Complement mediates nephrin redistribution and actin dissociation in experimental membranous nephropathy11See Editorial by Quigg, p. 2318

The role of complement in kidney disease: conclusions from a Kidney Disease: Improving Global Outcomes (KDIGO) Controversies Conference

Uncontrolled complement activation can cause or contribute to glomerular injury in multiple kidney diseases. Although complement activation plays a causal role in atypical hemolytic uremic syndrome and C3 glomerulopathy, over the past decade, a rapidly accumulating body of evidence has shown a role for complement activation in multiple other kidney diseases, including diabetic nephropathy and several glomerulonephritides. The number of available complement inhibitor therapies has also increased during the same period. In 2022, Kidney Diseases: Improving Global Outcomes (KDIGO) convened a Controversies Conference, "The Role of Complement in Kidney Disease," to address the expanding role of complement dysregulation in the pathophysiology, diagnosis, and management of various glomerular diseases, diabetic nephropathy, and other forms of hemolytic uremic syndrome. Conference participants reviewed the evidence for complement playing a primary causal or secondary role in progression for several disease states and considered how evidence of complement involvement might inform management. Participating patients with various complement-mediated diseases and caregivers described concerns related to life planning, implications surrounding genetic testing, and the need for inclusive implementation of effective novel therapies into clinical practice. The value of biomarkers in monitoring disease course and the role of the glomerular microenvironment in complement response were examined, and key gaps in knowledge and research priorities were identified.

C3d-Targeted factor H inhibits tissue complement in disease models and reduces glomerular injury without affecting circulating complement

Complement-mediated diseases can be treated using systemic inhibitors. However, complement components are abundant in circulation, affecting systemic inhibitors' exposure and efficacy. Furthermore, because of complement's essential role in immunity, systemic treatments raise infection risk in patients. To address these challenges, we developed antibody fusion proteins combining the alternative-pathway complement inhibitor factor H (fH1-5) with an anti-C3d monoclonal antibody (C3d-mAb-2fH). Because C3d is deposited at sites of complement activity, this molecule localizes to tissue complement while minimizing circulating complement engagement. These fusion proteins bind to deposited complement in diseased human skin sections and localize to activated complement in a primate skin injury model. We further explored the pharmacology of C3d-mAb-2fH proteins in rodent models with robust tissue complement activation. Doses of C3d-mAb-2fH >1 mg/kg achieved >75% tissue complement inhibition in mouse and rat injury models while avoiding circulating complement blockade. Glomerular-specific complement inhibition reduced proteinuria and preserved podocyte foot-process architecture in rat membranous nephropathy, indicating disease-modifying efficacy. These data indicate that targeting local tissue complement results in durable and efficacious complement blockade in skin and kidney while avoiding systemic inhibition, suggesting broad applicability of this approach in treating a range of complement-mediated diseases.

Complement activation and effector pathways in membranous nephropathy

Complement activation has long been recognized as a central feature of membranous nephropathy (MN). Evidence for its role has been derived from the detection of complement products in biopsy tissue and urine from patients with MN and from mechanistic studies primarily based on the passive Heymann nephritis model. Only recently, more detailed insights into the exact mechanisms of complement activation and effector pathways have been gained from patient data, animal models, and in vitro models based on specific target antigens relevant to the human disease. These data are of clinical relevance, as they parallel the recent development of numerous specific complement therapeutics for clinical use. Despite efficient B-cell depletion, many patients with MN achieve only partial remission of proteinuria, which may be explained by the persistence of subepithelial immune complexes and ongoing complement-mediated podocyte injury. Targeting complement, therefore, represents an attractive adjunct treatment for MN, but it will need to be tailored to the specific complement pathways relevant to MN. This review summarizes the different lines of evidence for a central role of complement in MN and for the relevance of distinct complement activation and effector pathways, with a focus on recent developments.

Metalloporphyrins Reduce Proteinuria in Podocyte Immune Injury: The Role of Metal and Porphyrin Moieties

Depending on their central metal atom, metalloporphyrins (MPs) can attenuate or exacerbate the severity of immune-mediated kidney injury, and this has been attributed to the induction or inhibition of heme oxygenase (HO) activity, particularly the inducible isoform (HO-1) of this enzyme. The role of central metal or porphyrin moieties in determining the efficacy of MPs to attenuate injury, as well as mechanisms underlying this effect, have not been assessed. Using an antibody-mediated complement-dependent model of injury directed against rat visceral glomerular epithelial cells (podocytes) and two MPs (FePPIX, CoPPIX) that induce both HO-1 expression and HO enzymatic activity in vivo but differ in their chelated metal, we assessed their efficacy in reducing albuminuria. Podocyte injury was induced using rabbit immune serum raised against the rat podocyte antigen, Fx1A, and containing an anti-Fx1A antibody that activates complement at sites of binding. FePPIX or CoPPIX were injected intraperitoneally (5 mg/kg) 24 h before administration of the anti-Fx1A serum and on days 1, 3, 6, and 10 thereafter. Upon completion of urine collection on day 14, the kidney cortex was obtained for histopathology and isolation of glomeruli, from which total protein extracts were obtained. Target proteins were analyzed by capillary-based separation and immunodetection (Western blot analysis). Both MPs had comparable efficacy in reducing albuminuria in males, but the efficacy of CoPPIX was superior in female rats. The metal-free protoporphyrin, PPIX, had minimal or no effect on urine albumin excretion. CoPPIX was also the most potent MP in inducing glomerular HO-1, reducing complement deposition, and preserving the expression of the complement regulatory protein (CRP) CD55 but not that of CD59, the expression of which was reduced by both MPs. These observations demonstrate that the metal moiety of HO-1-inducing MPs plays an important role in reducing proteinuria via mechanisms involving reduced complement deposition and independently of an effect on CRPs.

The role of the complement system in primary membranous nephropathy: A narrative review in the era of new therapeutic targets

Primary membranous nephropathy (MN) is an important cause of nephrotic syndrome and chronic kidney disease (CKD) in the adult population. Although the discovery of different autoantibodies against glomerular/podocytic antigens have highlighted the role of B cells in the pathogenesis of MN, suboptimal response or even resistance to B cell-directed therapies occurs, suggesting that other pathophysiological mechanisms are involved in mediating podocyte injury. The complement system plays an important role in the innate immune response to infection, and dysregulation of the complement system has been observed in various kidney diseases. There is compelling evidence of complement cascade activation in primary MN, with the mannose-binding lectin (MBL) and alternative pathways particularly implicated. With appropriate validation, assays of complements and associated activation products could hold promise as adjunctive tools for non-invasive disease monitoring and prognostication. While there is growing interest to target the complement system in MN, there is concern regarding the risk of infection due to encapsulated organisms and high treatment costs, highlighting the need for clinical trials to identify patients most likely to benefit from complement-directed therapies.

