David J. Salant

Membranous nephropathy: recent travels and new roads ahead

Insights from experimental studies have been recently translated into substantial advances in understanding the pathogenesis of human membranous nephropathy (MN). These include identification of neutral endopeptidase (NEP) as the target antigen in alloimmune MN resulting from fetomaternal immunization in NEP-deficient mothers, and our demonstration that a high proportion of patients with idiopathic MN (IMN) have circulating antibodies to the M-type phospholipase A2 receptor (PLA2R), a transmembrane protein located on podocytes. Here we highlight the studies that led to these discoveries and our current knowledge about the possible role of anti-PLA2R autoantibodies in the pathogenesis of IMN. Given that the sensitivity and specificity of anti-PLA2R for IMN are >75 and 100%, respectively, we foresee that a widely available assay for anti-PLA2R will prove to be valuable for diagnosing IMN, distinguishing it from secondary MN, and evaluating response to therapy. We suggest reasons why 25% of patients with IMN have tested negative for anti-PLA2R, and propose possible explanations for the presence of complement deposits in IMN despite the fact that immunoglobulin G4 (IgG4), the predominant anti-PLA2R IgG subclass, is incapable of activating the classical complement pathway. Finally, we point out avenues to be explored, including the events that induce production of anti-PLA2R, their ability to cause podocyte injury, the role of complement, and the nature of the antibodies in secondary forms of MN.

Role of MCP‐1 and RANTES in inflammation and progression to fibrosis during murine crescentic nephritis

The involvement of chemokines in inflammation is well established but their functional role in disease progression, and particularly in the development of fibrosis, is not yet understood. We have investigated the functional role that the chemokines monocyte chemotactic protein‐1 (MCP‐1) and RANTES play in inflammation and the progression to fibrosis during crescentic nephritis. During this disease inflammatory infiltrates are observed within glomeruli and interstitium in conjunction with increased expression of MCP‐1 and RANTES and a decrease in renal function. Disease progression is marked by formation of glomerular crescents and the deposition of type I collagen. Blocking the function of MCP‐1 or RANTES resulted in significant decreases in proteinuria as well as numbers of infiltrating leukocytes, indicating that both MCP‐1 and RANTES play an important role in the inflammatory phase of crescentic nephritis. In particular, neutralization of MCP‐1, but not RANTES, resulted in a dramatic decrease in glomerular crescent formation and deposition of type I collagen. These results highlight a novel role for MCP‐1 in crescent formation and development of interstitial fibrosis and indicate that in addition to recruiting inflammatory cells this chemokine is critically involved in irreversible tissue damage. J. Leukoc. Biol.62: 676–680; 1997.

Lysophosphatidic acid and renal fibrosis.

The development of fibrosis involves a multitude of events and molecules. Until now the majority of these molecules were found to be proteins or peptides. But recent data show significant involvement of the phospholipid lysophosphatidic acid (LPA) in the development of pulmonary, liver and renal fibrosis. The latest data on the role of LPA and the G-protein-coupled LPA1 receptor in the development of renal fibrosis will be discussed. LPA1-receptor activation was found to be associated with increased vascular leakage and increased fibroblast recruitment in pulmonary fibrosis. Furthermore, in renal fibrosis LPA1-receptor activation stimulates macrophage recruitment and connective tissue growth factor expression. The observations make this receptor an interesting alternative and new therapeutic target in fibrotic diseases.

Inhibition of Histone Deacetylase Activates Side Population Cells in Kidney and Partially Reverses Chronic Renal Injury

Bone morphogenic protein (BMP)‐7 is expressed in the adult kidney and reverses chronic renal injury when given exogenously. Here, we report that a histone deacetylase inhibitor, trichostatin A (TSA), attenuates chronic renal injury, in part, by augmenting the expression of BMP‐7 in kidney side population (SP) cells. We induced accelerated nephrotoxic serum nephritis (NTN) in C57BL/6 mice and treated them with TSA for 3 weeks. Compared with vehicle‐treated NTN mice, treatment with TSA prevented the progression of proteinuria, glomerulosclerosis, interstitial fibrosis, and loss of kidney SP cells. Basal gene expression of renoprotective factors such as BMP‐7, vascular endothelial growth factor, and hepatocyte growth factor was significantly higher in kidney SP cells as compared with non‐SP cells. Treatment with TSA significantly upregulated the expression of BMP‐7 in SP cells but not in non‐SP cells. Moreover, initiation of treatment with TSA after 3 weeks of NTN (for 3 weeks, until 6 weeks) partially but significantly reversed renal dysfunction. Our results indicate an important role of SP cells in the kidney as one of the possible generator cells of BMP‐7 and TSA as a stimulator of the cells in reversing chronic renal disease.

Membranous nephropathy associated with IgG4-related disease

Immunoglobulin G4 (IgG4)-related systemic disease is a rare condition characterized by high levels of circulating IgG4 and IgG4-positive plasma cell infiltrates in various organs, including the pancreas, salivary glands, biliary tract, liver, lung, and kidney. We describe a case of a 54-year-old man with IgG4-related systemic disease presenting with autoimmune pancreatitis and Mikulicz disease. Steroid therapy decreased circulating IgG4 levels and promoted regression of clinical signs. Thereafter, an increase in serum IgG4 values was followed by the occurrence of nephrotic-range proteinuria. Kidney biopsy showed membranous nephropathy with no IgG4-positive cell infiltrates. A search for circulating immune complexes was negative, and antibodies against M-type phospholipase A(2) receptor could not be detected. Western blot analyses identified circulating IgG3 reacting with superoxide dismutase 2. This case suggests that membranous nephropathy represents an additional renal manifestation of IgG4-related systemic disease, with a pathogenesis possibly associated with neoproduction of autoantibodies targeting podocyte antigen(s).

