Analysis of Na+-D-glucose cotransporter and other renal brush border proteins in human urine

Significance of urinary full-length megalin in patients with IgA nephropathy

Background and Objectives

Megalin is highly expressed at the apical membranes of proximal tubular epithelial cells. A urinary full-length megalin (C-megalin) assay is linked to the severity of diabetic nephropathy in type 2 diabetes. This study examined the relationship between levels of urinary C-megalin and histological findings in adult patients with IgA nephropathy (IgAN).

Design, Setting, Participants, & Measurements

Urine samples voided in the morning on the day of renal biopsy were obtained from 73 patients with IgAN (29 men and 44 women; mean age, 33 years) and 5 patients with membranous nephropathy (MN). Renal pathologic variables were analyzed using the Oxford classification of IgAN, the Shigematsu classification and the Clinical Guidelines of IgAN in Japan. The levels of urinary C-megalin were measured by sandwich ELISA.

Results

Histological analysis based on the Oxford classification revealed that the levels of urinary C-megalin were correlated with mesangial hypercellularity in IgAN patients (OR = 1.76, 95% CI: 1.04–3.27, P<0.05). There was a significant correlation between the levels of urinary C-megalin and the severity of chronic extracapillary abnormalities according to the Shigematsu classification in IgAN patients (β = 0.33, P = 0.008). The levels of urinary C-megalin were significantly higher in all risk levels of IgAN patients requiring dialysis using the Clinical Guidelines of IgAN in Japan than in the control group. The levels of urinary C-megalin were significantly higher in the high risk and very high risk grades than in the low risk grade (P<0.05). The levels of urinary C-megalin were significantly higher in MN patients compared to the control group.

Conclusions

The levels of urinary C-megalin are associated with histological abnormalities in adult IgAN patients. There is a possibility that urinary C-megalin is an independent predictor of disease progression of IgAN. In addition, our results suggest that urinary C-megalin is a marker of glomerular abnormalities in various glomerular diseases as well as IgAN.

Urinary megalin deficiency implicates abnormal tubular endocytic function in Fanconi syndrome

Normal reabsorption of glomerular filtrate proteins probably requires recycling of the endocytic receptors megalin (gp330) and cubilin. Both receptors are located on the luminal surface of the renal proximal tubule epithelium. Whether abnormal amounts of receptor are present in the urine of patients with Dent's disease, Lowe's syndrome, or autosomal dominant idiopathic Fanconi syndrome was explored. They are all forms of the renal Fanconi syndrome and are associated with tubular proteinuria. Urine samples of equal creatinine contents were dialyzed, lyophilized, and subjected to electrophoresis on nonreducing sodium dodecyl sulfate-5% polyacrylamide gels. Proteins were blotted and probed with anti-megalin IgG, anti-cubilin IgG, or receptor-associated protein. Megalin and cubilin levels detected by immunochemiluminescence were measured as integrated pixels and expressed as percentages of the normal mean values. A striking deficiency of urinary megalin, compared with normal individuals (n = 42), was observed for eight of nine families with Dent's disease (n = 10) and for the two families with Lowe's syndrome (n = 3). The family with autosomal dominant idiopathic Fanconi syndrome (n = 2) exhibited megalin levels within the normal range. The measured levels of cubilin were normal for all patients. These results are consistent with defective recycling of megalin to the apical cell surface of the proximal tubules and thus decreased loss into urine in Dent's disease and Lowe's syndrome. This defect would interfere with the normal endocytic function of megalin, result in losses of potential ligands into the urine, and produce tubular proteinuria.

