Circulating permeability factors in the nephrotic syndrome: a fresh look at an old problem

Soluble CD40 ligand directly alters glomerular permeability and may act as a circulating permeability factor in FSGS

CD40/CD40 ligand (CD40L) dyad, a co-stimulatory bi-molecular complex involved in the adaptive immune response, has also potent pro-inflammatory actions in haematopoietic and non-haematopoietic cells. We describe here a novel role for soluble CD40L (sCD40L) as modifier of glomerular permselectivity directly acting on glomerular epithelial cells (GECs). We found that stimulation of CD40, constitutively expressed on GEC cell membrane, by the sCD40L rapidly induced redistribution and loss of nephrin in GECs, and increased albumin permeability in isolated rat glomeruli. Pre-treatment with inhibitors of CD40-CD40L interaction completely prevented these effects. Furthermore, in vivo injection of sCD40L induced a significant reduction of nephrin and podocin expression in mouse glomeruli, although no significant increase of urine protein/creatinine ratio was observed after in vivo injection. The same effects were induced by plasma factors partially purified from post-transplant plasma exchange eluates of patients with focal segmental glomerulosclerosis (FSGS), and were blocked by CD40-CD40L inhibitors. Moreover, 17 and 34 kDa sCD40L isoforms were detected in the same plasmapheresis eluates by Western blotting. Finally, the levels of sCD40Lwere significantly increased in serum of children both with steroid-sensitive and steroid-resistant nephrotic syndrome (NS), and in adult patients with biopsy-proven FSGS, compared to healthy subjects, but neither in children with congenital NS nor in patients with membranous nephropathy.

Our results demonstrate that sCD40L directly modifies nephrin and podocin distribution in GECs. Moreover, they suggest that sCD40L contained in plasmapheresis eluates from FSGS patients with post-transplant recurrence may contribute, presumably cooperating with other mediators, to FSGS pathogenesis by modulating glomerular permeability.

Apolipoprotein C-I Levels Are Associated with the Urinary Protein/Urinary Creatinine Ratio in Pediatric Idiopathic Steroid-Sensitive Nephrotic Syndrome: A Case Control Study

Humoral factors may cause idiopathic steroid-sensitive nephrotic syndrome (ISSNS). In the present study, we analyzed serum proteins using mass spectrometry (MS) to identify proteins associated with the pathophysiology of pediatric ISSNS. We collected serial serum samples from 33 children during each ISSNS phase; Phase A1 is the acute phase prior to steroid treatment (STx), Phase A2 represents the remission period with STx, and Phase A3 represents the remission period after completion of STx. Children with normal urinalyses (Group B) and children with a nephrotic syndrome other than ISSNS (Group C) served as controls. No significant differences in urinary protein/urinary creatinine (UP/UCr) ratios were observed between the children with phase A1 ISSNS and Group C. We used surface-enhanced laser desorption/ionization time of flight MS for sample analysis. Four ion peaks with a mass-to-charge ratio (m/z) of 6,444, 6,626, 8,695, and 8,915 were significantly elevated during ISSNS Phase A1 compared to Phase A2, Phase A3, and Group C. The intensity of an m/z of 6,626 significantly correlated with the UP/UCr ratio and an m/z of 6,626 was identified as apolipoprotein C-I (Apo C-I). Apo C-I levels correlate with the UP/UCr ratio in pediatric ISSNS. Our findings provide new insights into the pathophysiology of ISSNS.

Permeability factors in idiopathic nephrotic syndrome: historical perspectives and lessons for the future

The term idiopathic nephrotic syndrome (iNS) traditionally covers minimal change disease and primary focal segmental glomerulosclerosis (FSGS), now thought to be separate disease entities. Clinical and experimental evidence suggest that circulating permeability factors are involved in their pathogenesis. In the past four decades, many investigators have searched for the responsible factors, thus far with little success. The recent report of the soluble urokinase plasminogen activator receptor as a causative factor in FSGS has received much attention, but again the initially promising findings were not confirmed. We describe the history of the search for permeability factors, discuss the pitfalls that are likely responsible for the lack of success and propose criteria that should be used in future studies when evaluating candidate permeability factors.

