments in cultured human podocytes showed that Gpr5cb regulated inflammatory response in podocytes via NF-kappa B signaling. Conclusions GPRC5b is a novel podocyte-specific receptor that regulates inflammatory response in the glomerulus by modulating the NF-kappa B signaling pathway. Upregulation of Gprc5b in human glomerulopathies suggests that it may play a role in their pathogenesis.

The kidneys constantly filter enormous amounts of fluid, with almost complete retention of albumin and other macromolecules in the plasma. Diseases of podocytes at the kidney filtration barrier reduce the glomerular capillary surface area available for filtration and alter the intrinsic permeability of the capillary wall resulting in albuminuria, however, direct quantitative assessment of the underlying morphological changes has not been possible so far. Here we developed a deep learning-based approach for segmentation of foot processes in images acquired with super-resolved stimulated emission depletion (STED) microscopy or confocal microscopy. Our method, Automatic Morphological Analysis of Podocytes (AMAP), detected 87-95% manually-annotated foot processes and additionally recognized 1.3 - 2.17-fold more. It also robustly determined morphometric parameters, at a Pearson correlation of r > 0.71 with a previously published semi-automated approach, across a large set of mouse tissue samples. The artificial intelligence algorithm was applied to a set of human kidney disease conditions allowing comprehensive quantifications of various underlying morphometric parameters. These data confirmed that when podocytes are injured, they take on a more simplified architecture and the slit-diaphragm length is much reduced, resulting in a reduction in the filtration slit area and a loss of the buttress force of podocytes which increases the permeability of the GBM to albumin.

Background and Aims Podocyte integrity is crucial for the maintenance of glomerular function in health and disease. Numerous studies have reported that Klotho overexpression, or treatment with recombinant Klotho, reduces glomerular and tubular damage in mouse models of renal disease. However, the mechanism(s) of action are not fully understood. Several recent studies have also reported that Klotho is expressed in podocytes, where it protects against various types of injury. These findings conflict with previous studies, which have shown that renal Klotho expression is exclusively confined to proximal and distal tubular cells. Method To address this discrepancy and enhance our understanding of the putative glomeruloprotective effects mediated by Klotho, we examined the expression pattern of Klotho in human and mouse kidney by several different methods, and explored its protective effects by overexpressing full-length human Klotho directly in podocytes or in a distant organ (i.e. liver). Results Data at the mRNA and protein levels all converged towards an absence or very low expression of Klotho in podocytes. The generation of a podocyte-specific Klotho knockout mouse further demonstrated that its deletion did not affect glomerular structure or function. Moreover, Klotho deficiency did not worsen glomerular injury in an experimental model of glomerulonephritis (anti-GBM). However, when Klotho was overexpressed in hepatocytes (Alb-cre;hKlothofl/+ - Alb-hKL), serum Klotho increased drastically with no changes in Fgf23 or phosphate metabolism. In mice challenged with anti-GBM, renal histology and ultrastructure of the filtration barrier was less severely affected in Alb-hKL compared to WT mice. There were also significantly less albuminuria, podocyte loss and interstitial fibrosis in Alb-hKL mice compared to their WT littermates. In contrast, mice which overexpressed Klotho in podocytes (Pod-hKL) were not protected from renal injury. Conclusion Taken together, these results strongly suggest that Klotho is not expressed in any substantial amounts in human or mouse podocytes, and that membrane-bound Klotho does not play a role in podocyte biology. Importantly, our results confirm a beneficial role for soluble Klotho in protecting podocytes against injury, and in maintaining glomerular integrity and function.

Molecular characterization of the individual cell types in human kidney as well as model organisms are critical in defining organ function and understanding translational aspects of biomedical research. Previous studies have uncovered gene expression profiles of several kidney glomerular cell types, however, important cells, including mesangial (MCs) and glomerular parietal epithelial cells (PECs), are missing or incompletely described, and a systematic comparison between mouse and human kidney is lacking. To this end, we use Smart-seq2 to profile 4332 individual glomerulus-associated cells isolated from human living donor renal biopsies and mouse kidney. The analysis reveals genetic programs for all four glomerular cell types (podocytes, glomerular endothelial cells, MCs and PECs) as well as rare glomerulus-associated macula densa cells. Importantly, we detect heterogeneity in glomerulus-associated Pdgfrb-expressing cells, including bona fide intraglomerular MCs with the functionally active phagocytic molecular machinery, as well as a unique mural cell type located in the central stalk region of the glomerulus tuft. Furthermore, we observe remarkable species differences in the individual gene expression profiles of defined glomerular cell types that highlight translational challenges in the field and provide a guide to design translational studies., The molecular identity of renal glomerular cells is poorly characterized and rodent glomerulopathy models translate poorly to humans. Here, the authors show molecular signatures of glomerulus-associated cells using single cell RNA sequencing and highlight differences between mouse and human cells.