Altered glycosylation of IgG4 promotes lectin complement pathway activation in anti-PLA2R1-associated membranous nephropathy

Primary membranous nephropathy (pMN) is a leading cause of nephrotic syndrome in adults. In most cases, this autoimmune kidney disease is associated with autoantibodies against the M-type phospholipase A2 receptor (PLA2R1) expressed on kidney podocytes, but the mechanisms leading to glomerular damage remain elusive. Here, we developed a cell culture model using human podocytes and found that anti-PLA2R1-positive pMN patient sera or isolated IgG4, but not IgG4-depleted sera, induced proteolysis of the 2 essential podocyte proteins synaptopodin and NEPH1 in the presence of complement, resulting in perturbations of the podocyte cytoskeleton. Specific blockade of the lectin pathway prevented degradation of synaptopodin and NEPH1. Anti-PLA2R1 IgG4 directly bound mannose-binding lectin in a glycosylation-dependent manner. In a cohort of pMN patients, we identified increased levels of galactose-deficient IgG4, which correlated with anti-PLA2R1 titers and podocyte damage induced by patient sera. Assembly of the terminal C5b-9 complement complex and activation of the complement receptors C3aR1 or C5aR1 were required to induce proteolysis of synaptopodin and NEPH1 by 2 distinct proteolytic pathways mediated by cysteine and aspartic proteinases, respectively. Together, these results demonstrated a mechanism by which aberrantly glycosylated IgG4 activated the lectin pathway and induced podocyte injury in primary membranous nephropathy.

Urinary Markers of Podocyte Dysfunction in Chronic Glomerulonephritis

Chronic glomerulonephritis (CGN) is a disease with a steady progressive course that involves the development of nephrosclerosis, which is especially evident in clinical courses with incidences of high proteinuria (PU). Currently, proteinuria is considered the main laboratory feature (sign) of CGN activity and progression because proteinuria is closely related to the process of tubulointerstitial fibrosis, which is correlated with the grade of renal insufficiency. The injury to podocytes, which are key components of the filtration barrier, plays a central role in proteinuria development. The detachment of podocytes from the glomerular basement membrane leading to podocytopenia is suggested to induce glomerulosclerosis and hyalinosis with obliteration of capillary loops and the progression of chronic kidney disease. Urinary markers of podocyte dysfunction could serve as useful tools while monitoring the activity and prognosis of CGN. In this chapter, the most important mechanisms of podocyte loss and urinary markers of this process are discussed.

Morphological analyses of nephrin expression in progressive glomerulonephropathy of common marmosets

In this study, we focused on nephrin, one of the key molecules within the slit diaphragm of podocytes, as although there have been reports on its expression in humans and rats, their presence in common marmosets has not been reported. We investigated nephrin expression and changes in glomeruli, depending on the development of spontaneous progressive glomerulonephropathy in common marmosets. Nineteen common marmosets at two to ten years of age were evaluated. The kidney was examined by microscopy with hematoxylin and eosin and immunohistochemical staining for nephrin. The lesions were classified into three grades according to a renal lesion grading system reported previously. The nephrin-positive area was measured by morphometric analysis, and the nephrin-positive ratio was calculated. Nephrin expression was observed along the glomerular capillary loop in a continuous linear pattern in renal lesion grades 0 to 2 and either discontinuous linear or coarse granular pattern in grade 3. Nephrin expression tended to decrease significantly depending on the grade of renal lesions. Alteration in nephrin expression has been suggested to play an important role in the progression of renal lesions.

Complement and Complement Targeting Therapies in Glomerular Diseases

The complement cascade is part of the innate immune system whose actions protect hosts from pathogens. Recent research shows complement involvement in a wide spectrum of renal disease pathogenesis including antibody-related glomerulopathies and non-antibody-mediated kidney diseases, such as C3 glomerular disease, atypical hemolytic uremic syndrome, and focal segmental glomerulosclerosis. A pivotal role in renal pathogenesis makes targeting complement activation an attractive therapeutic strategy. Over the last decade, a growing number of anti-complement agents have been developed; some are approved for clinical use and many others are in the pipeline. Herein, we review the pathways of complement activation and regulation, illustrate its role instigating or amplifying glomerular injury, and discuss the most promising novel complement-targeting therapies.

Podocyte-Specific Sialylation-Deficient Mice Serve as a Model for Human FSGS

BACKGROUND

The etiology of steroid-resistant nephrotic syndrome, which manifests as FSGS, is not completely understood. Aberrant glycosylation is an often underestimated factor for pathologic processes, and structural changes in the glomerular endothelial glycocalyx have been correlated with models of nephrotic syndrome. Glycans are frequently capped by sialic acid (Sia), and sialylation's crucial role for kidney function is well known. Human podocytes are highly sialylated; however, sialylation's role in podocyte homeostasis remains unclear.

METHODS

We generated a podocyte-specific sialylation-deficient mouse model (PCmas^-/- ) by targeting CMP-Sia synthetase, and used histologic and ultrastructural analysis to decipher the phenotype. We applied CRISPR/Cas9 technology to generate immortalized sialylation-deficient podocytes (asialo-podocytes) for functional studies.

RESULTS

Progressive loss of sialylation in PCmas^-/- mice resulted in onset of proteinuria around postnatal day 28, accompanied by foot process effacement and loss of slit diaphragms. Podocyte injury led to severe glomerular defects, including expanded capillary lumen, mesangial hypercellularity, synechiae formation, and podocyte loss. In vivo, loss of sialylation resulted in mislocalization of slit diaphragm components, whereas podocalyxin localization was preserved. In vitro, asialo-podocytes were viable, able to proliferate and differentiate, but showed impaired adhesion to collagen IV.

CONCLUSIONS

Loss of cell-surface sialylation in mice resulted in disturbance of podocyte homeostasis and FSGS development. Impaired podocyte adhesion to the glomerular basement membrane most likely contributed to disease development. Our data support the notion that loss of sialylation might be part of the complex process causing FSGS. Sialylation, such as through a Sia supplementation therapy, might provide a new therapeutic strategy to cure or delay FSGS and potentially other glomerulopathies.