Apolipoprotein AII levels are associated with the UP/UCr levels in idiopathic steroid-sensitive nephrotic syndrome

BACKGROUND

Various humoral factors have been proposed as causal agents of idiopathic steroid-sensitive nephrotic syndrome (ISSNS), resulting in varying data. We used mass spectrometry (MS) to analyze serum proteins in a search for proteins that might be involved in ISSNS pathophysiology.

METHODS

Serial serum samples were obtained from 33 children with ISSNS. Samples were collected during Phase A1 [the acute phase prior to steroid treatment (STx)], Phase A2 (remission with STx), and Phase A3 (remission without any medication). We also included age- and sex-matched two control groups comprising children with normal urinalysis (Group B) and children with a nephrotic syndrome other than ISSNS (Group C). The urinary protein/urinary creatinine (UP/UCr) ratios were not statistically different between Phase A1 and Group C. Samples were analyzed using surface-enhanced laser desorption/ionization time of flight MS.

RESULTS

A total of 207 peptide ion peaks were detected in the range of m/z 2000-10000. Four peptide ions (m/z 6444, 6626, 8695, and 8915) were detected at significant elevation during Phase A1 compared with Phase A2, Phase A3, and Group C. The intensities of m/z 6444 and 8695 were higher in Phase A3 than in Group B. There were significant correlations between the intensities of m/z 6626, 8695, and 8915 and UP/UCr levels. The m/z 8695 was identified as apolipoprotein AII.

CONCLUSIONS

Apolipoprotein AII was detected as a protein associated with the UP/UCr levels in pediatric ISSNS. Our findings present an interesting starting point for further investigation into the pathophysiology of ISSNS.

The plasma permeability factor in nephrotic syndrome: indirect evidence in pediatric peritoneal dialysis

BACKGROUND

Nephrotic syndrome (NS) in children has been associated with a systemic circulating permeability factor. Therefore, once peritoneal dialysis (PD) has been started, peritoneal protein losses should be higher in the nephrotic than in the non-nephrotic population.

OBJECTIVE

We compared peritoneal protein losses in children with and without NS on PD.

METHODS

Our retrospective 4-year study analyzed Hispanic patients with NS under PD. Data at dialysis entry and 6 months later were compared. Nutritional support was given according to recommended dietary allowances and recommendations from the Kidney Disease Outcomes Quality Initiative. Clinical and biochemical data were obtained, and 24-hour dialysate and urine samples were collected to measure protein losses. Dialysis dose (Kt/V), daily protein intake (DPI), normalized protein equivalent of nitrogen appearance (nPNA), peritoneal equilibration test (PET), and peritonitis rate were determined. All measurements took place at least 4 weeks after resolution of a peritonitis episode. All patients received automated PD using a HomeChoice PD System cycler (Baxter Healthcare Corporation, Deerfield, IL, USA), with an exchange volume of 1100 mL/m(2) and a dextrose concentration of 1.5% - 2.5%. A control group of non-NS children on PD matched by age and sex were also studied. Data are reported as mean ± standard deviation. Differences between groups were calculated using the Mann-Whitney U-test, and p < 0.05 was considered significant.

RESULTS

Each study group consisted of 10 patients [NS patients: 4 boys, mean age of 7.3 ± 4.1 years; control patients: 6 boys, mean age of 7.2 ± 4.7 years (p = nonsignificant)]. In the group with NS, 8 patients were diagnosed by biopsy as having focal segmental glomerulosclerosis, and 2 as having minimal-change disease. At study entry, patients with NS had hourly urinary protein losses of 398 ± 313 mg/m(2) and daily peritoneal protein losses of 3.4 ± 1.9 g/m(2), compared with 29.9 ± 31 mg/m(2) and 1.5 ± 1.1 g/m(2) respectively in the control group (p < 0.05). The same statistical difference was found 6 months later. We observed no statistical differences in PET results, daily exchange volume, and mean dextrose concentration of dialysate. Similarly, no significant between-group differences were observed for Kt/V, DPI, nPNA, and biochemical parameters.