Podocytes are critical for the maintenance of kidney ultrafiltration barrier and play a key role in the progression of glomerular diseases. Although mediator complex proteins have been shown to be important for many physiological and pathological processes, their role in kidney tissue has not been studied. In this study, we identified a mediator complex protein 22 (Med22) as a renal podocyte cell-enriched molecule. Podocyte-specific Med22 knockout mouse showed that Med22 was not needed for normal podocyte maturation. However, it was critical for the maintenance of podocyte health as the mice developed progressive glomerular disease and died due to renal failure. Detailed morphological analyses showed that Med22-deficiency in podocytes resulted in intracellular vacuole formation followed by podocyte loss. Moreover, Med22-deficiency in younger mice promoted the progression of glomerular disease, suggesting Med22-mediated processes may have a role in the development of glomerulopathies. This study shows for the first time that mediator complex has a critical role in kidney physiology.

In recent years, many light-microscopy protocols have been published for visualization of nanoscale structures in the kidney. These protocols present researchers with new tools to evaluate both foot process anatomy and effacement, as well as protein distributions in foot processes, the slit diaphragm and in the glomerular basement membrane. However, these protocols either involve the application of different complicated super resolution microscopes or lengthy sample preparation protocols. Here, we present a fast and simple, five-hour long procedure for three-dimensional visualization of kidney morphology on all length scales. The protocol combines optical clearing and tissue expansion concepts to produce a mild swelling, sufficient for resolving nanoscale structures using a conventional confocal microscope. We show that the protocol can be applied to visualize a wide variety of pathologic features in both mouse and human kidneys. Thus, our fast and simple protocol can be beneficial for conventional microscopic evaluation of kidney tissue integrity both in research and possibly in future clinical routines.

Many light-microscopy protocols have in recent years been published for visualization of ultrastructure in the kidney. These protocols present researchers with new tools to evaluate both foot process anatomy and effacement, as well as protein distributions in foot processes, the slit diaphragm and in the GBM. However, these protocols either involve the application of different complicated super-resolution microscopes or lengthy sample preparation protocols. We here present a fast and simple, 5-hour long procedure for the full three-dimensional visualization of podocyte foot processes using conventional confocal microscopy. The protocol combines and optimizes different optical clearing and tissue expansion concepts to produce a mild swelling, sufficient for resolving foot processes using a diffraction-limited confocal microscope. We further show that the protocol can be used for common visualization of large-scale renal histology, pathology, and kidney protein distributions. Thus, we believe that our fast and simple protocol can be beneficial for routine conventional microscopic evaluation of kidney tissue integrity both in research and in the clinic.

Diabetic kidney disease (DKD) is the most common cause of severe renal disease, and few treatment options are available today that prevent the progressive loss of renal function. DKD is characterized by altered glomerular filtration and proteinuria. A common observation in DKD is the presence of renal steatosis, but the mechanism(s) underlying this observation and to what extent they contribute to disease progression are unknown. Vascular endothelial growth factor B (VEGF-B) controls muscle lipid accumulation through regulation of endothelial fatty acid transport. Here, we demonstrate in experimental mouse models of DKD that renal VEGF-B expression correlates with the severity of disease. Inhibiting VEGF-B signaling in DKD mouse models reduces renal lipotoxicity, re-sensitizes podocytes to insulin signaling, inhibits the development of DKD-associated pathologies, and prevents renal dysfunction. Further, we show that elevated VEGF-B levels are found in patients with DKD, suggesting that VEGF-B antagonism represents a novel approach to treat DKD.

Podocytes have an important role in the pathogenesis of diabetic nephropathy (DN). Podocyte foot process effacement, mediated largely by the actin-based cytoskeleton of foot processes, is commonly detected in DN and is believed to be a key pathogenic event in the development of proteinuria. In this study, we identified coronin 2b (Coro2b), a member of known actin-regulating proteins, the coronins, as a highly podocyte-enriched molecule located at the cytoplasmic side of the apical plasma membrane. Studies in human renal biopsies show that glomerular Coro2b expression is significantly down-regulated in patients with DN. Studies in knockout mice indicate that Coro2b is not required for the development or maintenance of the glomerular filtration barrier. Moreover, inactivation of Coro2b specifically in podocytes does not affect the outcome of nephropathy in a streptozotocin-induced diabetes model. However, Coro2b seems to modulate the reorganization of foot processes under pathological conditions as Coro2b knockout podocytes are partially protected from protamine sulfate perfusion-induced foot process effacement. Taken together, our study suggests a role for Coro2b in the pathogenesis of glomerulopathies. Further studies regarding the involvement of Coro2b in podocyte health and diseases are warranted.