Respective Roles of Decay-Accelerating Factor and CD59 in Circumventing Glomerular Injury in Acute Nephrotoxic Serum Nephritis

Decay-accelerating factor (DAF or CD55) and CD59 are regulators that protect self cells from C3b deposition and C5b-9 assembly on their surfaces. Their relative roles in protecting glomeruli in immune-mediated renal diseases in vivo are unknown. We induced nephrotoxic serum (NTS) nephritis in Daf1−/−, CD59a−/−, Daf1−/−CD59a−/−, and wild-type (WT) mice by administering NTS IgG. After 18 h, we assessed proteinuria, and performed histological, immunohistochemical, and electron microscopic analyses of kidneys. Twenty-four mice in each group were studied. Baseline albuminuria in the Daf1−/−, CD59a−/−, and Daf1−/−CD59a−/− mice was 82, 83, and 139 as compared with 92 μg/mg creatinine in the WT controls (p > 0.1). After NTS, albuminuria in CD59a−/− and WT mice (186 ± 154 and 183 ± 137 μg/mg creatinine, p > 0.1) was similar. In contrast, Daf1−/− mice developed severe albuminuria (378 ± 520, p < 0.05) that was further exacerbated in Daf1−/−CD59a−/− mice (577 ± 785 μg/mg creatinine, p < 0.05). Glomerular histology showed essentially no infiltrating leukocytes in any group. In contrast, electron microscopy revealed prominent podocyte foot process effacement in Daf1−/− mice with more widespread and severe damage in the double knockouts compared with only mild focal changes in CD59a−/− or WT mice. In all animals, deposition of administered (sheep) NTS Ig was equivalent. This contrasted with marked deposition of both C3 and C9 in Daf1−/−CD59a−/− and Daf1−/− mice, which was evident as early as 2 h post-NTS injection. The results support the proposition that in autoantibody-mediated nephritis, DAF serves as the primary barrier to classical pathway-mediated injury, while CD59 limits consequent C5b-9-mediated cell damage.

Inhibitory Effects of Robo2 on Nephrin: A Crosstalk between Positive and Negative Signals Regulating Podocyte Structure

Robo2 is the cell surface receptor for the repulsive guidance cue Slit and is involved in axon guidance and neuronal migration. Nephrin is a podocyte slit-diaphragm protein that functions in the kidney glomerular filtration barrier. Here, we report that Robo2 is expressed at the basal surface of mouse podocytes and colocalizes with nephrin. Biochemical studies indicate that Robo2 forms a complex with nephrin in the kidney through adaptor protein Nck. In contrast to the role of nephrin that promotes actinxa0polymerization, Slit2-Robo2 signaling inhibits nephrin-induced actin polymerization. In addition, the amount of F-actin associated with nephrin is increased in Robo2 knockout mice that develop an altered podocyte foot process structure. Genetic interaction study further reveals that loss of Robo2 alleviates the abnormal podocyte structural phenotype in nephrin null mice. These results suggest that Robo2 signaling acts as a negative regulator on nephrin to influence podocyte foot process architecture.

Membranous Nephropathy: A Journey From Bench to Bedside.

Lessons from an animal model that faithfully resembles human membranous nephropathy (MN) have informed our understanding of the pathogenesis of this organ-specific autoimmune disease and common cause of nephrotic syndrome. After it was established that the subepithelial immune deposits that characterize experimental MN form in situ when circulating antibodies bind to an intrinsic podocyte antigen, it was merely a matter of time before the human antigen was identified. The M-type phospholipase A2 receptor 1 (PLA2R) represents the major target antigen in primary MN, and thrombospondin type 1 domain-containing 7A (THSD7A) was more recently identified as a minor antigen. Serologic tests for anti-PLA2R and kidney biopsy specimen staining for PLA2R show >90% specificity and 70% to 80% sensitivity for the diagnosis of primary MN in most populations. The assays distinguish most cases of primary MN from MN associated with other systemic diseases, and sequential anti-PLA2R titers are useful to monitor treatment response. A positive pretransplantation test result for anti-PLA2R is also helpful for predicting the risk for posttransplantation recurrence. Identification of target epitopes within PLA2R and the genetic association of primary MN with class II major histocompatibility and PLA2R1 variants are 2 additional examples of our evolving understanding of thisxa0disease.

Anuric Renal Failure Precipitated by Indomethacin and Triamterene

We report a patient with compensated congestive heart failure who developed acute anuric renal failure immediately after indomethacin and triamterene had been added to the treatment regimen. Renal function failed to improve promptly with discontinuation of these medications, anuria persisting for 11 days. While it is well known that patients suffering from edematous states are prone to develop renal insufficiency when given nonsteroidal anti-inflammatory agents, it is not generally appreciated that the specific combination of prostaglandin inhibitors with triamterene may carry a particularly high risk of acute renal failure, even in euvolemic subjects.

Genetic Variants in Membranous Nephropathy: Perhaps a Perfect Storm Rather than a Straightforward Conformeropathy?

Primary (or idiopathic) membranous nephropathy is an organ-specific autoimmune disease in which circulating antibodies to a conformation-dependent epitope in the target antigen, the M-type receptor for phospholipase A2 (PLA2R), are detectable in up to 80% of patients from various ethnic groups.[1][1