WT1 and PAX-2 podocyte expression in Denys-Drash syndrome and isolated diffuse mesangial sclerosis

Hidden genetics behind glomerular scars: an opportunity to understand the heterogeneity of focal segmental glomerulosclerosis?

Focal segmental glomerulosclerosis (FSGS) is a complex disease which describes different kinds of kidney defects, not exclusively linked with podocyte defects. Since nephrin mutation was first described in association with early-onset nephrotic syndrome (NS), many advancements have been made in understanding genetic patterns associated with FSGS. New genetic causes of FSGS have been discovered, displaying unexpected genotypes, and recognizing possible site of damage. Many recent large-scale sequencing analyses on patients affected by idiopathic chronic kidney disease (CKD), kidney failure (KF) of unknown origin, or classified as FSGS, have revealed collagen alpha IV genes, as one of the most frequent sites of pathogenic mutations. Also, recent interest in complex and systemic lysosomal storage diseases, such as Fabry disease, has highlighted GLA mutations as possible causes of FSGS. Tubulointerstitial disease, recently classified by KDIGO based on genetic subtypes, when associated with UMOD variants, may phenotypically gain FSGS features, as well as ciliopathy genes or others, otherwise leading to completely different phenotypes, but found carrying pathogenic variants with associated FSGS phenotype. Thus, glomerulosclerosis may conceal different heterogeneous conditions. When a kidney biopsy is performed, the principal objective is to provide an accurate diagnosis. The broad spectrum of phenotypic expression and genetic complexity is demonstrating that a combined path of management needs to be applied. Genetic investigation should not be reserved only to selected cases, but rather part of medical management, integrating with clinical and renal pathology records. FSGS heterogeneity should be interpreted as an interesting opportunity to discover new pathways of CKD, requiring prompt genotype-phenotype correlation. In this review, we aim to highlight how FSGS represents a peculiar kidney condition, demanding multidisciplinary management, and in which genetic analysis may solve some otherwise unrevealed idiopathic cases. Unfortunately there is not a uniform correlation between specific mutations and FSGS morphological classes, as the same variants may be identified in familial cases or sporadic FSGS/NS or manifest a variable spectrum of the same disease. These non-specific features make diagnosis challenging. The complexity of FSGS genotypes requires new directions. Old morphological classification does not provide much information about the responsible cause of disease and misdiagnoses may expose patients to immunosuppressive therapy side effects, mistaken genetic counseling, and misguided kidney transplant programs.

Multiplexed bulk and single-cell RNA-seq hybrid enables cost-efficient disease modeling with chimeric organoids

Disease modeling with isogenic Induced Pluripotent Stem Cell (iPSC)-differentiated organoids serves as a powerful technique for studying disease mechanisms. Multiplexed coculture is crucial to mitigate batch effects when studying the genetic effects of disease-causing variants in differentiated iPSCs or organoids, and demultiplexing at the single-cell level can be conveniently achieved by assessing natural genetic barcodes. Here, to enable cost-efficient time-series experimental designs via multiplexed bulk and single-cell RNA-seq of hybrids, we introduce a computational method in our Vireo Suite, Vireo-bulk, to effectively deconvolve pooled bulk RNA-seq data by genotype reference, and thereby quantify donor abundance over the course of differentiation and identify differentially expressed genes among donors. Furthermore, with multiplexed scRNA-seq and bulk RNA-seq, we demonstrate the usefulness and necessity of a pooled design to reveal donor iPSC line heterogeneity during macrophage cell differentiation and to model rare WT1 mutation-driven kidney disease with chimeric organoids. Our work provides an experimental and analytic pipeline for dissecting disease mechanisms with chimeric organoids.

Central role of podocytes in mediating cellular cross talk in glomerular health and disease

Podocytes are highly specialized epithelial cells that surround the capillaries of the glomeruli in the kidney. Together with the glomerular endothelial cells, these postmitotic cells are responsible for regulating filtrate from the circulating blood with their organized network of interdigitating foot processes that wrap around the glomerular basement membrane. Although podocyte injury and subsequent loss is the hallmark of many glomerular diseases, recent evidence suggests that the cell-cell communication between podocytes and other glomerular and nonglomerular cells is critical for the development and progression of kidney disease. In this review, we highlight these key cellular pathways of communication and how they might be a potential target for therapy in glomerular disease. We also postulate that podocytes might serve as a central hub for communication in the kidney under basal conditions and in response to cellular stress, which may have implications for the development and progression of glomerular diseases.

Mitochondrial Oxidative Stress and Cell Death in Podocytopathies

Podocytopathies are kidney diseases that are driven by podocyte injury with proteinuria and proteinuria-related symptoms as the main clinical presentations. Albeit podocytopathies are the major contributors to end-stage kidney disease, the underlying molecular mechanisms of podocyte injury remain to be elucidated. Mitochondrial oxidative stress is associated with kidney diseases, and increasing evidence suggests that oxidative stress plays a vital role in the pathogenesis of podocytopathies. Accumulating evidence has placed mitochondrial oxidative stress in the focus of cell death research. Excessive generated reactive oxygen species over antioxidant defense under pathological conditions lead to oxidative damage to cellular components and regulate cell death in the podocyte. Conversely, exogenous antioxidants can protect podocyte from cell death. This review provides an overview of the role of mitochondrial oxidative stress in podocytopathies and discusses its role in the cell death of the podocyte, aiming to identify the novel targets to improve the treatment of patients with podocytopathies.

The Pathology Lesion Patterns of Podocytopathies: How and why?

Podocytopathies are a group of proteinuric glomerular disorders driven by primary podocyte injury that are associated with a set of lesion patterns observed on kidney biopsy, i.e., minimal changes, focal segmental glomerulosclerosis, diffuse mesangial sclerosis and collapsing glomerulopathy. These unspecific lesion patterns have long been considered as independent disease entities. By contrast, recent evidence from genetics and experimental studies demonstrated that they represent signs of repeated injury and repair attempts. These ongoing processes depend on the type, length, and severity of podocyte injury, as well as on the ability of parietal epithelial cells to drive repair. In this review, we discuss the main pathology patterns of podocytopathies with a focus on the cellular and molecular response of podocytes and parietal epithelial cells.

Molecular Screening of PAX2 Gene Polymorphism in Primary Vesicoureteral Reflux Patients in Taif Governorate, KSA

<b>Background and Objective:</b> Primary Nonsyndromic Vesicoureteral Reflux (PVUR) is a widespread genetic malformation and considered a prevalent Congenital Abnormality of the Kidney and Urinary Tract (CAKUT). Mutations in the <i>PAX2 </i>gene have been associated with abnormalities in the kidney extending from CAKUT to oncogenic processes. The present study analyzes the <i>PAX2</i> polymorphisms and their association with primary VUR in Saudi children patients from the Taif governorate. <b>Materials and Methods:</b> Fifteen children with primary VUR were identified and screened for gene mutations in the <i>PAX2</i> gene by direct sequencing method of purified Polymerase Chain Reaction (PCR) products of all exons to elucidate the correlation between <i>PAX2</i> gene and VUR. <b>Results:</b> Seven new variants have been defined. Three polymorphic missense variants in homozygous genotype form were found in intron 8 and detected in eight patients, One missense mutation was found in exon 10 in the site of transactivation domain and detected in ten patients and <i>in-silico</i> analysis predicted it as a pathogenic one, Three mutations were found in exon 11 and detected in all patients as a compound homozygous. <b>Conclusion:</b> <i>PAX2</i>is important for normal kidney development and mutations in the gene possibly lead to disturbance in the protein structure and could be non-functional thus mutations in <i>PAX2</i> may be one of the causes of PVUR in Saudi Arabia. Further investigation is necessary to understand the aetiology of disease and maybe other genes implicated in VUR.