Background Steroid-resistant nephrotic syndrome (SRNS) is a common cause of end-stage renal disease in children but also occurs as an adult-onset condition. In a subset of SRNS patients, pathogenic variants are found in genes coding for podocyte foot process proteins. The aim of this study was to define the role of pathogenic variants in Finnish patients with familial and sporadic SRNS. Methods We analyzed SRNS-related genes NPHS1, NPHS2, NEPH1, ACTN4, TRPC6, INF2, WT1, CD2AP, LAMB2, and PLCE1 for disease-causing variants using direct sequencing of exons and intron/exon boundaries in all members of a family with dominant SRNS with early onset and slow progression to end-stage renal disease. We carried out a whole genome sequencing in two affected and two healthy family members. The function of found podocin variant was studied using co-immunoprecipitation and immunohistochemistry. Podocyte gene sequences were analyzed in a cohort of Finnish non-familial SRNS patients. Results A heterozygous de novo deletion, c. 988_989delCT in NPHS2, was found in all affected family members and in none of their healthy relatives, non-familial patients or controls. No other SRNS-related gene variant, coding or non-coding co-segregated with the disease phenotype in the family. While the truncated podocin remained able to bind nephrin, the expression of nephrin was fragmented and podocin expression reduced. The gene analysis of the non-familial SRNS patients revealed few variants. Conclusion The role of podocin variants in nephrotic syndrome may be more varied than previously thought.

BACKGROUND: Inflammatory processes play an important role in the pathogenesis of glomerulopathies. Finding novel ways to suppress glomerular inflammation may offer a new way to stop disease progression. However, the molecular mechanisms that initiate and drive inflammation in the glomerulus are still poorly understood. METHODS: We performed large-scale gene expression profiling of glomerulus-associated G protein–coupled receptors (GPCRs) to identify new potential therapeutic targets for glomerulopathies. The expression of Gprc5b in disease was analyzed using quantitative PCR and immunofluorescence, and by analyzing published microarray data sets. In vivo studies were carried out in a podocyte-specific Gprc5b knockout mouse line. Mechanistic studies were performed in cultured human podocytes. RESULTS: We identified an orphan GPCR, Gprc5b, as a novel gene highly enriched in podocytes that was significantly upregulated in common human glomerulopathies, including diabetic nephropathy, IgA nephropathy, and lupus nephritis. Similar upregulation of Gprc5b was detected in LPS-induced nephropathy in mice. Studies in podocyte-specific Gprc5b knockout mice showed that Gprc5b was not essential for normal development of the glomerular filtration barrier. However, knockout mice were partially protected from LPS-induced proteinuria and recruitment of inflammatory cells. Mechanistically, RNA sequencing in Gprc5b knockouts mice and experiments in cultured human podocytes showed that Gpr5cb regulated inflammatory response in podocytes via NF-κB signaling. CONCLUSIONS: GPRC5b is a novel podocyte-specific receptor that regulates inflammatory response in the glomerulus by modulating the NF-κB signaling pathway. Upregulation of Gprc5b in human glomerulopathies suggests that it may play a role in their pathogenesis.

Podocyte loss is central to the progression of proteinuric kidney diseases leading to end-stage kidney disease (ESKD), requiring renal replacement therapy, such as dialysis. Despite modern tools and techniques, the 5-year mortality of some patients requiring dialysis remains at about 70% to 80%. Thus, there is a great unmet need for podocyte-specific treatments aimed at preventing podocyte loss and the ensuing development of ESKD. Here, we show that ablation of the podocyte death-promoting protein dendrin delays the onset of ESKD, thereby expanding the life span of mice lacking the adapter protein CD2AP. Ablation of dendrin delays onset and severity of proteinuria and podocyte loss. In addition, dendrin ablation ameliorates mesangial volume expansion and up-regulation of mesangial fibronectin expression, which is mediated by a podocyte-secreted factor. In conclusion, onset of ESKD and death can be markedly delayed by blocking the function of dendrin.

