Characterization of plasma factors that alter the permeability to albumin within isolated glomeruli

State of the art in childhood nephrotic syndrome: concrete discoveries and unmet needs

Nephrotic syndrome (NS) is a clinical entity characterized by proteinuria, hypoalbuminemia, and peripheral edema. NS affects about 2-7 per 100,000 children aged below 18 years old yearly and is classified, based on the response to drugs, into steroid sensitive (SSNS), steroid dependent, (SDNS), multidrug dependent (MDNS), and multidrug resistant (MRNS). Forms of NS that are more difficult to treat are associated with a worse outcome with respect to renal function. In particular, MRNS commonly progresses to end stage renal failure requiring renal transplantation, with recurrence of the original disease in half of the cases. Histological presentations of NS may vary from minimal glomerular lesions (MCD) to focal segmental glomerulosclerosis (FSGS) and, of relevance, the histological patterns do not correlate with the response to treatments. Moreover, around half of MRNS cases are secondary to causative pathogenic variants in genes involved in maintaining the glomerular structure. The pathogenesis of NS is still poorly understood and therapeutic approaches are mostly based on clinical experience. Understanding of pathogenetic mechanisms of NS is one of the 'unmet needs' in nephrology and represents a significant challenge for the scientific community. The scope of the present review includes exploring relevant findings, identifying unmet needs, and reviewing therapeutic developments that characterize NS in the last decades. The main aim is to provide a basis for new perspectives and mechanistic studies in NS.

Mannan-binding lectin serine protease-2 (MASP-2) in human kidney and its relevance for proteolytic activation of the epithelial sodium channel

Proteolytic activation of the renal epithelial sodium channel (ENaC) is increased by aldosterone. The aldosterone-sensitive protease remains unidentified. In humans, elevated circulating aldosterone is associated with increased urinary extracellular vesicle (uEVs) excretion of mannan-binding lectin associated serine protease-2 (MASP-2). We hypothesized that MASP-2 is a physiologically relevant ENaC-activating protease. It was confirmed that MASP2 mRNA is abundantly present in liver but not in human and mouse kidneys. Aldosterone-stimulation of murine cortical colleting duct (mCCD) cells did not induce MASP-2 mRNA. In human kidney collecting duct, MASP-2 protein was detected in AQP2-negative/ATP6VB1-positive intercalated cells suggestive of MASP2 protein uptake. Plasma concentration of full-length MASP-2 and the short splice variant MAp19 were not changed in a cross-over intervention study in healthy humans with low (70 mmol/day) versus high (250 mmol/day) Na+ intake despite changes in aldosterone. The ratio of MAp19/MASP-2 in plasma was significantly increased with a high Na+ diet and the ratio correlated with changes in aldosterone and fractional Na+ excretion. MASP-2 was not detected in crude urine or in uEVs. MASP2 activated an amiloride-sensitive current when co-expressed with ENaC in Xenopus oocytes, but not when added to the bath solution. In monolayers of collecting duct M1 cells, MASP2 expression did not increase amiloride-sensitive current and in HEK293 cells, MASP-2 did not affect γENaC cleavage. MASP-2 is neither expressed nor co-localized and co-regulated with ENaC in the human kidney or in urine after low Na+ intake. MASP-2 does not mediate physiological ENaC cleavage in low salt/high aldosterone settings.

suPAR, a Circulating Kidney Disease Factor

Urokinase plasminogen activator receptor (uPAR) is a multifaceted, GPI-anchored three-domain protein. Release of the receptor results in variable levels of soluble uPAR (suPAR) in the blood circulation. suPAR levels have been linked to many disease states. In this mini-review, we discuss suPAR as a key circulating molecule mediating kidney disease with a particular focus on differently spliced isoforms.

Immune-mediated entities of (primary) focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) represents a glomerular scar formation downstream of various different mechanisms leading to podocytopathy and podocyte loss. Recently, significant advances were made in understanding genetic factors, podocyte intrinsic mechanisms, and adaptive mechanisms causing FSGS. However, while most cases of nephrotic FSGS are being treated with immunosuppressants, the underlying immune dysregulation, involved immune cells, and soluble factors are only incompletely understood. Thus, we here summarize the current knowledge of proposed immune effector cells, secreted soluble factors, and podocyte response in immune-mediated (primary) FSGS.

Renal and Hematological Effects of CLCF-1, a B-Cell-Stimulating Cytokine of the IL-6 Family

CLCF-1 is a cytokine known for B-cell stimulation and for neurotrophic properties. We have identified CLCF-1 as a potential injurious factor in the human renal disease focal segmental glomerulosclerosis (FSGS). We investigated its effects on renal cells and renal function in in vitro and in vivo studies. Methods include measurement of the effect of CLCF-1 on phosphorylation of target molecules of the JAK/STAT pathway, on cytoskeleton and cell morphology in cultured podocytes, on albumin permeability of isolated rat glomeruli, and on tissue phosphorylation and urine albumin after acute or chronic CLCF-1 injection. In addition, cell sorting was performed to determine the presence of cells expressing CLCF-1 in spleen and bone marrow of normal mice and the effect of CLCF-1 infusion on splenic B-cell populations. CLCF-1 increased phosphorylation of STAT3 in multiple cell types, activated podocytes leading to formation of lamellipodia and decrease in basal stress fibers, increased glomerular albumin permeability, and increased STAT3 phosphorylation of peripheral blood cells and renal cortex. CLCF-1 increased urine albumin/creatinine ratio in mice and increased B-cell expression of IgG in mouse spleen. We conclude that CLCF-1 has potentially important systemic effects, alters podocyte function, and may contribute to renal dysfunction and albuminuria.