A novel WT1 gene mutation in a chinese girl with denys-drash syndrome

OBJECTIVE

Denys-Drash syndrome (DDS) is defined by the triad of Wilms tumor, nephrotic syndrome, and/or ambiguous genitalia. Genetic testing may help identify new gene mutation sites and play an important role in clinical decision-making.

METHODS

We present a patient with an XY karyotype and female appearance, nephropathy, and Wilms tumor in the right kidney. Genomic DNA was extracted from peripheral blood cells according to standard protocols. "Next-generation" sequencing (NGS) was performed to identify novel variants. The variant was analyzed with Mutation Taster, and its function was explored by a cell growth inhibition assay.

RESULTS

We found the first case of Denys-Drash syndrome with the uncommon missense mutation (c.1420C>T, p.His474 Tyr) in the WT1 gene. In silico analysis, the variant was predicted "disease-causing" by Mutation Taster. The mutated variant showed a weaker effect in inhibiting tumor cells than wild-type WT1.

CONCLUSIONS

The uncommon missense mutation (c.1420C>T, p.His474 Tyr) in the WT1 gene may be a crucial marker in DDS.

Focal Segmental Membranoproliferative Glomerulonephritis: A Histological Variant of Denys-Drash Syndrome

Introduction: Denys-Drash Syndrome (DDS) consists of a triad of pseudohermaphroditism, Wilms'tumor and nephropathy. This condition may manifest as a complete triad or in an incomplete form; with either one or a combination of the above features. The characteristic glomerular abnormality in DDS is diffuse mesangial sclerosis (DMS).Case report: We report two cases of DDS with focal membranoproliferative glomerulonephritis (MPGN). Both of our cases were males with ambiguous genitalia. They had a similar heterozygous germline mutation in exon 9 of WT1, c.1180C>T, p.R394W; a known mutation hotspot for DDS. Case 1 had nephropathy at the age of 4 years and Case 2 at 2.5 years with different rates of progression to end-stage renal failure. Conclusion: Our findings, in combination with other reports, illustrate the clinicopathological heterogeneity of DDS. There are no universal recommendations for optimal management of patients with DDS due to the inability to accurately predict affected individuals' progress.

"Atrophic Kidney"-like Lesion: Clinicopathologic Series of 8 Cases Supporting a Benign Entity Distinct From Thyroid-like Follicular Carcinoma

Renal mass lesions with a follicular architecture resembling atrophic kidney have been described, but their distinction from thyroid-like follicular carcinoma of the kidney remains controversial. We collected 8 cases of this purported "atrophic kidney"-like lesion to fully describe their clinical and histologic spectrum, their possible etiology, and to discuss their distinction from other renal neoplasms. Eight total cases were identified with patient ages ranging from 9 to 48 years (mean: 29 y; median: 28.5 y). Four patients were female and 4 were male. The tumors were unifocal and size ranged from 1.6 to 4.9 cm (mean: 3.4 cm; median: 3.4 cm). All 8 tumors had a remarkably similar histology. Each was enveloped by a smooth muscle rich capsule and had an overall low power "follicular" architecture. The luminal spaces of the "follicles" (or cysts) contained eosinophilic secretions and the lining epithelium was often flattened and atrophic, but some had more rounded cells with a distinctive hobnail arrangement. Many cysts contained discohesive round cells floating within the eosinophilic material, and some contained small intraluminal tufts with features of markedly atrophic glomeruli. Periodic acid-Schiff stains highlighted basement membrane material extending into these glomerular-like tufts, and some contained small distinct capillaries surrounded by endothelial cells, interspersed mesangial-like cells, and rare surrounding podocyte-like cells, providing additional evidence for glomerulocystic structures. Scattered calcifications were present within cysts (or within cyst walls) in varying numbers and were characterized by 2 types: psammoma body-like or more amorphous deposits. The tissue between cystic glomeruli contained predominantly small atrophic tubular structures, but collagenized stroma and smaller collapsed glomeruli were also present. The 2 tumors from the oldest 2 patients (48 and 39 y) had a more striking degree of stromal hyalinization. Immunohistochemically, the cyst lining cells had a predominant WT-positive/PAX-8 negative/CK7-negative phenotype, while tubules were typically WT-1 negative/PAX-8 positive/CK7-positive. Upon comparison to a control group of 10 kidneys containing incidental non-mass-forming glomerulocystic change, the morphologic features and immunophenotype were identical. To date, no patient has had any recurrence or aggressive clinical behavior based on follow status in 7 of 8 cases (follow-up range: 9 to 168 mo; median: 24 mo; mean: 40 mo). In summary, we describe the clinicopathologic features of 8 unique, benign "atrophic kidney"-like lesions that may simply represent a non-neoplastic form of organizing tubular atrophy and glomerulocystic change, and emphasize their distinction from thyroid-like follicular carcinoma of the kidney.

Activation of podocyte Notch mediates early Wt1 glomerulopathy

The Wilms' tumor suppressor gene, WT1, encodes a zinc finger protein that regulates podocyte development and is highly expressed in mature podocytes. Mutations in the WT1 gene are associated with the development of renal failure due to the formation of scar tissue within glomeruli, the mechanisms of which are poorly understood. Here, we used a tamoxifen-based CRE-LoxP system to induce deletion of Wt1 in adult mice to investigate the mechanisms underlying evolution of glomerulosclerosis. Podocyte apoptosis was evident as early as the fourth day post-induction and increased during disease progression, supporting a role for Wt1 in mature podocyte survival. Podocyte Notch activation was evident at disease onset with upregulation of Notch1 and its transcriptional targets, including Nrarp. There was repression of podocyte FoxC2 and upregulation of Hey2 supporting a role for a Wt1/FoxC2/Notch transcriptional network in mature podocyte injury. The expression of cleaved Notch1 and HES1 proteins in podocytes of mutant mice was confirmed in early disease. Furthermore, induction of podocyte HES1 expression was associated with upregulation of genes implicated in epithelial mesenchymal transition, thereby suggesting that HES1 mediates podocyte EMT. Lastly, early pharmacological inhibition of Notch signaling ameliorated glomerular scarring and albuminuria. Thus, loss of Wt1 in mature podocytes modulates podocyte Notch activation, which could mediate early events in WT1-related glomerulosclerosis.

Diffuse mesangial sclerosis in a PDSS2 mutation-induced coenzyme Q10 deficiency

BACKGROUND

A 7-month-old male infant was admitted because he was suffering from nephrotic syndrome, along with encephalomyopathy, hypertrophic cardiomyopathy, clinically suspected deafness and retinitis pigmentosa, and an elevated serum lactate level.

METHODS

Coenzyme Q₁₀ supplementation was started because of the clinical suspicion of primary CoQ₁₀ deficiency. Despite intensive efforts, he passed away 4 weeks after admission.