The kidney ultrafiltration barrier is formed of endothelial cells, the glomerular basement membrane and podocytes. Podocytes have a central role in normal physiology and disease pathogenesis of the glomerulus. Signaling through epidermal growth factor receptor (EGFR) in podocytes mediates development of many glomerular disease processes. In this work, we have identified zinc finger FYVE-type containing 28 (ZFYVE28) as a novel highly podocyte-enriched gene. We localize ZFYVE28 in podocyte foot processes in adult kidney. During glomerulogenesis, Zfyve28 is first expressed at the early capillary loop glomerulus. In cultured podocytes, we show that overexpression of ZFYVE28 promotes EGF-signaling, possibly by up-regulating EGFR expression and by modulating its localization. To study the role of ZFYVE28 in vivo, we generated both conventional and podocyte-specific knockout mouse lines. Kidneys developed normally in ZFYVE28-deficient mice. In adult mice, the absence of ZFYVE28 did not affect the maintenance of the filtration barrier. Moreover, ZFYVE28-deficiency did not affect the outcome of glomerular damage induced by injection of nephrotoxic serum. Taken together, we have identified Zfyve28 as a new molecular component of podocyte foot processes and show that it mediates EGF-signaling in podocytes. However, ZFYVE28 is not essential for the development or maintenance of the glomerulus filtration barrier.

The evidence that gene mutations in the polarity determinant Crumbs homologs-2 (CRB2) cause congenital nephrotic syndrome suggests the functional importance of this gene product in podocyte development. Because another isoform, CRB3, was reported to repress the mechanistic/mammalian target of the rapamycin complex 1 (mTORC1) pathway, we examined the role of CRB2 function in developing podocytes in relation to mTORC1. In HEK-293 and MDCK cells constitutively expressing CRB2, we found that the protein localized to the apicolateral side of the cell plasma membrane and that this plasma membrane assembly required N-glycosylation. Confocal microscopy of the neonate mouse kidney revealed that both the tyrosine-phosphorylated form and non-phosphorylated form of CRB2 commence at the S-shaped body stage at the apicolateral side of podocyte precursor cells and move to foot processes in a capillary tuft pattern. The pattern of phosphorylated mTOR in developing podocytes was similar to that of CRB2 tyrosine phosphorylation. Additionally, the lack of a tyrosine phosphorylation site on CRB2 led to the reduced sensitivity of mTORC1 activation in response to energy starvation. CRB2 may play an important role in the mechanistic pathway of developing podocytes through tyrosine phosphorylation by associating with mTORC1 activation.

Background Renal glomeruli are the primary target of injury in diabetic nephropathy (DN), and the glomerular podocyte has a key role in disease progression. Methods To identify potential novel therapeutic targets for DN, we performed high-throughput molecular profiling of G protein–coupled receptors (GPCRs) using human glomeruli. Results We identified an orphan GPCR, Gprc5a, as a highly podocyte-specific gene, the expression of which was significantly downregulated in glomeruli of patients with DN compared with those without DN. Inactivation of Gprc5a in mice resulted in thickening of the glomerular basement membrane and activation of mesangial cells, which are two hallmark features of DN in humans. Compared with wild-type mice, Gprc5a-deficient animals demonstrated increased albuminuria and more severe histologic changes after induction of diabetes with streptozotocin. Mechanistically, Gprc5a modulated TGF-β signaling and activation of the EGF receptor in cultured podocytes. Conclusions Gprc5a has an important role in the pathogenesis of DN, and further study of the podocyte-specific signaling activity of this protein is warranted.

Development of physiologically relevant cellular models with strong translatability to human pathophysiology is critical for identification and validation of novel therapeutic targets. Herein we describe a detailed protocol for generation of an advanced 3-dimensional kidney cellular model using induced pluripotent stem cells, where differentiation and maturation of kidney progenitors and podocytes can be monitored in live cells due to CRISPR/Cas9-mediated fluorescent tagging of kidney lineage markers (SIX2 and NPHS1). Utilizing these cell lines, we have refined the previously published procedures to generate a new, higher throughput protocol suitable for drug discovery. Using paraffin-embedded sectioning and whole-mount immunostaining, we demonstrated that organoids grown in suspension culture express key markers of kidney biology (WT1, ECAD, LTL, nephrin) and vasculature (CD31) within renal cortical structures with microvilli, tight junctions and podocyte foot processes visualized by electron microscopy. Additionally, the organoids resemble the adult kidney transcriptomics profile, thereby strengthening the translatability of our in vitro model. Thus, development of human nephron-like structures in vitro fills a major gap in our ability to assess the effect of potential treatment on key kidney structures, opening up a wide range of possibilities to improve clinical translation.