CONCLUSIONS

Hispanic children with NS on PD show higher peritoneal protein losses than do their control counterparts. Such differences could be secondary to increased peritoneal permeability caused by a systemic permeability factor.

Therapeutic apheresis for renal disorders

This review summarizes the clinical evidence and practical details for the use of plasmapheresis and other apheresis modalities for each indication in nephrology. Updated information on the molecular biology and immunology of each renal disease is discussed in relation to the rationale for apheresis therapy and its place amid other available treatments. Autoantibody-mediated diseases, such as anti-GBM (anti-glomerular basement membrane) glomerulonephritis (GN), ANCA (antineutrophil cytoplasmic antibody)-related GN and the antibody-mediated type of TTP (thrombotic thrombocytopenic purpura), and alloantibody-mediated diseases such as kidney transplant sensitization and humoral rejection, can be treated by various plasmapheresis methods. These include standard plasmapheresis with a replacement volume, or plasmapheresis with online plasma purification using adsorption columns or secondary filtration. However, it should be noted that the pathogenic molecules implicated in FSGS (focal segmental glomerulosclerosis), myeloma cast nephropathy, and perhaps other diseases are too small to be removed by most online purification methods. A great majority of controlled trials and series on which evidence-based treatment recommendations are made were performed using centrifugal plasmapheresis; it is presumed that membrane-separation plasmapheresis is equally efficacious. For some rarer diseases, such as MPGN (membranoproliferative GN) type 2 with factor H abnormalities or C3Nef (C3 nephritic factor) autoantibodies, there are only a few case reports, but enough scientific understanding to warrant a trial of plasmapheresis in severe cases. Photopheresis, which is effective for cell-mediated rejection in heart and lung transplantation, has not yet found a place in the routine treatment of kidney transplant rejection.

Circulating urokinase receptor as a cause of focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a cause of proteinuric kidney disease, compromising both native and transplanted kidneys. Treatment is limited because of a complex pathogenesis, including unknown serum factors. Here we report that serum soluble urokinase receptor (suPAR) is elevated in two-thirds of subjects with primary FSGS, but not in people with other glomerular diseases. We further find that a higher concentration of suPAR before transplantation underlies an increased risk for recurrence of FSGS after transplantation. Using three mouse models, we explore the effects of suPAR on kidney function and morphology. We show that circulating suPAR activates podocyte β(3) integrin in both native and grafted kidneys, causing foot process effacement, proteinuria and FSGS-like glomerulopathy. Our findings suggest that the renal disease only develops when suPAR sufficiently activates podocyte β(3) integrin. Thus, the disease can be abrogated by lowering serum suPAR concentrations through plasmapheresis, or by interfering with the suPAR-β(3) integrin interaction through antibodies and small molecules targeting either uPAR or β(3) integrin. Our study identifies serum suPAR as a circulating factor that may cause FSGS.

Properties of the Glomerular Barrier and Mechanisms of Proteinuria

This review focuses on the intricate properties of the glomerular barrier. Other reviews have focused on podocyte biology, mesangial cells, and the glomerular basement membrane (GBM). However, since all components of the glomerular membrane are important for its function, proteinuria will occur regardless of which layer is affected by disease. We review the properties of endothelial cells and their surface layer, the GBM, and podocytes, discuss various methods of studying glomerular permeability, and analyze data concerning the restriction of solutes by size, charge, and shape. We also review the physical principles of transport across biological or artificial membranes and various theoretical models used to predict the fluxes of solutes and water. The glomerular barrier is highly size and charge selective, in qualitative agreement with the classical studies performed 30 years ago. The small amounts of albumin filtered will be reabsorbed by the megalin-cubulin complex and degraded by the proximal tubular cells. At present, there is no unequivocal evidence for reuptake of intact albumin from urine. The cellular components are the key players in restricting solute transport, while the GBM is responsible for most of the resistance to water flow across the glomerular barrier.