RESULTS

The results of genetic tests, available postmortem, explored two hitherto undescribed mutations in the PDSS2 gene. Both were located within the polyprenyl synthetase domain. Clinical exome sequencing revealed a heterozygous missense mutation in exon 3, and our in-house joint-analysis algorithm detected a heterozygous large 2923-bp deletion that affected the 5 prime end of exon 8. Other causative defects in the CoQ₁₀ and infantile nephrosis-related genes examined were not found. A postmortem histological, immunohistochemical, and electron microscopic evaluation of the glomeruli revealed collapsing-sclerosing lesions consistent with diffuse mesangial sclerosis. The extrarenal alterations included hypertrophic cardiomyopathy and diffuse alveolar damage. A histological evaluation of the central nervous system and skeletal muscles did not demonstrate any obvious abnormality.

CONCLUSIONS

Until now, the clinical features and the mutational status of 6 patients with a PDSS2 gene defect have been reported in the English literature. Here, we describe for the first time detailed kidney morphology features in a patient with nephrotic syndrome carrying mutations in the PDSS2 gene.

FAT1 mutations cause a glomerulotubular nephropathy

Steroid-resistant nephrotic syndrome (SRNS) causes 15% of chronic kidney disease (CKD). Here we show that recessive mutations in FAT1 cause a distinct renal disease entity in four families with a combination of SRNS, tubular ectasia, haematuria and facultative neurological involvement. Loss of FAT1 results in decreased cell adhesion and migration in fibroblasts and podocytes and the decreased migration is partially reversed by a RAC1/CDC42 activator. Podocyte-specific deletion of Fat1 in mice induces abnormal glomerular filtration barrier development, leading to podocyte foot process effacement. Knockdown of Fat1 in renal tubular cells reduces migration, decreases active RAC1 and CDC42, and induces defects in lumen formation. Knockdown of fat1 in zebrafish causes pronephric cysts, which is partially rescued by RAC1/CDC42 activators, confirming a role of the two small GTPases in the pathogenesis. These findings provide new insights into the pathogenesis of SRNS and tubulopathy, linking FAT1 and RAC1/CDC42 to podocyte and tubular cell function.

Understanding podocytopathy and its relevance to clinical nephrology

Podocytopathies are the most common group of glomerular disorder leading to proteinuria. On the basis of pathophysiology, light microscopic and ultrastructural evaluation, the podocytopathies include minimal change disease, diffuse mesangial sclerosis, focal segmental glomerulosclerosis and collapsing glomerulopathy. The present review summarizes the basic etiopathogenesis of podocytopthies, highlights the common genetic and acquired factors in its causation, puts forth various diagnostic modalities and discusses the role of emerging agents or treatment.

Pax8 and Pax2 are specifically required at different steps of Xenopus pronephros development

The respective role of Pax2 and Pax8 in early kidney development in vertebrates is poorly understood. In this report, we have studied the roles of Pax8 and Pax2 in Xenopus pronephros development using a loss-of-function approach. Our results highlight a differential requirement of these two transcription factors for proper pronephros formation. Pax8 is necessary for the earliest steps of pronephric development and its depletion leads to a complete absence of pronephric tubule. Pax2 is required after the establishment of the tubule pronephric anlage, for the expression of several terminal differentiation markers of the pronephric tubule. Neither Pax2 nor Pax8 is essential to glomus development. We further show that Pax8 controls hnf1b, but not lhx1 and Osr2, expression in the kidney field as soon as the mid-neurula stage. Pax8 is also required for cell proliferation of pronephric precursors in the kidney field. It may exert its action through the wnt/beta-catenin pathway since activation of this pathway can rescue MoPax8 induced proliferation defect and Pax8 regulates expression of the wnt pathway components, dvl1 and sfrp3. Finally, we observed that loss of pronephros in Pax8 morphants correlates with an expanded vascular/blood gene expression domain indicating that Pax8 function is important to delimit the blood/endothelial genes expression domain in the anterior part of the dorso-lateral plate.

Glomerular development--shaping the multi-cellular filtration unit

The glomerulus represents a highly structured filtration unit, composed of glomerular endothelial cells, mesangial cells, podocytes and parietal epithelial cells. During glomerulogenesis an intricate network of signaling pathways involving transcription factors, secreted factors and cell-cell communication is required to guarantee accurate evolvement of a functional, complex 3-dimensional glomerular architecture. Here, we want to provide an overview on the critical steps and relevant signaling cascades of glomerular development.

Glomerular epithelial cell phenotype in diffuse mesangial sclerosis: a report of 2 cases with markedly increased urinary podocyte excretion

We report 2 cases of diffuse mesangial sclerosis (DMS) accompanied by severe podocyte excretion in urine. Patient 1 was a 9-day-old girl with a WT1 mutation who developed Wilms tumor at 6 months of age and was subsequently diagnosed with Denys-Drash syndrome. Patient 2 was a 1-year-old boy without a WT1 abnormality but presenting with heavy proteinuria. In both patients, histological examination showed findings of DMS. Immunohistochemical staining for synaptopodin (a podocyte marker) revealed a reduced number of podocytes in the glomeruli with severe sclerosis; however, podocytes persisted in the relatively intact glomeruli. Some glomeruli were accompanied by sclerotic lesions surrounded by proliferating cells; immunofluorescence staining revealed a majority of these proliferating cells to be positive for claudin-1 (a parietal cell marker) but negative for synaptopodin. These findings suggest that podocyte loss and the consequent proliferation of parietal cells are common processes in the pathogenesis of DMS.

An unusual presentation of Denys-Drash syndrome due to bigenic disease

We report a case of Denys-Drash syndrome (DDS) in a 3-month-old girl presenting with bilateral renal cortical cysts mimicking polycystic kidney disease. Genetic analysis revealed a de novo heterozygous missense mutation c.1186G>A (p.Asp396Asn) in the WT1 gene, confirming the diagnosis of DDS. Because multiple renal cysts have never been reported in DDS, we explored several genes responsible for these renal manifestations, such as HNF-1β, PAX2, PKD1, and PKD2. Remarkably, we identified a heterozygous missense variant c.12439A>G (p.Lys4147Glu) in the PKD1 gene. The same variant was found in the patient's mother, who had no renal cysts, and in the grandfather, who had several renal cysts. Mutation prediction programs classified the c.12439A>G variant as being "likely pathogenic." We hypothesize that the severe cystic phenotype in the index patient could be due to the WT1 mutation, enhancing pathogenicity of the "hypomorph" PKD1 allele. A possible role for Wilms tumor suppressor 1 (WT1) in renal cyst development should be considered. From a conceptual point of view, this case shows that an unusual presentation of a known genetic syndrome might point to bigenic inheritance, with unexpected interference of mutated genes causing an uncommon clinical phenotype.

Insulin signalling to the kidney in health and disease

Ninety-one years ago insulin was discovered, which was one of the most important medical discoveries in the past century, transforming the lives of millions of diabetic patients. Initially insulin was considered only important for rapid control of blood glucose by its action on a restricted number of tissues; however, it has now become clear that this hormone controls an array of cellular processes in many different tissues. The present review will focus on the role of insulin in the kidney in health and disease.

PAX2 in human kidney malformations and disease

Human PAX2 mutations have been associated with abnormalities in the developing and adult kidney ranging from congenital abnormalities of the kidney and urinary tract (CAKUT) to oncogenic processes. Defining the relationship of PAX2 to human renal disease requires an appreciation of its fundamental role in renal development. Given the highly conserved nature of the PAX2 gene in vertebrates, it is not surprising that much of our understanding of PAX2 involvement in renal disease has been derived from animal models. The following review will outline the current evidence supporting involvement of PAX2 in the pathologic processes involving the kidney.