The glomerular filtration barrier, has historically only been spatially resolved using electron microscopy due to the nanometer-scale dimensions of these structures. Recently, it was shown that the nanoscale distribution of proteins in the slit diaphragm can be resolved by fluorescence based stimulated emission depletion microscopy, in combination with optical clearing. Fluorescence microscopy has advantages over electron microscopy in terms of multiplex imaging of different epitopes, and also the amount of volumetric data that can be extracted from thicker samples. However, stimulated emission depletion microscopy is still a costly technique commonly not available to most life science researchers. An imaging technique with which the glomerular filtration barrier can be visualized using more standard fluorescence imaging techniques is thus desirable. Recent studies have shown that biological tissue samples can be isotropically expanded, revealing nanoscale localizations of multiple epitopes using confocal microscopy. Here we show that kidney samples can be expanded sufficiently to study the finest elements of the filtration barrier using confocal microscopy. Thus, our result opens up the possibility to study protein distributions and foot process morphology on the effective nanometer-scale.

Background/Aims: The transmembrane proteins Neph1 and nephrin form a complex in the slit diaphragm (SD) of podocytes. As recent studies indicate an involvement of this complex in the polymerization of the actin cytoskeleton and proteinuria, we wanted to study the subcellular localization of Neph1 in the normal human kidney and its expression in focal segmental glomerulosclerosis (FSGS), minimal change nephrotic syndrome (MCNS), and the corresponding experimental models of Adriamycin-induced nephropathy (ADR) and puromycin aminonucleoside nephrosis (PAN). All these disorders are characterized by substantial foot process effacement (FPE) and proteinuria. Materials and Methods: Kidney biopsies from patients with primary FSGS (perihilar type) and MCNS were compared to normal renal tissue. Mouse and rat kidney cortices from days 7 and 14 after Adriamycin injection and days 2 and 4 after puromycin aminonucleoside injection, respectively, were compared to control mouse and rat kidney. Polyclonal antibodies against Neph1 and nephrin were used for immunoelectron microscopy, and semiquantification was performed. Results: We localized Neph1 mainly to, and in close proximity to, the SD. Double staining of Neph1 and nephrin showed the proteins to be in close connection in the SD. The total amount of Neph1 in the podocytes was significantly reduced in FSGS, MCNS, ADR, and PAN. The reduction of Neph1 was also seen in areas with and without FPE. Nephrin was reduced in MCNS and PAN but unchanged in FSGS. Conclusion: With nephrin (but not Neph1) unchanged in FSGS, there might be a disruption of the complex and an involvement of Neph1 in its pathogenesis.

Vertebrate life critically depends on renal filtration and excretion of low molecular weight waste products. This process is controlled by a specialized cell-cell contact between podocyte foot processes: the slit diaphragm (SD). Using a comprehensive set of targeted KO mice of key SD molecules, we provided genetic, functional, and high-resolution ultrastructural data highlighting a concept of a flexible, dynamic, and multilayered architecture of the SD. Our data indicate that the mammalian SD is composed of NEPHRIN and NEPH1 molecules, while NEPH2 and NEPH3 do not participate in podocyte intercellular junction formation. Unexpectedly, homo- and heteromeric NEPHRIN/NEPH1 complexes are rarely observed. Instead, single NEPH1 molecules appear to form the lower part of the junction close to the glomerular basement membrane with a width of 23 nm, while single NEPHRIN molecules form an adjacent junction more apically with a width of 45 nm. In both cases, the molecules are quasiperiodically spaced 7 nm apart. These structural findings, in combination with the flexibility inherent to the repetitive Ig folds of NEPHRIN and NEPH1, indicate that the SD likely represents a highly dynamic cell-cell contact that forms an adjustable, nonclogging barrier within the renal filtration apparatus.

Proprotein convertases (PCSKs) process matrix metalloproteases (MMPs) and cytokines. Apart from PCSK9, the role of these enzymes in vascular disease is largely unknown. Previously, we demonstrated upregulation of PCSK6 in carotid atherosclerosis, primarily localized to smooth muscle cells (SMCs) and positively correlated to inflammation, extracellular matrix remodeling and cytokines. Here, we extended these findings to determine the role of PCSK6 in vascular development and disease. Increased expression of PCSK6 in vascular disease was validated by microarrays from two non-overlapping cohorts of carotid plaques vs. non-atherosclerotic arteries (n=50 patients and n=32 patients, p0.7, p0.5, p-/- mice did not present an obvious vascular phenotype but showed reduced intimal hyperplasia compared to wild-type mice after carotid artery ligation (p=0.015). In vitro, PCSK6 overexpression markedly increased SMC migration upon PDGFBB stimulation (p

