Membranous Nephropathy: A Long Road but Well Traveled

Decay-Accelerating Factor Restrains Complement Activation and Delays Progression of Murine cBSA-Induced Membranous Nephropathy

Key Points

In a murine model of cationic bovine serum albumin (cBSA)–induced membranous nephropathy (MN), complement regulator decay-accelerating factor is upregulated and restrains complement activation. Studies using genetic deletion or pharmacological antagonism of C3aR indicate that the main effector mechanism of complement activation in cBSA-induced MN is C3a/C3aR signaling. C3a formation and/or C3aR-mediated signaling represent promising targets for hypothesis-driven therapies for MN.

Background

Complement activation is believed to play a major pathogenic role in membranous nephropathy (MN), but its effector mechanisms are still unclear. Even less investigated is the role of podocyte-expressed complement regulators, including decay-accelerating factor (DAF) in disease pathophysiology.

Methods

We induced MN by serial injections of cationic bovine serum albumin (cBSA) in WT, DAF−/−, and C3aR−/− BALB/c mice and measured disease severity (by albuminuria, BUN, serum albumin, and glomerular histologic changes) and signs of complement activation in the glomeruli (immunofluorescence for C1q, C3b, and membrane attack complex). We also treated DAF−/− mice with cBSA-induced MN with a selective C3aR antagonist and measured the same readouts.

Results

cBSA-induced MN was associated with increased glomerular expression of DAF. Genetic deletion of DAF resulted in increased complement activation and higher disease severity than in WT animals. Treating cBSA-injected DAF−/− mice with a C3aR antagonist reduced disease severity. Similarly, C3aR−/− animals were protected from cBSA-induced MN, despite IgG deposition in the glomeruli and complement activation. Evidence of C1q and C3b deposition in the glomeruli of these mice suggest that IgG-cBSA immune complex formation in the glomeruli activates complement through the classical pathway.

Conclusions

On cBSA-induced injury, podocytes upregulate DAF expression, which restrains complement activation. However, after prolonged injury, complement activation overcomes DAF regulatory effects leading to the formation of soluble anaphylatoxin C3a that, by signaling through C3aR, promotes glomerular injury and cBSA-induced MN disease progression. Considering the growing number of complement targeting therapies, our findings may have major translational effect on the treatment of patients with MN.

Membranous nephropathy

Membranous nephropathy is a glomerulopathy, which main affected target is the podocyte, and has consequences on the glomerular basement membrane. It is more common in adults, especially over 50 years of age. The clinical presentation is nephrotic syndrome, but many cases can evolve with asymptomatic non-nephrotic proteinuria. The mechanism consists of the deposition of immune complexes in the subepithelial space of the glomerular capillary loop with subsequent activation of the complement system. Great advances in the identification of potential target antigens have occurred in the last twenty years, and the main one is the protein "M-type phospholipase-A2 receptor" (PLA2R) with the circulating anti-PLA2R antibody, which makes it possible to evaluate the activity and prognosis of this nephropathy. This route of injury corresponds to approximately 70% to 80% of cases of membranous nephropathy characterized as primary. In the last 10 years, several other potential target antigens have been identified. This review proposes to present clinical, etiopathogenic and therapeutic aspects of membranous nephropathy in a didactic manner, including cases that occur during kidney transplantation.

How Times Have Changed! A Cornucopia of Antigens for Membranous Nephropathy

The identification of the major target antigen phospholipase A2 receptor (PLA2R) in the majority of primary (idiopathic) cases of membranous nephropathy (MN) has been followed by the rapid identification of numerous minor antigens that appear to define phenotypically distinct forms of disease. This article serves to review all the known antigens that have been shown to localize to subepithelial deposits in MN, as well as the distinctive characteristics associated with each subtype of MN. We will also shed light on the novel proteomic approaches that have allowed identification of the most recent antigens. The paradigm of an antigen normally expressed on the podocyte cell surface leading to in-situ immune complex formation, complement activation, and subsequent podocyte injury will be discussed and challenged in light of the current repertoire of multiple MN antigens. Since disease phenotypes associated with each individual target antigens can often blur the distinction between primary and secondary disease, we encourage the use of antigen-based classification of membranous nephropathy.