The bioactivity of plasma factors in focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a devastating form of nephrotic syndrome, often leading to end-stage renal failure after the failure of a succession of highly toxic therapies. It has long been thought to be caused by a circulating factor(s) that may be produced by cells of the immune system. Much research has focused on identifying such factor(s), including the development of a promising in vitro assay, which estimates glomerular permeability based on the swelling of isolated glomeruli in response to patients' plasma. This assay has also been used as the basis of testing plasma fractions for permeability activity, with no specific factor yet identified. Other studies have attempted to replicate proteinuria in whole animals, by injecting plasma or plasma fractions from focal segmental glomerulosclerosis patients, with inconsistent results. More recently there has been evidence that there may be either inhibitory or missing factor(s) in plasma, with respect to permeability. An additional major biological advance is a growing appreciation of the podocyte as the target cell in this disease, and an understanding of the key molecules involved. Putting together this knowledge, with the latest technological advances in protein identification, provides promising avenues towards finally solving the basis of this enigmatic disease.

Glucocorticoid receptor and vascular endothelial growth factor in nephrotic syndrome

AIM

The aim of the study was to assess plasma and urine concentrations of vascular endothelial growth factor (VEGF) in nephrotic syndrome children (NS) depending on the total dose of glucocorticoids (GC) and the percentage of lymphocytes with glucocorticoid receptor expression (CD3/GCR).

METHODS

We examined 51 children (2-15 years), allocated to three groups: group I: 13 children with the first NS onset, group II: 13 children with NS relapse, group C: 25 healthy children. The NS patients were examined: (A) before treatment and (B) 4-5 weeks after prednisone administration at a dose of 60 mg/m2/24 h. Plasma and urinary VEGF levels were determined using the immunoenzymatic ELISA method. Flow cytometry was applied to assess CD3/GCR expression.

RESULTS

Higher plasma and urinary VEGF concentrations were noted in NS children before treatment (A), as compared to control subjects (C). Following prednisone therapy (B), VEGF level was reduced but it was still higher than in the control group. Positive correlation was observed between VEGF and protein in the urine (group I r = 0.660, P < 0.05, group II r = 0.818, P<0.01) and a weak positive correlation between VEGF in plasma and urine (group I r = 0.531, P<0.05, group II - r = 0.581, P<0.05). CD3/GCR expression was lower in group II. In both groups, the correlation between plasma VEGF and CD3/GCR was positive (P<0.05).

CONCLUSIONS

1. Plasma and urinary VEGF levels increase during nephrotic syndrome onset. 2. Glucocorticoid treatment reduces plasma and urinary VEGF levels in NS children.

Renal pathology and retinol status in multiple myeloma patients

Renal dysfunction is a common and serious complication in multiple myeloma (MM) patients. Renal proximal tubule injury is characteristic in MM, and may result in disturbed renal handling of various vitamins. The abnormal excretion of vitamins in urine may result in their low serum levels. The goal of this study was to investigate the urinary excretion of retinol in MM and its relationship with serum retinol concentration. For this purpose, 24 MM patients and 10 healthy individuals were studied. Serum and urinary retinol and retinol-binding protein (RBP) were measured by the high-performance liquid chromatography method and enzyme-linked immunosorbent assay, respectively. The study showed that 58% of MM patients excreted retinol in urine, while only 29% had elevated serum creatinine (P<0.05). There was a strong and highly significant correlation between urinary retinol and RBP (r=0.973, P<0.006). Patients with normal and mildly elevated serum creatinine who excreted retinol in urine had a marked decrease in serum retinol (P<0.007). On the other hand, serum retinol was not decreased in patients with moderate or severe renal failure, despite its urinary loss. Our data indicate that (i) urinary retinol is a more frequent marker of renal dysfunction than elevated serum creatinine in MM patients, (ii) serum retinol is decreased in MM with normal or mildly elevated serum creatinine, but not in patients with moderate/severe renal failure, and (iii) urinary retinol may serve as a diagnostic marker of renal proximal tubule dysfunction in MM patients.