Progress in pathogenesis of proteinuria

Aims. Proteinuria not only is a sign of kidney damage, but also is involved in the progression of renal diseases as an independent pathologic factor. Clinically, glomerular proteinuria is most commonly observed, which relates to structural and functional anomalies in the glomerular filtration barrier. The aim of this paper was to describe the pathogenesis of glomerular proteinuria. Data Sources. Articles on glomerular proteinuria retrieved from Pubmed and MEDLINE in the recent 5 years were reviewed. Results. The new understanding of the roles of glomerular endothelial cells and the glomerular basement membrane (GBM) in the pathogenesis of glomerular proteinuria was gained. The close relationships of slit diaphragm (SD) molecules such as nephrin, podocin, CD2-associated protein (CD2AP), a-actinin-4, transient receptor potential cation channel 6 (TRPC6), Densin and membrane-associated guanylate kinase inverted 1 (MAGI-1), α3β1 integrin, WT1, phospholipase C epsilon-1 (PLCE1), Lmx1b, and MYH9, and mitochondrial disorders and circulating factors in the pathogenesis of glomerular proteinuria were also gradually discovered. Conclusion. Renal proteinuria is a manifestation of glomerular filtration barrier dysfunction. Not only glomerular endothelial cells and GBM, but also the glomerular podocytes and their SDs play an important role in the pathogenesis of glomerular proteinuria.

Podocyte biology in segmental sclerosis and progressive glomerular injury

During the past 2 decades, progress has been made in understanding the biology and mechanisms of podocyte injury and the relationship of these processes to glomerulosclerosis. In addition, studies of human biopsies and animal models have provided insights into the mechanisms of glomerular disease progression and repair. These new developments are critical for establishing better therapeutic guidelines that target specific pathways, which otherwise would lead to irreversible injury.

De novo expression of podocyte proteins in parietal epithelial cells in experimental aging nephropathy

A progressive decrease in podocyte number underlies the development of glomerulosclerosis and reduced kidney function in aging nephropathy. Recent data suggest that under certain disease states, parietal epithelial cells (PECs) begin to express proteins considered specific to podocytes. To determine whether this phenomenon increases in aging kidneys, 4-, 12-, and 20-mo ad libitum-fed and 20-mo calorie-restricted (CR) rats were studied. Single and double immunostaining were performed with antibodies to the PEC protein paired box gene 2 (PAX2) and tight junction protein claudin-1, the podocyte-specific protein Wilms' tumor 1 (WT-1), and the proliferating cell protein (Ki-67). ImageJ software measured Bowman's basement membrane (BBM) length and glomerular tuft area in individual glomeruli from each animal to assess glomerular size. The results showed that in aged ad libitum rats, the decrease in number of podocytes/glomerular tuft area was accompanied by an increase in the number of PECs/BBM length at 12 and 20 mo (P < 0.01 vs. 4 mo). The increase in PEC number was due to proliferation (increase in PAX2/Ki-67 double-positive cells). Aging was accompanied by a progressive increase in the number of glomerular cells double staining for PAX2 and WT-1. In contrast, the control 20-mo-old CR rats had no increase in glomerular size, and podocyte and PEC number were not altered. These results suggest that although the number of PECs and PECs expressing podocyte proteins increase in aging nephropathy, they are likely not sufficient to compensate for the decrease in podocyte number.

Molecular and genetic basis of inherited nephrotic syndrome

Nephrotic syndrome is an heterogeneous disease characterized by increased permeability of the glomerular filtration barrier for macromolecules. Podocytes, the visceral epithelial cells of glomerulus, play critical role in ultrafiltration of plasma and are involved in a wide number of inherited and acquired glomerular diseases. The identification of mutations in nephrin and other podocyte genes as causes of genetic forms of nephrotic syndrome has revealed new important aspects of the pathogenesis of proteinuric kidney diseases and expanded our knowledge of the glomerular biology. Moreover, a novel concept of a highly dynamic slit diaphragm proteins is emerging. The most significant discoveries in our understanding of the structure and function of the glomerular filtration barrier are reviewed in this paper.

Glomerular proteinuria: a complex interplay between unique players

Protein leak in the urine is a harbinger of disruption of the glomerular filtration barrier. It also correlates with disease progression and development of ESRD. At present, therapies are aimed at decreasing proteinuria to decrease further damage to the filter and as a marker of remission. Understanding the mechanism of molecular events that lead to protein leak is vital to developing new therapeutic interventions. There has been tremendous progress over the last decade in identifying gene defects which result in hereditary proteinuric defects. This has led to identifying pathways by which these genes regulate the structure and function of the components of the filtration barrier, namely the podocytes, mesangial cells, endothelial cells, and the basement membrane. Using gene knockout mouse models, a role of tubular cells in regulating proteinuria is also emerging. In this review, we have attempted to present some of the prevailing understanding of the underlying mechanisms and physiology of proteinuria.

PAX2 mutations in fetal renal hypodysplasia

Papillorenal syndrome also known as renal-coloboma syndrome (OMIM 120330) is an autosomal dominant condition comprising optic nerve anomaly and renal oligomeganephronic hypoplasia. This reduced number of nephron generations with compensatory glomerular hypertrophy leads towards chronic insufficiency with renal failure. We report on two fetuses with PAX2 mutations presenting at 24 and 18 weeks' gestation, respectively, born into two different sibships. In our first patient, termination of pregnancy was elected for anhydramnios and suspicion of renal agenesis in the healthy couple with an unremarkable previous clinical history. This fetus had bilateral asymmetric kidney anomalies including a small multicystic left kidney, and an extremely hypoplastic right kidney. Histology showed dysplastic lesions in the left kidney, contrasting with rather normal organization in the hypoplastic right kidney. Ocular examination disclosed bilateral optic nerve coloboma. The association of these anomalies, highly suggestive of the papillorenal syndrome, led us to perform the molecular study of the PAX2 gene. Direct sequencing of the PAX2 coding sequence identified a de novo single G deletion of nucleotide 935 in exon 3 of the PAX2 resulting in a frameshift mutation (c.392delG, p.Ser131Thrfs*28). In the second family, the presence of a maternally inherited PAX2 mutation led to a decision for termination of pregnancy. The 18-week gestation fetus presented the papillorenal syndrome including hypoplastic kidneys and optic nerve coloboma. In order to address the PAX2 involvement in isolated renal "disease," 18 fetuses fulfilling criteria were screened: 10/18 had uni- or bilateral agenesis, 6/18 had bilateral multicystic dysplasia with enlarged kidneys, and 2/18 presented bilateral severe hypodysplasia confirmed on fetopathological examination. To the best of our knowledge, our first patient represents an unreported fetal diagnosis of papillorenal syndrome, and another example of the impact of oriented fetopathological examination in genetic counseling of the parents.

The PAX258 gene subfamily: a comparative perspective

Whole genome duplication events are thought to have substantially contributed to organismal complexity, largely via divergent transcriptional regulation. Members of the vertebrate PAX2, PAX5 and PAX8 gene subfamily derived from an ancient class of paired box genes and arose from such whole genome duplication events. These genes are critical in establishing the midbrain-hindbrain boundary, specifying interneuron populations and for eye, ear and kidney development. Also PAX2 has adopted a unique role in pancreas development, whilst PAX5 is essential for early B-cell differentiation. The contribution of PAX258 genes to their collective role has diverged across paralogues and the animal lineages, resulting in a complex wealth of literature. It is now timely to provide a comprehensive comparative overview of these genes and their ancient and divergent roles. We also discuss their fundamental place within gene regulatory networks and the likely influence of cis-regulatory elements over their differential roles during early animal development.