Pathogenic pathways are activated in each major cell type of the glomerulus in the Cd2ap mutant mouse model of focal segmental glomerulosclerosis

Background

Mutations in several genes expressed in podocytes, including Cd2ap, have been associated with focal segmental glomerulosclerosis in humans. Mutant mouse models provide an opportunity to better understand the molecular pathology that drives these diseases.

Methods

In this report we use a battery of transgenic-GFP mice to facilitate the purification of all three major cell types of the glomerulus from Cd2ap mutant mice. Both microarrays and RNA-seq were used to characterize the gene expression profiles of the podocytes, mesangial cells and endothelial cells, providing a global dual platform cross-validating dataset.

Results

The mesangial cells showed increased expression of profibrotic factors, including thrombospondin, Tgfb2 and Tgfb3, as well as the angiogenesis factor Vegf. They also showed upregulation of protective genes, including Aldh1a2, involved in retinoic acid synthesis and Decorin, a Tgfb antagonist. Of interest, the mesangial cells also showed significant expression of Wt1, which has generally been considered podocyte specific. The Cd2ap mutant podocytes showed upregulation of proteases as well as genes involved in muscle and vasculature development and showed a very strong gene expression signature indicating programmed cell death. Endothelial cells showed increased expression of the leukocyte adhesion associated factors Vcam1 and Sele, as well as Midkine (promoting angiogenesis), endothelin and many genes responsive to cytokines and interferons.

Conclusions

This study provides a comprehensive analysis of the changing properties of the three cell types of the glomerulus in Cd2ap mutants, identifying activated and repressed pathways and responsible genes, thereby delivering a deeper molecular understanding of this genetic disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s12882-015-0063-z) contains supplementary material, which is available to authorized users.

Proteomic analysis of renal diseases: unraveling the pathophysiology and biomarker discovery

Current biomedical applications of proteomics have been conducted with four main objectives: to better understand the normal biology and physiology of cells, microorganisms, tissues and organs; to explore the pathogenic mechanisms and better understand the pathophysiology of medical diseases; to identify novel biomarkers for early disease detection, prediction and prognosis; and to define new therapeutic targets, drugs and vaccines. This review focuses predominantly on proteomic applications to unravel the pathophysiology and to define novel biomarkers for various renal diseases (i.e., glomerular diseases, tubulointerstitial diseases, renal vascular disorders and renal cancers). In addition, proteomic evaluations of renal transplantation and renal replacement therapy (for acute renal failure and end-stage renal disease) are summarized. Personal opinion, future perspectives and information resources for the field of renal and urinary proteomics are provided.

Active proteases in nephrotic plasma lead to a podocin-dependent phosphorylation of VASP in podocytes via protease activated receptor-1

Focal segmental glomerulosclerosis (FSGS) is associated with glomerular podocyte injury. Podocytes undergo dramatic changes in their actin structure, with little mechanistic insight to date into the human disease. Post-transplantation recurrence of FSGS is the archetypal form of the disease caused by unknown circulating plasma 'factors'. There is increasing indication that plasma protease activity could be central to this disease. Using clinical plasma exchange material, collected from patients in relapse and remission stages of disease, the effects of FSGS plasma on human conditionally immortalized podocytes (ciPods) were studied. We show that vasodilator stimulated phosphoprotein (VASP) is phosphorylated in response to relapse plasma from ten consecutively tested patients, and not in response to paired remission plasma or non-FSGS controls. The phosphorylation signal is absent in human podocytes carrying a pathological podocin mutation. To test for a plasma ligand, inhibition of proteases in relapse plasma leads to the loss of VASP phosphorylation. By the use of siRNA technology, we show that proteases in the plasma signal predominantly via protease activated receptor-1 (PAR1) to VASP. Mechanistically, FSGS plasma increases podocyte motility, which is dependent on VASP phosphorylation. These data suggest a specific biomarker for disease activity, as well as revealing a novel and highly specific receptor-mediated signalling pathway to the actin cytoskeleton.

