Giovanni Candiano

Autoimmunity in Membranous Nephropathy Targets Aldose Reductase and SOD2

Glomerular targets of autoimmunity in human membranous nephropathy are poorly understood. Here, we used a combined proteomic approach to identify specific antibodies against podocyte proteins in both serum and glomeruli of patients with membranous nephropathy (MN). We detected specific anti-aldose reductase (AR) and anti-manganese superoxide dismutase (SOD2) IgG(4) in sera of patients with MN. We also eluted high titers of anti-AR and anti-SOD2 IgG(4) from microdissected glomeruli of three biopsies of MN kidneys but not from biopsies of other glomerulonephritides characterized by IgG deposition (five lupus nephritis and two membranoproliferative glomerulonephritis). We identified both antigens in MN biopsies but not in other renal pathologies or normal kidney. Confocal and immunoelectron microscopy (IEM) showed co-localization of anti-AR and anti-SOD2 with IgG(4) and C5b-9 in electron-dense podocyte immune deposits. Preliminary in vitro experiments showed an increase of SOD2 expression on podocyte plasma membrane after treatment with hydrogen peroxide. In conclusion, our data support AR and SOD2 as renal antigens of human MN and suggest that oxidative stress may drive glomerular SOD2 expression.

Repetitive Fragmentation Products of Albumin and α1-Antitrypsin in Glomerular Diseases Associated with Nephrotic Syndrome

Even if nephrotic syndrome is characterized by massive urinary loss of major plasma proteins, a clear structural characterization based on proteomics has never been reported. Urine and plasma of 23 patients with different idiopathic nephrotic syndromes (10 steroid-sensitive minimal-change nephropathy, seven steroid-resistant FSGS, and six membranous glomerulonephritis) were analyzed with two-dimensional electrophoresis in soft gel, Western blot, and matrix-assisted laser desorption/ionization time of flight mass spectrometry; 72 urinary components corresponded to fragments of albumin and/or of alpha1-antitrypsin. Several repetitive fragmentation motives and a few differences among different pathologies were found. Several (21 of 72) urinary albumin fragments also were detected in plasma, although in lower concentration, suggesting a preferential excretion. The bulk of components with low molecular weight were detected only in urine, suggesting an in situ formation; zymograms with albumin as substrate showed the presence in urine of specific proteases. A final but not secondary point was the characterization of albumin adducts that harbor both the COOH and NH2 terminal parts of the protein, suggesting the formation of new covalent chemical groups. Altogether, these new findings reveal unexpected structural and functional aspects of proteinuria that may play a key role in pathogenesis. Characterization of urinary fragmentation patterns should be extended to other renal diseases.

Combinatorial peptide ligand libraries for urine proteome analysis: investigation of different elution systems.

Proteome treatments with peptide libraries in view of reducing high‐abundance proteins and increasing the concentration of rare species involve the adsorption on solid‐phase material. Subsequent elution of captured proteins may not be fully effective except when sequences of eluting agents are used. The standard way utilized up to the present has been a three‐ to four‐step, sequential elution system consisting of various agents mixed together such as urea, thiourea, CHAPS, sodium chloride, citric or acetic acid and some polar solvents such as ACN and isopropanol. Elution sequences produce distinct fractions adding to the burden of having to analyze all of them. An alternative, highly effective, single elution to reduce the workload is here reported for the first time, namely elution in boiling 10% SDS added with 3% DTE. This single step elutes almost quantitatively the adsorbed proteins, thus ensuring, for all practical purposes, a full recovery. This high efficiency is believed to be due to the fact that the SDS micelles bury the polypeptide chains within their hydrophobic core, thus shielding them from the surroundings and impeding accidental adsorption to surfaces. Suggestions for selecting the best method to eliminate the excess of SDS for further protein analysis are also evaluated. The merits and limits of this novel system are assessed and discussed.

Coexistence of Different Circulating Anti-Podocyte Antibodies in Membranous Nephropathy

BACKGROUND AND OBJECTIVES The discovery of different podocyte autoantibodies in membranous nephropathy (MN) raises questions about their pathogenetic and clinical meaning. This study sought to define antibody isotypes and correlations; to compare levels in MN, other glomerulonephritides, and controls; and to determine their association with clinical outcomes. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS Serum IgG(1), IgG(3), and IgG(4) against aldose reductase (AR), SOD2, and α-enolase (αENO) were measured at diagnosis in 186 consecutive MN patients, in 96 proteinuric controls (36 with FSGS, and 60 with IgA nephropathy), and in 92 healthy people recruited in four Italian nephrology units. Anti-phospholipase A2 receptor (PLA2r) and anti-neutral endopeptidase (NEP) IgG(4) were titrated in the same specimens. Association with 1-year follow-up clinical parameters was studied in 120 patients. RESULTS IgG(4) was the most common isotype for all antibodies; IgG(1) and IgG(3) were nearly negligible. IgG(4) levels were positive in a significant proportion of MN patients (AR, 34%; SOD2, 28%; αENO, 43%). Antibody titers were higher in MN than in healthy and pathologic controls (P<0.005). Anti-NEP IgG(4) did not differ from normal controls (P=0.12). Anti-PLA2r IgG(4) was detected in 60% of patients and correlated with anti-AR, anti-SOD2, and anti-αENO IgG(4) (P<0.001). In MN patients negative for the whole antibody panel (20%), 1-year proteinuria was lower compared with patients with at least one antibody positivity (P<0.05). CONCLUSIONS Our data suggest that IgG(4) is the prevalent isotype for antibodies against cytoplasmic antigens of podocytes (AR, SOD2, αENO). Their levels were higher than in other proteinuric glomerulonephritides and in normal controls and were correlated with anti-PLA2r. Only baseline negativity for all known antibodies predicted lower 1-year proteinuria.