The Crumbs family of transmembrane proteins has an important role in the differentiation of the apical membrane domain in various cell types, regulating such processes as epithelial cell polarization. The mammalian Crumbs protein family is composed of three members. Here, we inactivated the mouse Crb2 gene with gene-targeting techniques and found that the protein is crucial for early embryonic development with severe abnormalities appearing in Crb2-deficient embryos at late-gastrulation. Our findings indicate that the primary defect in the mutant embryos is disturbed polarity of the epiblast cells at the primitive streak, which affects epithelial to mesenchymal transition (EMT) during gastrulation, resulting in impaired mesoderm and endoderm formation, and embryonic lethality by embryonic day 12.5. These findings therefore indicate a novel role for the Crumbs family of proteins. Developmental Dynamics 240:2646–2656, 2011. © 2011 Wiley Periodicals, Inc.

The slit diaphragm and the apical and basal membrane domains of podocytes are connected to each other by an actin-based cytoskeleton critical to the maintenance of the glomerular filtration barrier. In an effort to discover novel regulatory proteins of the podocyte foot process, we identified and characterized pdlim2, a member of the actin-associated LIM protein subfamily of cytosolic proteins typified by an N-terminal PDZ domain and a C-terminal LIM domain. In the kidney, the pdlim2 protein is highly specific for the glomerulus and podocyte foot processes as shown by RT-PCR, western blotting, immunofluorescence, and immunoelectron microscopy. In cultured podocytes, pdlim2 was associated with stress fibers and cortical actin. Pdlim2 seems to regulate actin dynamics in podocytes since stress fibers were stabilized in its presence. Mechanistically, pdlim2 interacts with two actin-associated podocyte proteins, α-actinin-4 and angiomotin-like-1, as shown by immunoprecipitation and yeast two-hybrid analyses. By semi-quantitative immunoelectron microscopy, there was a reduced expression of pdlim2 in podocytes of patients with minimal change nephrotic syndrome and membranous nephropathy, whereas its expression was unchanged in patients with focal segmental glomerulosclerosis. Hence, pdlim2 is a novel actin-regulating protein of podocyte foot processes that may have a role in the pathogenesis of glomerular diseases.

Unbiased transcriptome profiling and functional genomics approaches identified glucocorticoid-induced transcript 1 (GLCCI1) as being a transcript highly specific for the glomerulus, but its role in glomerular development and disease is unknown. Here, we report that mouse glomeruli express far greater amounts of Glcci1 protein compared with the rest of the kidney. RT-PCR and Western blotting demonstrated that mouse glomerular Glcci1 is approximately 60 kD and localizes to the cytoplasm of podocytes in mature glomeruli. In the fetal kidney, intense Glcci1 expression occurs at the capillary-loop stage of glomerular development. Using gene knockdown in zebrafish with morpholinos, morphants lacking Glcci1 function had collapsed glomeruli with foot-process effacement. Permeability studies of the glomerular filtration barrier in these zebrafish morphants demonstrated a disruption of the selective glomerular permeability filter. Taken together, these data suggest that Glcci1 promotes the normal development and maintenance of podocyte structure and function.

Kidney podocytes are highly differentiated epithelial cells that form interdigitating foot processes with bridging slit diaphragms (SDs) that regulate renal ultrafiltration. Podocyte injury results in proteinuric kidney disease, and genetic deletion of SD-associated CD2-associated protein (CD2AP) leads to progressive renal failure in mice and humans. Here, we have shown that CD2AP regulates the TGF-β1–dependent translocation of dendrin from the SD to the nucleus. Nuclear dendrin acted as a transcription factor to promote expression of cytosolic cathepsin L (CatL). CatL proteolyzed the regulatory GTPase dynamin and the actin-associated adapter synaptopodin, leading to a reorganization of the podocyte microfilament system and consequent proteinuria. CD2AP itself was proteolyzed by CatL, promoting sustained expression of the protease during podocyte injury, and in turn increasing the apoptotic susceptibility of podocytes to TGF-β1. Our study identifies CD2AP as the gatekeeper of the podocyte TGF-β response through its regulation of CatL expression and defines a molecular mechanism underlying proteinuric kidney disease.