Altered activity of plasma hemopexin in patients with minimal change disease in relapse

Since an active isoform of plasma hemopexin (Hx) has been proposed to be a potential effector molecule in minimal change disease (MCD), we tested plasma and urine samples from subjects with MCD in relapse (n = 18) or in remission (n = 23) (after treatment with prednisolone) for presence or activity of Hx. For comparison, plasma or urine from proteinuric subjects with focal and segmental glomerulosclerosis (FSGS, n = 11), membranoproliferative glomerulonephritis (MPGN, n = 9), IgA nephropathy (n = 5) or healthy control donors (n = 10), were incorporated into the study. Electrophoresis and Western blotting methods were used for evaluation of the Hx status, whereas protease activity of Hx was tested upon kidney tissue in vitro according to standard methods. The results show (1) a decreased mean titer of plasma Hx exclusively in MCD relapse subjects as compared with MCD in remission (0.21+/-0.14 mg/ml vs 0.44+/-0.06 mg/ml; p < 0.01). Mean Hx titers in other proteinuric subjects ranged from 0.38+/-0.05 mg/ml to 0.40+/- 0.06 mg/ml, whereas, the mean titer of healthy controls was 0.59+/-0.03 mg Hx/ml; (2) an increased Hx activity (expressed in arbitrary units) exclusively in plasma from MCD relapse subjects (3.3+/-0.72 vs 1.16+/-0.56, MCD remission; p < 0.01); (3) different Western blot patterns in MCD relapse vs remission plasma; (4) reduced stainability or virtual absence of the 80-kD Hx band in blots of urine from MCD relapse in contrast to urine samples from other proteinuric subjects with FSGS, MPGN, or IgA nephropathy. It is concluded that Hx in MCD relapse subjects may exist in an altered isoform, showing enhanced protease activity as compared with subjects in remission, subjects with other forms of primary glomerulopathy, or healthy control individuals.

Enhanced ecto-apyrase activity of stimulated endothelial or mesangial cells is downregulated by glucocorticoids in vitro

Endothelial as well as mesangial cells show enhanced activity of ecto-apyrase following pro-inflammatory stimulation in vitro. Since this ecto-enzyme appears to be able to regulate plasma hemopexin, which latter molecule plays a role in the pathogenesis of corticosteroid responsive nephrotic syndrome, the question was raised whether glucocorticoids are potentially able to downregulate ecto-apyrase activity of these cells. Therefore, cell cultures of endothelial or mesangial were stimulated with or without lipopolysaccharide (10 ng/ml). Parallel cultures were supplemented with prednisolone with or without the glucocorticoid receptor antagonist mifepristone in various concentrations. After 24 h, cytospins were prepared and cytochemically stained for ecto-apyrase activity. mRNA for apyrase of these cells was detected using reverse transcription-polymerase chain reaction (RT-PCR). Apyrase activity of either cells or soluble apyrase (0.16 U/ml buffer) with or without supplementation of prednisolone were biochemically assayed for their phosphatase activity. The results show significantly decreased ecto-apyrase activity of lipopolysaccharide-stimulated cells after treatment with prednisolone as compared to non-prednisolone-treated cells. Preincubation with mifepristone did not inhibit the effect of prednisolone. Identical mRNA signals for apyrase were found in prednisolone and non-prednisolone-treated cells. Interestingly, soluble apyrase also showed a significant decrease of activity following preincubation with prednisolone. It is concluded that prednisolone is able to downregulate ecto-apyrase of stimulated endothelial or mesangial cells, which may potentially inhibit the conversion of hemopexin to its pro-inflammatory isoform. As blocking of the cytosolic glucocorticoid receptor showed no effect upon the prednisolone action, whereas prednisolone is able to affect soluble apyrase per se, it is felt that this particular action of prednisolone may (at least partly) be mediated through a non-genomic pathway.