Upregulated microRNAs in membranous glomerulonephropathy are associated with significant downregulation of IL6 and MYC mRNAs

Membranous glomerulonephropathy (MGN) is a glomerulopathy characterized by subepithelial deposits of immune complexes on the extracapillary side of the glomerular basement membrane. Insertion of C5b-9 (complement membrane-attack complex) into the membrane leads to functional impairment of the glomerular capillary wall. Knowledge of the molecular pathogenesis of MGN is actually scanty. MicroRNA (miRNA) profiling in MGN and unaffected tissues was performed by TaqMan Low-Density Arrays. Expression of miRNAs and miRNA targets was evaluated in Real-Time polymerase chain reaction (PCR). In vitro transient silencing of miRNAs was achieved through transfection with miRNA inhibitors. Ten miRNAs (let-7a-5p, let-7b-5p, let-7c-5p, let-7d-5p, miR-107, miR-129-3p, miR-423-5p, miR-516-3p, miR-532-3p, and miR-1275) were differentially expressed (DE) in MGN biopsies compared to unaffected controls. Interleukin 6 (IL6) and MYC messenger RNAs (mRNAs; targets of DE miRNAs) were significantly downregulated in biopsies from MGN patients, and upregulated in A498 cells following let-7a-5p or let-7c-5p transient silencing. Gene ontology analysis showed that DE miRNAs regulate pathways associated with MGN pathogenesis, including cell cycle, proliferation, and apoptosis. A significant correlation between DE miRNAs and mRNAs and clinical parameters (i.e., antiphospholipid antibodies, serum creatinine, estimated glomerular filtration, proteinuria, and serum cholesterol) has been detected. Based on our data, we propose that DE miRNAs and their downstream network may be involved in MGN pathogenesis and could be considered as potential diagnostic biomarkers of MGN.

Membranous nephropathy-one morphologic pattern with different diseases

Since the discovery of the phospholipase A₂ receptor 1 (PLA₂R1) and thrombospondin type-1 domain-containing 7A (THSD7A) as endogenous antigens involved in the development of membranous nephropathy (MN) in over 80% of adult patients, substantial progress in the diagnosis, prognosis, and therapy of MN has been made. In most cases of patients with MN, it is now possible to specifically define the responsible pathogenic mechanisms of disease and make a diagnosis even without a renal biopsy. Moreover, the presence of antibodies in the blood and the detection of the antigens in renal biopsies allow the definite diagnosis without the morphologic uncertainties, which now still apply for only about 20% of all renal biopsies showing MN. The discovery that the expression of THSD7A in malignant tumors might serve as the site of primary antigen recognition for the immune system to start MN might lead to a better understanding of not only tumor-associated MN, which accounts for up to 10% of all patients with MN, but also of the pathomechanisms relevant for MN development in general.

Vanillic Acid Ameliorates Cationic Bovine Serum Albumin Induced Immune Complex Glomerulonephritis in BALB/c Mice

Preclinical Research Vanillic acid (VA) is a dihydroxybenzoic acid derivative widely used as a flavoring agent. It has chemopreventive effects on experimentally-induced carcinogenesis and in ulcerative colitis. The object of the present study was to investigate the effects of VA, alone and in combination with methylprednisolone (MP), on cationic bovine serum albumin (cBSA induced immune-complex glomerulonephritis in female BALB/c mice. Pre-immunization was carried out with cBSA in BALB/c mice and repeated (cBSA, 13 mg/kg, 3 times/week, i.v.) for 6 weeks to induce glomerulonephritis which was confirmed by the presence of severe proteinuria. The effect of VA (50, 100, and 200 mg/kg, p.o.) and its combination with MP (12.5 mg/kg, p.o.) was assessed in the nephrotic disease model. Treatment with VA decreased inflammatory nephrotic injury as evidenced by decreased proteinuria, serum creatinine, blood urea nitrogen, serum IgG1 and TNF-α levels. Co-administration of VA with MP showed an improvement in the immunohistochemistry of glomerular nephrin and podocin. The present results indicate that VA has a nephroprotective effect in the management of autoimmune nephritis. Drug Dev Res 77 : 171-179, 2016.   © 2016 Wiley Periodicals, Inc.

Recent advances in the understanding of immune-mediated nephrotic syndrome: diagnostic and prognostic implications

Glomerular diseases with severe defects in glomerular permeability give rise to heavy proteinuria and can present as nephrotic syndrome. There are many different causes of the nephrotic syndrome and a renal biopsy is nearly always needed to elucidate the underlying disease. During the last decade, substantial advances have occurred in the understanding of the pathophysiological mechanisms involved in immune-mediated glomerular diseases. Here, we review the diagnostic and prognostic implications of recent progress on the understanding of membranous nephropathy, minimal change disease, focal segmental glomerulosclerosis, amyloidosis, IgA nephropathy and membranoproliferative glomerulonephritis.

Molecules Great and Small: The Complement System

The complement cascade, traditionally considered an effector arm of innate immunity required for host defense against pathogens, is now recognized as a crucial pathogenic mediator of various kidney diseases. Complement components produced by the liver and circulating in the plasma undergo activation through the classical and/or mannose-binding lectin pathways to mediate anti-HLA antibody-initiated kidney transplant rejection and autoantibody-initiated GN, the latter including membranous glomerulopathy, antiglomerular basement membrane disease, and lupus nephritis. Inherited and/or acquired abnormalities of complement regulators, which requisitely limit restraint on alternative pathway complement activation, contribute to the pathogenesis of the C3 nephropathies and atypical hemolytic uremic syndrome. Increasing evidence links complement produced by endothelial cells and/or tubular cells to the pathogenesis of kidney ischemia-reperfusion injury and progressive kidney fibrosis. Data emerging since the mid-2000s additionally show that immune cells, including T cells and antigen-presenting cells, produce alternative pathway complement components during cognate interactions. The subsequent local complement activation yields production of the anaphylatoxins C3a and C5a, which bind to their respective receptors (C3aR and C5aR) on both partners to augment effector T-cell proliferation and survival, while simultaneously inhibiting regulatory T-cell induction and function. This immune cell-derived complement enhances pathogenic alloreactive T-cell immunity that results in transplant rejection and likely contributes to the pathogenesis of other T cell-mediated kidney diseases. C5a/C5aR ligations on neutrophils have additionally been shown to contribute to vascular inflammation in models of ANCA-mediated renal vasculitis. New translational immunology efforts along with the development of pharmacologic agents that block human complement components and receptors now permit testing of the intriguing concept that targeting complement in patients with an assortment of kidney diseases has the potential to abrogate disease progression and improve patient health.