Immortalized rat proximal tubule cells produce membrane bound and soluble megalin

Megalin (gp330), a glycoprotein receptor found on renal proximal tubule cells and several other epithelial cells, is deduced to be a type I integral membrane protein, but may also exist as a cell surface form lacking a cytoplasmic domain. Furthermore, soluble megalin products have been detected in urine, and in culture medium of a rat yolk sac carcinoma cell line, combined with receptor associated protein (RAP). Permanent renal cell lines expressing megalin were unavailable until the recent description of two immortalized rat proximal tubule cell lines (IRPTC). The present study demonstrated megalin on IRPTC surface by immunofluorescence, without surface staining for RAP, which was, however, readily detected within cells. Antibodies to ectodomain megalin epitopes immunoprecipitated megalin products both from cell lysates and culture medium, whereas antibodies to cytoplasmic domain epitopes precipitated megalin only from lysates. Western blots showed two major megalin products in medium, a prominent band at approximately 200 kDa, and a fainter band above 400 kDa, slightly below intact megalin in cell lysates. Anti-receptor associated protein antibodies immunoprecipitated megalin from IRPTC lysates, but not from media. We propose that portions of megalin are spontaneously produced by IRPTC, probably either by cleavage in the ectodomain or release of forms lacking a cytoplasmic domain.

Clinical significance of urinary human tissue non-specific alkaline phosphatase (hTNAP) in pre-eclampsia and eclampsia

The aim of this work was to determine the levels of urinary human tissue non-specific alkaline phosphatase (hTNAP) in pre-eclampsia and eclampsia in order to assess renal tubular damage. Urine samples were collected from 26 mild pre-eclamptic, 26 were pre-eclamptic, 20 eclamptic patients and 20 healthy pregnant women (controls) in their late third trimester. Urinary hTNAP/creatinine (hTNAP/cr) in severe pre-eclampsia and eclampsia were significantly higher than in controls. Urinary hTNAP/cr was increased in 23%, 77% and 90% of cases of mild pre-eclampsia, severe pre-eclampsia and eclampsia, respectively, indicating that the increase correlates with the severity of the disease. Marked elevation or urinary hTNAP/cr was also associated with bad fetal outcome. These results provide additional evidence for renal tubular damage in pre-eclampsia and eclampsia.

Protein adsorption by artificial membrane materials under filtration conditions

Elevated plasma levels of numerous low molecular weight proteins (LMWP) in renal insufficiency are likely to contribute to the uremic syndrome. Dialysis-related amyloidosis, caused by the accumulation of beta 2-microglobulin (beta 2M), has highlighted the need for a renal replacement therapy that allows the elimination of LMWP in addition to small solutes. Synthetic membrane materials employed under hemofiltration conditions proved to be most effective in lowering elevated beta 2M plasma levels. In addition to convection, protein adsorption to artificial membrane materials is an important mechanism for beta 2M removal. Using an in vitro setup, 12 commercially available hemofilters representing 11 different membrane materials were perfused with human blood containing 125I-labeled plasma proteins. Under filtration conditions, total protein adsorption ranged from 338-2,098 mg/m2 of membrane surface, and the fraction of adsorbed LMWP varied between 14-70% of total protein adsorption and was negatively correlated to total protein adsorption. beta 2M adsorption showed up to an 8-fold difference between membranes, and was negatively correlated with total protein adsorption and positively correlated with the adsorption of LMWPs.

Monoclonal antibodies against luminal membranes of renal proximal tubules which are kidney-specific

After immunization with porcine brush-border membrane proteins, 11 monoclonal antibodies were generated which react with proximal tubules. Their antigenic polypeptides were characterized with respect to apparent molecular weight, histochemical localization in porcine and human kidney, and tissue distribution in pig. In porcine kidney, six antibodies bind selectively to the proximal tubule whereas the others also react with other nephron segments. With the exception of one antibody which reacts with the luminal and the basolateral membrane of the porcine proximal tubule, the other antibodies specific for the proximal tubule only stain the brush-border membrane. Four of them react along the entire length of the porcine proximal tubule, whereas one (R1A2) binds to the S3-segment in pig and to the entire length of the proximal tubule in man. This indicates that segment-specific expression may be species-dependent. Testing the antibodies in 21 different extrarenal tissues it was found that three of the antibodies, specific for the brush-border membrane in renal proximal tubules, only react in kidney. Two of these are specific for pig kidney whereas one also reacts with human kidney. This antibody (N4A4) is directed against a polypeptide with an apparent molecular weight of 400,000. Electron microscopic immunohistochemistry showed that N4A4 binds to the intervillus region of the brush-border membrane and to subapical vesicles.