A novel WT1 gene mutation in a three-generation family with progressive isolated focal segmental glomerulosclerosis

BACKGROUND AND OBJECTIVES

Wilms tumor-suppressor gene-1 (WT1) plays a key role in kidney development and function. WT1 mutations usually occur in exons 8 and 9 and are associated with Denys-Drash, or in intron 9 and are associated with Frasier syndrome. However, overlapping clinical and molecular features have been reported. Few familial cases have been described, with intrafamilial variability. Sporadic cases of WT1 mutations in isolated diffuse mesangial sclerosis or focal segmental glomerulosclerosis have also been reported.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Molecular analysis of WT1 exons 8 and 9 was carried out in five members on three generations of a family with late-onset isolated proteinuria. The effect of the detected amino acid substitution on WT1 protein's structure was studied by bioinformatics tools.

RESULTS

Three family members reached end-stage renal disease in full adulthood. None had genital abnormalities or Wilms tumor. Histologic analysis in two subjects revealed focal segmental glomerulosclerosis. The novel sequence variant c.1208G>A in WT1 exon 9 was identified in all of the affected members of the family.

CONCLUSIONS

The lack of Wilms tumor or other related phenotypes suggests the expansion of WT1 gene analysis in patients with focal segmental glomerulosclerosis, regardless of age or presence of typical Denys-Drash or Frasier syndrome clinical features. Structural analysis of the mutated protein revealed that the mutation hampers zinc finger-DNA interactions, impairing target gene transcription. This finding opens up new issues about WT1 function in the maintenance of the complex gene network that regulates normal podocyte function.

Evaluation of podocyte lesion in patients with diabetic nephropathy: Wilms' tumor-1 protein used as a podocyte marker

INTRODUCTION

The reduction of podocyte number and density per glomerulus has been linked to the development of proteinuria and the progression of disease in patients with diabetic nephropathy (DN). However, it has been recognized that measurement of podocyte number by light microscope is quite difficult because of the complexity of both podocyte and glomerular structure, which is not suitable for clinical research. In our research institute, we used WT1 as podocyte marker to evaluate the podocyte lesion.

METHODS

In our experiment, we selected the C-terminal antibody of WT1 to stain the nuclei and the N-terminal antibody of WT1 to stain the cytoplasma of podocytes. Forty patients were enrolled with type 2 diabetes and proven to have DN by renal biopsy analysis. DN patients were classified into three groups based on the degree of proteinuria: microalbuminuria (n=10, 30-300mg/24h), overt proteinuria (n=15, 0.5-3.5g/24h), and heavy proteinuria (n=15, >3.5g/24h).

RESULTS

The results demonstrated that the podocyte number was markedly decreased in patients with DN (30-51% reduction). There was a significant negative correlation between the proteinuria and both podocyte density and number. The cover area density of podocyte cytoplasma in glomerulus was also significantly decreased in all DN patients (39-80% reduction). A significant inverse correlation was observed between the cover area density and the degree of proteinuria. The correlation coefficient (r=-0.85) was much higher than that between proteinuria and podocyte density (r=-0.56) or podocyte number (r=-0.36).

CONCLUSION

In conclusion, podocyte damage occurred in patients with DN, even in the early stage and became more dramatic during the course of proteinuria progression. WT1 staining, using the polyclonal antibody to stain the nuclei and monoclonal antibody to stain the cytoplasma of podocytes together, is a valuable alternative technique in the study of podocyte injury.

Impaired transcription factor interplay in addition to advanced glycation end products suppress podocalyxin expression in high glucose-treated human podocytes

Podocalyxin represents a Wilms’ tumor suppressor protein (WT1)-regulated differentiation marker for glomerular epithelium. We provide evidence concerning mechanisms involved in the regulation of podocalyxin expression following long-term exposure to increased (25 mM) glucose levels. Prolonged culture of conditionally immortalized human podocytes in 25 mM glucose induced suppression of podocalyxin expression both at the protein and mRNA levels, whereas WT1 protein levels remained unaltered. WT1 interacted with another transcription factor, CRE-binding protein (CBP). This association was decreased by 40% in the presence of 25 mM glucose. Chromatin immunoprecipitation assays on chromatin from podocytes cultured in 25 mM glucose revealed reduced WT1 binding to podocalyxin promoter sequences, probably resulting from impaired WT1-CBP interactions. We explored the possible role of glucose-induced adducts (advanced glycation end products; AGEs) in impairing interactions between WT1 and CBP, with the use of aminoguanindine, an inhibitor of AGE formation. Podocytes were cultured in the simultaneous presence of 20 mM aminoguanidine and 25 mM glucose, and podocalyxin protein levels were examined. Aminoguanidine effectively prevented downregulation of podocalyxin protein levels but could not restore podocalyxin levels once expression was suppressed. Thus increased glucose apparently impaired the ability of WT1 to initiate transcription in part by decreased association of WT1 with CBP. Administration of aminoguanidine concomitant with increasing glucose levels in our in vitro model system protected from glucose-induced “silencing” of the podocalyxin gene, suggesting that AGEs play an important role in suppressing its expression in diabetic conditions.

Nephrin deficiency activates NF-kappaB and promotes glomerular injury

Increasing evidence implicates activation of NF-kappaB in a variety of glomerular diseases, but the mechanisms involved are unknown. Here, upregulation of NF-kappaB in the podocytes of transgenic mice resulted in glomerulosclerosis and proteinuria. Absence of the podocyte protein nephrin resulted in NF-kappaB activation, suggesting that nephrin negatively regulates the NF-kappaB pathway. Signal transduction assays supported a functional relationship between nephrin and NF-kappaB and suggested the involvement of atypical protein kinase C (aPKCzeta/lambda/iota) as an intermediary. We propose that disruption of the slit diaphragm leads to activation of NF-kappaB; subsequent upregulation of NF-kappaB-driven genes results in glomerular damage mediated by NF-kappaB-dependent pathways. In summary, nephrin may normally limit NF-kappaB activity in the podocyte, suggesting a mechanism by which it might discourage the evolution of glomerular disease.

Advances in the biology and genetics of the podocytopathies: implications for diagnosis and therapy

CONTEXT

Etiologic factors and pathways leading to altered podocyte phenotype are clearly numerous and involve the activity of different cellular function.

OBJECTIVE

To focus on recent discoveries in podocyte biology and genetics and their relevance to these human glomerular diseases, named podocytopathies.

DATA SOURCES

Genetic mutations in genes encoding for proteins in the nucleus, slit diaphragm, podocyte cytoplasm, and cell membrane are responsible for podocyte phenotype and functional abnormalities. Podocyte injury may also derive from secondary stimuli, such as mechanical stress, infections, or use of certain medications. Podocytes can respond to injury in a limited number of ways, which include (1) effacement, (2) apoptosis, (3) arrest of development, and (4) dedifferentiation. Each of these pathways results in a specific glomerular morphology: minimal change nephropathy, focal segmental glomerulosclerosis, diffuse mesangial sclerosis, and collapsing glomerulopathy.

CONCLUSIONS

Based on current knowledge of podocyte biology, we organized etiologic factors and morphologic features in a taxonomy of podocytopathies, which provides a novel approach to the classification of these diseases. Current and experimental therapeutic approaches are also discussed.