Membranous nephropathy: a review on the pathogenesis, diagnosis, and treatment

In adults, membranous nephropathy (MN) is a major cause of nephrotic syndrome. However, the etiology of approximately 75% of MN cases is idiopathic. Secondary causes of MN are autoimmune diseases, infection, drugs, and malignancy. The pathogenesis of MN involves formation of immune complex in subepithelial sites, but the definite mechanism is still unknown. There are three hypotheses about the formation of immune complex, including preformed immune complex, in situ immune-complex formation, and autoantibody against podocyte membrane antigen. The formation of immune complex initiates complement activation, which subsequently leads to glomerular damage. Recently, the antiphospholipase A2 receptor antibody was found to be associated with idiopathic MN. This finding may be useful in the diagnosis and prognosis of MN. The current treatment includes best supportive care, which consists of the use of angiotensin-converting enzyme inhibitors/angiotensin II receptor blockers, lipid-lowering agents, and optimal control of blood pressure. Immunosuppressive agents should be used for patients who suffer from refractory proteinuria or complications associated with nephrotic syndrome. Existing evidence supports the use of a combination of steroid and alkylating agents. This article reviews the epidemiology, pathogenesis, diagnosis, and the treatment of MN.

Na+/H+ exchanger-1 reduces podocyte injury caused by endoplasmic reticulum stress via autophagy activation

Podocyte injury has a critical role in the pathogenesis of proteinuria. Induction of endoplasmic reticulum (ER) stress is thought to lead to podocyte injury; however, no effective strategy for reducing ER stress-induced injury has been identified. We investigated specific mechanisms for reducing podocyte injury caused by ER stress. We found that the induction of ER stress in podocytes was related to cytoskeleton injury and increased proteinuria, which was associated with autophagy activation and downregulation of Na(+)/H(+) exchanger-1 (NHE-1) in the rat model of passive Heymann nephritis. Using mouse podocyte cells (MPCs), we showed that ER stress could lead to podocyte injury accompanied by autophagy activation, and the disturbance of autophagy aggravated cytoskeleton loss under conditions of ER stress. The balance between autophagy activation and ER stress was critical to podocyte survival, in which the efficiency of autophagy could have a pivotal role. Strikingly, the overexpression and small interfering RNA knockdown of NHE-1 results suggested that NHE-1 exerts a protective effect by reducing the loss of synaptopodin in MPCs exposed to ER stress. This protective mechanism involves NHE-1 activation of autophagy via the PI3K/Akt pathway to reduce ER stress injury in podocytes. This mechanism may provide a new pathway to prevent podocyte injury.

The role of complement in membranous nephropathy

Membranous nephropathy (MN) describes a histopathologic pattern of injury marked by glomerular subepithelial immune deposits and collectively represents one of the most common causes of adult nephrotic syndrome. Studies in Heymann nephritis, an experimental model of MN, have established a paradigm in which these deposits locally activate complement to cause podocyte injury, culminating in cytoskeletal reorganization, loss of slit diaphragms, and proteinuria. There is much circumstantial evidence for a prominent role of complement in human MN because C3 and C5b-9 are found consistently within immune deposits. Secondary MN often shows the additional presence of C1q, implicating the classic pathway of complement activation. Primary MN, however, is IgG4-predominant and IgG4 is considered incapable of binding C1q and activating the complement pathway. Recent studies have identified the M-type phospholipase A2 receptor (PLA2R) as the major target antigen in primary MN. Early evidence hints that IgG4 anti-PLA2R autoantibodies can bind mannan-binding lectin and activate the lectin complement pathway. The identification of anti-PLA2R antibodies as likely participants in the pathogenesis of disease will allow focused investigation into the role of complement in MN. Definitive therapy for MN is immunosuppression, although future therapeutic agents that specifically target complement activation may represent an effective temporizing measure to forestall further glomerular injury.

An update on the pathomechanisms and future therapies of Alport syndrome

Alport Syndrome (AS) is an inherited progressive disease that is caused by mutations of the genes encoding the key collagen chains, α3, α4, and α5, which are necessary for the composition of collagen type IV to form a robust glomerular basement membrane (GBM), capable of withstanding the significant biomechanical strain to which the glomerulus is subjected. Progressive loss of the filtration barrier allows excessive proteinuria, which ultimately leads to end-stage kidney disease (ESKD). The evidence for a beneficial renoprotective effect of renin-angiotensin-aldosterone system (RAAS) blockade by angiotensin-converting enzyme (ACE) inhibition and/or angiotensin receptor blockers (ARBs) is well established in AS and recent evidence has shown that it can significantly delay the time to onset of renal replacement therapy and ESKD. Future potential treatments of AS disease progression are evaluated in this review.

Upregulation of c-mip is closely related to podocyte dysfunction in membranous nephropathy

Membranous nephropathy is a glomerular disease typified by a nephrotic syndrome without infiltration of inflammatory cells or proliferation of resident cells. Although the cause of the disease is unknown, the primary pathology involves the generation of autoantibodies against antigen targets on the surface of podocytes. The mechanisms of nephrotic proteinuria, which reflect a profound podocyte dysfunction, remain unclear. We previously found a new gene, c-mip (c-maf-inducing protein), that was associated with the pathophysiology of idiopathic nephrotic syndrome. Here we found that c-mip was not detected in the glomeruli of rats with passive-type Heymann nephritis given a single dose of anti-megalin polyclonal antibody, yet immune complexes were readily present, but without triggering of proteinuria. Rats reinjected with anti-megalin develop heavy proteinuria a few days later, concomitant with c-mip overproduction in podocytes. This overexpression was associated with the downregulation of synaptopodin in patients with membranous nephropathy, rats with passive Heymann nephritis, and c-mip transgenic mice, while the abundance of death-associated protein kinase and integrin-linked kinase was increased. Cyclosporine treatment significantly reduced proteinuria in rats with passive Heymann nephritis, concomitant with downregulation of c-mip in podocytes. Thus, c-mip has an active role in the podocyte disorders of membranous nephropathy.

Complement in glomerular disease

The role of the complement system in renal disease has long been recognized, but there have been major advances in our understanding of its role over the past decade. Complement plays a critical role not only in host's defense against infection and preventing damage to "self" tissues but also mediates tissue injury, both in the glomerulus and tubulointerstitium. Although injury may originate in the glomerulus, resulting proteinuria and complement activation within the tubular lumen may lead to tubulointerstitial damage and progressive renal disease. Recent advances in our understanding of the mechanisms by which complement mediates renal injury have led to the development of promising strategies with which complement may be targeted to prevent renal injury and its associated complications.