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.

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.

Evidence for aerobic metabolism in retinal rod outer segment disks.

The disks of the vertebrate retinal rod Outer Segment (OS), devoid of mitochondria, are the site of visual transduction, a very energy demanding process. In a previous proteomic study we reported the expression of the respiratory chain complexes I-IV and the oxidative phosphorylation Complex V (F(1)F(0)-ATP synthase) in disks. In the present study, the functional localization of these proteins in disks was investigated by biochemical analyses, oxymetry, membrane potential measurements, and confocal laser scanning microscopy. Disk preparations, isolated by Ficoll flotation, were characterized for purity. An oxygen consumption, stimulated by NADH and Succinate and reverted by rotenone, antimycin A and KCN was measured in disks, either in coupled or uncoupled conditions. Rhodamine-123 fluorescence quenching kinetics showed the existence of a proton potential difference across the disk membranes. Citrate synthase activity was assayed and found enriched in disks with respect to ROS. ATP synthesis by disks (0.7 micromol ATP/min/mg), sensitive to the common mitochondrial ATP synthase inhibitors, would largely account for the rod ATP need in the light. Overall, data indicate that an oxidative phosphorylation occurs in rod OS, which do not contain mitochondria, thank to the presence of ectopically located mitochondrial proteins. These findings may provide important new insight into energy production in outer segments via aerobic metabolism and additional information about protein components in OS disk membranes.

2D-electrophoresis and the urine proteome map: Where do we stand?

The discovery of urinary biomarkers is a main topic in clinical medicine. The development of proteomics has rapidly changed the knowledge on urine protein composition and probably will modify it again. Two-dimensional electrophoresis (2D-PAGE) coupled with mass spectrometry has represented for years the technique of choice for the analysis of urine proteins and it is time to draw some conclusions. This review will focus on major methodological aspects related to urine sample collection, storage and analysis by 2D-PAGE and attempt to define an advanced normal urine protein map. Overall, 1118 spots were reproducibly found in normal urine samples but only 275 were characterized as isoforms of 82 proteins. One-hundred height spots belonging to 30 proteins were also detected in plasma and corresponded to typical plasma components. The identity of most of the proteins found in normal urine by 2D-PAGE remains to be determined, the majority being low-molecular weight proteins (<30 kDa). Equalization procedures would also enhance sensitivity of the analysis and allow low abundance proteins to be characterized. Therefore, we are still on the way to define the normal urine composition. Technology advancements in concentrating procedure will improve sensitivity and give the possibility to purify proteins for mass spectrometry.

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

Focal segmental glomerulosclerosis (FSGS) is responsible for intractable proteinuria and has become the leading cause of renal insufficiency in children. Protenuria in FSGS is probably due to the effect of one or more permeability plasma factors which increase the glomerular permeability to proteins. We fractioned serum from children with FSGS using two mixed chromatographic‐electrophoretic approaches and have purified ten proteins among several hundreds which maintained the original permeability activity after renaturation, utilizing an isolated rat glomeruli assay. Six proteins were successfully characterized by mass spectometry as fibulin, apolipoprotein J, vitronectin, albumin isoforms, γ chain fibrinogen and mannan‐binding lectin‐associated serine protease. Both procedures utilized for purification were based on affinity chromatography with Protein A‐Sepharose and ended with two‐dimensional electrophoresis, whereas the intermediate steps were different. Cross inhibition with zinc and aprotinin of purified factors and whole FSGS serum indicate strong homology. These are the first data demonstrating permeability activity for serum proteins, an observation with important implications in pathogenesis of proteinuria. Determination of the serum levels of each protein and a careful differentiation of FSGS from normal serum could provide the basis for clarifying the mechanism of proteinuria.

Proteomic analysis of the retinal rod outer segment disks

The initial events of vision at low light take place in vertebrate retinal rods. The rod outer segment consists of a stack of flattened disks surrounded by the plasma membrane. A list of the proteins that reside in disks has not been achieved yet. We present the first comprehensive proteomic analysis of purified rod disks, obtained by combining the results of two-dimensional gel electrophoresis separation of disk proteins to MALDI-TOF or nLC-ESI-MS/MS mass spectrometry techniques. Intact disks were isolated from bovine retinal rod outer segments by a method that minimizes contamination from inner segment. Out of a total of 187 excised spots, 148 proteins were unambiguously identified. An additional set of 61 proteins (partially overlapping with the previous ones) was generated by one-dimensional (1D) gel nLC-ESI-MS/MS method. Proteins involved in vision as well as in aerobic metabolism were found, among which are the five complexes of oxidative phosphorylation. Results from biochemical, Western blot, and confocal laser scanning microscopy immunochemistry experiments suggest that F 1F o-ATP synthase is located and catalytically active in ROS disk membranes. This study represents a step toward a global physiological characterization of the disk proteome and provides information necessary for future studies on energy supply for phototransduction.