Background/Aims: It is still unclear what happens in the glomerulus when proteinuria starts. Using puromycin aminonucleoside nephrosis (PAN) rats, we studied early ultrastructural and permeability changes in relation to the expression of the podocyte-associated molecules nephrin, α-actinin, dendrin, and plekhh2, the last two of which were only recently discovered in podocytes. Methods: Using immune stainings, semiquantitative measurement was performed under the electron microscope. Permeability was assessed using isolated kidney perfusion with tracers. Possible effects of ACE inhibition were tested. Results: By day 2, some patchy foot process effacement, but no proteinuria, appeared. The amount of nephrin was reduced in both diseased and normal areas. The other proteins showed few changes, which were limited to diseased areas. By day 4, foot process effacement was complete and proteinuria appeared in parallel with signs of size barrier damage. Nephrin decreased further, while dendrin and plekhh2 also decreased but α-actinin remained unchanged. ACE inhibition had no significant protective effect. Conclusions: PAN glomeruli already showed significant pathology by day 4, despite relatively mild proteinuria. This was preceded by altered nephrin expression, supporting its pivotal role in podocyte morphology. The novel proteins dendrin and plekhh2 were both reduced, suggesting roles in PAN, whereas α-actinin was unchanged.

Disturbances in many different molecular pathways and interactions can lead to the same clinical end points of proteinuria and end-stage renal disease. Proteinuria is often accompanied by a cytopathological change in the glomerulus that is referred to as effacement (retraction) of the podocyte foot processes. The molecular mechanisms that lead to proteinuria and podocyte effacement are poorly understood; therefore, targeted therapies are lacking. During the past 5 years, however, a large body of data has emerged in this field. The discovery of podocyte gene defects that underlie some hereditary proteinuric syndromes has changed our understanding of the relative contributions of components of the glomerular filter. Furthermore, pathogenic pathways activated in podocytes during proteinuria have been identified. Together, these findings pinpoint the podocyte as the most obvious candidate for therapeutic intervention. In the near future, the use of large-scale expression profiling platforms, transgenic mouse lines, and other in vivo gene delivery methods will further expand our understanding of the pathology of the glomerular filtration barrier, and perhaps reveal novel target molecules for the therapy of proteinuric kidney diseases.

Background. Dendrin is an 81-kD cytosolic protein hitherto described in the brain, where it is associated with the actin cytoskeleton. Recently, we found dendrin in foot processes of mouse glomerular podocytes. Here we describe its expression both during mouse glomerulogenesis and in the normal and diseased human kidney for the first time. Methods. Dendrin expression was characterized using RT-PCR and immunohistochemistry and semi-quantified using immunoelectron microscopy. Results. In glomerulogenesis, dendrin mRNA and protein appeared first at the early capillary loop stage. It was concentrated to the pre-podocytes on the basal side of podocalyxin, an apical cell membrane marker. In human tissue, dendrin transcripts were detected in the brain and kidney. In the mature kidney dendrin localized solely in the podocytes, close to the filtration slit diaphragms. A comparison with the slit-associated protein zonula occludens- 1 (ZO-1) was done in minimal change nephrotic syndrome (MCNS). Dendrin and ZO-1 were re-distributed from slit regions to the podocyte cytoplasm in areas with foot process effacement (FPE). In areas without FPE, dendrin and ZO-1 distributions were unchanged compared to controls. The total amounts of dendrin or ZO-1 markers were unchanged. This differs from nephrin that, according to our previous results, is also decreased in non-effaced areas. Conclusions. The expression of dendrin during glomerulogenesis and in the normal human kidney is similar to that previously shown for nephrin, which suggests that dendrin associates with the slit diaphragm complex. In MCNS patients, dendrin and ZO-1 are re-distributed within the podocytes. Whether this is a cause or a consequence of FPE remains unclear.

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

Histopathology provides the current basis for classification and diagnosis of glomerular disorders. Molecular profiling methods, such as microarray analysis of mRNA expression, have rapidly emerged over the past years and are now applicable to minute amounts of tissue material, such as glomeruli from embryos or adult experimental animals, or from human renal needle biopsies. This review summarizes current efforts aiming at the determination of the glomerular transcriptome and proteome during development, in the healthy adult, and in disease. These studies are encouraging and show that comprehensive molecular profiling of the kidney glomerulus will most likely provide significant new insights into the normal structure and function of the glomerular filter, the molecular mechanisms of glomerular development, the diagnosis and classification of glomerular disease, and the pathogenic mechanisms underlying the stepwise breakdown of glomerular filter function that accompanies several common systemic disorders.