Podocyte main slit diaphragm proteins, nephrin and podocin, are affected at early stages of lupus nephritis and correlate with disease histology

Renal podocytes and their slit diaphragms ensure the integrity of the renal basement membrane that forms the barrier to urinary protein loss. A putative disruption of the slit diaphragm and its main protein components, nephrin and podocin, may be implicated in the pathogenesis of lupus nephritis (LN). We studied the glomerular protein expression of nephrin and podocin in NZB/W LN mice by Western blot and immunofluorescence; mRNA levels were measured by real-time PCR. Human kidney biopsies of class II (n = 5), IV (n = 4), V (n = 7) LN were evaluated for nephrin expression by immunohistochemistry. Glomerular protein expression of nephrin and podocin were significantly reduced in NZB/W LN, starting from the earlier stages (mild mesangial LN) and becoming pronounced at advanced histological forms (focal and diffuse proliferative LN). Nephrin and podocin mRNA levels were substantially decreased in diffuse proliferative disease. Decreased expression of both proteins correlated with electron microscopy findings of distorted slit diaphragms. In patients with LN, nephrin was decreased particularly in diffuse proliferative LN. The main slit diaphragm proteins, nephrin and podocin, are affected from the earlier stages of LN and their expression correlates with disease histology. Our findings suggest a novel role of podocytes and their structures in immune-mediated nephritis.

Nephrin-signature molecule of the glomerular podocyte?

In recent years there has been an explosion of interest in the glomerular podocyte, which plays a central role in control of glomerular filtration. A host of new molecules have been identified as playing essential roles in the maintenance of podocyte integrity in both humans and mouse models. Of all of these, arguably the most pivotal is nephrin, a transmembrane receptor molecule located at the specialized podocyte cell-cell junction, termed the slit diaphragm. Mutations in this gene cause the most severe form of congenital nephrotic syndrome, and many interacting proteins have now been described to form a large multiprotein complex with complex dynamics. There is little evidence of functional nephrin expression outside the glomerulus, and there are accumulating data that nephrin is essential for the unique properties of podocyte biology. Utilizing a powerful human cell culture model, comparing wild-type with nephrin-null podocytes, we can show that several crucial functional properties of podocytes depend on nephrin, including insulin responsiveness and cytoskeletal reorganization. Thus, it is reasoned that nephrin is a signature molecule required to define distinct podocyte characteristics.

Experimental Models of Membranous Nephropathy

Membranous nephropathy (MN) is one of the commonest glomerular diseases, typically presenting in older males with nephrotic syndrome. The development and characterization of animal models of MN, in particular, the passive Heymann nephritis model (PHN), has greatly advanced our understanding of this disease. In this review we discuss the different animal models of human MN that are available, with an emphasis on the PHN model, including technical issues, the typical disease course and its application to human disease.

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.

De novo expression of podocyte proteins in parietal epithelial cells during experimental glomerular disease

Studies have shown that certain cells of the glomerular tuft begin to express proteins considered unique to other cell types upon injury. Little is known about the response of parietal epithelial cells (PEC) to injury. To determine whether PECs change their phenotype upon injury to also express proteins traditionally considered podocyte specific, the following four models of glomerular disease were studied: the transforming growth factor (TGF)-beta1 transgenic mouse model of global glomerulosclerosis, the adriamycin model of focal segmental glomerulosclerosis (FSGS), the anti-glomerular basement membrane (GBM) model of crescentic glomerulonephritis, and the passive Heymann nephritis model of membranous nephropathy. Double immunostaining was performed with antibodies to podocyte-specific proteins (synaptopodin and Wilms' tumor 1) and antibodies to PEC specific proteins (paired box gene 8 and claudin-1). No double staining was detected in normal mice. In contrast, the results showed a statistical increase in the number of cells attached to Bowman basement membrane that were double-positive for both podocyte/PEC proteins in TGF-beta1 transgenic, anti-GBM, and membranous animals. Double-positive cells for both podocyte and PEC proteins were also statistically increased in the glomerular tuft in TGF-beta1 transgenic, anti-GBM, and FSGS mice. These results are consistent with glomerular cells coexpressing podocyte and PEC proteins in experimental glomerular disease, but not under normal circumstances.

Nephrin deficiency activates NF-kappaB and promotes glomerular injury

Increasing evidence implicates activation of NF-kappaB in a variety of glomerular diseases, but the mechanisms involved are unknown. Here, upregulation of NF-kappaB in the podocytes of transgenic mice resulted in glomerulosclerosis and proteinuria. Absence of the podocyte protein nephrin resulted in NF-kappaB activation, suggesting that nephrin negatively regulates the NF-kappaB pathway. Signal transduction assays supported a functional relationship between nephrin and NF-kappaB and suggested the involvement of atypical protein kinase C (aPKCzeta/lambda/iota) as an intermediary. We propose that disruption of the slit diaphragm leads to activation of NF-kappaB; subsequent upregulation of NF-kappaB-driven genes results in glomerular damage mediated by NF-kappaB-dependent pathways. In summary, nephrin may normally limit NF-kappaB activity in the podocyte, suggesting a mechanism by which it might discourage the evolution of glomerular disease.

Microarray and bioinformatics analysis of gene expression in experimental membranous nephropathy

BACKGROUND

Passive Heymann nephritis (PHN), the best characterized animal model of experimental membranous nephropathy, is characterized by subepithelial immune deposits, podocyte foot processes effacement and massive proteinuria beginning 4 days following disease induction. Although single genes involved in PHN have been studied, no whole genome-wide expression analysis of kidney tissue has been performed.

METHODS

Microarray analysis was performed to identify gene expression changes in PHN rat kidneys during the onset of proteinuria.

RESULTS

Our results showed that 234 transcripts were differentially expressed in diseased animals compared to controls. Genes exclusively upregulated in diseased animals were mainly required for cell structure and motility, immunity and defense, cell cycle, and developmental processes. The single most increased gene was transgelin (Tagln) showing a 70-fold upregulation in animals with PHN. Protein-protein interaction analysis revealed the following four processes of major relevance in disease manifestation: (i) DNA damage and repair; (ii) changes in the extracellular matrix; (iii) deregulation of cytokines and growth factors, as well as (iv) rearrangements of the cytoskeleton.

CONCLUSION

We show for the first time the complex interplay between multiple different genes in experimental membranous nephropathy, supporting a role for genomic approaches to better understanding and defining specific disease processes.