Roles of microRNAs in renal disorders related to primary podocyte dysfunction

Through the regulation of gene expression, microRNAs (miRNAs) are capable of modulating vital biological processes, such as proliferation, differentiation, and apoptosis. Several mechanisms control the function of miRNAs, including translational inhibition and targeted miRNA degradation. Through utilizing high-throughput screening methods, such as small RNA sequencing and microarray, alterations in miRNA expression of kidneys have recently been observed both in rodent models and humans throughout the development of chronic kidney disease (CKD) and acute kidney injury (AKI). The levels of miRNAs in urine supernatant, sediment, and exosomal fraction could predict novel biomarker candidates in different diseases of kidneys, including IgA nephropathy, lupus nephritis, and diabetic nephropathy. The therapeutic potential of administrating anti-miRNAs and miRNAs has also been reported recently. The present study is aimed at reviewing the state-of-the-art research with regards to miRNAs involved in renal disorders related to primary podocyte dysfunction by laying particular emphasis on Focal Segmental Glomerulosclerosis (FSGS), Minimal Change Disease (MCD) and Membranous Nephropathy (MN).

Application of miR-193a/WT1/PODXL axis to estimate risk and prognosis of idiopathic membranous nephropathy

Background: This investigation was managed to explore whether miR-193a in combination with two podocytes, namely, Wilms tumor type 1 (WT1) and podocalyxin (PODXL), were feasible in estimating onset and prognosis of idiopathic membranous nephropathy (IMN). Methods: We recruited a total of 189 healthy controls and 364 IMN patients, whose urine samples were prepared to measure the expression of miR-193a and PODXL. Meanwhile, renal tissues collected from above-mentioned IMN patients (n = 364) and renal cell carcinoma patients (n = 189) were arranged to determine the expression of WT1. Ultimately, receiver operating characteristic curves were constructed to appraise the performance of miR-193a, WT1, and PODXL in predicting renal survival of IMN patients. Results: The IMN patients were measured with up-regulated miR-193a expression and down-regulated WT1/PODXL expression, when compared with healthy controls (p < 0.05). Moreover, highly expressed miR-193a, lowly expressed WT1/PODXL, elevated amounts of proteinuria (>3.79 g/24 h)/serum creatinine (>174.63 μmol/L), and declined GFR (≤68.13 mL/min/1.73 m²) were implicated as prominent biomarkers for the poor renal survival of IMN patients (all p < 0.05). Notably, miR-193a combined with PODXL and WT1 generated optimal effects in differentiating IMN patients from healthy controls (AUC = 0.994) and also in anticipating the renal survival state of IMN patients (AUC = 0.824), when compared with strategies that merely employed ≤2 of the biomarkers. Conclusion: The combination of miR-193a, WT1, and PODXL might serve as a favorable strategy for expecting IMN prognosis.

miR-217 Is a Useful Diagnostic Biomarker and Regulates Human Podocyte Cells Apoptosis via Targeting TNFSF11 in Membranous Nephropathy

Background

MicroRNAs have recently been verified as useful diagnostic biomarkers in various diseases. In this study, we investigated whether miR-217 is a useful diagnostic biomarker and the possible pathological mechanism of miR-217 in this disease.

Methods

Patients with focal segmental glomerulosclerosis (FSGS), membranous nephropathy (MN), and diabetic nephropathy (DN) and control patients were enrolled in this study. The miR-217 inhibitor and mimics were transfected into human podocyte cells to investigate the pathological mechanism of miR-217 in this disease. Relevant indicators were detected and tested.

Results

Compared with control patients, miR-217 was significantly downregulated and TNFSF11 was significantly upregulated in MN. Then, miR-217 had obvious separation between patients with MN and control patients, with an AUC of 0.941, a cutoff value of <750.0 copies/ul, and sensitivity and specificity of 88.9% and 75.9%. In addition, the TNFSF11 was confirmed to be the target gene of miR-217. Finally, in in vitro experiments, the upregulation of miR-217 could decrease the expression of TNFSF11 and not induce human podocyte cells apoptosis; however, the downregulation of miR-217 could bring about an opposite change.