Advances in the biology and genetics of the podocytopathies: implications for diagnosis and therapy

CONTEXT

Etiologic factors and pathways leading to altered podocyte phenotype are clearly numerous and involve the activity of different cellular function.

OBJECTIVE

To focus on recent discoveries in podocyte biology and genetics and their relevance to these human glomerular diseases, named podocytopathies.

DATA SOURCES

Genetic mutations in genes encoding for proteins in the nucleus, slit diaphragm, podocyte cytoplasm, and cell membrane are responsible for podocyte phenotype and functional abnormalities. Podocyte injury may also derive from secondary stimuli, such as mechanical stress, infections, or use of certain medications. Podocytes can respond to injury in a limited number of ways, which include (1) effacement, (2) apoptosis, (3) arrest of development, and (4) dedifferentiation. Each of these pathways results in a specific glomerular morphology: minimal change nephropathy, focal segmental glomerulosclerosis, diffuse mesangial sclerosis, and collapsing glomerulopathy.

CONCLUSIONS

Based on current knowledge of podocyte biology, we organized etiologic factors and morphologic features in a taxonomy of podocytopathies, which provides a novel approach to the classification of these diseases. Current and experimental therapeutic approaches are also discussed.

Molecular pathology of nephrotic syndrome in childhood: a contemporary approach to diagnosis

Molecular and genetic studies in the last 2 decades have shed new light on the understanding of congenital and infantile nephrotic syndrome (NS). Glomerular pathology may appear as minimal change disease, focal segmental glomerulosclerosis, or diffuse mesangial sclerosis, glomerular diseases now recognized as podocyte injuries and in part caused by altered podocyte genes. Even though genetic mutations are not implicated in all infants with NS, the study of familial disease and congenital NS reveals that proteinuria is in many patients due to specific gene mutations. The most common mutations are in 4 genes, 3 of which are podocyte genes: NPHS1 (Finnish nephropathy), NPHS2 (podocin-induced focal segmental glomerulosclerosis), WT1 (diffuse mesangial sclerosis), and LAMB2 (Pierson syndrome). Furthermore, these studies have improved our understanding of steroid-resistant NS in older children, particularly girls, in whom proteinuria may be due to WT1 mutations. Availability of molecular genetic testing and antibodies to specific gene products are closing the gap between histopathology of pediatric glomerular disease and molecular genetic diagnosis. Recognition of NS variants, which may be reversible (eg, mitochondrial mutations, viral disease), is important. This review discusses the most common entities and the differential diagnosis of pediatric NS from the pathologist's point of view, with an emphasis on congenital (<3 months) and infantile (3 months to 1 year) NS in light of molecular and genetic studies.

New insights into the function of the Wilms tumor suppressor gene WT1 in podocytes

The Wilms tumor suppressor gene WT1 is essential for early urogenital development: homozygous mutations in WT1 result in embryonic lethality due to a failure in the development of kidneys and gonads. In the adult kidney, WT1 expression is limited to the glomerular podocytes. Several human nephrotic diseases arise from mutations of the WT1 gene, including mutations that affect its zinc-fingers and alternative splicing of +/−KTS isoforms. These include WAGR (for Wilms tumor, aniridia, genitourinary anomalies, and mental retardation), and Frasier and Denys-Drash syndromes. Recent advances including the development of transgenic mouse models and conditionally immortalized podocyte cell lines are beginning to shed light on WT1's crucial role in podocyte function.

Ectopic notch activation in developing podocytes causes glomerulosclerosis

Genetic evidence supports an early role for Notch signaling in the fate of podocytes during glomerular development. Decreased expression of Notch transcriptional targets in developing podocytes after the determination of cell fate suggests that constitutive Notch signaling may oppose podocyte differentiation. This study determined the effects of constitutive Notch signaling on podocyte differentiation by ectopically expressing Notch's intracellular domain (NOTCH-IC), the biologically active, intracellular product of proteolytic cleavage of the Notch receptor, in developing podocytes of transgenic mice. Histologic and molecular analyses revealed normal glomerular morphology and expression of podocyte markers in newborn NOTCH-IC-expressing mice; however, mice developed severe proteinuria and showed evidence of progressive glomerulosclerosis at 2 wk after birth. Features of mature podocytes were lost: Foot processes were effaced; expression of Wt1, Nphs1, and Nphs2 was downregulated; cell-cycle re-entry was induced; and the expression of Pax2 was increased. In contrast, mice with podocyte-specific inactivation of Rbpsuh, which encodes a protein essential for canonical Notch signaling, seemed normal. In addition, the damaging effects of NOTCH-IC expression were prevented in transgenic mice after simultaneous conditional inactivation of Rbpsuh in murine podocytes. These results suggest that Notch signaling is dispensable during terminal differentiation of podocytes but that constitutive (or inappropriate) Notch signaling is deleterious, leading to glomerulosclerosis.

Culture methods of glomerular podocytes

Podocytes (glomerular visceral epithelial cells) cover the exterior surface of the glomerular capillaries and contribute to the glomerular filtration membrane. Failure of podocyte function is involved in the progression of chronic glomerular disease; accordingly, research interest into podocyte biology is driven by the need for better protection and perhaps recovery of these cells in renal diseases. This review aims at summarizing available techniques for podocyte cell cultures from both the past and present, with special attention to the currently used methods. The establishment of classical primary cultures is based on isolation of glomeruli by differential sieving. Plating of glomeruli onto a collagen surface is followed by an outgrowth of cobblestone-like cells that, after replating, differentiate into arborized, mature podocytes. Currently, the majority of research studies use immortalized podocytic cell lines most often derived from transgenic mice bearing a conditional immortalizing gene. The podocytes can also be collected and cultured from healthy or diseased animal or patient urine. The urinary podocytes obtained from subjects with active glomerulopathies display higher proliferation potential and viability in vitro, perhaps due to disease-induced transdifferentiation. Finally, a list of phenotypic markers useful for identification and characterization of the cultured podocytic elements is provided.

Ontogenetic development of the filtration barrier

The development of the filtration barrier is part of a complex sequence of steps proceeding from the early nephron anlage (renal vesicle) via the comma- and S-shaped body to the capillary loop stage and mature glomerulus. The main players are the podocytes (already in the stage of presumptive podocytes), which hold the commander function in this process, and the endothelial and the mesangial cells. A decisive role is also played by the GBM; its change in composition during the developmental process is a precondition for the final maturation of the podocytes, i.e. for the formation of the foot processes and, clearly subsequent, the slit membrane. Failure in the consecutive developmental stages due to genetic mutations or manipulations leads to characteristic hereditary diseases of increasing severity. The last step in this development, the formation of the slit membrane, marks a caesura between diseases with early and late onset; all disorders without a properly developed slit membrane start prenatally or at birth.

A proposed taxonomy for the podocytopathies: a reassessment of the primary nephrotic diseases

A spectrum of proteinuric glomerular diseases results from podocyte abnormalities. The understanding of these podocytopathies has greatly expanded in recent years, particularly with the discovery of more than a dozen genetic mutations that are associated with loss of podocyte functional integrity. It is apparent that classification of the podocytopathies on the basis of morphology alone is inadequate to capture fully the complexity of these disorders. Herein is proposed a taxonomy for the podocytopathies that classifies along two dimensions: Histopathology, including podocyte phenotype and glomerular morphology (minimal-change nephropathy, focal segmental glomerulosclerosis, diffuse mesangial sclerosis, and collapsing glomerulopathy), and etiology (idiopathic, genetic, and reactive forms). A more complete understanding of the similarities and differences among podocyte diseases will help the renal pathologist and the nephrologist communicate more effectively about the diagnosis; this in turn will help the nephrologist provide more accurate prognostic information and select the optimal therapy for these often problematic diseases. It is proposed that final diagnosis of the podocytopathies should result from close collaboration between renal pathologists and nephrologists and should whenever possible include three elements: Morphologic entity, etiologic form, and specific pathogenic mechanism or association.

