Direct and indirect tests of pore size and charge selectivity in nephrotic syndrome

Nephrotic Syndrome in South African Children: Changing Perspectives in the New Millennium

The epidemiological landscape of nephrotic syndrome (NS) in South Africa has changed drastically in the New Millennium. Although the pattern of disease in the 3 main non-Black racial groups (White, Indian, and Mixed race) mirror that seen in Western countries, Black African children show a pattern of disease that is at variance with these 3 racial groups. The incidence of infectious diseases, particularly hepatitis B virus associated nephropathy has sharply declined to being almost extinct in Black children in the New Millennium whereas HIV-related nephropathy surfaced. However, following the widespread use of anti-retroviral therapy, its incidence has also decreased dramatically. Focal segmental glomerulosclerosis (FSGS), which was once uncommon, has, in the New Millennium, emerged as one of the most challenging forms of NS across all racial groups, particularly in Black children. Although the introduction of calcineurin inhibitors, mycophenolate mofetil and monoclonal antibodies (e.g., rituximab) has improved the outcome of children with FSGS, the reponse in Black children is less than optimal, with those having single gene mutations being universally unresponsive to all forms of immunosuppression.

Disruption of glomerular basement membrane charge through podocyte-specific mutation of agrin does not alter glomerular permselectivity

Glomerular charge selectivity has been attributed to anionic heparan sulfate proteoglycans (HSPGs) in the glomerular basement membrane (GBM). Agrin is the predominant GBM-HSPG, but evidence that it contributes to the charge barrier is lacking, because newborn agrin-deficient mice die from neuromuscular defects. To study agrin in adult kidney, a new conditional allele was used to generate podocyte-specific knockouts. Mutants were viable and displayed no renal histopathology up to 9 months of age. Perlecan, a HSPG normally confined to the mesangium in mature glomeruli, did not appear in the mutant GBM, which lacked heparan sulfate. Moreover, GBM agrin was found to be derived primarily from podocytes. Polyethyleneimine labeling of fetal kidneys revealed anionic sites along both laminae rarae of the GBM that became most prominent along the subepithelial aspect at maturity; labeling was greatly reduced along the subepithelial aspect in agrin-deficient and conditional knockout mice. Despite this severe charge disruption, the glomerular filtration barrier was not compromised, even when challenged with bovine serum albumin overload. We conclude that agrin is not required for establishment or maintenance of GBM architecture. Although agrin contributes significantly to the anionic charge to the GBM, both it and its charge are not needed for glomerular permselectivity. This calls into question whether charge selectivity is a feature of the GBM.

Increased excretion of urinary glycosaminoglycans in meningococcal septicemia and their relationship to proteinuria

OBJECTIVES

Meningococcal septic shock is a devastating illness associated with an increase in vascular permeability leading to hypovolemia and accumulation of plasma proteins and fluid in tissues. The capillary leak syndrome is often associated with widespread thrombosis in the skin, limbs, and digits. We postulated that the increase in vascular permeability and the intravascular thrombosis might be caused by an inflammation-induced loss of endothelial and basement membrane glycosaminoglycans (GAGs), which play a role in the permeability and thromboresistant properties of the microvasculature.

DESIGN

Prospective, single-center observational study.

SETTING

University-affiliated meningococcal research unit and pediatric intensive care unit.

PATIENTS

Eighteen children requiring intensive care for meningococcal sepsis, 18 children with steroid-responsive nephrotic syndrome, and 18 healthy control children.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Serum concentrations and urine excretion of glycosaminoglycans were measured and related to changes in glomerular permeability to plasma proteins. The size-distribution and nature of glycosaminoglycans were defined by Polyacrylamide Gel Electrophoresis and specific enzyme digestion. Urinary excretion of heparan sulfate, chondroitin-4-sulfate, and chondroitin-6-sulfate were significantly increased in meningococcal disease (MD) relative to healthy controls and children with steroid-responsive nephrotic syndrome. The urinary GAGs in MD were of similar size to those in controls when analyzed after pronase digestion. However, analysis of proteoglycan size before proteolytic digestion showed the urinary GAGs in MD were of lower molecular weight and unattached to proteins. The fractional excretion of albumin and immunoglobulin G in MD increased with severity of disease. Patients with severe or fatal MD had albumin clearances overlapping those seen in steroid-responsive nephrotic syndrome. There was a significant correlation between proteinuria in MD and urinary excretion of heparan sulfate (r2 = 0.611, p < .0001), chondroitin-4-sulfate (r2 = 0.721, p < .0001), and chondroitin-6-sulfate (r2 = 0.395, p < .0001).

CONCLUSIONS

The capillary leak in meningococcal disease is associated with increased plasma and urine concentrations of GAGs, which may be proteolytically cleaved from endothelial and basement membrane sites. The correlation between the severity of protein leakage and the urine excretion of GAGs suggests that loss of GAGs may be causally related to the increase in permeability to proteins.