Characteristics of renal Rhbg as an NH4+ transporter

Rhbg is one of two recently cloned nonerythroid glycoproteins belonging to the Rh antigen family. Rhbg is expressed in basolateral membranes of intercalated cells of the kidney cortical collecting duct and some other cell types of the distal nephron and may function as NH4+ transporters. The aim of this study was to characterize the role of Rhbg in transporting NH4+. To do so, we expressed Rhbg in Xenopus laevis oocytes. Two-electrode voltage-clamp and H+-selective microlectrodes were used to measure NH4+ currents, current-voltage plots, and intracellular pH (pHi). In oocytes expressing Rhbg, 5 mM NH4+ induced an inward current of 93 ± 7.7 nA ( n = 20) that was significantly larger than that in control oocytes of −29 ± 7.1 nA ( P < 0.005). Whole cell conductance, at all tested potentials (−60 to +60 mV), was significantly more in oocytes expressing Rhbg compared with H2O-injected oocytes. In Rhbg oocytes, 5 mM NH4+ depolarized the oocyte by 28 ± 3.6 mV and decreased pHi by 0.30 ± 0.04 at a rate of −20 ± 2.5 × 10−4 pH/s. In control oocytes, 5 mM NH4+ depolarized Vm by only 20 ± 5.8 mV and pHi decreased by 0.07 ± 0.01 at a rate of −2.7 ± 0.6 × 10−4 pH/s. Raising bath [NH4+] in increments from 1 to 20 mM elicited a proportionally larger decrease in pHi (ΔpHi), larger depolarization (Δ Vm), and a faster rate of pHi decrease. Bathing Rhbg oocytes in 20 mM NH4+ induced an inward current of 140 ± 7 nA that was not significantly different from 178 ± 23 nA induced in H2O-injected (control) oocytes. The rate of pHi decrease induced by increasing external [NH4+] was significantly faster in Rhbg than in H2O-injected oocytes at all external NH4+ concentrations. In oocytes expressing Rhbg, net NH4+ influx (estimated from NH4+-induced H+ influx) as a function of external [NH4+] saturated at higher [NH4+] with a Vmax of ∼30.8 and an apparent Km of 2.3 mM ( R2 = 0.99). These data strongly suggest that Rhbg is a specific electrogenic transporter of NH4+.

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.

Role of the immune system in the pathogenesis of idiopathic nephrotic syndrome

Idiopathic NS (nephrotic syndrome) is characterized by massive proteinuria, due to a leak in the glomerular barrier to proteins. Genetic defects that affect the function and the composition of the glomerular capillary wall, in particular of the visceral epithelial cells, have recently been recognized as the cause of familial forms of NS. MCNS (minimal change NS) and FSGS (focal and segmental glomerulosclerosis) are common non-familial forms of NS in which the causative defect has not yet been identified. Several studies have shown that non-familial NS is associated with the presence of circulating permeability factors and with complex disturbances in the immune system. Thus far, there is no direct evidence that these factors directly alter glomerular permeability to proteins, and some of these factors may be a consequence, rather than a cause, of NS. In this review, we will briefly highlight the mechanisms that underlie proteinuria in general and focus on the immunological disturbances associated with idiopathic NS, with attention to potential mechanisms whereby the immune system may directly act on the glomerular capillary filter.