The spectrum of podocytopathies: a unifying view of glomerular diseases

Glomerular diseases encompass a broad array of clinicopathologically defined syndromes which together account for 90% of end-stage kidney disease costing $20 billion per annum to treat in the United States alone. Recent insights have defined the central role of the podocyte as both the regulator of glomerular development as well as the determinant of progression to glomerulosclerosis. We can now place all glomerular diseases within this spectrum of podocytopathies with predictable outcomes based on podocyte biology impacted by temporal, genetic, and environmental cues. This simplified construct is particularly useful to rationalize clinical effort toward podocyte preservation and prevention of progression as well as to focus basic research effort on understanding podocyte biology and for clinical research toward development of practical monitoring strategies for podocyte injury, dysfunction, and loss.

Positional cloning uncovers mutations in PLCE1 responsible for a nephrotic syndrome variant that may be reversible

Nephrotic syndrome, a malfunction of the kidney glomerular filter, leads to proteinuria, edema and, in steroid-resistant nephrotic syndrome, end-stage kidney disease. Using positional cloning, we identified mutations in the phospholipase C epsilon gene (PLCE1) as causing early-onset nephrotic syndrome with end-stage kidney disease. Kidney histology of affected individuals showed diffuse mesangial sclerosis (DMS). Using immunofluorescence, we found PLCepsilon1 expression in developing and mature glomerular podocytes and showed that DMS represents an arrest of normal glomerular development. We identified IQ motif-containing GTPase-activating protein 1 as a new interaction partner of PLCepsilon1. Two siblings with a missense mutation in an exon encoding the PLCepsilon1 catalytic domain showed histology characteristic of focal segmental glomerulosclerosis. Notably, two other affected individuals responded to therapy, making this the first report of a molecular cause of nephrotic syndrome that may resolve after therapy. These findings, together with the zebrafish model of human nephrotic syndrome generated by plce1 knockdown, open new inroads into pathophysiology and treatment mechanisms of nephrotic syndrome.

WT1 and glomerular diseases

The WT1 gene encodes a zinc finger transcription factor involved in kidney and gonadal development and, when mutated, in the occurrence of kidney tumor and glomerular diseases. Patients with Denys-Drash syndrome present with early nephrotic syndrome with diffuse mesangial sclerosis progressing rapidly to end-stage renal failure, male pseudohermaphroditism, and Wilms' tumor. Incomplete forms of the syndrome have been described. Germline WT1 missense mutations located in exons 8 or 9 coding for zinc fingers 2 or 3 have been detected in nearly all patients with Denys-Drash syndrome and in some patients with isolated diffuse mesangial sclerosis. Patients with Frasier syndrome present with normal female external genitalia, streak gonads, XY karyotype and progressive nephropathy with proteinuria and nephrotic syndrome with focal and segmental glomerular sclerosis progressing to end-stage renal disease in adolescence or young adulthood. They frequently develop gonadoblastoma. Germline intronic mutations leading to the loss of the +KTS isoforms have been observed in all patients with Frasier syndrome. The same mutations have been observed in genetically female patients with isolated FSGS. Transmission of the mutation is possible. Frasier mutations have also been reported in children with Denys-Drash syndrome.

ZHX proteins regulate podocyte gene expression during the development of nephrotic syndrome

Transcriptional regulation of podocyte gene expression in primary glomerular disease is poorly understood. In this study, we demonstrate a prominent role of members of the ZHX (zinc fingers and homeoboxes) family of proteins in regulating podocyte gene expression during the development of nephrotic syndrome. While studying mechanisms of glomerular disease, rat ZHX3 was cloned from a down-regulated gene fragment; its cellular localization, DNA binding, and transcriptional repressor properties were characterized; and its ability to influence podocyte gene expression directly or via ZHX1 and ZHX2 was studied. In eukaryotic promoters, ZHX3 bound to the CdxA binding motif. ZHX proteins were mostly sequestered in the non-nuclear compartment in the normal in vivo podocyte by virtue of heterodimer formation, and loss of heterodimerization was associated with entry into the nucleus. In experimental minimal change disease, ZHX3 was transiently down-regulated prior to the onset of proteinuria, and recovery of expression was associated with migration of ZHX3 protein into the nucleus and the development of proteinuria. This expression pattern mirrored the increased nuclear ZHX3 expression noted in vivo in the podocytes in human minimal change disease biopsies. In vitro, migration of ZHX3 protein into the nucleus during recovery from transient ZHX3 knockdown reproduced the gene expression profile of in vivo minimal change disease. Severe sustained knockdown of ZHX3 caused down-regulation of genes involved in focal sclerosis, including WT1, mediated mostly by increased nuclear entry of ZHX2 and ZHX1. In summary, ZHX proteins are major transcriptional mediators of podocyte disease.

Mechanisms of disease: focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS), a subtype of "idiopathic nephrotic syndrome", is not a single disease, but a lesion that initially affects the glomerulus followed by the tubulointerstitium and renal vessels. The term 'FSGS' does not accurately encompass the various pathologic features of the glomerulus, which are not always focal, segmental or sclerotic. Particular variants of FSGS, such as collapsing glomerulopathy and the glomerular tip lesion, exemplify the nosologic uncertainty inherent in the classification of glomerular lesions. Pathologic variation notwithstanding, all pathologic processes that affect the podocyte lead to one of the histologic subtypes of FSGS. This specialized cell type has essential roles in maintaining the integrity of glomerular architecture, resisting endocapillary hydraulic pressure and hindering egress of proteins into the urinary space. Once initiated, podocyte lesions and ensuing fibrosis are usually irreversible, at least in human forms of FSGS. Remarkable progress has been made in unraveling the mechanisms of podocyte dysregulation that accompany the cellular variants of FSGS and in identifying genetic mutations affecting proteins of the slit diaphragm. Hopefully, this progress will drastically improve treatments for what is one of the most difficult therapeutic challenges to confront the nephrologist.

Parietal podocytes in normal human glomeruli

Although parietal podocytes along the Bowman's capsule have been described by electron microscopy in the normal human kidney, their molecular composition remains unknown. Ten human normal kidneys that were removed for cancer were assessed for the presence and the extent of parietal podocytes along the Bowman's capsule. The expression of podocyte-specific proteins (podocalyxin, glomerular epithelial protein-1, podocin, nephrin, synaptopodin, and alpha-actinin-4), podocyte synthesized proteins (vascular endothelial growth factor and novH), transcription factors (WT1 and PAX2), cyclin-dependent kinase inhibitor p57, and intermediate filaments (cytokeratins and vimentin) was tested. In addition, six normal fetal kidneys were studied to track the ontogeny of parietal podocytes. The podocyte protein labeling detected parietal podocytes in all of the kidneys, was found in 76.6% on average of Bowman's capsule sections, and was prominent at the vascular pole. WT1 and p57 were expressed in some parietal cells, whereas PAX2 was present in all or most of them, so some parietal cells coexpressed WT1 and PAX2. Furthermore, parietal podocytes coexpressed WT1 and podocyte proteins. Cytokeratin-positive cells covered a variable part of the capsule and did not express podocyte proteins. Tuft-capsular podocyte bridges were present in 15.5 +/- 3.7% of the glomerular sections. Parietal podocytes often covered the juxtaglomerular arterioles and were present within the extraglomerular mesangium. Parietal podocytes were present in fetal kidneys. Parietal podocytes that express the same epitopes as visceral podocytes do exist along Bowman's capsule in the normal adult kidney. They are a constitutive cell type of the Bowman's capsule. Therefore, their role in physiology and pathology should be investigated.