Characterization of glomerular diseases using proteomic analysis of laser capture microdissected glomeruli

The application of molecular techniques to characterize clinical kidney biopsies has the potential to provide insights into glomerular diseases that cannot be revealed by traditional renal pathology. The present work is a proof-of-concept approach to test whether proteomic analysis of glomeruli isolated from clinical biopsies by laser capture microdissection can provide unique information regarding differentially expressed proteins relevant to disease pathogenesis. The proteomes of glomeruli isolated by laser capture microdissection from biopsies of normal kidneys (living-related donor kidneys) were compared with those from patients with diabetic nephropathy, lupus nephritis, and fibronectin glomerulopathy. Glomerular proteins were extracted, trypsin digested, and subjected to liquid chromatography-tandem mass spectrometry for identification and quantitation. Relative to normal glomeruli, all disease-associated glomeruli showed an increased presence of complement components, a marked decline in podocyte-associated proteins, and a decrease in proteins associated with cellular metabolism. Additionally, fibronectin glomerulopathy glomeruli differed from all the other glomeruli because of a significant accumulation of fibronectin and fibulin. This study demonstrates that our method acquires reproducible and quantitative proteomic information from laser capture microdissection isolates that can be used to characterize the molecular features of glomerular diseases.

Differential proteome profiling using iTRAQ in microalbuminuric and normoalbuminuric type 2 diabetic patients

Diabetic nephropathy (DN) is a long-term complication of diabetes mellitus that leads to end-stage renal disease. Microalbuminuria is used for the early detection of diabetic renal damage, but such levels do not reflect the state of incipient DN precisely in type 2 diabetic patients because microalbuminuria develops in other diseases, necessitating more accurate biomarkers that detect incipient DN. Isobaric tags for relative and absolute quantification (iTRAQ) were used to identify urinary proteins that were differentially excreted in normoalbuminuric and microalbuminuric patients with type 2 diabetes where 710 and 196 proteins were identified and quantified, respectively. Some candidates were confirmed by 2-DE analysis, or validated by Western blot and multiple reaction monitoring (MRM). Specifically, some differentially expressed proteins were verified by MRM in urine from normoalbuminuric and microalbuminuric patients with type 2 diabetes, wherein alpha-1-antitrypsin, alpha-1-acid glycoprotein 1, and prostate stem cell antigen had excellent AUC values (0.849, 0.873, and 0.825, resp.). Moreover, we performed a multiplex assay using these biomarker candidates, resulting in a merged AUC value of 0.921. Although the differentially expressed proteins in this iTRAQ study require further validation in larger and categorized sample groups, they constitute baseline data on preliminary biomarker candidates that can be used to discover novel biomarkers for incipient DN.

MAp19, the alternative splice product of the MASP2 gene

The lectin pathway of complement is a central part of innate immunity, but as a powerful inducer of inflammation it needs to be tightly controlled. The MASP2 gene encodes two proteins, MASP-2 and MAp19. MASP-2 is the serine protease responsible for lectin pathway activation. The smaller alternative splice product, MAp19, lacks a catalytic domain but retains two of three domains involved in association with the pattern-recognition molecules (PRMs): mannan-binding lectin (MBL), H-ficolin, L-ficolin and M-ficolin. MAp19 reportedly acts as a competitive inhibitor of MASP-2-mediated complement activation. In light of a ten times lower affinity of MAp19, versus MASP-2, for association with the PRMs, much higher serum concentrations of MAp19 than MASP-2 would be required for MAp19 to exert such an inhibitory activity. Just four amino acid residues distinguish MAp19 from MASP-2, and these are conserved between man, mouse and rat. Nonetheless we generated monoclonal rat anti-MAp19 antibodies and established a quantitative assay. We found the concentration of MAp19 in serum to be 217 ng/ml, i.e., 11nM, comparable to the 7 nM of MASP-2. In serum all MASP-2, but only a minor fraction of MAp19, was associated with PRMs. In contrast to previous reports we found that MAp19 could not compete with MASP-2 for binding to MBL, nor could it inhibit MASP-2-mediated complement activation. Immunohistochemical analyses combined with qRT-PCR revealed that both MAp19 and MASP-2 were mainly expressed in hepatocytes. High levels of MAp19 were found in urine, where MASP-2 was absent.

Proteomic analysis of urine from proteinuric patients shows a proteolitic activity directed against albumin

BACKGROUND

Nephrotic syndrome is a condition that is clinically associated with poor outcome. In this study, we compared different techniques of urine sample preparation in order to develop a robust analytical protocol to define the differential urinary proteome of urinary abnormalities compared to nephrotic proteinuria.

METHODS

We recruited 5 normal control subjects, 16 patients with urinary abnormalities and 16 patients with nephrotic syndrome. Proteins from normal urine were processed using three different protocols [acetone, ultrafiltration and trichloroacetic acid (TCA) precipitation], depletion of albumin and IgGs and then analysed by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) gels and mass spectrometry.

RESULTS

Comparing the three extraction methods by visual inspection of gels after 2D gel electrophoresis, the acetone precipitation and TCA methods yielded the best quality of protein extraction, while the acetone precipitation method was the most efficient. Furthermore, we tested three commercial kits for albumin and IgG depletion. We applied the optimized acetone extraction protocol to compare urinary samples from nephrotic patients (NP) to urinary samples obtained from patients presenting with urinary abnormalities (UAP). We observed a proteolytic activity directed against albumin. This observation was more prevalent in urinary samples from NP than from UAP. Within both groups, there was some inter-individual variability in the observed proteolytic activity. An increased concentration of alpha1 antitrypsin was also observed in urine of NP. We analysed albumin fragmentation by 1D and 2D western blots in the same samples skipping the albumin and IgG depletion steps to avoid the possible confound of albumin fragment removal. The analysis confirmed a stronger proteolytic activity in the nephrotic group.

