Endogenous albumin as a marker of renal selectivity in steroid-unresponsive nephrotic syndrome

Albumin heterogeneity in low-abundance fluids. The case of urine and cerebro-spinal fluid

BACKGROUND

Serum albumin is a micro-heterogeneous protein composed of at least 40 isoforms. Its heterogeneity is even more pronounced in biological fluids other than serum, the major being urine and cerebrospinal fluid. Modification 'in situ' and/or selectivity of biological barriers, such as in the kidney, determines the final composition of albumin and may help in definition of inflammatory states.

SCOPE OF REVIEW

This review focuses on various aspects of albumin heterogeneity in low 'abundance fluids' and highlights the potential source of information in diseases.

MAJOR CONCLUSIONS

The electrical charge of the protein in urine and CSF is modified but with an opposite change and depending on clinical conditions. In normal urine, the bulk of albumin is more anionic than in serum for the presence of ten times more fatty acids that introduce equivalent anionic charges and modify hydrophobicity of the protein. At the same time, urinary albumin is more glycosylated compared to the serum homolog. Finally, albumin fragments can be detected in urine in patients with proteinuria. For albumin in CSF, we lack information relative to normal conditions since ethical problems do not allow normal CSF to be studied. In multiple sclerosis, the albumin charge in CSF is more cationic than in serum, this change possibly involving structural anomalies or small molecules bindings.

GENERAL SIGNIFICANCE

Massively fatty albumin could be toxic for tubular cells and be eliminated on this basis. Renal handling of glycosylated albumin can alter the normal equilibrium of filtration/reabsorption and trigger mechanisms leading to glomerulosclerosis and tubulo-interstitial fibrosis. This article is part of a Special Issue entitled Serum Albumin.

Repetitive fragmentation products of albumin and alpha1-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.

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.

Clinical analysis of human urinary proteins using high resolution electrophoretic methods

The application of isoelectric focusing (IEF), sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional electrophoresis (2-DE) and capillary electrophoresis (CE) for high resolution electrophoretic analysis of human urinary proteins is reviewed. In each case, the information is tabulated chronologically with details of sample preparation, electrophoretic system, detection method and clinical application. The text includes an historical perspective of the use of each method for urinalysis and a detailed review of the application of the methods to the investigation of renal disease, renal transplantation, Bence Jones proteinuria (BJP), diabetes mellitus, cadmium toxicity, nephrolithiasis and cancers of the urogenital tract.

Characterization of proteinuria in primary glomerulonephritides: urinary polymers of albumin

In 142 patients with primary glomerulonephritis (GN), there were polymers of albumin (PAs) in the urine samples of 87% of 15 minimal-change disease (MCD) patients, 52% of 27 focal segmental glomerulosclerosis (FSGS) patients, 51% of 47 membranous glomerulonephritis (MGN) patients, 55% of 20 membranoproliferative glomerulonephritis (MPGN) patients, and 9% of 33 immunoglobulin A nephropathy (IgAN) patients (P = 0.000). In IgAN, only three patients with nephrotic syndrome were PA positive. The PAs were significantly correlated with nephrotic syndrome (NS) (P = 0.000) and with the degree of proteinuria, ranging from 8% in patients with proteinuria less than 0.5 g/d to 58% in patients with proteinuria > or = 15.0 g/d (P = 0.001), but 40% of the nephrotic syndrome patients were PA-negative despite values of proteinuria comparable to those of PA-positive patients, suggesting that the presence of PAs is not simply related to protein loss, but probably to some other unidentified factor or lesion. For 72 patients (43 with NS) (22 FSGS, 36 MGN, and 14 MPGN patients) with normal renal function at entry (serum creatinine, 1.02 +/- 0.23 mg/dL) and a mean follow-up duration of 52 +/- 27 months, for whom PAs were determined and urinary protein characterized by sodium-dodecyl-sulphate polyacrylamide gel electrophoresis (SDS-PAGE) at the beginning of the follow-up period, the functional outcome was correlated with the patterns of proteinuria. Chronic renal failure (CRF) developed in 24% of all 72 patients, in 36% of the PA-positive patients, in 9% of the PA-negative patients (P = 0.007), in 44% of the SDS-PAGE 10-kd mixed glomerulotubular pattern patients, and in 17% of the SDS-PAGE 23-kd mixed-pattern patients (P = 0.001). The association of PAs with the 10-kd pattern enhanced the predictive value for CRF outcome: CRF developed in 62% of the PA-positive patients with the 10-kd pattern compared with 11% of the PA-negative patients with the 23-kd pattern (P = 0.0001). CRF developed in 32% of 43 patients with the nephrotic syndrome, in 48% of the PA-positive patients, and in 11% of the PA-negative patients (P = 0.037); in 50% of the 10-kd patients and in 24% of the 23-kd patients (P = 0.007); and in 70% of the PA-positive patients with the 10-kd pattern and 14% of the PA-negative patients with the 23-kd pattern (P = 0.001). In a retrospective study of 21 treated patients (11 FSGS, nine MGN, and one MPGN patient), a response to therapy with complete or partial remission was observed in 57% of all 21 patients; in 58% of patients with the nephrotic syndrome; in 88% of the PA-negative patients versus 38% of the PA-positive patients (P = 0.027); in 90% of the 23-kd patients versus 27% of the 10-kd patients (P = 0.004); and in 100% of the PA-negative patients with the 23-kd pattern versus 12% of the PA-positive patients with the 10-kd pattern (P = 0.001). In conclusion, urinary PAs are associated with GN characterized by lesions mainly localized in the glomerular capillary wall, with the presence of the nephrotic syndrome, and with the degree of proteinuria. In patients with FSGS, MGN, MPGN, and normal renal function at entry, the presence of polymers has a predictive value for CRF outcome; this value is enhanced by the contemporaneous presence of an SDS-PAGE proteinuric pattern with low molecular weight proteins up to 10-kd, which is known to be associated with diffuse tubulointerstitial lesions. Therefore, the best predictive value for either CRF outcome or for response to therapy was provided by a combination between a marker associated with the degree of proteinuria and the types of GN characterized by lesions mainly localized in the glomerular capillary wall and a marker associated with tubulointerstitial damage (SDS-PAGE mixed glomerulotubular pattern with low molecular weight proteins between 20 and 10 kd).

Immunocytochemical study of glomerular permeability to anionic, neutral and cationic albumins

The renal handling of albumin of various isoelectric points (pI) was studied in mice by high resolution quantitative immunocytochemistry. Bovine serum albumin (BSA) was tagged with dinitrophenol (DNP) and cationized to pI 6.5 to 7.5 and to pI over 8.5. These tracers, including the anionic BSA-DNP (pI 4.8) were injected in the iliac vein of mice and the post-embedding protein A-gold technique was used with antibodies against DNP to localize the different tracers in renal tissue. Morphometric analysis of the labeling over the glomerular wall has demonstrated variations in its distribution according to the pI of the markers. Anionic and neutral BSA molecules were detected on the endothelial side of the basement membrane, while labeling for cationic BSA was present mainly in the laminae rarae interna and externa known to display fixed polyanionic sites. Neutral and cationic BSA-DNP reached the urinary space and were detected within the endocytotic apparatus of the proximal tubule epithelium. Neutralization of the anionic sites of the basement membrane with cationic BSA resulted in an increase in the permeability towards anionic BSA-DNP. In addition to the demonstration of glomerular permeability properties towards various probes, the present study has demonstrated that dinitrophenylated albumin represents an excellent versatile tool for the quantitative morphological investigation of glomerular permeability.