Urinary loss of immunoglobulin G anti-F(ab)2 and anti-DNA antibody in systemic lupus erythematosus nephritis

The objective of this study was to determine whether the low levels of serum immunoglobulin G (IgG) anti-F(ab)2 seen in some patients with active systemic lupus erythematosus (SLE) were directly related to the deposition of antibody with this specificity in the kidney or alternatively to the urinary loss of IgG anti-F(ab)2. Serum Levels of IgG anti-F(ab)2, anti-tetanus toxoid, and anti-ds DNA antibody were measured in parallel with urinary excretion of these same 3 antibodies in 28 patients with SLE nephritis and in 28 control patients with other forms of chronic kidney disease. Low levels of both serum IgG anti-F(ab)2 or anti-tetanus antibody appeared to correlate with increased levels of urinary loss of these same antibodies in some patients with SLE and in control subjects with kidney disease. However, urinary loss could not account for low serum levels of either IgG antibody in many subjects. Quantitative 24-hour urinary losses of IgG anti-F(ab)2 and anti-DNA were much higher in patients with SLE than in control subjects with kidney disease (P < .05), whereas amounts of IgG urinary loss of anti-tetanus were similar in patients with SLE and in control subjects. In nearly 1 third of SLE nephritis patients, 13% to 53% of total excreted urinary IgG showed anti-DNA enzyme-linked-immunosorbent assay reactivity. Urinary IgG in many patients with SLE showed both anti-DNA and anti-F(ab)2 reactivity, but dual anti-DNA/F(ab)2 specificity was more pronounced in affinity-isolated serum IgG anti-DNA or anti-F(ab)2 than in excreted urinary IgG molecules. The affinity of urinary IgG for either DNA or F(ab)2 was much lower than the same antibody activities measured either in serum or in kidney biopsy eluates. When the relative affinity of anti-DNA antibody in serum, urine, and kidney biopsy eluate was measured in parallel, the highest affinity antibody was found in kidney biopsy eluates, followed by serum antibody with urine antibody affinity showing the lowest values. These findings suggest a relative concentration of the highest affinity, doubly reactive IgG anti-DNA/F(ab)2 in SLE kidney tissues during SLE nephritis and implicate this process as an important factor in ongoing tissue damage.

Effect of plasma from patients with idiopathic nephrotic syndrome on proteoglycan synthesis by human and rat glomerular cells

In vivo and in vitro findings have shown that plasma of patients with idiopathic nephrotic syndrome (INS) contain factors that increase glomerular permeability to proteins. The effects of these factors on proteoglycan synthesis by glomerular cells are unknown. To investigate the effect of plasma from patients with INS (n = 23) and other glomerulopathies (n = 12) on the amount of proteoglycans synthesized by cultured rat mesangial cells and human glomerular epithelial cells, glomerular cells were cultured for 24 h with plasma from patients or control subjects, and incorporation of Na2(35)SO4 in chondroitin dermatan sulfate and heparan sulfate was assessed using a cationic nylon membrane. The mean ratio of glycosaminoglycan produced by rat mesangial cells when in contact with plasma (5%) from INS patients to the amount produced when in contact with control plasma was 0.70+/-0.06. The mean ratio of heparan sulfate was 0.58+/-0.08. The decrease of heparan sulfate production was present in the cellular and in the extracellular fraction. It was observed when the cells were in contact with plasma from patients in relapse but not when in remission. No decrease of heparan sulfate production was observed with four of the five patients with membranous glomerulonephritis (ratio of 1.27+/-0.03), IgA nephropathy (n = 5, ratio of 1.27+/-0.03), and membranoproliferative glomerulonephritis (n = 2, ratio of 1.39+/-0.34). When human glomerular epithelial cells were exposed to 5% plasma from INS patients in relapse (n = 9), the mean ratio of heparan sulfate was 0.62+/-0.06 in the cellular fraction and 0.72+/-0.08 in the medium. When in contact with plasma from patients in remission, no difference of glycosaminoglycan production was observed. A factor present in plasma from patients with INS during initial episodes or relapses is able to decrease the proteoglycan production of glomerular cells.

Comparison of noninvasive methods for distinguishing steroid-sensitive nephrotic syndrome from focal glomerulosclerosis

Although renal biopsy is the definitive investigation in kidney disorders and is particularly helpful in distinguishing steroid-responsive nephrotic syndrome (SRNS) from focal glomerulosclerosis (FGS), it is attended by a small risk to the patient. Accordingly, noninvasive tests have been used to predict the response to steroids and the underlying renal histologic diagnosis in nephrotic syndrome. The performance of these tests has, however, not been encouraging. We have therefore compared the reliability of the conventional selectivity index (SI) of serum and urinary transferrin and immunoglobulin G (IgG) against other tests of urinary proteins, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE), and isoelectric focusing (IEF). SI, SDS PAGE, and IEF were carried out in children with nephrotic syndrome within 2 months of clinical presentation. Thirty-one children who had SRNS were compared with 26 who had biopsy-proved FGS and who were steroid resistant. SDS PAGE and IEF revealed excretion of albumin and transferrin only, with homogeneous anionic charge, respectively, in SRNS but unrestricted excretion of additional proteins IgG, beta2-microglobulin, and lysozyme with heterogeneity of electrical charge in FGS. With SDS PAGE and IEF we were able to predict all children who had SRNS and FGS; the SI test predicted all steroid-resistant patients with FGS but was able to predict only 41.7% of the patients with SRNS. Therefore the negative predictive value for steroid response was 58.8% by SI and 100% by SDS PAGE and IEF; the positive predictive value was 100% by SI, SDS PAGE, and IEF. We illustrate the value of SDS PAGE in guiding management in a further seven children with FGS in whom there was either an initial discordance between renal biopsy results and steroid responsiveness or when biopsy was delayed. Accordingly, SDS PAGE and IEF of urinary proteins appear to be useful tests in the diagnosis and management of SRNS and FGS.