CONCLUSIONS

The proteolytic activity against albumin and the anti-proteolytic activity of alpha1 antitrypsin are likely linked and could play an important role in the nephrotic process. If replicated in larger samples, this methodology may lead to a better understanding of the underlying pathophysiological process of nephrotic syndrome.

A primer on recurrent and de novo glomerulonephritis in renal allografts

Accumulating evidence indicates that recurrent glomerulonephritis is the third most important cause of renal allograft loss at 10 years after transplantation. The proteinuria and elevated serum creatinine levels that result from recurrent glomerulonephritis are associated with cardiovascular morbidity and mortality. The exact prevalence of either recurrent or de novo post-transplantation glomerulonephritis is unknown because a considerable number of patients never undergo allograft biopsy, meaning that glomerulonephritis remains undiagnosed and a diagnosis of 'chronic rejection/chronic allograft nephropathy' is sometimes presumed. The lack of consensus regarding evaluation of kidney transplant recipients who exhibit slow deterioration of graft function is a major reason for underdiagnosis. All forms of glomerular disease can recur after transplantation, but the likelihood of recurrence differs according to type. Focal segmental glomerulosclerosis, membranoproliferative glomerulonephritis, IgA nephropathy and idiopathic diarrhea-negative hemolytic uremic syndrome often recur. Membranous nephropathy, focal segmental glomerulosclerosis, anti-glomerular basement membrane nephritis associated with Alport syndrome, and drug-induced thrombotic microangiopathy are the most common forms of de novo glomerulonephritis. This Review discusses the prevalence, risk factors, pathogenesis, clinicopathological features, and effects on graft outcome of recurrent and de novo glomerulonephritis in renal allografts. Treatment options are briefly outlined.

Active focal segmental glomerulosclerosis is associated with massive oxidation of plasma albumin

The basic mechanism for idiopathic FSGS still is obscure. Indirect evidence in humans and generation of FSGS by oxidants in experimental models suggest a role of free radicals. In vitro studies demonstrate a main role of plasma albumin as antioxidant, its modification representing a chemical marker of oxidative stress. With the use of complementary liquid chromatography electron spray ionization tandem mass spectrometry (LC-ESI-MS/MS) and biochemical methods, plasma albumin was characterized in 34 patients with FSGS; 18 had received a renal transplant, and 17 had IgM mesangial deposition. Patients with FSGS that was in remission or without recurrence after transplantation had normal plasma albumin, and the same occurred in patients with primary and secondary nephrites and with chronic renal failure. In contrast, patients with active FSGS or with posttransplantation recurrence had oxidized plasma albumin. This finding was based on the characterization of albumin Cys 34 with an mass-to-charge ratio of 511.71 in triple charge that was consistent with the formation of a cysteic acid carrying a sulfonic group (alb-SO(3)(-)). The exact mass of albumin was increased accordingly (+48 Da) for incorporation of three oxygen radicals. Direct titration of the free sulfhydryl group 34 of plasma albumin and electrophoretic titration curves confirmed loss of free sulfhydryl group and formation of a fast-moving isoform in all cases with disease activity. This is the first demonstration of in vivo plasma albumin oxidation that was obtained with an adequate structural approach. Albumin oxidation seems to be specific for FSGS, suggesting some pathogenetic implications. Free radical involvement in FSGS may lead to specific therapeutic interventions.

Podocin-Related Mechanisms in Posttransplantation Recurrence of Focal Segmental Glomerulsclerosis

Posttransplantation recurrence of focal segmental glomerulosclerosis (FSGS) is one of the most disarming events in human pathology with important social and psychological consequences. It usually occurs in 30% to 50% of patients affected by the primary form of the disease with an abrupt onset in the majority of cases occurring within 1 month of the transplantation. Prediction of recurrent cases and early therapy with plasmapheresis are the main goals of the therapy. Although the mechanism of posttransplantation recurrence is still obscure, it has been proposed to be of a multifactorial origin, in which plasma factors determine the shedding of proteins of the slit-diaphragm, such as nephrin and podocin, with structural alterations of the ultra-filtering unit of the glomerulus. Low resynthesis of podocin and/or haplo-insufficiency due to heterozygous mutations should represent significant predisposing factors to proteinuria. In this review, the role of podocin in posttransplantation recurrence will be evaluated focusing on the possibility that resynthesis of the protein could represent a key step also for stable normalization of the renal filter. The recent characterization of the podocin promoter cis- and trans- acting elements and the possibility to characterize low- and high-podocin producer haplotypes offer opportunities to evaluate the capacity for podocin resynthesis in the donor kidney. A review of the literature on posttransplantation recurrence of FSGS in patients originally carrying homozygous and/or heterozygous NPHS2 mutations supports the general idea of a multifactorial origin of the primary disease that can be extended to the pathogenesis of posttransplantation recurrence.