Bone marrow transplantation can attenuate the progression of mesangial sclerosis

Bone marrow (BM) transplantation has been shown to provide beneficial effects in injured organs, including heart, liver, and kidney. We explored the therapeutic potential of BM transplantation (BMT) in Wilms' tumor suppressor 1 (Wt1) heterozygous mice, which represent a model of mesangial sclerosis. After transplantation of wild-type BM, there is statistically significantly lower urinary albumin and increased survival in Wt1+/- recipients. Control BMT using Wt1+/- donors showed no significant beneficial effects. The long-term beneficial effect of BMT was dependent on the dose of irradiation applied to the recipients before BMT. At a lethal dose of 1,000 cGy, the decrease in albuminuria and prolongation of lifespan in Wt1+/- mice were transient, with maximal amelioration at 12 weeks and resumption of albuminuria by 24 weeks after BMT. This was, at least in part, due to irradiation and not Wt1 heterozygosity because wild-type recipients also developed albuminuria within 24 weeks of BMT with 1,000 cGy. In contrast, Wt1+/- mice transplanted after 400 cGy showed long-term improvement in albuminuria and lifespan. Approximately 0.4% of podocytes were marrow derived, a level that is unlikely to be responsible for the therapeutic effects. In addition, donor BM cells formed rings surrounding the glomeruli, and approximately one third of the cells in these rings were macrophages. In conclusion, transplantation of wild-type BM attenuates progression of mesangial sclerosis in the Wt1+/- model of renal disease, and the mechanism by which this occurs may involve engraftment of BM-derived cells in the renal parenchyma.

Renal tubulointerstitial changes after internal irradiation with alpha-particle-emitting actinium daughters

The effect of external gamma irradiation on the kidneys is well described. However, the mechanisms of radiation nephropathy as a consequence of targeted radionuclide therapies are poorly understood. The functional and morphologic changes were studied chronologically (from 10 to 40 wk) in mouse kidneys after injection with an actinium-225 (225Ac) nanogenerator, a molecular-sized, antibody-targeted, in vivo generator of alpha-particle-emitting elements. Renal irradiation from free, radioactive daughters of 225Ac led to time-dependent reduction in renal function manifesting as increase in blood urea nitrogen. The histopathologic changes corresponded with the decline in renal function. Glomerular, tubular, and endothelial cell nuclear pleomorphism and focal tubular cell injury, lysis, and karyorrhexis were observed as early as 10 wk. Progressive thinning of the cortex as a result of widespread tubulolysis, collapsed tubules, glomerular crowding, decrease in glomerular cellularity, interstitial inflammation, and an elevated juxtaglomerular cell count were noted at 20 to 30 wk after treatment. By 35 to 40 wk, regeneration of simplified tubules with tubular atrophy and loss with focal, mild interstitial fibrosis had occurred. A lower juxtaglomerular cell count with focal cytoplasmic vacuolization, suggesting increased degranulation, was also observed in this period. A focal increase in tubular and interstitial cell TGF-beta1 expression starting at 20 wk, peaking at 25 wk, and later declining in intensity with mild increase in the extracellular matrix deposition was noticed. These findings suggest that internally delivered alpha-particle irradiation-induced loss of tubular epithelial cells triggers a chain of adaptive changes that result in progressive renal parenchymal damage accompanied by a loss of renal function. These findings are dissimilar to those seen after gamma or beta irradiation of kidneys.

Pathways to nephron loss starting from glomerular diseases-insights from animal models

Studies of glomerular diseases in animal models show that progression toward nephron loss starts with extracapillary lesions, whereby podocytes play the central role. If injuries remain bound within the endocapillary compartment, they will undergo recovery or be repaired by scaring. Degenerative, inflammatory and dysregulative mechanisms leading to nephron loss are distinguished. In addition to several other unique features, the dysregulative mechanisms leading to collapsing glomerulopathy are particular in that glomeruli and tubules are affected in parallel. In contrast, in degenerative and inflammatory diseases, tubular injury is secondary to glomerular lesions. In both of the latter groups of diseases, the progression starts in the glomerulus with the loss of the separation between the tuft and Bowman's capsule by forming cell bridges (parietal cells and/or podocytes) between the glomerular and the parietal basement membranes. Cell bridges develop into tuft adhesions to Bowman's capsule, which initiate the formation of crescents, either by misdirected filtration (proteinaceous crescents) or by epithelial cell proliferation (cellular crescents). Crescents may spread over the entire circumference of the glomerulus and, via the glomerulotubular junction, may extend onto the tubule. Two mechanisms concerning the transfer of a glomerular injury onto the tubulointerstitium are discussed: (1) direct encroachment of extracapillary lesions and (2) protein leakage into tubular urine, resulting in injury to the tubule and the interstitium. There is evidence that direct encroachment is the crucial mechanism. Progression of chronic renal disease is underlain by a vicious cycle which passes on the damage from lost and/or damaged nephrons to so far healthy nephrons. Presently, two mechanisms are discussed: (1) the loss of nephrons leads to compensatory mechanisms in the remaining nephrons (glomerular hypertension, hyperfiltration, hypertrophy) which increase their vulnerability to any further challenge (overload hypothesis); and (2) a proteinuric glomerular disease leads, by some way or another, to tubulointerstitial inflammation and fibrosis, accounting for the further deterioration of renal function (fibrosis hypothesis). So far, no convincing evidence has been published that in primary glomerular diseases fibrosis is harmful to healthy nephrons. The potential of glomerular injuries to regenerate or to be repaired by scaring is limited. The only option for extracapillary injuries with tuft adhesion is repair by formation of a segmental adherent scar (i.e., segmental glomerulosclerosis).

[Molecular mechanism of edema formation in nephrotic syndrome]

Nephrotic edema are the clinical feature of isolated interstitial expansion. Expanded interstitial compartment compensates sodium accumulation in the extracellular volume due to inappropriate renal sodium retention. Renal sodium retention is brought about by an activation of the molecular structures responsible for the reabsorption of sodium along the cortical collecting duct: amiloride-sensitive epithelial sodium channel at the apical face and sodium pump at the basolateral face of the principal cell. This activation is independent of aldosterone and vasopressin. The asymmetry of expansion between interstitium and plasma compartments is due to impaired Starling forces and increased fluid transfer through the capillary wall. The lack of significant changes in transcapillary oncotic and hydrostatic gradients suggests that increased hydraulic conductivity due to transconformation of endothelial intercellular junctions drives the leakage of fluid into the interstitium and allows to understand the mobility of nephrotic edema. Consistently with the site of renal sodium retention and the activation of the epithelial sodium channel, the association of amiloride and furosemide is efficient to increase urinary sodium excretion, to reverse sodium balance and to remove edema from patients with nephrotic syndrome.