Conclusions

miR-217 is a useful diagnostic biomarker and is involved in human podocyte cells apoptosis via targeting TNFSF11 in membranous nephropathy.

Integrated profiling of microRNA expression in membranous nephropathy using high-throughput sequencing technology

The present study analyzed microRNA (miRNA) expression profiles in peripheral blood lymphocyte cells (PBLCs) from patients with membranous nephropathy (MN) and normal controls (NC), in an effort to improve the understanding of the pathogenesis of MN. High-throughput sequencing was performed on 30 MN patients and 30 healthy individuals (NC group). Known and novel miRNAs were analyzed and the results were confirmed by quantitative reverse transcription PCR (qRT-PCR). In total, 326 miRNAs showed a significant difference in expression between the MN and NC groups. This included 286 downregulated miRNAs and 40 upregulated miRNAs. In addition, there were 6 novel miRNAs that presented differential levels of expression between the MN and NC groups. The miRNAs were mapped to the genome, using a short oligonucleotide alignment program (SOAP), to analyze their expression and distribution. Twenty-five percent of the unique miRNAs in the MN group and 52.1% in the NC group were mapped to the genome. One hundred and eight mismatches were identified. Seventy-seven mismatches were detected in a higher proportion of the MN samples, compared with the NC samples. Twenty-five mismatches were detected in a higher proportion of the NC samples than the MN samples. Differential miRNA expression was also detected between 10 randomly selected pair groups, as depicted in a cluster analysis diagram. These data indicate that differential miRNA expression may be involved in the pathogenesis of MN. In addition, the discrepancies between the MN and NC groups, in the mismatched miRNAs that were mapped to the genome, strongly suggest that miRNAs play an important role in the pathogenesis of human disorders. miRNAs may provide a potential breakthrough in the research of MN and may provide a novel biomarker for the diagnosis and treatment of the disease.

Lupus nephritis: role of antinucleosome autoantibodies

The discovery of autoantigen clustering in blebs at the surface of apoptotic cells boosted research on the role of apoptosis in systemic lupus erythematosus (SLE) and led to the discovery of autoantigen modification during apoptosis. Normally, apoptotic cells are cleared efficiently and swiftly. However, it became clear that in SLE insufficient removal of apoptotic material leads to the release of these modified autoantigens. This creates the danger that these modified autoantigens are recognized by the immune system. Indeed, dendritic cells, the professional antigen-presenting cells, phagocytose these modified autoantigens, which leads to maturation and induction of a proinflammatory state of these dendritic cells. As a consequence, they present these modified autoantigens to T cells in an immunogenic way, which become activated and stimulate autoreactive B cells to secrete autoantibodies. In this review the currently available evidence for the sequential steps in the pathogenesis of SLE is discussed. Furthermore, the mechanisms responsible for the nephritogenicity of antinucleosome antibodies are reviewed. This will reveal that nucleosomes are not only a major driving force in the formation of antinuclear antibodies, but also play a pivotal role in the development of tissue lesions by mediating binding of autoantibodies to basement membranes as exemplified for the kidney.

ISN Forefronts Symposium 2010 in Sylt, Germany: 'Induction and Resolution of Renal Inflammation'

The International Society of Nephrology (ISN) Forefronts Symposium 'Induction and Resolution of Renal Inflammation' took place in May 2010 on the Island of Sylt, Germany. The program included basic and clinical aspects of inflammation with a special focus on human and experimental glomerulonephritis. Distinguished scientists from different fields of inflammation research reported their recent discoveries and discussed emerging topics including the role of resolution for inflammatory processes; the 'new and old' cellular players of innate immunity and their mediators; the fundamental role of T-cell subtypes and chemokines; new aspects of B cell-mediated immune responses; and finally the potential implication of results from basic science for human inflammatory renal disease.

Association between genetic polymorphisms of the NPHS1 gene and membranous glomerulonephritis in the Taiwanese population

BACKGROUND

Membranous glomerulonephritis (MGN) is one of the most common causes of nephrotic syndrome in adults. NPHS1 encoding nephrin is a transmembrane protein of the immunoglobulin family. We clarified the relationship between NPHS1 gene polymorphisms and the susceptibility or progression of MGN.