Inducible rodent models of acquired podocyte diseases

Glomerular diseases remain the leading cause of chronic and end-stage kidney disease. Significant advances in our understanding of human glomerular diseases have been enabled by the development and better characterization of animal models. Diseases of the glomerular epithelial cells (podocytes) account for the majority of proteinuric diseases. Rodents have been extensively used experimentally to better define mechanisms of disease induction and progression, as well as to identify potential targets and therapies. The development of podocyte-specific genetically modified mice has energized the research field to better understand which animal models are appropriate to study acquired podocyte diseases. In this review we discuss inducible experimental models of acquired nondiabetic podocyte diseases in rodents, namely, passive Heymann nephritis, puromycin aminonucleoside nephrosis, adriamycin nephrosis, liopolysaccharide, crescentic glomerulonephritis, and protein overload nephropathy models. Details are given on the model backgrounds, how to induce each model, the interpretations of the data, and the benefits and shortcomings of each. Genetic rodent models of podocyte injury are excluded.

Preeclamptic sera induce nephrin shedding from podocytes through endothelin-1 release by endothelial glomerular cells

In preeclampsia (PE), proteinuria has been associated with a reduced expression of nephrin by podocytes. In the present study, we investigated in vitro on human cultured podocytes the mechanism responsible for nephrin loss in PE. Sera from patients with PE did not directly downregulate the expression of nephrin. In contrast, conditioned medium obtained from glomerular endothelial cells incubated with PE sera induced loss of nephrin and synaptopodin, but not of podocin, from podocytes. Nephrin loss was related to a rapid shedding of the protein from the cell surface due to cleavage of its extracellular domain by proteases and to cytoskeleton redistribution. The absence of nephrin mRNA downregulation together with nephrin reexpression within 24 h confirm that the loss of nephrin was not related to a reduced synthesis. Studies with an endothelin-1 (ET-1) receptor antagonist that abrogated the loss of nephrin triggered by glomerular endothelial conditioned medium of PE sera indicated that ET-1 was the main effector of nephrin loss. Indeed, ET-1 was synthesized and released from glomerular endothelial cells when incubated with PE sera, and recombinant ET-1 triggered nephrin shedding from podocytes. Moreover, VEGF blockade induced ET-1 release from endothelial cells, and in turn the conditioned medium obtained triggered nephrin loss. In conclusion, the present study identifies a potential mechanism of nephrin loss in PE that may link endothelial injury with enhanced glomerular permeability.

Target antigens and nephritogenic antibodies in membranous nephropathy: of rats and men

Membranous nephropathy, a disease characterized by an accumulation of immune deposits on the outer aspect of the glomerular basement membrane, is the most common cause of idiopathic nephrotic syndrome in white adults. In the rat model of Heymann nephritis, the target antigen of antibodies is megalin, a multiligand receptor expressed at the podocyte cell surface. This review summarizes key findings provided by this experimental model and by our discovery of neutral endopeptidase being the alloantigen involved in neonatal cases of membranous nephropathy. We discuss the role of alloimmunization as a new mechanism of renal disease and the approach that we use to identify new podocyte antigens. We also summarize current knowledge on the mechanism of proteinuria, with special emphasis on the role of complement. In conclusion, substantial progresses have been made in understanding molecular mechanisms of membranous nephropathy, which should lead to novel therapeutic approaches.

[Lupus membranous nephropathy]

The systemic lupus erythematosus associated renal hystopathological complexity and its clinical translation, are still a diagnostic challenge with therapeutical implications which, however, include new options in the last few years within the immunosupression compass. The new insights elicited by research work attempt to give some light on renal biopsy performance, its relationship with the arrogated clinical spectrum, its prognosis and on the lupus nephropathy new treatments currently under ongoing clinical trials, some of them showing encouraging results. The lupus membranous nephropathy, recognized as an anatomopathological entity more than 4 decades ago, means a specific pattern in the whole renal lupus histologycal range and, in many aspects, an etiopathogenic enigma.

Podocyte injury and glomerulosclerosis in hyperhomocysteinemic rats

BACKGROUND/AIMS

We previously reported that increase in plasma homocysteine (Hcys) levels by a 6-week methionine treatment produced remarkable glomerular injury. However, the mechanism by which hyperhomocysteinemia (hHcys) produces glomerular injury remains unknown. The present study was to observe when glomerular injury happens during hHcys and to explore the possible role of podocyte injury in the progression of glomerulosclerosis associated with hHcys.

METHODS

Uninephrectomized Sprague-Dawley rats treated with methionine were used to examine the time course of glomerular injury induced by hHcys.

RESULTS

Creatinine clearance was not different until rats were treated with methionine for 6 weeks, although plasma Hcys levels significantly increased at the 1st week of methionine treatment. However, urinary albumin excretion increased at the 2nd week of methionine treatment. Morphological examinations showed that mesangial expansion occurred at the 2nd week and podocyte effacement was also observed as processed glomerular damage during hHcys. Immunofluorescence analyses demonstrated that podocin and nephrin expressions were reduced, while alpha-actinin-4 increased during hHcys.

CONCLUSIONS

Increased plasma Hcys level is an important pathogenic factor resulting in glomerular injury even in the very early time of hHcys. These pathogenic effects of Hcys are associated with podocyte injury and changed expression and distribution of podocyte-associated proteins.

New insights into the pathogenesis of membranous glomerulonephritis

PURPOSE OF REVIEW

Membranous nephropathy is one of the most common glomerulopathies. Current treatments are entirely empirical, and concept-driven therapies are dramatically lacking. This review focuses on new pathophysiologic aspects of the disease, with special emphasis on the antigenic targets of pathogenic antibodies.

RECENT FINDINGS

Neutral endopeptidase - a podocyte antigen that can digest biologically active peptides - was recently identified as the target antigen of antibodies deposited in the subepithelial space of glomeruli in a subset of patients with antenatal membranous nephropathy. The mothers became immunized because they are deficient in neutral endopeptidase due to truncating mutations in the gene. Membranous nephropathy could be transferred to the rabbit by injection of mothers' immunoglobulin. Development of the renal disease was associated with anti-neutral endopeptidase IgG1.

SUMMARY

Membranous nephropathy most likely is a heterogeneous disease, although a common denominator may be that podocytes provide antigenic targets for in-situ formation of glomerular immune deposits. Identification of neutral endopeptidase and additional (podocyte) antigens and characterization of their epitopes should make it possible to design more effective and better tolerated therapies. Fetomaternal alloimmunization is a novel mechanism of renal disease that may apply to other organs as well.