Transitions of serum albumin in patients with glomerulosclerosis ‘in vivo’ characterization by electrophoretic titration curves

HSA functions as a physiological transporter of solutes and small molecules that induce structural transitions 'in vitro'. Analysis of these transitions requires prior purification of HSA that could introduce bias due to conformational changes. We utilized electrophoretic titration curves to describe a neutral to acid (N-A) transition of HSA directly in sera of seven patients with active focal segmental glomerulosclerosis (FSGS). The divergent electrophoretic profile of HSA was characterized by a shift in the range of pHs between 4.5 and 7.5 with an average variation of free electrophoretic mobility corresponding to loss of 1 positive charge in the pKa protonation range of histidyl residues and should involve domain I of HSA. 'In-gel' determination by maleimide-PEO2-biotin of free SH 34 of domain I showed inaccessibility of the dye at this site in pathological HSA and alkylation with the same complex induced N-A transition in normal HSA. Potential binders of free imidazoles such as Ca++ and/or of SH 34 such as NO were excluded on the basis of direct titration and studies on binding stimulation. This is the first report describing a transition of HSA directly 'in vivo', and the utilization of electrophoretic titration curves was critical to this purpose. This transition appears to be specific to FSGS and is unrelated to the nephrotic syndrome, Ca++ and NO binding. Spectroscopic analysis will elucidate the structural implication.

Nitric oxide preserves the glomerular protein permeability barrier by antagonizing superoxide

BACKGROUND

The interaction of nitric oxide with superoxide (O2-) is a major O2- scavenging mechanism that can minimize O2 (-)-mediated oxidative stress. Glomeruli produce both nitric oxide and O2- and generation of both radicals is increased in various forms of glomerular disease. O2- increases glomerular capillary permeability to albumin (P(alb)). The present studies tested the hypothesis that nitric oxide opposes this effect, thereby preserving the glomerular protein permeability barrier.

METHODS

P(alb) was determined in isolated rat glomeruli by measuring the change in glomerular volume in response to an experimental oncotic gradient. Changes in P(alb) in response to O2- generated by tumor necrosis factor-alpha (TNF-alpha) or xanthine/xanthine oxidase (X/XO) was assessed under conditions of nitric oxide depletion and repletion.

RESULTS

Incubation of rat glomeruli with the nitric oxide synthase (NOS) inhibitor L-N(G)-monomethyl arginine (L-NMMA) increased P(alb.) This effect was reversed by the nitric oxide donor diethylenetriamine NONOate (DETA-NONOate) and by the superoxide dismutase (SOD) mimetic Tempol. O2- generated after incubation with TNF-alpha or X/XO increased P(alb). This effect was blocked by DETA-NONOate.

CONCLUSION

We demonstrate that nitric oxide protects the glomerular filtration barrier from injury caused by O2- and suggest that inhibition of nitric oxide synthesis could enhance O2(-)-mediated oxidative injury under pathologic conditions.

Circulating anti-actin and anti-ATP synthase antibodies identify a sub-set of patients with idiopathic nephrotic syndrome

Idiopathic nephrotic syndrome (iNS) with resistance or dependence to steroids is a common disease in children but in spite of an increasing clinical impact its pathogenesis is unknown. We screened for the presence of circulating antibodies against glomerular (podocytes, mesangium) and tubular cells (tubular epithelia) a cohort of 60 children with iNS including 8 patients with a familial trait of iNS or with proven mutation of NPHS1-NPHS2 and 12 with good sensitivity to steroids. Positive sera were found in 8 cases, all belonging to the category without familial trait/molecular defects. The targets of antibodies were characterized with Western blot and MALDI-Mass utilizing beta-hexyl cell extracts separated with two-dimensional electrophoresis. In all cases antibodies of the IgM class were directed against ATP synthase beta chain alone (4 cases) or in combination with actin (3 cases); one child presented IgG against aldose reductase. The clinical picture was nephrotic syndrome with steroid resistance or dependence and variable cyclosporin sensitivity; 3 patients developed end stage renal failure. The basic pathology picture was focal segmental glomerulosclerosis (FSGS) in 4 cases and mesangial proliferative glomerulonephrites with deposition of IgM in 2. Overall, patients with circulating auto-antibodies could not be readely differentiated on clinical grounds with the exception of 3 children who developed positivity for antinuclear antibodies during the follow-up. Affinity-purified IgM from one patient who underwent plasmapheresis for therapeutical pourposes (but not from a normal pool) induced proteinuria in Sprague-Dawley rats and concomitant human IgM deposition within glomeruli. This is the first report of circulating anti-actin/ATP synthase beta chain antibodies in a subset of patients with iNS. Both pathological significance and clinical impact given by the presence of these antibodies and the relationship with other conditions such as lupus-erythematosus, characterized by their presence, must be defined.

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

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

T-cell transcriptome analysis points up a thymic disorder in idiopathic nephrotic syndrome

BACKGROUND

Idiopathic nephrotic syndrome is a proteinuric disease secondary to the release of a nonidentified circulating glomerular permeability factor by T cells. Because specificities of T-cell activation in idiopathic nephrotic syndrome remain unknown, we evaluated transcriptional activation of T cells in nephrotic patients during proteinuria.