The kidney glomerulus plays a crucial role in blood filtration but the molecular composition and physiology of the glomerulus is not well understood. We previously constructed and large-scale sequenced four mouse glomerular expressed sequence tag (EST) libraries from newborn and adult mouse glomeruli. Here, we compared glomerular EST profiles with whole kidney EST profiles, thereby identifying 497 transcripts corresponding to UniGene clusters that were glomerulus-enriched, that is expressed more abundantly in glomeruli than in whole kidney. These include several known protein-coding glomerulus-specific transcripts critical for glomerulus development and function, but also a large number of gene transcripts, which have not previously been shown to be expressed in the glomerulus, or implicated in glomerular functions. We used in situ hybridization to demonstrate glomerulus-specific RNA expression for six novel glomerular genes and the public Human Protein Atlas to verify glomerular protein expression for another two. The higher mRNA abundance for the eight genes in glomeruli compared with whole kidney was also verified by Taqman quantitative polymerase chain reaction. We surmise that the further characterization of these genes and proteins will increase our understanding of glomerular development and physiology.

The glomerular capillary tuft is a highly specialized microcapillary that is dedicated to function as a sophisticated molecular sieve. The glomerulus filter has a unique molecular composition, and several essential glomerular proteins are expressed in the kidney exclusively by glomerular podocytes. A catalog of >300 glomerulus-upregulated transcripts that were identified using expressed sequence tag profiling and microarray analysis was published recently. This study characterized the expression profile of five glomerulus-upregulated transcripts/proteins (ehd3, dendrin, sh2d4a, plekhh2, and 2310066E14Rik) in detail. The expression pattern of these novel glomerular transcripts in various mouse tissues was studied using reverse transcriptase-PCR, Northern blotting, and in situ hybridization. For studying the distribution of corresponding proteins, polyclonal antibodies were raised against the gene products, and Western blotting, immunofluorescence, and immunoelectron microscopic analyses were performed. Remarkably, it was discovered that all five transcripts/proteins were expressed in the kidney exclusively by glomerular cells. Ehd3 was expressed only by glomerular endothelial cells. Importantly, ehd3 is the first gene ever shown to be expressed exclusively by glomerular endothelial cells and not by other endothelial cells in the kidney. Dendrin, sh2d4a, plekhh2, and 2310066E14Rik, however, were transcribed solely by podocytes. With the use of polyclonal antibodies, dendrin, sh2d4a, and plekhh2 proteins were localized to the slit diaphragm and the foot process, whereas 2310066E14Rik protein was localized to the podocyte major processes and cell body. This study provides fresh insights into glomerular biology and uncovers new possibilities to explore the role of these novel proteins in the glomerular physiology and pathology.

Background Podocyte foot process effacement accompanied by actin cytoskeleton rearrangements is a cardinal feature of many progressive human proteinuric diseases. Results By microarray profiling of mouse glomerulus, SCHIP1 emerged as one of the most highly enriched transcripts. We detected Schip1 protein in the kidney glomerulus, specifically in podocytes foot processes. Functionally, Schip1 inactivation in zebrafish by morpholino knock-down results in foot process disorganization and podocyte loss leading to proteinuria. In cultured podocytes Schip1 localizes to cortical actin-rich regions of lamellipodia, where it forms a complex with Nherf2 and ezrin, proteins known to participate in actin remodeling stimulated by PDGFβ signaling. Mechanistically, overexpression of Schip1 in vitro causes accumulation of cortical F-actin with dissolution of transversal stress fibers and promotes cell migration in response to PDGF-BB stimulation. Upon actin disassembly by latrunculin A treatment, Schip1 remains associated with the residual F-actin-containing structures, suggesting a functional connection with actin cytoskeleton possibly via its interaction partners. A similar assay with cytochalasin D points to stabilization of cortical actin cytoskeleton in Schip1 overexpressing cells by attenuation of actin depolymerisation. Conclusions Schip1 is a novel glomerular protein predominantly expressed in podocytes, necessary for the zebrafish pronephros development and function. Schip1 associates with the cortical actin cytoskeleton network and modulates its dynamics in response to PDGF signaling via interaction with the Nherf2/ezrin complex. Its implication in proteinuric diseases remains to be further investigated.

Two papers, one in Nature (Jones et al., 2006) and the other in the Journal of Clinical Investigation (Verma et al., 2006) show that Nck adaptor proteins connect phosphorylated nephrin with actin polymerization in podocyte foot processes, structures important for slit-diaphragm formation in the kidney. Their results further our understanding of podocyte development and repair in glomerular disease.

This review summarizes recent advances in our knowledge of the glomerular filter and the causes of hereditary proteinuria syndromes.