Calpain activation and secretion promote glomerular injury in experimental glomerulonephritis: evidence from calpastatin-transgenic mice

Glomerular injury and albuminuria in acute glomerulonephritis are related to the severity of inflammatory process. Calpain, a calcium-activated cysteine protease, has been shown to participate in the development of the inflammatory process. Therefore, for determination of the role of calpain in the pathophysiology of acute glomerulonephritis, transgenic mice that constitutively express high levels of calpastatin, a calpain-specific inhibitor protein, were generated. Wild-type mice that were subjected to anti-glomerular basement membrane nephritis exhibited elevated levels of calpain activity in kidney cortex at the heterologous phase of the disease. This was associated with the appearance in urine of calpain activity, which originated potentially from inflammatory cells, abnormal transglomerular passage of plasma proteins, and tubular secretion. In comparison with nephritic wild-type mice, nephritic calpastatin-transgenic mice exhibited limited activation of calpain in kidney cortex and limited secretion of calpain activity in urine. This was associated with less severe glomerular injury (including capillary thrombi and neutrophil activity) and proteinuria. There was a reduction in NF-kappaB activation, suggesting that calpain may participate in inflammatory lesions through NF-kappaB activation. There also was a reduction in nephrin disappearance from the surface of podocytes, indicating that calpain activity would enhance proteinuria by affecting nephrin expression. Exposure of cultured podocytes to calpain decreased nephrin expression, and, conversely, exposure of these cells to calpastatin prevented TNF-alpha from decreasing nephrin expression, demonstrating a role for the secreted form of calpain. Thus, both activation and secretion of calpains participate in the development of immune glomerular injury.

The relationship among nephrin, podocin, CD2AP, and alpha-actinin might not be a true 'interaction' in podocyte

The abnormality of a single podocyte molecule, caused by a single gene mutation, such as NPHS1, NPHS2, CD2AP, and ACTN4, can lead to the hereditary/congenital nephrotic syndromes (NS). Further studies suggested that more than one podocyte molecule were together involved in acquired or experimental NS. However, we do not know much on the relationship among these podocyte molecules, and the molecular response induced by the change of each podocyte protein to the remaining ones. We respectively knockdown the nephrin, podocin, CD2AP, or alpha-actinin-4 mRNA by using reconstructed RNA interference vector--psiRNA-hH1GFPzeo in mouse podocyte clone. The molecular behavior or response was revealed by the quantitative expression both at mRNA and protein levels with RT-PCR and Western blot, and by the molecular distribution detected with confocal microscopy. With nephrin knockdown, only CD2AP increased, whereas podocin showed no change. Contrarily, with podocin or CD2AP knockdown, nephrin decreased, while CD2AP or podocin increased. Nephrin, podocin, or CD2AP knockdown did not change the expression of alpha-actinin-4, whereas alpha-actinin-4 knockdown begetted the reduction of nephrin, and the increment of podocin and CD2AP. The redistributions of nephrin, podocin, and CD2AP were revealed around a predominant nuclear staining compared with the membrane surface staining in the control podocytes. Our data imply that the response between the four podocyte molecules is very complicated and evidently different. There is not always an interaction between podocyte molecules. The normal localization of podocyte molecules would depend on their normal expression quantity and the molecular reactions between them.

Treatment with 1,25-dihydroxyvitamin D3 preserves glomerular slit diaphragm-associated protein expression in experimental glomerulonephritis

In this study, we investigated the effect of 1,25(OH)2D3 on proteinuria and on the alteration of slit diaphragm-associated proteins induced by anti-Thy 1.1 in Wistar rats. Four groups of animals were studied: group I, anti-Thy 1.1 treated rats; group II, anti-Thy1.1 treated group that at day 2, after the onset of overt proteinuria, started the treatment with 1,25(OH)2D3; group III, normal control rats injected with vehicle alone; group IV, rats that received only 1,25(OH)2D3. At day 2, in group I and II, before the administration of 1,25(OH)2D3, protein excretion was significantly increased when compared to controls. Overt proteinuria was maintained until day 14 in group I whereas in group II protein excretion was significantly reduced from day 3 to day 14. Moreover, treatment with 1,25(OH)2D3 abrogated podocytes injury, detected as desmin expression and loss of nephrin and zonula occludens-1 (ZO-1), two slit diaphragm-associated proteins, and glomerular polyanion staining, that were observed in group I. In conclusion, these results suggest that 1,25(OH)2D3 administrated with a therapeutic regiment may revert proteinuria, counteracting glomerular podocyte injury.

Experimental membranous nephropathy redux

Membranous nephropathy (MN) is a common cause of nephrotic syndrome in adults. Active and passive Heymann nephritis (HN) in rats are valuable experimental models because their features so closely resemble human MN. In HN, subepithelial immune deposits form in situ as a result of circulating antibodies. Complement activation leads to assembly of C5b-9 on glomerular epithelial cell (GEC) plasma membranes and is essential for sublethal GEC injury and the onset of proteinuria. This review revisits HN and focuses on areas of substantial progress in recent years. The response of the GEC to sublethal C5b-9 attack is not simply due to disruption of the plasma membrane but is due to the activation of specific signaling pathways. These include activation of protein kinases, phospholipases, cyclooxygenases, transcription factors, growth factors, NADPH oxidase, stress proteins, proteinases, and others. Ultimately, these signals impact on cell metabolic pathways and the structure/function of lipids and key proteins in the cytoskeleton and slit-diaphragm. Some signals affect GEC adversely. Thus C5b-9 induces partial dissolution of the actin cytoskeleton. There is a decline in nephrin expression, reduction in F-actin-bound nephrin, and loss of slit-diaphragm integrity. Other signals, such as endoplasmic reticulum stress, may limit complement-induced injury, or promote recovery. The extent of complement activation and GEC injury is dependent, in part, on complement-regulatory proteins, which act at early or late steps within the complement cascade. Identification of key steps in complement activation, the cellular signaling pathways, and the targets will facilitate therapeutic intervention in reversing GEC injury in human MN.

Possible role of autoantibodies against nephrin in an experimental model of chronic graft-versus-host disease

Nephrin, a product of the NPHS1 gene, is a component of the slit diaphragms that are found between glomerular foot processes and is a crucial element for glomerular filtration barrier. Recently, nephrin has been focused in a number of studies of proteinuria development including various types of acquired glomerular diseases including minimal change nephrotic syndrome and membranous nephropathy. However, the precise role of nephrin in such acquired glomerular diseases is still unknown. To analyse the role of nephrin further, two kinds of anti-nephrin antibodies were raised in the rabbits and applied to an experimental mouse model of chronic graft-versus-host disease, in which (C57BL/10 x DBA/2) F1 mice developed clinically apparent severe proteinuria with significant glomerular lesions 7 weeks after parental DBA/2 cell transfer. Antibody-sandwich ELISA detected anti-nephrin antibodies during week 2 to week 6, with the peak at week 2 or week 4. Colocalization of nephrin and IgG on week 4, week 6, and week 8 was revealed by confocal microscopic analysis, suggesting that in situ immune complex formation with nephrin in glomerular lesion. Taken together, it seems to be suggested nephrin and its autoantibody have a certain role in the development of glomerular lesion in our model mice.