METHODS

Transcriptomes of CD2+ cells were analyzed by serial analysis of gene expression (SAGE) in a nephrotic child during proteinuria relapse and after remission, away from any immunosuppressive treatment. Expression of specific transcripts overexpressed during proteinuria relapse was compared by reverse transcription-polymerase chain reaction (RT-PCR) in CD2+ cells from 11 nephrotic patients during relapse and remission and 11 non nephrotic patients during infection and after recovery.

RESULTS

Differential analysis of CD2+ cell transcriptome identified >200 mRNA tags overexpressed during proteinuria relapse, including many T-cell markers. RT-PCR analysis of expression of specific transcripts indicated that (1) under remission conditions, nephrotic children displayed induction of four transcripts, including IKBKB, and repression of NFKBIA as compared to non nephrotic children after recovery, and (2) proteinuria relapse was associated with induction of L-selectin and T-lymphocyte maturation-associated protein, two markers of T-cell differentiation and recent emigrant/naive T cells.

CONCLUSION

Results indicate that circulating T cells from relapsing nephrotic patients include a significant population of low-mature cells while those from nephrotic patients in remission are characterized by constitutive activation of nuclear factor-kappaB (NF-kappaB), altogether suggesting a thymic dysregulation of apoptosis in nephrotic patients.

Renal glomerular permselectivity and vascular endothelium

The glomerular barrier is the kidney's physical block to the unrestricted flow of molecules from the plasma into the urinary space. Its exquisite selectivity allows solutes and water in the glomerular capillaries to pass through but it prevents the bulk of plasma proteins, most notably albumin, from crossing. Classically the barrier consists of three components: glomerular endothelium, glomerular basement membrane and glomerular epithelium (podocytes) with slit diaphragm. A lot of investigations are experimental but they are sufficient to show the pivotal role of endothelium in glomerular proteinuria. In this study the author discuss glomerular endothelium with particular emphasis on the barrier presumed to be imparted by endothelium-glomerular basement membrane-podocyte interactions. By specialized glomerular endothelial structure (caveolae, tight junctions, endothelium glycocalyx) and by circulating permeability factors (albumin, orosomucoid, apolipoproteins, Amadori's products). Concluding remarks underline the significance to study the glomerular vascular endothelial dysfunction.

Proteomics: a novel tool to unravel the patho-physiology of uraemia

BACKGROUND

Uraemic toxicity results in the dysfunction of many organ systems, provoking an increase in morbidity and mortality. To date, only approximately 90 uraemic retention solutes have been described. To examine unknown uraemic substances thoroughly, the identification of as many compounds as possible in the ultrafiltrate and/or plasma of patients would lead to a less biased definition of the uraemic retention process compared with what is proposed today.

METHODS

We describe the application of a novel proteomic tool for the identification of a large number of molecules present in ultrafiltrate from uraemic and normal plasma obtained with high- or low-flux membranes. Separation by capillary electrophoresis was coupled on-line to a mass spectrometer, yielding identification of polypeptides based on their molecular weight.

RESULTS

Between 500 and >1000 polypeptides with a molecular weight ranging from 800 to 10,000 Da could be detected in individual samples, and were identified via their mass and their particular migration time in capillary electrophoresis. In ultrafiltrate from uraemic plasma, 1394 polypeptides were detected in the high-flux vs 1046 in the low-flux samples, while in ultrafiltrate from normal plasma, 544 polypeptides vs 490 were found in ultrafiltrate from normal plasma obtained from membranes with comparable cut-off. In addition, polypeptides >5 kDa were virtually only detected in the uraemic ultrafiltrate from the high-flux membrane (n = 28 vs n = 5 with the low-flux membrane). To demonstrate the feasibility of further characterizing the detected molecules, polypeptides present exclusively in uraemic ultrafiltrate were chosen for sequencing analyses. A 950.6 Da polypeptide was identified as a fragment of the salivary proline-rich protein. A 1291.8 Da fragment was derived from alpha-fibrinogen.

CONCLUSION

The data presented here strongly suggest that the application of proteomic approaches such as capillary electrophoresis and mass spectrometry will result in the identification of many more uraemic solutes than those known at present. This could enable the introduction of more direct elimination strategies, since it is possible to obtain an extended appreciation of the removal capacities of particular dialyser membranes.

Glomerular albumin permeability as an in vitro model for characterizing the mechanism of focal glomerulosclerosis and predicting post-transplant recurrence

The putative mechanisms of proteinuria in idiopathic focal glomerulosclerosis and of its post-transplant recurrence are discussed. It is proposed that a balance between circulating factors with permeability activity on glomeruli and putative inhibitors play a key role. The characterization of inductors is currently in progress; most inhibitors appear to be apolipoproteins (mainly apoJ and apo E) but we cannot exclude other substances. The goal is now to evaluate the concentration of both inducers and inhibitors of glomerular permeability in vivo. Permeability activity in plasma of patients with FSGS with and without recurrence of the disease may be evaluated by an in vitro functional essay with isolated glomeruli. Published data on permeability activity evaluated with this method in different proteinuric states gave, however, controversial results and this test cannot be readily considered of clear clinical utility. Only the definitive characterization and quantification in vivo of the different molecules that play a role in FSGS may furnish adequate answer.