METHODS

We recruited a cohort of 132 biopsy-diagnosed MGN patients and 257 healthy subjects. Genotyping of three SNPs (rs401824, rs437168 and rs3814995) at chromosome positions 41034749 (5'UTR), 41026259(exon17) and 41034052 (exon 3) was performed using a Taqman SNP genotyping assay.

RESULTS

There was a significant difference in genotype frequency distribution of rs437168 polymorphism between MGN patients and controls. The results also showed that the frequency of the G allele was significantly higher in the patient group. Among the polymorphisms rs437168, rs401824 and rs3814995, no significant haplotype was shown in MGN patients. A stratified analysis revealed that a high disease progression in the AA genotype of rs401824 and GG genotype of rs437168 patients were associated with a low rate of remission.

CONCLUSIONS

The presence of the different genotypes of NPHS1 was associated with susceptibility of MGN and the remission of proteinuria during disease progression after the therapy.

Effects of rituximab on morphofunctional abnormalities of membranous glomerulopathy

BACKGROUND AND OBJECTIVES

In idiopathic membranous nephropathy (IMN), CD(20) B-cell depletion by rituximab may induce nephrotic syndrome (NS) remission. Whether this is associated with kidney function restoration and regression of the glomerular pathology was evaluated.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Treatment-induced morphofunctional changes were evaluated in 7 IMN patients consenting to repeat functional and morphologic evaluations after stable disease remission achieved by four weekly rituximab (375 mg/m(2)) infusions.

RESULTS

Over a median of 21 mo from rituximab administration, NS remission was associated with 8.5-fold increase versus baseline in sodium fractional clearance from 1.56 to 13.25, decrease in renal plasma flow from 440.8 to 276.6 ml/min per 1.73 m(2), stable glomerular filtration rate, and increased renal vascular resistances. Changes in sodium fractional clearance and hemoglobin concentration were positively correlated (r = 0.82). Biopsy reevaluations showed complete or partial reabsorption of subepithelial deposits. Median (interquartile range) IgG4 staining score decreased from 3 (3-3) to 1 (0-2), whereas total numbers of slit diaphragms (0.27; range, 0.19 to 0.30 versus 0.86; range, 0.53 to 1.16 slits/mum glomerular basement membrane) and percentages of those with electron-dense diaphragm (55.2; range, 42.0 to 62.0 versus 78.5; range, 73.0 to 82.7 of all slits) significantly increased in parallel with amelioration of glomerular ultrastructural changes. Changes in slit frequency and albumin fractional clearance were negatively correlated (r = -0.79).

CONCLUSIONS

In human IMN, treatment-induced NS remission is associated with restoration of sodium homeostasis and kidney hemodynamics, and regression of the glomerular changes underlying proteinuria. These effects are likely to translate into long-term renoprotection.

A new mouse model of immune-mediated podocyte injury

Podocytes play a major role in the initiation and progression of glomerular diseases and are a target of both immune-mediated and non-immune-mediated injury. To establish a mouse model of such injury, we preimmunized mice with Freunds adjuvant 5 days before intravenous injection of a rabbit polyclonal antibody directed against a murine podocyte cell line. For the next 7 weeks, we collected urine, serum, and kidney samples. Nephritic animals developed severe albuminuria, which was maximal on day 10. Histochemistry revealed diffuse mesangial matrix expansion. Mouse immunoglobulin G and complement were detected in a linear pattern along the glomerular filtration barrier and in the mesangial hinge region. Complement depletion, however, did not prevent proteinuria. Glomerular T cells were increased, whereas podocytes were significantly reduced. Glomerular foot processes were flattened in regions with mesangial matrix deposition as viewed by electron microscopy. Immunohistochemistry detected the injected anti-podocyte antibody exclusively at the glomerular tuft on all days examined. Immunoelectron microscopy localized the antibody to podocyte foot processes and the glomerular basement membrane, which was morphologically intact. This suggests that the podocyte was the main target of the antiserum. Our study establishes a new mouse model of immune-mediated podocyte injury.