Complement and glomerulonephritis: new insights

PURPOSE OF REVIEW

The last few years have seen a huge increase in our understanding of the role of the complement system and its regulation in glomerular disease. Our aim is to summarize the most important advances in this field.

RECENT FINDINGS

The role of complement in systemic lupus erythematosus continues to be elucidated. Classical pathway components protect from the development of autoimmunity, at least in part, through their role in the clearance of apoptotic cells. In contrast, the alternative pathway plays a direct role in exacerbating glomerular injury. Anti-C1q antibodies are related to activity in lupus nephritis and recent studies have shown that they are directly pathogenic in animal models. Proteinuria, whatever the cause, may lead to tubulointerstitial injury and complement activation adds to this process. In particular, deposition of terminal components of complement in the tubular lumen contributes to interstitial myofibroblast activation. There is increasing evidence for the role of complement regulatory proteins in glomerular injury. In particular, abnormalities of factor H or of CD46 may predispose to atypical haemolytic uraemic syndrome. The control proteins also protect against injury in immune complex glomerulonephritis.

SUMMARY

Advances in our understanding of the role of complement in glomerular injury point to the likely therapeutic benefits of targeting the complement system. Many new drugs are becoming available. Careful dissection of the pro and antiinflammatory effects of the complement system which the experimental models allow will assist in designing directed therapy that will avoid the detrimental effects of nonspecific systemic complement inhibition.

Molecular pathomechanisms of membranous nephropathy: from Heymann nephritis to alloimmunization

Membranous nephropathy (MN), the most common cause of idiopathic nephrotic syndrome in white adults, is characterized by an accumulation of immune deposits on the outer aspect of the glomerular basement membrane. In Heymann nephritis, the rat experimental model for MN, megalin--the target antigen of the nephritogenic antibodies--is expressed on the surface of podocytes, where immune complexes are formed, leading to complement activation and nephrotic-range proteinuria. However, megalin cannot be held responsible for human MN because it has not been found in human podocytes or detected in subepithelial immune deposits in patients with MN. Several potential antigens have been identified in so-called secondary forms of MN, but there is no real proof that these antigens are pathogenic. In a subgroup of infants with antenatal MN, neutral endopeptidase (NEP) has been identified as the first protein target on human podocytes of nephritogenic antibodies. The infants' mothers became immunized during pregnancy against NEP expressed on syncytiotrophoblastic cells because they were NEP deficient as a result of truncating mutations in the MME gene. Severity of neonatal renal disease was determined by the mothers' IgG response that led to the formation of the membrane attack complex of complement in the subepithelial deposits. Alloimmunization against NEP is a novel pathomechanism of MN that might also account for some cases of MN after renal or bone marrow transplantation. Other types of alloimmunization should be investigated in MN but also in other renal and nonrenal diseases, particularly those that affect the pediatric age.

Cellular Response to Injury in Membranous Nephropathy

The pathogenesis of membranous nephropathy (MN) involves in situ formation of subepithelial immune deposits that produce glomerular injury by damaging and/or activating podocytes through complement-dependent processes. C5b-9 formation and insertion into podocyte cell membranes causes glomerular injury in MN. C5b-9 in sublytic quantities stimulates podocytes to produce proteases, oxidants, prostanoids, extracellular matrix components, and cytokines including TGF-beta. C5b-9 also causes alterations of the cytoskeleton that lead to abnormal distribution of slit diaphragm protein and detachment of viable podocytes that are shed into Bowman's space. These events result in disruption of the functional integrity of the glomerular basement membrane and the protein filtration barrier of podocytes with subsequent development of massive proteinuria. Complement components in proteinuric urine also induce tubular epithelial cell injury and mediate progressive interstitial disease in MN. Measurements of urinary C5b-9 or podocyte excretion in the urine may be useful in the diagnosis of MN and as measures of disease activity and response to therapy. Recent studies of cell-cycle proteins and DNA damage in podocytes have clarified why podocytes fail to proliferate in response to C5b-9-mediated injury and podocyte loss in MN, resulting in the development of glomerular sclerosis and renal failure. Improved understanding of the role of complement in the pathogenesis of MN and of the cellular response to C5b-9 attack creates several new opportunities for therapeutic intervention that may benefit patients with MN in the future.

The podocyte: from bench to bedside

The understanding of the molecular structure of the podocyte has increased considerabely in recent 5 years by genetic research in inherited glomerulopathies. Injury to the podocyte often leads to a reorganisation of the slit diaphragm and to foot process effacement, which can be noted in all forms of the nephrotic syndrome. Since the recent progress in our understanding of the molecular biology of the podocyte, it has become obvious that the podocyte is crucial in the pathogenesis of not only genetic but also of many acquired glomerulopathies such as focal segmental glomerulosclerosis, membranous glomerulopathy and diabetic nephropathy. In this review we provide a summary of the recent physiological and molecular insights into the podocyte and of its significance in different glomerulopathies.

Nephrotic plasma alters slit diaphragm-dependent signaling and translocates nephrin, Podocin, and CD2 associated protein in cultured human podocytes

Podocytes are critical in maintaining the filtration barrier of the glomerulus and are dependent on the slit diaphragm (SD) proteins nephrin, podocin, and CD2-associated protein (CD2AP) to function optimally. The effects of normal human plasma and nephrotic plasma on podocytes were tested, focusing particularly on the SD complex. With the use of a conditionally immortalized human podocyte cell line, it first was shown that exposure to normal and non-nephrotic human plasma leads to a concentration of nephrin, podocin, CD2AP, and actin at the cell surface. Next, the effects of plasma from patients with nephrotic conditions to non-nephrotic conditions were compared. When exposed to all nephrotic plasma samples (and a non-human serum control), nephrin podocin and CD2AP assumed a cytoplasmic distribution; nephrin and synaptopodin were selectively downregulated, and the relocation of nephrin induced by nephrotic plasma could be rescued back to the plasma membrane by co-incubation with non-nephrotic plasma. Furthermore, intracellular calcium signaling was altered by nephrotic plasma, which was mediated by tyrosine kinase phosphorylation. With the use of nephrin mutant human cell lines, it was shown that this signaling and translocation response to normal plasma is nephrin dependent. This work demonstrates that nephrotic plasma seems to be deficient in factors that act via the podocyte SD complex, which are essential in maintaining its physiologic function.