The effect of proteinase inhibitors on glomerular albumin permeability induced in vitro by serum from patients with idiopathic focal segmental glomerulosclerosis

BACKGROUND

The putative circulating factor responsible for the glomerular permeability alterations induced in vitro by serum from patients affected by focal segmental glomerulosclerosis (FSGS) remains unidentified. We have observed that a serine proteinase isolated from patient serum increases albumin permeability in isolated glomeruli. The objective of the present study was to determine the effect of various proteinase inhibitors on glomerular albumin permeability (P(alb)) in isolated glomeruli incubated with FSGS serum.

METHODS

The study population consisted of 12 FSGS patients (eight males; mean age: 21+/-10 years) previously shown to have elevated serum albumin permeability activity. P(alb) was determined by measuring the change in glomerular volume induced by applying oncotic gradients to isolated healthy rat glomeruli treated with patient serum in comparison to control serum. Solutions of seven different proteinase inhibitors (0.5 mg/ml) were added to the incubation media with the sera (1:1 vol/vol): serine proteinase inhibitors (PMSF, leupeptin, aprotinin, gabexate mesylate), the cysteine proteinase inhibitor E-64, the metalloproteinase inhibitor EDTA and the aspartate proteinase inhibitor pepstatin. Sera from the same patients were also tested with the addition to the incubation media of quinaprilat, an inhibitor of the metalloproteinase angiotensin-converting enzyme.

RESULTS

Mean P(alb) of the sera was 0.86+/-0.11, with the addition of PMSF 0.41+/-0.09, leupeptin 0.30+/-0.17, aprotinin 0.09+/-0.14, gabexate mesylate 0.27+/-0.25, E-64 0.81+/-0.09, EDTA 0.68+/-0.10 or pepstatin 0.76+/-0.11. The mean P(alb) of the sera combined with quinaprilat was reduced to 0.34+/-0.35. Thus, only the serine proteinase inhibitors consistently blocked the increased P(alb) induced by the FSGS sera.

CONCLUSIONS

In the cascade of events that lead to the initiation of glomerular fibrosis in FSGS, the putative glomerular permeability factor associated with FSGS may require a serine proteinase to effect its activity.

Proteomics in nephrology: current status and future directions

Proteomics is one among various 'OMICS' fields that have been growing rapidly in the postgenomic era. During the past few years, proteomics has been extensively applied to several fields of medicine to better understand normal physiology, to define the pathophysiology of diseases, and to identify novel biomarkers and new therapeutic targets. This review focuses on current status and future directions of proteomics in the nephrology field. Recent studies of renal proteome, proteomes of individual intrarenal structures (i.e., glomerular, vascular, tubular, brush border membrane, mesangial, and podocyte proteomes), urinary proteome, and protein profiles in dialysate or ultrafiltrate removed by renal replacement therapy are summarized.

The focal segmental glomerulosclerosis permeability factor: biochemical characteristics and biological effects

Focal segmental glomerulosclerosis (FSGS) is characterized by steroid resistant nephrotic syndrome and progression to end-stage renal disease. Proteinuria in certain patients with FSGS may be caused by a circulating factor (FSGS permeability factor [FSPF]). The current report documents the biochemical characteristics and the biological and molecular effects of 70% ammonium sulfate supernatant of plasma from patients with recurrence of FSGS after transplantation (FSGS 70% supernatant). FS permeability activity, defined as the capacity of plasma from patients with FSGS to increase albumin permeability (P(alb)) of isolated glomeruli, was assessed in vitro. Permeability activity was not affected by lyophilization. FSPF bound strongly to matrices containing Mono-Q anion exchanger or protein A. It eluted from matrix-bound Cibacron blue F3GA over a wide range of salt concentrations, indicating a potential binding with other proteins, such as albumin. FSPF caused a maximal increase in P(alb) within 2 mins of incubation in vitro. Cellular proteins isolated from glomeruli with increased P(alb) showed decreased tyrosine phosphorylation of focal adhesion kinase, paxillin, and other proteins. Tyrosine phosphatase ]inhibition prevented the increase in P(alb). Intravenous administration of as little as 3 mg protein in FSGS 70% supernatant increased P(alb), while 9 mg or more were required to produce proteinuria. We conclude that FSPF is a low-molecular-weight protein, carries an anionic charge, and binds to protein A. Effects of FSPF on the glomerular permeability barrier are rapid and dose dependent and involve signaling through altered phosphorylation of cellular proteins. Identification of these biochemical and biological characteristics may be used to design strategies for removing FSPF from circulation and for purification and identification of this factor.

Broadening the spectrum of diseases related to podocin mutations

A total of 179 children with sporadic nephrotic syndrome were screened for podocin mutations: 120 with steroid resistance, and 59 with steroid dependence/frequent relapses. Fourteen steroid-resistant patients presented homozygous mutations that were associated with early onset of proteinuria and variable renal lesions, including one case with mesangial C3 deposition. Single mutations of podocin were found in four steroid-resistant and in four steroid-dependent; five patients had the same mutation (P20L). Among these, two had steroid/cyclosporin resistance, two had steroid dependence, and one responded to cyclosporin. The common variant R229Q of podocin, recently associated with late-onset focal segmental glomerulosclerosis, had an overall allelic frequency of 4.2% versus 2.5% in controls. To further define the implication of R229Q, a familial case was characterized with two nephrotic siblings presenting the association of the R229Q with A297V mutation that were inherited from healthy mother and father, respectively. Immunohistochemistry with anti-podocin antibodies revealed markedly decreased expression of the protein in their kidneys. All carriers of heterozygous coding podocin mutation or R229Q were screened for nephrin mutation that was found in heterozygosity associated with R229Q in one patient. Finally, podocin loss of heterozygosity was excluded in one heterozygous child by characterizing cDNA from dissected glomeruli. These data outline the clinical features of sporadic nephrotic syndrome due to podocin mutations (homozygous and heterozygous) in a representative population with broad phenotype, including patients with good response to drugs. The pathogenetic implication of single podocin defects per se in proteinuria must be further investigated in view of the possibility that detection of a second mutation could have been missed. A suggested alternative is the involvement of other gene(s) or factor(s).

Permeability factors in focal segmental glomerulosclerosis

The pathologic diagnosis of focal segmental glomerulosclerosis (FSGS) is associated with a syndrome of steroid-resistant nephrotic syndrome and progressive renal insufficiency. The incidence of FSGS has increased in recent years. Known causes of FSGS include genetic abnormalities, viral infections, decreased nephron number, and hyperperfusion/hyperfiltration. The etiology is unknown in the majority of cases. FSGS recurs after initial renal transplantation in as many as 30% to 50% of patients. Recent studies have verified the hypothesis that plasma of patients with FSGS contains a factor or factors that increase permeability of glomerular capillaries and cause proteinuria after injection into rats. Patients who experience posttransplant recurrence of FSGS and those with rapidly progressive disease exhibit this activity. Permeability activity has been verified in functional assays and defined by measurement of albumin permeability (P(alb)) or glomerular volume variation (GVV). Permeability activity is decreased by plasmapheresis or immunoadsorption and can be recovered from discarded plasma or eluate from adsorption materials. Studies from our laboratory indicate that permeability activity is carried by small, highly glycosylated, hydrophobic protein(s)/peptide(s). Normal plasma contains substances capable of blocking or inactivating the FSGS permeability factor. Pharmacologic agents including cyclosporine, indomethacin, and derivatives of Trypterigium wilfordii also block permeability activity in vitro. The observation that permeability activity can be blocked by diverse agents raises hope that specific therapy may be designed for FSGS. Future investigations will permit identification of the active FSGS permeability factor, of mechanisms that initiate and perpetuate proteinuria, and of interventions to prevent renal failure in native kidneys and recurrence of disease in renal allografts.

Serum glomerular permeability activity in patients with podocin mutations (NPHS2) and steroid-resistant nephrotic syndrome

A plasma factor displaying permeability activity in vitro and possibly determining proteinuria has been hypothesized in idiopathic focal segmental glomerulosclerosis (FSGS). In vitro permeability activity (P(alb)) was determined in sera of five patients with autosomal recessive steroid-resistant nephrotic syndrome (NPHS2), an inherited condition indistinguishable from idiopathic FSGS on clinical grounds, but in which proteinuria is determined by homozygous mutations of podocin, a key component of the glomerular podocyte. All patients had presented intractable proteinuria with nephrotic syndrome; four developed renal failure and received a renal allograft. For comparison, sera from 31 children with nephrotic syndrome were tested. Pretransplant P(alb) was high in all cases (mean 0.81 +/- 0.06), equivalent to levels observed in idiopathic FSGS. Overall, P(alb) did not correlate with proteinuria. The posttransplant outcome was complicated in two patients by recurrence of proteinuria after 10 and 300 d, respectively, that responded to plasmapheresis plus cyclophosphamide. P(alb) levels were high at the time of the recurrence episodes and steadily decreased after plasmapheresis, to reach normal levels in the absence of proteinuria after the seventh cycle. In an attempt to explain high P(alb) in these patients, putative inhibitors of the permeability activity were studied. Coincubation of serum with homologous nephrotic urine reduced P(alb) to 0, whereas normal urine did not determine any change, which suggests loss of inhibitory substances in nephrotic urine. The urinary levels of the serum P(alb) inhibitors apo J and apo E were negligible in all cases, thus suggesting that other urinary inhibitors were responsible for the neutralizing effect. These data indicate that P(alb) is high in NPHS2, probably resulting from loss of inhibitors in urine. Lack of correlation of P(alb) with proteinuria suggests a selective loss of inhibitors. As in idiopathic FSGS, proteinuria may also recur after renal transplantation in NPHS2 patients, and post-transplant proteinuria is associated with high P(alb). The relationship between elevated P(alb) and proteinuria in NPHS2 remains to be determined.